Catalyst Deactivation Reactions: The Role of Tertiary Amines Revisited

Decamethylzirconocene cation [Cp*2ZrMe](+) (2) decomposes in bromobenzene-d(5) solution to generate sigma-aryl species [Cp*Zr-2(2-C6H4Br-kappa Br,C)][B(C6F5)(4)] (3). This a-bond metathesis reaction is catalyzed by tertiary amines via a two-step mechanism, in which the amine acts as a proton relay. In benzene-d(6) compound 2 decomposes via C H bond activation of one of the Cp* ligands to generate tucked-in compound [Cp*{eta(5):eta(1)-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*Zr-2{C(Me)NEt-kappa C,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 beta-amide elimination. Attempted synthesis of this species by treatment of Cp*Zr-2(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)(4)NEt3}](+) (7). The results show that amine coproducts, often generated in the synthesis of cationic transition-metal complexes, are not necessarily innocent.