Phosphine oxide complexes of the actinide(IV) thiocyanates

J. inorg,nucl.Chem., 1973,Vol.35, pp. 150I-1507. PergamonPress. Printedin Great Britain

PHOSPHINE OXIDE COMPLEXES OF THE ACTINIDE(IV) THIOCYANATES Z. M. S. A L - K A Z Z A Z and K. W. B A G N A L L

Chemistry Department, The University of Manchester, M 13 9PL and D. BROWN

Chemistry Division, U.K.A.E.A., A.E.R.E., Harwell, Didcot, Berks (Received 3 A ugust 1972)

Al~a'aet-Phosphine oxide complexes of composition M(NCS)4(R3PO)4 (R = Me, M = U, Np, Pu; R = NMe2, M = Th,U,Np,Pu; R = Ph, M = Th,U,Np), Th(NCS)4(tmpo)6 and M(NCS)4(ompa)2 (M = Th,U,Np) have been prepared from the corresponding phosphine oxide complexes of the actinide tetrachlorides. The tppo and ompa complexes of Pu(NCS)4 were prepared in the same way but could not be obtained pure. Tiae i.r. and u.v./visible spectra of these compounds are reported. INTRODUCTION

ANIONIC thorium(IV)[1] and uranium(IV)[2, 3] octa-N-thiocyanatocomplexes are quite well known, but there is little published information[4] concerning oxygen donor complexes of the actinide tetrathiocyanates, the only examples of which appear to be the N,N-dimethylacetamide(dma) complexes, M(NCS)4(dma)4 (M = Th [5], U [6]) and hydrated thorium(IV) thiocyanate, [M(NCS)4(H20)4] [1]. Monodentate phosphine oxides usually form complexes of the type MCI4(R3PO)2 with the actinide tetrachlorides [7-10], except where R is CH3, in which case the hexakis complexes MCl4(tmpo)6 can be obtained [11]. In this work we have prepared a number of thorium(IV), uranium(IV), neptunium(IV) and plutonium(IV) analogues of these chloride complexes in order to ascertain whether the metal atom in the thiocyanate complexes can achieve a higher coordination number than is the case with the chloride compounds. Such a result is likely on size grounds if the thiocyanate group is nitrogen bonded to the metal atom.

The complexes The anhydrous actinide(IV) thiocyanates are not known; the thiocyanatephosphine oxide complexes were therefore prepared by treating the actinide 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

A. K. Molodkin and G. A. Skotnikova, Russ. J. inorg. Chem. 9, 32 (1964). V. P. Markov and E. N. Traggeim, R u s s . J . inorg. Chem. 6, 1175 (1961). I. E. Grey and P. W. Smith, A ust. J. Chem. 22, 311 (1969). K.W. Bagnali, M.T.P. Int. Rev. Sci. 7, 139 (1972). K.W. Bagnall, D. Brown, P. J. Jones and P. S. Robinson,J. chem. Soc. 2531 (1964). K. W. Bagnall, D. Brown and R. Colton, J. chem. Soc. 2527 (1964). P. Gans and B. C. Smith, J. chem. Soc. 4172 (1964). J. P. Day and L. M. Venanzi, J. chem. Soc. (A), 197 (1966). K. W. Bagnall, D. Brown, P. J. Jones and J. G. H. du Preez, J. chem. Soc. (A), 737 (1966). D. Brown, J. Hill and C. E. F. Rickard, J. chem. Soc. (A), 497 (1970). Z. M. S. AI-Kazzaz, K. W. Bagnall and D. Brown, J. inorg, nucl. Chem. 35, 1493 (1973). 1501

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z . M . S . AL-KAZZAZ, K. W. BAGNALL and D. BROWN

tetrachloride-phosphine oxide complex with potassium thiocyanate in a nonaqueous solvent in the presence of an excess of the ligand. T h e resulting thiocyanate complexes remained in solution, from which they crystallised on evaporation to small volume. T h e attempted preparation of Pu(NCS)4(tppo)4 and Pu(NCS)4 (ompa)2 by this procedure yielded a dark brown solid and a dark brown oil respectively. Neither compound could be obtained pure because of their very high solubility in organic solvents. All of the compounds prepared in this way were stable to at least 140°(2, but all of them decomposed at their melting points (Table 5). Some decomposition appears to occur when they are exposed to sunlight; the thorium compounds, for example, b e c o m e noticeably yellow on such exposure. N o n e of the complexes is noticeably hygroscopic and they are soluble in a variety o f organic solvents (Table l). Table 1. Solubilityof the complexesin organic solvents Complex Th(NCS)4(tmpo)e U(NCS)4(tmpo)4 Np(NCS)4(tmpo)4 Pu(NCS)4(tmpo)4 Th(NCS)4(hmpa)4 U(NCS)4(hmpa)4 Np(NCS)4(hmpa)4 Pu(NCS)4(hmpa)4 Th(NCS)4(tppo)4 U(NCS)4(tppo)4 Np(NCS)4(tppo)4 Th(NCS)4(ompa)2 U(NCS)4(ompa)2 Np(NCS)4(ompa)2

