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Magnetic study on radical-gadolinium(III) complexes. Relationship between the exchange coupling and coordination structure
Polyhedron 66 (2013) 183–187
Contents lists available at SciVerse ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Magnetic study on radical-gadolinium(III) complexes. Relationship between the exchange coupling and coordination structure Takayuki Ishida a,⇑, Rina Murakami a, Takuya Kanetomo a, Hiroyuki Nojiri b a b
Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 18 December 2012 Accepted 2 April 2013 Available online 9 April 2013 Keywords: Single-molecule magnet Lanthanide Lanthanoid Rare earth metal Exchange interaction
a b s t r a c t Antiferro- and ferromagnetic couplings were observed in [Gd(hfac)3(2pyNO)(H2O)] and [Gd(hfac)3(HNN)2], respectively, where 2pyNO stands for tert-butyl 2-pyridyl nitroxide and HNN for 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide. The magnitude of the former is the largest in Gd-nitroxide complexes (2J/kB = 13.8(3) K). An empirical relation between the exchange coupling and torsion angle Gd–O–N– C is drawn, where large torsion (>36°) favors ferromagnetic coupling. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Although the exchange coupling parameter (J) is one of the most important magnetic parameters in magnetic solids, little is known about guiding principle for constructing magnetic coupling in relation with the structures of 4f/2p- (4f/p⁄-) based magnets [1]. Among the Ln series, Gd3+ is often chosen as an initial attempt because the spin only character with SGd = 7/2 facilitates magnetic analysis. There have been a number of reports on ferromagnetic coupling between Gd3+ and organic spins [1–5]. On the other hand, Lescop et al. have claimed that antiferromagnetic Gd-radical complexes are relatively rare [4,6]. In this communication, a possible magneto-structure relation will be proposed, after two new Gd3+radical complexes are structurally and magnetically characterized. 2. Results and discussion We prepared [Gd(hfac)3(2pyNO)(H2O)] (Gd-2pyNO) (Hhfac and 2pyNO stand for 1,1,1,5,5,5-hexafluoropentane-2,4-dione and tert-butyl 2-pyridyl nitroxide [7], respectively). Complexation was carried out immediately after the generation of 2pyNO, because 2pyNO is unisolable. A polycrystalline product of Gd-2pyNO as synthesized was subjected to X-ray crystallographic analysis and magnetic study. Polycrystalline Gd-2pyNO is stable up to the melting point (76–78 °C). Several 3d-chelates involving 2pyNO showed ferromagnetic couplings as large as the order of room ⇑ Corresponding author. E-mail address: [email protected] (T. Ishida). 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.04.004
temperature [8,9]. The present work provides the first example of 4f-chelates with 2pyNO. The Gd ion in Gd-2pyNO is nine-coordinate (Fig. 1a), where the 4f and 2p (p⁄) spin centers are directly bonded and a fivemembered chelate ring was formed as expected. The geometry of 2pyNO is quite normal in comparison with several 2pyNO-ligated transition metal ion complexes [7–9]. The out-of-plane Gd dislocation from the radical p-conjugation plane is evaluated by Gd–O–N– C2py torsion. In Gd-2pyNO, the Gd1–O1–N2–C5 torsion angle is 19.5(8)°, relatively small owing to the steric reason due to the chelation. Another Gd3+-radical complex was obtained by using HNN, a 2-hydro derivative of nitronyl nitroxide (NN; the systematic name is 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) [10]. Complexation of HNN and Gd(hfac)3 followed by recrystallization gave [Gd(hfac)3(HNN)2] (Gd-HNN2). The 1/3 Gd-HNN complex (Gd-HNN3) has been recently reported [5], and the selective preparation has now been established. The Gd ion in Gd-HNN2 is eight-coordinate (Fig. 1b). The two HNN ligands are crystallographically independent, but the geometries around the HNN coordinations are similar to each other. The structure of the HNN portion is comparable to the known compounds [5,11]. The Gd1–O1–N1–C1 and Gd1–O3–N3–C8 torsion angles are 37.8(6)° and 52.6(6)°, respectively. Magnetic susceptibilities of randomly oriented polycrystalline specimens were measured at 500 Oe, as a function of temperature. The vmT value of Gd-2pyNO was decreased on cooling, indicating the presence of antiferromagnetic coupling, and reached a plateau at ca. 5.9 cm3 K mol 1 around 8 K (Fig. 2a). The ground Stotal = 3
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Fig. 1. X-ray crystal structures of (a) Gd-2pyNO and (b) Gd-HNN2 with thermal ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. Structural formulas are also shown. Selected geometrical parameters are: (a) Gd1–O1, 2.464(4) Å; Gd1–O1–N2, 124.5(4)°; (b) Gd1–O1, 2.384(4); Gd1–O3, 2.342(4) Å; Gd1–O1–N1, 136.2(3); Gd1–O3–N3, 136.1(3)°.
