A new dinuclear complex [{Fe(tpc-OBn)(NCS)(μ-NCS)}] (1) based on the tripodal tpc-OBn ligand (tpc-OBn = tris(2-pyridyl)benzyloxymethane), containing bridging μ-κN:κS-SCN and terminal κN-SCN thiocyanate ligands, has been prepared and characterized by single crystal X-ray diffraction, magnetic studies, and DFT theoretical calculations. This complex represents the first example of dinuclear Fe complex with double μ-κN:κS-SCN bridges in a head-to-tail configuration that exhibits ferromagnetic coupling between metal ions (J = +1.08 cm). Experimental and theoretical magnetostructural studies on this kind of infrequent Fe dinuclear complex containing a centrosymmetrically [Fe(μ-SCN)] bridging fragment show that the magnitude and sign of the magnetic coupling parameter, J, depend to a large extent on the Fe-N-C (α) angle, so that J decreases linearly when α decreases. The calculated crossover point below which the magnetic interactions change from ferromagnetic to antiferromagnetic is found at 162.3°. In addition, experimental results obtained in this work and those reported in the literature suggest that large N-Fe distances and bent N-bound terminal κN-SCN ligands favor the high spin state of the Fe ions, while short N-Fe distances and almost linear Fe-N-C angles favor a stronger ligand field, which enables the Fe ions to show spin crossover (SCO) behavior.

We present here a novel example of spin crossover phenomenon on a Fe(II) one-dimensional chain with unusual NS coordination sphere. The [{Fe(tpc-OMe)(NCS)(μ-NCS)} ] (1) compound was prepared using the tridentate tpc-OMe ligand (tpc-OMe = tris(2-pyridyl)methoxymethane), FeCl·4HO, and the KSCN salt. Crystallographic investigations revealed that the Fe(II) ions are connected by a single bridging NCS ligand (μ-κN:κS-SCN coordination mode) to afford a zigzag neutral chain running along the [010] direction, in which the thiocyanato bridging groups adopt a cis head-to-tail configuration. The (NS) metal environment arises from one thiocyanato-κS and two thiocyanato-κN ligands and from three pyridine of the fac-tpc-OMe tripodal ligand. This compound presents a unique extension of Fe(II) binuclear complexes into linear chains built on similar tripodal ligands and bridging thiocyanate anions. Compound 1 shows a spin crossover (SCO) behavior which has been evidenced by magnetic, calorimetric, and structural investigations, revealing a sharp cooperative spin transition with a transition temperature of ca. 199 K. Temperature scan rate studies revealed a very narrow hysteresis loop (∼1 K wide). Photoswitching of this compound was also performed, evidencing a very fast relaxation process at low temperature. Among other factors, the linearity of the N-bound terminal thiocyanato ligand appears as the main structural characteristic at the origin of the presence of the SCO transition in compound 1 and in the two others Fe(II) previous systems involving thiocyanato-bridges and tripodal tris(2-pyridyl)methane ligands.

Two new mononuclear Fe(II) polymorphs, [(C2H5)4N]2[Fe(py3C-OEt)(NCS)3]2 (1) and [(C2H5)4N][Fe(py3C-OEt)(NCS)3] (2) (py3C-OEt = tris(pyridin-2yl)ethoxymethane) have been synthesized and characterized by single crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable temperature infrared spectroscopy. The molecular structure, in both complexes, is composed by the same anionic [Fe(py3C-OEt)(NCS)3] -complex (two units for 1, and one unit for 2) generated by a coordination, to the Fe(II) metal centre, of one tridentate donor molecules such tetrathiafulvalene (TTF) derivatives for the design of multifunctional systems, such as fluorescent or conducting switchable materials. 6,60 ■ ASSOCIATED CONTENT Supporting informationThe Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX/acs.inorg-chem.XXXXXXX X-ray crystallographic data in CIF format: CCDC numbers 2025933-2025936 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. Syntheses details and figures S1-S19 (pdf)