Nonplanar, noninnocent, and chiral: a strongly saddled metallocorrole

Inverse Hypercorroles

Ground-state and time-dependent density functional theory (TDDFT) calculations with the long-range-corrected, Coulomb-attenuating CAMY-B3LYP exchange-correlation functional and large, all-electron STO-TZ2P basis sets have been used to examine the potential "inverse hypercorrole" character of meso-p-nitrophenyl-appended dicyanidocobalt(III) corrole dianions. The effect is most dramatic for 5,15-bis(p-nitrophenyl) derivatives, where it manifests itself in intense NIR absorptions. The 10-aryl groups in these complexes play a modulatory role, as evinced by experimental UV-visible spectroscopic and electrochemical data for a series of 5,15-bis(p-nitrophenyl) dicyanidocobalt(III) corroles. TDDFT (CAMY-B3LYP) calculations ascribe these features clearly to a transition from the corrole's a2u-like HOMO (retaining the D4h irrep used for metalloporphyrins) to a nitrophenyl-based LUMO. The outward nature of this transition contrasts with the usual phenyl-to-macrocycle direction of charge transfer transitions in many hyperporphyrins and hypercorroles; thus, the complexes studied are aptly described as inverse hypercorroles.

Molecular Geometry and Electronic Structure of Copper Corroles

Copper corroles are known for their unique multiconfigurational electronic structures in the ground state, which arise from the transfer of electrons from the π orbitals of the corrole to the d-orbital of copper. While density functional theory (DFT) provides reasonably good molecular geometries, the determination of the ground spin state and the associated energetics is heavily influenced by functional choice, particularly the percentage of the Hartree-Fock exchange. Using extended multireference perturbation theory methods (XMS-CASPT2), the functional choice can be assessed. The molecular geometries and electronic structures of both the unsubstituted and the meso-triphenyl copper corroles were investigated. A minimal active space was employed for structural characterization, while larger active spaces are required to examine the electronic structure. The XMS-CASPT2 investigations conclusively identify the ground electronic state as a multiconfigurational singlet (S0) with three dominant electronic configurations in its lowest energy and characteristic saddled structure. In contrast, the planar geometry corresponds to the triplet state (T0), which is approximately 5 kcal/mol higher in energy compared to the S0 state for both the bare and substituted copper corroles. Notably, the planarity of the T0 geometry is reduced in the substituted corrole compared with that in the unsubstituted one. By analyzing the potential energy surface (PES) between the S0 and T0 geometries using XMS-CASPT2, the multiconfigurational electronic structure is shown to transition toward a single electron configuration as the saddling angle decreases (i.e., as one approaches the planar geometry). Despite the ability of the functionals to reproduce the minimum energy structures, only the TPSSh-D3 PES is reasonably close to the XMS-CASPT2 surface. Significant deviations along the PES are observed with other functionals.

A DMRG/CASPT2 Investigation of Metallocorroles: Quantifying Ligand Noninnocence in Archetypal 3d and 4d Element Derivatives

Hybrid density functional theory (B3LYP) and density matrix renormalization group (DMRG) theory have been used to quantitatively compare the degree of ligand noninnocence (corrole radical character) in seven archetypal metallocorroles. The seven complexes, in decreasing order of corrole noninnocent character, are Mn[Cor]Cl > Fe[Cor]Cl > Fe[Cor](NO) > Mo[Cor]Cl2 > Ru[Cor](NO) ≈ Mn[Cor]Ph ≈ Fe[Cor]Ph ≈ 0, where [Cor] refers to the unsubstituted corrolato ligand. DMRG-based second-order perturbation theory calculations have also yielded detailed excited-state energetics data on the compounds, shedding light on periodic trends involving middle transition elements. Thus, whereas the ground state of Fe[Cor](NO) (S = 0) is best described as a locally S = 1/2 {FeNO}7 unit antiferromagnetically coupled to a corrole A' radical, the calculations confirm that Ru[Cor](NO) may be described as simply {RuNO}6-Cor3-, that is, having an innocent corrole macrocycle. Furthermore, whereas the ferromagnetically coupled S = 1{FeNO}7-Cor•2- state of Fe[Cor](NO) is only ∼17.5 kcal/mol higher than the S = 0 ground state, the analogous triplet state of Ru[Cor](NO) is higher by a far larger margin (37.4 kcal/mol) relative to the ground state. In the same vein, Mo[Cor]Cl2 exhibits an adiabatic doublet-quartet gap of 36.1 kcal/mol. The large energy gaps associated with metal-ligand spin coupling in Ru[Cor](NO) and Mo[Cor]Cl2 reflect the much greater covalent character of 4d-π interactions relative to analogous interactions involving 3d orbitals. As far as excited-state energetics is concerned, DMRG-CASPT2 calculations provide moderate validation for hybrid density functional theory (B3LYP) for qualitative purposes, but underscore the possibility of large errors (>10 kcal/mol) in interstate energy differences.

