Investigations of the mono- and dicycloaddition reactions of [SNS][MF6] (M = As, Sb) with the dinitriles NCC(O)CN and NCC(Cl)2CN — Energetics and the preference for [SNS]+ dicycloaddition products in solution and solid state

Abstract

The stepwise symmetry-allowed cycloaddition reactions of [SNS]+ with the dinitriles NC–X–CN (X = CCl2, CO) were investigated. The reaction of [SNS]+ and dinitrile in 2:1 ratio yielded the dicycloaddition product quantitatively after 2 weeks (in situ 13C NMR). At a 1:1 ratio, mixtures of mono- and dicycloaddition products were obtained that could not be completely separated from one another. The energetics of the reaction were estimated in the gas phase, in solution (SO2 and CH2Cl2), and solid phases using DFT calculations (PBE0/6-311G*) and a "volume-based thermodynamics" approach for lattice enthalpies. Monocycloaddition products are stable in the gas phase and in solution with respect to disproportionation reactions, but in the solid state the disproportionation of 2 [NC–X–(CNSNS)][AsF6] into [X(CNSNS)2][AsF6]2 and NC–X–CN is strongly favored because of the higher lattice enthalpy of the 1:2 salt. In the gas phase, the second cycloaddition is endothermic because of the charge repulsion of the two positive charges, but in the solid state the reaction is strongly favored because of the lattice enthalpy gain on the formation of the 1:2 ([X(CNSNS)2][AsF6]2) from two 1:1 salts ([NC–X–(CNSNS)][AsF6] and [SNS][MF6]). The crystal structures of the dicycloaddition products [Cl2C(CNSNS)2][AsF6]2 and [OC(CNSNS)2][AsF6]2 and the monocycloaddition product [Cl2C(CNSNS)CN][AsF6] were obtained. In [Cl2C(CNSNS)CN][AsF6], the free nitrile groups interact with neighboring heterocycles further demonstrating that potential of free nitrile groups for introducing intermolecular interactions. [OC(CNSNS)2]2+ is a planar dication, which adopts the conformation with Sδ+···Oδ– contacts to optimize intramolecular electrostatic interactions.Key words: crystal structure, sulfur-nitrogen, 1,3,2,4-dithiadiazolyium, nitriles, cycloaddition, DFT calculations, thermodynamics, energetics.