Magnetic Circular Dichroism of Porphyrinoid Silver Complexes: Evidence of the Electronic Structure Inversion upon Protonation of the N-Confused Core

We report a systematic investigation of a series of Ag(II) and Ag(III) complexes of porphyrins and their analogues using UV-vis magnetic circular dichroism (MCD) spectroscopies and theoretical calculations. Ag(II) and Ag(III) octaethyl- and tetraarylporphyrins show the usual sign sequence in the Q-band region (i.e., negative to positive intensities with increasing energy) of their MCD spectra, indicative of the ΔHOMO > ΔLUMO relationship (ΔHOMO is the energy difference between Michl's a and s orbitals, and ΔLUMO is the energy difference between Michl's -a and -s pair of MOs). In contrast, Ag(II) complexes of β,β'-pyrrole-modified porphyrins (with an effective chlorin-type π-system) and Ag(III) corroles have sign reverse features in the MCD spectra of their Q-band region (ΔHOMO < ΔLUMO relationships). The Ag(III) complex of N-confused porphyrin shows the ΔHOMO > ΔLUMO relationship in the neutral state and the ΔHOMO < ΔLUMO relationship in the protonated form.

Ag(III)···Ag(III) Argentophilic Interaction in a Cofacial Corrole Dyad

Metallophilic interactions between closed-shell metal centers are exemplified by d¹⁰ ions, with Au(I) aurophilic interactions as the archetype. Such an interaction extends to d⁸ species, and examples involving Au(III) are prevalent. Conversely, Ag(III) argentophilic interactions are uncommon. Here, we identify argentophilic interactions in silver corroles, which are authentic Ag(III) species. The crystal structure of a monomeric silver corrole is a dimer in the solid state, and the macrocycle exhibits an atypical domed conformation. In order to evaluate whether this represents an authentic metallophilic interaction or a crystal-packing artifact, the analogous cofacial or "pacman" corrole was prepared. The conformation of the monomer was recapitulated in the silver pacman corrole, exhibiting a short 3.67 Å distance between metal centers and a significant compression of the xanthene backbone. Theoretical calculations support the presence of a rare Ag(III)···Ag(III) argentophilic interaction in the pacman complex.

Metal Complexes of Singly, Doubly and Triply Linked Porphyrins and Corroles: An Insight into the Physicochemical Properties

Metal complexes of multi-porphyrins and multi-corroles are unique systems that display a host of extremely interesting properties. Availability of free meso and β positions allow formation of different types of directly linked bis-porphyrins giving rise to intriguing optical and electronic properties. While the fields of metalloporphyrin and corroles monomer have seen exponential growth in the last decades, the chemistry of metal complexes of bis-porphyrins and bis-corroles remain rather underexplored. Therefore, the impact of covalent linkages on the optical, electronic, (spectro)electrochemical, magnetic and electrocatalytic activities of metal complexes of bis-porphyrins and -corroles has been summarized in this review article. This article shows that despite the (still) somewhat difficult synthetic access to these molecules, their extremely exciting properties do make a strong case for pursuing research on these classes of compounds.

Ligand Non-innocence and Single Molecular Spintronic Properties of AgII Dibenzocorrole Radical on Ag(111)

A facile method for the quantitative preparation of silver dibenzo-fused corrole Ag-1 is described. In contrast to the saddle conformation resolved by single-crystal X-ray analysis for Ag-1, it adopts an unprecedented domed geometry, with up and down orientations, when adsorbed on an Ag(111) surface. Sharp Kondo resonances near Fermi level, both at the corrole ligand and the silver center were observed by cryogenic STM, with relatively high Kondo temperature (172 K), providing evidence for a non-innocent Ag^II -corrole^.2- species. Further investigation validates that benzene ring fusion and molecule-substrate interactions play pivotal roles in enhancing Ag(4d(x² -y² ))-corrole (π) orbital interactions, thereby stabilizing the open-shell singlet Ag^II -corrole^.2- on Ag(111) surface. Moreover, this strategy used for constructing metal-free benzene-ring fused corrole ligand gives rise to inspiration of designing novel metal-corrole compound for multichannel molecular spintronics devices.

