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Photochemical CO2-reduction catalyzed by mono- and dinuclear phenanthroline-extended tetramesityl porphyrin complexes† Corinna Matlachowski and Matthias Schwalbe* We here present a comprehensive study on the light-induced catalytic CO2 reduction employing a number of mono- and dinuclear complexes with a phenanthroline-extended tetramesityl porphyrin ligand (H2-1). A stepwise synthesis of heterodinuclear complexes is possible because the phenanthroline moiety of the ligand H2-1 can selectively coordinate a second metal center, e.g. Ru(tbbpy)22+ fragment, while any other metal can reside in the porphyrin cavity. We expanded our former studies to cobalt and iron compounds and synthesized the complexes Co-1, FeCl-1 and Co-1-Ru, FeCl-1-Ru. Thorough catalytic investigation on the light-driven CO2 reduction of all M-1(-Ru) compounds (M = 2H, Cu, Pd, Co, FeCl) was performed in a DMF solution in the presence of triethylamine (TEA) as a sacrificial electron donor. A very surprising wavelength dependence of the catalytic performance was observed. Turnover numbers (TONs) of CO were quantified and showed that redox active metals (i.e. M = Co and FeCl) in the porphyrin
Received 15th December 2014, Accepted 25th February 2015
cavity caused the highest catalytic activity. After 24 hours of illumination with light λ > 305 nm FeCl-1-Ru
DOI: 10.1039/c4dt03846k
reached a TONCO of 11.4 with our experimental setup without showing much decomposition. This value is twice as high as the TONCO determined for CoTPP (5.8) under the same conditions, which represented
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the most active porphyrinic system so far for photocatalytic CO2 reduction.
Introduction The catalytic reduction of CO2 to CO or into liquid fuels presents a crucial challenge due to the global warming problem that is forced by the burning of fossil fuels for energy production. The energy demanding process of CO2 reduction is favorably driven using sunlight as the energy source to avoid a deleterious effect from the continuous usage of fossil fuels.1–3 In the last two decades photochemical molecular devices (PMDs) were developed that are supramolecular complexes composed of covalently coupled individual components with specific functions designed to perform a catalytic reaction induced by light.4,5 PMDs consist of a light harvesting unit, a linker and a reaction center at which the substrate transformation (e.g. proton reduction, carbon dioxide reduction, etc.) takes place.6–14 At the beginning of the catalytic cycle, a photoelectron is created on the light harvesting unit (e.g. a
Institute of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-St. 2, 12489 Berlin, Germany. E-mail: [email protected]; Fax: +49-30-2093-6966; Tel: +49-30-2093-7571 † Electronic supplementary information (ESI) available: 1H-NMR-, EPR- and mass spectra of the synthesized compounds as well as a full comparison of the UV-vis absorption spectra and electrochemical data of M-1(-Ru) compounds. See DOI: 10.1039/c4dt03846k
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Ru(bpy)22+-fragment) and transferred via the linker to the catalytic site (if a photo-reduction process is considered). Because many important reduction reactions are of multi-electron nature (e.g. two electron processes), a linker with the partial electron storage ability to accumulate those electron equivalents is desirable. Ligands like tetraazatetrapyridopentacene (tatpp)15–17 and tetrapyridophenazine (tpphz)14,18,19 possess an extended π-system and, therefore, are very suitable for the storage of up to four electrons (in the case of tatpp). Unfortunately, the synthesis of heterodinuclear metal complexes proved to be difficult for these ligands. This is because of (a) the low solubility of the ligands in organic solvents and (b) the same phenanthroline-based coordination sphere on both ends of the molecules that causes selectivity problems when different metal centers need to be used. Nevertheless, in recent years several heterodinuclear complexes with the ligand tpphz have been synthesized and successfully tested in the photodriven proton reduction.14,20,21 There is only one example of a heterotrinuclear complex with the ligand tatpp in the literature. Lehn and coworkers published the synthesis of ruthenium–palladium molecular rods containing tatpp which are supposed to act as molecular wires. But no catalytic application was reported in these studies.22 In 1987, Crossley and coworkers connected two porphyrinic units using the tatpp motif.23,24 They could show that both
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porphyrin moieties are in electrochemical communication with each other and that multiple reversible reduction events can occur, making the system suitable for multi-electron reduction catalysis. However, no ( photo)catalytic application was tested for this system yet. Other groups succeeded in the synthesis of a bridging ligand that consists of a porphyrin moiety fused to a phenanthroline ( phen) moiety to allow the connection of two different metal centers in a straightforward manner.25–27 Albeit multiple spectroscopic and electrochemical investigations were performed no photocatalytic reactions have been investigated. Till now, the only examples of porphyrinic systems used in photocatalytic reduction reactions, in which the catalytic reaction takes place at the porphyrin unit, are mononuclear compounds that contain a redox-active metal center like iron or cobalt. These are able to drive the reduction of protons, dioxygen or CO2.28–35 The groups of Fujita and Neta showed for the first time that cobalt and iron porphyrins display a moderate catalytic activity for photochemical CO2reduction.32,33 These studies were extended just recently by photochemical reduction experiments by Bonin et al. on modified iron tetraphenylporphyrins bearing phenolic groups.34,35 To the best of our knowledge, these are the only studies about porphyrin compounds used in photocatalytic CO2-reduction so far. Photolysis afforded the formation of CO with a turnover number (TONCO) of 80 for CoTPP and 65 for FeClTPP (TPP = tetraphenylporphyrin) both after 200 h.32,33 A TONCO of 30 was reported for chloro iron(III) 5,10,15,20-tetrakis(2′,6′-dihydroxyphenyl)porphyrin (FeHPP) after 10 h of illumination.34 CoTPP and FeClTPP act therefore as reference catalysts and their activity in photochemical CO2 reduction is compared with that of the complexes described in this work. We recently published the synthesis and full characterization of Cu-1, Pd-1, Cu-1-Ru and Pd-1-Ru (for structures see Fig. 1 and 2, M = Cu or Pd) including their electron storage capability.36 The phenanthroline-extended tetramesityl porphyrin ligand H2-1 is very convenient for the synthesis of heterodinuclear complexes. The phenanthroline moiety of the ligand H2-1 allows the coordination of a Ru(tbbpy)22+ fragment (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) which broadens the absorption properties in the visible region and shifts the reduction potentials to more positive values. The latter is expected to be more suitable for efficient CO2-reduction because it should be easier to produce the catalytically active reduced species by photochemical quenching reaction with the sacrificial electron donor. One goal of our studies was to
Fig. 1
investigate whether the positive influence of the ruthenium fragment on photo-reduction reactions observed before would be confirmed here. We extend our former studies to cobalt and iron compounds, because Neta, Fujita and coworkers revealed the catalytic activity of cobalt and iron porphyrins. Therefore the synthesis of Co-1, FeCl-1 and Co-1-Ru, FeCl-1-Ru is reported as well as catalytic investigations of all M-1(-Ru) compounds. Herein, the photolysis of M-1 and M-1-Ru (M = H2, Cu, Pd, Co, FeCl) in DMF saturated with CO2 and in the presence of triethylamine (TEA) as a sacrificial electron donor is described. The amount of the solely detected reduction product CO is quantified by gas chromatography and the different reactivities of the complexes are compared. Photolysis studies using different long-wave pass cutoff filters in wavelength-dependent experiments for the first time revealed that an increase in the fraction of UV-light used for excitation causes a higher amount of CO to be formed.
Results and discussion Synthesis and characterization The multistep synthesis of ligand H2-1 was reported earlier.36 The complexation of H2-1 with a cobalt or iron salt led first to oligonuclear species due to the additional metalation of the phenanthroline moiety. Hence, the crude reaction product was treated with EDTA-buffer under aerobic conditions to demetalate the phen-moiety in order to obtain pure Co-1 and FeCl-1 (Fig. 1), which was confirmed by ESI-MS and UV-Vis-spectroscopy. Both complexes were formed in only moderate yields of around 40% due to formation of multiple side-products that were not characterized. The subsequent reaction of Co-1 and FeCl-1 with bis-(4,4′-di-tert-butyl-2,2′-bipyridine) ruthenium(II) dichloride (Ru(tbbpy)2Cl2) in DMF–water (∼10 : 1) led to the formation of Co-1-Ru and FeCl-1-Ru in about 54% yield. Due to the moderate overall yield based on H2-1 (22%) another synthetic strategy was followed. It starts first with the reaction of H2-1 and Ru(tbbpy)2Cl2 to form H2-1-Ru and in a subsequent step the second metal is inserted into the porphyrin moiety to obtain Co-1-Ru or FeCl-1-Ru in high yield (80% based on H2-1, Fig. 2). Compounds M-1-Ru (M = H2, Co or FeCl) were characterized by high-resolution ESI-MS (Fig. S13–S22†) and UV-Vis-spectroscopy (Fig. 3; Table S1†). H2-1-Ru was also characterized by 1H-NMR-spectroscopy (Fig. S2†) while Co- and FeCl-1-(Ru) are paramagnetic species.
Synthesis of Co-1 and FeCl-1.
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Fig. 2
Synthesis of Co-1-Ru and FeCl-1-Ru (MW = microwave irradiation).
Fig. 3
UV-Vis spectra of M-1 (left) and M-1-Ru (right) with M = H2, Co or FeCl.
The EPR-spectra of FeCl-1 and FeCl-1-Ru exhibit a typical Fe(III) porphyrin signal at 77 K (g = 6.0, Fig. S1†).37 In order to obtain an EPR signal of Co(II) species temperatures below 20 K are required,38 but these measurements are not possible with our experimental setup. Hence, Co-1-(Ru) was not investigated by EPR spectroscopy. To prove the paramagnetism of Co-1-(Ru), the magnetic moment was determined by the Evans-method.39 For both compounds, Co-1 and Co-1-Ru, the µeff is about 2.2 which is higher than the theoretical value µs.o. = 1.73 for a spin system of S = 1/2. This deviation can be explained by the spin– orbit coupling which is not considered for the theoretical µs.o.40 and experimental error. Electrochemical investigations (including cyclic voltammetry and square-wave voltammetry) in dichloromethane for most of the M-1-(Ru) compounds were published earlier.36
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Table 1 provides an overview of all the redox potentials measured for the compounds M-1 and M-1-Ru (see also Fig. S7–S10†). Other solvents like acetonitrile, DMF or THF could not be used because of their low solubility. The metal center inside the porphyrin cavity has a marked influence on the oxidation potentials of M-1 (M = Zn, Cu, Pd) but almost no influence on the two reduction potentials, which appeared at similar values of −1.8 and −2.2 V vs. Fc/Fc+. Attaching an electron withdrawing Ru(tbbpy)22+ fragment only had a small influence on the oxidation potentials but clearly affected the reduction potentials. Hence, the first and the second reduction potential of M-1-Ru (M = H2, Cu, Pd) are shifted anodically by about 0.2–0.3 V and the third reduction event appeared. Furthermore, a RuII/RuIII redox event is observable in the oxidation region and, in some cases, overlaps with a ligand-based oxidation.
