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A novel copper(II)-radical complex with ferromagnetic interaction: Synthesis, crystal structure and magnetic properties
Inorganica Chimica Acta 359 (2006) 4655–4659 www.elsevier.com/locate/ica
Note
A novel copper(II)-radical complex with ferromagnetic interaction: Synthesis, crystal structure and magnetic properties Yue Ma a, Dong-Zhao Gao a, Wei Zhang b,c, Kazuyoshi Yoshimura b, Dai-Zheng Liao a,*, Zong-Hui Jiang a, Shi-Ping Yan a a Department of Chemistry, Nankai University, Tianjin 300071, PR China Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan Research Center for Low Temperature and Materials Sciences, Kyoto University, Kyoto 606–8502, Japan b
c
Received 20 March 2006; received in revised form 15 May 2006; accepted 20 May 2006 Available online 27 June 2006
Abstract A novel copper(II)-radical complex [Cu(NITmPy)(PDA)(H2O)] Æ (H2O) (1) (NITmPy = 2-(3 0 -pyridyl)-4,4,5,5-tetramethylimidazoline1-oxyl-3-oxide, H2PDA = 2,6-pyridinedicarboxylic acid) has been synthesized and structurally characterized by X-ray diffraction methods. It crystallizes in the triclinic space group P 1. The Cu(II) ion exists in a distorted square pyramid environment. The molecules of [Cu(NITmPy)(PDA)(H2O)] Æ (H2O) are connected as a two-dimensional structure by the intermolecular hydrogen bonds. Magnetic measurements show intramolecular ferromagnetic interactions between NITmPy and Cu(II) ion and intermolecular antiferromagnetic interactions in 1. 2006 Elsevier B.V. All rights reserved. Keywords: Copper(II); Nitronyl nitroxide radical; PDA; Crystal structure; Ferromagnetic interaction
1. Introduction In the field of molecular-based magnetic materials, transition metal complexes with nitronyl nitroxide ligands (NITR) have attracted much attention in recent years [1–3], because NITR, stable and easy coordinating organic radicals, can act as not only good building blocks but also spin carriers. Among NITR, considerable work has been done on pyridyl-substituted nitronyl nitroxide radicals, especially on their Cu(II) complexes. Most of the NIToPy/NITpPy-Cu(II) complexes exhibit antiferromagnetic interactions between radicals and Cu(II) ions [4,5], while the interactions between Cu(II) and NITmPy are mostly ferromagnetic [4,6]. So we can conclude that NITmPy is an ‘‘easier ferromagnetic building block’’ in the Cu(II)-pyridyl-substituted nitronyl nitroxide radical complexes, compared with NIToPy and NITpPy. *
Corresponding author. Tel.: +86 22 23509957; fax: +86 22 23502779. E-mail address: [email protected] (D.-Z. Liao).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.05.044
Meanwhile, the PDA is a versatile ligand not only used in bridging style but in tridentate and bidentate modes in the chemical design of molecular assemblies. A lot of work has been done on PDA–metal complexes [7,8]. However, the PDA acts as a co-ligand in metal-nitroxide radical complexes and has seldom been reported before. Two antiferromagnetic complexes [M(NIT2Py)(PDA)(H2O)] Æ (CH3OH)(H2O) (M = Cu(II), Ni(II)) [9,10] have been synthesized by some of us. However, no ferromagnetic complexes containing both pyridyl-substituted nitronyl nitroxide radicals and PDA have so far been reported. With the hope of getting a complex with ferromagnetic interaction and gaining insight into magnetostructural correlation of this kind of system, in this paper, we present the synthesis, structure and magnetic property of a novel copper(II) complex containing both PDA and NITmPy, [Cu(NITmPy) (PDA) (H2O)] Æ (H2O), which has a two-dimensional structure connected by the intermolecular hydrogen bonds and shows intramolecular ferromagnetic interactions between the Cu(II) ion and NITmPy as we predicted.
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Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659
2. Experimental
Table 2 ˚ ) and angles () for title complex Selected bond lengths (A
2.1. General
Bond lengths Cu(1)–N(4) Cu(1)–N(1) Cu(1)–O(3) Cu(1)–O(6) Cu(1)–O(7) O(1)–N(2)
All starting materials were of analytical grade and used without further purification. Elemental analyses for C, H and N were carried out on Perkin–Elmer elemental analyzer (model 240). The infrared spectrum was obtained on a Bruker Tensor 27 Fourier transform infrared spectroscopy in the 4000–400 cm1 regions, using KBr pellets. EPR spectra were recorded as a powder of the compound with a ER 200D-SRC spectrometer. Variable temperature magnetic susceptibilities were measured on SQUID magnetometer between 2.0 and 300 K in a magnetic field of 5000 G. The molar magnetic susceptibility was corrected from the sample holder and diamagnetic contributions of all constituent atoms by using Pascal’s constants. 2.2. X-ray crystallography X-ray diffractions were measured on a APEX II CCD area detector equipped with a graphite-monochromated Mo Ka ˚ ). A summary of crystallographic radiation (k = 0.71073 A data is given in Table 1. The empirical absorption corrections by semi-empirical from equivalents were carried out. The structure was solved by direct methods using SHELXS-97 program and refined with SHELXL-97 [11] by full-matrix leastsquares techniques on F2. All non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were located geometrically and refined isotropically. The selected bond lengths and angles are listed in Table 2. 2-(3 0 -Pyridyl)4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITmPy) was prepared by the literature method [12].
