[28]Hexaphyrin Ge and Sn complexes were synthesized in high yields by reactions of [28]hexaphyrin with GeCl or SnCl in the presence of triethylamine. Both complexes display distinct 28π Möbius aromatic character and possess a trigonal bipyramidal geometry at the central Ge or Sn atom. The equatorial hydroxy group of the Ge complex was smoothly exchanged with neutral nucleophiles, such as phenol derivatives and thiophenol, with retention of configuration. In the Sn complex, intersystem crossing to the T state is remarkably enhanced owing to the effective heavy-atom effect, thus allowing the formation of the T state in high yield. The T states of the Ge and Sn complexes were found to be antiaromatic on the basis of the transient absorption features in line with the Baird rule.
Highly twisted structures of expanded porphyrin provide a prominent basis to unravel the relationship between aromaticity and chirality. Here we report the synthesis of bis-Ge(IV) complexes of [38]octaphyrin that display rigid figure-eight structures. Two bis-Ge(IV) [38] octaphyrin isomers with respect to the stereochemistry of the axial hydroxy groups on the germanium ions were obtained and found to be aromatic. Upon oxidation with MnO 2 , these [38]octaphyrin complexes were converted to a single syntype isomer of [36]octaphyrin with retained figure-eight conformation. The enantiomers have been successfully separated by HPLC equipped with a chiral stationary phase. While aromatic [38]octaphyrin Ge(IV) complexes showed quite large molar circular dichroism of up to Δɛ = 1500 M À 1 cm À 1 with a dissymmetry factor g abs of 0.035, weakly antiaromatic [36] octaphyrin Ge(IV) complexes underscored moderate values; Δɛ = 540 M À 1 cm À 1 with g abs of 0.023. Thus, the figure-eight octaphyrin scaffold has been proved to be an attractive platform for novel chiroptical materials with tunable aromaticity.
[28]Hexaphyrin GeIV and SnIV complexes were synthesized in high yields by reactions of [28]hexaphyrin with GeCl4 or SnCl4 in the presence of triethylamine. Both complexes display distinct 28π Möbius aromatic character and possess a trigonal bipyramidal geometry at the central GeIV or SnIV atom. The equatorial hydroxy group of the GeIV complex was smoothly exchanged with neutral nucleophiles, such as phenol derivatives and thiophenol, with retention of configuration. In the SnIV complex, intersystem crossing to the T1 state is remarkably enhanced owing to the effective heavy‐atom effect, thus allowing the formation of the T1 state in high yield. The T1 states of the GeIV and SnVI complexes were found to be antiaromatic on the basis of the transient absorption features in line with the Baird rule.
Metal complexation of octaphyrin(1.1.1.1.1.1.1.1) triggers unique ring-fixation aptitudes or unexpected rearrangement (cleavage) reactions. In this paper, a unique complexation behavior of [38]octaphyrin upon tin(IV) metalation is showcased. Two new [38]octaphyrin bis-Sn(IV) complexes 2Sn and 3Sn were isolated and characterized as weakly aromatic molecules. While 2Sn with the [Formula: see text] molecular symmetry displayed a similar characteristic to octaphyrin bis-Si(IV) and bis-Ge(IV) complexes reported previously, 3Sn showed a different coordination mode that is fixed by intramolecular hydrogen bondings between pyrrolic NH and axially ligated OH on the tin ion as revealed by X-ray diffraction analysis. An unexpected dimeric structure was also observed during an attempt to grow crystals of 2Sn. These characteristic behaviors indicate that the ring-fixation aptitude of octaphyrin is quite sensitive to the nature of metal ions even for the same group 14 elements.
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