Photo-catalytic Water Oxidation by a Mixed-Valent MnIII3MnIVO3 Manganese-Oxo-Core mimicking the Natural Oxygen Evolving Center

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi-redoxMnIII/MnIV manifold that evolves through five electronic states (Si , where i=0–4). The synthetic model system of this catalytic cycle and of its S0!S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [MnIII 3MnIVO3(CH3COO)3(A-a-SiW9O34)]6 (Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S0 state; it features a hybrid organic– inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)3]2+ and S2O8 2 is obtained by nanosecond laser flash photolysis; its S0!S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7%

Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [Cr^III(HX^VW₇O₂₈)₂]^13– (X = P, As) with Exceptionally Large Magnetic Anisotropy

Two monochromium(III)-containing heteropolytungstates, [CrIII(HPVW7O28)2]13- (1a) and [CrIII(HAsVW7O28)2]13- (2a), were prepared via simple, one-pot reactions in aqueous, basic medium, by reaction of the composing elements, and then isolated as hydrated sodium salts, Na13[CrIII(HPVW7O28)2]·47H2O (1) and Na13[CrIII(HAsVW7O28)2]·52H2O (2). Polyanions 1a and 2a comprise an octahedrally coordinated CrIII ion, sandwiched by two {PW7} or {AsW7} units. Both compounds 1 and 2 were fully characterized in the solid state by single-crystal XRD, IR spectroscopy, thermogravimetric and elemental analyses, magnetic susceptibility, and EPR measurements. Magnetic studies on 1 and 2 demonstrated that both compounds exhibit appreciable deviation from typical paramagnetic behavior, and have a ground state S = 3/2, as expected for a CrIII ion, but with an exceptionally large zero-field uniaxial anisotropy parameter (D). EPR measurements on powder and single-crystal samples of 1 and 2 using 9.5, 34.5, and 239.2 GHz frequencies and over 4–295 K temperature fully support the magnetization results and show that D = +2.4 cm–1, the largest and sign-assigned D-value so far reported for an octahedral CrIII-containing, molecular compound. Ligand field analysis of results from CASSCF and NEVPT2-correlated electronic structure calculations on Cr(OH)63– model complexes allowed to unravel the crucial role of the second coordination sphere of CrIII for the unusually large magnetic anisotropy reflected by the experimental value of D. The newly developed theoretical modeling, combined with the synthetic procedure for producing such unusual magnetic molecules in a well-defined and essentially magnetically isolated environment, appears to be a versatile new research area

Photo-assisted water oxidation by high-nuclearity cobalt-oxo cores: tracing the catalyst fate during oxygen evolution turnover

Multi-nuclear cobalt cores have been proposed as molecular analogues of the natural oxygen evolving complex, enabling water oxidation for artificial photosynthesis schemes and the production of solar fuels. In particular, cobalt containing polyoxometalates (Co-POMs) display a record activity as water oxidation catalysts (WOCs) in terms of the turnover number, turnover frequency, and quantum yield, when combined in a light activated oxygen evolving cycle with Ru(bpy)32+ (bpy = 2,2’-bipyridine) as the photosensitizer. The unique behavior of high-nuclearity cobalt clusters is addressed herein by employing Co-POMs with Co ≥9 as molecular WOCs. The temporal dissection of the catalytic events is framed herein to investigate the initial photo-induced electron transfer (ET) occurring in the micro-to-millisecond time domain, and followed by the oxygen evolution kinetics taking place within a minute-to-hours regime. In particular, flash photolysis shows ET from the Co-POM to photogenerated Ru(bpy)33+ with well-behaved diffusional kinetics (bimolecular rate constants in the range kET = 2.1–5.0 × 109 M−1 s−1) and counting up to 32 ET events in a 60 ms timeframe. The evolution of the Co-POMs is then traced under oxygenic conditions, where infrared and X-ray absorption spectroscopy (XAS) indicate that POM based structures are competent catalysts under the photo-assisted turnover regime