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 minor modulatory role. 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. The complexes studied, therefore, are aptly described as inverse hypercorroles.

Trisubstituted porphyrin derivatives [(TriMPyP)M][Formula: see text]([Formula: see text]) 3 [Formula: see text][Formula: see text] containing three [Formula: see text]-methyl-4-pyridyl ([Formula: see text][Formula: see text] groups were synthesized and characterized electrochemically and spectroscopically in DMSO containing 0.1 M tetrabutylammonium salts. The effect of specific counter-anions, type of metal ion and number of meso-[Formula: see text][Formula: see text] groups on the redox behavior and spectroscopic properties of the neutral and reduced species are examined before and after controlled potential reduction in a thin-layer cell. Each tri-cationic derivative initially undergoes a global two-electron reduction to give a two-banded absorption spectrum with a broad near-IR band ranging from 750–807 nm depending on the type of metal ion. The position of the near-IR band for the doubly reduced porphyrin was also found to correlate with the number of meso-[Formula: see text][Formula: see text] groups on the macrocycle which varied from 1 to 4 while the first reduction potential remained essentially unchanged upon going from macrocycles with two to three to four meso-[Formula: see text][Formula: see text] groups.

The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co-II porphyrin(2.1.2.1) complexes bearing Ph or F5Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co-II/Co-III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e(-) ORR pathway giving H2O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e(-) ORR as compared to the 2e(-) pathway, consistent with experimental data.

A mono-DMSO cobalt meso-CF3 corrole, formulated as (CF3)3CorCo(DMSO), where (CF3)3Cor is the trianion of 5,10,15-tris(trifluoromethyl)corrole, was synthesized and characterized as to its spectral and electrochemical properties in nonaqueous media with a focus on its coordination chemistry and electronic structure. Cyclic voltammetric measurements showed more facile reductions and difficult oxidations compared to the cobalt triarylcorrole possessing p-CF3Ph units at the meso-positions, a result consistent with the enhanced inductive effect of the electron-withdrawing trifluoromethyl substituents linked directly at the meso-carbon atoms of the macrocycle. The effects of DMSO, pyridine, and cyanide anions (CN-) on the electrochemistry and spectral properties of the compound were investigated, and it was found that only two molar equivalents are needed to form the bis-CN adduct, which exhibited two 1-electron oxidations at 0.27 and 0.95 V vs saturated calomel electrode (SCE) in CH2Cl2/0.1 M TBAP. The sites of electron transfer in the first oxidation and reduction were investigated by spectroelectrochemistry and confirmed that the first electron addition affords a Cor3-CoII complex under all solution conditions independent of the initial coordination and/or electronic configuration (i.e., innocent Cor3-CoIII or noninnocent Cor•2-CoII). In contrast, data for the first oxidation suggests that the site of electron abstraction (ligand or metal) depended upon coordination of the neutral and in situ generated complexes under the various solution conditions, leading to a Co(IV)-corrole3- product for both the bis-pyridine and bis-cyanide adducts.

We have revisited the electrochemistry of metallocorrole dimers with low-temperature cyclic voltammetry and UV-visible-NIR spectroelectrochemistry, with the aim of determining the sites of the redox processes undergone by these compounds. The systems studied include the metal-metal triple-bonded complexes {Ru[TpOMePC]}(2) and {Os[TpOMePC]}(2) and the metal-metal quadruple-bonded complex {Re[TPC]}(2), where TpOMePC and TPC refer to trianionic meso-tris(p-methoxyphenyl)corrole and meso-triphenylcorrole ligands. For all three compounds, the first oxidation potentials are found at 0.52 +/- 0.04 V vs SCE in CH2Cl2/0.1 M TBAP and are accompanied by major changes in the optical spectra, especially the appearance of broad, low-energy bands, suggesting macrocycle-centered oxidation in each case. In contrast, the reduction potentials span an 800 mV range, occurring at E-1/2 = -0.52 V for {Re[TPC]}(2), -0.81 V for {Ru[TpOMePC]}(2), and -1.32 V for {Os[TpOMePC]}(2), with more modest changes in the optical spectra, implying a significant metal-centered character in the reduction process. Density functional theory (DFT) calculations largely (but not entirely) bear out these expectations. The combined experimental and theoretical data indicate that one-electron addition to the Re dimer involves the Re-Re delta* LUMO, while one-electron addition to the Ru dimer largely involves the Ru-Ru pi* LUMO. In contrast, the calculations suggest that one-electron reduction of the Os dimer occurs largely on the corrole ligands, a phenomenon attributed to the relativistic destabilization of the Os-Os pi* MOs.

