CoIII Complex: An All ... - ACS Publications

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The Journal of Physical Chemistry

SnIV Metalloporphyrin + CoIII Complex: an AllAbundant-Element System for the Photocatalytic Production of H2 in Aqueous Solution Luise Mintrop, a Johannes Windisch, b Carla Gotzmann, a Roger Alberto,

b

Benjamin Probst b,*

and Philipp Kurza,* a

Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität Freiburg,

Albertstraße 21, 79104 Freiburg, Germany. Email: [email protected] b

Department Chemie, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland.

Email: [email protected]

ABSTRACT. A new, molecular system for the light-driven production of hydrogen in aqueous solution was developed by combining a water-soluble tin porphyrin ([SnIVCl2TPPC], A) acting as photosensitizer with a cobalt-based proton-reduction catalyst ([CoIIICl(dmgH)2(py)], C). Under visible light illumination and with triethanolamine (TEOA) as electron source, the system evolves H2 for hours and is clearly catalytic in both dye and catalyst. A detailed analysis of the relevant redox potentials in combination with time-resolved spectroscopy resulted in the development of a Z-scheme type model for the flow of electrons in this system. Key intermediates of the proposed mechanism for the pathway leading to H2 are the porphyrin dye’s highly oxidizing singlet excited state

1

A* (E~+1.3 V vs. NHE), its strongly reducing

isobacteriochlorin analogue (E18MΩ·cm. The coordination compounds [SnIVCl2TPPC] (A)21, [ReI(CO)3(bpy)(py)]OTf (B·OTf-)20, [CoIIICl(dmgH)2(py)] (C)24 and [CoIIBr(aPPy)]Br (D·Br-)25 (Scheme 2) were synthesized following published procedures and all products gave satisfactory results when analyzed by NMR (A-C), IR, elemental analysis and UV/Vis spectroscopy. UV/Vis spectroscopy. UV/Vis-spectra were recorded using a Jasco V-570 UV/VIS/NIR spectrophotometer. For the detection of catalytic reaction intermediates, solutions containing mixtures of photosensitizer, catalyst and TEOA donor in phosphate buffer were filled into a 10 mm septum-capped cuvette and purged with argon. The cuvette was irradiated using a lowcost Solar Simulator 300 W model 96000 of Newport Spectra-Physics equipped with a 300 W Xe-bulb and an Air Mass 1.5 Global (AM 1.5G) filter. The cuvette was irradiated with 0.1 W/cm2 (distance ~60 cm) while stirring the reaction mixture and spectra were measured every 10 min. Photocatalytic H2 evolution. 10 ml of a freshly prepared reaction mixture were filled into a 12 ml Schlenk tube, which was connected to an argon line with a controlled gas flow of 6 ml(Ar)/min passing through the solution. The dried exhaust gas was analyzed by GC (for

