Article pubs.acs.org/JPCC
Spectroscopic and Quantum Chemical Study of the Ni(PPh2NC6H4CH2P(O)(OEt)22)2 Electrocatalyst for Hydrogen Production with Emphasis on the NiI Oxidation State Amélie Kochem,* Frank Neese, and Maurice van Gastel* Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany S Supporting Information *
ABSTRACT: The bis(diphosphine)nickel catalyst first investigated by DuBois and co-workers [DuBois, M. R.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62] is arguably one of the most promising molecular catalysts for hydrogen production. It features a low overpotential and, in its most recent variation, a high turnover number of 105 s−1 [Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. Science 2011, 333, 863]. The complex features two reversible oneelectron reductions. It is believed that all accessible oxidation states (2+, 1+, 0) of nickel are involved in the proposed catalytic cycle. In this article we focus on the paramagnetic NiI state, for which few experimental studies have been performed. By a combination of modern EPR and quantum chemical methods, it is established that the stable NiI species does not feature a hydride ligand. Furthermore, hydrogen evolution already starts upon addition of acid to the NiI state even without the presence of additional reducing equivalents. The implications for the catalytic cycle are discussed.
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INTRODUCTION In a time where our society is exploring alternatives for fossil fuels, the search for catalysts that can quickly and efficiently convert renewable energy sources into chemical energy, in particular by using chemical bonds of small molecules, is the focus of research of many research groups.1 A vast multitude of catalysts for small molecule activation, in either the homogeneous or heterogeneous phase or extracted from nature, has been investigated.2 In general, investigations typically focus on catalysts that catalyze a half-reaction. For example, in the case of the prototypical clean fuel cell in which water is split into molecular hydrogen and oxygen, one catalyst ideally efficiently oxidizes water into molecular oxygen and protons and electrons, and another catalyst serves to reduce the protons to form molecular hydrogen. Obviously, since the reaction enthalpy of the reaction 2H 2O → 2H 2 + O2 (1)
materials. From the point of view of activation overpotential, platinum black has a vanishing overpotential and is thus an ideal catalytic material. However, in addition to being very expensive and in limited global supply, the platinum surface degrades owing to changing surface morphology, platinum particle growth, and catalyst poisoning by carbon monoxide.5,6 In sharp contrast to the multitude of systems under investigation, very little is known about the mechanisms of action of these catalysts at the molecular level. These include aspects such as substrate binding, bond activation reactions, bond cleavage reactions, or bond formation reactions. In the heterogeneous phase, direct structural information at the atomic level of surface morphologies can be probed by a variety of microscopic techniques.7−10 It is generally accepted that the microscopic surface morphology is a determining factor for the catalytic activity.11 These include terrace edges, steps, and defects, all of which feature coordinatively unsaturated metal atoms with free valence electron pairs. In the field of homogeneous catalysis it is often possible to obtain an in-depth structural and mechanistic picture through a combination of various kinetic and spectroscopic techniques. A leading example is provided by the case of [NiFe] hydrogenases for which all postulated reaction intermediates have been observed and extensively characterized.12−14 A key feature of the mechanism is the involvement of a coordinately unsaturated Ni center that can readily accept a hydride ligand.15−18
amounts to +116 kcal/mol,3 minimally a third component that supplies the required energy is necessary. In the case of water electrolysis, an interesting system has been known for some time that uses a solar cell to supply the energy from sunlight to two carbon electrodes.4 The efficiency of such a cell is limited, and requires a much larger voltage than the minimum value of 1.23 V, which corresponds to the energy required to split water into molecular oxygen and four protons and electrons. The minimization of the overpotential at the electrodes by using catalysts is an essential requirement for an efficiently operating system. Ideally these catalysts should themselves be comprised of cheap, readily available, stable, air- and water-tolerant © XXXX American Chemical Society
Received: November 28, 2013 Revised: January 10, 2014
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dx.doi.org/10.1021/jp411710b | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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Figure 1. (a, b) Schematic representation of the geometry optimized model for (1)2+. The four 31P atoms, labeled 1−4, are inequivalent and form a distorted square planar ligand field around the nickel atom. The orientation of the molecule is chosen such that the molecular z axis, representing the axial direction along which hydride binding can occur, is perpendicular to the plane of the paper. (c) Side view, endo conformation; (d) side view, exo conformation; (e) [2Fe2S] cluster in [FeFe] hydrogenase of C. pasteurianum.54 The hydride binding site is indicated with an arrow. Color coding of the atoms: carbon, gray; hydrogen, white; phosphorus, orange; nickel, cyan; nitrogen, blue; iron, brown; oxygen, red.
