Crystal Structures and Magnetic Properties of Two New Cobalt (II ...

Hindawi Publishing Corporation International Journal of Inorganic Chemistry Volume 2013, Article ID 272156, 5 pages http://dx.doi.org/10.1155/2013/272156

Research Article Crystal Structures and Magnetic Properties of Two New Cobalt(II) Complexes with Triazole-Substituted Nitronyl and Imino Nitroxide Radicals Jing Chen,1,2 You-Juan Zhang,1 Kun-Tao Huang,2 Qiang Huang,2 and Jun-Jie Wang1 1 2

College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455002, China School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450052, China

Correspondence should be addressed to Jing Chen; [email protected] Received 23 March 2013; Revised 29 May 2013; Accepted 8 June 2013 Academic Editor: Maurizio Peruzzini Copyright © 2013 Jing Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Two new cobalt(II) complexes of formula [Co(hfac)2 (NITphtrz) 1 and Co(hfac)2 (IMphtrz) 2] have been prepared and characterized structurally [where NITphtrz = 4,4,5,5-tetramethyl-2-(2-phenyl-1,2,3-triazole-4-yl)imidazoline-1-oxyl-3-oxide and IMphtrz = 4,4,5,5-tetramethyl-2-(2-phenyl-1,2,3-triazole-4-yl)imidazoline-1-oxyl]. All of the complexes crystallize in an isomorphous triclinic space group 𝑃1 with the Co(II) ion octahedrally coordinated via the bidentate chelating mode. The magnetic measurements show that two complexes exhibit antiferromagnetic interactions between the metal ions and the nitroxide radicals.

1. Introduction Metal-radical complexes have been well investigated toward molecule-based magnets, where a radical center is directly bonded to the metal ion and affording appreciable magnetic exchange coupling [1]. The predominant magnetic features of metal-nitroxide complexes are determined by the donor properties of the nitronyl group and the coordination geometry of the resulting complex. In the past decades, growing interest has been paid to metal complexes with nitronyl nitroxide (NITR: 4,4,5,5-tetramethyl-2-R-imidazolin-1-oxyl3-oxide) and imino nitroxide (IMR: 4,4,5,5-tetramethyl-2R-imidazolin-1-oxyl) radicals [2–5]. The feature of R-group linked to the nitronyl nitroxide not only influences the intermolecular spin-spin interactions but also affects the coordination mode of the nitronyl nitroxides with metal ions [6, 7]. Because the auxiliary heteroaromatic N-donor forces the nitroxide O-atom to bind to a poor electrophilic metal center by the chelating effect, the complexes of nitronyl or imino nitroxides substituted by N-heteroaromatic groups have drawn great interests. Cobalt(II) complexes have been rarely investigated due to large spin-orbital coupling, although cobalt(II) is a very effective spin carrier. In order to extend our knowledge of the extremely rich chemistry of such

systems, it was thus of interest to further explore the reactions between Co(hfac)2 and nitroxide radicals. In this paper, we will report two new nitroxide ligands L1 and L2 (Scheme 1). Due to the presence of the N-triazole moieties, L1 and L2 can coordinate to metal ions as chelate ligands. Reactions of L1 and L2 with Co(hfac)2 (hfac = hexafluoroacetylacetonate) afforded two heterospin complexes. Crystal structures and magnetic properties of the related compounds (Co(hfac)2 (NITphtrz) 1 and Co(hfac)2 (IMphtrz) 2) will be described and discussed.

2. Experimental 2.1. Materials and Equipment. 2-phenyl-1,2,3-triazole-4-carboxaldehyde was prepared according to the literature method [8]. Co(hfac)2 ⋅2H2 O (hfac = hexafluoroacetylacetone) was obtained as of previously reported [9]. All the other chemicals purchased were of reagent grade and used without purification. Elemental analyses (C, H, and N) were carried out with a Perkin Elmer 240C elemental analyzer. IR spectra were recorded from 400 to 4000 cm−1 on an Avatar-360 spectrophotometer using KBr pellets. Variable-temperature magnetic susceptibilities were measured with an MPMS-7

