Co(0)-Arene Complexes Derived from a Co4 Cluster: Synthesis, Characterization, and Hydrogenation Catalysis

Abstract

Low-valent cobalt complexes have emerged as effective earth-abundant pre-catalysts for olefin hydrogenation reactions. Here, we report a series of heteroleptic Co(0) arene cyclic (alkyl)(amino)carbene (CAAC) complexes that catalytically hydrogenate olefins. Whereas direct reduction of CAACCo(II) precursors affords precedented Co(0) dimers, formation and subsequent reduction of a tetracobalt cluster generates mononuclear Co(0) arenes. These complexes are characterized by X-ray crystallography, 1H NMR and EPR spectroscopy, cyclic voltammetry, and density functional theory calculations. CAACCo(C6H6) is an activator-free pre-catalyst for olefin hydrogenation, achieving high turnovers for styrenic substrates. Mechanistic studies support H2 activation prior to olefin binding, preceding a Co(II)/Co(0) catalytic cycle.