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Crystal structures and magnetic properties of two complexes synthesized from manganese and halogenophenyl-substituted nitronyl nitroxide
Inorganica Chimica Acta 367 (2011) 135–140
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Crystal structures and magnetic properties of two complexes synthesized from manganese and halogenophenyl-substituted nitronyl nitroxide Chen-Xi Zhang a,⇑, Xiang-Yu Zhao a,b, Na-Na Sun a,b, Yan-Ling Guo a,b, Yuying Zhang b a b
College of Science, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China Department of Chemistry, NanKai University, Tianjin 300071, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 July 2010 Received in revised form 29 November 2010 Accepted 9 December 2010 Available online 21 December 2010 Keywords: Crystal structure Nitronyl nitroxide Manganese complex Magnetic property
a b s t r a c t One-dimensional complex (1), [Mn(hfac)2(NITPhF)]2 and one binuclear radical complex (2), [Mn(hfac)2(NITPhBr)]1 have been synthesized. Here hfac stands for hexafluoroacetylacetonate, NITPhF for 2-(40 -fluorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, and NITPhBr for 2-(40 -bromophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). All the two precursors were prepared and characterized by single-crystal X-ray diffraction analysis, IR, and magnetic analysis. In complex 1, the NITPhF radical acts as a bridge ligand linking two Mn(II) ions through the oxygen atom of the N–O group to form cyclic dimer. The dimers further connect two oxygen atoms of uncoordinated nitroxides of two adjacent radicals and yield one-dimensional chain sections. Instead in complex 2, the Mn(II) ions are bridged by the NITPhBr radicals through their N–O groups giving infinite one-dimensional chains. Magnetic susceptibility measurements indicate that both complex 1 and 2 behave ferrimagnetically. The Mn(II) ions interact antiferromagnetically with the direct bonding nitroxide group of the radicals although the structures of two complexes are different. The magnetic behaviors can be satisfactorily explained on the basis of the structural data. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The design of molecule structure with practical magnetic properties has been interesting and full of challenge during the past two decades. One very promising strategy is to combine paramagnetic cations with organic open-shell molecules to make hybrid systems [1–5]. Stable organic radicals Nitronyl Nitroxide Radicals (NITR) have been widely employed as magnetic couplers to coordinate metal ions and affording variety of structural topologies thus new functional materials [6–16]. Recently, some cyclic dimer complexes with quadrangle geometry, such as nitronyl nitroxide and iminoyl nitroxide have been studied [17–22]. For these cyclic complexes, most studies have focused on the complexes constructed from nitroxide radicals substituted by nitrogen-containing group which can coordinate to metal ion. In general, the oxygen atoms of nitroxide groups acting as bridging ligands can also form cyclic dimer complexes with metal ions. The first single chain magnet [Co(hfac)2(NITPhOMe)] was discovered by Gatteschi and co-workers [25]. One-dimensional metal nitroxide complexs have attracted much attention for the design and synthesis of molecules based magnetic materials [23,24]. However, very few such complexes have been reported. Thus, detailed studies on low-dimensional metal-nitroxide compounds ⇑ Corresponding author. E-mail address: [email protected] (C.-X. Zhang). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.12.019
are essential not only to the design and preparation molecular magnet, but also to better understanding of the relationship between the structure and magnetic property. As we know, a small change in the structure of nitroxide radicals or in the coordination sphere of such complexes can lead to the large changes in the magnetic properties. We have designed and synthesized two kinds of similar halogen phenyl-substituted nitronyl nitroxide ligand, 2-(40 -fluorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide(NITPhF) and 2-(40 -bromophenyl)-4,4,5,5-tetramethylimidazoline1-oxyl-3-oxide(NITPhBr). Through changing the halogen group in the radical, two halogen phenyl-substituted radicals were successfully used to synthesize two different structural complexes: cyclic dimer M2L2 [Mn(hfac)2(NITPhF)]2 named as complex 1 and onedimensional radical [Mn(hfac)2(NITPhBr)]1 named as complex 2. In complex 1 the oxygen atom from nitroxide group acts as a l1,1 bridging. In this study, we report the crystal structure and magnetic properties of the two low dimensional complexes.
2. Experimental 2.1. Synthesis of the complex 1 [Mn(hfac)2(NITPhF)]2 and 2 [Mn(hfac)2(NITPhBr)]1 2-(4 0 -Fluorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide and 2-(40 -bromophenyl)-4,4,5,5-tetramethylimidazoline-
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1-oxyl-3-oxide were prepared by using of the procedure reported in references [26–28]. Complex 1 [Mn(hfac)2(NITPhF)]2 30.4 mg (0.06 mmol) of Mn(hfac)22H2O was dissolved in 30 ml of boiling heptanes for azeotropically removing hydration water of molecules. Then the solution was cooled to 80 °C, and a solution of NITPhF (12.0 mg, 0.04 mmol) in 2 ml CH2Cl2 was added. The resulting green solution was stirred for 5 min and cooling down to room temperature. The filtrate was allowed for 2 days, and dark green crystals were collected. Yield: 68%. Anal. Calc. for C46H36F26Mn2 N2O12 (1412.62): C, 39.11; N, 1.98; H, 2.57. Found: C, 39.51; N, 2.08; H, 2.51%. FTIR(KBr, cm1): 1656(m), 1527(s), 1359(w), 1254(s), 1140(s), 789(w), 660(w). Complex 2 [Mn(hfac)2(NITPhBr)]1 was prepared in the similar manner of complex 1 by replacing NITPhBr with NITPhF. Yield: 62%. Anal. Calc. for C49.5H44Br2F24Mn2N4O12 (1612.59): C, 36.87; N, 3.47; H, 2.75. Found: C, 36.75; N, 3.41; H, 2.72% FT-IR(KBr, cm1): 1658(m), 1530(s), 1361(w), 1258(s), 1141(s), 790(w), 658(w).
