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Nicolini, Alessio, (2021)  - EMAC (Extended Metal Atom Chains) a base di Ferro(II) come Magneti Molecolari: Sintesi, Struttura e Comportamento Magnetico  - , Tesi di dottorato - (, , Universitą degli studi di Modena e Reggio Emilia ) - pagg. -

Abstract: The annual size of the Global DataSphere (the total amount of data created across the world) has experienced an exponential increase during the last decade, and it will exceed 150 trillion of gigabytes within 2025. Storage, transfer and elaboration of data became one of the most appealing targets in emerging fields such as quantum technologies and spintronics (spin electronics). A primary goal is the control of magnetic properties and spin at the atomic and molecular level. Interesting are specific nanostructures called Single-Molecule Magnets (SMMs), which possess similar magnetic properties to those of bulk magnets but of pure molecular origin. SMMs can feature magnetic bistability and long relaxation times, which make them attractive to be implemented as single bits. Their physical properties can be tuned by chemical design. Iron(II)-based Extended Metal Atom Chains (EMACs) are appealing synthetic targets as SMMs, because of the large spin and magnetic anisotropy of high-spin iron(II). EMACs are linear arrays of at least 3 metal ions wrapped together by polydentate organic ligands, which promote short separations between the metal centers and, sometimes, metal-metal bonds. However, these compounds proved to be very elusive, due to the exceeding tendency of iron(II) to undergo oxidation and hydrolysis processes. In fact, before our report of a tetrairon(II) chain [Fe4(tpda)3Cl2] (1) based on oligo--pyridylamido ligand tpda2– in 2018, the only known iron(II)-based EMAC was a triiron(II) complex with formamidinato ligands (2) described by Cotton et al. in 1998. In 2020, Guillet et al. reported a new triiron(II) chain (3) supported by silylated diaminopyridines. Complexes 1, 2 and 3 exhibit ferromagnetic interactions, while 3 also contains metal-metal bonds. The aim of this Thesis was the design, the synthesis and the characterization of transition metal compounds with highly magnetic electronic states, containing ferromagnetic interactions and, possibly, metal-metal bonds. In particular, it describes a systematic study of a series of iron-based EMACs, obtained and handled in strictly anaerobic and anhydrous conditions. The novel synthesized compounds were firstly characterized by single crystals X-ray diffraction, to define their molecular structures with atomic precision. Afterwards, their properties were investigated in solution (mass spectrometry, electronic and NMR spectroscopy, cyclic voltammetry) and in solid state (Mössbauer spectroscopy, DC and AC magnetometry). The effect of replacing axial Cl– − with Br– ligands in 1 was firstly investigated. Complex [Fe4(tpda)3Br2] (4) exhibits dominant ferromagnetic coupling at room temperature and similar magnetic behavior to 1. Surprisingly, AC experiments pointed out a significant difference: although both 1 and 4 showed SMM behavior, slow magnetic relaxation in 4 was observable even in zero applied field. To attempt activating the double-exchange mechanism, which could strengthen the ferromagnetic interaction in mixed valent compounds, the chemical oxidation of 1 was carried out, isolating two linear triiron mixed-valence species: one- ([Fe2IIFeIII(tpda)3]PF6, 5) and two-electron oxidized ([FeIIFe2III(tpda)3Cl]PF6, 6). Variable temperature 57Fe Mössbauer and electronic spectra suggest that 5 is best classified as a Robin-Day Class II mixed-valence system at 298 K, while no delocalization occurs at 10 and 77 K. Furthermore, 5 exhibits zero-field SMM behavior. In order to better stabilize these chain like structures, a challenging new tripodal ligand, N(CH2CH2NH-Py-NH-Py)3, based on three covalently linked oligo-α-pyridylamido units, was designed and synthesized. Its coordination chemistry towards iron and cobalt was preliminary explored, although no new EMACs were obtained so far.