Sterically hindered solvent extractants—III

.L inorg, nucL Chem.. 1978.Vol. 40. pp, 663--665. PergamonPress. Printed in Great Britain

STERICALLY HINDERED SOLVENT EXTRACTANTS--III THE MOLECULAR AND CRYSTAL STRUCTURE AND HEAVY-ELEMENT EXTRACTION PROPERTIES OF THE DI-t-PENTYLPHOSPHINIC ACID DIMER[1]

J. L. SOLKA[2], A. H. REIS, Jr.,* G. W. MASON, S. M. LEWEY and D. F. PEPPARD Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A. (Received 28 April 1977)

Abstract--Di-t-pentylphosphinic acid, [C(CH3h(CH2CH3)]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(CH3)3hPO(OH), H[Dt-BPL 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 1.088g/cm3 is in good agreement with a calculated value of 1.074g/cm3 for a unit cell volume of 2549.3 (/~)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 O

H. . . . O

0 ....

H--O

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 UP22÷ follows the stoichiometry: UO~ + + 2(HY)2o ~

UO2(HY2)2o + 2HA ÷

where (HYh 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 UP22+ is: 1.0 F (NaCI+ HCI)

k = K, F21[H+I2,

where F is the concentration in formality units of H[l)t-PeP] in the organic phase, [H+] is the molar concentration of H+ in the aqueous phase, and K, is a constant characteristic of the system, i.e. benzene diluent, aqueous phase 1.0 F in chloride. The Ks value for UO22+is 3 x 104.This value is 1.5 times that for the corresponding extraction of UO22+by H[Dt-BP] reported elsewhere.

INTRODUCTION Phosphorus-based extractants of the general type (Xt) (X2)PO(OH) are widely used in metals separations chemistry. Reviews of their applications and pertinent stoichiometries in the liquid-liquid extraction (LLE)[3,4] and liquid-liquid chromatography (LLC)[4] separation of actinides and lanthanides document the versatility of this type of extractant. In general, these compounds exist as dimers in non-polar diluents such as benzene; the dimers are believed to be strongly hydrogen-bonded eightmembered rings [5-7], Xf

/

0

H. . . . 0

x2" / ~ o .

.

.

.

H--O/

X2

"~x,

Letting H represent the ionizable hydrogen of the monomeric extractant and Y the remainder of the mole]iNC VoL 40No. 4.--F

663

cule the dimer may be represented as (HY)2 and the extraction of M p+ as MAp+ + q(HY)2o ~

M(H2q-pY2q)o + pHA +

where p + is the charge on the metal and A and O refer to the mutually-equilibrated aqueous and organic phases. In some specific instances the non-committal formulation of the extracted species, M(H2q-pY2q), has been replaced by postdated formulations in terms of dimers and monomers, e.g. M(HY2)2, p = q = 2; M(HY2)3, p = q = 3 ; M(HY2hY, p =3, q =2.5; M(HY2)2(Y)2, p =4, q=3. Two important parameters in LLE systems embodying these extractants are the inherent acidity of the extractant and the steric hindrance within it. Another important parameter is the carrier diluent in which the extractant is dissolved. For example, in general (XI)(X2)PO(OH) compounds are monomeric in alcohols [7].

664

J . L . SOLKA et al.

