Kinetic Studies of the Reduction of - American Chemical Society

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Kinetic Studies of the Reduction of [Co(dmgH)2(py)(Cl)] Revisited: Mechanisms, Products, and Implications Axel Kahnt,*,† Katrin Peuntinger,† Claudia Dammann,† Thomas Drewello,† Ralf Hermann,‡ Sergej Naumov,§ Bernd Abel,‡,§ and Dirk M. Guldi*,† †

Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstr. 3, 91058 Erlangen, Germany ‡ Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, Universität Leipzig, Linne-Strasse 2, 04103 Leipzig, Germany § Chemical Department, Leibniz Institute of Surface Modification (IOM), Permoserstrasse 15, 04318 Leipzig, Germany S Supporting Information *

ABSTRACT: We report on a mechanistic investigation regarding the reduction of [CoIII(dmgH)2(py)(Cl)] (dmg = dimethylglyoxime) by several complementary techniques. The reduction of [CoIII(dmgH)2(py)(Cl)] was initiated by either electrochemical, photochemical, or pulse radiolytical techniques, and the corresponding products were analyzed by ESI mass spectrometry. In addition, all of the rate constants for each step were determined. We have found solid experimental as well as theoretical evidence for the appearance of a dinuclear complex [CoIICoIII(dmgH)4(py)2(H2O)2]+ to be the final product of reduction, implying the initially reduced form of [CoIII(dmgH)2(py)(Cl)] undergoes a dimerization with the starting material in solution.

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INTRODUCTION The properties of Co(dmgH)2-based complexes have attracted strong interest in the past decades. Here, a particular focus was set on alkyl and alkenyl cobaloximes as cobalamin model compounds1 as well as on Co(dmgH)2 based systems as vitamin B12 analogues2,3 or as vitamin B12 models.4−8 Two decades later, new interest arose regarding this class of compounds owing to the fact that Co(dmgBF2)2 catalyzes the reduction of protons in acidic solutions.9 The latter led subsequently to the investigation of electronic structure properties.10 In this regard, cobaloxime complexes are considered as promising candidates in the field of renewable energy, that is, the formation of fuel from solar energy to potentially meet the future energy demands without the use of fossil fuel.11 As a matter of fact, cobaloxime complexes exhibit great potential for photocatalytic water splitting. The lack of significant absorption in the visible part of the solar spectrum constitutes, however, a bottleneck. To this end, several organic12,13 and/or inorganic14−16,13 chromophores have been coordinated to the cobalt center of cobaloxime complexes and have successfully been tested during recent years. But plenty of precedents prompt a several hours lasting induction period for the photocatalytic reduction of water.17,18 Surprisingly, the reasons for such a phenomenon remain largely unknown.18 Taking the latter observation and experimental facts into consideration, the photocatalytic active species may not be [CoIII(dmgH)2(py)(Cl)] (Figure 1) or the corresponding chromophore containing analogue. Instead, the photocatalytic active species may be © 2014 American Chemical Society

Figure 1. Structure of [CoIII(dmgH)2(py)(Cl)].

formed in situ upon photoexcitation followed by a subsequent process such as reduction, etc. We therefore conducted a combined spectroscopic, kinetic, electrochemical, theoretical, and mass spectrometric study of the elementary processes that are associated with the photoexcitation/reduction of [CoIII(dmgH)2(py)(Cl)] to shed light onto the nature of the photocatalytically active species.

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EXPERIMENTAL AND THEORETICAL METHODS For the detailed description of the experimental and theoretical methods, please see Experimental and Theoretical Methods in the Supporting Information. Received: February 25, 2014 Revised: May 4, 2014 Published: May 12, 2014 4382

dx.doi.org/10.1021/jp501947y | J. Phys. Chem. A 2014, 118, 4382−4391

The Journal of Physical Chemistry A

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RESULTS AND DISCUSSION Pulse Radiolysis Studies. To investigate the reactions following the reduction of [CoIII(dmgH)2(py)(Cl)], we employed radiolytic reductive conditions. Here, two different strategies were followed. In the first approach, solvated electrons (eaq−) were generated, employed, and probed as very powerful reductants. These investigations necessitated N2-saturated, dilute aqueous solutions containing 5 vol % of 2-propanol. Such conditions lead to the production of three highly reactive species, namely •H, •OH, and eaq−, besides the molecular products H2 and H2O2. Under our experimental conditions, the function of 2propanol is to efficiently scavenge two of the radical species, namely •H and •OH, via hydrogen abstraction. The resulting (CH3)2•COH radical is yet another powerful reductant (−1.39 V vs NHE).19 Notably, the latter is, however, insufficient to reduce [CoIII(dmgH)2(py)(Cl)].20 H 2O ⇝ •OH + •H + eaq − •

OH + (CH3)2 CH(OH) → H 2O + (CH3)2 •C(OH)

(1) (2)

In the second approach, CO2•− was employed like eaq− as a powerful reductant (−2.0 V vs NHE for CO2•− compared to −2.9 V vs NHE for eaq−).19 CO2•− is formed in a standard procedure by irradiating HCOONa containing N2O saturated aqueous solutions. Under such conditions, solvated electrons formed during the radiolysis of water (eq 1) are quenched and converted to •OH (eq 3). •OH, originating from the primary radiolysis (eq 1) as well as from the secondary conversion of eaq− to •OH (eq 3), abstract hydrogen from HCOO− and form CO2•− (eq 4).21 eaq − + H 2O + N2O → •OH + OH− + N2

