A Comparative XAS and X-ray Diffraction Study of New Binuclear Mn(III) Complexes with Catalase Activity. Indirect Effect of the Counteranion on Magnetic Properties

Rationalizing the sign and magnitude of the magnetic coupling and anisotropy in dinuclear manganese(iii) complexes

We have synthesised twelve manganese(iii) dinuclear complexes, 1-12, in order to understand the origin of magnetic exchange (J) between the metal centres and the magnetic anisotropy (D) of each metal ion using a combined experimental and theoretical approach. All twelve complexes contain the same bridging ligand environment of one μ-oxo and two μ-carboxylato, that helped us to probe how the structural parameters, such as bond distance, bond angle and especially Jahn-Teller dihedral angle affect the magnetic behaviour. Among the twelve complexes, we found ferromagnetic coupling for five and antiferromagnetic coupling for seven. DFT computed the J and ab initio methods computed the D parameter, and are in general agreement with the experimentally determined values. The dihedral angle between the two Jahn-Teller axes of the constituent MnIII ions are found to play a key role in determining the sign of the magnetic coupling. Magneto-structural correlations are developed by varying the Mn-O distance and the Mn-O-Mn angle to understand how the magnetic coupling changes upon these structural changes. Among the developed correlations, the Mn-O distance is found to be the most sensitive parameter that switches the sign of the magnetic coupling from negative to positive. The single-ion zero-field splitting of the MnIII centres is found to be negative for complexes 1-11 and positive for complex 12. However, the zero-field splitting of the S = 4 state for the ferromagnetic coupled dimers is found to be positive, revealing a significant contribution from the exchange anisotropy - a parameter which has long been ignored as being too small to be effective.

High-spin enforcement in first-row metal complexes of a constrained polyaromatic ligand: synthesis, structure, and properties

The coordination chemistry of a rigid tetradentate polypyridyl ligand has been developed with first-row transition metals Mn(ii), Fe(ii), Co(ii), Ni(ii), and Zn(ii).

Influence of the Disposition of the Anisotropy Axes into the Magnetic Properties of MnIII Dinuclear Compounds with Benzoato Derivative Bridges

The two new Mn^III dinuclear compounds [{Mn(H₂O)(phen)}₂(μ-4-CH₃C₆H₄COO)₂(μ-O)](ClO₄)₂·3CH₃CN·H₂O (1·3CH₃CN·H₂O) and [{Mn(H₂O)(phen)}(μ-O)(μ-2-BrC₆H₄COO)₂{Mn(NO₃)(phen)}]NO₃ (2) have been synthesized. Their structural data reveal significant differences in the shape of the coordination octahedron around the Mn^III ions in both compounds. The different distortions from ideal geometry incite a very different magnetic behavior, affecting both the zero-field splitting parameters of the Mn^III ions (D_Mn and E_Mn) and the magnetic interaction between them. Compound 1, with elongation in the monodentate ligand direction, shows antiferromagnetic coupling (ground state S = 0) and local D_Mn < 0, while compound 2, with compression in the oxo bridge direction, displays a ferromagnetic interaction (ground state S = 4) and local D_Mn > 0. Theoretical CASSCF and DFT calculations corroborate the different magnetic anisotropy and exchange coupling found in both compounds. Moreover, with the help of theoretical calculations, some interesting magneto-structural correlations have been found between the degree of distortion of the coordination octahedra and the magnetic coupling; it becomes more antiferromagnetic when the elongation parameter, Δ, in absolute value is increased.

Temperature-induced single-crystal-to-single-crystal transformation of a binuclear Mn(ii) complex into a 1D chain polymer

The new binuclear Mn(ii) complex can transform into a new 1D chain structure in a SCSC transformation via dehydration.

Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

The diMn(III) complexes [Mn2(5-Me-salpentO)(mu-MeO)(mu-AcO)(H2O)Br] (1) and [Mn2(3-Me-salpentO)(mu-MeO)(mu-AcO)(MeOH)2]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(micro-alkoxo)(micro-acetato) triply-bridged diMn(III) core with an Mn...Mn separation of 2.93-2.94 A, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H2O2, with first-order dependence on the catalyst, and saturation kinetics on [H2O2], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H2O2 without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn(II) species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn2(III) oxidation state during cycling. The correlation between log(k(cat)) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H2O2 dismutation by these complexes converge at a catalytic cycle that involves the Mn2(III) and Mn2(IV) oxidation states.