Supramolecular interactions between Co(CN)₅(SO₃)^4- and polyammonium macrocyclic receptors

The acid-base properties as well as the photochemical reactivity of the co-ordination compound K₄[Co(CN)₅(SO₃)] in the presence of three polyammonium macrocyclic receptors were studied in aqueous solution. The pK_a of the free complex (3.9) (sulfite deprotonation) changed to pK_a <0.5 upon complexation with the receptors. The quantum yield for sulfite photoaquation of the free complex in the basic form (Φ = 0.85 ± 0.09) decreased to 0.05 ± 0.01, 0.12 ± 0.03 and 0.45 ± 0.09 in the presence of [24]aneN₈H₈ ⁸⁺, [30]aneN₁₀H₁₀ ¹⁰⁺ and [32]aneN₈H₈ ⁸⁺, respectively. For the acidic form of the free complex (Φ = 0.40 ± 0.05) the quantum yield was not affected by supercomplexation with [32]aneN₈H₈ ⁸⁺. For the adducts formed from the other two macrocyclic receptors it was not possible to evaluate the quantum yields of the acidic forms, because protonation was not complete even at very high proton concentrations. The results were interpreted in terms of second-sphere interactions involving hydrogen bonding between the complex and the macrocycles. In the case of [32]aneN₈H₈ ⁸⁺, the experimental results are compatible with a structure in which the cyanides are involved in hydrogen bonding but the sulfite ligand is not. In the two other supercomplexes the sulfite ligand seems to be involved in hydrogen bonding.