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Synthesis, structure and magnetic properties of lanthanide(III) complexes with new imidazole-substituted imino nitroxide radical
Inorganica Chimica Acta 358 (2005) 1253–1257 www.elsevier.com/locate/ica
Note
Synthesis, structure and magnetic properties of lanthanide(III) complexes with new imidazole-substituted imino nitroxide radical Toshiaki Tsukuda, Takayoshi Suzuki, Sumio Kaizaki
*
Department of Chemistry, Graduate School of Science, Faculty of Science, Osaka University, 1-1 Machikaneyamachyo, Toyonaka, Osaka 5600043, Japan Received 13 January 2004; accepted 6 November 2004
Abstract Synthesis, characterization and magnetic properties of new lanthanide–radical complexes, [LnIII(hfac)3(IM2imH)] (Ln = Gd, Tb; IM2imH = 2-(2-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy), are described. The molecular structure of the [Tb(hfac)3(IM2imH)] has been determined by the X-ray diffraction. The magnetic susceptibility data for [Gd(hfac)3(IM2imH)] show that the Gd–IM2imH magnetic interaction is antiferromagnetic with an exchange coupling constant J = 2.59 cm1 in contrast to the ferromagnetic interaction in most of Gd(III) complexes containing paramagnetic center, which will be examined in connection with planarity of the IM2imH chelate. 2004 Elsevier B.V. All rights reserved. Keywords: Lanthanide complexes; Imino nitroxide radical ligand; Magnetic interaction
1. Introduction In the last few years, much attention has been paid to metal–radical complexes for the design of molecularbased magnetic materials in terms of the so-called radical approach [1]. In this context, there have been reported several nitroxide radical gadolinium(III) complexes [2]. The intramolecular magnetic interaction between Gd(III) and other spin (e.g., 3d or radical electron) was considered to be generally ferromagnetic [2a,3], which are elucidated by the experimental approach [3c]. However, the exceptional examples were recently found subsequently; the antiferromagnetic interaction in the nitronyl nitroxide radical Gd(III) complexes reported by Luneau and co-workers [2b] and by Gatteschi and co-workers [4], following our recent result *
Corresponding author. Tel./fax: +81 668 505 408. E-mail address: [email protected] (S. Kaizaki).
0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.11.001
[5] for [Gd(hfac)3(IM2py)] (IM2py = 2-(2 0 -pyridyl)4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-1-oxy; Chart 1). Such a magnetic interaction may be elucidated in terms of the electron transfer of the unpaired electron of the organic radical ligands into the empty 5d orbitals of Gd(III) ions, whereby the extent of ferro- or antiferromagnetic interaction may depend on the overlap between the SOMO p* orbital of the radical ligands and the 5d orbitals of the metal. Thus, the antiferromagnetic interaction is assumed in the case of the planarity of SOMO p* orbital and N,N-five-membered chelate ring as found for transition metal complexes. This might be realized for the IM2py complex [6]. An attempt to test such an assumption qualitatively is invaluable to illuminate the magnetic interaction of lanthanide(III) complexes, though analytical determination has been made for the quantitative evaluation [7]. For this purpose, the IM2imH complex (IM2imH = 2-(4-imidazolyl)-4,5-dihydro-4,4,5,5-tetramethyl-imidazoline-1-oxyl)
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2.2. Magnetic measurement
N O N
N
Magnetic susceptibility data were collected at 2000 Oe between 2 and 300 K by using a SQUID susceptometer (MPMS-5S, QUANTUM design). Pascals constants were used to determine the continuent atom diamagnetism. 2.3. X-ray structural analysis
Chart 1.
forming a N(IM),N(im)-five-membered chelate ring is a suitable candidate for comparison with the IM2py complex. In addition, it is anticipated that polymeric chains could be formed by deprotonating the imidazolyl moiety to coordinate to the other metal centers [8]. In this paper, we describe the preparation and the crystal structure of [Ln(hfac)3(IM2imH)] complexes containing IM2imH as such building block. The magnetic property of the Gd complex has also been reported.
