Anion and Cation Complexes of Expanded Porphyrins

Synthesis and Studies of Structural Isomers of meso-Fused Dicarbahexaphyrins

Two close structurally related isomers of nonaromatic meso -fused dicarbahexaphyrins were synthesized by condensing one equivalent of fluorene based tripyrrane with one equivalent of pentafluorobenzaldehyde in CH 2 Cl 2 under BF 3 .OEt 2 catalyzed conditions. The cis and trans isomers of meso -fused dicarbahexaphyrins were separated by preparative thin-layer chromatography and isolated pure macrocycles as green solids in 6-7% yields. NMR spectra of cis and trans isomers are quite distinct from each other and trans isomer was very symmetric and showed fewer resonances than cis isomer in NMR. The NMR study supported the nonaromatic nature of both cis and trans isomers of meso -fused dicarbahexaphyrins. DFT optimized structures revealed that the cis isomer adopted a singly twisted puckered conformation whereas the trans isomer displayed a saddle like conformation. Both cis and trans isomers almost showed similar nonaromatic absorption features with slight differences in their peak maxima. However, the protonated derivative of cis isomer showed absorption bands in visible-NIR region with bands extended upto 1000 nm whereas the trans isomer showed strong bands in the visible region. Both cis and trans macrocycles were easier to oxidize and reduce and TD-DFT studies corroborated with the experimental findings.

Porphyrinoid actinide complexes

The diverse coordination modes and electronic features of actinide complexes of porphyrins and related oligopyrrolic systems (referred to as "porpyrinoids") have been the subject of interest since the 1960s. Given their stability and accessibility, most work with actinides has focused on thorium and uranium. This trend is also seen in the case of porphyrinoid-based complexation studies. Nevertheless, the diversity of ligand environments provided by porphyrinoids has led to the stabilization of a number of unique complexes with the early actinides that are often without structural parallel within the broader coordination chemical lexicon. This review summarizes key examples of prophyrinoid actinide complexes reported to date, including the limited number of porphyrinoid systems involving transuranic elements. The emphasis will be on synthesis and structure; however, the electronic features and reactivity pattern of representative systems will be detailed as well. Coverage is through December of 2021.

Kinetic trapping of a cobalt(ii) metallocage using a carbazole-containing expanded carbaporphyrinoid ligand

The meso-unsubstituted expanded porphyrinoid 3, incorporating two carbazole moieties, acts as an effective ligand for Co(ii) and permits the isolation and X-ray diffraction-based characterization of a 6 : 3 metal-to-ligand metallocage complex that converts spontaneously to the constituent 2 : 1 metal-to-ligand metalloring species in chloroform solution. The discrete metalloring is formed directly when the Co(ii) complex is crystallized from supersaturated solutions, whereas crystallization from more dilute solutions favors the metallocage. Studies with two other test cations, Pd(ii) and Zn(ii), revealed exclusive formation of the monomeric metalloring complexes with no evidence of higher order species being formed. Structural, electrochemical and UV-vis-NIR absorption spectral studies provide support for the conclusion that the Pd(ii) complex is less distorted and more effectively conjugated than its Co(ii) and Zn(ii) congeners, an inference further supported by TD-DFT calculations. The findings reported here underscore how expanded porphyrins can support coordination modes, including bimetallic complexes and self-assembled cage structures, that are not necessarily easy to access using more traditional ligand systems.

Carbazole containing expanded carbaporphyrinoid ligand supports the formation of 2 : 1 metal-to-ligand complexes with Pd, Co, and Zn. Solid-state studies also revealed formation of a 6 : 3 metal-to-ligand metallocage in the case of Co complexation.

A chromophore-supported structural and functional model of dinuclear copper enzymes, for facilitating mechanism of action studies

Type III dicopper centres are the heart of the reactive sites of enzymes that catalyze the oxidation of catechols. Numerous synthetic model complexes have been prepared to uncover the fundamental chemistry involved in these processes, but progress is still lagging much behind that for heme enzymes. One reason is that the latter gain very much from the informative spectroscopic features of their porphyrin-based metal-chelating ligand. We now introduce sapphyrin-chelated dicopper complexes and show that they may be isolated in different oxidation states and coordination geometries, with distinctive colors and electronic spectra due to the heme-like ligands. The dicopper(i) complex 1-Cu2 was characterized by ¹H and ¹⁹F NMR spectroscopy of the metal-chelating sapphyrin, the oxygenated dicopper(ii) complex 1-Cu2O2 by EPR, and crystallographic data was obtained for the tetracopper(ii)-bis-sapphyrin complex [1-Cu2O2]2. This uncovered a non-heme [Cu₄(OH)₄]⁴⁻ cluster, held together with the aid of two sapphyrin ligands, with structural features reminiscent of those of catechol oxidase. Biomimetic activity was demonstrated by the 1-Cu2O2 catalyzed aerobic oxidation of catechol to quinone; the sapphyrin ligand aided very much in gaining information about reactive intermediates and the rate-limiting step of the reaction.

Di-copper chelation by sapphyrin facilitates reaction mechanism investigations and characterization of reactive intermediates regarding biomimetic catechol oxidation.

Molecular “Texas Longhorn”: An expanded Schiff base oligopyrrolic macrocycle

We report here the synthesis and structural characterization of a novel expanded Schiff base oligopyrrolic macrocycle TxLH ([Formula: see text] compound 2) along with its smaller congener hemi-TxLH ([Formula: see text] compound 1). The solid-state structure of TxLH is reminiscent of the shape of a Texas Longhorn[Formula: see text]. It thus defines a new architectural form for porphyrin analogues. The present study thus underscores the potential of using functionalized oligopyrroles as readily accessible molecular building blocks for the construction of structurally non-trivial molecules.

The bimetallic and the anchoring group effects on both optical and charge transport properties of hexaphyrin amethyrin

Bimetallic Cu(ii)-hexaphyrin amethyrin proposed as a molecular switch operated by the application of an external magnetic field.

Tripyrrin-armed isosmaragdyrins: synthesis, heterodinuclear coordination, and protonation-triggered helical inversion

Oxidative ring closure of linear oligopyrroles is one of the synthetic approaches to novel porphyrinoids with dinuclear coordination sites and helical chirality. The spatial arrangement of the pyrrolic groups of octapyrrole (P8) affected the position of the intramolecular oxidative coupling of the pyrrolic units; tripyrrin-armed isosmaragdyrin analogue (1) containing a β,β-linked bipyrrole moiety was synthesized regioselectively in a high yield by using FeCl₃. Ni^II-coordination at the armed tripyrrin site of 1 allowed the formation of diastereomeric helical twisted complexes (2A and 2B) and succeeding Cu^II-coordination at the macrocyclic core afforded heterodinuclear Ni^II/Cu^II-complexes (3A and 3B). Each of them comprised a pair of separable enantiomers, exhibiting P- and M-helices, respectively. Notably, diastereomeric interconversion from 2A to 2B was quantitatively achieved as a consequence of helical transformation under acidic conditions.