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

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: Cu(11)H(3)(Tf-dpf)(6)(OAc)(2) (Cu(11)) with three interfacial μ(5)-H and [Cu(12)H(3)(Tf-dpf)(6)(OAc)(2)]·OAc (Cu(12)) with three interstitial μ(6)-H. A solvent-triggered reversible interconversion between Cu(11) and Cu(12) has been observed thanks to the flexibility of Tf-dpf. Cu(11) shows high activity in the reduction of 4-nitrophenol to 4-aminophenol, while Cu(12) displays very low activity. Deuteration experiments prove that the type of hydride is the key in dictating the catalytic activity, for the interfacial μ(5)-H species in Cu(11) are involved in the catalytic cycle whereas the interstitial μ(6)-H species in Cu(12) are not. This work highlights the role of hydrides with regard to catalytic hydrogenation activity.