Linear chain and mononuclear tri-spin compounds based on the lanthanide-nitronyl nitroxide radicals: structural design and magnetic properties

Abstract

Four Ln(III) complexes based on two nitronyl nitroxide radicals with different steric hindrance have been synthesized, characterized structurally and magnetically: [Ln(hfac)₃(NITPhOEt)]_n (Ce (1), Pr (2); hfac = hexafluoroacetylacetonate; and NITPhOEt = 4′-ethoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide); [Ln(hfac)₃(NITPhOCH₂Ph)₂] (Ce (3), Pr (4); NITPhOCH₂Ph = 4′-benzyloxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). The X-ray crystal structure analyses show that small steric hindrance of the radical makes complexes 1 and 2 show similar linear chain structures composed of Ln(hfac)₃ units bridged by NITPhOEt radicals through their N–O groups. While the larger steric hindrance of the radical induces complexes 3 and 4 to be mononuclear tri-spin compounds, in which central Ln(III) ions are coordinated by three hfac and two NITPhOCH₂Ph radicals. The variable-temperature magnetic susceptibility studies reveal that there are antiferromagnetic interactions between the paramagenetic ions (Ln(III) and radical) in all four complexes.

---

Introduction

Nitronyl nitroxides, stable organic radicals, have played a prominent role in the design and construction of molecular magnetic materials.^1,2 As they can act as not only spin carriers but also as building blocks, so far they are widely used.^3–9 Among the researches, transition metal-radical complexes have been studied greatly, while much less is known for lanthanide-radical complexes.¹⁰ This may be attributed to the effective shielding by the outer-shell electrons and the rather large and anisotropic magnetic moments of the rare-earth metal ions, which make their magnetic properties difficult to be treated. However, in recent years, lanthanide compounds involving organic radicals (2p-4f) have attracted much attention,^11–14 because several lanthanide compounds involving organic radicals have been reported to exhibit single-molecule and single-chain magnets behavior^14a,15–19 owing to the Ising anisotropy of their 4f centers, and the strength of magnetic interaction promoted by organic radicals.^2c,3,20 Furthermore, the researches show that the properties and structures of the lanthanide-radical complexes are affected greatly by modifying the radical ligand.^14c,d,30

In order to explore how modifying the nitronyl nitroxide radical ligand affects the structure of lanthanide-radical complexes, in this paper, we decided to use two radicals: NITPhOEt and NITPhOCH₂Ph (Scheme 1). In NITPhOCH₂Ph, the larger aromatic ring has a larger steric hindrance to coordinate with the lanthanide ions than ethyl in NITPhOEt.^4,21,22 By using these two different nitronyl nitroxide radical ligands, four new lanthanide-radical complexes were obtained. Among them, two are one-dimensional lanthanide-radical chain complexes bridged by the NITPhOEt radical, and the other two are mononuclear tri-spin compounds with the NITPhOCH₂Ph radical, due to the steric hindrance effect. In addition, the temperature dependence of the magnetic susceptibilities for the four complexes are also studied, and the results suggested that there are antiferromagnetic interactions between Ln(III) (Ce(III) or Pr(III)) ions and the radicals in all four Ln(III)-radical complexes.

Scheme 1 Structures of the two nitronyl nitroxide radicals with different steric hindrance.

Experimental

Materials

All of the reagents used in the syntheses were of analytical grade, the hexafluoroacetylacetone, 4-(ethoxy)benzaldehyde and 4-(benzyloxy)benzaldehyde were purchased from Alfa Chemical Company. Syntheses of the starting radicals²³ and Ce(hfac)₃·2H₂O and Pr(hfac)₃·2H₂O,²⁴ have been performed according to the literature methods.

