Self-Assembled Structurally and Functionally Mimetic Metalloclusters Inspired by the Oxygen-Evolving Complex and the Iron-Molybdenum Cofactor

Abstract

As society continues to develop, demand for energy is destined to increase. Energy production and consumption are responsible for a majority of greenhouse gas emissions that cause climate change, biodiversity loss, and can generate toxic waste. Today, much of our energy demands are met via nonrenewable resources, particularly by the combustion of fossil fuels. In order to correct recent fluxes of manmade species into the geochemical cycles, we must transition to alternate forms of energy production and consumption that do not imbalance the environment. The transition to a solar hydrogen economy is attractive because it utilizes sunlight and water—virtually unlimited resources, to produce hydrogen fuel and gaseous oxygen as a by-product. Efficient and affordable catalytic systems are necessary to establish a solar-hydrogen economy. Industrial nitrogen fixation plants require an immense amount of energy for maintaining operations aimed at the production of ammonia, an essential chemical for society. Further development of catalytic materials may serve to facilitate the process under less taxing conditions.Nature has resulted in the evolution of a wide variety of enyzmes to catalytically carry out chemical reactions. Billions of years of evolution have resulted in the incorporation of metals readily available in the environment. Enzymes may provide inspiration for the design of novel catalytic systems utilizing abundant elements to result in efficient and affordable materials. To this breath, to understand the structure and function of enzymes of interest, particularly the active sites of metalloenzymes. A useful tool is the comparison of native systems to molecular model complexes. It is thus imperative to synthesize and characterize model complexes that are both structurally and functionally relevant. Of interest to this work are the oxygen-evolving complex (OEC) of photosystem II and the iron-molybdenum cofactor (FeMoco) of molybdenum dependent nitrogenase enzymes. Model complexes of these active sites have been widely investigated, however, OEC and FeMoco model complexes generally fall into one category or the other: structural or functional models. The aim of this work is to synthesize and study synthetic multinuclear metallocomplexes to model the structure and function of the OEC and FeMoco. The lessons learned from these systems could contribute to the understanding and design of catalysts and help progress toward viable catalysts. Presented here is the synthesis and characterization of a series of OEC structural models with hemicubane geometry, derived from self-assembled precursor fragments. In chapter 2, a series of Ca-Mn clusters, with the ligand 2-pyridinemethoxide (Py-CH2O), have been prepared with varying degrees of topological similarity to the CaMn4 oxygen evolving complex (OEC) of photosystem II. Through variation of reagent stoichiometry, Mn4, CaMn3, CaMn5, Ca2Mn4, and Ca4Mn3 clusters are prepared via self-assembled precursor fragments and fully characterized. All products feature a hemicubane motif (a heterocubane structure minus one corner metal ion) with Mn or Ca atoms bridged by oxygen, while containing water-accessible metal coordination sites. In chapter 3, the catalytic activation of water as a substrate in the oxidative degradation of stable propylene carbonate into CO2 is discussed. The propensity of these clusters to activate water in this oxidation reaction is correlated with increasing topological similarity to the OEC. In chapter 4, a series of analogous molecules have been explored with the incorporation of calcium, cobalt, and iron metal centers. Preliminary studies indicate the ability to perform water oxidation by a calcium complex with redox-active ligands. The synthesis, characterization, and electrocatalytic water activation chemistry will be discussed. Chapter 5 focuses on the synthesis and characterization of a [FeII(NR2)2]2CS3 complex, resulting from the activation of CS2 by an FeI(NR2)3 precursor species. This work is an extension of attempts toward achieving a synthetic central carbide included within a synthetic Fe-S cluster, which previously resulted in the activation of CS2 by a reduced FeI(NR2)3 species to obtain {[Fe(NR2)2]2CS2}[K(18-crown-6)1.5]2, where a bridging CS2 group assumes a bent geometry. Understanding the electronic structure and magnetic properties of this model complex is relevant towards the design and development of catalysts used in the production of ammonia via industrial nitrogen fixation. Presented in chapter 6 is the discovery of a layered Fe-S material that consists of alternating sheets of an Fe-S layer and a potassium layer. The crystallographic structure suggests the inclusion of periodic carbon atoms with in the Fe-S layer, although these sites may also be modeled with partial Fe occupancy. X-ray diffraction methods (single crystal and powder diffraction) and X-ray photoelectron spectroscopy were employed to investigate the composition of this material.