Molecular Cobalt Catalysts for H2 Generation with Redox Activity and

Dec 26, 2018 - The activity and mechanism of two new molecular Co(II) macrocycles for catalytic aqueous H2 generation are investigated. Photocatalysis...
1 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/IC

Cite This: Inorg. Chem. 2019, 58, 1697−1709

Molecular Cobalt Catalysts for H2 Generation with Redox Activity and Proton Relays in the Second Coordination Sphere Lars Kohler,† Jens Niklas,† Ryan C. Johnson,† Matthias Zeller,‡ Oleg G. Poluektov,† and Karen L. Mulfort*,† †

Downloaded via EASTERN KENTUCKY UNIV on January 25, 2019 at 10:20:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Division of Chemical Sciences and Engineering, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, Illinois 60439, United States ‡ Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Two new Co(II) complexes have been synthesized and investigated as catalysts for H2 generation. These catalysts were designed to incorporate redox-active bipyridine components and nitrogen groups, which can participate in electron and proton transfer steps in the catalytic cycle. The two catalysts differ by only one amino group, yielding a completely closed macrocycle and an open “macrocycle” complex. Removing just one nitrogen linker between the Co(II)-binding bipyridine groups has a profound impact on the molecular geometry observed by single crystal analysis. Photocatalysis experiments show that both catalysts are highly active for aqueous proton reduction at moderate pH levels, with the closed macrocycle reaching almost 2 × 104 turnovers of H2 when photodriven by [Ru(2,2′-bipyridine)3]2+ using ascorbate as an electron relay and a phosphine compound as the terminal electron donor. Measurements of the electrocatalytic activity were used to investigate key steps in the mechanism of proton reduction by the molecular catalysts. The formation of a new reversible peak on addition of moderately strong acids in organic solvents suggests that protonation of the macrocycle plays an important role in H2 generation. Onset of the catalytic current occurs near the reduction potential of the bipyridine components, suggesting that catalysis is mediated by electron transfer from the macrocycle to the cobalt center. From these observations, we propose a mechanism for catalytic proton reduction to H2, which involves both intramolecular proton and electron transfer steps from the macrocycle ligand to the cobalt center. The vital role of the second coordination sphere in the catalytic cycle places these relatively simple complexes on the pathway toward molecular catalysts that mimic the valuable features of enzymatic catalysis.

■

INTRODUCTION The ability to convert highly abundant but relatively inert substrates such as water, CO2, and N2 into valuable chemicals and fuels using mild conditions (pH, pressure, and temperature) and reactants (such as light and catalysts based on earthabundant elements) would represent a revolution in energy technology.1−4 Toward this goal, the thoughtful design and study of homogeneous, molecular catalysts presents the opportunity to understand the single electron and proton transfer steps that constitute the mechanisms underpinning the complex, multielectron redox transformations required to convert solar energy into chemical energy.5−8 In particular, molecular cobalt complexes have been heavily pursued because of their clear promise as electro- and photocatalysts for the catalytic generation of hydrogen from water.9−12 Cobalt stands out among first row transition metals for its ability to support multiple oxidation states, occupy both high and low spin electronic states, and adopt multiple coordination geometries depending on its surrounding ligands. Therefore, macrocycles that accommodate cobalt such as Co(II)tetraazo-macrocycles,13 cobalt porphyrins,14 and cobaloximes15−17 are often © 2018 American Chemical Society

targeted for catalyst development because of their typically high metal binding affinity and consequently high stability under catalytic conditions. More recently, Co(II)poly(pyridyl) complexes have been integrated into both electro- and photocatalytic proton reduction catalyst systems and demonstrated remarkable stability and activity under a wide range of typically harsh conditions.18−20 The synthetic versatility of transition metal coordination complexes provides the ability to investigate the role of the ligands in the catalytic cycle for multiproton, multielectron redox transformations. Perhaps the most prominent example of a ligand playing an active role in proton reduction catalysis comes from the family of [Ni(P2RN2R′)2]2+ proton reduction catalysts (the “DuBois catalysts”).21−24 The installation of proton-binding tertiary amines in the second coordination sphere of these complexes has provided an opportunity to investigate the mechanism and activity in response to amine pKa and solution pH and convincingly prove the participation Received: November 26, 2018 Published: December 26, 2018 1697

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry

CAT) or an open “macrocycle” (O-CAT; Figure 1). The molecular catalysts were expressly designed with these two

of the second coordination sphere in catalytic proton reduction.25−27 An advanced design in this family of molecular catalysts contains only one pendant amine in each ligand, which prevents unproductive proton binding steps and yields a molecular proton reduction catalyst that turns over an amazing 100 000 H2 per second.28 This work clearly shows that the integration of proton relays in the second coordination sphere of a molecular catalyst can substantially impact its ability to manage critical proton transfer steps. The ligands of transition metal coordination complexes may also play a role in managing the multiple electron transfer steps that are necessary for solar fuels generation. Well-designed redox-active ligands can provide a mechanism to distribute the accumulation of multiple electrons and circumvent the need to access unstable or destructive metal oxidation states. In a series of molecular Co(II)poly(pyridyl) catalysts for H2 generation, the integration of redox-accessible pyrazine components was shown to reduce the overpotential for electrocatalytic proton reduction and allow the catalytic cycle to pass through the Co(I) state rather than the Co(0) state.29 In an earlier and related example, different mechanisms were proposed in response to variation in pH for a molecular cobaloxime analog containing the redox noninnocent bis(iminopyridine) ligand.30 The noninnocence of metal chelating ligands is a growing field of molecular catalysis in general,31 and there is enormous potential for solar fuel catalysis. A complete photocatalytic water-splitting system will couple molecular water oxidation and proton reduction by effective proton and electron transfer between the components that accomplish each half-reaction. However, a common strategy during photocatalyst development and mechanistic analysis is to employ a so-called “three component system,” composed of a catalyst, molecular photosensitizer, and a sacrificial electron donor/acceptor in large excess to rapidly regenerate the photosensitizer ground state.32 For reductive catalysis, typical sacrificial electron donors (for example, ascorbic acid, AA, under acidic conditions and tertiary amines for neutral to basic conditions) are thermodynamically compatible with the photoexcited state of benchmark molecular photosensitizers (i.e., [Ru(2,2′-bipyridine)3]2+) but suffer to some extent from unproductive back electron transfer steps from their oxidized products.33 In the case of AA, the formation and accumulation of dehydroascorbic acid (DHA) following reductive quenching of [Ru(2,2′-bipyridine)3]2+* by AA is proposed to significantly limit photocatalytic activity. Recently, Alberto and co-workers discovered that the addition of tris(2-carboxyethyl)phosphine (TCEP) more than doubled molecular photocatalyst activity as compared to the analogous three component system, purportedly by regenerating AA.34,35 However, in a closely related molecular photocatalyst system, adding TCEP resulted in an inefficient system, which was attributed to phosphine coordination to Co(II) and precipitation of the complex from the reaction mixture.36 These conflicting observations highlight the need to pay careful attention to the structural and mechanistic implications of all of the components in newly developed multimolecular photocatalyst systems, even the sacrificial reagents. In this work, we describe the synthesis, structural characterization, H2 electro- and photocatalytic activity, and proposed mechanisms for two new highly active Co(II) bis(bipyridine) macrocycles. The molecular catalysts are composed of Co(II)chelating 2,2′-bipyridine (bpy) groups linked by two or one nitrogen sites to yield a completely closed macrocycle (C-

Figure 1. Chemical structures of closed macrocycle and open “macrocycle” Co(II) proton reduction catalysts.

structural factors in cobalt’s second coordination sphere. Bipyridine groups have reversible reduction potentials which are accessible by common molecular photosensitizers and at moderate applied potentials. The nitrogen sites serve as an easily tuned handle for synthetic modifications between the bpy groups but also as a potential proton relay to the cobalt center. These new macrocycles are highly active H2 photocatalysts in near-neutral water, and introduction of the additional sacrificial reagent TCEP yields exceptional stability, with turnovers observed for several days under constant illumination. Interestingly, TCEP increases the observed activity for C-CAT but has the opposite effect for O-CAT. Investigation into the mechanism of proton reduction to H2 in organic solvents suggests that the macrocycle plays a critical role in both proton and electron transfer to the cobalt center. The work presented here demonstrates that well-designed metal-binding ligands can play an active role in molecular solar fuel catalysis and that even seemingly minor changes in molecular structure can induce enormous differences in activity.

