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Ferromagnetic interactions in copper(II) and nickel(II) coordination polymers containing nitronyl nitroxide radical and terephthalate
Inorganica Chimica Acta 357 (2004) 405–410 www.elsevier.com/locate/ica
Ferromagnetic interactions in copper(II) and nickel(II) coordination polymers containing nitronyl nitroxide radical and terephthalate Licun Li
a,b,*
, Daizheng Liao a, Zonghui Jiang a, Shiping Yan
a
a
b
Department of Chemistry, Nankai University, Tianjin 300071, PR China State Key Laboratory of Structural chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China Received 17 February 2003; accepted 14 May 2003
Abstract Two new complexes [Cu(NITmPy)2 (tp)] 1 and [Ni(NITmPy)2 (tp)(H2 O)2 ] 2 (NITmPy ¼ 2-(30 -pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and tp ¼ terephthalato dianion) were synthesized and structurally and magnetically characterized. The structure of 1 is a neutral infinite chain where Cu(NITmPy)2 units are linked by terephthalate ligands. In complex 2, the 1-D chains of Ni(NITmPy)2 (H2 O)2 units connected by tp develop into 2-D network via hydrogen bond interactions. The magnetic properties of 1 and 2 have been investigated in the temperature range 2–300 K. Both complexes exhibit ferromagnetic coupling and antiferromagnetic interactions dominate at low temperature. The magnetic behavior is discussed based on their structures. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Nitronyl nitroxide; Copper complexes; Nickel complexes; Terephthalate; Crystal structures; Magnetic properties
1. Introduction In the field of molecular-based magnetic materials, transition metal complexes with nitroxide radical ligands have found widespread interest in recent years [1– 4]. Considerable efforts have directed in preparing and characterising metal–radical multi-dimensional systems which can exhibit ferromagnetic properties [4–9]. One of synthetic strategies is that transition metal ions coordinated by paramagnetic nitroxide radicals are linked by inorganic or organic bridging ligands to give coordination polymers or networks [10–14]. The metal complexes, based on pyridyl-substituted nitronyl nitroxides, have attracted considerable attention due to the ability of these radicals to coordinate metal ions and act as magnetic couplers, giving rise to
*
Corresponding author. E-mail address: [email protected] (L. Li).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.05.006
new functional materials with a variety of structural topology [15–20]. The terephthalato ligand is a versatile ligand with good binding ability as exemplified by the formation of polymeric structures [21–24]. Terephthalato-bridged complexes have been actively studied to elucidate the limiting distance of magnetic exchange between paramagnetic metal centers. Long tp bridges which typically produce M–M separation of about 11 A, generally leads to weak antiferromagnetic interactions between metal centers for M ¼ Mn(II), Co(II), Ni(II), Cu(II) [25]. However, when both the metal basal plane and the tp ligand are coplanar, stronger antiferromagnetic interactions are observed [20,26]. With the purpose of obtaining materials with unusual magnetic properties derived from nitronyl nitroxide radicals and terephthalate ligand, we present here two new metal– radical coordination polymers with terephthalate bridging ligand, [Cu(NITmPy)2 (tp)] (1) and [Ni(NITmPy)2 (tp)(H2 O)2 ] (2) (NITmPy ¼ 2-(30 -pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and tp ¼ terephthalato dianion).
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L. Li et al. / Inorganica Chimica Acta 357 (2004) 405–410
2. Experimental 2.1. Materials All reagents and chemicals were purchased from commercial sources and used as received. 2-(30 -pyridyl)4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITmPy) was prepared by the literature method [27]. 2.2. Synthesis