Heavy-element-ligand covalence: ligand noninnocence in molybdenum and tungsten Viking-helmet Corroles

Extensive DFT calculations with several exchange-correlation functionals indicate that molybdenum-dichlorido Viking helmet corroles are noninnocent with significant Mo^IV-corrole˙^2- character. The effect is mediated by a Mo(4d)-corrole(π) orbital interaction similar to that postulated for MnCl, FeCl and FeNO corroles. The effect also appears to operate in tungsten-dichlorido corroles but is weaker relative to that for Mo. In contrast, MoO triarylcorroles do not exhibit a significant degree of corrole radical character. Furthermore, the Soret absorption maxima of a series of MoCl₂ tris(para-X-phenyl)corrole derivatives were found to redshift dramatically with increasing electron-donating character of the para substituent X, essentially clinching the case for a noninnocent macrocycle in MoCl₂ corroles.

The Story of 5d Metallocorroles: From Metal-Ligand Misfits to New Building Blocks for Cancer Phototherapeutics

Porphyrin chemistry is Shakespearean: over a century of study has not withered the field’s apparently infinite variety. Heme proteins continually astonish us with novel molecular mechanisms, while new porphyrin analogues bowl us over with unprecedented optical, electronic, and metal-binding properties. Within the latter domain, corroles occupy a special place, exhibiting a unique and rich coordination chemistry. The 5d metallocorroles are arguably the icing on that cake.

New Zealand chemist Penny Brothers has used the word “misfit” to describe the interactions of boron, a small atom with a predilection for tetrahedral coordination, and porphyrins, classic square-planar ligands. Steve Jobs lionized misfits as those who see things differently and push humanity forward. Both perspectives have inspired us. The 5d metallocorroles are misfits in that they encapsulate a large 5d transition metal ion within the tight cavity of a contracted porphyrin ligand.

Given the steric mismatch inherent in their structures, the syntheses of some 5d metallocorroles are understandably capricious, proceeding under highly specific conditions and affording poor yields. Three broad approaches may be distinguished.

(a) In the metal–alkyl approach, a free-base corrole is exposed to an alkyllithium and the resulting lithio-corrole is treated with an early transition metal chloride; a variant of the method eschews alkyllithium and deploys a transition metal–alkyl instead, resulting in elimination of the alkyl group as an alkane and insertion of the metal into the corrole. This approach is useful for inserting transition metals from groups 4, 5, and, to some extent, 6, as well as lanthanides and actinides.

(b) In our laboratory, we have often deployed a low-valent organometallic approach for the middle transition elements (groups 6, 7, 8, and 9). The reagents are low-valent metal–carbonyl or −olefin complexes, which lose one or more carbon ligands at high temperature, affording coordinatively unsaturated, sticky metal fragments that are trapped by the corrole nitrogens.

(c) Finally, a metal acetate approach provides the method of choice for gold and platinum insertion (groups 10 and 11).

This Account provides a first-hand perspective of the three approaches, focusing on the last two, which were largely developed in our laboratory. In general, the products were characterized with X-ray crystallography, electrochemistry, and a variety of spectroscopic methods. The physicochemical data, supplemented by relativistic DFT calculations, have provided fascinating insights into periodic trends and relativistic effects.