Dichloromethane Nitromethane -sol. -v. sol. -sol. sol. v. sol: v. sol. v. sol. sol. -sol. sol.

Methyl cyanide Acetone

sol. sol sol. v. sol. sol. sol. sol. v. sol. sol. sol. sol. sol. sol. sol.

sol. sol. sol. v. sol. sol. sol. sol. v. sol. sol. sol. sol. sol. sol. sol.

sol. sol. sol. v. sol. sol. sol. sol. v. sol. sol. sol. sol. sol. sol. sol.

Ethyl acetate insol. insol. insol. sl. sol. insol. sol. sol. -insol. sl. sol. -insol. sl. sol. --

Sol. = soluble; sl. = slightly; v. = very; insol. = insoluble. It is probable that the metal atom is 8 coordinate in the complexes M(NCS)4 (R3PO)4 and M(NCS)4(ompa)2. This compares with 6-coordination in the analogous MCI4(R3PO)2 and with possible 7-coordination in MCl4(ompa)l.5(M= U, Np, Pu), although this is not proved. T h e thorium(IV) N-thiocyanate complex with tmpo has the same tmpo: metal ratio, 6, as the analogous chloride complex, but there is a marked difference in stoicheiometry between the tmpo complexes of the chlorides and N-thiocyanates o f uranium(IV), neptunium(IV) and plutonium(IV), the former being of composition MC14(tmpo)6 and the latter M(NCS)4 (tmpo)4, which does not fit the pattern observed with the other phosphine oxide ligands. H o w e v e r , the coordination number of the metal atoms in the complexes MCl4(tmpo)6 is unknown, and it may be that in the thiocyanates the linear N C S ions are acting as a framework within which only four phosphine oxide ligands can normally be packed in the cases o f the smaller actinide(IV) ions from uranium onwards.

Phosphine oxide complexes of the actinide(IV) thiocyanates

1503

X-ray powder photography showed that the tmpo complexes, M(NCS)4(tmpo)4, where M = U, Np and Pu, were isostructural and the plutonium compound was identified by this means. X-ray powder photographs of the other thiocyanate compounds, however, were of very poor quality and of no value for identification purposes.

Infrared spectra The i.r. spectra of the complexes (Table 2) show an appreciable shift in the P--O stretching vibration to lower frequency as compared with the free ligand. These shifts are appreciably less than those observed for the analogous chloride Table 2. Infrared spectra of the complexes Ve=o

colour tmpo Th(NCS)4(tmpo)e U(NCS)4(tmpo)4 Np(NCS)4(tmpo)4 Pu(NCS)4(tmpo)4 hmpa Th(NCS)4(hmpa)4 U(NCS)4(hmpa)4 Np(NCS)4(hmpa)4 Pu(NCS)4(hmpa)4

tppo Th(NCS)4(tppoh U(NCS)4(tppo)4 Np(NCS)4(tppo)4 ompa Th(NCS)4(ompa)2 U(NCS)4(ompa)2 Np(NCS)4(ompah

white grey-green yellow-brown light-brown white grey-green yellow-brown darkbrown white yellow-green yellow white yellow-green pale yellow

cm-' l161s 1093s, br 1100sh, 1088s, br 1082br 1100br 1203br 1088s, hr 1085s 1088s,br l130m, 1055m 1190s 1123br, 1063s 1068s l123br, 1063s 1233br 1180br l170w 1155m

Avp=o cm -~

YeN cm-'

