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Porphyrin–cobaloxime complexes for hydrogen production, a photo- and electrochemical study, coupled with quantum chemical calculations† Jennifer C. Manton, Conor Long, Johannes G. Vos and Mary T. Pryce* Two porphyrin–cobaloxime complexes; [{Co(dmgH)2Cl}{MPyTPP}] (1a) and [{Co(dmgH)2Cl}{ZnMPyTPP}] (2a) (dmgH = dimethylglyoxime, MPyTPP = 5-(4-pyridyl)-10,15,20-triphenylporphyrin) have been synthesised as model systems for the generation of hydrogen from water. Although initially envisaged as photocatalytic systems neither complex catalysed the reduction of water to hydrogen following irradiation. However, both complexes are molecular precursors for hydrogen evolution under electrochemical conditions. Turnover numbers for hydrogen production of 1.8 × 103 and 5.1 × 103 were obtained for 1a and 2a respectively following potentiostatic electrolysis at −1.2 V vs. Ag/AgCl while cobaloxime alone produced a turnover-number of 8.0 × 103. The photophysical properties of 1a and 2a

Received 8th November 2013, Accepted 30th December 2013

were examined to provide an explanation for the lack of photochemical activity. These results, coupled with quantum chemical calculations, confirm that porphyrins fail to act as light-harvesting units for these

DOI: 10.1039/c3dt53166j

systems and that the lowest energy excited states are in fact cobaloxime-based rather than porphyrin

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based.

Introduction There is a growing need for alternatives to scarce and polluting fossil fuels.1–3 One possible technology is to use hydrogen gas, particularly in automotive applications. However hydrogen is an energy carrier rather than a fuel and must be produced from efficient renewable energy sources. Hydrogen can either be used directly by combustion in air or it can be converted to electrical power in a fuel cell.4 Provided the efficiency of hydrogen generation can be improved, the switch to a hydrogen based economy will provide a cost effective and energy efficient alternative to fossil fuels in the transport sector. Water electrolysis is a well-established method to produce hydrogen and oxygen, from water. First utilised in the late 1700s this method involves the use of expensive electrodes such as platinum and is carried out at high temperatures up to 100 °C. Developing an electrocatalytic system which produces net energy and cost gains is the next step to sustainable energy which can be produced consistently. The sun provides approximately 450 W m−2 of visible radiation at the Earth’s surface. The efficient harnessing of this energy represents an important technological goal. In particular harnessing solar energy to produce hydrogen is

School of Chemical Sciences, Dublin City University, Dublin 9, Ireland. E-mail: [email protected]; Fax: +353 1 7005503; Tel: +353 1 7008005 † Electronic supplementary information (ESI) available. See 10.1039/c3dt53166j

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DOI:

an attractive alternative to the current production methods of steam reforming or electrolysis. Despite the vast amount of energy provided by the sun and the fact that efficient harnessing of visible light is possible, sunlight remains an intermittent source of energy for many parts of the globe. Natural photosynthesis provides an ideal model to design a system which can cleanly and efficiently generate hydrogen by harnessing sunlight. Photosynthesis uses chlorophyll as a light harvester, where the chromophore consists of magnesium–porphyrin units. Using this as a model, we have studied the use of porphyrins as a light harvester for a nonnoble-metal catalytic centre, specifically a cobalt glyoxime centre. Cobalt glyoximes (cobaloximes) are an inexpensive alternative to centres based on Ru,5,6,7 Ir8 or Pt.9,10 Porphyrins are ideal chromophores11–14 as they absorb strongly in the visible region15,16 and have been shown to transfer energy and electrons to acceptor sites with high efficiencies.17–19 Initially, it was proposed that efficient electron transfer from the porphyrin unit through the meso-pyridyl ring to the cobalt catalytic centre could activate the cobalt centre towards water reduction (Fig. 1). However the results of these studies were disappointing, showing poor efficiencies of H2 production under photolytic conditions and limited stability of the proposed photocatalytic systems. Electrocatalysis however showed greater promise in this regard. Both porphyrin and cobaloxime complexes are known to be electroactive and undergo multiple reduction processes.20,21 Electrode surfaces modified with porphyrin complexes22–24

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experiments were carried out in anhydrous DCM and solutions were initially purged with argon for 20 minutes. Argon was used to degas samples for CV and electrocatalytic studies, and was purchased from BOC Ltd.

