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  • Title: Activation barriers to meso-aryl group rotation in titanyl tetraaryltetrapyrroles. An investigation of the out-of-plane flexibility of hydroporphyrins.
    Author: Stolzenberg AM, Haymond GS.
    Journal: Inorg Chem; 2002 Jan 28; 41(2):300-8. PubMed ID: 11800618.
    Abstract:
    The free-base and titanyl (Ti(IV)O) complexes of meso-tetratolyl- and meso-tetra(3,5-xylyl)hydroporphyrins were synthesized and characterized. Metalation of the hydroporphyrins with titanium was achieved by reaction of the lithium salts of the hydroporphyrin with TiCl(4). Other methods used to metalate porphyrins with titanium required harsher reaction conditions and led to substantial oxidative dehydrogenation of the macrocycle when applied to hydroporphyrins. The titanyl group differentiates the two faces of the macrocycle and consequently the two sides of the meso-aryl groups, which are tilted nearly perpendicular to the macrocycle plane. The (1)H NMR signals for the nonequivalent ortho protons and nonequivalent meta protons averaged on the NMR time scale at elevated temperatures due to aryl group rotation. Activation barriers for aryl group rotation in the para-substituted and meta-disubstituted titanyl hydroporphyrin complexes and in related titanyl porphyrin complexes were determined from variable-temperature NMR spectra and ranged from 15.6 to 18.4 kcal/mol. In chlorin compounds, barriers for rotation of aryl groups located between a pyrrole and a pyrroline (reduced) ring are greater than those of aryl groups located between two pyrrole rings. Comparisons of barriers in complexes with different macrocycle saturation levels show that the increased barriers for aryl groups adjacent to pyrroline rings cannot be attributed solely to the increased steric bulk of the pyrroline beta-CH(2) group relative to the pyrrole beta-CH group. Variations in flexibility and electronic environments at meso carbons in the hydroporphyrins may also contribute. Rotation barriers for meta-disubstituted aryl groups, which are higher than those for para-substituted aryl groups, increase with the size and mass of the substituent.
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