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Characterization, luminescence and crystal structure of uranyl nitrate complexes with diphenyl-N-ethylphosphine amide and tripiperidinephosphine oxide
www.elsevier.nl/locate/ica Inorganica Chimica Acta 306 (2000) 102 – 106
Note
Characterization, luminescence and crystal structure of uranyl nitrate complexes with diphenyl-N-ethylphosphine amide and tripiperidinephosphine oxide Alfonso R. de Aquino a, Gabriella Bombieri b,c,*, Paulo C. Isolani a, Geraldo Vicentini a, Julio Zukerman-Schpector a b
a Instituto de Quı`mica, Uni6ersidade de Sa˜o Paulo, Sa˜o Paulo, Brazil Istituto di Chimica Farmaceutica e Tossicologica, Uni6ersita` di Milano, Viale Abruzzi, 42, I-20131 Milan, Italy c Istituto di Chimica Farmaceutica, Uni6ersita` di Milano, Viale Abruzzi, 42, I-20131 Milan, Italy
Received 31 December 1999; accepted 27 March 2000
Abstract The uranyl complexes UO2(NO3)2L2 (L=DPEPA (1) diphenyl-N,N-ethylphosphine amide and TPPPO (2) tripiperidine oxide) have been prepared. Spectroscopic studies indicate that L is bound to the uranyl group via the phosphoryl oxygen atom. The crystal structures of (1) and (2) show a hexagonal-bipyramidal coordination about uranium. The luminescence properties of the compounds have been also investigated. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Uranyl complexes; Luminescence; Crystal structures
1. Introduction The spectroscopy of uranium compounds, particularly related to the uranyl ion, has been studied since the decade of 1940, especially because of the use of uranium as a nuclear fuel. Photoexcitation of the uranyl ion leads to a very strong oxidizer, and this fact allowed the operation of a successful actinometer [1]. Other applications of uranyl photoprocesses include solar collectors [2], photochemical conversion of solar energy [3] and photocatalysts [4,5]. Several reviews describe the spectroscopy [6], photophysics and photochemistry of this ion [7,8]. Analytical applications of the uranyl ion fluorescence resulted in very sensitive and selective methods for the determination of this ion in the environment and geologic samples [9].
* Corresponding author. Tel.: + 39-2-2950 2224; fax: + 39-2-2951 4197. E-mail address: [email protected] (G. Bombieri).
As a part of a study of luminescence properties of uranyl nitrate derivatives with phosphoryl ligands we have synthesized the title compounds. They have been characterized by infrared and luminescence spectra and X-ray structure. Thermal analyses have also been carried out, showing in both cases uranyl pyrophosphate as final products.
2. Experimental
2.1. Synthesis The title compounds were synthesized by reaction of ethanolic solutions of uranyl nitrate and the ligands. These solutions were mixed in a 1:2 molar ratio at room temperature (r.t.) and allowed to crystallize. The crystals were separated by decantation, washed with ethanol and dried in vacuo over anhydrous calcium chloride. Anal. Calc. for C28H32N4O10P2U (1): C, 38.1; H, 3.4; N, 6.3. Found: C, 37.9; H, 3.3; N, 6.1%. IR
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 1 4 1 - 9
A.R. de Aquino et al. / Inorganica Chimica Acta 306 (2000) 102–106
(KBr): n(PO) 1063s (1174s free ligand); n(UO2): 932s; n(NO3): n(1) 1278s; n(2) 1034s; n(3) 730m; n(4) 1462s; n(5) 812w. Anal. Calc. for C30H60N8O10P2U (2): C, 36.5; H, 6.1; N, 11.3. Found: C, 36.3; H, 6.0; N, 11.3%. IR (KBr): n(PO) 1070s; n(UO2) 953s; n(NO3): n(1) 1278m; n(2) 1028m; n(3) 725s; n(4) 1442m; n(5) 835m. Elemental analyses (CHN) were carried out on a Perkin–Elmer 240 elemental analyzer. IR spectra of the solid complexes were recorded on a Perkin – Elmer 1750 FTIR spectrometer, using KBr pellets and Nujol mulls between KBr windows. Luminescence spectra were recorded on a modified Hitachi-Perkin – Elmer MPF4 spectrofluorimeter at 293 and 77 K, using 390 and 410 nm exciting radiations for the DPEPA and TPPPO complexes, respectively. Thermogravimetric analyses were carried out on a Shimadzu TGA 50 thermobalance, under air flow (50 ml min − 1) and heating rate of 10.0°C min − 1.
