MOLECULAR AND ELECTRONIC-STRUCTURES OF THE PURPLE CHROMOPHORE RC(NH2)(NSEPH) (R=H, 4-CH3C6H4)

The reaction of Li[4-CH3C6H4C(NSiMe(3))(2)] with PhECl (E = S, Se) in a 1:1 molar ratio produces the monosubstituted derivatives 4-CH3C6H4C( NEPh)[N(SiMe(3))(2)] (3b, E = Se; 3d, E = S). The thermolysis of 3b at 130 degrees C produces PhSeN(SiMe(3))(2) and 4-CH3C6H4CN. Crystals of 3b slowly acquire a purple stain upon prolonged standing at 23 degree s C owing to the formation of 4-CH3C6H4C(NH2)(NSePh) (4c, lambda(max) = 551 nm). The formamidine derivative HC(NH2)(NSePh) (4a, lambda(max) = 472 nm) was isolated from the reaction of HCN2(SiMe(3))(3) with PhSe Cl (1:3 molar ratio) in dichloromethane. The structure of 4a was shown by X-ray crystallography to involve a syn configuration at the drop C =N- bond and hydrogen bonding between amino nitrogen atoms. The crysta ls of 4a are monoclinic, space group P2(1) (No. 4), with a = 7.514(1), b = 5.575(1), c = 8.925(1) Angstrom, beta = 100.71(1)degrees V = 367. 40(6) Angstrom(3), and Z = 2. The final R and R(W) values were 0.055 a nd 0.060, respectively. The H-1 and Se-77 NMR spectra indicate that bo th 4a and 4c exist as a mixture of tautomers in THF solution. DFT calc ulations for the model tautomers HC(NH2)(NER) and HC(NH)(NHER) (E = S, Se; R = H, Ph) show that the former is more stable in ail cases, but the difference in total bonding energy is smaller for selenium than fo r sulfur (17.5 vs. 29 kJ mol(-1)). The visible absorption band for 4a is assigned to the HOMO-LUMO transition(Delta E = 2.90 eV, lambda(max) = 427 nm) of the minor tautomer.