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Characterization and crystal structure of lanthanide cryptates of a ligand derived from 2,6-diformyl-pyridine
Polyhedron 23 (2004) 49–53 www.elsevier.com/locate/poly
Characterization and crystal structure of lanthanide cryptates of a ligand derived from 2,6-diformyl-pyridine Xue-Lei Hu, Yi-Zhi Li, Qin-Hui Luo
*
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University, 22 Hankou Rd., 210093, PR China Received 19 June 2003; accepted 3 September 2003
Abstract Seven lathanide cryptates [LnL](NO)3 nH2 O (n ¼ 1–3; Ln ¼ La, Ce, Pr, Sm, Eu, Gd, Tb) were synthesized by the reactions of lanthanide nitrates with free ligand L (L was derived from the (2 + 3) condensation of 2,6-diformyl-pyridine (dfp) with tris-(2aminoethyl) amine (tren)) and characterized by physical methods. ES-MS spectra show that except for La(III) cryptate, the other six are all stable in solution. The crystal structure of the Eu(III) cryptate showed a tricapped prismatic coordination geometry. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Lanthanide; Cryptate; Crystal structure; ES-MS
1. Introduction There has been a great deal of interest in design and synthesis of lanthanide complexes [1–5] due to their unique properties and many valuable applications [6], such as fluorescent probes in biological systems [7], magnetic resonance imaging and as contrast agents in clinical NMR images [8] as well as chemical logics in molecular information processing [9]. Cryptand ligands containing appropriate binding sites of suitable size and shape have been designed to form lanthanide cryptates [10,11]. The europium(III) cryptates are of particular interest in view of their novel photophysical properties [12], and several reports on their syntheses and properties have appeared [13,14]; for example, the luminescence properties of solutions of Eu(III) cryptates with 2,2-bipyridine and pyridine N-oxide units have been studied in detail [15–18]. A cryptand L synthesized by condensation of 2,6-diformyl-pyridine (dfp) with tris-(2aminoethyl) amine (tren) has been reported [19], but its lanthanide cryptates and their crystal structure reported to date are few. Generally, the lanthanide cryptates are obtained by template synthesis involving alkali, alkaliearth [20] or lanthanide ions [21] as templating agents. *
Corresponding author. Tel.: +86253594030; fax: +86253317761. E-mail address: [email protected] (Q.-H. Luo).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.09.024
We failed to obtain the lanthanide cryptates of ligand L by the template method. On the other hand, we obtained seven lanthanide cryptates [LnL](NO)3 nH2 O (Ln ¼ La, Ce, Pr, Sm, Eu, Gd, Tb) (Scheme 1) by directly reacting the free ligand L with the corresponding lanthanide nitrates. The crystal structure of the Eu(III) cryptate displays a tricapped prismatic coordination geometry which is different from the structure of the sodium cryptate with the same ligand [22] and those of other lanthanide cryptates [20]. The ES-MS spectra of the seven cryptates show that, except for La (III) cryptate, all complex cations are stable in the solution.
2. Experimental 2.1. Materials All starting chemicals were of reagent grade. Hydrated lanthanide nitrate was prepared by dissolving Ln2 O3 (99.99%) in an excess amount of nitric acid. Tris(2-aminoethyl)amine [23] and 2,6-diformyl-pyridine [24] were prepared by literature methods. Cryptand L was prepared by the condensation of dfp with tren as described in the literature [19]. Their physical constants and spectroscopic data are in agreement with previously reported values.
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X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
Scheme 1. The chemical formulas of the complex cations, Ln ¼ Eu (III), La (III), Ce (III), Pr (III), Sm (III), Gd (III), Tb(III).
2.2. Preparation of the compounds [EuL](NO3 )3 H2 O (1): Eu(NO3 )3 6H2 O (0.054 g, 0.12 mmol) in 20 ml MeCN was added to a boiling solution of L (0.059 g 0.1 mmol) in 150 ml MeCN. After refluxing for 12 h, the orange product was filtered out and washed with MeCN and Et2 O, and then dried. Red solid 0.078 g were obtained. Yield: 82%. (Anal. Calc. for C33 H41 N14 O10 Eu (Mw 945.74): C, 41.91; H, 4.37; N, 20.73. Found: C, 41.63; H, 4.75; N, 21.11%.) IR (cm1 ): 1641 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 215 (1 00 200), 246 (40 800), 317 (20 700), 330 (17 000). KM (H2 O, 293 K): 412 S cm2 mol1 . Red block-like crystals suitable for X-ray diffraction were obtained by diffusion of Et2 O into MeOH/ DMF solution over 1 week. The cryptates [LaL](NO3 )3 2H2 O (2), [CeL](NO3 )3 2H2 O (3), [PrL](NO3 )3 3H2 O (4), [SmL](NO3 )3 2H2 O (5), [GdL](NO3 )3 2H2 O (6), [TbL](NO3 )3 2H2 O (7) were prepared by a similar procedure as for 1, except that the corresponding lanthanide(III) nitrates were used instead of Eu(NO)3 6H2 O. [LaL](NO3 )3 2H2 O (2): Yellow tiny crystals. Yield: 75%. (Anal. Calc. for C33 H43 N14 O11 La (Mw 951.91): C, 41.69; H, 4.56; N, 20.63. Found: C, 41.25; H, 4.18; N, 20.66%.), IR (cm1 ): 1641 (s, m(C@N)); 1384 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 208 (68 700), 246 (20 430), 316 (10 200), 330 (8300). KM (H2 O, 293 K): 409 S cm2 mol1 . [CeL](NO3 )3 2H2 O (3): Orange solid. Yield: 75%. (Anal. Calc. for C33 H43 N14 O11 Ce (Mw 970.71) C, 41.64; H, 4.55; N, 20.60. Found: C, 41.85; H, 4.89; N, 20.35%.), IR (cm1 ): 1640 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 207 (97 600), 249 (25 400), 316 (12 200), 329 (9700). KM (H2 O, 293 K): 425 S cm2 mol1 . [PrL](NO3 )3 3H2 O (4): Yellow solid. Yield: 80%. (Anal. Calc. for C33 H45 N14 O12 Pr (Mw 970.71) C, 40.83; H, 4.67; N, 20.20. Found: C, 41.15; H, 4.25; N, 20.68%.) IR (cm1 ): 1642 (s, m(C@N)); 1384 (s, m(NO3 )), UV–Vis
