The [Mn₂(2-OHsalpn)₂]2-,-,0,+ System: Synthesis, Structure, Spectroscopy, and Magnetism of the First Structurally Characterized Dinuclear Manganese Series Containing Four Distinct Oxidation States

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Abstract

The series of complexes [Mn(2)(2-OH(Xsal)pn)(2)](2-,-,0,+) [where 2-OH(Xsal)pn represents substituted-phenyl-ring derivatives (X = H, 5-Cl, 3,5-Cl(2), 5-NO(2)) of 1,3-bis(salicylideneamino)-2-propanol] allow for the first detailed structural, magnetic, and spectroscopic study of a series of complexes that are the most active functional models for the manganese catalases. Representative examples of each oxidation state of the series (mimicking all of the known oxidation states for the enzyme's reaction chemistry) have been crystallographically characterized. The molecules presented herein are described as symmetric derivatives because they form dimers with both of the ligands spanning both Mn ions with the alkoxide on the backbone of the ligand bridging the metals. The variation in Mn-Mn separation across the four structures is 0.11 Å [Mn(II)-Mn(II) = 3.33 Å; Mn(II)-Mn(III) = 3.25 Å; Mn(III)-Mn(III) = 3.36 Å; Mn(III)-Mn(IV) = 3.25 Å], showing that the basic core structure is highly invariant. Nonetheless, significant structural changes in the polyhedra of each manganese ion can be observed across the range of metal oxidation states. These symmetric structures are distinct from the previously described asymmetric {[Mn(2)(2-OH(Xsal)pn)(2)](sol)}(0,+) structures which have only one bridging alkoxide and one monodentate solvent bound to the Mn(III) ion. These two forms (symmetric and asymmetric) are reminiscent of the carboxylate shift in metal carboxylate chemistry and illustrate how alkoxide ligands can participitate in an analogous alkoxide shift in order to generate a binding site for an incoming ligand, such as methanol, or substrate, such as hydrogen peroxide. This is the first series that allows the observation of the effect of subtle changes in geometry on the sign if not the magnitude of magnetic exchange in dimeric systems across a range of oxidation states. Regardless of the symmetric or asymmetric nature of the complex, the exchange parameter J was found to be very low; however, both ferro- and antiferromagnetic exchange can be realized with these dimers.

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