Cyclic systems of phosphorus V

Ph2P(NSiMc3)(NHSiMe3) -l- I/2S4N4 Ph2PN3S2 -l- (Mc3Si)2NH (13.1) 

The heterocycles 13.1 are isolated as purple crystals (R = Mc3SiNH, Ph) or as a thermally unstable, purple oil (R = Me), which decomposes at room temperature to give 13.3 (R = Me, Ph). The structure of 13.1 (R = Ph) consists of a six-membered ring in which the phosphorus atom lies 0.26 A out of the S2N3 plane. The S-N bond lengths fall within a narrow 

Coordination results in a lengthening of the S-N bond involving the coordinated nitrogen atom by ca. 0.1 A this N atom is also displaced out of the PN2S2 plane by 0.63 A. The Pt-N bond in 13.6 is weak and dissociation of the adduct occurs in solution. 

The diphosphadithiatetrazocines 13.2 (E = S) and 13.3 (E = S) contain one more R2PN unit in the ring than 13.1. The structure of the orange-red 1,3-isomer (13.2, E = S, R = Ph) consists of an essentially planar N3S2 unit with the two phosphorus atoms located on opposite sides of the plane. Like 13.1 this isomer forms a 1 1 adduct with norbornadiene via 1,3-addition across the sulfur atoms. 

5-isomers 13.3 (E = S) are colourless, air-stable solids. They are prepared by the cyclocondensation reaction of R2PN2(SiMe3)3 with sulfur dichloride or thionyl chloride. A similar cyclocondensation process, using a mixture of SeCU and Sc2Cl2 as a source of selenium, produces a mixture of the isomers 13.2 and 13.3 (E = Se, R = Ph). The structures of 13.3 (E = S, R = alkyl, aryl) are folded eight-membered rings with a cross-ring S S distance of ca. 2.50 This structural 

---