Fig. 2. Temperature dependence of vmT for polycrystalline specimens of (a) Gd-2pyNO and (b) Gd-HNN2 measured at 500 Oe. Solid lines show the theoretical fit. The inset shows the magnetization curve measured at 1.8 K. Solid lines show the Brillouin function.
state, which should have the spin-only value of 6.0 cm3 K mol 1, is confirmed. A small drop was found below 7 K, indicating the presence of weak intermolecular antiferromagnetic coupling. The magnetization curve of Gd-2pyNO was measured at 1.8 K (the inset of Fig. 2a). The magnetization reached 6.0 NAlB at 7 T, being consistent with Stotal = 3 with g = 2. The van Vleck equation is available from the Heisenberg spin Hamiltonian H = 2JSGdS2pyNO [12]. A Weiss mean field parameter h was introduced as an intermolecular interaction factor [T/(T h)].
The parameters were optimized, giving 2J/kB = 13.8(3) K and gavg = 2.0363(16) with h = 0.442(8) K. To the best of our knowledge, the exchange coupling parameter obtained here is the largest in magnitude among the Gd-nitroxide compounds known so far [1–6]. In contrast, as Fig. 2b shows, the vmT value of Gd-HNN2 was increased on cooling and reached the maximum around 7 K, but it was considerably smaller than that of the ferromagnetic Stotal = 9/ 2 state. This finding suggests the presence of intermolecular anti-
T. Ishida et al. / Polyhedron 66 (2013) 183–187
ferromagnetic interaction. The magnetization of Gd-HNN2 at 1.8 K was 8.2 NAlB at 7 T (the inset of Fig. 2b), being compatible with Stotal = 9/2. The vm(T) result on Gd-HNN2 was analyzed on the spin Hamiltonian H = 2JSGd(SHNN1 + SHNN2) [13], assuming the molecular symmetry. The parameters were optimized as 2J/kB = +2.61(2) K and gavg = 2.019(1) with intermolecular antiferromagnetic interaction of h = 1.363(7) K. Some Gd-radical chelates, such as [Gd(L)2(NO3)3] (L = 2-benzimidazolyl-NN), have been reported to exhibit antiferromagnetic couplings. On the other hand, Gd-HNN3 [5] and Gd-HNN2 possess ferromagnetic coupling, where the radical ligands do not form a chelate. Similarly, the known Gd-radical compounds without chelation are likely to show ferromagnetic couplings [1]. Now a possible magneto-structure relation is discussed, including the literature data. We had to pay attention to the definition of the exchange parameter as +J, J, or 2J. A meaningful relation was drawn (Fig. 3), when the exchange parameter J was plotted against the Gd–O–N–Ca torsion angle (|/|). For the 2pyNO ligand, / is defined with Gd–O–N–C2py. The J value of Gd-2pyNO was reduced by a factor of 0.73 to superpose the data of the NN-coordinated complexes [8], because the magnetic exchange coupling is proportional to the spin densities at the interacting atoms [14]. The spin density at the ligating N–O oxygen atom is assumed to be proportional to the aN value determined in electron spin resonance spectroscopy [15] (aN = 1.0 mT for 2pyNO and 0.73 mT for HNN [10]). Fig. 3 also includes the data on the Gd complexes having the imino nitroxide (IN) radical (the systematic name is 2-substituted 4,4,5,5-tetramethylimidazolin1-oxyl) [16]. The factor of 1.2 is applied to draw the data on the Gd-IN chelate system [8] on the same plot. The spin densities at the NN oxygen atoms and those at the IN nitrogen atoms have been directly measured for closely related compounds by means of polarized neutron diffraction study (for example, 0.277(13) for the Ph-NN oxygen atom [17] and 0.236(7) for the imino nitrogen atom in m-nitrophenyl IN [18]). We can find that smaller |/| favors antiferromagnetic coupling. The critical |/|, at which the sign of the Gd-radical exchange changes from positive to negative, is 36(2)° (see the line in Fig. 3). There is no exception in all the data so far known. This picture also holds for the antiferromagnetic coupling in the Gd-osemiquinonate complex having the five-membered planar structure [19]. Furthermore, the antiferromagnetic coupling in the