Table 1 Crystallographic data 1 Compound Empirical formula Molecular weight Crystal system Space group Unit cell dimensions a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) V (A, 3) Formula units (unit cell) Dcalc (Mg m−3) F(000) Crystal dimensions (mm) Radiation (l, A, ) m (cm−1) Diffractometer/scan Scan width u Range (°) T (K) Number of reflections collected Number of reflections observed [I]2.5s(I)] R = Fo − Fc / Fo Rw (on F 2) Goodness-of-fit
2
UO2(NO3)2 UO2(NO3)2 (DPEPA)2 (TPPPO)2 C28H32O10N4P2U C30H60O10N8P2U 884.56 triclinic P1(
992.84 monoclinic P21/n
10.171(1) 10.530(1) 16.750(1) 103.52(1) 102.32(1) 96.94(1) 1676.3(3) 2 1.780 860 0.30×0.22×0.18 Mo Ka (0.71073) 49.97 Enraf–Nonius CAD-4 (v/2u) 1.2+0.35 tan u 3–26 293(2) 6603
9.303(1) 13.653(1) 16.450(1)
2064.1(2) 2 1.549 936 0.28×0.24×0.20 Mo Ka (0.71073) 40.66 Enraf–Nonius CAD-4 (v/2u) 1.2+0.35 tan u 3–33 293(2) 8587
4163
3481
0.023 0.060 1.08
0.030 0.065 0.99
98.93(1)
103
3. X-ray measurements and structure determination Crystal data, collected reflections and parameters of the final refinement for UO2(NO3)2DPEPA and UO2(NO3)2TPPPO compounds are reported in Table 1. Reflections were collected using an Enraf– Nonius CAD4 Mach3 diffractometer, using graphitemonochromated MoKa radiation (l= 0.71073 A, ). The orientation matrix and cell dimensions were determined by least-square refinement of the angular positions of 25 reflections. Intensities data were collected at r.t. Three standard reflections were monitored every 2 h. The diffracted intensities were corrected for Lorentz polarization and absorption [10]. The positions of the heavy atoms were obtained from Patterson syntheses [11]. All non-H atoms were located in the subsequent Fourier maps. Structures were refined by full-matrix least-square using anisotropic temperature factors for all non-hydrogen atoms. Hydrogen atoms were introduced at calculated positions in their described geometries and during refinement were allowed to ride on the attached carbon atoms with fixed isotropic thermal parameters (1.2 Ueq. of the parent carbon atom). Calculations were performed with the SHELX-93 program [12], using the scattering factors enclosed therein. The program for the ORTEP drawing was taken from Ref. [13]
4. Results and discussion The complexes were characterized by several techniques. The IR spectra of 1 show a n(PO) aT 1063 cm − 1 (free ligand 1174) while in 2 occurs at 1070 cm − 1 (free ligand 1120). These shifts to lower frequencies evidence coordination to the metal ion through the phosphoryl oxygen of both ligands. All the bands of the nitrate anion (see Section 2) are indicative of coordinated bidentate nitrate anions in both complexes [14– 21]. The emission spectra in the solid state consist in a series of bands, which are often vibronically structured. The two compounds show maximum emission around 510 nm, which accounts for their green color. Despite some controversy [22,23] the emitting state is nowadays considered as one in which an electron has been transferred from an UO22 + molecular orbital to a 5f U atomic orbital (charge transfer) [24]. A strong argument in favor of this excited state lies in its radicallike reactivity [7]. The DPEPA complex at 77 K shows bands with the most intense lines at 486, 510, 533 and 557 nm, with average energy separation of 828 cm − 1, while the TPPPO presents these lines at 497, 513, 536 and 562 nm, with average separation of 850 cm − 1. The different values of these average energy separations suggest some
A.R. de Aquino et al. / Inorganica Chimica Acta 306 (2000) 102–106
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The UO2(NO3)(DPEPA)2 unit shows in the same crystal cell two different orientation of the N-ethyl chain which is responsible for two conformational isomers. A comparison of the structural parameters of the two units shows that the orientation of the N-ethyl group is in one characterized by the P(1)N(2)C(13)C(14) torsion angle of −148.4(5)° while is − 156.6(5) for the corresponding P(1A)N(2A)C(13A)C(14A) (see Fig. 1). Other differences concern the angle UOP which is 173.1(3)° for UO(2)P(1) and 155.5(2)° for UO(2A)P(1A) with an UO(2) bond slightly shorter (2.347(5) A, ) than the corresponding U(2)O(2A) (2.379(4) A, ). In the UO2(OPPh3)2(NO3)2 [26] crystal structure the UO bond distance is 2.359(7) A, and the UOP angle is 160.0(4)°. It seems that a correlation exists between the UO distance and the UOP angle: to a greater linearity of the UOP moiety corresponds a shorter UO(P) distance consequent to higher covalency in the UO bond. Significant differences are present also in the angles around the P atom in the two molecules (see Table 2).
