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.