Reaction-path calculations and crystal structures of 1,1'-(ethylene-1,2-diyl)dipyridinium dichloride dihydrate and 1,1'-(ethylene-1,2-diyl)dipyridinium dibromide.

The design of new organic-inorganic hybrid ionic materials is of interest for various applications, particularly in the areas of crystal engineering, supramolecular chemistry and materials science. The monohalogenated intermediates 1-(2-chloroethyl)pyridinium chloride, C5H5NCH2CH2Cl(+)·Cl(-), (I'), and 1-(2-bromoethyl)pyridinium bromide, C5H5NCH2CH2Br(+)·Br(-), (II'), and the ionic disubstituted products 1,1'-(ethylene-1,2-diyl)dipyridinium dichloride dihydrate, C12H14N2(2+)·2Cl(-)·2H2O, (I), and 1,1'-(ethylene-1,2-diyl)dipyridinium dibromide, C12H14N2(2+)·2Br(-), (II), have been isolated as powders from the reactions of pyridine with the appropriate 1,2-dihaloethanes. The monohalogenated intermediates (I') and (II') were characterized by multinuclear NMR spectroscopy, while (I) and (II) were structurally characterized using powder X-ray diffraction. Both (I) and (II) crystallize with half the empirical formula in the asymmetric unit in the triclinic space group P-1. The organic 1,1'-(ethylene-1,2-diyl)dipyridinium dications, which display approximate C2h symmetry in both structures, are situated on inversion centres. The components in (I) are linked via intermolecular O-H...Cl, C-H...Cl and C-H...O hydrogen bonds into a three-dimensional framework, while for (II), they are connected via weak intermolecular C-H...Br hydrogen bonds into one-dimensional chains in the [110] direction. The nucleophilic substitution reactions of 1,2-dichloroethane and 1,2-dibromoethane with pyridine have been investigated by ab initio quantum chemical calculations using the 6-31G** basis. In both cases, the reactions occur in two exothermic stages involving consecutive SN2 nucleophilic substitutions. The isolation of the monosubstituted intermediate in each case is strong evidence that the second step is not fast relative to the first.