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

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.

Properties and antioxidant activity of water-soluble iron catalysts with Schiff base ligands. Comparison with their manganese counterparts

“…16b This can be explained considering that in basic medium, deprotonation of the alcohol takes place and facilitates complex dimerization with the ligand acting through a N 2 O 3 donor set. Such a behavior has previously been observed for metal complexes formed with salpnOH, 17 in which the ligand adopts an asymmetrical disposition around the metal. In this type of dimer, protons of the two phenol moieties are not longer equivalent and phenolate occupies equatorial and axial positions around iron and σ/π contact interactions should be operative.…”

The UDP-diacylglucosamine Pyrophosphohydrolase LpxH in Lipid A Biosynthesis Utilizes Mn2+ Cluster for Catalysis

“…Purified HiLpxH was diluted in the standard buffer lacking Triton X-100, added to the reactions covering the mentioned range of detergent concentrations, and assayed for activity. Reported total detergent concentrations accounted for the small amount of detergent (0.01 mM, 0.0007% w/v) present in both the [␤- 32 Another form of the modified autoradiographic assay was used to assess HiLpxH dependence on MnCl 2 in which the protein was diluted in standard reaction buffer containing no MnCl 2 and then diluted into reactions in which the MnCl 2 concentration ranged from 0 to 5 mM. Protein concentration was 980 pM in the 0 mM assay condition and 10 pM in all others.…”

The Possible Role of Proton‐Coupled Electron Transfer (PCET) in Water Oxidation by Photosystem II

“…This occurs, for example, in the stepwise oxidation of a series of oxido and sulfido metal clusters, for which E8' increases by 0.3-0.4 V at each step. [32,130] …”