Versatile and Resilient Hydrogen-Bonded Host Frameworks

Abstract

Low-density molecular host frameworks, whether equipped with persistent molecular-scale pores or virtual pores that are sustainable only when occupied by guest molecules, have emerged as a promising class of materials owing to the ability to tailor the size, geometry, and chemical character of their free space through the versatility of organic synthesis. As such, molecular frameworks are promising candidates for storage, separations of commodity and fine chemicals, heterogeneous catalysis, and optical and electronic materials. Frameworks assembled through hydrogen bonds, though generally not stable toward collapse in the absence of guests, promise significant chemical and structural diversity, with pores that can be tailored for a wide range of guest molecules. The utility of these frameworks, however, depends on the resilience of n-dimensional hydrogen-bonded motifs that serve as reliable building blocks so that the molecular constituents can be manipulated without disruption of the anticipated global solid-state architecture. Though many hydrogen-bonded frameworks have been reported, few exist that are amenable to systematic modification, thus limiting the design of functional materials. This Account reviews discoveries in our laboratory during the past decade related to a series of host frameworks based on guanidinium cations and interchangeable organosulfonate anions, in which the 3-fold symmetry and hydrogen-bonding complementarity of these ions prompt the formation of a two-dimensional (2-D) quasi-hexagonal hydrogen-bonding network that has proven to be remarkably resilient toward the introduction of a wide range of organic pendant groups attached to the sulfonate. Since an earlier report in this journal that focused primarily on organodisulfonate host frameworks with lamellar architectures, this unusually persistent network has afforded an unparalleled range of framework architectures and hundreds of new crystalline materials with predictable solid-state architectures. These range from the surprising discovery of inclusion compounds in organomonosulfonates (GMS), as well as organodisulfonates (GDS), structural isomerism reminiscent of microstructures observed in soft matter, a retrosynthetic approach to the synthesis of polar crystals, separation of molecular isomers, storage of unstable molecules, formation of a zeolite-like hydrogen-bonded framework, and postsynthetic pathways to inclusion compounds through reversible guest swapping in flexible GS frameworks. Collectively, the examples described in this Account illustrate the potential for hydrogen-bonded frameworks in the design of molecular materials, the prediction of solid-state architecture from simple empirical parameters, and the importance of design principles based on robust high dimensional networks. These and other emerging hydrogen-bonded frameworks promise new advanced materials that capitalize fully on the ability of materials chemists to manipulate solid-state structure through molecular design.