Syntheses of [Ce(hfac)₃(NITPhOEt)]_n (1), [Pr(hfac)₃(NITPhOEt)]_n (2), [Ce(hfac)₃(NITPhOCH₂Ph)₂] (3) and [Pr(hfac)₃(NITPhOCH₂Ph)₂] (4)

All of the four complexes were synthesized by the same method. Therefore, the synthesis of compound 1 is detailed herein. Ce(hfac)₃·2H₂O (0.1 mmol) was dissolved in boiling n-heptane (20 mL). After stirring for 1 h, the solution was cooled to 65 °C, to which NITPhOEt (0.1 mmol) in CH₂Cl₂ (5 mL) was added with refluxing for 30 min. Then the solution was cooled to room temperature, filtrated and the filtrate was stored in a refrigerator at 4 °C for several days to give blue-violet crystals, which were suitable for X-ray analysis.²⁵ The compound of 2 was obtained in a similar manner using Pr(hfac)₃·2H₂O instead of Ce(hfac)₃·2H₂O; compound 3 was obtained using the NITPhOCH₂Ph radical instead of the NITPhOEt radical; and compound of 4 was obtained in a similar manner using NITPhOCH₂Ph and Pr(hfac)₃·2H₂O.

Anal. Calcd (1) for C₃₀H₂₄F₁₈CeN₂O₉ (Yield: 0.0099 g, 9.5%): C, 34.69; H, 2.33; N, 2.70%. Found: C, 34.81; H, 2.38; N, 2.81%. IR (KBr cm⁻¹): 1651(vs), 1608(w), 1558(w), 1394(w), 1379(w), 1255(vs), 1198(vs), 1093(w); Anal. Calcd (2) for C₃₀H₂₄F₁₈PrN₂O₉ (Yield: 0.0284 g, 27.3%): C, 34.67; H, 2.33; N, 2.70%. Found: C, 35.35; H, 2.42; N, 2.89%. IR (KBr cm⁻¹): 1651(vs), 1607(w), 1554(w), 1395(w), 1375(w), 1254(vs), 1202(vs), 1099(w); Anal. Calcd (3) for C₅₅H₄₉F₁₈CeN₄O₁₂ (Yield: 0.0171 g, 11.9%): C, 45.87; H, 3.43; N, 3.89%. Found: C, 44.76; H, 3.34; N, 3.79%. IR (KBr cm⁻¹): 1655(vs), 1605(w), 1556(w), 1397(w), 1376(w), 1256(vs), 1208(vs), 1093(w); Anal. Calcd (4) for C₅₅H₄₉F₁₈PrN₄O₁₂ (Yield: 0.0289 g, 20.1%): C, 45.85; H, 3.43; N, 3.89%. Found: C, 44.67; H, 3.32; N, 3.72%. IR (KBr cm⁻¹): 1652(vs), 1607(w), 1550(w), 1398(w), 1373(w), 1255(vs), 1205(vs), 1096(w).

Crystal structure determination

Determination of the unit cell and data collection for the four complexes were performed with Mo-Kα radiation (λ = 0.71073 Å) on a Bruker Smart 1000 diffractometer and equipped with a CCD camera. Both of the structures were solved primarily by direct methods and refined by the full-matrix least squares method. The computations were performed with the SHELXL-97 program. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter.^26,27Crystal data and details of structural determination refinement are summarized in Table 1 (the selected bond lengths and angles have been placed in the Electronic Supplementary Information†).

