N-Heterocyclic Carbene Based Tri-organyl-Zn-Alkyl Cations: Synthesis, Structures, and Use in CO₂ Functionalization

Tri-organyl and tricoordinate N-heterocyclic carbene (NHC) Zn-NHC alkyl cations [(nNHC)₂ Zn-Me]⁺ (nNHC=C2-bonded-IMes/-IDipp; 3⁺ and 4⁺ ; IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were first synthesized and structurally characterized by ionization of the corresponding neutral precursors [(nNHC)ZnMe₂ ] with [Ph₃ C][B(C₆ F₅ )₄ ] in the presence of one equivalent of free NHC. Whereas cation [(nIMes)₂ Zn-Me]⁺ (3⁺ ) is stable, its sterically congested analogue [(nIDipp)₂ Zn-Me]⁺ (4⁺ ) readily undergoes an nNHC-to-aNHC isomerization in the presence of THF or IDipp to afford the more thermodynamically stable [(aIDipp)(nIDipp)Zn-Me]⁺ (aIDipp=C4-bonded IDipp, 5⁺ ), reflecting the adaptable-to-sterics coordination chemistry of these cations for improved stability. Cations 3⁺ -5⁺ are the first Zn cations of the type Zn(C)(C')(C'')⁺ (C, C', C''=σ-donor carbyl ligand). Kinetic studies combined with DFT calculations agree with an nNHC-to-aNHC process proceeding through the initial deprotonation of 4⁺ (at a Zn-bonded C4-IDipp moiety) by IDipp. Unlike 3⁺ and 4⁺ , the rearranged cation 5⁺ reacts with CO₂ through insertion into the Zn-Me bond yielding the corresponding Zn(κ² -OAc)⁺ cation 6⁺ . Both cations 5⁺ and 6⁺ were successfully used in CO₂ hydrosilylation catalysis for silylformate formation.