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, C5H5NSeBr2, 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 SeBr2 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, C10H12NSe + ·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). © 2019.