Coordination chemistry of 2,2′-biphenylenedithiophosphinate and diphenyldithiophosphinate with U, Np, and Pu

Abstract

New members of the dithiophosphinic acid family of potential actinide extractants were prepared: heterocyclic 2,2′-biphenylenedithiophosphinic acids of stoichiometry HS₂P(R₂C₁₂H₆) (R = H or ^tBu). The time- and atom-efficient syntheses afforded multigram quantities of pure HS₂P(R₂C₁₂H₆) in reasonable yields (∼60%). These compounds differed from other diaryldithiophosphinic acid extractants in that the two aryl groups were connected to one another at the ortho positions to form a 5-membered dibenzophosphole ring. These 2,2′-biphenylenedithiophosphinic acids were readily deprotonated to form S₂P(R₂C₁₂H₆)¹⁻ anions, which were crystallized as salts with tetraphenylpnictonium cations (ZPh₄¹⁺; Z = P or As). Coordination chemistry between [S₂P(^tBu₂C₁₂H₆)]¹⁻ and [S₂P(C₆H₅)₂]¹⁻ with U, Np, and Pu was comparatively investigated. The results showed that dithiophosphinate complexes of U^IV and Np^IV were redox stable relative to those of U^III, whereas reactions involving Pu^IV gave intractable material. For instance, reactions involving U^IV and Np^IV generated An[S₂P(^tBu₂C₁₂H₆)]₄ and An[S₂P(C₆H₅)₂]₄ whereas reactions between Pu^IV and [S₂P(C₆H₅)₂]¹⁻ generated a mixture of products from which we postulated a transient Pu^III species based on UV-Vis spectroscopy. However, the trivalent Pu[S₂P(C₆H₅)₂]₃(NC₅H₅)₂ compound is stable and could be isolated from reactions between [S₂P(C₆H₅)₂]¹⁻ and the trivalent PuI₃(NC₅H₅)₄ starting material. Attempts to synthesize analogous trivalent compounds with U^III provided the tetravalent U[S₂P(C₆H₅)₂]₄ oxidation product.