Influence of Fluorinated Substituents on the Near-Infrared Phosphorescence of 5d Metallocorroles.

The influence of fluorinated substituents on the luminescent properties of rhenium-oxo, osmium-nitrido, and gold triarylcorroles was studied via a comparison of four ligands: triphenylcorrole (TPC), tris(p-trifluoromethylphenyl)corrole (TpCF3PC), tris{3,5-bis(trifluoromethyl)phenyl}corrole (T3,5-CF3PC), and tris(pentafluorophenyl)corrole (TPFPC). For each metal series examined, fluorinated substituents were found to enhance the luminescent properties, with the phosphorescence quantum yields and triplet decay times increasing in the order TPC < TpCF3PC < T3,5-CF3PC < TPFPC. Among the 11 complexes examined, the highest phosphorescence quantum yield, 2.2%, was recorded for Re[TPFPC](O).

Rhenium Biscorrole Sandwich Compounds: XAS Evidence for a New Coordination Motif.

The interaction of three free-base meso-tris(p-X-phenyl)corroles H3[TpXPC] (X = H, CH3, OCH3) with Re2(CO)10 at 235 °C in the presence of K2CO3 in o-dichlorobenzene has led to putative rhenium biscorrole sandwich compounds with the formula ReH[TpXPC]2. Density functional theory calculations and Re L3-edge extended X-ray absorption fine structure measurements suggest a seven-coordinate metal center, with the "extra" hydrogen located on one of the corrole nitrogens. The complexes can be deprotonated by a base such as 1,8-diazabicyclo[5.4.0]undec-7-ene, resulting in a substantial sharpening of the UV-vis spectra and split Soret bands, consistent with the generation of C2-symmetric anions. Both the seven-coordinate neutral and eight-coordinate anionic forms of the complexes represent a new coordination motif in the field of rhenium-porphyrinoid interactions.

Phenol- and resorcinol-appended metallocorroles and their derivatization with fluorous tags.

Boron tribromide-mediated demethylation of rhenium-oxo and gold meso-tris(4-methoxyphenyl)corrole and meso-tris(3,5-dimethoxyphenylcorrole), M[TpOMePC] and M[T(3,5-OMe)PC] (M = ReO, Au), have yielded the corresponding phenol- and resorcinol-appended metallocorroles, M[TpOHPC] and M[T(3,5-OH)PC], in good yields. The latter compounds proved insoluble in dichloromethane and chloroform but soluble in THF. The M[T(3,5-OH)PC] derivatives also proved moderately soluble in 0.05 M aqueous KOH. Unlike oxidation-prone aminophenyl-substituted corroles, the phenol- and resorcinol-appended metallocorroles could be readily handled in air without special precautions. The phenolic metallocorroles could be readily alkylated with 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyl iodide ("FtI") to afford the fluorous-tagged metallocorroles M[TpOFtPC] and M[T(3,5-OFt)PC] in > 90% yields. The simplicity of the synthetic protocols promise a wide range of phenolic and fluorous-tagged porphyrin analogues with potential applications to diverse fields such as sensors, catalysis, and photodynamic therapy, among others.

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 MoIV-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.

Rhenium Corrole Dimers: Electrochemical Insights into the Nature of the Metal-Metal Quadruple Bond.

The interaction of free-base triarylcorroles with Re2(CO)10 in 1,2-dichlorobenzene in the presence of 2,6-lutidine at 180 °C under strict anerobic conditions afforded approximately 10% yields of rhenium corrole dimers. The compounds exhibited diamagnetic 1H NMR spectra consistent with a metal-metal quadruple bond with a σ2π4δ2 orbital occupancy. One of the compounds proved amenable to single-crystal X-ray structure determination, yielding a metal-metal distance of ∼2.24 Å, essentially identical to that in triple-bonded osmium corrole dimers. On the other hand, the electrochemical properties of Re and Os corrole dimers proved to be radically different. Thus, the reduction potentials of the Re corrole dimers are some 800 mV upshifted relative to those of their Os counterparts. Stated differently, the Re corrole dimers are dramatically easier to reduce, reflecting electron addition to δ* versus π* molecular orbitals for Re and Os corrole dimers, respectively. The data also imply electrochemical HOMO-LUMO gaps of only 1.0-1.1 V for rhenium corrole dimers, compared with values of 1.85-1.90 V for their Os counterparts. These HOMO-LUMO gaps rank among the first such values reported for quadruple-bonded transition-metal dimers for any type of supporting ligand, porphyrin-type or not.

X-ray absorption spectroscopy of exemplary platinum porphyrin and corrole derivatives: metal- versus ligand-centered oxidation.