Fig. 1. Perspective view UO2(NO3)2(DPEPA)2 (1).
of
the
two
conformations
of
Table 2 Selected bond distances (A, ) and angles (°) for UO2(NO3)2(DPEPA)2 (1) Molecule 1
difference in the uranyl immediate environments in the two crystal systems. Vibronic structures are partially resolved for both compounds at 77 K. While at 293 K the emission spectra are different: the DPEPA presents the same four bands as broad peaks, while TPPPO exhibits a single, less intense and rather broad band with a flat maximum between 460 and 510 nm. This result IS evidence that TPPPO, as a larger ligand, causes a significant broadening of the emission bands at r.t. This effect may also be coupled with the wellknown self-quenching of uranyl luminescence [25]. A thermogravimetric study showed that the TPPPO compound undergoes decompositions at 315 and 600°C, producing a stable residue. Mass losses and IR spectra suggest (UO2)2P2O7 as final product. The DPEPA complex decomposition is more complex, showing mass losses at 215, 259 and 516°C. These mass losses and IR spectra of the residue point out to the formation of (UO2)2P3O9. The crystal structure determination of the compounds 1 and 2 show that they are isostructural but not isomorphous. The uranium atom is eight-coordinated and the linear uranyl group is equatorially linked to the two trans bidentate nitrates and to the two oxygens of the neutral ligands. The UO2(NO3)2·2L units lie on a crystallographic inversion center in both.
Bond lengths U(1)O(1) U(1)O(2) U(1)O(3) U(1)O(4) P(1)N(2) P(1)O(2) P(1)C(1) P(1)C(7) N(1)O(3) N(1)O(4) N(1)O(5) N(2)C(13) Bond angles O(3)U(1)O(4) O(2)U(1)O(4) O(2)U(1)O(3) O(1)U(1)O(4) O(1)U(1)O(3) O(1)U(1)O(2) C(1)P(1)C(7) O(2)P(1)C(7) O(2)P(1)C(1) N(2)P(1)C(7) N(2)P(1)C(1) N(2)P(1)O(2) P(1)N(2)C(13) U(1)O(2)P(1) N(2)C(13)C(14)
Molecule 2
1.743(5) 2.347(5) 2.512(4) 2.522(4) 1.553(4) 1.485(5) 1.780(6) 1.796(6) 1.254(6) 1.251(7) 1.199(6) 1.425(8) 49.3(1) 65.3(2) 114.4(1) 88.0(2) 91.1(2) 90.9(2) 109.1(3) 110.4(3) 111.6(3) 107.5(2) 103.1(2) 114.8(2) 122.5(4) 173.1(3) 111.0(6)
U(2)O(1A) U(2)O(2A) U(2)O(3A) U(2)O(4A) P(1A)N(2A) P(1A)O(2A) P(1A)C(1A) P(1A)C(7A) N(1A)O(3A) N(1A)O(4A) N(1A)O(5A) N(2A)C(13A) O(3A)U(2)O(4A) O(2A)U(2)O(3A) O(2A)U(2)O(4A) O(1A)U(2)O(4A) O(1A)U(2)O(3A) O(1A)U(2)O(2A) C(1A)P(1A)C(7A) O(2A)P(1A)C(7A) O(2A)P(1A)C(1A) N(2A)P(1A)C(7A) N(2A)P(1A)C(1A) N(2A)P(1A)O(2A) P(1A)N(2A)C(13A) U(2)O(2A)P(1A) N(2A)C(13A)C(14A)
1.746(4) 2.379(4) 2.527(4) 2.531(5) 1.559(4) 1.490(4) 1.785(6) 1.783(5) 1.267(8) 1.269(7) 1.201(7) 1.434(7) 50.1(1) 65.5(1) 115.3(1) 87.2(2) 91.3(2) 89.1(2) 109.8(3) 113.8(2) 108.5(3) 102.2(2) 107.2(2) 114.9(2) 125.6(4) 155.5(2) 108.0(5)
A.R. de Aquino et al. / Inorganica Chimica Acta 306 (2000) 102–106
Fig. 2. Perspective view of UO2(NO3)2(TPPPO)2 (2). Table 3 Selected bond distances (A, ) and angles (°) for UO2(NO3)2(TPPPO)2 (2) Bond lengths UO UO(41) PO(1P) PN(2) O(41)N(4) O(43)N(4) N(1)C(16) N(2)C(26) N(3)C(36) Bond angles O(41)UO(42) O(1P)UO(41) OUO(41) N(2)PN(3) N(1)PN(2) O(1P)PN(2) UO(1P)P UO(42)N(4) PN(1)C(12) PN(2)C(26) C(22)N(2)C(26) PN(3)C(32) O(42)N(4)O(43) O(41)N(4)O(42)
1.758(3) 2.557(4) 1.498(3) 1.637(4) 1.260(6) 1.203(6) 1.462(6) 1.477(7) 1.474(7) 49.2(1) 114.3(1) 93.1(1) 105.1(2) 114.2(2) 107.9(2) 146.7(2) 98.7(3) 120.1(3) 121.2(3) 111.7(4) 120.0(3) 122.7(4) 114.5(4)
UO(1P) UO(42) PN(1) PN(3) O(42)N(4) N(1)C(12) N(2)C(22) N(3)C(32)