(kmax /nm (e/dm3 mol1 cm1 ), H2 O): 207 (120 000), 245 (30 600), 316 (14 200), 329 (10 400). KM (H2 O, 293 K): 406 S cm2 mol1 . [SmL](NO3 )3 2H2 O (5): Orange solid. Yield: 83%. (Anal. Calc. for C33 H43 N14 O11 Sm (Mw 962.15) C, 41.20; H, 4.50; N, 20.38. Found: C, 41.35; H, 4.55; N, 20.55%.) IR (cm1 ): 1641 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 209 (92 000), 247 (24 700), 316 (11 500), 328 (9800). KM (H2 O, 293 K): 399 S cm2 mol1 . [GdL](NO3 )3 2H2 O (6): Yellow solid. Yield: 75%. (Anal. Calc. for C33 H43 N14 O11 Sm (Mw 969.04) C, 40.90; H, 4.47; N, 20.24. Found: C, 40.58; H, 4.56; N, 20.70%.) IR (cm1 ): 1634 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 211 (97 300), 249 (21 500), 318 (11 600), 329 (10 200). KM (H2 O, 293 K): 405 S cm2 mol1 . [TbL](NO3 )3 2H2 O (7): Orange solid. Yield: 74%. (Anal. Calc. for C33 H43 N14 O11 Tb (Mw 970.72) C, 40.84; H, 4.46; N, 20.20. Found: C, 41.40; H, 4.43; N, 20.67%.) IR (cm1 ): 1638 (s, m(C@N)); 1384 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 205 (94 500), 246 (22 500), 317 (15 600), 330 (9500). KM (H2 O, 293 K): 441 S cm2 mol1 . 2.3. Physical measurements Elemental analysis of C, H, N were performed on a Perkin–Elmer 240c analytical instrument. The molar electrical conductivities in aqueous solution containing 104 mol l1 complexes were measured at 25 0.1 °C using a BSD-A conductometer (Jangsu, China). Electronic spectra were recorded on a UV-3100 spectrophotometer. IR spectra were measured using KBr discs with a vector 22 FI-IR spectrophotometer. Electrospray mass spectrum (ES-MS) was determined on a Finnigan LCQ ES-MS mass spectrograph using methanol as the mobile phrase with a sample concentration of about 1.0 mmol dm3 . The diluted solution was electrosprayed at a flow rate of 5 106 dm3 min1 with a needle voltage of +4.5 kV. The temperature of the heated capillary in the interface was 200 °C and a fuse silica sprayer was used. 2.4. Crystal structure determination Diffraction intensity data were collected on a SMART-CCD area-detector diffractometer at 293 K using graphite monochromatic Mo Ka radiation ). Data reduction and cell refinement (k ¼ 0:71073 A were performed by S M A R T and S A I N T programs [25]. The structure was solved by direct methods (Bruker SHELXTL) and refined on F 2 by full-matrix leastsquares (Bruker SHELXTL) using all unique data [26]. The non-H atoms in the structure were treated as anisotropic. Hydrogen atoms were located geometrically and refined in the riding mode.
X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
3. Results and discussion 3.1. Synthesis and characterization We have tried to synthesize lanthanide cryptates by the condensation of dfp with tren in the presence of Ln3þ or alkali-earth metal ions as templates by previous methods, but pure products could not be obtained. An effective method has been developed by reacting free ligand with the lanthanide nitrate. However, by the same procedure, we could not get pure products of the cryptates of the heavier lanthanides, which may be due to mismatch of their ionic size with the cavity of the ligand. The conductivities of all the seven lanthanide cryptates are in the range of 399–412 S cm2 mol1 , typical for a 1:3 electrolyte [27] and consistent with the chemical formulas from elemental analysis. The IR spectra of all cryptates show the characteristic vibrations of the C@N bond (1634–1642 cm1 ) and nitrate ions (1384 cm1 ). The vibration frequency of the observed C@N bond shifts to longer wavelength by 5–13 cm1 in contrast to that of the free ligand (1647 cm1 ) [19]. The intense absorptions in the electronic spectra of the cryptates at 200–240 and 310–330 nm are designated to the p–p transitions of the pyridyls and C@N groups, respectively.
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one to two nitrate anions forming [ML(NO3 )]2þ or [ML(NO3 )2 ]þ . The simplicity of the spectra can be attributed to the thermodynamic and kinetic stability of the cryptates. For example, Fig. 1(a) shows the positiveion ES mass spectrum of 1 [EuL](NO3 )3 H2 O. The base peak at m=z ¼ 247:5 corresponds to [EuL]3þ . The peaks at m=z ¼ 402:1 can be assigned to the species [EuL(NO3 )]2þ . The isotopic distribution of the peak at m=z ¼ 247:5 is shown in Fig. 1(a). From the numbers
3.2. Electrospray mass spectra ES-MS data are listed in Table 1. There, except for 2, the ES-MS spectra of all cryptates show common features, and the spectra are composed of two to three simple peaks. The dominant peaks are assigned to the formation of [ML]3þ (M ¼ Eu, Ce, Pr, Sm, Gd, Tb) and the other peak clusters are due to [ML]3þ that captures
Fig. 1. The ES-MS spectra of cryptates 1 and 2 in MeOH solution: (a) cryptate 1 (inset: the isotopic distribution of the peak at m=z ¼ 247:5; left: experimental pattern; right: calculated pattern); (b) cryptate 2.
Table 1 ES-MS spectroscopic data for the cryptates 1–7 No.
