Postsynthetic Metal and Ligand Exchange in MFU-4l: A Screening Approach toward Functional Metal-Organic Frameworks Comprising Single-Site Active Centers

The isomorphous partial substitution of Zn2+ ions in the secondary building unit (SBU) of MFU-4l leads to frameworks with the general formula [MxZn(5-x)Cl4(BTDD)(3)], in which x approximate to 2, M=Mn-II, Fe-II, Co-II, Ni-II, or Cu-II, and BTDD=bis(1,2,3-triazolato-[4,5-b],[4,5-i])dibenzo-[1,4]-dioxin. Subsequent exchange of chloride ligands by nitrite, nitrate, triflate, azide, isocyanate, formate, acetate, or fluoride leads to a variety of MFU-4l derivatives, which have been characterized by using XRPD, EDX, IR, UV/Vis-NIR, TGA, and gas sorption measurements. Several MFU-4l derivatives show high catalytic activity in a liquid-phase oxidation of ethylbenzene to acetophenone with air under mild conditions, among which Co- and Cu derivatives with chloride side-ligands are the most active catalysts. Upon thermal treatment, several side-ligands can be transformed selectively into reactive intermediates without destroying the framework. Thus, at 300 degrees C, Co-II-azide units in the SBU of Co-MFU-4l are converted into Co-II-isocyanate under continuous CO gas flow, involving the formation of a nitrene intermediate. The reaction of Cu-II-fluoride units with H-2 at 240 degrees C leads to Cu-I and proceeds through the heterolytic cleavage of the H-2 molecule.