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Dalton Transactions Redox potentials of M-1 with M = Zn, Cu, Pd, H2, Co and FeCl and M-1-Ru with M = Cu, Pd, H2 in DCM containing 0.1 M NBu4PF6 (TBAP)a
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Potential vs. Fc/Fc+ in DCM Zn-136 Cu-136 Pd-136 H2-136 Cu-1-Ru36 Pd-1-Ru36 H2-1-Ru [Ru(tbbpy)2(phen)](PF6)236
−2.25 −2.21 −2.25 −2.01 −2.16 −2.09 −1.95 −2.08
−1.80 −1.78 −1.80 −1.74 −1.93 −1.85 −1.82 −1.80
Co-1 FeCl-1
−2.25 Ep,c −2.18 Ep,c
−1.78 −1.47
0.44 0.62 0.67 0.59 0.60 0.76 0.66
−1.57 −1.50 −1.54 −1.25 Ep,c −1.23
−0.84 Ep,c
0.51 0.72b
0.70 0.89 1.13 0.88b 0.91 0.85 0.84
1.03
0.79 1.15
a
Ep,c stands for cathodic peak potentials in the case of irreversible reduction events. Note that almost all redox events of FeCl-1-Ru and Co-1-Ru are irreversible and broad. Thus, determination and assignment of the events is very difficult and we do not denote the potentials here. b 2-Electron processes because of overlapping events.
Additional experiments on the cobalt and iron complexes were conducted in this work and are discussed in more detail in the ESI (Fig. S10†). In both cases, with M = Co or FeCl, the first reduction is irreversible and metal-based, but further reduction events are either irreversible or quasi-reversible and not easily assigned (a mix of ligand- and metal-based reductions occurs). On the other hand, two quasi-reversible oxidation events can be observed for Co-1 as well as FeCl-1 that are assigned to metal and ligand oxidations (a full discussion is included in the ESI†). The UV-Vis absorption spectra of M-1 and M-1-Ru (M = H2, Co or FeCl) in dichloromethane (DCM) are shown in Fig. 3. The spectra of M-1 depict typical porphyrin spectra with a strong absorption band around 420 nm (Soret-band) and weaker bands in the 500–650 nm region (Q-bands). The metal center has a great influence on the position of the Q-bands, whereas the position of the Soret-band does not strongly depend on the metal center (M = H2, Co or FeCl). However, the Soret-band structure of Co-1 and FeCl-1 differs from that of metal free H2-1. The Soret-band is broadened, less intense and shows splitting into two bands (which has also been observed in the literature before),26 albeit this splitting is very small for Co-1. This Soret-band shape is in accordance with other M-1 compounds with M = Cu, Pd or Zn synthesized before (see also Table S1 for a complete list of the absorption maxima and Fig. S3 and S4†).36 The Soret-band structure of compounds M-1-Ru (M = H2, Co, FeCl) is broadened further in comparison with that of parent compounds M-1 (M = H2, Co, FeCl), while the influence of the ruthenium moiety on the position of the Q-bands is quite small. A band at 290 nm arises that represents π–π*-transitions of the bipyridine ligands of the ruthenium moiety. It becomes clear that the UV-Vis spectra are not a simple addition of the properties of the individual components (see Fig. S5† for a comparison between M-1 and M-1Ru for M = Cu, Co and FeCl).36 The emission properties of the compounds were studied under steady state conditions. Only the metal-free and zinc compounds show fluorescence (see Fig. S6†). Emission
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maxima in DCM can be found at 655 and 726 nm for H2-1 and they are blue-shifted for Zn-1 (615 and 673 nm).36 H2-1-Ru exhibits, compared to H2-1, red-shifted emission maxima at 665 and 734 nm. The fluorescence spectra resemble in any case (i.e. changing excitation wavelengths) a porphyrin-based emission. Thus, even with the Ru(tbbpy)22+ fragment attached, the lowest-lying excited state is presumably located on the porphyrin moiety. No steady state emission is observed for M-1 and M-1-Ru compounds with M = Cu, Pd, Co or FeCl. Photolysis experiments Photochemical CO2 reduction experiments were conducted in DMF as a solvent and TEA (5%) as a sacrificial electron donor under 1 atmosphere of CO2. DMF was used due to solubility reasons – compounds M-1 are not well soluble in water, acetonitrile or THF, but show good solubility in DCM and reasonably well in DMF (up to a concentration of ca. 0.1 mM). In preliminary experiments we realized a strong dependence of the catalytic activity on the light source (the type of lamp) and finally used a 200 W high-pressure-mercury-vapor lamp for all catalytic experiments. Several (long-wave pass) cutoff filters were utilized for wavelength-dependent experiments. Furthermore, the formation of CO was determined and quantified by gas chromatography. To estimate the amount of CO that was produced by other sources like solvent decomposition, blank experiments were conducted without the catalyst (and/or under an argon atmosphere) under the same experimental conditions and all turnover numbers of CO were corrected by these values (further details can be found in the Experimental section). Irradiation of the catalyst (e.g. Co-1) in DMF and 5% TEA under an argon atmosphere leads to the same small amount of CO within the experimental error as an experiment without the catalyst. If the catalyst or TEA is not present under a CO2 atmosphere, illumination of the solution leads to the formation of the same amount of “background” CO as experiments under argon. Furthermore, no CO is produced in the dark or due to heating the reaction mixture at 60 °C in the dark for 24 h under a CO2 atmosphere. These results illustrate
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that neither the catalyst supports decomposition of the solvent nor photodecomposition of the catalyst leads to substantial CO formation. Hence, the corrected TONCO values given below refer only to the gas-phase detected amount of CO formed from CO2 reduction after a certain time period. CoTPP was investigated earlier under similar conditions by another group.32,33 However, a different setup was used (e.g. a 300 W Xenon lamp as the light source) and hence the TONCO given in the literature cannot be compared directly with our results. Thus, we determined the TONCO of CoTPP with our setup and discuss these values in relation to our catalysts (the same applies to FeClTPP). In Fig. 4A the TONCO values for CoTPP and Co-1 are shown after different photolysis times and using different long-wave pass cutoff filters. CoTPP and Co-1 are not active towards photochemical CO2 reduction using a >400 nm cutoff filter. This reveals clearly that only visible light is not sufficient to produce CO. A similar observation was reported by Bonin just recently.34 Thus, we worked with cutoff filters that also transmit near UV-light. In this case, a constant CO evolution can be detected for the first 60 hours of irradiation after which the curve seems to flatten. The flattening is stronger for CoTPP, which might indicate better stability of Co-1 in long-term experiments. However, long-term experiments have not been conducted so far and are planned in the near future together with more mechanistic
Fig. 4 A: Time course of TONCO for CoTPP and Co-1 using different cutoff filters. B: TONCO after 24 h of photolysis for MTPP and M-1 with M = Co or FeCl depending on the cutoff filter (experimental conditions: a 5 mL solution of the catalyst (5 × 10−5 M) in DMF and TEA (5%) was photolyzed under 1 atm. CO2 in a Schlenk tube (the gas volume above the solution: 20 mL)).
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experiments. In addition, decomposition processes and poisoning of the catalyst by produced CO cannot be excluded.32 In the case of Co-1, we performed mass spectroscopic studies after 24 h of photolysis. No additional peaks can be detected in the MALDI or ESI-MS spectrum and the spectrum looks basically the same as the one for Co-1 before the reaction (Fig. S16†) indicating good stability of the catalyst. The TONCO of CoTPP and Co-1 differs only marginally using the >320 nm cutoff filter; after 93 hours the TONCO is 4–5 in both cases, whereas the TONCO values are significantly higher using the >305 nm cutoff filter. The TONCO of CoTPP after 93 hours reaches 16 and, therefore, CoTPP is one-fourth times more active than Co-1 that reaches a TONCO of 12. Because borosilicate glass does not transmit light with wavelengths lower than 300 nm and because of possibly more side reactions no filter with a lower cutoff wavelength was used. The wavelength dependence of the TONCO for cobalt and iron chloride complexes of TPP and 1 was investigated further with different filters between 300 and 400 nm (Fig. 4B). Each photolysis experiment was carried out under the same experimental conditions and the TONCO values were determined after 24 hours. Although the catalysts are still active at that time and the catalytic reaction still proceeds, a clear comparison between the catalytic activities of the different catalysts can be obtained. When the >400 nm, >375 nm and >350 nm cutoff filters are used the TONCO values of all regarded compounds are in the range between 0 and 1. This picture changes when lower wavelength filters are used. The TONCO values of CoTPP and Co-1 for the >320 nm filter are very similar, but this is surprisingly not valid for the iron complexes. FeCl-1 is more active than FeClTPP at all wavelengths below 350 nm. Furthermore, the TONCO of FeCl-1 is around 8 using the >305 nm cutoff filter and is twice as high as the TONCO of FeClTPP in contrast to the cobalt complexes where Co-1 is only half as active as CoTPP. The TONCO for CoTPP using the >305 nm filter and our experimental setup is around 6 compared to the previously reported value of 18.33 We want to stress again that the latter value was determined with a different experimental setup and irradiation source. Nevertheless, the TONCO of 6 for CoTPP after 24 h of illumination will define the reference point for the following comparison of the catalytic activity of the new compounds. In summary, FeCl-1 shows the highest catalytic activity towards photochemical CO2 reduction of all the four compounds investigated so far (even a higher TONCO than CoTPP). The reason why we see higher activity using light in the 300 nm region than using visible light is vague at the moment. We want to note, however, that similar results could be observed in photocatalytic proton reduction and neither was an explanation presented in those studies.20,41,42 Bonin et al. suggested that efficient reductive quenching of an excited iron porphyrin catalyst by triethylamine can only occur with UV-light.34 However, they used illumination wavelengths λ > 280 nm and did not use any other filters to support their proposition. In future work we want to try to shed light on this behavior with the help of extended photophysical studies.
