An Interconverting Family of Coordination Cages and a meso-Helicate; Effects of Temperature, Concentration, and Solvent on the Product Distribution of a Self-Assembly Process

The self-assembly between a water-soluble bis-bidentate ligand L18w and Co(II) salts in water affords three high-spin Co(II) products: a dinuclear meso-helicate [Co2(L18w)3]X4; a tetrahedral cage [Co4(L18w)6]X8; and a dodecanuclear truncated-tetrahedral cage [Co12(L18w)18]X24 (X = BF4 or ClO4). All three products were crystallized under different conditions and structurally characterized. In [Co2(L18w)3]X4 all three bridging ligands span a pair of metal ions; in the two larger products, there is a metal ion at each vertex of the Co4 or Co12 polyhedral cage array with a bridging ligand spanning a pair of metal ions along every edge. All three structural types are known: what is unusual here is the presence of all three from the same reaction. The assemblies Co2, Co4, and Co12 are in slow equilibrium (hours/days) in aqueous solution, and this can be conveniently monitored by 1H NMR spectroscopy because (i) the paramagnetism of Co(II) disperses the signals over a range of ca. 200 ppm and (ii) the different symmetries of the three species give characteristically different numbers of independent 1H NMR signals, which makes identification easy. From temperature- and concentration-dependent 1H NMR studies it is clear that increasing temperature and increasing dilution favors fragmentation to give a larger proportion of the smaller assemblies for entropic reasons. High concentrations and low temperature favor the larger assembly despite the unfavorable entropic and electrostatic factors associated with its formation. We suggest that this arises from the hydrophobic effect: reorganization of several smaller complexes into one larger one results in a smaller proportion of the hydrophobic ligand surface being exposed to water, with a larger proportion of the ligand surface protected in the interior of the assembly. In agreement with this, 1H NMR spectra in a nonaqueous solvent (MeNO2) show formation of only [Co2(L18w)3]X4 because the driving force for reorganization into larger assemblies is now absent. Thus, we can identify the contributions of temperature, concentration, and solvent on the result of the metal/ligand self-assembly process and have determined the speciation behavior of the Co2/Co4/Co12 system in aqueous solution.