Intramolecular Chalcogen−Nitrogen Interactions: Molecular and Electronic Structures of Geometrical Isomers of the Diazenes RSNC(R‘)NNC(R‘)NSR

The sulfur-containing diazenes ArSN C(Ar‘)N NC(Ar‘) NSAr ( 1d, Ar = Ar‘ = 4-CH 3 C 6 H 4; 3b, Ar = Ph, Ar‘ = 2-BrC 6 H 4; 3c, Ar = Ph, Ar‘ = 2-CF 3 C 6 H 4 ) are obtained by the reaction of Ar‘CN 2 (SiMe 3 ) 3 with 3 molar equivs of ArSCl in CH 2 Cl 2 . X-ray structural determinations have shown that 1d exists as a Z, E, Z isomer with a weak intramolecular S···N interaction [2.607(10) Å], whereas 3b adopts an E, E, E configuration. Crystals of 1d are monoclinic, space group P 2 1 / n, with a = 6.140(2) Å, b = 10.492(6) Å, c = 20.728(9) Å, β = 96.56(4)°, V = 1325(1) Å 3, Z = 2, R = 0.056, and R w = 0.052. Crystals of 3b are orthorhombic, space group Ccca, with a = 13.884(5) Å, b = 24.763(7) Å, c = 14.500(3) Å, V = 4985(2) Å 3, Z = 8, R = 0.043, and R w = 0.044. Density functional theory calculations for the model diazenes HEN C(H)N NC(H) NEH (E = S, Se, Te) show that (a) Z, E, Z isomers with an intramolecular E···N interaction are more stable than the E, E, E isomers, (b) the intramolecular interaction involves donation from the σ(N) lone pairs into both σ*(S−H) and σ*(S−N) and back-donation from a chalcogen π lone pair into the π*(N N) orbital, and (c) the intense visible absorption bands (λ max 500−550 nm, ε = 1 × 10 4 M - 1 cm - 1 ) can be attributed to the π-HOMO (3a u ) → π*-LUMO (3b g ) transition. Variable-temperature 1 H NMR spectra of PhSN C(H)N NC(H) NSPh in toluene- d 8 and in CD 2 Cl 2 provide evidence for the co-existence and interconversion of several geometrical isomers.