Crystal Structures and Magnetic Properties of Two New Cobalt(II) Complexes with Triazole-Substituted Nitronyl and Imino Nitroxide Radicals

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.


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-oxyl-3-oxide) and imino nitroxide (IMR: 4,4,5,5-tetramethyl-2-R-imidazolin-1-oxyl) radicals [2][3][4][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.

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 ⋅2H 2 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 International Journal of Inorganic Chemistry

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. 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 N   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   and    versus  are represented in Figures 3 and 4, respectively.
For complex 1, the value of    at 300 K is 2.59 cm 3 ⋅K⋅mol −1 , lower than the spin-only value expected for the uncorrelated spin system with Co(II) ( = 3/2) ion and the nitronyl nitroxide unit ( = 1/2).The    values decreased steadily with decreasing temperature to give a plateau around 150 K, where the    value is about 2.11 cm 3 ⋅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    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    at 300 K is 2.80 cm 3 ⋅K⋅mol −1 , slightly lower than the spin-only value expected for the uncorrelated spin system with Co(II) ( = 3/2) ion and the nitronyl nitroxide unit ( = 1/2).The    values decreased steadily with decreasing temperature to give a plateau around 50 K, where the    value is about 2.32 cm 3 ⋅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    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 ( = 3/2) and nitroxide ligand can be analyzed with (1) derived from the Hamiltonian: Ĥ = −2 ŜCo Ŝ −   2 for the two spin units with 1 = 3/2 and 2 = 1/2, where  refers to the magnetic exchange between Co(II) ions ( = 3/2) and the nitroxides and  is the zero-field splitting parameter for Co(II) ions [12].Condsider the following: The best-fit parameters for complex 1 are  = −158.18cm −1 ,  = −3.56cm −1 , and  = 2.84 with  = 1.74 × 10 −4 , where  is defined as  = ∑ [(  ) calc − (  ) expt ] 2 / ∑ [(  ) expt ] 2 .The result ( = −158.18cm −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  = −142.03cm −1 ,  = −3.44 cm −1 , and  = 2.92 with  = 1.49× 10 −4 .The magnitudes of the exchange integrals of Co II -radical complexes are basically dependent on the extent of overlap of the SOMO  * orbital of the radical with the magnetic orbital of metal ions [13].

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.

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

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

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