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Synthesis and characterization of a trinuclear iridium(III) based catalyst for the photocatalytic reduction of CO2† R. O. Reithmeier,a S. Meister,a A. Siebelb and B. Rieger*a A trimetallic Ir(III) based complex (3) was synthesized and fully characterized by spectroscopic and electrochemical methods. A detailed comparison to its mono- (1) and bimetallic (2) analogue regarding the photocatalytic reduction of CO2 is outlined. In particular, the effect of intramolecular quenching, provided by ethyl tethers, was investigated. Moreover, the relationship between the photophysical properties, the lifetime of the excited state, the quenching efficiency and the catalytic performance is presented.

Received 27th January 2015, Accepted 26th February 2015

Notably, the covalent linkage of the Ir(III) moieties within the three-armed ligand structure (complex 3) leads to a twofold increase of the turn over number (TON) compared to its monometallic analogue 1.

DOI: 10.1039/c5dt00370a

Taking in account the quantum efficiency of 10% and the TONCO = 60 ( per Ir(III) center), complex 3 is a

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highly active Ir(III) based photocatalyst.

Introduction The homogeneous photocatalytic reduction of CO2 developed remarkably during the last three decades.1–4 A wide variety of chromophoric ligand systems in combination with transition metals, ranging from Ni,5–7 Co,8–10 Ru,11,12 Re13–19 to Ir,20,21 were found to act as photocatalysts. Among these, Ir(III) based complexes combine outstanding advantages, as they act as light absorbing unit and CO2 reduction site, show high absorption in the visible light and are active in aqueous solution.20,21 Beside the development of new photocatalysts, various articles deal with mechanistic studies gaining insights into the catalytic cycle of CO2 reduction, which is still not verified completely.9,22–26 In this regard Ishitani et al. made an interesting finding in 2009.27 By connecting a ruthenium(II) photosensitizer and a rhenium(I) catalyst, a through bond electron transfer for ethyl tethers, which leads to an increased quantum efficiency, was observed. Remarkably, this weak electronic communication on the one hand allows electron transfer, but on the other hand does not diminish the reducing power, as would be the case for conjugated linkers.28 Recently, we obtained the same electronic communication through ethyl spacers by comparing bimetallic Ir(III) based photocatalysts with various bridging length.21 However, due to the fact that in a WACKER-Chair of Macromolecular Chemistry, Technische Universität München, Lichtenbergstr. 4, 85748 Garching b. München, Germany. E-mail: [email protected] b Department of Technical Electrochemistry, Technische Universität München, Lichtenbergstr. 4, 85748 Garching b. München, Germany † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt00370a

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our case both metal sites act as photosensitizer and CO2 reduction catalyst, the through bond interaction lowers the quantum efficiency, caused by intramolecular quenching. Herein, we deepen the insight into this quenching process by comparing the properties of complexes 1–3. Besides, the influence of increasing nuclearity from complex 1 (one Ir(III) center) over complex 2 (two Ir(III) centers) to complex 3 (three Ir(III) centers) towards the catalytic performance is outlined.

Results and discussion Synthesis and characterization Three photocatalysts were synthesized to examine the effect of ethyl bridged metal centers, whereby complexes 1 and 2 serve as reference systems (Fig. 1).21 The title complex 3 was synthesized by reaction of [Ir(Cl)3(tpy)]29 with the corresponding bridging ligand (L3), developed by Zysman-Colman et al. in 2011,30 in ethylene glycol solution at 190 °C overnight (Fig. 2). The three-armed structure of L3 is perfectly suited to further investigate the described through bond interaction, as the aromatic ligand system of the catalyst centers is disconnected by two ethyl bridges and a central benzene unit. Therefore, an electronic communication, which is slightly lower than in case of complex 2, but still higher than for complex 1 should be expected, if the theory of an ethyl tether provided electron transfer holds true. Characterization of the presented compounds was achieved via NMR, ESI-MS, IR and elemental analysis (see Experimental data). According to the NMR data no permanent geometric isomers are present in solution.