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Instrumentation

Fig. 1

Molecular structures of 1a and 2a.

and cobaloxime derivatives25–27 have been studied and have been shown to induce an anodic shift in the onset of the hydrogen generating catalytic process, and therefore generating higher turnover numbers (TONs) of hydrogen production (where TON refers to moles of H2 produced/moles of catalyst), when compared with unmodified electrodes. In this paper we report the results of photophysical and electrochemical studies on porphyrin-coupled cobaloxime complexes. The porphyrin chosen for this study is based on the tetraphenyl porphyrin, where one phenyl substituent at the meso-position was replaced by a pyrid-4-yl group (5-(4pyridyl),10,15,20-triphenylporphyrin) (1). These complexes are similar to those recently studied by Sun et al.,11 Guldi et al.28 and Scandola et al.29 We report the results of photophysical studies and explain the quenching of the porphyrin emission following complexation to the cobaloxime centre. Calculations presented here, indicate the presence of a low energy cobaloxime-based excited state which is repulsive to the Co–N(pyridyl) bond. Electrochemical studies confirm that these systems can catalyse the reduction of water to hydrogen.

Experimental details Materials All reagents used in the syntheses were obtained commercially and were used without further purification. The mobile phases for column chromatography were dried over MgSO4 before use. Column chromatography was carried out using neutral silica gel (Merck) pH 6.5–7.5. Solvents were supplied by the Aldrich Chemicals Co. Solutions and were deoxygenated by purging with argon or nitrogen for ∼5–10 min before commencing synthetic reactions. Syntheses were conducted under an inert atmosphere using standard Schlenk techniques. UV-vis absorption, fluorescence, quantum yields and cyclic voltammetry experiments were carried out using spectrophotometric grade solvents which were used as received. Photocatalytic experiments were carried out in deionised water, anhydrous THF, distilled from benzophenone and sodium, and triethylamine distilled from sodium. The cyclic voltammetry

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Proton NMR spectra were recorded on a Brüker AC 400 spectrophotometer in CDCl3 using the residual solvent signal as an internal calibration. All UV spectra were measured on an Agilent 8453 UV-Vis spectrophotometer in 1 cm quartz cells. Emission spectra were recorded on a Perkin Elmer LS50B luminescence spectrophotometer using a 1 cm quartz cell. All cyclic voltammetry or bulk electrolysis experiments were carried out using a CH Instruments CHI600a potentiostat. The electrodes used during the experiments were a glassy carbon working electrode (area 0.07 cm2), a platinum wire counter electrode and an Ag/AgCl reference electrode (filled with 0.1 M TBAPF6 in DCM during CV measurements and an aqueous 3 M KCl solution during electrocatalytic experiments) were obtained from CH Instruments. All gaseous samples were analysed using a Varian GC-P-3800 gas chromatograph equipped with both a Hayesep C polysiloxane column and a 5 Å molecular sieve column and tandem thermal conductivity and flame ionisation detectors. Standard gas mixtures of 0.01% (100 ppm), 0.1% (1000 ppm), 1% (10 000 ppm) and 5% (50 000 ppm) were used to calibrate the instrument response.