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Now, we wanted to investigate the catalytic activity of all the M-1-Ru compounds synthesized to determine the influence of the metal center in the porphyrin moiety as well as that of the attached Ru(tbbpy)22+ fragment. Photolysis experiments were carried out under the same experimental conditions as used before either using the >305 nm filter or the >375 nm filter to screen all the potential catalysts. The TONCO values were determined after 24 hours and are summarized in Table 2. The error range for the TONCO is estimated to be around 10%. For all the investigated compounds the TON values obtained with a >375 nm cutoff filter are below 1 after 24 hours of illumination but they can still be used to compare the influence of different metal centers. No CO could be detected for Cu-1 and Pd-1 when working with the >375 nm filter, but both show a TONCO around 1 if a >305 nm filter is applied. As expected, using zinc or no metal in the porphyrin moiety gives no catalytic activity at all. The inactivity of Zn-1 and H2-1 reveals that the porphyrin central metal plays a crucial role in the photochemical CO2 reduction and it seems that especially the redox-activity of the metal is a key factor. The attached ruthenium fragment had a positive influence on the catalytic performance for all the investigated M-1-Ru compounds compared to their parent M-1 compounds. Pd-1Ru and Cu-1-Ru display a 3 times higher catalytic activity than Pd-1 and Cu-1 using the >305 nm filter and working with the >375 nm filter, a small amount of CO gas could be detected for the ruthenium complexes. Furthermore, FeCl-1-Ru and Co1-Ru exhibit a 1.5 times higher TONCO than the corresponding ruthenium free complexes using the >305 nm filter and an improved catalytic performance was also observed when using the >375 nm filter. Thus, as intended, the ruthenium-moiety increases the catalytic activity of the whole complex. A simple antenna effect of the ruthenium chromophore in the near UV region might assist the photochemical reaction; however, we do not have conclusive evidence for that yet (see also Fig. S6†). In summary, FeCl-1-Ru represents the best porphyrinic photocatalyst reported to date with roughly twice the activity as
Table 2 TONCO of M-1 and M-1-Ru complexes after 24 hours of photolysis (in DMF/TEA (5%), CO2 atmosphere)
Catalyst
TONCO (>305 nm cutoff filter)
TONCO (>375 nm cutoff filter)
CoTPP Co-1 Co-1-Ru FeClTPP FeCl-1 FeCl-1-Ru Cu-1 Cu-1-Ru Pd-1 Pd-1-Ru H2-1 H2-1-Ru Zn-1
5.8 3.2 4.7 3.8 7.6 11.4 0.7 3.0 1.0 3.1 0 0 0
0.4 0.1 0.4 0.1 0.1 0.7 0 0.1 0 0.1 0 0 0
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CoTPP after 24 hours of illumination (TONCO of 12 vs. 6). It is important to note that the final TON values are not very high, however they are obtained only after 24 hours with a limited setup (light source). Thus, in comparison with the best system reported so far, CoTPP, we could observe a marked increase in activity even if we just focus on a short reaction time. Unfortunately, they still cannot be compared with other systems based totally on rare elements7 that show a much higher catalytic performance. For example, Tanaka et al. presented a system composed of [Ru(bpy)3]2+ and [Ru(bpy)2(CO)2]2+ using BNAH as the sacrificial electron donor in H2O–DMF that showed a TONCO of 120 after 10 hours of irradiation43 producing considerable amounts of formate too. Formate can be produced as a major product with a TONHCOOH > 500 if the two single units are covalently connected to form a PMD.45 Carbon monoxide can be formed almost exclusively with TONs > 100 after 8 hours of irradiation with Ru–Re dyads and BNAH in DMF as introduced by Ishitani and coworkers.4,13,44 We further investigated the influence of different sacrificial electron donors on the photochemical catalysis. Photolysis experiments were conducted with Co-1 as the catalyst (5 × 10−5 M) dissolved in DMF with 5% TEA, 5% triethanolamine (TEOA) or 1-benzyl-1,4-dihydronicotinamide (BNAH, 0.1 M) as sacrificial electron donors. The TONCO using TEOA is about 3 after 24 h of irradiation, which is the same value as using TEA. In contrast, when BNAH is used as the sacrificial electron donor, no photochemical CO2-reduction took place after 24 h. BNAH is normally used as a sacrificial electron donor for Ru(II) based-photocatalysts45 and not for porphyrins. Hence, BNAH seems to be unsuitable to act as a sacrificial electron donor in combination with a metal porphyrin complex. In order to detect other catalytic products (like formic acid) we performed solvent suppression NMR experiments with the reaction solution. So far, no formate could be found using TEA as the sacrificial electron donor and Co-1 as the catalyst. In the GC analysis we are also unable to observe dihydrogen formation. Thus, with our setup, CO is selectively formed and the only catalytic product detected. Similar results were published in the literature for other porphyrin compounds, although a small amount of dihydrogen could be detected in those studies.32,34 The results obtained raise a few questions, which we cannot answer right now. Why do we see an opposite catalytic performance for M-1 vs. MTPP (for M = Co, FeCl)? How is CO2 activated (e.g. which oxidation state of the metal center is needed)? Mechanistic investigations are on the way and with additional spectroscopic experiments we expect to answer some of the questions in the near future. At present, we can only draw some parallels to already published work about the reaction mechanism. Fujita, Neta and coworkers performed extended studies on the reaction mechanism of cobalt and iron macrocycles with carbon dioxide.32,33,46–48 Triethylamine functions in every case as a reductive quencher that reduces the excited macrocyclic compound during photocatalysis. In the case of the iron and cobalt compounds it is very likely that the metal center in the
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porphyrin cavity needs to be reduced to at least the metal(I) state (maybe even the metal(0)-state).32,33,46 Bonin et al. conclusively showed that in the case of iron porphyrins, e.g. FeHPP, the iron(0) state is the one that activates CO2.34 It might be that the efficient reduction of e.g. the double reduced species to the triple reduced species can only occur under near UV-light irradiation. But this point needs further studies by photophysical experiments in the near future. At this point, the “highly” reduced metal center can attack a CO2 molecule to form the initial adduct. Now, depending on the experimental conditions, there are two ways how CO2 can be transformed to CO.49 In the beginning of the catalysis the proton concentration is very low and thus we assume that an oxygen atom is probably transferred to a second CO2 molecule to produce carbonate and CO.47 The decomposition of TEA leads to the formation of protons32,47,50 that can react with activated CO2 to yield water and CO.34,46 As the reaction time prolongs the concentration of protons increases and this pathway likely becomes dominant. Because of the non-activity of H2-1, Zn-1 and H2-1-Ru we do not believe that the ruthenium fragment plays a direct role in the reduction of CO2. As discussed earlier (and can clearly be deduced from the electrochemical investigations), the ruthenium center shifts the reduction potentials of the compounds to less negative values, which positively influences the catalytic performance. Note that the catalysts need to be reduced at least twice (see Fig. S11 and S12† for Cu-1) or threefold (as is likely the case for FeClTPP or FeCl-132–34) before they become active in CO2 reduction and depending on the metal center in the porphyrin cavity one or two of these reduction events should be metal-based. The ligand 1 might function as an electron reservoir during the catalysis and, in doing so, assists the metal center in the porphyrin moiety to convert CO2. Finally, we want to emphasize that photocatalytic reactions performed in different groups are not easy to compare because of a strong influence of the light source, filters used, solvent and other reaction conditions on the catalytic performance, which seems to be specially complicated when using porphyrinic compounds. Conclusion We successfully developed mononuclear phenanthrolineextended metal porphyrin complexes M-136 containing various metals (M = FeCl, Co, Cu, Pd) for the photocatalytic reduction of CO2 to CO. Their dinuclear analogues M-1-Ru were synthesized by simple attachment of a Ru(tbbpy)22+ fragment and represent a new class of PMD. In every case, the ruthenium moiety increases the catalytic activity of the whole molecule. This is very likely due to the fact that the Ru(tbbpy)22+ fragment influences the reduction potentials of the catalyst so that they become less negative and thus the photochemical quenching reaction with the sacrificial electron donor becomes more efficient which finally raises the CO2-reduction activity. This result should in principle be achievable with any other positively charged metal fragment. We do not think that the ruthenium fragment otherwise takes part in the
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photochemical reaction. However, a simple antenna effect of the ruthenium chromophore in the near UV region cannot be excluded too. In addition, we can imagine that the electron storage capability of the ligand framework 1 is helpful for the accumulation of reducing equivalents. Furthermore, we revealed that near UV-light is necessary for the photochemical CO2 reduction by means of wavelength dependent photolysis experiments. Merely visible light is not sufficient to catalyze the CO2 reduction by metal porphyrin complexes as has been observed by others just recently34 as well. In addition, we found out that the photocatalytic activity is dependent on the redox activity of the porphyrin metal. The highest TONCO could be achieved using Co or FeCl porphyrin complexes, whereas Zn or free base porphyrins are inactive. FeCl-1 and FeCl-1-Ru display the highest catalytic activity and show even higher TONsCO in our setup than the already known compounds CoTPP32 and FeClTPP33, which presented the most catalytically active porphyrins up to this point. Although these values are still lower than the best reported CO2 reduction photocatalysts as stated above,7,43–45 they show the potential of non-precious metal catalysts in light-driven CO2 reduction.
Experimental section General methods Silica gel for column chromatography was obtained from Acros (Silica Gel 60, 230–400 μm mesh). All solvents (tetrahydrofuran (THF), toluene, dichloromethane (DCM), dimethylformamide (DMF), dimethylsulfoxide (DMSO), hexane, acetonitrile (ACN), acetic acid (HAc) and ethanol) and starting materials were obtained from Sigma-Aldrich or abcr. If necessary, solvents were purified employing an MBraun Solvent Purification System. DMF for photolysis experiments was fractionally distilled with benzene and water, dried with CaH2 and finally distilled under argon. Triethylamine for photocatalytic reactions was refluxed over KOH, filtered, dried with CaH2 and finally distilled under argon. Starting materials were used without further purification. Bis(4,4′-di-tert-butyl-2,2′-bipyridine) ruthenium(II) chloride (Ru(tbbpy)2Cl2),51 CoTPP,52 FeClTPP,52 H2-1, Cu-1, Pd-1, Zn-1, Pd-1-Ru, and Cu-1-Ru36 were synthesized according to literature procedures. 1 H NMR spectra were recorded at ambient temperature either on a Bruker DPX-300 or an AV-400 spectrometer. All spectra were referenced to tetramethylsilane (TMS) or deuterated chloroform (CDCl3) as an internal standard (measured values for δ are given in ppm and for J in Hz). Assignment of signals was done with the help of 2D experiments and is presented in the ESI (Fig. S2†). Elemental analysis was performed by the microanalytical laboratory of the Institute of chemistry at the Humboldt-Universität zu Berlin using a HEKAtech EURO 3000. ESI mass spectra were obtained in a methanolic solution using Thermo Finnigan LCQ XP and LTQ FT instruments. EPR spectra were recorded on an X-band spectrometer ERS 300 (ZWG/Magnettech Berlin/Adlershof, Germany)
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equipped with a fused quartz Dewar for measurements at liquid nitrogen temperature. The g-values were calculated with respect to a 2,2-diphenyl-1-picrylhydrazyl (dpph) reference (g′ = 2.0036). Absorption spectroscopic measurements were made in DCM solutions of each compound using a Cary 100 UV-VisNIR spectrometer from Varian employing the software Cary WinUV. SUPRASIL® Quartz cells from Hellma Analytics with a 10 mm path length were used. Microwave reactions were carried out at 50 W in a CEM monomode microwave (model: Discover Bench mate). Emission spectra were recorded on a Cary eclipse fluorescence spectrophotometer using SUPRASIL® Quartz cells from Hellma Analytics with a 10 mm path length. Irradiation experiments were performed in DMF and 5% TEA with 5 × 10−5 M catalyst concentration. Schlenk flasks with a 5 mL solution and 20 mL headspace were used. Fresh solutions were prepared before each experiment. The solutions were bubbled with CO2 for 20 minutes to remove argon. Photolysis was performed with a 200 W high-pressure-mercuryvapor lamp using a water filter to absorb IR radiation and different long-wave pass cutoff-filters to absorb wavelengths below the given values. The filters were purchased from LOTQuantumDesign GmbH and have an edge rise of 2% (an example of a “filter spectrum” is given in the ESI, Fig. S23†). CO evolved was determined by gas chromatography (Shimadzu GC-17A with a thermal conductivity detector and Resteks ShinCarbon packed column ST 80/100 (2 m, 1/8″ OD, 2 mm ID) and quantified using a calibration curve. The TONCO was then calculated by dividing the amount of CO formed (corrected by the amount of CO formed in blank experiments) by the amount of catalyst used. In blank experiments, no CO was detected using a >350 nm cutoff-filter and higher, while TONCO values obtained with the >305 nm filter were corrected by a value of 0.8. Measurements were performed by manual injection of gas phase samples (250 μl) and in triplicate. CO dissolved in the solvent was not considered, thus the TON of CO is only related to the amount detected in the gas phase above the solution. {(Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrin-bis(4,4′-di-tert-butyl-2,2′-bipyridine)-ruthenium(II)} dihexafluorophosphate (H2-1-Ru). H2-1 (40 mg, 41 µmol) and Ru(tbbpy)2Cl2 (44 mg, 62 µmol, 1.5 eq.) were dissolved in 30 mL DMF and 3.3 mL water. This solution was heated in a microwave cavity at 120 °C for 6 hours. The solvent was subsequently removed under reduced pressure and the product was purified by column chromatography with acetonitrile– water = 9 : 1. The product was eluted after additional 0.05 vol.% of saturated aqueous KNO3 was added. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The brown precipitate was filtered off with Celite, washed with water, dried in a vacuum and the product was washed off using DCM. The solvent was subsequently removed under reduced pressure to yield H2-1-Ru (77 mg, 40 µmol, 99%).