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Gd–N23 –Gd triad [20] may be related with the completely planar core due to the centrosymmetry. The / parameter is a convenient indicator for the dislocation of the metal ion out of the p⁄ nodal plane, without definition of the principal axis of the Gd ion. The present result shows a sharp contrast with those of 3d-p⁄ compounds, where exactly coplanar configuration favors orbital orthogonality and ferromagnetic coupling [7,8]. In the present case, an appreciable orbital interaction such as 5d-p⁄ is expected in the largely twisted Gd–O–N–Ca systems. Namely, the presence of transfer integral b5d-p⁄ would stabilize the high-spin configuration (Fig. 4), just like the role of b5d-3d proposed for the well-investigated ferromagnetic Gd–Cu systems [12,21,22]. The unpaired-electron transfer from the paramagnetic ligand to the empty 5d orbital brings about the parallel alignment of the 4f and 5d electrons by Hund’s rule. This electron configuration is admixed with the ground state, leading to the high-spin state favored. Charge transfer from p⁄ to 6s seems to be less likely from the angular dependence of J. The breakdown of the orthogonal arrangement between the 5d and p⁄ orbitals is essential for the ferromagnetic Gd–nitroxide coupling. Although a subset showing the data around |/| = 90° in Fig. 3 might have a peculiar interaction yet unclear, the proposed model holds also for them with respect to the 5d-p⁄ interaction. Difference between the 4f-3d and 4f-p systems is stated briefly. Kahn et al. proposed that JGd–Cu in the diphenoxo-bridged Gd–Cu complexes should be a function of the dihedral angle between the GdO2 and CuO2 planes [12]. Costes et al. studied the diatomic Gd–O–N–Cu bridge systems [23]. The ferromagnetic exchange originates in the planar structure to gain b5d-3d for both case, in contrast to the Gd-radical result. It can be simply explained in terms of the difference between r- and p-characters in the Cu2+ and nitroxide magnetic orbitals, respectively. Superexchange mechanism tells us that an electron can be transferred along the r-electron system in Gd–Cu compounds. 3. Summary The interaction involving Gd3+ ion is a key to understand those of other Ln3+ ions. The magnitude of the coupling can be quantitatively estimated according to the chemical trend found throughout the Ln-complex analogs [13,22,24]. In summary, we have proposed here a magneto-structure relation based on a charge-transfer model in 4f-p complexes, which seems to be totally consistent with the interactions including 3d-p [8] and 4f-3d [21,22] compounds. Calculation work is a future study, but the present empirical relation may be a strong guiding principle to predict exchange coupling in Ln-radical heterospin systems. 4. Experimental 4.1. Preparation of [Gd(hfac)3(2pyNO)(H2O)] (Gd-2pyNO) The precursory hydroxylamine [7] (16 mg; 0.10 mmol) was oxidized with Ag2O (0.12 g; 0.51 mmol) in dichloromethane (6 mL) at room temperature for 0.5 h. The resultant red mixture was filtered,
Fig. 3. Plot of the exchange coupling parameter vs. Gd–O–N–Ca torsion angle on the Gd3+-NN basis. The results on Gd-2pyNO and three IN chelates (open triangles) are shown after normalization (see the text). The averaged parameters are used when the crystallographically independent geometries are present. Open circles and triangles stand for the data in the literature [1–6]. A line implies the best fit for the data in |/| < 70°. The definition of / is drawn in the inset.
Fig. 4. Charge transfer configuration of the Gd-radical system with large torsion. b implies a transfer integral.