Synthesis The syntheses of the porphyrin compounds 5-(4-pyridyl), 10,15,20-triphenylporphyrin (1) and zinc-5-(4-pyridyl),10,15,20triphenylporphyrin (2) and the dichlorobis(dimethylglyoximato)cobalt(III) (cobaloxime) complex were achieved using literature methods.30–32 Synthesis of 1a and 2a. 0.06 g of 1 or 0.066 g of 2 (0.0975 mmol) was dissolved in 5 mL of DCM. 0.035 g of cobaloxime (0.0975 mmol) in 25 mL of methanol was added with mixing under an N2 atmosphere over 10 minutes. 20 μL of TEA was then added and the vessel was sealed. The reaction mixture was allowed to stir for a further 1 hour in the dark. The solvents were then removed under reduced pressure and the crude products were purified by column chromatography on silica, using ethanol–chloroform 97 : 3 as the mobile phase. Spectroscopic data are in agreement with that reported in the literature.11,12 Yield (1a) 79%. 1H NMR (1a) (400 MHz, CDCl3), 12.03 ppm (s, 2 –OH), 8.90 ppm (d, J = 4.8 Hz, 2H), 8.85 ppm (dd, Ja = 7.6 Hz, Jb = 4.8 Hz, 4H), 8.65 ppm (d, J = 6.8 Hz, 2H), 8.573 ppm (d, J = 4.4 Hz), 8.18 ppm (m, 6H), 8.09 ppm (dd, Ja = 5.2 Hz, Jb = 1.2 Hz, 2H), (7.78 ppm (m, 9H), 2.594 ppm (s, 12H), −2.858 ppm (s, 2H). Yield (2a) 87%. 1H NMR (2a) (400 MHz, CDCl3), 12.24 ppm (s, 2 –OH), 8.96 ppm (d, J = 4.8 Hz, 2H), 8.93 ppm (dd, Ja = 6.8 Hz, Jb = 4.8 Hz, 4H), 8.638 ppm (d, J = 4.4 Hz, 2H), 8.60 ppm (d, J = 6.4 Hz, 2H), 8.17 ppm (m, 6H), 8.07 ppm (d, J = 6.4 Hz, 2H), 7.74 ppm (m, 9H), 2.58 ppm (s, 12H).

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Photocatalytic studies

Results and discussion

Experiments to detect photocatalytic H2 generation were carried under an Ar atmosphere. A 1.2 mL sample of a 1 × 10−4 M solution of the appropriate complex in THF was placed in a 5 mL septa capped gas tight vial. The sacrificial donor, triethylamine (33% (v/v) 0.6 mL), was then added along with 0.2 mL of deionised water (10% (v/v)) as a proton source or 0.2 mL THF to make a 0% water mixture for comparison. The solutions were photolysed for 20 hours using 470 nm, or 350 nm and >470 nm LED arrays (see ESI† for UV-vis spectra at these concentrations). After 20 hours a 0.5 mL sample of the headspace in the reaction vial was collected using a gas-tight syringe and analysed by gas chromatography. The peak-area response was measured and compared to standards to calculate the number of moles of H2 produced during photolysis. Each experiment was conducted in triplicate. The average of the three results is reported.

Absorbance spectra of 1a and 2a

Electrocatalytic studies Electrocatalytic hydrogen generation experiments were carried out at room temperature in a sealed v-shaped cell. A 1 × 10−4 M solution of each compound was prepared in DMF and a 1.5 µL sample of each solution was drop-cast onto the surface of a glassy carbon electrode (0.07 cm2) and left overnight in the dark at 22 °C to dry. A 15 mL aliquot of a pH 2.0 buffer solution (sodium hydrogen phosphate/ortho-phosphoric acid) was added to the electrochemical cell. The solutions were then purged with argon for 20 minutes, and were maintained under an argon atmosphere throughout the experiment. Bulk electrolysis experiments were carried out at a potentiostatic potential of −1.2 V for 1 hour, following which a 1 mL sample of the headspace was collected and analysed by gas chromatography. Each experiment was conducted in triplicate. The average of the three results is reported. Computational studies All quantum chemical calculations were performed at the Irish Centre for High End Computing (ICHEC) using the Gaussian09 program suite.33 The hybrid B3LYP34 functional coupled with the 6-31G(d) basis set35 was used for all quantum chemical calculations. Time Dependent Density Functional Theory methods (TDDFT) estimated the vertical excitation energy for various species along a relaxed potential energy path.36 AOMix37 was used to estimate the contribution of various molecular fragments to specific orbitals and GaussView38 was used to visualise the molecular structure, the shapes of orbitals, and electron density difference maps. Quantum yield studies for cleavage of the Co–N( pyridyl) bond To replicate the conditions of the photolysis experiments the solutions studied contained 1a in THF (5 × 10−7 M), triethylamine (33% (v/v)) and deionised water (10% (v/v)). The quantum yield for cleavage of the Co–N(pyridyl) bond in 1a was carried out at through irradiating at 405 and 546 nm, for periods of up to 15 min.

