| Crystal engineering has been defined as “the understanding of intermolecular interactions in the context of crystal packing and in the utilization of such understanding in the design of new solids with desired physical and chemical properties.” The field of crystal engineering is growing rapidly and is generally centered on the use of hydrogen bonding or coordination polymers in the design and synthesis of new materials. Other interactions, such as halogen bonding also lend themselves to crystal design.; In halogen-bonding, donation of a lone pair of electrons into the σ and σ* orbitals of an acceptor molecule, like elemental iodine or bromine, allows for relatively strong and highly directional interactions. Charge-transfer interactions involving aromatic nitrogen heterocycles and diiodine (I 2) will be discussed. With weaker donors, such as pyrazine, we have prepared target complexes with infinite chain structures consisting of alternating donor and acceptor molecules. But with stronger donors, such as 4,4 ′-bipyridine, only simple adducts are formed. Strong donation at one end of the I2 reduces the Lewis acid character at the other end and extended interactions do not form. With some strong donors, reduction in Lewis acidity occurs to the point that the I2 exhibits amphoteric behavior. In these systems, the non-complexed end serves as a Lewis base to a second-bridging I2 molecule to form neutral polyiodine systems.; To reduce electronic communication between the two iodine atoms, organoiodides, in which an organic spacer has been inserted between the two iodine atoms, have been employed. The organic spacer also offers a manifold for adjusting the Lewis acidity of the acceptors in these compounds. Acceptors involving more than two iodines provide the possibility of multidirectional extended interactions leading to interesting layered or network solids. Useful acceptors include 1,4-diiodobenzene, 4,4′-diiodobiphenyl, 1,4-diiodotetraflurobenzene, and tetraiodoethylene.; Organoiodides can also be used to build very interesting layered ‘pseudo polyiodides’, as well. Traditional coordination polymers consist of a Lewis acid metal center and Lewis base bridging ligands. The shapes that can be made using these coordination polymers include, but are not limited to, chains, ladders, bricks, square or rectangular grids and hexagonal grids. We have been able to build many of these same motifs using a Lewis base iodide center and Lewis acid organoiodide. The diverse structures seen are dependent on factors such as the nature of the cation, the bridging organoiodide, stoichiometry, and the nature of the anion (e.g. I− vs. I3 −). These complexes may exhibit ionic conductivity as well as the possibility of cation exchange.; Polymorphism can be a major problem for crystal engineering as it can make prediction of structure very difficult. George Whitesides states “truly rational solid-state design will not be possible until polymorphism is understood and controlled.” We have used halogen bonding interactions for interconversion between polymorphic forms of a donor molecule. Polymorphic forms of tetrapyridylpyrazine and acridine, their charge-transfer complexes with iodine, and their interconversion through decomposition of the complexes will be discussed.; Through systematic studies of halogen bonding, numerous donors and acceptors have been used to show the structural diversity possible. Our purpose is to define the fundamental structural motifs adopted by these systems. An understanding of these patterns is essential for ongoing crystal engineering projects. |