2. Experimental All Chemicals were of reagent grade and used without purification. LnCl3 Æ 6H2O (Tb and Gd), Hhfac and 4-imidazolcarbaldehyde were purchased from Wako Pure Chemical Industries Co. Ltd., Tokyo Chemical Industry Co. Ltd. and Aldrich Co. Ltd., respectively. The starting complexes [Ln(hfac)3(H2O)2] were prepared by the literature method [9]. 2-(4-imidazolyl)-4,5-dihydro-4,4,5,5-tetramethyl-imidazoline-1-oxyl (IM2imH) was prepared by the similar procedure by using 4-imidazolcarbaldehyde instead of 2-pyridinecarbaldehyde [10]. 2.1. Preparation of [Ln(hfac)3(IM2imH)] (Ln = Gd, Tb) These complexes were obtained according to the literature procedures [8]. [Ln(hfac)3(H2O)2] (1.0 mmol) was suspended in 10 ml CH2Cl2. IM2imH (1.10 mmol) in CH2Cl2 (10 ml) was added to this suspension. After stirring for ca. 2 h, the red solution was obtained. The solution was poured into 20 ml of n-heptane and allowed to evaporate slowly. Red orange crystals were obtained on standing for overnight. Yield: 0.43 g (44%) for Gd; 0.55 g (56%) for Tb. Elemental Anal. Calc. for C25H18N4F18O7Gd, [Gd(hfac)3(IM2imH)]: C, 30.46; H, 1.84; N, 5.68. Found: C, 30.79; H, 1.81; N, 5.31%. Anal. Calc. for C25H18N4F18O7Tb, [Tb(hfac)3(IM2imH)]: C, 30.40; H, 1.84; N, 5.67. Found: C, 30.32; H, 1.93; N, 5.55%.
Red orange crystals of [Tb(hfac)3(IM2imH)] suitable for X-ray crystal analysis were obtained by slowly evaporating the complex solution in a mixture of dichloromethane and n-heptane. The obtained crystal was mounted in a glass capillary. The X-ray intensities were measured at 23 C with graphite-monochromated Mo ˚ ) on a Rigaku AFC-5R or Ka radiation (k = 0.71073 A four-circle diffractometer using x–2h scan technique was employed at scan rate. Final lattice constants were determined by least-squares refinements of the orientation angles of 25 centered reflections in the range 25 < 2h < 30. Three standard reflections were monitored every 150 reflections and showed no serious decomposition. (|Fo|/(|Fo|)initial > 99%). The intensities collected for (h, +k, ±l) octants at 2h 6 60 were corrected for Lorentz-polarization effects and absorption corrections were made by the Gauss numerical integration method [11]. The structure could be solved reasonably by using direct method (SHELXS -97 [12]) and refined by SHELXL -97 program [13]. The positions of hydrogen atoms were fixed at calculated positions and only their isotropic displacement parameters were refined. All calculations were carried out by CRYSTAL STRUCTURE software [14]. 3. Results and discussion 3.1. Molecular structure of [Tb(hfac)3(IM2imH)] Crystallographic data for each complex are listed in Table 1. There are two independent structure molecules in an unit cell, which are enantiomers with each other (Fig. 1(a)). One of the independent structure molecules of [Tb(hfac)3(IM2imH)] is illustrated in Fig. 1(b). The Tb atom is eight-coordinated with three didentate hfac and one didentate IM2imH ligand. The N(1) and N(3) donors were coordinated to Tb atom, whereas the N(4) was not coordinated to any metal center. Selected bond lengths and angles are also summarized in Table 2. The bond length of N(2)–O(1) ˚ ) in the Tb complex is similar to that in (1.291(13) A the other lanthanide complexes containing IM2py ˚ ) [5], which is much shorter than that of (1.263–1.295 A ˚ ). This fact indicates average N–O single bond (1.41 A the existence of a radical unpaired spin on complexa-
T. Tsukuda et al. / Inorganica Chimica Acta 358 (2005) 1253–1257 Table 1 Crystallographic data for [Tb(hfac)3(IM2imH)] Formula Formula weight Crystal system Space group Crystal size (mm) ˚) a (A ˚) b (A ˚) c (A () ˚ 3) V (A Z F (0 0 0) Dcalc (g cm3) l (Mo Ka) (cm1) R1 wR2 Goodness-of-fit