■

RESULTS Synthesis. C-CAT and O-CAT were prepared following the multistep synthesis presented in Schemes S1 and S2, and complete details of the synthesis procedures are described in the Supporting Information. The closed macrocycle of C-CAT was obtained in good yield using a previously reported procedure.37 Surprisingly, no metal complexes containing this macrocycle (with amine substitution) have been previously described in the literature, but C-CAT was readily obtained by metalation of the macrocycle using the tetrafluoroborate salt of Co(II) in methanol. The synthesis of O-CAT proceeded similarly to that of C-CAT, starting with an asymmetric analog formed from two bpy groups linked by only one tertiary amine. The open “macrocycle” was prepared via palladium catalyzed Buchwald−Hartwig coupling of 6-amino-2,2′-bipyridine with 6-bromo-2,2′-bipyridine.38 Butyl substitution at the amine position was accomplished by first deprotonation of the secondary amine with sodium hydroxide in DMSO followed by nucleophilic attack of 1-bromobutane. O-CAT was then obtained by reacting the bis(bpy) open “macrocycle” with either the tetrafluoroborate or perchlorate salt of Co(II) in methanol or acetonitrile. Single Crystal Analysis. X-ray diffraction quality single crystals of C-CAT were obtained by slowly cooling a concentrated methanol solution, and the crystal structure is shown in Figure 2, with the relevant crystallographic data summarized in Tables S1−S3. The central cobalt atom is coordinated in its equatorial plane by the four bpy nitrogens of the macrocycle with typical Co−N bond lengths of 1.894(1) Å. 1698

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry

Figure 2. Crystal structures of C-CAT and O-CAT. Ellipsoids are depicted at 50% probability. Atom labels: carbon, gray; nitrogen, blue; oxygen, red; cobalt, magenta. Hydrogen atoms, counterions, and disordered solvent molecules are omitted for clarity.

However, the axial bond lengths in the dinuclear complex are much shorter than those observed in either the mononuclear O-CAT or C-CAT, with Co−Favg = 2.02 Å and Co−Oavg = 2.11 Å. Ground State Characterization. The ground state properties of C-CAT and O-CAT were characterized by UV−vis absorbance spectroscopy, cyclic voltammetry, and Xband EPR spectroscopy. Since we obtained two different solidstate structures of O-CAT, depending on the counterion used during synthesis and crystallization (ClO4 vs BF4), a primary goal was to detect any differences in the optical and electronic properties between O-CAT prepared using different counterions and to test whether the dinuclear F-bridged O-CAT species persists in solution. The UV−vis spectra of C-CAT and O-CAT in dimethylformamide are presented in Figure S23. The spectrum of CCAT features a broad, relatively low-energy band centered at 468 nm, which we assign as a ligand-based transition in agreement with similar spectral features previously observed for related macrocyclic complexes.37,43,44 This assignment in our complex is corroborated by comparison with the spectra of the unmetalated macrocycle, as well as that of the Zn(II) analog to C-CAT (Figure S24), and likely arises from the imine-like character of the bridging nitrogen observed in the crystal structure. On the basis of comparison with literature complexes, we assign the higher energy bands with peaks at 380 and 401 nm to MLCT and intraligand based transitions, respectively. The absorbance spectrum of O-CAT is quite different from that of C-CAT, with only two intense bands in the UV region present. We assign the band centered at 340 nm to ligand to metal charge transfer (LMCT) and the band at 280 nm to the n−π* and π−π* transitions of the nonelectronically coupled bipyridine subunits of the macrocycle, both based on analogous structures described previously.45 Cyclic voltammetry (CV) was used to measure the reduction potential of C-CAT and O-CAT in deoxygenated, anhydrous dimethylformamide (Figure 3A, summarized in Table S4). The Co(II/I) potential of both complexes was readily assigned by comparison with the Zn(II) analog to CCAT (Figure S25). C-CAT is the most easily reduced complex, with Co(II/I) = −0.65 V vs SCE. Removal of one linking nitrogen and introducing more flexibility into the tetradentate ligand drives the Co(II/I) reduction potential nearly 150 mV more negative to −0.79 V vs SCE for O-CAT. Two reversible one-electron reduction waves occur at approximately −1.25 and −1.60 V vs SCE for both C-CAT and O-CAT, and we assign these to the reductions of the bpy units of each macrocycle. We also investigated the electrochemical response of C-CAT and O-CAT in aqueous solutions in the interest of understanding their activity under conditions relevant to H2

Two loosely bound methanol atoms (Co−O = 2.295(1) Å) in the axial positions complete the cobalt coordination environment, which is a Jahn−Teller distorted octahedral geometry. The two bpy subunits of the macrocycle form one plane, and the two linking nitrogens are slightly above/below the plane defined by the bpy nitrogens. The C−N bond lengths of the butyl-substituted nitrogens which link the bpy groups are shorter than would be expected for a tertiary amine (1.394(4) Å), and the planarity of the nitrogen geometry suggests that the bonding of the linking nitrogens has significant imine character. A partial positive charge on the nitrogen due to an imine structure being present would yield a partial negative charge on the bpy nitrogens, comparable to a pyrrole or porphyrin-like ring. X-ray diffraction quality single crystals of O-CAT synthesized with Co(ClO4)2 were obtained following toluene precipitation from an acetonitrile solution, and the structure is shown in Figure 2. The Co(II) center is penta-coordinate in a distorted square pyramidal environment with the four bpy nitrogens of the open macrocycle occupying the equatorial positions and one acetonitrile molecule in the axial position. Unlike the structure of C-CAT, the two bpy subunits are twisted with respect to each other, confirming a high degree of structural flexibility about the bpy-linking nitrogen. The average Co−N distance in the equatorial plane is 1.932(2) Å (from 1.908(2) to 1.959(2)), just slightly longer than that in C-CAT. The axial positions of the Co(II) coordination environment of O-CAT are filled by one acetonitrile molecule (Co−N 2.133(2) Å) and a semibound perchlorate ion (Co−O 2.699(2) Å). In the course of exploring synthesis conditions for O-CAT, X-ray diffraction quality single crystals of the complex prepared using Co(BF4)2 were grown by vapor diffusion of a diethyl ether/pentane mixture into a concentrated methanol solution. Surprisingly, this solid-state structure revealed a dinuclear complex where the two octahedral Co(II) centers are bridged by a fluoride atom (Figure S22). We postulate that the bridging fluoride originates from methanol-induced fluoride abstraction from BF4−, which is supported by literature examples of F-bridged multinuclear complexes of cobalt39 and other transition metals.40−42 The dinuclear structure in the solid state is stabilized by π−π interactions between the two open macrocyclic ligands which occupy the equatorial positions of each Co(II). Each macrocycle is bowed toward the fluoride anion, and they are rotated approximately 120° with respect to each other. The exterior axial positions of the two Co(II) centers are occupied by methanol molecules. In general, the macrocycle is expanded as compared to the mononuclear structure: the average equatorial Co−N distance and C(pyridine)N bond lengths are 0.10 and 0.02 Å larger, respectively, than in the mononuclear structure of O-CAT. 1699

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry

comparison to O-CAT (−1.04 V vs SHE). Lowering the pH resulted in no change of the Co(II/I) and first ligand reduction potentials of both cobalt complexes (Figures S30, S32). In contrast, the Co(III/II) potential did show a dependence on pH. The Co(III/II) potential of O-CAT shifted linearly to more negative potentials with increasing pH by about −53 mV per pH unit, which is close to the expected 59 mV for a proton-coupled electron-transfer process and suggests protonation of an axial water ligand (Figure S31). The Co(III/II) redox wave of C-CAT was not affected by pH for acidic solutions with a pH below 5.5 but then linearly decreased in solutions with a pH higher than about 5.5 (−55 mV/pH unit; Figure S29). Continuous wave (cw) X-band EPR spectroscopy was performed on frozen glasses of C-CAT and O-CAT dissolved in a waterglycerol solution (Figure 3B), and confirms the low-spin d7 Co(II) electronic state of both macrocycles.46−51 We also measured the EPR spectra of C-CAT and O-CAT in the presence of ascorbate and TCEP to simulate photocatalytic conditions (Figure S33). Even with a large excess of TCEP, the spectra show that one and only one phosphorus atom binds quantitatively to the Co(II) site in an axial position,51−56 which agrees with a recently described crystal structure of TCEP bound to a similar Co(II) macrocycle.36 Photocatalytic Proton Reduction. The activity of CCAT and O-CAT for photocatalytic H2 generation was studied by illuminating aqueous solutions of the catalyst molecules with the benchmark molecular photosensitizer, [Ru(bpy)3Cl2], and a sacrificial electron donor (ascorbic acid). The photocatalyst containing solutions were illuminated with a 455 nm LED and maintained at 20 °C, and H2 evolution was monitored with a pressure transducer and confirmed by gas chromatography analysis of the reactor headspace (full details can be found in the Supporting Information). Our assessment of the photocatalytic activity of C-CAT and O-CAT began with defining “baseline” conditions for concentration of the catalyst, photosensitizer, and sacrificial electron donors, which were established consulting similar studies in the literature. Using [Ru(bpy)3]Cl2 (500 μM) as a photosensitizer and AA (0.1 M) as the sacrificial electron donor, both C-CAT and O-CAT (5 μM) were found to drive photocatalytic H2 generation, with an optimum activity at pH 4.5 (Figures 4, S38−S40). Under these conditions, O-CAT had a faster initial rate of H2 production than C-CAT (2070

Figure 3. Ground state characterization of C-CAT and O-CAT. (A) Cyclic voltammetry in DMF on glassy carbon electrode with 0.1 M TBAPF6 supporting electrolyte. (B) Comparison of cw X-band EPR spectra in glycerol/water under cryogenic conditions.

photocatalysis. Both catalysts were studied in aqueous phosphate-buffered-saline (PBS) in the range of pH 4 to 7, and under these conditions almost all of the observed redox events are irreversible. At pH 7, a Co(III/II) potential is observed for C-CAT at 0.43 V vs SHE and for O-CAT at 0.54 V vs SHE (Figure S28, Table S5). Similar to that found in an organic medium, the Co(II/I) potential of C-CAT (−0.43 V vs SHE) is about 200 mV more positive than the potential of OCAT (−0.64 V vs SHE), and the first ligand reduction of CCAT (−1.09 V vs SHE) is slightly more negative in

