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A New Adamantane-like Tetranuclear Manganese(III) Complex Based on Flexible Schiff-base Ligand: Synthesis, Crystal Structure and Magnetic Property
A New Adamantane-like Tetranuclear Manganese(III) Complex Based on Flexible Schiff-base Ligand: Synthesis, Crystal Structure and Magnetic Property
Bulletin of the Korean Chemical Society. 2014. Jul, 35(7): 1965-1969
Copyright © 2014, Korea Chemical Society
  • Received : February 21, 2014
  • Accepted : March 06, 2014
  • Published : July 20, 2014
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About the Authors
Ran Zhang
Zhong-Hai Ni
Li-Fang Zhang
Hui-Zhong Kou
Department of Chemistry, Tsinghua University, Beijing 10084, P.R. China.

Abstract
A new tetranuclear Mn(III) complex [Mn III 4 (sae) 4 3 -O)(μ 1,1 -N 3 )(OH)(H 2 O) 2 ]·H 2 O ( 1 ) (H 2 sae = 2-salicylideneamino-1-ethanol) has been synthesized by the reaction of MnCl 2 ·4H 2 O, H 2 sae and sodium azide in the mixed solvent of methanol, acetonitrile and water. The X-ray diffraction analysis shows that the four Mn(III) ions in complex 1 have a unique adamantine arrangement, whereas the coordination environment of each Mn(III) ions is different. Magnetic studies indicate that complex 1 manifests antiferromagnetic behaviors. The magnetic susceptibilities of complex 1 have been fitted by two magnetic models based on the suitable analysis of its magnetic structural topology.
Keywords
Introduction
In the field of molecular magnetic materials, the famous manganese molecular clusters complexes Mn 12 O 12 (CH 3 CO 2 ) 16 (H 2 O) 4 and its derivatives have attracted intense interest due to their relatively excellent single molecule magnets behaviors (SMMs). 1-4 Therefore, in very recent years, the synthesis and assembly of manganese multinuclear complexes with high-spin ground states and large magnetic anisotropy have been one hot topic for constructing new and amusing molecular magnets. 5-8
Based on the understanding on the nature of magnetic coupling in Mn 12 , new polynuclear manganese complexes containing oxygen bridges are more expected. 9 Along this idea, many new manganese clusters have been synthesized based on O -containing ligands and some of them interesting exhibit single molecule magnets behaviors. 10-12 The O -containing ligand used for the synthesis of polynuclear molecular clusters not only include classical carboxylic acid, 13 but also include Schiff base ligands, 14 phenolic ligands and their derivatives. 15 Among the numerous O -containing ligands, Schiff base ligands have attracted durative much attention because they manifest more diversity in assembly multinuclear complexes resulting from both nitrogen and oxygen atoms. 16,17
Compared with the rigid Schiff base ligands, the flexible Schiff base ligands are more suitable for the assembly of polynuclear complexes with unique molecular structures and fascinating properties. 18 Up to date, several tens of clusters involving Ni II , Cu II , Fe II , Fe III , Mn II and Mn III ions based on flexible Schiff base ligands have been reported. 14,17,19-21 Recently, we selected a flexible Schiff base H 2 sae as the main ligand, and small azide anion was introduced as the terminal or bridging ligand, a new adamantane-shaped tetranuclar Mn(III) complex with formula [Mn III 4 (sae) 4 3 -O)(μ 1,1 -N 3 )(OH)(H 2 O) 2 ]·H 2 O was synthesized. Herein, we report the synthesis, crystal structure and magnetic property of the title complex.
Experimental Section
Measurements. Elemental analyses (C, H and N) were carried out on an Elemental Vario E1 instrument. The infrared spectroscopy on Nicolet 7199B FTIR spectrophotometer in the 4000–400 cm –1 regions. Magnetic properties measurements on crystal samples were performed on a Quantum Design MPMS SQUID magnetometer. The experimental susceptibilities were corrected for the diamagnetism estimated based on Pascal’s constants.
