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, the steric effects of aryl substituents on the Schlenk-type equilibria of their lighter congeners, arylberyllium bromides, are systematically studied. Combination of diarylberyllium complexes Ar₂ Be(OEt₂ ) (1Ar, Ar=Tip, Tcpp; Tip=2,4,6-iPr₃ C₆ H₃ , Tcpp=2,4,6-Cyp₃ C₆ H₃ , Cyp=c-C₅ H₉ ), containing sterically demanding aryl groups, and BeBr₂ (OEt₂ )₂ (2) affords the Grignard-analogous arylberyllium bromides ArBeBr(OEt₂ ) (3Ar, Ar=Tip, Tcpp). In contrast, Mes₂ Be(OEt₂ ) (1Mes, Mes=2,4,6-Me₃ C₆ H₃ ) and 2 exist in a temperature-dependent equilibrium with MesBeBr(OEt₂ ) (3Mes) and free OEt₂ . Ph₂ Be(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, IPr₂ Me₂ (IPr₂ Me₂ =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 (¹ H, ⁹ Be, ¹³ C) NMR spectroscopic methods and single-crystal X-ray diffraction, while DFT calculations were employed to support the experimental findings.