Wolves in Sheep's Clothing: μ-Oxo-Diiron Corroles Revisited

Iron Corrole-Catalyzed Intramolecular Amination Reactions of Alkyl Azides. Spectroscopic Characterization and Reactivity of [FeV(Cor)(NAd)]

As nitrogen analogues of iron-oxo species, high-valent iron-imido species have attracted great interest in the past decades. FeV-alkylimido species are generally considered to be key reaction intermediates in Fe(III)-catalyzed C(sp3)─H bond aminations of alkyl azides but remain underexplored. Here, it is reported that iron-corrole (Cor) complexes can catalyze a wide range of intramolecular C─H amination reactions of alkyl azides to afford a variety of 5-, 6- and 7-membered N-heterocycles, including alkaloids and natural product derivatives, with up to 3880 turnover numbers (TONs) and excellent diastereoselectivity (>99:1 d.r.). Mechanistic studies including density functional theory (DFT) calculations and intermolecular hydrogen atom abstraction (HAA) reactions reveal key reactive FeV-alkylimido intermediates. The [FeV(Cor)(NAd)] (Ad = adamantyl) complex is independently prepared and characterized through electron paramagnetic resonance (EPR), resonance Raman (rR) measurement, and X-ray photoelectron spectroscopy (XPS). This complex is reactive toward HAA reactions with kinetic isotope effects (KIEs) similar to [Fe(Cor)]-catalyzed intramolecular C─H amination of alkyl azides.

Diversifying the functions of heme proteins with non-porphyrin cofactors

Heme proteins perform diverse biochemical functions using a single iron porphyrin cofactor. This versatility makes them attractive platforms for the development of new functional proteins. While directed evolution and metal substitution have expanded the properties, reactivity, and applications of heme proteins, the incorporation of porphyrin analogs remains an underexplored approach. This review discusses the replacement of heme with non-porphyrin cofactors, such as porphycene, corrole, tetradehydrocorrin, phthalocyanine, and salophen, and the attendant properties of these conjugates. While structurally similar, each ligand exhibits distinct optical and redox properties, as well as unique chemical reactivity. These hybrids serve as model systems to elucidate the effects of the protein environment on the electronic structure, redox potentials, optical properties, or other features of the porphyrin analog. Protein encapsulation can confer distinct chemical reactivity or selectivity of artificial metalloenzymes that cannot be achieved with the small molecule catalyst alone. Additionally, these conjugates can interfere with heme acquisition and uptake in pathogenic bacteria, providing an inroad to innovative antibiotic strategies. Together, these examples illustrate the diverse functionality that can be achieved by cofactor substitution. The further expansion of this approach will access unexplored chemical space, enabling the development of superior catalysts and the creation of heme proteins with emergent properties.

The Hyperporphyrin Concept: A Contemporary Perspective

The Gouterman four-orbital model conceptualizes porphyrin UV-visible spectra as dominated by four frontier molecular orbitals-two nearly degenerate HOMOs and two exactly degenerate LUMOS under D 4h symmetry. These are well separated from all the other molecular orbitals, and normal spectra involve transitions among these MOs. Unusual spectra occur when additional orbitals appear in this energy range, typically as a consequence of the central coordinated atom. For example, metals with empty d orbitals in a suitable energy range may lead to charge transfer from porphyrin (ligand) to metal, that is, so-called LMCT transitions. Metals with filled p or d orbitals may lead to charge transfer from metal to porphyrin, MLCT transitions. These cases lead to additional peaks and/or significant redshifts in the spectra and were classified as hyperporphyrins by Gouterman. Cases in which spectra are blueshifted were classified as hypsoporphyrins; they are common for relatively electronegative late transition metal porphyrins. Many of the same principles apply to porphyrin analogues, especially corroles. In this Perspective, we focus on two newer classes of hyperporphyrins: one reflecting substituent effects in protonated or deprotonated free-base tetraphenyporphyrins and the other reflecting "noninnocent" interactions between central metal ions and corroles. Hyperporphyrin effects on spectra can be dramatic, yet they can be generated by relatively simple changes and subtle structural variations, such as acid-base reactions or the selection of a central metal ion. These concepts suggest strategies for engineering porphyrin or porphyrinoid dyes for specific applications, especially those requiring far-red or near-infrared absorption or emission.

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.