Figure 4. Photocatalytic H2 generation from water by C-CAT (black) and O-CAT (red). Conditions: 5 μM catalyst, 500 mM [Ru(bpy)3]Cl2, 0.1 M AA, 455 nm LED illumination, 120 mW/cm2. Turnovers presented as mol H2/mol catalyst. (A) Direct comparison of the photocatalytic activity of C-CAT and O-CAT at pH 4.5. (B) Photocatalytic H2 evolution activity of C-CAT following the addition of fresh sacrificial electron donors at pH 4.5. Fresh aliquots of AA (black) and TCEP (green) added to reactor at 3 h; dashed lines represent continuation of H2 evolution. (C) Photocatalytic activity of C-CAT and O-CAT with 0.1 M TCEP added at beginning of illumination at the optimum pH (pH 5 for C-CAT and pH 6.5 for O-CAT). 1700

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry H2 h−1 vs 1220 H2 h−1) and yielded higher total turnovers after 4 h of illumination (1380 H2/Co vs 800 H2/Co). Control measurements conducted without one of the reagents (i.e., without catalyst, photosensitizer, sacrificial electron donor, light) resulted in little or no hydrogen evolution and confirm that all components are required for a successful photocatalytic system (Figure S41). Following the initial pH screen for activity, we explored other variables to optimize photocatalytic activity. Reducing the catalyst concentration from 5 μM to 1 μM led to an approximately 3-fold increase of the initial TOF for both CCAT and O-CAT, reaching the highest TOF of 6870 TON h−1 for O-CAT and yielding a significant increase in the TON observed for both catalysts by approximately 1000 H2/Co (Figures S42−S45). We also surveyed the light intensity as a variable to influence the rate and total activity of H2 photocatalysis. We observed that decreasing the light intensity yielded the same total TON of H2 produced, but the initial rate of H2 evolution decreased. For example, using 1 μM O-CAT, the initial TOF decreased from 6870 h−1 for 120 mW/cm2 to 3381 h−1 for 60 mW/cm2 light intensity (Figure S46). To investigate the stability of C-CAT and O-CAT, and to understand why H2 generation ceases after only about 2 h of illumination (as in Figure 4A, for example), we systematically added additional components to the photocatalyst solution following the plateau in activity. Using O-CAT, we found that injection of either fresh catalyst or photosensitizer resulted in minimal additional H2 generation, suggesting that this slowdown in photocatalytic response is not a result of decomposition of O-CAT or [Ru(bpy)3]Cl2 (Figure S47). Next, working from the hypothesis that consumption of AA is the limiting factor in photocatalysis longevity, we added a fresh aliquot of pH 4.5 AA to the photocatalysis solution containing C-CAT to re-establish a concentration of 0.1 M, but this also did not yield recovery of photocatalytic activity (Figure 4B, black trace). In previous work, the accumulation of oxidized AA (DHA, dehydroascorbic acid) has been postulated to selfinhibit [Ru(bpy)3]2+-driven photocatalysis by back electron transfer between [Ru(bpy)2(bpy•−)]+ and DHA.57,58 This unproductive pathway can be avoided using tris(2carboxylethyl)phosphine (TCEP) as the terminal electron donor, which can reduce DHA back to AA, so that AA can be recycled up to 50 times as the electron donor to the photoexcited state of [Ru(bpy)3]Cl2.34,59 Here, we observe that H2 generation resumed immediately following the addition of TCEP after approximately 3 h of illumination (Figure 4B, green trace), although with a slightly slower rate than initially observed (887 h−1 with just 0.1 M AA, 255 h−1 after addition of 0.1 M TCEP). Nevertheless, the observation that catalytic activity is restored following addition of TCEP supports its previously proposed role of regenerating AA from DHA, therefore avoiding unproductive back electron transfer processes. Encouraged by these findings, TCEP (0.1 M) was introduced to our standard photocatalyst solution (5 μM catalyst, 500 μM [Ru(bpy)3]Cl2, Figure 4C). For C-CAT, continuous H2 generation was observed for much longer than without TCEP (over 10 h), and the total turnovers increased approximately five times as compared to the activity with just AA, reaching almost 4000 TON H2/Co. A screen of activity versus pH found that the maximum H2 production occurred at higher pH values than with only AA, between pH 5 and 6.5, and a prolonged induction period was observed starting at pH 5.5 (Figure S48). When TCEP was added to photocatalyst

solutions containing O-CAT, the largest H2 production was observed at pH 6.5, and even at pH 7 the TONs reached near 2000, although at a much lower rate (TOF ∼ 175 h−1; Figures S49, S50). Interestingly, the introduction of TCEP in photocatalytic solutions of O-CAT only showed a slight increase in H2 TON as compared to using just AA, in contrast to the behavior of C-CAT. Given the impressive increase in photocatalytic activity using TCEP with C-CAT, we explored other conditions to investigate catalyst stability. As shown in Figure 4C, H2 evolution ceases at approximately 15 h of constant illumination. However, in the presence of TCEP, we find that this is not because of the degradation of C-CAT or depletion of the sacrificial electron donor. Rather, we reason that the decrease in activity is because of decomposition of the photosensitizer since addition of fresh [Ru(bpy)3]Cl2 led to the immediate recovery of H2 evolution (Figure 5A). This

Figure 5. Stability of C-CAT under conditions for aqueous H2 photocatalysis. Conditions: 1−5 μM catalyst; 500 μM [Ru(bpy)3]Cl2; 0.1 M AA/0.1 M TCEP; 455 nm LED. (A) Addition of fresh [Ru(bpy)3]Cl2 to photocatalyst solution (5 μM C-CAT, 120 mW/ cm2) at 20 and 45 h, when H2 evolution had largely ceased. (B) Effect of light intensity on initial kinetics and activity of H2 photocatalysis (1 μM C-CAT).

experiment was conducted twice until no further H2 generation was observed, achieving 7800 H2 TONs over approximately 60 h. We found that decreasing the concentration of C-CAT from 5 μM to 1 μM resulted in faster rates of H2 evolution (from 713 h−1 to 4528 h−1), but at the lowest catalyst concentration, H2 evolution ended after only 2 h, yielding approximately 4000 H2 TONs (Figures S51−S54). In an effort to increase the longevity of both the catalyst and photosensitizer, we investigated the effect of the intensity of illumination (Figure 1701

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry S55). Using the low concentration of C-CAT (1 μM), we found that reducing the light intensity resulted in slower H2 generation, with a significant induction period observed for the lowest intensity achievable with our experimental setup (Figure 5B). However, these conditions yielded activity for more than 72 h, nearly achieving an outstanding 20 000 H2 TONs. Electrocatalytic Proton Reduction. C-CAT and O-CAT were subjected to cyclic voltammetry with the addition of a proton source to investigate the mechanism of electrocatalytic proton reduction. Figure 6 presents the cyclic voltammetry of

Fc+. On scanning to further negative potentials, we observe a strong current response that indicates the catalytic reduction of the added protons in solution. The catalytic current response increases with acid concentration for all potentials negative of the Co(II/I) potential, but with distinctly different rates depending on the applied potential (Figures S65, S66). For both C-CAT and O-CAT, the mildly weaker acid anilinium was also used as a proton source, and similar behavior was observed as for p-cyanoanilinium (Figures S67, S68). However, when the same measurements were performed with pmethoxyanilinium, which has a pK a of nearly 12 in CH3CN,60 the additional peak at about −1400 mV vs Fc/ Fc+ is not observed prior to the onset of catalytic current response at more negative potentials (Figure S71). We performed several experiments to investigate the role of the ligand in the electrocatalytic mechanism. First, we subjected the Zn(II) macrocycle analog of O-CAT to the same conditions in the cyclic voltammetry measurements. In these experiments using the redox-inert Zn(II) metal center and the stronger acids p-cyanoanilinium and anilinium (Figures 6B, S70), we still observe this new feature at approximately −1400 mV vs Fc/Fc+, suggesting that this is a ligand-based species. The addition of p-methoxyanilinium to the Zn(II) O-CAT analog does not yield any activity prior to the background electrode activity (Figure S73). Second, we synthesized the carbon-bridged analog to O-CAT, denoted OCAT(C), to isolate the effect of the bridging nitrogen in the electrocatalytic activity (Scheme S3, details of synthesis and characterization in the Supporting Information). The electrochemical responses of O-CAT(C) and its Zn(II) analog were measured following the addition of aliquots of p-cyanoanilinium, anilinium, and p-methoxyanilinium (Figures S75−S80). For O-CAT(C), a large current response is observed near the first bpy reduction potential without the formation of a new peak like that observed for the nitrogen-bridged O-CAT. Electrocatalytic measurements were also performed in aqueous solution to more closely mimic the conditions experienced during H2 photocatalysis. Cyclic voltammograms of C-CAT and O-CAT were recorded in phosphate buffer, and the pH was adjusted between 3 and 7 using hydrochloric acid or sodium hydroxide. The onset of catalytic current was found for both complexes at the first ligand reduction at about −1.10 V vs SHE, resulting in an onset overpotential of 0.7 V considering a hydrogen potential of −0.42 V vs SCE at pH 7 (Figures S84, S85). The onset of the catalytic current also showed a dependence on pH for both C-CAT and O-CAT. The applied potential decreased linearly with pH at a catalytic current of 80 μA, clearly showing the importance of proton concentration on catalytic proton reduction as expected for such a process.