Table 1 Crystal data and structure refinements for complexes 1–4

	1	2	3	4
Empirical formula	C30H24F18CeN2O9	C30H24F18PrN2O9	C55H49F18CeN4O12	C55H49F18PrN4O12
Formula weight	1038.63	1039.42	1440.10	1440.89
T/K	113(2)	293(2)	113(2)	293(2)
Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic
Space group	P21/c	P21/c	P1̄	P1̄
a/Å	10.953(5)	11.259(2)	12.056(2)	12.421(3)
b/Å	17.082(8)	17.207(3)	15.763(3)	15.856(3)
c/Å	23.293(9)	23.884(6)	17.490(4)	18.210(4)
α (°)	90	90	73.04(3)	69.13(3)
β (°)	116.58(17)	117.99(2)	74.49(3)	73.10(3)
γ (°)	90	90	78.36(3)	89.59(3)
Volume/Å^3	3897(3)	4085.9(14)	3030.5(10)	3187.2(12)
Z	4	4	2	2
ρ c/g cm^−3	1.770	1.690	1.578	1.501
μ/mm^−1	1.305	1.323	0.868	0.876
F(000)	2040	2044	1446	1448
Reflections collected	35 785	32 606	17 709	26 632
Unique/parameters	6852/546	7291/658	10 626/819	11 197/879
R(int)	0.0481	0.0796	0.0312	0.0549
Completeness to θ = 27.48	100.0%	99.8%	99.2%	99.7%
Max./min. transmission	1.000/0.745	0.899/0.852	0.8881/0.8319	0.858/0.804
Goodness-of-fit on F^2	1.045	1.128	1.060	1.102
Final R indices [I > 2σ(I)]	R 1 = 0.0344, wR2 = 0.0815	R 1 = 0.0666, wR2 = 0.1244	R 1 = 0.0325, wR2 = 0.0702	R 1 = 0.0796, wR2 = 0.2258
R indices (all data)	R 1 = 0.0366, wR2 = 0.0830	R 1 = 0.1000, wR2 = 0.1369	R 1 = 0.0381, wR2 = 0.0726	R 1 = 0.1007, wR2 = 0.2400

---

Materials and physical techniques

Elemental analysis for C, H, and N were obtained on a Perkin-Elmer elemental analyzer model 240. Variable-temperature magnetic susceptibilities were measured on a SQUID MPMS XL-7 magnetometer in the range of 2–300 K. Diamagnetic corrections were made with Pascal's constants for all of the constituent atoms.

Results and discussion

Description of the crystal structure

The molecular structures of 1 and 2 are similar, which crystallize in the monoclinic P2₁/c space group, and both consist of linear chains built by Ce(hfac)₃ or Pr(hfac)₃ units bridged by NITPhOEt radicals through their N–O groups. The structure of complex 1 is shown in Fig. 1. Each Ce(III) atom is eight-coordinated by three bidentate hfac ligands and two nitroxide groups from the radicals. The Ce–O bond lengths are in the range of 2.417(10)–2.472(8) Å. The intrachain Ce⋯Ce distance is 8.607 Å, and the nearest interchain Ce⋯Ce contact is 12.493 Å. For complex 2, [Pr(hfac)₃(NITPhOEt)]_n is isomorphic to complex 1 except for the substitution of Ce(III) with the Pr(III) ion, which makes the bond distances and angles vary a little.

Fig. 1 Crystal structure of [Ce(hfac)₃(NITPhOEt)]_n (1). All hydrogen and fluorine atoms are omitted for clarity.

Both complexes 3 and 4 crystallize in the triclinic P1̄ space group, and consist of isolated molecules where the nitronyl nitroxide radical acts as a monodentate ligand towards Ce(III) or Pr(III) through the oxygen atom of the N–O group to form the monometallic Radical–Ln(III)–Radical complexes. The structure of complex 3 is shown in Fig. 2, where the Ce(III) ion is in the center of a distorted dodecahedron with triangular faces^28,29 and coordinated by six oxygen atoms of three hfac molecules with the Ce–O bond lengths in the range of 2.4421(18)–2.4875(19) Å, and two oxygen atoms from two NITPhOCH₂Ph radicals with the Ce–O bond lengths of 2.4060(19) and 2.4504(19) Å, respectively.

