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

Bromination of bis­(pyridin-2-yl) diselenide in methyl­ene chloride was carried out and the mechanism is proposed. The mol­ecular and crystal structures of 1 H -pyridine-2-selenenyl dibromide and its cyclo­adduct with cyclo­pentene were studied by X-ray diffraction. 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).