Synthesis, Characterization, and Magnetic Properties of Copper(II)-Lanthanide(III) Heterobinuclear Complexes Bridged by N,N′-Oxamidobis(3,5-dibromobenzoato)cuprate(II)
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 4 481-485
Yan-Tuan Li,*,† Cui-Wei Yan,‡ and Jie Zhang§ Marine Drug & Food Institute, College of Marine Life Sciences, and College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yushan Road, Qingdao, Shandong, 266003, P. R. China Received August 14, 2002
ABSTRACT: Twelve new copper(II)-lanthanide(III) heterobinuclear complexes bridged by N,N′-oxamidobis(3,5dibromobenzoato)cuprate(II) [Cu(Br4obbz)]2- and end-capped with 4,4′-dimethyl-2,2′-bipyridine (Me2bpy), namely, Cu(Br4obbz)Ln(Me2bpy)2NO3 (Ln ) Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb), have been synthesized and characterized by elemental analyses, molar conductivity measurements, and spectroscopic (IR, UV, ESR) studies. The temperature dependence of the magnetic susceptibility of the Cu(Br4obbz)Gd(Me2bpy)2NO3 complex was further measured over the range 4.2-300 K. The least-squares fit of the experimental susceptibilities based on the spin Hamiltonian operator, H ˆ ) - 2JS ˆ 1‚S ˆ 2, yielded J ) +2.75 cm-1. This result indicates the presence of a weak ferromagnetic spin-exchange interaction between Gd(III) and Cu(II) ions. A plausible mechanism for the ferromagnetic coupling between Gd(III) and Cu(II) is discussed in terms of spin polarization. 1. Introduction The study of synthesis and magnetic interactions of heteropolymetallic systems with two different paramagnetic centers has attracted considerable attention.1 The aim is to understand fundamental factors governing the electronic and geometric structure of metalloproteins and enzymes and thus the correlating structure with biological function, and to obtain useful information about designing and synthesizing molecule-based magnets and investigating the spin-exchange mechanism between paramagnetic metals ions. Compared with the number of studies dealing with heteronuclear systems comprising d-transition metal ions,2-10 relatively few studies dealing with heterometal complexes containing d-transition metal ions and lanthanide(III) ions (socalled d-f heteronuclear complexes) have been reported due to the very weak interaction and large anisotropic effect of lanthanide ions.11-14 However, since Vidali and co-workers11 and Abid and Fenton12 reported d-f heteronuclear complexes in 1984, much effort has been also devoted to magnetic studies of d-f heteronuclear complexes.15-20 The main reason is that the d-f mixed oxides have been used as a variety of functional materials and/or have been expected as promising functional materials.21 Taking into account the above facts, it is of considerable interest to synthesize and study heteronuclear complexes possessing new functions associated some d-f heterometal centers to gain some insight into the magnetic properties of this kind of complex. In this paper, 12 new Cu(II)-Ln(III) binuclear complexes of the general formula Cu(Br4obbz)Ln(Me2bpy)2NO3 (Ln ) Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) have been prepared by the reaction of sodium N,N′-oxamido-bis* Corresponding author. E-mail:
[email protected]. † Marine Drug & Food Institute. ‡ College of Marine Life Sciences. § College of Chemistry and Chemical Engineering.