trophotometer model 408, using KBr pellets. Variable temperature magnetic susceptibilities on polycrystalline samples were measured on a Maglab system 2000 magnetometer. Diamagnetic corrections were made with PascalÕs constants for all constitute atoms. Experimental susceptibilities of 1 were also corrected for the temperature-independent paramagnetism (60 106 cm3 mol1 per copper(II) ion). 2.4. Crystallographic data collection and structure determination
2.2.1. [Cu(NITmPy)2 (tp)] (1) An aqueous solution (10 ml) of dipotassium terephthalate (0.121 g, 0.5 mmol) was added to a solution of Cu(ClO4 )2 6H2 O (0.185 g, 0.5 mmol) and NITmPy (0.234 g. 1 mmol) in H2 O (10 ml). The reaction mixture was then stirred for 1 h. The solution was then filtered and the blue microcrystals was washed with methanol (Yield: ca. 70%). (Found: C, 54.87; H, 5.04; N, 11.52%. Calc. for C32 H36 N6 O8 Cu: C, 55.24; H, 5.17; N, 12.06%); IR (KBr): m: 1580 (s), 1400 (s), 1365 (s) cm1 . Blue single crystals suitable for X-ray analysis were obtained by slow evaporation of above filtrate at room temperature. 2.2.2. [Ni(NITmPy)2 (tp)(H2 O)2 ] (2) This complex was prepared as dark blue crystals in a way similar to that of 1. (Yield: ca. 55%). (Found: C, 51.21; H, 5.39; N, 11.81%. Calc. for C32 H40 N6 O10 Ni: C, 52.84; H, 5.54; N, 11.55%); IR (KBr): m: 1570 (s), 1370 (s) cm1 . 2.3. Physical measurements Elemental analysis for C, H and N were carried out on Perkin–Elmer elemental analyzer model 240. The infrared spectrum was taken on a Shimadzu IR spec-
Single crystals of the complexes 1 and 2 were mounted on a Bruker Smart 1000 diffractometer with a CCD Table 1 Summary of crystallographic data for complexes 1 and 2
Formula Formula weight T (K) Crystal system Space group a (A) b (A) c (A) a (°) b (°) c (°) 3 ) U (A Z l (mm1 ) Total reflections/unique Rint Observed ½I > rðIÞ R1 ½wR2 I > rðIÞ R1 wR2 (all data)
1
2
C32 H36 N6 O8 Cu 696.21 293(2) triclinic P 1 7.350(6) 10.214(7) 12.348(9) 73.986(14) 80.295(13) 75.429(12) 857.4(11) 1 0.694 3132/2731 0.0767 1638 0.0891 0.2034 0.1580 0.2501
C32 H40 N6 O10 Ni 727.41 293(2) monoclinic P 21 =n 10.604(4) 11.094(3) 16.570(5) 90 106.712(7) 90 1867.0(10) 2 0.579 7453/3261 0.0893 1524 0.0591 0.1219 0.1467 0.1457
Table 2 and angles (°) for 1 and 2 Selected bond lengths (A) Complex 1 Cu(1)–O(3) Cu(1)–O(4) N(3)–O(2) O(3)–C(13) O(3)–Cu(1)–O(3A) O(3A)–Cu(1)–N(1) O(3A)–Cu(1)–N(1A) O(4)–C(13)–O(3)
2.012(4) 2.539(5) 1.275(9) 1.276(9) 180.0 88.4(2) 91.6(2) 124.1(6)
Complex 2 Ni(1)–O(3) Ni(1)–N(1) O(2)–N(3) O(4)–C(13) O(3A)–Ni(1)–O(3) O(3)–Ni(1)–O(5) O(5)–Ni(1)–O(5A) O(3)–Ni(1)–N(1) O(5A)–Ni(1)–N(1)
2.020(3) 2.102(4) 1.277(6) 1.257(7) 180.0(3) 90.69(14) 180.00(17) 89.68(14) 92.61(15)
Cu(1)–N(1) N(2)–O(1)
2.036(6) 1.217(12)
O(4)–C(13) O(3)–Cu(1)–N(1) O(3)–Cu(1)–N(1A) N(1)–Cu(1)–N(1A) C(13)–O(3)–Cu(1)
1.259(9) 91.6(2) 88.4(2) 180.0(3) 100.7(4)
Ni(1)–O(5) O(1)–N(2) O(3)–C(13)
2.079(3) 1.272(6) 1.250(7)
O(3A)–Ni(1)–O(5) N(1)–Ni(1)–N(1A) O(3A)–Ni(1)–N(1) O(5)–Ni(1)–N(1)
89.31(14) 180.0(2) 90.32(14) 87.39(15)
L. Li et al. / Inorganica Chimica Acta 357 (2004) 405–410
area detector and a graphite monochromated Mo Ka Data collection was radiation source (k ¼ 0:71073 A). performed at room temperature. Empirical absorption corrections by SADABS were carried out [28]. The structures were solved by direct methods using the S H E L X S -97 program [29] and refined with S H E L X L -97 [30] by full matrix least-squares method on F 2 . All nonhydrogen atoms were refined anisotropically, while the hydrogen atoms were located geometrically and refined isotropically. Crystallographic data are summarized in Table 1, selected bond distances and bond angles are given in Table 2.