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Anal. Calcd for C104H106F12N12P2Ru·CH2Cl2: C, 62.61; H, 5.62; N, 8.30. Found: C, 63.06; H, 5.44; N, 8.40; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 287 (95.4), 428 (140.1), 538 (19.7), 608 (12.7), 658 (1.5); 1H NMR (400 MHz, CD3CN, δ): −2.39 (s, 2H), 1.34 (s, 18H), 1.48 (s, 18H), 1.75–1.83 (m, 24H), 2.58 (s, 6H), 2.79 (s, 6H), 7.20 (dd, J = 1.5 and 4.8 Hz, 2H), 7.32 (s, 4H), 7.44 (s, 2H), 7.49–7.57 (m, 6H), 7.75 (d, J = 5.0 Hz, 2H), 7.89 (dd, J = 3.9 and 6.3 Hz, 2H), 8.14 (dd, J = 0.9 and 4.2 Hz, 2H), 8.50 (s, 2H), 8.51 (d, J = 1.5 Hz, 2H), 8.57 (d, J = 1.5 Hz, 2H), 8.78 (d, J = 3.6 Hz, 2H) 8.87 (d, J = 3.6 Hz, 2H), 9.00 (dd, J = 1.1 and 6.2 Hz, 2H), 13C NMR (125 MHz, CD3CN, δ): 20.71, 29.43, 29.56, 35.27, 35.42, 115.74, 119.59, 121.48, 121.57, 124.46, 124.63, 126.78, 127.72, 127.81, 128.09, 128.14, 131.31, 133.15, 133.93, 137.29, 137.45, 137.98, 138.12, 138.25, 138.42, 138.58, 139.02, 139.54, 139.61, 143.19, 149.65, 151.18, 151.44, 152.20, 152.92, 155.49, 156.76, 157.13, 162.59, 162.78. ESI-MS m/z: [M − 2PF6−]2+ calcd for C104H106N12Ru 812.3848. Found 812.3852. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-cobalt(II) (Co-1). H2-1 (25 mg, 30 μmol) and Co(Ac)2·4H2O (31 mg, 130 μmol) were dissolved in 15 mL DMF and the reaction mixture was heated at reflux overnight. The solution was subsequently evaporated to dryness. The solid residue was dissolved in 10 mL DCM, washed with water and dried over Na2SO4. After removal of the solvent under vacuum the solid residue was dissolved in 5 mL DCM and stirred vigorously with 3.5 mL EDTA-buffer (10 mM H4EDTA, 27 mM NaAc and 0.04 mM HAc) for 24 h. The organic layer was separated and washed with 5% aqueous NaHCO3, water and dried over Na2SO4. The solid residue was then purified using silica column chromatography with DCM and 3% MeOH as the eluent. This method afforded Co-1 as a red/brown solid (10 mg, 10 μmol, 40%). Anal. Calcd for C68H56N8Co·0.5 × CH2Cl2: C, 75.71; H, 10.31; N, 5.29. Found: C, 75.29; H, 10.46; N, 5.38; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 411 (99.3), 557 (20.4), 587 (17.4); ESI-MS m/z: [M + H+] calcd for C68H57N8Co 1044.4033. Found 1044.4030. {(Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-cobalt(II) bis(4,4′-di-tert-butyl-2,2′-bipyridine)ruthenium(II)} dihexafluorophosphate (Co-1-Ru). Variant 1: a solution of H2-1-Ru (30 mg, 16 µmol) and Co(Ac)2·4H2O (20 mg, 80 µmol) in 15 mL DMF was prepared and heated at reflux overnight under an argon atmosphere. An aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added and the precipitate was filtered off. The crude product was subsequently purified by column chromatography with acetonitrile–water = 9 : 1. The product was eluted with additional 0.05 vol.% of saturated aqueous KNO3. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The violet-brown precipitate was filtered off with Celite, washed with water and dried in a vacuum. The remaining solid was washed using DCM. The solvent was subsequently removed under reduced pressure to yield Co-1-Ru (30 mg, 15 µmol, 80%).
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Variant 2: to a solution of Co-1 (27 mg, 26 µmol) in 30 mL DMF and 3.8 mL H2O was added Ru(tbbpy)2Cl2 (27 mg, 39 µmol, 1.5 eq.). This solution was heated for 72 h under reflux. The solvent was subsequently removed under reduced pressure and the product was purified by silica column chromatography with acetonitrile–water = 9 : 1. The product was eluted after additional 0.05 vol.% of saturated aqueous KNO3 was added. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The violet-brown precipitate was filtered off with Celite, washed with water, dried in a vacuum and the product was washed off using DCM. The solvent was subsequently removed under reduced pressure to yield Co-1-Ru (27 mg, 14 µmol, 53%). Anal. Calcd for C104H104N12P2F12RuCo·1.75 × CH2Cl2: C, 60.15; H, 5.12; N, 7.96; Found: C, 59.70; H, 5.52; N, 8.38; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 287 (92.4), 423 (86.7), 470 (76.4), 574 (18.8); ESI-MS m/z: [M − 2PF6−]2+ calcd for C104H104N12CoRu 840.8436. Found 840.8445. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-iron(III) chloride (FeCl-1). H2-1 (30 mg, 30 μmol) and FeCl2 (15 mg, 121 μmol) were dissolved in 15 mL DMF. This solution was heated in a microwave cavity at 130 °C for 3.5 h and after cooling to room temperature subsequently evaporated to dryness. The solid residue was dissolved in 10 mL DCM, washed with water and dried over Na2SO4. After removal of the solvent under vacuum the solid residue was dissolved in 6 mL DCM and stirred vigorously with 4.5 mL EDTAbuffer for 24 h. The organic layer was separated and washed with 5% aqueous NaHCO3, water and dried over Na2SO4. The material was then purified using silica column chromatography with DCM and 3% MeOH as the eluent. The product was subsequently dissolved in 20 mL DCM and stirred over 7% HCl for 1.5 hours. The organic layer was separated, washed with water and dried over Na2SO4 to yield FeCl-1 as a violet/ brown solid (13 mg, 12 μmol, 40%). Anal. Calcd for C68H56N8FeCl·CH2Cl2: C, 71.35; H, 9.16; N, 5.03 Found: C, 71.73; H, 9.26; N, 5.60; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 380 (75.0), 438 (100.4), 524 (19.0), 734 (sh, 3.2); MALDI-MS m/z: [M + H+] calcd for C68H57FeN8Cl 1076.4. found 1076.4; ESI-MS m/z: [M − Cl− + MeOH]+ calcd for C69H60FeN8O 1072.4234. Found 1072.4233. {(Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-iron(III) chloride-bis(4,4′-di-tert-butyl-2,2′-bipyridine)-ruthenium(II)} dihexafluorophosphate (FeCl-1-Ru). Variant 1: H2-1-Ru (30 mg, 16 µmol) and FeCl2 (12 mg, 63 µmol) were dissolved in 15 mL DMF under an argon-atmosphere. This solution was heated in a microwave cavity at 130 °C for 3.5 h. An aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added and the precipitate was filtered off. The crude product was subsequently purified by column chromatography with acetonitrile–water = 9 : 1. The product was eluted with additional 0.05 vol.% of saturated aqueous KNO3. After removing the solvent, the product was dissolved in 10 mL DCM and stirred over 7% HCl for 1.5 h. The organic layer was separated, washed with water and dried over Na2SO4.
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After removing the solvent, the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The precipitate was filtered off with Celite, washed with water and dried in a vacuum. The remaining solid was washed off by changing the solvent to DCM. The solvent was subsequently removed under reduced pressure to yield FeCl-1-Ru (30 mg, 15 µmol, 80%). Variant 2: to a solution of FeCl-1 (27 mg, 19 µmol) in 20 mL DMF and 2 mL H2O was added Ru(tbbpy)2Cl2 (20 mg, 29 µmol, 1.5 eq.). This solution was heated under an argon atmosphere for 72 h under reflux. The solvent was subsequently removed under reduced pressure and the product was purified by silica column chromatography with acetonitrile–water = 9 : 1. The product was eluted after additional 0.05 vol.% of saturated aqueous KNO3 was added. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The violet-brown precipitate was filtered off with Celite, washed with water, dried in a vacuum and the product was washed off using DCM. The solvent was subsequently removed under reduced pressure to yield FeCl-1-Ru (20 mg, 10 µmol, 55%). Anal. Calcd for C104H104N12P2F12ClRuFe·2 × CH2Cl2: C, 60.36; H, 5.11; N, 8.05. Found C, 60.18; H, 5.43; N, 8.06; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 287 (83.6), 423 (83.3), 530 (sh, 22.3), 622 (sh, 7.0), 690 (sh, 4.0); ESI-MS m/z: [M − Cl− − 2PF6− + CH3O−]2+ calcd for C105H107N12OFeRu 854.8536. Found 854.8533.
Acknowledgements We thank the German Science Foundation (DFGSCHW 1454/32) for financial support and Prof. Stößer for his help acquiring EPR spectra.
