Communication pubs.acs.org/IC
Unexpectedly Strong Magnetic Anisotropy in a Mononuclear EightCoordinate Cobalt(II) Complex: a Theoretical Exploration Jin-Mei Wei and Yi-Quan Zhang* Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R. China S Supporting Information *
n the field of single-molecule magnets (SMMs), lanthanidebased SMMs have attracted much attention for their large unquenched orbital moments and strong spin−orbit coupling, which can produce large single-ion anisotropies.1 The largest relaxation barrier among the lanthanide-containing SMMs has been up to 652 cm−1.2 Compared to f-element SMMs, however, the energy barriers of those transition-metal-based SMMs are small because of the weak spin−orbit coupling of d-element ions. All the same, for the strong spin−orbit coupling effect of the CoII ion, which can provide a considerable anisotropy energy, CoIIbased SMMs have made great progress in recent years.3 Although a lot of CoII-based SMMs have been reported,3 the energy barriers are still far smaller than those of lanthanide-based SMMs. To our surprise, Vas and co-workers reported a cobalt-based single-chain magnet with a high blocking temperature of around 14 K, which is the largest one among molecule-based magnets.4 Recently, Cheng and we5 reported a mononuclear eightcoordinate [CoII(12-crown-4)2]2+, which is shown in Figure 1. It is a new kind of CoII-based complex exhibiting SMM properties. α, the angle between the eight metal−ligand directions and the S8 axis passing through the metal atom, is
54.74° for the ideal square antiprism (Figure 1a).6 A twist angle φ, defined as the rotation angle of one coordination square away from the eclipsed configuration to the other, is 45° for an ideal square antiprism (Figure 1b).6 Compared to the ideal squareantiprism geometry, the angles α are in the range of 55.69(5)− 56.55(5)° in 1, and the twist angle φ of 39.07(7)° deviates significantly from 45°. As usual, low-coordinate complexes may provide more opportunities to obtain the highly axial-coordinated geometry, facilitating the high magnetic anisotropy of a metal ion. Recently, a series of low-coordinate lanthanide complexes were explored in the pursuit of SMMs.7 Similarly, a transition-metal complex with a lower coordination number also tends to have a larger magnetic anisotropy.3 However, our reported complex 1, which has a coordination number of 8, exhibits SMM properties. To probe the origin of the high magnetic anisotropy of 1 and the possible structure with much larger magnetic anisotropy, we thoroughly investigated the magnetostructural correlations between the angles α and φ and the magnetic anisotropy parameter D of 1 using two changing ways. The first one is to change the angles α from 48° to 64°, with the twist angle φ of 39.07(7)° unchanged. The second is to change φ from 33° to 45°, with α in the range of 55.69(5)−56.55(5)° unchanged. Complete-active-space second-order perturbation theory (CASPT2) considering the effect of the dynamical electronic correlation based on the complete-active-space self-consistentfield (CASSCF) method using the MOLCAS 7.8 program package8 was performed on the complete structure of complex 1 (see Figure 1a) and the corresponding models to obtain the parameters D and E [see the Supporting Information (SI) for computational details]. To investigate the influence of the angles α and φ on the magnetic anisotropy of 1, we used two ways to change the structure of complex 1. The calculated D and E of complex 1 changed by the two ways are shown in Table 1. Also, we give the dependence of parameter D on the angles of α and φ in Figure S1 in the SI. From Table 1 and Figure S1 in the SI, the absolute D value of 1 decreases with an increase of α but increases with an increase of the twist angle φ. Moreover, we observed that the dependence of the D values on α is almost linear. When changing φ from 33° to 45°, however, the variations of D are much larger than those from 25° to 33°. In addition, it is unexpected that the |D| value of 1 with φ = 45° is not the largest one, while that of 1 with φ = 43° is up to the limit. To probe the
Figure 1. Structure of complex 1. H atoms are omitted for clarity.
Received: November 27, 2014
ABSTRACT: Ab initio methods have been used to explore the unexpectedly strong magnetic anisotropy and the magnetostructural correlations in mononuclear eightcoordinate complex [CoII(12-crown-4)2]2+. Our calculations showed that both decreasing α and increasing φ may enhance its magnetic anisotropy, which was rationalized by the qualitative theory proposed by Long and co-workers. Moreover, we deduced that the |D| value of [CoII(12crown-4)2]2+ with α = 52° and φ = 43° is the largest one.
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© XXXX American Chemical Society
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DOI: 10.1021/ic502840s Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Table 1. Calculated D and E Values (cm−1) Using CASPT2 with the Variations of α and φ (deg) of Complex 1 Changed Using the Two Ways of I and II I α 48 50 52 54 56 58 60 62 64
D −106.3 −97.0 −87.6 −80.8 −70.1 −62.6 −56.3 −42.4 −30.5
It is obvious that the crystal field around CoII tends to be axially elongated with decreasing α (see Figure 4).