Synthesis of Complex 1. MnCl 2 ·4H 2 O (3 mmol) was added to a solution of H 2 sae (3 mmol) in methanol (15 mL), the mixture was stirred at room temperature and then acetonitrile (10 mL) was added. To the above solution, an aqueous solution (5 mL) of NaN 3 (10 mmol) was added carefully. The resulting mixture was stirred for 15 min and filtered, the filtrate was left undisturbed at room temperature and brown cubic block crystals suitable for X-ray single crystal diffraction were obtained after several days. Yield: 60%. Elemental analysis calculated for: C 36 H 41 Mn 4 N 7 O 13 , (999.52): C, 43.26; H, 4.13; N, 9.81%. Found: C, 43.23; H, 4.09; N, 9.83%. Selected IR frequencies (KBr disk, cm –1 ): 3347, 2044 (νN 3 ), 2859 (νCH 2 ), 2938(νCH 2 ), 1625, 1600, 1541.
Crystal Structure Determination. Crystals data of complex 1 was selected on a Bruker APEX II CCDC diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K using the ω-scan technique. The structure was solved by direct methods with the SHELXS-97 computer program, and refined by full-matrix least-squares methods (SHELXL-97) on F 2 . Detailed information about the crystal data and structure determination for complex 1 is summarized in Table 1 . The selected bond distances and bond angles are given in Table 2 . Crystallographic data for complex 1 has been deposited with CCDC-987816. This data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. E-mail: deposit@ccdc.cam.ac.uk.
Crystal Data and Structure Refinement for1
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Crystal Data and Structure Refinement for 1
Selected Bond Distances (Å) and Angles (°) for1
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Selected Bond Distances (Å) and Angles (°) for 1
Results and Discussion
Crystal Structure of Complex 1. Single crystal X-ray diffraction analysis reveals that complex 1 crystallizes in monoclinic space group P 2 1 / n . The crystal structure of complex 1 is shown in Figure 1 . Complex 1 has a adamantaneshaped structure consisting of four independent Mn(III) ions which were linked by four doubly deprotonated sae ligands, one azide anion and one oxygen atom. The coordination geometries of the four Mn(III) ions is shown in Figure 2 . The crystal structure of complex 1 clearly shows that the coordination environments of four Mn(III) ions are different from each other, but it is in common that each of them has a trident sae ligand at equatorial plane. Mn(1) is coordinated by two oxygen atoms (O(1) and O(2)) and one nitrogen atom (N(1)) from the sae ligand and one oxygen atom (O(6)) from another sae ligand belongs to Mn(3) at the equatorial plane. Mn(1) and the four coordination atoms in the equatorial position are almost in the same plane with the maximum deviation of 0.051 Å (N(1)), the bond lengths of the Mn-O are range from 1.858(4) to 1.963(3) Å and Mn-N is 1.986(4) Å. At the axial position, Mn(1) is coordinated by one water molecule and one oxygen atom (O(4)) from the sae ligand which also coordinates to Mn(2) with the bond lengths of 2.359(6) Å for Mn(1)-O(1w) and 2.250(3) Å for Mn(1)- O(4), respectively. Mn(2) is bonded to two oxygen atoms (O(3) and O(4)) and one nitrogen atom (N(2)) from the sae ligand and one independent oxygen atom (O(9)) at the equatorial plane, which form a plane with a maximum deviation of 0.041 Å (O(9) and O(3)). Mn(2) is completed by one hydroxyl oxygen and one nitrogen atom (N(5)) from the bridging N 3 ion at axial direction. Mn(3) is surrounded by two oxygen atoms (O(5) and O(6)) and one nitrogen atom (N(3)) from the sae ligand along with one independent oxygen atom (O(9)) at equatorial plane, which form a plane with a maximum deviation of 0.042 Å (O(9)) and a distance of 0.037 Å from Mn(3). Mn(3) is axially occupied by one oxygen atom (O(8)) from the sae ligand belongs to Mn(4) and one nitrogen atom (N(5)) from the bridging N 3 ion. Mn(4) is chelated by two oxygen atoms (O(7) and O(8)) and one nitrogen atom (N(4)) from the sae ligand and one independent oxygen atom (O(9)) at the equatorial plane. Mn(4) is axially chelated by one oxygen atom (O(2)) from the sae ligand and weak-coordinated oxygen atom from water with the bond distance of Mn(4)-O(2w) = 2.751 Å. Finally, an elongated octahedral coordination environment is created for each Mn(III) ion. Thus, each of alcohol oxygen (O(2), O(4), O(6) and O(8) from the sae ligands is connected to the two Mn(III) ions. Furthermore, three Mn(III) ions are bridged by the independent O 2– (O(9)), the Mn(2) and Mn(3) ions are linked to N(5) from the bridging N 3 ion. The coconnection of the three bridging modes formed the Mn 4 O 4 N adamantane structure of complex 1 ( Figure 2 ).