68 61, 73 79 61

2050s, br 2022w, 2047s, 2073m 2030m, 2053s, 2080m 2020sh, 2045s, 2065w

115 118 115 73, 148

2058s, br 2050s 2072s, br 2015m, 2040w, 2060m

67, 127 122 67, 127

2025w, 2050s, 2070sh 2020m, 2045sh, 2055s, 2070w 2025s, 2055s

53 63 78

2050s, 2065sh 2025m, 2050m 2060s, br

s = strong; m = medium; w = weak; br = broad; sh = shoulder; v = very.

complexes, except in the case of Np(NCS)4(ompa)2 where the shift is almost identical with that in NpCl4(ompa),.s. Apart from this instance, the difference between the shifts in the chloride and thiocyanate complexes is consistent with the higher coordination number of the metal atom in the latter. It is difficult to establish unambiguously from the i.r. spectra whether the thiocyanate group is N or S bonded to the actinide metal. According to the "soft" and "hard" concepts of Pearson[12], one would expect the N C S - ion, in which nitrogen is "hard", to coordinate by that atom to "hard" acids, such as ChattAhrland Class A metals like actinides, whereas sulphur in the S C N - ion is "soft" and should therefore be the atom coordinated to class B metals. The C - N stretching frequency in the actinide(IV) thiocyanate complexes appears in the range 20152080 cm-' and is often a broad feature which is, in many cases, split into two, 12. R. G. Pearson, J. chem. Educ. 45, 581,643 (1968).

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Z . M . S . AL-KAZZAZ, K. W. B A G N A L L and D. BROWN

three or four componeiats. This C - N frequency range lies on the borderline for distinguishing between sulphur and nitrogen bonding in the thiocyanates[13], although the high relative intensity of the band in each case suggests that the thiocyanate groups are nitrogen bonded[14, 15]. The frequency of the C - S stretching vibration has also been used to diagnose the bonding mode in thiocyanates[16, 17]; in S bonded thiocyanates the frequency decreases to about 700 cm -1 whereas in N bonded thiocyanates it increases to between 760 and 880 cm -1. Unfortunately, the C-S band is nearly always of very low intensity and it is difficult to identify it in the p~esence of organic ligands [13]. We have tentatively identified this vibration in Th(NCS)4(tmpo)6, Th(NCS)4(tppo)4 and Th(NCS)4 (ompa)2 at 730, 760 and 790 cm -1 respectively, but have not been able to identify it in other complexes, so this means of identification is rather inconclusive. However, on the basis of the high relative intensity of the C - N bands, we consider that the thiocyanate group is almost certainly N bonded and have written the complexes in this form.

Ultraviolet/visible spectra The absorption spectra of the uranium(IV) complexes in solution in dichloromethane or nitromethane (Table 3) were very similar to those reported[6] for U(NCS)4(dma)4 and the complexes[18] U C I 4 . 4 R C N , and to the solid transmission spectrum of UCI4. The extinction coefficients for the most intense bands in these spectra were highest for the ompa complex and decreased in the order ompa > dma > tmpo > tppo > hmpa. The spectra of the analogous neptunium compounds (Table 4) resemble each other quite markedly and also resemble the solid transmission spectrum of NpC14. The ompa complex again has the highest Table 3. U ltraviolet/visible spectra of the uranium(IV) complexes (band positions 10-3 × frequency/cm-1) U(NCS),(hmpa)4 Band

~

14"81 15-50 15"80 16-81 18-02 20"20 20"61 21"51 21"98 22"73

93 55 42 28 20 36 33 33 35 30

13. 14. 15. 16. 17. 18.

U(NCS)4(tppo)4 U(NCS)4(ompa)z (in dichloromethane) Band ~ Band ~

U(NCS)4(tmpo), U(NCS)4(dma),e (in nitromethane) Band c Band

14.70 15"40 15"72 16-72 17"92 20"12

95 62 40 24 23 45

14.53 15'20

191 72

14'70 15"63

113 75

14"60 15-27

125 57

17-60 19.88

36 76

17"95 20"20

27 42

17"70 20.00

23 46

21"51 21"98 22.57

37 41 36

21"28 22-22 22-99

28 47 44

22.73

33

22"22

28

A. Sabatini and I. Bertini,lnorg. Chem. 4, 1665 (1965); 5, 1025 (1966). S. Fronaeus and R. Larsson, Acta chem. scand. 16, 1447 (1962). C. Pecile, lnorg. Chem. 5, 210 (1966). J. Lewis, R. S. Nyholm and P. W. Smith,J. chem. Soc. 4590 (1961). A. Turco and C. Pecile, Nature Lond. 191, 66 (1961). P. Gans and J. Marriage, J.C.S. Dalton 46 (1972).