3578 | Dalton Trans., 2014, 43, 3576–3583

Complexes 1a and 2a exhibit strong absorptions in the UV-vis region of the spectrum. The Soret band occurs as a prominent feature at approximately 418 nm, corresponding to the porphyrin-localised symmetry-allowed transition.15 The absorbances between 510 and 645 nm for each of the porphyrin complexes are generally attributed to symmetry forbidden or quasi allowed transitions such as π to π* or Q-bands of the porphyrin. The metallated porphyrins exhibit two Q-bands while the free base porphyrins exhibit four. The absorbance spectra of 1 and 2 are similar and consistent with previous studies.39 The λmax of the Soret bands of 1 and 2 occur at 416 nm and 418 nm respectively (Table 1). The Soret bands of 1a and 2a are red shifted to a very minor extent and occur at 418 nm and 419 nm respectively. A similar small red-shift was observed11,29 for related porphyrin–cobaloxime complexes. The spectral changes observed close to the Q-bands are more significant however. As is evident from the inset expanded view of the Q-bands in Fig. 2 (and the extinction coefficients, Table 1) the lowest energy absorbance for both complexes 1a and 2a is significantly more intense relative to the corresponding absorbance in the spectra of 1 and 2 (Fig. 2). This increase in absorbance is attributed to the addition of a symmetry forbidden cobaloxime based transition in the two cobalt complexed porphyrins. There is also a marginal red shift of up to 10 nm for the Q-bands in complexes 1a and 2a. Fluorescence spectra of 1 and 2 Excitation of 1 or 2 at 418 nm (i.e. into the Soret band of the porphyrin unit) gives rise to two poorly resolved emission bands as expected for porphyrins of this type.40 The emission features of the Zn–porphyrins are less well resolved when compared to the free base porphyrins. The presence of the Zn atom reduces the emission intensity for the un-complexed species.41 The fluorescence spectra of 1 and 2 are shown in Fig. 3. In the case of 2, the emission maximum occurs at higher energy compared to the non-metallated porphyrin. Thus metallation of the porphyrin increases the energy of the emissive states. The complete quenching of the emission in 1a and 2a suggests the existence of a competitive process which reduces the efficiency by which the emissive state is populated. This process results predominantly in the population of a nonemissive state for both complexes. Free cobaloxime itself does not emit. Quantum chemical calculations The molecular structures of 1a and 2a optimised using tight convergence criteria (see ESI†) and these structures were then used to estimate the energy of the twenty lowest energy singlet excited states using Time-Dependent DFT methods (TDDFT). The intensity and sharpness of the Soret band facilitates the calibration of the calculated vertical excitation with the observed band position from UV/vis measurements. The calculated vertical excitation transitions for 1a or 2a correspond to

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Table 1 UV-Vis absorption data, extinction coefficient data and emission data for all compounds in this study. All spectra were recorded in spectrophotometric grade DCM

Compound

Soret band, λmax (nm), (ε × 104 M−1 cm−1)

1

Oscillator strength

Calculated wavelength (nm)

