Synthesis, crystal structure and magnetic properties of a novel manganese(III) cluster

Semiempirical Magnetostructural Correlation for High-Nuclearity MnIII-Oxo Complexes: Accommodation of Different Relative Jahn-Teller Axis Orientations

The previous development of a magnetostructural correlation (MSC) for polynuclear FeIII/oxo clusters has now been extended to one for polynuclear MnIII/oxo clusters. A semiempirical model estimating each pairwise Mn2 exchange constant (Jij) from the Mn-O bond lengths and Mn-O-Mn angles has been formulated based on the angular overlap model. The extra complication, compared with the FeIII/oxo MSC, of different relative orientations of the Jahn-Teller distortion axes typical of high-spin MnIII in near-octahedral geometry was accommodated by developing a separate MSC variant for each possible situation. The final coefficients of the three MSC variants were determined by using reliable crystal structure data and experimentally determined Jij values from the literature. The estimated JMSC values from the new MnIII/oxo MSC have been employed to successfully rationalize the magnetic properties of a number of MnIII clusters in the nuclearity range Mn3-Mn10. These properties include relative spin vector alignments in the ground state, the presence of spin frustration effects, and the resulting overall ground state spin. In addition, the JMSC values can be used to simulate the direct-current magnetic susceptibility versus temperature data and provide realistic input values for fits of these data to minimize false-fit problems. A protocol for the use of the new MSC is also reported.

Role of Oxido Incorporation and Ligand Lability in Expanding Redox Accessibility of Structurally Related Mn4 Clusters

Photosystem II supports four manganese centers through nine oxidation states from manganese(II) during assembly through to the most oxidized state before O₂ formation and release. The protein-based carboxylate and imidazole ligands allow for significant changes of the coordination environment during the incorporation of hydroxido and oxido ligands upon oxidation of the metal centers. We report the synthesis and characterization of a series of tetramanganese complexes in four of the six oxidation states from Mn^II₃Mn^III to Mn^III₂ Mn^IV₂ with the same ligand framework (L) by incorporating four oxido ligands. A 1,3,5-triarylbenzene framework appended with six pyridyl and three alkoxy groups was utilized along with three acetate anions to access tetramanganese complexes, Mn₄O _x , with x = 1, 2, 3, and 4. Alongside two previously reported complexes, four new clusters in various states were isolated and characterized by crystallography, and four were observed electrochemically, thus accessing the eight oxidation states from Mn^II₄ to Mn^IIIMn^IV₃. This structurally related series of compounds was characterized by EXAFS, XANES, EPR, magnetism, and cyclic voltammetry. Similar to the ligands in the active site of the protein, the ancillary ligand (L) is preserved throughout the series and changes its binding mode between the low and high oxido-content clusters. Implications for the rational assembly and properties of high oxidation state metal-oxido clusters are presented.