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] 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.一类新型[CCC]型X₃-供体钳形配体（pincer ligand），通过对2,6-二(对甲苯基)苯基([ArTol₂])衍生物进行环金属化（cyclometalation）构建得到。具体而言，将三甲基膦(PMe₃)加入[ArTol₂]TaMe₃Cl中，可诱导甲烷消除反应，生成钳形配合物[κ³-ArTol′₂]Ta(PMe₃)₂MeCl（其中Tol′=C₆H₃Me）；该配合物也可通过Ta(PMe₃)₂Me₃Cl₂与[ArTol₂]Li反应制得。 [κ³-ArTol′₂]Ta(PMe₃)₂MeCl的溶液会发生配体重分布反应，生成[κ³-ArTol′₂]Ta(PMe₃)₂Me₂与[κ³-ArTol′₂]Ta(PMe₃)₂Cl₂；这两种配合物也可分别通过[κ³-ArTol′₂]Ta(PMe₃)₂MeCl与甲基溴化镁(MeMgBr)、氯化锌(ZnCl₂)反应合成。 用八石墨化钾(KC₈)在苯中还原[κ³-ArTol′₂]Ta(PMe₃)₂Cl₂，可得到苯配位配合物[κ³-ArTol′₂]Ta(PMe₃)₂(η⁶-C₆H₆)，该配合物更适合被描述为1,4-环己二烯二基衍生物。 采用Ta(PMe₃)₂(CD₃)₃Cl₂进行的氘标记实验表明，该钳形配体是通过两次Ar–H/Ta–Me σ键复分解（sigma-bond metathesis）反应生成的，而非经由钽甲基配体夺取α-氢的机制形成。