Two-Legged Cp*Co(III)(N,O) Complex as Stable Precatalyst for Selective Catalytic Deoxygenation of Secondary Amides

Abstract

In this work, we report the synthesis of stable half-sandwich two-legged Cp*Co(III)(N,O) complexes using donor-flexible pyridylidene-amine ligands, a deviation from the typical three-legged configuration of Cp*Co(III) complexes. In comparison to our previous report, wherein Cp*Co(III)-hydride species was inaccessible from the Cp*Co(III)(N,O)-alkoxy species ( ACS Catal. 2021, 11, 11906–11920), this two-legged complex C-1 created a vacant coordination site at the cobalt(III) center, which was crucial for the formation of Cp*Co(III)(N,O)-hydride species (C-1-H) when treated with phenylsilane as a hydride donor. C-1-H exhibits versatile reactivity, enabling the selective deoxygenation of a variety of secondary amides, including aryl amides, acetanilide, and cyclic substrates. Moreover, the system demonstrates impressive chemoselectivity, as evidenced by the selective reduction of nitroarenes, N-phenyl urea, and the targeted deoxygenation of pharmaceutical derivatives such as paracetamol, indomethacin, and oxaprozin derivatives. The study also provides insight into the reaction mechanism, with DFT calculations confirming the role of in situ-generated cobalt hydride species as an active catalyst. This cobalt hydride is marginally more stable at triplet electronic state compared to that at the singlet state, which accounts for the experimental observation of the short-lived nature of the diamagnetic hydride species. Control experiments and kinetic profile agreed with the proposed mechanism.