Bromination of bis-(pyridin-2-yl) diselenide in methyl-ene chloride: the reaction mechanism and crystal structures of 1-pyridine-2-selenenyl dibromide and its cyclo-adduct with cyclo-pentene (3a,9a)-2,3,3a,9a-tetra-hydro-1-cyclo-penta-[4,5][1,3]selenazolo[3,2-]pyridinium bromide.

1-Pyridine-2-selenenyl dibromide, CHNSeBr, , is a product of the bromination of bis-(pyridin-2-yl) diselenide in methyl-ene chloride recrystallization from methanol. Compound is essentially zwitterionic: the negative charge resides on the SeBr moiety and the positive charge is delocalized over the pyridinium fragment. The C-Se distance of 1.927 (3) Å is typical of a single bond. The virtually linear Br-Se-Br moiety of 178.428 (15)° has symmetrical geometry, with Se-Br bonds of 2.5761 (4) and 2.5920 (4) Å, and is twisted by 63.79 (8)° relative to the pyridinium plane. The Se atom forms an inter-molecular Se⋯Br contact of 3.4326 (4) Å, adopting a distorted square-planar coordination. In the crystal, mol-ecules of are linked by inter-molecular N-H⋯Br and C-H⋯Br hydrogen bonds, as well as by non-covalent Se⋯Br inter-actions, into a three-dimensional framework. (3a,(9a)-2,3,3a,9a-Tetra-hydro-1-cyclo-penta[4,5][1,3]selenazolo[3,2-]pyridinium-9 bromide, CHNSe·Br, , is a product of the cyclo-addition reaction of with cyclo-pentene. Compound is a salt containing a selenazolopyridinium cation and a bromide anion. Both five-membered rings of the cation adopt envelope conformations. The dihedral angle between the basal planes of these rings is 62.45 (11)°. The Se atom of the cation forms two additional non-covalent inter-actions with the bromide anions at distances of 3.2715 (4) and 3.5683 (3) Å, attaining a distorted square-planar coordination. In the crystal, the cations and anions of form centrosymmetric dimers by non-covalent Se⋯Br inter-actions. The dimers are linked by weak C-H⋯Br hydrogen bonds into double layers parallel to (001).