A New Class of Transition Metal Pincer Ligand: Tantalum Complexes that Feature a [CCC] X3-Donor Array Derived from a Terphenyl Ligand

A new class of [CCC] X-3-donor pincer ligand for transition metals has been constructed via cyclometalation of a 2,6-di-p-tolylphenyl ([Ar-Tol2]) derivative. Specifically, addition of PMe3 to [Ar-Tol2]TaMe3Cl induces elimination of methane and formation of the pincer complex, [kappa(3)-Ar-Tol2]-Ta(PMe3)(2)MeCl (Tol' = C6H3Me), which may also be obtained by treatment of Ta(PMe3)(2)Me3Cl2 with [Ar-Tol2]Li. Solutions of [kappa(3)-Ar-Tol2]Ta(PMe3)(2)MeCl undergo ligand redistribution with the formation of [kappa(3)-Ar-Tol2]Ta(PMe3)(2)Me-2 and [kappa(3)-Ar-Tol2]Ta(PMe3)(2)Cl-2, which may also be synthesized by the reactions of [kappa(3)-Ar-Tol2]Ta(PMe3)(2)MeCl with MeMgBr and ZnCl2, respectively. Reduction of [kappa(3)-Ar-Tol2]Ta(PMe3)(2)Cl-2 with KC8 in benzene gives the benzene complex [kappa(3)-Ar-Tol2]-Ta(PMe3)(2)(eta(6)-C6H6) that is better described as a 1,4-cyclohexadienediyl derivative. Deuterium labeling employing Ta(PMe3)(2)-(CD3)(3)Cl-2 demonstrates that the pincer ligand is created by a pair of Ar-H/Ta-Me sigma-bond metathesis transformations, rather than by a mechanism that involves alpha-H abstraction by a tantalum methyl ligand.