(3)

HCOO− + •OH(•H) → CO2•− + H 2O(H 2)

(4)

The advantage of the latter approach, namely employing CO2•− instead of eaq−, is the higher yield of strongly reducing species. The G-value as the formed transient species per 100 eV absorbed energy is 5.9 for CO2•−, while it is only 2.7 for eaq−, and, as such, increased by more than a factor of 2.22 Regardless of the approach, immediately after the radiation pulse, the transient absorption of the primary reducing species, that is, either eaq− or CO2•−, is recorded. On one hand, for CO2•− a well-established transient absorption in the blue region of the spectrum with a maximum around 320 nm is discernible, see black spectrum in Figure 2a.21 On the other hand, eaq− gives rise to a broad transient absorption that maximizes around 720 nm (see Figure S1a in the Supporting Information). Common to both approaches is that the transients due to CO2•− and eaq− decay rapidly in the presence of [CoIII(dmgH)2(py)(Cl)]− and give rise to a new transient. Features of the newly developing absorption, Figures 2 and S1 in the Supporting Information, include a major maximum at 470 nm and a minor maximum at 320 nm accompanied by a shoulder at around 630 nm. We assign this transient in accordance with a previously reported transient absorption investigation to [Co II (dmgH)2(py)(Cl)]−.23,24 Still, a minor difference prevails in the form of the maximum at 470 nm relative to the maximum observed by Hoshino et al.23 at 500 nm. A likely explanation involves the different solvent and temperature conditions, 2methyl-THF as a transparent frozen glass at 77 K versus liquid H2O at room temperature.

Figure 2. (a) Differential absorption spectra obtained upon electron pulse radiolysis (100 Gy, 15 ns fwhm) of [CoIII(dmgH)2(py)(Cl)] (5 × 10−4 M) in N2O saturated aqueous solution in the presence of 5 × 10−3 M HCOONa with time delays of 350 ns (black spectrum) and 7.0 μs (red spectrum) after the electron pulse. (b) Corresponding absorption time profiles at 350 (black), 470 (red), and 630 nm (green). (c) Plot of the pseudo-first-order rate constant versus [CoIII(dmgH)2(py)(Cl)] concentration in N2O saturated aqueous solution in the presence of 5 × 10−3 M HCOONa. Pseudo-first-order rate constants taken from the 350 nm (black □), the 470 nm (red ○), and the 630 nm (green △) absorption time profiles.

The rate constants for the reduction of [CoIII(dmgH)2(py)(Cl)] were determined via the analysis of the plot of the pseudofirst-order rate constant versus the [CoIII(dmgH)2(py)(Cl)] concentration, as shown in Figure 2c. For both reduction approaches, linear relationships between the observed pseudofirst-order rate constants for the reduction of [CoIII(dmgH)2(py)(Cl)] and the concentration of [CoIII(dmgH)2(py)(Cl)], which was increased from 0.9 × 10−4 to 5 × 10−4 M, were derived. The pseudo-first-order rate constants for the CO2•− induced reduction were taken, on one hand, from the decay of the CO2•− transient absorption at 350 nm and, on the other hand, from the growth of the [CoII(dmgH)2(py)(Cl)]− transient absorption at either 470 or 630 nm. In this regard, the bimolecular rate constant for the reduction of [CoIII(dmgH)2(py)(Cl)] by CO2•− derived from the plot of 4383



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the pseudo-first-order rate constant versus [CoIII(dmgH)2(py)(Cl)] concentration is 1.3 × 109 M−1 s−1 and is in line with the corresponding value for eaq− that was recently reported to be 2.8 × 1010 M−1 s−1.20,24 A closer look at the transient absorption of [Co II (dmgH)2(py)(Cl)]− shown in Figure 3, more specifically the secondary growth of the 470 nm maximum and the decay of the 630 nm shoulder, reveals that commencing with its initial formation, a secondary process sets in. Both exhibit the same rate constants. To disclose the nature of this secondary rise, the initial concentration of [CoIII(dmgH)2(py)(Cl)] as well as the applied radiation dose per pulse was varied. Please note that the dependence of the rate constant of the 630 nm decay and the 470 nm growth from the initial [CoIII(dmgH)2(py)(Cl)] concentration would indicate a reaction of [CoII(dmgH)2(py)(Cl)]− with [CoIII(dmgH)2(py)(Cl)]. On the contrary, a dependence on the applied radiation dose, which is proportional to the concentration of the formed [CoII(dmgH)2(py)(Cl)]−, would be indicative for a bimolecular reaction between two [CoII(dmgH)2(py)(Cl)]−. This could involve the formation of a dimer or the reaction via disproportionation. But no evidence for any of the aforementioned was seen. In fact, the secondary growth at 470 nm and the decay at 630 nm is neither dependent on the initial [CoIII(dmgH)2(py)(Cl)] concentration nor on the applied radiation dose (20, 50, and 100 Gy), resulting in clear first-order conditions kinetics with a rate constant of 7.5 × 104 s−1. The most reasonable explanation for this finding is the release of Cl− from [CoII(dmgH)2(py)(Cl)]− to afford [CoII(dmgH)2(py)] as previously proposed by Hoshino et al. in 2-methyl THF23 and suggested by our quantum chemical calculations (see Quantum Chemical Calculations). This is further corroborated by the ESI-MS analysis, vide inf ra, of the final product of the radiolysis, which confirms the formation of a product lacking Cl− ligands. The rise at 470 nm and the decay at 630 nm in the transient absorption spectrum, vide inf ra, are followed yet by another process. In particular, a decay of the transient absorption at 470 nm and an ongoing decay at 630 nm are accompanied by an increased transient bleaching in the region below 400 nm (Figure 4). In this range of the spectrum, the initial [CoIII(dmgH)2(py)(Cl)] shows strong absorptions as demonstrated in Figure S6 in the Supporting Information. Again, to shed more light onto the nature of this process, the initial [CoIII(dmgH)2(py)(Cl)] concentration and the applied dose were altered. Whereas the applied dose (20, 50, and 100 Gy) exerts no notable impact on the overall kinetics, a systematic increase of the pseudo-firstorder rate constant for this decay evolves as the initial [CoIII(dmgH)2(py)(Cl)] concentration is increased. From the evaluation of the plot of the pseudo-first-order rate constant versus [CoIII(dmgH)2(py)(Cl)] concentration (Figure 4c), a bimolecular rate constant of 4.2 × 106 M−1 s−1 was obtained for the reaction of [CoII(dmgH)2(py)] with [CoIII(dmgH)2(py)(Cl)]. The fact that [Co II (dmgH) 2 (py)] reacts with [CoIII(dmgH)2(py)(Cl)] is not unexpected, since the formation of a dinuclear complex was observed in a similar system.25 Our finding that [CoII(dmgH)2(py)] reacts with [CoIII(dmgH)2(py)(Cl)] fits well with the ESI-MS product analysis and our quantum chemical calculations, vide inf ra. The key product ion observed by ESI/MS analysis has a mass of m/z 771. Implicit in such a mass is the formation of a binuclear complex featuring two cobalt centers. Moreover, no evidence in the ESI-MS product analysis is gathered that would support the presence of Cl− ligands. Instead, there is no doubt that the product coordinates

