Quantification of site-specific cation exchange in metal-organic frameworks using multi-wavelength anomalous X-ray dispersion

We employed multiwavelength anomalous X-ray dispersion to determine the relative cation occupation at two crystallographically distinct metal sites in Fe²⁺-, Cu²⁺-, and Zn²⁺-exchanged versions of the microporous metal-organic framework (MOF) known as MnMnBTT (BTT = 1,3,5-benzenetristetrazolate). By exploiting the dispersive differences between Mn, Fe, Cu, and Zn, the extent and location of cation exchange were determined from single crystal X-ray diffraction data sets collected near the K edges of Mn²⁺ and of the substituting metal, and at a wavelength remote from either edge as a reference. Comparing the anomalous dispersion between these measurements indicated that the extent of Mn²⁺ replacement depends on the identity of the substituting metal. We contrasted two unique methods to analyze this data with a conventional approach and evaluated their limitations with emphasis on the general application of this method to other heterometallic MOFs, where site-specific metal identification is fundamental to tuning catalytic and physical properties.