Bond angles N(4)–Cu(1)–N(1) N(4)–Cu(1)–O(3) N(1)–Cu(1)–O(3) N(4)–Cu(1)–O(6) N(1)–Cu(1)–O(6) O(3)–Cu(1)–O(6) N(4)–Cu(1)–O(7) N(1)–Cu(1)–O(7) O(3)–Cu(1)–O(7) O(6)–Cu(1)–O(7) C(13)–O(3)–Cu(1)
1.904(2) 1.976(2) 2.002(2) 2.009(2) 2.270(2) 1.278(3) 165.54(8) 81.46(8) 96.33(8) 80.76(7) 99.76(7) 161.66(7) 102.49(8) 91.78(9) 90.35(7) 97.87(7) 114.70(15)
O(2)–N(3) O(3)–C(13) O(4)–C(13) O(5)–C(19) O(6)–C(19)
C(19)–O(6)–Cu(1) C(18)–N(4)–C(14) C(18)–N(4)–Cu(1) C(14)–N(4)–Cu(1) O(1)–N(2)–C(6) O(1)–N(2)–C(11) O(2)–N(3)–C(6) O(2)–N(3)–C(7) O(3)–C(13)–C(14) O(6)–C(19)–C(18)
1.287(3) 1.279(3) 1.230(3) 1.231(3) 1.283(3)
115.45(14) 123.7(2) 118.60(15) 117.48(17) 125.85(19) 122.69(19) 125.31(18) 122.73(18) 114.60(19) 113.9(2)
0.1 mmol) and NITmPy (0.023 g, 0.1 mmol) with stirring. After the resulting solution had been filtered and evaporated slowly at room temperature for several weeks in a dark place, blue crystals suitable for X-ray analysis were obtained (yield 61%). Anal. Calc. for C19H23CuN4O8: C, 45.70; H, 4.61; N, 11.22. Found: C, 45.40; H, 4.71; N, 11.00%. IR (KBr disc, cm1): 1638 and 1613 (mas(CO2)), 1403 (ms(CO2)), 1369 cm1(m(NO)). The difference between mas(CO2) and ms(CO2) is about 230 cm1, which is in accordance with the monodentate coordinated carboxyl, as shown in the crystal structure. 3. Results and discussion
2.3. Preparation of [Cu(NITmPy)(PDA)(H2O)] Æ (H2O)
3.1. Crystal structure of 1
An aqueous solution (10 mL) of Na2(PDA) (0.021 g, 0.1 mmol) was slowly added to a blue methanol solution (10 mL) containing both Cu(ClO4)2 Æ 6H2O (0.037 g,
The crystal structure of complex 1 was determined by X-ray diffraction analyses [11]. The ORTEP drawing is illustrated in Fig. 1. The structure consists of [Cu(NITmPy)(PDA)(H2O)] moiety and one solvent water molecule. In this compound, copper atom is five-coordinated in a distorted square pyramid CuN2O3 coordination environment, in which the basal coordination sites are occupied by two carboxyl oxygen atoms (O3, O6) from PDA, two nitrogen atoms (N4, N1) from PDA and NITmPy ligands, with the ˚ , respecbond lengths of 2.002, 2.009, 1.904, and 1.976 A tively. The axial position is occupied by one oxygen atom ˚ . The Cu(II) (O7) of H2O, with the bond length of 2.270 A ion is nearly coplanar with the basal plane N1–O6–N4–O3, ˚ out of the basal plane. The with a displacement of 0.1809 A ring of pyridine from NITmPy ligand makes an angle of 24.1 with the basal coordination plane of the Cu(II) ion. The dihedral angle between pyridine ring and nitroxide group (O1–N2–C6–N3–O2) of the radical is 41. The hydrogen bond interactions, as shown in Fig. 2, firstly connect the coordinated water molecules, the uncoordinated water molecules and the oxygen atoms of the
Table 1 Summary of crystallographic data for title complex Empirical formula Formula weight T (K) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z l (mm1) Reflections/unique Final R indices [I > 2r] R indices (all data) ˚ 3) Largest difference in peak and hole (e A
C19H23CuN4O8 498.95 293(2) triclinic P 1 8.316(6) 10.634(8) 12.092(9) 86.231(9) 76.608(9) 89.779(10) 1037.9(14) 2 1.108 5618/3617 R1 = 0.0301, wR2 = 0.0806 R1 = 0.0363, wR2 = 0.0833 0.259 and 0.370
Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659
Fig. 1. An ORTEP drawing of [Cu(NITmPy)(PDA)(H2O)] Æ H2O with 30% thermal ellipsoids.
