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Unusual magnetism and thermochromism of mononuclear iron(III) complex

A new mononuclear iron(III) octahedral complex [FeL6](BF4)3, where ligand L is 2-hydroxy-6-methylpyridine, has been synthesized and characterized by single crystal X-ray diffraction (SCXRD), magnetization measurements, electron spin/paramagnetic resonance (ESR/EPR) spectroscopy and DFT calculation.[1] The complex shows thermochromism i. e. color of the complex reversibly changes from red at room temperature to yellow-orange at the liquid nitrogen temperature. SQUID magnetization measurements reveal there is no spin-crossover of the complex ; iron(III) is in a high-spin state S = 5/2 in all temperature range 1.8295 K. The high-spin ground state of iron(III) is also confirmed by DFT calculations. X-band and multifrequency high-field/high-frequency ESR spectroscopy shows rather unusual iron(III) spectra.[2] The spectra can be simulated by temperature independent axial zero-field splitting parameter D while the value of rhombic parameter E increases on lowering the temperature. SCXRD study shows there is no temperature changes in iron(III) coordination geometry while supramolecular interactions are temperature dependent, influencing the iron(III) rhombicity. DFT calculations show temperature variation of the HOMO–LUMO gap, in agreement with the temperature changes of color and ESR spectra. References 1. K. Garg, Y. Goriya, S. Manojveer, S. Muratović, D. Pajić, M. Cetina, I. Petreska, Y. Krupskaya, V. Kataev, M. T. Johnson, O. F. Wendt and D. Žilić, Dalton Trans. 51, 2338 (2022). 2. A. Solano-Peralta, J. P. Saucedo-Vázquez, R. Escudero, H. Höpfl, H. El-Mkami, G. M. Smith and M. E. Sosa-Torres, Dalton Trans.1668 (2009).

iron(III) complex ; ESR/EPR ; magnetism ; thermochromism ; DFT

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