A low-dimensional molecular spin system with two steps of magnetic transitions and liquid crystal property

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Science, Nanjing University of Technology, Nanjing, 210009, PR China.
Dalton Transactions (Impact Factor: 4.2). 03/2011; 40(14):3622-30. DOI: 10.1039/c0dt01704c
Source: PubMed


A low-dimensional compound [C(6)-Apy][Ni(mnt)(2)] (1, where mnt(2-) = maleonitriledithiolate, C(6)-Apy(+) = 4-amino-1-hexylpyridinium) has been designed and synthesized, which has layer arrangement of anions and cations and shows two steps of magnetic transitions. The low temperature magnetic transition has an uncommon hysteresis loop, while the crystal structure investigations disclosed no structural transition with the magnetic transition. The high temperature magnetic transition exhibits two remarkable features: (1) it synchronously occurs with a crystalline-to-mesophase transition in the first heating process and (2) the structural changes that accompany the solid-mesophase transition are irreversible. A diamagnetic and isostructural compound, [C(6)-Apy][Cu(mnt)(2)], is further characterized by structure, DSC and POM techniques, which revealed also the existence of an irreversible crystalline-to-mesophase transition in the same temperature interval of [C(6)-Apy][Ni(mnt)(2)]. Therefore, the high-temperature magnetic transition in 1 is driven by release of the structural strains, but not magnetoelastic interactions. The mesophase exhibits the characteristic of smectic A phase, and the alkyl chain melting in the cation layers probably lead to the formation of mesophase. It is noticeable that the finding of a mesophase occurring in a hexyl hydrocarbon chain molecular system is in contrast to a suggested rule that at least a dodecyl chain is required. Our results will shed a light on the design and preparation of a new low-dimensional molecular system combining magnetic transition and liquid crystal properties.

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    ABSTRACT: Two metal-dithiolene compounds with a formula of [Cn-py][Ni(mnt)2] (Cn-py+ represents the 1-alkyl-pyridinium cation, n = 8 (denoted as compound 1), and 9 (denoted as compound 2); mnt2− = maleonitriledithiolate) were synthesized and characterized. The crystal structures determined for two compounds disclosed that these are isostructural with similar cell parameters and packing structures. The cation adopts a bent conformation and the direction of the hydrocarbon chain is approximately parallel to the long molecular axis of the [Ni(mnt)2]− anions, and the [Ni(mnt)2]− anions and cations of 1 and 2 form segregated irrequidistant columns stacks along the crystallographic c-axis direction. It is worth noting that there existed charge assisted C–H···N interactions between the adjacent anion and cation stacks in crystal of 1 and 2. DSC measurement of 1 and 2 exhibited two exothermic peaks in the first and second cooling process, and showed supercooling behavior and the thermal hysteresis of 18 and 50 K were observed for 1 and 2, respectively. Furthermore, 1 exhibits cold crystallization around 342 K during the second heating scan. Compound 1 shows the magnetic characteristics of a low-dimension antiferromagnetic coupling spin system in the high temperature and spin gap in the low-temperature phase, and compound 2 exhibits weak paramagnetism over the temperature range of 2–400 K. Graphical Abstract Compound 1 shows the magnetic characteristics of spin gap in the low-temperature phase and 2 exhibits weak paramagnetism over the temperature range of 2–400 K.
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    ABSTRACT: The asymmetric unit of the title complex, (C9H15N2)2[Ni(C4N2S2)2], comprises one 4-dimethyl­amino-1-ethyl­pyri­din­ium cation and one half of a [Ni(mnt)2]2− (mnt2− = maleo­nitrile­dithiol­ate) anion; the complete anion is generated by the application of a centre of inversion. The NiII ion is coordinated by four S atoms of two mnt2− ligands and exhibits a square-planar coordination geometry.
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