Assignment

641 (0.34) 583 (0.47) 542 (0.70) 507 (1.85) 416 (24.2)

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Q-bands, λmax (nm), (ε × 104 M−1 cm−1)

1a

643 (0.42)a 584 (0.55) 546 (0.77) 510 (1.43)

418 (34.0) 2 418 (37.0) 2a

550 (1.3) 597 (0.60) 605 (0.33)a 605 (0.33) 561 (1.19)

419 (23.7)

Emission maxima (nm) 709 650

0.0000 0.0315 0.0494 0.0020 0.6460

642 598 542 498 420

Cobaloxime Porphyrin Porphyrin Porphyrin Porphyrin

b

609 638 0.0000 0.0315 0.0494 0.6460

642 598 560 420

Cobaloxime Porphyrin Porphyrin Porphyrin

b

a

Cobaloxime-based absorbance bands overlaps with the lowest energy Q-band absorbance of the porphyrin. The extinction coefficient is a combination of both Q-band and cobaloxime absorbance as they are not resolved. The corrected calculated vertical excitation wavelength (nm), the oscillator strength, the observed λmax (nm) and a proposed assignment for the transition based on electron density difference maps. b No emission observed.

Fig. 2 A comparison of the UV-vis spectra of 2 (red line) and 2a (black line) in DCM. Inset is an expanded view of the spectral region close to the Q-bands.

photons of 409 or 403 nm respectively in the gas phase. This indicates that the calculations overestimate the observed transition energy in solution by some 0.07 eV. Consequently the calculated values presented in Table 1, were corrected by applying a 0.07 eV reduction to all calculated transition energies. The data contained in Table 1 indicates the presence of a low energy cobaloxime-based excited state. In the uncomplexed porphyrins 1 or 2, emission occurs from the lowest energy singlet excited state,15 however the presence of a cobaloximebased excited state at lower energy to all porphyrin-based excited states in 1a or 2a provides a non-emissive path to the deactivation of the excited species thereby quenching the emission. The nature of the deactivation process was then investigated by performing a relaxed potential energy scan on 1a as

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Fig. 3 Emission spectrum of 1 (black line), 2 (red line. All complexes are 2 × 10−7 M in spectrophotometric grade DCM, λexc = 418 nm.

the Co–N( pyridyl) bond length was increased from 1.905 to 3.905 Å in steps of 0.1 Å. The equilibrium Co–N( pyridyl) bond distance is 2.005 Å so this range represents a slight compression of the Co–N( pyridyl) bond followed by a considerable lengthening to non-bonding dimensions. This reaction coordinate was then used to estimate the excited state energies at each point along this coordinate using the TDDFT method. The resulting plots are presented in Fig. 4. Quantum yield of cleavage of Co–N( pyridyl) bond The photochemical quantum yield for Co–N( pyridyl) bond cleavage was measured using potassium ferrioxalate actinometry (ESI, S2†). As complex 1a is non-emissive (Fig. 3), monitoring of the fluorescence spectrum, following irradiation, indicated release of free 1 in solution. Thus monitoring an increase in the emission intensity at the monitoring

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Fig. 4 The energy of the ground-state (blue) and the ten lowest energy singlet excited states (non-adiabatic) as the Co–N( pyridyl) bond is stretched from 2.00471 Å (equilibrium) to 2.70471 Å in 0.1 Å steps, the red, green, magenta, and orange curves represent the behaviour of the 1st, 4th, 9th and 10th singlet excited states which are either cobaloxime based or porphyrin to cobaloxime charge-transfer in nature and are unbound with respect to the Co–N( pyridyl) interaction.

