• New Synthetic Approaches and Structural Models of the Oxygen-Evolving Complex in Photosystem II from the Use of Oximato-Based Ligands

      Alaimo, Alysha; Department of Chemistry
      The employment of the chelating/bridging ligands salicylhydroxime (shiH3), quinoline-2-aldoxime (qaoH) and 2,6-diacetylpyridine dioxime (dapdoH2) in heterometallic Mn‒Ca chemistry has afforded various compounds with diverse topologies, metal stoichiometries and Mn oxidation state descriptions. Chapter 1 provides a general introduction to the oxygen-evolving complex (OEC) of Photosystem II (PSII) including discussions of fundamental aspects such as composition, structural proposals, mechanism of O‒O bond formation and synthetic approaches. My research results are reported in Chapter 2, 3 and 4. In the first project (Chapter 2), one-pot reactions between Mn(ClO4)2∙6H2O, Ca(ClO4)2∙4H2O and the potentially tetradentate chelating/bridging ligand salicylhydroxime (shiH3), resulting from the in situ metal ion-assisted amide-iminol tautomerism of salicylhydroxamic acid (shaH2), in the presence of various fluorescence carboxylate groups (2-naphthoic acid = L1-H; 9-anthracenecarboxylic acid = L2-H; 1-pyrenecarboxylic acid = L3-H) and base NEt3 has led to a family of structurally similar {MnIII4Ca} clusters (1‒4¬) with distorted square pyramidal topologies. The combined results demonstrate the ability of shiH3 and fluorescence carboxylates to yield new heterometallic Mn‒Ca clusters with (i) the same Mn‒Ca ratio as the OEC of PSII, (ii) structural stability in solution, (iii) a pronounced redox and optical activity and (iv) predominant antiferromagnetic exchange interactions with S = 0 spin ground states. These complexes may be relevant to lower oxidation level species of the catalytic cycle of the OEC. The second project of this thesis, discussed in Chapter 3, involved one-pot reactions between the [Mn3O(O2CPh)6(py)x]+/0 triangular precursors and either CaBr2∙xH2O or CaCl2∙6H2O in the presence of shaH2. This afforded the heterometallic complexes [MnIII4Ca2(O2CPh)4(shi)4(H2O)3(Me2CO)] (5) and (pyH)[MnII2MnIII4Ca2Cl2(O2CPh)7(shi)4(py)4] (6), respectively, in good yields. Further reactions but using a more flexible synthetic scheme comprising the Mn(NO3)2∙4H2O/Ca(NO3)2∙4H2O and Mn(O2CPh)2∙2H2O/Ca(ClO4)2∙4H2O “metal blends” and shaH2 in the presence of external base NEt3, led to the new complexes (NHEt3)[MnIII4MnIV4Ca2(OEt)2(shi)10(EtOH)2] (7) and (NHEt3)4[MnIII8Ca2(CO3)4(shi)8] (8), respectively. Solid-state dc magnetic susceptibility studies of 5‒8 revealed the presence of predominant antiferromagnetic exchange interactions between the Mn centers, leading to S = 0 spin ground state values. From a bioinorganic chemistry perspective, these compounds may demonstrate some relevance to both the high-valent scheme (7) and lower oxidation level species (5, 6 and 8) of the catalytic cycle of the OEC. In the last chapter of this thesis (Chapter 4), the ligands quinoline-2-aldoxime (qaoH) and 2,6-diacetylpyridine dioxime (dapdoH2) were introduced for a first time in heterometallic Mn‒Ca chemistry. This afforded a mixed-valence {MnII/III22Ca2} (9) cluster containing several {Mn4CaOx} subunits and a butterfly-like {MnIV2Ca2} (10) complex, respectively. These compounds demonstrate structural and magnetic relevance to both the low- and high-valent states of the OEC. All research-based Chapters (Chapter 2‒4) are divided into subsections in order to facilitate the understanding of the research concepts by the familiar and non-familiar readers and contextualize the messages, goals and conclusions of each individual project. I felt it was appropriate to begin each Chapter with a short preface of the work that summarizes the most important aspects of the specific project, followed by the complete experimental work and discussion of the results, and end with conclusions and some future perspectives.