Synthesis, Experimental/Theoretical Characterization, and Thermolysis Chemistry of CpBe(SiMe3), a Molecule Containing an Unprecedented Beryllium−Silicon Bond

Abstract

The synthesis, characterization, and thermal decomposition of CpBe(SiMe3) are presented as part of an exploratory investigation designed to obtain more effective chemical vapor deposition precursors of metallic beryllium. The title compound provides the first example of a direct bond between beryllium and a non-carbenoid group 14 element. The base-free reaction of LiSiMe3 with CpBeCl in pentane affords the air-sensitive, volatile solid CpBe(SiMe3) (ca. 70% yield based on CpBeCl), which was characterized by single-crystal CCD X-ray diffraction, multinuclear NMR, and mass spectrometric studies, and theoretically by DFT/NBO analysis. The solid-state molecular geometry of CpBe(SiMe3) ideally conforms to C3v symmetry (under assumed cylindrical symmetry for the C5H5 ring); the Be−Si bond length of 2.185(2) Å is markedly longer than the sum of covalent radii (2.01 Å). The DFT-optimized molecular geometry closely conforms to that determined crystallographically. Total fragment charges (based upon atomic charge NBO calculations) of −0.79 e for C5H5, +1.26 e for Be, +0.81 e for Si, and −1.28 e for the three Me groups constitute a polarity pattern consistent with the Be−Cp bonding interaction being mainly ionic and with the Be−Si bonding pair being polarized toward the more electronegative SiMe3 fragment. Beryllium-9 and 29Si NMR spectra exhibit a large J(Be−Si) coupling constant of 51 Hz; the 9Be chemical shift of δ −27.70 ppm, the highest field value recorded to date, is in accordance with the calculated bond-polarity pattern, as well as a bond to Si. Mass spectra (EI) exhibited peaks for the molecular ion and its isotopomers. Thermal decomposition of CpBe(SiMe3) gives rise to trimethylsilane, CpBeMe, and CpBe(SiMe2SiMe3) as the major products, as determined by multinuclear NMR. The latter species is likewise formed by the reaction of CpBeCl with LiSiMe2SiMe3.