Probing the Nature of the CoIII Ion in Corrins: The Structural and Electronic Properties of Dicyano- and Aquacyanocobyrinic Acid Heptamethyl Ester and a Stable Yellow Dicyano- and Aquacyanocobyrinic Acid Heptamethyl Ester

A stable yellow derivative of cobyrinic acid heptamethyl ester, (5R,6R)-Coα,Coβ-dicyano-5,6-dihydro-5-hydroxy-heptamethylcob(III)yrinate-c,6-lactone (DCSYCbs), was prepared from dicyanocobyrinic acid heptamethyl ester (DCCbs). The C5 carbon is oxidized and the c side chain cyclized to form a lactone at C6; the 13 atom, 14 π-e– delocalized system of corrins is interrupted, giving a triazamethine system with four conjugated double bonds between N22 and N24 and an isolated double bond between N21 and C4. Stable yellow aquacyanocobyrinic acid heptamethyl ester (ACSYCbs) was prepared by driving off HCN with N2 in a methanol/acetic acid solution. The electronic spectra of DCCbs and DCSYCbs appear similar except that the bands in DCSYCbs are shifted to shorter wavelengths and the γ-band is much less intense. The experimental spectra were adequately modeled using TD-DFT at the PBE1PBE/6-311G(d,p) level of theory. DCSYCbs crystallizes in the space group P212121 (R1 = 6.08%) with Z = 4, including one methanol solvent molecule and one water molecule per cobester. The addition of a hydroxyl group at C5 causes loss of the double bond between C5 and C6 and elongation of the C5–C6 bond. From a combination of two-dimensional 1H TOCSY and ROESY NMR spectra and 1H/13C HSQC and HMBC data, the complete 1H and 13C NMR assignments of DCSYCbs were possible, except for two of the ester methyl groups and the 13C resonances of the two axial cyanide ligands. The latter were assigned using relative chemical shifts calculated by GIAO-DFT methods. The 59Co resonance of DCCbs was observed at 4074 ppm while that of DCSYCbs is shifted downfield to 4298 ppm. Comparison with available 59Co data of analogous systems suggests that the more π-conjugated corrin of DCCbs interacts more strongly with the metal than the less extensively conjugated macrocycle of DCSYCbs. As the strength of the interaction between Co(III) and an equatorial macrocycle increases, νCN of axially coordinated CN– shifts to lower frequency; in DCSYCbs and DCCbs νCN occurs at 2138 and 2123 cm–1, respectively. Hence the corrin ligand in DCCbs interacts more strongly with the metal than the stable yellow corrin ligand, with its diminished conjugation. The UV–vis spectral data and DFT-calculated MOs are consistent with greater overlap between the corrin and the metal orbitals in DCCbs relative to DCSYCbs, which gives the metal in the former a softer, more covalent character.