Postsynthetic Modification of Metal–Organic Frameworks through Nitrile Oxide–Alkyne Cycloaddition

Postsynthetic modification of metal–organic frameworks is an important method to tailor their properties. We report on the nitrile oxide–alkyne cycloaddition (NOAC) as a modification tool, a reaction requiring neither strained alkynes nor a catalyst. This is demonstrated with the reaction of nitrile oxides with PEPEP-PIZOF-15 and -19 at room temperature. PIZOF-15 and -19 are porous Zr-based MOFs (BET surface areas 1740 and 960 m² g^–1, respectively) consisting of two mutually interpenetrating UiO-type frameworks with linkers of the type ^–O₂C­[PE-P­(R¹,R²)-EP]­CO₂^– (P, phenylene; E, ethynylene; R¹ and R², side chains at the central benzene ring with R¹ = R² = OCH₂CCH or R¹ = OCH₂CCH and R² = O­(CH₂CH₂O)₃Me). Their syntheses, using benzoic acid as a modulator, and their characterization are reported herein. The propargyloxy (OCH₂CCH) side chains contain the ethyne moieties needed for NOAC. Formation of nitrile oxides through oxidation of oximes in aqueous ethanolic solution in the presence of PEPEP-PIZOF-15 and -19 resulted in the reaction of 96–100% of the ethyne moieties to give isoxazoles. Thereby the framework was preserved. The type of nitrile oxide RCNO was greatly varied with R being isopentyl, tolyl, 2-pyridyl, and pentafluorophenyl. A detailed NMR spectroscopic investigation showed the formation of the 3,5-disubstituted isoxazole to be clearly favored (≥96%) over that of the constitutional isomeric 3,4-disubstituted isoxazole, except for one example