Molecular structure of (5,10,15,20-tetrabutyl-2,3,7,8,12,13,17,18-octaethylporphyrinato)nickel(II)-correlation of nonplanarity with frontier orbital shifts

Highly substituted porphyrins exhibit significantly distorted, nonplanar co nformations in the solid state. The crystallographically determined degree of nonplanarity correlates with a bathochromic shift of the absorption maxi ma in solution. In addition, nonplanar porphyrins with meso aryl groups sho w increasing in-plane rotation of the meso aryl groups, which potentially c ould account for the observed changes in spectroscopic and physicochemical properties of nonplanar porphyrins. A crystal structure analysis of the tit le compound NiTBuOEP reveals a highly nonplanar conformation with an averag e deviation from planarity for the 24 macrocycle atoms of 0.462 Angstrom an d displacements of the meso carbon atoms from the 4N-plane of 1.044 Angstro m. The average Ni-N bond distance in the crystal (1.873(3) Angstrom) is in good agreement with the Ni-N bond distance in solution (1.87 Angstrom) that was determined by EXAFS. Compared to more planar reference compounds, NiTB uOEP exhibits significantly red-shifted absorption spectra in solution, cor rectly predicted by INDO/s calculations. As the shortness of the Ni-N bonds has been shown to be an excellent indicator for the degree of conformation al distortion in porphyrins, this proves that the highly nonplanar conforma tion of sterically strained porphyrins is maintained in solution. Thus, the physical and chemical properties measured in solution do indeed reflect th e stereochemistry of the single crystals. In addition, the use of only alky l substituents in NiTBuOEP to cause nonplanarity obviously circumvents pote ntial electronic effects due to aryl ring interactions.