A combination of Pt L3-edge X-ray absorption spectroscopy (EXAFS and XANES) and DFT (TPSS) calculations have been performed on powder samples of the archetypal platinum porphyrinoid complexes PtII[TpCF3PP], PtIV[TpCF3PP]Cl2, and PtIV[TpCF3PC](Ar)(py), where TpCF3PP2- = meso-tetrakis(p-trifluoromethylphenyl)porphyrinato and TpCF3PC3- = meso-tris(p-trifluoromethylphenyl)corrolato. The three complexes yielded Pt L3-edge energies of 11 566.0 eV, 11 567.2 eV, and 11 567.6 eV, respectively. The 1.2 eV blueshift from the Pt(ii) to the Pt(iv) porphyrin derivative is smaller than expected for a formal two-electron oxidation of the metal center. A rationale was provided by DFT-based Hirshfeld which showed that the porphyrin ligand in the Pt(iv) complex is actually substantially oxidized relative to that in the Pt(ii) complex. The much smaller blueshift of 0.4 eV, going from PtIV[TpCF3PP]Cl2, and PtIV[TpCF3PC](Ar)(py), is ascribable to the significantly stronger ligand field in the latter compound.

Regioselective formylation of rhenium-oxo and gold corroles: substituent effects on optical spectra and redox potentials.

Vilsmeier-Haack formylation of ReO and Au meso-triarylcorroles over 16-18 hours affords moderate to good yields (47-65%) of the ReO-3-formyl and Au-3,17-diformyl derivatives in a highly regioselective manner. Formylation was found to effect substantial upshifts for redox potentials (especially the reduction potentials) as well as significant to dramatic redshifts for both the Soret and Q bands.

Correction: X-ray absorption spectroscopy of exemplary platinum porphyrin and corrole derivatives: metal- versus ligand-centered oxidation.

[This corrects the article DOI: 10.1039/D1RA06151H.].

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 ß-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, ß-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 ß-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 ß-unsubstituted starting materials, the ß-octahalogenated products were found to exhibit sharp 1H NMR signals at room temperature, indicating that the aryl groups are locked in place by the ß-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 ß-octabromination protocols, which seems best attributed to the deactivating influence of the high-valent Re center.

Amphiphilic Rhenium-Oxo Corroles as a New Class of Sensitizers for Photodynamic Therapy.

A set of rhenium(V)-oxo meso-triarylcorroles bearing ester and carboxylic acid functionalities were synthesized with a view to determining their potential for photodynamic therapy. Toward this end, we measured their near-IR phosphorescence and their ability to sensitize singlet oxygen formation. The two esters studied, ReVO 5,10,15-tris(meta-carbomethoxyphenyl)corrole and ReVO 5,10,15-tris(para-carbomethoxyphenyl)corrole, were found to exhibit phosphorescence quantum yields of around 1% and fairly long phosphorescence lifetimes of about 60 µs in toluene. The corresponding carboxylic acids, which were examined in ethanolic/aqueous media, in contrast, showed much lower phosphorescence quantum yields on the order of 0.01% and somewhat shorter phosphorescent lifetimes. The quantum yields for singlet oxygen formation, on the other hand, turned out to be equally high (0.72 ± 0.02) for the esters and corresponding carboxylic acids. For the two carboxylic acids, we also carried out photocytotoxicity measurements on rat bladder cancer cells (AY27) and human colon carcinoma cells (WiDr). Cell viability measurements (MTT assays) indicated 50% cell death (LD50) for AY27 cells upon 5 min of blue light exposure with the meta carboxylic acid and upon 7 min of exposure with the para carboxylic acid; complete cell death resulted after 20 min for both compounds. The WiDr cells proved less sensitive, and LD50 values were reached after 8 and 12 min illumination with the meta and para carboxylic acids, respectively.

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-Nimido 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.

Electrophilic Activation of Osmium-Nitrido Corroles: The OsN Triple Bond as a π-Acceptor Metallaligand in a Heterobimetallic OsVIN-PtII Complex.

Presented herein is a first investigation of the chemical reactivity of osmium-nitrido corroles, which are known for their unusual thermal, chemical, and photochemical stability. Elemental chlorine perchlorinates the ß-positions of the triarylcorrole but leaves the OsN unit untouched. The OsN unit is also unaffected by a variety of other electrophilic and nucleophilic reagents. Upon photolysis, however, the anion of Zeise's salt associates with the nitrido ligand to generate an OsVI≡N-PtII complex. The very short OsN-Pt linkage [1.895(9)-1.917(8) Å] and the downfield 195Pt NMR resonance (-2702 ppm) suggest that the OsN corrole acts as a π-accepting ligand toward the Pt(II) center. This finding represents a rare example of the successful photochemical activation of a metal-ligand multiple bond that is too kinetically inert to exhibit any appreciable reactivity under thermal conditions.

