J. Chil. Chem. Soc., 51, Nº 1 (2006)

MAGNETO-STRUCTURAL STUDIES OF A COPPER (II)-ALKYLPYRAZOLONE ENAMINE COMPLEX

P. HERMOSILLA¹, Y. MORENO¹, A. BULJAN¹, J. BELMAR¹, F. BROVELLI², O. PEÑA³, R. BAGGIO⁴

1. Universidad de Concepción, Concepción, Chile; 2. Universidad del Bío-Bío, Concepción, Chile; 3. Université de Rennes-1, Francia; 4. Comisión Nacional de Energía Atómica, Buenos Aires, Argentina.

E-mail: ymoreno@udec.cl

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ABSTRACT

The magnetic behaviour for copper-pirazolone complex, bis{1-n-hexyl-3-methyl-4-[1-(phenylimino)propyl]-1 H-pyrazol-5-olato}-copper(II), [(C₁₉H₂₆ON₃)₂ Cu], was studied. This compound is paramagnetic, with uncorrelated copper(II) ions having one unpaired electron in a square structure. No magnetic interactions were observed between copper centres situated at distances larger than 8 Å.

Keywords: paramagnetism, alkylpirazolone, copper complex

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INTRODUCTION

The physical properties of the molecular systems, in particular magnetism, strongly depend on the structural parameters, for example, through the distances between metallic centres or through the angle values between molecular groups (i.e. dihedral planes)^.[1] In many cases the magnetic phenomena are closely related to symmetry of the molecular fragment.^[1-5]. In order to better understand the interconnections between structural and magnetic properties it becomes necessary to develop flexible molecular systems with real possibilities to reach any favourable geometrical configuration. Several research groups have been working in this direction to obtain templates of organic molecules.^[6] In this sense, pirazolones are very interesting systems. They constitute an important family of organic compounds, widely studied because of theoretical and practical reasons,^[7-10] in particular their numerous applications in the fields of kinetic chemistry, ionic extraction, pharmaceutical, etc.,^[11]. Pirazolones are very flexible systems, whose structure can be easily modulated in order to establish magneto-structural dependencies.

Most pyrazolone derivatives so far reported bear an aryl ring at position 1, which usually diminishes their solubility. Schiff bases and enamines derived from pyrazolones are known but similar solubility problems may be expected. Therefore, their applications might be limited. To overcome this problem, convenient syntheses of 1-n-alkyl derivatives of pyrazolones were reported a few years ago^[12] Most of the obtained products exhibited good solubility in regular solvents, (i.e., acetone, ethanol, chloroform, ethyl acetate, ethyl ether, THF, toluene), although their crystallization may become a lengthy procedure in some cases.

In this work we report the magnetic properties of a copper-pirazolone complex, bis{1-n-hexyl-3-methyl-4-[1-(phenylimino)propyl]-1 H-pyrazol-5-olato}-copper(II),[(C₁₉H₂₆ON ₃)₂ Cu], and discuss the magneto-structural relationship. The copper(II) ion has one unpaired electron in an square structure ; therefore it is a paramagnetic ion but it may undergo the influence of other copper(II) ions in the vicinity.

II EXPERIMENTAL

Copper complex synthesis

The synthesis was done with 0.1 g (0.32mmol) of the protonated ligand, 1-n-hexyl-3-methyl-4-[1-(phenylimino) propyl]-1H-pyrazol-5(4H)-one, HL (C₁₉H₂₇N₃O), and Cu(O₂CCH₃)₂.H ₂O; they were dissolved in 10 ml of ethanol and heated to reflux during 1 h. The solution was then evaporated to a final volume of 5.0 ml and allowed to cool down to room temperature. The solid was filtered and crystallized from a chloroform-hexane mixture. Yield 0.08 g (73%). M.P. : 400 (2)K. Anal. Calc. for C₁₉H₂₇N₃O (HL): C, 72.81; H, 8.68; N, 13.41%. Found: C, 72.70; H, 8.70; N, 13.10%.

Structural study

This compound is monomeric and consists of a tetracoordinated copper(II) atom lying on an inversion centre, coordinated to two chelating (symmetry related) bidentate L ligand (see Figure 1).^[13] The resulting CuO₂N₂ core is perfectly planar, due to the restraints imposed by symmetry. The distances between Cu-N = 2.036 (3) Å, Cu-O = 1.879 (21) Å. The distance between the copper centres is larger than 8 Å (see Figure 2)

Magnetic study

The magnetic susceptibility (Quantum Design MPMS XL5 susceptometer) was measured at an applied field of 5 kOe (0.5 Tesla), between 2 K and 300 K, under warming and cooling conditions. Powder samples were placed and compacted inside a gelatine capsule. The temperature independent magnetic contribution due to the sample holder was subtracted at all temperatures. The diamagnetic contribution due to the core electrons was estimated using Pascal constants and subtracted from the experimental values. The magnetization as a function of the applied field was measured at 2 K, with increasing and decreasing fields, between 0 and 50 kOe.

In order to clarify the magnetic properties at low temperature, we measured the magnetization versus applied field at T = 2K. The insert of figure 3 shows a reversible behaviour and an approximate value for the magnetic moment gS, at 50 kOe, of 1 BM, in agreement with the presence of one unpaired electron. The reversibility of M(H) confirms the paramagnetic nature of copper(II), with no magnetic interactions between copper centres (distances larger than 8 Å, without connectivity).

Figure 1. Molecular structure of the copper complex

Figure 2. Packing of units. Units are only schematized.

DISCUSSION

This complex, [(C₁₉H₂₆ON₃)₂ Cu], has a paramagnetic behaviour, as evidenced by magnetic susceptibility measurements. Figure 3 shows the inverse of the magnetic susceptibility versus temperature for this complex. Full reversibility of the susceptibility with increasing and decreasing temperatures confirmed the paramagnetic character of the independent copper(II) ions. The effective magnetic moment was calculated as usual, from the slope of the c^-1(T) variation, and it [BM]. The intercept of c^-1 with the temperature axis gives a value for the Weiss parameter Q of approximately 0.5 K (± 0.5 K).

Figure 3. Magnetic susceptibility vs. temperature for complex [(C₁₉H₂₆ON₃)₂ Cu]. The insert shows the magnetization at 2 K

CONCLUSION

This functionalised pyrazole is a good ligand to coordinate copper(II) ions. The distance between metallic centres is large, precluding any magnetic interaction between them. The system is a simple paramagnet, its magnetic moment being 1.77 [BM].

ACKNOWLEDGEMENTS

The authors thank Fondecyt (Grant No 1040461) and ECOS-CONICYT (Grant NºE05E01) for financial support. The authors are greatly indebted to the Spanish Research Council (CSIC) for provision of a free-of-charge license for the CSD and ConQuest and IsoStar software.

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