A manganese oxido complex bearing facially coordinating trispyridyl ligands - is coordination geometry crucial for water oxidation catalysis?Electronic supplementary information (ESI) available: details of magnetic susceptibility measurements, resonance Raman spectra with different excitation wavelength, cyclic voltammogram of 1 at different scan rates, UV/Vis OTTLE spectra of electrochemical re-reduction at 500 mV after oxidation at 1200 mV, UV/Vis spectra after oxidation with tBuOOH, UV/Vis OT

• Main Authors: Berends, Hans-Martin, Manke, Anne-Marie, Näther, Christian, Tuczek, Felix, Kurz, Philipp
• Format: Journal Article
• Language: English
• Published: 01.05.2012
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c2dt30129f

In this work the synthesis of the novel manganese complex [Mn 2 III,III (tpdm) 2 (μ-O)(μ-OAc) 2 ] 2+ ( 1 ) is reported, containing two manganese centres ligated to the unusual, facially coordinating, all-pyridine ligand tpdm (tris(2-pyridyl)methane). The geometric and electronic properties of complex 1 were characterised by X-ray crystallography, vibrational (IR and Raman) and optical spectroscopy (UV/Vis and MCD). Cyclic voltammograms of 1 showed a quasi-reversible oxidation event at 950 mV and an irreversible reduction wave at −250 mV vs. Ag/Ag + . The redox behaviour of the compound was investigated in detail by UV/Vis- and X-band EPR-spectroelectrochemistry. Both electrochemical (+1200 mV) and chemical ( t BuOOH) oxidations transform 1 into the singly oxidized di-μ-oxido species [Mn 2 III,IV (tpdm) 2 (μ-O) 2 (μ-OAc)] 2+ . Further electrochemical oxidation at the same potential results in the removal of a second electron to obtain a Mn 2 IV,IV -species. The ability of compound 1 to evolve O 2 was studied using different reaction agents. While reactions with both hydrogen peroxide and peroxomonosulfate yield O 2 , homogeneous water-oxidation using Ce IV was not observed. Nevertheless, the oxidation reactions of 1 are very interesting model processes for oxidation state (S-state) transitions of the natural manganese water-oxidation catalyst in photosynthesis. However, despite its favourable coordination geometry and multielectron redox chemistry, complex 1 fails to be a catalytically active model for natural water-oxidation. A novel dimanganese( iii , iii ) complex has been synthesised and fully characterised in order to elucidate the importance of the geometrical arrangement of the complex for water oxidation catalysis.