3. Results and discussion 3.1. Crystal structure 3.1.1. Structure of complex 1 An ORTEP [31] drawing of 1 is shown in Fig. 1. Each terephthalato dianion binds two copper(II) ions in bisdidentate mode that leads to 1-D chain structure. Each copper(II) ion is located in a centrosymmetric 4 + 2 coordination environment in which the NITmPy radicals are trans-coordinated to Cu(II) ion via the nitrogen atoms of the pyridyl rings. The basal plane is formed by two nitrogen atoms (N(1), N(1A)) from the pyridyl rings of NITmPy ligands with Cu–N bond length 2.036(6) A and two carboxylato oxygen atoms (O(3), O(3A)) from The two tp ligands with Cu–O bond length 2.012(4) A. apical sites are occupied by other two carboxylato oxygen atoms (O(4), O(4A)) with Cu–O bond length The phenyl ring of tp ligand makes angle of 2.539(5) A. 79.6° with the equatorial plane of the copper(II) ion. The dihedral angle between pyridyl ring and nitroxide group (O(1)–N(2)–C(6)–N(3)–O(2)) is 6.4°. The shortest However, contact between nitroxide groups is 3.825 A. it can be noted that the closest contact is observed between the oxygen atom (O(2)) of nitroxide group and
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the carbon atom (C0 (3)) belonging to the adjacent chain (shown in Fig. 2). The intrachain (O(2)–C0 (3)) ¼ 3.192 A and the shortest inCu(II)–Cu(II) distance is 10.980 A which terchain Cu(II)–Cu(II) separation is 7.350 A, corresponds to cell length (a). 3.1.2. Structure of complex 2 The ORTEP drawing of 2 is shown in Fig. 3. Each Ni(II) ion is located on an inversion center and adopts distorted octahedral geometry, completed by the two nitrogen atoms (N(1), N(1A)) of pyridyl rings of NIT two carboxylato oxmPy radicals (Ni–N: 2.102(4) A), ygen atoms (O(3), O(3A)) of two tp ligands (Ni–O: and two water molecules (O(5), O(5A), Ni– 2.020(3) A) Each terephthalate ligand links two O(w): 2.079(3) A). Ni(II) ions in a bis-monodentate mode, which results in the linear coordination chain. The dihedral angle between pyridyl ring and nitroxide group (O(1)–N(2)– C(6)–N(3)–O(2)) is 35.6°. The shortest contact between With the linear chain, the nitroxide groups is 4.810 A.
Fig. 2. The closest contact in [Cu(NITmPy)2 (tp)] showing exchange coupling through NO–C pathway.
Fig. 1. An ORTEP drawing of [Cu(NITmPy)2 (tp)] 1 with atomlabeling and 30% thermal ellipsoids.
Fig. 3. An ORTEP drawing of [Ni(NITmPy)2 (tp)(H2 O)2 ] 2 with atom-labeling and 30% thermal ellipsoids.
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distance between successive Ni(II) ions is 11.904 A, corresponding to the cell length (b). The shortest inter Among the chain Ni(II)–Ni(II) separation is 10.112 A. 1-D chains, the coordinated water molecules and the oxygen atoms of the NO groups of NITmPy radicals Thus the form hydrogen bonds (O(2)–O0 (5) ¼ 2.780 A). whole molecular structure becomes 2-D network (Fig. 4). 3.2. Magnetic properties The magnetic susceptibilities of complexes 1 and 2 were measured in the temperature range 2–300 K. The plots vM and leff versus T are shown in Figs. 5 and 6 for complexes 1 and 2, respectively. At room temperature for complex 1, the effective magnetic moment leff is equal to 3.10 lB , which is slightly higher than that expected for three uncorrelated S ¼ 1=2 spins. When the temperature is lowered, this value gradually increases and reaches a maximum of 4.35 lB at 10.46 K, then it decreases quickly. For the present magnetic system, there are four kinds of magnetic interactions, namely: (i) Cu(II) interacting with the coordinated NITmPy radical, (ii) magnetic coupling between neighboring NITmPy radicals through the (iii) Cu(II)–Cu(II) interacNO C pathway (3.192 A), tion via the terephthalate bridge and (iv) interaction between neighboring NO groups of NITmPy radicals Since the tp ligand makes angle through space (3.825 A). of 79.6° with the equatorial plane of the copper(II) ion, the magnetic coupling between copper(II) ions through tp bridge should be weakly antiferromagnetic [25]. The magnitude of magnetic coupling of the two adjacent NO groups depends on their distance and the relative orientation of the p orbitals [32]. In complex 1, the
Fig. 4. The 2-D structure of [Ni(NITmPy)2 (tp)(H2 O)2 ].
Fig. 5. Plots of vM ðsÞ and leff ðMÞ vs. T for [Cu(NITmPy)2 (tp)]. The solid line corresponds to the best theoretical fits.
Fig. 6. Plots of vM ðsÞ and leff ðMÞ vs. T for [Ni(NITmPy)2 (tp)(H2 O)2 ]. The solid line corresponds to the best theoretical fits.
shortest O–O separation between nitroxide groups is and the planes of the p systems of rather large (3.825 A) the nitroxide radicals form an angle of 3.9° with the plane defined by the two adjacent NO groups, which results in the p-type overlap of two adjacent NO groups. These geometrical parameters lead to weakly antiferromagnetic interaction [32–34] Therefore, the last two kinds of magnetic interactions correspond to the antiferromagnetic behavior of complex 1 at the low temperature. The crystal structure of 1 shows that the closest contact involves the oxygen atom (O(2)) of uncoordinated NO group which carries a large positive spin density and the carbon atom (C0 (3)) of the adjacent NITmPy radical that carries a small negative These two atoms spin density (O(2)–C0 (3) ¼ 3.192 A). carrying opposite sign of the spin density alternate leading to "N(3)–O(2)" #C0 (3)–"C0 (4)–#C0 (6)–"N0 (3)– "O0 (2) #C00 (3), which matches McConnellÕs criteria [35]. Thus the exchange coupling between neighboring
L. Li et al. / Inorganica Chimica Acta 357 (2004) 405–410
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Table 3 Selected magneto-structural data reported for relevant metal–NITmpy complexes Complex
J (cm1 )
Coordination sites
b (°)
Ref.