Cryptate
Peak (m=z)
Relative abundance (%)
Assignment
1
[EuL](NO3 )3 H2 O
2
[LaL](NO3 )3 2H2 O
3
[CeL](NO3 )3 2H2 O
4
[PrL](NO3 )3 3H2 O
5
[SmL](NO3 )3 2H2 O
6
[GdL](NO3 )3 2H2 O
7
[TbL](NO3 )3 2H2 O
247.5 402.1 242.9 395.2 612.6 696.5 852.3 243.2 395.7 243.7 396.3 854.3 247.3 402.7 249.1 403.8 249.5 405.7
100 29 100 23 20 47 19 100 11 100 18 10 100 22 100 19 100 29
[EuL]3þ [EuL(NO3 )]2þ [LaL]3þ [LaL(NO3 )]2þ [NaL]þ [Na2 L(NO3 )]þ [LaL(NO3 )2 ]þ [CeL]3þ [CeL(NO3 )]2þ [PrL]3þ [PrL(NO3 )]2þ [PrL(NO3 )2 ]þ [SmL]3þ [SmL(NO3 )]2þ [GdL]3þ [GdL(NO3 )]2þ [TbL]3þ [TbL(NO3 )]2þ
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X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
and isotopic abundances of the atoms in [EuL]3þ , a revised program is used to calculated the pattern which is in good agreement with the experimental one (Fig. 1(a) inset). For the six cryptates, no peak of free ligand or ligand fragments is observed, showing that the cryptates and the ligand are rather stable in solution. For 2, the ES-MS spectra (Fig. 1(b)) are more complicated. Besides the species of [LaL]3þ and [LaL(NO3 )]2þ , two peaks at
m=z ¼ 612:5 and at m=z ¼ 696:5 are designed to [NaL]þ and [Na2 L(NO3 )]þ , respectively, implying that the cryptand can bind the Naþ ion to form an inclusion complex. The Naþ is present in a trace amount. The dissociation of the Laþ ion implies that the cryptate is less stable than the other six. Because of its larger radius, La3þ in the ligand cage is not so stable as Eu3þ so that the binding of the ligand to La3þ is weaker than those of other lanthanide ions.
Table 2 Crystal data of the complex [EuL]NO3 0.5H2 O
3.3. Crystal structure of the cryptate
Empirical formula Crystal system Space group ) a, b, c (A a, b, c (°) 3 ) V (A Dcalcd: (g/cm3 ) F ð0 0 0Þ Crystal size (mm) Temperature (K) l(Mo Ka) (mm1 ) 2h range (°) Reflections collected Independent reflections Observed reflections Index ranges w1 Goodness-of-fit on F 2 Final R1 ; wR2 ½I > 2r
C33 H40 N14 O9:5 Eu trigonal R-3c 14.709(1), 14.709(1), 32.167(3) 90, 90, 120 6027.1(8) 1.548 2850 0.20 0.20 0.30 293 1.631 2–25 9636 1189 757 17 6 h 6 17k; 17 6 k 6 9; 38 6 l 6 37 1=½r2 ðFo2 Þ þ ð0:0240P Þ2 þ 1:880P ; P ¼ ðFo2 þ 2Fc2 Þ=3 1.02 0.0419, 0.0919
The crystal data of 1 are listed in Table 2. Selected bond distances and bond angles are listed in Table 3. In the complex 1 (Fig. 2(a)), the central cation Eu1 is lo-
Table 3 ) and angles (°) of [EuL](NO3 )3 0.5H2 Oa Selected bond distances (A Bond distances Eu1–N1 Eu1–N1A Eu1–N1B Eu1–N1C Eu1–N1D Eu1–N1E Bond angles N1–Eu1–N2 N1–Eu1–N1A N1–Eu1–N2A N1–Eu1–N1B N1–Eu1–N2B N1–Eu1–N1C N1–Eu1–N1E N1–Eu1–N1D N1B–Eu1–N1E N1A–Eu1–N2A N1A–Eu1–N1B N1A–Eu1–N1C N1A–Eu1–N1E N1B–Eu1–N1D
2.597(3) 2.597(3) 2.597(3) 2.597(3) 2.597(3) 2.597(3) 63.28(8) 80.78(12) 72.96(7) 80.78(12) 137.97(7) 145.91(13) 126.56(15) 84.07(15) 84.07(15) 63.28(8) 80.78(12) 84.07(15) 145.91(13) 145.91(13)
Eu1–N2 Eu1–N2A Eu1–N2B Eu1 N3 Eu1 N3A
N1C–Eu1–N2B N1E–Eu1–N2B N1D–Eu1–N2B N1C–Eu1–N1E N1C–Eu1–N1D N1E–Eu1–N1D N1A–Eu1–N2 N2–Eu1–N2A N1B–Eu1–N2 N2–Eu1–N2B N1C–Eu1–N2 N1E–Eu1–N2 N1D–Eu1–N2 N1A–Eu1–N2B
2.559(5) 2.559(5) 2.559(5) 3.768(5) 3.768(5)
63.28(8) 72.95(7) 137.96(7) 80.78(12) 80.78(12) 80.78(12) 137.97(7) 120.00(1) 72.96(7) 120.00(1) 137.97(7) 63.28(8) 72.96(7) 72.96(7)
a Symmetry transformation used to generate equivalent atoms: (i) 1 y; 1 þ x y; z (N1A, N2A); (ii) x þ y; 1 x; z (N1B, N2B); (iii) 1=3 þ y; 1=3 þ x; 5=6 z (N1C, N3A); (iv) 2=3 þ x y; 4=3 y; 5=6 z (N1D); (v) 2=3 x; 1=3 x þ y; 5=6 z (N1E).
Fig. 2. (a) A perspective view of the [EuL]3þ cation; (b) the structure of the coordination polyhedron. Symmetry transformation used to generate equivalent atoms: (i) 1 y; 1 þ x y; z (N1A, N2A); (ii) x þ y; 1 x; z (N1B, N2B); (iii) 1=3 þ y; 1=3 þ x; 5=6 z (N1C, N3A); (iv) 2=3 þ x y, 4=3 y; 5=6 z (N1D); (v) 2=3 x; 1=3 x þ y; 5=6 z (N1E).
