Reaction of tertiary phosphine selenides, R₃PSe (R = Me₂N, Et₂N or C₆H₁₁), with dibromine. The first reported examples of 1∶1 addition

Abstract

The R₃PSeBr₂ compounds (R = Me₂N, Et₂N or C₆H₁₁) have been prepared and characterised by ³¹P-{H} and infrared spectroscopy. The compounds R₃PSeBr₂ (R = Me₂N or C₆H₁₁) have also been crystallographically characterised. In contrast to the analogous diiodo compounds R₃PSeI₂ (which have a molecular Ψ-tetrahedral charge-transfer structure, R₃PSeI–I), the R₃PSeBr₂ compounds adopt Ψ-trigonal bipyramids at the selenium centre (taking account of the stereochemically active lone pairs). The crystal structure of (Me₂N)₃PSeBr₂ exhibits very different d (Se–Br), 2.602(2) and 2.544(2) Å. This phenomenon is reasoned to be due to the fact that both staggered and eclipsed Se–Br bonds are observed in the structure. The crystal structure of (C₆H₁₁)₃PSeBr₂ shows two crystallographically independent molecules in the asymmetric unit, d (Se–Br) being identical in one molecule, 2.568(3) and 2.566(3) Å, but significantly different in the second molecule, 2.591(3) and 2.556(3) Å. A possible explanation for this is the presence of a close non-bonded Br  · · ·  Br contact in this second (C₆H₁₁)₃PSeBr₂ molecule. The compounds R₃PSeBr₂ (R = Me₂N or C₆H₁₁) both exhibit P–Se bonds typical of those expected for single bonds, 2.262(2) and 2.263(2) average, respectively, again in contrast to the analogous diiodo compounds, R₃PSeI₂, in which significant P–Se double bond character was retained. The ³¹P-{H} NMR and infrared spectroscopic data for R₃PSeBr₂ (R = Me₂N, Et₂N or C₆H₁₁) are discussed with respect to those of the parent tertiary phosphine selenide, R₃PSe.

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