INTRAMOLECULAR CHALCOGEN-NITROGEN INTERACTIONS - MOLECULAR AND ELECTRONIC-STRUCTURES OF GEOMETRICAL-ISOMERS OF THE DIAZENES RSN=C(R')N=NC(R')=NSR

The sulfur-containing diazenes ArSN=C(Ar')N=NC(Ar')=NSAr (1d, Ar = Ar' = 4-CH3C6H4; 3b, Ar = Ph, Ar' = 2-BrC6H4; 3c, Ar = Ph, Ar' = 2-CF3C6H 4) are obtained by the reaction of Ar'CN2(SiMe(3))(3) with 3 molar equ ivs of ArSCl in CH2Cl2. X-ray structural determinations have shown tha t 1d exists as a Z,E,Z isomer with a weak intramolecular S ... N inter action [2.607(10) Angstrom], whereas 3b adopts an E,E,E configuration. Crystals of 1d are monoclinic, space group P2(1)/n, with a = 6.140(2) Angstrom, b = 10.492(6) Angstrom, c = 20.728(9) Angstrom, beta = 96.5 6(4)degrees, V = 1325(1) Angstrom(3), Z = 2, R = 0.056, and R(w) = 0.0 52. Crystals of 3b are orthorhombic, space group Ccca, with a = 13.884 (5) Angstrom, b = 24.763(7) Angstrom, c = 14.500(3) Angstrom, V = 4985 (2) Angstrom(3), Z = 8, R = 0.043, and R(w) = 0.044. Density functiona l 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 intramolec ular interaction involves donation from the sigma(N) lone pairs into b oth sigma(S-H) and sigma*(S-N) and back-donation from a chalcogen pi lone pair into the pi(N=N) orbital, and (c) the intense visible absor ption bands (lambda(max) 500-550 nm, epsilon = 1 x 10(4) M(-1) cm(-1)) can be attributed to the pi-HOMO (3a(u)) --> pi-LUMO (3b(g)) transit ion. Variable-temperature H-1 NMR spectra of PhSN=C(H)N=NC(H)=NSPh in toluene-d(8) and in CD2Cl2 provide evidence for the co-existence and i nterconversion of several geometrical isomers.