Conformational preferences for isomeric N,N′-bis(pyridin-n-ylmethyl)ethanedithiodiamides, n = 2, 3 and 4: a combined crystallographic and DFT study

Abstract

Abstract Crystal structure analysis of the isomeric N,N′-bis(pyridin-n-ylmethyl)ethanedithioamides, n = 2 (1), 3 (2) and 4 (3), show a planar conformation for 1 and conformations whereby the pyridyl rings lie orthogonal and to either side of the central residue for each of 2 and 3. The universal adoption of the all ZZ conformation about the central C–N bonds, which have double bond character, is ascribed to the presence of intramolecular N–H…S hydrogen bonds that close S(5) rings. The gas-phase geometry optimised structure for 1 is the same as the experimental structure which features intramolecular amine-N–H…N(pyridyl) hydrogen bonds. The open structures found for 2 and 3 differ from the somewhat flattened optimised structures. Systematic variations in the geometric parameters characterising the central C2N2S2 residue, in particular the double-bond character of the C–N bond and the elongation of the central C–C bond are shown by theory to be due to conjugative nN → π*C=S interactions and nS → σ*C-C hyperconjugation, respectively.