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Photochemical CO2-reduction catalyzed by mono- and dinuclear phenanthroline-extended tetramesityl porphyrin complexes† Corinna Matlachowski and Matthias Schwalbe* We here present a comprehensive study on the light-induced catalytic CO2 reduction employing a number of mono- and dinuclear complexes with a phenanthroline-extended tetramesityl porphyrin ligand (H2-1). A stepwise synthesis of heterodinuclear complexes is possible because the phenanthroline moiety of the ligand H2-1 can selectively coordinate a second metal center, e.g. Ru(tbbpy)22+ fragment, while any other metal can reside in the porphyrin cavity. We expanded our former studies to cobalt and iron compounds and synthesized the complexes Co-1, FeCl-1 and Co-1-Ru, FeCl-1-Ru. Thorough catalytic investigation on the light-driven CO2 reduction of all M-1(-Ru) compounds (M = 2H, Cu, Pd, Co, FeCl) was performed in a DMF solution in the presence of triethylamine (TEA) as a sacrificial electron donor. A very surprising wavelength dependence of the catalytic performance was observed. Turnover numbers (TONs) of CO were quantified and showed that redox active metals (i.e. M = Co and FeCl) in the porphyrin
Received 15th December 2014, Accepted 25th February 2015
cavity caused the highest catalytic activity. After 24 hours of illumination with light λ > 305 nm FeCl-1-Ru
DOI: 10.1039/c4dt03846k
reached a TONCO of 11.4 with our experimental setup without showing much decomposition. This value is twice as high as the TONCO determined for CoTPP (5.8) under the same conditions, which represented
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the most active porphyrinic system so far for photocatalytic CO2 reduction.
Introduction The catalytic reduction of CO2 to CO or into liquid fuels presents a crucial challenge due to the global warming problem that is forced by the burning of fossil fuels for energy production. The energy demanding process of CO2 reduction is favorably driven using sunlight as the energy source to avoid a deleterious effect from the continuous usage of fossil fuels.1–3 In the last two decades photochemical molecular devices (PMDs) were developed that are supramolecular complexes composed of covalently coupled individual components with specific functions designed to perform a catalytic reaction induced by light.4,5 PMDs consist of a light harvesting unit, a linker and a reaction center at which the substrate transformation (e.g. proton reduction, carbon dioxide reduction, etc.) takes place.6–14 At the beginning of the catalytic cycle, a photoelectron is created on the light harvesting unit (e.g. a
Institute of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-St. 2, 12489 Berlin, Germany. E-mail: [email protected]; Fax: +49-30-2093-6966; Tel: +49-30-2093-7571 † Electronic supplementary information (ESI) available: 1H-NMR-, EPR- and mass spectra of the synthesized compounds as well as a full comparison of the UV-vis absorption spectra and electrochemical data of M-1(-Ru) compounds. See DOI: 10.1039/c4dt03846k
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Ru(bpy)22+-fragment) and transferred via the linker to the catalytic site (if a photo-reduction process is considered). Because many important reduction reactions are of multi-electron nature (e.g. two electron processes), a linker with the partial electron storage ability to accumulate those electron equivalents is desirable. Ligands like tetraazatetrapyridopentacene (tatpp)15–17 and tetrapyridophenazine (tpphz)14,18,19 possess an extended π-system and, therefore, are very suitable for the storage of up to four electrons (in the case of tatpp). Unfortunately, the synthesis of heterodinuclear metal complexes proved to be difficult for these ligands. This is because of (a) the low solubility of the ligands in organic solvents and (b) the same phenanthroline-based coordination sphere on both ends of the molecules that causes selectivity problems when different metal centers need to be used. Nevertheless, in recent years several heterodinuclear complexes with the ligand tpphz have been synthesized and successfully tested in the photodriven proton reduction.14,20,21 There is only one example of a heterotrinuclear complex with the ligand tatpp in the literature. Lehn and coworkers published the synthesis of ruthenium–palladium molecular rods containing tatpp which are supposed to act as molecular wires. But no catalytic application was reported in these studies.22 In 1987, Crossley and coworkers connected two porphyrinic units using the tatpp motif.23,24 They could show that both
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porphyrin moieties are in electrochemical communication with each other and that multiple reversible reduction events can occur, making the system suitable for multi-electron reduction catalysis. However, no ( photo)catalytic application was tested for this system yet. Other groups succeeded in the synthesis of a bridging ligand that consists of a porphyrin moiety fused to a phenanthroline ( phen) moiety to allow the connection of two different metal centers in a straightforward manner.25–27 Albeit multiple spectroscopic and electrochemical investigations were performed no photocatalytic reactions have been investigated. Till now, the only examples of porphyrinic systems used in photocatalytic reduction reactions, in which the catalytic reaction takes place at the porphyrin unit, are mononuclear compounds that contain a redox-active metal center like iron or cobalt. These are able to drive the reduction of protons, dioxygen or CO2.28–35 The groups of Fujita and Neta showed for the first time that cobalt and iron porphyrins display a moderate catalytic activity for photochemical CO2reduction.32,33 These studies were extended just recently by photochemical reduction experiments by Bonin et al. on modified iron tetraphenylporphyrins bearing phenolic groups.34,35 To the best of our knowledge, these are the only studies about porphyrin compounds used in photocatalytic CO2-reduction so far. Photolysis afforded the formation of CO with a turnover number (TONCO) of 80 for CoTPP and 65 for FeClTPP (TPP = tetraphenylporphyrin) both after 200 h.32,33 A TONCO of 30 was reported for chloro iron(III) 5,10,15,20-tetrakis(2′,6′-dihydroxyphenyl)porphyrin (FeHPP) after 10 h of illumination.34 CoTPP and FeClTPP act therefore as reference catalysts and their activity in photochemical CO2 reduction is compared with that of the complexes described in this work. We recently published the synthesis and full characterization of Cu-1, Pd-1, Cu-1-Ru and Pd-1-Ru (for structures see Fig. 1 and 2, M = Cu or Pd) including their electron storage capability.36 The phenanthroline-extended tetramesityl porphyrin ligand H2-1 is very convenient for the synthesis of heterodinuclear complexes. The phenanthroline moiety of the ligand H2-1 allows the coordination of a Ru(tbbpy)22+ fragment (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) which broadens the absorption properties in the visible region and shifts the reduction potentials to more positive values. The latter is expected to be more suitable for efficient CO2-reduction because it should be easier to produce the catalytically active reduced species by photochemical quenching reaction with the sacrificial electron donor. One goal of our studies was to
Fig. 1
investigate whether the positive influence of the ruthenium fragment on photo-reduction reactions observed before would be confirmed here. We extend our former studies to cobalt and iron compounds, because Neta, Fujita and coworkers revealed the catalytic activity of cobalt and iron porphyrins. Therefore the synthesis of Co-1, FeCl-1 and Co-1-Ru, FeCl-1-Ru is reported as well as catalytic investigations of all M-1(-Ru) compounds. Herein, the photolysis of M-1 and M-1-Ru (M = H2, Cu, Pd, Co, FeCl) in DMF saturated with CO2 and in the presence of triethylamine (TEA) as a sacrificial electron donor is described. The amount of the solely detected reduction product CO is quantified by gas chromatography and the different reactivities of the complexes are compared. Photolysis studies using different long-wave pass cutoff filters in wavelength-dependent experiments for the first time revealed that an increase in the fraction of UV-light used for excitation causes a higher amount of CO to be formed.
Results and discussion Synthesis and characterization The multistep synthesis of ligand H2-1 was reported earlier.36 The complexation of H2-1 with a cobalt or iron salt led first to oligonuclear species due to the additional metalation of the phenanthroline moiety. Hence, the crude reaction product was treated with EDTA-buffer under aerobic conditions to demetalate the phen-moiety in order to obtain pure Co-1 and FeCl-1 (Fig. 1), which was confirmed by ESI-MS and UV-Vis-spectroscopy. Both complexes were formed in only moderate yields of around 40% due to formation of multiple side-products that were not characterized. The subsequent reaction of Co-1 and FeCl-1 with bis-(4,4′-di-tert-butyl-2,2′-bipyridine) ruthenium(II) dichloride (Ru(tbbpy)2Cl2) in DMF–water (∼10 : 1) led to the formation of Co-1-Ru and FeCl-1-Ru in about 54% yield. Due to the moderate overall yield based on H2-1 (22%) another synthetic strategy was followed. It starts first with the reaction of H2-1 and Ru(tbbpy)2Cl2 to form H2-1-Ru and in a subsequent step the second metal is inserted into the porphyrin moiety to obtain Co-1-Ru or FeCl-1-Ru in high yield (80% based on H2-1, Fig. 2). Compounds M-1-Ru (M = H2, Co or FeCl) were characterized by high-resolution ESI-MS (Fig. S13–S22†) and UV-Vis-spectroscopy (Fig. 3; Table S1†). H2-1-Ru was also characterized by 1H-NMR-spectroscopy (Fig. S2†) while Co- and FeCl-1-(Ru) are paramagnetic species.
Synthesis of Co-1 and FeCl-1.
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Fig. 2
Synthesis of Co-1-Ru and FeCl-1-Ru (MW = microwave irradiation).
Fig. 3
UV-Vis spectra of M-1 (left) and M-1-Ru (right) with M = H2, Co or FeCl.
The EPR-spectra of FeCl-1 and FeCl-1-Ru exhibit a typical Fe(III) porphyrin signal at 77 K (g = 6.0, Fig. S1†).37 In order to obtain an EPR signal of Co(II) species temperatures below 20 K are required,38 but these measurements are not possible with our experimental setup. Hence, Co-1-(Ru) was not investigated by EPR spectroscopy. To prove the paramagnetism of Co-1-(Ru), the magnetic moment was determined by the Evans-method.39 For both compounds, Co-1 and Co-1-Ru, the µeff is about 2.2 which is higher than the theoretical value µs.o. = 1.73 for a spin system of S = 1/2. This deviation can be explained by the spin– orbit coupling which is not considered for the theoretical µs.o.40 and experimental error. Electrochemical investigations (including cyclic voltammetry and square-wave voltammetry) in dichloromethane for most of the M-1-(Ru) compounds were published earlier.36
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Table 1 provides an overview of all the redox potentials measured for the compounds M-1 and M-1-Ru (see also Fig. S7–S10†). Other solvents like acetonitrile, DMF or THF could not be used because of their low solubility. The metal center inside the porphyrin cavity has a marked influence on the oxidation potentials of M-1 (M = Zn, Cu, Pd) but almost no influence on the two reduction potentials, which appeared at similar values of −1.8 and −2.2 V vs. Fc/Fc+. Attaching an electron withdrawing Ru(tbbpy)22+ fragment only had a small influence on the oxidation potentials but clearly affected the reduction potentials. Hence, the first and the second reduction potential of M-1-Ru (M = H2, Cu, Pd) are shifted anodically by about 0.2–0.3 V and the third reduction event appeared. Furthermore, a RuII/RuIII redox event is observable in the oxidation region and, in some cases, overlaps with a ligand-based oxidation.