Syntheses of Water-Soluble Silver(II)-Phthalocyanines toward Optical Sensing for Thiol Detection

Water-soluble silver(II)-phthalocyanine complexes (AgPcs), tetrakis{4-(N-alkylpyridinium)thio}phthalocyaninato silver(II) tetrafluoroborate, [Ag(tRpySpc)](BF₄)₄, (R = Me and Et), have been synthesized for the first time by quaternization of pyridyl groups of tetrakis(4-pyridylthio)phthalocyaninato silver(II) by using Meerwein reagents and characterized by ESI-MS, elemental analyses, and optical absorption spectroscopy. Although they strongly aggregate in water, the presence of appropriate surfactants, such as polyethyleneglycol-monooleyl ether (n = approximately 50; PEG50) and sodium dodecyl sulfate, effectively disaggregates them to monomeric species. The spectral properties of the AgPcs and their aggregates in aqueous and nonaqueous solutions have been investigated by optical absorption, emission, and magnetic circular dichroism spectroscopy. These AgPcs rapidly react with thiols such as cysteine, glutathione, homocysteine, and sodium 2-sulfanylethanesulfonate (even on the order of 0.01 mM) in aqueous PEG50 solutions at room temperature to liberate the corresponding macrocyclic ligand, H₂Pc, but not with the other amino-acid analogs without sulfhydryl groups. The molar ratio of thiol to AgPc has been determined to be 1:1. Since AgPcs are essentially nonfluorescent at room temperature, while H₂Pcs emit intense red fluorescence, AgPcs can be a potent thiol-sensor toward bioimaging.

Spectral investigation of phthalocyanine complexes of high-valence silver and their aggregates

Several novel silver(II) complexes ligating a tetra-substituted phthalocyaninate, [Ag(tbpc)] (where tbpc denotes tetra-tert-butylphthalocyaninate), [Ag(tppc)] (tppc = tetrakis(2,6-dimethylphenoxy)phthalocyaninate), [Ag(tObpc)] (tObpc = tetra-n-butoxyphthalocyaninate), and [Ag(tpySpc)] (tpySpc = tetrakis(4-pyridylthio)phthalocyaninate) have been synthesized and characterized by elemental analyses, MALDI-TOF MS, optical absorption, and magnetic circular dichroism (MCD) spectroscopy. Although all the compounds are well soluble in common organic solvents, concentration studies on their optical spectra in solutions have found that they are prone to strongly aggregate in a cofacial manner (i.e., H-aggregate). Silver(II) complexes, which are essentially non-fluorescent, are readily demetallated in the presence of appropriate reductant (e.g., I^- or BH₄^-) to liberate the corresponding macrocyclic ligand, which emits intense red fluorescence. Chemical oxidation by using NOBF₄ generates the corresponding silver(III) species.

Photocatalytic C-H Thiocyanation of Corroles: Development of Near-Infrared (NIR)-Emissive Dyes

A new method of activating corrole macrocycles via an in situ generated SCN radical has been developed at very mild conditions at room temperature. This photoredox reaction resulted in the generation of tetrathiocyanatocorroles in good yields. The synthesis of tetrathiocyanatocorroles was never reported earlier. Single-crystal XRD analysis reveals that the insertion of four thiocyanate moieties at the four β-pyrrolic positions has imparted significant distortion to the corrole macrocycle. The generated tetrathiocyanatocorroles are different from the parent corroles in many ways. The photophysical properties of the newly synthesized tetrathiocyanatocorroles are dramatically altered from the parent corroles. The absorption feature of these modified corrole derivatives (both position and intensity) bears a nice similarity with the chlorophyll-a macrocycle. Thus, these newly synthesized molecules can be considered as spectroscopic model systems for chlorophyll-a pigments. The observed absorption and emission spectra of these tetrathiocyanatocorroles certainly point out that these newly developed ligand scaffolds and their various metal complexes will have immense potential as pigments in solar cells and also as NIR-emissive dyes. The observed C-H···Au weak interactions in a representative Au(III)-corrole complex point out that these complexes are capable of activating the unfunctionalized C-H groups and thus will have potential implications in C-H activation reactions.