Figure 6. (A) Electrocatalytic H2 generation by O-CAT in dimethylformamide using p-cyanoanilinium as a proton source. Conditions: 1 mM O-CAT, 1 mM ferrocene, 0.1 M TBABF4, aliquots of p-cyanoanilinium (0, 1, 5, 10, 20, 30 mM shown, full titration shown in Figure S59), glassy carbon working electrode, 100 mV/s. Inset shows relatively constant response at Co(II/I) potential (about −1200 mV vs Fc/Fc+) and growth of new feature following addition of p-cyanoanilinium (about −1400 mV vs Fc/Fc+). (B) Cyclic voltammetry of Zn(II) O-CAT analog with increasing concentration of p-cyanoanilinium (0, 1, 5, 10, 20, 30 mM shown, full titration shown in Figure S61). Inset compares increase in current response of Co(II) and Zn(II) O-CAT macrocycles at about −1400 mV vs Fc/Fc+.

■

DISCUSSION Molecular Catalyst Design and Structure. In this study, we present two new and closely related molecular catalysts, CCAT and O-CAT, which were designed with several structural features to achieve high stability and activity for catalytic proton reduction from water. The first design element is the use of bpy as a building block within the macrocycle. For both C-CAT and O-CAT, the first bpy reduction occurs at approximately −1.2 V vs SCE (Figure 3), which is thermodynamically equivalent to the reduction potential of the reduced photosensitizer, [Ru(bpy)2(bpy•−)]+, with a reduction potential of −1.23 V vs SCE.61 Therefore, the bpy

O-CAT in dimethylformamide in response to the addition of aliquots of p-cyanoanilinium; Figure S62 presents the same measurements for C-CAT. For both molecular catalysts, on addition of the proton source, we observe the growth of an additional reductive wave immediately following the Co(II/I) response (inset of Figure 6A). This new reductive wave is welldefined and semireversible, and the current response is linear to the square root of the scan rate (Figures S63, S64), suggesting the formation of a new homogeneous species that has a reduction potential at approximately −1400 mV vs Fc/ 1702

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry

investigate the influence on activity with atomic-level resolution. In this spirit, we designed C-CAT and O-CAT with only one linking nitrogen group different between their molecular structures. We originally hypothesized that removing one of the linking nitrogens between the Co(II)-binding bpy groups would yield a catalyst with similar activity and potential for bpy participation in electron transfer, but also with some structural flexibility to accommodate the multiple oxidation states of cobalt which are generally understood to play a part in the mechanism of proton reduction. Indeed, as seen in Figure 2, the solid state structure of O-CAT exhibits a large deviation from the almost perfectly planar geometry of C-CAT. And, as shown in Figure 4A, this structural flexibility may be responsible for the increase in photocatalytic activity (to be discussed in more detail below). Structurally similar open “macrocycle” Co(II) catalysts have been reported in the literature. One recent example from Queyriaux et al. describes a proton reduction catalyst where the opening of the macrocycle breaks one of the bpy components into two pyridine units.67 Schnidrig et al. reported a catalyst that has an identical bis(bpy) “macrocycle” to O-CAT but lacking butyl substitution at the linking nitrogen and Br ligands axial to the Co(II) center.68 Despite the structural similarity of O-CAT to these two catalysts, their photo- and electrocatalytic activity are strikingly different, reinforcing the potential for a molecular design approach. Surprisingly, O-CAT yielded two different solid state structures based on the counterion involved during the last step of the synthesis (ClO4 vs BF4) and conditions used for crystallization. However, in comparing the ground state characterization (UV−vis, CV, EPR) of solution samples prepared from both, we observe an identical response (Figures S23, S26, S34). Furthermore, we did not observe any significant difference in the photocatalytic activity when comparing the catalyst synthesized using either anion (Figures S35−S37). Therefore, we conclude that the fluoride-bridged dimer structure of O-CAT is a structure unique to the solid state and is not retained in any of the homogeneous, solution based analyses. Photocatalytic Activity and Mechanism. Both C-CAT and O-CAT are highly active for photocatalytic H2 generation in fully aqueous solution and compare well with related molecular catalysts described in the literature. Given the diversity in experimental setups for measuring photocatalysis across different research laboratories (including variations in light source and intensity, optical path length, head space, temperature, etc.), we note with caution that a comparison of activity between various reports is semiquantitative at best. However, it is instructive to put this work in the context of similar molecular complexes to highlight the excellent stability and activity of C-CAT and O-CAT (Table S7).18,29,34−36,45,62,67−70 At the optimal conditions of pH, catalyst concentration, and light intensity identified here, CCAT and O-CAT are among the most active molecular photocatalysts in generating H2 from near neutral aqueous solution. Table S7 lists our results as well as those from the literature at the optimal reported pH for each photocatalyst system. Like most other photocatalytic systems, the activity of C-CAT and O-CAT shows a sharp dependence on solution pH (Figure S40). For both catalysts, the highest activity is observed at pH 4.5 when using just AA as the sacrificial electron donor. As has been proposed for related multimolecular photocatalyst

groups may be able to accept an electron from a reductively quenched [Ru(bpy)3]2+* and introduce the possibility to share some of the burden to manage the two electron, two proton transfer required for H2 generation from water. Ligand noninnocence has been explored previously in the design of molecular catalysts for proton and CO2 reduction, as well as water oxidation. In an example specifically related to the work here, Nippe et al. introduced bpy groups into the ligand framework of molecular Co(II)penta(pyridyl) catalysts.62 The bpy-containing catalysts demonstrated significantly improved photo- and electrocatalytic proton reduction as compared to their redox-inert analogs. The second relevant design element of C-CAT and O-CAT is the integration of nitrogen groups to link the Co(II)chelating bpy components into macrocycles. We note that very similar Co(II) macrocycles have been recently described by the Alberto group, which links the Co(II)-chelating bpy units with either methylene or carbonyl groups, and these molecular catalysts also displayed excellent activity and stability for catalytic proton reduction from aqueous solutions.35,36 In our work, however, we employed the nitrogen group for both synthetic versatility and as a potential site for protonation adjacent to the metal center. The bis- or monoamine precursors (6,6′-amino-2,2′-bpy or 6-amino-2,2′-bpy) to the unmetalated macrocycles of C-CAT and O-CAT (structures 1 and 2 in Schemes S1 and S2) are readily obtained in high yields using well-established literature techniques and therefore are desirable synthetic intermediates. Furthermore, the secondary amines of the unmetalated macrocycles are easy targets for functionalization through nucleophilic substitution under relatively mild conditions. With butyl groups off the nitrogen linkers, both C-CAT and O-CAT are adequately soluble in neutral to acidic aqueous conditions, important for proton reduction from water. However, this versatile synthetic handle could certainly be used to tune redox properties, solubility, or connect the molecular catalysts to other homogeneous or heterogeneous components. The nitrogen groups were also included as structural elements of C-CAT and O-CAT for their potential to participate in the mechanism of proton reduction as a proton relay to the metal center. As elegantly shown by DuBois and colleagues, protonatable nitrogen groups in the second coordination sphere of molecular catalysts can participate as proton relays to the metal center. Other molecular catalysts that propose a ligand protonation step in the mechanism for proton reduction include Co(II)bis(dithiolene)63−65 and Ni(II)tris(pyridinethiolate)66 complexes. However, these examples may be of less practical utility since the proposed protonation step occurs in the first coordination sphere and involves ligand dissociation, which would negatively impact long-term stability. Joliat-Wick et al. described a molecular Co(II) macrocycle very similar in structure to C-CAT, but with bridging keto groups, which could plausibly perform in a proton relay capacity.36 While the in-depth study of any proton-coupled electron transfer (PCET) behavior by C-CAT or O-CAT is outside the scope of the current work, we anticipate that their synthetic versatility will render these molecular catalysts an ideal platform toward investigating how further elaboration at the nitrogen link impacts the primary mechanistic steps. A chief advantage of molecular catalysts over heterogeneous materials approaches to catalyst development is the ability to modify the molecular structure one atom at a time and 1703