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Dalton Transactions Redox potentials of M-1 with M = Zn, Cu, Pd, H2, Co and FeCl and M-1-Ru with M = Cu, Pd, H2 in DCM containing 0.1 M NBu4PF6 (TBAP)a
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Potential vs. Fc/Fc+ in DCM Zn-136 Cu-136 Pd-136 H2-136 Cu-1-Ru36 Pd-1-Ru36 H2-1-Ru [Ru(tbbpy)2(phen)](PF6)236
−2.25 −2.21 −2.25 −2.01 −2.16 −2.09 −1.95 −2.08
−1.80 −1.78 −1.80 −1.74 −1.93 −1.85 −1.82 −1.80
Co-1 FeCl-1
−2.25 Ep,c −2.18 Ep,c
−1.78 −1.47
0.44 0.62 0.67 0.59 0.60 0.76 0.66
−1.57 −1.50 −1.54 −1.25 Ep,c −1.23
−0.84 Ep,c
0.51 0.72b
0.70 0.89 1.13 0.88b 0.91 0.85 0.84
1.03
0.79 1.15
a
Ep,c stands for cathodic peak potentials in the case of irreversible reduction events. Note that almost all redox events of FeCl-1-Ru and Co-1-Ru are irreversible and broad. Thus, determination and assignment of the events is very difficult and we do not denote the potentials here. b 2-Electron processes because of overlapping events.
Additional experiments on the cobalt and iron complexes were conducted in this work and are discussed in more detail in the ESI (Fig. S10†). In both cases, with M = Co or FeCl, the first reduction is irreversible and metal-based, but further reduction events are either irreversible or quasi-reversible and not easily assigned (a mix of ligand- and metal-based reductions occurs). On the other hand, two quasi-reversible oxidation events can be observed for Co-1 as well as FeCl-1 that are assigned to metal and ligand oxidations (a full discussion is included in the ESI†). The UV-Vis absorption spectra of M-1 and M-1-Ru (M = H2, Co or FeCl) in dichloromethane (DCM) are shown in Fig. 3. The spectra of M-1 depict typical porphyrin spectra with a strong absorption band around 420 nm (Soret-band) and weaker bands in the 500–650 nm region (Q-bands). The metal center has a great influence on the position of the Q-bands, whereas the position of the Soret-band does not strongly depend on the metal center (M = H2, Co or FeCl). However, the Soret-band structure of Co-1 and FeCl-1 differs from that of metal free H2-1. The Soret-band is broadened, less intense and shows splitting into two bands (which has also been observed in the literature before),26 albeit this splitting is very small for Co-1. This Soret-band shape is in accordance with other M-1 compounds with M = Cu, Pd or Zn synthesized before (see also Table S1 for a complete list of the absorption maxima and Fig. S3 and S4†).36 The Soret-band structure of compounds M-1-Ru (M = H2, Co, FeCl) is broadened further in comparison with that of parent compounds M-1 (M = H2, Co, FeCl), while the influence of the ruthenium moiety on the position of the Q-bands is quite small. A band at 290 nm arises that represents π–π*-transitions of the bipyridine ligands of the ruthenium moiety. It becomes clear that the UV-Vis spectra are not a simple addition of the properties of the individual components (see Fig. S5† for a comparison between M-1 and M-1Ru for M = Cu, Co and FeCl).36 The emission properties of the compounds were studied under steady state conditions. Only the metal-free and zinc compounds show fluorescence (see Fig. S6†). Emission
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maxima in DCM can be found at 655 and 726 nm for H2-1 and they are blue-shifted for Zn-1 (615 and 673 nm).36 H2-1-Ru exhibits, compared to H2-1, red-shifted emission maxima at 665 and 734 nm. The fluorescence spectra resemble in any case (i.e. changing excitation wavelengths) a porphyrin-based emission. Thus, even with the Ru(tbbpy)22+ fragment attached, the lowest-lying excited state is presumably located on the porphyrin moiety. No steady state emission is observed for M-1 and M-1-Ru compounds with M = Cu, Pd, Co or FeCl. Photolysis experiments Photochemical CO2 reduction experiments were conducted in DMF as a solvent and TEA (5%) as a sacrificial electron donor under 1 atmosphere of CO2. DMF was used due to solubility reasons – compounds M-1 are not well soluble in water, acetonitrile or THF, but show good solubility in DCM and reasonably well in DMF (up to a concentration of ca. 0.1 mM). In preliminary experiments we realized a strong dependence of the catalytic activity on the light source (the type of lamp) and finally used a 200 W high-pressure-mercury-vapor lamp for all catalytic experiments. Several (long-wave pass) cutoff filters were utilized for wavelength-dependent experiments. Furthermore, the formation of CO was determined and quantified by gas chromatography. To estimate the amount of CO that was produced by other sources like solvent decomposition, blank experiments were conducted without the catalyst (and/or under an argon atmosphere) under the same experimental conditions and all turnover numbers of CO were corrected by these values (further details can be found in the Experimental section). Irradiation of the catalyst (e.g. Co-1) in DMF and 5% TEA under an argon atmosphere leads to the same small amount of CO within the experimental error as an experiment without the catalyst. If the catalyst or TEA is not present under a CO2 atmosphere, illumination of the solution leads to the formation of the same amount of “background” CO as experiments under argon. Furthermore, no CO is produced in the dark or due to heating the reaction mixture at 60 °C in the dark for 24 h under a CO2 atmosphere. These results illustrate
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that neither the catalyst supports decomposition of the solvent nor photodecomposition of the catalyst leads to substantial CO formation. Hence, the corrected TONCO values given below refer only to the gas-phase detected amount of CO formed from CO2 reduction after a certain time period. CoTPP was investigated earlier under similar conditions by another group.32,33 However, a different setup was used (e.g. a 300 W Xenon lamp as the light source) and hence the TONCO given in the literature cannot be compared directly with our results. Thus, we determined the TONCO of CoTPP with our setup and discuss these values in relation to our catalysts (the same applies to FeClTPP). In Fig. 4A the TONCO values for CoTPP and Co-1 are shown after different photolysis times and using different long-wave pass cutoff filters. CoTPP and Co-1 are not active towards photochemical CO2 reduction using a >400 nm cutoff filter. This reveals clearly that only visible light is not sufficient to produce CO. A similar observation was reported by Bonin just recently.34 Thus, we worked with cutoff filters that also transmit near UV-light. In this case, a constant CO evolution can be detected for the first 60 hours of irradiation after which the curve seems to flatten. The flattening is stronger for CoTPP, which might indicate better stability of Co-1 in long-term experiments. However, long-term experiments have not been conducted so far and are planned in the near future together with more mechanistic
Fig. 4 A: Time course of TONCO for CoTPP and Co-1 using different cutoff filters. B: TONCO after 24 h of photolysis for MTPP and M-1 with M = Co or FeCl depending on the cutoff filter (experimental conditions: a 5 mL solution of the catalyst (5 × 10−5 M) in DMF and TEA (5%) was photolyzed under 1 atm. CO2 in a Schlenk tube (the gas volume above the solution: 20 mL)).
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experiments. In addition, decomposition processes and poisoning of the catalyst by produced CO cannot be excluded.32 In the case of Co-1, we performed mass spectroscopic studies after 24 h of photolysis. No additional peaks can be detected in the MALDI or ESI-MS spectrum and the spectrum looks basically the same as the one for Co-1 before the reaction (Fig. S16†) indicating good stability of the catalyst. The TONCO of CoTPP and Co-1 differs only marginally using the >320 nm cutoff filter; after 93 hours the TONCO is 4–5 in both cases, whereas the TONCO values are significantly higher using the >305 nm cutoff filter. The TONCO of CoTPP after 93 hours reaches 16 and, therefore, CoTPP is one-fourth times more active than Co-1 that reaches a TONCO of 12. Because borosilicate glass does not transmit light with wavelengths lower than 300 nm and because of possibly more side reactions no filter with a lower cutoff wavelength was used. The wavelength dependence of the TONCO for cobalt and iron chloride complexes of TPP and 1 was investigated further with different filters between 300 and 400 nm (Fig. 4B). Each photolysis experiment was carried out under the same experimental conditions and the TONCO values were determined after 24 hours. Although the catalysts are still active at that time and the catalytic reaction still proceeds, a clear comparison between the catalytic activities of the different catalysts can be obtained. When the >400 nm, >375 nm and >350 nm cutoff filters are used the TONCO values of all regarded compounds are in the range between 0 and 1. This picture changes when lower wavelength filters are used. The TONCO values of CoTPP and Co-1 for the >320 nm filter are very similar, but this is surprisingly not valid for the iron complexes. FeCl-1 is more active than FeClTPP at all wavelengths below 350 nm. Furthermore, the TONCO of FeCl-1 is around 8 using the >305 nm cutoff filter and is twice as high as the TONCO of FeClTPP in contrast to the cobalt complexes where Co-1 is only half as active as CoTPP. The TONCO for CoTPP using the >305 nm filter and our experimental setup is around 6 compared to the previously reported value of 18.33 We want to stress again that the latter value was determined with a different experimental setup and irradiation source. Nevertheless, the TONCO of 6 for CoTPP after 24 h of illumination will define the reference point for the following comparison of the catalytic activity of the new compounds. In summary, FeCl-1 shows the highest catalytic activity towards photochemical CO2 reduction of all the four compounds investigated so far (even a higher TONCO than CoTPP). The reason why we see higher activity using light in the 300 nm region than using visible light is vague at the moment. We want to note, however, that similar results could be observed in photocatalytic proton reduction and neither was an explanation presented in those studies.20,41,42 Bonin et al. suggested that efficient reductive quenching of an excited iron porphyrin catalyst by triethylamine can only occur with UV-light.34 However, they used illumination wavelengths λ > 280 nm and did not use any other filters to support their proposition. In future work we want to try to shed light on this behavior with the help of extended photophysical studies.