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details, see SI), and as soon as the solution was degassed (no more oxygen and nitrogen detectable in the purge gas), irradiation was started with current-controlled, water-cooled lightemitting diodes (LEDs, see SI, Figure S1) yielding 390 nm light (95 mW, 3.1 +/- 0.2 ·10-7 mol/s, LED model LZ4-40B200 provided by LED engine, San Jose, USA) for the rhenium and 410 nm light (85 mW, 2.9 +/- 0.2 ·10-7 mol/s, LED model LZC-70UA00-U8) for the tin photosensitizer, respectively. The illuminations were carried out for 24 h or until hydrogen evolution had ceased. The GC data were then analyzed following the procedure described in ref.20 to calculate the total amount of H2 produced as a function of time. Electrochemistry was performed in H2O containing 0.5 M H3PO4, adjusted to pH 7 with NaOH, as conducting electrolyte and buffer. A Metrohm 797 VA Computrace electrochemical analyzer was used with a standard three electrode setup of hanging mercury drop (HMDE) or glassy carbon (ID = 2 mm) working electrodes, Pt auxiliary electrodes and an Ag/AgCl reference electrodes (3 M KCl). K3[Fe(CN)6] served as internal standard and all potentials were converted to the NHE scale by using E1/2 (K3[Fe(CN)6]) = 430 mV vs NHE.14 The sweep rate for all cyclic voltammetry (CV) measurements was 100 mV⋅s-1. Excited state lifetimes. Luminescence lifetime measurements were performed either on an Edinburgh Instrument FLS920 system equipped with an nF900 ns flash lamp and a TCSPC or on an Edinburgh Instrument LP920-K system equipped with a pumped Q-switched Nd:YAG laser (Continuum Surelite), monochromator (Czerny-Turner with triple grating turret, FL 300 mm) and a PMT. Data analysis was performed using either the F900 package provided by Edinburgh Instruments or Origin by OriginLab Corp. RESULTS AND DISCUSSION Light absorption properties of photosensitizers. Aqueous solutions of the two studied dyes appear magenta ([SnCl2TPPC], A) or yellow ([Re(CO)3(bpy)(py)]+, B) in color. A comparison of the UV/Vis spectra of the compounds in neutral buffer (Figure 1) reveals large differences of the light absorption features. This is expected because the molecular architectures of the two coordination compounds differ greatly (Scheme 2). The spectrum of [Re(CO)3(bpy)(py)]+ below ~330 nm is characterized by various overlapping π → π* transitions of the coordinated aromatic ligands bpy and py. Above 330 nm, the dominant


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feature is a broad MLCT band with an absorption maximum at ~330 nm, which tails off at ~450 nm and B shows no significant light absorption for wavelength higher than this value. In contrast, light absorption by [SnCl2TPPC] exhibits the classic properties of porphyrin molecules.26 The by far most intense feature of the spectrum is the Soret band centered at 421 nm with an extinction coefficient of ε > 200.000 M-1cm-1 (Figure 1), more than 10 times higher than any part of the spectrum of B. The Soret band is also about 50 times more intense than the MLCT transition of the rhenium dye at ~450 nm, which is the light-excitation process relevant for the photo-redox chemistry of B (see below). In addition, the spectrum of A shows a broad absorption around ~320 nm as well as three characteristic Q bands at 518, 557 and 596 nm of which the latter two are also quite intense with ε > 5.000 M-1cm-1. All these absorption features are best described as different “ligand-centered” excitations of the extended π-system of the porphyrin macrocycle and e.g. do not involve a reduction or oxidation of the SnIV metal center.26 So there are two major differences between the light absorption events of the two photosensitizers used in this study: a) the extinction coefficients for the SnIV metalloporphyrin are much higher than those of the ReI bipyridine complex and b) the most important process causing the absorption of visible light by [Re(CO)3(bpy)(py)]+ is a MLCT event (and thus a formal oxidation of the metal ion), while for [SnCl2TPPC] all excitations are ligand-centered and the central ion can always be characterized as Sn4+.

Figure 1. UV/Vis spectra for aqueous solutions of [SnIVCl2TPPC] (A, blue) and [Re(CO)3(bpy)(py)]+ (B, black).


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Photocatalytic H2 production. Compounds A - D were now tested for their abilities to act as parts of light-driven electron-transfer chains for H2 evolution. By doing so, we succeeded in identifying the combination A / C as the first example for a synthetic, truly homogenous and precious metal free photocatalytic system for proton reduction in pure H2O. A comparison with the performance of the alternative combination B / D allowed us to identify the rate limiting factor (photoreaction of the Sn-porphyrin) and the performance limiting component (instability of the Co HER catalyst). The combinations investigated for photocatalysis involved both a variation of the dye (A vs. B) and the added HER catalysts (PtO2, [CoCl(dmgH)2(py)] (C), [CoBr(aPPy)]+ (D)). In all cases the pH of the solutions at the start was set to pH 8.5 and TEOA served as sacrificial electron donor. Table 1 provides an overview of the different photocatalysis runs carried out in this study.

Figure 2. Time courses of hydrogen evolution turnover frequencies (dots with error bars) and integrated turnover numbers (lines) for reactions of the tin(IV) porphyrin A with the HER catalysts C (black) or PtO2 (red). Conditions (see also Table 1): dye: 1 mM A; cat.: 0.1 mM C or PtO2; [TEOA]=60 mM; 410nm LED illumination.