˚; PDA (with the bond lengths of O7C O8A, 2.783 A ˚ ˚ O8A O5A, 2.767 A; and O7C O4B, 2.774 A) to form a 1D-chain along the c-axis, and then the hydrogen bonds between uncoordinated water and the oxygen atoms of the NO groups from NITmPy radicals (bond length: O8A ˚ ) connect the former 1D-chains along the O2E, 2.909 A b-axis, forming a 2D plane. The two completely parallel pyridine rings from neighbouring NITmPy units have a ˚ between the p–p interaction with the distance of 3.49 A two pyridine rings. Two adjacent NITmPy groups also stack to form a structure with parallel arrangements, the ˚. close inter-radical (O O) distances of which is 3.89 A 3.2. Magnetic properties Variable temperature magnetic susceptibility measurements were carried out with a SQUID magnetometer in the temperature range 2.0–300 K at a magnetic field of
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5000 Oe. The magnetic properties of 1 in the form of vMT versus T plots are presented in Fig. 3. At room temperature, the vMT is 0.83 cm3 mol1 K, which is slightly higher than the value (0.75 cm3 mol1 K) expected for an uncoupled system with one Cu(II) ion (S = 1/2) and one NITmPy (S = 1/2). As the temperature decreases, vMT firstly increases and reaches a maximum of 0.87 cm3 K mol1 at 14 K, then decreases sharply to 0.52 cm3 K mol1 at 2.0 K. The increasing of magnetic susceptibility is typical for a ferromagnetic interaction between Cu(II) and NITmPy, while its decreasing at low temperature may be due to the intermolecular antiferromagnetic exchange interactions and/or the zero-field splitting (ZFS) of S = 1 ground state. To evaluate the exchange coupling constants in such a magnetic system, Eq. (1) derived from the spin Hamiltob ¼ 2J b nian H S Cu b S Z 2 was applied [13]. S Rad þ D b Ng2 b2 2 expðD=KT Þ vbi ¼ þ Na 1 þ 2 expðD=KT Þ þ expð2J =KT Þ KT ð1Þ where J and D correspond to the magnetic interactions between Cu(II) and NITmPy and the axial zero-field splitting parameter for the S = 1 state, respectively. In addition, a mean field correction (Eq. (2)) was added to account for the intermolecular interactions (zJ 0 ) vbi vM ¼ ð2Þ 0 1 ð2zJ =Ng2 b2 Þvbi By assuming, Na = 60 · 106 cm3 mol1, the best fitting parameters were obtained as, g = 2.07, J = 6.83 cm1, jDj = 0.24 cm1, zJ 0 = 0.93 cm1, R = 8.52 · 104 (R value is defined as R = R[(vM)obs (vM)calc]2/R[(vM)obs]2). The positive J confirms the weak intramolecular ferromagnetic
Fig. 2. Packing diagram for [Cu(NITmPy)(PDA)(H2O)] Æ H2O, a 2-D network formed by hydrogen bond interactions. Hydrogen atoms are omitted for clarity.
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Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659 a
0.90
0.30
b
0.85 0.25 0.20
0.75
-1
0.15
0.70
3
χMT/cm mol K
3
χM/cm mol
-1
0.80
0.65 0.10 0.60 0.05
0.55 0.50
0.00
0.45 0
100
50
150
200
250
300
T/K
Fig. 3. Temperature dependence of the molar susceptibility (vM) and vMT vs. T for 1.
2000
2500
3000
3500
4000
4500
H/G
coupling between Cu(II) and NITmPy, and the negative zJ 0 value shows the weak intermolecular antiferromagnetic exchange interactions. The sign of the exchange interaction between Cu(II) ion and NITmPy can be explained by a spin polarization mechanism of the p-electrons [14,15] and the orthogonality of 3d magnetic orbital of Cu(II) ion and the 2pp orbital on the pyridine rings [16]. As seen clearly in Scheme 1, owing to spin polarization by radical center, the sign alternation of spin density at the carbon atoms of pyridine leads to significant positive spin density at N atom of pyridine, which has the same sign as the radical center. Furthermore, the geometry around copper(II) ion is a distorted square pyramid, consequently, the dx2 y 2 magnetic orbital should point towards the N atom of the pyridine ring. Because the dihedral angel between the pyridine ring and the xy plane of the copper(II) ion is small (24.1), 2pp orbital at the nitrogen of the pyridine ring and the magnetic orbital dx2 y 2 of Cu(II) ion have no significant overlap. Thus, this orthogonality of dx2 y 2 of copper(II) ion and 2pp orbital of pyridine should lead to the ferromagnetic interaction between Cu(II) ion and NITmPy, as shown in other Cu(II)-NITmPy compounds [4,6]. The weak intermolecular antiferromagnetic exchange interactions revealed by the negative zJ 0 can also be explained by the relative arrangement of
Z
Fig. 4. The powder EPR spectra of 1 at room temperature; the experiment spectrum (circle, microwave frequency 9.7763 GHz); the simulated spectrum (solid line, see text for parameters). The inset is the comparing of band intensity of the powder EPR spectra of 1 at different temperature (a for 110 K, b for room temperature).