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Now, we wanted to investigate the catalytic activity of all the M-1-Ru compounds synthesized to determine the influence of the metal center in the porphyrin moiety as well as that of the attached Ru(tbbpy)22+ fragment. Photolysis experiments were carried out under the same experimental conditions as used before either using the >305 nm filter or the >375 nm filter to screen all the potential catalysts. The TONCO values were determined after 24 hours and are summarized in Table 2. The error range for the TONCO is estimated to be around 10%. For all the investigated compounds the TON values obtained with a >375 nm cutoff filter are below 1 after 24 hours of illumination but they can still be used to compare the influence of different metal centers. No CO could be detected for Cu-1 and Pd-1 when working with the >375 nm filter, but both show a TONCO around 1 if a >305 nm filter is applied. As expected, using zinc or no metal in the porphyrin moiety gives no catalytic activity at all. The inactivity of Zn-1 and H2-1 reveals that the porphyrin central metal plays a crucial role in the photochemical CO2 reduction and it seems that especially the redox-activity of the metal is a key factor. The attached ruthenium fragment had a positive influence on the catalytic performance for all the investigated M-1-Ru compounds compared to their parent M-1 compounds. Pd-1Ru and Cu-1-Ru display a 3 times higher catalytic activity than Pd-1 and Cu-1 using the >305 nm filter and working with the >375 nm filter, a small amount of CO gas could be detected for the ruthenium complexes. Furthermore, FeCl-1-Ru and Co1-Ru exhibit a 1.5 times higher TONCO than the corresponding ruthenium free complexes using the >305 nm filter and an improved catalytic performance was also observed when using the >375 nm filter. Thus, as intended, the ruthenium-moiety increases the catalytic activity of the whole complex. A simple antenna effect of the ruthenium chromophore in the near UV region might assist the photochemical reaction; however, we do not have conclusive evidence for that yet (see also Fig. S6†). In summary, FeCl-1-Ru represents the best porphyrinic photocatalyst reported to date with roughly twice the activity as
Table 2 TONCO of M-1 and M-1-Ru complexes after 24 hours of photolysis (in DMF/TEA (5%), CO2 atmosphere)
Catalyst
TONCO (>305 nm cutoff filter)
TONCO (>375 nm cutoff filter)
CoTPP Co-1 Co-1-Ru FeClTPP FeCl-1 FeCl-1-Ru Cu-1 Cu-1-Ru Pd-1 Pd-1-Ru H2-1 H2-1-Ru Zn-1
5.8 3.2 4.7 3.8 7.6 11.4 0.7 3.0 1.0 3.1 0 0 0
0.4 0.1 0.4 0.1 0.1 0.7 0 0.1 0 0.1 0 0 0
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CoTPP after 24 hours of illumination (TONCO of 12 vs. 6). It is important to note that the final TON values are not very high, however they are obtained only after 24 hours with a limited setup (light source). Thus, in comparison with the best system reported so far, CoTPP, we could observe a marked increase in activity even if we just focus on a short reaction time. Unfortunately, they still cannot be compared with other systems based totally on rare elements7 that show a much higher catalytic performance. For example, Tanaka et al. presented a system composed of [Ru(bpy)3]2+ and [Ru(bpy)2(CO)2]2+ using BNAH as the sacrificial electron donor in H2O–DMF that showed a TONCO of 120 after 10 hours of irradiation43 producing considerable amounts of formate too. Formate can be produced as a major product with a TONHCOOH > 500 if the two single units are covalently connected to form a PMD.45 Carbon monoxide can be formed almost exclusively with TONs > 100 after 8 hours of irradiation with Ru–Re dyads and BNAH in DMF as introduced by Ishitani and coworkers.4,13,44 We further investigated the influence of different sacrificial electron donors on the photochemical catalysis. Photolysis experiments were conducted with Co-1 as the catalyst (5 × 10−5 M) dissolved in DMF with 5% TEA, 5% triethanolamine (TEOA) or 1-benzyl-1,4-dihydronicotinamide (BNAH, 0.1 M) as sacrificial electron donors. The TONCO using TEOA is about 3 after 24 h of irradiation, which is the same value as using TEA. In contrast, when BNAH is used as the sacrificial electron donor, no photochemical CO2-reduction took place after 24 h. BNAH is normally used as a sacrificial electron donor for Ru(II) based-photocatalysts45 and not for porphyrins. Hence, BNAH seems to be unsuitable to act as a sacrificial electron donor in combination with a metal porphyrin complex. In order to detect other catalytic products (like formic acid) we performed solvent suppression NMR experiments with the reaction solution. So far, no formate could be found using TEA as the sacrificial electron donor and Co-1 as the catalyst. In the GC analysis we are also unable to observe dihydrogen formation. Thus, with our setup, CO is selectively formed and the only catalytic product detected. Similar results were published in the literature for other porphyrin compounds, although a small amount of dihydrogen could be detected in those studies.32,34 The results obtained raise a few questions, which we cannot answer right now. Why do we see an opposite catalytic performance for M-1 vs. MTPP (for M = Co, FeCl)? How is CO2 activated (e.g. which oxidation state of the metal center is needed)? Mechanistic investigations are on the way and with additional spectroscopic experiments we expect to answer some of the questions in the near future. At present, we can only draw some parallels to already published work about the reaction mechanism. Fujita, Neta and coworkers performed extended studies on the reaction mechanism of cobalt and iron macrocycles with carbon dioxide.32,33,46–48 Triethylamine functions in every case as a reductive quencher that reduces the excited macrocyclic compound during photocatalysis. In the case of the iron and cobalt compounds it is very likely that the metal center in the
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porphyrin cavity needs to be reduced to at least the metal(I) state (maybe even the metal(0)-state).32,33,46 Bonin et al. conclusively showed that in the case of iron porphyrins, e.g. FeHPP, the iron(0) state is the one that activates CO2.34 It might be that the efficient reduction of e.g. the double reduced species to the triple reduced species can only occur under near UV-light irradiation. But this point needs further studies by photophysical experiments in the near future. At this point, the “highly” reduced metal center can attack a CO2 molecule to form the initial adduct. Now, depending on the experimental conditions, there are two ways how CO2 can be transformed to CO.49 In the beginning of the catalysis the proton concentration is very low and thus we assume that an oxygen atom is probably transferred to a second CO2 molecule to produce carbonate and CO.47 The decomposition of TEA leads to the formation of protons32,47,50 that can react with activated CO2 to yield water and CO.34,46 As the reaction time prolongs the concentration of protons increases and this pathway likely becomes dominant. Because of the non-activity of H2-1, Zn-1 and H2-1-Ru we do not believe that the ruthenium fragment plays a direct role in the reduction of CO2. As discussed earlier (and can clearly be deduced from the electrochemical investigations), the ruthenium center shifts the reduction potentials of the compounds to less negative values, which positively influences the catalytic performance. Note that the catalysts need to be reduced at least twice (see Fig. S11 and S12† for Cu-1) or threefold (as is likely the case for FeClTPP or FeCl-132–34) before they become active in CO2 reduction and depending on the metal center in the porphyrin cavity one or two of these reduction events should be metal-based. The ligand 1 might function as an electron reservoir during the catalysis and, in doing so, assists the metal center in the porphyrin moiety to convert CO2. Finally, we want to emphasize that photocatalytic reactions performed in different groups are not easy to compare because of a strong influence of the light source, filters used, solvent and other reaction conditions on the catalytic performance, which seems to be specially complicated when using porphyrinic compounds. Conclusion We successfully developed mononuclear phenanthrolineextended metal porphyrin complexes M-136 containing various metals (M = FeCl, Co, Cu, Pd) for the photocatalytic reduction of CO2 to CO. Their dinuclear analogues M-1-Ru were synthesized by simple attachment of a Ru(tbbpy)22+ fragment and represent a new class of PMD. In every case, the ruthenium moiety increases the catalytic activity of the whole molecule. This is very likely due to the fact that the Ru(tbbpy)22+ fragment influences the reduction potentials of the catalyst so that they become less negative and thus the photochemical quenching reaction with the sacrificial electron donor becomes more efficient which finally raises the CO2-reduction activity. This result should in principle be achievable with any other positively charged metal fragment. We do not think that the ruthenium fragment otherwise takes part in the
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photochemical reaction. However, a simple antenna effect of the ruthenium chromophore in the near UV region cannot be excluded too. In addition, we can imagine that the electron storage capability of the ligand framework 1 is helpful for the accumulation of reducing equivalents. Furthermore, we revealed that near UV-light is necessary for the photochemical CO2 reduction by means of wavelength dependent photolysis experiments. Merely visible light is not sufficient to catalyze the CO2 reduction by metal porphyrin complexes as has been observed by others just recently34 as well. In addition, we found out that the photocatalytic activity is dependent on the redox activity of the porphyrin metal. The highest TONCO could be achieved using Co or FeCl porphyrin complexes, whereas Zn or free base porphyrins are inactive. FeCl-1 and FeCl-1-Ru display the highest catalytic activity and show even higher TONsCO in our setup than the already known compounds CoTPP32 and FeClTPP33, which presented the most catalytically active porphyrins up to this point. Although these values are still lower than the best reported CO2 reduction photocatalysts as stated above,7,43–45 they show the potential of non-precious metal catalysts in light-driven CO2 reduction.
Experimental section General methods Silica gel for column chromatography was obtained from Acros (Silica Gel 60, 230–400 μm mesh). All solvents (tetrahydrofuran (THF), toluene, dichloromethane (DCM), dimethylformamide (DMF), dimethylsulfoxide (DMSO), hexane, acetonitrile (ACN), acetic acid (HAc) and ethanol) and starting materials were obtained from Sigma-Aldrich or abcr. If necessary, solvents were purified employing an MBraun Solvent Purification System. DMF for photolysis experiments was fractionally distilled with benzene and water, dried with CaH2 and finally distilled under argon. Triethylamine for photocatalytic reactions was refluxed over KOH, filtered, dried with CaH2 and finally distilled under argon. Starting materials were used without further purification. Bis(4,4′-di-tert-butyl-2,2′-bipyridine) ruthenium(II) chloride (Ru(tbbpy)2Cl2),51 CoTPP,52 FeClTPP,52 H2-1, Cu-1, Pd-1, Zn-1, Pd-1-Ru, and Cu-1-Ru36 were synthesized according to literature procedures. 1 H NMR spectra were recorded at ambient temperature either on a Bruker DPX-300 or an AV-400 spectrometer. All spectra were referenced to tetramethylsilane (TMS) or deuterated chloroform (CDCl3) as an internal standard (measured values for δ are given in ppm and for J in Hz). Assignment of signals was done with the help of 2D experiments and is presented in the ESI (Fig. S2†). Elemental analysis was performed by the microanalytical laboratory of the Institute of chemistry at the Humboldt-Universität zu Berlin using a HEKAtech EURO 3000. ESI mass spectra were obtained in a methanolic solution using Thermo Finnigan LCQ XP and LTQ FT instruments. EPR spectra were recorded on an X-band spectrometer ERS 300 (ZWG/Magnettech Berlin/Adlershof, Germany)
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equipped with a fused quartz Dewar for measurements at liquid nitrogen temperature. The g-values were calculated with respect to a 2,2-diphenyl-1-picrylhydrazyl (dpph) reference (g′ = 2.0036). Absorption spectroscopic measurements were made in DCM solutions of each compound using a Cary 100 UV-VisNIR spectrometer from Varian employing the software Cary WinUV. SUPRASIL® Quartz cells from Hellma Analytics with a 10 mm path length were used. Microwave reactions were carried out at 50 W in a CEM monomode microwave (model: Discover Bench mate). Emission spectra were recorded on a Cary eclipse fluorescence spectrophotometer using SUPRASIL® Quartz cells from Hellma Analytics with a 10 mm path length. Irradiation experiments were performed in DMF and 5% TEA with 5 × 10−5 M catalyst concentration. Schlenk flasks with a 5 mL solution and 20 mL headspace were used. Fresh solutions were prepared before each experiment. The solutions were bubbled with CO2 for 20 minutes to remove argon. Photolysis was performed with a 200 W high-pressure-mercuryvapor lamp using a water filter to absorb IR radiation and different long-wave pass cutoff-filters to absorb wavelengths below the given values. The filters were purchased from LOTQuantumDesign GmbH and have an edge rise of 2% (an example of a “filter spectrum” is given in the ESI, Fig. S23†). CO evolved was determined by gas chromatography (Shimadzu GC-17A with a thermal conductivity detector and Resteks ShinCarbon packed column ST 80/100 (2 m, 1/8″ OD, 2 mm ID) and quantified using a calibration curve. The TONCO was then calculated by dividing the amount of CO formed (corrected by the amount of CO formed in blank experiments) by the amount of catalyst used. In blank experiments, no CO was detected using a >350 nm cutoff-filter and higher, while TONCO values obtained with the >305 nm filter were corrected by a value of 0.8. Measurements were performed by manual injection of gas phase samples (250 μl) and in triplicate. CO dissolved in the solvent was not considered, thus the TON of CO is only related to the amount detected in the gas phase above the solution. {(Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrin-bis(4,4′-di-tert-butyl-2,2′-bipyridine)-ruthenium(II)} dihexafluorophosphate (H2-1-Ru). H2-1 (40 mg, 41 µmol) and Ru(tbbpy)2Cl2 (44 mg, 62 µmol, 1.5 eq.) were dissolved in 30 mL DMF and 3.3 mL water. This solution was heated in a microwave cavity at 120 °C for 6 hours. The solvent was subsequently removed under reduced pressure and the product was purified by column chromatography with acetonitrile– water = 9 : 1. The product was eluted after additional 0.05 vol.% of saturated aqueous KNO3 was added. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The brown precipitate was filtered off with Celite, washed with water, dried in a vacuum and the product was washed off using DCM. The solvent was subsequently removed under reduced pressure to yield H2-1-Ru (77 mg, 40 µmol, 99%).
