Synthesis, structure, and reactivity of terminal cobalt imido complexes

The synthesis and structural characterization of imido ligated cobalt complexes that employ the sterically hindered hydrotris(3- tBu-5-Me-pyrazolyl)borate ligand, i.e., Tp tBu,MeCoNR (R = Me, Et, tBu, Ad) have been accomplished. These terminal imido complexes possess relatively short Co-N bond distances in the range of 1.64-1.67 A, indicating a multiple bond to the metal. Reactivity studies have been undertaken with a variety of substrate ranging from protonation using acids such as HCl and lutidinium-BARF (BARF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to ligand transfer of the imido fragment to carbon monoxide.; Kinetic studies of the thermal decomposition of Tpt Bu,MeCoNAd which undergoes C-H activation of the Tp ligand yielding BptBu,Me(Me-pz-CMe2CH 2N(Ad)H)Co, have been closely monitored by 1H NMR spectroscopy. Determination of the rate of C-H insertion of the imido ligand of Tp tBu,MeCoNAd at a variety of temperatures led to the generation of an Erying Plot which showed curvature. Kinetic isotope measurements were also conducted showing a temperature dependent kinetic isotope effect indicating that breaking of the C-H bond was involved in the rate determining step.