Table 2 Selected bond lengths (Å) and angles (°) for complex 1 and 2. Complex 1
Complex 2
Bond distances Mn(1)–O(5) Mn(1)–O(6) Mn(1)–O(3) Mn(1)–O(4) Mn(1)–O(1) Mn(1)–O(1)#1 O(1)–N(1) O(2)–N(2)
2.148(2) 2.154(2) 2.162(2) 2.170(2) 2.177 (2) 2.191 (2) 1.344(3) 1.277(5)
Mn(1)–O(1) Mn(1)–O(2)#1 Mn(1)–O(5) Mn(1)–O(6) Mn(1)–O(4) Mn(1)–O(3) O(1)–N(1) O(2)–N(2)
2.108(3) 2.130(3) 2.143(3) 2.151(3) 2.156(3) 2.169(3) 1.293(4) 1.300(4)
Bond angles O(5)–Mn(1)–O(6) O(5)–Mn(1)–O(3) O(6)–Mn(1)–O(3) O(5)–Mn(1)–O(4) O(6)–Mn(1)–O(4) O(3)–Mn(1)–O(4) O(5)–Mn(1)–O(1) O(6)–Mn(1)–O(1) O(3)–Mn(1)–O(1) O(4)–Mn(1)-O(1)
81.37(8) 97.25(8) 81.53(8) 82.76(8) 153.22(8) 79.24(8) 162.89(7) 95.78(8) 99.01(7) 105.47(7)
O(1)–Mn(1)–O(2)#1 O(1)–Mn(1)–O(5) O(2)#1–Mn(1)–O(5) O(1)–Mn(1)–O(6) O(2)#1–Mn(1)–O(6) O(5)–Mn(1)–O(6) O(1)–Mn(1)–O(4) O(2)#1–Mn(1)–O(4) O(5)–Mn(1)–O(4) O(6)–Mn(1)–O(4)
79.99(10) 165.21(10) 88.48(10)) 92.52(11) 108.06(11) 82.21(11) 105.00(10) 92.14(11) 84.50(11) 155.40(10)
2.2. X-ray crystallography Diffraction intensity data for single crystals 1 and 2 were collected at room temperature on a Bruker Smart 1000 CCDC area detector equipped with graphite-mono-chromated Mo Ka radiation (k = 0.71073 Å). The structure was determined first by the direct method and then refined by the full-matrix least-squares method on F2 with anisotropic thermal parameters for all nonhydrogen atoms. The hydrogen atoms of solvent molecules were not added, and the other hydrogen atoms were located geometrically and refined isotropically. All calculations were carried out using SHELXS-97 and SHELXL-97 programs [29]. Crystal data are displayed in Table 1. Selected bond lengths and angles are listed in Table 2. 2.3. Materials and equipment All reagents and solvents were purchased commercially and used without further purification. Elemental analyses for carbon, Table 1 Crystallographic data and processing parameters for complex 1 and 2. Compound
1
2
Empirical formula Formula weight Temperature(K) Wavelength(Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) Volume (Å3) Z Calculated density (mg/m3) Absorption coefficient(mm1) F(0 0 0) Crystal size (mm3) h Range for data collection (°) Reflections Collected/unique Data/restraints/parameters Goodness-of-fit on F2 R1[I > 2r(I)] wR2[I > 2r(I)] R1 (all data) wR2 (all data)
C23H18F13N2O5.50Mn 712.33 113(2) 0.71073 monoclinic P2(1)/n
C49.50H44Br2F24N4O1Mn2 1612.59 113(2) 0.71073 monoclinic C2/c
11.719(2) 13.282(3) 17.825(4) 99.41(3) 2737.2(10) 4 1.729 0.611
26.591(5) 13.630(3) 22.839(5) 124.57(3) 6816(2) 4 1.571 1.663
1424 0.20 0.18 0.16 2.32–25.02
3204 0.24 0.20 0.16 1.76–25.02
19784/4822 [R(int) = 0.0290] 4822/270/522 1.091 0.0449 0.1191 0.0492 0.1224
22778/5997 [R(int) = 0.0469] 5997/170/502 1.049 0.0527 0.1573 0.0674 0.1690
Symmetry transformations used to generate equivalent atoms: #1: x + 1, y + 1, z + 2 for 1. #1: x + 1/2, y + 1/2, z + 3/2; #2: x + 1/2, y 1/2, z + 3/2; #3: x + 1, y, z + 3/2 for 2.
hydrogen, and nitrogen were carried out on a Model 240 Perkin– Elmer elemental analyzer. The infrared spectrum was taken on a Shimadzu IR spectrophotometer model 408 in the region of 4000–600 cm1 of KBr pellets. Temperature dependence of magnetic susceptibilities was measured on a MPMS-7 SQUID magnetometer in the temperature range 2.0–300 K. Diamagnetic corrections were made with Pascal’s constants for all constituent atoms. The magnetic moments were calculated using the equation leff = 2.828(vMT)1/2. 5. Results and discussion 5.1. Determination of the crystal structure An ORTEP drawing of the complex 1 is shown in Fig. 1. The complex consists of netural dimer units. Each NITPhF radical ligand
Fig. 1. The molecular structure of complex 1. Hydrogen atoms are not shown for the sake of clarity.
C.-X. Zhang et al. / Inorganica Chimica Acta 367 (2011) 135–140
137
Fig. 2. View of the one-dimensional crystal structure of complex 1 by weak nitroxide–nitroxide bonding along b direction.