Phosphine oxide complexes of the actinide(IV) thiocyanates Table 4. Ultraviolet/visible

WNCSMmpoh

spectra of the neptunium(IV) 1O-3 X frequency/cm-‘)

49 167 82 86 116 92 129 116 92 129 169

14.08 15.04 15.87 16.26 18.02 18.59 19.41 20.20

163 67 61 63 35 94 104 73

complexes (band positions

NpWCSMhmpah W(NCSMtppoh NpW34@w43

(in nitromethane) Band l 998 IO.20 10.63 10.75 1099 11.36 11.76 12.08 12.35 12.82 1348

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(in dichloromethane) Band e

Band

E

998 10.26

107 83

10.20

97

10.85 11.11 11.34 11.74 1198 12.35

163 149 123 120 128 189

10.85 11.05 11.27 11.68 12.00 12.22

152 139 111 113 107 156

13.24 13.79 13.89 14.93 16.00 17.45

209 149 144 72 75 49

13.28 13.72 13.91 14.93 15.92 17.39

172 137 130 67 61 35

18.87 19.61 20.41

99 85 88

18.59 19.08

35 98

Band

l

10.15

225

11.33 11.63 1199

119 103 134

12.74 13.39

134 213

13.89 14.71 16.08

217 106 %

19.23 20.00

161 130

extinction coefficients and for the most intense bands these decrease in the same order as in the uranium complexes. The extinction coefficients for the corresponding tetrachloride complexes, MCl,(R,PO),, were much smaller, as would be expected for a coordination geometry in which there is a centre of symmetry, and it is reasonable to conclude that the environment of the metal atom in the thiocyanate complexes is of low symmetry. Conductivity

measurements

The thorium(IV) thiocyanate complexes with tppo, hmpa and ompa are essentially non-electrolytes in nitromethane (AJoo,14f 1 ohm-’ mole-’ cm2 at 20°C) and the thorium atom is presumably 8 coordinate in these complexes. EXPERIMENTAL The compounds were prepared and handled in inert (nitrogen or argon) atmosphere dry boxes because of the radioactive hazards associated with the a-emitting nuclides z3’Np and psBpuwhich were used in this work. Materials The actinide tetrachloride

complexes with phosphine oxides [7-l 11, tmpo [ 191 and tppo [ 201 were prepared by published methods. Solvents were purified as previously described [l 1,2 l] and the liiands 19. A. B. Burgeand W. E. McKee, J. Am. them. Sot. 73,459O (1951). 20. P. Gans, Thesis, London University (1964). 21. D. Brown and P. J. Jones, J. them. Sot. (A), 247 (1967).

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Z . M . S . AL-KAZZAZ, K. W. BAGNALL and D. BROWN

hmpa and ompa were obtained from BDH Ltd. and Koch-Light Ltd., respectively and were used as supplied.

Preparation of the complexes The phosphine oxide complex of the actinide tetrachloride (0.160--0-445 g) was treated with the stoicheiometric quantity of potassium thiocyanate in acetone/methanol (2 : I v/v; 5 ml, ThCh(ompa)2, NpCh(tppo)2), methyl cyanide (5 ml, UCL~(tppo)z) or acetone (5 ml, all others) in the presence of an excess of the ligand. After removal of the precipitated potassium chloride, the supernatant was evaporated to small volume, whereupon the thiocyanate complexes crystallized on cooling. The products were recrystallized from acetone (tmpo, hmpa and ompa complexes of Th(NCSh, and tppo complexes of U(NCS)4 and Np(NCS)4), methyl cyanide (U(NCS)4(tmpo)4, Np(NCS)4(tmpo)4, Th(NCS)4(tppo)4), ethyl acetate (U(NCS)4(hmpa)4, Np(NCSh(hmpa)4) or a mixture of isopentane with dichloromethane (U(NCSh(ompa)z, Np(NCS)4(ompah) or with acetone (Pu(NCS)4(tmpo)4). Preparative yields, starting from the chloride complex, are given in Table 5. Table 5. Melting points, preparative yields and analysis

mp,°C Th(NCSh(tmpo)6 U(NCS)4(tmpo)4 Np(NCS)~(tmpoh Pu(NCS)4(tmpoh Th(NCS)4(hmpa)4 U(NCS)4(hmpa)4 Np(NCS)4(hmpa)4 Pu(NCS)4(hmpa)4 Th(NCS)4(tppo)4 U(NCS)4(tppo)4 Np(NCS)4(tppo)4 Th(NCS)4(ompa)z U(NCS)4(ompa)2 Np(NCS)4(ompa)2

