Mimic Of Oxygen Elvolution Center (OEC): Synthesis, Properties And Catalytic Water Oxidation Of Metal Corrole Complexes

Solar energy is converted into chemical energy by photosynthesis in nature, a processwhich keeps all living forms to survive on the earth. The electrons used to sustain the energycome from the light driven water oxidation. This happens in a so called reaction center inPhotosystemâ…¡(PSâ…¡), where a tetrameric chlorophylls (P680) absorb the solar light andinitiate photoinduced electron transfer reaction to the acceptors, quinone A and B. Thephoto-oxidized P680 retrieves the electron from a Mn4Ca cluster via a tyrosine(Tyr-Z), thelatter components are also called oxygen evolving center (OEC). After 4 photo-processes, twomolecular water are oxidized to a molecular oxygen. In this thesis, efforts to make syntheticmodels of OEC aiming for catalytic water oxidation driven either by electrochemistry or bylight have been made. Three different model systems have been designed and synthesized: 1.two corrole xanthene ligands 5, 6 and four corresponding high valence Mn^â…£(7, 9) and Cu^â…¢(8, 10) complexes for the mimic of oxygen evolution center (OEC); 2. Boc-protectedtyrosine-attached corrole compounds 13, 14 and corresponding high valence copper (15, 17)and manganese (16, 18, 18') complexes for the mimic of Tyr-Z and Mn4Ca in OEC; 3. Acopper-corrole complex linked to a ruthenium(â…¡) tris(bipyridine) complex 22 with an amidebond to mimic the P₆₈₀ and OEC. Totally 20 new complexes are synthesized, andcharacterized by ¹H NMR, MS, UV-vis, mass-spectrometry, elemental analysis, et al.The synthesis starts from a nitro-corrole compound 2 which was prepared according to theliterature method. After reduction of the nitro group in 2, the obtained amine compound isused to synthesize the desired single armed corrole compound 5 (HCX) and the double armedcorrole compound 6 (BCX). A subsequent coordination of metal ions to 5 and 6 givescorresponding complexes 7, 8, 9 and 10. Through the measurements of MS, UV-vis andelectrochemistry, the valence state for the central metal ion of manganese complexes 7 and 9is confirmed to be Mn(â…£). By studying the cyclic voltammetry (CV) of metal complexes7～10, it was found that the manganese complexes can be relatively easily oxidized to highervalence and the corrole ligands are stable enough to steady high valent metal ions. Bothmanganese complexes 7 and 9 have shown catalytic water oxidation to evolve molecularoxygen at low potential (about 0.80 V vs Ag/Ag+) by electrochemical method and thegenerated O₂ was further confirmed by oxygen electrode detection. The complex 9 has highercatalytic activity than complex 7 on oxygen evolution. We also studied the catalytic functionsof copper corrole complexes, but no molecular oxygen was detectedin similar experimentalcondition. In order to mimic the function of Tyr in OEC, tyrosine-attached complexes 15～18, 18' werealso synthesized. In the same manner, we can conform that the valence of Mn ion in complex16, 18 is Mn(IV) and 18' is Mn(â…¢). We studied the CV of metal Tyr-corrole complexes, andfound the oxidation potential of Tyr units was higher than the metal corrolering. The resultssuggested that the Tyr radical can oxidize metal corrole ring through electron transfer.Furthermore, we obtained a crystal structure of 17, which displays the relative position of Tyrto the high valent metal center and provided more useful information on mimicking the activecenter of OEC. We also studied the catalyzing water oxidation of metal complexes byelectrochemical, chemical, photochemical methods, but no oxygen was detected.Our final target is to obtain light-driven water oxidation. Therefore, we combined aphotosensitizer Ru(bpy)₃ with the Cu-corrole complex and we synthesized theRu(bpy)₃-Cu-corrole complex 22 in two different way. We found that the oxidation potentialof [Ru(bpy)₃]²⁺ unit was higher than the metal corrole unit. In addition, the emission ofRu(bpy)₃Cu-corrole complex 22 was found to have a substantial quenching when the MLCTof [Ru(bpy)₃]²⁺ was selectively photoexcited compared to the parent [Ru(bpy)₃]²⁺. Thisobservation suggested a quenching mechanism with possible intramolecular electron transferor energy transfer between the [Ru(bpy)₃]²⁺ moiety and the Cu corrole moiety.Insummary, we built up a relatively integrated model system to mimic either thestructure or the function of P₆₈₀, Tyr_z and OEC by using high valence Mn(â…£)-corrolecomplexes. Catalytic water oxidation to evolve molecular oxygen in a basic condition byelectrochemical method has been achieved. However, no catalytic water oxidation has beenfound with Cu(â…¢)-corrole complexes in the similar experimental condition.