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Fig. 1 Synthesized Ir(III) photocatalysts. Left: reference complex 1, middle: reference complex 2, right: title complex 3.

Fig. 2

Fig. 3 Stern–Volmer plots of complexes 1–3 (1.8 mM Ir(III) centers), excitation wavelength 365 nm, 25 °C.

Synthesis of the trimetallic title complex.

Photophysical properties The photophysical properties of complexes 1–3 are very similar, regarding their absorbance and emission behaviour (Table 1). The triplet MLCT absorption bands are located at around 470 and 505 nm,20 with almost identical extinction coefficients, calculated per Ir(III) center. Besides, the emission band of all three structures was obtained at 562–563 nm. The determination of the quenching efficiency of the photocatalysts 1–3 was achieved via Stern–Volmer plots, measured in acetonitrile solution under stepwise addition of aliquots of TEOA as sacrificial electron donor (Fig. 3).20 The most efficient quenching was obtained in case of the bimetallic complex 2, whereas kQτ of complex 1 has the smallest value. Therefore, the Stern–Volmer plot of 3 is located in between 1 and 2. The differences of kQτ mainly result from

Table 1

Fig. 4 Illustration of the TEOA provided quenching (I) and the intramolecular quenching process by electron donation from an OER (II).

diverse quenching rate constants, as the lifetimes of emission from the triplet MLCT excited state are within the same range of 2.5–2.8 microseconds for all presented structures (Table 1). This clearly demonstrates that intramolecular quenching contributes to the quenching efficiency in case of the multinuclear complexes 2 and 3. Hereby, an electron is transferred from an OER to an excited state of another Ir(III) unit (Fig. 4). As

Photophysical properties of complexes 1–3

Complex

λ(3MLCT)a,b [nm] (ε [M−1 cm−1])

λ(3MLCT)a,b [nm] (ε [M−1 cm−1])

λem a [nm]

kQτa [M−1]

τa [ns] (τc [ns])

kQ a [M−1 ns−1]

1 2 3

474 (783) 471 (946) 474 (875)

506 (749) 503 (816) 505 (815)

562 563 563

524 733 610

820 (2690) 865 (2481) 863 (2748)

0.639 0.847 0.707

a

Measured in acetonitrile. b Calculated per Ir(III) center. c Measured in degassed acetonitrile.

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expected, this electron transfer is less efficient for complex 3, compared to 2, as the electron has to pass two ethyl spacers (and the benzene unit), whereby the catalyst centers of complex 2 are separated by only one ethyl linker.

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Electrochemistry Cyclic voltammograms of the mono- and trinuclear complexes are presented in Fig. 5. Both compounds show three reduction waves (C1–C3), whereas only two corresponding oxidation features (A1 and A2) are observed in the positive-going scan. For complex 3 the reduction potentials of C1 and C2 occur at slightly more positive potentials than in the mononuclear complex. In accordance to Ishitani et al., who observed a comparable shift in potential for bimetallic complexes containing a Ru(II) and a Re(I) metal center,27 this indicates a weak interaction between the Ir(III) centers through the three-armed bridging ligand (L3). In case of complex 2 this electronic communication is even stronger due to the short ethyl linker and thus causes a greater shift in potential (Table 2). In a previous study, we have shown that the LUMO of these complexes is the π* molecular orbital, mainly located at the terpyridine (tpy) ligand and hence, the first reduction wave at ca. −1.5 V vs. Fc+/0 likely corresponds to the one electron reduction of the tpy ligand forming a radical intermediate.21 In strongly coordinating solvents like acetonitrile, this intermediate can be stabilized and thus, the reversibility of this redox process is increased. However, such behaviour is only observed for the mononuclear complex 1 if the negative-going potential scan is

