Immobilized Cobalt Bis(benzenedithiolate) Complexes: Exceptionally Active Heterogeneous Electrocatalysts for Dihydrogen Production from Mildly Acidic Aqueous Solutions

Abstract

A series of cobalt bis(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt) ring substitutions (SCX) were prepared and adsorbed on inexpensive electrodes composed of (a) reduced graphene oxide (RGO) electrodeposited on fluorine-doped tin oxide (FTO) and (b) highly ordered pyrolytic graphite (HOPG). The catalyst-adsorbed electrodes are characterized by X-ray photoelectron spectroscopy. Catalyst loading across the ligand series improved notably with increasing halide substitution [from 2.7 × 10 mol cm for TBA[Co(SCH)] (1) to 6.22 × 10 mol cm for TBA[Co(SCCl)] (3)] and increasing ring size of the benzenedithiolate ligand [up to 3.10 × 10 mol cm for TBA[Co(SCH)] (6)]. Electrocatalytic analysis of the complexes immobilized on HOPG elicits a reductive current response indicative of dihydrogen generation in the presence of mildly acidic aqueous solutions (pH 2-4) of trifluoroacetic acid, with overpotentials of around 0.5 V versus SHE (measured vs platinum). Rate constant (k) estimates resulting from cyclic voltammetry analysis range from 24 to 230 s with the maximum k for TBA[Co(SCHCl)] (2) at an overpotential of 0.59 V versus platinum. Controlled-potential electrolysis studies performed in 0.5 M HSO at -0.5 V versus SHE show impressive initial rate constants of over 500 s under bulk electrolysis conditions; however, steady catalyst deactivation over an 8 h period is observed, with turnover numbers reaching 9.1 × 10. Electrolysis studies reveal that halide substitution is a central factor in improving the turnover stability, whereas the ring size is less of a factor in optimizing the long-term stability of the heterogeneous catalyst manifolds. Catalyst deactivation is likely caused by catalyst desorption from the electrode surfaces.

Synthesis of Mono- and Diiron Dithiolene Complexes as Hydrogenase Models by Dithiolene Transfer Reactions, Including the Crystal Structure of [{Ni(S2C2Ph2)}6]

“…[32][33][34][35][36][37][38][39] It should also be mentioned that bdt complexes of other metals, notably cobalt, nickel and molybdenum, have also shown activity for proton reduction. [40][41][42][43][44][45] Over fifty years after the initial breakthroughs of the 1960s, the efficient synthesis of transition metal dithiolene complexes continues to be an important objective in inorganic chemistry. 46 We were therefore interested in exploring iron complexes similar to those in Figure 2 but containing related dithiolene ligands to take advantage of the unique properties conferred by the non-innocence of these ligands, in particular the accessibility of a range of redox states and the possibility of incorporating additional redox-active substituents.…”

“…This is particularly the case for five-coordinate complexes of the type [Fe­(bdt)­(CO)­(L 2 )], where L 2 is a diphosphine ligand (Figure ). − It should also be mentioned that bdt complexes of other metals, notably cobalt, nickel, and molybdenum, have also shown activity for proton reduction. − …”

Electrocatalytic Hydrogen Production by a Nickel Complex Containing a Tetradentate Phosphine Ligand

“…An equally important metric is the catalyst lifetime, particularly under strongly acidic conditions (pH 0–1) that are technologically relevant for use in polymer electrolyte membrane (PEM) electrolyzers. Molecular catalysts are notable for decomposing under highly acidic conditions and, in some cases, decompose into catalytically active heterogeneous films. − Examples of catalysts that can operate under highly acidic conditions include Ni­(diphosphine) 2 − and Co­(azamacrocycle) complexes, as well as Co­(dithiolate) 2 complexes that are adsorbed to a carbon electrode , or incorporated into coordination polymers. − …”

“…Molecular catalysts are notable for decomposing under highly acidic conditions and, in some cases, decompose into catalytically active heterogeneous films. 16−20 Examples of catalysts that can operate under highly acidic conditions include Ni(diphosphine) 2 21−23 and Co(azamacrocycle) 6 complexes, as well as Co(dithiolate) 2 complexes that are adsorbed to a carbon electrode 24,25 or incorporated into coordination polymers. 26−28 Tetradentate "P 4 N 2 " ligands have previously been used to design catalysts for H 2 production, 29,30 CO 2 hydrogenation, 31 and N 2 silylation 32 (Figure 1).…”

Design of a One-Dimensional Stacked Spin Peierls System with Room-Temperature Switching from Quantum Mechanical Predictions

“…Bis-1, 2-dithiolene transition metal complexes show novel properties that are promisingly applied in the areas of conducting and magnetic materials, dyes, non-linear optics, and catalysis. [1][2][3][4][5][6] These applications arise from a combination of functional properties, specific geometries, and intermolecular interactions. [7][8][9] The core is composed of a central metal, four sulfurs and the C=C units in the metal-bis-1,2-dithiolenes anion (denoted as [M(dithiolato) 2 ] − and M = Ni, Pd or Pt ion), and this leads to delocalization of the frontier orbitals over much or all of the molecule so that the negative charge is distributed over the anion skeleton.…”