2

International Journal of Inorganic Chemistry O−

N

N

+

N

N

N

N

N N

N

N

O∙

O∙ L2 = IMphtrz

L1 = NITphtrz

Scheme 1

SQUID magnetometer. Diamagnetic corrections were made with Pascal’s constants for all constituent atoms. 2.2. Synthesis of NITphtrz (L1) and IMphtrz (L2). NITphtrz (L1) was prepared as previously described [10]. Yield: 76%. Anal. Calcd. for C15 H18 N5 O2 : C, 60.00; H, 6.00; N, 23.33%. Found: C, 60.31; H, 6.12; N, 23.18%. Important IR absorptions (KBr, cm−1 ): 1368 (s, ]N–O ). IMphtrz (L2) was prepared according to [11]. Yield: 52%. Anal. Calcd. for C15 H18 N5 O: C, 63.38; H, 6.34; N, 24.65%. Found: C, 63.51; H, 6.21; N, 24.50%. Important IR absorptions (KBr, cm−1 ): 1360 (s, ]N–O ). Reactions of Co(hfac)2 ⋅2H2 O with L1 or L2 yielded complexes (Co(hfac)2 (NITphtrz) 1 and Co(hfac)2 (IMphtrz) 2), respectively. 2.3. Synthesis of Co(hfac)2 (NITphtrz) 1 and Co(hfac)2 (IMphtrz) 2 Co(hfac)2 (NITphtrz) 1. Co(hfac)2 ⋅2H2 O (0.1 mmol) was dissolved in boiling 𝑛-heptane (30 mL). The solution was left to boil for 30 min and then cooled down to 60∘ C, whereupon 0.1 mmol NITphtrz (L1) was added under stirring followed by the addition of 5 mL of CH2 Cl2 . The mixture was stirred for 40 min and then filtered. The deep brown filtrate was set aside at a refrigerator for two weeks. Crystals of complex 1 suitable for X-ray crystallographic analysis were obtained and filtered. Yield: 58%. Anal. Calcd. for C25 H22 F12 CoN5 O6 1: C, 38.73; H, 2.86; N, 9.03%. Found: C, 38.68; H, 2.83; N, 8.95%. Important IR absorptions (KBr, cm−1 ): 1370 (s, ]N–O ). Co(hfac)2 (IMphtrz) 2 was synthesized using the same procedure as that of complex 1, which started from IMphtrz and Co(hfac)2 ⋅2H2 O. Yield: 36%. Anal. Calcd. for C25 H22 F12 CoN5 O5 2: C, 39.55; H, 2.92; N, 9.22%. Found: C, 39.42; H, 2.83; N, 9.13%. Important IR absorptions (KBr, cm−1 ): 1345 (s, ]N–O ).

3. Results and Discussion 3.1. Descriptions of the Structure. For two title complexes Co(hfac)2 (NITphtrz) 1 and Co(hfac)2 (IMphtrz) 2, the corresponding crystallographic and refinement data are listed in Table 1. Selected bond distances and angles are given in Table 2.

Figure 1: ORTEP drawing of complex 1 showing the atom labelling scheme with 30% thermal ellipsoids, and H atoms are omitted for clarity.

ORTEP drawings of complex 1 and complex 2 are depicted in Figures 1 and 2, respectively. In complex 1, the Co(II) atom is six-coordinated in a distorted octahedron N1 O5 environment. The equatorial plane is formed by N(3) from NITphtrz and by O(3), O(5), and O(6) from two hfac ligands. The Co–O bond lengths ˚ in the basal plane are 2.031(2), 2.057(2), and 2.054(2) A, ˚ The axial respectively. The Co–N3 distance is 2.251(2) A. positions are occupied by two oxygen atoms from hfac and NITphtrz, respectively. The Co–O bond lengths are 2.025(3) ˚ for Co–O(NITphtrz) and Co–O(hfac) , respectively. and 2.030(3) A The O4–Co–O1 angle is 177.82∘ , and the N3–Co–O1 angle is 91.03∘ . The dihedral angle between the triazole rings from the phtrz part and the O–N–C–N moieties (O1, N1, C9, and N2) is 11.9∘ . The fragment O(1)–N(1)–C(9)–N(2)–O(2) forms a dihedral angle of 54.5∘ with the equatorial plane. In complex 2, the Co(II) ion is also in a distorted octahedron N2 O4 environment. The equatorial plane is formed by N(1) and N(5) from IMphtrz and by O(3) and O(4) from two hfac molecules. The Co–O bond lengths in the basal plane are ˚ respectively. The Co–N distances are 2.045(2) and 2.069(2) A, ˚ respectively. The axial positions are 2.287(2) and 2.088(2) A,

International Journal of Inorganic Chemistry

3

Table 1: Crystal data and structure refinement for complex 1 and complex 2.

