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Dimeric uranyl complexes with bridging perrhenates†

Gordon H. John,b Iain May,b Mark J. Sarsfield,‡b David Collison*a and Madeleine Helliwella

Received 4th October 2006, Accepted 20th February 2007First published as an Advance Article on the web 12th March 2007DOI: 10.1039/b614481k

The reaction between [UO2(ReO4)2·H2O] and two equivalents of either tri-n-butyl phosphine oxide(TBPO) or tri-iso-butyl phosphate (TiBP) results in the formation of [UO2(l2-ReO4)(ReO4)(TBPO)2]2

(1) and [UO2(l2-ReO4)(ReO4)(TiBP)2]2 (2) respectively. Both complexes crystallise as two structurallysimilar centrosymmetric dimers, the cores containing two uranyl moieties linked by bridging perrhenates.Two P=O donor ligands and one monodenatate perrhenate complete the pentagonal bipyramidalcoordination sphere at each metal centre. Both complexes have also been characterised in the solid state byvibrational and absorption spectroscopy. Solution spectroscopic characterisation indicates that both per-rhenate and phosphine oxide (1) or phosphate (2) remain coordinated, although it is not possible to stateconclusively that the dimeric species remain intact. A low resolution structural study of a minor productfrom the reaction that yielded 2 revealed a monomeric complex with only monodentate perrhenate coordi-nation, [UO2(ReO4)2(H2O)(TiBP)2] (2′). These results represent the first structural evidence for the bridg-ing coordination mode of perrhenate on coordination to an actinide and yields further insight into thepossible solvent phase pertechnetate complexes that may exist in PUREX process phosphate rich solvent.

Introduction

The behaviour of 99Tc during the course of irradiated nuclearfuel processing is of interest. Most current reprocessing plantsstill operate using a tri-n-butyl phosphate (TBP) based extrac-tion process—plutonium and uranium recovery by extractions(PUREX)—with legacy solvent waste also an issue. It haspreviously been shown that the pertechnetate anion, [TcO4]−, willco-extract with a range of metal cations, including {UO2}2+, ZrIV

and PuIV, contaminating product streams and catalysing unwantedside reactions through Tc redox chemistry.1

In an attempt to gain insight into [TcO4]− process chemistrywe have studied the coordination chemistry of both pertechnetateand non-radioactive perrhenate with a range of actinide cations,{UO2}2+, {NpO2}+, ThIV and UIV in the presence of phosphateand phosphine oxide donor ligands as mimics for the TBPextractant.2–7 Monodentate coordination of both [TcO4]− and[ReO4]− was observed to {UO2}2+ and ThIV in the solid state,with UIV–[ReO4]− complexes also structurally characterised.In addition, [UO2(TcO4)(DPPMO2)2]+, where DPPMO2 =bis(diphenylphosphino)methane dioxide, was characterised insolution by both 99Tc and 31P NMR, again with monodentate[TcO4]−.4 In all cases [ReO4]− proved to be an effective analoguefor [TcO4]−. However, while DPPMO2 acts as a bidentate chelateligand the extractant that we wish to mimic, TBP, is monodentate.Solution NMR evidence (99Tc) suggested that [TcO4]− remains co-

aSchool of Chemistry, The University of Manchester, Manchester, M13 9PL,UK. E-mail: David.Collison@manchester.ac.uk; Fax: +44 (0)161 275 4616;Tel: +44 (0)161 275 4660bCentre for Radiochemistry Research, School of Chemistry, The Universityof Manchester, Manchester, M13 9PL, UK† Electronic supplementary information (ESI) available: Solid state vibra-tional spectra and 31P NMR solution spectra for both 1 and 2. See DOI:10.1039/b614481k‡ Current address: Nexia Solutions, Sellafield, Seascale, Cumbria, CA201PG UK

ordinated to ThIV in solution in the presence of monodentate P=Odonor ligands, although the coordination mode of pertechnetatecould not be ascertained.3,7 Of greater interest was the solutionIR (MeOH) study of [U(ReO4)4(TEP)4] and [U(ReO4)4(TiBP)4]which revealed multiple m3 [ReO4]− bands,5 indicative of eithermore than one species in solution or bridging perrhenates (TEP =triethyl phosphate and TiBP = tri-iso-butyl phosphate).8,9

To complete our structural study of [TcO4]− and [ReO4]−

coordination to actinide cations in the presence of P=O donorligands it would be of interest to obtain a solid state complexwith bridging Group 7 oxoanion, thus linking in with solutionstate studies. Certainly IR evidence points to bridging perrhenatein [UO2(ReO4)2·H2O],2,10 U(ReO4)4·5H2O,5 and Th(ReO4)4·4H2O,7

as well as bridging pertechnate in Th(TcO4)4·4H2O.7 Defini-tive structural evidence for bridging [TcO4]− is observed inthe solid state for [(NpO2)2(TcO4)4·3H2O].11 However, onlymonodenate perrhenate coordination to neptunyl(V) is ob-served in the presence of the 1,10-phenanthroline co-ligand in[(NpO2(ReO4)(phen)(H2O)2].12 Of greater relevance is a previousreport on the synthesis of [UO2(ReO4)2(TBPO)2], where TBPO =tri-n-butylphosphine oxide, for which no structural study hadbeen undertaken. Intriguingly the authors assigned four bandsin the solid state infrared spectrum of this complex to [ReO4]−

m3 transitions which they attributed to the presence of bothmonodentate and bidentate coordinated perrhenate.13 We havefollowed up this investigation, and extended the study to TiBPand now report the structural and spectroscopic characterisationof two dimeric uranyl complexes with bridging perrhenate.