coordinats to two metal ions through the oxygen atom of the nitronyl nitroxide group, so that the NITPhF radicals act as a l1,1 bridging ligands and form a rectangle-like centrosymmetric dimer structure. The Mn(II) ions of the cyclic dimer exhibit a slightly distorted octahedral coordination sphere with four oxygen atoms from two hfac anions and two oxygen atoms of two different NITPhF radical ligands. The Mn–O(hfac) bond lengths are fairly similar to each other, ranging from 2.148(2) to 2.170(2) Å, while the Mn– O(Radical) bond lengths are longer, 2.177(2) and 2.191(2) Å. These bond lengths are comparable to the reported cyclic metal–nitronyl nitroxide dimmers [20,21]. In NITPhF radicals, the O–N–C–N–O group containing unpaired electrons has 36.1° angle with the benzene ring. In the dimmer unit, the distance between two Mn(II) ions is 3.522 Å. It should be noted that the shortest distance of (2.001 Å) was observed between the two oxygen atoms of uncoordinated nitroxide groups of two adjacent radicals. The way of the molecular coordination yields one-dimensional chain consisting of cyclic dimer units. These are demonstrated in Fig. 2. Single crystal X-ray crystallographic analysis reveal that complex 2 consists of linear chains building with Mn(hfac)2 units and bridged by N–O groups of NITPhBr radicals. The structure is shown in Fig. 3. The chains run along the crystallographic b-axis. Each Mn(II) ion has six-coordination sites which are respectively
occupied by two oxygen atoms of nitroyl nitroxide radicals and four oxygen atoms of two different hfac ligands. The radical acts as bridge ligand and through two oxygen atoms of nitronyl nitroxide connect the two Mn(II) ions. The ONCNO moiety is almost planar, with a mean deviation (0.0050 Å) from the least-squares plane. This geometry makes the free electrons within ONCNO moiety easily delocalizing. The dihedral angle between the nitronyl nitroxide moiety and the benzene ring is 42.2°. The Mn–O(radical) bond lengths are in the range of 2.108(3)–2.169(3) Å, comparable to those dimensions of Mn(hfac)2 with nitronyl nitroxide radicals reported in references [23,24]. The angle was formed by the two bridging nitronyl nitroxide with Mn(II) ion is 79.99(10)° for O(1)–Mn(1)–O(2A). The distance between two adjacent intrachain Mn ions is 7.419 Å. The shortest interchain MnMn contact is 11.627 Å. 5.2. Magnetic properties The magnetic susceptibilities, vM, of the complex 1 was measured in the range 2.0–300 K at a magnetic field of 1000 G, and the plots of vM and leff versus T are showed in Fig. 4. The leff value per molecule at room temperature is 7.85 B.M. When the temperature was cooling down to 77 K, the leff value increases gradually
Fig. 3. The molecular structure of complex 2. Fluorine and hydrogen atoms are not shown for the sake of clarity.
C.-X. Zhang et al. / Inorganica Chimica Acta 367 (2011) 135–140
1.0
7.8
0.8 7.2 0.6 6.6 0.4
μeff [ B.M.]
χM[cm3 mol-1]
A mean-field parameter zJ0 , was added in the formula for taking into account the magnetic interaction between units of cyclic dimmer. The theoretical expression of the molar magnetic susceptibility is
8.4
6.0 0.2 5.4 0.0 0
50
100
150
200
250
300
T/ K Fig. 4. Temperature dependence of leff (4) and vM (s) for the complex 1 and their corresponding theoretical curves (solid lines).
Chart 1. Magnetic exchange pathways in complex 1.
up to 8.40 B.M. which agrees well with that expected for S = 4 and then rapidly decreases. This behavior indicates that ferrimagnetic interactions with alternating S = 5/2 and S = 1/2 spins are predominant in the complex. In the crystal structure of the complex, two pathways for the exchange mechanism can be existing: (i) interaction between units of cyclic dimmer through the two uncoordinated oxygen atoms of nitroxide groups of the two adjacent radicals, (ii) Mn(II)–NITPhF interaction. To evaluate the exchange coupling constants, the complex was treated as a one-dimensional chain of Mn(II)–Rad binuclear units. The magnetic data were fitted by using of the isotropic model ^ ¼ 2J^ H SMn ^ SRad . Where J is the interaction parameter between Mn(II) ion and the NITPhF (Chart 1). The theoretical expression of the magnetic susceptibility of the binuclear unit is
vbi ¼
2Ng 2 b2 A B kT
10J 2J 12J A ¼ 91 exp þ 105 exp þ 30 exp kT kT kT 8J 10J 6J þ 14 exp þ 30 exp þ 55 exp kT kT kT 8J 4J 6J þ 5 exp þ 14 exp þ exp kT kT kT 2J þ 105 þ 5 exp kT 10J 2J 12J 8J þ 35 exp þ 9 exp þ 11 exp B ¼ 13 exp kT kT kT kT 10J 6J 8J 4J þ 9 exp þ 5 exp þ 7 exp þ 7 exp kT kT kT kT 6J 2J 4J þ 3 exp þ 5 exp þ exp þ 39 kT kT kT
vM ¼
vbi 1 ð2zJ 0 =Ng 2 b2 Þvbi
The best-fit parameters are J = 50.34 cm1, zJ0 = 0.76 cm1, P g = 2.00 with R = 8.66 104. R is defined as R ¼ ½ðvM Þobs 2 P 2 ðvM Þcalc = ðvM Þobs . The negative J and zJ0 implies antiferromagnetic interaction between the Mn(II) and NITPhF. The interactions between the cyclic dimmer units are also weak antiferromagnetic. The magnetic behavior of complex 2 is shown in Fig. 5. At 300 K leff is 6.54 B.M., which is larger than the spin-only value expected for uncoupled spin systems (6.16 B.M.) with one S = 5/2 and one S = 1/2 respectively. As the temperature was going down, leff increases first and then decreases rapidly. At 12 K leff reaches to 4.95 B.M which is corresponding to S = 2, as expected. This magnetic behavior indicates that ferrimagnetic interactions with alternating S = 5/2 and S = 1/2 spins are predominant in the complex. In order to quantitatively understand the magnetic interaction, following calculation has been done. The complex was treated as a one-dimensional Mn(II)–Rad chain, and molecular field correction has been used to fit the experimental data. The magnetic data were ^ ¼ 2J ^ fitted to the isotropic model H SMn ^ SRad . Where J is the interaction parameter between Mn(II) ion and the NITPhBr. The theoretical expression of the magnetic susceptibility is 4:75 1:62370X þ 2:05042X 2 4:52588X 3 8:64256X 4
vchain T ¼ ðg 2 =4Þ
2
3
1 þ 0:77968X 1:56527X 1:57333X 0:11666X
!
4:5
Where X = |J|/kT and the other parameters have their usual meaning. An additional coupling parameter, zJ0 , added in the formula as mean field correction, has been applied for taking into account the magnetic behavior between two chains of Mn(II)–Rad. The theoretical expression of the molar magnetic susceptibility is
vM ¼
vchain 1 ð2zJ =Ng 2 b2 Þvchain 0
The best-fit parameters are J = 129.4 cm1, zJ0 = 0.10 cm1, g = 2.02 with R = 4.21 103, where R is defined as R ¼ P P ½ðvM Þobs ðvM Þcalc 2 = ðvM Þ2obs . The large negative J implies strong antiferromagnetic interaction between the Mn(II) ions and radicals. The small zJ0 implies weak antiferromagnetic interaction between Mn(II)–Rad chains.