The only known distinct dimeric phosphinic acid in the solid state is di-t-butylphosphine acid, H[Dt-BP], which has been studied by X-ray [8] and neutron diffraction[9] methods. Similarly, di-t-pentylphosphinic acid [C(CHs),(CH,CHs)]ePO(OH), H[Dt-PeP], is severely stericaily hindered and should form discrete dimers. Therefore, in order to correlate liquid-liquid extraction data on several sterically hindered extractant molecules of widely differing inherent acidities, the molecular and crystal structure of H[Dt-Pep] was investigated and its extraction selectivity and porperties determined. EXPERIMENTAL In accordance with previous usage[10, 11], a phosphinic acid, (G)2PO(OH), containing two like groups is represented as H[DGP] where H represents a theoretically ionizable hydrogen, D the prefix di and G a generalized organic group. (The brackets distinguish (G)2(PO(OH) from (GO)2PO(OH) which is represented as HDGP.) Letting t-Pe represent the t-CsHH group, ditertiary pentylphosphinic acid, (t-CsH~)zPO(OH), is represented as H[Dt-PeP]. The concentration unit formality, F, is defined as the number of formula weights of solute contained in one liter of solution. The distribution ratio, K, of the 233U nuclide is defined as the :concentration of that nuclide in the organic phase divided by the concentration of nuclide in the aqueous phase of two mutuallyequilibrated sensibly-immiscible liquid phases, the concentration of nuclide being expressed on an atom basis as reflected in alpha counting rates per given aliquot of liquid phase. Alpha-active 1.6 × 10Lyear 233U tracer (23su:23sU mass ratio less than 0.03) was obtained from ANL stock and purified using the system reported by Peppard et al.[12]. The reagent grade benzene, used as carrier diluent, was obtained from Mallinckrodt Chemical Works. The PC13 and (tCsHH)CI used in the preparation of the H[Dt-PeP] were obtained from Aldrich Chemical Company. The pure grade n-heptane used in the recrystallizntion of the H[Dt-PeP] was obtained from Phillips Petroleum Company. Ready-Solv EP Liquid Scintillation Cocktail was obtained from Beckman Instrument Company. Preparation and purification of di-tertiary pentylphosphinic acid, (t-CsHu)2PO(OH), H[Dt-PeP]. The H[Dt-PeP] was synthesized and purified by a modification of the method of Crofts and Parker[13] in the manner previously described in detail for di-tertiary-butylphosphinic acid, H[Dt-BP][14] except that tertiary-pontylchloride was used instead of tertiary-butylchloride. A 30% yield of H[Dt-PeP] product suitable for liquid-liquid extraction studies was obtained. The m.p., determined by using a Thomas-Hoover Capillary Melting Point Apparatus, was 148-M8.5°C (literature 132-136°C). The PKA in 75% ethanol-25% water, determined as described previously[15], was found to be 6.60.' Determination of extraction of 233U by H[Dt-PeP]. The distribution ratio, K, of z33U was determined as described previously[16, 17] with the following exceptions. The extractant phase was a solution of H[Dt-PeP] in benzene, and the aqueous phase was a solution of HCI with or without NaCI. The aqueous phases were pre-equllibrated before use by cycling each aqueous phase through two one-half volume portions of the extractant phase. The H[Dt-PeP]. extractant dependency data were obtained by forward extraction except for the 0.15 F and 0.106 F H[Dt-PeP] points which were obtained by reverse extraction. The hydrogen-ion dependency data were obtained by reverse extraction. The extractant and aqueous phases were assayed by liquid scintillation counting of a lO0-micro-liter portion of a given phase dissolved in 10 ml of liquid-scintillation cocktail. Counting was done by a Beckman Model LS-100 Liquid Scintillation Counter. X-Ray data collection. Single crystals of H[Dt-PeP], obtained by several recrystallizations from n-heptane, were used initially in this study. An experimental density of 1.088 gJcm~, determined by flotation in an ethylacetate-chloroform mixture, is in good agreement with a ~c,~c of 1.0"/4g/cms. However, the mosaic