= C6H4X, with X being different functional groups in the para position that fine-tune the basicity, has indicated that the turnover number can be changed by approximately 1 order of magnitude.27 Moreover, also the structure of the acid that is used in the reaction as a proton donor has a substantial effect on the turnover number. The cyclic voltammogram (CV) features two reversible reductions, in which the oxidation state of the nickel atom formally changes among NiII, NiI, and Ni0. The observation that the complex can be isolated in three different oxidation states allows a detailed investigation of the electronic structure by spectroscopic techniques. X-ray crystallography has been performed as well as investigations by NMR spectroscopy for the diamagnetic states NiII and Ni0 in solution,26 providing information about the crystal structure and the structure in solution. On a theoretical level, calculations have been performed for the molecule with R, R′ = cyclohexyl, benzyl, where the exact geometry of the complex, in particular the position of amines with respect to nickel, being in either the catalytically favored endo (Figure 1c) or the kinetically favored exo (Figure 1d) positions, has been identified as having an important impact on the catalytic rates.23 In particular, intramolecular proton exchange was found to be absent in the exo form.23 A reaction mechanism has recently been proposed for hydrogen oxidation as well as for proton reduction, in which the NiII, NiI, and Ni0 oxidation states are involved.22 The catalytic activity is proposed to be exclusively related to a proton transfer pathway that involves the amine in the endo position and the axial coordination position of nickel, where hydride binding can occur.22 Spectroscopically, the NiI oxidation state is paramagnetic and not amenable to NMR investigations. Investigations by advanced EPR spectroscopy37 and hyperfine resolving methods such as electron nuclear double resonance (ENDOR) spectroscopy are thus feasible. In this article, we investigate the NiI state of the DuBois complex by magnetic resonance techniques,
The design of molecular catalysts for hydrogen production proceeding by a heterolytic bond formation pathway and the understanding of the mechanism of action is crucial for future applications where these catalysts will be attached to an electrode. The heterolytic reaction (H+ + H− → H2) has clear advantages over the homolytic reaction (2H• → H2) in that the latter pathway presumably requires two metal centers that will be difficult to bring together when immobilized on the electrode surface.19,20 In the field of bioinspired chemistry, a series of bis(diphosphine)nickel complexes has been synthesized in the group of DuBois et al.21−35 The molecules feature a NiP4 core with two free coordination positions at nickel, where a hydride may bind. A schematic picture of one such complex is shown in Figure 1a. The design of these complexes has been inspired by the active site of [FeFe] hydrogenase (Figure 1e), and features a coordinatively unsaturated metal and the presence of a base in the second coordination sphere. Additionally, these molecules fulfill most of the abovementioned criteria in that they feature an inexpensive 3d transition metal and a relatively low overpotential. The complex tolerates H2O up to concentrations of up to 0.3 M (the activity is even increased in the presence of H2O). Depending on the choice of substituents on the ligand, it features a relatively low overpotential of down to 370 mV and a turnover of up to 1850 s−1.26 Recently, even a turnover number of up to 105 s−1 has been reported,36 thus making this class of molecules the presently best-performing known molecular catalysts with the lowest overpotential and highest turnover number even exceeding that of hydrogenases. The change of the functional groups (cf. Figure 1b) from R, R′ = phenyl, phenyl to R, R′ = phenyl, benzyl reduces the turnover number from 350 to 5 s−1,30 indicating that the increased basicity and concomitant reduced proton donor ability of the amine lowers the activity. A systematic study of the overpotential and turnover number with R = phenyl and R′ B
dx.doi.org/10.1021/jp411710b | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
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electrode and counter electrode and a Ag wire placed in a AgNO3 (0.01 M in MeCN)/NBu4PF6 (0.2 M in MeCN) solution as a pseudoreference electrode. The system was systematically calibrated against ferrocene after each experiment, and all the potentials are therefore given versus the Fc/ Fc+ redox potential. For the bulk electrolysis experiments, a glassy carbon stick electrode (d = 4.1 mm, l = 25 mm, A ∼ 3.4 cm2) was used and the solutions were stirred during the electrolysis experiment to achieve better homogeneity. Bulk electrolysis was performed at −25 °C on solutions of (1)2+ (1 mmol L−1) in acetonitrile (5 mL) at the constant potential of −0.9 or −1.2 V versus Fc/Fc+ to give rise to the one- or twoelectron-reduced species, respectively. Quantum