⁵⁹Co NMR of Six-Coordinate Cobalt(III) Tetraphenylporphyrin Complexes. 4. The Effect of Phenyl Ortho Substituents on Chemical Shift, Line Width, and Structure

The ⁵⁹Co NMR spectra have been recorded for a series of sterically congested six-coordinate cobalt(III) porphyrins of the type Co(por)(RIm)₂⁺, where Rim is imidazole or 1-methylimidazole and por is the dianion of tetramesitylporphyrin (TMP) or tetrakis(2,6-dichlorophenyl)porphyrin (TDCPP). The presence of the ortho chloro or methyl substituents on the meso phenyl groups in the six-coordinate complexes Co(TDCPP)(RIm)₂⁺ and Co(TMP)(RIm)₂⁺ has a large effect on the ⁵⁹Co NMR chemical shifts and line widths when compared to analogous cobalt complexes lacking ortho substituents on the phenyls, e.g., Co(TPP)(RIm)₂⁺. Extensive studies of solvent effects indicate that the TDCPP and TMP complexes do not experience specific solvation, even with small solvent molecules; the solvent influence is limited to long-range polarity effects. The TPP complexes, on the other hand, exhibit specific, short-range solvation and are able to discriminate among solvents on the basis of size. The X-ray structure of [Co(TDCPP)(1-MeIm)₂]BF₄ (13) shows that the ortho chloro groups effectively block the space above and below the porphyrin core. The two axial imidazole ligand planes adopt a perpendicular relative orientation with each imidazole plane nearly eclipsing a N_p-Co-N_p bond axis. Additionally, the porphyrin core is substantially ruffled with approximate S4 symmetry. The distortions produced by the bulky ortho groups in these “hindered” complexes account for the observed solvent dependence of the NMR parameters. Furthermore, strong evidence is presented that the electric field gradient changes sign in going from unhindered to hindered complexes. The structural results provide a ready rationale for this in terms of decreased axial and increased equatorial interaction between the cobalt d-orbitals and the ligand orbitals in the hindered complexes. Compound 13 crystallizes in the monoclinic space group P2₁/n with a = 12.229 (4) Å, b = 21.344 (7) Å, c = 21.227 (7) Å, β = 105.50 (2)°, V= 5339 (3) ų, Z = 4, 7622 observed reflections, R = 0.090, and R_w = 0.101.