X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
cated in the center of the macrocycle and coordinated by three pyridyl nitrogen atoms (N2, N2A and N2B) and six imino nitrogens (N1, N1A, N1B, N1C, N1D, N1E) to form a tricapped prism (Fig. 2(b)). The complex cation has a threefold axis through Eu1 and the two bridgehead nitrogen atoms (N3, N3A). The average bond length between Eu1 and nitrogen atoms of the , being slightly shorter than pyridine groups is 2.559 A ). The average length those for the imino groups (2.598 A . No interaction beof Eu1–N (bridgehead) is 3.768 A tween Eu1 and N(bridgehead) is observed. The coordination polyhedron of 1 can be described approximately as a tricapped prism. The upper and basal planes of the prism, which are approximately parallel, are composed of the imino nitrogen atoms (N1, N1A, N1B and N1C, N1D, N1E, respectively). The distance between the two . The distance between Eu1 and the planes is 3.446 A . The three pyridyl nitrogen atoms upper plane is 1.723 A are located at the capping positions. Eu1 is located in the symmetrical center of the prism. The distances of Eu1 to each least squares prism plane is approximately . The tricapped prism structure is different from 0.964 A the unsymmetrical structure of sodium cryptate with the same ligand [22] and those of phenol-based lanthanide cryptates [13,20,21], in which the central ions are located at one side of the macrocycles. The present Eu(III) cryptate is located at the inversion center of the cavity of the macrocycle in order to satisfy the requirement of a higher coordination number of the lanthanide ion. To our knowledge, this is the first crystal structure of a lanthanide cryptate derived from 2,6-pyridyl-dicarboxaldehyde.
4. Supplementary data Supplementary data are available from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; E-mail: [email protected] or www: http://www.ccdc.cam.ac.uk) on request, quoting the deposition number CCDC-208092.
Acknowledgements This work was supported by the National Natural Science Foundation of China.
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References [1] V. Alexander, Chem. Rev. (1995) 273. [2] C. Piguet, J.-C.G. B€ unzli, Chem. Soc. Res. 18 (1999) 347. [3] Q.-Y. Chen, Q.-H. Luo, X.-L. Hu, M.-C. Shen, J.-T. Chen, Chem. Eur. J. 8 (2002) 3984. [4] G.F. de Sa, O.L. Malta, C. de Mello Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165. [5] B.H. Bakker, M. Goes, N. Hoebe, H.J. Van Ramesdonk, J.W. Verhoeven, M.H.V. Werts, J.W. Hofstraat, Coord. Chem. Res. 208 (2000) 3. [6] S. Aime, M. Botta, M. Fasano, E. Terreno, Chem. Soc. Rev. 27 (1998) 19. [7] J.-M. Lehn, D.D. Macnicol, J.L. Atwood, J.E. Davies, D.N. Reinhoudt, F. Vogtl (Eds.), Comprehensive Supramolecular Chemistry, vol. 10, Pergamon, Oxford, 1996 (Ch. 17). [8] A. Beeby, S.W. Botchway, I.M. Clarkson, S. Faulkner, A.W. Parker, D. Parker, J.A. Gareth Williams, J. Photochem. Photobiol. B 57 (2000) 83. [9] T. Gunnlaugsson, D.A. MacDonail, D. Parker, Chem. Commun. (2000) 93. [10] Q.-Y. Chen, Q.-H. Luo, Z.-L. Wang, J.-T. Chen, Chem. Commun. (2000) 1033. [11] Q.-Y. Chen, Q.-H. Luo, L.-M. Zheng, Z.-L. Wang, J.-T. Chen, Inorg. Chem. 41 (2002) 605. [12] G. Vicentini, L.B. Zinner, J. Zukerman-Schpector, K. Zinner, Coord. Chem. Rev. 196 (2000) 353. [13] Q.-Y. Chen, C.-J. Feng, Q.-H. Luo, C.-Y. Duan, X.-S. Yu, Y.-J. Liu, Eur. J. Inorg. Chem. (2001) 1067. [14] C. Bazzicalupi, A. Bencini, E. Berni, A. Bianchi, C. Giorgi, V. Fusi, B. Valtancoli, C. Lodeiro, A. Roque, F. Pina, Inorg. Chem. 40 (2001) 6172. [15] B. Alpha, J.-M. Lehn, G. Mathis, Angew. Chem., Int. Ed. Engl. 26 (1987) 266. [16] C.O. Paul-Roth, J.-M. Lehn, J. Guilhem, C. Pascard, Helv. Chim. Acta 78 (1995) 1895, and references therein. [17] J.-M. Lehn, J.-B.R. de Vains, Helv. Chim. Acta 75 (1992) 1221. [18] C. Galaup, C. Picard, H. Autiero, D. Aspe, Helv. Chim. Acta 82 (1999) 544. [19] D.J. Marrs, V. Mckee, J. Nelson, Q. Lu, C.J. Harding, Inorg. Chim. Acta 211 (1993) 195. [20] M.G. Drew, O.W. Howarth, C.J. Harding, N. Martin, J. Nelson, J. Chem. Soc., Chem. Commun. (1995) 903. [21] C.-J. Feng, Q.H. Luo, C.-Y. Duan, M.-C. Shen, Y.-J. Liu, J. Chem. Soc., Dalton Trans. (1998) 1377. [22] V. Mckee, M.R.J. Dorrity, J.F. Malone, D. Marrs, J. Nelson, J. Chem. Soc., Chem. Commun. (1992) 383. [23] E. Kimura, S. Young, J.P. Collman, Inorg. Chem. 9 (1970) 1187. [24] C.J. Gray, F.A. Hart, J. Chem. Soc., Dalton Trans. (1987) 2289. [25] S M A R T and S A I N T , Area Detector Control and Integration Software. Siemens Analytical X-Ray Systems, Inc., Madison, WI, USA, 1996. [26] G.M. Sheldrick, S H E L X T L V5.1 Software Reference Manual, Bruker AXS, Madison, WI, USA, 1997. [27] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
Characterization and crystal structure of lanthanide cryptates of a ligand derived from 2,6-diformyl-pyridine Xue-Lei Hu, Yi-Zhi Li, Qin-Hui Luo
*
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University, 22 Hankou Rd., 210093, PR China Received 19 June 2003; accepted 3 September 2003