[Cu(Cl2 CHCO2 )2 (NITmPy)2 (H2 O)2 ] [Cu(tta)2 (NITmPy)2 ] C7 H8 [Cu(NITmPy)2 (tp)] [Ni(NITmPy)2 (tp)(H2 O)2 ]
12.0 6.1 17.00 4.38
equatorial equatorial equatorial equatorial
24.91, 25.26
[36] [37] this work this work
6.4 35.6
b is defined as the dihedral angle between pyridyl ring and nitroxide group for NITmPy radical; tta ¼ thenoyltrifluoroacetonate.
NITmPy radicals through an NO–C pathway should be weakly ferromagnetic. The first two kinds of magnetic interactions lead to the ferromagnetic behavior of complex 1. It would be very difficult to incorporate all of these into a model to fit the magnetic data in the temperature range 2–300 K. An P Eq. (1) derived from the spin Hamiltonian H^ ¼ 2J ðS^Cu S^R þ S^Cu S^R Þ was applied to reproduce the magnetic data from 10 to 300 K. Considering the magnetic coupling between neighboring NITmPy radicals through the NO C pathway, the mean-field approximation, zJ 0 was introduced. vtri ¼
N b2 g32 þ g22 expð2J =kT Þ þ 10g12 expðJ =kT Þ ; 1 þ expð2J =kT Þ þ 2 expðJ =kT Þ 4kT ð1aÞ
vM ¼ vtri
1 vtri 2zJ 0 =Ng2 b2 ;
ð1bÞ
with g1 ¼ ð2grad þ gCu Þ=3; g2 ¼ ð4grad gCu Þ=3; g3 ¼ gCu : The best fit to the experimental data (10–300 K) yielded J ¼ 17:00 cm1 , zJ 0 ¼ 0:81 cm1 , gCu ¼ 2:02, grad ¼ 2:0 4 (fixed) and R ¼ 9:35 (R value is defined as P10 P ½ðvM Þobs ðvM Þcalc 2 = ½ðvM Þobs 2 ). For complex 2, the leff value is 3.77 lB at room temperature, which is consistent with the expected value for isolated SNi ¼ 1 and two Srad ¼ 1=2 spins. This value almost is a constant from 300 to 150 K, then smoothly increase as the temperature decreases and reaches a maximum of 4.09 lB at 18.27 K, and then decreases rapidly to 2 K. According to the crystal structure, the uncoordinated NO groups are well isolated (4.810 A), thus the magnetic interaction between neighboring NO groups of NITmPy radicals can be neglected. The magnetic coupling between nickel(II) ion and the NO groups through NO OH2 pathway should be very weakly antiferromagnetic. The magnetic interaction between nickel(II) ions through tp bridging ligand also should be weakly antiferromagnetic. Therefore, the magnetic behavior of complex 2 can be explained by a theoretical expression P (2) deduced from the spin Hamiltonian H^ ¼ 2J ðS^Ni S^R þ S^Ni S^R Þ, where J represents the magnetic interaction between Ni(II) ion and the
coordinated NITmPy radical. The weak antiferromagnetic interactions and the zero-field splitting of the ground spin state S ¼ 2 were considered in mean-field approximation as zJ 0 . 2
2 N b2 ðgNi þ gR Þ ð5 þ e4x Þ þ 4gNi e2x ; 2kT 5 þ 3 e4x þ e6x þ 3 e2x 0 ¼ vM 1 vM 2zJ =Ng2 b2 :
vM ¼ vtotal
ð2Þ
The best fit for magnetic data led to gNi ¼ 2:04, grad ¼ 2:0 (fixed), J ¼ 4:38 cm1 , zJ 0 ¼ 0:61 cm1 , R ¼ 6:83 105 . For complexes 1 and 2, the fitting results indicate the magnetic interactions between metal ions and the coordinated NITmPy radicals are ferromagnetic interactions. Assuming both complexes with C2v symmetry as shown from the crystal structures, the magnetic orbital of NITmPy radical (p ) has b2 symmetry while the magnetic orbitals of the metal ions have a1 and a1 þ a01 symmetry for Cu(II) (dx2 y 2 ) and Ni(II) (dx2 y 2 , dz2 ), respectively, and in both complexes, the overlap is symmetry forbidden. The magnetic interactions are predicted to be ferromagnetic as observed. In order to compare the results of the present two complexes with other metal–NITmPy complexes found in the literature, the relevant magneto-structural data of these complexes are listed in Table 3. It can be seen that the magnetic coupling between metal ion and NITmPy radical ligand is dependent on the value of the dihedral angle between pyridyl ring and nitroxide group: the larger the value of the dihedral angle, the weaker is the ferromagnetic coupling. This can be understood by spin polarization. The large dihedral angle between the nitroxide moiety and pyridyl ring results in difficult delocalization of the spin density of the NO group along the pyridyl ring in the radical ligand. Acknowledgements This work was supported by the National Science Foundation of China (Nos. 20271029 and 20171025).
