~~~~~ ~~~~ ~~~ ~~Stability constants have been measured for a series of ligands based on a 2,2'-(pyridine-2.6-diyl)bis[lH-benzimidazole] unit which forms dinuclear double-stranded helical complexes with copper(1). Variation of different structural parameters confirms the importance of the coordinate bond, the stacking interactions, and the weakly bridging pyridine units observed by X-ray crystallography. The stabilities of the complexes depend strongly on the solvent, and in MeCN, which is a good solvent for copper(1). the complexes are less stable and assemble in a stepwise manner. The interconversion of the enantiomers may be followed by 'H-NMR and takes place on a millisecond time scale around room temperature. The trends in lability are similar to those found for the stability of the complexes.Introduction. ~ There is currently considerable interest in the use of the coordinate bond to assemble supramolecular structures [I], since, compared to the other forces used to combine the modular units of a supermolecule, the coordinate bond offers a number of advantages: the interaction between a metal ion and a coordination site is frequently stronger than intermolecular forces, and, by virtue of the stereochemical preferences of the metal ion, possesses a directional character which can control the structure of the assembled product. Despite its strength, the coordinate bond is frequently labile, and it is, therefore, possible for rapid rearrangements to occur, and for a system held together by such bonds to reach the free-energy minimum corresponding to strict self-assembly as defined by Lindsey [2]. Polynuclear helical complexes, or helicates [3], occupy a privileged position in metallosupramolecular chemistry. A considerable body of research now exists and has established the structural principles of the formation of polynuclear helicates with a wide variety of metals and ligands. With these principles, it should be possible to design new systems which maximize the interactions leading to helication and enhance the stability of the complexes. To d o this, we need data on the quantitative importance of effects such as stacking interactions, the strength of the coordinate bond, rtc. The techniques used to study helicate formation, typically X-ray crystallography, NMR, and mass spectroscopy, are powerful structure-analysis techniques but do not generally yield quantitative data about the stability of the helices. Thermodynamic studies of helicate formation in solution are quite rare and have generally been concerned with the identification of the formation pathways for complexes [4-61 rather than with the relative stability of helicates containing closely related ligands.Our first objective in this paper is to compare the stabilities of a series of dicopper double helicates containing closely related ligands in order to evaluate the importance of the various factors proposed from X-ray structure analysis to be important for helicate formation. The second aim is to study the dynamics of these systems. Since the coor...
