Studies in the Organic Solid State

This paper describes an approach to crystal engineering based on designing and analyzing hierarchical levels of crystal l ine architecture. The system under study consists of 1:1 co-crystals of melamines and barbituric acids that selfassemble into crystallographically infinite hydrogen-bonded tapes. The formation of structural elements can be rat ional ized and control led using famil iar molecular concepts such as steric repulsion. INTRODUCTION AND BACKGROUND Control l ing the packing of organic molecules in crystals presents a major design chal lenge for molecular engineering.l-3 The ult imate goal of patterned, functional arrays of organic molecules is of potential ly great interest and rer,vard. Fundamental investigations of the physical-organic chemistry of the sol id state should f ind application throughout materials science. Design of Our System.a-6 Our approach attempts to simpli fy the packing of molecules by constraining them to adopt only a handful of orientat ions. Thc problem of rat ional ly stuclying ( let alone designing) molecular crystals rvould be intractable without some kind of control of this sort: the number of possible threedimensional orientat ions of molecules in a crystal could be very large. Competing packin_g arrangements might have similar energies, and although progress is beirrg made,7-9 computational routines to predict the most energetical ly favorable arrangements are in their infancy. The geometries that result from our constraints simpli fy comparison of packing motifs and render the overal l crystal structures analyzable in terms of successive levels of organization. The part icular system that we used as a conceptual guide is the 1:1 complex betr,veen cyanuric acid and melamine (Fig. 1) 10,1t The planar molecules in this structure are proposed to hyclrogen bond with each other to give inf ini te hydrogen-bonr-lecl sheets. We hypothesized that by appending substi tuents to certain posit ions of these molecules, r, t 'e could prevent hydrogen bonding in some directions and obtain structural elements that would be responsive to physicalorganic-type investigations. Hydrogen bonds possess several prc-rpert ies that recommend them as stiuctural l inkage units in molecular sol ids: they are direct ional, their formation is reversible at room temperature, and they can be incorporated into many different organic structures. Norr-cl irect ional, dispersion-type interactions such as the stacking of van der Waals surfaces coulcl lead to numerous options for the geometries of aclducts. The three-fold hydrogen bonding of these planar molecules (or analogues of them) pcrmi ts , by cont rast , on ly two in termolecu lar or ienta t ions, lr 'hich are related by rotat ion arouud the ccrrtral Ni l---N corrtact. The planar, r igid