An unexpected feature of many 5d metallocorroles, given their misfit character, is their remarkable stability under thermal, chemical, and photochemical stimulation. Many of them also exhibit long triplet lifetimes on the order of 100 μs and effectively sensitize singlet oxygen formation. Many exhibit phosphorescence in the near-infrared under ambient conditions. Furthermore, water-soluble ReO and Au corroles exhibit impressive photocytotoxicity against multiple cancer cell lines, promising potential applications as cancer phototherapeutics. We thus envision a bright future for the compounds as rugged building blocks for new generations of therapeutic and diagnostic (theranostic) agents.

Synthesis and molecular structure of perhalogenated rhenium-oxo corroles

As part of our efforts to develop rhenium-oxo corroles as photosensitizers for oxygen sensing and photodynamic therapy, we investigated the potential β-perhalogenation of five ReO meso-tris(para-X-phenyl)corroles, Re[TpXPC](O) (X = CF₃, H, F, CH₃, and OCH₃), with elemental chlorine and bromine. With Cl₂, β-octachlorinated products Re[Cl₈TpXPC](O) were rapidly obtained for X = CF₃, H, and CH₃, but X = OCH₃ resulted in overchlorination on the meso-aryl groups. Full β-octabromination proved slower relative to Cu and Ir corroles, but the desired Re[Br₈TpXPC](O) products were finally obtained for X = H and F after a week at room temperature. For X = CH₃ and OCH₃, these conditions led to undecabrominated products Re[Br₁₁TpXPC](O). Compared to the β-unsubstituted starting materials, the β-octahalogenated products were found to exhibit sharp ¹H NMR signals at room temperature, indicating that the aryl groups are locked in place by the β-halogens, and substantially redshifted Soret and Q bands. Single-crystal X-ray structures of Re[Cl₈TpCF₃PC](O), Re[Cl₈TpCH₃PC](O), and Re[Br₈TpFPC](O) revealed mild saddling for one Cl₈ structure and the Br₈ structure. These structural variations, however, appear too insignificant to explain the slowness of the β-octabromination protocols, which seems best attributed to the deactivating influence of the high-valent Re center.

Molecular Structure of Copper and μ-Oxodiiron Octafluorocorrole Derivatives: Insights into Ligand Noninnocence

Single-crystal X-ray structures were obtained for the copper and μ-oxodiiron complexes of 2,3,7,8,12,13,17,18-octafluoro-5,10,15-triphenylcorrole, hereafter denoted as Cu[F₈TPC] and {Fe[F₈TPC]}₂O. A comparison with the crystal structures of other undecasubstituted Cu corroles, including those with H, Ar, Br, I, and CF₃ as β-substituents, showed that the degree of saddling increases in the order: H ≲ F < Ar ≲ Br ≲ I < CF₃. In other words, Cu[F₈TPC] is marginally more saddled than β-unsubstituted Cu triarylcorroles, but substantially less saddled than Cu undecaarylcorroles, β-octabromo-meso-triarylcorroles, and β-octaiodo-meso-triarylcorroles, and far less saddled than Cu β-octakis(trifluoromethyl)-meso-triarylcorroles. As for {Fe[F₈TPC]}₂O, the moderate quality of the structure did not allow us to draw firm conclusions in regard to bond length alternations in the corrole skeleton and hence also the question of ligand noninnocence. The Fe-O bond distances, 1.712(8) and 1.724(8), however, are essentially identical to those observed for {Fe[TPFPC]}₂O, where TPFPC^3- is the trianion of 5,10,15-tris(pentafluorophenyl)corrole, suggesting that a partially noninnocent electronic structural description may be applicable for both compounds.

