Phase control in the modulated self-assembly of lanthanide MOFs of a flexible tetratopic bis-amide linker

The targeted synthesis of phase-pure metal-organic frameworks (MOFs) is often complicated by the fact that a number of different products can result from the same metal-ligand combination. The self-assembly of lanthanide-based MOFs can be particularly challenging due to the versatile coordination chem. of the f-block allowing for diverse coordination spheres with irregular geometries driven by steric effects. Herein we show that modulated self-assembly - the addition of small mols. to solvothermal synthesis in order to control pH and coordination equilibrium - can control phase formation when preparing lanthanide MOFs from the tetratopic bis-(3,5-dicarboxyphenyl)terephthalamide ligand. We report 27 new MOFs, of which 21 are characterised by single crystal X-ray diffraction, that can be categorised into six structure types. The formation of different phases is largely driven by the contraction in ionic radius across the lanthanide series, but we show how careful choice of both solvent and modulator can influence reaction outcome. As an example, phase pure samples of three Gd MOFs of the same ligand can be selected by simply changing the modulator - acetic acid, benzoic acid, or nitric acid - used in closely related syntheses. These results show that modulation can control phase in f-block MOFs, just as it can in d-block MOFs, and provide design rules for uncovering structural diversity by carefully exploring the chem. parameter space of lanthanide MOFs in single ligand systems.