Results and discussion

Solid state structures

Both [UO2(l2-ReO4)(ReO4)(TBPO)2]2 (1) and [UO2(l2-ReO4)(ReO4)(TiBP)2]2 (2) could be readily prepared from

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Table 1 Crystallographic data for compounds 1·2CCl4 and 2

1 2

Empirical formula C50H108Cl8O24P4Re4U2 C48H108Re4O36P4U2

Formula weight 2721.70 2606.08Temperature/K 100(2) 100(2)Wavelength/A 0.71069 0.71069Crystal system Triclinic MonoclinicSpace group P−1 P21/nUnit cell dimensionsa/A 15.002(3) 26.129(5)b/A 16.580(3) 10.1329(18)c/A 17.205(3) 30.099(5)a/◦ 83.219(3) 90b/◦ 86.142(3) 93.113(5)c /◦ 78.133(3) 90Volume/A3 4154.6(12) 7957(2)Z 2 4Absorption coefficient/mm−1 10.084 10.276F(000) 2560 4912Crystal size/mm 0.25 × 0.2 × 0.15 0.22 × 0.06 × 0.04Theta range for data collection/◦ 1.64–28.42 2.12–26.39Reflections collected/unique 46219/18983 57489/16241Absorption correction Multi-scan Multi-scanData/restraints/parameters 18983/906/825 16241/0/871Goodness-of-fit on F 2 1.053 0.998Final R indices [I >2r(I)]R1 0.0723 0.0360wR2 0.1729 0.0715R indices (all data)R1 0.0962 0.0572wR2 0.1876 0.0781

the reaction between [UO2(ReO4)2·H2O] and two equivalentsof either TBPO or TiBP in EtOH. Crystals suitable for X-raydiffraction could be grown from CCl4, for 1 or hexane, for 2.Selected crystallographic data for both complexes are presentedin Table 1.

[UO2(l2-ReO4)(ReO4)(TBPO)2]2·2CCl4, 1·2CCl4 crystalliseswith two half molecules of structurally similar dimeric, uranylcomplexes in the unit cell, see Fig. 1 (1a) and Fig. 2 (1b). Thepentagonal bipyramidal uranyl groups are bridged by a pair

Fig. 1 Structure of 1a. The asterisk in the atom labels representssymmetry generated atoms (2 − x, 2 − y, −z).

Fig. 2 Structure of 1b. The asterisk in the atom labels representssymmetry generated atoms (2 − x, 2 − y, −z).

of [ReO4]− anions in a centrosymmetric dimer. The equatorialcoordination at each U is completed by two TBPO ligands adjacentto each bridging [ReO4]− and a terminal monodentate [ReO4]−

approximately trans to the U–U direction. There are two moleculesof CCl4 in the unit cell, remote from the coordination sphere ofthe U. Selected bond distances and angles are given in Table 2 (1a)and Table 3 (1b).

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Table 2 Selected bond lengths [A] and angles [◦] for 1a

U(1)–O(1) 2.300(9) Re(1)–O(3) 1.727(10)U(1)–O(2) 2.329(9) Re(1)–O(4) 1.708(10)U(1)–O(3) 2.451(10) Re(1)–O(5) 1.693(15)U(1)–O(6*) 2.457(11) Re(1)–O(6) 1.737(12)U(1)–O(7) 2.355(10) Re(2)–O(7) 1.757(11)U(1)–O(11) 1.763(10) Re(2)–O(8) 1.695(12)U(1)–O(12) 1.751(11) Re(2)–O(9) 1.701(11)

Re(2)–O(10) 1.740(10)

O(11)–U(1)–O(12) 178.0(5) U(1)–O(3)–Re(1) 155.3(6)O(12)–U(1)–O(1) 89.9(4) U(1)–O(6*)–Re(1*) 157.1(7)O(12)–U(1)–O(2) 90.6(4) O(3)–Re(1)–O(4) 107.9(5)O(12)–U(1)–O(7) 90.0(5) O(3)–Re(1)–O(5) 109.1(8)O(12)–U(1)–O(3) 89.7(5) O(3)–Re(1)–O(6) 110.4(6)O(12)–U(1)–O(6*) 88.1(5) O(4)–Re(1)–O(5) 108.8(8)O(1)–U(1)–O(7) 74.5(4) O(4)–Re(1)–O(6) 109.7(6)O(7)–U(1)–O(2) 74.9(3) O(5)–Re(1)–O(6) 111.0(7)O(2)–U(1)–O(3) 71.5(3) O(7)–Re(2)–O(8) 110.2(6)O(1)–U(1)–O(6*) 71.0(4) O(7)–Re(2)–O(9) 109.9(6)O(3)–U(1)–O(6*) 68.1(4) O(7)–Re(2)–O(10) 108.5(5)U(1)–O(7)–Re(2) 153.5(6) O(8)–Re(2)–O(9) 109.0(6)U(1)–O(1)–P(1) 145.4(6) O(8)–Re(2)–O(10) 109.5(5)U(1)–O(2)–P(2) 150.8(6) O(9)–Re(2)–O(10) 109.8(5)