In Figure 2, two hydrogen evolution curves are shown comparing H2 formation for the tin(IV) porphyrin A in combination with the alternative HER catalysts PtO2 or the cobalt complex C. Clearly, H2 formation with turnover numbers larger than 1 is detected for both combinations and thus also for the combination Sn-dye A plus Co-catalyst C. This mixture thus represents a functioning precious-metal-free photocatalytic system for the production of H2 in water and its identification was one of the main objectives of our study. However, by substituting the PtO2 catalyst from the already known reaction sequence21 by [CoCl(dmgH)2(py)], the H2 evolution


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rate is reduced to about half of its already low value for the system A plus PtO2. Looking closer at Figure 2, one also realizes that the HER of these mixtures in both cases requires significant “induction periods” of 1-4 h to reach their maximum turnover frequencies which then subsequently slow down again during the following hours of reaction. Successively, all combinations of the two dyes and the three HER catalysts were tested for their photocatalytic performances. The six resulting H2 formation curves for these runs are shown in Figure 3. H2 evolution could be detected for a number of combinations (four out of six), but the traces fall into three, clearly different categories (see also Table 1): a) Reactions of the rhenium photosensitizer B with the molecular cobalt catalysts C or D are the fastest and turnover numbers of >5 H per dye molecule are reached after ~1h in both cases. However, these rates cannot be maintained for extended times and instead the reactions stop when TON maxima of ~5.5 or ~7.5 have been reached after few hours. b) Photocatalysis by the tin porphyrin A in combination with [CoCl(dmgH)2(py)] or PtO2 yield H2, but at much slower rates than found for the two Re/Co systems just mentioned. Interestingly, the reactions involving dye A level off only if the illuminations are continued for a day or more, thus indicating that the system remains functional for long time periods. If the reaction is monitored for 50 h, the combination A / PtO2 reaches about the same TON maximum as the better of the two B / Co combinations (~7.5 per A). c) Thirdly, the two PS / catalyst pairs A / D and B / PtO2 both do not yield significant amounts of H2 at all (maximum TON >1. Even higher TONs per A can be reached if the ratio A : C is decreased. In a series of experiments where we did so, we found a maximum TON per dye molecule of >25 when the A : C ratio was “reversed” from the initial 10 : 1 (experiments 1 and 3) to 1 : 10 ([A] = 10 µM, see SI, Figure S2). This observation is in agreement with our previous results on the system B / C which was also clearly limited by the maximum TON of the HER catalysts C and not the photosensitizer B.20 On the other hand, better catalytic performances are obviously possible for both compounds, e.g. if A is combined with PtO2 and especially if the rhenium dye B drives hydrogen evolution by C (Figure 3, Table 1). In the following, we thus set out to identify some factors that might explain the great differences in the detected photocatalysis rates. To do so, we started with a closer look at the thermodynamic basis of the photoreactions in form of the redox potentials of the different species present in the reaction mixture. Redox potentials. At neutral pH, the reduction of protons to molecular hydrogen takes place at potentials of E < -0.41 V. On the other hand, triethanolamine, the electron source for HER in the studied photocatalytic reaction chains, requires at least +1.1 V for its irreversible oxidation at pH 7.21 Thus, the light-driven reaction chain has to pump electrons very significantly “uphill” so that H2 can be formed. Additionally, the redox potentials of the involved species (donor, dye and catalyst) have to match to establish a functional photocatalytic reaction sequence.14


