Lattice-imposed geometry in metal-organic frameworks: Lacunary Zn ₄O clusters in MOF-5 serve as tripodal chelating ligands for Ni ²⁺

The inorganic clusters in metal-organic frameworks can be used to trap metal ions in coordination geometries that are difficult to achieve in molecular chemistry. We illustrate this concept by using the well-known basic carboxylate clusters in Zn ₄O(1,4-benzenedicarboxylate) ₃ (MOF-5) as tripodal chelating ligands that enforce an unusual pseudo-tetrahedral oxygen ligand field around Ni ²⁺. The new Ni-based MOF-5 analogue is characterized by porosity measurements and a suite of electronic structure spectroscopies. Classical ligand field analysis of the Ni ²⁺ ion isolated in MOF-5 classifies the Zn ₃O(carboxylate) ₆ "tripodal ligand" as an unusual, stronger field ligand than halides and other oxygen donor ligands. These results may inspire the widespread usage of MOFs as chelating ligands for stabilizing site-isolated metal ions in future reactivity and electronic structure studies.