NMR spectroscopy and structural characterization of dithiophosphinate ligands relevant to minor actinide extraction processes

Abstract

Synthetic routes to alkyl and aryl substituted dithiophosphinate salts that contain non-coordinating PPh 4+ counter cations are reported. In general, these compounds can be prepared via a multi-step procedure that starts with reacting secondary phosphines, i.e. HPR 2, with two equivalents of elemental S. The synthetic transformation proceeds by oxidation of the phosphine followed by insertion of S into the H-P bond. This approach was used to synthesize a series of dithiophosphinic acids that were fully characterized, namely HS 2P(p-CF 3C 6H 4) 2, HS 2P(m-CF 3C 6H 4) 2, HS 2P(o-MeC 6H 4) 2 and HS 2P(o-MeOC 6H 4) 2. Although the insertion step was found to be much slower than the oxidation reaction, the formation of (NH 4)S 2PR 2 from HPSR 2 occurred rapidly upon addition of NH 4OH. Subsequent cation exchange reactions proceeded readily with PPh 4Cl in water, under air and at ambient conditions to provide analytically pure samples of [PPh 4][S 2PR 2] (R = p-CF 3C 6H 4, m-CF 3C 6H 4, o-CF 3C 6H 4, o-MeC 6H 4, o-MeOC 6H 4, Ph, and Me, 1b-7b, respectively), which were characterized by elemental analysis, multinuclear NMR, and IR spectroscopy. In addition, S 2PPh 2- and dithiophosphinates with ortho-substituted aryl groups (3b-6b) were characterized by X-ray crystallography. As opposed to the acids, which have short PS double bonds and long P-SH single bonds, the metric parameters for the S atoms in S 2PR 2- are equivalent. In addition, the presence of large non-coordinating PPh 4+ cations guard against intermolecular P-S⋯X interactions and ensure that the P-S bond is isolated. These S 2PR 2- anions, which can be prepared in large quantities and isolated in crystalline form, are attractive for spectroscopic and theoretical studies because the P-S interaction can be probed independently in the absence of intermolecular interactions. © 2012 The Royal Society of Chemistry.