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

Abstract

The isomorphous partial substitution of Zn(2+) ions in the secondary building unit (SBU) of MFU-4l leads to frameworks with the general formula [M(x)Zn(5-x)Cl4(BTDD)3], in which x≈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 °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 H2 at 240 °C leads to Cu(I) and proceeds through the heterolytic cleavage of the H2 molecule.