Synthesis of new Mn₁₉ analogues and their structural, electrochemical and catalytic properties

Abstract

We report the synthesis, structural characterisation, electrochemistry and catalytic activity in the selective oxidation of benzyl alcohol of new Mn19 and Mn18M analogues: [MnIII12MnII7(μ4-O)8(μ3-OCH3)2(μ3-Br)6(HLMe)12(MeOH)6]Br2 (2) and [MnIII12MnII6Sr(μ4-O8(μ3-Cl)8(HLMe)12(MeCN)6]Cl2 cluster (3), where H3LMe is 2,6-bis(hydroxymethyl)-p-cresol.

Bioinspired Water Oxidation Using a Mn-Oxo Cluster Stabilized by Non-Innocent Organic Tyrosine Y161 and Plastoquinone Mimics

“…The use of the tridentate 2,6-bis­(hydroxymethyl)- p -cresol ligand (H 3 L Me ) (Figure C) in the presence of MnBr 2 in an acetonitrile/methanol mixture results in the formation of a variant of Mn cluster with supertetrahedral core structure, [Mn III 12 Mn II 7 (μ 4 -O) 8 (μ 3 -OCH 3 ) 2 (μ 3 -Br) 6 (HL Me ) 12 (MeOH) 5 (MeCN)]­Br 2 ·9MeCN·MeOH ( Mn 19 Br ). {Mn 19 } coordination clusters with supertetrahedral core structures were previously reported by Powell co-workers, revealing ferromagnetic interactions between the spin centers and leading to a maximum-spin ground state of S = 83/2. , Noteworthy, the use of Mn 19 Br as catalysts for the oxidation of benzyl alcohol to benzaldehyde was recently reported …”

“…The origin of these, cumulatively, high activity characteristics warranted a more detailed analysis. In agreement with other electrochemical studies using biomimetic cubane-type Mn clusters and other Mn II compounds, the Mn 19 Br /CP electrode shows an irreversible redox process centered at 1.22 V vs NHE which was previously attributed to the irreversible oxidation of the seven Mn II atoms to Mn III of the Mn 19 Br cluster . Interatomic distances within the cluster are expected to vary upon oxidation as the electronic structure of the oxo-cluster changes.…”

Tuning the Catalytic Water Oxidation Activity through Structural Modifications of High-Nuclearity Mn-oxo Clusters [Mn18M] (M = Sr2+, Mn2+)

“…The modified carbon paste electrodes were prepared using ALS Co. The synthesis of Mn18Sr was performed following a previously reported procedure [50]. MnCl2•4H2O (0.2 g, 1 mmol), Et3N (0.1 g, 1 mmol) and 2,6-bis(hydroxymethyl)-4methylphenol (H3L Me ) (0.17 g, 1 mmol) were added into a solvent mixture of MeCN and MeOH (25 mL/5 mL) and stirred for 30 min at room temperature.…”

“…Synthesis [50]. MnCl 2 •4H 2 O (0.2 g, 1 mmol), Et 3 N (0.1 g, 1 mmol) and 2,6-bis(hydroxymethyl)-4-methylphenol (H 3 L Me ) (0.17 g, 1 mmol) were added into a solvent mixture of MeCN and MeOH (25 mL/5 mL) and stirred for 30 min at room temperature.…”

“…Additionally, the presence of Sr II is also intriguing, as this is the only metal known which can replace Ca II in the OEC without rendering the cluster catalytically inactive [49]. Moreover, in a recent study by Mameri et al, Mn18Sr and Mn19 were successfully employed in the electrocatalytic oxidation of benzyl alcohol into benzaldehyde using O2 as an oxidant and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) as a co-catalyst [50]. Therefore, given the previously reported high WOC activity for the homologous Mn19 cluster, its resemblance with the OEC, its insolubility in aqueous solutions and facile preparation from inexpensive and environmentally benign starting materials, Mn18Sr is an attractive candidate for heterogeneous H2O oxidation catalysis.…”