Mn And Fe Clusters Modeling The Oxygen-evolving Center And The β-diketone Cleaving Dioxygenase

More than one-third of enzymes and protein require metals.The most common redox-active metals are Fe and Mn,which play essential roles in the most important biological processes,such as photosynthesis,respiration and nitrogen fixation.Metalloenzymes have attracted much attention for their mild reaction conditions,specific selectivity and high catalytic efficiency.Since the first discovery of metal centers in its active sites of enzymes,more and more structures of metalloenzymes and metalloproteins have been determined with the rapid development of various techniques.While the steric configuration and coordination environment of proteins are so complicated and important that the active sites often greatly reduce or completely eliminate function when isolated from their local environment.People begun to synthesize and optimize artificial model compounds to simulate the properties of natural enzymes.The approach of biomimetic chemistry can be divided into structural simulation and functional simulation.A structural analogue focuses on constituent elements,oxidation state（s）,ligand binding,and stereochemistry.A functional analogue sustains a stoichiometric,or better,a catalytic reaction which transforms substrate to product as does the enzyme.In natural enzymes,metal ions are mostly coordinated with amino acid residues.Therefore,regardless of structural or functional analogue,the corresponding model compounds are often synthesized by coordinative methods.The skeleton of ligand plays a vital role in coordination chemistry,and C₃ symmetry ligands frequently appeared in biomimetic synthesis,such as the application of substituted1,3,5-triphenylbenzene ligands in the mimic of oxygen evolving center（OEC）.In the past years,2-pyridine methanol（hmp）and 1,3-diol ligands are often utilized in the synthesis of transition metal clusters and have formed various structures.The coordinating complementarity of these two independent motifs enlightened us to‘‘merge”them together into a new polydentate ligand system.The aim of this dissertation is to synthesize Mn and Fe clusters which can be used as model compounds using the merged pseudo C₃ symmetric ligands.Chapter 1 briefly introduces the research background of biomimetic simulation and the selection of ligands.In chapter 2,four structural model compounds of OEC have been synthesized:[Fe₃^ⅢZn₂^Ⅱ（L₁）₃O（OAc）₃（CH₃OH）]·（Cl O₄）₂·（CH₃OH）_1.5（compound1）,[Fe₃^ⅢCo₂^Ⅱ（L₁）₃O（OAc）₃（CH₃OH）]·（Cl O₄）₂·（sol）_x（compound2）,[Fe₃^ⅢMn₂^Ⅱ（L₁）₃O（OAc）₃（CH₃OH）]·（Cl O₄）₂·（CH₃OH）_1.5（compound 3）,and[Fe₃^ⅢNi₂^Ⅱ（L₁）₃O（OAc）₂（OCH₃）（CH₃OH）（H₂O）]·（Cl O₄）₂（compound 4）.Compound 1,2,and 3 are isostructural,consisting of a[MFe₃O₄]cubane linked to a pentacoordinated‘‘dangler’’M(M=Zn^Ⅱ,Co^Ⅱ,and Mn^Ⅱ).ESI-MS measurements were conducted and demonstrated that the core structures of all compounds remain intact.Electrochemical investigation revealed that the reduction potentials of the iron clusters can be tuned by the Lewis acidity of the incorporated heterometal ions.Replacement of Zn^Ⅱ by Ni^Ⅱproduced a new model compound that perfectly mirrors the‘‘cubane-plus-dangler’’skeleton and the connectivity between the dangling ion and the parent cubane core of the native OEC.Chapter 3 mainly mimics the photoactivation process of OEC after light induced damage.First,we obtained[Mn₂^Ⅱ（HL₁）₂Cl₂（CH₃OH）₂]（compound 5）under anaerobic ambient.When exposed to the air,[Mn₃^ⅡMn^Ⅲ（L₁）₃（OCH₃）Cl₂]·（CH₃OH）₄（compound6）and[Mn₃^ⅡMn^Ⅳ（L₁）₃（OCH₃）Cl₃]·（CH₃OH）_2.25（compound 7）were generated subsequently.Thus,the self-assembly process undergoes[Mn₂^Ⅱ]-[Mn₃^ⅡMn^Ⅲ]-[Mn₃^ⅡMn^Ⅳ]in the same ligand system without introduction of extra oxidant.After treated with quaternary ammonium permanganate,only 7 was obtained,and the oxidation state of manganese clusters did not continue to increase.The mononuclear compound[Mn^Ⅳ（L₁）₂]·（CH₃CN）（compound 8）was formed by using high valent manganese salt as the initial material.[Mn₆^ⅡMn₂^Ⅲ（L₁）₆O₂（N₃）₂（CH₃OH）₂]（compound9）with double"cubane"skeleton was obtained by changing the counter-ion and introducing co-ligand.Magnetic measurements of the mixed valence manganese clusters（6,7,and 9）were conducted subsequently.In chapter 4,we synthesized[Mn ₃^Ⅲ（L₂）₂（HL₂）（acac）]·（CH₃OH）_2.5·（H₂O）（compound10）in anaerobic,which exhibits defected-cubane configuration with a acetylacetone chelated to the motif.When operated in the air,[Mn₄^ⅢMn₂^ⅡK₂（L₂）₄（HL₂）₂（oxa）（CH₃OH）₂（H₂O）₂]（compound 11）was obtained and an oxalate anion was identified in the structure determined by single crystal x-ray diffraction.GC-MS and ¹⁸O labeled experiment indicated that oxalate was derived from twice dioxygenated cleavage of acetylacetone,with the formation and decomposition of pyruvate as intermediate product.The mechanism resembles native acac oxidative cleavagecatalyzedby（？）-diketonecleavingdioxygenase.[Mn^ⅢMn₂^Ⅱ（L₁）₂（acac）₂（OAc）]·（H₂O）（compound 12）was obtained via systematic ligand modification and solvent variation,and the stepwise degradation can be terminated at the stage of pyruvate.Chapter 5 briefly summarizes the above chapters.