two water ligands. In other words, the reaction of [CoII(dmgH)2(py)] with [CoIII(dmgH)2(py)(Cl)] is followed by hydrolysis on a timescale, which seems to be slower than the 1.5 ms detectable with our pulse radiolysis setup (eq 8).

Figure 3. (a) Differential absorption spectra obtained upon electron pulse radiolysis (100 Gy, 15 ns fwhm) of [CoIII(dmgH)2(py)(Cl)] (5 × 10−4 M) in N2O saturated aqueous solution in the presence of 5 × 10−3 M HCOONa with time delays of 7.0 μs (black spectrum) and 30 μs (red spectrum) after the electron pulse. (b) Corresponding absorption time profiles at 350 nm (black), 470 nm (red), and 630 nm (green).

In summary and in line with our quantum chemical calculations (see Quantum Chemical Calculations), our findings suggest that it is reasonable to assume the following reaction mechanism upon reducing [CoIII(dmgH)2(py)(Cl)] with either CO2•− or eaq−: [CoIII(dmgH)2 (py)(C1)] + CO2•− → CO2 [CoII(dmgH)2 (py)(C1)]−

k 2 = 1.3 × 109 M−1s−1 (5)

−

[Co (dmgH)2 (py)(C1)] → [Co (dmgH)2 (py)] + C1− II

II

k1 = 7.5 × 104 s−1

(6)

[CoIII(dmgH)2 (py)(C1)] + [CoII(dmgH)2 (py)] → [CoIICoIII(dmgH)4 (py)2 ]+ + C1− k 2 = 4.2 × 106 M−1s−1

(7)

[CoIICoIII(dmgH)4 (py)2 ]+ + 2H 2O → [CoIICoIII(dmgH)4 (py)2 (H 2O)2 ]+

(8)

In an alternative mechanism, we nevertheless might have to consider the coordination of CO2•− to [CoIII(dmgH)2(py)(Cl)] by a fast exchange of Cl− by CO2•−, as the initial step prior to the reductive elimination of CO2. This alternative is, however, 4384



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unlikely to happen and is ruled out on the basis that eaq− rather than CO2•− as a reducing species evokes the same reaction steps. In this regard, please see Figures S1, S2, and S3 in the Supporting Information.

Figure 5. Absorption time profiles (black ■) at 630 nm with different time delays up to (a) 20 μs and (b) 2.0 ms of [CoIII(dmgH)2(py)(Cl)] (5 × 104 M) upon pulse radiolysis (100 Gy, 15 ns fwhm) in N2O saturated aqueous solution in the presence of 5 × 10−3 M HCOONa and corresponding kinetic simulations of the absorption time profiles (red □) using ACCUCHEM26 with the rate constants stated above (eaq−, green ▲; [CoII(dmgH)2(py)(Cl)]−, blue ▽; [CoII(dmgH)2(py)], cyan ●; and [CoIICoIII(dmgH)4(py)2]+, magenta ○).

