Can enantiomer ligands produce structurally distinct homochiral MOFs?

Abstract

Here, we report a self-assembled homochiral metal–organic framework [Cu_1.5(H₂L^L-leu)(Ac)H₂O]_n·3H₂O (1) obtained from an L-leucine-derived ligand (H₄L^L-leu) and Cu(Ac)₂·H₂O in a 1 : 1 ratio. Coordination-induced conformational change in the ligand has been monitored by circular dichroism which has been further attested by synthesizing a D-leucine-containing enantiomer H₄L^D-leu and its Cu(II) complex [Cu_1.5(H₂L^D-leu)H₂O]_n·10H₂O (2). Structure determination revealed entirely different structures for the homochiral MOFs (1 and 2) obtained from the L/D-leucine-derived enantiomer ligands under analogous reaction conditions. Further, structural dissimilarity in these MOFs has been judicially supported by proton conductance studies. MOF 1 shows higher proton (10⁻⁵ S cm⁻¹) conductance in comparison to 2 (10⁻⁶ S cm⁻¹) due to dissimilar alignment of the hydrogen-bonded water molecules in the hydrophilic pocket as well as crystal packing.