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

A new class of [CCC] X3-donor pincer ligand for transition metals has been constructed via cyclometalation of a 2,6-di-p-tolylphenyl ([ArTol2]) derivative. Specifically, addition of PMe3 to [ArTol2]­TaMe3Cl induces elimination of methane and formation of the pincer complex, [κ3-ArTol′2]­Ta­(PMe3)2MeCl (Tol′ = C6H3Me), which may also be obtained by treatment of Ta­(PMe3)2Me3Cl2 with [ArTol2]­Li. Solutions of [κ3-ArTol′2]­Ta­(PMe3)2MeCl undergo ligand redistribution with the formation of [κ3-ArTol′2]­Ta­(PMe3)2Me2 and [κ3-ArTol′2]­Ta­(PMe3)2Cl2, which may also be synthesized by the reactions of [κ3-ArTol′2]­Ta­(PMe3)2MeCl with MeMgBr and ZnCl2, respectively. Reduction of [κ3-ArTol′2]­Ta­(PMe3)2Cl2 with KC8 in benzene gives the benzene complex [κ3-ArTol′2]­Ta­(PMe3)2(η6-C6H6) that is better described as a 1,4-cyclohexadienediyl derivative. Deuterium labeling employing Ta­(PMe3)2(CD3)3Cl2 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.