Self-assembled, labile, multinuclear metal complexes inspired by nature's oxygen-evolving complex of photosystem II and iron-molybdenum cofactor

Abstract

The aim of this work is to synthesize and study novel multinuclear manganese systems to model the structure and function of the oxygen-evolving complex (OEC). With the synthesis and study of these model complexes, a greater understanding of nature’s OEC mechanism and processes will come to fruition as well as a viable homogenous water-oxidation catalyst. In addition, small molecule activation was investigated using an FeI precursor. A tetramanganese “pinned-butterfly” cluster with the chemical formula Mn4(µ3-N2Ph2)2(µ-N2Ph2)(µ-NHPh)2(L)4, was synthesized via self-assembly with the addition of N, N’-diphenylhydrazine to Mn(N(SiMe3)2)2). The reaction proceeds over the course of a few hours with a visible color change pale yellow to yellow, black and finally red. The self-assembly mechanism was elucidated with methods such as ligand labeling, kinetic isotope effect, IR spectroscopy, X-ray diffractometry (single crystal and powder), UV/vis kinetic studies and absorbance studies, hydrogen-atom transfer (HAT) competition studies, NMR studies, GC studies and freezing point depression studies. The “twisted basket” cluster is a two-hydrogen-atom reduced analogue of the aforementioned “pinned-butterfly” cluster with a chemical formula of Mn4(µ-NHPh)4(µ-PhNNPh-2N,N’)2(py)4. Conversion between the clusters was investigated and achieved with the addition of an equivalent of N,N’-diphenylhydrazine and heat to a solution of the “pinned-butterfly” complex. This conversion between the clusters displays similarities to the OEC in the sense that it is undergoing proton-coupled electron transfer (PCET), cluster rearrangement and N-N bond formation. While these novel tetramanganese clusters provide us unique, reactive, and flexible clusters, they are far too sensitive to air and water to perform any useful catalysis. Due to the ligands’ lack of stabilization, alternate ligand platforms were investigated that would be able to form more rigid complexes, but retain lability. Bi- and tridentate ligands were investigated that resulted in the synthesis of several novel multinuclear homo- and hetero- metallic complexes. The ligands include polyoligimeric silsesquioxanes and substituted pyridines. These multinuclear Mn clusters show similarities to the OEC in their composition and structures. Upon exposure to air, a color change is observed without the precipitation of a manganese oxide insoluble species. This observation supports the increased stability, yet retained reactivity of the chelated clusters. Lastly, an FeI precursor was reacted with CS2 in attempts to isolate an Fe-S carbide complex and model the iron-molybdenum cofactor (FeMoco). Instead, a CS2 bridging dimer was formed and isolated. The activation of CS2 led us to attempt the reaction of the FeI precursor with other analogues such as CO2, diisopropylcarbodiimide, methylisothiocyanate, and phenylacetylene. CO2 and acetylene have been shown to be reactive substrates to the native FeMoco. These small molecule activated Fe complexes were characterized using X-ray diffraction technqiues, UV/visible spectroscopy, electron paramagnetic resonance spectroscopy and infrared spectroscopy.