In(III) Metal–Organic Framework Incorporated with Enzyme-Mimicking Nickel Bis(dithiolene) Ligand for Highly Selective CO2 Electroreduction

Inspired by the exciting physical/chemical properties in metal-organic frameworks (MOFs) of the redox-active tetrathiafulvalene (TTF) ligands, nickel bis(dithiolene-dibenzoic acid), [Ni(C2S2(C6H4COOH)2)2], has been designed and developed as an inorganic analogue of the corresponding TTF-type donors (such as tetrathiafulvalene-tetrabenzoate, TTFTB), where a metal site (Ni) replaces the central C═C bond. In this work, [Ni(C2S2(C6H4COOH)2)2] and In3+ have been successfully assembled into a three-dimensional MOF, (Me2NH2+){InIII-[Ni(C2S2(C6H4COO)2)2]}·3DMF·1.5H2O (1, DMF = N,N-dimethylformamide), with satisfying chemical and thermal stabilities. With the combination of reversible redox activity and unsaturated metal sites originated from [Ni(C2S2(C6H4COOH)2)2], 1 showed a significantly enhanced performance in electrocatalytic CO2 reduction compared with the isomorphic MOF, (Me2NH2+)[InIII-(TTFTB)]·0.7C2H5OH·DMF (2, with TTFTB ligand). More importantly, by mimicking the active [NiS4] sites of formate dehydrogenase and CO-dehydrogenase, a prominently higher conversion rate and Faradaic efficiency (FE), with FEHCOO- increasing from 54.7% to 89.6% (at -1.3 V vs RHE, jHCOO- = 36.0 mA cm-2), were achieved in 1. Mechanistic investigations further confirm that [NiS4] can serve as a CO2 binding site and efficient catalytic center. This unprecedented effect of redox-active nickel dithiolene-based MOF catalysts on the performance of electroreduction of CO2 provides an important strategy for designing stable and efficient crystalline enzyme-mimicking catalysts for the conversion of CO2 into high-value chemical stocks.