Studies on Manganese(II) Complexes ofN-Benzimidazole-Functionalized 1,4,7-Triazacyclononane: Crystal Structures, Properties and Combined Superoxide Dismutase and Catalase Functions

Understanding the Effect of the Electron Spin Relaxation on the Relaxivities of Mn(II) Complexes with Triazacyclononane Derivatives

Investigating the relaxation of water ¹H nuclei induced by paramagnetic Mn(II) complexes is important to understand the mechanisms that control the efficiency of contrast agents used in diagnostic magnetic resonance imaging (MRI). Herein, a series of potentially hexadentate triazacyclononane (TACN) derivatives containing different pendant arms were designed to explore the relaxation of the electron spin in the corresponding Mn(II) complexes by using a combination of ¹H NMR relaxometry and theoretical calculations. These ligands include 1,4,7-triazacyclononane-1,4,7-triacetic acid (H₃NOTA) and three derivatives in which an acetate group is replaced by sulfonamide (H₃NO2ASAm), amide (H₂NO2AM), or pyridyl (H₂NO2APy) pendants. The analogue of H₃NOTA containing three propionate pendant arms (H₃NOTPrA) was also investigated. The X-ray structure of the derivative containing two acetate groups and a sulfonamide pendant arm [Mn(NO2ASAm)]^- evidenced six-coordination of the ligand to the metal ion, with the coordination polyhedron being close to a trigonal prism. The relaxivities of all complexes at 20 MHz and 25 °C (1.1-1.3 mM^-1 s^-1) are typical of systems that lack water molecules coordinated to the metal ion. The nuclear magnetic relaxation profiles evidence significant differences in the relaxivities of the complexes at low fields (<1 MHz), which are associated with different spin relaxation rates. The zero field splitting (ZFS) parameters calculated by using DFT and CASSCF methods show that electronic relaxation is relatively insensitive to the nature of the donor atoms. However, the twist angle of the two tripodal faces that delineate the coordination polyhedron, defined by the N atoms of the TACN unit (lower face) and the donor atoms of the pendant arms (upper face), has an important effect in the ZFS parameters. A twist angle close to the ideal value for an octahedral coordination (60°), such as that in [Mn(NOTPrA)]^-, leads to a small ZFS energy, whereas this value increases as the coordination polyhedron approaches to a trigonal prism.

Dimerization, redox properties and antioxidant activity of two manganese(III) complexes of difluoro- and dichloro-substituted Schiff-base ligands

Two mononuclear Mn^III complexes [Mn(3,5-F₂salpn)(H₂O)₂][B(C₆H₅)₄]·2H₂O (1·2H₂O) and [Mn(3,5-Cl₂salpn)(OAc)(H₂O)]·H₂O (2·H₂O), where H₂salpn=1,3-bis(salicylidenamino)propane, have been prepared and characterized. The crystal structure of 1·H₂O shows that this complex forms μ-aqua dimers with a short Mn⋯Mn distance of 4.93Å. Under anaerobic conditions, the two complexes are stable in solution and possess trans-diaxial symmetry with the tetradentate Schiff base ligand symmetrically arranged in the equatorial plane. When left in air, these complexes slowly dimerize to yield high-valent [Mn^IV₂(3,5-X₂-salpn)₂(μ-O)₂] in which each X₂-salpn ligand wraps the two Mn ions. This process is favored in basic medium where the deprotonation of the bound water molecule is concomitant with air oxidation. The two complexes catalyze the dismutation of superoxide (superoxide dismutase (SOD) activity) and peroxide (catalase (CAT) activity) in basic medium. The phenyl-ring substituents play an important role on the CAT reaction but have little effect on SOD activity. Kinetics and spectroscopic results indicate that 1 and 2 catalyze H₂O₂ disproportionation through a cycle involving Mn^III₂ and Mn^IV₂ dimers, unlike related complexes with a more rigid and smaller chelate ring, which employ Mn^III/Mn^VO monomers.

A new mononuclear manganese(III) complex of an unsymmetrical hexadentate N3O3 ligand exhibiting superoxide dismutase and catalase-like activity: synthesis, characterization, properties and kinetics studies

A mononuclear Mn(III) complex MnL·4H2O (H3L=1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(4-methylbenzyl)amino]propan-2-ol) has been prepared and characterized. This complex catalyzes the dismutation of superoxide efficiently, with catalytic rate constant kcat=1.7×10(6)M(-1)s(-1) and IC50 1.26μM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH7.8. MnL is also able to disproportionate more than 300 equivalents of H2O2 in CH3CN, with initial rate of H2O2 decomposition given by ri=kcat [MnL](2) [H2O2] and kcat=1.32(2)mM(-2)min(-1). The accessibility of the Mn(IV) state (E(p)=0.53V vs. saturated calomel electrode), suggests MnL employs a high-valent catalytic cycle to decompose O2(-) and H2O2.

COMPARATIVE INVESTIGATION OF THE EFFECT OF TYPE OF DENSITY FUNCTIONAL IN THE DETERMINATION OF GEOMETRICAL PARAMETERS IN A Cu COMPLEX

The effects of short-range electron correlation, long-range electron exchange, local and nonlocal parts of density, higher order gradients of density, and adding some percentage of Hartree–Fock exchange to the functional on the prediction of geometrical parameters were investigated. A copper complex namely 1,2-bis(1,4,7-triaza-1-cyclononyl) ethane copper (II) with Jahn–Teller distortion in octahedral geometry was used to evaluate the performance of 50 commonly available density functionals. The standard 3-21G basis set was used for all light elements, while pseudo potential LANL2DZ was used for the copper atom. The best bond lengths and bond angles were obtained using M05-2x and OP functionals respectively. Also in order to more accurate survey the performance of B3LYP, we used this functional with two all-electron basis sets (6-31G and 3-21G) and three basis sets involving effective core potentials (LANL2DZ/3-21G, LANL2DZ, and LACVP).

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.