O(1P)UO(42) OUO(42) OUO(1P) N(1)PN(3) O(1P)PN(3) O(1P)PN(1) UO(41)N(4) PN(1)C(16) C(12)N(1)C(16) PN(2)C(22) PN(3)C(36) C(32)N(3)C(36) O(41)N(4)O(43)
2.340(3) 2.536(4) 1.635(4) 1.629(4) 1.260(5) 1.483(6) 1.477(7) 1.474(7)
65.9(1) 87.0(1) 92.2(1) 104.9(2) 116.9(2) 108.2(2) 97.7(3) 123.2(3) 111.9(4) 118.2(3) 120.7(3) 111.4(4) 122.8(4)
The presence of two conformational isomers in the same crystal suggests that same degree of fluxionality between the two forms must be present in solution and in the crystal the packing forces are able to freeze the two forms here described. This observation find support in the structure of the cis and of the two a,b forms in trans dichlorodioxobis(triphenylphosphineoxide)uranium(VI) [27,28] where the two trans a,b forms have different packing, the unit cell volumes per molecule are 872 A, 3 (a form), 908 A, 3 (b form), respectively. However with nitrate the possibility of having a
105
cis form can be discarded on the basis of the different steric hindrance with respect to the chlorine. In the complex 2 only one conformer is present in the structure (Fig. 2). Significant bond distances and angles are reported in Table 3. The UOP moiety is significantly bent with an angle of 146.7(2)° and a UO bond length of 2.340(3) A, . Also in this case a comparison with the structure of the analogous UO2(NO3)2(HMPA)2 (HMPA= hexamethylphosphoramide) [29] shows that with an about linear UOP group the UO(P) (2.27(2) A, ) bond length tends to be shorter. The same UO bond distance 2.272(9) A, with an almost linear UOP moiety (178.5°) has been found in the trans UO2Cl2(HMPA)2 [30] An electrostatic repulsion between the positively-charged uranium and the phosphorus center and the steric requirement of the complexes seems to play a minor role on the bending of the UOP angles as shown by a study of the possible correlation between packing density of similar uranium compounds in terms of mean volume of the neutral ligand [30].
5. Supplementary material Tables of additional material, including atomic coordinates, full listing of bond lengths/angles and anisotropic thermal parameters are available from the Cambridge Crystallographic Data Centre, CCDC No. 142195 and No. 142196. Copies of this information can be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: + 44-1223-336-033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
References [1] C.P. Baird, T.J. Kemp, Progr. React. Kinetics 22 (1997) 87. [2] G. Folcher, N. Keller, J. Paris, Solar Energy Mater. 10 (1984) 303. [3] C.K. Jo¨rgensen, R. Reisfeld, Struct. Bonding (Berlin) 50 (1982) 121. [4] C.G. Roffey, Photopolymerisation of surface coatings, Wiley, Chichester, 1985, p. 123. [5] S.L. Suib, K.A. Carrado, Inorg. Chem. 24 (1984) 200. [6] E. Rabinovitch, R.L. Belford, Spectroscopy and Photochemistry of Uranyl Compounds, Pergamon, Oxford, 1964. [7] H.D. Burrows, T.J. Kemp, Chem. Soc. Rev. 3 (1974) 179. [8] H.G.F. Gu¨sten, Gmelin Handbook of Inorganic Chemistry, Springer-Verlag, Berlin-Heidelberg, vol. A6, 1983, chapter 3. [9] C. Moulin, P. Decambox, V. Moulin, J.G. Decaillon, Anal. Chem. 67 (1995) 348. [10] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968) 351. [11] G.M. Sheldrick, SHELXS-86, Acta Crystallogr., Sect. A 46 (1990) 467. [12] G.M. Sheldrick, SHELXL-93 Program for the Refinement of Crystal Structures, University of Go¨ttingen, Germany, 1993.
