Porphyrins and Corroles with 2,6-Pyrimidyl Substituents

Milestones in corrole chemistry: historical ligand syntheses and post-functionalization

Corroles are synthetic porphyrin analogs that contain one meso carbon atom lesser and bear a trianionic N4 metal-chelating core. They require in-depth preparative chemistry, demonstrate unique coordination chemistry and have impressive and diverse physical properties, and these are commonly compared to their respective porphyrins. The corrole's macrocyclic system is inherently electron rich and chelates metal ions in a more compact, less symmetric tetranitrogen cavity compared to that of porphyrins. Herein, we cover the highlights of the corrole research through the decades by first reviewing, in a chronological sense, multi-step syntheses; some routes have since been discontinued. This is followed by describing post-functionalization of already formed corroles via reactions performed on either the macrocycle's periphery or the inner nitrogen atoms or on the existing substituents. We do also mention milestones in literature reviewing, publication of encyclopedias, and the creation of professional organizations and conferences (ICPP) which make up the corrole/porphyrin research landscape. Also highlighted are still existing challenges and future perspectives.

Highly reducible π-extended copper corroles

Di- and octa-phenylethynyl (PE) substituted π-extended copper corroles were synthesized and characterized as to their structural, electrochemical and spectroscopic properties. The addition of two or eight PE groups to the β-pyrrole positions of the corrole results in dramatic red shifts in the electronic absorption spectra and new reductions which are not seen for the parent compound lacking PE substituents. CuCor(PE)₈ is reduced in four reversible one-electron transfer steps to give derivatives of [CuCor(PE)₈]^n- where n = 1, 2, 3 or 4. Variable temperature ¹H NMR and EPR measurements were carried out and suggest that the octa- and di-PE substituted Cu-corroles can both be described as an antiferromagnetically coupled Cu^II corrole cation radical which is in equilibrium with a triplet state, possibly due to a lower singlet-triplet energy gap as compared to 1 and 2 at room temperature. The EPR spectra of one-electron oxidized and one electron reduced species exhibited the characteristics of Cu(ii) corroles. The products generated in the first two reductions of each π-extended corrole were characterized by thin-layer spectroelectrochemistry, thus providing new insights into how UV-vis spectra of highly reduced corroles vary as a function of the number of PE groups and overall charge on the molecule. The singly reduced and singly oxidized copper corroles were also chemically generated in CH₃CN and shown to have UV-visible spectra almost identical to the spectra obtained by electroreduction or electrooxidation in PhCN or THF containing 0.1 M tetrabutylammonium perchlorate.

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations

Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV-vis-NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole^•2- radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(d_z2)-corrole("a_2u") interaction (most commonly observed in Mn and Fe corroles) and the metal(d_x2-y2)-corrole(a_2u) interaction in coinage metal corroles. Less commonly encountered is the metal(d_π)-corrole("a_1u") interaction, a unique feature of formal d⁵ metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic-structural research on corrole derivatives.

Synthesis of Corroles and Their Heteroanalogs

Corroles have come a long way from being a curiosity to being a mainstream research topic. They are now regularly synthesized in numerous research laboratories worldwide with diverse specific aims in mind. In this review we present a comprehensive description of corroles' synthesis, developed both before and after 1999. To aid the investigator in developing synthetic strategies, some of the sections culminate in tables containing comparisons of various methodologies leading to meso-substituted corroles. The remaining challenges are delineated. In the second part of this review, we also describe the syntheses of isocorroles and heteroanalogs of corroles such as triazacorroles (corrolazines), 10-heterocorroles, 21-heterocorroles, 22-heterocorroles, N-confused corroles, as well as norcorroles. The review is complemented with a short outlook.

The evolution of corrole synthesis — from simple one-pot strategies to sophisticated ABC-corroles

This review highlights synthesis procedures to obtain A3-, cis- and trans- A2B- and ABC-corroles. Synthesis methods from the early beginning of “corrole chemistry”, acid-catalyzed condensation methods of various building blocks (pyrrole, aldehyde, dipyrromethane, dipyrromethane-carbinol, and dipyrromethane-dicarbinol etc.), possible side reaction during Brønsted acid catalyzed reactions (scrambling), one-pot synthesis of corroles, and post-macrocyclization modification reactions of meso-substituted A3-corroles are discussed.

Stabilization of hexa-coordinated P(v) corroles by axial silyloxy groups

We report the stabilization of the hexa-coordination environment for P(v) corroles using alkyl/aryl substituted silyloxy groups as axial ligands. The P(v) corroles are highly fluorescent and highly stable under electrochemical conditions.