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and the filtrate was concentrated to ca. 4 mL. After a heptane solution (50 mL) of [Gd(hfac)3(H2O)2] [25] (86 mg; 0.11 mmol) was boiled and concentrated to ca. 15 mL, the previous 2pyNO solution was added while hot (ca. 50 °C). The mixture was allowed to stand at room temperature and further cooled in a refrigerator for a day. A polycrystalline product of Gd-2pyNO (48 mg; 0.050 mmol) was precipitated and separated on a filter. The yield was 56%. The product is stable up to the melting point (76–78 °C). IR (neat, attenuated total reflection) 1651, 1253, 1190, 1135, 1098, 1060, 796, 660, 583 cm 1. Anal. Calc. for C24H18F18GdN2O8: C, 29.98; H, 1.89; N, 2.91. Found: C, 30.35; H, 2.03; N, 2.95%.
unique reflection data. Selected crystallographic data are listed in Table 1. 4.4. Magnetic measurements Magnetic susceptibilities of polycrystalline specimens of Gd2pyNO and Gd-HNN2 were measured on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T coil in a temperature range 1.8–300 K. The magnetic responses were corrected with diamagnetic blank data of the sample holder measured separately. The diamagnetic contribution of the sample itself was estimated from Pascal’s constants.
4.2. Preparation of [Gd(hfac)3(HNN)2] (Gd-HNN2) Acknowledgements
After a heptane solution (45 mL) of [Gd(hfac)3(H2O)2] [25] (82 mg; 0.10 mmol) was boiled and concentrated to ca. 6 mL, a dichloromethane solution (4 mL) involving HNN [10] (32 mg; 0.20 mmol) was added to the above solution while hot (ca. 50 °C). The mixture was stirred for 30 min and cooled to room temperature. After being concentrated under reduced pressure, a reddish orange fine polycrystalline product was obtained. To remove a small amount of impurity such as Gd-HNN3 [5], the product was purified by crystallization from 2 mL of dichloromethane and 12 mL of heptane. Reddish orange plates were collected. The yield was 51.4 mg (47%). The polycrystalline Gd-HNN2 is stable up to the melting point (85–87 °C). IR (neat, attenuated total reflection) 583, 624, 660, 798, 1095, 1139, 1193, 1249, 1461, 1647 cm 1. Anal. Calc. for C29H29F18GdN4O10: C, 31.87; H, 2.67; N, 5.13. Found: C, 32.93; H, 2.72; N, 5.76%.
CCDC numbers 851804 and 914126 contain the supplementary crystallographic data for complexes Gd-2pyNO and Gd-HNN2, respectively. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
4.3. X-ray crystallographic analysis
References
X-ray diffraction data of Gd-2pyNO and Gd-HNN2 were collected on Rigaku Saturn70 CCD and R-axis Rapid diffractometers with graphite monochromated MoKa radiation (k = 0.71073 Å). The structures were directly solved by a heavy-atom method and expanded using Fourier techniques in the CRYSTALSTRUCTURE program package [26]. Numerical absorption correction was used. All of the hydrogen atoms were located at calculated positions and the parameters were refined as ‘‘riding.’’ The thermal displacement parameters of non-hydrogen atoms were refined anisotropically. Full-matrix least-squares methods were applied using all of the
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Table 1 Selected crystallographic data for Gd-2pyNO and Gd-HNN2.
a b
Compounds
Gd-2pyNO
Gd-HNN2
Formula Formula weight Habit Dimension (mm3) T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm 3) Unique data l(Mo Ka) (mm 1) R (F)a (I > 2r(I)) Rw(F2)b (all data)
C24H18F18GdN2O8 961.64 orange platelet 0.24 0.15 0.015 100 triclinic P1 11.456(4) 11.959(5) 13.300(4) 66.749(13) 71.042(14) 80.015(15) 1581.1(10) 2 2.020 7208 2.259 0.0549 0.0763
C29H29F18GdN4O10 1092.79 orange platelet 0.21 0.14 0.05 100 monoclinic P21/n 11.969(3) 16.559(4) 20.691(5) 90 90.015(11) 90 4100.9(17) 4 1.770 9390 1.758 0.0553 0.0528
R = R||Fo| |Fc||/R|Fo|. Rw = [Rw(Fo2 Fc2)2/Rw(Fo2)2]1/2.