Comparing the structure of the two systems of complexes, here 1 and 3 for example, they are both cerium(III) complexes consisting of nitronyl nitroxide radicals and hfac ligands. The only difference between them is the substituent group of the nitronyl nitroxide radical ligand, which makes their structures significantly different. In complex 1, the radical of NITPhOEt with the small steric hindrance makes a chain structure with Ce(hfac)₃, whereas when the steric hindrance is enlarged as big as NITPhOCH₂Ph, the mononuclear tri-spin structure was obtained for complexe 3. The larger steric hindrance effect of NITPhOCH₂Ph in the mononuclear tri-spin complex 3 induces the nearest Ce⋯Ce distance to be as large as 14.341 Å, which is much larger than the nearest intrachain Ce⋯Ce distance (8.607 Å) and the interchain Ce⋯Ce contact (12.493 Å) in chain complex 1 (with the smaller steric hindrance effect of NITPhOEt). It can be concluded that the chains are to be assigned to a dynamical regime dominated by steric hindrance effects.^3,31 The smaller steric hindrance benefits forming the chain complex, while the larger steric hindrance enlarges the Ce⋯Ce distance and destroys the stability of the possible formation of the chain complex, and eventually forms the mononuclear complex.

Fig. 2 Perspective view of complex [Ce(hfac)₃(NITPhOCH₂Ph)₂] (3). All hydrogen and fluorine atoms are omitted for clarity.

Magnetic properties

The temperature dependence of magnetic susceptibilities for complexes 1 and 2 with chain structure are studied and shown in Fig. 3 and 4, and the χ_MT values at room temperature are 1.34 and 1.98 cm³ K mol⁻¹ for Ce(III) and Pr(III) complexes, respectively. Both the values are close to the expected values of 1.18 and 1.97 cm³ K mol⁻¹ for an uncoupled system of one Ln(III) ion (²F_5/2, g = 6/7, for Ce(III) and ³H₄, g = 4/5 for Pr(III)) plus one radical (S = 1/2, 0.375 cm³ K mol⁻¹). Upon cooling, the χ_MT values of both 1 and 2 decrease gradually and reach a minimum of 0.2 and 0.1 cm³ K mol⁻¹ at 2.0 K, respectively, which could arise from a selective depopulation of the excited crystal field state and antiferromagnetic interaction between Ln(III) ion and radical. There is no available expression to determine the magnetic susceptibility of such a 1D system with large anisotropy. However, to obtain a rough quantitative estimation of the magnetic interactions between paramagnetic species (Ce(III) [or Pr(III)] ion and radical), the magnetic susceptibility χ_total of the complex can be assumed as the sum of χ_Ln of the isolated Ce(III) or Pr(III) ion and χ_Rad of one radical (eqn (1)). The Ce(III) [or Pr(III)] ion may be assumed to exhibit a splitting of the m_j energy levels (Ĥ = ΔĴ_z²) in an axial crystal field.^14e,32 Thus χ_Ce and χ_Pr can be described as eqn (3) and (4), respectively. In the expression, Δ is the zero-field-splitting parameter, g is the Lande factor, k is the Boltzmann constant, β is the Bohr magneton constant, and N is Avogadro's number. The Weiss constant, θ, is introduced (eqn (5)) to roughly simulate the magnetic interactions between all the paramagnetic species in the system.^32a,33 Thus the magnetic data can be analyzed by the following approximate treatment of eqn (1)–(5).

χ_(total) = χ_Ln + χ_Rad (1)

Fig. 3 χ _M T (○) versus T and χ_M (□) versus T plots for complex 1 in the range 2–300 K in the field of 2000 Oe. The solid lines represent the theoretical.

Fig. 4 χ _M T (○) versus T and χ_M (□) versus T plots for complex 2 in the range 2–300 K in the field of 1000 Oe. The solid lines represent the theoretical.

The observed χ_MT data were well reproduced (Fig. 3 and 4) by using the above approximate eqn (1)–(5), giving the best fitting parameters of g = 0.93, Δ = 6.99, θ = −0.79 K for Ce(III) complex 1 and g = 0.84, Δ = 20.22, θ = −11.72 K for Pr(III) complex 2, which indicates that there are antiferromagnetic interactions between the paramagenetic ions (Ln(III) [Ce(III) or Pr(III)] and radical) in the 1D chain system.

