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

Tri-organyl and tricoordinate N-heterocyclic carbene (NHC) Zn-NHC alkyl cations [(nNHC) 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)ZnMe ] with [Ph C][B(C F ) ] in the presence of one equivalent of free NHC. Whereas cation [(nIMes) Zn-Me] (3 ) is stable, its sterically congested analogue [(nIDipp) 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 CO through insertion into the Zn-Me bond yielding the corresponding Zn(κ -OAc) cation 6 . Both cations 5 and 6 were successfully used in CO hydrosilylation catalysis for silylformate formation.