Protonation-Induced Hyperporphyrin Spectra of meso-Aminophenylcorroles

UV-vis spectrophotometric titrations have been carried out on meso-tris(o/m/p-aminophenyl)corrole (H₃[o/m/p-TAPC]) and meso-triphenylcorrole (H₃[TPC]) in dimethyl sulfoxide with methanesulfonic acid (MSA). Monoprotonation was found to result in hyperporphyrin spectra characterized by new, red-shifted, and intense Q bands. The effect was particularly dramatic for H₃[p-TAPC] for which the Q band red-shifted from ∼637 nm for the neutral species to 764 nm in the near-IR for H₄[p-TAPC]⁺. Upon further protonation, the Q band was found to blue-shift back to 687 nm. A simple explanation of the phenomena has been offered in terms of quinonoid resonance forms.

Rhenium-Imido Corroles

Metallocorroles involving 5d transition metals are currently of interest as near-IR phosphors and as photosensitizers for oxygen sensing and photodynamic therapy. Their syntheses, however, are often bedeviled by capricious and low-yielding protocols. Against this backdrop, we describe rhenium-imido corroles, a new class of 5d metallocorroles, synthesized simply and in respectable (∼30%) yields via the interaction of a free-base corrole, Re₂(CO)₁₀, K₂CO₃, and aniline in 1,2,4-trichlorobenzene at ∼190 °C in a sealed vial under strict anaerobic conditions. The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H₃[TPFPC], and five different meso-tris(p-X-phenyl)corroles, H₃[TpXPC], where X = CF₃, F, H, CH₃, OCH₃. Single-crystal X-ray structures obtained for two of the complexes, Re[TpFPC](NPh) and Re[TpCF₃PC](NPh), revealed relatively unstrained equatorial Re-N distances of ∼2.00 Å, a ∼ 0.7-Å displacement of the Re from the mean plane of the corrole nitrogens, and an Re-N_imido distance of ∼1.72 Å. Details of the corrole skeletal bond distances, diamagnetic ¹H NMR spectra, relatively substituent-independent Soret maxima, and electrochemical HOMO-LUMO gaps of ∼2.2 V all indicated an innocent corrole macrocycle. Surprisingly, unlike several other classes of 5d metallocorroles, the Re-imido complexes proved nonemissive in solution at room temperature and also failed to sensitize singlet oxygen formation, indicating rapid radiationless deactivation of the triplet state, presumably via the rapidly rotating axial phenyl group. By analogy with other metal-oxo and -imido corroles, we remain hopeful that the Re-imido group will prove amenable to further elaboration and thereby contribute to the development of a somewhat challenging area of coordination chemistry.

Gold dipyrrin-bisphenolates: a combined experimental and DFT study of metal–ligand interactions

Given that noninnocent and metalloradical-type electronic structures are ubiquitous among dipyrrin-bisphenolate (DPP) complexes, we synthesized the gold(iii) derivatives as potentially innocent paradigms against which the properties of other metallo-DPP derivatives can be evaluated. Electronic absorption spectra, electrochemical studies, a single-crystal X-ray structure, and DFT calculations all suggest that the ground states of the new complexes indeed correspond to an innocent Au^III–DPP³⁻, paralleling a similar description noted for Au corroles. Interestingly, while DFT calculations indicate purely ligand-centered oxidations, reduction of AuDPP is predicted to occur across both the metal and the ligand.

The first gold dipyrrin-bisphenolates have been synthesized. Like their corrole analogues, they exhibit Au^III–L³⁻ ground states, providing rare innocent paradigms for a class of complexes that commonly occur as metalloradicals.