16
10 9
14
8
12
7 6
10
5
8
4
μeff [ B.M.]
1.2
χM [cm3mol-1]
138
6
3 2
4
1
2
0 0
50
100
150
200
250
300
0
T/K Fig. 5. Temperature dependence of leff (4) and vM (s) of complex 2 and their corresponding theoretical curves (solid lines).
C.-X. Zhang et al. / Inorganica Chimica Acta 367 (2011) 135–140
3
M / N μΒ
2
1
0
0
1
2
3
4
5
H/ T Fig. 6. Field dependence of the magnetization of complex 2 measured at 1.8 K.
To verify the nature of the interaction observed in the temperature dependence of magnetization, the field dependence of the magnetization of complex 2 was measured in the range of 0–5 T at 1.8 K. The plot of M versus H is shown in Fig. 6. The experimental saturation value is very closer to 2 which confirms that complex 2 has an ground state of S = 2 resulting from the antiferromagnetic interaction between the Mn(II) ions and nitroxide radicals.
6. Discussion In order to understand the magnetic properties observed in complexes 1 and 2, it is necessary to consider the involved magnetic orbital. Based on the orbital symmetry, the Mn(II) ion has two magnetic orbitals with r symmetry (dx2 y2 , dz2 ), which are orthogonal to the magnetic orbital (p⁄) of the radical, leading to a ferromagnetic interaction and three magnetic orbital with p symmetry (dxy, dxz, dyz) that will be able to overlap with the magnetic orbital of the radical leading to antiferromagnetic coupling. The observed antiferromagnetic coupling in complexes 1 and 2 can be attributed to the good overlap of the magnetic orbitals which is capable of overcoming the ferromagnetic component. Therefore, the spin on both the Mn(II) ion and radicals is expected to interact magnetically through the p-conjugated system, which lead to an antiferromagntic interaction. Many published data on Mn(II)– radical complexes showed a larger antiferromagnetic interaction.
Table 3 Selected magnetic parameters for Mn (II) and nitroxide radical compounds. Complex
J (cm1)
g
References
[Mn(hfac)2(NITphIm)]2 [Mn(hfac)2(NITphPyrim)]2 Mn(hfac)2[oPONit] Mn(hfac)2[pPONit] [1 Mn(hfac)2] 0.5C6H6
200.38 87.61 213 218 148 (J/K)
2.0 1.99 2.0 2.0 2.0081
[17] [18] [19] [19] [20]
1 = 2-(5-Pyrimidinyl)-substituted nitronyl nitroxide [2 Mn(hfac)2] 0.5C6H6 140 (J/K) 2.0086
[20]
2 = 2-(5-Pyrimidinyl)-substituted [Mn(F3bzac)2(NITMe)]2 [[Mn(hfac)2(NITPhF)]2 [Mn(hfac)(NIT-i-Pr)] [Mn(hfac)(NIT-Et)] [Mn(hfac)(NIT-Me)] [Mn(hfac)(NIT-Ph)] [Mn(hfac)(NIT-PhNEt2)] [Mn(hfac)2(NITPhF)]2
[21] Present work [23] [23] [23] [23] [24] Present work
nitronyl nitroxide 2.0 142 2.0 –50.34 2.0 –329.8 2.0 –259.5 2.0 –216.7 2.0 –208.2 2.0 –34.2 2.02 –129.4
139
In complex 1, the value of J is significantly smaller compared to the similar compounds. The relevant magnetic data of some complexes are listed in Table 3. The reason could be the weaker antiferromagnetic interaction is due to the larger distance between the Mn(II) ion and the N–O group of NITPhF (Mn(1)–O(1), 2.1774(18) Å; Mn(1)–O(1A), 2.1912(18) Å). Also, the large angle between the metal and radical (Mn–O–N (133.18(14)°)), thus a poor orbital overlapping can just make less contribution. However, the large J value of complex 2 suggests the antiferromagnetic interaction between the Mn(II) ion and the nitroxide unit is very strong, which may attribute to small angle of Mn–O–N(123.6°) and the short distance between the Mn(II) ion and the N–O group of NITPhBr (Mn(1)–O(1), 2.108(3) Å). Those geometric parameters cause larger overlap between magnetic orbital with p symmetry of the Mn(II) ion and the magnetic orbital (p⁄) of the radical. Using two different halogen phenyl-substituted nitroxide radicals forms two complexes with different structures (cyclic dimer and one-dimensional chain structures). It may due to the steric hindrance effect in which the atom radius of Br is larger than that of F. Those different structures lead to the different magnitude antiferromagnetic interaction in the two complexes. 7. Conclusion In conclusion, we report two complexes with different structures synthesized from two similar new halogen phenyl-substituted nitronyl nitroxide radicals. The results show that one complex forms cyclic dimer structure, and another has one-dimensional chain structure. The nitronyl nitroxide radical acts as bridge ligand linking two Mn(II) ions through the oxygen atom of the N–O group. In complex 1, the dimer-dimer contact was observed. This occurred between the two oxygen atoms of uncoordinated nitroxide groups of two adjacent radicals in the nearest dimer to yield one-dimensional chain structure along b direction. To our knowledge, it is the shortest distance between the cyclic dimers in metal-radical complexes. The temperature dependencies of magnetic susceptibilities of two complexes are studied. The magnetic couplings between Mn(II) ion and nitronyl nitroxide radical are antiferromagnetic in nature although two complexes exhibit ferrimagnetic properties individually. The J value of complex 2 is significantly larger than that of complex 1. This can be explained on the basis of the structural data. Orbital symmetry is used to explain the magnetic properties of the two complexes and their precursors. Our work shows that a slightly difference in radical ligands can lead to significant different structures of the resulting complexes, which again drastically affects the magnetic properties of the complexes. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20771081). Appendix A. Supplementary material CCDC 744747 and 768614 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. References [1] O. Kahn, Molecule Magnetism, Verlag Chemie, New York, 1993. [2] R. Sessoli, Angew. Chem., Int. Ed. 47 (2008) 5508. [3] H. Oshio, T. Watanabe, A. Ohto, T. Ito, U. Nagashima, Angew. Chem., Int. Ed. 6 (1994) 670.
