Synthesis and Catalytic Reactivity of Cobalt Pincer Nitrosyl Hydride Complexes

The synthesis, characterization, and catalytic activity of low-spin {CoNO}8 pincer complexes of the type [Co­(PCP)­(NO)­(H)] are described. These compounds are obtained either by reacting [Co­(PCP)­(κ2-BH4)] with NO and Et3N or, alternatively, by reacting [Co­(PCP)­(NO)]+ with boranes, such as NH3·BH3 in solution. The five-coordinate, diamagnetic Co­(III) complex [Co­(PCPNMe-iPr)­(NO)­(H)] was found to be the active species in the hydroboration of alkenes with anti-Markovnikov selectivity. A range of aromatic and aliphatic alkenes were efficiently converted with pinacolborane (HBpin) under mild conditions in good to excellent yield. Mechanistic insight into the catalytic reaction is provided by means of isotope labeling, NMR spectroscopy, and APCI/ESI-MS as well as DFT calculations.