Abstract Seven lathanide cryptates [LnL](NO)3 nH2 O (n ¼ 1–3; Ln ¼ La, Ce, Pr, Sm, Eu, Gd, Tb) were synthesized by the reactions of lanthanide nitrates with free ligand L (L was derived from the (2 + 3) condensation of 2,6-diformyl-pyridine (dfp) with tris-(2aminoethyl) amine (tren)) and characterized by physical methods. ES-MS spectra show that except for La(III) cryptate, the other six are all stable in solution. The crystal structure of the Eu(III) cryptate showed a tricapped prismatic coordination geometry. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Lanthanide; Cryptate; Crystal structure; ES-MS
1. Introduction There has been a great deal of interest in design and synthesis of lanthanide complexes [1–5] due to their unique properties and many valuable applications [6], such as fluorescent probes in biological systems [7], magnetic resonance imaging and as contrast agents in clinical NMR images [8] as well as chemical logics in molecular information processing [9]. Cryptand ligands containing appropriate binding sites of suitable size and shape have been designed to form lanthanide cryptates [10,11]. The europium(III) cryptates are of particular interest in view of their novel photophysical properties [12], and several reports on their syntheses and properties have appeared [13,14]; for example, the luminescence properties of solutions of Eu(III) cryptates with 2,2-bipyridine and pyridine N-oxide units have been studied in detail [15–18]. A cryptand L synthesized by condensation of 2,6-diformyl-pyridine (dfp) with tris-(2aminoethyl) amine (tren) has been reported [19], but its lanthanide cryptates and their crystal structure reported to date are few. Generally, the lanthanide cryptates are obtained by template synthesis involving alkali, alkaliearth [20] or lanthanide ions [21] as templating agents. *
Corresponding author. Tel.: +86253594030; fax: +86253317761. E-mail address: [email protected] (Q.-H. Luo).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.09.024
We failed to obtain the lanthanide cryptates of ligand L by the template method. On the other hand, we obtained seven lanthanide cryptates [LnL](NO)3 nH2 O (Ln ¼ La, Ce, Pr, Sm, Eu, Gd, Tb) (Scheme 1) by directly reacting the free ligand L with the corresponding lanthanide nitrates. The crystal structure of the Eu(III) cryptate displays a tricapped prismatic coordination geometry which is different from the structure of the sodium cryptate with the same ligand [22] and those of other lanthanide cryptates [20]. The ES-MS spectra of the seven cryptates show that, except for La (III) cryptate, all complex cations are stable in the solution.
2. Experimental 2.1. Materials All starting chemicals were of reagent grade. Hydrated lanthanide nitrate was prepared by dissolving Ln2 O3 (99.99%) in an excess amount of nitric acid. Tris(2-aminoethyl)amine [23] and 2,6-diformyl-pyridine [24] were prepared by literature methods. Cryptand L was prepared by the condensation of dfp with tren as described in the literature [19]. Their physical constants and spectroscopic data are in agreement with previously reported values.
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X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
Scheme 1. The chemical formulas of the complex cations, Ln ¼ Eu (III), La (III), Ce (III), Pr (III), Sm (III), Gd (III), Tb(III).
2.2. Preparation of the compounds [EuL](NO3 )3 H2 O (1): Eu(NO3 )3 6H2 O (0.054 g, 0.12 mmol) in 20 ml MeCN was added to a boiling solution of L (0.059 g 0.1 mmol) in 150 ml MeCN. After refluxing for 12 h, the orange product was filtered out and washed with MeCN and Et2 O, and then dried. Red solid 0.078 g were obtained. Yield: 82%. (Anal. Calc. for C33 H41 N14 O10 Eu (Mw 945.74): C, 41.91; H, 4.37; N, 20.73. Found: C, 41.63; H, 4.75; N, 21.11%.) IR (cm1 ): 1641 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 215 (1 00 200), 246 (40 800), 317 (20 700), 330 (17 000). KM (H2 O, 293 K): 412 S cm2 mol1 . Red block-like crystals suitable for X-ray diffraction were obtained by diffusion of Et2 O into MeOH/ DMF solution over 1 week. The cryptates [LaL](NO3 )3 2H2 O (2), [CeL](NO3 )3 2H2 O (3), [PrL](NO3 )3 3H2 O (4), [SmL](NO3 )3 2H2 O (5), [GdL](NO3 )3 2H2 O (6), [TbL](NO3 )3 2H2 O (7) were prepared by a similar procedure as for 1, except that the corresponding lanthanide(III) nitrates were used instead of Eu(NO)3 6H2 O. [LaL](NO3 )3 2H2 O (2): Yellow tiny crystals. Yield: 75%. (Anal. Calc. for C33 H43 N14 O11 La (Mw 951.91): C, 41.69; H, 4.56; N, 20.63. Found: C, 41.25; H, 4.18; N, 20.66%.), IR (cm1 ): 1641 (s, m(C@N)); 1384 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 208 (68 700), 246 (20 430), 316 (10 200), 330 (8300). KM (H2 O, 293 K): 409 S cm2 mol1 . [CeL](NO3 )3 2H2 O (3): Orange solid. Yield: 75%. (Anal. Calc. for C33 H43 N14 O11 Ce (Mw 970.71) C, 41.64; H, 4.55; N, 20.60. Found: C, 41.85; H, 4.89; N, 20.35%.), IR (cm1 ): 1640 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 207 (97 600), 249 (25 400), 316 (12 200), 329 (9700). KM (H2 O, 293 K): 425 S cm2 mol1 . [PrL](NO3 )3 3H2 O (4): Yellow solid. Yield: 80%. (Anal. Calc. for C33 H45 N14 O12 Pr (Mw 970.71) C, 40.83; H, 4.67; N, 20.20. Found: C, 41.15; H, 4.25; N, 20.68%.) IR (cm1 ): 1642 (s, m(C@N)); 1384 (s, m(NO3 )), UV–Vis