wavelength (650 nm) provides a measurement for decomplexation of the porphyrin cobalt oxime complexes in this study. The quantum yield of the Co–N( pyridyl) bond cleavage was calculated at various wavelengths (see Experimental) for 1a to determine if the photochemical process was wavelength dependent. Wavelengths at which the complexes absorb strongly (405 nm) and minimally (546 nm) were investigated. A quantum yield of 56% for cleavage of the Co–N( pyridyl) bond was measured following irradiation at 405 nm, which irradiates into the Soret absorbance band of the porphyrin moiety (see ESI†). Following irradiation 546 nm, no significant change was observed in the fluorescence spectrum over the length of the experiment (15 min). Control experiments were allowed to sit in the dark for 20 h. A comparison of the emission profile at T = 0 h and T = 20 h showed that cleavage of the Co–N( pyridyl) bond also occurs slowly in the dark over this time period. Photocatalysis experiments Porphyrin rings are known to be efficient light absorbing compounds16,17 and have been used in a variety of systems as electron transfer agents.17,18,42,43 TONs of up to 63 have been reported using porphyrin–pyrene systems.14 Cobaloxime complexes have been reported to efficiently produce hydrogen photochemically44–46 and achieved TON of up to ∼300. Recently hydrogen production has been reported using a porphyrin photosensitiser and a cobaloxime catalytic centre similar to the complexes reported in this study.47 Zhang et al.11,12 have shown photoinduced H2 evolution using a mono-pyridylporphyrin complex with meso appended

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p-phenyl(C(CH3)3) groups, similar to 1a and 2a, with TEA as a sacrificial donor. Following irradiation with λexc > 400 nm, a TON of 22 was reported. As the catalytic conditions employed in this study are similar to those reported by Zhang et al., the increase in catalytic activity for the latter study may be due to the electron donating nature of the C(CH3)3 groups. More recently Guldi et al.28 have reported modest rates of hydrogen production, although no TON figures are given, using monopyridylporphyrin–cobaloxime derivatives with meso appended p-phenyl(COOMe) groups, and have stated that selective irradiation of the first singlet state of the porphyrin leads to charge separation, however, it is short lived preventing accumulation of the reduced cobalt species needed for the water splitting reaction. During the course of our studies on photoinduced hydrogen evolution the suitability of 1a and 2a for this reaction was studied.47 Corresponding experiments, under identical conditions, were carried out using complexes 1 and 2 and an appropriate concentration of free cobaloxime. This was to investigate if the H2 production may occur both as an intra and inter-molecular process. However, in our studies no H2 was detected following irradiation of solutions containing mixtures of catalyst and photosensitiser, or in the case of the intramolecular porphyrin–cobalt oxime complexes. This lack of photocatalytic activity is attributed to the cleavage of the Co–N( pyridyl) bond between the porphyrin and the cobaloxime in the experiments using 1a and 2a. Cyclic voltammetry Reductive electrochemistry of a porphyrin generally consists of two mono-electronic processes which result in the formation of a radical anion, [ porph]•− and dianion species, [ porph]2−. Oxidation processes generally consist of two mono-electronic oxidations of the porphyrin ring yielding the radical cation, [ porph]•+ and the dication, [ porph]2+.15 1 possesses these two, mono-electronic reversible reductions at E1/2 = −1.545 V and −1.875 V vs. Ag/AgCl representing a cathodic shift of approximately 0.15 V compared to the reductions observed for tetraphenylporphyrin. This is attributed to electron donation from the pyridyl group.48,49 Addition of Zn to the centre of the porphyrin ring shifts the reduction potentials in the region of −1.22 V and −1.68 V for complex 2. The first ring-based reduction to occur is irreversible while the second reduction is completely reversible, Ia/Ic = 1.03. Two reductions were also observed for 1a and 2a. The ring based reductions occur at more negative potentials than those observed for 1 and 2 yielding a reversible wave at E1/2 = −1.675 V and −1.78 V for 1a and 2a respectively. Another irreversible redox process is observed in the electrochemical window of 1a at −1.40 V and 2a at −1.51 V and is attributed to the reduction of the cobalt metal centre, Co3+/Co2+ as it is similar to the reduction of cobaloxime itself (Table 3).29,47 Compound 1 possesses two, mono-electronic oxidations at 0.99 V and 1.74 V. The coordination of Zn shifts the oxidations to the more anodic potentials of 0.93 V and 1.185 V yielding reversible processes. A similar effect was observed for the reduction processes upon metallation of 1a. The ring-based