Synthesis, Characterization, and Application of Oxo-Molybdenum(V)-Corrolato Complexes in Epoxidation Reactions

Sharpless et al. have described, while performing the molybdenum-catalyzed epoxidation reaction of olefins using alkyl hydroperoxides, that the molybdenum-oxo moiety is an active catalytic species. Thus, continuous efforts have been made to synthesize molybdenum-oxo complexes of different ligand environments. While plenty of such works on molybdenum porphyrins are reported in the literature, related molybdenum corroles are very less reported. The synthesis and characterization of two new oxo-molybdenum(V)-corrolato complexes are described herein. Both the complexes have been fully characterized by several spectroscopic techniques in conjunction with single-crystal X-ray diffraction analysis. The efficacy of the oxo-molybdenum(V)-corrolato complexes for the catalytic epoxidation reaction of olefins with the help of hydroperoxides has also been explored. The catalytic application of oxo-molybdenum(V)-corrolato complexes in the epoxidation reaction has not been reported earlier. A mechanism has been proposed to explain the experimental findings.

Meso-alkynyl corroles and their cobalt(III), manganese(III) and gallium(III) complexes

We report routes towards synthesis of novel [Formula: see text]-conjugated freebase cobalt, copper, gallium and manganese meso-alkynylcorroles. UV-vis spectra show that extensive peak broadening, red shifts, and changes in the oscillator strength of absorptions increase with the extension of [Formula: see text]-conjugation. Using density functional theory (DFT), we have carried out a first theoretical study of the electronic structure of these metallocorroles. Decreased energy gaps of about 0.3–0.4 eV between the HOMO and LUMO orbitals compared to the corresponding copper, gallium and manganese meso-5,10,15 triphenylcorrole are observed. In all cases, the HOMO energies are nearly unperturbed as the [Formula: see text]-conjugation is expanded. The contraction of the HOMO–LUMO energy gaps is attributed to the lowered LUMO energies.

A new synthesis of porphyrins via a putative trans-manganese(iv)-dihydroxide intermediate

A new method for the synthesis of meso-substituted porphyrins was developed. In this two-step methodology, the first step involves the condensation of pyrroles/dipyrromethanes with aldehydes in a water-methanol mixture under acidic conditions. The second step involves manganese induced cyclization followed by oxidation via PhIO/O2. This methodology has been useful for the synthesis of a wide range of trans-A2B2 porphyrins and also symmetric porphyrins in moderate to good yields. A detailed investigation of the manganese induced cyclization reaction has allowed us to characterize a Mn-porphyrinogen complex. A series of analytical and spectroscopic techniques and DFT calculations have led us to the conclusion that the putative intermediate species are trans-manganese(iv)-dihydroxide complexes. EPR and magnetic susceptibility measurements helped us to assign the oxidation state of the manganese complexes in their native state. The assumption of trans-manganese(iv)-dihydroxide as the true intermediate for this porphyrin synthesis has been authenticated via in situ UV-Vis experiments. This new methodology is certainly different from other previously reported methodologies in many aspects and most importantly these reactions can be easily performed on a gram scale for the synthesis of porphyrins.

Regioselective thiocyanation of corroles and the synthesis of gold nanoparticle-corrole assemblies

Herein we demonstrate a synthetic protocol for the regioselective thiocyanation of corroles. To the best of our knowledge, thiocyanato appended corrole has never been reported earlier. The resulting thiocyanato appended corrole turned out to be a good corrole based precursor for the facile synthesis of thiol protected gold nanoparticles (Au NPs). The ligand system acts as a good bidentate framework and passivates the gold surface. A strong electronic interaction between the corrole and the gold nanoparticles is manifested by their unique photo physical properties and it also confirms that the binding through β-substitutions has a more pronounced effect even though the corrole rings are face-off to the gold surface.

Asymmetric zinc porphyrin derivatives bearing three pseudo-pyrimidine meso-position substituents and their photosensitization for H2 evolution

ZnPy-6 has lower steric hindrance than ZnPy-5 when the N atoms of the pseudo-pyrimidines combine with g-C₃N₄ and AA, and thus it exhibits better photosensitization.

