Copper Corroles Research Articles

Electronic absorption spectra of copper corroles: unexpected substituent effects in trans-meso-A2B-triarylcorrole complexes

The Soret maxima of copper triarylcorroles are exquisitely sensitive to substitutions on the meso phenyl groups. Similar substituent effects are also known, in more muted form, for Mn and Fe corroles. In contrast, the Soret maxima Ag , CrO and MoO triarylcorroles are essentially invariant with respect to phenyl-group substituents. TDDFT calculations suggest that the unique substituent sensitivity in the copper case results from phenyl-to- Cu (dx2-y2) charge transfer character in the main peaks of the Soret region. To probe the phenonmenon in even greater depth, we have examined here the electrochemistry and electronic absorption spectra of a series of copper trans-A2B-triarylcorrole complexes, where the para substituents on the aryl groups include CF3 , H , CH3 , and OCH3 . Although both 5,15- and 10-substituents appear to influence the oxidation and reduction potentials of the copper corroles in a simple, additive manner, they affect the Soret maxima very differently. Thus, for a given 10-substituent, 5,15-aryl substituents exert a strong, "normal" substituent effect on the Soret maximum. For given 5,15-substituents, however, the 10-substituent has almost no effect on the Soret maximum. An attempt has been made to relate these somewhat curious observations to a molecular orbital picture based on copper triphenylcorrole.

The Structural Chemistry of Metallocorroles: Combined X-ray Crystallography and Quantum Chemistry Studies Afford Unique Insights

Although they share some superficial structural similarities with porphyrins, corroles, trianionic ligands with contracted cores, give rise to fundamentally different transition metal complexes in comparison with the dianionic porphyrins. Many metallocorroles are formally high-valent, although a good fraction of them are also noninnocent, with significant corrole radical character. These electronic-structural characteristics result in a variety of fascinating spectroscopic behavior, including highly characteristic, paramagnetically shifted NMR spectra and textbook cases of charge-transfer spectra. Although our early research on corroles focused on spectroscopy, we soon learned that the geometric structures of metallocorroles provide a fascinating window into their electronic-structural characteristics. Thus, we used X-ray structure determinations and quantum chemical studies, chiefly using DFT, to obtain a comprehensive understanding of metallocorrole geometric and electronic structures. This Account describes our studies of the structural chemistry of metallocorroles. At first blush, the planar or mildly domed structure of metallocorroles might appear somewhat uninteresting particularly when compared to metalloporphyrins. Metalloporphyrins, especially sterically hindered ones, are routinely ruffled or saddled, but the missing meso carbon apparently makes the corrole skeleton much more resistant to nonplanar distortions. Ruffling, where the pyrrole rings are alternately twisted about the M-N bonds, is energetically impossible for metallocorroles. Saddling is also uncommon; thus, a number of sterically hindered, fully substituted metallocorroles exhibit almost perfectly planar macrocycle cores. Against this backdrop, copper corroles stand out as an important exception. As a result of an energetically favorable Cu(d(x2-y2))-corrole(π) orbital interaction, copper corroles, even sterically unhindered ones, are inherently saddled. Sterically hindered substituents accentuate this effect, sometimes dramatically. Thus, a crystal structure of a copper β-octakis(trifluoromethyl)-meso-triarylcorrole complex exhibits nearly orthogonal, adjacent pyrrole rings. Intriguingly, the formally isoelectronic silver and gold corroles are much less saddled than their copper congeners because the high orbital energy of the valence d(x2-y2) orbital discourages overlap with the corrole π orbital. A crystal structure of a gold β-octakis(trifluoromethyl)-meso-triarylcorrole complex exhibits a perfectly planar corrole core, which translates to a difference of 85° in the saddling dihedral angles between analogous copper and gold complexes. Gratifyingly, electrochemical, spectroscopic, and quantum chemical studies provide a coherent, theoretical underpinning for these fascinating structural phenomena. With the development of facile one-pot syntheses of corrole macrocycles in the last 10-15 years, corroles are now almost as readily accessible as porphyrins. Like porphyrins, corroles are promising building blocks for supramolecular constructs such as liquid crystals and metal-organic frameworks. However, because of their symmetry properties, corrole-based supramolecular constructs will probably differ substantially from porphyrin-based ones. We are particularly interested in exploiting the inherently saddled, chiral architectures of copper corroles to create novel oriented materials such as chiral liquid crystals. We trust that the fundamental structural principles uncovered in this Account will prove useful as we explore these fascinating avenues.

