Research paperMagnetic properties of the anion-radical salts [M(bipy)3](TCNQ)4·X, where M = Co, Ni, Zn and X = H2O or (CH3)2CO
Graphical abstract
Stacking of TCNQ molecules in anion-radical salt [Co(bipy)3](TCNQ)4·H2O.
Introduction
Charge-transfer compounds have a rich history dating back to the early 1970s, and presently, they are receiving renewed attention due to their emerging role in applications [1], [2]. For example, 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules are one example of organic anion-radical salts (ARS) that possess a wide range of magnetic, electrical, and optical properties. The additional incorporation of metal ions in ARS compounds leads to materials combining the properties of organic and inorganic components for use in various applications, including nanoelectronics [2] and quantum-computing [3], [4]. More specifically, ARS complexes based on TCNQ can form low-dimensional magnets, and many of them form dimerized magnetic systems with alternating exchange interactions. It is particularly noteworthy that small modifications of the crystal structure have considerable impact on their charge transfer and magnetic properties [5], [6], [7], [8]. This intimate “form-function” correlation between crystal structure and exchange interaction between anion radicals and transition metal ions have been investigated in ARS based on TCNQ containing transition metal ions [9], [10]. Recently, ARS [Mn(phen)3](TCNQ)2·H2O and [Co(phen)3](TCNQ)2·H2O were studied and a weak interaction was found between the TCNQ anion-radicals and transition metal ions [11].
The present work was initiated to explore the magnetic and thermodynamic properties of [Ni(bipy)3](TCNQ)4·(CH3)2CO, whose structural properties were already reported [12]. In order to better understand the magnetic response arising from the Ni2+ ion, [Zn(bipy)3](TCNQ)4·H2O [12] was magnetically characterized. In addition, in an attempt to expand the transition metal ions being incorporated in these types of complexes, [Co(bipy)3](TCNQ)4·H2O was synthesized for the first time, and its structural and magnetic properties are reported herein. The magnetic characterization of each complex involved temperature-dependent and isothermal magnetic-field dependent magnetometry and temperature-dependent X-band (9.4 GHz) electron paramagnetic resonance (EPR). The thermodynamic response of [Ni(bipy)3](TCNQ)4·(CH3)2CO was also studied by specific heat techniques. Briefly stated, the magnetic response of [Co(bipy)3](TCNQ)4·H2O is characterized by the presence of Co2+ ions in a spin-admixed state, while the magnetic response of [Ni(bipy)3](TCNQ)4·(CH3)2CO is dominated by the contribution of single-ion anisotropy of Ni2+ ions at low-temperatures.
Section snippets
Experimental methods
The synthesis [Zn(bipy)3](TCNQ)4·H2O (1) and [Ni(bipy)3](TCNQ)4·(CH3)2CO (2) followed the previously published protocols [12], [13]. In fact, the samples used in this work are from the same batches as the ones used previously [12], [13]. Whereas [Co(bipy)3](TCNQ)4·H2O (3) was synthesized in the manner used for (1) [12], with the only exception being the transition metal starting materials.
X-ray diffraction studies were performed using an automatic “Xcalibur 3” diffractometer (graphite
Synthesis of [Co(bipy)3](TCNQ)4·H2O
Hot solution of [Co(bipy)3]I2 (1.5 g, 1.92 mmol) in 30 ml of acetone was added in one portion to hot solution of TCNQ in 50 ml of acetone (1.05 g, 5.12 mmol). The black solution obtained was left for 3 days in dark place. Black crystals precipitated were collected, washed with ether and dried. This gave the title compound in 55% yield (0.95 g). Elemental analysis: calc. C – 69.38%, N – 21.98%, H – 3.31%; found C – 68.70%, N – 21.45%, H – 3.35%.
The crystal structure
The crystal and molecular structures of (1) and (2)
Conclusion
In this paper, the results of experimental investigation of ARS [Zn(bipy)3](TCNQ)4·H2O, [Ni(bipy)3](TCNQ)4·(CH3)2CO and [Co(bipy)3](TCNQ)4·H2O by the measurement of the temperature dependence of magnetic susceptibility, the field dependence of magnetization and EPR spectra are presented. All studied ARS are isostructural and contain structural TCNQ tetramers with S = 0 ground state (AFM spin dimers with S = 1/2). The contribution of uncoupled spins only is observed in magnetic properties in the
Acknowledgments
This work was supported by the Slovak Research and Development Agency under contract No. APVV-14-0073 and APVV-0132-11, by ERDF EU projects under the contract Nos. ITMS26220120005, ITMS26220120047, and ITMS26220220186. We thank J. Kuchár for single crystal orientation using X-ray diffraction and prof. M.W. Meisel for the discussion and comments that improved the manuscript. A.V. Fedorchenko thanks P.J. Šafárik University for the opportunity to carry out research.
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