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Cyclic voltammograms (CVs) of A and C in neutral, aqueous solution show irreversible reduction events for both complexes within the potential range of interest (-1.5-0 V, Figure 4). All occur at potentials well below the thermodynamic potential of the HER and thus proton reduction by both the reduced forms of A and C clearly is energetically feasible. The CV signal detected for the reduction of C at ~-0.80 V has an unusual shape and could only be resolved in form of a well-defined peak at a very low concentration of C (25µM). It has been known for a very long time that [Co(dmgH)2]-complexes adsorb on mercury electrodes if reducing potentials are applied and the resulting adsorption-enhanced electrochemical signals are routinely used in analytical chemistry for the detection of trace amounts of cobalt by adsorptive stripping voltammetry.27 The origins of the large currents at ~-0.80 V are complex, but previous, very detailed studies in ammonia buffer (pH~10) indicated that the electrochemical reaction most likely involve a) the reduction of the cobalt center from its CoII state to amalgamated Co0Hg (probably via a CoI intermediate28) as well as b) the proton-coupled reduction of coordinated dmgH to either 2,3-bis(hydroxylamino)butane (4e-/4H+) or 2,3-diaminobutane (8e-/8H+).28,29

Figure 4. Cyclic voltammograms for reductive scans of [CoCl(dmgH)2(py)] (C, black) and [SnCl2TPPC] (A, grey) in aqueous phosphate buffer (0.5 M, pH 7) using a hanging mercury drop electrode. The thermodynamic potential for the HER at this pH is indicated as dashed vertical line for comparison. Measurement conditions: [C] = 25 µM, [A] = 500 µM; sweep rate: 100 mV⋅s-1. The dashed curve at the top shows the electrolyte background.

We investigated the reduction of [CoCl(dmgH)2(py)] further and found that for the phosphate buffer system used here both the concentration of C and the pH clearly influence the irreversible reduction event: for larger [C] the detected current increases while a variation of the pH from 8 to 6 results in a shift of the peak maximum to higher E by about 70 mV per pH-unit (see SI, Fig. S3). Both observations are in agreement with the scenario of a combined, proton-dependent


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reduction process of metal and ligand as described above and thus we conclude that the irreversible reduction of C in phosphate buffer at pH 7 most likely represents the transfer of multiple electrons to adsorbed [Co(dmgH)2] resulting in the formation of CoI/Co0 species and reduced forms of the ligand. In previous work, it has been found that HER catalysis by cobalt catalysts most likely involves CoI species,9,18 but also that cobalt complexes bearing two dmgH ligands decompose after 10 min.30 In the presence of protons, the chlorin form of the metalloporphyrin is then apparently accessible from these singly reduced complexes via phlorin formation and consecutive molecular rearrangement (Scheme 3).30 Lacking any further accessible data on this issue, we currently think it plausible to assign the event at ~-0.70 V as the 1e-- reduction of A to form the π-radical anion A• - which is then slowly transformed to the chlorin form via disproportionation / rearrangement. In our previous study, we have estimated the zero-zero energy difference E00 between the ground and the singlet excited states of A as ~2 eV.21 Using the Rehm-Weller approach,14 it was possible to estimate the potential for the reduction of the excited singlet state 1A* to the π-radical


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anion A• - as E(1A*) ~ +1.3 V, and thus high enough to make a reduction of the excited tin(IV) metalloporphyrin by the electron donor TEOA possible. In contrast, the triplet excited states of tin(IV) metalloporphyrins are weaker oxidation agents: the triplet excited state energy of a related SnIV-porphyrin has been determined as E00Tt~ 1.7 V31 and we can therefore estimate the reduction potential of the triplet state as E(3A*) ~ +1.0 V, so that no/little reductive quenching of 3

A* by TEOA is expected.

Figure 5. A “half-Z-scheme” for the reaction system TEOA / A / C / H2 illustrating likely electron transfer events and the thermodynamic feasibility of light-driven hydrogen production for the studied combination of molecular compounds. “h+” represents the generation of a strongly oxidizing electron hole for the exited state of the SnIV photosensitizer and Ared / Cred reduction products of the reductions of [SnCl2TPPC] / [CoCl(dmgH)2(py)] in water (like (isobacterio)chlorin or CoI complexes).

