Reaction of dimethylselenourea and selenourea with dibromine to produce selenourea–dibromine, the ‘T’-shaped 1:1 molecular adduct N,N-dimethyl-2-selenourea–dibromine, its solvent of crystallisation-containing analogue and the unusual ionic compound 5[(H2N)(Me2N)CBr]+[SeBr6]2−[Se2Br9]−2[Br3]−. A low temperature crystallographic reinvestigation of N-methylbenzothiazole-2-selone–dibromine

Abstract

The reactions of the selenoamides N,N-dimethyl-2-selenourea, dmsu, selenourea, su and N-methylbenzothiazole-2-selone, mbts, with dibromine has been studied. In all reactions the bulk product formed is the 1∶1 T-shaped addition compound, selenoamide·Br2. The crystal structure of dmsu·Br2 1 has been determined and compared to its solvated analogue, dmsu·Br2·CH2Cl2. Despite the different crystal packing in these two adducts, an asymmetry in d(Se–Br) is exhibited by both, ruling out the possibility that differences in Se–Br bond lengths are attributed solely to crystal packing forces. Both structures are essentially zwitterionic, a negative charge resides on the SeBr2 moiety and the positive charge is supported by the two nitrogen atoms. The recently reported mbts·Br2, previously described as a carbene interacting with SeBr2, was reinvestigated. Considering findings from a low temperature X-ray study and the 13C NMR spectrum, this compound is in fact also best described as zwitterionic, analogous to 1 and previously described structures. An interesting minor product from the reaction of two equivalents of dibromine with dmsu has also been characterised crystallographically. This complicated ionic structure of formula 5[(H2N)(Me2N)CBr]+[SeBr6]2–[Se2Br9]–2[Br3]– illustrates the susceptibility of certain selenoamides to carbon–selenium bond cleavage when a dihalogen that is more strongly oxidising than I2 or IBr is treated with them in a stoichiometric ratio greater than 1∶1. This curious (and disordered) structure represents the first report of the anion [Se2Br9]–.