Electron spin resonance studies of anion radicals of phenylmethylsilanes

Abstract

Anion radicals of various phenylsilanes were prepared by potassium and electrolytic reductions. Their esr spectra were analyzed. A large value of the silicon-hydrogen coupling constant (a[subscript Si-H]) was consistently obtained for a series of phenylsilanes containing silicon-hydrogen bonds. This shows that a significant amount of spin density resides at the silicon atom probably through use of silicon's 3d-oribitals. This is particularly apparent when these anion radicals are compared to the anion radicals of their carbon analogs. For example, the a[subscript Si-H] for the anion radical of pheylsilane is 8.04 G, whereas a[subscript CH₃] for the anion radical of toluene is only 0.59 G. Anion radicals of phenylmethylsilanes containing no silicon-hydrogen were examined. A total splitting of approximately 16 G was observed in all cases. The considerable narrowing of the total splitting means that there is less spin density at the ring carbons because there is significant spin density at the silicon atom. Also the hyperfine coupling constants obtained for ring protons in these anion radicals are directly proportional to the number of phenyl attached to silicon indicating that the unpaired electron is delocalized into all rings. The contrast between the results reported by Makarov and the results obtained in this study is explained. It is believed that Makarov observed only the anion radical of pheyltrimethylsilane. This anion radical appears to be a common paramagnetic product of many silanes due to the reaction between the silanes and solvent, DME. Metallic reduction of phenylsilanes appears to be a stepwise process. In addition to the anion radical of the starting material, the anion radicals of various 1,4-bis(silyl)benzene, biphenyl, tetrapheylsilane, and phenyltrimethylsilane were frequently obtained. Possible reaction paths are suggested.