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(Tol(2))]) derivative. Specifically, addition of PMe(3) to Ar(Tol(2))]TaMe(3)Cl induces elimination of methane and formation of the pincer complex, κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl (Tol' = C(6)H(3)Me), which may also be obtained by treatment of Ta(PMe(3))(2)Me(3)Cl(2) with Ar(Tol(2))]Li. Solutions of κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl undergo ligand redistribution with the formation of κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Me(2)and κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Cl(2), which may also be synthesized by the reactions of κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)MeCl with MeMgBr and ZnCl(2), respectively. Reduction of κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)Cl(2) with KC(8) in benzene gives the benzene complex κ(3)-Ar(Tol'(2))]Ta(PMe(3))(2)(η(6)-C(6)H(6)) that is better described as a 1,4-cyclohexadienediyl derivative. Deuterium labeling employing Ta(PMe(3))(2)(CD(3))(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 α-H abstraction by a tantalum methyl ligand.