Schlenk-type Equilibria of Grignard-analogous Arylberyllium Complexes: Steric Effects

The presence of complex Schlenk equilibria is a central property of synthetically invaluable Grignard reagents substantially affecting their reactivity and selectivity in chemical transformations. In this work, we systematically studied the steric effects of aryl substituents on the Schlenk-type equilibria of their lighter congeners, arylberyllium bromides. Combination of diarylberyllium complexes Ar2Be(OEt2) (1Ar, Ar = Tip, Tcpp; Tip = 2,4,6-iPr3C6H3, Tcpp = 2,4,6-Cyp3C6H3, Cyp = c-C5H9), containing sterically demanding aryl groups, and BeBr2(OEt2)2 (2) affords the Grignard-analogous arylberyllium bromides ArBeBr(OEt2) (3Ar, Ar = Tip, Tcpp). In contrast, Mes2Be(OEt2) (1Mes, Mes = 2,4,6-Me3C6H3) and 2 exist in a temperature-dependent equilibrium with MesBeBr(OEt2) (3Mes) and free OEt2. Ph2Be(OEt2) (1Ph) reacts with 2 to afford dimeric [PhBeBr(OEt2)]2 ([3Ph]2). Moreover, the influence of replacing the OEt2 donor by an N-heterocyclic carbene, IPr2Me2 (IPr2Me2 = C(iPrNCMe)2), on the redistribution reactions was investigated. The solution- and solid-state structures of the diarylberyllium and arylberyllium bromide complexes were comprehensively characterized using multinuclear (1H, 9Be, 13C) NMR spectroscopic methods and single-crystal X-ray diffraction, while DFT calculations were employed to support the experimental findings.