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry systems,71 this is likely the pH that balances the competing effects of optimal reductive quenching rate with stability of the reduced catalyst. Interestingly, the pH dependence shifts to more neutral levels when TCEP is added to remove accumulated DHA and regenerate AA (Figure S50). Lowtemperature EPR spectroscopy shows that both C-CAT and O-CAT accommodate one phosphine ligand in the axial position (Figure S33). This observation suggests that TCEP is playing a dual role in the multimolecular catalyst mixture: regenerating AA by removing accumulated DHA as well as stabilizing the Co(I) oxidation state by coordination to the cobalt center of each macrocycle. The stabilizing effect proposed here is supported by previous work that demonstrated a similar effect in a cobaloxime-based multimolecular photocatalyst system.72 Even the minor structural modification between C-CAT and O-CAT results in substantial differences in the photocatalytic activity for proton reduction. Under the “standard” conditions using just AA as the sacrificial electron donor (Figure 4A), OCAT generates nearly twice as many turnovers of H2 as CCAT. As mentioned in the above discussion of the catalyst design approach, the accepted mechanism for proton reduction of most Co(II)-based molecular catalysts passes through several oxidation states and coordination environments.10,73,74 Therefore, we interpret the higher activity of O-CAT under these conditions as the open “macrocycle” providing a higher degree of flexibility, which can more readily accommodate multiple oxidation states and coordination environments than the more rigid ligand of C-CAT. However, when we introduce TCEP to regenerate oxidized AA and serve as the terminal electron acceptor in the photocatalytic mechanism, we observe an inverse in activity with C-CAT performing much better than O-CAT, clearly shown in Figure 4C. As discussed above and in previous literature,57−59 TCEP regenerates oxidized AA and prevents back electron transfer from [Ru(bpy)2(bpy•−)]+ to DHA, and the total H2 turnovers for both catalysts increases on addition of TCEP to the photocatalytic solution. However, the increase observed for O-CAT (1380 (pH 4.5, AA) vs 2075 (pH 6.5, AA/TCEP) H2 TON) is much less dramatic than that for C-CAT (810 (pH 4.5, AA) vs 3800 (pH 5.0, AA/ TCEP) H2 TON). We reason that this difference in behavior is that the structural flexibility of O-CAT is a liability in the presence of TCEP. In the flexible open “macrocycle” of OCAT, Co−N bond dissociation could be quickly followed by rotation of one of the pyridine rings, coordination to cobalt by the central phosphine of TCEP, and finally complete dissociation of cobalt from the polypyridine ligand (depicted in Scheme S4). This opportunistic phosphine coordination to cobalt is much less likely to occur in the closed macrocycle of C-CAT, and therefore the addition of TCEP yields a remarkable increase of activity for this catalyst. Under some of the photocatalytic conditions explored for both C-CAT and O-CAT, there is an early period of little to no H2 generation following the start of illumination. This is observed most dramatically when TCEP is present and at the “extreme” conditions tested; specifically at very low catalyst concentration and low light intensity (Figure 5B), or at relatively high pH values (pH 6.4 in Figure S48). Importantly, we confirmed the homogeneous nature of the molecular catalysts by the standard Hg poisoning test, and identical kinetics and activity were confirmed, indicating that the molecular structure is retained and H2 generation is not the result of cobalt nanoparticle or colloid formation (Figure S56).

Therefore, we propose that the initially slow H2 activity of this multimolecular system is a result of the slow accumulation of the catalytic intermediates in favor of unproductive, back electron transfer steps. For example, at low catalyst concentration, the cumulative yield of the two-electron reduced complex is likely quite low. At low light intensity, the yield of [Ru(bpy)3]2+* is small, resulting in a minimal fraction that a catalyst might encounter through diffusion interactions. At relatively high values of pH, protonation of the Co(I) state is disfavored, precluding the continuation of the catalytic cycle.71 This reasoning is generally consistent with similar findings observed for other molecular or multinuclear proton reduction catalysts where an induction period is associated with the light induced accumulation of the catalytically active species.14,75−78 We propose that H2 photocatalysis proceeds by a reductive quenching mechanism in the multimolecular systems containing C-CAT or O-CAT (Figure 7). The Co(II) ground state for

Figure 7. Proposed mechanism for H2 photocatalysis by the multimolecular system containing the water reduction catalyst (WRC) C-CAT or O-CAT, photosensitizer (PS) [Ru(bpy)3]2+, electron relay AA, and terminal electron donor TCEP. TCEPO and DHA refer to the oxidized forms of TCEP and AA, respectively. Reproduced with permission from ref 34. Copyright 2014 Royal Society of Chemistry.

both catalysts is thermodynamically able to accept an electron from either [Ru(bpy)3]2+* via oxidative quenching or [Ru(bpy)2(bpy•−)]+ following reductive quenching of the visible-light excited photosensitizer by AA. To assess the kinetics of both pathways, we performed a Stern−Volmer analysis of the [Ru(bpy)3]2+* emission following titration of AA, TCEP, C-CAT, or O-CAT, at several pH values. The rate of [Ru(bpy)3]2+* emission quenching by AA (kq) is (1−2) × 107 M−1 s−1, depending on pH, in good agreement with extensive previous analysis of this interaction (Figure S57). TCEP had no effect on the emission of [Ru(bpy)3]2+*. The emission quenching rate for C-CAT and O-CAT (3 × 109 M−1 s−1, 9 × 108 M−1 s−1, respectively, see Figure S58) is 1 to 2 orders of magnitude greater than that observed for AA. However, given the much greater concentration of AA vs catalyst used in the multimolecular system, we reason that reductive quenching of the photosensitizer by AA is the first step in the photocatalytic mechanism following excitation. This thermodynamic and kinetic analysis is consistent with previously reported photocatalyst systems based on molecular cobalt catalysts. 62 However, it is difficult to use the photocatalysis results alone to comment on the mechanism following photoinduced electron transfer and the intermediate species of the catalyst. Electrocatalytic Activity and Mechanism. Electrochemical analysis has been used extensively to elucidate key mechanistic information for a number of molecular catalysts for critical reactions including the reduction of protons, CO2, and N2, as well as water oxidation.74,79 Here, we used measurements of the electrocatalytic activity to isolate the 1704

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry mechanistic steps at the catalyst center (i.e., the blue arrow in Figure 7) and specifically examine the role of the macrocycle. The electrocatalytic analysis in aqueous solution is useful to establish the overpotential for proton reduction by C-CAT and O-CAT, which varies from 0.65 to 0.73 V, depending on pH (Figures S84, S85). These values are comparable to what is reported for similar molecular proton reduction catalysts.9,35,67 An initial investigation into the mechanistic pathway for catalytic proton reduction was performed using cyclic voltammetric measurements in dimethylformamide with pcyanoanilinium, a moderately strong acid which is also relatively inert and resistant to homoconjugation or dimerization.60 The cyclic voltammetry response of both catalysts displays a feature at approximately 1400 mV negative of Fc/ Fc+ which is not present in the scans prior to the addition of acid but grows in as the H+ concentration increases (Figure S62 for C-CAT, Figures 6A and S59 for O-CAT). As this feature has little to no precedent in related molecular proton reduction catalysts, we carried out several experiments in an effort to understand its nature. First, the current response of this feature follows a linear dependence on the square root of the scan rate (Figures S63, S64), suggesting that this is a homogeneous species interacting diffusionally with the electrode, as opposed to an electrode-bound species. Second, we ruled out the possibility of this species being cobalt-based (for example, a stable cobalt hydride) since we observed the same feature in the Zn(II) analog to O-CAT (Figures 6B and S61). Also, cyclic voltammetry in the presence of 4aminobenzonitrile excludes the possibility that this feature represents another population of macrocycles with Co(II) coordinated to the cyano group of the proton source (Figures S82, S83). Third, we synthesized the carbon-bridged analog to O-CAT, designated O-CAT(C), which we also investigated by cyclic voltammetry in the presence of p-cyanoanilinium to isolate the structural features of the macrocycles and their contributions to the mechanism of catalytic proton reduction. In the cyclic voltammogram of O-CAT(C) using pcyanoanilinium, there were no additional features like those observed for C-CAT, O-CAT, and the Zn(II) analog to OCAT (Figures S75−S77, S81). From all of these experiments taken together, we assign the new feature at −1400 mV vs Fc/ Fc+ to protonation and reduction of the bridging bpy nitrogens. Importantly, the new feature in the cyclic voltammograms of C-CAT, O-CAT, and the Zn(II) analog to O-CAT using p-cyanoanilinium is also present in the electrocatalytic measurements using anilinium but not following the addition of p-methoxyanilinium (Figures S67− S74). Therefore, we can use the known pKa values of anilinium and p-methoxyanilinium60 to estimate the pKa of the bpybridging nitrogens of C-CAT and O-CAT at roughly 11 in organic solvents. From cyclic voltammetry in the presence of three proton sources, we propose the mechanism shown in Figure 8 for the catalytic reduction of protons to H2 by C-CAT and O-CAT. First, in the presence of the relatively strong acids used here, the ground state of the molecular complex (depicted as Co(II)L) is protonated, which likely occurs at the bpylinking nitrogen groups. Next, at ∼1200 mV negative of Fc/ Fc+, the Co(II) center is reduced to Co(I), which shifts only slightly positive as acid is added to the system. We propose that the peak observed at ∼1400 mV negative of Fc/Fc+ is the reduction of the protonated ligand. These are the steps in the catalytic cycle that we can propose using the cyclic

Figure 8. Proposed mechanism for electrocatalytic proton reduction by C-CAT and O-CAT. L represents macrocycle ligand, dashed box indicates ground state species, ET = electron transfer, pT = proton transfer.