Seven Clues to Ligand Noninnocence: The Metallocorrole Paradigm

Noninnocent ligands do not allow an unambiguous definition of the oxidation state of a coordinated atom. When coordinated, the ligands also cannot be adequately represented by a classic Lewis structure. A noninnocent system thus harbors oxidizing (holes) or reducing equivalents (electrons) that are delocalized over both the ligand and the coordinated atom. To a certain degree, that is true of all complexes, but the phenomenon is arguably most conspicuous in complexes involving ligands with extended π-systems. The electronic structures of such systems have often been mischaracterized, thereby muddying the chemical literature to the detriment of students and newcomers to the field. In recent years, we have investigated the electronic structures of several metallocorrole families, several of which have turned out to be noninnocent. Our goal here, however, is not to present a systematic account of the different classes of metallocorroles, but rather to focus on seven major tools (in a nod to A. G. Cairns-Smith's Seven Clues to the Origin of Life) that led us to recognize noninnocent behavior and subsequently to characterize the phenomenon in depth. (1) The optical probe: For a series of noninnocent meso-triarylcorrole derivatives with different para substituents X, the Soret maxima are typically exquisitely sensitive to the nature of X, red-shifting with increasing electron-donating character of the group. No such substituent sensitivity is observed for the Soret maxima of innocent triarylcorrole derivatives. (2) Quantum chemistry: Spin-unrestricted density functional theory calculations permit a simple and quick visualization of ligand noninnocence in terms of the spin density profile. Even for an S = 0 complex, the broken-symmetry method often affords a spin density profile that, its fictitious character notwithstanding, helps visualize the intramolecular spin couplings. (3) NMR and EPR spectroscopy: In principle, these two techniques afford experimental probes of the electronic spin density. (4) Structure/X-ray crystallography. Ligand noninnocence in metallocorroles is often reflected in small but distinct skeletal bond length alternations in and around the bipyrrole part of the macrocycle. In addition, for Cu and some Ag corroles, ligand noninnocence manifests itself via a strong saddling of the macrocycle. (5) Vibrational spectroscopy. Unsurprisingly, the aforementioned bond length alternations translate to structure-sensitive vibrational marker bands. (6) Electrochemistry. Noninnocent metallocorroles exhibit characteristically high reduction potentials, but caution should be exercised in turning the logic around. A high reduction potential does not necessarily signify a noninnocent metallocorrole; certain high-valent metal centers also undergo metal-centered reduction at quite high potentials. (7) X-ray absorption spectroscopy (XAS). By focusing on a given element, typically the central atom in a coordination complex, X-ray absorption near-edge spectroscopy (XANES) can provide uniquely detailed local information on oxidation and spin states, ligand field strength, and degree of centrosymmetry. For metallocorroles, some of the most clear-cut distinctions between innocent and noninnocent systems have come from the K-edge XANES of Mn and Fe corroles. For researchers faced with a new, potentially noninnocent system, the take-home message is to employ a good majority (i.e., at least four) of the above methods to arrive at a reliable conclusion vis-à-vis noninnocence.

X-ray Absorption Spectroscopy as a Probe of Ligand Noninnocence in Metallocorroles: The Case of Copper Corroles

The question of ligand noninnocence in Cu corroles has long been a topic of discussion. Presented herein is a Cu K-edge X-ray absorption spectroscopy (XAS) study, which provides a direct probe of the metal oxidation state, of three Cu corroles, Cu[TPC], Cu[Br8TPC], and Cu[(CF3)8TPC] (TPC = meso-triphenylcorrole), and the analogous Cu(II) porphyrins, Cu[TPP], Cu[Br8TPP], and Cu[(CF3)8TPP] (TPP = meso-tetraphenylporphyrin). The Cu K rising-edges of the Cu corroles were found to be about 0-1 eV upshifted relative to the analogous porphyrins, which is substantially lower than the 1-2 eV shifts typically exhibited by authentic Cu(II)/Cu(III) model complex pairs. In an unusual twist, the Cu K pre-edge regions of both the Cu corroles and the Cu porphyrins exhibit two peaks split by 0.8-1.3 eV. Based on time-dependent density functional theory calculations, the lower- and higher-energy peaks were assigned to a Cu 1s → 3d x2- y2 transition and a Cu 1s → corrole/porphyrin π* transition, respectively. From the Cu(II) porphyrins to the corresponding Cu corroles, the energy of the Cu 1s → 3d x2- y2 transition peak was found to upshift by 0.6-0.8 eV. This shift is approximately half that observed between Cu(II) to Cu(III) states for well-defined complexes. The Cu K-edge XAS spectra thus show that although the metal sites in the Cu corroles are more oxidized relative to those in their Cu(II) porphyrin analogues, they are not oxidized to the Cu(III) level, consistent with the notion of a noninnocent corrole. The relative importance of σ-donation versus corrole π-radical character is discussed.