C25H18O7N4F18Tb 973.72 monoclinic P21/c 0.50 · 0.10 · 0.10 18.469(8) 17.744(3) 22.314(3) 105.13(2) 7059(3) 4 3832 1.858 17.54 0.067 0.245 0.87
˚ ) is tion. The bond length of the Tb–N(3) (2.504(10) A considerably shorter than the Tb–N(1) one (2.582(10) ˚ ), whereas in [Ln(hfac)3(IM2py)] (Ln = Sm, Gd, Dy, A Er, Yb), the bond lengths of Ln–N(pyridine) are almost similar to that of Ln–N(imidazole) [3]. The Tb–O(hfac) ˚ (Tb–O(7A)) to bond lengths range from 2.327(9) A ˚ 2.419(8) A (Tb–O(6b)). The torsion angle for N(1)–C(1)–C(2)–N(3) (1.9(14)) is smaller than the corresponding one (8.6(8)) for the IM2py complex, showing that the IM2imH chelate is more planar than the IM2py one. And the torsion angles for Tb(1)–N(1)–C(1)–C(2) (3.3(16)) and Tb(1)–N(3)– C(2)–C(1) show that the chelate plane of the Tb– IM2imH is nearly planar. The semi-quantitative method of polytopal analysis is examined in which the coordination geometry of this
Table 2 Selected bond distances [Tb(hfac)3(IM2imH)] ˚) Bond lengths (A
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˚) (A
and
bond
angles
()
of
Tb(1a)–O(2a) Tb(1a)–O(3a) Tb(1a)–O(4a) Tb(1a)–O(5a) Tb(1a)–O(6a) Tb(1a)–O(7a) Tb(1a)–N(1a) Tb(1a)–N(3a) O(1a)–N(2a)
2.373(9) 2.385(9) 2.338(9) 2.36(1) 2.39(1) 2.33(1) 2.57(1) 2.51(1) 1.30(2)
Tb(1b)–O(2b) Tb(1b)–O(3b) Tb(1b)–O(4b) Tb(1b)–O(5b) Tb(1b)–O(6b) Tb(1b)–O(7b) Tb(1b)–N(1b) Tb(1b)–N(3b) O(1b)–N(2b)
2.364(8) 2.371(9) 2.342(9) 2.336(9) 2.42(1) 2.35(1) 2.60(1) 2.50(1) 1.28(2)
Bond angles () O(2a)–Tb(1a)–O(3a) O(2a)–Tb(1a)–O(5a) O(2a)–Tb(1a)–O(7a) O(2a)–Tb(1a)–N(3a) O(3a)–Tb(1a)–O(5a) O(3a)–Tb(1a)–O(7a) O(3a)–Tb(1a)–N(3a) O(4a)–Tb(1a)–O(6a) O(4a)–Tb(1a)–N(1a) O(5a)–Tb(1a)–O(6a) O(5a)–Tb(1a)–N(1a) O(6a)–Tb(1a)–O(7a) O(6a)–Tb(1a)–N(3a) O(7a)–Tb(1a)–N(3a)
71.7(3) 145.6(3) 106.6(3) 141.2(4) 139.8(3) 76.0(4) 73.1(3) 74.1(4) 77.7(4) 76.5(4) 110.2(4) 71.8(4) 142.1(4) 79.7(4)
O(2a)–Tb(1a)–O(4a) O(2a)–Tb(1a)–O(6a) O(2a)–Tb(1a)–N(1a) O(3a)–Tb(1a)–O(4a) O(3a)–Tb(1a)–O(6a) O(3a)–Tb(1a)–N(1a) O(4a)–Tb(1a)–O(5a) O(4a)–Tb(1a)–O(7a) O(4a)–Tb(1a)–N(3a) O(5a)–Tb(1a)–O(7a) O(5a)–Tb(1a)–N(3a) O(6a)–Tb(1a)–N(1a) O(7a)–Tb(1a)–N(1a) N(1a)–Tb(1a)–N(3a)
83.9(3) 72.6(4) 88.9(4) 143.1(4) 121.5(3) 74.7(3) 73.2(3) 139.0(4) 116.8(3) 77.2(4) 73.1(3) 147.6(4) 140.2(4) 66.4(4)
Torsion angles () N(1a)–C(1a)–C(2a)–N(3a) Tb(1a)–N(1a)–C(1)–C(2) Tb(1a)–N(3a)–C(2)–C(1) N(1a)–C(1a)–N(2a)–O(1a)
4(1) 3(1) 9(1) 1(1)
The bond angles and torsion angles of the other molecules are omitted.
Table 3 d values and / values of [Tb(hfac)3(IM2imH)] Tb complex
SAPR-8
TPRS-8
DD-8
N(3) [O(5) N(1)] O(4)a N(3) [O(3) N(1)] O(2)a O(7) [O(5) O(6)] O(4)a O(7) [O(3) O(6)] O(2)a
6.70 53.89 51.39 14.51
0.0 52.4 52.4 0.0
21.8 48.2 48.2 0.0
29.5 29.5 29.5 29.5
/1: N(1)–O(6)–N(3)–O(7)b /2: O(5)–O(3)–O(4)–O(2)b
22.63 27.61
24.5 24.5
14.1 14.1
0.0 0.0
d1: d2: d3: d4:
a
A [B C] D is dihedral angle between ABC plane and BCD plane. A–B–C–D is dihedral angle between (AB)CD and AB(CD)plane, where (AB) is the center of A and B. b
Fig. 1. (a) Ortep view of [Tb(hfac)3(IM2imH)] (50% probability ellipsoids), in which H and F atoms are omitted. (b) The polyhedron showing SAPR-8 with donor atoms around Tb metal center.
complex is square antiprism (SAPR), dodecahedral (DD) or bicapped trigonal prism (TPRS) [15]. The d and / values are summarized in Table 3. The d1 and d2 values showing planarity of the squares are 6.70 and 14.51, respectively. But the d3 and d4 values are 53.89 and 51.39, which are relatively larger. The /1 and /2 values are 22.63 and 27.61, which are fairly close to the angle (24.5) of the ideal SAPR polyhedron. Thus, the most reasonable geometry around the Tb atom is SAPR as shown in Fig. 1.