In consequence, electron transfer events for the photocatalytic system A / C can be visualized in form of an energy diagram similar in appearance to one half of the well-known Z-scheme for oxygenic photosynthesis (Figure 5).32 From such a drawing, one can see that the presented reaction sequence represents a crude thermodynamic mimic of biological electron transfer events as it features a) an electron source with an oxidation potential in the range of H2O (E = +0.82 V at pH 7); b) a bio-inspired photosensitizer which is structurally much closer related to chlorophylls than e.g. [Ru(bpy)3]2+ or B and c) a reaction product (H2) with a reduction potential similar to that of the biological electron storage molecule NADPH (E ~ -0.35 V). On the other hand, there are of course fundamental differences between this and the biological redox chain: the initial charge separation in the photosystems takes place by oxidative (and not reductive) quenching and the fine-tuning of the redox potentials in the biological sequence is much better,


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e.g. avoiding large potential drops like the ∆E ~ 0.35 V found here between Cred and protonreduction. The thermodynamic basis for the previously known photocatalytic mixture B / D can be rationalized in a similar way. Here, the first, irreversible reduction of the rhenium photosensitizer B is can be detected by CV at -0.95 V, while the CoII HER catalyst D accepts electrons at 1.00 V and so already at a slightly lower potential than catalyst C (see SI, Figure S4). Furthermore, the oxidation potential of the excited state B* can be estimated as >+1.20 V and thus reductive quenching of B* by TEOA is thermodynamically similar to reductive quenching of 1A* (E*=+1.3 V) and more facile than for 3A* (E*=+1.0 V). A comparison of all discussed redox potentials is provided in Table 2. From this data we conclude that while differences between the dyes and catalysts clearly exist, H2 formation according to reaction sequences as outlined in Figure 5 are thermodynamically possible for all combinations of A / B with C / D. Table 2. Summary of physicochemical data for the molecular dyes / catalysts A - D in H2O (pH 7-8). compound A B C D a

λabsa / nm

ε / M cm-1

421 557 596c 350 (sh) ‒ ‒

224’000 8’000 5’400c 3’600 ‒ ‒

λabs for bands > 350nm;

b

-1

߶௤,௠௔௫ =

Ered /V

E*red /V

τ / ns

kq, TEOA / M-1 s-1

φq,maxb

-0.70 -0.95

+1.3S,c +1.0T,d

1.8S 3.64⋅105 T

3 ⋅ 108 S 2 ⋅ 104 T

0.03S 0.28T

-0.95 -0.80 -1.00

>+1.2e ‒ ‒

120 ‒ ‒

5 ⋅ 107 ‒ ‒

0.27

ଵ

షభ

ଵା൫௞೜ ఛሾொሿ൯

for [TEOA] = 60 mM, see ref.20;

c

E00 from

absorption / emission intersection21; d E00T ~ 1.7 eV31; e E00,min from phosphorescence maximum; data for singlet / triplet excited states.

S/T

Excited state lifetimes and quenching. As a first important parameter concerning the dynamics of the electron transfer between the dyes A or B and the electron donor TEOA, we measured the excited state lifetimes τ for both photosensitizers (Table 2). From lifetime and transient absorption measurements for aqueous solutions, τ’s for A* (singlet / triplet) and B* were determined as 1.8±0.1 ns (1A*), 364±7 µs (3A*) and 122±8 ns (B*). These values are in good agreement with literature data.14,33,34 The short-lived singlet state A* shows a fluorescence quantum yield of 3.6 % and intersystem crossing to the long-lived, non-emitting triplet is known to be very efficient (>60 %).35 From a thermodynamic point of view, TEOA (E = +1.1 V) should be able to reductively quench both 1A* and B*, but hardly 3A*. Experimentally, quenching by