products following reduction were obtained from cyclic voltammetry measurements. The reductive side of the voltammogram of [CoIII(dmgH)2(py)(Cl)] in acetonitrile showed three reductions namely two irreversible reductions at −0.8 and −1.1 V versus Fc+/Fc and one reversible reduction at −1.5 V versus Fc+/Fc. In line with the literature,27 we infer that the first and second reduction is metal centered and involves the CoIII/CoII and the CoII/CoI redox couples, while the third reduction is ligand centered and comprises the dmgH/dmg•− redox couple (Figure S4 in the Supporting Information). Most importantly, the finding that the [CoIII(dmgH)2(py)(Cl)] to [CoII(dmgH)2(py)(Cl)]− reduction is irreversible is in good agreement with the pulse radiolysis study described above. It corroborates that the [CoIII(dmgH)2(py)(Cl)] reduction initiates a reaction cascade, which, in turn, hampers any reoxidation of [CoII(dmgH)2(py)(Cl)]− to yield the original [CoIII(dmgH)2(py)(Cl)]. For details see electrochemical measurements in the Supporting Information. Photochemical Studies. In order to mimic the photocatalytic activity of [CoIII(dmgH)2(py)(Cl)] and also to provide further evidence for the formation of [CoIICoIII(dmgH)4(py)2(H2O)2]+ under photocatalytic conditions, we performed steadystate photolysis experiments with [CoIII(dmgH)2(py)(Cl)] in the presence and absence of TEOA (triethanolamine) as a sacrificial electron donor. The samples were irradiated using a 1000 W xenon lamp with a water filter shielding the sample from the heat of the excitation source. To avoid excitation of TEOA and decomposition by UV light, a 389 nm long-pass filter was used. Under such conditions, exclusively exciting the

Figure 4. (a) Differential absorption spectra obtained upon electron pulse radiolysis (100 Gy, 15 ns fwhm) of [CoIII(dmgH)2(py)(Cl)] (5 × 10−4 M) in a N2O saturated aqueous solution in the presence of 5 × 10−3 M HCOONa with time delays of 100 μs (black spectrum) and 1500 μs (red spectrum) after the electron pulse. (b) Corresponding absorption time profiles at 350 (black), 470 (red), and 630 nm (green). (c) Plot of the pseudo-first-order rate constant versus [CoIII(dmgH)2(py)(Cl)] concentration in N2O saturated aqueous solution with the addition of 5 × 10−3 M HCOONa. Pseudo-first-order rate constants taken from 350 nm (black □) and 630 nm (green △) absorption time profiles.

With the aforementioned reaction mechanism in hand (eq 3−7), we performed the kinetic simulations with ACCUCHEM26 for the 630 nm absorption time profiles, which are displayed in Figure 5. Overall, reasonable matches between the simulated absorption time profiles and the measured ones are noted. Complementary insights into the reduction of [CoIII(dmgH)2(py)(Cl)] and into the electrochemical properties of the formed product came from electrochemical measurements. Electrochemical Measurements. Insights into the electrochemical features of [CoIII(dmgH)2(py)(Cl)] as well as the 4385



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[CoIII(dmgH)2(py)(Cl)] complex, the UV−vis spectrum in the presence of TEOA shows increasing absorptions with increasing illumination. This new absorption band is maximizing at 275 nm and is accompanied by two shoulders around 310 and 370 nm (Figure 6).

dominated by signals that relate to the starting material [CoIII(dmgH)2(py)(Cl)]. Evidence came in the form of signals at mass-to-charge ratios of 289 [Co III (dmgH) 2 ] + , 368 [CoIII(dmgH)2(py)]+, and 447 [CoIII(dmgH)2(py)2]+. See, for example, Figure 7 for the radiolytic reduction approach and

Figure 6. Differential absorption spectra of [CoIII(dmgH)2(py)(Cl)] (9.1 × 10−5 M) in H2O containing 1 vol % TEOA upon different illumination times ranging from 10 to 60 min.

Figure 7. (a) ESI-MS spectrum of 2 × 10−4 M [CoIII(dmgH)2(py)(Cl)] in aqueous solution containing 5 vol % 2-propanol before radiolytic reduction. (b) ESI-MS spectrum of 2 × 10−4 M [CoIII(dmgH)2(py)(Cl)] in aqueous solution containing 5 vol % 2-propanol after radiolytic reduction with a dose of 3000 Gy.

Upon comparison with Figure S7 in the Supporting Information, which illustrates the behavior in the absence of TEOA, we ascribe the 275 nm absorption to the oxidized form of TEOA produced after electron transfer to [CoIII(dmgH)2(py)(Cl)]. Crucial is a broad absorption band around 470 nm, which features a shape similar to that observed in the pulse radiolysis experiments. Again, implicit is the presence of dinuclear [CoIICoIII(dmgH)4(py)2(H2O)2]+. Further evidence for the formation of dinuclear [CoIICoIII(dmgH)4(py)2(H2O)2]+ came from ESI-MS spectra. This finding corroborates that the product of the illumination is, indeed, the same as previously observed upon radiolytic and electrochemical reduction. On the contrary, performing illumination experiments in pristine pyridine instead of water hampers the [CoIII2(dmgH)3(dmg)(py)2(H2O)2]+ formation as inferred from ESI-MS spectra (Figure S9 in the Supporting Information). This corroborates the postulated mechanism by pulse radiolysis, due to the fact that pyridine is blocking the free coordination site at the cobalt. Instead, decomposition of the complex took place. Moreover, the ESI-MS studies with m/z 698 prompt to a ligand exchange, in which one oxime ligand was displaced by two pyridine ligands. ESI-MS Studies. ESI-MS studies were performed from all of the samples to gather insights into the final product. [CoIII(dmgH)2(py)(Cl)] was reduced in aqueous solutions in three different ways. First, [CoIII(dmgH)2(py)(Cl)] was subjected to radiolytic reduction, as described in Pulse Radiolysis Studies, vide supra, such conditions induce reduction with eaq−.28 Second, we turned to electrochemical reduction of [CoIII(dmgH)2(py)(Cl)] dissolved in H2O/acetonitrile (1:1 v/ v) with potassium chloride as supporting electrolyte.29 Reduction of the latter was conducted by applying a potential of −1.0 V (versus Fc+/Fc) for up to 45 min. Third, photochemical reduction with TEOA under illumination was performed. In any of the three scenarios, ESI-MS spectra were taken before and after reduction. All approaches led to the same general pattern in the ESI-MS spectra. All mass spectra, before and after the reduction, were

