(Pyridinylmethylidene)dithioles: synthesis, structure, and reactivity

Spectral characteristics, an X-ray crystal structure, and molecular orbital calculations indicate that (pyridinylmethylidene)dithioles 3, prepared from butadiynyl precursors 4 and hydrogen sulfide in basic methanol, are analogous in their bonding to the parent 1-aza-6,6a-lambda-4-dithiapentalenes 1 which, according to ab initio studies at the Hartree-Fock level, is best represented as an (iminomethylidene)dithiole. An interaction between the sulfur and the nitrogen atoms is evident from the significant polarization of the S-S bond in those isomers where the nitrogen is proximal to the dithiole moiety. A comparison of the calculated structures of 1 and 3 with experimental data, as well as preliminary calculations on the related system trithiapentalene 2, suggest that ab initio methods using a single determinant are inadequate to describe these hypervalent sulfur systems. For example, an asymmetric structure is calculated to be the minimum energy form for 2 in contradiction to experimental results. Electrophilic substitutions, ring rearrangement, and reaction with electron acceptors are described for 3.