(3,5-dibromobenzoato)cuprate(II), Na2[Cu(Br4obbz)], with lanthanide(III) ions and the terminal ligand 4,4′-dimethyl-2,2′-bipyridine (Me2bpy). The ESR and magnetic properties of the complex Cu(Br4obbz)Gd(Me2bpy)2NO3 have also been studied. The main result of this investigation is that the Cu(II) and Gd(III) ions are coupled in a ferromagnetic fashion via isotropic coupling (J ) +2.75 cm-1), thus indicating that weak coupling can be established between transition-metal and rare-earth ions. 2. Experimental Section 2.1. Materials. All the reagents used in the synthesis were of analytical grade. The hydrated lanthanide(III) nitrates were prepared by general methods.22 The terminal ligand 4,4′dimethyl-2,2′-bipyridine (Me2bpy) was purchased from the Perking Chemical Company. The starting material sodium N,N′-oxamidobis(3,5-dibromobenzoato)cuprate(II), Na2[Cu(Br4obbz)], was synthesized as previously described.6 2.2. Synthesis of Binuclear Cu(II)-Ln(III) Complexes. The methods used to prepare the 12 copper(II)-lanthanide(III) heterobinuclear complexes were virtually identical and are exemplified by Cu(Br4obbz)Gd(Me2bpy)2NO3. To a solution of Na2[Cu(Br4obbz)] (749.4 mg, 1 mmol) in absolute methanol (10 mL) was added successively a solution of Gd(NO3)3‚6H2O (451.4 mg, 1 mmol) in absolute methanol (10 mL), followed by a solution of ethyl orthofomate (10 mL) with stirring. The mixture was continuously stirred until the solution became limpid. This solution was then filtered. To the filtrate was added an absolute methanol solution (20 mL) of Me2bpy (368.5 mg, 2 mmol). The color of the solution turned from violet-red to blue immediately and a small amount of precipitate formed. After refluxing for ca. 12 h, the mixture was then allowed to cool to room temperature and the blue microcrystals thus obtained were removed by filtration, washed several times with methanol, water, and diethyl ether, and dried over P2O5 under reduced pressure. All analytical data, colors, yields, and melting points of the binuclear complexes are collected in Table 1. 2.3. Physical Measurements. Carbon, hydrogen, and nitrogen elemental analyses were performed with a Perkin-
10.1021/cg020037q CCC: $25.00 © 2003 American Chemical Society Published on Web 05/01/2003
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Table 1. Elemental Analyses, Yields, Colors, and Melting Points (mp) of the Binuclear Complexesa complex (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
empirical formula (formula weight) CuYC40H28N7O9Br4 (1222.77) CuLaC40H32N7O11Br4 (1308.80) CuPrC40H30N7O10Br4 (1292.79) CuNdC40H28N7O9Br4 (1278.06) CuSmC40H28N7O9Br4 (1284.26) CuEuC40H28N7O9Br4 (1285.82) CuGdC40H28N7O9Br4 (1291.11) CuTbC40H30N7O10Br4 (1310.81) CuDyC40H28N7O9Br4 (1296.36) CuHoC40H28N7O9Br4 (1298.79) CuErC40H28N7O9Br4 (1301.12) CuYbC40H30N7O10Br4 (1324.92)
color
found (calcd) (%) N Cu
yield (%)
mp (°C)
C
H
light-blue
81
331
light-blue
75
309
blue