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Molecular structure of complex 1, free water molecules and hydrogen atoms are omitted for clarity.
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Adamantane-shaped structure formed by oxygen and nitrogen atoms linking four Mn(III) ions in complex 1 (Red for oxygen atom, pink for Mn atom and blue for nitrogen atom).
Azide group in complex 1 is end-on type with the bond lengths of Mn(2)-N(5) = 2.474(4) Å, Mn(3)-N(5) = 2.474(3) Å, and the bridging angle Mn(2)-N(5)-Mn(3) is 81.26(17)°. The reason for the occurrence of such a low bridging angle is that the coordination of N(5) for Mn(2) and Mn(3) ions are axial. To the best of our knowledge, it is the lowest bridging angle for azide-bridged complexes. In the complex {[Mn III 4 3 -O)(sae) 4 (μ-N 3 )(CH 3 OH) 2 ] 2 (μ-N 3 )}(N 3 ), the bridging angle of azide with EO mode is 84.04(8)° which is considered as the lowest linking angle previously. 22 Azide ion is approximately linear with the bond angle of N(5)- N(6)-N(7) = 177.7(8)° in complex 1 . Other coordination bond lengths are in the normal range. 19,21,23,24 The shortest intramolecular distance between the two metal atoms is 2.9707(11) Å for Mn(3)---Mn(4). It is worth noting that the supramolecular structure of complex 1 is relatively simple due to the lack of relatively strong intermolecular interactions. Two clusters of complex 1 are linked together by π-π interactions between the two neighboring benzene rings, forming dimeric supramolecular structure ( Figure S1 ).
Magnetic Properties. The magnetic susceptibilities of complex 1 have been measured in the temperature range of 1.8−300 K under the field of 1000 Oe. A pot of χ m T and 1/ χ m T versus T are shown in Figure 3 , in which χ m is molar susceptibility per Mn III 4 unit. The χ m T values of 9.42 emu K mol –1 at 300 K is lower than the spin-only values of 12.00 emu K mol –1 for four uncoupled Mn(III) ions based on g = 2.0, which is normal for antiferromagnetic systems with relatively strong antiferromagnetic coupling. On lowering the temperature, the χ m T values relatively quickly decline to a minimum of 0.56 emu K mol –1 at 1.8 K. The changing tendency of this curve clearly suggests the existence of dominant antiferromagnetic coupling in complex 1 . The magnetic susceptibilities in the range of 25–300 K agree well with the Curie-Weiss law with a large negative Weiss constant θ = –94.59 K and Curie constant C = 12.27 emu K mol –1 which further confirms the presence of a stronger overall antiferromagnetic interactions in complex 1 . The magnetic field ( H ) dependence of magnetization ( M ) of complex 1 was measured at 1.8 K with fields up to 50 kOe. The data are presented in Figure 4 as plot of M / ( N represents Avogadro's constant, β is Bohr magneton) versus H . The experimental curve is far lower than the Brillouin curve with Lande factor g = 2.0, which also confirms the overall antiferromagnetic interactions in complex 1 .
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Curves of βmT–T and 1/βm–T for complex 1 (The solid lines represent the best fitting based on the parameters discussed in the text).
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Field dependence of magnetization complex 1 (The line represents the Brillouin function based on g = 2.0 and four noninteracting Mn(III) ions).
In complex 1 , the magnetic coupling between each pair of Mn(III) ions are extraordinary complicated due to the fact that the coordination environments of the four Mn(III) ions are different and the linkage of each pair of Mn(III) ions are not completely the same.
In order to qualitatively evaluate the magnitude of the magnetic interaction between the Mn(III) ions, the magnetic model of complex 1 was simplified by employing the same intramolecular magnetic coupling constant ( J ) for the comparatively complicated case. In addition, the intermolecular magnetic interaction ( zJ' ) was taken into account.