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TEOA is observed for all cases (Table 2 and Figure 6), and quench rates match the driving force for the respective reaction well with fast rates of kq > 107 M-1 s-1 for 1A* and B*, and a more than 1000fold smaller value for 3A* (2·104 M-1 s-1), which strongly indicates that reductive quenching occurs in accordance with redox potential estimates.14 In the case of B, reductive quenching by TEOA is well studied and known to yield [ReI(CO)3(bpy•-)X] species, where the additional electron is mainly localized on the noninnocent bipyridine ligand.36 For SnIV-porphyrins, the situation is much less clear: in a study from 1980 on a related SnIV metalloporphyrin, Kalyanasundaram and Grätzel arrived at the conclusion that not the singlet but the much longer lived triplet state (τ > 100 µs) of the dye is the species involved in the photoreduction event, which they studied in water using EDTA as electron source.37 Similarly, reductive quenching of a SnIV porphyrin triplet, but not the singlet, by SnCl2 as electron source has been observed.33 On the other hand, Manassen finds singlet quenching of [SnCl2(TPP)] by amines38 and we concluded previously21 (and find here again, see Table 2) that 3A* is not a strong enough oxidation agent to extract an electron from TEOA. Time-resolved transient absorption spectroscopy was thus performed on the tin(IV) metalloporphyrin in order to elucidate the initial reactions that ultimately lead to the formation of hydrogen in the photocatalytic system when A is used as dye. As shown below, continuous wave irradiations of mixtures of A and TEOA lead to the prompt appearance of the chlorin, and subsequently to the bacteriochlorin form of A, thus clearly indicating a reductive interaction between TEOA and the excited tin porphyrin. Indeed, we observed both a decrease in the lifetime of 1A* and 3A* upon increasing [TEOA], deriving quench rates of 3 ⋅ 108 M-1s-1 for the singlet state, and 2 ⋅ 104 M-1s-1 for the triplet state, respectively (Figure 6 and SI, Figures S5). Whereas these rates are in agreement with the redox potentials of the respective excited states and TEOA, only small theoretical yields of reduced metalloporphyrin (φq,max) can be expected if the quencher concentration is [TEOA] = 60 mM as in the catalytic experiments (φq,max = 0.03 and 0.28 for the quenching of the singlet and the triplet states, respectively, Table 2). So while a clear decrease in triplet lifetime results upon increasing [TEOA] (Fig. 6), no significant differences between the time-resolved absorption spectra with / without added TEOA are observed (comp. Figures 6 and S6), indicating that the observed quenching of the triplet state by TEOA is, if caused by electron transfer, occurring at a rather low cage escape yield. On the


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other hand, the reaction of the singlet state 1A* is expected to give (even if it would occur at 100% cage escape yield) only 3% of A-, thus preventing a detection of A- in the TA experiments. To identify the spectral signature of a possibly formed reduced form of A, we undertook a series of experiments using ascorbic acid (AscOH), which can be oxidized much easier than TEOA (E~+0.3 V at pH 723). Indeed, the prompt formation of A- was observed spectroscopically if AscOH is used as electron donor instead of TEOA (see SI, Figure S7) and an absorption at 730 nm, indicative of a delocalized radical on the porphyrin macrocycle,39 was detected immediately within the time resolution of the setup. This new absorption feature is not found in experiments with TEOA, and thus it appears that the reduced form of the porphyrin is produced at a very low yield in the TEOA-experiments. Taken together, we suggest that a diffusional reaction of the singlet state with TEOA to produce A- in low yields (below 3%) is the initial event of the photocatalysis chain studied here, because this interpretation is consistent with a) the continuous wave experiments using A and TEOA showing chlorin formation (see below), b) the relative position of the redox potentials (Figure 5), c) the fact that quenching rates and yields match well with the observed (low) H2 production rates and d) the decrease in both singlet lifetime and triplet yield upon increasing [TEOA] (Figures 6 and S5).

Figure 6. TA experiment using 10 µM A, 1 M phosphate buffer, adjusted to pH = 8 with KOH, 60 mM TEOA, H2O (inset: transients at 480 nm vs time for 0, 60 and 200 mM TEOA; Excitation at 532 nm, 3.7 mJ).


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Detection of reduced porphyrin species. From the results presented in the last paragraph, we conclude that there are strong indications for the photoreduction of [SnCl2TPPC] by TEOA in water, but these are no real proof for the hypothesis that reduced forms of A are involved in photocatalytic H2 production as outlined in Figure 5. However, in following experiments we were able to directly observe the formation of reduced A in the reaction mixture by UV/Vis spectroscopy.

Figure 7. Absorption spectra (Q band region) of an aqueous mixture of A, C and TEOA exposed to visible light. Spectra were recorded every 10 min during a total experiment time of 1 h. Reaction conditions: [A] = 100 µM, [TEOA] = 6 mM, [C] = 10 µM, light source: solar simulator, 0.1 W/cm2.