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Fig. 5 Cyclic voltammogram of 1 (blue), 1a (red) and cobaloxime (green) in 0.1 M TBAPF6–DCM illustrating their reduction processes (scan rate = 0.1 V s−1).

oxidations were observed at 1.30 V and 1.57 V for 1a. A Co based redox process is quasi-reversible at 0.1 V s−1 and is observed in the electrochemical window of 1a at 0.85 V. This is attributed to the oxidation of the cobalt metal centre, Co3+/ Co4+ as it is similar to the oxidation of cobaloxime itself. The addition of a cobaloxime group forming 2a had a similar effect on the redox processes of the porphyrin. The ring based oxidation processes were shifted cathodically when compared to the same processes in the electrochemical window of 2 (Fig. 5).

This process is assigned to the catalytic production of H2 and this was confirmed by GC analysis of the headspace. The onset of the catalytic curve using a bare glassy carbon electrode was observed at −1.15 V vs. Ag/AgCl. Upon modification of the electrode surface with 1a or 2a the onset of the H2 generating catalytic curve shifted anodically to −1.0 V and −0.9 V respectively. The addition of the cobaloxime group to the 1 ligand introduces a Co3+/Co2+ reduction at approximately −1.4 V vs. Ag/ AgCl in DCM (Table 2). The Co based process in organic solvents for 1a occurs at a similar potential to the onset of the catalytic current, thereby implying a Co metal catalysed reaction leading to the production of H2. Glassy carbon surfaces modified with 1a, 2a and cobaloxime were tested using an applied potential of −1.2 V for 1 hour and the resulting hydrogen TONs generated were determined. The current density measured over the course of the hour long bulk electrolysis experiment at −1.2 V was measured to be 1.07 mA cm−2, producing an GC determined TON of 1.8 × 103 for 1a, while the current density measured for 2a was slightly higher at 1.15 mA cm−2 yielding a TON of 5.1 × 103, indicating greater catalytic activity for the metallated porphyrin derivative. The 1a and 2a based systems yielded efficiencies of 49 and 65% respectively indicating that 2a is a viable electrocatalytic system with higher TONs. Cobaloxime yielded a TON of 8.0 × 103, with a higher current density of 2.18 mA cm−2 and an efficiency of 51% during the H2 generating experiment.50 The nature of the

Table 3 The cyclic voltammetry results for the processes of 1, 2, 1a and 2a vs. Ag/AgCl in 0.1 M TBAPF6–DCM

Electrocatalysis experiments Although initially envisaged as photocatalytic systems the efficiency with which 1a, 2a and cobaloxime can electrocatalytically produce H2 was measured by taking into account both the onset potential of the H2 generating catalytic curve, and the overall efficiency of the system to produce hydrogen. A series of cyclic voltammogram experiments were carried out using both bare glassy carbon electrodes and glassy carbon electrodes with surfaces modified with 1a, 2a or cobaloxime. The scans were performed with a scan rate of 0.1 V s−1 from 0 V to −1.2 V at a pH of 2.0 (0.1 M NaH2PO4 buffer). A substantial new process is visible in these CVs when the cyclic voltammograms produced were compared to similar CVs obtained in DCM. These CVs indicate the occurrence of a new process in water, which is not present in anhydrous media.