Table 3 Selected bond lengths [A] and angles [◦] for 1b

U(2)–O(13) 2.297(9) Re(4)–O(19) 1.727(10)U(2)–O(14) 2.293(9) Re(4)–O(20) 1.704(11)U(2)–O(15) 2.406(10) Re(4)–O(21) 1.698(10)U(2)–O(19) 2.463(9) Re(4)–O(22) 1.738(11)U(2)–O(22*) 2.443(11) Re(3)–O(15) 1.748(11)U(2)–O(23) 1.772(10) Re(3)–O(16) 1.715(11)U(2)–O(24) 1.765(9) Re(3)–O(17) 1.703(12)

Re(3)–O(18) 1.697(12)

O(23)–U(2)–O(24) 179.3(5) U(2)–O(19)–Re(4) 174.5(6)O(24)–U(2)–O(13) 91.1(4) U(2)–O(22*)–Re(4*) 169.8(6)O(24)–U(2)–O(14) 92.3(14) O(19)–Re(4)–O(20) 109.9(5)O(24)–U(2)–O(15) 88.1(4) O(19)–Re(4)–O(21) 109.4(5)O(24)–U(2)–O(19) 87.1(4) O(19)–Re(4)–O(22) 111.2(5)O(24)–U(2)–O(22*) 90.9(4) O(20)–Re(4)–O(21) 107.7(5)O(13)–U(2)–O(15) 74.3(4) O(20)–Re(4)–O(22) 109.4(6)O(15)–U(2)–O(14) 74.6(4) O(21)–Re(4)–O(22) 109.2(5)O(14)–U(2)–O(19) 71.8(3) O(15)–Re(3)–O(16) 109.9(5)O(13)–U(2)–O(22*) 71.7(4) O(15)–Re(3)–O(17) 109.2(6)O(19)–U(2)–O(22*) 68.1(3) O(15)–Re(3)–O(18) 110.2(6)U(2)–O(15)–Re(3) 144.5(6) O(16)–Re(3)–O(17) 109.3(6)U(2)–O(13)–P(3) 154.3(6) O(16)–Re(3)–O(18) 109.3(6)U(2)–O(14)–P(4) 153.7(6) O(17)–Re(3)–O(18) 108.9(7)

The U=O distances of 1a and 1b (average 1.757 and1.768 A, respectively) are essentially the same, and compare wellwith the equatorially 6-coordinate trans-[UO2(NO3)2(TBPO)2](1.742(7) A)14 and equatorially 5-coordinate [UO2(ReO4)2-(TPPO)3] (1.759(4) and 1.756(4) A).2 The uranyl ion is slightlydistorted from linear in 1a (178.0(5)◦), although the O=U=Obond angle is essentially linear in 1b (179.3(5)◦). The U–OTBPO

distances in compounds 1a and 1b are statistically equivalent(2.293(9)–2.329(9) A), although only the longest bond length,found in 1a, (U1–O2, 2.329(9) A) is comparable to U–O(TBPO) inthe related compound, trans-[UO2(NO3)2(TBPO)2]14 (2.347(6) A),suggesting that overall, the TBPO groups are more tightly boundin 1a and 1b than in the nitrate analogue. This may be a resultof the differences in equatorial coordination number for theuranyl complexes in 1, five, and trans-[UO2(NO3)2(TBPO)2], six.The bridging U–O(perrhenate) and Re–O(uranyl) bond distances for 1a

and 1b are the same within error (U–O, 2.443(11)–2.463(9) A;Re–O, 1.727(10)–1.738(11) A). In 1a the U–O(perrhenate) bondlength for monodentate coordinated perrhenate, 2.355(10) A, issignificantly shorter than the two U–O(perrhenate) bond lengths forthe two bridging perrhenate anions. A comparison of the bridgingO–Re–O bond angles shows that the perrhenate molecules areslightly distorted, but to a similar degree (1a, 107.9(5)–111.0(7)◦;1b, 107.7(5)–111.2(5)◦). This is expected, and the range iscomparable to that in the bridging perrhenate bond angles inpolymeric [(1,5-C8H12)Rh(l-g2-ReO4)]n

15 (107.4(10)–111.8(9)◦).1a and 1b differ in the degree of twisting in the perrhenate

bridge, and this is directly shown by the separation of the uraniumcentres (1a, 6.802(12) A; 1b, 6.909(9) A). The U–O–Re(bridging) bondangles in 1a and 1b (1a, 155.3(6) and 157.1(7)◦; 1b, 169.8(8) and174.5(6)◦) are significantly different from each other, and also, thetwo angles in 1b are dissimilar. This is not unexpected as the twoRh–O–Re(bridging) angles in [(1,5-C8H12)Rh(l-g2-ReO4)]n (157.0(1)and 176.0(1)◦)15 are also significantly different, one being almostlinear and the other substantially bent. Shapley et al.15 hypothesisethat this is consistent with a largely r-bonding interaction ofone O-atom with the metal centre, and a largely p-bondinginteraction from the other. However, there are no conclusiveexperimental data, and this is not likely to be the case for uranyl.A recent theoretical study indicates that there is no evidencefor p-bonding in the equatorial plane of uranyl when stronglycoordinating hydroxide ligands are present thus this is veryunlikely to be the case with more weakly coordinating perrhenate.16

The dissimilarity of the Rh–O–Re(bridging) angles appears to besuitable for maintaining favourable O–M–O bond angles at themetal centres, although the existence of two similar U–O–Re(bridging)

bond angles in 1a shows that further studies of compoundscontaining bridging perrhenates are necessary. In the case of 1b,the dissimilarity in the perrhenate bridge could suggest that thecoordination sphere at the uranium centre is slightly strained. Thiswould account for the slightly longer terminal U–O(perrhenate) bondfound in 1b (2.406(10) A) compared to the corresponding bond in1a (2.355(10) A).

