Similarities of artificial photosystems by ruthenium oxo complexes and native water splitting systems

Abstract

The nature of chemical bonds of ruthenium(Ru)-quinine(Q) complexes, mononuclear ½RuðtrpyÞð3,5-t-Bu 2 QÞðOH 2 Þ ðClO 4 Þ 2 (trpy ¼ 2,2 0 : 6 0 ,2 0 0 -terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone) (1), and binuclear ½Ru 2 ðbtpyanÞð3,6-di-Bu 2 QÞ 2 ðOH 2 Þ 2þ ðbtpyan ¼ 1,8-bisð2,2 0 : 6 0 ,2 0 0 -terpyrid-4 0 -ylÞanthracene, 3,6-t-Bu 2 Q ¼ 3,6-di-tertbutyl-1,2-benzoquinone) (2), has been investigated by broken-symmetry (BS) hybrid density functional (DFT) methods. BS DFT computations for the Ru complexes have elucidated that the closed-shell structure (2b) Ru(II)-Q complex is less stable than the open-shell structure (2bb) consisting of Ru(III) and semiquinone (SQ) radical fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates that may be generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model for these species has been derived to elucidate six different effective exchange interactions (J) for four spin systems. Six J values have been determined using total energies of the eight (or seven) BS solutions for different spin configurations. The natural orbital analyses of these BS DFTsolutions have also been performed in order to obtain natural orbitals and their occupation numbers, which are useful for the lucid understanding of the nature of chemical bonds of the Ru complexes. Implications of the computational results are discussed in relation to the proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems and the similarity between artificial and native water splitting systems.four redox center | manganese clusters | water oxidation | oxyl radical | O-O bond formation P hotosynthesis is one of the most important chemical processes on our planet. Extensive experimental studies (1-6) on the process have revealed that oxygenic photosynthesis involves several protein-cofactor complexes embedded in the photosynthetic thylakoid membranes of plants, green algae, and cyanobacteria. Among these complexes, photosystem II (PSII) has a prominent role because it catalyzes the oxidation of water (, which is the prerequisite for all aerobic life. The main cyclic process to catalyze the water-oxidation consists of successive four steps, which is referred to as the Kok cycle (6). During this process, the oxygen-evolving complex (OEC), the catalyst of the water oxidation reaction, takes five oxidation states (S 0 -S 4 ). The OEC in PSII contains an inorganic cluster consisting of four manganese ions and one calcium ion that are bridged by at least five oxygens; the active site is therefore expressed with the CaMn 4 O 5 cluster (3) (5). Very recently, the electronic structure and reactivity of 3 (7-10) have been elucidated based on the new high-resolution X-ray structure (5).In the past decades, a number of experimental and theoretical studies (11-29) have been performed to design artificial photosynthetic systems that mimic native PSII systems. Many binuclear transition-metal catalysts s...

Artificial Molecular Photosynthetic Systems: Towards Efficient Photoelectrochemical Water Oxidation

“…Unlike other Ru‐based molecular catalysts, the Tanaka catalyst 6 , which is a non‐innocent quinone complex, maintains its Ru II centers during the redox process at the quinone and water units in achieving formation of oxyl radicals at a relatively low potential (0.4 V vs. Ag/AgCl). Theoretical and experimental results revealed that the removal of four protons from two molecules of water bound to ruthenium centers affords the corresponding two oxyl radicals and semiquinones via intramolecular charge transfer (Scheme ), and the subsequent four‐electron oxidation and the spin inversion achieved the O−O bond formation via intramolecular radical coupling of two oxyl species …”

“…molecular charge transfer (Scheme 4), and the subsequent four-electron oxidation and the spin inversiona chieved the OÀ Ob ond formationv ia intramolecular radicalc oupling of two oxyl species. [55][56][57] Thus, the quinone ligand functions as an efficient redox mediator in the oxidation of water by delocalizing electrons over the complex scaffold,a nd the radical coupling facilitates the formationo fabridged superoxide. One problem with that is that the replacement of the bridging peroxideb yw ater molecules is difficult within the rigid scaffold.…”

Loss of Telomere Protection: Consequences and Opportunities

“…Moreover, in breast cancer the presence of short telomeres coincides with a sudden rise in genomic instability at the transition from ductal hyperplasia to carcinoma in situ and before the activation of telomerase (Chin et al, 2004). This, together with the recent PCR-based detection of telomere fusions in early breast cancer, provides strong evidence for the occurrence of telomere crisis during breast cancer development (Tanaka et al, 2012). In addition, syndromes associated with telomere replication defects, such as dyskeratosis congenita and Werner syndrome, are characterized by short telomeres, genomic instability, and an elevated incidence of spontaneous cancer (Armanios and Blackburn, 2012).…”

Heterogene molekulare Systeme für eine photokatalytische CO2‐Reduktion mit Wasseroxidation

“…Wiei nA bbildung 2c dargestellt, folgt der Mechanismus der Wasseroxidation im PSII anerkanntermaßen einem S-Zyklus-Modell, an dem fünf Oxidationszustände beteiligt sind, die mit S 0 -S 4 bezeichnet werden (Kok-Zyklus). [72][73][74] [79][80][81][82][83][84][85][86][87][88] Iridium (Ir), [89][90][91][92][93][94] Mangan (Mn), [95,96] Molybdän( Mo), [97] Eisen (Fe), [98,99] Cobalt (Co), [100][101][102] Nickel (Ni) [103,104] und Kupfer (Cu). [105] Die Entwicklung von homogenen WOCw urde in vielen ausgezeichneten Übersichten bereits detailliert diskutiert.…”