A new family of β-dicyanovinyl (DCV)-appended corroles represented as MTPC(MN) (where M = 3H, Cu, Ag, and Co(PPh3) and MN = malononitrile and TPC = 5,10,15-triphenylcorrole) were synthesized starting from the free base mono β-formyl corrole, H3TPC(CHO), and characterized along with their respective MTPC(CHO) and MTPC complexes as to their spectroscopic and electrochemical properties in nonaqueous media. Comparisons between the two series of corroles demonstrate a pronounced substituent effect of the β-DCV group on the physicochemical properties making the MTPC(MN) derivatives substantially easier to reduce and more difficult to oxidize than the formyl or unsubstituted corroles. In addition, the colorimetric and spectral detection of 11 different anions (X) in the form of tetrabutylammonium salts (TBAX, X = PF6-, OAc-, H2PO4-, CN-, HSO4-, NO3-, ClO4-, F-, Cl-, Br-, and I-) were also investigated in nonaqueous media. Of the investigated anions, only CN- was found to induce changes in the UV-vis and 1H NMR spectra of the β-DCV metallocorroles. This data revealed that CuTPC(MN) and AgTPC(MN) act as chemodosimeters for selective cyanide ion detection via a nucleophilic attack at the vinylic carbon of the DCV substituent, while (PPh3)CoTPC(MN) acts as a chemosensor for cyanide ion sensing via axial coordination to the cobalt metal center. A low-limit detection of cyanide ions was observed at 1.69 ppm for CuTPC(MN) and 1.17 ppm for AgTPC(MN) in toluene.

A series of mono- and dirhodium(I) meso-diaryl dibenzoporphyrin(2.1.2.1) complexes containing [Formula: see text]-tolyl, phenyl or pentafluorophenyl meso-substiuents were synthesized and characterized as to their electrochemical and spectroscopic properties in organic solvents and the data compared to that for same series of free base dibenzoporphyrins in their neutral and protonated forms. The redox behavior of the free base, mono- and di-metallic porphyrins was measured by cyclic voltammetry (CV) in methylene chloride or pyridine before and after the addition of acid in the form of TFA and the products of electron transfer are proposed on the basis of the CV data combined with results from thin-layer UV-visible spectroelectrochemistry. X-ray crystal structures of the dirhodium(I) complexes are also reported.

A novel cobalt corrole bearing 4-vinylphenyl groups at the 5,10,15-meso-positions of the macrocycle has been synthesized from tris(4-bromophenyl)corrole using a Suzuki coupling reaction. The spectral and electrochemical properties are reported in CH2Cl2 along with its ability to form a highly stable six-coordinate complex and cross-linked corrole-based polymer in a 59% yield.