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[13] J.K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge, National Laboratory, Tennessee, 1976. [14] G. Vicentini, O.J. Fentanes, Ann. Acad. Bras. Cienc. 51 (1979) 451. [15] N.F. Curtis, Y.M. Curtis, Inorg. Chem. 4 (1965) 804. [16] A.B.P. Lever, E. Mantovani, B.S. Ramaswamy, Can. J. Chem. 49 (1971) 1957. [17] B.M. Gatehouse, S.E. Livingstone, R.S. Nyholm, J. Chem. Soc. (1957) 4222. [18] C.C. Addison, B.M. Gatehouse, J. Chem. Soc. (1960) 613. [19] B.O. Field, C.J. Hardy, Quart. Rev. 18 (1964) 361. [20] C.C. Addison, W.B. Simpson, J. Chem. Soc. (1965) 598. [21] C.C. Addison, N. Logan, S.C. Wallwork, D.C. Garner, Quart. Rev. 25 (1971) 289.
.
[22] R.G. Denning, T.R. Snellgrove, D.R. Woodwark, Mol. Phys. 37 (1979) 1109. [23] R.G. Denning, I.D. Morrison, Chem. Phys. Lett. 180 (1991) 101. [24] C.K. Jo¨rgensen, J. Lumines. 18 (1979) 63. [25] P. Benson, A. Cox, T.J. Kemp, Q. Sultana, Chem. Phys. Lett. 35 (1975) 195. [26] N.W. Alcock, M.M. Roberts, J. Chem. Soc. (1982) 25 [27] G. Bombieri, E. Forsellini, E. Day, W.I. Azeez, J. Chem. Soc., Dalton Trans. (1978) 677. [28] S.B. Akona, J. Fawcett, J.H. Holloway, D.R. Russell, Acta Crystallogr., Sect. C 47 (1991) 45. [29] P. Charpin, M. Lance, E. Soulie, D. Vigner, Acta Crystallogr., Sect. C 41 (1985) 884. [30] J.C. Russell, M.P. Du Plessis, L.R. Nasimbeni, Acta Crystallogr., Sect. B 33 (1977) 2062.
.
Note
Characterization, luminescence and crystal structure of uranyl nitrate complexes with diphenyl-N-ethylphosphine amide and tripiperidinephosphine oxide Alfonso R. de Aquino a, Gabriella Bombieri b,c,*, Paulo C. Isolani a, Geraldo Vicentini a, Julio Zukerman-Schpector a b
a Instituto de Quı`mica, Uni6ersidade de Sa˜o Paulo, Sa˜o Paulo, Brazil Istituto di Chimica Farmaceutica e Tossicologica, Uni6ersita` di Milano, Viale Abruzzi, 42, I-20131 Milan, Italy c Istituto di Chimica Farmaceutica, Uni6ersita` di Milano, Viale Abruzzi, 42, I-20131 Milan, Italy
Received 31 December 1999; accepted 27 March 2000
Abstract The uranyl complexes UO2(NO3)2L2 (L=DPEPA (1) diphenyl-N,N-ethylphosphine amide and TPPPO (2) tripiperidine oxide) have been prepared. Spectroscopic studies indicate that L is bound to the uranyl group via the phosphoryl oxygen atom. The crystal structures of (1) and (2) show a hexagonal-bipyramidal coordination about uranium. The luminescence properties of the compounds have been also investigated. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Uranyl complexes; Luminescence; Crystal structures
1. Introduction The spectroscopy of uranium compounds, particularly related to the uranyl ion, has been studied since the decade of 1940, especially because of the use of uranium as a nuclear fuel. Photoexcitation of the uranyl ion leads to a very strong oxidizer, and this fact allowed the operation of a successful actinometer [1]. Other applications of uranyl photoprocesses include solar collectors [2], photochemical conversion of solar energy [3] and photocatalysts [4,5]. Several reviews describe the spectroscopy [6], photophysics and photochemistry of this ion [7,8]. Analytical applications of the uranyl ion fluorescence resulted in very sensitive and selective methods for the determination of this ion in the environment and geologic samples [9].
* Corresponding author. Tel.: + 39-2-2950 2224; fax: + 39-2-2951 4197. E-mail address: [email protected] (G. Bombieri).
As a part of a study of luminescence properties of uranyl nitrate derivatives with phosphoryl ligands we have synthesized the title compounds. They have been characterized by infrared and luminescence spectra and X-ray structure. Thermal analyses have also been carried out, showing in both cases uranyl pyrophosphate as final products.
