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 elec-tro/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,. 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 pro-moting electrocatalytic HER. Mechanistic investigations by DFT and multi-reference variational 2-electron reduced density matrix (v2RDM)-CASSCF implicate a sulfur-based hydrogen evolution pathway in which the bridging 2-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 2-sulfides. Reduction and protonation of two adjacent Mo(2-S) moieties leads to H2 elimination with a low 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 2-SH species via H2 splitting leads to spin density being shared across the MoIII centers. These results provide an atomistic perspec-tive into the role of Cu in modulating the 2-S pKa and facilitating HER via a formal MoVI/IV cycle with significant elec-tron delocalization between Mo, Cu, and 2-S.