Axial Ligand Effects in Sterically Strained Porphyrins: A Crystallographic Study of Five- and Six-coordinated Metal Complexes of 2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetranitroporphyrin

Porphyrins with steric hindrance at the periphery are known to exhibit severely non-planar macrocycle conformations. Among other dodecasubstituted porphyrins, the title compound has been studied widely and shows a typical saddle-distorted macrocycle. The specific conformation of the porphyrin leads to the formation of distinct cavities on both sides of the macrocycle. Compared to planar porphyrins this should increase steric interactions between the macrocycle and axial ligands in five- and six-coordinated metal derivatives. In order to study the influence of different axial ligands on the conformation (or vice versa) a variety of five- and six-coordinated metal derivatives of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetranitroporphyrin (oetnp) were prepared and their conformation investigated by X-ray crystallography. Structural data for Zn II oetnp (L) where L is imidazole, 1-methylimidazole, 2-methylimidazole, 4-methylimidazole and 3,5-lutidine clearly indicated that a mutual influence exists between axial ligands and non-planar porphyrins. An asymmetric macrocycle distortion and axial ligand orientation was found in the sterically hindered 2- and 4-methylimidazole derivatives. The presence of four electron-withdrawing nitro-groups led to the formation of the novel polymeric porphyrins ( Zn II oetnp )n and ( Co II oetnp )n where polymer formation was achieved via the utilization of nitro oxygen atoms as axial ligands to the metal centre of neighbouring porphyrins. The structure of the six-coordinated Ni II oetnp ( pyr )2 (where pyr is pyridine) yielded first structural data on neutral, non-planar, high-spin Ni ( II ) porphyrins. A comparison with the respective low-spin Ni II oetnp clearly showed that a change to the high-spin form leads to a less non-planar macrocycle conformation further proving that spin state changes can have a profound effect on the conformation in distorted porphyrins.