2. Experimental
2.1. Synthesis The title compounds were synthesized by reaction of ethanolic solutions of uranyl nitrate and the ligands. These solutions were mixed in a 1:2 molar ratio at room temperature (r.t.) and allowed to crystallize. The crystals were separated by decantation, washed with ethanol and dried in vacuo over anhydrous calcium chloride. Anal. Calc. for C28H32N4O10P2U (1): C, 38.1; H, 3.4; N, 6.3. Found: C, 37.9; H, 3.3; N, 6.1%. IR
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 1 4 1 - 9
A.R. de Aquino et al. / Inorganica Chimica Acta 306 (2000) 102–106
(KBr): n(PO) 1063s (1174s free ligand); n(UO2): 932s; n(NO3): n(1) 1278s; n(2) 1034s; n(3) 730m; n(4) 1462s; n(5) 812w. Anal. Calc. for C30H60N8O10P2U (2): C, 36.5; H, 6.1; N, 11.3. Found: C, 36.3; H, 6.0; N, 11.3%. IR (KBr): n(PO) 1070s; n(UO2) 953s; n(NO3): n(1) 1278m; n(2) 1028m; n(3) 725s; n(4) 1442m; n(5) 835m. Elemental analyses (CHN) were carried out on a Perkin–Elmer 240 elemental analyzer. IR spectra of the solid complexes were recorded on a Perkin – Elmer 1750 FTIR spectrometer, using KBr pellets and Nujol mulls between KBr windows. Luminescence spectra were recorded on a modified Hitachi-Perkin – Elmer MPF4 spectrofluorimeter at 293 and 77 K, using 390 and 410 nm exciting radiations for the DPEPA and TPPPO complexes, respectively. Thermogravimetric analyses were carried out on a Shimadzu TGA 50 thermobalance, under air flow (50 ml min − 1) and heating rate of 10.0°C min − 1.
Table 1 Crystallographic data 1 Compound Empirical formula Molecular weight Crystal system Space group Unit cell dimensions a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) V (A, 3) Formula units (unit cell) Dcalc (Mg m−3) F(000) Crystal dimensions (mm) Radiation (l, A, ) m (cm−1) Diffractometer/scan Scan width u Range (°) T (K) Number of reflections collected Number of reflections observed [I]2.5s(I)] R = Fo − Fc / Fo Rw (on F 2) Goodness-of-fit
2
UO2(NO3)2 UO2(NO3)2 (DPEPA)2 (TPPPO)2 C28H32O10N4P2U C30H60O10N8P2U 884.56 triclinic P1(
992.84 monoclinic P21/n
10.171(1) 10.530(1) 16.750(1) 103.52(1) 102.32(1) 96.94(1) 1676.3(3) 2 1.780 860 0.30×0.22×0.18 Mo Ka (0.71073) 49.97 Enraf–Nonius CAD-4 (v/2u) 1.2+0.35 tan u 3–26 293(2) 6603
9.303(1) 13.653(1) 16.450(1)
2064.1(2) 2 1.549 936 0.28×0.24×0.20 Mo Ka (0.71073) 40.66 Enraf–Nonius CAD-4 (v/2u) 1.2+0.35 tan u 3–33 293(2) 8587
4163
3481
0.023 0.060 1.08
0.030 0.065 0.99
98.93(1)
103
3. X-ray measurements and structure determination Crystal data, collected reflections and parameters of the final refinement for UO2(NO3)2DPEPA and UO2(NO3)2TPPPO compounds are reported in Table 1. Reflections were collected using an Enraf– Nonius CAD4 Mach3 diffractometer, using graphitemonochromated MoKa radiation (l= 0.71073 A, ). The orientation matrix and cell dimensions were determined by least-square refinement of the angular positions of 25 reflections. Intensities data were collected at r.t. Three standard reflections were monitored every 2 h. The diffracted intensities were corrected for Lorentz polarization and absorption [10]. The positions of the heavy atoms were obtained from Patterson syntheses [11]. All non-H atoms were located in the subsequent Fourier maps. Structures were refined by full-matrix least-square using anisotropic temperature factors for all non-hydrogen atoms. Hydrogen atoms were introduced at calculated positions in their described geometries and during refinement were allowed to ride on the attached carbon atoms with fixed isotropic thermal parameters (1.2 Ueq. of the parent carbon atom). Calculations were performed with the SHELX-93 program [12], using the scattering factors enclosed therein. The program for the ORTEP drawing was taken from Ref. [13]