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Ferromagnetic interactions in copper(II) and nickel(II) coordination polymers containing nitronyl nitroxide radical and terephthalate Licun Li
a,b,*
, Daizheng Liao a, Zonghui Jiang a, Shiping Yan
a
a
b
Department of Chemistry, Nankai University, Tianjin 300071, PR China State Key Laboratory of Structural chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China Received 17 February 2003; accepted 14 May 2003
Abstract Two new complexes [Cu(NITmPy)2 (tp)] 1 and [Ni(NITmPy)2 (tp)(H2 O)2 ] 2 (NITmPy ¼ 2-(30 -pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and tp ¼ terephthalato dianion) were synthesized and structurally and magnetically characterized. The structure of 1 is a neutral infinite chain where Cu(NITmPy)2 units are linked by terephthalate ligands. In complex 2, the 1-D chains of Ni(NITmPy)2 (H2 O)2 units connected by tp develop into 2-D network via hydrogen bond interactions. The magnetic properties of 1 and 2 have been investigated in the temperature range 2–300 K. Both complexes exhibit ferromagnetic coupling and antiferromagnetic interactions dominate at low temperature. The magnetic behavior is discussed based on their structures. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Nitronyl nitroxide; Copper complexes; Nickel complexes; Terephthalate; Crystal structures; Magnetic properties
1. Introduction In the field of molecular-based magnetic materials, transition metal complexes with nitroxide radical ligands have found widespread interest in recent years [1– 4]. Considerable efforts have directed in preparing and characterising metal–radical multi-dimensional systems which can exhibit ferromagnetic properties [4–9]. One of synthetic strategies is that transition metal ions coordinated by paramagnetic nitroxide radicals are linked by inorganic or organic bridging ligands to give coordination polymers or networks [10–14]. The metal complexes, based on pyridyl-substituted nitronyl nitroxides, have attracted considerable attention due to the ability of these radicals to coordinate metal ions and act as magnetic couplers, giving rise to
*
Corresponding author. E-mail address: [email protected] (L. Li).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.05.006
new functional materials with a variety of structural topology [15–20]. The terephthalato ligand is a versatile ligand with good binding ability as exemplified by the formation of polymeric structures [21–24]. Terephthalato-bridged complexes have been actively studied to elucidate the limiting distance of magnetic exchange between paramagnetic metal centers. Long tp bridges which typically produce M–M separation of about 11 A, generally leads to weak antiferromagnetic interactions between metal centers for M ¼ Mn(II), Co(II), Ni(II), Cu(II) [25]. However, when both the metal basal plane and the tp ligand are coplanar, stronger antiferromagnetic interactions are observed [20,26]. With the purpose of obtaining materials with unusual magnetic properties derived from nitronyl nitroxide radicals and terephthalate ligand, we present here two new metal– radical coordination polymers with terephthalate bridging ligand, [Cu(NITmPy)2 (tp)] (1) and [Ni(NITmPy)2 (tp)(H2 O)2 ] (2) (NITmPy ¼ 2-(30 -pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and tp ¼ terephthalato dianion).
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L. Li et al. / Inorganica Chimica Acta 357 (2004) 405–410
2. Experimental 2.1. Materials All reagents and chemicals were purchased from commercial sources and used as received. 2-(30 -pyridyl)4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITmPy) was prepared by the literature method [27]. 2.2. Synthesis
trophotometer model 408, using KBr pellets. Variable temperature magnetic susceptibilities on polycrystalline samples were measured on a Maglab system 2000 magnetometer. Diamagnetic corrections were made with PascalÕs constants for all constitute atoms. Experimental susceptibilities of 1 were also corrected for the temperature-independent paramagnetism (60 106 cm3 mol1 per copper(II) ion). 2.4. Crystallographic data collection and structure determination
2.2.1. [Cu(NITmPy)2 (tp)] (1) An aqueous solution (10 ml) of dipotassium terephthalate (0.121 g, 0.5 mmol) was added to a solution of Cu(ClO4 )2 6H2 O (0.185 g, 0.5 mmol) and NITmPy (0.234 g. 1 mmol) in H2 O (10 ml). The reaction mixture was then stirred for 1 h. The solution was then filtered and the blue microcrystals was washed with methanol (Yield: ca. 70%). (Found: C, 54.87; H, 5.04; N, 11.52%. Calc. for C32 H36 N6 O8 Cu: C, 55.24; H, 5.17; N, 12.06%); IR (KBr): m: 1580 (s), 1400 (s), 1365 (s) cm1 . Blue single crystals suitable for X-ray analysis were obtained by slow evaporation of above filtrate at room temperature. 2.2.2. [Ni(NITmPy)2 (tp)(H2 O)2 ] (2) This complex was prepared as dark blue crystals in a way similar to that of 1. (Yield: ca. 55%). (Found: C, 51.21; H, 5.39; N, 11.81%. Calc. for C32 H40 N6 O10 Ni: C, 52.84; H, 5.54; N, 11.55%); IR (KBr): m: 1570 (s), 1370 (s) cm1 . 2.3. Physical measurements Elemental analysis for C, H and N were carried out on Perkin–Elmer elemental analyzer model 240. The infrared spectrum was taken on a Shimadzu IR spec-