Variable-temperature magnetic susceptibilities for mononuclear complexes of 3 and 4 were also studied and shown in Fig. 5 and 6. The χ_MT values at 300 K are 1.54 and 2.18 cm³ K mol⁻¹ for Ce(III) and Pr(III) mononuclear complexes, respectively. Both the values are very close to the expected values of 1.55 and 2.35 cm³ K mol⁻¹ for an uncoupled system of one Ln(III) ion (Ce(III) or Pr(III)) plus two radicals. On decreasing the temperature, the χ_MT values of both 3 and 4 decrease gradually and reach a minimum of 0.18 cm³ K mol⁻¹ at 2.0 K for both the compounds. To obtain a rough quantitative estimate of the magnetic interaction parameters between paramagnetic species (Ln(III) and radical) in the mononuclear tri-spin system, we assumed that the total magnetic susceptibility χ_total is given by the sum of the isolated Ln(III) ion and two radicals (eqn (6)), and the Weiss constant, θ, is also introduced to roughly simulate the magnetic interactions between all the paramagnetic species in the system (eqn (5)).^32a,33

χ_(total) = χ_Ln + 2χ_Rad (6)

Fig. 5 χ _M T (○) versus T and χ_M (□) versus T plots for complex 3 in the range 2–300 K in the field of 1000 Oe. The solid lines represent the theoretical.

Fig. 6 χ _M T (○) versus T and χ_M (□) versus T plots for complex 4 in the range 2–300 K in the field of 1000 Oe. The solid lines represent the theoretical.

Giving the best fitting parameters of g = 0.87, Δ = 13.82, θ = −0.89 K for Ce(III) complex 3 and g = 0.83, Δ = 22.96, θ = −9.13 K for Pr(III) complex 4, which indicates the antiferromagnetic interaction between the paramagenetic ions (Ln(III) and radicals) in both of the two mononuclear tri-spin systems.

Conclusions

In conclusion, by modifying the nitroxide radicals with different steric hindrance effects, we have successfully prepared two novel one-dimensional lanthanide-radical chain complexes bridged by NITPhOEt radicals and two mononuclear tri-spin compounds with NITPhOCH₂Ph, which indicate that the small steric hindrance of the radical will benefit the formation of the chain structure of lanthanide-radical complexes. The rough magnetic studies show that there are antiferromagnetic interactions between the paramagenetic ions (Ln(III) and radical) in all the four Ln(III)-radical complexes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China 21101096, 90922032, 21071085, 20971072, National Basic Research Program of China (973 Program, 2007CB815305), NSF of Tianjin 09JCYBJC05500, Research Fund for the Doctoral Program of Higher Education 20100031120013, and Fundamental Research Funds for the Central Universities, National Undergraduates Innovating Experimentation Project 101005533.