Synthesis and electrochemical properties of meso-phenyl substituted copper corroles: Solvent effect on copper oxidation state

Two meso-phenyl chloro-substituted copper corroles were synthesized and characterized by a variety of spectroscopic techniques. The investigated compounds are represented as ((ClPh)3Cor)Cu and ((Cl2Ph)3Cor)Cu , where (ClPh)3Cor and (Cl2Ph)3Cor are the trianions of the 5,10,15-tri- (4-chlorophenyl)corrole and 5,10,15-tri(2,4-dichlorophenyl)corrole, respectively. UV-visible and ESR spectroscopy revealed that the electronic ground state of each copper corrole is solvent dependent. Both compounds contain Cu(III) corroles in the solid state and in solutions of CH2Cl2 , but an equilibrium exists between Cu(III) and its Cu(II) form in pyridine, DMSO or DMF. The Cu(II) corrole could also be generated by controlled potential reduction in a thin-layer cell, which is reversible in CH2Cl2 or DMF containing 0.1 M TBAP. The structure of ((ClPh)3Cor)Cu was determined by a single-crystal X-ray study.

Substituent effects on metallocorrole spectra: insights from chromium-oxo and molybdenum-oxo triarylcorroles

The Soret maxima of a number of metallotriarylcorroles shift sensitively in response to varying substituents on the meso-aryl groups. The effect is most pronounced for copper corroles but is not seen for silver and gold corroles. In the copper case, the effect has been attributed to a small HOMO–LUMO gap. Chromium-oxo corroles share a small HOMO–LUMO gap with copper corroles, as well as a substantially metal-based LUMO, yet the Soret maxima of chromium-oxo triarylcorroles do not shift in response to substitution on the meso-aryl groups. Molybdenum-oxo corroles are substituent-insensitive in the same sense. TDDFT calculations, focusing on chromium-oxo and molybednum-oxo triarylcorroles, are reported here in an attempt to explain the divergent spectroscopic behavior of different metallocorrole families.

Amination reaction on copper and germanium β-nitrocorrolates.

Copper and germanium complexes of β-substituted nitrocorroles were reacted with 4-amino-4H-1,2,4-triazole to give the corresponding β-amino-β-nitro derivatives, in moderate to good yields. This is the first successful example of a vicarious nucleophilic substitution performed on corrole derivatives, because the same reaction carried out on silver complexes afforded the corresponding 6-azahemiporphycenes by way of corrole ring expansion. The first step of this work is related to the modification of a synthetic protocol for preparation of the β-substituted nitro corroles. The nitration reaction was carried out on a copper corrole using NaNO(2) as the primary source of NO(2)(-) coupled with AgNO(2) used as oxidant. By variation of the molar ratio of the reagents it was possible to direct the product distribution toward mono- and dinitro derivatives. The reaction between mono- and dinitro derivatives of (TtBuCorrCu) with 4-amino-4H-1,2,4-triazole gave good results, leading to the isolation of 2-(NH(2))-3-(NO(2))-TtBuCorrCu and 2,18-(NH(2))(2)-3,17-(NO(2))(2)-TtBuCorrCu in moderate yields. To elucidate factors that influence the reaction, and to highlight the different behavior observed for different metal complex substrates, the electrochemistry of three copper complexes, TtBuPCorrCu, (NO(2))TtBuPCorrCu, and (NO(2))(2)TtBuPCorrCu, was studied by cyclic voltammetry and thin-layer UV-visible spectroelectrochemistry. The nitro groups on (NO(2))(x)TtBuPCorrCu are highly electron-withdrawing, which leads not only to a substantial positive shift of all redox potentials but also to a unique redox behavior and UV-vis spectrum of the singly reduced product as compared to the parent compound, TtBuPCorrCu. Finally, the amination reaction was carried out on a Ge(IV) nitrocorrolate, giving in good yield the 2-amino-3-nitroderivative, which was structurally characterized by single crystal X-ray crystallography.