Single crystals of the complexes 1 and 2 were mounted on a Bruker Smart 1000 diffractometer with a CCD Table 1 Summary of crystallographic data for complexes 1 and 2
Formula Formula weight T (K) Crystal system Space group a (A) b (A) c (A) a (°) b (°) c (°) 3 ) U (A Z l (mm1 ) Total reflections/unique Rint Observed ½I > rðIÞ R1 ½wR2 I > rðIÞ R1 wR2 (all data)
1
2
C32 H36 N6 O8 Cu 696.21 293(2) triclinic P 1 7.350(6) 10.214(7) 12.348(9) 73.986(14) 80.295(13) 75.429(12) 857.4(11) 1 0.694 3132/2731 0.0767 1638 0.0891 0.2034 0.1580 0.2501
C32 H40 N6 O10 Ni 727.41 293(2) monoclinic P 21 =n 10.604(4) 11.094(3) 16.570(5) 90 106.712(7) 90 1867.0(10) 2 0.579 7453/3261 0.0893 1524 0.0591 0.1219 0.1467 0.1457
Table 2 and angles (°) for 1 and 2 Selected bond lengths (A) Complex 1 Cu(1)–O(3) Cu(1)–O(4) N(3)–O(2) O(3)–C(13) O(3)–Cu(1)–O(3A) O(3A)–Cu(1)–N(1) O(3A)–Cu(1)–N(1A) O(4)–C(13)–O(3)
2.012(4) 2.539(5) 1.275(9) 1.276(9) 180.0 88.4(2) 91.6(2) 124.1(6)
Complex 2 Ni(1)–O(3) Ni(1)–N(1) O(2)–N(3) O(4)–C(13) O(3A)–Ni(1)–O(3) O(3)–Ni(1)–O(5) O(5)–Ni(1)–O(5A) O(3)–Ni(1)–N(1) O(5A)–Ni(1)–N(1)
2.020(3) 2.102(4) 1.277(6) 1.257(7) 180.0(3) 90.69(14) 180.00(17) 89.68(14) 92.61(15)
Cu(1)–N(1) N(2)–O(1)
2.036(6) 1.217(12)
O(4)–C(13) O(3)–Cu(1)–N(1) O(3)–Cu(1)–N(1A) N(1)–Cu(1)–N(1A) C(13)–O(3)–Cu(1)
1.259(9) 91.6(2) 88.4(2) 180.0(3) 100.7(4)
Ni(1)–O(5) O(1)–N(2) O(3)–C(13)
2.079(3) 1.272(6) 1.250(7)
O(3A)–Ni(1)–O(5) N(1)–Ni(1)–N(1A) O(3A)–Ni(1)–N(1) O(5)–Ni(1)–N(1)
89.31(14) 180.0(2) 90.32(14) 87.39(15)
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area detector and a graphite monochromated Mo Ka Data collection was radiation source (k ¼ 0:71073 A). performed at room temperature. Empirical absorption corrections by SADABS were carried out [28]. The structures were solved by direct methods using the S H E L X S -97 program [29] and refined with S H E L X L -97 [30] by full matrix least-squares method on F 2 . All nonhydrogen atoms were refined anisotropically, while the hydrogen atoms were located geometrically and refined isotropically. Crystallographic data are summarized in Table 1, selected bond distances and bond angles are given in Table 2.
3. Results and discussion 3.1. Crystal structure 3.1.1. Structure of complex 1 An ORTEP [31] drawing of 1 is shown in Fig. 1. Each terephthalato dianion binds two copper(II) ions in bisdidentate mode that leads to 1-D chain structure. Each copper(II) ion is located in a centrosymmetric 4 + 2 coordination environment in which the NITmPy radicals are trans-coordinated to Cu(II) ion via the nitrogen atoms of the pyridyl rings. The basal plane is formed by two nitrogen atoms (N(1), N(1A)) from the pyridyl rings of NITmPy ligands with Cu–N bond length 2.036(6) A and two carboxylato oxygen atoms (O(3), O(3A)) from The two tp ligands with Cu–O bond length 2.012(4) A. apical sites are occupied by other two carboxylato oxygen atoms (O(4), O(4A)) with Cu–O bond length The phenyl ring of tp ligand makes angle of 2.539(5) A. 79.6° with the equatorial plane of the copper(II) ion. The dihedral angle between pyridyl ring and nitroxide group (O(1)–N(2)–C(6)–N(3)–O(2)) is 6.4°. The shortest However, contact between nitroxide groups is 3.825 A. it can be noted that the closest contact is observed between the oxygen atom (O(2)) of nitroxide group and
407
the carbon atom (C0 (3)) belonging to the adjacent chain (shown in Fig. 2). The intrachain (O(2)–C0 (3)) ¼ 3.192 A and the shortest inCu(II)–Cu(II) distance is 10.980 A which terchain Cu(II)–Cu(II) separation is 7.350 A, corresponds to cell length (a). 3.1.2. Structure of complex 2 The ORTEP drawing of 2 is shown in Fig. 3. Each Ni(II) ion is located on an inversion center and adopts distorted octahedral geometry, completed by the two nitrogen atoms (N(1), N(1A)) of pyridyl rings of NIT two carboxylato oxmPy radicals (Ni–N: 2.102(4) A), ygen atoms (O(3), O(3A)) of two tp ligands (Ni–O: and two water molecules (O(5), O(5A), Ni– 2.020(3) A) Each terephthalate ligand links two O(w): 2.079(3) A). Ni(II) ions in a bis-monodentate mode, which results in the linear coordination chain. The dihedral angle between pyridyl ring and nitroxide group (O(1)–N(2)– C(6)–N(3)–O(2)) is 35.6°. The shortest contact between With the linear chain, the nitroxide groups is 4.810 A.
