Enantiomers of conformation-flexible cyclopentane-1,2,3,4-tetracarboxylate in metal–organic frameworks

Abstract

Four novel coordination polymers, [Mn₂(cptc)(2,2′-bpy)(H₂O)]·H₂O (1), [Mn₂(cptc)(phen)(H₂O)]·H₂O (2), Mn₄(cptc)₂(phen)₄(H₂O)₂]·3H₂O (3) and [Cd₂(cptc)(H₂O)₃] (4) (H₄cptc = cyclopentane-1,2,3,4-tetracarboxylic acid, 2,2′-bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline), have been synthesized by hydrothermal reactions of flexible (1S,2S,3R,4R)-H₄cptc with MnCl₂/CdCl₂. By breaking the mirror plane of symmetry, the meso-cptc ligand is successfully converted into two pairs of enantiomers, the (1R,2S,3S,4R)- and (1S,2R,3R,4S)-cptc groups in 1–3 are unprecedented while the documented (1R,2R,3R,4S)- and (1S,2S,3S,4R)-cptc enantiomers exhibit an unprecedented μ₆-mode in 4. Layered 1 and 2 show similar structures and are constructed by centrosymmetric tetranuclear units. 3 is a binodal (3,4)-connected layer with Schläfli symbols of (5³)₂(5⁴·8²), in which cptc exhibits two types of μ₅-mode bonding. 4 has a 3-D pillar layered structure composed of neutral [Cd(cptc)]_n layers and octahedral CdO₆ pillars. The presence probability of different enantiomers is in agreement with the relative stability of different conformations of cptc calculated by time-dependent density functional theory (TD-DFT).