232-240(d) 220-225(d) 238-242(d) 252-256(d) 164-167(d) 195-198(d) 226-230(d) 155-158(d) 145-147(d) 143-147(d) 164-166(d) 155-160(d)

Analysis Found (%) Required (%) M NCS M NCS

Preparative yield (%) 87 78 86 70 65 47 72 59 70 62 56 59 65 81

24.4 14.9 25.1 28.5 2 8 . 0 2 8 . 4 28"8 2 8 . 0 28.3 * 20.0 19.9 19.7 20.0 19.8 20.1 20.1 19.6 20-0 19.4 20.8 19.6 14-6 14.8 14.7 15"1 15-0 15.0 14-8 15.1 15.0 22.2 22.5 2 2 . 4 22.5 22-4 2 2 - 8 22.9 22.0 2 2 . 8

15-3 27-7 27.7 19.7 19.6 19-6 20.1 14.7 14-7 14.7 22-4 22.3 22.3

*Identified by X-ray powder photography; isostructural with the uranium and neptunium analogues. Table 6. Carbon, hydrogen, nitrogen, sulphur and phosphorus analyses

Th(NCS)4(hmpa)4 Found Required U(NCS)4(hmpa)4 Found Required Th(NCS)4(tppo)4 Found Required U(NCS)4(tppo)4 Found Required Th(NCS)4(ompa)2 Found Required

C

H

N

S

P%

28.6 28.5 28-0 28-3 58.8 57.9 57.6 57.7 22-9 23.2

6.1 6.1 6.0 6-1 3.9 3.8 3.7 3-8 4.9 4-6

18-8 19.0 19.0 18.9 3.8 3.5 3.6 3-5 16.1 16-2

9-8 10-5 10.9 10.8 8.9 8.1 8.4 8-1 ---

10-0 10.9 10.5 10.5 8.6 7.9 7-8 7-8 12-3 12-0

Phosphine oxide complexes of the actinide(IV) thiocyanates

1507

Analysis The metals were determined as previously described [22] and thiocyanate, which remained in the ammoniacal supernatant following precipitation of the metal as its hydroxide, was weighed as silver thiocyanate, precipitated from the acidified solution [23]. The results are summarised in Table 5. Carbon, hydrogen, nitrogen, sulphur and phosphorus were also determined for some of the thorium and uranium complexes by microcombustion techniques. These results are given in Table 6. Physical measurements I.R. spectra were recorded using either an Infrascan (4000-650 cm -1) or Grubb-Parsons DM4 spectrometer (650-225 cm -1) with the samples mounted as Nujol mulls between potassium bromide or polythene plates. Visible and near i.r. solution spectra were recorded (1800-400 nm) using a Carey model 14 spectrophotumeter; solid transmission spectra were recorded by means of the technique devised by Brown and Edwards [24] using this last instrument. X-ray powder diffraction photographs were obtained using a Debye-Scherrer 19 cm camera with CuK~ radiation (a~ = 1-54051 A); capillaries of neptunium and plutonium compounds were coated with Bostikote to contain the a-activity [25]. Conductivities were measured as described previously [26]. Acknowledgements-The authors are indebted to Dr. J. P. Day for helpful discussions, to Mr. W. I. Azeez for assistance with the preparation of U(NCS)4(tppo), and Mr. M. Hart for the combustion analyses. They also wish to thank the Science Research Council for a grant (BISR/7325) which covered the cost of using U.K.A.E.A., A.E.R.E, facilities for the work with neptunium and plutonium, and one of us (Z.M.S.A.-K) thanks the Iraq Ministry of Oil for the award of a scholarship. 22. 23. 24. 25. 26.

K.W. Bagnall, D. Brown, D. G. Holah and F. Lux, J. chem. Soc. (A), 465 (1968). A. L Vogel, Quantitative lnorganicAnalysis, p. 569. Longmans, London (1953). D. Brown and J. Edwards, To be published. K.W. Bagnall andJ. H. Freeman, J. chem. Soc. 4579 (1956). K. W. Bagnall, A. M. Dean, T. Markin, P. S. Robinson and M. A. A. Stewart, J. chem. Soc. 1611 (1961).