Spectroscopic examination of host-guest interactions in metalloporphyrin-based metal-organic frameworks
Creators: name: Lahanas, Nicole-Irene Olivia
Publisher: name: No Publisher Supplied
Year: 2019

description: At the center of heme-based protein systems, the metalloporphyrin has been extensively studied to yield better understanding of biological systems and their ability to reversibly bind small molecules. When metalloporphyrin units are strategically used as linkers in MOF materials, their associated functionality can be harnessed for potential catalytic processes occurring within the pores of these solid-state networks. The MOF format affords high densities of accessible metal reaction sites while preventing porphyrin dimerization and other undesirable deactivation processes that would normally occur in solution environments, thus promoting a new generation of heterogenous catalytic materials. Despite significant literature precedent for metalloporphyrin-based MOF catalysis, the underlying host-guest chemistry and catalytic reaction mechanisms are often unclear. Thus, a comprehensive understanding of how these frameworks interact with various guest molecules on a molecular level is crucial. Conventional characterization methods of crystalline solid-state materials such as single crystal XRD are helpful for examining local structure but have their limitations. Therefore, in this dissertation, more structurally sensitive characterization methods such as Raman, X-ray absorption, and X-ray emission spectroscopy techniques are utilized in addition to conventional methods like XRD to obtain electronic and structural information of the host-guest interaction on a molecular level. A brief summary of each chapter is provided below.Chapter 1 introduces relevant background information for the research topics in this thesis. The overview starts with a general introduction of host-guest systems, followed by an introduction to metal-organic frameworks, porphyrins, and finally porphyrin-based metal-organic frameworks. Lastly, the chapter concludes with a summary of the spectroscopic techniques employed in this research, namely Raman, X-ray absorption and X-ray emission spectroscopy. Chapter 2 focuses on two isostructural metal-organic frameworks based on cobalt(II) and nickel(II) metalloporphyrin linkers, Co-PCN222 and Ni-PCN222, which are investigated using resonance Raman and X-ray absorption spectroscopy. The spectroscopic consequences of framework formation and host–guest interaction with weakly and strongly coordinating guest molecules (acetone and pyridine) are assessed. Structure sensitive vibrational modes of the resonance Raman spectra provide insights on the electronic and structural changes of the porphyrin linkers upon framework formation. XANES and EXAFS measurements reveal axial binding behavior of the metalloporphyrin units in Co-PCN222, but almost no axial interaction with guest molecules at the Ni porphyrin sites in Ni-PCN222. Chapter 3 discusses how probing small-molecule interactions at the metalloporphyrin sites within MOF materials on a molecular level under ambient conditions is crucial for both understanding and ultimately harnessing this functionality for potential catalytic purposes. Co-PCN-222, a metal−organic framework based on cobalt(II) porphyrin linkers, is investigated using in-situ UV−vis diffuse reflectance and X-ray absorption spectroscopy. Spectroscopic evidence for the axial interaction of diatomic oxygen with the framework’s open metalloporphyrin sites at room temperature is presented and discussed.Chapter 4 is a systematic comparison of host−guest interactions in two iron porphyrin-based metal−organic frameworks, FeCl-PCN222 and FeCl-PCN224, with drastically different pore sizes and geometries. Guest molecules (acetone, imidazole, and piperidine) of different sizes, axial binding strengths, and reactivity with the iron porphyrin centers are employed to demonstrate the range of possible interactions that occur at the porphyrin sites inside the pores of the MOF. Binding patterns of these guest species under the constraints of the pore geometries in the two frameworks are established using multiple spectroscopy methods, including UV−vis diffuse reflectance, Raman, X-ray absorption, and X-ray emission spectroscopy. Line shape analysis applied to the latter method provides quantitative information on axial ligation through its spin state sensitivity. The observed coordination behaviors derived from the spectroscopic analyses of the two MOF systems are compared to those predicted using space-filling models and relevant iron porphyrin molecular analogues. While the space-filling models show the ideal axial coordination behavior associated with these systems, the spectroscopic results provide powerful insight into the actual binding interactions that occur in practice. Evidence for potential side reactions occurring within the pores that may be responsible for the observed deviation from model coordination behavior in one of the MOF/guest molecule combinations is presented and discussed in the context of literature precedent.Chapter 5 is primarily a crystallographic study. This study was necessary for the evaluation of the coordination environment of manganese-porphyrin MOFs under various guest environments. Studying the axial ligation behavior of metalloporphyrins with nitrogenous bases helps to better understand not only the biological function of heme-based protein systems, but also the catalytic properties of porphyrin-based reaction sites in other biomimetic synthetic support environments, like MOFs. Unlike iron porphyrin complexes, little is known about the axial ligation behavior of Mn porphyrins, particularly in the solid state with Mn in the +3 oxidation state. Here, the syntheses and crystal and molecular structures of three new high-spin manganese (III) porphyrin complexes with the different amine-based axial ligands imidazole (im), piperidine (pip), and 1,4 diazabicyclo[2.2.2]octane(DABCO) is presented. These results, in conjunction with on-going TD-DFT calculations, will be used to explain the coordination of Mn-MOF materials with various guest molecules. XANES data suggests significant deviation from their analogous reference complexes. Chapter 6 details the current status of research studying the liquid phase diffusion of guest molecules imidazole (Im) and 1-methylimidazole (MeIm) into the iron porphyrin-based MOFs, FeCl-PCN-222 and FeCl-PCN-224. MOF suspensions of varying particle size are measured to evaluate the impact of their porous structures on this process. Taking advantage of its element specificity and bulk penetration properties, in-situ hard X-ray absorption spectroscopy is used to assess the degree of Fe-imidazole (or Fe-MeIm) coordination in real time. Qualitative evaluation of these results shows surprisingly fast diffusion kinetics in these materials, which have interesting implications for their use as catalysts. The future direction of this project will be discussed with an emphasis on extracting quantitative diffusion rates from XANES data.