β-Arylethynyl substituted silver corrole complexes

Silver corrolates are attractive compounds from both practical and theoretical points of view. Indeed, they play a key role in peripheral functionalization reactions occurring at the macrocycle, enabling high-yield and regioselective group insertions useful to further elaborate the molecular skeleton. In parallel, the Janus innocent or noninnocent behavior of the corrole ligand in these complexes makes their description particularly challenging. Herein, we report properties for a series of silver 3,17-disubstituted triarylcorrole complexes with various functionalities (halogens or different phenylethynyl units) that deeply affect the electron density in the macrocyclic ligand, with obvious repercussions on the observed spectral characteristics. The compounds were obtained in yields of 54-92% by applying the Stille coupling reaction with the appropriate tributylethynyl stannane. Among the complexes prepared was a derivative bearing two terminal acetylenic units which opens the way to "click" reactions for new corrole-based architectures. This corrole was structurally characterized by single crystal X-ray crystallography. The addition of substituted ethynyl groups resulted in red-shifts of the electronic absorption spectra, the largest of which was observed for the compound with two β-NO2-Ph-C[triple bond, length as m-dash]C substituents. The remarkable influence of the NO2 groups on the electron density of this macrocycle was further demonstrated by electrochemical measurements, where an easier reduction of this complex derivative was observed as compared to the others. DFT calculations showed full delocalization over the entire p-nitrophenylethynyl unit of 5, largely affecting orbital distributions and the corresponding electronic absorptions. Although a variation of the β-substituents dramatically modifies the Soret- and Q-band positions towards lower energies for all the examined complexes, the saddling of the macrocycle resulting from functionalization is only moderate. The collected results suggest the description of these compounds as AgIII-corrolate3-, a metallocorrole with an innocent macrocyclic ligand.

Chromium Complexes with Oxido and Corrolato Ligands: Metal-Based Redox Processes versus Ligand Non-Innocence

Metal- versus ligand-centered redox processes and the effects of substituents on the ligands on the spectroscopic properties of the metal complexes are at the heart of research on metal complexes with non-innocent ligands. This work presents three examples of chromium complexes that contain both oxido and corrolato ligands, with the substituents on the corrolato ligands being different in the three cases. Combined X-ray crystallographic, electrochemical, UV/Vis/NIR/EPR spectroelectrochemical, and EXAFS/XANES measurements, together with DFT calculations, have been used to probe the complexes in three different redox forms. This combined approach makes it possible to address questions related to chromium- versus corrolato-centered redox processes, and the accessibility (or not) of Cr^IV , Cr^V , and Cr^VI in these complexes, as well as their spin states. To the best of our knowledge, these are the first EXAFS/XANES investigations on Cr-corrolato complexes in different redox forms, and hence these data should set benchmarks for future investigations on such complexes by this method.

Synthesis and characterization of meso-substituted A2B corroles with extended π-electronic structure

Abstract

We report the chemical synthesis and characterization of the stable 5,15-bis(pentafluorophenyl)-10-(trimethylsilylethynyl)corrole which serves as a precursor for the subsequent in situ sila-Sonogashira-cross-coupling reaction and metalation with copper(II) acetate. Under ambient conditions and a common catalyst system the reaction with 1-iodopyrene occurred within five hours. Due to the direct conjugation of the 18π-electronic system of the corrole macrocycle over the alkynyl group to the pyrene moiety the optical transitions in the Soret (B-) band Q-band region are significantly altered. The copper corrole exhibited complex hyperfine and superhyperfine structure in the EPR spectrum. The assignment of the EPR spectrum reveals the existence of an axial [CuII-cor^∙+] species.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1007/s00706-017-2114-6) contains supplementary material, which is available to authorized users.

Metal coordination induced ring contraction of porphyrin derivatives

Ferrocenyl-porphyrin derivatives upon treatment with silver acetate at room temperature show ring contraction followed by Ag-chelation.

Strategies for Corrole Functionalization

This review covers the functionalization reactions of meso-arylcorroles, both at the inner core, as well as the peripheral positions of the macrocycle. Experimental details for the synthesis of all known metallocorrole types and for the N-alkylation reactions are presented. Key peripheral functionalization reactions such as halogenation, formylation, carboxylation, nitration, sulfonation, and others are discussed in detail, particularly the nucleophilic aromatic substitution and the participation of corroles in cycloaddition reactions as 2π or 4π components (covering Diels-Alder and 1,3-dipolar cycloadditions). Other functionalizations of corroles include a large diversity of reactions, namely Wittig reactions, reactions with methylene active compounds, formation of amines, amides, and imines, and metal catalyzed reactions. At the final section, the reactions involving oxidation and ring expansion of the corrole macrocycle are described comprehensively.