Figure S8 in the Supporting Information for the electrochemical approach. The dominating m/z 447 signal reveals efficient substitution of the Cl− ligand by a pyridine ligand. This exchange is most likely taking place during the ESI-MS process. All these signals relate to Co(III), owing to the fact that the mass-to-charge ratios show only singly charged peaks, proofed by the isotope pattern. After radiolytic, electrochemical, or photochemical reduction (see, for example Figures 7 and S8 in the Supporting Information), additional signals were seen in the ESI mass spectra, which were identical in all three approaches. The additional key feature in the ESI mass spectra comprises a newly developing set of signals at m/z 771 accompanied by peaks with m/z 753, m/z 735, m/z 692, m/z 674, and m/z 656. It is important to note that all of these signals relate to ions that are singly charged, similar to those seen in the spectra prior to any reduction. Taking the isotope pattern of m/z 771 ion into account, we lack support for the presence of Cl− ligands and propose from this the formation of [CoIII2(dmgH)3(dmg)(py)2(H2O)2]+. The actual structure of this ion has been deduced from quantum chemical calculations (vide inf ra). It implies one extra ligand and theoretical studies (see Quantum Chemical Calculations) helped to corroborate this pathway. The mechanism, which was derived from the pulse radiolysis study, implies a signal relating to [CoIICoIII(dmgH)4(py)2(H2O)2]+ (m/z 772). Here, practical aspects are likely to be responsible for the disagreement. The reduced form is a positively charged ion, which requires detection using the positive-ion mode. However, positive-ion ESI can have an oxidizing influence on the species of interest. Therefore, it is not unexpected that the reduced form may undergo certain changes during ESI.30 The key observation is, however, the detection of a dinuclear cobalt species following the initial reduction. The nature of the ion at m/z 771 was evaluated further in a collision-induced dissociation (CID) experiment. The ion at m/z 771 was isolated and allowed to collide with He gas to promote its dissociation inside the ion trap. 4386



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The resulting daughter ion spectrum (MS/MS or MS2) is shown in Figure 8. It supports the assumed precursor ion composition.

pattern. The low abundant fragment ions with m/z values higher than m/z 771 could not be assigned and are rationalized on the basis of forming cluster ions accompanying the ESI process. Our finding that the reduction of [CoIII(dmgH)2(py)(Cl)] by either electrochemical, photochemical, or pulse radiolytical techniques produces similar signals of higher molecular masses other than those seen for untreated solutions corroborates the observation based on the pulse radiolysis experiments, in which the formation of a dinuclear complex emerges. The fact that no Cl− ligand ions were detected is well in line with our quantum chemical calculations, vide inf ra, which suggest that the formation of [CoIICoIII(dmgH)4(py)2(H2O)2]+ occurs with a simultaneous release of the Cl− ligand (see Pulse Radiolysis Studies, eq 7). This is not unexpected, since even for [CoIII(dmgH)2(py)(Cl)], a slow exchange of the Cl− ligand by water is well-established.31 Taken the aforementioned into concert and in line with our quantum chemical calculations (vide inf ra), we postulate that the final product of the reduction is [CoIICoIII(dmgH)4(py)2(H2O)2]+. Quantum Chemical Calculations. Since none of the applied methods provided more information regarding the possible structure and reaction pathways of the experimentally observed dinuclear complex [CoIICoIII(dmgH)4(py)2(H2O)2]+, we turned to computational methods. Density Functional Theory (DFT) calculations were carried out in water at the M06-D3/LACVP*/PBF level of theory as implemented in Jaguar 8.1.32 Additionally, results of calculations on possible dimeric structures were proved with those calculated with a larger LACV3P** basis set and compared with common B3LYP and BH&HLYP functionals. The details of computational methods are given in the Supporting Information. In accordance with the calculations, four stable dimeric structures could be formed in water (Figure 10). The relative stability of dinuclear complexes [CoIICoIII(dmgH)4(py)2(H2O)2]+and most relevant bond lengths are given in Table 1. The first structure (A), which is energetically most favorable, is formed through strong interaction between a positively charged Co(III) from one side and a negatively charged oxygen from the other side with a Co(III)−O bond length of 2.00 Å (shown in Figure 10). The Co(II), with a strongly localized spin (S) (unpaired electron, {S[Co(II)] = 1.04}), indicates only weak interactions with other parts of the dimer, apparently due to Coulomb repulsions between the unpaired electron on Co(II) and the negatively charged oxygen from the other side. The calculated Co(II)−O bond length is 2.65 Å. The second structure (B), which is energetically slightly less favorable than structure (A), is formed again through strong interactions between a positively charged Co(III) from one side and a negatively charged oxygen from the other side with the Co(III)−O bond length of 1.99 Å. But in this case, Co(II) with a strongly localized spin {S[Co(II)] = 1.05} is bound to the oxygen from the water molecule with a bond length of 2.47 Å. The third structure (C) is formed through strong interaction between a positively charged Co(III) from one side and a negatively charged nitrogen of N−OH group from the other side with a Co(III)−N bond length of 2.21 Å. Co(II), with a strongly localized unpaired electron, indicates only weak interactions with other parts of the dimer. The calculated Co(II)−N bond length is 3.08 Å. The structure (D) is formed through the direct interactions between a Co(II) and a Co(III) with a Co(II)−Co(III) bond length of 3.21 Å. The electronic structure of (D) is completely

Figure 8. MS/MS spectrum of [CoIII2(dmgH)3(dmg)(py)2(H2O)2]+ (selected ion) in which m/z 771 was fragmented with a fragmentation amplitude of 0.5 V.

