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

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 . Br-2. The crystal structure of dmsu . Br-2 1 has been determined and compared to its solvated analogue, dmsu . Br-2. 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 . Br-2, previously described as a carbene interacting with SeBr2, was reinvestigated. Considering findings from a low temperature X-ray study and the C-13 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[Br-3](-) illustrates the susceptibility of certain selenoamides to carbon-selenium bond cleavage when a dihalogen that is more strongly oxidising than I-2 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](-).