When an aqueous solution containing A, C and TEOA in concentrations similar to the catalytic mixture used for H2 evolution was illuminated by visible light, the color turns from purple to green and profound changes of the Q band region of the UV/Vis can be detected (see SI, Figure S8). Similar to our previous experiments on the system A / PtO2 / TEOA, a decrease of the Q band at 557 nm is found while a new band at 624 nm evolves, which has been assigned to the formation of the SnIV chlorin complex.21 However, if HER catalysis is “slowed down” by a reduction of the concentration of the catalyst C to one tenth of its previous value, a new set of absorption bands at 497 and 610 nm starts to appear (Figure 7, dashed arrows). Especially the feature at 610 nm is very intense. It dominates the spectrum after 60 min of illumination time at these conditions and a comparison to literature data makes a clear assignment to the isobacteriochlorin form of the metalloporphyrin possible. Similar spectral changes have been found earlier for photoreductions of SnIV porphyrins and were in each case interpreted as the


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signature of the stepwise reduction of the porphyrin via the chlorin to the isobacteriochlorin form of the macrocyclic ligand.40,41 Chlorin and isobacteriochlorin can be described as reduction products of the initial porphyrin macrocycle to which one or two equivalents of H2 have formally been added (Scheme 3).42 By detecting these species, UV/Vis spectroscopy provides strong evidence that a photoreduction of A* (most likely 1A*, see above) by TEOA indeed occurs. Furthermore, it is apparently possible to successively transfer up to four electrons and protons to A to form the isobacteriochlorin as final reaction product, but isobacteriochlorin only accumulates in solution if little HER catalyst C is present, as the differences between Figures 7 an S6 show. We see no reason why isobacteriochlorin should not form as well in mixtures containing high [C]. Therefore this result is a strong indication that the back-reaction of the isobacteriochlorin to the chlorin is catalyzed by the cobalt catalyst and this process appears to be a (if not the) reaction step yielding the H2 product (Scheme 3). Scheme 3. Photoreduction products of the tin(IV) metalloporphyrin.42 With the exception of phlorin, all forms could be detected by UV/Vis spectroscopy for photoreductions of A with TEOA in water.

This series of events leading to H2 is also reasonable from a thermodynamic point of view. Chlorins are more electron-rich than their porphyrin counterparts42 and correctly we did not observe isobacteriochlorin formation earlier when reducing A electrochemically at -1.05 V.21 In consequence, we expect the reducing power of the isobacteriochlorin to be larger than the one for the corresponding chlorin and so there should be even more driving force for the reduction of the HER catalyst by the isobacteriochlorin than for the chlorin (Ared in Figure 5, which could be either of the two forms). Finally, we would like to mention that this “isobacteriochlorin model” for the photocatalytic HER using tin(IV) porphyrin dyes was already correctly proposed by Fuhrhop and coworkers in two hardly cited papers in 1983 where a photocatalytic system composed of a SnIV-dye similar to


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A, EDTA as electron donor and Pt colloid as HER catalyst is described.22,40 However, the available spectroscopic data of these early studies is rather limited and in addition to the isobacteriochlorin route, alternative pathways involving the phlorin or chlorinphlorin forms of the dye were proposed as well. Nevertheless, we think it remarkable that our study using today’s modern equipment arrives at a very similar reaction mechanism as proposed over 30 years ago.


