Structural transformation and catalytic hydrogenation activity of amidinate-protected copper hydride clusters

Abstract

Copper hydrides are important hydrogenation catalysts, but their poor stability hinders the practical applications. Ligand engineering is an effective strategy to tackle this issue. An amidinate ligand, N,N′-Di(5-trifluoromethyl-2-pyridyl)formamidinate (Tf-dpf) with four N-donors has been applied as a protecting agent in the synthesis of stable copper hydride clusters: Cu11H3(Tf-dpf)6(OAc)2 (Cu11) with three interfacial μ5-H and [Cu12H3(Tf-dpf)6(OAc)2]·OAc (Cu12) with three interstitial μ6-H. A solvent-triggered reversible interconversion between Cu11 and Cu12 has been observed thanks to the flexibility of Tf-dpf. Cu11 shows high activity in the reduction of 4-nitrophenol to 4-aminophenol, while Cu12 displays very low activity. Deuteration experiments prove that the type of hydride is the key in dictating the catalytic activity, for the interfacial μ5-H species in Cu11 are involved in the catalytic cycle whereas the interstitial μ6-H species in Cu12 are not. This work highlights the role of hydrides with regard to catalytic hydrogenation activity.