Conformational flexibility of meso-tetrakis(perfluoroalkyl)porphyrins: Energetics of ruffling versus saddling

To better understand the existence of multiple nonplanar conformations of certain porphyrins, we have carried out geometry optimizations with nonlocal density functional theory and triple-zeta plus polarization basis sets for the ruffled and saddled conformations of four metalloporphyrins, M(TCF3P) and M(TC2F5P), where M is Ni and Zn, TCF3P is meso-tetrakis(trifluoromethyl)porphyrin, and TC2F5P is meso-tetrakis(pentafluoroethyl)porphyrin. Short F ... H-beta contacts lead to strongly nonplanar geometries for every optimized structure. Ruffling is significantly favored over saddling for NI(II) complexes, whereas the ruffled and saddled geometries are essentially equienergetic for Zn(II) complexes, consistent with experimental observation of both predominantly ruffled and saddled conformations for meso-tetrakis(perfluoroalkyl)porphyrins. Contrastingly, for dodecaphenylporphyrins, which generally exhibit saddled conformations, ruffling and saddling appear to be most competitive for Ni(II) complexes. Despite shorter F ... H-beta contacts in the TC2F5P complexes, the TCF3P and TC2F5P complexes exhibit comparable nonplanar distortions for a particular metal.