Synthesis and reactivity of zirconium and hafnium complexes incorporating chelating diamido-N-heterocyclic-carbene ligands

Early transition metal complexes employing a diamido N-heterocyclic carbene (NHC) ligand set (denoted [NCN]) render the centrally disposed NHC moiety stable to dissociation. Aminolysis reactions with the mesityl-substituted ligand precursor (Mes[NCN]H2) and M(NMe2)4 (M = Zr, Hf) provide bis(amido)-NHC-metal complexes that can be further converted to chloro and alkyl derivatives. Activation of Mes[NCN]M(CH3)2 with [Ph3C][B(C6F5)4] yields {Mes[NCN]MCH3}{B(C6F5)4}, which is surprisingly inactive for the polymerization of 1-hexene. The zirconium cation did, however, show moderate ability to catalytically polymerize ethylene. The hafnium dialkyls are thermally stable with the exception of the diethyl complex, Mes[NCN]Hf(CH2CH3)2, which undergoes β-hydrogen transfer and subsequent C–H bond activation with an ortho-methyl substituent on the mesityl group. The hafnium dialkyl complexes also insert carbon monoxide and substituted isocyanides to yield η2-acyls and η2-iminoacyls, respectively. In some circumstances, further C–C bond coupling occurs to yield enediolates and eneamidolate metallocycles. The molecular structures of Mes[NCN]Hf(CH2CHMe2)2, Mes[NCN]Hf(η2-(2,6-Me2C6H3NCCH3)(CH3), Mes[NCN]Hf(η2-(2,6-Me2C6H3NCCH3)2, Mes[NCN]Hf(OC(CH3)double bond; length as m-dashC(CH3)NXy), and [Mes[NCN]Hf(OC(iBu)double bond; length as m-dashC(iBu)O)]2 are included.