Molecular Models of Layered Ternary Cu₂MoS₄ Reveal Sulfur-Based H₂ Evolution Pathway

Highly crystalline ternary copper-molybdenum sulfide (Cu₂MoS₄) has been demonstrated to promote the electro/photocatalytic hydrogen evolution reaction (HER). Theoretical calculations have suggested that Cu₂MoS₄ outperforms molybdenum disulfide (MoS₂) because of its more moderate free energy of hydrogen adsorption, ΔG_H*. However, the elucidation of intermediates involved in this process remains challenging, rendering the mechanistic details and specific role of Cu in improving activity uncertain. Herein, we describe the isolation and characterization of [NEt₄][Tp*MoS₃(CuSPh)₂] (Tp* = tris(3,5-dimethylpyrazolyl)borate), a molecular model of Cu₂MoS₄ capable of promoting electrocatalytic HER. Mechanistic investigations by DFT and multireference variational 2-electron reduced density matrix (v2RDM)-CASSCF implicate a sulfur-based hydrogen evolution pathway in which the bridging μ₂-sulfides are rendered more basic by one-electron reduction, which populates a Mo-4d/S-3p based orbital with significant electron delocalization on the two μ₂-sulfides. Reduction and protonation of two adjacent Mo(μ₂-S) moieties leads to H₂ elimination with an accessible energy barrier of +20.2 kcal/mol. This pathway contrasts recent studies of stilbene hydrogenation by [Mo₃S₄Cl₃(ImNH₂)₃]⁺, in which formation of the corresponding one-electron reduced μ₂-SH species via H₂ splitting leads to spin density being shared across the Mo^III centers. These results provide an atomistic perspective into the role of Cu in modulating the μ₂-S pK_a and facilitating HER via a formal Mo^VI/IV cycle with significant electron delocalization between Mo, Cu, and μ₂-S.