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CONCLUSIONS A new, molecular system for the light-driven production of hydrogen in near-neutral aqueous solution was found. As key components it contains a tin(IV) metalloporphyrin photosensitizer ([SnCl2TPPC]) and a cobalt-based HER catalyst ([CoCl(dmgH)2(py)]) and thus operates without use of precious metals like Ru or Pt in 100% aqueous solution. Reduction equivalents are obtained from the thermodynamically demanding electron source triethanolamine (E=+1.1 V) and the redox potential difference from donor to acceptor in this photoreaction is an impressive ∆E~1.5 V. Long-term experiments clearly show that this system is catalytic in both dye and catalysts. An analysis of thermodynamic and kinetic data enabled us to develop a model for the flow of electrons in this process. Reductive quenching of the tin(IV) porphyrin singlet by the electron donor triethanolamine is the most likely event initializing the catalytic cycle and should result in the formation of a π-radical form of the dye as first reduction product according to the literature. However, the [SnCl2TPPC]- radicals (A-) are only formed in a very low yield due to the short excited state lifetime of 1A* and as a result A- could not be detected by UV/Vis spectroscopy here. Instead, the reduced dye apparently rearranges in water into the chlorin which can be photoreduced further to the corresponding isobacteriochlorin. Both intermediates were identified by their characteristic UV/Vis absorption features. On the other hand, an accumulation of isobacteriochlorin is only observed for low concentrations of the [CoCl(dmgH)2(py)] catalyst, while chlorin formation is independent of the presence of the Co complex. From this observation we conclude that a reaction of the SnIV isobacteriochlorin with the Co HER catalyst is one (and probably the main) pathway leading to the H2 product. This mechanistic model is in agreement with the redox potentials determined here and additionally confirms an interpretation of the events described by the Fuhrhop group already over 30 years ago. In comparison to the combination Sn-porphyrin / Co-catalyst, much faster photocatalytic hydrogen evolution was observed if [SnCl2TPPC] was replaced by [ReI(CO)3(bpy)(py)]+ as photosensitizer. The rhenium complex possesses much longer excited state lifetimes and also a simpler redox chemistry (only 1e--reduction takes place) and both probably make this system much more efficient. Interestingly, the two dye / catalyst combinations eventually yield similar amounts of H2 which confirms conclusions from former studies which identified the decomposition of the cobalt catalyst – and not the stability of the dye – as the main factor


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limiting maximum turnover numbers. Additionally, the shift to a fully aqueous reaction medium and the use of TEOA, an electron donor which is rather hard to oxidize, most likely explains why the presented photocatalytic system produces H2 much slower than recently reported alternative combinations using Zn-/Al-metalloporphyrins in mixtures of water with organic solvents.15,16 The photoreduction of [SnCl2TPPC] and its subsequent reaction with Co-catalysts like [CoCl(dmgH)2(py)] is thus an interesting, molecular (and also conceptually biomimetic) reaction chain to photocatalytically generate H2. However, many details of the process like the molecular mechanism of the initial reductive quenching event or the reduced cobalt species involved in HER catalysis are currently unknown. In the future, we will therefore look at the system more closely e.g. using EPR spectroscopy or pH-dependent catalysis studies in order to reveal further mechanistic details.


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ASSOCIATED CONTENT Supporting Information. Details of the experimental set-up for photocatalysis, additional H2 evolution curves, cyclic voltammograms, transient absorption spectra for A and UV/Vis monitoring of the photochemical formation of the chlorin form of A. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author * Email: [email protected] (B.P.), [email protected] (Ph.K.) ACKNOWLEDGMENTS AND DEDICATION Ulf Sachs assisted with the electrochemical measurements in Freiburg, where work on this project was generally made possible by a grant from the Deutsche Forschungsgemeinschaft (DFG project KU 2885/1-1). In Zurich, we thank Cyril Bachmann for the synthesis of catalyst D and the University of Zurich for financial support through the University Research Priority Program LightChEC. This work is dedicated to Prof. Dr. Wolfgang Lubitz, expert on light-driven charge separation in biological photosynthesis, on the occasion of his 65th birthday.


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Table of Contents Graphic and Synopsis

If visible light shines on an aqueous reaction mixture containing the water-soluble tin porphyrin [SnIVCl2TPPC], the cobalt complex [CoIIICl(dmgH)2(py)] and triethanolamine (TEOA), molecular hydrogen is formed photocatalytically. The precious-metal free system was closely investigated by a combination of electrochemistry, time-resolved UV/Vis spectroscopy and variations of catalysis conditions, resulting in the detection of likely reaction intermediates and an electron-transfer pathway for the catalytic cycle.


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CoIII Complex: An All ... - ACS Publications

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