Elsevier

Polyhedron

Volume 170, 15 September 2019, Pages 51-59
Polyhedron

Synthesis, crystal structure and magnetic properties of the new Cu(II)/Mn(II) coordination polymer [{Cu(cyclam)MnCl3(H2O)2}Cl]n

https://doi.org/10.1016/j.poly.2019.05.024Get rights and content

Abstract

The synthesis, characterization and crystal structures of the copper complex [Cu(cyclam)Cl2]·H2O (1) and bimetallic compound [{Cu(cyclam)MnCl3(H2O)2}Cl]n (2), where cyclam = 1,4,8,11-tetraazacyclotetradecane, are reported. The crystal structure of 1 consists of complexes where Cu(II) is hexacoordinated and has elongated tetragonal-bipyramidal coordination, axial positions are occupied by the oxygen atoms from coordinated water molecules and the basal ones by the nitrogen atoms from the macrocyclic ligand. The reaction of 1 with manganese(II) chloride gives compound 2 with an ionic structure consisting of polymeric cation where Mn(II) and Cu(II) ions are bridged by chlorido ligands. Both central atoms are hexacoordinated and have distorted octahedral coordination environment. In the crystal structure, the formation of two types of chains is observed, one of them contains Cu(II) and Mn(II) ions arranged in alternating manner and perpendicular to them another type of chains consisting of chlorido-bridged Mn(II) atoms is formed. The magnetic properties are characterized by the dominant antiferromagnetic interaction forming Mn(II) chains along c-axis with JMn/kB = −3.38 K. A long-range magnetic order is formed at 4.23 K in zero magnetic field through the interaction with the Cu(II) subsystem. The analysis of the Arrott plots and critical exponent n describing the field dependence of the entropy change during adiabatic magnetization of the system revealed that the application of a weak magnetic field changes the order of the phase transition from second to the first order.

Graphical abstract

Cu(II)/Mn(II) bimetallic compound with the interesting 3D cation in the structure prepared using two different synthetic routes and characterized by elemental analysis, spectroscopy and single crystal X-ray diffraction. Measurements of magnetic properties revealed a formation of antiferromagnetic Mn(II) chains interconnected by Cu(II) ions with long-range magnetic order below 4.23 K.

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Introduction

In recent years, coordination polymers have been widely prepared and studied because of their interesting properties and their applicability in many areas, such as crystal engineering, material chemistry and also practical applications like gas sorption and catalysis [1], [2]. One broad field of research is the study of magnetic properties in three-dimensional (3D) systems. Due to the diversity of applicable metal ions, ligands and coordination geometry, a great number of coordination polymers with various structures and magnetic properties have been synthesized and characterized [1], [3], [4].

Interesting properties can be achieved in heterometallic complexes where metal centres have various spin value, as can be demonstrated by Cu(II)/Mn(II) bimetallic complexes. Several compounds exhibiting ferrimagnetic behavior have been prepared, where, from a structural point of view, Cu(II) and Mn(II) atoms are located within the chains in an alternating manner [5], [6], [7]. Such alternating structural motif seems to be convenient by preparation of complexes with a three-dimensional structure, where the structure is consisting of connected alternating bimetallic chains. This could be a useful strategy for the preparation of molecular magnets with higher Tc, what is one of the main goals in the field of molecular materials. It was predicted that the possible way to increase Tc in prepared molecular magnets could be increasing of dimensionality in prepared complexes [8].

Moreover, the synthesis of heterobimetallic complexes with three-dimensional structure is a remaining challenge for chemists, because of the difficult synthetic strategies and a large number of potential products. Widely used synthetic strategies are a one-pot synthesis, where ligand design plays a crucial role and building-block synthesis, where metal complexes are used as starting materials for the synthesis of heterometallic complexes. Here we present the preparation and characterization of Cu(II) complex [Cu(cyclam)Cl2]·H2O (1) and bimetallic Cu(II)/Mn(II) complex [{Cu(cyclam)MnCl3(H2O)2}Cl]n (2) prepared using 1 as precursor, and also successful preparation of 2 by one-pot synthesis. In the crystal structure of 2 alternating chains of Cu(II) and Mn(II) atoms are present and the connection between the chains are mediated by [MnCl4(H2O)2]-2 particle. This leads to the formation of homometallic chains perpendicular to the bimetallic ones and a polymeric cation with interesting structure is obtained.

Despite the complicated 3D structure a dominant antiferromagnetic (AFM) interaction was identified forming Mn(II) chains along c-axis, JMn/kB = −3.38 K. The presence of the λ-anomaly in the specific heat confirmed the presence of a long-range magnetic order 4.23 K in zero magnetic field through the interaction of Mn(II) chains with the Cu(II) subsystem. The analysis of the magnetocaloric effect (MCE) confirmed the presence of the AFM interaction. This is evidenced by the suppression of the maximum value of entropy change during the adiabatic magnetization from 0 to 5 T to 2.04 J·K−1·mol−1, which is only 1/10 of the possible entropy change for paramagnetic system. The analysis of the Arrott plots and critical exponent n describing the field dependence of the entropy change during adiabatic magnetization of the system revealed the change of the order of the phase transition with the application of magnetic field.

Section snippets

General

All reagents were purchased from commercial sources and used as received. Infrared spectra were recorded on a Nicolet 6700 FT-IR spectrometer using KBr pellets technique in the range of 4000–400 cm−1. Electronic absorption spectra were recorded on an AnalyticJena Specord 250 spectrometer in nujol suspension in the range of 400–800 nm. Elemental analyses (C, H, N) were performed on a CHNS Elementar Analyzer VarioMICRO by Elementar Analysensysteme GmbH.

Synthesis of [{Cu(cyclam)MnCl3(H2O)2}Cl]n using one-pot procedure (2)

A solution of cyclam in methanol (0.8 g,

Synthesis and characterization

The synthetic strategy for preparing bimetallic complexes with composition {Cu(L)nMnCl4}n was used according to the literature [14]. For the synthesis of 2 two slightly different modification was used. In the first case, 2 was prepared by one-pot synthesis where ethanolic solution of cyclam was added to CuCl2 dissolved in the same solvent and to resulting solution MnCl2 dissolved in ethanol was directly added. The second way can be considered as a synthesis of building blocks. In the first

Conclusions

The article presents the preparation, characterization by elemental analysis and spectroscopic methods and also magnetic properties of Cu(II) complex [Cu(cyclam)Cl2]∙H2O (1) and new bimetallic Cu(II)/Mn(II) polymer [{Cu(cyclam)MnCl3(H2O)2}Cl]n (2). Structure of both complexes was determined using single crystal X-ray diffraction analysis. The compound 2 was prepared using two synthetic strategies: one-pot synthesis and so-called building block synthesis. The second mentioned procedure consists

Acknowledgements

This work was supported by the Slovak Research and Development Agency [contract No. APVV-14-0073 and APVV-18-0016], Scientific Grant Agency of Ministry of Education, Science, Research and Sport of the Slovak Republic [contract No. VEGA 1/0426/19 and VEGA 1/0063/17] and internal grant of UPJŠ [vvgs-2017-652]. The crystallographic part was supported by the project of the Czech Science Foundation [project 18-10504S] using instruments of the ASTRA laboratory established within the Operation program

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