the stacked NITmPy groups. In complex 1, two adjacent NITmPy groups stack to form a structure with parallel arrangements, the close inter-radical (O O) distances of ˚ , so a r-type overlap which is short enough to be 3.89 A (SOMO–SOMO overlap) of the nitronyl nitroxide p* orbitals from the adjacent radicals can occur and favor the intermolecular antiferromagnetic interaction [17]. 3.3. EPR spectrum The powder EPR spectrum of 1 was measured at room temperature, as shown in Fig. 4, which displays an asymmetric absorption. The simulated spectrum (S = 1) is obtained with the following parameters: g^ = 2.04 and gi = 2.14, with the average g value 2.07, which agrees well with that (2.07) obtained from susceptibility data. Moreover, the intensity of the band increases with decreasing temperature to 110 K (see the inset of Fig. 4). This behavior is in accordance with temperature dependence of the magnetic susceptibility and therefore further supports the occurrence of an ferromagnetic interaction between Cu(II) and NITmPy.
Y
4. Conclusions N
Cu
O N +
N O
-
Scheme 1.
X
In summary, a new complex, formulated as [Cu(NITmPy)(PDA)(H2O)] Æ (H2O) was obtained and characterized structurally and magnetically. It has a two-dimensional structure connected by the intermolecular hydrogen bonds, and shows intramolecular ferromagnetic interactions between the Cu(II) ion and NITmPy because of the orthogonality of dx2 y 2 of copper(II) ion and 2pp orbital of pyridine. The SOMO–SOMO overlap of the nitronyl
Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659
nitroxide p* orbitals from the adjacent radicals favors the intermolecular antiferromagnetic interaction. 5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 284554. 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; email: [email protected] or http://www.ccdc. cam.ac.uk)
[6]
[7]
Acknowledgements This project was supported by the National Natural Science Foundation of China (Nos. 20471031, 20331010, 90501002 and 20571045) and Natural Science Key Foundation of Tianjin. This study was also partly supported by a Grant-in-Aid for Science Research on Priority Area, ‘‘Invention of anomalous quantum materials’’, from the Ministry of Education, Science, Sports and Culture of Japan (Grant No. 16076210).
[8] [9] [10] [11]
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(c) Z.L. Liu, Z.L. Lu, D.Q. Zhang, Z.H. Jiang, L.C. Li, C.M. Liu, D.B. Zhu, Inorg. Chem. 43 (2004) 6620; (d) L.Y. Wang, L.C. Li, D.Z. Liao, Z.H. Jiang, S.P. Yan, Chin. J. Chem. 21 (2003) 500. (a) L.C. Li, Z.L. Liu, S.S. Turner, D.Z. Liao, Z.H. Jiang, S.P. Yan, New J. Chem. 27 (2003) 752; (b) C.X. Zhang, Z.Q. Liu, D.Z. Liao, Z.H. Jiang, S.P. Yan, Inorg. Chim. Acta 357 (2004) 376; (c) I. Yeltsov, V. Ovcharenko, V. Ikorskii, G. Romanenko, S. Vasilevsky, Polyhedron 20 (2001) 1215; (d) L.C. Li, D.Z. Liao, Z.H. Jiang, S.P. Yan, Inorg. Chim. Acta 357 (2004) 405; (e) L.C. Li, D.Z. Liao, Z.H. Jiang, S.P. Yan, J. Chem. Soc., Dalton Trans. (2002) 1350. (a) B. Zhao, X.Y. Chen, P. Cheng, D.Z. Liao, S.P. Yan, Z.H. Jiang, J. Am. Chem. Soc. 126 (2004) 15394; (b) B. Zhao, P. Cheng, X.Y. Chen, C. Cheng, W. Shi, D.Z. Liao, S.P. Yan, Z.H. Jiang, J. Am. Chem. Soc. 126 (2004) 3012; (c) M.B. Zhang, J. Zhang, S.T. Zheng, G.Y. Yang, Angew. Chem., Int. Ed. 44 (2005) 1385; (d) J.Y. Lu, A.M. Babb, Chem. Commun. (2003) 1346. C. Brouca-Cabarrecq, A. Fernandes, J. Jaud, J.P. Costes, Inorg. Chim. Acta 322 (2002) 54. C.X. Zhang, Z.Q. Liu, D.Z. Liao, Z.H. Jiang, S.P. Yan, J. Mol. Struct. 650 (2003) 21. C.X. Zhang, D.Z. Liao, Z.H. Jiang, S.P. Yan, B. Zhao, Transit. Met. Chem. 28 (2003) 621. (a) G.M. Sheldrick, SHELXS-97: Program for the Solution of Crystal Structures, University of Gottingen, Germany, 1997; (b) G.M. Sheldrick, SHELXL-97: Program for the Refinement of Crystal Structures, University of Gottingen, Germany, 1997. M.S. Davis, K. Morokuma, R.W. Kreilick, J. Am. Chem. Soc. 94 (1972) 5588. (a) T. Kamiusuki, H. Okawa, E. Kitaura, M. Koikawa, N. Matsumoto, S. Kida, H. Oshio, J. Chem. Soc., Dalton Trans. (1989) 2077; (b) M. Fondo, A.M. Garcı´a-Deibe, M. Corbella, J. Ribas, A. Llamas-Saiz, M.R. Bermejo, J. Sanmartı´n, Dalton Trans. (2004) 3503. H.M. McConnell, J. Chem. Phys. 39 (1963) 1910. J. Cirujeda, M. Mas, E. Molins, F.L. de Panthou, J. Laugier, J.G. Park, C. Pauson, P. Rey, C. Rovira, J. Veciana, J. Chem. Soc., Chem. Commun. (1995) 709. O. Kahn, J. Galy, Y. Journaux, J. Jaud, I. Morgenstern-Badarau, J. Am. Chem. Soc. 104 (1982) 2165. (a) H. Oshio, T. Ito, Coord. Chem. Rev. 198 (2000) 329; (b) A. Caneschi, F. Ferraro, D. Gatteschi, P. Rey, R. Sessoli, Inorg. Chem. 29 (1990) 1756.
