This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012 New J. Chem., 2012, 36, 1149–1152 1149 Cite this: New J. Chem., 2012, 36, 1149–1152 A tridentate Ni pincer for aqueous electrocatalytic hydrogen productionw Oana R. Luca, a Steven J. Konezny, a James D. Blakemore, a Dominic M. Colosi, b Shubhro Saha, a Gary W. Brudvig, a Victor S. Batista* a and Robert H. Crabtree* a Received (in Gainesville, FL, USA) 25th October 2011, Accepted 29th February 2012 DOI: 10.1039/c2nj20912h A Ni II complex with a redox-active pincer ligand reduces protons at a low overpotential in aqueous acidic conditions. A combined experimental and computational study provides mechanistic insights into a putative catalytic cycle. H 2 is currently produced by steam reforming of fossil fuels 1,2 which is both expensive and detrimental to the environment. If H 2 is to be a fuel in environmentally friendly alternative energy strategies, 3,4 more sustainable sources of H 2 are required. Elemental Pt is currently the best catalyst for the reduction of protons to H 2 5 but its low abundance and high cost make it unsuitable for global use. 3 A range of different transition metal complexes can act either as electrocatalysts or photocatalysts, including systems involving Co, 6 Mo 7,8 and Ni. 9–11 In a recent report, a tetradentate Co system with a redox active ligand also operates in aqueous conditions. 12 Improved systems that can operate in aqueous conditions with abundant first row transition metals are of current interest. Pincer ligands are attractive because they are easy to assemble from readily available materials and impart high stability to the resulting complexes. Furthermore, their modular nature facilitates tuning of ligand properties. 13 Here we report that a Ni pincer gives good activity as an operationally homo- geneous electrocatalyst for proton reduction in aqueous conditions. Electrochemistry in acetonitrile was used to pinpoint redox events of the metal complex in the presence of added acid and subsequently aqueous conditions. Density functional calculations (DFT) calculations offer insight into a possible mechanism. Results and discussion One electron reduction of catalyst 1 (Scheme 1) is known to give a ligand-centered reduction of the NNN pincer ligand, 14 as shown by EPR data. In inital voltammetry in MeCN, we observe a first reduction wave just above 0 V vs. NHE for complex 1 (Fig. 1) that we tentatively assign a ligand-centered process given the literature precedent. The second reduction at B À0.5 V vs. NHE is therefore assigned as a Ni II /Ni I couple. To the precatalyst in acetonitrile increasing amounts of acid were added and an increase in current response was observed by cyclic voltammetry (ESI: S1-2w). Assuming a rate-limiting chemical step, we applied the voltammetric kinetic treatment of DuBois et al., 15a,b originally developed by Nicholson et al., 15c (S7w) that leads to kinetic parameters for the H 2 evolution reaction. Like the DuBois case, our rate law follows eqn (1) and an apparent rate constant of 1.05 (Æ0.21) Â 10 4 has been determined for 0.1 M H + , 5 mM 1 at À250 mV vs. NHE. This corresponds to a voltammetric rate of hydrogen formation of 105 s À1 . 16 rate = k[H + ] 2 [cat] (1) Further information was obtained at variable potential where we find sustained H 2 evolution in a series of chronoamperometry experiments. A plot of current density vs. overpotential (Fig. S3-1w) shows that the overpotential for 1 is a very satisfactory 140 mV at a current density of 1 mA cm À2 , assuming a thermodynamic potential of À84 mV vs. NHE under our conditions (Details in S3w). The compound can also operate in water. A high surface area reticulated vitreous carbon working electrode was used to determine quantitative H 2 evolution from 1 via mass spectro- scopy (see ESIw). Specifically, a 50 mL 0.1 M KCl/HCl solution (pH 1) containing 0.2 mM precatalyst was held at À1.1 V vs. NHE for one hour. The charge passed in the catalytic experiment was 212 C. After subtraction of the relevant 80 C background current, 132 C were consumed by the catalytic chemistry. This is equivalent to a predicted production of 0.68 millimoles of H 2 . From quantitative mass Scheme 1 Catalyst 1. a Department of Chemistry, Yale University, 225 Prospect St., New Haven, CT 06520-8107, USA. E-mail: robert.crabtree@yale.edu, victor.batista@yale.edu, gary.brudvig@yale.edu; Fax: (+)1 203 432 6144 b Department of Geology and Geophysics: Earth Systems Center for Stable Isotopic Studies, Yale University, 210 Whitney Ave., New Haven, CT 06511, USA. E-mail: dcolosi@wisc.edu w Electronic supplementary information (ESI) available: All relevant experimental details, NMR spectra, kinetics and computational results. See DOI: 10.1039/c2nj20912h NJC Dynamic Article Links www.rsc.org/njc LETTER Published on 19 March 2012. Downloaded by California Institute of Technology on 10/08/2013 21:04:11. View Article Online / Journal Homepage / Table of Contents for this issue