Chemical Science

Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue A mechanistic study of proton reduction catalyzed by a pentapyridine cobalt complex: evidence for Chem. Sci. Cite this: , 2013, 4, 1578 involvement of an anation-based pathway† Amanda E. King,a Yogesh Surendranath,a Nicholas A. Piro,ae Julian P. Bigi,ae Jeffrey R. Long*ad and Christopher J. Chang*abce 2+ The pentapyridine cobalt complex [Co(PY5Me2)] and its congeners have been shown to catalyze proton reduction to hydrogen in aqueous solution over a wide pH range using electrical or solar energy input. Here, we employ electrochemical and spectroscopic studies to examine the mechanisms of proton reduction by this parent complex under soluble, diffusion-limited conditions in acetonitrile with acetic acid as the proton donor. Two pathways for proton reduction are identified via cyclic voltammetry: one pathway occurring from an acetonitrile-bound CoII/I couple and the other pathway operating from an acetate-bound CoII/I couple. Kinetics studies support protonation of a CoI species as the rate- determining step for both processes, and additional electrochemical measurements further suggest that the onset of catalysis from the acetonitrile-bound CoII/I couple is highly affected by catalyst electronics. Received 14th December 2012 Taken together, this work not only establishes the CoPY5Me unit as a unique molecular platform that Accepted 12th February 2013 2 catalyzes the reduction of protons under soluble, diffusion-limited conditions in both aqueous and DOI: 10.1039/c3sc22239j organic media, but also highlights the participation of anation processes that are likely relevant for a www.rsc.org/chemicalscience wide range of hydrogen-producing and related catalytic systems. Introduction number of structural and functional mimics.3,4 In this regard, inorganic model complexes of hydrogenase active sites have The concomitant rise in global energy demands and growing greatly contributed to our mechanistic understanding of these Downloaded by University of California - Berkeley on 05 March 2013 concerns over climate change have led to increased interest in enzymes, but their common requirement for organic solvents, Published on 14 February 2013 http://pubs.rsc.org | doi:10.1039/C3SC22239J exploring alternative energy resources.1 In this context, instability to air, and the need for strong acids has precluded hydrogen has garnered considerable attention as a potential their widespread use in functional device systems.3 In addition, molecular fuel whose use could result in a closed, carbon- many elegant abiotic monometallic H2 production systems have neutral energy production system.2 In nature, both mono- and been developed; however, high reactivity at low overpotentials bimetallic hydrogenase enzymes mediate the equilibrium in aqueous media is largely restricted to the use of low-abun- 5 between H2 and protons/electrons in aqueous media at the dance and expensive precious metal centers, whereas molec- thermodynamic overpotential and with turnover frequencies as ular proton reduction catalysts based on earth-abundant metals high as 10 000 per mole catalyst per second.3 As such, the search oen require organic solvents or additives, strong acids, and/or 6,7 for robust synthetic H2 production systems that match the relatively high overpotentials. Notable elegant exceptions efficiency of hydrogenases has resulted in the design of a include tethered molecular catalysts that show tolerance to aqueous media,7e,7j,8 as well as hydrogenase mimics that operate in aqueous media.9 Thus, the invention of water-stable molec- a Departments of Chemistry, University of California, Berkeley, California 94720, USA. ular catalysts for H production using earth-abundant metals E-mail: [email protected]; [email protected] 2 remains a signicant challenge. bDepartments of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA To meet this need, we have initiated a broad-based program to cHoward Hughes Medical Institute, University of California, Berkeley, California create earth-abundant metal catalyst platforms that can operate 94720, USA in clean aqueous media.10 As part of these investigations, we dMaterials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, recently reported the reactivity of a family of hydrogen catalysts California 94720, USA supported by chelating polypyridine ligand frameworks eChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, (Chart 1).11 The molybdenum(IV)-oxo catalyst employing 2,6- California 94720, USA bis(1,1-bis(2-pyridyl)ethyl)pyridine (PY5Me ) was shown to † Electronic supplementary information (ESI) available. CCDC 915137, 915338, 2 11a 915155 and 915156. For ESI and crystallographic data in CIF or other electronic catalyze robust H2 production in both seawater and acetoni- format see DOI: 10.1039/c3sc22239j trile solutions.11b The related molybdenum(IV) disulde, 1578 | Chem. Sci., 2013, 4, 1578–1587 This journal is ª The Royal Society of Chemistry 2013 View Article Online Edge Article Chemical Science in the design of systems