Redox‐Controlled Magnetic {Mn₁₃} Keggin Systems

Abstract

Polyoxometalates (POMs) have been widely reported in recent years. These molecular metal oxides, or polyanions, are most commonly constructed of tungsten, molybdenum, or vanadium ions in their highest oxidation state, bridged by oxide ligands to form clusters which can range in size from low-nuclearity building blocks to large-scale protein-like superstructures.[1] An archetypical POM structural motif is the {XM 12 O 40 } nÀ species (X = P, Si…) known as the Keggin anion and Keggin structures have been successfully shown to act as catalysts [2] among other potential applications.[3] POMs are inorganic materials that can be functionalized through their combination with organic ligands and/or the introduction of paramagnetic heterometal ions which leads to magnetic heterometallic POMs. [4][5][6] In addition, there are a few studies of related species consisting exclusively of late first-row transition-metal ions such as some mixed-valence manganese Keggin-related clusters described by Lampropoulos et al, [7] the uncapped {Fe 13 } cluster reported by Bino et al., [8] and the reverse-Keggin structures presented by Baskar et al. [9] To the best of our knowledge, there are no other examples of POM-type complexes consisting exclusively of open-shell transition metals, and so far their physical properties have been barely investigated. In contrast, transition-metal oxide materials are widely used and their properties such as magnetic ordering, semi-and superconductivity, giant magnetoresistance, and ferroelectricity are much studied. Their electronic properties can be understood by their band structures and changed to show the desired characteristics. [10,11] Replication or improvement of metal oxide properties in discrete molecules can be extremely difficult. However, molecular metal clusters can possess wellseparated energy levels and their characteristic electronic structures can be altered to show, for example, valence tautomerism, multi-bistability with spin crossover, and singlemolecule-magnetic (SMM) behavior by tuning the frontier orbitals and the electronic interactions between the metal centers. [12][13][14][15] The band filling in solids is readily controlled in their syntheses by altering the ratio of the constituent elements which drastically changes the physical properties. The question arises whether the chemist can synthesize metal oxide clusters displaying controllable redox states which can perturb the physical properties.Herein, a polyoxometalate-type cluster was synthesized by using exclusively first-row transition-metal ions in combination with organic capping ligands. In the resultant system the spin state and magnetic properties were tuned without substantial change to the molecular structure, and its SMM behavior was perturbed through manipulation of the cluster oxidation state. Herein, the synthesis, magnetic properties, and redox behavior of three mixed-valence {Mn 13 } Keggintype clusters are reported.The one-pot reaction of Mn(NO 3 ) 2 ·6 H 2 O with 2,6-bis[N-(2-hydroxyethyl)iminomethyl]-4-met...

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