Organometallics [SRCO / LCQ]

Abstract

The present study details the synthesis and characterization of novel Zn(II)-based organocations of the type [IPr*-Zn-R]⁺ (IPr* = 1,3-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene; R = alkyl, aryl) and their use in styrene, alkyne, and carbonyl hydrosilylation catalysis. The neutral IPr* adducts [IPr*-ZnR₂] (1, R = Me; 2, R = Et; 3, R = Ph) were prepared by the reaction of IPr* with an equimolar amount of ZnR₂ and isolated in good yields. Despite the severe steric hindrance of IPr*, compounds 1–3 are robust in solution, reflecting the bulky-yet-flexible nature of carbene IPr*. Adducts 1 and 2 can be readily ionized by [Ph₃C][B(C₆F₅)₄] to produce two-coordinate Zn(II) cations [IPr*-ZnMe]⁺ ([4]⁺) and [IPr*-ZnEt]⁺ ([5]⁺), both isolated in high yields (>85%) as [B(C₆F₅)₄]⁻ salts. Interestingly, the more Lewis acidic cation [IPr*-ZnC₆F₅]⁺ ([6]⁺), prepared by reaction of [4][B(C₆F₅)₄] with a B(C₆F₅)₃/HSiEt₃ mixture, is further stabilized through π arene interactions with Zn(II), indicating that IPr* may provide steric and electronic stabilization to Zn(II). The latter certainly explains the improved hydrolytic stability of the salt [6][B(C₆F₅)₄]. Zn cations of the [IPr*-ZnR]⁺ series are less Lewis acidic than their [IPr-ZnR]⁺, yet they display a distinct reactivity in hydrosilylation catalysis. Thus, cation [6]⁺ catalyzes at room-temperature styrene and alkyne hydrosilylation with HSiEt₃ as the silane source. Remarkably, it is also a highly effective ketone/aldehyde hydrosilylation catalysis for a rather broad silane and ketone scope and performs much better than [IPr-ZnR]⁺ systems. The density functional theory (DFT)-estimated mechanism for the hydrosilylation of benzophenone by [6]⁺ suggests that Si–H activation by the cationic Zn(II) center is required for the catalysis to proceed. Overall, the improved hydrolytic stability and straightforward synthesis of a well-defined Zn-based Lewis acid such as [6][B(C₆F₅)₄] may promote its further use in various Lewis-acid-mediated transformations.

 

Reference

Robust two-coordinate Zn(II) organocations supported by bulky-yet-flexible IPr* carbene: synthesis, structure and distinct reactivity in hydrosilylation catalysis

Xuejuan Xu, Christophe Gourlaouen, Béatrice Jacques and Samuel Dagorne

Organometallics 2023, 42, 19, 2813–2825 - DOI: 10.1021/acs.organomet.3c00282

 

Contacts

Samuel Dagorne & Xuejuan Xu (team SRCO), Institut de Chimie de Strasbourg (UMR 7177).