Single crystal X-ray diffraction has revealed two separatestructures for compound 2. The connectivity has been estab-lished for [UO2(ReO4)2(H2O)(TiBP)2], 2′, see below, a disordered,monomeric, pentagonal bipyramidal uranyl complex containingtwo TiBP, one H2O and two cisoid [ReO4]− ligands in anequatorially 5-coordinate plane. However, the structure could notbe fully resolved and this species was observed to be the minorproduct.

A second structure, the major product, has been fully crys-tallographically characterised as [UO2(l2-ReO4)(ReO4)(TiBP)2]2

(2). 2 crystallises with two half molecules of structurally similardimeric, cationic uranyl complexes in the unit cell, see Fig. 3 (2a)

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Fig. 3 Structure of 2a. The asterisk in the atom labels representssymmetry generated atoms (2 − x, 2 − y, −z).

and Fig. 4 (2b). The pentagonal bipyramidal uranyl groups areconnected by a pair of bridging perrhenates in a centrosymmetricdimer. Two TiBP ligands adjacent to each bridging [ReO4]−, anda terminal monodentate [ReO4]− approximately trans to the U–U direction complete the equatorial coordination sphere of eachU. The bond connectivity is very similar to that found in 1aand 1b, the difference being the coordination of TiBP insteadof TBPO. Selected bond distances and angles are given in Table 4(2a) and Table 5 (2b).

Fig. 4 Structure of 2b. The asterisk in the atom labels representssymmetry generated atoms (2 − x, 2 − y, −z).

The U=O distances of 2a and 2b (averages 1.760 and1.759 A, respectively) are essentially the same and comparewell with the U=O distances in equatorially 6-coordinate trans-[UO2(NO3)2(TiBP)2] (1.758(3) A)17 and 1a and 1b. The uranylion is distorted from strict linearity in both 2a (177.5(2)◦)and 2b (177.5(2)◦), as is expected for the majority of uranylcomplexes.18 Compounds 2a and 2b each contain one long andone short U–O(TiBP) bond (2a, 2.327(5) and 2.376(4) A; 2b,2.355(4) and 2.391(4) A). Only the shortest distance is significantlydifferent from that in the analogous nitrate complex, trans-[UO2(NO3)2(TiBP)2] (2.371(4) A).17 The U–O(TiBP) distance in therelated equatorially 5-coordinate complex, [UO2(TTA)2(TiBP)]

Table 4 Selected bond lengths [A] and angles [◦] for 2a

U(1)–O(1) 1.757(5) Re(1)–O(3) 1.745(5)U(1)–O(2) 1.763(5) Re(1)–O(4) 1.741(5)U(1)–O(3) 2.464(5) Re(1)–O(5) 1.707(6)U(1)–O(4*) 2.444(5) Re(1)–O(6) 1.691(5)U(1)–O(7) 2.327(5) Re(2)–O(7) 1.754(5)U(1)–O(11) 2.376(4) Re(2)–O(8) 1.698(5)U(1)–O(15) 2.366(5) Re(2)–O(9) 1.695(6)U(1*)–O(4) 2.444(5) Re(2)–O(10) 1.707(6)

O(1)–U(1)–O(2) 177.5(2) O(3)–Re(1)–O(4) 110.7(2)O(1)–U(1)–O(7) 90.1(2) O(3)–Re(1)–O(5) 108.8(3)O(1)–U(1)–O(11) 91.00(19) O(3)–Re(1)–O(6) 109.6(2)O(1)–U(1)–O(15) 91.10(19) O(4)–Re(1)–O(5) 109.6(3)O(7)–U(1)–O(11) 74.09(16) O(4)–Re(1)–O(6) 109.0(3)O(7)–U(1)–O(15) 74.41(17) O(5)–Re(1)–O(6) 109.1(3)O(1)–U(1)–O(3) 87.7(2) O(7)–Re(2)–O(8) 109.9(3)O(1)–U(1)–O(4*) 87.3(2) O(7)–Re(2)–O(9) 109.7(3)O(3)–U(1)–O(4*) 68.67(16) O(7)–Re(2)–O(10) 111.1(3)U(1)–O(3)–Re(1) 138.2(2) O(8)–Re(2)–O(9) 109.7(3)U(1*)–O(4)–Re(1) 150.6(3) O(8)–Re(2)–O(10) 107.7(3)U(1)–O(7)–Re(2) 170.2(3) O(9)–Re(2)–O(10) 108.7(4)U(1)–O(11)–P(1) 151.6(3)U(1)–O(15)–P(2) 163.0(3)

