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 †

Abstract

Highly substituted porphyrins exhibit significantly distorted, nonplanar conformations in the solid state. The crystallographically determined degree of nonplanarity correlates with a bathochromic shift of the absorption maxima in solution. In addition, nonplanar porphyrins with meso aryl groups show increasing in-plane rotation of the meso aryl groups, which potentially could account for the observed changes in spectroscopic and physicochemical properties of nonplanar porphyrins. A crystal structure analysis of the title compound NiTBuOEP reveals a highly nonplanar conformation with an average deviation from planarity for the 24 macrocycle atoms of 0.462 Å and displacements of the meso carbon atoms from the 4N-plane of 1.044 Å. The average Ni–N bond distance in the crystal (1.873(3) Å) is in good agreement with the Ni–N bond distance in solution (1.87 Å) that was determined by EXAFS. Compared to more planar reference compounds, NiTBuOEP exhibits significantly red-shifted absorption spectra in solution, correctly 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 conformational distortion in porphyrins, this proves that the highly nonplanar conformation of sterically strained porphyrins is maintained in solution. Thus, the physical and chemical properties measured in solution do indeed reflect the stereochemistry of the single crystals. In addition, the use of only alkyl substituents in NiTBuOEP to cause nonplanarity obviously circumvents potential electronic effects due to aryl ring interactions.