First step in preparation of multifunctional spin crossover material based on Fe(II) complex of cyclam-based ligand. Magnetism and DFT studies

A multifunctional magnetic material based on a solid solution of Fe(ii)/Co(ii) complexes with a macrocyclic cyclam-based ligand

In order to prepare a multifunctional magnetic material combining spin crossover together with single-molecular magnetism, co-crystallization of Fe(ii) and Co(ii) complexes of the pyridine derivative of cyclam (Py₂-C = 1,8-bis(pyridin-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane) was performed. Complexes with the general formula [M^II(Py₂-C)](ClO₄)₂·H₂O (M^II = Fe (1), Co (2) or Fe_0.4Co_0.6 (3)) were prepared and thoroughly characterized. Based on X-ray molecular structures, they formed octahedral complexes with cis-arrangement of the coordinated pyridine moieties. Magnetic data revealed that the Fe(ii) complex 1 shows complete SCO with the transition temperature T_1/2 = 141 K, which is preserved also in the mixed Fe/Co system 3 (T_1/2 = 128 K). Co(ii) complex 2 behaves as a field-induced single-molecule magnet as well as the mixed system 3 with a direct and phonon bottleneck relaxation process, respectively. This is the first example of such Fe/Co solid solution providing SCO in combination with field-induced SMM properties. Unfortunately, the light-induced excited spin-state trapping (LIESST) effect was not observed either for the Fe(ii) complex 1 or the mixed system 3 and thus, the effect of SCO on SMM properties at low temperature could not be investigated in detail. Nevertheless, the obtained results clearly document the success of the solid solution methodology for the preparation of multifunctional magnetic materials.

Low-spin and spin-crossover iron(II) complexes with pyridyl-benzimidazole ligands: synthesis, and structural, magnetic and solution study

Two tridentate ligands (L1 = 2,6-bis(1-(3,5-di-tert-butylbenzyl)-1H-benzimidazol-2-yl)pyridine and L2 = 2,6-bis(1-(4-tert-butylbenzyl)-1H-benzimidazol-2-yl)pyridine) and one didentate ligand (L3 = 1-(4-tert-butylbenzyl)-2-pyridine-2-yl-1H-benzimidazol) were used for the synthesis of eight mononuclear Fe(ii) compounds 1-8 containing miscellaneous counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported coordination compounds and revealed the octahedral geometry of metal centres in the complex dications of 1-8. Compounds 1-6 prepared from tridentate ligands were low-spin and, therefore, diamagnetic up to 400 K. On the other hand, compounds 7 and 8, in which the Fe(ii) centre was coordinated with didentate ligand L3, exhibited temperature and light triggered spin-crossover behaviour. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal and light induced spin state switching. In addition, the solution redox properties of compounds 1-8 were investigated by cyclic voltammetry.

Magnetic Properties of Fe(II) Complexes of Cyclam Derivative with One p-Aminobenzyl Pendant Arm

In order to prepare an Fe(II) spin crossover (SCO) complex that could be consequently modified to a bimetallic coordination compound that possesses another magnetic property of interest, a specially designed ligand L-NH2 (1-(4-aminobenzyl)-4,11-bis(pyridine-2-ylmethyl)- 1,4,8,11-tetraazacyclotetradecane) was prepared. This ligand consists of a macrocyclic cyclam part containing two 2-pyridylmethyl pendant arms (expecting SCO upon Fe(II) complexation) and one p-aminobenzyl pendant arm with an NH2 group. The presence of this group enables the consequent transformation to various functional groups for the selective complexation of other transition metals or lanthanides (providing the second property of interest). Furthermore, the performed theoretical calculations (TPSSh/def2-TZVP) predicted SCO behavior for the Fe(II) complex of L-NH2. Thus, Fe(II) complexes [Fe(L-NH2)](ClO4)2 (1) and [Fe(L-NH2)]Cl2·6H2O (2) were synthesized and thoroughly characterized. Based on the crystal structure of an isostructural analogous Ni(II) complex [Ni(L-NH2)]Cl2·6H2O (3), the coordination number six was confirmed with an octahedral coordination sphere and a cis-arrangement of the pyridine pendant arms. The measured magnetic data confirmed the high-spin behavior of both compounds with large magnetic anisotropy (D = 17.8 for 1 and 20.9 cm−1 for 2 complemented in both cases also with large rhombicity), though unfortunately without any indication of the SCO behavior with decreasing temperature. The lack of SCO can be ascribed to the crystal packing and/or the non-covalent intermolecular interactions stabilizing the high-spin state in the solid state.