Fig. 2. The closest contact in [Cu(NITmPy)2 (tp)] showing exchange coupling through NO–C pathway.
Fig. 1. An ORTEP drawing of [Cu(NITmPy)2 (tp)] 1 with atomlabeling and 30% thermal ellipsoids.
Fig. 3. An ORTEP drawing of [Ni(NITmPy)2 (tp)(H2 O)2 ] 2 with atom-labeling and 30% thermal ellipsoids.
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distance between successive Ni(II) ions is 11.904 A, corresponding to the cell length (b). The shortest inter Among the chain Ni(II)–Ni(II) separation is 10.112 A. 1-D chains, the coordinated water molecules and the oxygen atoms of the NO groups of NITmPy radicals Thus the form hydrogen bonds (O(2)–O0 (5) ¼ 2.780 A). whole molecular structure becomes 2-D network (Fig. 4). 3.2. Magnetic properties The magnetic susceptibilities of complexes 1 and 2 were measured in the temperature range 2–300 K. The plots vM and leff versus T are shown in Figs. 5 and 6 for complexes 1 and 2, respectively. At room temperature for complex 1, the effective magnetic moment leff is equal to 3.10 lB , which is slightly higher than that expected for three uncorrelated S ¼ 1=2 spins. When the temperature is lowered, this value gradually increases and reaches a maximum of 4.35 lB at 10.46 K, then it decreases quickly. For the present magnetic system, there are four kinds of magnetic interactions, namely: (i) Cu(II) interacting with the coordinated NITmPy radical, (ii) magnetic coupling between neighboring NITmPy radicals through the (iii) Cu(II)–Cu(II) interacNO C pathway (3.192 A), tion via the terephthalate bridge and (iv) interaction between neighboring NO groups of NITmPy radicals Since the tp ligand makes angle through space (3.825 A). of 79.6° with the equatorial plane of the copper(II) ion, the magnetic coupling between copper(II) ions through tp bridge should be weakly antiferromagnetic [25]. The magnitude of magnetic coupling of the two adjacent NO groups depends on their distance and the relative orientation of the p orbitals [32]. In complex 1, the
Fig. 4. The 2-D structure of [Ni(NITmPy)2 (tp)(H2 O)2 ].
Fig. 5. Plots of vM ðsÞ and leff ðMÞ vs. T for [Cu(NITmPy)2 (tp)]. The solid line corresponds to the best theoretical fits.
Fig. 6. Plots of vM ðsÞ and leff ðMÞ vs. T for [Ni(NITmPy)2 (tp)(H2 O)2 ]. The solid line corresponds to the best theoretical fits.
shortest O–O separation between nitroxide groups is and the planes of the p systems of rather large (3.825 A) the nitroxide radicals form an angle of 3.9° with the plane defined by the two adjacent NO groups, which results in the p-type overlap of two adjacent NO groups. These geometrical parameters lead to weakly antiferromagnetic interaction [32–34] Therefore, the last two kinds of magnetic interactions correspond to the antiferromagnetic behavior of complex 1 at the low temperature. The crystal structure of 1 shows that the closest contact involves the oxygen atom (O(2)) of uncoordinated NO group which carries a large positive spin density and the carbon atom (C0 (3)) of the adjacent NITmPy radical that carries a small negative These two atoms spin density (O(2)–C0 (3) ¼ 3.192 A). carrying opposite sign of the spin density alternate leading to "N(3)–O(2)" #C0 (3)–"C0 (4)–#C0 (6)–"N0 (3)– "O0 (2) #C00 (3), which matches McConnellÕs criteria [35]. Thus the exchange coupling between neighboring
L. Li et al. / Inorganica Chimica Acta 357 (2004) 405–410
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Table 3 Selected magneto-structural data reported for relevant metal–NITmpy complexes Complex
J (cm1 )
Coordination sites
b (°)
Ref.