Suzuki–Miyaura cross-coupling reaction on copper-trans-A2B corroles with excellent functional group tolerance

The palladium-catalyzed Suzuki–Miyaura cross-coupling reaction has been investigated on meso-substituted trans-A2B-corrole using tailored Pd-catalyst systems.We present the first examples of Suzuki–Miyaura cross-coupling reactions on meso-substituted trans-A2B-corrole derivatives with neutral, sterically hindered, inactivated and heteroaromatic boronic acids and esters, alkenylboronic acids, as well as quickly deboronating aryl boronic acids and benzo-condensated five membered heterocyclic boronic acids. In addition, we established a high-yield procedure for the Suzuki–Miyaura cross-coupling reaction of corroles with neutral boronic acids.Due to the lability of the free-base corrole macrocycles, functionalization of the corrole periphery was performed with the corresponding Cu-metallated species. meso-Substituted trans-A2B-corrole can hence be regarded as highly versatile platform towards more sophisticated corrole systems.X-ray structure analysis of a functionalized meso-substituted trans-A2B copper corrole exhibited the typical features of such a Cu-complex: short N–Cu distances and a saddled corrole configuration.Moreover, we observed a sensitivity of the formal oxidation state of the coordinated copper ions towards Suzuki–Miyaura cross-coupling reaction conditions, where the central copper(III) ion approaches the characteristic features of a copper(II) species. This redox behaviour was examined by UV/vis absorption spectra, nuclear magnetic resonance (NMR) experiments and time-dependent density functional theoretical calculations.

A Metallocorrole with Orthogonal Pyrrole Rings (Eur. J. Inorg. Chem. 12/2011)

AbstractThe cover picture shows a variety of nonplanar aromatics, including corannulene, fullerenes, helicenes and nonplanar porphyrins against a background of the midnight sun near Tromsø, Norway. The molecule at the center is a new member of this intriguing family of compounds – an exceedingly strongly saddled, chiral copper corrole. These have been synthesized by Prof. A. Ghosh and co‐workers at the University of Tromsø; details are discussed in the Short Communication on p. 1865 ff.

Graphical Abstract: Eur. J. Inorg. Chem. 12/2011

AbstractThe cover picture shows a variety of nonplanar aromatics, including corannulene, fullerenes, helicenes and nonplanar porphyrins against a background of the midnight sun near Tromsø, Norway. The molecule at the center is a new member of this intriguing family of compounds – an exceedingly strongly saddled, chiral copper corrole. These have been synthesized by Prof. A. Ghosh and co‐workers at the University of Tromsø; details are discussed in the Short Communication on p. 1865 ff.

A First TDDFT Study of Metallocorrole Electronic Spectra: Copper meso‐Triarylcorroles Exhibit Hyper Spectra

AbstractThe Soret maxima of a number of metallotriarylcorrole derivatives, notably the copper corroles, are exquisitely sensitive to substitutions on the meso phenyl groups. In contrast, the Soret maxima of silver triarylcorroles are essentially invariant with respect to phenyl‐group substituents. TDDFT calculations indicate that the substituent sensitivity in the copper case results from significant phenyl‐to‐metal charge‐transfer character in the main peaks of the Soret region. In other words, copper triarylcorroles exhibit so‐called hyper spectra. By contrast, the Soret region in the silver case is largely π→π* in nature. DFT calculations suggest that the difference in nature of the Soret bands reflects the energy difference between the LUMOs of the copper and silver complexes, which, in both cases, may be described as an antibonding metal(d)‐corrole(π)‐based MO. Strong support for this picture comes from electrochemistry: the reduction potentials of silver triarylcorroles are about 0.7–0.8 V more negative than for the analogous copper complexes. In the same vein, the “electrochemical HOMO–LUMO gaps” are ca. 1 eV for the silver corroles and only 0.8 eV for their copper analogues.

