ESR Investigations of the Radicals {Li3[E(NtBu)3]2}•(E = S, Se) and the Radical Anions SOx(NtBu)3-x•-(x= 1, 2)

Abstract

The air oxidation of the cluster compounds [Li₂E(N^tBu)₃]₂ (E = S, Se) in toluene produces deep blue (E = S) or green (E = Se) solutions. The ESR spectra of these solutions consist of a septet (1:3:6:7:6:3:1) of decuplets. The simulation of these spectra shows that the secondary hyperfine splitting results from interaction of the unpaired electron with three equivalent ⁷Li ions consistent with the formation of the neutral radicals {Li₃[E(N^tBu)₃]₂}^• (4a, E = S, g = 2.0039, a(¹⁴N) = 5.69 G, a(⁷Li) = 0.82 G; 4b, E = Se, g = 2.00652, a(¹⁴N) = 5.41 G, a(⁷Li) = 0.79 G). Over a period of 25 h the seven line pattern of 4b is replaced first by a five line (1:2:3:2:1) spectrum (g = 2.009, a(¹⁴N) = 13.4 G) and, subsequently, by a three line (1:1:1) spectrum (g = 2.00946, a(¹⁴N) = 15.4 G, a(⁷⁷Se) = 4.3 G), neither of which exhibit ⁷Li hyperfine splitting. These spectra are tentatively assigned to the radical anions SeO(N^tBu)₂^•- and SeO₂(N^tBu)^•-, respectively. The cluster {Li₂[O₂S(N^tBu)]}_n (3) is prepared by the reaction of sulfur dioxide with 2 equiv of LiNH^tBu in toluene. The air oxidation of toluene solutions of {Li₂[OS(N^tBu)₂]}₆ (2a) or 3 produces deep blue species. In the former case the initial ESR spectrum is a 1:2:3:2:1 quintet (g = 2.009, a(¹⁴N) = 13.3 G) which, after 16 h, evolves into a 1:1:1 triplet (g = 2.0088, a(¹⁴N) = 15.9 G). The same triplet is observed in the ESR spectrum of oxidized solutions of 3 leading to the assignments OS(N^tBu)₂^•- and O₂S(N^tBu)^•- for the quintet and triplet, respectively. The disproportionation 2OS(N^tBu)₂^•- → O₂S(N^tBu)^•- + S(N^tBu)₃^•- is indicated by the changes observed for the ESR spectra of oxidized solutions of 2a.