(kmax /nm (e/dm3 mol1 cm1 ), H2 O): 207 (120 000), 245 (30 600), 316 (14 200), 329 (10 400). KM (H2 O, 293 K): 406 S cm2 mol1 . [SmL](NO3 )3 2H2 O (5): Orange solid. Yield: 83%. (Anal. Calc. for C33 H43 N14 O11 Sm (Mw 962.15) C, 41.20; H, 4.50; N, 20.38. Found: C, 41.35; H, 4.55; N, 20.55%.) IR (cm1 ): 1641 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 209 (92 000), 247 (24 700), 316 (11 500), 328 (9800). KM (H2 O, 293 K): 399 S cm2 mol1 . [GdL](NO3 )3 2H2 O (6): Yellow solid. Yield: 75%. (Anal. Calc. for C33 H43 N14 O11 Sm (Mw 969.04) C, 40.90; H, 4.47; N, 20.24. Found: C, 40.58; H, 4.56; N, 20.70%.) IR (cm1 ): 1634 (s, m(C@N)); 1383 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 211 (97 300), 249 (21 500), 318 (11 600), 329 (10 200). KM (H2 O, 293 K): 405 S cm2 mol1 . [TbL](NO3 )3 2H2 O (7): Orange solid. Yield: 74%. (Anal. Calc. for C33 H43 N14 O11 Tb (Mw 970.72) C, 40.84; H, 4.46; N, 20.20. Found: C, 41.40; H, 4.43; N, 20.67%.) IR (cm1 ): 1638 (s, m(C@N)); 1384 (s, m(NO3 )), UV–Vis (kmax /nm (e/dm3 mol1 cm1 ), H2 O): 205 (94 500), 246 (22 500), 317 (15 600), 330 (9500). KM (H2 O, 293 K): 441 S cm2 mol1 . 2.3. Physical measurements Elemental analysis of C, H, N were performed on a Perkin–Elmer 240c analytical instrument. The molar electrical conductivities in aqueous solution containing 104 mol l1 complexes were measured at 25 0.1 °C using a BSD-A conductometer (Jangsu, China). Electronic spectra were recorded on a UV-3100 spectrophotometer. IR spectra were measured using KBr discs with a vector 22 FI-IR spectrophotometer. Electrospray mass spectrum (ES-MS) was determined on a Finnigan LCQ ES-MS mass spectrograph using methanol as the mobile phrase with a sample concentration of about 1.0 mmol dm3 . The diluted solution was electrosprayed at a flow rate of 5 106 dm3 min1 with a needle voltage of +4.5 kV. The temperature of the heated capillary in the interface was 200 °C and a fuse silica sprayer was used. 2.4. Crystal structure determination Diffraction intensity data were collected on a SMART-CCD area-detector diffractometer at 293 K using graphite monochromatic Mo Ka radiation ). Data reduction and cell refinement (k ¼ 0:71073 A were performed by S M A R T and S A I N T programs [25]. The structure was solved by direct methods (Bruker SHELXTL) and refined on F 2 by full-matrix leastsquares (Bruker SHELXTL) using all unique data [26]. The non-H atoms in the structure were treated as anisotropic. Hydrogen atoms were located geometrically and refined in the riding mode.
X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
3. Results and discussion 3.1. Synthesis and characterization We have tried to synthesize lanthanide cryptates by the condensation of dfp with tren in the presence of Ln3þ or alkali-earth metal ions as templates by previous methods, but pure products could not be obtained. An effective method has been developed by reacting free ligand with the lanthanide nitrate. However, by the same procedure, we could not get pure products of the cryptates of the heavier lanthanides, which may be due to mismatch of their ionic size with the cavity of the ligand. The conductivities of all the seven lanthanide cryptates are in the range of 399–412 S cm2 mol1 , typical for a 1:3 electrolyte [27] and consistent with the chemical formulas from elemental analysis. The IR spectra of all cryptates show the characteristic vibrations of the C@N bond (1634–1642 cm1 ) and nitrate ions (1384 cm1 ). The vibration frequency of the observed C@N bond shifts to longer wavelength by 5–13 cm1 in contrast to that of the free ligand (1647 cm1 ) [19]. The intense absorptions in the electronic spectra of the cryptates at 200–240 and 310–330 nm are designated to the p–p transitions of the pyridyls and C@N groups, respectively.
51
one to two nitrate anions forming [ML(NO3 )]2þ or [ML(NO3 )2 ]þ . The simplicity of the spectra can be attributed to the thermodynamic and kinetic stability of the cryptates. For example, Fig. 1(a) shows the positiveion ES mass spectrum of 1 [EuL](NO3 )3 H2 O. The base peak at m=z ¼ 247:5 corresponds to [EuL]3þ . The peaks at m=z ¼ 402:1 can be assigned to the species [EuL(NO3 )]2þ . The isotopic distribution of the peak at m=z ¼ 247:5 is shown in Fig. 1(a). From the numbers
3.2. Electrospray mass spectra ES-MS data are listed in Table 1. There, except for 2, the ES-MS spectra of all cryptates show common features, and the spectra are composed of two to three simple peaks. The dominant peaks are assigned to the formation of [ML]3þ (M ¼ Eu, Ce, Pr, Sm, Gd, Tb) and the other peak clusters are due to [ML]3þ that captures
Fig. 1. The ES-MS spectra of cryptates 1 and 2 in MeOH solution: (a) cryptate 1 (inset: the isotopic distribution of the peak at m=z ¼ 247:5; left: experimental pattern; right: calculated pattern); (b) cryptate 2.
Table 1 ES-MS spectroscopic data for the cryptates 1–7 No.
