PREPARATION AND SPECTROSCOPIC CHARACTERIZATION OF 1,5-DIPHOSPHADISELENATETRAZOCINES AND 1,3-DIPHOSPHADISELENATETRAZOCINES - X-RAY STRUCTURE OF [N(PME2NH2)2]2SE

The cyclocondensation of R2PN2(SiMe3)3 (R = Me, Et, Ph) with a mixture of selenium monochloride and selenium tetrachloride that gives a Se:Cl ratio of 1:3 in acetonitrile at 23-degrees-C produces 1,5-Ph4P2N4Se2, 2a, in 85% yield or mixtures of 1,5- and 1,3-R4P2N4Se2 in combined isolated yield of 79% (R = Me) or 96% (R = Et). The heterocycles 1,3-Me4P2N4Se2, 4b, and 1,5-Et4P2N4Se2, 2c, were separated from these mixtures by fractional crystallization and gel permeation chromatography, respectively. The pure compounds 2a,c and 4b were characterized by elemental analyses, IR spectroscopy, FAB mass spectrometry, and P-31 and Se-77 NMR spectroscopy. A solution of 2a in dichloromethane is pale yellow at -20-degrees-C and green at 23-degrees-C. This green solution exhibits visible absorption bands at 634 and 653 nm and a five line (1:2:3:2:1) ESR signal (g = 2.01 1, a(N) = 0.67 mT) tentatively attributed to the cyclic radical Ph2PN2Se.. Density functional calculations for the model anion H2PN2Se- indicate that the corresponding radical is a planar five pi-electron ring system. The singly occupied molecular orbital of H2PN2Se. is based mainly on selenium with strong antibonding interactions with both nitrogen atoms. The decomposition of 4b in dichloromethane yields [N(PMe2NH2)2]2Se, 5, in which the selenide ion is involved in strong Se- -NH hydrogen bonding to four NH2 groups of the surrounding cations. The crystals of 5 are tetragonal, space group P4(1)2(1)2 with a = 8.532(2) angstrom, c = 14.869(3) angstrom, V = 1082.4 angstrom3, and Z = 2. The final R and R(w) values were 0.055 and 0.061, respectively.