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

Abstract

The synthesis and characterisation of Ph₄Se₄Br₄ (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) Å] into a Se₄ square, and is structurally analogous to the previously reported Ph₄Te₄I₄. The reactions of Ph₄Se₄Br₄ with a variety of tertiary phosphines have been undertaken, resulting in the formation of compounds of formula R₃PSe(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) Å], with an essentially linear P–Se–Br bond angle, 172.15(4)° and T-shaped geometry at selenium. Me₃PSe(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) Å is considerably longer than observed for 2. In contrast, Cy₃PSe(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.