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p* orbital results in the decrease in ferromagnetic contribution to the Jobs or increase in the antiferromagnetic interaction.
4. Conclusion
Fig. 2. Temperature dependence of vMT in the 2–250 K range for [Gd(hfac)3(IM2imH)]. The solid line represents the best fit calculation (see text).
3.2. Magnetic properties of [Gd(hfac)3(IM2imH)] The variable temperature magnetic susceptibilities of [Gd(hfac)3(IM2imH)] are shown in the form of the vMT versus T plots in Fig. 2. Since the ground state of Gd(III) (8S7/2) is orbitally non-degenerate and well separated from the excited state, Gd(III) gives a simple single ion magnetic properties. The observed vMT value (8.17 cm3 mol1 K) at room temperature is similar to the uncoupled value for Gd(III) and IM2imH (the calculated value is 8.12 cm3 mol1 K). The vMT value of this complex slightly decreases by lowering the temperature, and then below 100 K, the drastic decrease occurs, following a slight increase below 4 K which is probably due either to weak intermolecular magnetic interaction between the IM2imH radicals or to experimental artifact. The magnetic data were analyzed in consideration of only the intramolecular interaction between the IM2imH spin and 4f spins. The usual van Vlecks equation for this type of dimer was derived by using the isotropic Hamiltonian H = 2JS1 Æ S2 [16]. A good fit to the experimental data is obtained for J = 2.59 cm1 (g = 1.97), which is comparable to that of the corresponding IM2py complex. The magnetic interaction between Gd(III) ion and paramagnetic species is claimed to involve the electron transfer of the unpaired electron in the organic radical ligand into the empty 5d orbitals of the metal, resulting in the parallel alignment of 4f and 5d electrons according to Hunds rule [17]. Thereby, the magnetic interaction is generally ferromagnetic. Since the planarity of the chelated plane in IM2imH, demonstrated by the X-ray structural study of the IM2imH Tb complex, is assumed to be not so much different from that in the Gd complex, the SOMO p* orbital in IM2imH ligand of the Gd complex is predicted to be less overlapped or orthogonal with the 5d orbital as similarly presumed for the Gd–IM2py complex. Since the observed magnetic coupling constant Jobs represents the sum of the antiferromagnetic one JAF and ferromagnetic one JF (Jobs = JAF + JF), the decreasing overlap of 5d with the IM2imH SOMO
Newly prepared IM2imH Tb(III) and Gd(III) complexes were characterized by the X-ray analysis and/or the magnetic susceptibility. The magnetic interaction between Gd(III) and IM-type radical, where the planar chelate is formed, is always antiferromagnetic. IM2imH could be a promising ligand to build new polymeric magnetic materials as deprotonation of imidazole moiety in this ligand enables the ligand to coordinate onto the other lanthanide ions.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version at doi:10.1016/ j.ica.2004.11.001.
References [1] (a) J.S. Miller, A.J. Epstein, Angew. Chem. Int. Ed. 33 (1994) 385; (b) J.S. Miller, Inorg. Chem. 39 (2000) 4392, and references therein; (c) A. Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22 (1989) 392; (d) A. Caneschi, D. Gatteschi, P. Rey, Prog. Inorg. Chem. 39 (1991) 331, and references therein; (e) K.E. Vostrikova, D. Luneau, W. Wernsdorfer, P. Rey, M. Verdaguer, J. Am. Chem. Soc. 122 (2000) 718; (f) J.-P. Sutter, M.L. Kahn, K.P. Mo¨rtl, R. Ballou, P. Porcher, Polyhedron (2001) 1593; (g) J.-P. Sutter, M.L. Kahn, O. Kahn, Adv. Mater. 11 (1999) 863. [2] (a) C. Benelli, D. Gatteschi, Chem. Rev. 102 (2002) 2369; (b) C. Lescop, D. Luneau, E. Belorizky, P. Fries, M. Guillot, P. Rey, Inorg. Chem. 38 (1999) 5472. [3] (a) M.L. Kahn, P. Lecante, M. Verelst, C. Mathonie`re, O. Kahn, Chem. Mater. 12 (2000) 3073; (b) J.-P. Costes, J.-M.C. Juan, F. Dahan, F. Nicode`me, J. Chem. Soc., Dalton Trans. (2003) 1272; (c) M.L. Kahn, J.-P. Sutter, S. Golhen, P. Guionneau, L. Ouahab, O. Kahn, D. Chasseau, J. Am. Chem. Soc. 122 (2000) 3413. [4] A. Caneschi, A. Dei, D. Gatteschi, L. Sorace, K. Vostrikova, Angew. Chem. Int. Ed. 39 (2000) 246. [5] (a) T. Tsukuda, T. Suzuki, S. Kaizaki, J. Chem. Soc., Dalton Trans. (2002) 1721; (b) T. Tsukuda, T. Suzuki, S. Kaizaki, Mol. Cryst. Liq. Cryst. 379 (2002) 159. [6] (a) T. Yoshida, T. Suzuki, K. Kanamori, S. Kaizaki, Inorg. Chem. 38 (1999) 1054; (b) Y. Tsukahara, A. Iino, T. Yoshida, T. Suzuki, S. Kaizaki, J. Chem. Soc., Dalton Trans. (2002) 181; (c) Y. Tsukahara, T. Kamatani, A. Iino, T. Suzuki, S. Kaizaki, Inorg. Chem. 41 (2002) 4363; (d) S. Kaizaki, Bull. Chem. Soc. Jpn. 76 (2003) 673.