Cryptate
Peak (m=z)
Relative abundance (%)
Assignment
1
[EuL](NO3 )3 H2 O
2
[LaL](NO3 )3 2H2 O
3
[CeL](NO3 )3 2H2 O
4
[PrL](NO3 )3 3H2 O
5
[SmL](NO3 )3 2H2 O
6
[GdL](NO3 )3 2H2 O
7
[TbL](NO3 )3 2H2 O
247.5 402.1 242.9 395.2 612.6 696.5 852.3 243.2 395.7 243.7 396.3 854.3 247.3 402.7 249.1 403.8 249.5 405.7
100 29 100 23 20 47 19 100 11 100 18 10 100 22 100 19 100 29
[EuL]3þ [EuL(NO3 )]2þ [LaL]3þ [LaL(NO3 )]2þ [NaL]þ [Na2 L(NO3 )]þ [LaL(NO3 )2 ]þ [CeL]3þ [CeL(NO3 )]2þ [PrL]3þ [PrL(NO3 )]2þ [PrL(NO3 )2 ]þ [SmL]3þ [SmL(NO3 )]2þ [GdL]3þ [GdL(NO3 )]2þ [TbL]3þ [TbL(NO3 )]2þ
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X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
and isotopic abundances of the atoms in [EuL]3þ , a revised program is used to calculated the pattern which is in good agreement with the experimental one (Fig. 1(a) inset). For the six cryptates, no peak of free ligand or ligand fragments is observed, showing that the cryptates and the ligand are rather stable in solution. For 2, the ES-MS spectra (Fig. 1(b)) are more complicated. Besides the species of [LaL]3þ and [LaL(NO3 )]2þ , two peaks at
m=z ¼ 612:5 and at m=z ¼ 696:5 are designed to [NaL]þ and [Na2 L(NO3 )]þ , respectively, implying that the cryptand can bind the Naþ ion to form an inclusion complex. The Naþ is present in a trace amount. The dissociation of the Laþ ion implies that the cryptate is less stable than the other six. Because of its larger radius, La3þ in the ligand cage is not so stable as Eu3þ so that the binding of the ligand to La3þ is weaker than those of other lanthanide ions.
Table 2 Crystal data of the complex [EuL]NO3 0.5H2 O
3.3. Crystal structure of the cryptate
Empirical formula Crystal system Space group ) a, b, c (A a, b, c (°) 3 ) V (A Dcalcd: (g/cm3 ) F ð0 0 0Þ Crystal size (mm) Temperature (K) l(Mo Ka) (mm1 ) 2h range (°) Reflections collected Independent reflections Observed reflections Index ranges w1 Goodness-of-fit on F 2 Final R1 ; wR2 ½I > 2r
C33 H40 N14 O9:5 Eu trigonal R-3c 14.709(1), 14.709(1), 32.167(3) 90, 90, 120 6027.1(8) 1.548 2850 0.20 0.20 0.30 293 1.631 2–25 9636 1189 757 17 6 h 6 17k; 17 6 k 6 9; 38 6 l 6 37 1=½r2 ðFo2 Þ þ ð0:0240P Þ2 þ 1:880P ; P ¼ ðFo2 þ 2Fc2 Þ=3 1.02 0.0419, 0.0919
The crystal data of 1 are listed in Table 2. Selected bond distances and bond angles are listed in Table 3. In the complex 1 (Fig. 2(a)), the central cation Eu1 is lo-
Table 3 ) and angles (°) of [EuL](NO3 )3 0.5H2 Oa Selected bond distances (A Bond distances Eu1–N1 Eu1–N1A Eu1–N1B Eu1–N1C Eu1–N1D Eu1–N1E Bond angles N1–Eu1–N2 N1–Eu1–N1A N1–Eu1–N2A N1–Eu1–N1B N1–Eu1–N2B N1–Eu1–N1C N1–Eu1–N1E N1–Eu1–N1D N1B–Eu1–N1E N1A–Eu1–N2A N1A–Eu1–N1B N1A–Eu1–N1C N1A–Eu1–N1E N1B–Eu1–N1D
2.597(3) 2.597(3) 2.597(3) 2.597(3) 2.597(3) 2.597(3) 63.28(8) 80.78(12) 72.96(7) 80.78(12) 137.97(7) 145.91(13) 126.56(15) 84.07(15) 84.07(15) 63.28(8) 80.78(12) 84.07(15) 145.91(13) 145.91(13)
Eu1–N2 Eu1–N2A Eu1–N2B Eu1 N3 Eu1 N3A
N1C–Eu1–N2B N1E–Eu1–N2B N1D–Eu1–N2B N1C–Eu1–N1E N1C–Eu1–N1D N1E–Eu1–N1D N1A–Eu1–N2 N2–Eu1–N2A N1B–Eu1–N2 N2–Eu1–N2B N1C–Eu1–N2 N1E–Eu1–N2 N1D–Eu1–N2 N1A–Eu1–N2B
2.559(5) 2.559(5) 2.559(5) 3.768(5) 3.768(5)
63.28(8) 72.95(7) 137.96(7) 80.78(12) 80.78(12) 80.78(12) 137.97(7) 120.00(1) 72.96(7) 120.00(1) 137.97(7) 63.28(8) 72.96(7) 72.96(7)
a Symmetry transformation used to generate equivalent atoms: (i) 1 y; 1 þ x y; z (N1A, N2A); (ii) x þ y; 1 x; z (N1B, N2B); (iii) 1=3 þ y; 1=3 þ x; 5=6 z (N1C, N3A); (iv) 2=3 þ x y; 4=3 y; 5=6 z (N1D); (v) 2=3 x; 1=3 x þ y; 5=6 z (N1E).
Fig. 2. (a) A perspective view of the [EuL]3þ cation; (b) the structure of the coordination polyhedron. Symmetry transformation used to generate equivalent atoms: (i) 1 y; 1 þ x y; z (N1A, N2A); (ii) x þ y; 1 x; z (N1B, N2B); (iii) 1=3 þ y; 1=3 þ x; 5=6 z (N1C, N3A); (iv) 2=3 þ x y, 4=3 y; 5=6 z (N1D); (v) 2=3 x; 1=3 x þ y; 5=6 z (N1E).
