Half-Metallocene 1-Aza-1,3-butadiene Complexes of Tantalum: Auxiliary Ligand Effects on Controlling Coordination Modes of 1-Aza-1,3-butadiene Ligand

Abstract

Abstract We prepared some half-metallocene complexes of tantalum bearing ortho-substituted aryl derivatives of 1-aza-1,3-butadiene ligand. The coordination mode of the 1-aza-1,3-diene (abbr. AD) ligand highly depended on the substituent(s) on the aryl group of the AD ligand and also on the metal center. The number of methyl substituents on the aryl group of the AD ligand differentiated two coordination modes: one methyl substituent, TaCl2Cp*(η4-supine-o-Tol-AD) (1) (Cp* = pentamethylcyclopentadienyl; o-Tol-AD = 1-(2-methylphenyl)-4-phenyl-1-aza-1,3-butadiene), favored an η4-supine-coordination mode, while two methyl substituents, TaCl2Cp*(η2-C, N-Xyl-AD) (5) (Xy1-AD = 2,6-dimethylphenyl-4-phenyl-1-aza-1,3-butadiene), adopted an η2-C, N-imine coordination fashion. Furthermore, the preferential coordination mode of the AD ligand was delicately affected by the substituent(s) on the tantalum center. Dialkylation of 1 led to the formation of TaR2Cp*(o-Tol-AD) (6: R = Me; 8: R = CH2Ph) in the dimethyl complex 6. The equilibrium between η4-coordination mode and η2-C, N-coordination one was observed and the latter mode was thermally more feasible in solution and as solid state, and the o-Tol-AD ligand predominantly coordinated in an η2-C, N-imine fashin to the tantalum center of 8. In contrast, the monobenzylation of 1 did not change the η4-supine coordination mode, giving a monobenzyl complex Cp*TaCl(CH2Ph)(η4-supine-o-Tol-AD) (9). The dibenzyl complex 8 gradually turned to a benzylidene complex Ta(=CHPh)Cp*(η4-supine-o-Tol-AD) (10) on heating at 70 °C with the change of the coordination mode from η2 to η4. Introduction of a 1,3-butadiene ligand to the metal center gave a mixed-ligand complex TaCp*(η2-C,N-o-Tol-AD)(η4-s-cis-1,3-butadiene) (12), in which the metal center adopted an η2-C,N-fashion.