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

Abstract

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] = [(HCNCPh₃)₂C:]; [IAd] = [(HCNAd)₂C:], 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)₂] (1, R = Me; 2, R = Et) and [(IAd)Zn(R)₂] (3, R = Me; 4, R = Et) were synthesized by reaction of carbene [ITr] or [IAd] with a stoichiometric amount of [ZnR₂] 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 [Ph₃C][B(C₆F₅)₄] 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(C₆F₅)₄]⁻ salts, which were fully characterized. Remarkably, cation [(ITr)Zn(C₆F₅)]⁺ ([9]⁺), prepared by reaction of [5][B(C₆F₅)₄] with [B(C₆F₅)₃], 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(C₆F₅)]⁺ ([9]⁺) towards hydrolysis at RT, as it only coordinates H₂O to afford an unprecedented stable Zn–OH₂ organocation [10]⁺. Also noteworthy, H₂O coordination is reversible allowing recovery of [(ITr)Zn(C₆F₅)]⁺ cation, even after prolonged air exposure. Yet, controlled hydrolysis of [(ITr)Zn(C₆F₅)]⁺ may occur upon heating with selective protonolysis of the Zn–C₆F₅ bond to afford structurally characterized dication [(ITr)Zn(OH)]₂²⁺ [11]²⁺. Interestingly, despite steric hindrance, the air-/water-tolerant cation [(ITr)Zn(C₆F₅)]⁺ is an effective CO₂ hydrosilylation catalyst, and was also shown to mediate imine hydrogenation catalysis.