Table 5 Selected bond lengths [A] and angles [◦] for 2b

U(2)–O(19) 1.758(5) Re(3)–O(21) 1.758(5)U(2)–O(20) 1.760(5) Re(3)–O(22) 1.718(6)U(2)–O(21) 2.349(5) Re(3)–O(23) 1.704(6)U(2)–O(25) 2.432(4) Re(3)–O(24) 1.709(5)U(2)–O(26*) 2.447(5) Re(4)–O(25) 1.752(5)U(2)–O(29) 2.391(4) Re(4)–O(26) 1.738(5)U(2)–O(33) 2.355(4) Re(4)–O(27) 1.715(5)U(2*)–O(26) 2.447(5) Re(4)–O(28) 1.697(5)

O(19)–U(2)–O(20) 177.5(2) O(21)–Re(3)–O(22) 110.9(3)O(19)–U(2)–O(21) 92.2(2) O(21)–Re(3)–O(23) 109.9(3)O(19)–U(2)–O(29) 91.16(19) O(21)–Re(3)–O(24) 108.9(3)O(19)–U(2)–O(33) 89.00(19) O(22)–Re(3)–O(23) 108.7(3)O(21)–U(2)–O(29) 72.46(16) O(22)–Re(3)–O(24) 109.2(3)O(21)–U(2)–O(33) 72.42(17) O(23)–Re(3)–O(24) 109.2(3)O(19)–U(2)–O(25) 89.93(19) O(25)–Re(4)–O(26) 110.1(2)O(19)–U(2)–O(26*) 89.97(19) O(25)–Re(4)–O(27) 109.3(2)O(25)–U(2)–O(26*) 70.03(15) O(25)–Re(4)–O(28) 108.8(2)U(2)–O(25)–Re(4) 146.1(2) O(26)–Re(4)–O(27) 109.2(3)U(2*)–O(26)–Re(4) 165.1(3) O(26)–Re(4)–O(28) 109.8(2)U(2)–O(21)–Re(3) 159.3(3) O(27)–Re(4)–O(28) 109.5(3)U(2)–O(29)–P(3) 146.9(3)U(2)–O(33)–P(4) 147.7(3)

(2.403(7) A)19 (where TTA = thenoyltrifluoroacetonate) is onlysimilar to the longest bond in 2b, while the Eu–O(TiBP) distancein [Eu(PMBP)3(TiBP)2] (2.35(3) A)20 (where PMBP = 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone) is similar to all of the U–O(TiBP)

distances in 2a and 2b.Both 2a and 2b have effectively symmetrical perrhenate bridges

(U–O(perrhenate) 2a, 2.444(5) and 2.464(5) A; 2b, 2.432(4) and2.447(5) A), and a comparison of the bridging O–Re–O bondangles shows that as in 1a and 1b, the perrhenate molecules areslightly distorted but to a similar degree (2a, 108.8(3)–110.7(2)◦;2b, 108.8(2)–110.1(2)◦). In a situation similar to that found for1a and 1b, the difference between 2a and 2b lies primarily in thedegree of twisting in the perrhenate bridge, which can be seenin the separation of the uranium centres (2a, 6.580(13) A; 2b,6.686(11) A). 2a has slightly more acute U–O–Re bridging angles

1606 | Dalton Trans., 2007, 1603–1610 This journal is © The Royal Society of Chemistry 2007

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than 2b (2a, 138.2(2) and 150.6(3)◦; 2b, 146.1(2) and 165.1(3)◦),and it can be noted that unlike in 1a and 1b, the bridging anglesin both compounds are all significantly different from each other.The perrhenate bridge appears to be quite flexible, twisting toaccommodate more favourable solid state packing. The terminalU–O(perrhenate) bonds in 2a and 2b are significantly different fromeach other (2.327(5) and 2.349(5) A, respectively), but are bothsimilar to the terminal U–O(perrhenate) distance in 1a (2.355(10) A),and are significantly shorter than the equivalent distance in 1b(2.406(10) A). They are also significantly shorter than the U–O(perrhenate) bond lengths for the bridging perrhenates in 2a and 2b(range between 2.432(4)–2.464(5) A). The U–O–Re(terminal) bond an-gles are markedly different from each other in 2a and 2b (170.2(3)and 159.3(3)◦, respectively) while the terminal perrhenates in both2a and 2b have regular Re–O(uncoordinated) bond lengths (2a, 1.695(6)–1.707(6) A; 2b, 1.704(6)–1.718(6) A), as also observed in 1a and1b. The O–Re–O bond angles are slightly distorted in 2a and 2b(2a, 107.7(3)–111.1(3)◦; 2b, 108.7(3)–110.9(3)◦), unlike those in 1aand 1b that are essentially the same (1a, 108.5(5)–110.2(6)◦; 1b,108.9(7)–110.2(6)◦). The [ReO4]− geometry found in 2a and 2b isobserved in a number of complexes containing terminally bondedperrhenates, [Cd(ReO4)2·(thiourea)4]21 (1.705(5)–1.722(4) A and107.9(2)–111.5(2)◦) and [Fe(ReO4)4(H2O)2]− 22 (1.69(1)–1.70(2) Aand 106.0(8)–111.7(6)◦). We have also observed varying degrees ofdistortion for monodentate coordinated perrhenate to {UO2}2+,UIV and ThIV.2–7

Vibrational spectroscopy

The full solid state infrared (in transmittance) and Raman (inabsorbance) spectra for compounds 1 and 2 are given in the ESI,†with selected infrared and Raman vibrations shown in Table 6.The P=O stretches for uncoordinated TBPO and TiBP are foundat 1155 and 1284 cm−1, respectively. On coordination to {UO2}2+,these values are expected to decrease significantly for both thephosphine oxide and phosphate.23 Coordinated P=O stretchesare assigned in the infrared spectrum at 1077 and 1166 cm−1 forcompounds 1 and 2, respectively. Other characteristic vibrationsfor the coordinated P=O donor ligands could be observed, butwill not be discussed further. For 1 and 2 bands associated withboth the symmetric and asymmetric uranyl stretching vibrations(m1 and m3) could also be assigned.