Chemistry. Density functional theory (DFT) calculations of models consisting of (1) with the metal in the divalent, monovalent, or neutral oxidation state and with varying protonation state of the amines and nickel were performed with the ORCA program package.38 Geometry optimization was carried out with the BP86 functional39 and a polarized triple-ζ basis set (Def2-TZVP).40 After geometry optimization, the B3LYP functional was employed to calculate g values41 and hyperfine coupling constants42 within a spinunrestricted Kohn−Sham formalism. Scalar relativistic corrections are included within the zero order regular approach (ZORA).43 NMR chemical shifts were calculated with individual gauge for localized orbitals (IGLO)44 and the Pipek−Mezey orbital localization scheme.45 The Cartesian coordinates of the geometry optimized structures are provided in the Supporting Information. Solvent effects were included within the conductor-like screening model (COSMO) model.46 Preparation of the NiI, Ni0, and NiII Hydride States. (1)+. One equivalent of 5% Na/Hg amalgam (52.0 mg, 0.113 mmol) was added to a solution of (1)2+(BF4)2 (0.20 g, 0.113 mmol) in 6 mL of CH3CN under Ar atmosphere. The mixture was vigorously stirred overnight at room temperature until a white precipitate and liquid mercury were formed. The resulting deep brown solution was filtered through degassed Celite to remove the amalgam and NaBF4. The Celite was washed with 4 mL of CH3CN and the combined CH3CN filtrates were concentrated to dryness. The residue was washed with Et2O (2 × 2 mL) to yield (1)+(BF4) as a brown powder. Yield: 0.17 g (91%). ESI-MS m/z: 793.3 [M + Na]+. UV−vis− NIR (CH3CN, T = 298 K) [λmax/nm (ε/M−1 cm−1)]: 407 (2487), 903 (180). (1). This complex was synthesized in an identical manner to (1)+ with 3 equivalents of 5% Na/Hg amalgam to yield (1) as a brown-yellow powder. Yield: 0.166 g (92%). ESI-MS m/z: 793.4 [M + Na]+. UV−vis−NIR (CH3CN, T = 298 K) [λmax/ nm (ε/M−1 cm−1)]: 382 (8791). 31 1 P{ H} NMR (CD3CN): δ 5.63 (CH2PPhCH2); 26.7 (P(O)(OEt)2).1H NMR (CD3CN): δ 7.77 (m, 8H, Ph); 7.21 (t, J = 7 Hz, 4H, Ph); 7.13−7.08 (m, 16H, Ph); 6.91 (d, J = 9 Hz, 8H, Ph); 3.96 (d of q, J = 8, 4 Hz, 16H, OCH2CH3); 3.90 (m, 8H, PCH2N); 3.50 (d, J = 13 Hz, 8 H, PCH2N), 3.01 (d, J = 21 Hz, 8H, PhCH2P); 1.19 (t, J = 7 Hz, 24H, OCH2CH3). (H1)+. A solution of p-cyanoanilinium tetrafluoroborate (520 μL, 0.102 mmol) in CH3CN was added to a stirred solution of (1) (200 mg, 0.127 mmol) in CH3CN (5 mL) at room temperature. After it was stirred for a few minutes, the mixture was concentrated to dryness. The residue was washed with Et2O (2 × 2 mL) to afford (H1)·(BF4) as a brown powder. 31 1 P{ H} NMR (CD3CN): δ 14.84 (CH2PPhCH2); 26.3 (P(O)(OEt)2). 1H NMR (CD3CN): 7.54−6.65 (m, 36H,
which up to now have not been performed. We have chosen the molecule with R = phenyl and R′ = C4H4CH2P(O)(OEt)2 as a representative molecule since it was recently shown that this molecule features a high turnover number (1850 s−1) and an overpotential of 370 mV when used with protonated dimethylformamide triflate [(DMF)H+]OTf− as a proton source.26 Analysis of the EPR and ENDOR data extended with theoretical calculations allows us to obtain a very detailed picture of the electronic and geometric structure of the catalytically active NiI intermediate in this fascinating class of molecules.
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MATERIALS AND METHODS Materials. All reactions were carried out under an inert atmosphere of argon using standard Schlenk techniques or in a dry argon glovebox (MBraun LabMaster130). The solvents used for chemical reactions were purified by the solvent purification system MBraun MB SPS-800 Auto. Deoxygenated CD3CN used for NMR spectroscopy was purchased from Euriso-top and stored in the glovebox. The ligand PPh2NC6H4CH2P(O)(OEt)2 and the corresponding [NiII(PPh2N C6H4CH2P(O)(OEt)22) 2](BF4) 2 complex, hereafter called (1)2+(BF4)2, were synthesized according to published procedures.26 The supporting electrolyte NBu4BF4 used for electrochemistry was purchased from Sigma-Aldrich and dried overnight at 100 °C under vacuum before use. Anhydrous acetonitrile (>99.8%) used for electrochemistry was purchased from Sigma-Aldrich and stored in the glovebox. Spectroscopy. Fourier transform infrared (FTIR) spectra were recorded at room temperature on a Perkin-Elmer 2000 NIR FT-Raman spectrometer. Measurements in the solid state were carried out in KBr pellets (3 mg sample in 300 mg of KBr). NMR spectra were recorded at room temperature using a Bruker DRX 400 spectrometer operating at 400.13 MHz for 1H and 161.96 MHz for 31P. Solvent peaks are used as internal references for 1H and 13C chemical shifts (listed in parts per million (ppm)). NMR samples were prepared in the glovebox. Absorption spectra were obtained using a diode-array UV−vis spectrometer (HP 8453). Absorption spectra of samples prepared in an anaerobic chamber were measured in 1 cm path length cuvettes sealed with a silicon stopper to retain anaerobic conditions, and spectra were measured quickly (
