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.