T. Tsukuda et al. / Inorganica Chimica Acta 358 (2005) 1253–1257 [7] M. Kahn, R. Ballou, P. Porcher, O. Kahn, J.-P. Sutter, Chem. Eur. J. 8 (2002) 525. [8] C. Aoki, T. Ishida, T. Nogami, Inorg. Chem. 42 (2003) 7616. [9] M.F. Richardson, W.F. Wagner, D.E. Sands, J. Inorg. Nucl. Chem. 30 (1968) 1275. [10] T. Akita, Y. Mazaki, K. Kobayashi, J. Org. Chem. 60 (1995) 2092. [11] W.L. Busing, H.A. Levy, Acta Crystallogr. 10 (1957) 180. [12] G.M. Sheldrick, SHELXL -97: Program for Crystal Structure Determination, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
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[13] G.M. Sheldrick, SHELXL -97: Program for the Refinement of Crystal Structures, University of Go¨ttingen, Go¨ttingen, Germany, 1997. [14] (a) Crystal Structure Ver. 2.00; Molecular Structure Cooperation and Rigaku Co. Ltd; (b) teXsan ver.1.11; Crystal Structure Analysis Package, Molecular Structure Cooperation. [15] M.G.B. Drew, Coord. Chem. Rev. 24 (1977) 179. [16] O. Kahn, Molecular Magnetism, VCH, New York, 1993. [17] J.-P. Sutter, M.L. Kahn, S. Golhen, L. Ouahab, O. Kahn, Chem. Eur. J. 4 (1998) 571.
Note
Synthesis, structure and magnetic properties of lanthanide(III) complexes with new imidazole-substituted imino nitroxide radical Toshiaki Tsukuda, Takayoshi Suzuki, Sumio Kaizaki
*
Department of Chemistry, Graduate School of Science, Faculty of Science, Osaka University, 1-1 Machikaneyamachyo, Toyonaka, Osaka 5600043, Japan Received 13 January 2004; accepted 6 November 2004
Abstract Synthesis, characterization and magnetic properties of new lanthanide–radical complexes, [LnIII(hfac)3(IM2imH)] (Ln = Gd, Tb; IM2imH = 2-(2-pyridyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy), are described. The molecular structure of the [Tb(hfac)3(IM2imH)] has been determined by the X-ray diffraction. The magnetic susceptibility data for [Gd(hfac)3(IM2imH)] show that the Gd–IM2imH magnetic interaction is antiferromagnetic with an exchange coupling constant J = 2.59 cm1 in contrast to the ferromagnetic interaction in most of Gd(III) complexes containing paramagnetic center, which will be examined in connection with planarity of the IM2imH chelate. 2004 Elsevier B.V. All rights reserved. Keywords: Lanthanide complexes; Imino nitroxide radical ligand; Magnetic interaction
1. Introduction In the last few years, much attention has been paid to metal–radical complexes for the design of molecularbased magnetic materials in terms of the so-called radical approach [1]. In this context, there have been reported several nitroxide radical gadolinium(III) complexes [2]. The intramolecular magnetic interaction between Gd(III) and other spin (e.g., 3d or radical electron) was considered to be generally ferromagnetic [2a,3], which are elucidated by the experimental approach [3c]. However, the exceptional examples were recently found subsequently; the antiferromagnetic interaction in the nitronyl nitroxide radical Gd(III) complexes reported by Luneau and co-workers [2b] and by Gatteschi and co-workers [4], following our recent result *
Corresponding author. Tel./fax: +81 668 505 408. E-mail address: [email protected] (S. Kaizaki).