4. Results and discussion The complexes were characterized by several techniques. The IR spectra of 1 show a n(PO) aT 1063 cm − 1 (free ligand 1174) while in 2 occurs at 1070 cm − 1 (free ligand 1120). These shifts to lower frequencies evidence coordination to the metal ion through the phosphoryl oxygen of both ligands. All the bands of the nitrate anion (see Section 2) are indicative of coordinated bidentate nitrate anions in both complexes [14– 21]. The emission spectra in the solid state consist in a series of bands, which are often vibronically structured. The two compounds show maximum emission around 510 nm, which accounts for their green color. Despite some controversy [22,23] the emitting state is nowadays considered as one in which an electron has been transferred from an UO22 + molecular orbital to a 5f U atomic orbital (charge transfer) [24]. A strong argument in favor of this excited state lies in its radicallike reactivity [7]. The DPEPA complex at 77 K shows bands with the most intense lines at 486, 510, 533 and 557 nm, with average energy separation of 828 cm − 1, while the TPPPO presents these lines at 497, 513, 536 and 562 nm, with average separation of 850 cm − 1. The different values of these average energy separations suggest some
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The UO2(NO3)(DPEPA)2 unit shows in the same crystal cell two different orientation of the N-ethyl chain which is responsible for two conformational isomers. A comparison of the structural parameters of the two units shows that the orientation of the N-ethyl group is in one characterized by the P(1)N(2)C(13)C(14) torsion angle of −148.4(5)° while is − 156.6(5) for the corresponding P(1A)N(2A)C(13A)C(14A) (see Fig. 1). Other differences concern the angle UOP which is 173.1(3)° for UO(2)P(1) and 155.5(2)° for UO(2A)P(1A) with an UO(2) bond slightly shorter (2.347(5) A, ) than the corresponding U(2)O(2A) (2.379(4) A, ). In the UO2(OPPh3)2(NO3)2 [26] crystal structure the UO bond distance is 2.359(7) A, and the UOP angle is 160.0(4)°. It seems that a correlation exists between the UO distance and the UOP angle: to a greater linearity of the UOP moiety corresponds a shorter UO(P) distance consequent to higher covalency in the UO bond. Significant differences are present also in the angles around the P atom in the two molecules (see Table 2).
Fig. 1. Perspective view UO2(NO3)2(DPEPA)2 (1).
of
the
two
conformations
of
Table 2 Selected bond distances (A, ) and angles (°) for UO2(NO3)2(DPEPA)2 (1) Molecule 1
difference in the uranyl immediate environments in the two crystal systems. Vibronic structures are partially resolved for both compounds at 77 K. While at 293 K the emission spectra are different: the DPEPA presents the same four bands as broad peaks, while TPPPO exhibits a single, less intense and rather broad band with a flat maximum between 460 and 510 nm. This result IS evidence that TPPPO, as a larger ligand, causes a significant broadening of the emission bands at r.t. This effect may also be coupled with the wellknown self-quenching of uranyl luminescence [25]. A thermogravimetric study showed that the TPPPO compound undergoes decompositions at 315 and 600°C, producing a stable residue. Mass losses and IR spectra suggest (UO2)2P2O7 as final product. The DPEPA complex decomposition is more complex, showing mass losses at 215, 259 and 516°C. These mass losses and IR spectra of the residue point out to the formation of (UO2)2P3O9. The crystal structure determination of the compounds 1 and 2 show that they are isostructural but not isomorphous. The uranium atom is eight-coordinated and the linear uranyl group is equatorially linked to the two trans bidentate nitrates and to the two oxygens of the neutral ligands. The UO2(NO3)2·2L units lie on a crystallographic inversion center in both.
Bond lengths U(1)O(1) U(1)O(2) U(1)O(3) U(1)O(4) P(1)N(2) P(1)O(2) P(1)C(1) P(1)C(7) N(1)O(3) N(1)O(4) N(1)O(5) N(2)C(13) Bond angles O(3)U(1)O(4) O(2)U(1)O(4) O(2)U(1)O(3) O(1)U(1)O(4) O(1)U(1)O(3) O(1)U(1)O(2) C(1)P(1)C(7) O(2)P(1)C(7) O(2)P(1)C(1) N(2)P(1)C(7) N(2)P(1)C(1) N(2)P(1)O(2) P(1)N(2)C(13) U(1)O(2)P(1) N(2)C(13)C(14)
Molecule 2
