Robust Two-Coordinate Zn(II) Organocations Supported by Bulky-Yet-Flexible IPr* Carbene: Synthesis, Structure, and Distinct Reactivity in Hydrosilylation Catalysis

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 pi 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.