limited to −2.0 V vs. Fc+/0 or above (i.e. before the onset of the third reduction wave C3) where the current ratio, ipa/ipc, reaches 0.85 (Table 2). Then, another electron is transferred to the mppy ligand at ca. −1.7 V vs. Fc+/0 and the electrochemical response exhibits a similar behaviour to A1/C1 with a peak current ratio of 0.92. In the trinuclear compound, this second reduction leads to an up to three-fold negatively charged complex which is no longer soluble in acetonitrile. Hence, the species stay adsorbed on the electrode surface if the potential window is limited to −2.0 V vs. Fc+/0 or above, leading to a sharp, intense feature coinciding with A1 upon reversing the direction of the potential scan. This adsorption behaviour was confirmed by rotating disk electrode (RDE) experiments in the same solvent, where the peak was still observed when the electrode was rotated at 400, 900 or 1600 rpm (Fig. S5†). If this feature would correspond to oxidation of species in solution, these species would be removed from the vicinity of the electrode surface through convective mass transport induced by rotation and hence, an oxidation peak would not appear in the positive-going potential scan. Instead, the magnitude of this peak rises with increasing rotation rate, which is related to the increased mass transport leading to a higher concentration of the reduced species on the electrode. Extending the potential window to include the third reduction process (C3), which likely corresponds to a two-electron conversion of Ir(III) to Ir(I), this large oxidation peak (A1) is not observed. This is, because the chloro ligand is eliminated from the complex and the structure collapses, yielding different fragments that are sub-

Fig. 5 Cyclic voltammograms of complex 1 (1 mM, left) and complex 3 (1 mM Ir centers, right) recorded in de-aerated CH3CN containing 0.1 M TBABF4 at room temperature using a scan rate of 100 mV s−1.

Table 2 Cyclic voltammetric potentials and peak current ratios of the complexes 1, 2 and 3 in V vs. Fc+/0 and when limiting the potential scan to 2.0 V vs. Fc+/0 (in brackets)

Complex

E1/2, 1 [V]

ΔE [mV]

ipa/ipc

E1/2, 2 [V]

ΔE [mV]

ipa/ipc

1 2 3

−1.52 (−1.52) −1.49 (−1.45) −1.49 (−1.50)

70 (60) 40 (104) 70 (51)

0.40 (0.85) 0.22 (1.32) 0.82 (0.85)a

−1.68 (−1.70) (−1.63) −1.67

180 (120) (125) 80

0.54 (0.92) (0.87) 0.76 (0.80)a

a

Values obtained in DMF electrolyte.

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Fig. 6 Cyclic voltammograms of complex 3 recorded in Ar-saturated DMF solution containing 0.1 M TBABF4 at room temperature using a scan rate of 100 mV s−1 (left) and under CO2 atmosphere (right).

sequently desorbed. Since the reduced complexes are directly adsorbed on the electrode surface, the magnitude of C3 is increased. In order to avoid adsorption onto the electrode surface, further CVs were recorded in DMF electrolyte (Fig. 6), where this highly reduced complex remains soluble. Hereby, the obtained peak current ratios of A1/C1 and A2/C2 are 0.85 and 0.80, respectively (Table 2). In the presence of carbon dioxide, a catalytic wave appears at an onset potential of circa −2.0 V vs. Fc+/0, which demonstrates the catalytic activity of complex 3 towards the reduction of CO2, similar to what was previously reported for the reference systems 1 and 2.21 In summary, the shifts of the reduction potentials C1 and C2 towards more positive potentials in case of the multinuclear complexes 2 and 3 are in line with the results described before and thus suggest a weak electronic communication between the covalently linked catalyst centers.21,27 In addition to their photocatalytic activity the presented structures appear to be good catalysts for the electrochemical reduction of CO2 as indicated by the moderate electrical bias, which is necessary to drive this reaction. Photocatalysis The photocatalytic performance of complexes 1–3 is shown in Fig. 7. All compounds selectively catalyze the reduction of carbon dioxide to carbon monoxide. Notably, no other reduction products, such as formic acid (2 e−), formaldehyde (4 e−) or methanol (6 e−) are observed. Control experiments in the absence of any of the components (TEOA, CO2, complex 1–3, or in the dark) showed no amounts of carbon monoxide. In a typical run a CO2 saturated acetonitrile solution (5 ml), containing 1 ml (1.26 M) triethanolamine (TEOA) as sacrificial electron donor and the photocatalyst (0.1–0.9 mM Ir(III) centers), is irradiated at a wavelength of 450 nm and the CO evolution is monitored via micro-GC of the gas phase. At a catalyst concentration of 0.34 mM the turn over frequency (TOF) of 1 is 35 h−1, whereas 2 is limited to a TOF of 12 h−1 (Table 3). Conspicuously, the TOF of 3 is situated right between the values of 1 and 2, indicating a strong relation

