SYNTHESIS, CATALYTIC ACTIVITY AND ELECTRONIC STRUCTURES OF PYRIDINE BIS-N-HETEROCYCLIC CARBENE IRON AND COBALT HYDRIDE COMPLEXES

Abstract

The catalytic hydrogenation activity of the electron rich bis(arylimidazole-2-ylidene)pyridine iron dinitrogen complex, (iPrCNC)Fe(N¬2)2 and its more sterically open variants have been evaluated. In general, this class of iron catalysts is highly active towards the hydrogenation of hindered, unfunctionalized tri- and tetra-substituted olefins under mild conditions. In continuation to the study on the electronic effect of the bis(arylimidazol-2-ylidene)pyridine chelate on the catalytic hydrogenation activity of reduced iron dinitrogen complexes, the variant bearing the saturated imidazole fragments, (H4-iPrCNC)Fe(N2)2¬ was synthesized and structurally characterized. Under H2 atmosphere in benzene-d6 solution, (H4-iPrCNC)Fe(N2)2 undergoes facile oxidative addition of H2 to form an equilibrium mixture of the non-classical polyhydride species (H4-iPrCNC)Fe(H2)(H)2 and the classical hydride species, (H4-iPrCNC)Fe(N2)(H)2, respectively, both of which are structurally and spectroscopically characterized. Solution studies on the hydride species revealed facile aryl C-H activation chemistry. In full collaboration with Merck, the catalytic activity for hydrogen isotope exchange was investigated. This study ultimately led to the discovery of a novel catalyst system for the application of tritium labeling of pharmaceuticals.

The catalytic hydrogenation activity of the bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, (iPrCNC)CoCH3, was also evaluated. Under mild reaction conditions, (iPrCNC)CoCH3¬ is an effective pre-catalyst for the hydrogenation of sterically hindered, unactivated alkenes, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (iPrCNC)CoH was accomplished by hydrogenation of (iPrCNC)CoCH3¬. A combination of structural, spectroscopic and computational studies was conducted and provided definitive evidence for the presence of ligand centered radicals in these reduced cobalt complexes, unambiguously identifying the redox active nature of the bis(arylimidazol-2-ylidene)pyridine chelate.

The bis(imino)pyridine cobalt catalyzed selective anti-Markovnikov hydrosilylation of olefins has been discovered based on continued mechanistic investigations. A catalytically active cobalt silyl-olefin complex was also discovered during this study which exhibits good air and moisture tolerance allowing brief handling in air, representing the first example of activator-free, bench-stable cobalt hydrosilylation pre-catalyst. Other ligand platforms exhibiting similar electronic and steric environment to bis(imino)pyridine were also demonstrated as potentially viable ligand classes for cobalt catalyzed hydrosilylation chemistry.