Half-metallocene 1-aza-1,3-butadiene complexes of tantalum:auxiliary ligand effectson controlling coordination modes of 1-aza-1,3-butadiene ligand

摘要

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 substituentson the aryl group of the AD ligand differentiated two coordination modes:one methyl substituent,TaCl_2Cp*(eta~4-supine-o-Tol-AD)(1) (Cp*=pentamethylcyclopentadienyl;o-Tol-AD=1(2-methylphenyl)-4-phenyl-1-aza-1,3-butaidene),favored an eta~4-supine-coordination mode,while two methyl substituents,TaCl_2Cp*(eta~2-C,N-Xyl-AD)(5)(Xyl-AD=2,6-dimethylphenyl-4-phenyl-1-aza-1,3-butadiene),adopted an eta~2-C,N-imine coordination fashion.Furthermore,the peferential 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 TaR_2Cp*(o-Tol-AD)(6:R=Me;8R=CH_2Ph) in the dimethyl complex 6.The equilibrium between eta~4-coordination mode and eta~2-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 predominatly coordinated in an eta~2-C,N-imine fashin to the tantalum center of 8.In contrast,the monobenzylation of 1 did not change the eta~4-supine coordination mode,giving a monobenzyl complex Cp*TaCl(CH_2Ph)(eta~4-supine-o-Tol-AD)(9).The dibenzyl complex 8 gradually turned to a benzylidene complex Ta(=CHPh)Cp* (eta~4-supine-o-Tol-AD) (10) on heating at 70degC with the changeof the coordination mode from eta~2 to eta~4.Introduction of a 1,3-butadiene ligand to the metal center gave a mixed-ligand complex TaCp*(eta~2-C,N-fashion.