Spectroscopic interrogations of isostructural metalloporphyrin-based metal-organic frameworks with strongly and weakly coordinating guest molecules

<p>Two isostructural metal-organic frameworks based on cobalt(II) and nickel(II) metalloporphyrin linkers, Co-PCN222 and Ni-PCN222, 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.</p

Spectroscopic Evidence for Room Temperature Interaction of Molecular Oxygen with Cobalt Porphyrin Linker Sites within a Metal–Organic Framework

Metalloporphyrin-based metal–organic frameworks offer a promising platform for developing solid-state porous materials with accessible, coordinatively unsaturated metal sites. Probing small-molecule interactions at the metalloporphyrin sites within these 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

Spectroscopic Evidence of Pore Geometry Effect on Axial Coordination of Guest Molecules in Metalloporphyrin-Based Metal Organic Frameworks

A systematic comparison of host–guest interactions in two iron porphyrin-based metal–organic frameworks (MOFs), FeCl-PCN222 and FeCl-PCN224, with drastically different pore sizes and geometries is reported in this fundamental spectroscopy study. 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

Probing Framework-Restricted Metal Axial Ligation and Spin State Patterns in a Post-Synthetically Reduced Iron-Porphyrin-Based Metal–Organic Framework

An iron-porphyrin-based metal organic framework PCN-222­(Fe) is investigated upon postsynthetic reduction with piperidine. Fe K-edge X-ray absorption and Kβ mainline emission spectroscopy measurements reveal the local coordination geometry, oxidation, and spin state changes experienced by the Fe sites upon reaction with this axially coordinating reducing agent. Analysis and fitting of these data confirm the binding pattern predicted by a space-filling model of the structurally constrained pore environments. These results are further supported by UV–vis diffuse reflectance, IR, and resonance Raman spectroscopy data