Note
A novel copper(II)-radical complex with ferromagnetic interaction: Synthesis, crystal structure and magnetic properties Yue Ma a, Dong-Zhao Gao a, Wei Zhang b,c, Kazuyoshi Yoshimura b, Dai-Zheng Liao a,*, Zong-Hui Jiang a, Shi-Ping Yan a a Department of Chemistry, Nankai University, Tianjin 300071, PR China Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan Research Center for Low Temperature and Materials Sciences, Kyoto University, Kyoto 606–8502, Japan b
c
Received 20 March 2006; received in revised form 15 May 2006; accepted 20 May 2006 Available online 27 June 2006
Abstract A novel copper(II)-radical complex [Cu(NITmPy)(PDA)(H2O)] Æ (H2O) (1) (NITmPy = 2-(3 0 -pyridyl)-4,4,5,5-tetramethylimidazoline1-oxyl-3-oxide, H2PDA = 2,6-pyridinedicarboxylic acid) has been synthesized and structurally characterized by X-ray diffraction methods. It crystallizes in the triclinic space group P 1. The Cu(II) ion exists in a distorted square pyramid environment. The molecules of [Cu(NITmPy)(PDA)(H2O)] Æ (H2O) are connected as a two-dimensional structure by the intermolecular hydrogen bonds. Magnetic measurements show intramolecular ferromagnetic interactions between NITmPy and Cu(II) ion and intermolecular antiferromagnetic interactions in 1. 2006 Elsevier B.V. All rights reserved. Keywords: Copper(II); Nitronyl nitroxide radical; PDA; Crystal structure; Ferromagnetic interaction
1. Introduction In the field of molecular-based magnetic materials, transition metal complexes with nitronyl nitroxide ligands (NITR) have attracted much attention in recent years [1–3], because NITR, stable and easy coordinating organic radicals, can act as not only good building blocks but also spin carriers. Among NITR, considerable work has been done on pyridyl-substituted nitronyl nitroxide radicals, especially on their Cu(II) complexes. Most of the NIToPy/NITpPy-Cu(II) complexes exhibit antiferromagnetic interactions between radicals and Cu(II) ions [4,5], while the interactions between Cu(II) and NITmPy are mostly ferromagnetic [4,6]. So we can conclude that NITmPy is an ‘‘easier ferromagnetic building block’’ in the Cu(II)-pyridyl-substituted nitronyl nitroxide radical complexes, compared with NIToPy and NITpPy. *
Corresponding author. Tel.: +86 22 23509957; fax: +86 22 23502779. E-mail address: [email protected] (D.-Z. Liao).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.05.044
Meanwhile, the PDA is a versatile ligand not only used in bridging style but in tridentate and bidentate modes in the chemical design of molecular assemblies. A lot of work has been done on PDA–metal complexes [7,8]. However, the PDA acts as a co-ligand in metal-nitroxide radical complexes and has seldom been reported before. Two antiferromagnetic complexes [M(NIT2Py)(PDA)(H2O)] Æ (CH3OH)(H2O) (M = Cu(II), Ni(II)) [9,10] have been synthesized by some of us. However, no ferromagnetic complexes containing both pyridyl-substituted nitronyl nitroxide radicals and PDA have so far been reported. With the hope of getting a complex with ferromagnetic interaction and gaining insight into magnetostructural correlation of this kind of system, in this paper, we present the synthesis, structure and magnetic property of a novel copper(II) complex containing both PDA and NITmPy, [Cu(NITmPy) (PDA) (H2O)] Æ (H2O), which has a two-dimensional structure connected by the intermolecular hydrogen bonds and shows intramolecular ferromagnetic interactions between the Cu(II) ion and NITmPy as we predicted.