Control of clustering behavior in anionic cerium(iii) corrole complexes: from oligomers to monomers

The first synthesis of anion capped cerium corrole complexes is reported. Unusual clustering of the lanthanide corrole units has been found and the degree of aggregation can be controlled by the choice of the capping ligand. A polymeric structure 1a, with the general formula [Cor-Ce(THF)-Cp-Na]n (Cor = 5,15-bis(2,4,6-trimethylphenyl)-10-(4 methoxyphenyl)-corrole, THF = tetrahydrofuran), is formed using sodium cyclopentadienide (NaCp) and a dimeric structure 2a, with the general formula [Cor-Ce-Tp]2, is formed when potassium tris(pyrazolyl)borate (KTp) is used. Encapsulation of the counter-cation leads to the isolation of the monomeric structures 1b and 2b, with the general formulas [AM(2.2.2-cryptand)][Cor-Cp-X] (AM = Na or K, X = Cp or Tp). The structural and spectroscopic properties of the complexes have been investigated.

Structural, electrochemical and spectroelectrochemical study on the geometric and electronic structures of [(corrolato)AuIII]n (n = 0, +1, −1) complexes

UV-vis-NIR/EPR spectroelectrochemical investigations on Au^III corroles display NIR electrochromism, the origins of which are all on corrole-centered processes.

NMR spectroscopy of the phenyl derivative of germanium(IV) 5,10,15-tritolylcorrole

A thoroughly structural characterization of (TTC)GePh (TTC [Formula: see text] 5,10,15-tritolylcorrole; Ph [Formula: see text] phenyl) in solution has been carried out through a combination of 2D NMR (1H-1H COSY, 1H-1H ROESY, 1H-[Formula: see text]C HSQC and 1H-[Formula: see text]C HMBC) experiments and density functional theory (DFT) calculations of the molecular and electronic structure and the shielding constants. The 1H and [Formula: see text]C chemical shifts computed at DFT-S12g and DFT-SAOP levels of theory nicely reproduce the experimental values, the agreement between theory and experiment being especially good for the DFT-S12g results. The calculations prove to be able to capture the fine details of the NMR spectra and to resolve some assignment ambiguities related to the inherent conformational flexibility of the macrocycle. The calculations also provide an explanation of the observed chemical shift trends in terms of diamagnetic and paramagnetic components of the shielding tensor.

Experimental and Theoretical Investigations of the Existence of Cu(II), Cu(III), and Cu(IV) in Copper Corrolato Complexes

The most common oxidation states of copper in stable complexes are +I and +II. Cu(III) complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu(IV) species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu(IV) complexes being non-existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu(IV) species, and describe a redox series where Cu(II), Cu(III), and Cu(IV) are discussed within the same molecular platform.

Ligand Noninnocence in Coinage Metal Corroles: A Silver Knife-Edge

A silver β-octabromo-meso-triarylcorrole has been found to exhibit a strongly saddled geometry, providing the first instance of a strongly saddled corrole complex involving a metal other than copper. The Soret maxima of the Ag octabromocorroles also redshift markedly in response to increasingly electron-donating para substituents on the meso-aryl groups. In both these respects, the Ag octabromocorroles differ from simple Ag triarylcorrole derivatives, which exhibit only mild saddling and substituent-insensitive Soret maxima. These results have been rationalized in terms of an innocent M(III)-corrole(3-) description for the simple Ag corroles and a noninnocent M(II)-corrole(·2-) description for the Ag octabromocorroles. In contrast, all copper corroles are thought to be noninnocent, while all gold corroles are innocent. Uniquely among metallocorroles, silver corroles thus seem poised on a knife-edge, so to speak, between innocent and noninnocent electronic structures and may tip either way, depending on the exact nature of the corrole ligand.

NIR-emissive iridium(iii) corrole complexes as efficient singlet oxygen sensitizers

The observed phosphorescence of the studied Ir(iii)corroles at ambient temperature appears at much longer wavelengths than the previously reported Ir(iii) porphyrin/corrole derivatives. Efficiencies of these compounds in the generation of singlet oxygen are also studied for the first time.

Asymmetric zinc porphyrin-sensitized nanosized TiO2 for efficient visible-light-driven CO2 photoreduction to CO/CH4

Asymmetric zinc porphyrin was used as a novel sensitizer for the visible-light-driven activity of CO₂ photoreduction to CO/CH₄ in the gas phase.

A comparative study of optical nonlinearities of trans-A2B-corroles in solution and in aggregated state

The NLO properties of a series of FB corroles are studied in the solution as well as in the aggregated state (in the form of thin films).

Synthesis, electron transport, and charge storage properties of fullerene–zinc porphyrin hybrid nanodiscs

A MIS-type diode (Ag/C₆₀–ZnTANP circular discs/p-Si/Ag) has been fabricated and its charge storage properties have been explored.