the influence of phosphate on structure and activity

Two types of manganese oxides have been prepared by hydrolysis of tetranuclear Mn(III) complexes in the presence or absence of phosphate ions. The oxides have been characterized structurally using X-ray absorption spectroscopy and functionally by O2 evolution measurements. The structures of the oxides prepared in the absence of phosphate are dominated by di-μ-oxo bridged manganese ions that form layers with limited long-range order, consisting of edge-sharing MnO6 octahedra. The average manganese oxidation state is +3.5. The structure of these oxides is closely related to other manganese oxides reported as water oxidation catalysts. They show high oxygen evolution activity in a light-driven system containing [Ru(bpy)3]2+ and S2O82− at pH 7. In contrast, the oxides formed by hydrolysis in the presence of phosphate ions contain almost no di-μ-oxo bridged manganese ions. Instead the phosphate groups are acting as bridges between the manganese ions. The average oxidation state of manganese ions is +3. This type of oxide has much lower water oxidation activity in the light-driven system. Correlations between different structural motifs and the function as a water oxidation catalyst are discussed and the lower activity in the phosphate containing oxide is linked to the absence of protonable di-μ-oxo bridges

First turnover analysis of water-oxidation catalyzed by Co-oxide nanoparticles

Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Presently, several different proposals exist for how they catalyze O2 formation from water. Knowledge about this process at molecular detail will be required for their further improvement. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet mass spectrometry (MIMS) experiments to study the mechanism of water oxidation in Co/methylenediphosphonate (Co/M2P) oxide nanoparticles using [Ru(bpy)3]3+ (bpy = 2,2'-bipyridine) as chemical oxidant. We show that 16O–Co/M2P-oxide nanoparticles produce 16O2 during their first turnover after simultaneous addition of H218O and [Ru(bpy)3]3+, while sequential addition with a delay of 3 s yields oxygen reflecting bulk water 18O-enrichment. This result is interpreted to show that the O–O bond formation in Co/M2P-oxide nanoparticles occurs via intramolecular oxygen coupling between two terminal Co–OHn ligands that are readily exchangeable with bulk water in the resting state of the catalyst. Importantly, our data allow the determination of the number of catalytic sites within this amorphous nanoparticular material, to calculate the TOF per catalytic site and to derive the number of holes needed for the production of the first O2 molecule per catalytic site. We propose that the mechanism of O–O bond formation during bulk catalysis in amorphous Co-oxides may differ from that taking place at the surface of crystalline materials

Light-induced multistep oxidation of dinuclear manganese complexes for artificial photosynthesis

Two dinuclear manganese complexes, [Mn2BPMP(@m-OAc)2].ClO4 (1, where BPMP is the anion of 2,6-bis{[N,N-di(2-pyridinemethyl)amino]methyl}-4-methylphenol) and [Mn2L(@m-OAc)2].ClO4 (2, where L is the trianion of 2,6-bis{[N-(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2-pyridinemethyl)amino]met hyl}-4-methylphenol), undergo several oxidations by laser flash photolysis, using rutheniumII-tris-bipyridine (tris(2,2-bipyridyl)dichloro-ruthenium(II) hexahydrate) as photo-sensitizer and penta-amminechlorocobalt(III) chloride as external electron acceptor. In both complexes stepwise electron transfer was observed. In 1, four Mn-valence states from the initial Mn2II,II to the Mn2III,IV state are available. In 2, three oxidation steps are possible from the initial Mn2III,IIIstate. The last step is accomplished in the Mn2IV,IV state, which results in a phenolate radical.For the first time we provide firm spectral evidence for formation of the first intermediate state, Mn2II,III, in 1 during the stepwise light-induced oxidation. Observation of Mn2II,III is dependent on conditions that sustain the @m-acetato bridges in the complex, i.e., by forming Mn2II,III in dry acetonitrile, or by addition of high concentrations of acetate in aqueous solutions. We maintain that the presence of water is necessary for the transition to higher oxidation states, e.g., Mn2III,III and Mn2III,IV in 1, due to a bridging ligand exchange reaction which takes place in the Mn2II,III state in water solution. Water is also found to be necessary for reaching the Mn2IV,IV state in 2, which explains why this state was not reached by electrolysis in our earlier work (Eur. J. Inorg. Chem (2002) 2965).In 2, the extra coordinating oxygen atoms facilitate the stabilization of higher Mn valence states than in 1, resulting in formation of a stable Mn2IV,IV without disintegration of 2. In addition, further oxidation of 2, led to the formation of a phenolate radical (g=2.0046) due to ligand oxidation. Its spectral width (8 mT) and very fast relaxation at 15 K indicates that this radical is magnetically coupled to the Mn2IV,IV center

FTIR Study of Manganese Dimers with Carboxylate Donors As Model Complexes for the Water Oxidation Complex in Photosystem II

The carboxylate stretching frequencies of two high-valent, di-μ-oxido bridged, manganese dimers has been studied with IR spectroscopy in three different oxidation states. Both complexes contain one monodentate carboxylate donor to each Mn ion, in one complex, the carboxylate is coordinated perpendicular to the Mn-(μ-O)₂-Mn plane, and in the other complex, the carboxylate is coordinated in the Mn-(μ-O)₂-Mn plane. For both complexes, the difference between the asymmetric and the symmetric carboxylate stretching frequencies decrease for both the Mn₂^IV,IV to Mn₂^III,IV transition and the Mn₂^III,IV to Mn₂^III,III transition, with only minor differences observed between the two arrangements of the carboxylate ligand versus the Mn-(μ-O)₂-Mn plane. The IR spectra also show that both carboxylate ligands are affected for each one electron reduction, i.e., the stretching frequency of the carboxylate coordinated to the Mn ion that is not reduced also shifts. These results are discussed in relation to FTIR studies of changes in carboxylate stretching frequencies in a one electron oxidation step of the water oxidation complex in Photosystem II