Agostic Hydridoborate Complexes as Masked Coinage-Metal Hydrides: Hydride Migration Between Boron and Gold Unlocks CO2 Reactivity.

Hydride addition to diphosphine-borane (DPB) coinage-metal complexes affords an unprecedented series of Cu(I), Ag(I), and Au(I) hydridoborate complexes 2-M featuring bridging B-H···M interactions. Combined multinuclear NMR, IR, single-crystal x-ray diffraction, and DFT studies reveal the presence of agostic-type σ(B-H)→M interactions across the series, with the strongest interaction observed for copper. These hydridoborate species are thermodynamically favored over the corresponding metal hydrides. Markedly distinct reactivity is observed depending on the coinage metal. Whereas, the Cu and Ag congeners proved to be unreactive toward CO2, the Au complex 2-Au cleanly and reversibly fixes CO2 to form the isolable Au(I) formate complex 3-Au. In addition to quantitative and reversible CO2 uptake and release, the Au hydridoborate complex 2-Au is an efficient and robust catalyst for the hydroboration of CO2 to HCO2Bpin and the dehydrogenation of formic acid, both with complete selectivity. Experimental and computational studies support a cooperative mechanism in which the B─H─Au motif acts as a masked gold hydride, thanks to hydride transfer.