Spontaneous resolution of two chiral metal–organic frameworks through local geometric and lattice frustration effects

Abstract

We highlight two key examples of where the achirality of reagents used in MOF syntheses transposes to chiral structures, thereby expanding the potential ligand pool used in the future design of chiral MOFs. Herein, we report the characterisation of two new chiral Cd^II MOFs: (1) a pair of enantiomorphous structures produced from the achiral bicyclo[1.1.1]pentane-1,3-dicarboxyate linker, [Cd(pdc)(DMF)] (pdc = bicyclo[1.1.1]pentane-1,3-dicarboxylate; DMF = N,N-dimethylformamide) (3DL-MOF-3), and (2) a similar chiral network featuring cubane-1,4-dicarboxylate, [Cd₃(cdc)₃(DMF)₃] (cdc = cubane-1,4-dicarboxylate) (3DL-MOF-4). Conventional solvothermal synthesis of 3DL-MOF-3 yields a conglomerate of two enantiomorphs that spontaneously resolve into their respective right- or left-handed chiral hexagonal space groups (space groups P6₁/P6₅). Similarly, 3DL-MOF-4 resolves into a non-centrosymmetric orthorhombic phase (P2₁2₁2). Both 3DL-MOF-3 and 3DL-MOF-4 possess infinitely-linked, chiral, and 1D Cd–O helical chains which are observed to be the apparent cause of chirality in each system. In both cases, homochirality is preserved within each individual crystal where only one type of helical handedness is observed. We consider a general mechanism for this preservation of homochirality to include an avoidance of a physical analogue of geometric frustration which manifests due to the tripartite nature of the underlying triangular lattice upon which each chiral chain is located.