Hexanuclear Copper(I) Hydride from the Reduction-Induced Decarboxylation of a Dicopper(II) Formate

Copper­(I) hydride complexes represent a promising entry into formic acid dehydrogenation catalysis. Herein we present the spontaneous decarboxylation of a μ_1,3-formate-bridged dicopper­(II) complex (1^H) to a hexacopper­(I) hydride cluster (2^H) upon reduction. Isotopic labeling studies revealed that both the H^– and CO₂ originate from the bound μ_1,3-formate in 1^H, which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1^H to 2^H is established. The structure of 2^H features two Cu₃ triangles, each capped by a hydride ligand. Typical hydride reactivity of 2^H is demonstrated by the addition of phenylacetylene, leading to the replacement of the hydrides by alkynide ligands ⁻CCPh (3) while retaining the hexacopper­(I) core. Temperature-dependent dynamic behavior in solution on the NMR time scale was observed for both 2^H and 3, reflecting the rich structural landscape of the bis­(pyrazolate)-bridged hexacopper­(I) core (four isomers each for 2^H and 3) predicted by DFT calculations.