spread of these crystals (half height-peak width on ~o--1.40 °) proved to be large rendering any data unusable. A second suitable crystal was obtained by recrystallization from ethyl acetate. This method produced oily, plate-like crystals with a decreased mosaic spread (half height-poak width on ~o--0.60°). A crystal with approximate dimensions 0.30 x 0.16 × 0.27 mm was mounted on the end of a glass fiber for data collection (see Table I). Preliminary oscillation, Weissenberg, and precession photographs revealed mmm Laue symmetry and the systematic absences hkl for h + k -- 2n + l, hkO for h = 2n + 1, and h01 for I = 2n + 1, indicative of either the centrosymmetric space group Cmca [D~, No. 64], or the noncentrosymmetric space group Aba2 [C~, No. 41]. The crystal was placed on a GE XRD-490 automatic diffractometer for data collection and determination of unit-cell constants. The cell parameters were determined by a least-squares calculation of the angular coordinates of 13 reflections distributed over the hk/ octant with a 20 range of 17°<20<30 °. The corresponding cell parameters at 26°C are: a = 17.694(7), b-11.021(4), c = 13.073(5) A, and Z = 8. Intensity data were collected with a coupled 0-20 scan over a range of 0° < 20 < 40°. A total of 646 reflections were collected. The (200) and (020) reflections were used as check reflections and were monitored every 100 reflections; the relative standard deviation from the mean intensity was 3.3%. The weights, w, of the intensity data were assigned using the relation w = 1/¢2 where ¢ is the standard deviation of the net count; crrno, was obtained by:

¢ , ,,t = [ l, ot + ~ Btot+ O.O3Inet] 'n where ln¢t = Itot - Btot and/to t is the total count, Btot is the total background equal to (tJtb)(B~ + B2), tb = time counting background counts, t~ = time scanning peak, and 0.03 is a fractional systematic error. Details specific to the data collection have been given previously[8]. The data were reduced and absorption, Lorentz, and polarization corrections applied using the pro~am DATAL1B [18]. Solution and refinement of the X-ray structure. Programs used in elucidating the structure were MULTAN-74[19], SSFOUR[20], S5XFLS[20], and S5FFE[20]. Scattering factors for the phosphorus, oxygen, and carbon atoms were taken from the compilation of Cromer and Waber[21]. The scattering curves were adjusted for anomalous dispersion using the values of Cromer [22]. A Patterson map calculated for point group mmm was consistent with the centrosymmetric space group Cmca with the P and two O atoms located in a mirror plane. A Fourier map based on these positions revealed the positions of four additional carbon atoms, but it failed to Tesolve the position of the ethylene carbon. Because of this ambiguity, the direct-methods program MULTAN-74[19] was used to predict an initial phasing model. The Wilson plot, [E2- I[ values, and Phillips, Howells and Rogers [23] statistics clearly indicated that the structure is consistent with the centrosymmetric space group, Cmca. Eight atomic positions were derived from the solution with the highest combined Figure of Merit which were equivalent to the Patterson and Fourier atomic positions plus a position which seemed possible for the missing ethyl carbon. Three cycles of full-matrix least-squares isotropic refinement of all non-hydrogen atoms converged to an Re = 0.28, where R,~ = :~llPol-IPcll~lFol.

Higher than normal isotropic thermal parameters were observed, especially for the errant ethylene carbon. A further three cycles of anisotropic full-matrix least-squares refinement, with C(5) removed due to its unusually high isotropic thermal parameter, con~'erged at an R F - 0.23. A series of subsequent differenceFourier and Fourier maps failed to resolve an accurate C(5) position or any possible hydrogen positions. A listing of the final positional parameters is given in Table 2. A fisting of the anisotropic temperature factors is given in Table 3. Bond distances

665

The di-t-pentylphosphinic acid dimer Table 1. Experimental parameters for the H[Dt-PeP] X-ray data collection o

T = 26°C,

CELL CONSTANTS:

a = 17.694(7),

b = 11.021(4),

c = 13.073(5)A

o

CELL VOLUME: MOLECULAR

WEIGHT

CALCULATED MEASURED

2549.3

A3

OF ASYMMETRIC

DENSITY:

DENSITY:

1.074 1,088

UNIT:

206.25

g/eq.

g/cm 3

g/cm 3

BY F L O T A T I O N

IN

ethyl

acetate-chloroform.

Z = 8 SPACE

18 Cmc___aa, [D2h,

GROUP:

RADIATION: ATTENUATOR:

MoKa

2%:

40 °

S C A N TYPE:

cps.