Table 6 Selected infrared and Raman vibrations for compounds 1 and 2

Vibration Technique 1a/cm−1 2a/cm−1

m1(U=O) IR 837 s 835 sRaman 841 s 835 s

m3(U=O) IR 933 s 933 sRaman 929 m 933 m

m1(Re-O) IR 966 vw 976 vwRaman 979 s, 987 s 980 s, 991 s

m2(Re-O) IR 332 w, sh —Raman 337 m 337 m

m3(Re-O) IR 862 s, 915 s, 950 m 860 s, 907 s, 954 mRaman 861 m, 869 m, 947 w 870 m, 952 w

m4(Re-O) IR 317 w 308 w, 318 wm(P=O)(coord) IR 1077 m 1166 m

a ‘−’ = not observed, ‘s’ = strong, ‘m’ = medium, ‘w’ = weak, ‘sh’ =shoulder, ‘b’ = broad, ‘v’ = very.

Perhaps of most interest were the four [ReO4]− vibrations, m1–m4.In the infrared spectrum of 1, m1 is weakly visible at 966 cm−1,m3 is split into two broad (862 and 915 cm−1) and one sharp(950 cm−1) band and in the far-IR, m2 and m4 are observed at 332(shoulder) and 317 cm−1, respectively. The broadness of many ofthese transitions may be due to the presence of two structurallydistinct complexes (1a and 1b). The Raman spectrum is betterresolved and complements the IR assignments with peaks at337 cm−1 for m2, 979 and 987 cm−1 for m1, and 861, 869 and 947 cm−1

for m3. A previous study of a {UO2}2+–[ReO4]−–TBPO complex13

gave unassigned IR vibrations at 837, 863, 912, 933 and 953 cm−1,which are consistent with the values obtained for 1 in the presentstudy and can be assigned to perrhenate and uranyl stretches. Itis also interesting to note that they assign the coordinated P=Ostretch to a band at 1080 cm−1, again in very good agreement withthis current study.

[UO2(l2-ReO4)(ReO4)(TiBP)2]2 (2) also contains both bridgingand terminal perrhenate groups, and both 1 and 2 have verysimilar IR and Raman spectra. The m1 region in the mid-IRis heavily masked by a strong absorption from the TiBP group(1019 cm−1), although a small peak at 976 cm−1 can be assigned tothe m1 vibration. Peaks at 860, 907 and 954 cm−1 for m3 are clearlyvisible, but shoulders on some of these peaks suggest substantialoverlapping of vibrations. Weak vibrations in the far-IR at 308 and318 cm−1 are both low enough energy to be assigned to m4. In thecomplementary Raman spectrum, m2 is clearly visible at 337 cm−1,m1 at 980 and 991 cm−1, and m3 at 870 and 952 cm−1. In both 1 and 2the observed [ReO4]− transitions are consistent with the presenceof both monodentate and bidentate perrhenate.

Compound 1 is very soluble in CD2Cl2 and produces rela-tively clear and well-defined solution state spectra (Fig. 5). Acoordinated m(P=O) stretch is found in the IR spectrum at1086 cm−1, compared to the same stretch in the solid state complexat 1077 cm−1. The m(P=O) stretch for uncoordinated TBPO inCD2Cl2 solution has been observed at 1150 cm−1. The symmetricuranyl stretch (m1) is clearly visible at 839 and 840 cm−1 in theinfrared and Raman spectra, respectively, at values similar to thosefound in the solid state spectra (837 and 841 cm−1, respectively).The asymmetric uranyl stretch (m3) is visible at 938 and 930 cm−1

in the infrared and Raman spectra, respectively. This compareswell with the stretches found in the equivalent solid state IR andRaman spectra at 933 and 929 cm−1, respectively. m1(Re–O), a veryweak vibration in the solid state infrared spectrum is not observedin the solution spectrum, but is clearly split into two peaks in theRaman spectrum at 980 and 988 cm−1, complementing the solidstate splitting at 979 and 987 cm−1. m3(Re–O) can be attributed tovibrations in the infrared spectrum at 864, 910 and 954 cm−1 andin the Raman spectrum at 867 and 952 cm−1, again similar to thosefound in the solid state.

