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 [(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.