Electron spin resonance studies of a range of stannane radical cations
Abstract
Exposure of dilute solutions of stannanes in freon (CFCl3) at 77 K to 60Co γ-rays gave the parent radical cations. SnH4 gave two well defined centres, both exhibiting very large hyperfine coupling to 117Sn and 119Sn, One, with C3v symmetry, showing strong coupling to a unique proton, is thought to have the unpaired electron confined to one σ Sn–H orbital with a large 5s contribution from tin. The other (C2v symmetry), with strong coupling to two equivalent protons, and formed initially in almost equal yield, is slowly converted into the C3v form. The main cations SnMeH3+, SnMe2H2+, and SnMe3H + have comparable C2v structures with strongly s–p hybridised tin orbitals, but that for SnMe4 has its major spin density on a single methyl group, with almost no 5s-orbital character on tin. In this case, the SnMe3 unit appears to have become planar. Studies using 13CH3 labelling confirm that this radical has extensive Me3Sn+⋯ĊH3 character. In accord with this, methyl radicals were detected on annealing above 77 K. This agrees with the well established tendency for such organometallic compounds to give alkyl radicals in the liquid phase on chemical or electrochemical reduction. When methyl groups were progressively replaced by chlorine, good yields of methyl radicals were obtained at 77 K. For SnMe3Cl+ the parent cation spectrum closely resembled that of SnMe4, showing that electron loss from chlorine is disfavoured relative to loss from a tin–carbon σ bond. Other stannanes studied in this way include SnMe3Et, which gave high yields of ethyl radicals at 77 K. The t-butyl derivative gave a stable cation at 77 K, having a nearly planar SnMe3 unit but a pyramidal –CMe3 unit. This signal was lost reversibly on annealing, in contrast with that for SnMe4+ cations, with the reversible growth of broad features assigned to ˙CMe3 radicals.