Copper Corroles: the Question of Noninnocence

In this paper, the results are presented from a comparative study of the electronic and geometric structure of copper corroles by means of either density functional theory (DFT, using both pure and hybrid functionals) and multiconfigurational ab initio methods, starting from either a complete active space (CASSCF) or restricted active space (RASSCF) reference wave function and including dynamic correlation by means of second-order perturbation theory (CASPT2/RASPT2). DFT geometry optimizations were performed for the lowest singlet and triplet states of copper corrole, both unsubstituted and meso-substituted with three phenyl groups. The effect of saddling on the electronic structure was investigated by comparing the results obtained for planar (C(2v)) and saddled (C(2)) structures. With DFT, the origin of the saddling distortion is found to be dependent on the applied functional: covalent Cu 3d-corrole π interactions with pure functionals (BP86, OLYP), antiferromagnetic exchange coupling between an electron in the corrolate (C(2)) b type π orbital, and an unpaired Cu(II) 3d electron with hybrid functionals (B3LYP, PBE0). The CASPT2 results essentially confirm the suggestion from the hybrid functionals that copper corroles are noninnocent, although the contribution of diradical character to the copper-corrole bond is found to be limited to 50% or less. The lowest triplet state is calculated at 0-10 kcal/mol and conform with the experimental observation (variable temperature NMR) that this state should be thermally accessible.

A Biomimetic Copper Corrole – Preparation, Characterization, and Reconstitution with Horse Heart Apomyoglobin

AbstractA copper corrole with biomimetic propionate side chains was prepared as a novel heme analog by metalation of the respective ligand as the dimethylester with copper(II) acetate hydrate and subsequent saponification with LiOH. The metalated dimethylester was characterized by spectroscopic and crystallographic means and shown by comparison to contain a divalent copper ion, antiferromagnetically coupled to the radical‐dianionic organic ligand. The molecular structure of this complex is characterized by a saddled macrocycle conformation. In addition, the methylpropionate side chains are not stretched out but bent to the same side of the mean copper corrole plane. In buffered solution, the saponified propionate complex slowly forms an equilibrium mixture with small, insoluble aggregates. The compound cleanly binds in 1:1 stoichiometry to horse heart apomyoglobin without a support by proximal histidine binding. The binding in the protein pocket is rather weak, and the biohybrid compound tends to decompose under the slightly acidic conditions of mass spectrometry. Reduction of the copper corrole was achieved with sodium dithionite inside the protein pocket and yielded an air‐ and buffer stable, monoanionic, and protein bound copper corrolate complex, which could be identified by its EPR spectrum.

β-octabromo-meso-tris(pentafluorophenyl)corrole: reductive demetalation-based synthesis of a heretofore inaccessible, perhalogenated free-base corrole

Long known in various metal-complexed forms, β-octabromo-meso-tris(pentafluorophenyl)corrole, H3[Br8TPFPC ], has not been available as a free ligand until now. It can be prepared in high yield (86%) via reductive demetalation (conc. H2SO4 , FeCl2 ) of Mn[Br8TPFPC ]. Interestingly, the same conditions did not result in demetalation of the analogous copper complex, which may be contrasted to the behavior of many other copper corroles, which demetalate cleanly. X-ray crystallographic analysis revealed a unique "half-saddled" conformation, wherein two of the pyrrole rings on one side of the direct pyrrole-pyrrole linkage are strongly tilted up and down relative to each other, whereas the other two pyrrole rings are roughly in the mean plane of the macrocycle.