[Cu(Cl2 CHCO2 )2 (NITmPy)2 (H2 O)2 ] [Cu(tta)2 (NITmPy)2 ] C7 H8 [Cu(NITmPy)2 (tp)] [Ni(NITmPy)2 (tp)(H2 O)2 ]
12.0 6.1 17.00 4.38
equatorial equatorial equatorial equatorial
24.91, 25.26
[36] [37] this work this work
6.4 35.6
b is defined as the dihedral angle between pyridyl ring and nitroxide group for NITmPy radical; tta ¼ thenoyltrifluoroacetonate.
NITmPy radicals through an NO–C pathway should be weakly ferromagnetic. The first two kinds of magnetic interactions lead to the ferromagnetic behavior of complex 1. It would be very difficult to incorporate all of these into a model to fit the magnetic data in the temperature range 2–300 K. An P Eq. (1) derived from the spin Hamiltonian H^ ¼ 2J ðS^Cu S^R þ S^Cu S^R Þ was applied to reproduce the magnetic data from 10 to 300 K. Considering the magnetic coupling between neighboring NITmPy radicals through the NO C pathway, the mean-field approximation, zJ 0 was introduced. vtri ¼
N b2 g32 þ g22 expð2J =kT Þ þ 10g12 expðJ =kT Þ ; 1 þ expð2J =kT Þ þ 2 expðJ =kT Þ 4kT ð1aÞ
vM ¼ vtri
1 vtri 2zJ 0 =Ng2 b2 ;
ð1bÞ
with g1 ¼ ð2grad þ gCu Þ=3; g2 ¼ ð4grad gCu Þ=3; g3 ¼ gCu : The best fit to the experimental data (10–300 K) yielded J ¼ 17:00 cm1 , zJ 0 ¼ 0:81 cm1 , gCu ¼ 2:02, grad ¼ 2:0 4 (fixed) and R ¼ 9:35 (R value is defined as P10 P ½ðvM Þobs ðvM Þcalc 2 = ½ðvM Þobs 2 ). For complex 2, the leff value is 3.77 lB at room temperature, which is consistent with the expected value for isolated SNi ¼ 1 and two Srad ¼ 1=2 spins. This value almost is a constant from 300 to 150 K, then smoothly increase as the temperature decreases and reaches a maximum of 4.09 lB at 18.27 K, and then decreases rapidly to 2 K. According to the crystal structure, the uncoordinated NO groups are well isolated (4.810 A), thus the magnetic interaction between neighboring NO groups of NITmPy radicals can be neglected. The magnetic coupling between nickel(II) ion and the NO groups through NO OH2 pathway should be very weakly antiferromagnetic. The magnetic interaction between nickel(II) ions through tp bridging ligand also should be weakly antiferromagnetic. Therefore, the magnetic behavior of complex 2 can be explained by a theoretical expression P (2) deduced from the spin Hamiltonian H^ ¼ 2J ðS^Ni S^R þ S^Ni S^R Þ, where J represents the magnetic interaction between Ni(II) ion and the
coordinated NITmPy radical. The weak antiferromagnetic interactions and the zero-field splitting of the ground spin state S ¼ 2 were considered in mean-field approximation as zJ 0 . 2
2 N b2 ðgNi þ gR Þ ð5 þ e4x Þ þ 4gNi e2x ; 2kT 5 þ 3 e4x þ e6x þ 3 e2x 0 ¼ vM 1 vM 2zJ =Ng2 b2 :
vM ¼ vtotal
ð2Þ
The best fit for magnetic data led to gNi ¼ 2:04, grad ¼ 2:0 (fixed), J ¼ 4:38 cm1 , zJ 0 ¼ 0:61 cm1 , R ¼ 6:83 105 . For complexes 1 and 2, the fitting results indicate the magnetic interactions between metal ions and the coordinated NITmPy radicals are ferromagnetic interactions. Assuming both complexes with C2v symmetry as shown from the crystal structures, the magnetic orbital of NITmPy radical (p ) has b2 symmetry while the magnetic orbitals of the metal ions have a1 and a1 þ a01 symmetry for Cu(II) (dx2 y 2 ) and Ni(II) (dx2 y 2 , dz2 ), respectively, and in both complexes, the overlap is symmetry forbidden. The magnetic interactions are predicted to be ferromagnetic as observed. In order to compare the results of the present two complexes with other metal–NITmPy complexes found in the literature, the relevant magneto-structural data of these complexes are listed in Table 3. It can be seen that the magnetic coupling between metal ion and NITmPy radical ligand is dependent on the value of the dihedral angle between pyridyl ring and nitroxide group: the larger the value of the dihedral angle, the weaker is the ferromagnetic coupling. This can be understood by spin polarization. The large dihedral angle between the nitroxide moiety and pyridyl ring results in difficult delocalization of the spin density of the NO group along the pyridyl ring in the radical ligand. Acknowledgements This work was supported by the National Science Foundation of China (Nos. 20271029 and 20171025).
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