PREPARATION AND SPECTROSCOPIC CHARACTERIZATION OF 1,5-DIPHOSPHADISELENATETRAZOCINES AND 1,3-DIPHOSPHADISELENATETRAZOCINES - X-RAY STRUCTUREOF [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:C l 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 i solated yield of 79% (R = Me) or 96% (R = Et). The heterocycles 1,3-Me 4P2N4Se2, 4b, and 1,5-Et4P2N4Se2, 2c, were separated from these mixtur es by fractional crystallization and gel permeation chromatography, re spectively. The pure compounds 2a,c and 4b were characterized by eleme ntal analyses, IR spectroscopy, FAB mass spectrometry, and P-31 and Se -77 NMR spectroscopy. A solution of 2a in dichloromethane is pale yell ow at -20-degrees-C and green at 23-degrees-C. This green solution exh ibits 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 t he model anion H2PN2Se- indicate that the corresponding radical is a p lanar five pi-electron ring system. The singly occupied molecular orbi tal of H2PN2Se. is based mainly on selenium with strong antibonding in teractions with both nitrogen atoms. The decomposition of 4b in dichlo romethane yields [N(PMe2NH2)2]2Se, 5, in which the selenide ion is inv olved in strong Se- -NH hydrogen bonding to four NH2 groups of the sur rounding 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 an gstrom3, and Z = 2. The final R and R(w) values were 0.055 and 0.061, respectively.