voltammetric measurements since the onset of the catalytic current begins shortly after this new H+-induced feature, at approximately −1500 mV vs Fc/Fc+. Therefore, the final steps which complete the catalytic cycle (formation of hydride, reduction of hydride, and H2 formation) are based on extensive experimental and computational mechanistic analysis on related molecular cobalt catalysts in the literature.73,75,80−84 Since these steps cannot be directly observed, we caution that there is a certain level of speculation in the proposed mechanistic pathway for the later steps. An in-depth analysis of these steps exceeds the scope of this initial catalyst development study, but ongoing work is planned to investigate this behavior. The proposed mechanism for H2 generation by C-CAT and O-CAT occurs through two intramolecular steps: intramolecular electron transfer to reduce the hydride from Co(III) to Co(II), followed by intramolecular proton transfer which generates the H−H bond between the metal hydride and ligand proton. The intramolecular mechanism is highly unusual for molecular catalysts. In general, the electron transfer steps in similar mechanistic evaluations occur either via interfacial electron transfer, from an applied potential at an electrode, or intermolecular electron transfer, from an excited photosensitizer molecule. One exception for a molecular catalyst that likely proceeds via intramolecular electron transfer is found in a supramolecular Ru(II)−Rh(II)−Ru(II) catalyst in which the metal-bridging pyrazine-based ligands are postulated to act as an electron reservoir during catalysis.85 Heterogeneous electrocatalysts for O2 or CO2 reduction follow similar design principles by providing a fast source of electrons through carbon black supports to molecular transition metal catalyst sites.86,87 Intramolecular proton transfer is a key step in the catalytic cycle for the family of [Ni(P 2 R N 2 R ′ ) 2 ] 2+ catalysts,21−28 and variation of amine substitution in a recently described CO2 reduction catalyst suggests that amine protonation influences catalyst activity and provides a controllable solvation shell.88,89 Tandem electron and proton transfer activity from a dangling pyridine of a Co(II)tetra(pyridyl) complex has been recently proposed,90 but the inclusion of both intramolecular electron and proton transfer features remains quite rare in molecular catalysis. 1705

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Inorganic Chemistry

■

CONCLUSIONS In conclusion, we have described the synthesis, structural characterization, and catalytic behavior of two new, redoxactive Co(II) molecular complexes which are active for proton reduction. The optimal conditions found here for photocatalytic H2 production driven by [Ru(bpy)3]2+ from nearneutral water yields almost 2 × 104 H2 turnovers, which places these catalysts among the most active molecular catalysts based on first-row transition metals. We propose that this exceptional activity arises because the coordinating ligand does more than just support the metal center and plays an active role in the multielectron, multiproton redox catalysis. Analysis of the electrochemical response under acidic conditions suggests unprecedented behavior in which both intramolecular electron transfer and intramolecular proton transfer steps between the ligand and the cobalt center are key components in the proposed mechanism for H2 catalysis. Therefore, these complexes represent a step in the path toward molecular catalysts which mimic the valuable features of enzymatic catalysis that are typically extraordinarily difficult to access synthetically. Future studies will benefit from the synthetic versatility of C-CAT and O-CAT, which enables a relatively straightforward way to modify the ligand potential and amine pKa to perform detailed inquiries into the individual mechanistic steps. These highly robust, active, and synthetically tractable molecular catalysts can be connected to lightharvesting and electrode surfaces with relative ease and will enable integration into a full water-splitting system based on molecular architectures.

■

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors gratefully acknowledge support by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, through Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Funding for the single crystal X-ray diffractometer at Purdue University was made possible through funding by the National Science Foundation through the Major Research Instrumentation Program under Grant No. CHE 1625543.

(1) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474−6502. (2) Blankenship, R. E.; Tiede, D. M.; Barber, J.; Brudvig, G. W.; Fleming, G.; Ghirardi, M.; Gunner, M. R.; Junge, W.; Kramer, D. M.; Melis, A.; Moore, T. A.; Moser, C. C.; Nocera, D. G.; Nozik, A. J.; Ort, D. R.; Parson, W. W.; Prince, R. C.; Sayre, R. T. Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement. Science 2011, 332, 805−809. (3) Nocera, D. G. The Artificial Leaf. Acc. Chem. Res. 2012, 45, 767− 776. (4) Chen, J. G.; Crooks, R. M.; Seefeldt, L. C.; Bren, K. L.; Bullock, R. M.; Darensbourg, M. Y.; Holland, P. L.; Hoffman, B.; Janik, M. J.; Jones, A. K.; Kanatzidis, M. G.; King, P.; Lancaster, K. M.; Lymar, S. V.; Pfromm, P.; Schneider, W. F.; Schrock, R. R. Beyond fossil fuel− driven nitrogen transformations. Science 2018, 360, 6391. (5) Morris, A. J.; Meyer, G. J.; Fujita, E. Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42, 1983−1994. (6) Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J. Chemical approaches to artificial photosynthesis. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15560−15564. (7) Eckenhoff, W. T.; Eisenberg, R. Molecular systems for light driven hydrogen production. Dalton Trans 2012, 41, 13004−13021. (8) Mulfort, K. L.; Utschig, L. M. Modular Homogeneous Chromophore−Catalyst Assemblies. Acc. Chem. Res. 2016, 49, 835− 843. (9) Sun, Y.; Bigi, J. P.; Piro, N. A.; Tang, M. L.; Long, J. R.; Chang, C. J. Molecular Cobalt Pentapyridine Catalysts for Generating Hydrogen from Water. J. Am. Chem. Soc. 2011, 133, 9212−9215. (10) Artero, V.; Chavarot-Kerlidou, M.; Fontecave, M. Splitting Water with Cobalt. Angew. Chem., Int. Ed. 2011, 50, 7238−7266. (11) 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. (12) Roy, S.; Bacchi, M.; Berggren, G.; Artero, V. A Systematic Comparative Study of Hydrogen-Evolving Molecular Catalysts in Aqueous Solutions. ChemSusChem 2015, 8, 3632−3638. (13) McCrory, C. C. L.; Uyeda, C.; Peters, J. C. Electrocatalytic Hydrogen Evolution in Acidic Water with Molecular Cobalt Tetraazamacrocycles. J. Am. Chem. Soc. 2012, 134, 3164−3170. (14) Natali, M.; Luisa, A.; Iengo, E.; Scandola, F. Efficient photocatalytic hydrogen generation from water by a cationic cobalt(ii) porphyrin. Chem. Commun. 2014, 50, 1842−1844. (15) Du, P.; Knowles, K.; Eisenberg, R. A Homogeneous System for the Photogeneration of Hydrogen from Water Based on a Platinum(II) Terpyridyl Acetylide Chromophore and a Molecular Cobalt Catalyst. J. Am. Chem. Soc. 2008, 130, 12576−12577. (16) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. Kinetics of Electron Transfer Reactions of H2-Evolving Cobalt Diglyoxime Catalysts. J. Am. Chem. Soc. 2010, 132, 1060−1065. (17) Kaeffer, N.; Chavarot-Kerlidou, M.; Artero, V. Hydrogen Evolution Catalyzed by Cobalt Diimine−Dioxime Complexes. Acc. Chem. Res. 2015, 48, 1286−1295.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03297. Full experimental details including synthetic procedures and structural characterization; details of single crystal X-ray analysis and tables of important bond lengths and angles; cyclic voltammetry, EPR spectroscopy, and UV− vis absorbance of molecular catalysts; emission quenching titrations; supporting photo- and electrocatalytic experiments; table comparing activity of similar molecular H2 catalysts (PDF) Accession Codes

CCDC 1824468−1824470 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jens Niklas: 0000-0002-6462-2680 Matthias Zeller: 0000-0002-3305-852X Oleg G. Poluektov: 0000-0003-3067-9272 Karen L. Mulfort: 0000-0002-3132-1179 Notes