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Anal. Calcd for C104H106F12N12P2Ru·CH2Cl2: C, 62.61; H, 5.62; N, 8.30. Found: C, 63.06; H, 5.44; N, 8.40; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 287 (95.4), 428 (140.1), 538 (19.7), 608 (12.7), 658 (1.5); 1H NMR (400 MHz, CD3CN, δ): −2.39 (s, 2H), 1.34 (s, 18H), 1.48 (s, 18H), 1.75–1.83 (m, 24H), 2.58 (s, 6H), 2.79 (s, 6H), 7.20 (dd, J = 1.5 and 4.8 Hz, 2H), 7.32 (s, 4H), 7.44 (s, 2H), 7.49–7.57 (m, 6H), 7.75 (d, J = 5.0 Hz, 2H), 7.89 (dd, J = 3.9 and 6.3 Hz, 2H), 8.14 (dd, J = 0.9 and 4.2 Hz, 2H), 8.50 (s, 2H), 8.51 (d, J = 1.5 Hz, 2H), 8.57 (d, J = 1.5 Hz, 2H), 8.78 (d, J = 3.6 Hz, 2H) 8.87 (d, J = 3.6 Hz, 2H), 9.00 (dd, J = 1.1 and 6.2 Hz, 2H), 13C NMR (125 MHz, CD3CN, δ): 20.71, 29.43, 29.56, 35.27, 35.42, 115.74, 119.59, 121.48, 121.57, 124.46, 124.63, 126.78, 127.72, 127.81, 128.09, 128.14, 131.31, 133.15, 133.93, 137.29, 137.45, 137.98, 138.12, 138.25, 138.42, 138.58, 139.02, 139.54, 139.61, 143.19, 149.65, 151.18, 151.44, 152.20, 152.92, 155.49, 156.76, 157.13, 162.59, 162.78. ESI-MS m/z: [M − 2PF6−]2+ calcd for C104H106N12Ru 812.3848. Found 812.3852. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-cobalt(II) (Co-1). H2-1 (25 mg, 30 μmol) and Co(Ac)2·4H2O (31 mg, 130 μmol) were dissolved in 15 mL DMF and the reaction mixture was heated at reflux overnight. The solution was subsequently evaporated to dryness. The solid residue was dissolved in 10 mL DCM, washed with water and dried over Na2SO4. After removal of the solvent under vacuum the solid residue was dissolved in 5 mL DCM and stirred vigorously with 3.5 mL EDTA-buffer (10 mM H4EDTA, 27 mM NaAc and 0.04 mM HAc) for 24 h. The organic layer was separated and washed with 5% aqueous NaHCO3, water and dried over Na2SO4. The solid residue was then purified using silica column chromatography with DCM and 3% MeOH as the eluent. This method afforded Co-1 as a red/brown solid (10 mg, 10 μmol, 40%). Anal. Calcd for C68H56N8Co·0.5 × CH2Cl2: C, 75.71; H, 10.31; N, 5.29. Found: C, 75.29; H, 10.46; N, 5.38; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 411 (99.3), 557 (20.4), 587 (17.4); ESI-MS m/z: [M + H+] calcd for C68H57N8Co 1044.4033. Found 1044.4030. {(Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-cobalt(II) bis(4,4′-di-tert-butyl-2,2′-bipyridine)ruthenium(II)} dihexafluorophosphate (Co-1-Ru). Variant 1: a solution of H2-1-Ru (30 mg, 16 µmol) and Co(Ac)2·4H2O (20 mg, 80 µmol) in 15 mL DMF was prepared and heated at reflux overnight under an argon atmosphere. An aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added and the precipitate was filtered off. The crude product was subsequently purified by column chromatography with acetonitrile–water = 9 : 1. The product was eluted with additional 0.05 vol.% of saturated aqueous KNO3. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The violet-brown precipitate was filtered off with Celite, washed with water and dried in a vacuum. The remaining solid was washed using DCM. The solvent was subsequently removed under reduced pressure to yield Co-1-Ru (30 mg, 15 µmol, 80%).
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Variant 2: to a solution of Co-1 (27 mg, 26 µmol) in 30 mL DMF and 3.8 mL H2O was added Ru(tbbpy)2Cl2 (27 mg, 39 µmol, 1.5 eq.). This solution was heated for 72 h under reflux. The solvent was subsequently removed under reduced pressure and the product was purified by silica column chromatography with acetonitrile–water = 9 : 1. The product was eluted after additional 0.05 vol.% of saturated aqueous KNO3 was added. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The violet-brown precipitate was filtered off with Celite, washed with water, dried in a vacuum and the product was washed off using DCM. The solvent was subsequently removed under reduced pressure to yield Co-1-Ru (27 mg, 14 µmol, 53%). Anal. Calcd for C104H104N12P2F12RuCo·1.75 × CH2Cl2: C, 60.15; H, 5.12; N, 7.96; Found: C, 59.70; H, 5.52; N, 8.38; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 287 (92.4), 423 (86.7), 470 (76.4), 574 (18.8); ESI-MS m/z: [M − 2PF6−]2+ calcd for C104H104N12CoRu 840.8436. Found 840.8445. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-iron(III) chloride (FeCl-1). H2-1 (30 mg, 30 μmol) and FeCl2 (15 mg, 121 μmol) were dissolved in 15 mL DMF. This solution was heated in a microwave cavity at 130 °C for 3.5 h and after cooling to room temperature subsequently evaporated to dryness. The solid residue was dissolved in 10 mL DCM, washed with water and dried over Na2SO4. After removal of the solvent under vacuum the solid residue was dissolved in 6 mL DCM and stirred vigorously with 4.5 mL EDTAbuffer for 24 h. The organic layer was separated and washed with 5% aqueous NaHCO3, water and dried over Na2SO4. The material was then purified using silica column chromatography with DCM and 3% MeOH as the eluent. The product was subsequently dissolved in 20 mL DCM and stirred over 7% HCl for 1.5 hours. The organic layer was separated, washed with water and dried over Na2SO4 to yield FeCl-1 as a violet/ brown solid (13 mg, 12 μmol, 40%). Anal. Calcd for C68H56N8FeCl·CH2Cl2: C, 71.35; H, 9.16; N, 5.03 Found: C, 71.73; H, 9.26; N, 5.60; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 380 (75.0), 438 (100.4), 524 (19.0), 734 (sh, 3.2); MALDI-MS m/z: [M + H+] calcd for C68H57FeN8Cl 1076.4. found 1076.4; ESI-MS m/z: [M − Cl− + MeOH]+ calcd for C69H60FeN8O 1072.4234. Found 1072.4233. {(Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-iron(III) chloride-bis(4,4′-di-tert-butyl-2,2′-bipyridine)-ruthenium(II)} dihexafluorophosphate (FeCl-1-Ru). Variant 1: H2-1-Ru (30 mg, 16 µmol) and FeCl2 (12 mg, 63 µmol) were dissolved in 15 mL DMF under an argon-atmosphere. This solution was heated in a microwave cavity at 130 °C for 3.5 h. An aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added and the precipitate was filtered off. The crude product was subsequently purified by column chromatography with acetonitrile–water = 9 : 1. The product was eluted with additional 0.05 vol.% of saturated aqueous KNO3. After removing the solvent, the product was dissolved in 10 mL DCM and stirred over 7% HCl for 1.5 h. The organic layer was separated, washed with water and dried over Na2SO4.
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After removing the solvent, the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The precipitate was filtered off with Celite, washed with water and dried in a vacuum. The remaining solid was washed off by changing the solvent to DCM. The solvent was subsequently removed under reduced pressure to yield FeCl-1-Ru (30 mg, 15 µmol, 80%). Variant 2: to a solution of FeCl-1 (27 mg, 19 µmol) in 20 mL DMF and 2 mL H2O was added Ru(tbbpy)2Cl2 (20 mg, 29 µmol, 1.5 eq.). This solution was heated under an argon atmosphere for 72 h under reflux. The solvent was subsequently removed under reduced pressure and the product was purified by silica column chromatography with acetonitrile–water = 9 : 1. The product was eluted after additional 0.05 vol.% of saturated aqueous KNO3 was added. After removing the solvent the product was dissolved in ethanol and an aqueous ammonium hexafluorophosphate solution (ca. 0.1 M) was added. The violet-brown precipitate was filtered off with Celite, washed with water, dried in a vacuum and the product was washed off using DCM. The solvent was subsequently removed under reduced pressure to yield FeCl-1-Ru (20 mg, 10 µmol, 55%). Anal. Calcd for C104H104N12P2F12ClRuFe·2 × CH2Cl2: C, 60.36; H, 5.11; N, 8.05. Found C, 60.18; H, 5.43; N, 8.06; UV (DCM) λmax, nm (ε × 10−3, dm3 mol−1 cm−1): 287 (83.6), 423 (83.3), 530 (sh, 22.3), 622 (sh, 7.0), 690 (sh, 4.0); ESI-MS m/z: [M − Cl− − 2PF6− + CH3O−]2+ calcd for C105H107N12OFeRu 854.8536. Found 854.8533.
Acknowledgements We thank the German Science Foundation (DFGSCHW 1454/32) for financial support and Prof. Stößer for his help acquiring EPR spectra.
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