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.

Stable Platinum(IV) Corroles: Synthesis, Molecular Structure, and Room-Temperature Near-IR Phosphorescence

A series of stable Pt(IV) corrole complexes with the general formula Pt^IV[TpXPC](m/p-C₆H₄CN)(py), where TpXPC^3- is the trianion of a tris(p-X-phenyl)corrole and X = CF₃, H, and CH₃, has been synthesized, affording key physicochemical data on a rare and elusive class of metallocorroles. Single-crystal X-ray structures of two of the complexes revealed very short equatorial Pt-N distances of 1.94-1.97 Å, an axial Pt-C distance of ∼2.03 Å, and an axial Pt-N distance of ∼2.22 Å. The complexes exhibit Soret maxima at ∼430 nm, which are essentially independent of the meso-aryl para substituents, and strong Q bands with the most intense peak at 595-599 nm. The substituent-independent Soret maxima are consistent with an innocent Pt^IV-corrole^3- description for the complexes. The low reduction potentials (-1.45 ± 0.08 V vs saturated calomel reference electrode) also support a highly stable Pt(IV) ground state as opposed to a noninnocent corrole^•2- description. The reductions, however, are irreversible, which suggests that they involve concomitant cleavage of the Pt-aryl bond. Unlike Pt(IV) porphyrins, two of the complexes, Pt^IV[TpXPC](m-C₆H₄CN)(py) (X = CF₃ and CH₃), were found to exhibit room-temperature near-IR phosphorescence with emission maxima at 813 and 826 nm, respectively. The quantum yield of ∼0.3% is comparable to those observed for six-coordinate Ir(III) corroles.

Molecular structure of gold 2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(4′-pentafluorosulfanylphenyl)corrole: Potential insights into the insolubility of gold octabromocorroles

X-ray structures of gold corroles, which are still rather uncommon, afford significant insights into intermolecular interactions involving Au(III), a subject that has been much less studied than aurophilic and metallophilic interactions involving Au(I). The X-ray structure of gold [Formula: see text]-octabromo-meso-tris(para-pentafluorosulfanylphenyl)corrole, reported herein, has revealed two Au···Br interactions (∼4.2 Å) per Au atom. We suggest that analogous but somewhat stronger Au···Br interactions are a key factor underlying the remarkable insolubility of gold octabromocorrole derivatives.

Synthesis and Molecular Structure of a Copper Octaiodocorrole

Although rather delicate on account of their propensity to undergo deiodination, β-octaiodoporphyrinoids are of considerable interest as potential precursors to novel β-octasubstituted macrocycles. Presented herein are early results of our efforts to synthesize β-octaiodocorrole derivatives. Oxidative condensation of 3,4-diiodopyrrole and aromatic aldehydes failed to yield free-base octaiodocorroles. Treatment of copper meso-tris(p-cyanophenyl)corrole with N-iodosuccinimide and trifluoroacetic acid over several hours, however, yielded the desired β-octaiodinated product in ∼22% yield. Single-crystal X-ray structure determination of the product revealed a strongly saddled corrole macrocycle with metrical parameters very close to those of analogous Cu octabromocorrole complexes. The compound was also found to exhibit an exceptionally red-shifted Soret maximum (464 nm in dichloromethane), underscoring the remarkable electronic effect of β-octaiodo substitution.