63
337
pink
67
326
pale-violet
86
311
blue
66
329
blue
76
298
pale-brown
85
319
blue
79
276
pale-red
68
308
pale-red
88
317
blue
77
322
39.2 (39.3) 36.8 (36.7) 37.1 (37.2) 37.7 (37.6) 37.5 (37.4) 37.3 (37.4) 37.1 (37.2) 36.6 (36.7) 37.2 (37.1) 37.1 (37.0) 37.1 (36.9) 37.2 (36.3)
2.2 (2.3) 2.4 (2.5) 2.1 (2.3) 2.1 (2.2) 2.1 (2.2) 2.3 (2.2) 2.2 (2.2) 2.1 (2.3) 2.1 (2.2) 2.1 (2.2) 2.1 (2.2) 2.2 (2.3)
7.9 (8.0) 7.6 (7.5) 7.5 (7.6) 7.8 (7.7) 7.5 (7.6) 7.4 (7.6) 7.7 (7.6) 7.4 (7.5) 7.4 (7.6) 7.4 (7.5) 7.4 (7.5) 7.2 (7.4)
5.0 (5.2) 4.8 (4.9) 5.0 (4.9) 5.2 (5.0) 4.7 (4.9) 4.8 (4.9) 4.8 (4.9) 4.9 (4.8) 4.8 (4.9) 5.0 (4.9) 4.8 (4.9) 4.7 (4.8)
Ln 7.1 (7.3) 10.5 (10.6) 10.8 (10.9) 11.2 (11.3) 11.6 (11.7) 11.6 (11.8) 12.1 (12.2) 12.0 (12.1) 12.6 (12.5) 12.6 (12.7) 12.7 (12.9) 13.0 (13.1)
a (1) ) Cu(Br obbz)Y(Me bpy) NO , (2) ) Cu(Br obbz)La(Me bpy) NO ‚2H O, (3) ) Cu(Br obbz)Pr(Me bpy) NO ‚H O, (4) ) 4 2 2 3 4 2 2 3 2 4 2 2 3 2 Cu(Br4obbz)Nd(Me2bpy)2NO3, (5) ) Cu(Br4obbz)Sm(Me2bpy)2NO3, (6) ) Cu(Br4obbz)Eu(Me2bpy)2NO3, (7) ) Cu(Br4obbz)Gd(Me2bpy)2NO3, (8) ) Cu(Br4obbz)Tb(Me2bpy)2NO3‚H2O, (9) ) Cu(Br4obbz)Dy(Me2bpy)2NO3, (10) ) Cu(Br4obbz)Ho(Me2bpy)2NO3, (11) ) Cu(Br4obbz)Er(Me2bpy)2NO3, (12) ) Cu(Br4obbz)Yb(Me2bpy)2NO3‚H2O.
Table 2. Physical Data for the Ligand and Binuclear Complexes IR (cm-1) υ(NO3-)
UV (nm)
complex
ΛM a
υas(CO2-)
υ(CdO)
υ(Ln-N)
υ4
υ1
υ2
d-d
Na2[Cu(Br4obbz)] (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
4.0 5.5 4.3 4.8 4.7 4.5 4.1 5.0 4.6 4.0 4.9 5.0
1582 1582 1582 1582 1582 1582 1582 1582 1582 1582 1582 1582 1582
1615 1660 1650 1645 1648 1660 1655 1658 1658 1660 1665 1655 1658
388 395 380 385 385 390 390 380 385 382 381 380
1492 1492 1490 1493 1495 1490 1495 1490 1492 1490 1491 1490
1312 1315 1312 1305 1319 1309 1310 1312 1310 1308 1305 1310
1030 1032 1030 1032 1935 1032 1030 1031 1032 1030 1030 1032
514 530 542 545 535 538 539 540 542 540 536 540 537
a
f-f
assignment
730 420
(4I9/2f7F7/2) (4H5/2f4I9/2)
455 420 642 650
(7F6f5F4) (6H5/2f4I15/2) (5I8f4F5) (4I15/2f7F9/2)
CT 325 330 329 320 327 318 324 325 319 322 331 328
Values for 1 × 10-3 mol L-1 DMF solution, unit: Ω-1 cm2 mol-1.
Elmer elemental analyzer model 240. Metal contents were determined by EDTA titration. IR spectra were recorded with a Nicolet FT-IR spectrophotometer using KBr pellets. The electronic spectra (DMF solution) were measured on a PerkinElmer Hitachi-240 spectrophotometer. Molar conductances were measured with a DDS-11A conductometer. Variable temperature (4.2-300 K) magnetic susceptibilities were measured using a Quantum Design MPMS-5 SQUID magnetometer. Diamagnetic corrections were made with Pascal’s constants23 for all the constituent atoms and effective magnetic moments were calculated by the equation µeff ) 2.828(χMT)1/2, where χM is the molar magnetic susceptibility corrected for diamagnetisms of the constituting atoms. The ESR spectra were measured with a JES-FEIXG ESR apparatus using the X-band.