with minimized overpotential under ionic aqueous media. Results and discussion n+ Synthesis and characterization of [Co(PY5Me2)] complexes In the absence of a proton source in acetonitrile, the cyclic voltammogram of 1 exhibits two reversible redox processes (Fig. 1). III 3+ The one-electron oxidation of 1 to form a [Co (PY5Me2)] species (2) occurs at E½ ¼ 815 mV, while the one-electron I + reduction of 1 to furnish a [Co (PY5Me2)] species (3) occurs at E½ ¼830 mV (all potentials reported herein are referenced to SHE). The reversible nature of the redox processes observed for 1 Chart 1 Water-compatible molecular polypyridine catalysts for proton suggested that the CoI and CoIII congeners could be indepen- reduction. dently synthesized. Accordingly, exposure of 1 to NOBF4 yields III 3+ [Co (NCMe)(PY5Me2)] (2) (Scheme 1). The UV-visible spectrum of 2 shows a weak absorption at 490 nm and a shoulder at 360 (Fig. S1†), and the diamagnetic 1H NMR spectrum indicates obtained by exposure of a molybdenum(II) PY5Me2 precursor to a low-spin d6 complex (Fig. S1†). Compound 2 proved amenable elemental sulfur, is also capable of generating H2 from acidic aqueous or organic media, and provides a structural and func- to structure determination using X-ray crystallographic tech- 11c niques and displayed the six-coordinate octahedral geometry tional mimic for the triangular edge active sites of MoS2. In 6 – rst-row transition metal chemistry, we showed that a cobalt(II) expected for a low-spin d complex (Fig. 2). All Co N bond – complex ligated by 2-bis(2-pyridyl)(methoxy)methyl-6-pyr- distances contract upon oxidation of 1, with the Co Naxial bond ˚ ˚ idylpyridine (PY4) can effect catalytic proton reduction in 1 : 1 length showing the largest decrease from 2.09(6) A to 1.93(6) A 11d and the Co–N bond distances decreasing from an H2O : MeCN mixtures, where the onset of catalysis occurred at equatorial ˚ ˚ 14 ˚ the potential observed for the CoII/CoI redox couple. Finally, the average of 2.12(8) A to 1.98(2) A. The 0.09(7) A out-of-plane 2+ shi observed in 1 decreases to 0.00(1) Ain˚ 2. complex [Co(PY5Me2)] (1) displayed catalytic H2 production in aqueous solutions at neutral pH with a turnover frequency of 300 Adding equimolar amounts of Co(PPh3)3Cl to a benzene solution containing PY5Me yields [CoI(PY5Me )]+ (3). mmol H2 per mole catalyst per second at a 660 mV over- 2 2 potential.11e Introduction of various functional groups in the Compound 3 also proved amenable to structure determination ff para position of the central pyridine ring resulted in rational using single-crystal X-ray di raction analysis (Fig. 2). Here, the shis for both the CoII/CoI reduction potential and onset of complex adopts a square pyramidal geometry at the cobalt center, Downloaded by University of California - Berkeley on 05 March 2013 catalytic proton reduction; indeed, the least reducing derivative with an empty coordination site due to the synthesis and recrys- Published on 14 February 2013 http://pubs.rsc.org | doi:10.1039/C3SC22239J tallization in a weakly coordinating solvent. The contraction bearing a CF3 functionality was capable of mediating both elec- – ˚ – tro- and photocatalytic hydrogen production under diffusion- of the Co Naxial bond length in 3 to 1.96(8) A and two trans Co ˚ ˚ limited conditions in aqueous media at physiological pH.12 Nequatorial bonds to 2.01(3) A and 2.02(8) A upon reduction results in the cobalt center residing in a ‘saddle-shaped’ environment. During our initial investigations of the Co(PY5Me2) platform, we noted that the catalytic activity of 1 in water is unusual Additionally, 3 displayed a broad, paramagnetically shi ed 1 8 compared to the activity of the vast majority of other well- H-NMR spectrum consistent with a high-spin d electronic studied cobalt(II) proton-reduction systems, in that the catalytic onset potential is not at the CoII/CoI couple but rather from a species with a more negative reduction potential.9c,11e,13 Intrigued by this result, we sought to obtain more insight into the mechanisms of proton reduction by 1 by performing experiments under homogenous, diffusion-limited conditions that allow systematic control of acid strength and concentra- tion. In this report, we present electrochemical and spectro- photometric studies on 1 in acetonitrile solution with acetic acid as the proton donor, showing that this platform is a competent hydrogen-producing electrocatalyst under soluble, diffusion-limited conditions in both organic and aqueous media. Interestingly, we observe two mechanistic pathways: one pathway occurring from an acetonitrile-bound CoII/I couple and II/I the other pathway operating from an acetate-bound Co Fig. 1 Cyclic voltammogram of 1 in acetonitrile. Conditions: [1] ¼ 10 mM, couple. The identi cation of an anation-assisted pathway has [NBu4PF6] ¼ 100 mM, working/auxiliary/reference electrodes ¼ glassy carbon À broader implications for proton reduction catalysts, particularly disk/Pt wire/silver wire, scan rate ¼ 100 mV s 1.

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