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.