3. Results and Discussion 3.1. Synthetic Route and Coordination Environment of the Binuclear Complexes. Two synthetic strategies are generally available for the preparation
of discrete heterobinuclear complexes. The first is to use the heterobinucleating ligand, which offers either the coordination geometry or the ligand field strength suitable for dissimilar metal ions.5 The second is to use a “complex ligand” that contains a potential donor group capable of coordinating to another metal ion.6,7,24,25 In this study, our purpose was to obtain Cu(II)-Ln(III) heterobinuclear complexes; therefore, the latter synthetic method was adopted. As the “ligand complex”, we chose Na2[Cu(Br4obbz)], which was first used to prepare alternating Cu(II)-Mn(II) bimetallic chain complexes6 by Kahn et al., as a bidentate mononuclear fragment, because it can coordinate to another metal ion through the carbonyl oxygens of oxamido.6 Simultaneously, 4,4′dimethyl-2,2′-bipyridine (Me2bpy) was used as terminal ligand. Indeed, elemental analyses and physical data (see Tables 1 and 2) indicate that the reaction of Na2[Cu(Br4obbz)] with Ln(NO3)3‚6H2O (Ln ) Y, La, Pr,
Copper(II)-Lanthanide(III) Heterobinuclear Complexes
Figure 1. Plausible coordination environment of the binuclear complexes (Ln ) Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb; NN bonds ) Me2bpy).
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) and Me2bpy in 1:1:2 mole ratio yielded the heterobinuclear complexes of the general formula Cu(Br4obbz)Ln(Me2bpy)2NO3, as expected. These complexes are the first examples Cu(II)Ln(III) heterobinuclear complexes bridged by N,N′oxamidobis(3,5-dibromo-benzoato)cuprate(II). On the basis of the conductivity measurements, spectroscopic (IR, ESR, and UV) characterization, and magnetic studies (vide infra), these complexes are presumed to have the coordination environment as shown in Figure 1. 3.2. General Properties of the Binuclear Complexes. These binuclearcomplexes are very soluble in acetonitrile, DMF, and DMSO to give stable solutions at room temperature, moderately soluble in water, methanol, and acetone, and practically insoluble in carbon tetrachloride, chloroform, and benzene. The Cu(Br4obbz)Gd(Me2bpy)2NO3 complex can be recrystallized from a DMF/ethanol (1:1) mixture. In the solid state, all of the Cu(II)-Ln(III) binuclear complexes are fairly stable in air, thus allowing physical measurements. For the 12 Cu(II)-Ln(III) binuclear complexes, the observed molar conductance values in DMF solution (in the range 4.0 ∼ 5.5.0 Ω-1 cm2 mol-1) are given in Table 2. These values are indicative of the nonelectrolytic nature of these complexes,26 suggesting the weak coordination of nitrate anion. This is consistent with the measured IR data of the heterobinuclear complexes. 3.3. Infrared Spectra. Since the IR spectra of all the 12 binuclear complexes are similar, the discussion is confined to the most important vibrations of the 200 ∼ 4000 cm-1 region in relation to the structure. The most relevant IR absorption bands from the IR spectra of the binuclear complexes and the mononuclear fragment Na2[Cu(Br4obbz)], along with their assignments, are shown in Table 2. We will only discuss the selected infrared bands. The carbonyl stretching vibration at 1615 cm-1 for Na2[Cu(Br4obbz)] is considerably shifted toward higher frequencies (ca. 30 ∼ 50 cm-1) in the binuclear complexes. Therefore, in general, when the deprotonated amide nitrogen is coordinated to the metal ion, its amide I band shifts considerably toward lower wavenumbers. In the case of an oxamide dianion coordinated to two metal ions as a bridging ligands, the amide I band reverts to near its original position (in the protonated species).19 Although the amide I is due to a
