Mononuclear Manganese(III) Complexes as Building Blocks for the Design of Trinuclear Manganese Clusters: Study of the Ligand Influence on the Magnetic Properties of the [Mn₃(μ₃-O)]⁷⁺ Core

Abstract

The synthesis, crystal structure, and magnetic properties of three new manganese(III) clusters are reported, [Mn 3(mu 3-O)(phpzH) 3(MeOH) 3(OAc)] (1), [Mn 3(mu 3-O)(phpzMe) 3(MeOH) 3(OAc)].1.5MeOH (2), and [Mn 3(mu 3-O)(phpzH) 3(MeOH) 4(N 3)].MeOH (3) (H 2phpzH = 3(5)-(2-hydroxyphenyl)-pyrazole and H 2phpzMe = 3(5)-(2-hydroxyphenyl)-5(3)-methylpyrazole). Complexes 1- 3 consist of a triangle of manganese(III) ions with an oxido-center bridge and three ligands, phpzR (2-) (R = H, Me) that form a plane with the metal ions. All the complexes contain the same core with the general formula [Mn 3(mu 3-O)(phpzR) 3] (+). Methanol molecules and additional bridging ligands, that is, acetate (complexes 1 and 2) and azide (complex 3), are at the terminal positions. Temperature dependent magnetic susceptibility studies indicate the presence of predominant antiferromagnetic intramolecular interactions between manganese(III) ions in 1 and 3, while both antiferromagnetic and ferromagnetic intramolecular interactions are operative in 2.

Oxidation of phenothiazine dyes by manganese(III) in sulfuric acid solution

“…Although most of the manganese(II)-manganese(III) complexes belong to the Class I according to the Robin-Day classification [29], the found optical properties suggest rather a Class II case for the Mn(III)-SO 4 -Mn(II) mixed-valence complex. The changes within the visible spectrum range reported by Pinto et al [23], namely an absorbance decrease with an increase in the sulfuric acid concentration within the 1-2 M range, can be explained by the transformation of the [Mn(H 2 O) 5 [17], azure B and thionine [18] Degradation of the phenothiazine dyes by manganese(III) could be examined only at sulfuric acid concentrations higher than 2 M because, as outlined above, at lower H 2 SO 4 concentrations, the sulfato manganese(III) complexes are not stable. On the other hand, within the 2-5 M range, sulfuric acid concentration affects the reaction rate only slightly.…”

“…Thus, manganese(III) is employed in water splitting during the light reactions of green plant photosynthesis [1] and in enzymatic and nonenzymatic redox catalysis in mammalian systems [2][3][4]. It is also a powerful one-electron oxidant and a versatile, selective redox catalyst frequently used in laboratory and industrial organic synthesis involving oxygen transfer reactions to alkanes, alkenes and compounds containing nitrogen and sulfur atoms as well as carbon-carbon bond generation [5][6][7][8][9][10][11][12][13][14][15][16].…”

An Oxide‐Centered Trinuclear Manganese(III) Compound with a Bulky Phenol‐Pyrazolate Ligand

“…[6,16,17,25] When the Mn-O-Mn angle is smaller than 120°(value for an equilateral triangle), a switch from antiferromagnetic to ferromagnetic exchange is observed. [16,17] Another relevant factor is the displacement of the µ 3 -oxide ligand from the plane of the three manganese(III) ions. [6,25] In order to establish magneto-structural correlations it is useful to compare a series of compounds.…”

“…[3,8,9,[15][16][17] In all the cases, the compounds are formed by three doubly deprotonated phenol-pyrazole ligands that are in the same plane of the [Mn 3 (µ 3 -O)] 7+ core, whereas bridging ligands such as azide or carboxylates and alcoholic solvents are present at the axial positions of the manganese(III) ions. In most cases, these trinuclear manganese(III) units are linked by a bridging ligand, or just by hydrogen bonding to form 1-D chains.…”

“…[3,8,9,[15][16][17] In all trinuclear manganese(III) compounds containing phenol-pyrazole ligands, overall antiferromagnetic interactions are present in the trinuclear unit, [3,8,9,[15][16][17] whereas ferromagnetic properties are operative between the trinuclear units of the 1-D chains in most of the cases. [3,8,9,[15][16][17] The substituents in the phenol-pyrazole ligands (Scheme 1) have been crucial in determining the topology and dimensionality of these trinuclear compounds. [3,8,9,[15][16][17] These substituents are found at the 4-position (R 1 ) of the phenol ring and also at the 5(3)-position (R 2 ) of the pyrazole ring.…”