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Crystal structures and magnetic properties of two complexes synthesized from manganese and halogenophenyl-substituted nitronyl nitroxide Chen-Xi Zhang a,⇑, Xiang-Yu Zhao a,b, Na-Na Sun a,b, Yan-Ling Guo a,b, Yuying Zhang b a b
College of Science, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China Department of Chemistry, NanKai University, Tianjin 300071, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 July 2010 Received in revised form 29 November 2010 Accepted 9 December 2010 Available online 21 December 2010 Keywords: Crystal structure Nitronyl nitroxide Manganese complex Magnetic property
a b s t r a c t One-dimensional complex (1), [Mn(hfac)2(NITPhF)]2 and one binuclear radical complex (2), [Mn(hfac)2(NITPhBr)]1 have been synthesized. Here hfac stands for hexafluoroacetylacetonate, NITPhF for 2-(40 -fluorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, and NITPhBr for 2-(40 -bromophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). All the two precursors were prepared and characterized by single-crystal X-ray diffraction analysis, IR, and magnetic analysis. In complex 1, the NITPhF radical acts as a bridge ligand linking two Mn(II) ions through the oxygen atom of the N–O group to form cyclic dimer. The dimers further connect two oxygen atoms of uncoordinated nitroxides of two adjacent radicals and yield one-dimensional chain sections. Instead in complex 2, the Mn(II) ions are bridged by the NITPhBr radicals through their N–O groups giving infinite one-dimensional chains. Magnetic susceptibility measurements indicate that both complex 1 and 2 behave ferrimagnetically. The Mn(II) ions interact antiferromagnetically with the direct bonding nitroxide group of the radicals although the structures of two complexes are different. The magnetic behaviors can be satisfactorily explained on the basis of the structural data. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The design of molecule structure with practical magnetic properties has been interesting and full of challenge during the past two decades. One very promising strategy is to combine paramagnetic cations with organic open-shell molecules to make hybrid systems [1–5]. Stable organic radicals Nitronyl Nitroxide Radicals (NITR) have been widely employed as magnetic couplers to coordinate metal ions and affording variety of structural topologies thus new functional materials [6–16]. Recently, some cyclic dimer complexes with quadrangle geometry, such as nitronyl nitroxide and iminoyl nitroxide have been studied [17–22]. For these cyclic complexes, most studies have focused on the complexes constructed from nitroxide radicals substituted by nitrogen-containing group which can coordinate to metal ion. In general, the oxygen atoms of nitroxide groups acting as bridging ligands can also form cyclic dimer complexes with metal ions. The first single chain magnet [Co(hfac)2(NITPhOMe)] was discovered by Gatteschi and co-workers [25]. One-dimensional metal nitroxide complexs have attracted much attention for the design and synthesis of molecules based magnetic materials [23,24]. However, very few such complexes have been reported. Thus, detailed studies on low-dimensional metal-nitroxide compounds ⇑ Corresponding author. E-mail address: [email protected] (C.-X. Zhang). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.12.019
are essential not only to the design and preparation molecular magnet, but also to better understanding of the relationship between the structure and magnetic property. As we know, a small change in the structure of nitroxide radicals or in the coordination sphere of such complexes can lead to the large changes in the magnetic properties. We have designed and synthesized two kinds of similar halogen phenyl-substituted nitronyl nitroxide ligand, 2-(40 -fluorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide(NITPhF) and 2-(40 -bromophenyl)-4,4,5,5-tetramethylimidazoline1-oxyl-3-oxide(NITPhBr). Through changing the halogen group in the radical, two halogen phenyl-substituted radicals were successfully used to synthesize two different structural complexes: cyclic dimer M2L2 [Mn(hfac)2(NITPhF)]2 named as complex 1 and onedimensional radical [Mn(hfac)2(NITPhBr)]1 named as complex 2. In complex 1 the oxygen atom from nitroxide group acts as a l1,1 bridging. In this study, we report the crystal structure and magnetic properties of the two low dimensional complexes.
2. Experimental 2.1. Synthesis of the complex 1 [Mn(hfac)2(NITPhF)]2 and 2 [Mn(hfac)2(NITPhBr)]1 2-(4 0 -Fluorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide and 2-(40 -bromophenyl)-4,4,5,5-tetramethylimidazoline-
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1-oxyl-3-oxide were prepared by using of the procedure reported in references [26–28]. Complex 1 [Mn(hfac)2(NITPhF)]2 30.4 mg (0.06 mmol) of Mn(hfac)22H2O was dissolved in 30 ml of boiling heptanes for azeotropically removing hydration water of molecules. Then the solution was cooled to 80 °C, and a solution of NITPhF (12.0 mg, 0.04 mmol) in 2 ml CH2Cl2 was added. The resulting green solution was stirred for 5 min and cooling down to room temperature. The filtrate was allowed for 2 days, and dark green crystals were collected. Yield: 68%. Anal. Calc. for C46H36F26Mn2 N2O12 (1412.62): C, 39.11; N, 1.98; H, 2.57. Found: C, 39.51; N, 2.08; H, 2.51%. FTIR(KBr, cm1): 1656(m), 1527(s), 1359(w), 1254(s), 1140(s), 789(w), 660(w). Complex 2 [Mn(hfac)2(NITPhBr)]1 was prepared in the similar manner of complex 1 by replacing NITPhBr with NITPhF. Yield: 62%. Anal. Calc. for C49.5H44Br2F24Mn2N4O12 (1612.59): C, 36.87; N, 3.47; H, 2.75. Found: C, 36.75; N, 3.41; H, 2.72% FT-IR(KBr, cm1): 1658(m), 1530(s), 1361(w), 1258(s), 1141(s), 790(w), 658(w).