Notes and references

1. (a) K. E. Vostrikova, Coord. Chem. Rev., 2008, 252, 1409; (b) A. Caneschi, D. Gatteschi and R. Sessoli, Acc. Chem. Res., 1989, 22, 392; (c) A. Caneschi and D. Gatteschi, Prog. Inorg. Chem., 1991, 39, 331.
2. (a) O. Kahn, Molecular Magnetism, VCH Publishers, Inc., New York, 1993; (b) A. Cogne, J. Laugier, D. Luneau and P. Rey, Inorg. Chem., 2000, 39, 5510; (c) R. Sessoli and A. K. Powell, Coord. Chem. Rev., 2009, 253, 2328.
3. K. Bernot, L. Bogani, A. Caneschi, D. Gatteschi and R. Sessoli, J. Am. Chem. Soc., 2006, 128, 7947.
4. L. Bogani, C. Sangregorio, R. Sessoli and D. Gatteschi, Angew. Chem., Int. Ed., 2005, 44, 5817.
5. G. Poneti, K. Bernot, L. Bogani, A. Caneschi, R. Sessoli, W. Wernsdorferc and D. Gatteschia, Chem. Commun., 2007, 1807.
6. D. Luneau and P. Rey, Coord. Chem. Rev., 2005, 249, 2591.
7. T. Tsukuda, T. Suzuki and S. Kaizaki, J. Chem. Soc., Dalton Trans., 2002, 1721.
8. C. Train, L. Norel and M. Baumgarten, Coord. Chem. Rev., 2009, 253, 2342.
9. (a) L. Y. Wang, L. F. Ma, Z. H. Jiang, D. Z. Liao and S. P. Yan, Inorg. Chim. Acta, 2005, 358, 820; (b) K. Hayakawa, D. Shiomi, T. Ise, K. Sato and T. Takui, J. Mater. Chem., 2006, 16, 4146; (c) J. Y. Zhang, C. M. Liu, D. Q. Zhang, S. Gao and D. B. Zhu, Inorg. Chim. Acta, 2007, 360, 3553; (d) R. N. Liu, L. C. Li, X. Y. Xing and D. Z. Liao, Inorg. Chim. Acta, 2009, 362, 2253.
10. (a) C. Lescopa, G. Bussiereb, R. Beaulacb, H. Belisleb, E. Belorizkyc, P. Rey, C. Reberb and D. Luneau, J. Phys. Chem. Solids, 2004, 65, 773; (b) T. Tsukuda, T. Suzuki and S. Kaizaki, Polyhedron, 2007, 26, 3175; (c) C. Benelli, Inorg. Chim. Acta, 2008, 361, 4157; (d) K. Bernot, L. Bogani, R. Sessoli and D. Gatteschi, Inorg. Chim. Acta, 2007, 360, 3807.
11. C. Benelli, A. Caneschi, D. Gatteschi and R. Sessoli, Inorg. Chem., 1993, 32, 4797.
12. M. L. Kahn, J. P. Sutter, S. Golhen, P. Guionneau, L. Ouahab, O. Kahn and D. Chasseau, J. Am. Chem. Soc., 2000, 122, 3413.
13. C. Benelli, A. Caneschi, D. Gatteschi, L. Pardi and P. Rey, Inorg. Chem., 1990, 29, 4223.
14. (a) R. N. Liu, L. C. Li, X. L. Wang, P. P. Yang, C. Wang, D. Z. Liao and J. P. Sutter, Chem. Commun., 2010, 46, 2566; (b) C. Lescop, E. Belorizky, D. Luneau and P. Rey, Inorg. Chem., 2002, 41, 3375; (c) N. Zhou, Y. Ma, C. Wang, G. F. Xu, J. K. Tang, J. X. Xu, S. P. Yan, P. Cheng, L. C. Li and D. Z. Liao, Dalton Trans., 2009, 8489; (d) R. N. Liu, Y. Ma, P. P. Yang, X. Y. Song, G. F. Xu, J. K. Tang, L. C. Li, D. Z. Liao and S. P. Yan, Dalton Trans., 2010, 39, 3321; (e) J. X. Xu, Y. Ma, D. Z. Liao, G. F. Xu, J. K. Tang, C. Wang, N. Zhou, S. P. Yan, P. Cheng and L. C. Li, Inorg. Chem., 2009, 48, 8890.
15. (a) L. Bogani, C. Sangregorio, R. Sessoli and D. Gatteschi, Angew. Chem., Int. Ed., 2005, 44, 5817; (b) G. Poneti, K. Bernot, L. Bogani, A. Caneschi, R. Sessoli, W. Wernsdorferc and D. Gatteschi, Chem. Commun., 2007, 43, 1807.
