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

摘要

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.