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

Abstract

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 PS 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⋯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.