Copper Corroles Are Inherently Saddled

X-ray crystallographic analyses of two sterically unhindered copper meso-triarylcorroles, Cu[5,15-P(2)-10-(4-MeOP)C] and Cu[5,15-(4-CF(3)P)(2)-10-(4-MeOP)C] (P = phenyl and C = corrole), revealed substantially saddled corrole rings. These results are in marked contrast to those on highly sterically hindered cobalt(III) and iridium(III) corroles, which exhibit planar corrole macrocycles. The solution to this conundrum is that copper corroles are inherently saddled, as a result of a specific copper(d)-corrole(pi) orbital interaction. This orbital interaction results in a noninnocent corrole ligand, and the overall electronic structure may thus be described as Cu(II)-corrole(*2-). While many specific metal(d)-macrocycle(pi) orbital interactions are known for nonplanar metalloporphyrins, this work provides a rare example of such an orbital interaction providing the actual driving force for a significant nonplanar distortion. Our findings on copper corroles, along with those of others on cobalt and iridium corroles, thus constitute an intriguing and somewhat counterintuitive chapter in the structural chemistry of metallocorroles.

Copper β-Octakis(trifluoromethyl)corroles: New Paradigms for Ligand Substituent Effects in Transition Metal Complexes

The reaction of copper beta-octabromo- meso-triarylcorrole derivatives with methyl 2,2-difluoro-2-(fluorosulfonyl)acetate has provided four beta-octakis(trifluoromethyl)corrole complexes, Cu[(CF 3) 8T( p-XP)C] (X = F, H, Me, OMe), in moderate yields. The new complexes present a conglomeration of remarkable substituent effects, both steric and electronic. DFT (OLYP/TZP) geometry optimization of Cu[(CF 3) 8TPC] (i.e., X = H) indicates a sterically hindered, strongly saddled geometry, with numerous short F...F nonbonded contacts of 2.5-2.9 A and certain beta carbons displaced by over 1.5 A relative to the mean corrole plane. The CF 3 groups generally appear as quartets in the (19)F NMR spectra, with unexpectedly large (5) J FF coupling constants of about 14 Hz, apparently a manifestation of the highly crowded structure. The eight CF 3 groups together exert a powerful influence on the redox potentials of the copper corrole core. Thus, the E 1/2ox of Cu[(CF 3) 8TPC] (1.4 V vs saturated calomel electrode) is a full half of a volt above that of Cu(TPC) (0.9 V) and a quarter of a volt above that of Cu(Br 8TPC) (1.14 V). Intriguingly, the beta CF 3 groups also greatly intensify the influence of the meso aryl substituents on the redox potentials, relative to the other Cu[Y 8T( p-XP)C] series, where Y = H, F, and Br. The Cu[(CF 3) 8T( p-XP)C] complexes also exhibit the most red-shifted optical spectra of any series of metallocorroles synthesized to date. Thus, between Cu(TPC) and Cu[(CF 3) 8T( p-MeO-P)C], the Soret maximum shifts by nearly 100 nm. The observed red-shifts are attributed in part to charge-transfer transitions of the Soret region and in part to the extreme nonplanar distortions.

Cationic Metal–Corrole Complexes: Design, Synthesis, and Properties of Guanine‐Quadruplex Stabilizers

A series of pyridinium and quaternary ammonium copper corroles has been designed and synthesized. All new compounds have been fully characterized by NMR spectroscopy, high-resolution mass spectrometry, UV/Vis spectrscopy, and elemental analysis. Biochemical studies have indicated that all of these corrole derivatives can stabilize G-quadruplex structures, with corrole 4 being the most effective according to the results of circular dichroism (CD) melting experiments, polymerase chain reaction (PCR) stop assays, and surface plasmon resonance (SPR) experiments. Moreover, both corroles 3 and 4 tend to induce the human telomeric sequence to form hybrid G-quadruplex structures, whereas corroles 8 and 9 are more inclined to induce the human telomeric sequence to form antiparallel G-quadruplex structures.

