Studies on binuclear hydroxo/carboxylato-bridged manganese(III) complexes and a mononuclear manganese(III) complex involving salen type ligands

Abstract

Synthesis of six hydroxo-bridged binuclear manganese(III) complexes of formulae [MnL-X-MnL](ClO 4 ) [X = OH(1-6)] along with a mononuclear manganese(III) complex (7) [Mn(L)(L¢)(MeOH) 2 ] [HL¢ ¼ 2-(2-hydroxyphen-yl)benzimidazole] and two carboxylate-bridged binuclear manganese(III) complexes (8) and (9) are described. The complexes have been characterized by the combination of i.r., u.v.-vis spectroscopy, magnetic moments and by their redox properties. The electronic spectra of all the complexes exhibit almost identical features consisting of two d-d bands at ca. 550 and 600 nm, one MLCT band at ca. 400 nm, together with two p-p * intra-ligand transitions at ca. 250 nm and ca. 300 nm. Room temperature magnetic data range from l = 2.7-3.0 BM indicates some superexchange between the binuclear metal centers via bridging hydroxo/carboxylato groups. The X-ray crystal structure of the binuclear complex (5) revealed that it has a symmetric Mn III N 2 O 2 core bridged by a hydroxyl group. The Xray analysis of the mononuclear complex (7) showed that the manganese-center possesses a distorted octahedral geometry. Electrochemical properties of hydroxo-bridged manganese(III) complexes (1-6) show identical features consisting of an irreversible and a quasi-reversible reduction corresponding to the Mn 2 III fi Mn II Mn III fi Mn II Mn II couples in the voltammogram. It was found that electron withdrawing substituents on the ligand result in easier reduction. Complex (7) displays an irreversible reduction at 0.08 V and a reversible oxidation at 0.45V assignable to the Mn III fi Mn II reduction and Mn III fi Mn IV oxidation, respectively. The carboxylate-bridged compound (8) exhibits two irreversible oxidations at 0.4 and 0.6 V, probably due to Mn 2 III fi Mn III Mn IV fi Mn IV Mn IV oxidations and shows a quasi-reversible reductive wave at )0.85 V, tentatively assigned to Mn 2 III fi Mn II Mn III reduction.

The Critical Role of Ligand Flexibility on the Activity of Free and Immobilized Mn Superoxide Dismutase Mimics

“…From elemental analysis, the solid can be formulated as Mn 2 (salbn) 2 (OH)ClO 4 . The strong band at 774 cm −1 (Figure S4) together with the absorption at 3420 cm −1 in the FT-IR spectrum of this complex, suggest hydroxide may bridge the two Mn(III) ions [48,49]. The formation of the µ-OH bridged dimer in a mildly basic medium has precedent in previous reports for related Mn(III) complexes under similar conditions [50,51].…”

Dioxygen Activation and Catalytic Reduction to Hydrogen Peroxide by a Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol

“…The Mn III –S distanceswith S being unprotonatedare shorter than those in Mn II 2 SH , in agreement with the increase of the Mn oxidation state from +II to +III. The Mn–O­(H)–Mn angles of 146.56(17)° and 146.11(17)° and the Mn···Mn separations of 3.902(1) Å and 3.903(1) Å are in the range found in the other reported dinuclear μ-OH Mn III complexes (141° < Mn–O–Mn angle < 160° and 3.87 Å < Mn···Mn distance < 3.99 Å). − A mono-oxo bridge can be excluded on the basis of the Mn···Mn distance that is notably shorter in dinuclear mono-μ-oxo Mn III complexes (3.4–3.6 Å). ,, …”

“…The Mn III 2 OH complex crystallizes as a mixed salt, [(Mn III L) 2 (OH)]­(PF 6 ) 0.81 (ClO 4 ) 0.19 ( Mn III 2 OH # ), with two crystallographically independent molecules with similar geometries. Mn III 2 OH is a rare example of a dinuclear Mn III complex with a unique hydroxo bridge and the first with thiolate ligands. − Each manganese­(III) ion is surrounded by an N2S2O donor set in a distorted square pyramidal coordination sphere. The Mn III –S distanceswith S being unprotonatedare shorter than those in Mn II 2 SH , in agreement with the increase of the Mn oxidation state from +II to +III.…”

Electronic Structure and Spectroscopic Properties of Mononuclear Manganese(III) Schiff Base Complexes: A Systematic Study on [Mn(acen)X] Complexes by EPR, UV/vis, and MCD Spectroscopy (X = Hal, NCS)

“…In consequence, the presence of dimers, chains, or aggregates can be ruled out in the case of the [Mn( acen )X] complexes at the used concentrations of 2 mM. However, at higher concentrations (≥5 mM) the parallel mode EPR spectra of the [Mn( acen )X] complexes show additional lines which can be compared to the EPR spectra of [Mn( salen* )Cl] without additives (Supporting Information, Figure S2) and therefore suggest that μ-carboxido- or μ-halogenido-bridged dimers, [Mn( acen )X] x clusters, chains, or other di- and polynuclear species may be formed at high concentrations. − …”