X.-L. Hu et al. / Polyhedron 23 (2004) 49–53
cated in the center of the macrocycle and coordinated by three pyridyl nitrogen atoms (N2, N2A and N2B) and six imino nitrogens (N1, N1A, N1B, N1C, N1D, N1E) to form a tricapped prism (Fig. 2(b)). The complex cation has a threefold axis through Eu1 and the two bridgehead nitrogen atoms (N3, N3A). The average bond length between Eu1 and nitrogen atoms of the , being slightly shorter than pyridine groups is 2.559 A ). The average length those for the imino groups (2.598 A . No interaction beof Eu1–N (bridgehead) is 3.768 A tween Eu1 and N(bridgehead) is observed. The coordination polyhedron of 1 can be described approximately as a tricapped prism. The upper and basal planes of the prism, which are approximately parallel, are composed of the imino nitrogen atoms (N1, N1A, N1B and N1C, N1D, N1E, respectively). The distance between the two . The distance between Eu1 and the planes is 3.446 A . The three pyridyl nitrogen atoms upper plane is 1.723 A are located at the capping positions. Eu1 is located in the symmetrical center of the prism. The distances of Eu1 to each least squares prism plane is approximately . The tricapped prism structure is different from 0.964 A the unsymmetrical structure of sodium cryptate with the same ligand [22] and those of phenol-based lanthanide cryptates [13,20,21], in which the central ions are located at one side of the macrocycles. The present Eu(III) cryptate is located at the inversion center of the cavity of the macrocycle in order to satisfy the requirement of a higher coordination number of the lanthanide ion. To our knowledge, this is the first crystal structure of a lanthanide cryptate derived from 2,6-pyridyl-dicarboxaldehyde.
4. Supplementary data Supplementary data are available from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; E-mail: [email protected] or www: http://www.ccdc.cam.ac.uk) on request, quoting the deposition number CCDC-208092.
Acknowledgements This work was supported by the National Natural Science Foundation of China.
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References [1] V. Alexander, Chem. Rev. (1995) 273. [2] C. Piguet, J.-C.G. B€ unzli, Chem. Soc. Res. 18 (1999) 347. [3] Q.-Y. Chen, Q.-H. Luo, X.-L. Hu, M.-C. Shen, J.-T. Chen, Chem. Eur. J. 8 (2002) 3984. [4] G.F. de Sa, O.L. Malta, C. de Mello Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165. [5] B.H. Bakker, M. Goes, N. Hoebe, H.J. Van Ramesdonk, J.W. Verhoeven, M.H.V. Werts, J.W. Hofstraat, Coord. Chem. Res. 208 (2000) 3. [6] S. Aime, M. Botta, M. Fasano, E. Terreno, Chem. Soc. Rev. 27 (1998) 19. [7] J.-M. Lehn, D.D. Macnicol, J.L. Atwood, J.E. Davies, D.N. Reinhoudt, F. Vogtl (Eds.), Comprehensive Supramolecular Chemistry, vol. 10, Pergamon, Oxford, 1996 (Ch. 17). [8] A. Beeby, S.W. Botchway, I.M. Clarkson, S. Faulkner, A.W. Parker, D. Parker, J.A. Gareth Williams, J. Photochem. Photobiol. B 57 (2000) 83. [9] T. Gunnlaugsson, D.A. MacDonail, D. Parker, Chem. Commun. (2000) 93. [10] Q.-Y. Chen, Q.-H. Luo, Z.-L. Wang, J.-T. Chen, Chem. Commun. (2000) 1033. [11] Q.-Y. Chen, Q.-H. Luo, L.-M. Zheng, Z.-L. Wang, J.-T. Chen, Inorg. Chem. 41 (2002) 605. [12] G. Vicentini, L.B. Zinner, J. Zukerman-Schpector, K. Zinner, Coord. Chem. Rev. 196 (2000) 353. [13] Q.-Y. Chen, C.-J. Feng, Q.-H. Luo, C.-Y. Duan, X.-S. Yu, Y.-J. Liu, Eur. J. Inorg. Chem. (2001) 1067. [14] C. Bazzicalupi, A. Bencini, E. Berni, A. Bianchi, C. Giorgi, V. Fusi, B. Valtancoli, C. Lodeiro, A. Roque, F. Pina, Inorg. Chem. 40 (2001) 6172. [15] B. Alpha, J.-M. Lehn, G. Mathis, Angew. Chem., Int. Ed. Engl. 26 (1987) 266. [16] C.O. Paul-Roth, J.-M. Lehn, J. Guilhem, C. Pascard, Helv. Chim. Acta 78 (1995) 1895, and references therein. [17] J.-M. Lehn, J.-B.R. de Vains, Helv. Chim. Acta 75 (1992) 1221. [18] C. Galaup, C. Picard, H. Autiero, D. Aspe, Helv. Chim. Acta 82 (1999) 544. [19] D.J. Marrs, V. Mckee, J. Nelson, Q. Lu, C.J. Harding, Inorg. Chim. Acta 211 (1993) 195. [20] M.G. Drew, O.W. Howarth, C.J. Harding, N. Martin, J. Nelson, J. Chem. Soc., Chem. Commun. (1995) 903. [21] C.-J. Feng, Q.H. Luo, C.-Y. Duan, M.-C. Shen, Y.-J. Liu, J. Chem. Soc., Dalton Trans. (1998) 1377. [22] V. Mckee, M.R.J. Dorrity, J.F. Malone, D. Marrs, J. Nelson, J. Chem. Soc., Chem. Commun. (1992) 383. [23] E. Kimura, S. Young, J.P. Collman, Inorg. Chem. 9 (1970) 1187. [24] C.J. Gray, F.A. Hart, J. Chem. Soc., Dalton Trans. (1987) 2289. [25] S M A R T and S A I N T , Area Detector Control and Integration Software. Siemens Analytical X-Ray Systems, Inc., Madison, WI, USA, 1996. [26] G.M. Sheldrick, S H E L X T L V5.1 Software Reference Manual, Bruker AXS, Madison, WI, USA, 1997. [27] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.