The authors declare no competing financial interest. 1706

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry (18) Khnayzer, R. S.; Thoi, V. S.; Nippe, M.; King, A. E.; Jurss, J. W.; El Roz, K. A.; Long, J. R.; Chang, C. J.; Castellano, F. N. Towards a comprehensive understanding of visible-light photogeneration of hydrogen from water using cobalt(ii) polypyridyl catalysts. Energy Environ. Sci. 2014, 7, 1477−1488. (19) Queyriaux, N.; Jane, R. T.; Massin, J.; Artero, V.; ChavarotKerlidou, M. Recent developments in hydrogen evolving molecular cobalt(II)−polypyridyl catalysts. Coord. Chem. Rev. 2015, 304−305, 3−19. (20) Eckenhoff, W. T. Molecular catalysts of Co, Ni, Fe, and Mo for hydrogen generation in artificial photosynthetic systems. Coord. Chem. Rev. 2018, 373, 295. (21) Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.; Rakowski DuBois, M.; DuBois, D. L. Hydrogen Oxidation and Production Using Nickel-Based Molecular Catalysts with Positioned Proton Relays. J. Am. Chem. Soc. 2006, 128, 358−366. (22) Rakowski Dubois, M.; Dubois, D. L. Development of Molecular Electrocatalysts for CO2 Reduction and H2 Production/Oxidation. Acc. Chem. Res. 2009, 42, 1974−1982. (23) Rakowski DuBois, M.; DuBois, D. L. The roles of the first and second coordination spheres in the design of molecular catalysts for H2 production and oxidation. Chem. Soc. Rev. 2009, 38, 62−72. (24) Rakowski DuBois, M.; DuBois, D. L. The roles of the first and second coordination spheres in the design of molecular catalysts for H2 production and oxidation. Chem. Soc. Rev. 2009, 38, 62−72. (25) Kilgore, U. J.; Roberts, J. A. S.; Pool, D. H.; Appel, A. M.; Stewart, M. P.; DuBois, M. R.; Dougherty, W. G.; Kassel, W. S.; Bullock, R. M.; DuBois, D. L. [Ni(PPh2NC6H4 × 2)2]2+ Complexes as Electrocatalysts for H2 Production: Effect of Substituents, Acids, and Water on Catalytic Rates. J. Am. Chem. Soc. 2011, 133, 5861− 5872. (26) Wiese, S.; Kilgore, U. J.; DuBois, D. L.; Bullock, R. M. [Ni(PMe2NPh2)2](BF4)2 as an Electrocatalyst for H2 Production. ACS Catal. 2012, 2, 720−727. (27) Yang, J. Y.; Smith, S. E.; Liu, T.; Dougherty, W. G.; Hoffert, W. A.; Kassel, W. S.; DuBois, M. R.; DuBois, D. L.; Bullock, R. M. Two Pathways for Electrocatalytic Oxidation of Hydrogen by a Nickel Bis(diphosphine) Complex with Pendant Amines in the Second Coordination Sphere. J. Am. Chem. Soc. 2013, 135, 9700−9712. (28) Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 s−1 for H2 Production. Science 2011, 333, 863−866. (29) Jurss, J. W.; Khnayzer, R. S.; Panetier, J. A.; El Roz, K. A.; Nichols, E. M.; Head-Gordon, M.; Long, J. R.; Castellano, F. N.; Chang, C. J. Bioinspired design of redox-active ligands for multielectron catalysis: effects of positioning pyrazine reservoirs on cobalt for electro- and photocatalytic generation of hydrogen from water. Chem. Sci. 2015, 6, 4954−4972. (30) Stubbert, B. D.; Peters, J. C.; Gray, H. B. Rapid Water Reduction to H2 Catalyzed by a Cobalt Bis(iminopyridine) Complex. J. Am. Chem. Soc. 2011, 133, 18070−18073. (31) Berben, L. A.; de Bruin, B.; Heyduk, A. F. Non-innocent ligands. Chem. Commun. 2015, 51, 1553−1554. (32) Pellegrin, Y.; Odobel, F. Sacrificial electron donor reagents for solar fuel production. C. R. Chim. 2017, 20, 283. (33) Shan, B.; Baine, T.; Ma, X. A. N.; Zhao, X.; Schmehl, R. H. Mechanistic Details for Cobalt Catalyzed Photochemical Hydrogen Production in Aqueous Solution: Efficiencies of the Photochemical and Non-Photochemical Steps. Inorg. Chem. 2013, 52, 4853−4859. (34) Bachmann, C.; Probst, B.; Guttentag, M.; Alberto, R. Ascorbate as an electron relay between an irreversible electron donor and Ru(ii) or Re(i) photosensitizers. Chem. Commun. 2014, 50, 6737−6739. (35) Joliat, E.; Schnidrig, S.; Probst, B.; Bachmann, C.; Spingler, B.; Baldridge, K. K.; von Rohr, F.; Schilling, A.; Alberto, R. Cobalt complexes of tetradentate, bipyridine-based macrocycles: their structures, properties and photocatalytic proton reduction. Dalton Trans 2016, 45, 1737−1745.

(36) Joliat-Wick, E.; Weder, N.; Klose, D.; Bachmann, C.; Spingler, B.; Probst, B.; Alberto, R. Light-Induced H2 Evolution with a Macrocyclic Cobalt Diketo-Pyrphyrin as a Proton-Reducing Catalyst. Inorg. Chem. 2018, 57, 1651−1655. (37) Ogawa, S.; Shiraishi, S. A tautomerisable macrocyclic compound containing two aza-bridged 2,2[prime or minute]bipyridine moieties. J. Chem. Soc., Perkin Trans. 1 1980, 1, 2527− 2530. (38) Zheng, S.; Reintjens, N. R. M.; Siegler, M. A.; Roubeau, O.; Bouwman, E.; Rudavskyi, A.; Havenith, R. W. A.; Bonnet, S. Stabilization of the Low-Spin State in a Mononuclear Iron(II) Complex and High-Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding. Chem. - Eur. J. 2016, 22, 331−339. (39) Cho, Y. I.; Ward, M. L.; Rose, M. J. Substituent effects of N4 Schiff base ligands on the formation of fluoride-bridged dicobalt(ii) complexes via B-F abstraction: structures and magnetism. Dalton Trans 2016, 45, 13466−13476. (40) Gobeze, W. A.; Milway, V. A.; Moubaraki, B.; Murray, K. S.; Brooker, S. Solvent control: dinuclear versus tetranuclear complexes of a bis-tetradentate pyrimidine-based ligand. Dalton Trans 2012, 41, 9708−9721. (41) Reger, D. L.; Pascui, A. E.; Smith, M. D.; Jezierska, J.; Ozarowski, A. Dinuclear Complexes Containing Linear M−F−M [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)] Bridges: Trends in Structures, Antiferromagnetic Superexchange Interactions, and Spectroscopic Properties. Inorg. Chem. 2012, 51, 11820−11836. (42) Gobeze, W. A.; Milway, V. A.; Chilton, N. F.; Moubaraki, B.; Murray, K. S.; Brooker, S. Di- and Tetranuclear Complexes of BisTetradentate Pyrimidine-Based Ligands with All-Methylene- Versus Mixed Methylene/Ethylene-Linked Arms. Eur. J. Inorg. Chem. 2013, 2013, 4485−4498. (43) Estiú, G. L.; Jubert, A. H.; Molina, J.; Costamagna, J.; Canales, J.; Vargas, J. Semiempirical SCF/CI interpretation of the origin of the catalytic activity of transition metal-macrocycle complexes. J. Mol. Struct.: THEOCHEM 1995, 330, 201−210. (44) Morita, J.; Tsuchiya, S.; Yoshida, N.; Nakayama, N.; Yokokawa, S.; Ogawa, S. The unique properties observed for the unsymmetrical macrocyclic compounds with the highly distorted structure. Tetrahedron 2007, 63, 9522−9530. (45) Tong, L.; Kopecky, A.; Zong, R.; Gagnon, K. J.; Ahlquist, M. S. G.; Thummel, R. P. Light-driven proton reduction in aqueous medium catalyzed by a family of cobalt complexes with tetradentate polypyridine-type ligands. Inorg. Chem. 2015, 54, 7873−7884. (46) 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. (47) Nishida, Y.; Kida, S. Splitting of d-orbitals in square planar complexes of copper(II), nickel(II) and cobalt(II). Coord. Chem. Rev. 1979, 27, 275−298. (48) Bakac, A.; Brynildson, M. E.; Espenson, J. H. Characterization of the structure, properties, and reactivity of a cobalt(II) macrocyclic complex. Inorg. Chem. 1986, 25, 4108−4114. (49) Baumgarten, M.; Lubitz, W.; Winscom, C. J. EPR and ENDOR studies of cobaloxime(II). Chem. Phys. Lett. 1987, 133 (2), 102−108. (50) 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. (51) Mukherjee, A.; Kokhan, O.; Huang, J.; Niklas, J.; Chen, L. X.; Tiede, D. M.; Mulfort, K. L. Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly. Phys. Chem. Chem. Phys. 2013, 15, 21070−21076. (52) Pezeshk, A.; Greenaway, F. T.; Vincow, G. Electron paramagnetic resonance of low-spin cobalt(II) complexes: effect of axial ligation upon the ground state. Inorg. Chem. 1978, 17, 3421− 3425. (53) Pezeshk, A.; Greenaway, F. T.; Dabrowiak, J. C.; Vincow, G. EPR studies of axial ligation of a low-spin cobalt(II) macrocyclic Schiff base complex. Inorg. Chem. 1978, 17, 1717−1725. 1707