Table 2 Selected bond lengths (Å) and angles (°) for complex 1 and 2. Complex 1
Complex 2
Bond distances Mn(1)–O(5) Mn(1)–O(6) Mn(1)–O(3) Mn(1)–O(4) Mn(1)–O(1) Mn(1)–O(1)#1 O(1)–N(1) O(2)–N(2)
2.148(2) 2.154(2) 2.162(2) 2.170(2) 2.177 (2) 2.191 (2) 1.344(3) 1.277(5)
Mn(1)–O(1) Mn(1)–O(2)#1 Mn(1)–O(5) Mn(1)–O(6) Mn(1)–O(4) Mn(1)–O(3) O(1)–N(1) O(2)–N(2)
2.108(3) 2.130(3) 2.143(3) 2.151(3) 2.156(3) 2.169(3) 1.293(4) 1.300(4)
Bond angles O(5)–Mn(1)–O(6) O(5)–Mn(1)–O(3) O(6)–Mn(1)–O(3) O(5)–Mn(1)–O(4) O(6)–Mn(1)–O(4) O(3)–Mn(1)–O(4) O(5)–Mn(1)–O(1) O(6)–Mn(1)–O(1) O(3)–Mn(1)–O(1) O(4)–Mn(1)-O(1)
81.37(8) 97.25(8) 81.53(8) 82.76(8) 153.22(8) 79.24(8) 162.89(7) 95.78(8) 99.01(7) 105.47(7)
O(1)–Mn(1)–O(2)#1 O(1)–Mn(1)–O(5) O(2)#1–Mn(1)–O(5) O(1)–Mn(1)–O(6) O(2)#1–Mn(1)–O(6) O(5)–Mn(1)–O(6) O(1)–Mn(1)–O(4) O(2)#1–Mn(1)–O(4) O(5)–Mn(1)–O(4) O(6)–Mn(1)–O(4)
79.99(10) 165.21(10) 88.48(10)) 92.52(11) 108.06(11) 82.21(11) 105.00(10) 92.14(11) 84.50(11) 155.40(10)
2.2. X-ray crystallography Diffraction intensity data for single crystals 1 and 2 were collected at room temperature on a Bruker Smart 1000 CCDC area detector equipped with graphite-mono-chromated Mo Ka radiation (k = 0.71073 Å). The structure was determined first by the direct method and then refined by the full-matrix least-squares method on F2 with anisotropic thermal parameters for all nonhydrogen atoms. The hydrogen atoms of solvent molecules were not added, and the other hydrogen atoms were located geometrically and refined isotropically. All calculations were carried out using SHELXS-97 and SHELXL-97 programs [29]. Crystal data are displayed in Table 1. Selected bond lengths and angles are listed in Table 2. 2.3. Materials and equipment All reagents and solvents were purchased commercially and used without further purification. Elemental analyses for carbon, Table 1 Crystallographic data and processing parameters for complex 1 and 2. Compound
1
2
Empirical formula Formula weight Temperature(K) Wavelength(Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) Volume (Å3) Z Calculated density (mg/m3) Absorption coefficient(mm1) F(0 0 0) Crystal size (mm3) h Range for data collection (°) Reflections Collected/unique Data/restraints/parameters Goodness-of-fit on F2 R1[I > 2r(I)] wR2[I > 2r(I)] R1 (all data) wR2 (all data)
C23H18F13N2O5.50Mn 712.33 113(2) 0.71073 monoclinic P2(1)/n
C49.50H44Br2F24N4O1Mn2 1612.59 113(2) 0.71073 monoclinic C2/c
11.719(2) 13.282(3) 17.825(4) 99.41(3) 2737.2(10) 4 1.729 0.611
26.591(5) 13.630(3) 22.839(5) 124.57(3) 6816(2) 4 1.571 1.663
1424 0.20 0.18 0.16 2.32–25.02
3204 0.24 0.20 0.16 1.76–25.02
19784/4822 [R(int) = 0.0290] 4822/270/522 1.091 0.0449 0.1191 0.0492 0.1224
22778/5997 [R(int) = 0.0469] 5997/170/502 1.049 0.0527 0.1573 0.0674 0.1690
Symmetry transformations used to generate equivalent atoms: #1: x + 1, y + 1, z + 2 for 1. #1: x + 1/2, y + 1/2, z + 3/2; #2: x + 1/2, y 1/2, z + 3/2; #3: x + 1, y, z + 3/2 for 2.
hydrogen, and nitrogen were carried out on a Model 240 Perkin– Elmer elemental analyzer. The infrared spectrum was taken on a Shimadzu IR spectrophotometer model 408 in the region of 4000–600 cm1 of KBr pellets. Temperature dependence of magnetic susceptibilities was measured on a MPMS-7 SQUID magnetometer in the temperature range 2.0–300 K. Diamagnetic corrections were made with Pascal’s constants for all constituent atoms. The magnetic moments were calculated using the equation leff = 2.828(vMT)1/2. 5. Results and discussion 5.1. Determination of the crystal structure An ORTEP drawing of the complex 1 is shown in Fig. 1. The complex consists of netural dimer units. Each NITPhF radical ligand
Fig. 1. The molecular structure of complex 1. Hydrogen atoms are not shown for the sake of clarity.
C.-X. Zhang et al. / Inorganica Chimica Acta 367 (2011) 135–140
137
Fig. 2. View of the one-dimensional crystal structure of complex 1 by weak nitroxide–nitroxide bonding along b direction.