Formula Formula weight Crystal system Space group ˚ ∘) Unit cell dimensions (A, a b c 𝛼 𝛽 𝛾 ˚ 3 ), Z Volume (A F(000) 𝜃 range for data collection (∘ ) Limiting indices Reflections collected Independent reflections Completeness (%) Goodness-of-fit on 𝐹2 Final R indices (I > 2 𝜎 (I)) R indices (all data)

Complex 1

Complex 2

C25 H22 F12 Co N5 O6 775.11 Triclinic 𝑃1

C25 H22 F12 Co N5 O5 759.106 Triclinic 𝑃1

10.208(2) 11.678(9) 14.680(3) 79.02(3) 76.53(3) 67.98(3) 1567.4(5), 2 1160 1.89 to 28.37 −13 ≤ ℎ ≤ 13 −15 ≤ 𝑘 ≤ 15 −19 ≤ 𝑙 ≤ 19 37545 7808 99.3 1.097 𝑅1 = 0.0622; 𝑤𝑅2 = 0.1864 𝑅1 = 0.0782; 𝑤𝑅2 = 0.2035

10.107(2) 10.983(2) 15.168(3) 86.19(3) 76.17(3) 77.53(3) 1596.2(5), 1 748 1.90 to 28.37 −13 ≤ ℎ ≤ 12 −14 ≤ 𝑘 ≤ 14 −20 ≤ 𝑙 ≤ 17 28519 7899 (𝑅int = 0.0223) 98.7 1.515 𝑅1 = 0.0621; 𝑤𝑅2 = 0.1973 𝑅1 = 0.0781; 𝑤𝑅2 = 0.2100

˚ and angles (∘ ) for complex 1 and complex 2. Table 2: Selected bond distances (A) Complex 1 Co(1)–O(1) Co(1)–O(4) Co(1)–O(3) N(1)–O(1) O(1)–Co(1)–O(6) O(6)–Co(1)–O(4) O(6)–Co(1)–O(3) Complex 2 Co(2)–O(5) Co(2)–O(2) Co(2)–N(1) N(2)–O(1) O(5)–Co(2)–O(3) O(4)–Co(2)–N(5) O(3)–Co(2)–O(4)

2.025(3) 2.030(3) 2.057(2) 1.289(3) 89.30(10) 91.26(10) 175.71(8)

Co(1)–O(6) Co(1)–O(5) Co(1)–N(3) N(2)–O(2) O(1)–Co(1)–O(4) O(1)–Co(1)–O(5) O(5)–Co(1)–O(3)

2.031(2) 2.054(2) 2.251(2) 1.265(4) 177.82(10) 86.09(11) 167.92(10)

2.028(2) 2.044(2) 2.088(2) 1.275(3) 89.69(9) 173.43(9) 87.28(11)

Co(2)–O(3) Co(2)–O(4) Co(2)–N(5)

2.045(2) 2.069(2) 2.287(2)

O(5)–Co(2)–O(2) O(3)–Co(2)–N(1) N(1)–Co(2)–N(5)

174.56(7) 170.37(9) 77.07(9)

occupied by two oxygen atoms from two hfac ligands. The ˚ respectively. Co–O bond lengths are 2.044(2) and 2.028(2) A, ∘ The O2–Co–O5 angle is 174.56 , and the N1–Co–N5 angle is 77.07∘ . The dihedral angle between the triazole rings for the phtrz part and the O–N–C–N moieties (O1, N2, C9, and N1)

is 1.4∘ . The fragment O(1)–N(2)–C(9)–N(1) forms a dihedral angle of 7.5∘ with the equatorial plane. 3.2. Descriptions of the Magnetic Properties. The temperature dependence of the magnetic susceptibility for complexes 1

4

International Journal of Inorganic Chemistry 0.008

𝜒M (cm3 mol−1 )

2.0

0.006 0.005

1.5

0.004

1.0

0.003 0.002

0.5

0.001 0.000

Figure 2: ORTEP drawing of complex 2 showing the atom labelling scheme with 30% thermal ellipsoids, and H atoms are omitted for clarity.