This work was partly supported by KAKENHI (Grant Numbers JSPS/22350059 and MEXT/23110711) and the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University. Appendix A. Supplementary data
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Contents lists available at SciVerse ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Magnetic study on radical-gadolinium(III) complexes. Relationship between the exchange coupling and coordination structure Takayuki Ishida a,⇑, Rina Murakami a, Takuya Kanetomo a, Hiroyuki Nojiri b a b
Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 18 December 2012 Accepted 2 April 2013 Available online 9 April 2013 Keywords: Single-molecule magnet Lanthanide Lanthanoid Rare earth metal Exchange interaction
a b s t r a c t Antiferro- and ferromagnetic couplings were observed in [Gd(hfac)3(2pyNO)(H2O)] and [Gd(hfac)3(HNN)2], respectively, where 2pyNO stands for tert-butyl 2-pyridyl nitroxide and HNN for 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide. The magnitude of the former is the largest in Gd-nitroxide complexes (2J/kB = 13.8(3) K). An empirical relation between the exchange coupling and torsion angle Gd–O–N– C is drawn, where large torsion (>36°) favors ferromagnetic coupling. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Although the exchange coupling parameter (J) is one of the most important magnetic parameters in magnetic solids, little is known about guiding principle for constructing magnetic coupling in relation with the structures of 4f/2p- (4f/p⁄-) based magnets [1]. Among the Ln series, Gd3+ is often chosen as an initial attempt because the spin only character with SGd = 7/2 facilitates magnetic analysis. There have been a number of reports on ferromagnetic coupling between Gd3+ and organic spins [1–5]. On the other hand, Lescop et al. have claimed that antiferromagnetic Gd-radical complexes are relatively rare [4,6]. In this communication, a possible magneto-structure relation will be proposed, after two new Gd3+radical complexes are structurally and magnetically characterized. 2. Results and discussion We prepared [Gd(hfac)3(2pyNO)(H2O)] (Gd-2pyNO) (Hhfac and 2pyNO stand for 1,1,1,5,5,5-hexafluoropentane-2,4-dione and tert-butyl 2-pyridyl nitroxide [7], respectively). Complexation was carried out immediately after the generation of 2pyNO, because 2pyNO is unisolable. A polycrystalline product of Gd-2pyNO as synthesized was subjected to X-ray crystallographic analysis and magnetic study. Polycrystalline Gd-2pyNO is stable up to the melting point (76–78 °C). Several 3d-chelates involving 2pyNO showed ferromagnetic couplings as large as the order of room ⇑ Corresponding author. E-mail address: [email protected] (T. Ishida). 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.04.004
temperature [8,9]. The present work provides the first example of 4f-chelates with 2pyNO. The Gd ion in Gd-2pyNO is nine-coordinate (Fig. 1a), where the 4f and 2p (p⁄) spin centers are directly bonded and a fivemembered chelate ring was formed as expected. The geometry of 2pyNO is quite normal in comparison with several 2pyNO-ligated transition metal ion complexes [7–9]. The out-of-plane Gd dislocation from the radical p-conjugation plane is evaluated by Gd–O–N– C2py torsion. In Gd-2pyNO, the Gd1–O1–N2–C5 torsion angle is 19.5(8)°, relatively small owing to the steric reason due to the chelation. Another Gd3+-radical complex was obtained by using HNN, a 2-hydro derivative of nitronyl nitroxide (NN; the systematic name is 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) [10]. Complexation of HNN and Gd(hfac)3 followed by recrystallization gave [Gd(hfac)3(HNN)2] (Gd-HNN2). The 1/3 Gd-HNN complex (Gd-HNN3) has been recently reported [5], and the selective preparation has now been established. The Gd ion in Gd-HNN2 is eight-coordinate (Fig. 1b). The two HNN ligands are crystallographically independent, but the geometries around the HNN coordinations are similar to each other. The structure of the HNN portion is comparable to the known compounds [5,11]. The Gd1–O1–N1–C1 and Gd1–O3–N3–C8 torsion angles are 37.8(6)° and 52.6(6)°, respectively. Magnetic susceptibilities of randomly oriented polycrystalline specimens were measured at 500 Oe, as a function of temperature. The vmT value of Gd-2pyNO was decreased on cooling, indicating the presence of antiferromagnetic coupling, and reached a plateau at ca. 5.9 cm3 K mol 1 around 8 K (Fig. 2a). The ground Stotal = 3
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Fig. 1. X-ray crystal structures of (a) Gd-2pyNO and (b) Gd-HNN2 with thermal ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. Structural formulas are also shown. Selected geometrical parameters are: (a) Gd1–O1, 2.464(4) Å; Gd1–O1–N2, 124.5(4)°; (b) Gd1–O1, 2.384(4); Gd1–O3, 2.342(4) Å; Gd1–O1–N1, 136.2(3); Gd1–O3–N3, 136.1(3)°.
