The characterization of [HSe] − and [Se x] 2− ions by 77Se NMR

Abstract

Preparative reactions for the ions Se 2−, HSe − and [Se x ] 2− in protic and aprotic solvent systems are established, and these ions in solution have been characterized by the most informative probe, 77Se NMR. Fully reduced solutions in DMF contain HSe − for which δ Se ranges between −530 and −390 ppm (vs Me 2Se) depending strongly on solvent and temperature: 1 J(SeH) = 26 Hz. Although HSe − and Se 2− are in fast equilibrium, there is no direct NMR evidence for the conjugate strong base Se 2− in any of these solutions, and it is predicted that δ Se(Se 2−) < −650 ppm. There is no NMR evidence for [Se 2] 2−, which is believed to disproportionate to HSe − plus [Se x ] 2− ( x > 2) or Se (s). The polyselenide ions [Se x ] 2−, x = 3−6, all exist in DMF, and the resonances for all selenium atoms in each of these ions have been observed and assigned. Using α, β, γ to signify selenium positions relative to the equivalent ends of the chain, the δ Se(α), δ Se(β), δ Se(γ) data [in DMF plus 38% (vol) EtOH, 230°K] are: [Se 3] 2−, 193, 214, -; [Se 4] 2−, 256, 581, -; [Se 5] 2−, 354, 570, 868; [Se 6] 2−, 404, 636, 807. The chemical shifts of the Se(α) in these ions are very responsive to the solvent proticity, becoming more negative in the range DMF → ethanol, while the Se(β) atoms respond much less, and the effect at the Se(γ) atoms is small and can be in the other direction. Protic solvents retard the selenium exchange reactions of polyselenide ions. Comparative analysis is made of the δ Se values for related compounds of type XSe − or with chains of selenium atoms. There is no NMR evidence for ions with x > 6. The spirocyclic ion [Se(Se 5) 2] 2− with many oxidants, including NO 3 − in DMF, and possesses only one exchanging 77Se resonance at 720 ppm.