Halterman Corroles and Their Use as a Probe of the Conformational Dynamics of the Inherently Chiral Copper Corrole Chromophore

Halterman corroles have been synthesized for the first time from pyrrole and Halterman's aldehyde via Gryko's "water-methanol method". These were derivatized to the corresponding copper complexes and subsequently to the β-octabromo complexes. Electronic circular dichroism spectra were recorded for the enantiopure copper complexes, affording the first such measurements for the inherently chiral Cu corrole chromophore. Interestingly, for a given configuration of the Halterman substituents, X-ray crystallographic studies revealed both P and M conformations of the Cu-corrole core, proving that the substituents, even in conjunction with β-octabromination, are unable to lock the Cu-corrole core into a given chirality. The overall body of evidence strongly indicates a dynamic equilibrium between the P and M conformations. Such an interconversion, which presumably proceeds via saddling inversion, provides a rationale for our failure so far to resolve sterically hindered Cu corroles into their constituent enantiomers by means of chiral HPLC.

Metallocorroles as inherently chiral chromophores: resolution and electronic circular dichroism spectroscopy of a tungsten biscorrole

An inherently chiral metallocorrole has been resolved for the first time by means of HPLC on a chiral stationary phase. For the compound in question, a homoleptic tungsten biscorrole, the absolute configurations of the enantiomers were assigned using online HPLC-ECD measurements in conjunction with time-dependent CAM-B3LYP calculations, which provided accurate simulations of the ECD spectra.

Ligand Noninnocence in Iron Corroles: Insights from Optical and X-ray Absorption Spectroscopies and Electrochemical Redox Potentials

Two new series of iron meso-tris(para-X-phenyl)corrole (TpXPC) complexes, Fe[TpXPC]Ph and Fe[TpXPC]Tol, in which X=CF3 , H, Me, and OMe, and Tol=p-methylphenyl (p-tolyl), have been synthesized, allowing a multitechnique electronic-structural comparison with the corresponding FeCl, FeNO, and Fe2 (μ-O) TpXPC derivatives. Optical spectroscopy revealed that the Soret maxima of the FePh and FeTol series are insensitive to the phenyl para substituent, consistent with the presumed innocence of the corrole ligand in these compounds. Accordingly, we may be increasingly confident in the ability of the substituent effect criterion to serve as a probe of corrole noninnocence. Furthermore, four complexes-Fe[TPC]Cl, Fe[TPC](NO), {Fe[TPC]}2 O, and Fe[TPC]Ph-were selected for a detailed XANES investigation of the question of ligand noninnocence. The intensity-weighted average energy (IWAE) positions were found to exhibit rather modest variations (0.8 eV over the series of corroles). The integrated Fe-K pre-edge intensities, on the other hand, vary considerably, with a 2.5 fold increase for Fe[TPC]Ph relative to Fe[TPC]Cl and Fe[TPC](NO). Given the approximately C4v local symmetry of the Fe in all the complexes, the large increase in intensity for Fe[TPC]Ph may be attributed to a higher number of 3d holes, consistent with an expected FeIV -like description, in contrast to Fe[TPC]Cl and Fe[TPC](NO), in which the Fe is thought to be FeIII -like. These results afford strong validation of XANES as a probe of ligand noninnocence in metallocorroles. Electrochemical redox potentials, on the other hand, were found not to afford a simple probe of ligand noninnocence in Fe corroles.

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.

One-Pot Synthesis of a bis-Pocket Corrole through a 14-fold Bromination Reaction

A one-pot protocol, effecting 14-fold bromination with elemental bromine, has afforded copper β-octabromo-meso-tris(2,6-dibromo-3,5-dimethoxyphenyl)corrole, a new bis-pocket metallocorrole. The Cu complex underwent smooth demetalation under reductive conditions, affording the free corrole ligand, which in turn could be readily complexed to MnIII and AuIII. A single-crystal X-ray structure was obtained for the MnIII complex.

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.

Facile Conversion of Ni(II) Cyclopropylchlorins into Novel β-Substituted Porphyrins through Acid-Catalyzed Ring-Opening Reaction

The conversion of cyclopropylchlorins into porphyrins represents a key step in the synthetic manipulation of macrocycles with tunable physical and chemical properties. Herein, we report a facile method for the synthesis of novel β-substituted porphyrins from cyclopropylchlorins. A series of Ni(II) cyclopropylchlorins was converted into the corresponding Ni(II) and free base porphyrins using TFA and H₂SO₄ under mild reaction conditions in good yields (75-86%). The new chlorins and porphyrins were characterized by various spectroscopic techniques and the single-crystal X-ray diffraction method. The reaction proceeds very fast (<5 min.) with complete conversion of chlorin into porphyrin with distinct color change. Facile conversion, shorter reaction time scale, and good yield (75-86%) without any side products are the significant features of this new protocol. These porphyrinoids exhibited red-shifted electronic spectral features with varying degrees nonplanar conformation, tunable redox properties, and porphyrin core basicity.