Fig. 2. Temperature dependence of vmT for polycrystalline specimens of (a) Gd-2pyNO and (b) Gd-HNN2 measured at 500 Oe. Solid lines show the theoretical fit. The inset shows the magnetization curve measured at 1.8 K. Solid lines show the Brillouin function.
state, which should have the spin-only value of 6.0 cm3 K mol 1, is confirmed. A small drop was found below 7 K, indicating the presence of weak intermolecular antiferromagnetic coupling. The magnetization curve of Gd-2pyNO was measured at 1.8 K (the inset of Fig. 2a). The magnetization reached 6.0 NAlB at 7 T, being consistent with Stotal = 3 with g = 2. The van Vleck equation is available from the Heisenberg spin Hamiltonian H = 2JSGdS2pyNO [12]. A Weiss mean field parameter h was introduced as an intermolecular interaction factor [T/(T h)].
The parameters were optimized, giving 2J/kB = 13.8(3) K and gavg = 2.0363(16) with h = 0.442(8) K. To the best of our knowledge, the exchange coupling parameter obtained here is the largest in magnitude among the Gd-nitroxide compounds known so far [1–6]. In contrast, as Fig. 2b shows, the vmT value of Gd-HNN2 was increased on cooling and reached the maximum around 7 K, but it was considerably smaller than that of the ferromagnetic Stotal = 9/ 2 state. This finding suggests the presence of intermolecular anti-
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ferromagnetic interaction. The magnetization of Gd-HNN2 at 1.8 K was 8.2 NAlB at 7 T (the inset of Fig. 2b), being compatible with Stotal = 9/2. The vm(T) result on Gd-HNN2 was analyzed on the spin Hamiltonian H = 2JSGd(SHNN1 + SHNN2) [13], assuming the molecular symmetry. The parameters were optimized as 2J/kB = +2.61(2) K and gavg = 2.019(1) with intermolecular antiferromagnetic interaction of h = 1.363(7) K. Some Gd-radical chelates, such as [Gd(L)2(NO3)3] (L = 2-benzimidazolyl-NN), have been reported to exhibit antiferromagnetic couplings. On the other hand, Gd-HNN3 [5] and Gd-HNN2 possess ferromagnetic coupling, where the radical ligands do not form a chelate. Similarly, the known Gd-radical compounds without chelation are likely to show ferromagnetic couplings [1]. Now a possible magneto-structure relation is discussed, including the literature data. We had to pay attention to the definition of the exchange parameter as +J, J, or 2J. A meaningful relation was drawn (Fig. 3), when the exchange parameter J was plotted against the Gd–O–N–Ca torsion angle (|/|). For the 2pyNO ligand, / is defined with Gd–O–N–C2py. The J value of Gd-2pyNO was reduced by a factor of 0.73 to superpose the data of the NN-coordinated complexes [8], because the magnetic exchange coupling is proportional to the spin densities at the interacting atoms [14]. The spin density at the ligating N–O oxygen atom is assumed to be proportional to the aN value determined in electron spin resonance spectroscopy [15] (aN = 1.0 mT for 2pyNO and 0.73 mT for HNN [10]). Fig. 3 also includes the data on the Gd complexes having the imino nitroxide (IN) radical (the systematic name is 2-substituted 4,4,5,5-tetramethylimidazolin1-oxyl) [16]. The factor of 1.2 is applied to draw the data on the Gd-IN chelate system [8] on the same plot. The spin densities at the NN oxygen atoms and those at the IN nitrogen atoms have been directly measured for closely related compounds by means of polarized neutron diffraction study (for example, 0.277(13) for the Ph-NN oxygen atom [17] and 0.236(7) for the imino nitrogen atom in m-nitrophenyl IN [18]). We can find that smaller |/| favors antiferromagnetic coupling. The critical |/|, at which the sign of the Gd-radical exchange changes from positive to negative, is 36(2)° (see the line in Fig. 3). There is no exception in all the data so far known. This picture also holds for the antiferromagnetic coupling in the Gd-osemiquinonate complex having the five-membered planar structure [19]. Furthermore, the antiferromagnetic coupling in the