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composite N-CdO vibration, it can essentially be seen as υ(CdO). It is likely that the bond order of CdO (carbonyl) in the binuclear complexes is higher than that in the corresponding mononuclear complex Na2[Cu(Br4obbz)]. This shift has often been used as a diagnostic indicator for oxamido-bridged structures.25 On the other hand, the CdO deformation vibration at 758 cm-1 of the ligand complex, Na2[Cu(Br4obbz)], disappeared in the spectra of the binuclear complexes. This fact may be attributed to the coordination of the carbonyl oxygens to the Ln(III) ion.27 This coordination mode of the complex ligand, Na2[Cu(Br4obbz)], has been revealed by X-ray diffraction analysis of an analogous complex.6 In addition, the -NdC- stretching vibration for the terminal ligand (Me2bpy) was shifted to higher frequences (1520 cm-1) in these binuclear complexes, suggesting that the N atoms of the terminal ligand (Me2bpy) coordinated with the Ln(III) ion. The additional band observed at around 380 ∼ 395 cm-1 due to υ(LnN) further supports this view. On the other hand, the antisymmetric stretching vibration of the carboxylate group for Na2[Cu(Br4obbz)] remains at 1582 cm-1 in their binuclear complexes, indicating that the carboxylate group is not coordinated with the Ln(III) ion. However, the spectra of the binuclear complexes exhibit characteristic vibrational frequencies of coordinated nitrate group, which is suggested by two bands observed around 1490 and 1310 cm-1 due to the υ4 and υ1 vibrations of the nitrate group of C2v symmetry.28 The medium band at 1030 cm-1 due to the υ2 vibration of the nitrate group (C2v) stands as additional evidence for the presence of coordinated nitrate group. The difference in wavenumbers between the two highest frequency bands (υ4-υ1) of nitrate (C2v) is about 180 cm-1, indicating that the coordinated nitrate ion is bidentate.29 Τhus, the above spectral observations, together with the molar conductance data, confirm that the nitrate ion is coordinated to the Ln(III) ions in a bidentate fashion in these binuclear complexes. 3.4. Electronic Spectra. To clarify the mode of bonding, the electron spectra of the mononuclear fragment Na2[Cu(Br4obbz)] and Cu(II)-Ln(III) heterobinuclear complexes were studied and assigned on the basis of a careful comparison of the latter with the former. As shown in Table 2, the electronic spectra of all the binuclear complexes exhibit a band at 530 ∼ 545 nm, which may be attributed to the d-d transition of the “inside” copper(II) in a square-planar environment. The frequency is lower than that for the mononuclear copper(II) complex (514 nm). Such a red-shift of copper(II) ion in the d-d band may be attributed to the decreased planarity of the [CuN2O2] chromophore on forming a binuclear complex with Ln(III) ion.30 In addition, a strong absorption in the short wavelength range (see Table 2) may be attributable to the chargetransfer absorption bands, which may be due to the spin-exchange interaction between the copper(II) and lanthanide(III) ions through the π-path orbital set up by an oxamido bridge.25 Further investigation of these and similar systems is required to obtain more detailed assignment for charge transfer. Besides, in the electronic spectra of Cu(Br4obbz)Ln(Me2bpy)2NO3 (Ln ) Nd, Sm, Tb, Dy, Ho, Er), f-f transitions of Ln(III) were also observed. These data are listed in Table 2 along with
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Figure 2. Magnetic susceptibility (χM) and effective magnetic moment (µeff) vs T for the Cu(Br4obbz)Gd(Me2bpy)2NO3 complex. The solid line denotes the least-squares fit the data to eq 1 given in the text.
