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 compoun

摘要

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 centre dot Br_2. The crystal structure of dmsu centre dot Br_2 1 has been determined and compared to its solvated analogue, dmsu centre dot Br_2 centre dot CH_2Cl_2. 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. Boh structures are essentially zwitterionic, a negative charge resides on the SeBr_2 moiety and the positive charge is supported by the two nitrogen atoms. The recently reported mbts centre dot Br_2, previously described as a carbene interacting with SeBr_2, was reinvestigated. Considering findings from a low temperature X-ray study and the ~(13)C 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[(H_2N)(Me_2N)CBr]~+[SeBr_6]~(2-)[Se_2Br_9]~-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 [Se_2Br_9]~-.