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.

Tungsten Biscorroles: New Chiral Sandwich Compounds

The oxidative metalation method, involving the interaction of free-base meso-triarylcorroles and W(CO)6 in refluxing decalin, led to a set of three tungsten(VI) biscorroles, the first homoleptic sandwich compounds involving corroles. Single-crystal X-ray structures of two of the complexes revealed square-antiprismatic coordination and strongly domed corroles with long W-N distances of 2.15-2.22 Å and a substantial displacement of ∼1.17 Å of the metal relative to the mean N4 planes of the ligands. The structures correspond to approximate C2 symmetry and are thus chiral. DFT calculations strongly indicate that the enantiomers are configurationally stable and hence amenable to chiral resolution. Their other notable properties include a strongly blueshifted Soret band at (357±2) nm, a relatively intense π→W(dz2 ) near-IR feature at (781±3) nm, and a low electrochemical HOMO-LUMO gap of approximately 1.3 V. The results obtained herein suggest that metallobiscorroles may emerge as a new class of inherently chiral chromophores with novel optical and electrochemical properties.

Ligand noninnocence in FeNO corroles: insights from β-octabromocorrole complexes

With a noninnocent {FeNO}⁷-(corrole˙²⁻) formulation, the first FeNO octabromocorrole derivatives deepen our appreciation of ligand noninnocence.

Metal-Ligand Misfits: Facile Access to Rhenium-Oxo Corroles by Oxidative Metalation

With the exception of a single accidental synthesis, rhenium corroles are unknown, but of great interest as catalysts and potential radiopharmaceuticals. Oxidative metalation of meso-triarylcorroles with [Re2 (CO)10 ] in refluxing decalin has provided a facile and relatively high-yielding route to rhenium(V)-oxo corroles. The complexes synthesized could all be fully characterized by single-crystal X-ray structure analyses.

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.

Synthesis and functionalization of β-alkyl-meso-triarylcorroles

After the definition of efficient synthetic routes for the preparation of triarylcorroles, the functionalization of these macrocycles is becoming a necessary and challenging field of research. One important synthetic step is the introduction of substituents able to influence the electronic distribution in the macrocyclic ring. A valuable target would be a corrole macrocycle with some β-pyrrole positions occupied by methyl groups, while exploiting other positions to introduce electron-withdrawing substituents. To explore the scope of this approach, we investigated the bromination and the nitration of the corrole ring and the desired products have been obtained in moderate to good yield. The successful preparation of selectively halogenated corroles is particularly interesting since they are suitable substrates for the preparation of more complex partially alkylated structures using modern cross coupling methodologies.

Platinum corroles

Platinum has been inserted into corroles for the first time and three oxidized Pt(IV)(corrole˙(2-))ArAr' complexes have been structurally characterized. The Soret maxima of these complexes exhibit an unusually strong dependence on the meso-aryl substituents on the corrole, indicating aryl → corrole˙(2-) charge transfer character in these transitions.

Mono- and tri-β-substituted unsymmetrical metalloporphyrins: synthesis, structural, spectral and electrochemical properties

Mono-/tri-β-substituted metalloporphyrins have been synthesized and characterized. Dramatic reduction in the HOMO–LUMO gap with tunable electronic, spectral and electrochemical redox potentials were observed as the number of electron withdrawing groups increased.

Light absorption and hole-transport properties of copper corroles: from aggregates to a liquid crystal mesophase

A room temperature corrole-based liquid crystal phase is described, fully characterized and compared to assemblies produced by simpler corrole derivatives.