(hkZ) 8-2@ 2.0 °

SCAN

0.1 ° s t e p

COUNTING

(Zr Filter)

2"

SCAN WIDTH: SPEED:

64]

0.71073,

C u foil at i 0 , 0 0 0

TAKE OFF ANGLE: MAX

No.

i =

TIME:

4 sec/step

(BACKGROUND

16 S E C

EACH

S I D E OF PEAK)

CRYSTAL: c AXIS

MOUNTED

VOLUME

= 9.69 x 1 0 - 5 c m 3

ABSORPTION

COEFFICIENT

= 1.86

MAX TRANSMISSION

FACTOR

= 98

MIN TRANSMISSION

FACTOR

= 96

NUMBER

OF R E F L E C T I O N S

cm

COLLECTED

-i

= 646

RF = 0 . 2 3

Table 2. Final positionM p~ameters for H[Dt-PeP]a

Atom

x

y

Z

P

0

0,6488(11)

0.3979(7)

0(I)

0

0.653(2)

0.483(i)

0(2)

0

0.518(2)

0.633(1)

C(1)

-.078(1)

0.765(3)

0.643(2)

C(2)

-.119(4)

0.873(2)

0.611(3)

C(3)

-.098(1)

0.707(2)

0.770(1)

C(4)

-.149(2)

0.647(3)

0.601(2)

C(5)

-.171(5)

0.862(7)

0.636(5)

aEstimated

standard

deviations

are given

in p a r e n t h e s e s .

666

J. L. SOLKA et aL Table 3. Anisotropic thermal parameters" (x 104) for H[Dt-PeP] b Atom

Bl I

822

812

813

P

139(1)

191(2)

82(7)

0

O

11(12)

0(i)

151(14)

155(23)

97(14)

O

0

8(21)

0(2)

102(12)

153(21)

89(16)

0

O

4(1)

C(1)

40(2)

700(85)

385(54)

-12(36)

6(25)

353(58)

C(2)

1225(161)

91(23)

419(49)

4(44)

388(60)

51(32)

C(3)

207(21)

350(41)

65(15)

-89(24)

-26(18)

-27(20)

C(4)

260(26)

304(43)

233(29)

154(32)

48(26)

21(36)

aThese parameters

833

823

are given in the form of exp [-(h2811 + k2822 + £2833 +

2hk812 + 2h£813 + 2k£823)] bEstimated

standard deviations

are given in parentheses.

Table 4. Interatomic bond distances and anOes for H[Dt-PeP] = o

Atoms

Distances

o

(A)

Atoms

Angles

P-O (i)

i. 56 (9)

O (2)-P-O (1)

iii (5)

P-O (2)

1.50(4)

O (2)-P-C (1)

107(2)

P-C (I)

I. 82 (5)

0 (1)-P-C (I)

i08 (9)

C(I)-C(2)

1.58(4)

P-C(1)-C(3)

108 (2)

C (1)-C (3)

i. 60 (4)

P-C (1)-C (4)

i00 (6)

C (1)-C (4)

i. 64 (3)

C (2)-C (1)-C (3)

iii (2)

C (2)-C (5)

i. 52 (4)

C (3)-C (1)-C (4)

109 (5)

O(i)---O (2) 1

2.53 (3)

C (4)-C (1)-C (2)

118 (2)

C (1)-C (2)-C (5)

99 (3)

aEstimated standard deviations

are given in parentheses.

(Z/) c(5)

R,-.....p/

O(I). . H. . 0(2)-

.....

/ ~

R"~~ ~O(2)"'H~O(1)/

.C~(I) , ~,(3)

~R~

Fig. 1. The H[Dt-PeP] molecule showing atom labeling: R: = - C(CH3)2(CH2CH3).

