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 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.