Catalyst Deactivation Reactions: The Role of Tertiary Amines Revisited

Abstract

Decamethylzirconocene cation [Cp*2ZrMe]+ (2) decomposes in bromobenzene-d5 solution to generate σ-aryl species [Cp*2Zr(2-C6H4Br-κBr,C)][B(C6F5)4] (3). This σ-bond metathesis reaction is catalyzed by tertiary amines via a two-step mechanism, in which the amine acts as a proton relay. In benzene-d6 compound 2 decomposes via C−H bond activation of one of the Cp* ligands to generate tucked-in compound [Cp*{η5:η1-C5Me4(CH2)}Zr]+ (4). In the presence of Et3N, no formation of tucked-in compound 4 is observed, but instead an overall double C−H bond activation and C−N bond cleavage of the tertiary amine is observed, resulting in [Cp*2Zr{C(Me)NEt-κC,N}]+ (6). A mechanism is proposed that nominates [Cp*2ZrNEt2]+ as an intermediate, the result of a C−H bond activation of Et3N, followed by β-amide elimination. Attempted synthesis of this species by treatment of Cp*2Zr(NEt2)Me with [Ph3C][B(C6F5)4] results, again, in formation of compound [6]+. The presence of Et3N also has an effect on the stability of THF adduct [Cp*2ZrMe(THF)]+ as the amine performs a nucleophilic THF ring-opening to generate [Cp*2ZrMe{O(CH2)4NEt3}]+ (7). The results show that amine coproducts, often generated in the synthesis of cationic transition-metal complexes, are not necessarily innocent.