Transition-metal Complexes of Pyrrole Pigments. XII. Optical and Magnetic Resonance Spectra of the Cobalt(II) and Nickel(II) Complexes of 1,19-Disubstituted Tetradehydrocorrins

The cobalt(II) and nickel(II) complexes of 1,19-dimethyl- and 1,19-diethoxycarbonyltetradehydrocorrins were prepared by the template reaction as functioned by metal ions from the corresponding 1,19-disubstituted 1,19-dideoxybiladiene-ac’s. The absorption bands appearing in the energy region greater than 15000 cm−1 were attributed to the π→π* transitions within a ligand molecule. The ligand-field bands were observed in the lower energy region (<15000 cm−1) for the cobalt(II) complexes in which cobalt(II) assumes the C2v coordination symmetry: 2A1←2A1, 2A2←2A1,2B2←2A1, and 2B1←2A1 in an increasing order of energy. The vibrational spectra provided evidences for the formation of the tetradehydrocorrin complexes with perchlorate ion as the counter ion. The angular methyl groups placed at the 1- and 19-positions of tetradehydrocorrins showed the abnormally large up-field shift (2.0 ppm) of their proton signal relative to the parent biladiene-ac’s due to the magnetic anisotropy of the adjacent pyrrole rings A and D. For a 1,19-diethyoxycarbonyltetradehydrocorrin complex, the 1,19-substituents were also placed in the shielding zone of the neighboring pyrrole rings. In the case of the cyclized 2,8,12,18-tetra(2-ethoxycarbonylethyl) derivative, the ethyl ester groups of the 2,18-substituents were presumably placed in a conformational space different from that occupied by those of the 8,12-substituents according to the proton NMR behaviors. The parent biladiene-ac which carries ethoxycarbonyl groups at the 1,19-positions caused a up-field shift of its methine (at bridge positions) proton signal upon cyclization due to the loss of electronic conjugation of carbonyl groups with pyrrole rings, in consistent with the vibrational data. The ESR spectra were measured in chloroform-benzene (2 : 1) at 77 K in the presence and absence of pyridine. The cobalt(II) complexes were identified as of low-spin square-planar type with the unpaired electron in the cobalt dz2 orbital. The coordination capability of 2-ethoxycarbonylethyl groups, which are directly bound to the pyrrole rings in the cyclized metal complex system, was also suggested.