The most pronounced dissociations led to signals at m/z 753, which are caused by the loss of one water molecule, as well as m/z 674 and m/z 656. The latter two, which are daughter ions, are formed in a successive reaction following the first water loss and correspond to the additional loss of one pyridine molecule and one pyridine and water molecule, respectively. These signals strongly suggest a reaction sequence of water, pyridine, and, again, water loss. Moreover, three less-abundant peaks were detected at m/z 735, m/z 692, and m/z 577. Also these fragment ions correspond to the loss of water, pyridine, or a mixture of both ligands (Figure 9). Evidently, the dimeric ion releases peripheral ligands in an ordered fashion, while the dimeric core resists the decomposition into monomeric fragments under the applied collision conditions.

Figure 9. Fragmentation scheme starting from the m/z 771 as the selected ion.

The observed dissociation behavior suggests that the dimeric ion observed in the gas phase is structurally related to the dimers proposed by our DFT approach. It is important to emphasize that all fragment ions observed in the CID experiment of m/z 771 were also detected in the mass spectrum after reduction. In accordance with the MS/MS spectrum of m/z 771 the abovementioned fragments did not show any Cl− ligand in the isotope 4387



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Figure 10. Four stable structures of the [CoIICoIII(dmgH)4(py)2(H2O)2]+ dimers with structures A and B being energetically most favorable. Please note L stands for the pyridine ligand.

From the calculated data (Table 1), it can be seen that all four computational models give very similar results for dimers (A), (B), and (C), indicating applicability of the used level of theory. However, in the case of dimer (D), in contrast to the M06-D3 functional, the common B3LYP and BH&HLYP functionals are not able to optimize a reasonable structure for a symmetrical dimer formed through the direct interactions between a Co(II) and a Co(III). The B3LYP functional is not able to calculate properly weak interactions between two positively charged Co atoms and gives a Co−Co bond length (5.16 Å), which is too long. All attempts to optimize structure (D) with the BH&HLYP functional were not successful because the structure (D) transforms into the most stable structure (A) during geometry optimization. It seems that the London dispersion interactions included in the M06-D3 functional are indeed very important in the case of weak long-range interactions. The comparison of the relative stabilities of the different complexes (Table 1) shows that all computational models reveal that structures (C) and (D) are essentially less favorable (>20 kcal mol−1) than the most stable structure (A). As a result, only the possible reaction pathways of the formation of dimers (A) and (B) will be discussed in the following. To get insight into possible reaction pathways toward the energetically most favorable dinuclear [CoIICoIII(dmgH)4(py)2(H2O)2]+ complexes (A) and (B), the calculations were performed by starting with the primary species (1) [CoIII(dmgH)2(py)(Cl)]. Possible reaction pathways and their energetics are shown in Figure 11. As calculated, the primary species (1) can undergo heterolytic dissociation into (3+) and Cl− with reasonable bond dissociation energy (Gibbs free energy of the dissociation is 21 kcal mol−1). The homolytic dissociation leading to structure (2) and Cl• is energetically rather impossible due to the very high dissociation

Table 1. Comparison of the Calculated Relative Energies and Relevant Bond Lengths in Water at Different Level of Theory for Four Dinuclear Complexes of [CoIICoIII(dmgH)4(py)2(H2O)2]+ (According to Figure 10)a structure

functional basis set

dimer A

dimer B

dimer C

dimer D

ΔH ΔG d(CoIII−O) d(CoII−O) ΔH ΔG d(CoIII−O) d(CoII−OH2) ΔH ΔG d(CoIII−N) d(CoII−N) ΔH ΔG d(CoIII−CoII)

M06-D3

M06-D3

LACVP* LACV3P** 0.0 0.0 2.00 Å 2.65 Å 3.7 4.3 1.99 Å 2.47 Å 20.7 17.9 2.21 Å 3.08 Å 16.3 11.8 3.21 Å

0.0 0.0 1.99 Å 2.65 Å 4.5 2.1 2.00 Å 2.45 Å 22.3 19.7 2.18 Å 3.05 Å 21.2 17.7 3.21 Å

B3LYP

BH&HLYP

LACVP*

LACVP**

0.0 0.0 2.03 Å 2.69 Å 3.7 4.3 1.99 Å 2.47 Å 27.1 26.1 2.34 Å 3.35 Å 26.2 27.5 5.16 Å

0.0 0.0 1.95 Å 2.44 Å 0.4 0.0 1.96 Å 2.28 Å 31.0 28.5 2.17 Å 3.13 Å unstableb

Here ΔH and ΔG are the stabilities (kcal mol−1) of the different structures relative to the most stable structure (A), calculated as difference of enthalpies (H) and Gibbs free energies (G), respectively; d(Å), corresponding bond length. bStructure transforms into the most stable dimer A during geometry optimization.