Unexpected Molecular Structure of a Putative Rhenium-Dioxo-Benzocarbaporphyrin Complex. Implications for the Highest Transition Metal Valence in a Porphyrin-Type Ligand Environment.

A combination of quantum chemical calculations and synthetic studies was used to address the possibility of very high (>6) valence states of transition metals in porphyrin-type complexes. With corrole as a supporting ligand, DFT calculations ruled out Re(VII) and Ir(VII) dioxo complexes as stable species. Attempted rhenium insertion into benzocarbaporphyrin (BCP) ligands on the other hand led to two products with different stoichiometries - Re[BCP]O and Re[BCP]O2. To our surprise, single-crystal structure determination of one of the complexes of the latter type indicated an ReVO center with a second oxygen bridging the Re-C bond. In other words, although the monooxo complexes Re[BCP]O are oxophilic, the BCP ligand cannot sustain a trans-ReVII(O)2 center. The search for metal valence states >6 in porphyrin-type ligand environments must therefore continue.

Relativistic Effects on a Metal-Metal Bond: Osmium Corrole Dimers.

A series of metal-metal bonded osmium corrole dimers, {Os[T pXPC]}2, were synthesized in reasonably good yields (35-46%) via the interaction of the corresponding free-base meso-tris( p-X-phenyl)corroles (H3[T pXPC], X = CF3, H, CH3, and OCH3), Os3(CO)12, and potassium carbonate in 1,2,4-trichlorobenzene under an inert atmosphere at 180 °C over several hours. The complexes are only the second class of Os corroles reported to date (the first being OsVIN corroles) and also the second class of metal-metal bonded metallocorrole dimers (the other being Ru corrole dimers). Comparison of the X-ray structures, redox potentials, and optical spectra of analogous Ru and Os corrole dimers, along with scalar-relativistic DFT calculations, has provided an experimentally calibrated account of relativistic effects in these complexes. Three of the Os corrole dimers (X = CF3, H, and OCH3) were analyzed with single-crystal X-ray diffraction analysis, revealing inversion-related corrole rings with eclipsed Os-N bonds and Os-Os distances of ∼2.24 Å that are ∼0.06 Å longer than the Ru-Ru distances in the analogous Ru corrole dimers. Interestingly, a comparison of scalar-relativistic and nonrelativistic DFT calculations indicates that this difference in metal-metal bond distance does not, in fact, reflect a differential relativistic effect. For a given corrole ligand, the Ru and Os corrole dimers exhibit nearly identical oxidation potentials but dramatically different reduction potentials, with the Os values ∼0.5 V lower relative to Ru, suggesting that whereas oxidation occurs in a ligand-centered manner, reduction is substantially metal-centered, which indeed was confirmed by scalar-relativistic calculations. The calculations further indicate that approximately a third of the ∼0.5 V difference in reduction potentials can be ascribed to relativity. The somewhat muted value of this relativistic effect appears to be related to the finding that reduction of an Os corrole dimer is not exclusively metal-based but that a significant amount of spin density is delocalized over to the corrole ligand; in contrast, reduction of an Ru corrole dimer occurs exclusively on the Ru-Ru linkage. For isoelectronic complexes, the Ru and Os corrole dimers exhibit substantially different UV-vis spectra. A key difference is a strong near-UV feature of the Os series, which in energy terms is blue-shifted by ∼0.55 V relative to the analogous feature of the Ru series. TDDFT calculations suggest that this difference may be related to higher-energy Os(5d)-based LUMOs in the Os case relative to analogous MOs for Ru.

Ambient-temperature near-IR phosphorescence and potential applications of rhenium-oxo corroles.

Presented herein is a first photophysical investigation of a series of rhenium(v)-oxo tris(p-X-phenyl)corroles, Re[TpXPC](O), where X = CF3, F, H, CH3, and OCH3. The complexes all exhibit near-IR phosphorescence in anoxic toluene at room temperature, with emission maxima at 780 ± 10 nm, phosphorescence decay times of 56-74 µs and quantum yields of 1.07-1.52%, the highest reported to date for phosphorescent corrole derivatives. We have also demonstrated that the dyes may serve as indicators in optical oxygen sensors and as sensitizers in triplet-triplet-annihilation-based upconversion. Although the ReO corroles are not superior to benchmark benzoporphyrin complexes that are currently used in these applications, they may prove useful in certain areas such as optical multianalyte sensing owing to improved flexibility in the available spectral properties of the dyes. The high thermal and photochemical stability of the complexes also bodes well for their deployment as new, phosphorescent sensitizers in photodynamic therapy.

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 PtIV[TpXPC](m/p-C6H4CN)(py), where TpXPC3- 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 PtIV-corrole3- 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, PtIV[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.

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.