Metathesis in Single Crystal: Complete and Reversible Exchange of Metal Ions Constituting the Frameworks of Metal-Organic Frameworks

We report for the first time the complete and reversible exchange of metal ions constituting a robust microporous framework while maintaining not only the structural integrity of the framework but also single crystallinity. Solvothermal reaction of Cd(NO3)(2)center dot 4H(2)O with H(3)hett in DMF produced a new crystalline MOF I having a cubic network with 3D channels. The basic building unit of the framework 1 is an octahedron where the square planar tetrametallic SBUs ({Cd4O}(6+)) and tricarboxylate linkers (hett(3-)) occupy the vertices and triangular faces of the octahedron, respectively. The cubic framework is generated by sharing the vertices of the octahedra. The framework is exceptionally stable in open air as well as in various solutions including aqueous medium. Most interestingly, the framework constituting Cd(II) ions undergo complete and reversible exchange with Pb(II) in aqueous solution at room temperature without altering structural integrity and losing single crystallinity. We have also studied the complete exchange of framework Cd(II) ions with lanthanide ions (Dy(III) and Nd(III)) with the retention of the framework topology. The reversible exchange of framework constituting metal ions while maintaining the framework integrity and single crystallinity was confirmed by a combination of various methods including ICP-AES, in situ PXRD, TGA, IR, optical microscopy, elemental analysis, N-2 sorption and in situ single crystal X-ray structural analysis.