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

Synthetic routes to alkyl and aryl substituted dithiophosphinate salts that contain non-coordinating PPh4+ counter cations are reported. In general, these compounds can be prepared via a multi-step procedure that starts with reacting secondary phosphines, i.e. HPR2, 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 HS2P(p-CF3C6H4)(2), HS2P(m-CF3C6H4)(2), HS2P(o-MeC6H4)(2) and HS2P(o-MeOC6H4)(2). Although the insertion step was found to be much slower than the oxidation reaction, the formation of (NH4)S2PR2 from HPSR2 occurred rapidly upon addition of NH4OH. Subsequent cation exchange reactions proceeded readily with PPh4Cl in water, under air and at ambient conditions to provide analytically pure samples of [PPh4][S2PR2] (R=p-CF3C6H4, m-CF3C6H4, o-CF3C6H4, o-MeC6H4, o-MeOC6H4, Ph, and Me, 1b-7b, respectively), which were characterized by elemental analysis, multinuclear NMR, and IR spectroscopy. In addition, S2PPh2- and dithiophosphinates with ortho-substituted aryl groups (3b-6b) were characterized by X-ray crystallography. As opposed to the acids, which have short P=S double bonds and long P-SH single bonds, the metric parameters for the S atoms in S2PR2- are equivalent. In addition, the presence of large non-coordinating PPh4+ cations guard against intermolecular P-S center dot center dot center dot X interactions and ensure that the P-S bond is isolated. These S2PR2- 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.