Synthesis And Properties Of Metal Corrole Complexes Modified By Tyrosine

The four-electron oxidation of water in photosynthetic organisms is achieved by anoxygen-evolving complex (OEC) in photosynthesis, which is one of the most fundamentalchemical processes in nature. A series of X-ray structure analysis of photosystemâ…¡(PSâ…¡)indicated that the active site in a photosynthetic OEC protein contains a Mn₄Ca cluster and aneighboring tyrosine (Tyr_z) is linked between P₆₈₀ and OEC. By light absorption of theprimary electron donor P₆₈₀ consisting of chlorophyllαmolecules, electron transfer occursfrom the excited P₆₈₀^* to a primary electron acceptor pheophytin and subsequently to twoquinines, forming P₆₈₀ radical cation. The photo-oxidized P₆₈₀ retrieves the electron from aMn₄Ca cluster via Tyr_z. After four photo-processes, two molecular water are oxidized to amolecular oxygen. Tyr_Z is crucial for stabilization of the primary charge separation. As toelectron donor, oxidation of water at the Mn₄Ca cluster occurs in five steps and involves sixoxidation states (S₀-S₄). Though the mechanism of dioxygen evolution has not beendetermined completely, the stage of oxygen evolution would involve a high valent manganesespecies. How to obtain a stable ligand to stable high valent manganese is still a urgentproblem. Corroles are aromatic tetrapyrrole macrocycles as a ring-contracted analogue ofporphyrins, which have been proved to stabilize higher oxidation states for coordinated metalscompare to the more famous porphyrins. Therefore, the ligand 4 which contains a corrole anda tyrosine connected through an amide bond was designed, as well as its Mn complex 4b forthe study of water oxidation.The synthesis starts from a dipyrromethane compound 1 prepared according to theliterature. After three steps of corrole formation, reduction of the nitro group in 2 andformation of amido bond by DCC, the armed corrole compound 4 is synthesized. Asubsequent metal ions complexation gives corresponding complexes 4a and 4b. The coppercomplexes 2a and 3a of the intermediates 2 and 3 were also prepared for comparison. Most ofseven new complexes are characterized by ¹H NMR, MS, UV-vis, mass-spectrometry,elemental analysis. Through the measurements of MS, UV-vis, the valence state for thecentral metal ion of manganese complexes 4b is confirmed to be Mn(â…£). The emissionproperty indicates that the emission of ligands 2 and 3 is strong, but no emission is observedfor their metal complex due to the extermination of metal center. A remarkable dimerizationis found for 4a in contrast of no dimerization is observed for 2a and 3a in electrochemical oxidations in CH₂Cl₂. The first two oxidations of 4a occur via two stepwise abstractions ofone electron from each macrocycle in the electrogenerated corroleÏ€-Ï€dimer. Though nooxygen was detected in the catalyzing water oxidation of metal complexes 4a and 4b byelectrochemical, chemical, photochemical processes, the target tyrosine modified metalcorrole complexes can be also seemed as a model in simulating the structure of OEC.