Atom orientation in SBU-induced conformational isomerism in hydroxamate-based In-MOFs

Abstract

Although conformational isomerism is a common phenomenon in carboxylate-based metal–organic frameworks (MOFs), the conformational isomers of hydroxamate-based MOFs are still scarce. Herein, we report a rare example of conformational isomerism in MOFs with 1,4-benzenedihydroxamic acid (H₂L). Three novel In-MOFs, [InL_1.5]·H₂O·DMF (1), [InL_1.5]·4H₂O (2), and (Me₂NH₂)[InL(pzdc)] (3) have been successfully constructed under solvothermal conditions (DMF = N,N-dimethylformamide; H₂pzdc = 2,5-pyrazinedicarboxylic acid). Single-crystal structural analyses display that both 1 and 2 contain a distorted [InO₆] octahedron where the four O atoms (O2, O3ⁱ, O4ⁱ, and O6) in the equatorial plane are arranged in a counterclockwise direction in 1, whereas the corresponding atoms are located in a clockwise direction in 2. This minor difference in atom orientation in the secondary building units (SBUs) induces MOFs 1 and 2 to be conformational isomers, the first example of conformational isomerism in hydroxamate-based MOFs. Moreover, MOFs 1 and 2 exhibit the unique 1D double-chain structures, while 3 with an [InO₆N₂] square antiprism as a SBU indicates a 2D sql network. Additionally, MOFs 1 and 2 display low water and chemical stability, whereas MOF 3 reveals excellent water and chemical stability but a low proton conductivity (5.10 × 10⁻⁶ S cm⁻¹) at 95 °C and 98% relative humidity.