Analytical ultracentrifugation as a tool in supramolecular chemistry: a feasibility study using a metal coordination array

Self-assembling systems composed of organic molecules and metal ions represent one of the topics in modern supramolecular chemistry. Up to now, their physical characterization was essentially limited to crystals or to surface layers. Studies on solubilized systems, e.g., on their molecular mass distribution or association behavior, are rare. We have explored whether sedimentation equilibrium analysis in the Beckman Optima XL-A analytical ultra-centrifuge can be successfully applied. A grid-like cobalt coordination array (“[2×2]-Co(II)-grid”) was used as a model compound. The technical problem involved, concerning the chemical resistance of the cell components against organic solvents, was solved by using titanium centerpieces and polyethylene gaskets. Another potential problem, the high UV absorbance of the solvents, could be circumvented by measuring the absorbance versus radius profiles in the visible wavelength range, where “grids” and most other related compounds show absorption bands. Special efforts were made to solve the remaining problems: (1) the suppression of nonidealities in sedimentation behavior, which could prevent or greatly complicate a successful analysis of self-associating systems, and (2) the determination of the partial specific volume $$\bar v$$ of the complexes, in particular considering compounds either not available in sufficient quantities for density measurements or exhibiting low solubility. With respect to problem (1), solvent/salt systems were found in which the model compound apparently shows ideal sedimentation behavior. Procedures are suggested to overcome problem (2).