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Gd–N23 –Gd triad [20] may be related with the completely planar core due to the centrosymmetry. The / parameter is a convenient indicator for the dislocation of the metal ion out of the p⁄ nodal plane, without definition of the principal axis of the Gd ion. The present result shows a sharp contrast with those of 3d-p⁄ compounds, where exactly coplanar configuration favors orbital orthogonality and ferromagnetic coupling [7,8]. In the present case, an appreciable orbital interaction such as 5d-p⁄ is expected in the largely twisted Gd–O–N–Ca systems. Namely, the presence of transfer integral b5d-p⁄ would stabilize the high-spin configuration (Fig. 4), just like the role of b5d-3d proposed for the well-investigated ferromagnetic Gd–Cu systems [12,21,22]. The unpaired-electron transfer from the paramagnetic ligand to the empty 5d orbital brings about the parallel alignment of the 4f and 5d electrons by Hund’s rule. This electron configuration is admixed with the ground state, leading to the high-spin state favored. Charge transfer from p⁄ to 6s seems to be less likely from the angular dependence of J. The breakdown of the orthogonal arrangement between the 5d and p⁄ orbitals is essential for the ferromagnetic Gd–nitroxide coupling. Although a subset showing the data around |/| = 90° in Fig. 3 might have a peculiar interaction yet unclear, the proposed model holds also for them with respect to the 5d-p⁄ interaction. Difference between the 4f-3d and 4f-p systems is stated briefly. Kahn et al. proposed that JGd–Cu in the diphenoxo-bridged Gd–Cu complexes should be a function of the dihedral angle between the GdO2 and CuO2 planes [12]. Costes et al. studied the diatomic Gd–O–N–Cu bridge systems [23]. The ferromagnetic exchange originates in the planar structure to gain b5d-3d for both case, in contrast to the Gd-radical result. It can be simply explained in terms of the difference between r- and p-characters in the Cu2+ and nitroxide magnetic orbitals, respectively. Superexchange mechanism tells us that an electron can be transferred along the r-electron system in Gd–Cu compounds. 3. Summary The interaction involving Gd3+ ion is a key to understand those of other Ln3+ ions. The magnitude of the coupling can be quantitatively estimated according to the chemical trend found throughout the Ln-complex analogs [13,22,24]. In summary, we have proposed here a magneto-structure relation based on a charge-transfer model in 4f-p complexes, which seems to be totally consistent with the interactions including 3d-p [8] and 4f-3d [21,22] compounds. Calculation work is a future study, but the present empirical relation may be a strong guiding principle to predict exchange coupling in Ln-radical heterospin systems. 4. Experimental 4.1. Preparation of [Gd(hfac)3(2pyNO)(H2O)] (Gd-2pyNO) The precursory hydroxylamine [7] (16 mg; 0.10 mmol) was oxidized with Ag2O (0.12 g; 0.51 mmol) in dichloromethane (6 mL) at room temperature for 0.5 h. The resultant red mixture was filtered,
Fig. 3. Plot of the exchange coupling parameter vs. Gd–O–N–Ca torsion angle on the Gd3+-NN basis. The results on Gd-2pyNO and three IN chelates (open triangles) are shown after normalization (see the text). The averaged parameters are used when the crystallographically independent geometries are present. Open circles and triangles stand for the data in the literature [1–6]. A line implies the best fit for the data in |/| < 70°. The definition of / is drawn in the inset.
Fig. 4. Charge transfer configuration of the Gd-radical system with large torsion. b implies a transfer integral.