0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.11.001
[5] for [Gd(hfac)3(IM2py)] (IM2py = 2-(2 0 -pyridyl)4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazoline-1-oxy; Chart 1). Such a magnetic interaction may be elucidated in terms of the electron transfer of the unpaired electron of the organic radical ligands into the empty 5d orbitals of Gd(III) ions, whereby the extent of ferro- or antiferromagnetic interaction may depend on the overlap between the SOMO p* orbital of the radical ligands and the 5d orbitals of the metal. Thus, the antiferromagnetic interaction is assumed in the case of the planarity of SOMO p* orbital and N,N-five-membered chelate ring as found for transition metal complexes. This might be realized for the IM2py complex [6]. An attempt to test such an assumption qualitatively is invaluable to illuminate the magnetic interaction of lanthanide(III) complexes, though analytical determination has been made for the quantitative evaluation [7]. For this purpose, the IM2imH complex (IM2imH = 2-(4-imidazolyl)-4,5-dihydro-4,4,5,5-tetramethyl-imidazoline-1-oxyl)
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2.2. Magnetic measurement
N O N
N
Magnetic susceptibility data were collected at 2000 Oe between 2 and 300 K by using a SQUID susceptometer (MPMS-5S, QUANTUM design). Pascals constants were used to determine the continuent atom diamagnetism. 2.3. X-ray structural analysis
Chart 1.
forming a N(IM),N(im)-five-membered chelate ring is a suitable candidate for comparison with the IM2py complex. In addition, it is anticipated that polymeric chains could be formed by deprotonating the imidazolyl moiety to coordinate to the other metal centers [8]. In this paper, we describe the preparation and the crystal structure of [Ln(hfac)3(IM2imH)] complexes containing IM2imH as such building block. The magnetic property of the Gd complex has also been reported.
2. Experimental All Chemicals were of reagent grade and used without purification. LnCl3 Æ 6H2O (Tb and Gd), Hhfac and 4-imidazolcarbaldehyde were purchased from Wako Pure Chemical Industries Co. Ltd., Tokyo Chemical Industry Co. Ltd. and Aldrich Co. Ltd., respectively. The starting complexes [Ln(hfac)3(H2O)2] were prepared by the literature method [9]. 2-(4-imidazolyl)-4,5-dihydro-4,4,5,5-tetramethyl-imidazoline-1-oxyl (IM2imH) was prepared by the similar procedure by using 4-imidazolcarbaldehyde instead of 2-pyridinecarbaldehyde [10]. 2.1. Preparation of [Ln(hfac)3(IM2imH)] (Ln = Gd, Tb) These complexes were obtained according to the literature procedures [8]. [Ln(hfac)3(H2O)2] (1.0 mmol) was suspended in 10 ml CH2Cl2. IM2imH (1.10 mmol) in CH2Cl2 (10 ml) was added to this suspension. After stirring for ca. 2 h, the red solution was obtained. The solution was poured into 20 ml of n-heptane and allowed to evaporate slowly. Red orange crystals were obtained on standing for overnight. Yield: 0.43 g (44%) for Gd; 0.55 g (56%) for Tb. Elemental Anal. Calc. for C25H18N4F18O7Gd, [Gd(hfac)3(IM2imH)]: C, 30.46; H, 1.84; N, 5.68. Found: C, 30.79; H, 1.81; N, 5.31%. Anal. Calc. for C25H18N4F18O7Tb, [Tb(hfac)3(IM2imH)]: C, 30.40; H, 1.84; N, 5.67. Found: C, 30.32; H, 1.93; N, 5.55%.
Red orange crystals of [Tb(hfac)3(IM2imH)] suitable for X-ray crystal analysis were obtained by slowly evaporating the complex solution in a mixture of dichloromethane and n-heptane. The obtained crystal was mounted in a glass capillary. The X-ray intensities were measured at 23 C with graphite-monochromated Mo ˚ ) on a Rigaku AFC-5R or Ka radiation (k = 0.71073 A four-circle diffractometer using x–2h scan technique was employed at scan rate. Final lattice constants were determined by least-squares refinements of the orientation angles of 25 centered reflections in the range 25 < 2h < 30. Three standard reflections were monitored every 150 reflections and showed no serious decomposition. (|Fo|/(|Fo|)initial > 99%). The intensities collected for (h, +k, ±l) octants at 2h 6 60 were corrected for Lorentz-polarization effects and absorption corrections were made by the Gauss numerical integration method [11]. The structure could be solved reasonably by using direct method (SHELXS -97 [12]) and refined by SHELXL -97 program [13]. The positions of hydrogen atoms were fixed at calculated positions and only their isotropic displacement parameters were refined. All calculations were carried out by CRYSTAL STRUCTURE software [14]. 