Probing the nature of the Co(III) ion in corrins: The reactions of aquacyano-5-seco-cobyrinic acid heptamethyl ester with anionic ligands

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– < S2O32– < NO2– < SO32– < CN–); the first two behave as π donors towards metal ions, and the last three as π acceptors. Cleavage of the corrin, with a concomitant increase in hardness of the metal, decreases its affinity for π acceptors while the thermodynamic stability of its complexes with π donors increases as the metal ion becomes more Co(III)-like, and more capable of accepting electron density from the ligand. The temperature-dependence of log K values show very negative values for ΔH offset by ΔS values which are also large and negative. In the absence of crystal structures, DFT methods (BP86-D3/6-311G(d,p)) are used to assess the structural consequences of cleavage of the corrin. The topological properties of the electron density were assessed using QTAIM. The cleavage of the corrin does not unduly perturb the coordination sphere of the metal ion and the metal remains essentially octahedral. The sum of the partial charges on the metal and the entire coordination sphere, or the metal and the four equatorial donor N atoms, is less negative in the 5-seco-Cbs complexes than in the Cbs themselves and supports the supposition that cleavage of the corrin has made the metal and its immediate environment more positive. Cleavage of the corrin causes the bond lengths to the equatorial N donors closest to the site of ring cleavage to increase and allows the bonds to the axial ligands usually to be shorter, offering a rationalization for the observed very negative ΔH values for substitution of H2O by an exogenous ligand. The rate of substitution of H2O by CN– in [AC-5-seco-Cbs]+ is some two orders of magnitude slower than that of the substitution of H2O by CN– in [ACCbs]+. Cleavage of the corrin has therefore significantly decreased the lability of Co(III). This study demonstrates how perturbing the electronic structure of the corrin in cobalt corrin complexes significantly affects the coordination chemistry of the axial coordination sites and points to the importance of cis effects in the chemistry of the cobalt corrins.