Quo vadis niobium ? Divergent coordination behavior of early-transition metals towards MOF-5

Treatment of MOF-5 with NbCl₄(THF)₂ in acetonitrile leads to incorporation of Nb(iv) centers in a fashion that diverges from the established cation metathesis reactivity of this iconic material. A combination of X-ray absorption spectroscopy analysis and reactivity studies altogether supported by density functional theory computational studies document an unprecedented binding mode for the Zn₄O(O₂C-)₆ secondary building units (SBUs), which in Nb(iv)-MOF-5 function as κ²-chelating ligands for NbCl₄ moieties, with no exchange of Zn²⁺ observed. This unusual reactivity expands the portfolio of post-synthetic modification techniques available for MOFs, exemplified here by MOF-5, and underscores the diverse coordination environments offered by this and potentially other MOFs towards heterometal species.