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Fig. 7 Evolution of carbon monoxide, 0.34 mM Ir(III) centers (1–3), 5 ml CH3CN, 1 ml TEOA, light intensity 3.35 × 10−7 einstein s−1, 450 nm, CO2 atmosphere. Shown data points represent the average values of three measurements.

Table 3

Photocatalytic performance of complexes 1–3

Complex

ca [mM]

TONa

TOFa [h−1]

ϕCO b [%]

1 2 3

0.34 0.34 0.34

33 41 60

35 12 21

18 6 10

a

Calculated per Ir(III) center. b Determined at 0.9 mM of Ir(III) centers.

between the photocatalytic performance and the effect of intramolecular quenching. As shown in Fig. 4 the electron transfer from an OER1 to an excited state leads to the formation of an OER2 and a ground state. Hence, the intramolecular quenching process negatively influences the overall reaction rate of the catalytic cycle, because an excited state is converted into a ground state with simultaneously a constant concentration of OER. On the other hand, the covalent linkage of the photocatalysts within the multinuclear complexes (2 and 3) leads to increased turn over numbers (TON) (Table 3). The highest

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overall conversion of CO2 was obtained in case of catalyst 3 (TONCO = 60, calculated per Ir(III) center). Therefore, the trimetallic title complex (3) was found to be within the most active Ir(III) based photocatalysts for CO2 reduction, reported so far.20,21 In a previous study we were able to determine the OER as initial point of catalyst deactivation and revealed oxidation products of the sacrificial amine as the most probable deactivating reagent.21 In general, the direct quenching product TEOA•+ is converted into α- and β-aminoradicals (TEOA•) due to fast deprotonation.31–33 These species were found to deactivate the catalyst by radical recombination with the OER in case of Re(I) based photocatalysts.34 In contrast, donation of a second electron from TEOA• (or TEOA•+) to a catalyst center in its electronic ground state ([Re]), results in a gentle way of radical elimination.31,34–36 Owing to similar reaction mechanisms,20,34 these processes are likely for Ir(III) based photocatalysts as well. Consequently, the TON is strongly related to the quantity of OER, TEOA• and [Ir] ([Ir] = electronic ground state) within the reaction solution. This hypothesis is supported by literature34 and by a series of photoexperiments with different concentrations of complexes 1–3 (Fig. 8). Hereby, increasing the catalyst concentration, which comes along with a reduced absorption cross section and thus an increased concentration of [Ir], leads to rising TON values. On the contrary, high amounts of OER and TEOA• increase the probability of the above mentioned radical recombination reaction, and the TON rather decreases.34 In case of complexes 2 and 3 the concentrations of OER and TEOA• are limited by the intramolecular quenching process. Thus, an enhanced resistance against catalyst deactivation, compared to complex 1 was to be expected. Besides, spatial proximity of the Ir(III) units is assumed to contribute to the observed trend of the TONs. Bearing in mind the various reaction possibilities of TEOA•, especially the spatial distribution of the reactants seems to be crucial for subsequent processes, whereby the probability for gentle radical elimination (electron transfer from TEOA• to [Ir]) increases with [Ir] being close to the generated TEOA•. This might

