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 μ1,3-formate-bridged dicopper(II) complex (1H) to a hexacopper(I) hydride cluster (2H) upon reduction. Isotopic labeling studies revealed that both the H- and CO2 originate from the bound μ1,3-formate in 1H, which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1H to 2H is established. The structure of 2H features two Cu3 triangles, each capped by a hydride ligand. Typical hydride reactivity of 2H 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 2H and 3, reflecting the rich structural landscape of the bis(pyrazolate)-bridged hexacopper(I) core (four isomers each for 2H and 3) predicted by DFT calculations.