3. Results and discussion 3.1. Molecular structure of [Tb(hfac)3(IM2imH)] Crystallographic data for each complex are listed in Table 1. There are two independent structure molecules in an unit cell, which are enantiomers with each other (Fig. 1(a)). One of the independent structure molecules of [Tb(hfac)3(IM2imH)] is illustrated in Fig. 1(b). The Tb atom is eight-coordinated with three didentate hfac and one didentate IM2imH ligand. The N(1) and N(3) donors were coordinated to Tb atom, whereas the N(4) was not coordinated to any metal center. Selected bond lengths and angles are also summarized in Table 2. The bond length of N(2)–O(1) ˚ ) in the Tb complex is similar to that in (1.291(13) A the other lanthanide complexes containing IM2py ˚ ) [5], which is much shorter than that of (1.263–1.295 A ˚ ). This fact indicates average N–O single bond (1.41 A the existence of a radical unpaired spin on complexa-
T. Tsukuda et al. / Inorganica Chimica Acta 358 (2005) 1253–1257 Table 1 Crystallographic data for [Tb(hfac)3(IM2imH)] Formula Formula weight Crystal system Space group Crystal size (mm) ˚) a (A ˚) b (A ˚) c (A () ˚ 3) V (A Z F (0 0 0) Dcalc (g cm3) l (Mo Ka) (cm1) R1 wR2 Goodness-of-fit
C25H18O7N4F18Tb 973.72 monoclinic P21/c 0.50 · 0.10 · 0.10 18.469(8) 17.744(3) 22.314(3) 105.13(2) 7059(3) 4 3832 1.858 17.54 0.067 0.245 0.87
˚ ) is tion. The bond length of the Tb–N(3) (2.504(10) A considerably shorter than the Tb–N(1) one (2.582(10) ˚ ), whereas in [Ln(hfac)3(IM2py)] (Ln = Sm, Gd, Dy, A Er, Yb), the bond lengths of Ln–N(pyridine) are almost similar to that of Ln–N(imidazole) [3]. The Tb–O(hfac) ˚ (Tb–O(7A)) to bond lengths range from 2.327(9) A ˚ 2.419(8) A (Tb–O(6b)). The torsion angle for N(1)–C(1)–C(2)–N(3) (1.9(14)) is smaller than the corresponding one (8.6(8)) for the IM2py complex, showing that the IM2imH chelate is more planar than the IM2py one. And the torsion angles for Tb(1)–N(1)–C(1)–C(2) (3.3(16)) and Tb(1)–N(3)– C(2)–C(1) show that the chelate plane of the Tb– IM2imH is nearly planar. The semi-quantitative method of polytopal analysis is examined in which the coordination geometry of this
Table 2 Selected bond distances [Tb(hfac)3(IM2imH)] ˚) Bond lengths (A
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˚) (A
and
bond
angles
()
of
Tb(1a)–O(2a) Tb(1a)–O(3a) Tb(1a)–O(4a) Tb(1a)–O(5a) Tb(1a)–O(6a) Tb(1a)–O(7a) Tb(1a)–N(1a) Tb(1a)–N(3a) O(1a)–N(2a)
2.373(9) 2.385(9) 2.338(9) 2.36(1) 2.39(1) 2.33(1) 2.57(1) 2.51(1) 1.30(2)
Tb(1b)–O(2b) Tb(1b)–O(3b) Tb(1b)–O(4b) Tb(1b)–O(5b) Tb(1b)–O(6b) Tb(1b)–O(7b) Tb(1b)–N(1b) Tb(1b)–N(3b) O(1b)–N(2b)
2.364(8) 2.371(9) 2.342(9) 2.336(9) 2.42(1) 2.35(1) 2.60(1) 2.50(1) 1.28(2)
Bond angles () O(2a)–Tb(1a)–O(3a) O(2a)–Tb(1a)–O(5a) O(2a)–Tb(1a)–O(7a) O(2a)–Tb(1a)–N(3a) O(3a)–Tb(1a)–O(5a) O(3a)–Tb(1a)–O(7a) O(3a)–Tb(1a)–N(3a) O(4a)–Tb(1a)–O(6a) O(4a)–Tb(1a)–N(1a) O(5a)–Tb(1a)–O(6a) O(5a)–Tb(1a)–N(1a) O(6a)–Tb(1a)–O(7a) O(6a)–Tb(1a)–N(3a) O(7a)–Tb(1a)–N(3a)
71.7(3) 145.6(3) 106.6(3) 141.2(4) 139.8(3) 76.0(4) 73.1(3) 74.1(4) 77.7(4) 76.5(4) 110.2(4) 71.8(4) 142.1(4) 79.7(4)
O(2a)–Tb(1a)–O(4a) O(2a)–Tb(1a)–O(6a) O(2a)–Tb(1a)–N(1a) O(3a)–Tb(1a)–O(4a) O(3a)–Tb(1a)–O(6a) O(3a)–Tb(1a)–N(1a) O(4a)–Tb(1a)–O(5a) O(4a)–Tb(1a)–O(7a) O(4a)–Tb(1a)–N(3a) O(5a)–Tb(1a)–O(7a) O(5a)–Tb(1a)–N(3a) O(6a)–Tb(1a)–N(1a) O(7a)–Tb(1a)–N(1a) N(1a)–Tb(1a)–N(3a)
83.9(3) 72.6(4) 88.9(4) 143.1(4) 121.5(3) 74.7(3) 73.2(3) 139.0(4) 116.8(3) 77.2(4) 73.1(3) 147.6(4) 140.2(4) 66.4(4)
Torsion angles () N(1a)–C(1a)–C(2a)–N(3a) Tb(1a)–N(1a)–C(1)–C(2) Tb(1a)–N(3a)–C(2)–C(1) N(1a)–C(1a)–N(2a)–O(1a)
4(1) 3(1) 9(1) 1(1)
The bond angles and torsion angles of the other molecules are omitted.