On the basis of the regular tetrahedron arrangement of Mn III 4 model ( Scheme 1 ), the magnetic susceptibilities can be fitted accordingly by the following expressions given in Eqs. (1 and 2) derived from the spin Hamiltonian Ĥ = –2 J ( Ŝ 1 Ŝ 2 + Ŝ 1 Ŝ 4 + Ŝ 2 Ŝ 3 + Ŝ 3 Ŝ 4 ), where A = 408exp(48 J / kT ) + 840 exp(32 J / kT ) + 1092exp(18 J / kT ) + 1100exp(6 J / kT ) + 900exp (–4 J / kT ) + 476exp(–12 J / kT ) + 160exp(–18 J / kT ) + 24exp(–22 J / kT ), B = 17exp(48 J / kT ) + 45exp(32 J / kT ) + 78exp(18 J / kT ) + 110exp(6 J / kT ) + 135exp(–4 J / kT ) + 119exp(–12 J / kT ) + 80 exp(–18 J / kT ) + 36exp(–22 J / kT ) + 5exp(–24 J / kT ).
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Sketch map of magnetic coupling for regular tetrahedronal arrangement of MnIII4 system in complex 1.
In which χ t is molar susceptibility per Mn III 4 unit only including intramolecular magnetic coupling, k is Boltzmann constant. The least-square fit of the experimental data in the whole temperature range to the above expressions gives the following parameters J = –3.52(1) cm –1 , g = 1.96(1), zJ' = –0.12(2) cm –1 , R = Σ[( χ m T) obsd. – ( χ m T) calcd. ] 2 /Σ[( χ m T) 2 obsd. ] = 8.72 × 10 –4 . The small zJ' value suggests the intermolecular magnetic interaction is weak. The above fitting results for the magnetic susceptibilities of complex 1 seems acceptable based on the assumption that the four Mn(III) ions system accords with regular tetrahedron arrangement model (the blue line in Figure 3 ). However, the crystal structure parameters of complex 1 manifest that the coordination environments of four Mn(III) ions are largely different. Therefore, the above magnetic fitting model doesn’t reflect the magnetic nature of complex 1 .
In order to obtain the main parameters which can better reflect the actual situation of magnetic coupling of complex 1 , we have carried out a very detailed analysis on the crystal structure parameters of complex 1 and referred to some relevant reports on the structural-magneto correlations of similar Mn(III) systems, 22 then offered an improved magnetic model for complex 1 .
According to the crystal structure parameters of complex 1 , the bond angles of Mn(1)-O(2)-Mn(4), Mn(2)-O(4)- Mn(1) and Mn(3)-O(8)-Mn(1) are close to each other, which indicates that the magnetic coupling constants should be very similar between Mn(1) and Mn(2), Mn(3), and Mn(4). Hence, it’s reasonable to hypothesize that they have equal J 1 value. In addition, the bond angle of Mn(2)-O(9)-Mn(4) is 139.72(16)° which is obviously different with the angles mentioned above. Therefore, the magnetic coupling between Mn(2) and Mn(4) is another situation should be considered ( J 2 ). Based on the reports of the literature, 22 the angles of Mn(4)-O(9)-Mn(3) and Mn(2)-O(9)-Mn(3) are approximately equal to 110° which is in the range of weak ferromagnetic coupling between neighboring Mn(III) ions. Moreover, the azide EO bridge transfers weak ferromagnetic coupling between Mn(2) and Mn(3)). In addition, there exists a Mn(3)-O(8)-Mn(4) magnetic coupling route between Mn(3) and Mn(4) with the angle of 92.25(13)° which also should be in the range of ferromagnetic coupling. However, the measurement results of magnetic susceptibility indicate that the complex 1 shows an overall antiferromagnetic coupling interaction, which can be due to that the antiferromagnetic coupling interactions between Mn(2) and Mn(4), and between Mn(1) and Mn(2), Mn(3), and Mn(4) are dominated. Therefore, it is reasonable to fit the magnetic property of complex 1 mainly considering the four strong antiferromagnetic coupling interactions ( Scheme 2 ). The relatively weak ferromagnetic coupling between Mn(2) and Mn(3) and between Mn(3) and Mn(4) together with the intercluster magnetic coupling can be classified to intermolecular interactions ( zJ' ). Moreover, it is easily to understand that the two ferromagnetic coupling interactions above mentioned are dominating in zJ' .
