Bromination of bis­(pyridin-2-yl) diselenide in methyl­ene chloride: the reaction mechanism and crystal structures of 1H-pyridine-2-selenenyl dibromide and its cyclo­adduct with cyclo­pentene (3aSR,9aRS)-2,3,3a,9a-tetra­hydro-1H-cyclo­penta­[4,5][1,3]selenazolo[3,2-a]pyridinium bromide

1H-Pyridine-2-selenenyl dibromide, C(5)H(5)NSeBr(2), 1, is a product of the bromination of bis­(pyridin-2-yl) diselenide in methyl­ene chloride recrystallization from methanol. Compound 1 is essentially zwitterionic: the negative charge resides on the SeBr(2) 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 inter­molecular Se⋯Br contact of 3.4326 (4) Å, adopting a distorted square-planar coordination. In the crystal, mol­ecules of 1 are linked by inter­molecular N—H⋯Br and C—H⋯Br hydrogen bonds, as well as by non-covalent Se⋯Br inter­actions, into a three-dimensional framework. (3aSR,(9aRS)-2,3,3a,9a-Tetra­hydro-1H-cyclo­penta[4,5][1,3]selenazolo[3,2-a]pyridinium-9 bromide, C(10)H(12)NSe(+)·Br(−), 2, is a product of the cyclo­addition reaction of 1 with cyclo­pentene. Compound 2 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 inter­actions 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 2 form centrosymmetric dimers by non-covalent Se⋯Br inter­actions. The dimers are linked by weak C—H⋯Br hydrogen bonds into double layers parallel to (001).