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and the filtrate was concentrated to ca. 4 mL. After a heptane solution (50 mL) of [Gd(hfac)3(H2O)2] [25] (86 mg; 0.11 mmol) was boiled and concentrated to ca. 15 mL, the previous 2pyNO solution was added while hot (ca. 50 °C). The mixture was allowed to stand at room temperature and further cooled in a refrigerator for a day. A polycrystalline product of Gd-2pyNO (48 mg; 0.050 mmol) was precipitated and separated on a filter. The yield was 56%. The product is stable up to the melting point (76–78 °C). IR (neat, attenuated total reflection) 1651, 1253, 1190, 1135, 1098, 1060, 796, 660, 583 cm 1. Anal. Calc. for C24H18F18GdN2O8: C, 29.98; H, 1.89; N, 2.91. Found: C, 30.35; H, 2.03; N, 2.95%.
unique reflection data. Selected crystallographic data are listed in Table 1. 4.4. Magnetic measurements Magnetic susceptibilities of polycrystalline specimens of Gd2pyNO and Gd-HNN2 were measured on a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T coil in a temperature range 1.8–300 K. The magnetic responses were corrected with diamagnetic blank data of the sample holder measured separately. The diamagnetic contribution of the sample itself was estimated from Pascal’s constants.
4.2. Preparation of [Gd(hfac)3(HNN)2] (Gd-HNN2) Acknowledgements
After a heptane solution (45 mL) of [Gd(hfac)3(H2O)2] [25] (82 mg; 0.10 mmol) was boiled and concentrated to ca. 6 mL, a dichloromethane solution (4 mL) involving HNN [10] (32 mg; 0.20 mmol) was added to the above solution while hot (ca. 50 °C). The mixture was stirred for 30 min and cooled to room temperature. After being concentrated under reduced pressure, a reddish orange fine polycrystalline product was obtained. To remove a small amount of impurity such as Gd-HNN3 [5], the product was purified by crystallization from 2 mL of dichloromethane and 12 mL of heptane. Reddish orange plates were collected. The yield was 51.4 mg (47%). The polycrystalline Gd-HNN2 is stable up to the melting point (85–87 °C). IR (neat, attenuated total reflection) 583, 624, 660, 798, 1095, 1139, 1193, 1249, 1461, 1647 cm 1. Anal. Calc. for C29H29F18GdN4O10: C, 31.87; H, 2.67; N, 5.13. Found: C, 32.93; H, 2.72; N, 5.76%.
CCDC numbers 851804 and 914126 contain the supplementary crystallographic data for complexes Gd-2pyNO and Gd-HNN2, respectively. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
4.3. X-ray crystallographic analysis
References
X-ray diffraction data of Gd-2pyNO and Gd-HNN2 were collected on Rigaku Saturn70 CCD and R-axis Rapid diffractometers with graphite monochromated MoKa radiation (k = 0.71073 Å). The structures were directly solved by a heavy-atom method and expanded using Fourier techniques in the CRYSTALSTRUCTURE program package [26]. Numerical absorption correction was used. All of the hydrogen atoms were located at calculated positions and the parameters were refined as ‘‘riding.’’ The thermal displacement parameters of non-hydrogen atoms were refined anisotropically. Full-matrix least-squares methods were applied using all of the
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Table 1 Selected crystallographic data for Gd-2pyNO and Gd-HNN2.
a b
Compounds
Gd-2pyNO
Gd-HNN2
Formula Formula weight Habit Dimension (mm3) T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm 3) Unique data l(Mo Ka) (mm 1) R (F)a (I > 2r(I)) Rw(F2)b (all data)
C24H18F18GdN2O8 961.64 orange platelet 0.24 0.15 0.015 100 triclinic P1 11.456(4) 11.959(5) 13.300(4) 66.749(13) 71.042(14) 80.015(15) 1581.1(10) 2 2.020 7208 2.259 0.0549 0.0763
C29H29F18GdN4O10 1092.79 orange platelet 0.21 0.14 0.05 100 monoclinic P21/n 11.969(3) 16.559(4) 20.691(5) 90 90.015(11) 90 4100.9(17) 4 1.770 9390 1.758 0.0553 0.0528
R = R||Fo| |Fc||/R|Fo|. Rw = [Rw(Fo2 Fc2)2/Rw(Fo2)2]1/2.
This work was partly supported by KAKENHI (Grant Numbers JSPS/22350059 and MEXT/23110711) and the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University. Appendix A. Supplementary data
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