Series of Fluorinated Benzimidazole-Substituted Nitronyl Nitroxides: Synthesis, Structure, Acidity, Redox Properties, and Magnetostructural Correlations

Abstract

A special series of nitronyl nitroxides was synthesized: 2-(benzimidazol-2′-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1Himidazole-3-oxide-1-oxyls mono-, di-, tri-, or tetrafluorinated on the benzene ring. The structure of all paramagnets was unambiguously confirmed by single-crystal X-ray diffraction. It was found that in crystals, the radicals are assembled into chains due to intermolecular H-bonds between the benzimidazole moiety (H-bond donor) and the nitronyl nitroxide group or benzimidazole ring (H-bond acceptor). The magnetic properties of nitronyl nitroxides depend on the type of binding of radicals by H-bonds. The magnetic motif of 4-fluoro-, 5-fluoro-, 4,6-difluoro-, 4,5,6trifluoro-, 4,5,7-trifluoro-, and 4,5,6,7-tetrafluoro-derivatives, as well as the nonfluorinated compound, consists of ferromagnetic chains (J/k B ≈ 20−40 K) formed by the McConnell type I mechanism. In the 5,6-difluoro-and 4,5-difluoro-derivatives, the distances between the paramagnetic centers are large, as a result of which the exchange interactions are weak. According to cyclic voltammetry, paramagnets are oxidized reversibly, while their reduction is a quasi-reversible electron transfer (EC mechanism); experimental redox potentials of radicals correlate well with the calculated values. Quantum chemical assessment of the acidity of benzimidazolylsubstituted nitronyl nitroxides revealed that the introduction of fluorine atoms into the benzene ring enhances the acidity of the paramagnets by more than 5 orders of magnitude.

Effects of Difluorophenyl Substituents on Structural, Redox, and Magnetic Properties of Blatter Radicals

“…In a series of other works, a trifluoromethyl group was introduced to the C7 position of the Blatter radical to give radicals with significantly improved chemical stability [38,39]. Herein, as part of our increasing interest in "structure-property" correlations inherent in fluorinated organic radicals [40][41][42][43][44][45][46][47][48][49][50][51][52], we report synthesis, structure and complete characterization of novel difluorophenyl-substituted Blatter radicals, namely 1-(3,4-difluorophenyl)-3-phenyl-1,4-dihydrobenzo[e] [1,2,4]triazin-4-yl (1a) and 1-(2,4-difluorophenyl)-3-phenyl-1,4-dihydrobenzo[e] [1,2,4]triazin-4-yl (1b).…”

5-Fluoro-1-Methyl-Pyrazol-4-yl-Substituted Nitronyl Nitroxide Radical and Its 3d Metal Complexes: Synthesis, Structure, and Magnetic Properties

“…It has been demonstrated that transitions in Cu(II) complexes with nitronyl nitroxides are initiated by temperature changes and via irradiation; photoswitching between low-temperature and high-temperature magnetic states is extremely rapid, occurring within a nanosecond [22,23]. Among complexes of transition metal ions with nitroxides, the highest temperatures of magnetic ordering have been achieved in coordination compounds between Mn(II) and deprotonated benzimidazol-2-yl-substituted nitronyl nitroxides, and in this field, further research is needed to design new paramagnetic ligands for obtaining molecular magnets featuring high T C [24,25]. So far, various nitronyl nitroxides have been created to construct heterospin molecule-based magnetic systems, but a comprehensive series of paramagnetic metal ions' complexes with fluorinated radicals has not been investigated.…”

An Influence of Fluorinated Alkyl Substituents on Structure and Magnetic Properties of Mn(II) Complexes with Pyrazolyl-Substituted Nitronyl Nitroxides

“…In a preceding paper, we investigated the influence of step-by-step fluorination of different paramagnetic systems on their structure and magnetism [14][15][16]. The motivation was that the introduction of fluorine atom(s) into molecules alters physical and chemical properties of the compounds because of fluorine's electronegativity, its low polarisability and high bond strength [17].…”

“…The complexation reaction of [Mn(hfac) 2 ] with these radicals under identical conditions led to the formation of complexes of dimer structure, head-to-head chain-polymeric structure or head-to-tail chain-polymeric structure depending on the structure of the fluorinated alkyl substituent. Magnetic analyses showed that there is a strong antiferromagnetic interaction between the Mn(II) ion and the directly coordinated N-O group in the high-temperature region and weak ferro-or antiferromagnetic exchange interactions below 100 K. In a preceding paper, we investigated the influence of step-by-step fluorination of different paramagnetic systems on their structure and magnetism [14][15][16]. The motivation was that the introduction of fluorine atom(s) into molecules alters physical and chemical properties of the compounds because of fluorine's electronegativity, its low polarisability and high bond strength [17].…”