Exploration of N-heterocyclic carbene complexes of iron and cobalt in catalysis and non-traditional organometallic chemistry

Earth-abundant transition metal catalysts are not as well understood or developed as those involving precious metal systems. In order to develop a method of evaluating transition metal catalyzed reactions employing earth-abundant elements, several catalyst systems composed of N-heterocyclic carbene (NHC) supported Fe(II), Fe(III), and Co(II) compounds have been synthesized and characterized. The primary N-heterocyclic carbenes used were 1,3-dimesitylimidazole-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr). Selection of the appropriate NHC ligand with respect to steric bulk afforded both monomeric and chloride bridged dimeric halide species. Derivatives of these ligands, including a chlorinated (IPrCl, IMesCl) or saturated (SIPr, SIMes) backbone, were also employed. Low-coordinate hydrocarbyls of these Fe(II) and Co(II) compounds were isolated and crystallized including, but not limited to [Fe(IPr)(TMS(CH2))2], [Fe(IPr)Bn2], [Co(IPr)(TMS(CH2))2], and analogues involving carbene ligand variation. The NHC supported metal halide species were catalytically active and efficient in a variety carbon-carbon cross coupling reactions. Isolated metal-hydrocarbyls afforded the opportunity to investigate the mechanisms of carbon-carbon cross-coupling by this series of well-defined earth-abundant transition metal catalysts. Mechanistic studies of the Fe-NHC system were consistent with an FeII/III couple based mechanism. Low oxidation states of cobalt NHC compounds were explored, affording new Co(I) and Co(0) complexes [Co2(IPr)2] and [CoCl(IPr)2].