and 2 was investigated in the temperature range 1.8–300 K under a magnetic field of 2000 G. For both complexes, the variation curves of 𝜒𝑀 and 𝜒𝑀𝑇 versus 𝑇 are represented in Figures 3 and 4, respectively. For complex 1, the value of 𝜒𝑀𝑇 at 300 K is 2.59 cm3 ⋅K⋅mol−1 , lower than the spin-only value expected for the uncorrelated spin system with Co(II) (𝑆 = 3/2) ion and the nitronyl nitroxide unit (𝑆 = 1/2). The 𝜒𝑀𝑇 values decreased steadily with decreasing temperature to give a plateau around 150 K, where the 𝜒𝑀𝑇 value is about 2.11 cm3 ⋅K⋅mol−1 . This behavior can be accounted for by a strong intramolecular antiferromagnetic coupling between high-spin Co(II) and nitronyl nitroxide. Below 75 K, the 𝜒𝑀𝑇 value decreases markedly, coming close to zero at the lowest temperature. These results show that there exists strong antiferromagnetic coupling between the cobalt(II) ion and the directly coordinated nitroxide group. For complex 2, the value of 𝜒𝑀𝑇 at 300 K is 2.80 cm3 ⋅K⋅mol−1 , slightly lower than the spin-only value expected for the uncorrelated spin system with Co(II) (𝑆 = 3/2) ion and the nitronyl nitroxide unit (𝑆 = 1/2). The 𝜒𝑀𝑇 values decreased steadily with decreasing temperature to give a plateau around 50 K, where the 𝜒𝑀𝑇 value is about 2.32 cm3 ⋅K⋅mol−1 . This behavior can be accounted for by a strong intramolecular antiferromagnetic coupling between high-spin Co(II) and imino nitroxide. Below 30 K, the 𝜒𝑀𝑇 value decreases markedly, coming close to zero at the lowest temperature. These results show that there exists strong antiferromagnetic coupling between the cobalt(II) ion and the directly coordinated nitroxide group. The strict analysis of the magnetic data of Co(II) complexes needs to consider the effects of spin-orbit coupling and zero-field splitting. A more elaborate model, taking into account all of these factors, may be constructed, but it will be then overparameterized. The interaction between Co(II) ions (𝑆 = 3/2) and nitroxide ligand can be analyzed with (1) 2 ̂ ̂ = −2𝐽𝑆̂ derived from the Hamiltonian: 𝐻 Co 𝑆𝑅 − 𝐷𝑆𝑍 for the two spin units with 𝑆1 = 3/2 and 𝑆2 = 1/2, where 𝐽 refers

𝜒M T (cm3 mol−1 K)

2.5

0.007

0

50

100

150 200 T (K)

250

300

0.0

Figure 3: Plot of temperature dependence of 𝜒𝑀 𝑇 (I) and 𝜒𝑀 (◻) for complex 1.

to the magnetic exchange between Co(II) ions (𝑆 = 3/2) and the nitroxides and 𝐷 is the zero-field splitting parameter for Co(II) ions [12]. Condsider the following:

𝜒𝑀 =

2𝑁𝑔2 𝛽2 𝑘𝑇 ×

exp 𝐴 1 + 4 exp 𝐴 3 + exp 𝐴 5 , 2 exp 𝐴 1 + exp 𝐴 2 + 2 exp 𝐴 3 + exp 𝐴 4 + 2 exp 𝐴 5 1/2

𝐴1 =

[4𝐽 + 5𝐷/4 − (4𝐽2 − 2𝐷𝐽 + 𝐷2 ) 𝑘𝑇

𝐴2 =

(2𝐽 + 𝐷/4) , 𝑘𝑇

𝐴3 =

(6𝐽 + 9𝐷/4) , 𝑘𝑇

𝐴4 =

(6𝐽 + 𝐷/4) , 𝑘𝑇 1/2

𝐴5 =

]

[4𝐽 + 5𝐷/4 + (4𝐽2 − 2𝐷𝐽 + 𝐷2 ) 𝑘𝑇

]

,

. (1)

The best-fit parameters for complex 1 are 𝐽 = −158.18 cm−1 , 𝐷 = −3.56 cm−1 , and 𝑔 = 2.84 with 𝑅 = 1.74 × 10−4 , where 𝑅 is defined as 𝑅 = ∑ [(𝜒𝑀𝑇)calc − (𝜒𝑀𝑇)expt ]2 / ∑ [(𝜒𝑀𝑇)expt ]2 . The result (𝐽 = −158.18 cm−1 ) indicates that a strong antiferromagnetic interaction exists between Co(II) and NITphtrz via Co(II)⋅ ⋅ ⋅ O coordination bonding. For complex 2, the best-fit parameters are 𝐽 = −142.03 cm−1 , 𝐷 = −3.44 cm−1 , and 𝑔 = 2.92 with 𝑅 = 1.49 × 10−4 . The magnitudes of the exchange integrals of CoII -radical complexes are basically dependent on the extent of overlap of the SOMO 𝜋∗ orbital of the radical with the magnetic orbital of metal ions [13].

International Journal of Inorganic Chemistry

5 2.8 2.6

0.5

2.4

0.4

2.2 2.0

0.3

1.8

0.2

1.6

0.1

1.4

0.0

𝜒M T (cm3 mol−1 K)

𝜒M (cm3 mol−1 )

0.6

1.2 0

50

100

150

200

250

300

T (K)

Figure 4: Plot of temperature dependence of 𝜒𝑀 𝑇 (I) and 𝜒𝑀 (◻) for complex 2.

