LAUSR

Abstract The substitution of H2O in aquacyano-5,6-dioxo-5,6-seco-heptamethyl-cob(III)yrinate, (aquacyano-5-seco-cobester, [AC-5-seco-Cbs]+), in which the C5–C6 bond of the corrin ring of this vitamin B12 derivative is cleaved, by a variety of anionic ligands is reported. The pKa for ionization of coordinated water decreases from 9.8 ± 0.3 in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, [ACCbs]+) to 7.28 at 25 °C (ΔH = –88 ± 17 kJ mol−1 and ΔS = –434 ± 56 J K−1 mol−1) in [AC-5-seco-Cbs]+. The pKa, confirmed by determining the pH-dependence of coordination of SO32– by [AC-5-seco-Cbs]+, shows Co(III) in this complex behaves much more like Co(III) in simple hexacoordinate complexes than in intact cobalt corrins. A comparison of log K values for coordination of CN–, SO32–, NO2–, N3– and S2O32– by [ACCbs]+ and [AC-5-seco-Cbs]+ demonstrates that cleavage of the corrin ring significantly decreases the affinity of Co(III) for the softer ligands CN–, SO32– and, more marginally, NO2–. However, [AC-5-seco-Cbs]+ has a higher affinity for N3– and S2O32– than [ACCbs]+. These trends correlate with the position of the ligands in the spectrochemical series (N3–