Reductive demetalation of copper corroles: first simple route to free-base β-octabromocorroles

Although the chemistry of corroles has grown spectacularly in recent years, the field has been marred by the lack of convenient protocols for demetalation of metallocorroles. Reported herein is a superior procedure for demetalating copper corroles with concentrated H 2SO4 and 5-200 equiv FeCl 2 or SnCl 2. The yields obtained with this reductive procedure are generally substantially better than with CHCl 3/ H 2SO4, CH 2Cl2/ H 2SO4, or H 2SO4 alone. With an oxidation-prone metallocorrole such as Cu [ T (p- OMeP ) C ], the reductive protocol was essential for obtaining any measurable yield of the free base at all. Free-base β-octabromo-meso-triarylcorroles were also obtained in pure form, in good yields, and with relative ease via this procedure.

Functionalization of Corroles: The Nitration Reaction

The reaction of meso-triarylcorroles with AgNO2 proceeds with concomitant metalation and peripheral substitution to give the corresponding nitro-substituted silverIII corrole complex. The substitution is highly regioselective, giving only the corresponding 3-nitro derivative, among the different possible isomers. The results obtained indicate that the reaction intermediate is the pi-cation radical of the complex, which is then attacked by nitrite ion. This was proven by the reaction of the copper corrole complexes with NaNO2: in this case, the nitration reaction proceeded without the addition of an oxidant, because of the pi-cation radical character of the copper complex. The reaction is also successful in the case of 2,3,17,18-tetraethyl-8,12-diacetoxymethyl-7,13-dimethylcorrole (AMCorH3), with the formation of the meso-substituted silver corrole derivative (NO2)3AMCorAg (fully characterized by X-ray crystallography), the first of its kind to be reported. Two of the corroles are characterized by cyclic voltammetry and spectroelectrochemistry in dichloromethane, and the site of electron transfer is elucidated.

Revisiting the Electronic Ground State of Copper Corroles

Revisiting the Electronic Ground State of Copper Corroles

Synthesis and characterization of manganese and copper corrole xanthene complexes as catalysts for water oxidation

Two corrole xanthene ligands and four corresponding Mn IV and Cu III complexes have been synthesized and spectroscopically characterized. This kind of complexes, comprising of xanthene and corrole linked by an amide bond, were designed as bio-inspired models for the oxygen evolving complex (OEC) in Photosystem II. We find that both manganese complexes 4a and 5a have efficiency on catalyzing oxygen evolution at low potential (about 0.80 V) by electrochemical method, which is a significant progress in the study of dioxygen formation.

Biliverdine-Based Metalloradicals: Sterically Enhanced Noninnocence

This is a first density functional theory survey of transition-metal biliverdines (Blv), where we have chosen to focus on key Mn, Fe, Co, and Cu complexes. According to the calculations, the complexes are invariably noninnocent, featuring Blv*2- ligand radicals. In this, biliverdine complexes resemble metallocorroles, but the parallels are only approximate. Briefly, metallobiliverdines exhibit a much greater tendency to adopt noninnocent electronic structures than analogous metallocorroles. The O...O nonbonded contacts in biliverdines apparently preclude the formation of short metal-N bonds that, in turn, could stabilize high-valent metal ions. Thus, while most copper corroles (Cor) exhibit diamagnetic CuIII ground states, copper biliverdines are clearly Cu(II)Blv*2- species. In the same spirit, while chloroiron corroles are best described as FeIII(S = 3/2)Cor*2-,the analogous biliverdine derivative seems best described as Fe(III)(S = 5/2)Blv*2-, i.e., featuring a high-spin FeIII center with long (>2.0 A) Fe-N bond distances. Overall, the results highlight the important role that steric effects may play in modulating the electronic structures and the potentially noninnocent nature of transition-metal complexes.