Design And Synthesis Of High Valent Complexes As Model For OEC In PSII

Much research effort has been aimed at oxygen evolving center (OEC) in donor side of natural photosystem II which is responsible for the oxygenic atmosphere on the earth and provides the electrons needed for producing energy rich reduced carbon product by reducing CO2. Although the exact mechanism of photosynthetic water oxidation is not solved, it is clear that the Mn ions in the higher oxidation states of the S-cycle are at the MnⅢ state or higher. However, it is hard to prepare high valent Mn complexes with ligand with only neutral chelating nitrogen atom of two dpa arms in [Mn2(II,II)(bpmp)(μ-OAc)2]+. In order to get a Mn (Ⅲ,Ⅲ) dimmer complex with high oxidation states of manganese to increase the possibility of storing more oxidizing equivalents necessary to oxidize water evolving oxygen, some N atoms are replaced by anionic phenolate groups. Simultaneously, the stronger coordinate band O-Mn than N-Mn prevents dissociation of the Mn dimmer in aqueous solution. Introduction of tert-butyl groups to the ligand not only improves the solubility of complex, but also increases their electron donating effect that may result in lower redox potentials for the manganese redox complex. Simultaneously para orientation of tert-butyl than ortho orientation to the hydroxyl decreases the steric hindrance when water molecule attacks reaction center. The two protruding arms of morpholine on tert-butyl-phenol provide the function of anchoring water via a chain of "N-H-O~H-N" by hydrogen bound between two nitrogen in morpholine and two hydrogen in a water molecule, which is a very important improvement for the reason that the decreasing distance between anchored water and reaction center of Mn2Ⅲ,Ⅲ enhances their interaction which benefits the following water oxidation with less external influence. Thus, novel complexes has been designed and synthesized. The solubility of ligand is also improved remarkably by the introduction of tert-butyl groups. And the whole .ligand maintains the structural ascendancy of Hbpmp in analogy with Tyrz and His190.In functional center of OEC, four manganese ions were founded in the Mn cluster to mimic the OEC either on aspect of function or structure. We try to prepare multi-nuclear complexes with the ligand synthesized by organic method. Different with other models in inorganic chemistry, the new complex have a "whole" ligand which maybe very useful either for stability of the complex or the activation in catalytic oxidation.To confirm the structure of ligands, 2D-NMR technology was used including gCOSY, HSQC, HMBC. During the analysis, other meaningful information was obtained such as chemical microenvironment and three-dimensional structure, whichcould direct the design of other models.The reactions of ligand exchange were detected by ESI-MS. The results showed that OCH3 can take the place of OOCCH3 as bridge in complex, but only in the process of electrospray ionization.In the research of catalytic ability of the models in oxidation, the Fe2 complex gave good selectivity on cyclohexanone and cyclohexanol, and high activation in the oxidation of cyclohexane. Other substrates such as cyclohexene and styrene were studied with H2O2 or TBHP as oxidant in acetonitrile or acetone as solvent. The benzoic aldehyde is the main product of styrene oxidation, but when TBHP is employed, a little product of epoxidation is detected.