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Sketch map of critical magnetic coupling in complex 1.
On the basis of the above Mn III 4 model ( Scheme 2 ), the magnetic susceptibilities of the title complex can be fitted accordingly by the below expression deduced from the spin Hamiltonian Ĥ = –2 J 1 Ŝ 2 ( Ŝ 1 + Ŝ 3 + Ŝ 4 ) – 2 J 2 Ŝ 1 Ŝ 3 J 1 ( Ŝ 2 Ŝ B 2 Ŝ 2 2 ) – J 2 ( Ŝ A 2 –Ŝ 1 2 –Ŝ 3 2 ), in which S A = S 1 + S 3 , S B = S A + S 4 , S = S B + S 2.
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Where, the detailed values of A and B see SI. The best-fit parameters obtained are J 1 = –4.67(5) cm –1 , J 2 = –5.14(8) cm –1 , zJ' = 1.42(3) cm –1 , g = 2.02(1), R = 1.69 × 10 –5 . The theoretical values conform to the experimental values (the black line in Figure 3 ) and the best-fit parameters are also very reasonable, which manifested that the model well reflects the actual coupling situation of complex 1 .
In the complex 1 , due to the d x2 y2 orbit of the high-spin Mn(III)[d 4 , t 2g 3 e g ] ions have the highest energy, the four unpaired electrons occupied the four orbits of d xy (t 2g ), d xz ( t 2g ), d yz (t 2g ) and d z 2 (e g ), respectively. Based on the Kahn orbit model in complex 1 , the ferromagnetic ( t 2g + eg) and antiferromagnetic (t 2g + t 2g ) coupling coexist between the neighboring Mn(III) ions through oxygen atom bridge and the antiferromagnetic coupling contributions are dominated in the system. In addition, according to the spin polarization theory of the azido group, there may exist ferromagnetic coupling interaction between Mn(III) ions through EO azide bridge in complex 1 , but in general the ferromagnetic coupling interaction is too much weak to atonement the net contribution of antiferromagnetic coupling bridged by oxygen atom between Mn(III) ions. According to the above described results, we can see that the complex 1 exhibits overall antiferromagnetic coupling.
Conclusion
In summary, we have synthesized a new tetranuclear manganese(III) adamantane-like complex [Mn III 4 (sae) 4 3 -O)(μ 1,1 -N 3 )(OH)(H 2 O) 2 ]·H 2 O based on Schiff base ligand. The coordination environments of four Mn(III) ions are different from each other. The magnetic measurement indicated the presence of overall antiferromagnetic interaction in the title complex. The magnetic susceptibilities of the title complex have been fitted by two magnetic models based on the suitable analysis of its magnetic structural topology.
Acknowledgements
Supplementary Materials.The dimeric supramolecular structure Figure, the powder XRD Figure and the fitting expression for magnetic susceptibilities and the CIF file of the title complex can be found.
