SUPRAMOLECULAR INTERACTIONS BETWEEN CO(CN)(5)(SO3)(4-) AND POLYAMMONIUM MACROCYCLIC RECEPTORS

The acid-base properties as well as the photochemical reactivity of th e co-ordination compound K-4[Co(CN)(5)(SO3)] in the presence of three polyammonium macrocyclic receptors were studied in aqueous solution. T he 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 (Phi = 0. 85 +/- 0.09) decreased to 0.05 +/- 0.01, 0.12 +/- 0.03 and 0.45 +/- 0. 09 in the presence of[24]aneN(8)H(8)(8+), [30]aneN(10)H(10)(10+) and [ 32]aneN(8)H(8)(8+), respectively. For the acidic form of the free comp lex (Phi = 0.40 +/- 0.05) the quantum yield was not affected by superc omplexation with [32]aneN(8)H(8)(8+). For the adducts formed from the other two macrocyclic receptors it was not possible to evaluate the qu antum 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 bet ween the complex and the macrocycles. In the case of [32]aneN(8)H(8)(8 +) the experimental results are compatible with a structure in which t he cyanides are involved in hydrogen bonding but the sulfite ligand is not. In the two other supercomplexes the sulfite ligand seems to be i nvolved in hydrogen bonding.