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Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659
2. Experimental
Table 2 ˚ ) and angles () for title complex Selected bond lengths (A
2.1. General
Bond lengths Cu(1)–N(4) Cu(1)–N(1) Cu(1)–O(3) Cu(1)–O(6) Cu(1)–O(7) O(1)–N(2)
All starting materials were of analytical grade and used without further purification. Elemental analyses for C, H and N were carried out on Perkin–Elmer elemental analyzer (model 240). The infrared spectrum was obtained on a Bruker Tensor 27 Fourier transform infrared spectroscopy in the 4000–400 cm1 regions, using KBr pellets. EPR spectra were recorded as a powder of the compound with a ER 200D-SRC spectrometer. Variable temperature magnetic susceptibilities were measured on SQUID magnetometer between 2.0 and 300 K in a magnetic field of 5000 G. The molar magnetic susceptibility was corrected from the sample holder and diamagnetic contributions of all constituent atoms by using Pascal’s constants. 2.2. X-ray crystallography X-ray diffractions were measured on a APEX II CCD area detector equipped with a graphite-monochromated Mo Ka ˚ ). A summary of crystallographic radiation (k = 0.71073 A data is given in Table 1. The empirical absorption corrections by semi-empirical from equivalents were carried out. The structure was solved by direct methods using SHELXS-97 program and refined with SHELXL-97 [11] by full-matrix leastsquares techniques on F2. All non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were located geometrically and refined isotropically. The selected bond lengths and angles are listed in Table 2. 2-(3 0 -Pyridyl)4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITmPy) was prepared by the literature method [12].
Bond angles N(4)–Cu(1)–N(1) N(4)–Cu(1)–O(3) N(1)–Cu(1)–O(3) N(4)–Cu(1)–O(6) N(1)–Cu(1)–O(6) O(3)–Cu(1)–O(6) N(4)–Cu(1)–O(7) N(1)–Cu(1)–O(7) O(3)–Cu(1)–O(7) O(6)–Cu(1)–O(7) C(13)–O(3)–Cu(1)
1.904(2) 1.976(2) 2.002(2) 2.009(2) 2.270(2) 1.278(3) 165.54(8) 81.46(8) 96.33(8) 80.76(7) 99.76(7) 161.66(7) 102.49(8) 91.78(9) 90.35(7) 97.87(7) 114.70(15)
O(2)–N(3) O(3)–C(13) O(4)–C(13) O(5)–C(19) O(6)–C(19)
C(19)–O(6)–Cu(1) C(18)–N(4)–C(14) C(18)–N(4)–Cu(1) C(14)–N(4)–Cu(1) O(1)–N(2)–C(6) O(1)–N(2)–C(11) O(2)–N(3)–C(6) O(2)–N(3)–C(7) O(3)–C(13)–C(14) O(6)–C(19)–C(18)
1.287(3) 1.279(3) 1.230(3) 1.231(3) 1.283(3)
115.45(14) 123.7(2) 118.60(15) 117.48(17) 125.85(19) 122.69(19) 125.31(18) 122.73(18) 114.60(19) 113.9(2)
0.1 mmol) and NITmPy (0.023 g, 0.1 mmol) with stirring. After the resulting solution had been filtered and evaporated slowly at room temperature for several weeks in a dark place, blue crystals suitable for X-ray analysis were obtained (yield 61%). Anal. Calc. for C19H23CuN4O8: C, 45.70; H, 4.61; N, 11.22. Found: C, 45.40; H, 4.71; N, 11.00%. IR (KBr disc, cm1): 1638 and 1613 (mas(CO2)), 1403 (ms(CO2)), 1369 cm1(m(NO)). The difference between mas(CO2) and ms(CO2) is about 230 cm1, which is in accordance with the monodentate coordinated carboxyl, as shown in the crystal structure. 3. Results and discussion
2.3. Preparation of [Cu(NITmPy)(PDA)(H2O)] Æ (H2O)
3.1. Crystal structure of 1
An aqueous solution (10 mL) of Na2(PDA) (0.021 g, 0.1 mmol) was slowly added to a blue methanol solution (10 mL) containing both Cu(ClO4)2 Æ 6H2O (0.037 g,