II E
φ
D
E
0.77 1.07 0.63 0.78 1.05 1.17 0.99 1.12 1.82
25 29 31 33 35 37 39 41 43 45
−6.1 −7.9 −10.9 −15.3 −31.5 −46.5 −70.1 −90.2 −122.4 −101.1
0.46 0.22 −0.05 0.18 0.83 1.10 1.05 0.51 −0.04 1.03
Figure 4. Scheme of the combined β orbitals of dz2 and dxy in the ground state for complex 1 with α = 48°, 56°, and 64°, respectively.
This change leads to the larger distribution of the 3d-electron cloud along the local main magnetic axis on CoII (see Figure S4 in the SI), which has the same direction with the S8 axis (see Figure 1a), with α ranging from 64° to 48° (see Figure 4). Thus, the magnetic anisotropy of 1 will become stronger (see Table 1) with a decrease of α according to Rinehart and Long’s view.10 From Table 1, an increase of φ largely enhanced the magnetic anisotropy of 1. When the twist angle φ was changed to 25°, the obtained D value was only −6.1 cm−1, which indicates that the magnetic anisotropy almost disappears. This phenomenon can also be rationalized using the above analysis. As Figure 5 indicates, the combined β orbitals of dz2 and dxy in the ground state for complex 1 with φ = 25° are almost spherical, which results in its small D value.
mechanism of the magnetostructural correlations, we give the calculated d-orbital energy levels of 1 with variation of the angle α ranging from 58° to 64° using density functional theory (see the SI for computational details; the energy levels with variation of φ are shown in Figure S2 in the SI) in Figure 2.
Figure 2. 3d energy levels of complex 1 with α ranging from 58° to 64°.
The dependence of the calculated d-orbital energy levels of 1 with α ranging from 48° to 54° is shown in Figure S3 in the SI. From Figures 2 and S3 in the SI, we observe that the larger the difference of the d-orbital energy levels, the larger the parameter | D|, which is contrary to the usual condition for those lowcoordination CoII-based SMMs.9 However, we could not observe the obvious relationship between the d-orbital energy level and the magnetic anisotropy from Figure S2 in the SI. However, we found that almost all of the doubly occupied orbitals are dz2 and dxy for models changed by the two ways. Thus, the 3d-shell electron distribution (clearly, we used the combined β orbitals of dz2 and dxy to represent the 3d-shell electron distribution) in the ground state for 1 tends to be approximately prolate (axially elongated), which is shown in Figure 3.
Figure 5. Scheme of the combined β orbitals of dz2 and dxy (dx2−y2 for φ = 43°) in the ground state for complex 1 with φ = 25°, 39°, and 43°, respectively.
When increasing φ, the shape of the combined orbitals of dz2 and dxy (dx2−y2 for φ = 43°) tends to be more prolate (see Figure 5). Especially for the case with φ = 43°, its combined orbital has been twisted much more. Accordingly, it is safe to conclude that when we increase the twist angle φ, the magnetic anisotropy will be enhanced based on the above analysis. Also, we believe that our conclusion may be extended to other SMMs having similar structures, such as Ln-based systems, which usually have much higher coordination numbers. If we twist the up and down ligands around Ln ions, the zero-field splitting of the Ln-based systems may also be enhanced. The above results show that not only decreasing α but also increasing φ may increase the |D| value of 1. Thus, to probe the possible structure with the largest |D|, we decrease α and increase φ by 0−4° at the same time (this change is indicated as III). The calculated results are shown in Table S1 in the SI. From Table S1, it is expected that the calculated |D| value of 1 with α = 52° and φ = 43° is the largest one. Here, we also give the scheme of the
Figure 3. Scheme of the combined β orbitals of dz2 and dxy representing the 3d-shell electron distribution for complex 1. B
DOI: 10.1021/ic502840s Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry combined β orbitals of dz2 and dxy or dx2−y2 in the ground state for complex 1 with α = 54° and φ = 41° and with α = 52° and φ = 43° in Figure S5 in the SI. From Figure S5 in the SI, these combined orbitals tend to be more twisted and also to be more elongated along the local main magnetic axis on CoII from left to right. Moreover, we calculated the D value of complex 1 with α = 50° and φ = 45°, which is only −95.1 cm−1. Thus, we consider that the |D| value of 1 with α = 52° and φ = 43° is the largest one for this kind of CoII-based complex.
