Comparative Reactivity of Zr- and Pd-Alkyl Complexes with Carbon Dioxide

Structure/reactivity trends and DFT studies reveal mechanistic differences and parallels for the carboxylation of Zr and Pd alkyls. CO2 reacts with Cp2ZrMe(C6D5Cl)(+) >10(4) faster than with Cp2ZrMe2, yielding monoacetate products in both cases. These reactions proceed by insertion mechanisms in which Zr center dot center dot center dot O interactions activate the CO2. In contrast, CO2 reacts readily with [(PO-Pr-i)PdMe2]- (PO-iPr = 2-PiPr(2)-4-Me-C6H3SO3) to yield [(PO-iPr)PdMe(OAc)](-) but not with (PO-iPr)PdMe(L) species. Carboxylation of [(PO-iPr)PdMe2]- occurs by direct S(E)2 attack of CO2 at the PdMetrans-to-P group, and the nucleophilicity of the PdMe group controls the reactivity. However, the SE2 process is accelerated by a Li+center dot center dot center dot OCO interaction when Li+ is present.