N‐Heterocyclic Carbene Based Tri‐organyl‐Zn–Alkyl Cations: Synthesis, Structures, and Use in CO 2 Functionalization

Tri‐organyl and tricoordinate N‐heterocyclic carbene (NHC) Zn–NHC alkyl cations [( n NHC) 2 Zn‐Me] + ( n NHC=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 [( n NHC)ZnMe 2 ] with [Ph 3 C][B(C 6 F 5 ) 4 ] in the presence of one equivalent of free NHC. Whereas cation [( n IMes) 2 Zn‐Me] + ( 3 + ) is stable, its sterically congested analogue [( n IDipp) 2 Zn‐Me] + ( 4 + ) readily undergoes an n NHC‐to‐ a NHC isomerization in the presence of THF or IDipp to afford the more thermodynamically stable [( a IDipp)( n IDipp)Zn‐Me] + ( a IDipp=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 n NHC‐to‐ a NHC 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 CO 2 through insertion into the Zn–Me bond yielding the corresponding Zn( κ 2 ‐OAc) + cation 6 + . Both cations 5 + and 6 + were successfully used in CO 2 hydrosilylation catalysis for silylformate formation.