The di-t-pentylphosphinicacid dimer and angles for H[Dt-PeP]are given in Table 4, and the atoms are labeled as is shown in Fig. 1. DISCUSSIONOF STRUCTURALRESULTS The diffuse nature of many of the Bragg peaks, the low resolution of the data set, the high isotropic and anisotropic temperature factors, and the unresolved C(5) atom indicate a structural disorder which is probably thermal in origin. These observations are in accordance with similarly disordered phosphinic acid molecules[24]. In spite of these difficulties, a number of significant structural results have been obtained in the current study of H[Dt-PeP]; these are given below. H[Dt-PeP] is shown to be a discrete dimer bonded through an eight membered, . o

H ....

3

I

~0

I

o

2.0

0

-p ~

667

.... H - - O ~

I -2

~

I -I

0

Log _FH[Dt-PeP] Extractant dependency of the e~traction of U02a+ into H[Dt-PeP],

ring containing strong hydrogen bonds. The O(1)---O(2) hydrogen bonded distance is 2.53(3)/~. Thus, H[Dt-PeP] acid is the second characterized discrete dimeric (R2)PO(OH) compound, di-t-butylphosphinic acid, H[DtBP], being the first-reported dimer [8, 9]. H[Dt-BP] was shown to contain strong nearly symmetric hydrogen bonds with an O(1)--O(2)' distance of 2.486(2) ~[9]. The P, O(1), and 0(2) atoms of H[Dt-PeP] appeared, in all refinement models, to lie in a mirror plane, and if H is included, forming an eight membered planar ring with site symmetry 1. In the H[Dt-BP] dimer, the eight membered ring is distinctly non-planar and has a chair conformation. A non-planar ring may also be conjectured for H[Dt-PeP] if the possibility of disorder of the ring atoms is considered. The anisotropic thermal ellipsoids which have high x axis components of 0.22, 0.24, 0.16 A2 for the P, O(1), and 0(2), respectively, allow a small distortion of the planar ring (much smaller, however, than the 16.80 observed in H[Dt-BP]). The coordination about the phosphorus atom is tetrahedral as is the coordination about C(1) with bond distances and bond angles similar to those found in H[DtBP]. These bond lengths and angles are given in Table 4, and" the atom labeling shown in Fig. 1. In H[Dt-BP], the tertiary-butyl ligands were interrelated by a pseudo-mirror plane passing through the eight membered, dimeric ring. In H[Dt-PeP], a crystallographic mirror plane is imposed on the molecule causing the t-pentyl ligands to be mirrored. This yields an eclipsed conformation of the mirrored t-pentyl ligands and results in large C-H--H--C interactions. Any structural disorder of the t-pentyl ligands will reduce this repulsion. The large thermal motion of the carbon atoms is indicative of such a disorder. The elusive C(5), even though placed at several crystallographic positions, always gave "extremely large thermal parameters and must therefore be the most disordered of any of the atoms. LIQUID-LIQUIDEXTRACTIONRESULTS From Figs. 2 and 3 it is seen that the distribution ratio for UO22÷ is directly second-power dependent upon the concentration of extractant in the equilibrated organic phase and inversely second-power dependent upon the hydrogen-ion concentration in the equilibrated aqueous phase. The numerical values of the hydrogen-ion and

benzene diluent, from I.OF HCI.

Fig. 2. Extractant dependency of the extraction of UO22+into H[Dt-PeP], benzene diluent, from 1.0F HCI. I

I

20

v

o

I

-I

I

0 Log F_H÷ Hydrogen-ion dependency of the extraction of UO~+ into 0.0094F H lOt-PeP], benzene diluent, from 1.0E (HCI+NoCl).

