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

1 H -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. (3a SR ,(9a RS )-2,3,3a,9a-Tetrahydro-1 H -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).