16. C. Benelli, A. Caneschi, D. Gatteschi, L. Pardi, P. Rey, D. P. Shum and R. L. Carlin, Inorg. Chem., 1989, 28, 272.
17. C. Lescop, E. Belorizky, D. Luneau and P. Rey, Inorg. Chem., 2002, 41, 3375.
18. (a) C. Benelli, A. Caneschi, D. Gatteschi and L. Pardi, Inorg. Chem., 1992, 31, 741; (b) G. Poneti, K. Bernot, L. Bogani, A. Caneschi, R. Sessoli, W. Wernsdorfer and D. Gatteschi, Chem. Commun., 2007, 1807.
19. (a) J. P. Sutter, M. L. Kahn, S. Golhen, L. Ouahab and O. Kahn, Chem.–Eur. J., 1998, 4, 571; (b) M. L. Kahn, J. P. Sutter, S. Golhen, P. Guionneau, L. Ouahab, O. Kahn and D. Chasseau, J. Am. Chem. Soc., 2000, 122, 3413; (c) T. Tsukuda, T. Suzuki and S. Kaizaki, Polyhedron, 2007, 26, 3175.
20. K. Bernot, L. Bogani, R. Sessoli and D. Gatteschi, Inorg. Chim. Acta, 2007, 360, 3807.
21. C. Benelli, A. Caneschi, D. Gatteschi and R. Sessoli, J. Appl. Phys., 1993, 73, 5333.
22. A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio and R. Sessoli, J. Chem. Soc., Dalton Trans., 2000, 3907.
23. E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and R. Darcy, J. Am. Chem. Soc., 1972, 94, 7049.
24. M. F. Richardson, W. F. Wagner and D. E. Sands, J. Inorg. Nucl. Chem., 1968, 30, 1275.
25. Q. H. Zhao, Y. P. Ma, L. Du and R. B. Fang, Transition Met. Chem., 2006, 31, 593.
26. T. Shiga, M. Ohba and H. Okawa, Inorg. Chem. Commun., 2003, 6, 15.
27. S. Akine, T. Matsumoto, T. Taniguchi and T. Nabeshima, Inorg. Chem., 2005, 44, 3270.
28. C. Benelli, A. Caneschi, D. Gatteschi, J. Laugier and P. Rey, Angew. Chem., Int. Ed. Engl., 1987, 26, 913.
29. K. Bernot, J. Luzon, L. Bogani, M. Etienne, C. Sangregorio, M. Shanmugam, A. Caneschi, R. Sessoli and D. Gatteschi, J. Am. Chem. Soc., 2009, 131, 5573.
30. N. Zhou, Y. Ma, C. Wang, G. F. Xu, J. K. Tang, S. P. Yan and D. Z. Liao, J. Solid State Chem., 2010, 183, 927.
31. H. Miyasaka, R. Clerac, K. Mizushima, K. Sugiura, M. Yamashita, W. Wernsdorfer and C. Coulon, Inorg. Chem., 2003, 42, 8203.
32. (a) I. A. Kahwa, J. Selbin, C. J. O'Connor, J. W. Foise and G. L. McPherson, Inorg. Chim. Acta, 1988, 148, 265; (b) J. K. Tang, Q. L. Wang, S. F. Si, D. Z. Liao, Z. H. Jiang, S. P. Yan and P. Cheng, Inorg. Chim. Acta, 2005, 358, 325; (c) B. Li, W. Gu, L. Z. Zhang, J. Qu, Z. P. Ma, X. Liu and D. Z. Liao, Inorg. Chem., 2006, 45, 10425; (d) Y. Ouyang, W. Zhang, N. Xu, G. F. Xu, D. Z. Liao, K. Yoshimura, S. P. Yan and P. Cheng, Inorg. Chem., 2007, 46, 8454; (e) N. Xu, W. Shi, D. Z. Liao, S. P. Yan and P. Cheng, Inorg. Chem., 2008, 47, 8748; (f) N. Xu, C. Wang, W. Shi, S. P. Yan, P. Cheng and D. Z. Liao, Eur. J. Inorg. Chem., 2011, 15, 2387.
33. Y. Liao, W. W. Shum and J. S. Miller, J. Am. Chem. Soc., 2002, 124, 9336.

---

Footnote

† Electronic supplementary information (ESI) available: The selected bond lengths and angles for complexes 1–4. CCDC reference numbers 826308, 808012, 731494 and 808624. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce06016c

---