Directing both the Morphology and Packing of Chiral Metal-Organic Frameworks by Cation Exchange Mediated by Nanochannels

Abstract

Crystals are among the most challenging materials to design, both at the molecular and macroscopic levels. We show here that metal-organic frameworks, based on tetrahedral pyridyl ligands, can be used as a morpho-logical and structural mold to form a series of other isostructural crystals having different metal ions. The cati-on exchange is versatile, based on the use of diverse first-row metals; it occurs with retention of the morpholo-gy. Different morphologies were obtained by a direct reaction between the ligand and metal salts. An iterative crystal-to-crystal conversion has also been demonstrated by two consecutive cation exchange processes. The primary manganese-based crystals have a complex connectivity characterized by a rare space group (P622). The molecular structure generates two types of homochiral channels that span longitudinally the entire hex-agonal prism. These channels mediate the cation exchange, as indicated by energy-dispersive X-ray spectros-copy combined with scanning electron microscopy measurements on microtome-sectioned crystals. The occur-rence of the observed cation exchange is in excellent agreement with the Irving-Williams series (Mn < Fe < Co < Ni < Cu > Zn) that are associated with the relative stability of the resulting coordination nodes. The overall approach allows for the predictability of the structural properties of rare metal-organic frameworks based on tetrahedral pyridyl ligands at different hierarchies: from elemental composition, molecular packing, and mor-phology to the bulk properties.