Fig. 3, Hydrogen-iondependencyof the extraction of UO:2+into 0.0094F H[Dt-PeP], benzene diluent, from 1.0F (HCI+ NaCI). extractant dependencies are equal to the numerical value of the charge on the cation extracted so the measured distribution ratio, K, for UO22+ may be expressed as K = K , F21[H+] 2

where F equals 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, is a constant characteristic of the system. The stoichiometry of extraction of UO22. in H[DtPeP] in benzene vs an aqueous chloride phase may be represented as: UO2,~+ 2(HY)2o 4 ~ UO2(HY2)2o+ 2HA÷

668

J.L. SOLKA et al.

where (HYh represents the dimer of H[Dt-PeP] and A and O represent the mutually-equilibrated aqueous and organic phases. The Ks value for UO22+ is 3 x 104, a value that is 1.5 times that for the corresponding extraction of UO22+ by H[Dt-BP] reported elsewhere [25]. REFERENCES 1. Research supported under the auspices of the U.S. Energy Research and Development Administration. 2: Participant in the undergraduate research participation program directed by the Argo.nne Center for Educational Affairs. 3. D. F. Peppard, In Annual Reviews of Nuclear Science (Edited by E. Segre), Vol. 21, pp. 365-396. Annual Reviews Inc., Palo Alto, California (1971). 4. E. K. Hulet and D. D. Bode, In MTP International Review of Science, Lanthanides and Actinides (Edited by K. W. Bagnail), Vol. 7, pp. 1--45. Butterworths, London and University Park Press, Baltimore (1972). 5. D. Dyrssen, Acta Chem. Scand. 11, 1771 (1957). 6. D. F. Peppard, .l'. R. Ferraro and G. W. Mason, J. lnorg. Nucl. Chem. 7, 231 (1958). 7. J. R. Ferraro, G. W. Mason and D. F. Peppard, J, lnorg. Nucl. Chem. 22, 285 (1961). 8. M. E. Druyan, A. H. Reis, Jr., E. Gebert, S. W. Peterson, G. W. Mason and D. F. Peppard, J. Am. Chem. Soc. 98, 4801 (1976). (Paper I in the series). 9. A. H. Reis, Jr., S. W. Peterson, M. E. Druyan, E. Gebert, G. W. Mason and D. F. Peppard, lnorg. Chem. 15, 2748 (1976). (Paper II in this series).

I0. D. F. Peppard, G. W. Mason and S. Lewey, 3.. lnorg. Nucl. Chem. 27, 2065 (1965). 11. D. F. Peppard, G. W. Mason, W. J. Driscoll and R. J. Sironen, J'. lnorg. Nucl. Chem. 7, 276 (1958). 12. D. F. Peppard, G. W. Mason and M. V. Gergel, J. lnorg. Nucl. Chem. 3, 370 (1957). 13. P. C. Crofts and D. M. Parker, 3'. Chem. Soc. 2529 (C1970). 14. G, W. Mason and S. Lewey, J. lnorg. Nucl. Chem. 36, 911

(1974). 15. D. F. Peppard, G. W. Mason and C. M. Andrejasich, J. lnorg. Nucl. Chem. 27, 697 (1965). 16. D. F. Peppard, G. W. Mason, A. F. Bollmeier and S. Lewey, 3'. lnorg. Nucl. Chem. 33, 845 (1971). 17. D. F. Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll, J. lnorg. Nucl. Chem. 4, 334 (1957). 18. A. IBM 370/195 program written by H. A. Levy. 19. MULTAN-74 written by P. Main, M. M. Woolfson and G. Germain. 20. S5FOUR, S5XFLS, and S5FFE are Sigma V versions of the programs FOURIER by R. J. Dcllaco and W. T. Robinson, ORXFLS3 written by W, R. Busing and H. A. Levy and ORFFE3 written by W. R. Busing and H. A. Levy. 21. International Tables/or X-ray Crystallography, Vol. IV, p. 71. Kynoch Press, Birmingham, England (1974). 22. International Tables for X-ray Crystallography, Vol. IV, p. 148. Kynocb Press, Birmingham, England (1974). 23. E. R. Howells, D. C. Phillips and D. Rogers, Acta Cryst. 3, 210 (1950). 24. V. Giancotti, F. Giordano, L. Randaccio and A. Ripamonti, J. Chem. Soc. 757 (1968). 25. G. W. Mason, S. M. Lewey, D. M. Gilles and D. F. Peppard, in press.