Epa Red (V)

−1.6a

−1.64a −1.97a −1.22b −1.73a −1.40b −1.75a

−1.72a

−1.51 −1.84a

−1.45a −1.78a

2

−1.62a

1a

E1/2 Red (V)

−1.38b

Co-Ox 1

Epc Red (V)

−1.545 −1.875 −1.675 −1.675

b

2a

−1.78

Epa Ox (V)

Epc Ox (V)

E1/2 Ox (V)

1.00c 1.39 0.99b 1.74b 0.97a 1.24a 0.89c 1.30b 1.57b 0.75a 1.1a

0.81c

0.9

0.89a 1.13a 0.82c

0.93 1.185 0.85

0.57a 0.87a

0.66 0.985

a

Indicates a reversible oxidation wave. b Indicates an irreversible oxidation wave. c Indicates a quasi-reversible oxidation wave. Co-Ox refers to cobaloxime (Co(dmgH)2Cl2).

Table 2 Results of electrocatalytic studies for porphyrin–cobaloxime complexes 1a and 2a and cobaloxime (Co-Ox). Bulk electrolysis experiments were carried out at −1.2 V vs. Ag/AgCl. All TON are quoted ±30%

Complex

Onset of catalytic current (V)

Moles H2 (echem)a (×10−7)

TON (echem)b (×103)

Moles H2 (GC)c (×10−7)

TON (GC)d (×103)

Efficiencye (%)

Current density (mA cm−2)

1a 2a Co-Ox

−1.00 −0.90 −0.98

5.5 11.8 23.5

3.7 7.8 15.7

2.7 7.7 12.0

1.8 5.1 8.0

49 65 51

1.07 1.15 2.18

a

Number of moles of H2 produced was determined from the charge produced over the length of the experiment. b Electrochemically determined turnover number (TON). c Number of moles of H2 produced was determined using GC measurements. d GC determined TON. e Efficiency refers to (GC determined TON/electrochemically determined TON). Co-Ox refers to cobaloxime (Co(dmgH)2Cl2).

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electroactive species at this stage is unknown, and further work will focus on identifying this species.

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Conclusions Excitation into the Soret band of each of the porphyrin complexes (1, 2) gave rise to two emission bands with Stokes shifts in the range 7290–9879 cm−1. Coordination of the cobaloxime unit, however, completely quenched emission from the porphyrin unit. Recently Scandola et al.29 attributed quenching of the porphyrin in these dyads to photoinduced electron transfer to the cobaloxime unit. However our calculations indicate the presence of a low lying Co-based excited state which is repulsive to the Co–N( pyridyl) band. Population of the excited state from the higher energy Soret absorption will rupture the Co–N( pyridyl) bond. This process provides a facile deactivation route for the excited state, liberating the porphyrin from the coordination sphere of the cobalt atom. In our experiments no evidence was obtained to support photocatalytic generation of H2 either intramolecularly utilising complexes 1a or 2a or alternatively using an intermolecular approach containing a mix of 1 or 2 and the cobaloxime. Depending upon the excitation wavelength employed, efficient photo-cleavage (45%) of the bond linking the porphyrin to the cobalt centre is observed. The experiments reported here demonstrate that tethering a catalytic centre to a photosensitiser via a pyridyl linkage is not appropriate in the design of efficient photocatalysts for intramolecular generation of H2. In contrast, electrochemically hydrogen evolution was observed for electrodes modified with all complexes with TONs up to 5100 for 1a, while cobaloxime alone yielded a TON of 8000. Complex 2a exhibited the earliest onset of a catalytic current at −0.9 V.

Acknowledgements The authors would like to thank the EPA for their financial support (EPA grant 2008-ET-MS-3-S2).

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Paper

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Dalton Trans., 2014, 43, 3576–3583 | 3583

Porphyrin-cobaloxime complexes for hydrogen production, a photo- and electrochemical study, coupled with quantum chemical calculations.

Two porphyrin-cobaloxime complexes; [{Co(dmgH)2Cl}{MPyTPP}] () and [{Co(dmgH)2Cl}{ZnMPyTPP}] () (dmgH = dimethylglyoxime, MPyTPP = 5-(4-pyridyl)-10,15...
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