Framework-Controlled Axial Coordination of Guest Molecules in Metalloporphyrin-Based MOFs

Metalloporphyrin-based metal-organic frameworks (MOFs) offer a unique platform for probing how confined environments influence porphyrin metal site structure and reactivity. The MOF series, FeCl-PCN-222 and MnCl-PCN-222, are investigated under a series of guest environments, including acetone, imidazole, pyridine, and piperidine, to demonstrate how differences in metalloporphyrin site accessibility and reactivity influence guest molecule axial ligation patterns. Fe and Mn K-edge X-ray absorption spectroscopy (XAS) is employed to characterize the local coordination, oxidation state, and electronic structure of the porphyrin metal centers within the frameworks under those guest environments. A set of molecular metalloporphyrin complexes with well-defined axial coordination status, oxidation, and spin states is measured for comparison and to aid interpretation of the MOF porphyrin coordination patterns. XANES pre-edge features, assigned and interpreted with the help of time-dependent density functional theory (TD-DFT) calculations based on the molecular models, reveal that MOF metalloporphyrin geometries deviate from the molecular analogues for some metalloporphyrin linker/guest combinations. These findings are discussed in the context of framework-imposed pore size restrictions and differences in both porphyrin ring distortion and initial chloride ligand distribution, with the latter two factors further supported by X-ray crystallographic results.