Compound 2 also produces two reasonably well-resolved vibra-tional spectra in CD2Cl2 solution that have very similar spectralfeatures to compound 1 dissolved in the same solvent (Fig. 5).The “triplet” of peaks in the IR spectrum of 1 at 910, 938and 954 cm−1 are observed in 2 at 908, 941 and 957 cm−1,and can be assigned to perrhenate (m3), asymmetric uranyl (m3),and perrhenate (m3) vibrations, respectively. A complementaryasymmetric uranyl stretch (m3) is found in the Raman spectrumat 934 cm−1. The symmetric uranyl stretch (m1) is clearly visible inthe IR and Raman spectra at 839 and 840 cm−1, respectively. A

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Fig. 5 A comparison between the solution (continuous trace) and solid state (dashed line) IR spectra of 1 (left) and 2 (right) in the m1 and m3 {UO2}2+

and [ReO4]− stretching regions. * = CD2Cl2 peak.

coordinated m(P=O) stretch is found at 1186 cm−1, compared tothe equivalent stretch in the solid state complex at 1166 cm−1. Them(P=O) stretch for uncoordinated TiBP in CD2Cl2 solution hasbeen observed at 1282 cm−1 (and in the solid state at 1284 cm−1).Vibrations from CD2Cl2 appear at 896 cm−1 in the IR and Ramanspectra, and from TiBP in the Raman spectrum at 829 cm−1. Anadditional perrhenate m3 vibration is observed in the solution IRat 860 cm−1, with perrhenate vibrations in the solution Ramanspectrum observed at 982 and 992 cm−1 (m1) and 870 cm−1 (m3),consistent with those found in the solid state.

Both dimeric compounds 1 and 2 have very similar, well-resolvedsolution state spectra. Similarities between the number of bandsassigned to uranyl and perrhenate vibrations, and their positionsin the solution and solid state IR and Raman spectra, suggestthat in 1 and 2, perrhenate coordinates to uranyl in solution.Since the symmetric and asymmetric uranyl stretches of 1 and2 are assigned to similar positions in the solid and solution statespectra, the equatorial coordination number of the uranyl group islikely to remain the same. However, the coordinated P=O stretches(only visible in the IR spectra of 1 and 2) differ between solid andsolution states by 9 and 20 cm−1 for 1 and 2, respectively. This mayindicate that either the organic ligands are labile in solution and anaverage P=O environment is observed on the IR timescale, or thatthe ligands remain attached to the uranyl group, and the overallgeometry of the complexes changes, the most likely change beingdimer dissociation in solution to form monomers. However, bridgeopening and closing or ring flipping within the dimeric unit cannotbe ruled out. Clearly solution IR evidence alone does not provideconclusive evidence for the stability of the dimeric complexes 1and 2 in solution.

UV/vis absorption spectroscopy

In the solid state both 1 and 2 display the typical vibrationalfine structure associated with the uranyl ion charge-transfer band,in the region 350–500 nm with the highest intensity bands (kmax)at 431 and 430 nm. Nine other distinct bands are visible in therange 365–490 nm, with the average splitting between the highest

intensity bands and the two bands adjacent to these equal to 703and 705 cm−1 for compounds 1 and 2, respectively. Very similarspectral features are also observed in CD2Cl2 solution, kmax at 430and 429 nm for 1 and 2. Eleven other distinctive bands are observedwith the average splitting between the highest intensity bands andthe two bands adjacent to these equal to 704 and 708 cm−1 forcompounds 1 and 2, respectively. As the solid and solution statespectra are very similar for both 1 and 2 UV/vis spectroscopyprovides evidence that the uranyl coordination environment doesnot change greatly on dissolution.

31P NMR solution spectroscopy

The variable temperature 31P NMR spectra of 1 and 2 in CD2Cl2

have also been recorded (see ESI†). The spectrum of 1 at 273 Khas one broad 31P peak at 74.5 ppm. A single peak would expectedfrom the 4 phosphorus environments that would be essentiallyequivalent if 1 retained the solid state connectivity on dissolutionin CD2Cl2. This peak is not assigned to free TBPO (which hasbeen observed at 48.4 ppm), suggesting coordination of the TBPOligands to the metal centre. On lowering the temperature to 233 K,a minor resonance becomes visible at 71.2 ppm. The broadeningof this peak at 213 K and subsequent appearance of at least 5separate, low intensity, resonances between 70.6 and 73.8 ppm at183 K is typical of freezing out exchange processes on cooling, inwhich the rate of a dynamic process is altered by controlling thetemperature.

As with the vibrational spectra, 2 dissolved in CD2Cl2 producessimilar spectra to 1. Again, all 4 phosphorus environments areessentially the same in solution, and this is reflected by the singlebroad peak at 1.85 ppm at 253 K. This peak shifts downfieldslightly by 0.19 ppm as the temperature decreases to 183 K. Overthis temperature range, a number of minor peaks become visiblein the range 1.79–2.09 ppm, the result of indeterminate dynamicexchange processes. However, compared to 1, these minor peaksare at greater intensity in comparison with the major resonance,suggesting greater instability of the dimer in solution. Free TiBPin CD2Cl2 produces a single 31P peak at 0.2 ppm.

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Experimental

Caution

Natural uranium was used during the course of these experiments.As well as the radiological hazards uranium is a toxic metal andcare should be taken with all manipulations.