Iridium(VII)-Corrole Terminal Carbides Should Exist as Stable Compounds

Scalar-relativistic DFT calculations with multiple exchange-correlation functionals and large basis sets foreshadow the existence of stable iridium(VII)-corrole terminal carbide derivatives. For the parent compound Ir[Cor](C), OLYP/STO-TZ2P calculations predict a short Ir-C bond distance of 1.69 Å, a moderately domed macrocycle with no indications of ligand noninnocence, a surprisingly low electron affinity of ∼1.1 eV, and a substantial singlet-triplet gap of ∼1.8 eV. These results, and their essential invariance with respect to the choice of the exchange-correlation functional, lead us to posit that Ir(VII)-corrole terminal carbide complexes should be isolable and indefinitely stable under ambient conditions.

Comparing Isoelectronic, Quadruple-Bonded Metalloporphyrin and Metallocorrole Dimers: Scalar-Relativistic DFT Calculations Predict a >1 eV Range for Ionization Potential and Electron Affinity

A scalar-relativistic DFT study of isoelectronic, quadruple-bonded Group 6 metalloporphyrins (M = Mo, W) and Group 7 metallocorroles (M = Tc, Re) has uncovered dramatic differences in ionization potential (IP) and electron affinity (EA) among the compounds. Thus, both the IPs and EAs of the corrole derivatives are 1 eV or more higher than those of the porphyrin derivatives. These differences largely reflect the much lower orbital energies of the δ- and δ*-orbitals of the corrole dimers relative to those of the porphyrin dimers, which in turn reflect the higher (+III as opposed to +II) oxidation states of the metals in the former compounds. Significant differences have also been determined between Mo and W porphyrin dimers and between Tc and Re corrole dimers. These differences are thought to largely reflect greater relativistic destabilization of the 5d orbitals of W and Re relative to the 4d orbitals of Mo and Tc. The calculated differences in IP and EA should translate to major differences in electrochemical redox potentials-a prediction that in our opinion is well worth confirming.

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.

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.

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.

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.

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.

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.

Stepwise Deoxygenation of Nitrite as a Route to Two Families of Ruthenium Corroles: Group 8 Periodic Trends and Relativistic Effects

Given the many applications of ruthenium porphyrins, the rarity of ruthenium corroles and the underdeveloped state of their chemistry are clearly indicative of an area ripe for significant breakthroughs. The tendency of ruthenium corroles to form unreactive metal-metal-bonded dimers has been recognized as a key impediment in this area. Herein, by exposing free-base meso-tris(p-X-phenyl)corroles, H₃[TpXPC] (X = CF₃, H, Me, and OMe), and [Ru(COD)Cl₂]_x in refluxing 2-methoxyethanol to nitrite, we have been able to reliably intercept the series Ru[TpXPC](NO) in a matter of seconds to minutes and subsequently Ru^VI[TpXPC](N), the products of a second deoxygenation, over some 16 h. Two of the Ru^VIN complexes and one ruthenium corrole dimer could be crystallographically analyzed; the Ru-N_nitrido distance was found to be ∼1.61 Å, consistent with the triple-bonded character of the Ru^VIN units and essentially identical with the Os-N_nitrido distance in analogous osmium corroles. Spectroscopic and density functional theory (DFT) calculations suggest that the RuNO corroles are best viewed as innocent {RuNO}⁶ complexes, whereas the analogous FeNO corroles are noninnocent, i.e., best viewed as {FeNO}⁷-corrole^•2-. Both Ru^VIN and Os^VIN corroles exhibit sharp Soret bands, suggestive of an innocent macrocycle. A key difference between the two metals is that the Soret maxima of the Os^VIN corroles are red-shifted some 25 nm relative to those of the Ru^VIN complexes. Careful time-dependent DFT studies indicate that this difference is largely attributable to relativistic effects in Os^VIN corroles. The availability of two new classes of mononuclear ruthenium corroles potentially opens the door to new applications, in such areas as catalysis and cancer therapy.

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.

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.

Synthesis and Molecular Structure of 99 Tc Corroles

The first ⁹⁹ Tc corroles have been synthesized and fully characterized. A single-crystal X-ray structure of a ⁹⁹ TcO triarylcorrole revealed nearly identical geometry parameters as the corresponding ReO structure. A significant spectral shift between the Soret maxima of TcO (410-413 nm) and ReO (438-441 nm) corroles was observed and, based on two-component spin-orbit ZORA TDDFT calculations, ascribed to relativistic effects in the Re case. The syntheses reported herein potentially pave the way toward ^99m Tc-porphyrinoid-based radiopharmaceuticals.