Solution properties of supramolecular cobalt coordination arrays

Abstract

A number of chemically related gridlike Co coordination arrays were studied by UV/vis absorption spectroscopy, electron microscopy and, in particular, analytical ultracentrifugation and partial specific volume measurements, in order to determine their solubility, stability and association behavior in a variety of organic solvents. As judged by the naked eye, solubilization of the compounds occurred instantaneously or at least within minutes. In contrast, the UV/ vis absorbance of the samples distinctly changed for hours or even days, depending on the compound in question. In some cases, the spectral changes indicated dissociation events, probably involving dissolution of clusters or microcrystals. This was supported by ¹H NMR (on related Cd and Zn compounds) and electron microscopic observations at different time intervals after addition of the solvent. Under certain conditions, addition of 20–50 mM salt (necessary to obtain ultracentrifuge data not influenced by nonideal sedimentation behavior) again led to aggregation of the material. However, according to equilibrium sedimentation experiments in most solvents the solubilized Co coordination arrays finally were in the form of monomers, whereas in some solvents intermediate aggregates were predominant. Prolonged storage of the solubilized compounds at room temperature in most cases led to their decomposition or conversion. Reliably determining the partial specific volume, ν, of the compounds turned out to be the most difficult problem in our studies. Density measurements using a Paar density meter apparently suffered from disturbances (probably due to aggregation) at the relatively high compound concentrations required. v determinations applying the Edelstein—Schachman method to data collected in solvents of different density suffered from dependencies of v on the solvent. Combining measurements in nondeuterated and deuterated solvents (as in the original Edelstein—Schachman method) suffered from relatively low accuracy; in addition, it is applicable to a few solvents only. At present, weighted averages of the ν values from the different ultracentrifuge methods seem to yield the most reliable figures.