Rational Synthesis of 5,5,5-Tricyclic Fused Thia-heptaphyrin (1.1.1.1.1.1.0) From a Helical Oligopyrrin Hybrid

Oxidation of a thiophene-hexapyrrane hybrid S-P₆ afforded a stable conjugated open-chain thiaheptapyrrolic helix 1 with the terminal thiophene and confused pyrrole units lying at a long distance that is adverse for further cyclization. Chelation of 1 with copper(II) ion afforded 1-Cu, which exhibits more distant terminal units. Interestingly, further oxidation of 1 triggered an intramolecular C-N fusion reaction to afford a unique 5,5,5-tricyclic fused linear thiaheptapyrrin 2, with the two terminals positioned in proximity, which favors the oxidative ring-closure reaction to give a unique 5,5,5-tricyclic fused thiaheptaphyrin (1.1.1.1.1.1.0) 3 under air. The inner-fusion strategy for positioning the reactive sites in proximity to promote oxidative cyclization offers a new approach for constructing large porphyrinoids through conjugated oligopyrrins without the assistance of metal ions.

Rapid one-pot synthesis of pyrrole-appended isocorroles

Free-base meso-triarylcorroles have been found to undergo oxidative coupling with an excess of pyrrole in dichloromethane in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) affording 5/10-pyrrole-appended isocorroles in reasonable yields (35-60%) and in a matter of seconds. The free-base isocorrole ligands could all be complexed to copper with Cu(OAc)2·H2O in chloroform/methanol in 55-80% yields. Single-crystal X-ray structures of two of the new compounds (H2[5-pyr-TpOMePiC] and Cu[10-pyr-TpOMePiC]) revealed planar macrocycles with rms atomic displacements of only 0.02 and 0.06 Å relative to their respective best-fit C19N4 planes. Both free-base and Cu(ii)-complexed isocorroles exhibit richly featured UV-vis-NIR spectra with red/NIR absorption maxima at ∼650 nm and ∼725 nm for the free-bases and ∼800-850 nm for the copper complexes, suggesting potential applications in photodynamic therapy. Cyclic voltammetric analyses of five of the Cu complexes revealed fully reversible redox cycles with multiple oxidation and reduction features.

Magnetic Diversity in Heteroisocorroles: Aromatic Pathways in 10-Heteroatom-Substituted Isocorroles

A recent study on magnetically induced currents in 10-isocorrole derivatives indicated that both the free-base and metal-complexed forms of the unsubstituted macrocycle are homoaromatic. Furthermore, depending on the substituents at the 10-position, the aromatic character was found to swing between substantially homoaromatic to substantially antihomoaromatic. Heteroisocorroles, in which the saturated 10-position has been replaced by a heteroatom-containing group X, are predicted to exhibit even more dramatic variations in aromatic character, ranging from strongly aromatic (X = O, NH, PH, and S) to strongly antiaromatic (X = BH and CO). Interestingly, the experimentally studied X = SiMe2 case does not appear to sustain a significant global ring current.

Heterocorroles: corrole analogues containing heteroatom(s) in the core or at a meso-position

Corroles are 18 π aromatic macrocyclic systems having one direct pyrrole–pyrrole linkage leading to a contracted cavity compared to porphyrins. Corroles exhibit contrasting coordination chemistry and properties compared to porphyrins. Structural modification of corroles by introducing a heteroatom in their aromatic conjugation circuit i.e., either in the core or at a meso position leads to a new class of corrinoids called heterocorroles. The core modification strategy includes replacing one or two core nitrogen atom(s) with O, S or C atoms and meso-modification involves replacing the meso-carbon atom at the 10-position with NH, NR, O, S, Se or Si atoms. This review article presents an overview of the progress in heterocorrole chemistry including their syntheses, key structural aspects and properties.

This review article presents an overview of the progress in heterocorrole chemistry including their syntheses, key structural aspects and properties.

Heterocorrole Conformations: Little Saddling, Much Ruffling

10-Heterocorrole complexes with oxygen, sulfur, and selenium at position 10 of the macrocycle and with the divalent ions of nickel, copper, and palladium were prepared and investigated. The focus was set on the size adaptation and matching mechanisms of cavity size versus ionic radius in corrole-type macrocycles. A full set of single-crystal X-ray analytical data revealed that in all but one case the N₄ binding site of the ring-contracted tetrapyrrole was larger than necessary to bind the metal ion without deformation. In-plane size adaptation through M-N bond-length elongation by 2.5-3.2 % was effective, as well as pronounced out-of-plane ruffling of the macrocycle for those compounds with a more severe size mismatch. Such ruffling had been excluded for corroles previously, but is apparently the most efficient mechanism to adapt to small central ions.

The synthesis of NiII and AlIII 10-azacorroles through coordination-induced cyclisation involving 1,2-migration

We have synthesized Ni^II and Al^III 10-azacorroles through coordination-induced cyclisation of nitrogen-bridged bisdipyrrins. The α-substituent (X = Br or Ph) shifted to the adjacent β-position during cyclisation.

Prodigiosin analogue designed for metal coordination: stable zinc and copper pyrrolyldipyrrins

The pyrrolyldipyrrin motif is found in several naturally occurring prodigiosin pigments. The potential roles of the interactions of prodigiosins with transition metals and the properties of metal-bound pyrrolyldipyrrins, however, have been difficult to assess because of the very limited number of well-characterized stable complexes. Here, we show that the introduction of a meso-aryl substituent and an ethyl ester group during the sequential assembly of the three heterocycles affords a pyrrolyldipyrrin of enhanced coordinating abilities when compared to that of natural prodigiosins. UV-visible absorption studies indicate that this ligand promptly binds Zn(II) ions with 2:1 ligand-to-metal stoichiometry and Cu(II) ions with 1:1 stoichiometry. Notably, no addition of base is required for the formation of the resulting stable complexes. The crystal structures reveal that whereas the tetrahedral zinc center engages two nitrogen donors on each ligand, the pseudosquare planar copper complex features coordination of all three pyrrolic nitrogen atoms and employs the ester group as a neutral ligand. This first example of coordination of a redox-active transition metal within a fully conjugated pyrrolyldipyrrin framework was investigated spectroscopically by electron paramagnetic resonance to show that the 1:1 metal-to-ligand ratio found in the crystal structure is also maintained in solution.