coordinats to two metal ions through the oxygen atom of the nitronyl nitroxide group, so that the NITPhF radicals act as a l1,1 bridging ligands and form a rectangle-like centrosymmetric dimer structure. The Mn(II) ions of the cyclic dimer exhibit a slightly distorted octahedral coordination sphere with four oxygen atoms from two hfac anions and two oxygen atoms of two different NITPhF radical ligands. The Mn–O(hfac) bond lengths are fairly similar to each other, ranging from 2.148(2) to 2.170(2) Å, while the Mn– O(Radical) bond lengths are longer, 2.177(2) and 2.191(2) Å. These bond lengths are comparable to the reported cyclic metal–nitronyl nitroxide dimmers [20,21]. In NITPhF radicals, the O–N–C–N–O group containing unpaired electrons has 36.1° angle with the benzene ring. In the dimmer unit, the distance between two Mn(II) ions is 3.522 Å. It should be noted that the shortest distance of (2.001 Å) was observed between the two oxygen atoms of uncoordinated nitroxide groups of two adjacent radicals. The way of the molecular coordination yields one-dimensional chain consisting of cyclic dimer units. These are demonstrated in Fig. 2. Single crystal X-ray crystallographic analysis reveal that complex 2 consists of linear chains building with Mn(hfac)2 units and bridged by N–O groups of NITPhBr radicals. The structure is shown in Fig. 3. The chains run along the crystallographic b-axis. Each Mn(II) ion has six-coordination sites which are respectively
occupied by two oxygen atoms of nitroyl nitroxide radicals and four oxygen atoms of two different hfac ligands. The radical acts as bridge ligand and through two oxygen atoms of nitronyl nitroxide connect the two Mn(II) ions. The ONCNO moiety is almost planar, with a mean deviation (0.0050 Å) from the least-squares plane. This geometry makes the free electrons within ONCNO moiety easily delocalizing. The dihedral angle between the nitronyl nitroxide moiety and the benzene ring is 42.2°. The Mn–O(radical) bond lengths are in the range of 2.108(3)–2.169(3) Å, comparable to those dimensions of Mn(hfac)2 with nitronyl nitroxide radicals reported in references [23,24]. The angle was formed by the two bridging nitronyl nitroxide with Mn(II) ion is 79.99(10)° for O(1)–Mn(1)–O(2A). The distance between two adjacent intrachain Mn ions is 7.419 Å. The shortest interchain MnMn contact is 11.627 Å. 5.2. Magnetic properties The magnetic susceptibilities, vM, of the complex 1 was measured in the range 2.0–300 K at a magnetic field of 1000 G, and the plots of vM and leff versus T are showed in Fig. 4. The leff value per molecule at room temperature is 7.85 B.M. When the temperature was cooling down to 77 K, the leff value increases gradually
Fig. 3. The molecular structure of complex 2. Fluorine and hydrogen atoms are not shown for the sake of clarity.
C.-X. Zhang et al. / Inorganica Chimica Acta 367 (2011) 135–140
1.0
7.8
0.8 7.2 0.6 6.6 0.4
μeff [ B.M.]
χM[cm3 mol-1]
A mean-field parameter zJ0 , was added in the formula for taking into account the magnetic interaction between units of cyclic dimmer. The theoretical expression of the molar magnetic susceptibility is
8.4
6.0 0.2 5.4 0.0 0
50
100
150
200
250
300
T/ K Fig. 4. Temperature dependence of leff (4) and vM (s) for the complex 1 and their corresponding theoretical curves (solid lines).
Chart 1. Magnetic exchange pathways in complex 1.
up to 8.40 B.M. which agrees well with that expected for S = 4 and then rapidly decreases. This behavior indicates that ferrimagnetic interactions with alternating S = 5/2 and S = 1/2 spins are predominant in the complex. In the crystal structure of the complex, two pathways for the exchange mechanism can be existing: (i) interaction between units of cyclic dimmer through the two uncoordinated oxygen atoms of nitroxide groups of the two adjacent radicals, (ii) Mn(II)–NITPhF interaction. To evaluate the exchange coupling constants, the complex was treated as a one-dimensional chain of Mn(II)–Rad binuclear units. The magnetic data were fitted by using of the isotropic model ^ ¼ 2J^ H SMn ^ SRad . Where J is the interaction parameter between Mn(II) ion and the NITPhF (Chart 1). The theoretical expression of the magnetic susceptibility of the binuclear unit is
vbi ¼
2Ng 2 b2 A B kT
10J 2J 12J A ¼ 91 exp þ 105 exp þ 30 exp kT kT kT 8J 10J 6J þ 14 exp þ 30 exp þ 55 exp kT kT kT 8J 4J 6J þ 5 exp þ 14 exp þ exp kT kT kT 2J þ 105 þ 5 exp kT 10J 2J 12J 8J þ 35 exp þ 9 exp þ 11 exp B ¼ 13 exp kT kT kT kT 10J 6J 8J 4J þ 9 exp þ 5 exp þ 7 exp þ 7 exp kT kT kT kT 6J 2J 4J þ 3 exp þ 5 exp þ exp þ 39 kT kT kT
vM ¼
vbi 1 ð2zJ 0 =Ng 2 b2 Þvbi
The best-fit parameters are J = 50.34 cm1, zJ0 = 0.76 cm1, P g = 2.00 with R = 8.66 104. R is defined as R ¼ ½ðvM Þobs 2 P 2 ðvM Þcalc = ðvM Þobs . The negative J and zJ0 implies antiferromagnetic interaction between the Mn(II) and NITPhF. The interactions between the cyclic dimmer units are also weak antiferromagnetic. The magnetic behavior of complex 2 is shown in Fig. 5. At 300 K leff is 6.54 B.M., which is larger than the spin-only value expected for uncoupled spin systems (6.16 B.M.) with one S = 5/2 and one S = 1/2 respectively. As the temperature was going down, leff increases first and then decreases rapidly. At 12 K leff reaches to 4.95 B.M which is corresponding to S = 2, as expected. This magnetic behavior indicates that ferrimagnetic interactions with alternating S = 5/2 and S = 1/2 spins are predominant in the complex. In order to quantitatively understand the magnetic interaction, following calculation has been done. The complex was treated as a one-dimensional Mn(II)–Rad chain, and molecular field correction has been used to fit the experimental data. The magnetic data were ^ ¼ 2J ^ fitted to the isotropic model H SMn ^ SRad . Where J is the interaction parameter between Mn(II) ion and the NITPhBr. The theoretical expression of the magnetic susceptibility is 4:75 1:62370X þ 2:05042X 2 4:52588X 3 8:64256X 4
vchain T ¼ ðg 2 =4Þ
2
3
1 þ 0:77968X 1:56527X 1:57333X 0:11666X
!
4:5
Where X = |J|/kT and the other parameters have their usual meaning. An additional coupling parameter, zJ0 , added in the formula as mean field correction, has been applied for taking into account the magnetic behavior between two chains of Mn(II)–Rad. The theoretical expression of the molar magnetic susceptibility is
vM ¼
vchain 1 ð2zJ =Ng 2 b2 Þvchain 0
The best-fit parameters are J = 129.4 cm1, zJ0 = 0.10 cm1, g = 2.02 with R = 4.21 103, where R is defined as R ¼ P P ½ðvM Þobs ðvM Þcalc 2 = ðvM Þ2obs . The large negative J implies strong antiferromagnetic interaction between the Mn(II) ions and radicals. The small zJ0 implies weak antiferromagnetic interaction between Mn(II)–Rad chains.
