A Versatile Family of Interconvertible Microporous Chiral Molecular Frameworks: The First Example of Ligand Control of Network Chirality

Abstract

Two families of molecular frameworks which grow as homochiral single crystals are described. Both consist of multiple interpenetration of the three-connected chiral (10,3)-a (Y*) network and result from the tridentate coordination of the 1,3,5-benzenetricarboxylate (btc) ligand to octahedral metal centers which act as linear connectors. The nature of the interpenetration is controlled by the auxiliary ligands bound in the equatorial plane of the metal center. Ethylene glycol (eg) binds in a unidentate fashion to form phase A which has 28% accessible solvent volume and contains four interpenetrating (10,3)-a networks. 1,2-Propanediol (1,2-pd) coordinates as a bidentate ligand to yield a phase B with a greatly enhanced 51% of solvent accessible volume, because only two (distorted) (10,3)-a‘ networks interpenetrate. Ligands in the void space and bound to the metal center can both be liberated thermally: the kinetics of this process allow isolation of microporous desolvated crystalline A and B . The porous phases lose crystallinity reversibly upon further loss of ligands bound to the equatorial metal: crystallinity is restored upon exposure to the vapors of simple alcohols, which can also effect conversion of B to A . Both phases present interpenetrating network topologies that are unique to chemistry and adopt space groups that are new for molecular solids: A crystallizes in P 4 2 32 and B adopts I 4 1 32. B can be grown homochirally from enantiomerically pure diol template. The stereochemistry of the alcohol bound to the metal controls the helicity of the chiral framework. The structure determination of the 1,2-propanediol phase represents the first demonstration that chiral molecules can specifically template helix handedness in a chiral porous framework solid.