References
Novoa J. J. , Deumal M. , Jornet somoza 2011 J. Chem. Soc. Rev. 40 3182 -    DOI : 10.1039/c0cs00112k
Zhang P. , Guo Y.-N. , Tang J. 2013 Coord. Chem. Rev. 257 1728 -    DOI : 10.1016/j.ccr.2013.01.012
Sessoli R. , Gatteschi D. , Caneschi A. , Novak M. A. 1993 Nature 365 141 -    DOI : 10.1038/365141a0
Thomas L. , Lionti F. , Ballou R. , Gatteschi D. , Sessoli R. , Barbara B. 1996 Nature 383 145 -    DOI : 10.1038/383145a0
Cavallini M. , Gomez-Segura J. , Ruiz-Molina D. , Massi M. , Albonetti C. , Concepci C. , Rovira C. , Veciana J. , Biscarini F. 2005 Angew. Chem. Int. Ed. 44 888 -    DOI : 10.1002/anie.200461554
Yoo J. , Wernsdorfer W. , Yang E. C. , Nakano M. , Rheingold A. L. , Hendrickson D. N. 2005 Inorg. Chem. 44 3377 -    DOI : 10.1021/ic048212a
Chakov N. E. , Wernsdorfer W. , Abboud K. A. , Christou G. 2004 Inorg. Chem. 43 5919 -    DOI : 10.1021/ic0494049
Ge C. H. , Ni Z. H. , Liu C. M. , Cui A. L. , Zhang D. Q. , Kou H. Z. 2008 Inorg. Chem. Commun. 11 675 -    DOI : 10.1016/j.inoche.2008.03.005
Shah S. J. , Ramsey C. M. , Heroux K. J. , DiPasquale A. G. , Dalal N. S. , Rheingold A. L. , Barco E. , Hendrickson D. N. 2008 Inorg. Chem. 47 9569 -    DOI : 10.1021/ic801374u
Lecren L. , Roubeau O. , Coulon C. , Li Y. G. , Le Goff X. F. , Wernsdorfer W. , Miyasaka H. , Clérac R. 2005 J. Am. Chem. Soc. 127 17353 -    DOI : 10.1021/ja0551685
Yang C. , Wang Q. L. , Su C. Y. , Hu L. N. , Li L. C. , Liao D. Z. 2014 Inorg. Chem. Commun. 40 26 -    DOI : 10.1016/j.inoche.2013.11.028
Yang P. P. , Song X. Y. , Liu R. N. , Li L. C. , Liao D. Z. 2010 Dalton. Trans. 39 6285 -    DOI : 10.1039/c0dt00291g
Sivanesan D. , Son K. , Lee H. J. , Park K. T. , Jang Z. , Suh B. J. , Yoon S. 2013 Polyhedron 50 339 -    DOI : 10.1016/j.poly.2012.11.005
Zhang L. F. , Ni Z. H. , Zong Z. M. , Wei X. Y. , Ge C. H. , Kou H. Z. 2005 Acta Cryst. 61 542 -    DOI : 10.1107/S0108767305026760
Yang E. C. , Harden N. , Wernsdorfer W. , Zakharov L. , Brechin E. K. , Rheingold A. L. , Christou G. , Hendrickson D. N. 2003 Polyhedron 22 1857 -    DOI : 10.1016/S0277-5387(03)00173-6
Oshio H. , Hoshino N. , Ito T. 2000 J. Am. Chem. Soc. 122 12602 -    DOI : 10.1021/ja002889p
Oshio H. , Hoshino N. , Ito T. , Nakano M. , Renz F. , Gütlich P. 2003 Angew. Chem. Int. Ed. 42 223 -    DOI : 10.1002/anie.200390084
Yang X. P. , Chan C. , Lam D. , Schipper D. , Stanley J. M. , Chen X. Y. , Jones R. A. , Holliday B. J. , Wong W. K. , Chen S. C. , Chen Q. 2012 Dalton Trans. 41 11449 -    DOI : 10.1039/c2dt31268a
Boskovic C. , Bircher R. , Tregenna-Piggott P. L. W. , Güdel H. U. , Paulsen C. , Wernsdorfer W. , Barra A. L. , Khatsko E. , Neels A. , Stoeckli-Evans H. 2003 J. Am. Chem. Soc. 125 14046 -    DOI : 10.1021/ja0367086
Lu Z. L. , Fan T. , Guo W. W. , Lu J. , Fan C. H. 2013 Inorganica Chimica Acta 400 191 -    DOI : 10.1016/j.ica.2013.02.030
Oshio H. , Nihei M. , Yoshida A. , Nojiri H. , Nakano M. , Yamaguchi A. , Karaki Y. , Ishimoto H. 2005 Chem. Eur. J. 11 843 -    DOI : 10.1002/chem.200400368
Hoshino N. , Ito T. , Nihei M. , Oshio H. 2003 Inorg. Chem. Commun. 6 377 -    DOI : 10.1016/S1387-7003(02)00793-1
Miyasaka H. , Nezu T. , Sugimoto K. , Sugiura K. I. , Yamashita M. , Clérac R. 2005 Chem. Eur. J. 11 1592 -    DOI : 10.1002/chem.200400946
Nihei M. , Hoshino N. , Ito T. , Oshio H. 2003 Polyhedron 22 2359 -    DOI : 10.1016/S0277-5387(03)00159-1