Cyclic triphosphenium ions and related species

Abstract

A series of cyclic triphosphenium ions containing three adjacent phosphorus atoms linked by a hydrocarbon backbone has been synthesised. They vary in ring size (four- to seven- membered rings), substituents on the four-coordinate phosphorus atoms, and organic backbone, and have been characterised by (^31)P NMR spectroscopy. Variable temperature studies probing the mechanism of formation of cyclic triphosphenium ions from a reaction of PX(_3) with a diphosphane have shown a three-step mechanism: (i) the addition of PX(_3) to the diphosphane to form an acyclic intermediate, (ii) cyclisation to afford a heterocycle with a halogen still bonded to the central P atom, and (iii) removal of the halogen to afford the cyclic triphosphenium ion. The syntheses of P(_c)-alkyl and aryl derivatives of cyclic triphosphenium ions using two different methods are described. Direct ethylation of a cyclic triphosphenium ion using ethyl triflate is only possible when the substituents on the four-coordinate phosphorus atoms are small e.g. Et. Synthesis of Pc-alkyl and aryl derivatives with larger substituents on the four-coordinate P atoms, and/or Pc, have been achieved via reaction of a diphosphane with a dichlorophosphane in the presence of AICI(_3_ or SnCI(_2). A series of tetraphosphonium ions containing four adjacent P atoms linked by an organic backbone has also been synthesised. These derivatives have been characterised using (^31)P NMR spectroscopy and where possible X-ray diffractions studies and elemental analyses. The P-P bonds in these derivatives are typical values for single bonds. The synthesis of Pt(II) complexes containing cyclic triphosphenium ions has been achieved to afford cis and/or trans isomers via reaction of a cyclic triphosphenium ion with trans-[Pt(PR(_3))CI(µCl)](_2). However, if the cyclic triphosphenium ion contains phenyl groups on both of the four-coordinate phosphorus atoms, ring scission followed by complexation of the diphosphane to Pt is observed. In the Pt(II) complexes containing cyclic triphosphenium ions, an unusually small 'J(_pt-p) value to the phosphenium central P (981-1267 Hz) suggests that the bond between the phosphenium central P atom and the Pt centre is long. Hydrolysis reactions of several Pt(II) complexes containing chlorophosphane ligands have been monitored by (^31)P NMR spectroscopy.