An experimental and computational investigation of the site selective and lattice driven cycloaddition reactions of SNS+ salts with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 1,1,2,2-tetracyanoethylene:: Accounting for experimental findings for SNS+ cycloaddition reactions with other related multi cyano-containing compounds using DFT and VBT approaches

Reaction of SNSSbF6 and DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1)) in SO2 solution in a 2:1 ratio afforded the dicycloaddition product 1(SNS)(2)(SbF6)(2)center dot SO2 in 85% isolated yield. The dicycloaddition product 1(SNS)(2)(SbF6)(2)center dot SO2 was fully characterized by IR, Raman, NMR, elemental analysis, and single crystal X-ray crystallography. The product of the site selective dicycloaddition of SNS+ at the nitrile functionalities of I represents the first structurally characterized ortho-bis-1,3,2,4-dithiadiazolylium salt which adopts a conformation that utilizes the intramolecular S delta+ - O delta- electrostatic interactions. The reaction of 1 and SNS[Al(OC(CF3)(4)] in a 1: 1 ratio yielded only the monocycloaddition product 1(SNS)[Al(OC(CF3)(3))(4)] indentified by multinuclear C-13, N-14, F-19, Al-27 NMR in SO2 solution. The assignment of the C-13 NMR of 1(SNS)(+) in SO2 solution, were supported by comparison of the calculated (MPW1PW91/631G*) C-13 NMR and experimental chemical shifts, and by comparison of the observed chemical shifts of the related 2(SNS)+ produced on reacting the mono-nitrile CDMQ (2-cyano-5,6-dichloro-3-methoxy-1,4-benzoquinone (2)) with SNS+ (as the SbF6- salt). The reaction of SNSSbF6 with TCNE (1,1,2,2-tetracyanoethylene (3)) gave the tri-cycloaddition product 3(SNS)(3)(SbF6)(3)center dot SO2 (IR, Raman microscopy and X-ray crystallography) in 40% yield. The energetics of the cycloaddition reactions of SNS+ with 1 - 3 and other multifunctional unsaturated centers (e.g. HC C - CN, NC - CN, C(CN)(3)(-), o,m,p-(CN)(2)C6H4, 1,3,5-(NC)(3)C6H3, 4 - 10, previously reported, see Chart 1), were estimated in the gas phase (MPW1PW91/6-31G*), solution (PBE0/6-311G*) and in the solid state by the 'volume based thermodynamics' (VBT) approach. The general thermodynamic trends associated with the stepwise cycloaddition of SNS+ with multifunctional nitriles were established. The 1:1 cycloaddition products were calculated to be stable in the gas phase and solution, while in the solid state the larger lattice enthalpy of the 1:2 salt, relative to twice that of the 1:1 salt, favored the 1:2 cycloaddition product. (c) 2007 Elsevier B.V. All rights reserved.