1.743(5) 2.347(5) 2.512(4) 2.522(4) 1.553(4) 1.485(5) 1.780(6) 1.796(6) 1.254(6) 1.251(7) 1.199(6) 1.425(8) 49.3(1) 65.3(2) 114.4(1) 88.0(2) 91.1(2) 90.9(2) 109.1(3) 110.4(3) 111.6(3) 107.5(2) 103.1(2) 114.8(2) 122.5(4) 173.1(3) 111.0(6)
U(2)O(1A) U(2)O(2A) U(2)O(3A) U(2)O(4A) P(1A)N(2A) P(1A)O(2A) P(1A)C(1A) P(1A)C(7A) N(1A)O(3A) N(1A)O(4A) N(1A)O(5A) N(2A)C(13A) O(3A)U(2)O(4A) O(2A)U(2)O(3A) O(2A)U(2)O(4A) O(1A)U(2)O(4A) O(1A)U(2)O(3A) O(1A)U(2)O(2A) C(1A)P(1A)C(7A) O(2A)P(1A)C(7A) O(2A)P(1A)C(1A) N(2A)P(1A)C(7A) N(2A)P(1A)C(1A) N(2A)P(1A)O(2A) P(1A)N(2A)C(13A) U(2)O(2A)P(1A) N(2A)C(13A)C(14A)
1.746(4) 2.379(4) 2.527(4) 2.531(5) 1.559(4) 1.490(4) 1.785(6) 1.783(5) 1.267(8) 1.269(7) 1.201(7) 1.434(7) 50.1(1) 65.5(1) 115.3(1) 87.2(2) 91.3(2) 89.1(2) 109.8(3) 113.8(2) 108.5(3) 102.2(2) 107.2(2) 114.9(2) 125.6(4) 155.5(2) 108.0(5)
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Fig. 2. Perspective view of UO2(NO3)2(TPPPO)2 (2). Table 3 Selected bond distances (A, ) and angles (°) for UO2(NO3)2(TPPPO)2 (2) Bond lengths UO UO(41) PO(1P) PN(2) O(41)N(4) O(43)N(4) N(1)C(16) N(2)C(26) N(3)C(36) Bond angles O(41)UO(42) O(1P)UO(41) OUO(41) N(2)PN(3) N(1)PN(2) O(1P)PN(2) UO(1P)P UO(42)N(4) PN(1)C(12) PN(2)C(26) C(22)N(2)C(26) PN(3)C(32) O(42)N(4)O(43) O(41)N(4)O(42)
1.758(3) 2.557(4) 1.498(3) 1.637(4) 1.260(6) 1.203(6) 1.462(6) 1.477(7) 1.474(7) 49.2(1) 114.3(1) 93.1(1) 105.1(2) 114.2(2) 107.9(2) 146.7(2) 98.7(3) 120.1(3) 121.2(3) 111.7(4) 120.0(3) 122.7(4) 114.5(4)
UO(1P) UO(42) PN(1) PN(3) O(42)N(4) N(1)C(12) N(2)C(22) N(3)C(32)
O(1P)UO(42) OUO(42) OUO(1P) N(1)PN(3) O(1P)PN(3) O(1P)PN(1) UO(41)N(4) PN(1)C(16) C(12)N(1)C(16) PN(2)C(22) PN(3)C(36) C(32)N(3)C(36) O(41)N(4)O(43)
2.340(3) 2.536(4) 1.635(4) 1.629(4) 1.260(5) 1.483(6) 1.477(7) 1.474(7)
65.9(1) 87.0(1) 92.2(1) 104.9(2) 116.9(2) 108.2(2) 97.7(3) 123.2(3) 111.9(4) 118.2(3) 120.7(3) 111.4(4) 122.8(4)
The presence of two conformational isomers in the same crystal suggests that same degree of fluxionality between the two forms must be present in solution and in the crystal the packing forces are able to freeze the two forms here described. This observation find support in the structure of the cis and of the two a,b forms in trans dichlorodioxobis(triphenylphosphineoxide)uranium(VI) [27,28] where the two trans a,b forms have different packing, the unit cell volumes per molecule are 872 A, 3 (a form), 908 A, 3 (b form), respectively. However with nitrate the possibility of having a
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cis form can be discarded on the basis of the different steric hindrance with respect to the chlorine. In the complex 2 only one conformer is present in the structure (Fig. 2). Significant bond distances and angles are reported in Table 3. The UOP moiety is significantly bent with an angle of 146.7(2)° and a UO bond length of 2.340(3) A, . Also in this case a comparison with the structure of the analogous UO2(NO3)2(HMPA)2 (HMPA= hexamethylphosphoramide) [29] shows that with an about linear UOP group the UO(P) (2.27(2) A, ) bond length tends to be shorter. The same UO bond distance 2.272(9) A, with an almost linear UOP moiety (178.5°) has been found in the trans UO2Cl2(HMPA)2 [30] An electrostatic repulsion between the positively-charged uranium and the phosphorus center and the steric requirement of the complexes seems to play a minor role on the bending of the UOP angles as shown by a study of the possible correlation between packing density of similar uranium compounds in terms of mean volume of the neutral ligand [30].
5. Supplementary material Tables of additional material, including atomic coordinates, full listing of bond lengths/angles and anisotropic thermal parameters are available from the Cambridge Crystallographic Data Centre, CCDC No. 142195 and No. 142196. Copies of this information can be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: + 44-1223-336-033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
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