Cyclic(Alkyl)(Amino)Carbene (CAAC)-Supported Zn Alkyls: Synthesis, Structure and Reactivity in Hydrosilylation...

The reactivity of Zn^II dialkyl species ZnMe₂ with a cyclic(alkyl)(amino)carbene, 1-[2,6-bis(1-methylethyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene (CAAC, 1), was studied and extended to the preparation of robust CAAC-supported Zn^II Lewis acidic organocations. CAAC adduct of ZnMe₂ (2), formed from a 1:1 mixture of 1 and ZnMe₂ , is unstable at room temperature and readily undergoes a CAAC carbene insertion into the Zn-Me bond to produce the ZnX₂ -type species (CAAC-Me)ZnMe (3), a reactivity further supported by DFT calculations. Despite its limited stability, adduct 2 was cleanly ionized to robust two-coordinate (CAAC)ZnMe⁺ cation (5⁺ ) and derived into (CAAC)ZnC₆ F₅ ⁺ (7⁺ ), both isolated as B(C₆ F₅ )₄ ^- salts, showing the ability of CAAC for the stabilization of reactive [ZnMe]⁺ and [ZnC₆ F₅ ]⁺ moieties. Due to the lability of the CAAC-ZnMe₂ bond, the formation of bis(CAAC) adduct (CAAC)₂ ZnMe⁺ cation (6⁺ ) was also observed and the corresponding salt [6][B(C₆ F₅ )₄ ] was structurally characterized. As estimated from experimental and calculations data, cations 5⁺ and 7⁺ are highly Lewis acidic species and the stronger Lewis acid 7⁺ effectively mediates alkene, alkyne and CO₂ hydrosilylation catalysis. All supporting data hints at Lewis acid type activation-functionalization processes. Despite a lower energy LUMO in 5⁺ and 7⁺ , their observed reactivity is comparable to those of N-heterocyclic carbene (NHC) analogues, in line with charge-controlled reactions for carbene-stabilized Zn^II organocations.