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Fig. 9 Determination of the reaction order in catalyst for complex 3, 0.1–2.0 mM Ir(III) centers, 5 ml CH3CN, 1 ml TEOA, light intensity 3.35 × 10−7 einstein s−1, 450 nm, CO2 atmosphere.

explain the obtained TON values, as electron donation from TEOA• to [Ir] becomes more probable with an increasing number of Ir(III) centers per catalyst (1 < 2 < 3). Further investigations concerning the relation between a high local concentration of catalytic centers and the TON are the subject of ongoing research. Finally, the reaction order in catalyst was determined by varying the catalyst concentration at otherwise identical reaction conditions (Fig. 9). TEOA as well as CO2 are added in excess to the reaction solution and therefore the reaction rate is only influenced by the amount of catalyst at the beginning (0–30 min) of the catalysis. Besides, the number of emitted photons by the light source, stirring and the distance of the reaction vessel to the light source are kept constant. Plotting of ln(Ir centers [mM]) against ln(V(CO)/time [ml min−1]) gives an order of 1.1 in catalyst below catalyst concentrations of 0.9 mM Ir centers. Above this value the order switches to 0, because the number of emitted photons by the light source becomes the limiting factor for the overall reaction rate. All given quantum efficiencies are determined at the turning point of the reaction order (0.9 mM Ir centers), which corresponds to the highest achievable value. For more and detailed photoexperiments see ESI.†

Conclusion

Fig. 8 Dependence of TON and catalyst concentration, 5 ml CH3CN, 1 ml TEOA, light intensity 3.35 × 10−7 einstein s−1, 450 nm, CO2 atmosphere.

6470 | Dalton Trans., 2015, 44, 6466–6472

In conclusion, a highly active trimetallic Ir(III) based photocatalyst was synthesized and investigated regarding the catalytic performance towards CO2 reduction. Thereby, a quantum efficiency of 10% and a TONCO = 60 were obtained. In particular, the covalent linkage of three photocatalysts within complex 3 was found to affect both, the activity (TOF) and the stability (TON) of the individual catalyst center. Since the obtained intramolecular quenching process runs counter to the overall reaction rate, the TOF of complex 3 (TOF = 21 h−1) lies in between the values of 1 (TOF = 35 h−1, no intramolecular quenching) and 2 (TOF = 12 h−1, highest contribution of intramolecular quenching). In contrast, the TON of 3 exceeds

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the values of both reference systems. This is on the one hand due to intramolecular quenching, which causes reduced concentrations of OER and TEOA• and thus lowers the probability of catalyst deactivation by radical recombination, on the other hand our investigations indicate that gentle radical elimination via electron donation from TEOA• to [Ir] occurs to an increasing degree.