Table 3 d values and / values of [Tb(hfac)3(IM2imH)] Tb complex
SAPR-8
TPRS-8
DD-8
N(3) [O(5) N(1)] O(4)a N(3) [O(3) N(1)] O(2)a O(7) [O(5) O(6)] O(4)a O(7) [O(3) O(6)] O(2)a
6.70 53.89 51.39 14.51
0.0 52.4 52.4 0.0
21.8 48.2 48.2 0.0
29.5 29.5 29.5 29.5
/1: N(1)–O(6)–N(3)–O(7)b /2: O(5)–O(3)–O(4)–O(2)b
22.63 27.61
24.5 24.5
14.1 14.1
0.0 0.0
d1: d2: d3: d4:
a
A [B C] D is dihedral angle between ABC plane and BCD plane. A–B–C–D is dihedral angle between (AB)CD and AB(CD)plane, where (AB) is the center of A and B. b
Fig. 1. (a) Ortep view of [Tb(hfac)3(IM2imH)] (50% probability ellipsoids), in which H and F atoms are omitted. (b) The polyhedron showing SAPR-8 with donor atoms around Tb metal center.
complex is square antiprism (SAPR), dodecahedral (DD) or bicapped trigonal prism (TPRS) [15]. The d and / values are summarized in Table 3. The d1 and d2 values showing planarity of the squares are 6.70 and 14.51, respectively. But the d3 and d4 values are 53.89 and 51.39, which are relatively larger. The /1 and /2 values are 22.63 and 27.61, which are fairly close to the angle (24.5) of the ideal SAPR polyhedron. Thus, the most reasonable geometry around the Tb atom is SAPR as shown in Fig. 1.
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p* orbital results in the decrease in ferromagnetic contribution to the Jobs or increase in the antiferromagnetic interaction.
4. Conclusion
Fig. 2. Temperature dependence of vMT in the 2–250 K range for [Gd(hfac)3(IM2imH)]. The solid line represents the best fit calculation (see text).
3.2. Magnetic properties of [Gd(hfac)3(IM2imH)] The variable temperature magnetic susceptibilities of [Gd(hfac)3(IM2imH)] are shown in the form of the vMT versus T plots in Fig. 2. Since the ground state of Gd(III) (8S7/2) is orbitally non-degenerate and well separated from the excited state, Gd(III) gives a simple single ion magnetic properties. The observed vMT value (8.17 cm3 mol1 K) at room temperature is similar to the uncoupled value for Gd(III) and IM2imH (the calculated value is 8.12 cm3 mol1 K). The vMT value of this complex slightly decreases by lowering the temperature, and then below 100 K, the drastic decrease occurs, following a slight increase below 4 K which is probably due either to weak intermolecular magnetic interaction between the IM2imH radicals or to experimental artifact. The magnetic data were analyzed in consideration of only the intramolecular interaction between the IM2imH spin and 4f spins. The usual van Vlecks equation for this type of dimer was derived by using the isotropic Hamiltonian H = 2JS1 Æ S2 [16]. A good fit to the experimental data is obtained for J = 2.59 cm1 (g = 1.97), which is comparable to that of the corresponding IM2py complex. The magnetic interaction between Gd(III) ion and paramagnetic species is claimed to involve the electron transfer of the unpaired electron in the organic radical ligand into the empty 5d orbitals of the metal, resulting in the parallel alignment of 4f and 5d electrons according to Hunds rule [17]. Thereby, the magnetic interaction is generally ferromagnetic. Since the planarity of the chelated plane in IM2imH, demonstrated by the X-ray structural study of the IM2imH Tb complex, is assumed to be not so much different from that in the Gd complex, the SOMO p* orbital in IM2imH ligand of the Gd complex is predicted to be less overlapped or orthogonal with the 5d orbital as similarly presumed for the Gd–IM2py complex. Since the observed magnetic coupling constant Jobs represents the sum of the antiferromagnetic one JAF and ferromagnetic one JF (Jobs = JAF + JF), the decreasing overlap of 5d with the IM2imH SOMO
Newly prepared IM2imH Tb(III) and Gd(III) complexes were characterized by the X-ray analysis and/or the magnetic susceptibility. The magnetic interaction between Gd(III) and IM-type radical, where the planar chelate is formed, is always antiferromagnetic. IM2imH could be a promising ligand to build new polymeric magnetic materials as deprotonation of imidazole moiety in this ligand enables the ligand to coordinate onto the other lanthanide ions.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version at doi:10.1016/ j.ica.2004.11.001.
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