General

Solid and solution state infrared measurements were collected ona Bruker Equinox 55 spectrometer using a Golden Gate attenu-ated total reflectance (ATR) attachment (resolution 4 cm−1) fortypically 64 scans. A Bruker FRA 106/S Raman instrument witha coherent laser (1064 nm, 500 mW) was used to collect the Ramanscattering from the solid samples held in the laser beam path orsolution samples in a mirrored cuvette. Solid samples were run for32 scans with ca. 30 000 scans used for the solution measurements.Variable temperature 31P NMR solution spectra were recordedon a Bruker Avance UltrashieldTM 400 MHz spectrometer at162 MHz for 64 scans between 183 and 273 K. Both solid andsolution samples were referenced to H3PO4 (85%). Solution UV-vis spectra were recorded on a double-beam Cary Varian 500 ScanUV-vis-NIR spectrophotometer. Solid state diffuse reflectancespectra were recorded on an Ocean Optics SD2000 spectrometerwith a fibre optic probe using a deuterium–halogen DH-200 lightsource. BaSO4 was used as a reference. Elemental analysis wascarried out using a Carlo Erba Instruments CHNS-O EA1108Elemental Analyser (C, H and N analysis) and a Fisons HorizonElemental Analysis ICP-OED spectrometer (metal analysis). Allsolution spectroscopic measurements of 1 and 2 were all recordedin CD2Cl2 (0.2 mmol dissolved in 2 cm3).

Synthesis

All chemicals were obtained from standard chemical suppliersand used as received apart from UO3 which was supplied by NexiaSolutions. The synthesis of [UO2(ReO4)2·H2O] was undertaken inaccordance with the literature method through the addition of twoequivalents of HReO4 to UO3.10

Synthesis of bis[(l2-perrhenato)(perrhenato)bis(tri-n-butylphos-phine oxide)dioxouranium(VI)], [UO2(l2-ReO4)(ReO4)(TBPO)2]2

(1). [UO2(ReO4)2·H2O] (0.70 mmol, 0.55 g), was dissolved inEtOH (2 cm3) and subsequent dropwise addition of a stoichio-metric amount of TBPO (1.40 mmol, 0.305 g) dissolved in EtOH(2 cm3) led to the formation of a yellow solution, which onevaporation yielded a yellow precipitate. Crystallisation fromCCl4 (5 cm3) away from direct sunlight produced yellow platecrystals, which were collected by filtration and dried in air, yield0.72 g, 85%. (Elemental analysis of the powdered sample. Calc. forC48H108O24P4Re4U2: C, 23.84; H, 4.47; P, 5.13; Re, 30.96; U, 19.70.Found: C, 23.58; H, 4.39; P, 4.96; Re, 31.12; U, 19.76%.)

Synthesis of bis[(l2-perrhenato)(perrhenato)bis(tri-iso-butylphosphate)dioxouranium(VI)], [UO2(l2-ReO4)(ReO4)(TiBP)2]2 (2).[UO2(ReO4)2·H2O] (0.70 mmol, 0.55 g), was dissolved in EtOH(2 cm3) and subsequent dropwise addition of a stoichiometricamount of TiBP (1.40 mmol, 0.378 cm3) led to the formation ofa yellow solution, which on evaporation yielded an oily yellow

solid. Crystallisation from hexane (5 cm3) away from directsunlight produced yellow plate crystals, which were collected byfiltration and dried in air, yield 0.65 g, 70%. (Elemental analysis.Calc. for C24H54O18P2Re2U: C, 21.79; H, 4.24; P, 4.69; Re, 28.29;U, 18.00. Found: C, 22.09; H, 4.14; P, 4.75; Re, 28.68; U, 18.25%.)

Crystallography

Selected crystallographic data for compounds 1 and 2 are sum-marised in Table 1. The diffraction data were measured on a BrukerAPEX SMART platform CCD at 100 K. The structures weresolved by direct methods using SIR97 (1)24 and SHELXS97 (2)25

and refined by full-matrix least squares using SHELXL97.25 Allnon-hydrogen atoms were refined anisotropically, while hydrogenatoms were treated by constrained refinement. Restraints wereused on the anisotropic thermal motion in the structure ofcompound 1. CCDC reference numbers 623014 and 623015. Forcrystallographic data in CIF or other electronic format see DOI:10.1039/b614481k

Conclusions

Two structurally very similar dimeric uranyl complexes, withtwo bridging perrhenate anions and two monodentate perrhenateanions balancing the charge, have been synthesised. Two P=Odonor ligands also coordinate to each uranyl cation yielding 7-coordinate complexes, [UO2(l2-ReO4)(ReO4)(TBPO)2]2 (1) and[UO2(l2-ReO4)(ReO4)(TiBP)2]2 (2). These complexes represent thefirst structurally characterised examples of perrhenate bridgingtwo actinide metal cations in the presence of a P=O donorligand. Comparison of the solution and solid state vibrationand absorption spectroscopy, alongside a variable temperature31P solution NMR study, gives clear evidence for both perrhenateand P=O donor ligand coordination in solution. However, it wasnot possible to state definitively that both 1 and 2 remained intactin solution. In addition, the partial structural refinement of amonomeric complex, [UO2(ReO4)2(H2O)(TiBP)2] (2′), as a minorproduct in the reaction to form 2 indicates that monomeric speciescan also exist in solution. Coupled with our previous actinideperrhenate and pertechnetate coordination chemistry studies wenow have a better understanding of the potential behaviour ofpertechnetate as a coordinating ligand in PUREX process solventsystems.

Acknowledgements

We thank the EPSRC, the Research and Technology Group atBNFL (now Nexia Solutions) and the Nuclear DecommissioningAuthority for funding.

References

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