Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts.

The ever-growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO4 3- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na15 [Ge4 PCo4 (H2 O)2 W24 O94 ] ⋅ 38H2 O (Co4 ), Na2.5 K17.5 [Ge3 PCo9 (OH)5 (H2 O)4 W30 O115 ] ⋅ 45H2 O (Co9 ), Na6 K16 [Ge4 P4 Co20 (OH)14 (H2 O)18 W36 O150 ] ⋅ 61H2 O (Co20 ) with non-, quasi-, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4 Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT-scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well-known Na10 [Co4 (H2 O)2 (PW9 O34 )2 ] {Co4 P2 W18 } by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s-1 Co9 currently represents the fastest Co-GT-based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4 .

Reevaluation of the kinetics of polynuclear mimics for manganese catalases.

Considerable effort has been expended in order to understand the mechanism of manganese catalases and to develop functional mimics for these enzymes. For many years, the most efficient reactivity mimic was [MnIVsalpn(mu-O)]2 [H2salpn = 1,3-bis(salicylideneiminato)propane], a compound that cycles between the MnIV2 and MnIII2 oxidation levels instead of the MnII2 and MnIII2 oxidation states used by the enzyme, with kcat = 250 s(-1) and kcat/KM = 1000 M(-1) s(-1). Recently, a truly exceptional high value of kcat was reported for the complex [Mn(bpia)(mu-OAc)]22+ [bpia = bis(picolyl)(N-methylimidazol-2-yl)amine]. On the basis of a calculated kcat value of 1100 s(-1) and an efficiency kcat/KM of 34 000 M(-1) s(-1), this complex has been suggested to represent a significant breakthrough in catalytic efficiencies of manganese catalase mimics. However, a plot of ri/[cat]T vs [H2O2]0, where the saturation value approaches 1.5 s(-1), is inconsistent with the 1100 s(-1) value tabulated for kcat. Similar discrepancies are observed for two other families of manganese complexes containing either a Mn2(mu-OPh)22+ core and different substituted tripodal ligands or complexes of methyl and ethyl salicylimidate, with an Mn2(mu-OPh)24+ core. Reevaluation of the kinetic parameters for these three systems reveals that the originally reported values were overestimated by a factor of approximately 1000 for both kcat and kcat/KM. We discuss the origin of the discrepancy between the previously published kinetic parameters and the newly derived values. Furthermore, we provide a short analysis of the existing manganese catalase mimics in an effort to provide sound directions for future investigations in this field.

Comparison of the Manganese Cluster in Oxygen-Evolving Photosystem II with Distorted Cubane Manganese Compounds through X-ray Absorption Spectroscopy.

X-ray absorption spectroscopy has been employed to assess the degree of similarity between the oxygen-evolving complex (OEC) in photosystem II (PS II) and a family of synthetic manganese complexes containing the distorted cubane [Mn(4)O(3)X] core (X = benzoate, acetate, methoxide, hydroxide, azide, fluoride, chloride, or bromide). These [Mn(4)(&mgr;(3)-O)(3)(&mgr;(3)-X)] cubanes possess C(3)(v)() symmetry except for the X = benzoate species, which is slightly more distorted with only C(s)() symmetry. In addition, Mn(4)O(3)Cl complexes containing three or six terminal Cl ligands at three of the Mn were included in this study. The Mn K-edge X-ray absorption near edge structure (XANES) from the oxygen-ligated complexes begin to resemble general features of the PS II (S(1) state) spectrum, although the second derivatives are distinct from those in PS II. The extended X-ray absorption fine structure (EXAFS) of these Mn compounds also displays superficial resemblance to that of PS II, but major differences emerge on closer examination of the phases and amplitudes. The most obvious distinction is the smaller magnitude of the Fourier transform (FT) of the PS II EXAFS compared to the FTs from the distorted cubanes. Curve fitting of the Mn EXAFS spectra verifies the known core structures of the Mn cubanes, and shows that the number of the crucial 2.7 and 3.3 Å Mn-Mn distances differs from that observed in the OEC. The EXAFS method detects small changes in the core structures as X is varied in this series, and serves to exclude the distorted cubane of C(3)(v)() symmetry as a topological model for the Mn catalytic cluster of the OEC. Instead, the method shows that even more distortion of the cubane framework, altering the ratio of the Mn-Mn distances, is required to resemble the Mn cluster in PS II.