Experimental section General comments All reactions were performed under an atmosphere of dry argon in oven-dried glassware using standard Schlenk technique. All solvents were purified and dried by standard methods. Commercial available reagents were used without further purification. [IrCl3(tpy)], complex 1,21 complex 221 and 1,3,5-tris(2-(6-phenylpyridin-3-yl)ethyl)benzene30 were synthesized according to literature procedures. NMR spectra were obtained using a Bruker AVIII-300 spectrometer at 298 K. Signals were referenced to residual solvent signals. Elemental analyses were performed in the micro analytical laboratory at the Technische Universität München. Fluorescence spectra were recorded on an Avantes Avaspec-2048 spectrometer. ESI mass spectral analysis was performed on a Varian LCMS 500 spectrometer. UV/Vis spectra were recorded on a Varian Cary 50 Scan UV/Vis spectrometer. Emission lifetimes of the complexes were measured with a homebuilt fast-working Si-Photodiode with a bias voltage of 12 V. The signal was displayed, averaged and saved by an oscilloscope (LeCroy 9350A). For excitation of the complexes a pulsed laser (Nd:YAG laser, Lumonics HY 1200, pulse length: 10 ns, repetition rate: 20 Hz, 355 nm) with low power (50 μJ per pulse) was used. For a measurement an average over 200 laser pulses was taken. Cyclic voltammetry measurements were performed at the Technical Electrochemistry Department of Technische Universität München. The redox potentials of the photocatalysts (1 mM Ir centers) were measured in degassed acetonitrile solution, containing 0.1 M tetraethylammonium tetrafluoroborate. The electrochemical measurements were performed with a Pine AFCBP1 potentiostat. The reaction cell consists of a glass bubbler, a Pt-wire counter electrode, a glassy carbon disk working electrode and a 0.1 M Ag/AgNO3 reference electrode. All irradiation measurements were performed in a 160 ml septum capped Schlenk tube. For selective irradiation the reaction solution was irradiated by a homebuilt LED setup with an excitation wavelength of 450 nm.21 Incident light intensities were determined with a K3[Fe(C2O4)3] actinometer.37–39 The gaseous reaction products were determined by GC-TCD (Varian 490 GC). The liquid phase was examined for other reduction products by GC and NMR spectroscopy. Synthesis of 3 A solution of [IrCl3(tpy)] (100 mg, 0.19 mmol, 3.00 eq.) and 1,3,5-tris(2-(6-phenylpyridin-3-yl)ethyl)benzene (39 mg, 0.06 mmol, 1.00 eq.) in 15 ml of dry and degassed ethylene

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glycol was heated to 190 °C for 20 h. Afterwards the reaction solution was allowed to cool to room temperature and 10 ml of a saturated solution of NH4PF6 (aq.) were added. The precipitated product was separated from the solution via centrifugation, washed with water (20 ml), diluted in acetone (5 ml) and precipitated again in 250 ml of diethyl ether. Purification of the crude product was achieved via recrystallization from 20 ml ethanol–acetonitrile (9 : 1). 1H-NMR (300 MHz, CD3CN, 298 K): δ ( ppm) = 2.94–3.25 (m, 12H, CH2–CH2), 5.97 (dd, 3H, 3 J = 7.5 Hz, aromat. H), 6.72 (td, 3H, 3J = 7.5 Hz, aromat. H), 6.90 (td, 3H, 3J = 7.5 Hz, aromat. H), 7.16 (s, 3H, aromat. H), 7.28–7.42 (m, 6H, aromat. H), 7.46 (dt, 6H, 3J = 5.7 Hz, aromat. H), 7.63–7.75 (m, 3 H, aromat. H), 7.96–8.09 (m, 9H, aromat. H), 8.13 (d, 3H, 3J = 8.4 Hz, aromat. H), 8.27–8.44 (m, 9H, aromat. H), 8.51 (d, 6H, 3J = 8.4 Hz, aromat. H), 9.70 (br, 3H, aromat. H). ESI-MS: m/z = 771.0 ([M – 3 PF6− − IrCl(tpy) + H]2+), 1081.9 ([M – 3 PF6− – 2 IrCl(tpy) + 2H]+). Anal. Calcd for C90H69Cl3F18Ir3N12P3: C, 44.37; H, 2.85; N, 6.90. Found: C, 43.84; H, 3.00; N, 6.80. UV/Vis: λmax (CH3CN) = 474 nm, (ε = 2625 cm−1 M−1), 505 nm (ε = 2445 cm−1 M−1). Fluorescence: λem = 563 nm. All analytical spectra are given in the ESI.†

Acknowledgements Funding by the BMBF and by the Clariant AG. (subsidy indicator: 01RC1106A) is thankfully acknowledged. R. Reithmeier is thankful for proofreading by Peter Altenbuchner and Patrick Werz. S. Meister gratefully acknowledges the Beilstein Institute for a Ph.D. scholarship.

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Synthesis and characterization of a trinuclear iridium(III) based catalyst for the photocatalytic reduction of CO2.

A trimetallic Ir(iii) based complex () was synthesized and fully characterized by spectroscopic and electrochemical methods. A detailed comparison to ...
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