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 C_3v 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 (C_2v symmetry), with strong coupling to two equivalent protons, and formed initially in almost equal yield, is slowly converted into the C_3v form. The main cations SnMeH₃⁺, SnMe₂H₂⁺, and SnMe₃H ⁺ have comparable C_2v 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.