a

symmetrical with charge and spin (unpaired electron) distributed equally between the two parts of the dimer. All four structures are additionally stabilized through H-bonds formed between the two H2O molecules and the NO groups (as shown in Figure 10). 4388



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Figure 11. Possible reaction pathways leading to formation of the two energetically most favorable dinuclear complexes [CoIICoIII(dmgH)4(py)2(H2O)2]+. Here, ΔH and ΔG are reaction enthalpy and Gibbs free energy of the reaction (kcal mol−1) as calculated in water at the M06/LACVP(d)/ PBF level of theory (according to the literature,33,34 the error of these values is 5 kcal mol−1). The reaction pathway observed via pulse radiolysis and subsequent ESI-MS studies is marked in red.

leads to the formation of a dinuclear complex [CoIICoIII(dmgH)4(py)2(H2O)2]+. Theoretical calculations support the formation of such a dinuclear complex and propose a structure, which is in part held together by four noncovalent hydrogen bridges, allowing the detection in the present experiments but hampering further attempts of isolation and structural identification. Most notably, this finding has potential impact on the commonly proposed mechanisms for the CoIII(dmgH)2 type based water splitting systems. Typically, the formation of a doubly reduced species in the form of CoI/CoIII is postulated, which provides two electrons to photocatalytically reduce water in the presence of a sacrificial electron donor.35 Here, we lay the grounds for an alternative mechanism. As such, a dinuclear complex is considered, in which the two electrons for the reduction of water most likely originate from [CoIICoIII(dmgH) 4 (py) 2 (H 2 O) 2 ] + , which is further reduced to [CoIICoII(dmgH)4(py)2(H2O)2]. The latter is likely to be a photocatalytically active form. One may even speculate if the rather weak binding of such a structure, four hydrogen bonds to water molecules, is beneficial for its photocatalytic activity and

energy. However, if the anion of the primary species (1(−)) is formed, the formation of structure (2) will proceed very easily. Thus, structure (2) is the reactive intermediate which, in line with the pulse radiolysis measurements (see section Pulse Radiolysis Studies), undergoes further reaction with 1 forming 6+. This reaction is energetically favorable and is therefore occurring until the end of the detection time (1.5 ms) of our pulse radiolysis experiment. The subsequent addition of two water molecules leading to the formation of structure A is energetically less favorable and occurs with rate constants too small for being observed in the pulse radiolysis experiment but well-observed by the ESI-MS product analysis. In summary, the quantum chemical calculations strongly support the possible formation of dinuclear [CoIICoIII(dmgH)4(py)2(H2O)2]+ complexes with structure A as the most favorable isomer.

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CONCLUSION The combined experimental and theoretical approach, including pulse radiolysis, electrochemical, photochemical, ESI-MS, and quantum chemical studies, demonstrates that the reduction of [CoIII(dmgH)2(py)(Cl)] in aqueous media is not reversible and 4389



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the complex might fall apart into two [CoIII(dmgH)2(py)(H2O)]+ once electrons are transferred to protons. The finding of a dinuclear complex may even shed light onto the long induction periods, which are observed in photocatalytic water splitting by means of CoIII(dmgH)2.17,18 Such induction periods are indications for the in situ formation of the catalytic active species. A feasible rationale would imply that light illumination of CoIII(dmgH)2 leads to the sequential formation of a dimeric species similar to [CoIICoIII(dmgH)4(py)2(H2O)2]+ followed by [CoIICoII(dmgH)4(py)2(H2O)2], providing the two electrons needed for the reduction of protons in the presence of a sacrificial electron donor. Taking the latter into account, new paradigms in the design of photocatalytic water splitting systems could be possible and implemented. To this end, two photoactive centers as excited state electron donors and dimers of two CoIII(dmgH)2 type complexes as electron acceptors/catalytic active center would be likely candidates. We want to emphasize that such newly designed photocatalysts may display higher efficiencies without going through unwanted induction periods.

