One-electron reduction of di-imidosulphur compounds, S(NR)2, and some complexes of Group 6A metal carbonyl derivatives containing S(NR)2 ligands: studies of the radical products by electron spin resonance spectroscopy

Abstract

Reduction of the di-imidosulphur compounds, S(NR)₂(R = Bu^t, Ph, 4-MeC₆H₄, or 4-O₂NC₆H₄), in 1,2-dimethoxyethane solution with a potassium mirror in vacuo gives fairly stable solutions of the corresponding anion radicals [S(NR)₂]^– which have been studied by e.s.r. spectroscopy. The frozen-solution e.s.r spectrum of K[S(NBu^t)₂] has also been recorded. Isotropic coupling constants A(¹⁴N), A(¹H) and, for R = Bu^t, A(³³S) have been deduced and analysed in terms of the electronic structures of the radicals using empirical electron-density relationships. Some CNDO/2 and INDO calculations have been made and the experimental and calculated electron-spin densities compared. Variable-temperature e.s.r. measurements have given no indication of the presence of distinct geometric isomers of the anions in solution. The model calculations suggest that the E/E conformation is most stable and that interconversion to give the E/Z or Z/Z forms is unlikely. The experimental evidence in all cases indicates the two nitrogen atoms to the magnetically equivalent. Alkali-metal isotropic coupling constants A(M^I) for the species M^I[S(NBu^t)₂](M^I=⁷Li, ²³Na, or ³⁹K) have been estimated and indicate that the metal–anion interaction increases in the order K < Na < Li.

Potassium-metal reduction of [M(Co)₄{S(NBu^t)₂}](M = Cr, Mo, or W) produces the corresponding anions with well defined isotropic e.s.r spectra. Values for A(¹⁴N) and A(M)(M =^95/97Mo or ¹⁸³W) have been deduced and the spectra analysed. The results suggest that the unpaired electron is located mainly in the S(NBu^t)₂ ligand. CNDO/2 calculations for the chromium anion are consistent with this observation. Attempts to produce [M(CO)₄{S(NPh)₂}]^– have been unsuccessful except for M = Cr, when an unstable species is formed. A tentative analysis of the e.s.r spectrum of this radical anion is presented.