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry (54) Labauze, G.; Raynor, J. B. Electron spin resonance studies of axial ligation to cobalt(II) complexes. Part 1. Some oxo-, thio-, and seleno-Schiff bases. J. Chem. Soc., Dalton Trans. 1980, 2388−2394. (55) Labauze, G.; Raynor, J. B. Electron spin resonance studies of axial ligation to cobalt (II)complexes. Part 2. Interaction with phosphorus ligands. Analysis of the cobalt bonding parameters and the phosphorus hyperfine coupling. J. Chem. Soc., Dalton Trans. 1981, 590−598. (56) Rangel, M.; Leite, A.; Gomes, J.; de Castro, B. Photolysis Secondary Products of Cobaloximes and Imino/Oxime Compounds Controlled by Steric Hindrance Imposed by the Lewis Base. Organometallics 2005, 24, 3500−3507. (57) Guttentag, M.; Rodenberg, A.; Kopelent, R.; Probst, B.; Buchwalder, C.; Brandstätter, M.; Hamm, P.; Alberto, R. Photocatalytic H2 Production with a Rhenium/Cobalt System in Water under Acidic Conditions. Eur. J. Inorg. Chem. 2012, 2012, 59−64. (58) Guttentag, M.; Rodenberg, A.; Bachmann, C.; Senn, A.; Hamm, P.; Alberto, R. A highly stable polypyridyl-based cobalt catalyst for homo- and heterogeneous photocatalytic water reduction. Dalton Trans 2013, 42, 334−337. (59) Martindale, B. C. M.; Joliat, E.; Bachmann, C.; Alberto, R.; Reisner, E. Clean Donor Oxidation Enhances the H2 Evolution Activity of a Carbon Quantum Dot−Molecular Catalyst Photosystem. Angew. Chem., Int. Ed. 2016, 55, 9402−9406. (60) McCarthy, B. D.; Martin, D. J.; Rountree, E. S.; Ullman, A. C.; Dempsey, J. L. Electrochemical Reduction of Brønsted Acids by Glassy Carbon in AcetonitrileImplications for Electrocatalytic Hydrogen Evolution. Inorg. Chem. 2014, 53, 8350−8361. (61) Kalyanasundaram, K. Photophysics, photochemistry and solar energy conversion with tris(bipyridyl)ruthenium(II) and its analogues. Coord. Chem. Rev. 1982, 46, 159−244. (62) Nippe, M.; Khnayzer, R. S.; Panetier, J. A.; Zee, D. Z.; Olaiya, B. S.; Head-Gordon, M.; Chang, C. J.; Castellano, F. N.; Long, J. R. Catalytic proton reduction with transition metal complexes of the redox-active ligand bpy2PYMe. Chemical Science 2013, 4, 3934−3945. (63) McNamara, W. R.; Han, Z.; Alperin, P. J.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. A Cobalt-Dithiolene Complex for the Photocatalytic and Electrocatalytic Reduction of Protons. J. Am. Chem. Soc. 2011, 133, 15368−15371. (64) McNamara, W. R.; Han, Z.; Yin, C.-J.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15594−15599. (65) Solis, B. H.; Hammes-Schiffer, S. Computational Study of Anomalous Reduction Potentials for Hydrogen Evolution Catalyzed by Cobalt Dithiolene Complexes. J. Am. Chem. Soc. 2012, 134, 15253−15256. (66) Han, Z.; Shen, L.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. Nickel Pyridinethiolate Complexes as Catalysts for the LightDriven Production of Hydrogen from Aqueous Solutions in NobleMetal-Free Systems. J. Am. Chem. Soc. 2013, 135, 14659−14669. (67) Queyriaux, N.; Giannoudis, E.; Windle, C. D.; Roy, S.; Pécaut, J.; Coutsolelos, A. G.; Artero, V.; Chavarot-Kerlidou, M. A noble metal-free photocatalytic system based on a novel cobalt tetrapyridyl catalyst for hydrogen production in fully aqueous medium. Sustainable Energy & Fuels 2018, 2, 553−557. (68) Schnidrig, S.; Bachmann, C.; Müller, P.; Weder, N.; Spingler, B.; Joliat-Wick, E.; Mosberger, M.; Windisch, J.; Alberto, R.; Probst, B. Structure−Activity and Stability Relationships for Cobalt Polypyridyl-Based Hydrogen-Evolving Catalysts in Water. ChemSusChem 2017, 10, 4570−4580. (69) Singh, W. M.; Baine, T.; Kudo, S.; Tian, S.; Ma, X. A. N.; Zhou, H.; DeYonker, N. J.; Pham, T. C.; Bollinger, J. C.; Baker, D. L.; Yan, B.; Webster, C. E.; Zhao, X. Electrocatalytic and Photocatalytic Hydrogen Production in Aqueous Solution by a Molecular Cobalt Complex. Angew. Chem., Int. Ed. 2012, 51, 5941−5944. (70) Tong, L.; Zong, R.; Thummel, R. P. Visible Light-Driven Hydrogen Evolution from Water Catalyzed by A Molecular Cobalt Complex. J. Am. Chem. Soc. 2014, 136, 4881−4884.

(71) Natali, M. Elucidating the Key Role of pH on Light-Driven Hydrogen Evolution by a Molecular Cobalt Catalyst. ACS Catal. 2017, 7, 1330−1339. (72) Zhang, P.; Jacques, P.-A.; Chavarot-Kerlidou, M.; Wang, M.; Sun, L.; Fontecave, M.; Artero, V. Phosphine Coordination to a Cobalt Diimine−Dioxime Catalyst Increases Stability during LightDriven H2 Production. Inorg. Chem. 2012, 51, 2115−2120. (73) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. Mechanism of H2 Evolution from a Photogenerated Hydridocobaloxime. J. Am. Chem. Soc. 2010, 132, 16774−16776. (74) Elgrishi, N.; McCarthy, B. D.; Rountree, E. S.; Dempsey, J. L. Reaction Pathways of Hydrogen-Evolving Electrocatalysts: Electrochemical and Spectroscopic Studies of Proton-Coupled Electron Transfer Processes. ACS Catal. 2016, 6, 3644−3659. (75) Du, P.; Schneider, J.; Luo, G.; Brennessel, W. W.; Eisenberg, R. Visible Light-Driven Hydrogen Production from Aqueous Protons Catalyzed by Molecular Cobaloxime Catalysts. Inorg. Chem. 2009, 48, 4952−4962. (76) McCormick, T. M.; Calitree, B. D.; Orchard, A.; Kraut, N. D.; Bright, F. V.; Detty, M. R.; Eisenberg, R. Reductive Side of Water Splitting in Artificial Photosynthesis: New Homogeneous Photosystems of Great Activity and Mechanistic Insight. J. Am. Chem. Soc. 2010, 132, 15480−15483. (77) Huang, J.; Mulfort, K. L.; Du, P.; Chen, L. X. Photodriven Charge Separation Dynamics in CdSe/ZnS Core/Shell Quantum Dot/Cobaloxime Hybrid for Efficient Hydrogen Production. J. Am. Chem. Soc. 2012, 134, 16472−16475. (78) Yang, S.; Pattengale, B.; Kovrigin, E. L.; Huang, J. Photoactive Zeolitic Imidazolate Framework as Intrinsic Heterogeneous Catalysts for Light-Driven Hydrogen Generation. ACS Energy Letters 2017, 2, 75−80. (79) Rountree, E. S.; McCarthy, B. D.; Eisenhart, T. T.; Dempsey, J. L. Evaluation of Homogeneous Electrocatalysts by Cyclic Voltammetry. Inorg. Chem. 2014, 53, 9983−10002. (80) Hu, X.; Brunschwig, B. S.; Peters, J. C. Electrocatalytic Hydrogen Evolution at Low Overpotentials by Cobalt Macrocyclic Glyoxime and Tetraimine Complexes. J. Am. Chem. Soc. 2007, 129, 8988−8998. (81) Baffert, C.; Artero, V.; Fontecave, M. Cobaloximes as Functional Models for Hydrogenases. 2. Proton Electroreduction Catalyzed by Difluoroborylbis(dimethylglyoximato)cobalt(II) Complexes in Organic Media. Inorg. Chem. 2007, 46, 1817−1824. (82) Muckerman, J. T.; Fujita, E. Theoretical studies of the mechanism of catalytic hydrogen production by a cobaloxime. Chem. Commun. 2011, 47, 12456−12458. (83) Solis, B. H.; Hammes-Schiffer, S. Theoretical Analysis of Mechanistic Pathways for Hydrogen Evolution Catalyzed by Cobaloximes. Inorg. Chem. 2011, 50, 11252−11262. (84) Marinescu, S. C.; Winkler, J. R.; Gray, H. B. Molecular mechanisms of cobalt-catalyzed hydrogen evolution. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15127−15131. (85) Manbeck, G. F.; Canterbury, T.; Zhou, R.; King, S.; Nam, G.; Brewer, K. J. Electrocatalytic H2 Evolution by Supramolecular RuII− RhIII−RuII Complexes: Importance of Ligands as Electron Reservoirs and Speciation upon Reduction. Inorg. Chem. 2015, 54, 8148−8157. (86) Oh, S.; Gallagher, J. R.; Miller, J. T.; Surendranath, Y. GraphiteConjugated Rhenium Catalysts for Carbon Dioxide Reduction. J. Am. Chem. Soc. 2016, 138, 1820−1823. (87) Jackson, M. N.; Oh, S.; Kaminsky, C. J.; Chu, S. B.; Zhang, G.; Miller, J. T.; Surendranath, Y. Strong Electronic Coupling of Molecular Sites to Graphitic Electrodes via Pyrazine Conjugation. J. Am. Chem. Soc. 2018, 140, 1004−1010. (88) Chapovetsky, A.; Do, T. H.; Haiges, R.; Takase, M. K.; Marinescu, S. C. Proton-Assisted Reduction of CO2 by Cobalt Aminopyridine Macrocycles. J. Am. Chem. Soc. 2016, 138, 5765− 5768. (89) Chapovetsky, A.; Welborn, M.; Luna, J. M.; Haiges, R.; Miller, T. F.; Marinescu, S. C. Pendant Hydrogen-Bond Donors in Cobalt 1708

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709

Article

Inorganic Chemistry Catalysts Independently Enhance CO2 Reduction. ACS Cent. Sci. 2018, 4, 397−404. (90) Wang, J.; Li, C.; Zhou, Q.; Wang, W.; Hou, Y.; Zhang, B.; Wang, X. A polypyridyl Co(ii) complex-based water reduction catalyst with double H2 evolution sites. Catal. Sci. Technol. 2016, 6, 8482− 8489.

1709

DOI: 10.1021/acs.inorgchem.8b03297 Inorg. Chem. 2019, 58, 1697−1709