16
10 9
14
8
12
7 6
10
5
8
4
μeff [ B.M.]
1.2
χM [cm3mol-1]
138
6
3 2
4
1
2
0 0
50
100
150
200
250
300
0
T/K Fig. 5. Temperature dependence of leff (4) and vM (s) of complex 2 and their corresponding theoretical curves (solid lines).
C.-X. Zhang et al. / Inorganica Chimica Acta 367 (2011) 135–140
3
M / N μΒ
2
1
0
0
1
2
3
4
5
H/ T Fig. 6. Field dependence of the magnetization of complex 2 measured at 1.8 K.
To verify the nature of the interaction observed in the temperature dependence of magnetization, the field dependence of the magnetization of complex 2 was measured in the range of 0–5 T at 1.8 K. The plot of M versus H is shown in Fig. 6. The experimental saturation value is very closer to 2 which confirms that complex 2 has an ground state of S = 2 resulting from the antiferromagnetic interaction between the Mn(II) ions and nitroxide radicals.
6. Discussion In order to understand the magnetic properties observed in complexes 1 and 2, it is necessary to consider the involved magnetic orbital. Based on the orbital symmetry, the Mn(II) ion has two magnetic orbitals with r symmetry (dx2 y2 , dz2 ), which are orthogonal to the magnetic orbital (p⁄) of the radical, leading to a ferromagnetic interaction and three magnetic orbital with p symmetry (dxy, dxz, dyz) that will be able to overlap with the magnetic orbital of the radical leading to antiferromagnetic coupling. The observed antiferromagnetic coupling in complexes 1 and 2 can be attributed to the good overlap of the magnetic orbitals which is capable of overcoming the ferromagnetic component. Therefore, the spin on both the Mn(II) ion and radicals is expected to interact magnetically through the p-conjugated system, which lead to an antiferromagntic interaction. Many published data on Mn(II)– radical complexes showed a larger antiferromagnetic interaction.
Table 3 Selected magnetic parameters for Mn (II) and nitroxide radical compounds. Complex
J (cm1)
g
References
[Mn(hfac)2(NITphIm)]2 [Mn(hfac)2(NITphPyrim)]2 Mn(hfac)2[oPONit] Mn(hfac)2[pPONit] [1 Mn(hfac)2] 0.5C6H6
200.38 87.61 213 218 148 (J/K)
2.0 1.99 2.0 2.0 2.0081
[17] [18] [19] [19] [20]
1 = 2-(5-Pyrimidinyl)-substituted nitronyl nitroxide [2 Mn(hfac)2] 0.5C6H6 140 (J/K) 2.0086
[20]
2 = 2-(5-Pyrimidinyl)-substituted [Mn(F3bzac)2(NITMe)]2 [[Mn(hfac)2(NITPhF)]2 [Mn(hfac)(NIT-i-Pr)] [Mn(hfac)(NIT-Et)] [Mn(hfac)(NIT-Me)] [Mn(hfac)(NIT-Ph)] [Mn(hfac)(NIT-PhNEt2)] [Mn(hfac)2(NITPhF)]2
[21] Present work [23] [23] [23] [23] [24] Present work
nitronyl nitroxide 2.0 142 2.0 –50.34 2.0 –329.8 2.0 –259.5 2.0 –216.7 2.0 –208.2 2.0 –34.2 2.02 –129.4
139
In complex 1, the value of J is significantly smaller compared to the similar compounds. The relevant magnetic data of some complexes are listed in Table 3. The reason could be the weaker antiferromagnetic interaction is due to the larger distance between the Mn(II) ion and the N–O group of NITPhF (Mn(1)–O(1), 2.1774(18) Å; Mn(1)–O(1A), 2.1912(18) Å). Also, the large angle between the metal and radical (Mn–O–N (133.18(14)°)), thus a poor orbital overlapping can just make less contribution. However, the large J value of complex 2 suggests the antiferromagnetic interaction between the Mn(II) ion and the nitroxide unit is very strong, which may attribute to small angle of Mn–O–N(123.6°) and the short distance between the Mn(II) ion and the N–O group of NITPhBr (Mn(1)–O(1), 2.108(3) Å). Those geometric parameters cause larger overlap between magnetic orbital with p symmetry of the Mn(II) ion and the magnetic orbital (p⁄) of the radical. Using two different halogen phenyl-substituted nitroxide radicals forms two complexes with different structures (cyclic dimer and one-dimensional chain structures). It may due to the steric hindrance effect in which the atom radius of Br is larger than that of F. Those different structures lead to the different magnitude antiferromagnetic interaction in the two complexes. 7. Conclusion In conclusion, we report two complexes with different structures synthesized from two similar new halogen phenyl-substituted nitronyl nitroxide radicals. The results show that one complex forms cyclic dimer structure, and another has one-dimensional chain structure. The nitronyl nitroxide radical acts as bridge ligand linking two Mn(II) ions through the oxygen atom of the N–O group. In complex 1, the dimer-dimer contact was observed. This occurred between the two oxygen atoms of uncoordinated nitroxide groups of two adjacent radicals in the nearest dimer to yield one-dimensional chain structure along b direction. To our knowledge, it is the shortest distance between the cyclic dimers in metal-radical complexes. The temperature dependencies of magnetic susceptibilities of two complexes are studied. The magnetic couplings between Mn(II) ion and nitronyl nitroxide radical are antiferromagnetic in nature although two complexes exhibit ferrimagnetic properties individually. The J value of complex 2 is significantly larger than that of complex 1. This can be explained on the basis of the structural data. Orbital symmetry is used to explain the magnetic properties of the two complexes and their precursors. Our work shows that a slightly difference in radical ligands can lead to significant different structures of the resulting complexes, which again drastically affects the magnetic properties of the complexes. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20771081). Appendix A. Supplementary material CCDC 744747 and 768614 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. References [1] O. Kahn, Molecule Magnetism, Verlag Chemie, New York, 1993. [2] R. Sessoli, Angew. Chem., Int. Ed. 47 (2008) 5508. [3] H. Oshio, T. Watanabe, A. Ohto, T. Ito, U. Nagashima, Angew. Chem., Int. Ed. 6 (1994) 670.
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