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(6) Schrauzer, G. N.; Lee, L. P. Cobaloximes(II) and Vitamin B12r as Oxygen Carriers. Evidence for Monomeric and Dimeric Peroxides and Superoxides. J. Am. Chem. Soc. 1970, 92, 1551−1557. (7) Schrauzer, G. N. New Developments in the Field of Vitamin B12: Reactions of the Cobalt Atom in Corrins and in Vitamin B12 Model Compounds. Angew. Chem., Int. Ed. 1976, 15, 417−426. (8) Prince, R. H.; Segal, M. G. Cobaloxime Electron Transfer Reactions. Nature 1974, 249, 246−247. (9) Connolly, P.; Espenson, J. H. Cobalt-Catalyzed Evolution of Molecular Hydrogen. Inorg. Chem. 1986, 25, 2684−2688. (10) Niklas, J.; Mardis, K. L.; Rakhimov, R. R.; Mulfort, K. L.; Tiede, D. M.; Poluektov, O. G. The Hydrogen Catalyst Cobaloxime: A Multifrequency EPR and DFT Study of Cobaloxime’s Electronic Structure. J. Phys. Chem. B 2012, 116, 2943−2957. (11) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009, 42, 1995−2004. (12) Zhang, P.; Wang, M.; Li, C.; Li, X.; Dong, J.; Sun, L. Photochemical H2 Production with Noble-metal-Free Molecular Devices Comprising a Porphyrin Photosensitizer and a Cobaloxime Catalyst. Chem. Commun. 2010, 46, 8806−8808. (13) Frischmann, P. D.; Mahata, K.; Würthner, F. Powering the Future of Molecular Artificial Photosynthesis with Light-Harvesting Metallosupramolecular Dye Assemblies. Chem. Soc. Rev. 2013, 42, 1847− 1870. (14) Li, C.; Wang, M.; Pan, J.; Zhang, P.; Zhang, R.; Sun, L. Photochemical Hydrogen Production Catalyzed by Polypyridyl Ruthenium−Cobaloxime Heterobinuclear Complexes with Different Bridges. J. Organomet. Chem. 2009, 694, 2814−2819. (15) Fihri, A.; Artero, V.; Razavet, M.; Baffert, C.; Leibl, W.; Fontecave, M. Cobaloxime-Based Photocatalytic Devices for Hydrogen Production. Angew. Chem., Int. Ed. 2008, 47, 564−567. (16) Fihri, A.; Artero, V.; Pereira, A.; Fontecave, M. Efficient H2Producing Photocatalytic Systems Based on Cyclometalated Iridiumand Tricarbonylrhenium-Diimine Photosensitizers and Cobaloxime Catalysts. Dalton Trans. 2008, 5567−5569. (17) Du, P.; Eisenberg, R. Catalysts Made of Earth-Abundant Elements (Co, Ni, Fe) for Water Splitting: Recent Progress and Future Challenges. Energy Environ. Sci. 2012, 5, 6012−6021. (18) Zhang, P.; Wang, M.; Li, X. Q.; Cui, H. G.; Dong, J. F.; Sun, L. C. Photochemical Hydrogen Production with Molecular Devices Comprising a Zinc Porphyrin and a Cobaloxime Catalyst. Sci. China: Chem. 2012, 55, 1976−1981. (19) Wardman, P. Reduction Potentials of One-Electron Couples Involving Free Radicals in Aqueous Solution. J. Phys. Chem. Ref. Data 1989, 18, 1637−1755. (20) Peuntinger, K.; Lazarides, T.; Dafnomili, D.; Charalambidis, G.; Landrou, G.; Kahnt, A.; Sabatini, R. P.; McCamant, D. W.; Gryko, D. T.; Coutsolelos.; et al. Photoinduced Charge Transfer in PorphyrinCobaloxime and Corrole- Cobaloxime Hybrids. J. Phys. Chem. C 2013, 117, 1647−1655. (21) Neta, P.; Simic, M.; Hayon, E. Pulse Radiolysis of Aliphatic Acids in Aqueous Solutions. I. Simple Monocarboxylic Acids. J. Phys. Chem. 1969, 73, 4207−4213. (22) G (CO2•−) = G (•OH) + G(eaq−) + G(•H) with G (•OH) = 2.7, G (eaq−) = 2.6, and G (•H) = 0.6 under the assumptions that the solvated electrons are qualitatively converted into •OH by N2O as well as •H and • OH react exclusively with HCOO−. Under the chosen conditions, these assumptions are reasonable. (23) Hoshino, M.; Konishi, S.; Terai, Y.; Imamura, M. Optical and ESR Studies of One-Electron Reduction of Alkylcobaloximes in Rigid Matrixes. Inorg. Chem. 1982, 21, 89−93. (24) Kumar, M.; Natarajan, E.; Neta, P. Electron Transfer and Alkyl Transfer with Cobaloximes in Aqueous Solutions. Kinetic Studies by Pulse Radiolysis. J. Phys. Chem. 1994, 98, 8024−8029. (25) Gupta, B. D.; Kumar, K. Organo-Bridged Dicobaloximes: Synthesis, Structure and Nuclear Magnetic Resonance Study. Inorg. Chim. Acta 2011, 372, 8−16.

ASSOCIATED CONTENT

S Supporting Information *

Description of the experimental and theoretical methods, pulse radiolysis transient absorption spectra, CV spectra, absorption and differential absorption spectra, ESI-MS spectra, calculation of the structure of [CoIII(dmgH)2(py)(Cl)]. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K., B.A., and D.M.G. gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via Grant KA 3491/ 2-1/AB 63/14-1. B.A. is also grateful for funding of the project through an Sächsische Aufbaubank (SAB) Junior Research Group grant “Applied and Theoretical Molecular Electrochemistry” (SAB, Grant 100122082).

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REFERENCES

(1) Schrauzer, G. N.; Windgassen, R. J. Cobalamin Model Compounds. Preparation and Reactions of Substituted Alkyl- and Alkenylcobaloximes and Biochemical Implications. J. Am. Chem. Soc. 1967, 89, 1999−2007. (2) Schrauzer, G. N.; Windgassen, R. J. Alkylcobaloximes and their Relation to Alkylcobalamins. J. Am. Chem. Soc. 1966, 88, 3738−3743. (3) Schrauzer, G. N.; Sibert, J. W.; Windgassen, R. J. Photochemical and Thermal Cobalt-Carbon Bond Cleavage in Alkylcobalamins and Related Organometallic Compounds. A Comparative Study. J. Am. Chem. Soc. 1968, 90, 6681−6688. (4) Schrauzer, G. N.; Windgassen, R. J. Ü ber Cobaloxime(II) und deren Beziehung zum Vitamin B12r. Chem. Ber. 1966, 99, 602−610. (5) Schrauzer, G. N. Organocobalt Chemistry of Vitamin B12 Model Compounds (Cobaloximes). Acc. Chem. Res. 1968, 1, 97−103. 4390



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