Crystal Engineering with Soft and Topologically Adaptable Molecular Host Frameworks

The twodimensional (2D) hydrogenbonded sheet comprising guanidinium (G) cations and the sulfonate (S) moieties of organosulfonate anions displays a remarkable persistence that can be exploited for the synthesis of crystalline low-density host frameworks capable of including a variety of guest molecules. The organic portions of organodisulfonates function as "pillars" that connect opposing GS sheets to create porous galleries, occupied by guest molecules, between the sheets. The use of different pillars permits adjustment of pore size and character. The guest molecules, however, also play an important role in directing the assembly of the framework into two possible architectural isomers-a discrete pillared bilayer isomer and a lower density, more open, continuous brick isomer, the latter promoted by larger guest molecules. The GS host frameworks possess an intrinsic conformational "softness" associated with accordion-like flexibility of the GS sheet, and rotational and conformational freedom of the organic pillars. This conformational softness enables the host to "shrinkwrap" around included guests so that dense packing, required for achieving sufficient cohesive energy for crystallization, can be achieved. This structural adaptability bestows the GS hosts with the unique ability to include a variety of differently sized and functionalized guest molecules. Host systems with this property significantly facilitiate the design of new inclusion compounds with molecular scale cavities that can serve as miniature reaction chambers, catalytic environments and chemical storage compartments, while providing reliable architectures that enable control of the spatial organization of guest molecules, so that solid state properties can be systematically manipulated.