4. Conclusion Two chelate nitroxides ligands (L1 and L2) have been synthesized and characterized structurally. Reactions of L1 and L2 with Co(hfac)2 yielded two new cobalt(II) complexes. The temperature dependence of magnetic susceptibilities was studied and fitted according to suitable models. Strong antiferromagnetic couplings have been observed.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 21071006), the Natural Science Foundation of Henan Province (Grant no. 102102210457), and the Natural Science Foundation of the Henan Higher Education Institutions of China (Grant no. 2010B150001). Detailed crystallographic data in CIF format for the title complexes are available from Cambridge Crystallographic Data Center (CCDC IDs: 895284 (C25 H22 Co1 F12 N5 O5) and 895285 (C25 H22 Co1 F12 N5 O6)).

References [1] A. Caneschi, D. Gatteschi, R. Sessoli, and P. Rey, “Toward molecular magnets: the metal-radical approach,” Accounts of Chemical Research, vol. 22, no. 11, pp. 392–398, 1989. [2] F. M. Romero, D. Luneau, and R. Ziessel, “Structural control of ferromagnetic interactions in Nickel(Ii) complexes based on a tetradentate biradical,” Chemical Communications, vol. 5, pp. 551–552, 1998. [3] D. Luneau, G. Risoan, P. Rey et al., “Transition metal derivatives of a chelating nitronyl nitroxide ligand. Nickel(II) and manganese(II) complexes,” Inorganic Chemistry, vol. 32, no. 24, pp. 5616–5622, 1993. [4] D. Luneau, P. Rey, J. Laugier et al., “N-bonded copper(II)imino nitroxide complexes exhibiting large ferromagnetic interactions,” Journal of the American Chemical Society, vol. 113, no. 4, pp. 1245–1251, 1991. [5] D. Luneau, P. Rey, J. Laugier, E. Belorizky, and A. Cogne, “Ferromagnetic behavior of nickel(II)-imino nitroxide derivatives,” Inorganic Chemistry, vol. 31, no. 17, pp. 3578–3584, 1992.

[6] A. Caneschi, D. Gatteschi, and P. Rey, “The chemistry and magnetic properties of metal nitronyl nitroxide complexes,” Progress in Inorganic Chemistry, vol. 39, p. 331, 2007. [7] E. Coronado, P. Delha`es, D. Gatteschi, and J. S. Miller, Eds., Molecular Magnetism: from Molecular Assemblies to the Devices, NATO ASI Series E321, Kluwer Academic Publishers, Dodrecht, The Netherlands, 1996. [8] Q. Yan, L. Shao, F. M. Liu, and Z. F. Xie, “Synthesis of some new monocyclic 𝛽-lactams containing 2-Phenyl-1,2,3-triazolyl,” Chinese Journal of Organic Chemistry, vol. 251, p. 129, 2005. [9] F. A. Cotton and R. H. Holm, “Magnetic investigations of spin-free cobaltous complexes. III. On the existence of planar complexes,” Journal of the American Chemical Society, vol. 82, no. 12, pp. 2979–2983, 1960. [10] Y. J. Zhang, J. J. Wang, and J. Chen, “Three Nickel(II) and Zinc(II) complexes with two novel nitronyl nitroxide ligands: syntheses, crystal structures, and luminescent properties,” Zeitschrift f¨ur Anorganische und Allgemeine Chemie, vol. 638, no. 11, pp. 1849–1854, 2012. [11] J. N. Helbert, P. W. Kopf, E. H. Poindexter, and B. E. Wagner, “Complexing and protonation of free-radical imidazolin-1-oxyl and imidazolin-1-oxyl 3-oxide ligands: a magnetic-resonance investigation,” Journal of the Chemical Society, Dalton Transactions, no. 11, pp. 998–1006, 1975. [12] Y. X. Yu, H. M. Hu, D. Q. Zhang, Z. Y. Wang, and D. B. Zhu, “Two new nitronyl nitroxide radicals and their complexes with M(hfac)2 [M=Co(II), Ni(II), Mn(II)]: syntheses, crystal structures, and magnetic characterizations,” Chinese Journal of Chemistry, vol. 25, no. 9, pp. 1259–1266, 2007. [13] S. Fokin, V. Ovcharenko, G. Romanenko, and V. Ikorskii, “Problem of a wide variety of products in the Cu(hfac)2 nitroxide system,” Inorganic Chemistry, vol. 43, no. 3, pp. 969– 977, 2004.

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