Reactions of Ph4Se4Br4 with tertiary phosphines. Structural isomerism within a series of R3PSe(Ph)Br compounds

The synthesis and characterisation of Ph4Se4Br 4 (1) directly from the reaction of Ph2Se2 with dibromine is reported. The solid-state structure of 1 consists of four PhSeBr units linked by weak selenium-selenium bonds [3.004(2)-3.051(2) ] into a Se 4 square, and is structurally analogous to the previously reported Ph4Te4I4. The reactions of Ph 4Se4Br4 with a variety of tertiary phosphines have been undertaken, resulting in the formation of compounds of formula R 3PSe(Ph)Br. X-Ray crystallographic analysis of three of the compounds reveals different structural isomers. Ph3PSe(Ph)Br (2) is a charge-transfer (CT) compound [Se-Br 3.0020(8) ], with an essentially linear P-Se-Br bond angle, 172.15(4)°and T-shaped geometry at selenium. Me 3PSe(Ph)Br (5) also contains the selenium atom in a T-shaped geometry, consistent with a CT formulation, although the Se-Br distance of 3.327(3) is considerably longer than observed for 2. In contrast, Cy 3PSe(Ph)Br (6) is an ionic phosphonium salt, [Cy3PSePh]Br with no short Se-Br interactions. Geometry at selenium is bent, as expected for an ionic compound. These results are discussed with reference to the previously reported iodo-compounds Ph3PSe(Ph)I and [(Me2N) 3PSe(Ph)]I. © The Royal Society of Chemistry 2006.