The crystal structure of complex 1 was determined by X-ray diffraction analyses [11]. The ORTEP drawing is illustrated in Fig. 1. The structure consists of [Cu(NITmPy)(PDA)(H2O)] moiety and one solvent water molecule. In this compound, copper atom is five-coordinated in a distorted square pyramid CuN2O3 coordination environment, in which the basal coordination sites are occupied by two carboxyl oxygen atoms (O3, O6) from PDA, two nitrogen atoms (N4, N1) from PDA and NITmPy ligands, with the ˚ , respecbond lengths of 2.002, 2.009, 1.904, and 1.976 A tively. The axial position is occupied by one oxygen atom ˚ . The Cu(II) (O7) of H2O, with the bond length of 2.270 A ion is nearly coplanar with the basal plane N1–O6–N4–O3, ˚ out of the basal plane. The with a displacement of 0.1809 A ring of pyridine from NITmPy ligand makes an angle of 24.1 with the basal coordination plane of the Cu(II) ion. The dihedral angle between pyridine ring and nitroxide group (O1–N2–C6–N3–O2) of the radical is 41. The hydrogen bond interactions, as shown in Fig. 2, firstly connect the coordinated water molecules, the uncoordinated water molecules and the oxygen atoms of the
Table 1 Summary of crystallographic data for title complex Empirical formula Formula weight T (K) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z l (mm1) Reflections/unique Final R indices [I > 2r] R indices (all data) ˚ 3) Largest difference in peak and hole (e A
C19H23CuN4O8 498.95 293(2) triclinic P 1 8.316(6) 10.634(8) 12.092(9) 86.231(9) 76.608(9) 89.779(10) 1037.9(14) 2 1.108 5618/3617 R1 = 0.0301, wR2 = 0.0806 R1 = 0.0363, wR2 = 0.0833 0.259 and 0.370
Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659
Fig. 1. An ORTEP drawing of [Cu(NITmPy)(PDA)(H2O)] Æ H2O with 30% thermal ellipsoids.
˚; PDA (with the bond lengths of O7C O8A, 2.783 A ˚ ˚ O8A O5A, 2.767 A; and O7C O4B, 2.774 A) to form a 1D-chain along the c-axis, and then the hydrogen bonds between uncoordinated water and the oxygen atoms of the NO groups from NITmPy radicals (bond length: O8A ˚ ) connect the former 1D-chains along the O2E, 2.909 A b-axis, forming a 2D plane. The two completely parallel pyridine rings from neighbouring NITmPy units have a ˚ between the p–p interaction with the distance of 3.49 A two pyridine rings. Two adjacent NITmPy groups also stack to form a structure with parallel arrangements, the ˚. close inter-radical (O O) distances of which is 3.89 A 3.2. Magnetic properties Variable temperature magnetic susceptibility measurements were carried out with a SQUID magnetometer in the temperature range 2.0–300 K at a magnetic field of
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5000 Oe. The magnetic properties of 1 in the form of vMT versus T plots are presented in Fig. 3. At room temperature, the vMT is 0.83 cm3 mol1 K, which is slightly higher than the value (0.75 cm3 mol1 K) expected for an uncoupled system with one Cu(II) ion (S = 1/2) and one NITmPy (S = 1/2). As the temperature decreases, vMT firstly increases and reaches a maximum of 0.87 cm3 K mol1 at 14 K, then decreases sharply to 0.52 cm3 K mol1 at 2.0 K. The increasing of magnetic susceptibility is typical for a ferromagnetic interaction between Cu(II) and NITmPy, while its decreasing at low temperature may be due to the intermolecular antiferromagnetic exchange interactions and/or the zero-field splitting (ZFS) of S = 1 ground state. To evaluate the exchange coupling constants in such a magnetic system, Eq. (1) derived from the spin Hamiltob ¼ 2J b nian H S Cu b S Z 2 was applied [13]. S Rad þ D b Ng2 b2 2 expðD=KT Þ vbi ¼ þ Na 1 þ 2 expðD=KT Þ þ expð2J =KT Þ KT ð1Þ where J and D correspond to the magnetic interactions between Cu(II) and NITmPy and the axial zero-field splitting parameter for the S = 1 state, respectively. In addition, a mean field correction (Eq. (2)) was added to account for the intermolecular interactions (zJ 0 ) vbi vM ¼ ð2Þ 0 1 ð2zJ =Ng2 b2 Þvbi By assuming, Na = 60 · 106 cm3 mol1, the best fitting parameters were obtained as, g = 2.07, J = 6.83 cm1, jDj = 0.24 cm1, zJ 0 = 0.93 cm1, R = 8.52 · 104 (R value is defined as R = R[(vM)obs (vM)calc]2/R[(vM)obs]2). The positive J confirms the weak intramolecular ferromagnetic
Fig. 2. Packing diagram for [Cu(NITmPy)(PDA)(H2O)] Æ H2O, a 2-D network formed by hydrogen bond interactions. Hydrogen atoms are omitted for clarity.
