Magneto-structural characterization of tetranuclear copper(II) complex [Cu4(pz)4L2]·(ClO4) (LH=1,3-diamino-2-propanol, Hpz=pyrazole)

doi:10.1016/S0277-5387(02)01027-6

Polyhedron

Volume 21, Issue 16, 1 July 2002, Pages 1539-1545

Dedicated to Professor Hartmut Fuess on the occasion of his 60th birthday

https://doi.org/10.1016/S0277-5387(02)01027-6 Get rights and content

Abstract

The title compound C₁₈H₃₀ClCu₄N₁₂O₆ has been synthesized and its crystal structure and magnetic properties determined. The compound contains a tetranuclear copper(II) complex in which two dimeric units are bridged by two pyrazolate ligands and a perchlorate anion which is bound to the tetrameric unit by hydrogen bonds in the asymmetric unit. In each dimeric moiety the two copper(II) ions are bridged by the alkoxo group of the ligand and an additional pyrazolate ligand, these dimers are then bridged by two pyrazolate ions to form the tetranuclear complex. Magnetic susceptibilities for the compound in the solid state are measured over the temperature range 4.5–306.8 K. The experimental magnetic susceptibilities were fitted to the Bleaney–Bowers equation for a spin-coupled S=1/2 system in which the singlet–triplet separation is 2J. The title compound shows weak antiferromagnetic coupling with a best fit J value −98.5 cm⁻¹. The alkoxo oxygen bridge is one of the principal pathways for the super-exchange interaction and the weak antiferromagnetic coupling of the complex is interpreted in terms of the countercomplementary effect of the different bridging ligands which participate in the super-exchange interactions. The ab initio restricted Hartree–Fock molecular orbital calculation has shown that the pyrazolate bridge contributes to magnetic interaction countercomplementarily to the alkoxide bridge.

The tetranuclear copper(II) complex [Cu₄(pz)₄L₂]·(ClO₄) has been synthesized and its crystal structure and magnetic properties determined. The title compound shows weak antiferromagnetic coupling with a best fit J value −98.5 cm⁻¹. Molecular orbital analysis of the compound has been studied by the ab initio restricted Hartree–Fock molecular orbital method to explain the main reason to the weak antiferromagnetic coupling.

Introduction

Polynuclear copper(II) complexes have been investigated actively in the last two decades in order to understand the relationship between the structure and the magnetic properties [1]. In addition, copper(II) complexes are of interest because of their relevance as active site structures of metalloproteins [2], [3].

Magnetic exchange interactions in dinuclear complexes have been investigated for a long time. The sign and the magnitude of the exchange coupling constant is influenced by bridging ligands between the metal ions depending on the various types of overlap interactions between the metal d-orbitals and the ligand orbitals. Magneto-structural correlations have been determined for certain types of binuclear copper(II) complexes [4], [5]. In recent years, the magnetic properties of several higher nuclearity metal complexes such as the tetranuclear complexes have been investigated and considerable insight has been obtained [6], [7], [8], [9], [10], [11]. Ten Hoedt [12] and Manzur [13] reported the structures and magnetic properties of the tetranuclear copper complexes [Cu₄(MPZ)₄(AcMPZ)₂(NO₃)₂] (MPZ=3(5)-methylpyrazolato; AcMPZ=1-(1-ethanoyl)-5-methylpyrazolato) and [Cu₄(pz)₄(CH₃OH)₂L₂″](ClO₄)₂ (Hpz=pyrazole; L″H=1,1-Di-(2-pyridyl)-1-methoximethanol), respectively, in which copper(II) centers are bridged by oxygen and the pyrazolate ligands. In both complexes, weak antiferromagnetic coupling between two copper(II) ions takes place through the oxygen and pyrazolate bridge. In Ten Hoedt's study the distortion of the geometry around the Cu atom from square planar toward tetrahedral was considered as the main reason for the small J value, and in Manzur's study the exchange mechanism was explained by the countercomplementary effect without molecular orbital analysis [13]. In this study, we have synthesized a similar tetranuclear copper(II) complex [Cu₄(pz)₄L₂]·(ClO₄) (LH=1,3-diamino-2-propanol, Hpz=pyrazole) and investigated its crystal structure and magnetic properties. Molecular orbital analysis of the compound have been done by the ab initio restricted Hartree–Fock molecular orbital method [14] to determine the influence of bridging ligands between the metal ions on the magnetic super-exchange interaction and to explain the main reason of the weak antiferromagnetic coupling.

Section snippets

Synthesis of [Cu₄(pz)₄L₂]·(ClO₄)

The copper(II) complex was synthesized by mixing Cu(ClO₄)₂·6H₂O (0.741 g, 2 mmol) with pyrazole (0.136 g, 2 mmol) in methanol at room temperature (r.t.), followed by the addition of 1,3-diamino-2-propanol (0.090 g, 1 mmol) and triethylamine (0.304 g, 3 mmol) in a methanol-water mixture (5:1, 50 ml). The product is well formed dark blue crystals (yield 0.11 g ca. 49%). Anal. Found: C, 23.9; H, 3.5; N, 18.7. Calc. for C₁₈H₃₀ClCu₄N₁₂O₆: C, 24.0; H, 3.4; N, 18.8%. Absorption spectra [λ_max (nm) (ε,

X-ray crystal structure

A perspective drawing of the title compound is shown in Fig. 1 [19] and consists of a tetranuclear copper(II) complex and perchlorate anion which is bound to the tetrameric unit by hydrogen bonds. Selected bond lengths and angles are summarized in Table 1. In the solid state, the title compound resides on an inversion center so that only half of the molecule is crystallographically unique. The structure contains a pair of dimeric units. The two copper(II) centers are bridged by the alkoxide

Concluding remarks

The title compound can be considered as being composed of two weakly associated pairs of dimers. The magnetic exchange interaction occurs more possibly via the alkoxide oxygen and pyrazolate bridge. The observed relatively weak antiferromagnetic exchange within the tetranuclear compound is explained by the countercomplementary behavior of the bridging pyrazolate ion which participates in the super-exchange interaction with alkoxide oxygen by means of ab initio restricted Hartree–Fock molecular

Supplementary material

Further details of the structure determination, including atomic coordinates, bond lengths and angles, thermal parameters of all non-hydrogen atoms for the structure are available on request as supplementary material from The Director, Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail: [email protected] or www: http//www.ccdc.cam.ac.uk) (CCDC deposition number, 162126).

Acknowledgements

We thank Professor Dr. W. Haase for giving us the opportunity to do of magnetic susceptibility measurement. This work was supported by the Research Fund of the University of Ankara under grant number 98-25-00-03.

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Magneto-structural characterization of tetranuclear copper(II) complex [Cu4(pz)4L2]·(ClO4) (LH=1,3-diamino-2-propanol, Hpz=pyrazole)

Abstract

Introduction

Section snippets

Synthesis of [Cu4(pz)4L2]·(ClO4)

X-ray crystal structure

Concluding remarks

Supplementary material

Acknowledgements

Polyhedron

Inorg. Chim. Acta

Polyhedron

Molecular Magnetism

Prog. Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Inorg. Chem

shelxs-97, Program for the Solution of Crystal Structures

shelxl-97, Program for the Refinement of Crystal Structures

Inorg. Chem.

Magneto-structural characterization of tetranuclear copper(II) complex [Cu₄(pz)₄L₂]·(ClO₄) (LH=1,3-diamino-2-propanol, Hpz=pyrazole)

Synthesis of [Cu₄(pz)₄L₂]·(ClO₄)