Low coordinate Zn(II) organocations bearing extremely bulky NHC ligands: structural features, air-/water-tolerance and use in hydrosilylation and hydrogenation catalysis.

This work details the synthesis and characterization of low-coordinate Zn(II)-based organocations [(NHC)Zn(R)] incorporating extremely bulky NHCs [ITr] and [IAd] ([ITr] = ([ITr] = [(HCNCPh)C:]; [IAd] = [(HCNAd)C:], Ad = adamantyl)). Their structural features and particularities are thoroughly assessed as well as their air and water tolerance. Neutral ITr and IAd adducts [(ITr)Zn(R)] (1, R = Me; 2, R = Et) and [(IAd)Zn(R)] (3, R = Me; 4, R = Et) were synthesized by reaction of carbene [ITr] or [IAd] with a stoichiometric amount of [ZnR] and isolated in good yields. Despite the steric bulk of [ITr] and [IAd], neutral compounds 1-4 are robust and the solid state structure of adduct 3 was established through X-ray crystallographic studies as a trigonal monomer Zn(II) species. Adducts 1-4 may readily be ionized by [PhC][B(CF)] to afford two-coordinate Zn(II) alkyl cations [(ITr)Zn(Me)] ([5]) and [(ITr)Zn(Et)] ([6]), [(IAd)Zn(Me)] ([7]) and [(IAd)Zn(Me)] ([8]), all isolated in high yields (>80%) as [B(CF)] salts, which were fully characterized. Remarkably, cation [(ITr)Zn(CF)] ([9]), prepared by reaction of [5][B(CF)] with [B(CF)], features π-arene interactions with the electrophilic Zn(II), as deduced from solid state data and further completed by DFT-estimated non-covalent interactions (NCI), indicating that [ITr] may provide substantial steric and electrostatic stabilization. The latter certainly explains the remarkable stability of [(ITr)Zn(CF)] ([9]) towards hydrolysis at RT, as it only coordinates HO to afford an unprecedented stable Zn-OH organocation [10]. Also noteworthy, HO coordination is reversible allowing recovery of [(ITr)Zn(CF)] cation, even after prolonged air exposure. Yet, controlled hydrolysis of [(ITr)Zn(CF)] may occur upon heating with selective protonolysis of the Zn-CF bond to afford structurally characterized dication [(ITr)Zn(OH)] [11]. Interestingly, despite steric hindrance, the air-/water-tolerant cation [(ITr)Zn(CF)] is an effective CO hydrosilylation catalyst, and was also shown to mediate imine hydrogenation catalysis.