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 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.