Influences of molecular structure on supramolecular selection during cocrystallization of polypyridyl metal complexes

© 2014 American Chemical Society. Heteroleptic complexes of the type [RuL2L](PF6)2(L, L = combinations of 1,10-phenanthroline (phen) and 2,2′-bipyridine (bipy)) were found to cocrystallize with [Ni(phen)3](PF6)2to produce cocrystals of [Ni(phen)3]x[RuL2L]1-x(PF6)2. In this report we show that the ability of the complexes to cocrystallize is influenced by the number of common ligands between complexes in solution. Supramolecular selection is a phenomenon caused by molecular recognition through which cocrystals can grow from the same solution but contain different ratios of the molecular components. It was found that systems where L = phen displayed less supramolecular selection than systems where L = bipy. With increasing supramolecular selection, the composition of cocrystals was found to vary significantly from the initial relative concentration in the cocrystallizing solution, and therefore it was increasingly difficult to control the final composition of the resultant cocrystals. Consequently, modulation of concentration-dependent properties such as phase was also found to be less predictable with increasing supramolecular selection. Notwithstanding the complication afforded by the presence of supramolecular selection, our results reaffirm the robustness of the [M(phen)3](PF6)2structure because it was maintained even when ca. 90% of the complexes in the cocrystals were [Ru(phen)(bipy)2](PF6)2, which in its pure form is not isomorphous with [M(phen)3](PF6)2. Experiments between complexes without common ligands, i.e., [Ru(bipy)3](PF6)2cocrystallized with [Ni(phen)3](PF6)2, were found to approach the limit to which molecular recognition processes can be confused into cocrystallizing different molecules to form single cocrystals. For these systems the result was the formation of block-shaped crystals skewered by a needle-shaped crystals.