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 [<i>CCC</i>] X₃-donor pincer ligand for transition metals has been constructed <i>via</i> cyclometalation of a 2,6-di-<i>p</i>-tolylphenyl ([Ar^Tol₂]) derivative. Specifically, addition of PMe₃ to [Ar^Tol₂]­TaMe₃Cl induces elimination of methane and formation of the pincer complex, [κ³-Ar^Tol′₂]­Ta­(PMe₃)₂MeCl (Tol′ = C₆H₃Me), which may also be obtained by treatment of Ta­(PMe₃)₂Me₃Cl₂ with [Ar^Tol₂]­Li. Solutions of [κ³-Ar^Tol′₂]­Ta­(PMe₃)₂MeCl undergo ligand redistribution with the formation of [κ³-Ar^Tol′₂]­Ta­(PMe₃)₂Me₂ and [κ³-Ar^Tol′₂]­Ta­(PMe₃)₂Cl₂, which may also be synthesized by the reactions of [κ³-Ar^Tol′₂]­Ta­(PMe₃)₂MeCl with MeMgBr and ZnCl₂, respectively. Reduction of [κ³-Ar^Tol′₂]­Ta­(PMe₃)₂Cl₂ with KC₈ in benzene gives the benzene complex [κ³-Ar^Tol′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