K.; Zhang, H. J.; Chibotaru, L. F.; Powell, A. K. J. Am. Chem. Soc. 2011, 133, 11948−11951. (d) Lin, S. Y.; Wernsdorfer, W.; Ungur, L.; Powell, A. K.; Guo, Y. N.; Tang, J. K.; Zhao, L.; Chibotaru, L. F.; Zhang, H. J. Angew. Chem., Int. Ed. 2012, 51, 12767−12771. (e) Zhang, P.; Guo, Y. N.; Tang, J. K. Coord. Chem. Rev. 2013, 257, 1728−1763. (f) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Chem. Rev. 2013, 113, 5110− 5148. (g) Ungur, L.; Lin, S. Y.; Tang, J. K.; Chibotaru, L. F. Chem. Soc. Rev. 2014, 43, 6894−6905. (2) Ganivet, C. R.; Ballesteros, B.; de la Torre, G.; Clemente-Juan, J.; Coronado, M. E.; Torres, T. Chem.Eur. J. 2013, 19, 1457−1465. (3) (a) Zhu, Y. Y.; Cui, C.; Zhang, Y. Q.; Jia, J. H.; Guo, X.; Gao, C.; Qian, K.; Jiang, S. D.; Wang, B. W.; Wang, Z. M.; Gao, S. Chem. Sci. 2013, 4, 1802−1806. (b) Zadrozny, J. M.; Long, J. R. J. Am. Chem. Soc. 2011, 133, 20732−20734. (c) Vallejo, J.; Castro, I.; Ruiz-García, R.; Cano, J.; Julve, M.; Lloret, F.; Munno, G. D.; Wernsdorfer, W.; Pardo, E. J. Am. Chem. Soc. 2012, 134, 15704−15707. (4) Vaz, M. G. F.; Cassaro, R. A. A.; Akpinar, H. J.; Schlueter, A.; Lahti, P. M.; Novak, M. A. Chem.Eur. J. 2014, 20, 5460−5467. (5) Chen, L.; Wang, J.; Wei, J. M.; Wernsdorfer, W.; Chen, X. T.; Zhang, Y. Q.; Song, Y.; Xue, Z. L. J. Am. Chem. Soc. 2014, 136, 12213− 12216. (6) Sorace, L.; Benelli, C.; Gatteschi, D. Chem. Soc. Rev. 2011, 40, 3092−3104. (7) Zhang, P.; Zhang, L.; Wang, C.; Xue, S. F.; Lin, S. Y.; Tang, J. K. J. Am. Chem. Soc. 2014, 136, 4484−4487. (8) Karlstr€om, G.; Lindh, R.; Malmqvist, P.-Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P.-O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. Comput. Mater. Sci. 2003, 28, 222−239. (9) Habib, F.; Luca, O. R.; Vieru, V.; Shiddiq, M.; Korobkov, I.; Gorelsky, S. I.; Takase, M. K.; Chibotaru, L. F.; Hill, S.; Crabtree, R. H.; Murugesu, M. Angew. Chem., Int. Ed. 2013, 52, 11290−11293. (10) Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078−2085.
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CONCLUSIONS Ab initio methods have been performed to explore the origin of the magnetic anisotropy of complex 1 and the possible structure with the largest |D| value. Our analysis based on Rinehart and Long’s theory showed that the large twist angle φ far from zero may be responsible for the unexpected strong magnetic anisotropy of 1 because the combined β orbitals of dz2 and dxy were axially elongated along the local main magnetic axis on CoII under the twist. Moreover, we observed that decreasing α or increasing φ may increase the |D| value of 1, which was first rationalized by us using the qualitative theory. On the basis of the above results, we predicted that the possible structure having the largest |D| value is the one with α = 52° and φ = 43°. Furthermore, we suppose that if the doubly occupied orbitals of some similar CoII-based molecules are dz2 and dxz (or dyz) in the ground states, variation of α may have a much larger influence on the magnetic anisotropy because their 3d-electron clouds tend to be much more prolate. On the other hand, if the doubly occupied orbitals of the ground state of CoII-based SMMs are dx2−y2 and dxy, the shape of the 3d-electron cloud will be oblate (equatorially expanded), and we also believe that variation of α may have an important influence on their magnetic anisotropy. Given the results above, we conclude that changing α, φ, and the doubly occupied orbitals of the ground state may have an important role on the magnetic anisotropy of CoII-based SMMs having a structure similar to that of 1. Thus, for those highcoordination CoII-based SMMs and even Ln-based magnets, twisting, pulling, or hilling the up and down ligands around the magnetic ion will be very effective for obtaining higher energy barriers.
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ASSOCIATED CONTENT
S Supporting Information *
Computational details, 3d energy levels, and orientation of the local main magnetic axis on the CoII ion. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions for financial support. REFERENCES
(1) (a) Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078−2085. (b) Guo, Y. N.; Xu, G. F.; Gamez, P.; Zhao, L.; Lin, S. Y.; Deng, R. P.; Tang, J. K.; Zhang, H. J. J. Am. Chem. Soc. 2010, 132, 8538−8539. (c) Guo, Y. N.; Xu, G. F.; Wernsdorfer, W.; Ungur, L.; Guo, Y.; Tang, J. C
DOI: 10.1021/ic502840s Inorg. Chem. XXXX, XXX, XXX−XXX
