CONVERGENT USE OF PI-PI INTERACTIONS AND IRON(II) COORDINATION IN THE BINDING OF NITRILES AND OTHER SUBSTRATES TO A CONFORMATIONALLY FLEXIBLE MACROCYCLIC RECEPTOR, FE(DMGBPH(2))(2)

A versatile flexible host is described which makes convergent use of coordinate covalent binding and pi-pi interactions. Facile conversion between C-2h and C-2v conformations of bis ((dimethylglyoximato)diphenylborato)-iron (II) complexes, Fe(dmgBPh(2))(2), moves boron-linked phenyl groups into or out of intimate contact with iron-bound axial ligands. The pi-pi interactions between bound substrates and peripheral phenyl groups span some 8 kcal/mol in energy from repulsive (2-methylimidazole, pyridine, benzonitrile, 2-cyanopyridine, 9-cyanoanthracene) to attractive (phthalonitriles, nitrobenzonitriles, 2- and 4-cyano-N-methylpyridinium ion, methylpyrazinium ion, dichlorodicyanobenzoquinone, tetracyanoethylene). Both attractive (charge transfer) and repulsive pi-pi interactions are dominated by Coulombic forces. They add to or subtract from the stronger metal-ligand bonding interaction, producing net effects of 10(6) on substrate-binding equilibria and dissociation rates. The convergent dynamic use of several weak interactions is demonstrated via the controlled manipulation of fragment positioning with this molecular device.