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Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659 a
0.90
0.30
b
0.85 0.25 0.20
0.75
-1
0.15
0.70
3
χMT/cm mol K
3
χM/cm mol
-1
0.80
0.65 0.10 0.60 0.05
0.55 0.50
0.00
0.45 0
100
50
150
200
250
300
T/K
Fig. 3. Temperature dependence of the molar susceptibility (vM) and vMT vs. T for 1.
2000
2500
3000
3500
4000
4500
H/G
coupling between Cu(II) and NITmPy, and the negative zJ 0 value shows the weak intermolecular antiferromagnetic exchange interactions. The sign of the exchange interaction between Cu(II) ion and NITmPy can be explained by a spin polarization mechanism of the p-electrons [14,15] and the orthogonality of 3d magnetic orbital of Cu(II) ion and the 2pp orbital on the pyridine rings [16]. As seen clearly in Scheme 1, owing to spin polarization by radical center, the sign alternation of spin density at the carbon atoms of pyridine leads to significant positive spin density at N atom of pyridine, which has the same sign as the radical center. Furthermore, the geometry around copper(II) ion is a distorted square pyramid, consequently, the dx2 y 2 magnetic orbital should point towards the N atom of the pyridine ring. Because the dihedral angel between the pyridine ring and the xy plane of the copper(II) ion is small (24.1), 2pp orbital at the nitrogen of the pyridine ring and the magnetic orbital dx2 y 2 of Cu(II) ion have no significant overlap. Thus, this orthogonality of dx2 y 2 of copper(II) ion and 2pp orbital of pyridine should lead to the ferromagnetic interaction between Cu(II) ion and NITmPy, as shown in other Cu(II)-NITmPy compounds [4,6]. The weak intermolecular antiferromagnetic exchange interactions revealed by the negative zJ 0 can also be explained by the relative arrangement of
Z
Fig. 4. The powder EPR spectra of 1 at room temperature; the experiment spectrum (circle, microwave frequency 9.7763 GHz); the simulated spectrum (solid line, see text for parameters). The inset is the comparing of band intensity of the powder EPR spectra of 1 at different temperature (a for 110 K, b for room temperature).
the stacked NITmPy groups. In complex 1, two adjacent NITmPy groups stack to form a structure with parallel arrangements, the close inter-radical (O O) distances of ˚ , so a r-type overlap which is short enough to be 3.89 A (SOMO–SOMO overlap) of the nitronyl nitroxide p* orbitals from the adjacent radicals can occur and favor the intermolecular antiferromagnetic interaction [17]. 3.3. EPR spectrum The powder EPR spectrum of 1 was measured at room temperature, as shown in Fig. 4, which displays an asymmetric absorption. The simulated spectrum (S = 1) is obtained with the following parameters: g^ = 2.04 and gi = 2.14, with the average g value 2.07, which agrees well with that (2.07) obtained from susceptibility data. Moreover, the intensity of the band increases with decreasing temperature to 110 K (see the inset of Fig. 4). This behavior is in accordance with temperature dependence of the magnetic susceptibility and therefore further supports the occurrence of an ferromagnetic interaction between Cu(II) and NITmPy.
Y
4. Conclusions N
Cu
O N +
N O
-
Scheme 1.
X
In summary, a new complex, formulated as [Cu(NITmPy)(PDA)(H2O)] Æ (H2O) was obtained and characterized structurally and magnetically. It has a two-dimensional structure connected by the intermolecular hydrogen bonds, and shows intramolecular ferromagnetic interactions between the Cu(II) ion and NITmPy because of the orthogonality of dx2 y 2 of copper(II) ion and 2pp orbital of pyridine. The SOMO–SOMO overlap of the nitronyl
Y. Ma et al. / Inorganica Chimica Acta 359 (2006) 4655–4659
nitroxide p* orbitals from the adjacent radicals favors the intermolecular antiferromagnetic interaction. 5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 284554. 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; email: [email protected] or http://www.ccdc. cam.ac.uk)
[6]
[7]
Acknowledgements This project was supported by the National Natural Science Foundation of China (Nos. 20471031, 20331010, 90501002 and 20571045) and Natural Science Key Foundation of Tianjin. This study was also partly supported by a Grant-in-Aid for Science Research on Priority Area, ‘‘Invention of anomalous quantum materials’’, from the Ministry of Education, Science, Sports and Culture of Japan (Grant No. 16076210).
[8] [9] [10] [11]
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