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.