Sterically hindered solvent extractants—III: The molecular and crystal structure and heavy-element extraction properties of the di- t-pentylphosphinic acid dimer

Abstract

Di- t-pentylphosphinic acid, [C(CH 3) 2(CH 2CH 3)] 2PO(OH), H[Dt-PeP], has been shown by single-crystal X-ray diffraction data to be dimeric in the solid state. Previously, di- t-butylphosphinic acid, [C(CH 3) 3] 2PO(OH), H[Dt-BP], was the only phosphinic acid known to be dimeric in the solid state. H[Dt-PeP] crystallizes in the centro-symmetric orthorhombic space group, Cmca, with unit cell parameters, a = 17.694(7), b = 11.021(4), and c = 13.073(5) A ̊ , and Z = 8 , indicating that the molecule must conform to a crystallographic mirror plane or 2-fold axis. A measured density of 1.088 g/cm 3 is in good agreement with a calculated value of 1.074 g/cm 3 for a unit cell volume of 2549.3 (Å) 3 and a formula weight of 206.25 g. A total of 646 three-dimensional X-ray data were collected on an automated XRD-490 G.E. diffractometer. The structure was solved using a combination of direct methods, Patterson, Fourier, and least-squares refinement techniques. Refinement of the data indicates that H[Dt-PeP] is dimeric, and contains a mirror plane in which the hydrogen-bonded, eight-membered ring lies. A structural disorder ▪ involving principally the ethylene carbon but affecting the methyl carbons as well precluded a precise determination of the carbon positions and severely reduced the precision of the final refinement. In the liquid-liquid extraction system consisting of a solution of H[Dt-PeP] in benzene vs an acidic aqueous chloride phase, the extraction of UO 2 2+ follows the stoichiometry: UO 2 A 2+ + 2(HY 2 O ⇆ UO 2(HY 2) 2 O + 2H A + where (HY) 2 represents the dimer of H[Dt-PeP] and A and O represent the mutually equilibrated aqueous and organic phases. The expression for the distribution ratio, k, for UO 2 2+ is: k = K S F 2/[H +] 2, 1.0 F(NaCl+HCl) where F is the concentration in formality units of H[Dt-PeP] in the organic phase, [H +] is the molar concentration of H + in the aqueous phase, and K s is a constant characteristic of the system, i.e. benzene diluent, aqueous phase 1.0 F in chloride. The K s value for UO 2 2+ is 3 × 10 4. This value is 1.5 times that for the corresponding extraction of UO 2 2+ by H[Dt-BP] reported elsewhere.