Synthesis and magnetic properties of mixed crystals of paramagnetic transition metal thiocyanato coordination polymers.

• Applicant: Professor Dr. Christian Näther
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 290067379

Recently, the development of strategies for a more rational synthesis of new coordination compounds with defined magnetic properties has become of increasing interest. Most compounds consists of only one kind of metal cation but there is also increasing interest for the preparation of bimetallic compounds. This can be achieved, e.g., by the reaction of anionic metal complexes with different metal cations. In contrast, the targeted synthesis of solid solutions with the major goal to modify the magnetic properties of coordination compounds in more detail was rarely used and this is exactly the content of the present project. Mixed crystals of compositions [M1xM21-x(NCS)2(L1)2]n and [M1xM21-x(NCS)2(L2)]n (M1, M2 = Mn(II), Fe(II), Co(II) and Ni(II); L1 = monodentate N-donor ligand; L2 = bridging N-donor) should be prepared and investigated for their composition, their homogeneity and their magnetic properties. Preliminary investigations have shown, that from solution no homogenous samples of the desired compounds can be obtained. Therefore, an alternative synthetic route was used, which was never used before for the synthesis of mixed crystals of coordination compounds. This strategy is based on the synthesis of precursors of composition [M1M21-x(NCS)2(L1)4]n and [M1xM21-x(NCS)2(L2)2]n, which finally are transformed into the desired compound by thermal decomposition. First results shows, that with this method homogenous mixed crystals consisting of Co and Ni can be prepared, that exhibits a composition, which exactly corresponds to that, used in the synthesis. On heating they transform quantitatively into the desired compounds. As shown by magnetic measurements, on thermal decomposition no phase separation take place, because these samples shows a magnetic behavior, which is completely different from that of the homometallic compounds. In this context it is noted that [Co(NCS)2(pyridine)2]n exhibits a ferromagnetic ground state and shows a slow relaxation of the magnetization, whereas [Ni(NCS)2(pyridine)2]n exhibits an antiferromagnetic ground state and shows a metamagnetic transition. In contrast, mixed crystals of composition [Co0.5Ni0.5(NCS)2(Pyridin)2]n shows a metamagnetic transition, which is typical for the pure Ni compound but in the antiferromagnetic phase slow relaxations are observed, which are typical for the pure Co compound. In the course of this project, it should be investigated if this synthetic tool can be used in general for the synthesis of mixed crystals of this class of compounds and which changes in the magnetic properties occur on mixed crystal formation.

• DFG Programme: Research Grants