Rational syntheses and structural characterization of sulfur-rich phosphorus polysulfides: α-P2S7 and β-P2S7.

A wide range of binary compounds is known in the system phosphorus-sulfur. They consist of discrete cage-like molecules with the composition P4Sn (n= 3–10) except for P14S. [1] The cages can formally be derived from the P4 tetrahedron of white phosphorus either by insertion of sulfur atoms into P P bonds or by exocyclic addition to phosphorus atoms. The maximum amount of sulfur per molecule is present in P4S10. [2] The congruently melting phosphorus sulfides P4S3, P4S7, and P4S10 are synthesized by reaction of stoichiometric amounts of the elements in the melt. Other phosphorus sulfides are not accessible by this route. For instance, a-P4S5 is prepared by stirring a solution of P4S3, sulfur, and catalytic amounts of iodine in dry carbon disulfide at room temperature and diffuse daylight for three days. Jason used triphenylarsenic sulfide and triphenylantimony sulfide as sulfur transfer reagents in the synthesis of g-P4S6, for example. [5] Another very useful sulfur transfer reagent is [(CH3)3Sn]2S. [6] In contrast triphenylphosphine is suitable for the abstraction of sulfur atoms. Note that the syntheses of phosphorus sulfides often result in mixtures of different compounds. In addition, various modifications of phosphorus sulfides of the same composition are known. Unfortunately the nomenclature is far from systematic. In fact, it is a quite confusing mix of Greek letter prefixes and Roman numeral suffixes that are used to distinguish between both different constitution isomers and different crystal structures of identical cage molecules. Herein we use Greek letter prefixes, not only to distinguish between two polymorphs but also to consider the chronological order of their discovery. As already mentioned P4S10 is the most sulfur-rich phosphorus sulfide structurally characterized to date. However there is evidence for the existence of more sulfur-rich compounds P4Sn with n> 10. [8] As early as 1910 Stock suspected that P4S10 was not the most sulfur-rich phosphorus sulfide at all. D marcq prepared PSx (x= 3–75) by heating P4S10 with an excess of sulfur at 300 8C. [10] The corresponding solution P NMR spectra indicated a number of novel species. He termed these compounds as “phosphorus polysulfides” or “phosphorus persulfides” implying the presence of S S bonds. Though the system phosphorus-sulfur has been investigated for a long time, there are no structural data of phosphorus poly-(or per-) sulfides available. More than 150 years after the discovery of P4S10 by Berzelius in 1843, we herein present the first crystal structure determinations of phosphorus polysulfides, namely a-P2S7 and b-P2S7. The structures of a-P2S7 and b-P2S7 differ dramatically from those of common phosphorus sulfides. They do not consist of discrete cage-like molecules but of neutral polymeric strands featuring direct S S bonds. Both the polymers catena-P2S7 in a-P2S7 and in b-P2S7 are quite similar: each phosphorus atom is tetrahedrally surrounded by sulfur atoms. These PS4 tetrahedra share one common edge resulting in P2S6 units reminiscent of the thiophosphate ion P2S6 2 in M2P2S6 (M=Ag, K, Cs). [11] These units are interconnected by sulfur atoms to form P2S7 polymers, see Figure 1.