their assignments.31 Other f-f transitions which are expected to appear may be concealed by d-d, chargetransfer, or intraligand transitions. Despite our many efforts, single crystals suitable for X-ray crystallography have not yet been obtained for these complexes. However, based on the composition of these complexes, their IR, ESR, electronic spectra and conductivity measurements, magnetic characterization (vide infra), and the crystal structure of the analogous complex,6 these complexes are proposed to have an extended oxamido-bridged binuclear structure and to contain a lanthanide(III) ion and a copper(II) ion, which have the presumed coordination environment as shown in Figure 1. The plausible structure is further characterized by the following ESR and magnetic studies. 3.5. ESR and magnetic studies of Cu(Br4obbz)Gd(Me2bpy)2NO3. The X-band powder ESR spectra of the Cu(Br4obbz)Gd(Me2bpy)2NO3 and Na2[Cu(Br4obbz)] have been recorded at room temperature. It is noted that the spectrum of the former obviously is different from that of the latter and exhibits a dissymmetric broad band around g ) 2.1, which also indicates a magnetic spin-exchange interaction between Gd(III) and Cu(II) ions.14 Since there is a lack of structural data for this kind of complex, it is difficult to quantitatively interpret this broad band. Qualitatively, however, it is clear that these features reflect the exchange coupling between gadolinium(III) and Cu(II) ions. According to Kambe’s approach,32 two spins, SCu(II) ) 1/2 and SGd(III) ) 7/2, are coupled to yield the total spin states of S ) 3 and S ) 4. On the basis of the Boltzmann distribution, the two states are both populated at room-temperature owing to very weak interaction (vide infra). Thus, the dissymmetric broad signals may be ascribed to the complex in these spin states.14 To obtain further information on the structure of the binuclear complexes, variable-temperature (4.2 ∼ 300 K) magnetic susceptibility data were collected for the Cu(Br4obbz)Gd(Me2-bpy)2NO3 complex, as an example, and the results are shown in Figure 2 in the form of plots χM vs. T and µeff vs. T, where χM, µeff, and T denote magnetic susceptibility per molecule, effective magnetic moment per molecule, and absolute temperature, respectively. The reason for the choice of gadolinium is because Gd(III) ion and its complexes are quite simple for a magnetic study, since the ground state of Gd(III) is 8S7/2 and the energy level of the lowest excited state
Li et al.
is very high, that is, the magnetic data usually can be interpreted without considering the contribution of the orbital angular momentum and the anisotropic effect.14,30 On the basis of the inherent nature of the Gd(III), the Cu(Br4obbz)Gd(Me2bpy)2NO3 complex is an ideal model for studying the magnetic interaction between Cu(II) and Ln(III) ions among the 12 binuclear complexes. As shown in Figure 2, the effective magnetic moment at room temperature is 8.18 µB, which is slightly larger than the spin-only value (8.12 µB) in the absence of the magnetic interaction of this spin-system (SCu(II) ) 1/2, SGd(III) ) 7/2). As the temperature is lowered, the magnetic moment increases gradually from 8.18 µB at 300 K to 8.91 µB at 12.6 K. This is typical of a ferromagnetic coupling between the Cu(II) and Gd(III) ions within this complex. The expected value of µeff ) 8.94 µB for a spin S ) 4 ground state, which should arise from ferromagnetic spin-coupling of the spinsystem Gd(III)-Cu(II) (SCu(II) ) 1/2, SGd(III) ) 7/2), is not reached, which is indicative of intermolecular antiferromagnetic interactions and/or zero-field splitting effects in the ground state. To understand quantitatively the spin-exchange interaction, the magnetic analysis was carried out with the susceptibility equation based on the Heisenberg ˆ 2), where the spin-exchange operator (H ˆ ) - 2JS ˆ 1‚S exchange integral J is negative for an antiferromagnetic interaction and positive for a ferromagnetic interaction. For the Cu(II)-Gd(III) binuclear system (SCu(II) ) 1/2, SGd(III) ) 7/2), the theoretical expression of magnetic susceptibility is given in eq 1:
χM )
[
]
4Nβ2g2 15 + 7 exp(-8J/kT) kT 9 + 7 exp(-8J/kT)
(1)
where χM denotes the molecular susceptibility per binuclear complex and the other symbols have their usual meanings. As shown in Figure 2, good leastsquares fits to the experimental data were attained with eq 1. The magnetic parameters thus determined are J ) +2.75 cm-1 and g ) 2.09. The agreement factor F, defined here as F ) ∑[(χM)obs - (χM)calc]2/∑(χM)obs, is then equal to 2.8 × 10-5. The results have confirmed that the spin coupling between gadolinium(III) and copper(II) ions through an oxamido group is a ferromagnetic spin-exchange interaction. Moreover, the very good agreement factor (F) also indicates that the contribution of the intermolecular antiferromagnetic interactions and/or the zero-field splitting effects in the ground state is evidently weaker than the intramolecular spinexchange and can be neglected. The small J value for the complex is indicative of the very weak spin-exchange interaction between the two metal ions. However, the ferromagnetic coupling between gadolinium(III) and copper(II) ions is interesting because gadolinium(III) has unpaired electrons in all seven 4f orbitals, and it might be expected that at least one or one linear combination of these orbitals would overlap with the semi-occupied orbital on copper(II). Such an interaction between two half-occupied orbitals would be antiferromagnetic coupling,14,33 as it would create a molecular orbital that could contain both electrons. The fact that the coupling between Gd(III) and Cu(II) in many CuGd clusters is ferromagnetic14,30,34 may be due to the spin polarization33,34 that occurs when the magnetic orbital of Cu(II)
Copper(II)-Lanthanide(III) Heterobinuclear Complexes
overlaps with the empty 5d orbitals34 of Gd(III). The fraction of unpaired electrons thus polarized from Cu(II) to Gd(III) is parallel to the f electrons due to Hund’s rule, affording ferromagnetic coupling between Cu(II) and Gd(III) ions. The fact that the 4f orbital is shielded by the outer filled 5s and 5p orbitals, and lanthanide ions generally form complexes using 6s, 6p, and/or 5d orbitals, further supports the spin-polarization mechanism. Further investigations are in progress involving other lanthanides and different stoichiometries to get a reasonable explanation and deeper insight into this exciting field of magnetic interactions. Acknowledgment. This project was supported by the National Natural Science Foundation of China and the Natural Science Foundation of Shandong Province. References (1) Willett, R. D.; Gatteschi, D.; Kahn, O., Eds. MagnetoStructural Correlation in Exchange Coupled Systems; Reidel: Dordrecht, Holland, 1985; pp 523-554. (2) Gatteschi, D.; Kahn, O.; Miller, J. S.; Palacio F., Eds. Molecular Magnetic Materials; NATO ASI Series, Kluwer: Dordrecht, 1991. (3) Yonemura, M.; Matsumura, Y.; Furutachi, H.; Ohba, M.; Okawa, H.; Fenton, D. E. Inorg. Chem. 1997, 36, 27112717. (4) Li, Y. T.; Yan, C. W.; Miao, S. H.; Liao, D. Z. Polyhedron 1998, 15, 2491-2496. (5) Caneschi, A.; Gatteschi, D.; Malendri, M. C.; Rey, P.; Sessoli, R. Inorg. Chem. 1990, 29, 4228-4234. (6) Nakatani, K.; Sletten, J.; Hault-Desporte, S.; Jeannin, S.; Jeannin, Y.; Kahn, O. Inorg. Chem. 1991, 30, 164-171. (7) Yu, P.; Kahn, O.; Nakatani, K.; Codjovi, E.; Mathoniere, C.; Sletten, J. J. Am. Chem. Soc. 1991, 113, 6558-6564. (8) Zhuong, J. Z.; Okawa, H.; Matsumoto, N.; Sakiyama, H.; Kide, S. J. Chem. Soc., Dalton Trans. 1991, 479-483. (9) Lloret, F.; Julve, M.; Ruiz, R.; Journaux, Y.; Nakatani, K.; Kahn, O.; Sletten, J. Inorg. Chem. 1993, 32, 27-31. (10) Cortes, R.; Urtiaga, M. K.; Lezama, L.; Isabel, M.; Rajo, T. Inorg. Chem. 1994, 33, 829-832.
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