A series of cobalt nitrophenylcorroles were spectrally characterized in CH2Cl2, and under certain solution conditions, several compounds were shown to exhibit hypercorrole spectra resulting from charge transfer interactions from the corrole π-system to the redox-active meso-NO2Ph substituents. The resulting spectral pattern has not previously been reported for metallocorroles and in the case of the cobalt derivatives was shown to depend upon the number and position of the meso-nitrophenyl groups on the macrocycle, the position of the NO2 substituent on the meso-phenyl ring(s) (para or meta), and the electronic structure of the corrole, which can exist in its innocent or noninnocent form depending in large part upon the type and number of axial ligands. Cobalt corroles bearing p-nitrophenyl groups at the 5,15- or 5,10,15-positions of the macrocycle exhibited the most marked hypercorrole spectra under solution conditions where the complex was innocent (i.e., Cor3-CoIII), and a systematic analysis of the spectral data suggests the root of this perturbation to be a corrole-to-aryl interaction (i.e., ligand-to-ligand charge transfer or LLCT). The largest interaction between the π-system and the NO2Ph substituents was seen upon coordination of anionic cyanide (CN-) axial ligands to the Co(III) center of the bis-(CN-)-5,15-dinitrophenyl derivative, resulting in a cobalt hypercorrole spectrum where the broad Q-band was red-shifted even further into the NIR region and located at 795 nm in CH2Cl2 and 827 nm in pyridine. Cyclic voltammetry of the bis-CN- adducts showed that the first electrons are added to the LUMOs of the p-NO2Ph substituents rather than the corrole, while the same orbitals for the mono-CN- adducts are nearly degenerate. This redox behavior contrasts with what is seen for the noninnocent nitrophenyl corroles having "normal" unperturbed UV-vis spectra where the first reduction involves the π-system of the macrocycle, followed by reduction of the p-NO2Ph groups at more negative potentials.

Our laboratories have long been interested in the redox properties of four-, five- and six-coordinate metallocorroles (Chart 1) with different donor axial ligands and a variety of peripheral corrole substituents. In this work, we describe and compare our most recent results on different transition metal derivatives containing highly electron withdrawing substituents at the meso-positions of the macrocycle. Figure 1

Three copper dibenzoporphyrin(2.1.2.1) complexes having two dipyrromethene units connected through o-phenylen bridges and 4-MePh, Ph, or F5Ph substituents at the meso positions of the dipyrrins were synthesized and characterized according to their spectral, electrochemical, and structural properties. As indicated by the single-crystal X-ray structures, all three derivatives have highly bent molecular structures, with angles between each planar dipyrrin unit ranging from 89° to 85°, indicative of a nonaromatic molecule. The insertion of copper(II) into dibenzoporphyrins(2.1.2.1) induced a change in the macrocyclic cavity shape from rectangular in the case of the free-base precursors to approximately square for the metalated copper derivatives. Solution electron paramagnetic resonance (EPR) spectra at 100 K showed hyperfine coupling of the Cu(II) central metal ion and the N nucleus in the highly bent molecular structures. Electrochemical measurements in CH2Cl2 or N,N-dimethylformamide (DMF) containing 0.1 M tetrabutylammonium perchlorate (TBAP) were consistent with ring-centered electron transfers and, in the case of reduction, were assigned to electron additions involving two equivalent π centers on the bent nonaromatic molecule. The potential separation between the two reversible one-electron reductions ranged from 230 to 400 mV in DMF, indicating a moderate-to-strong interaction between the equivalent redox-active dipyrrin units of the dibenzoporphyrins(2.1.2.1). The experimentally measured highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps ranged from 2.14 to 2.04 eV and were smaller than those seen for the planar copper tetraarylporphyrins(1.1.1.1), (Ar)4PCu.

The coordination chemistry of corrole has witnessed a great improvement in the past few years and its Periodic Table has been widened to be so large that it is compared with that of porphyrins. However, Ni and Zn ions, commonly used with porphyrins for both synthetic and theoretical purposes, are sparsely reported in the case of corroles. Here, we report synthetic protocols for preparing Ni and Zn triarylcorrole complexes. In the case of Zn, the preliminary oxidation of the free base corrole in DMSO to the neutral corrole radical is a necessary step to obtain the coordination of the metal ion, because the direct reaction led to the formation of an open-chain tetrapyrrole. The Ni complex could be directly obtained by heating the free base corrole and Ni(II) salt to 100 °C in a DMSO solution containing FeCl3. The non-innocent nature of the corrole ligand for both complexes has been elucidated by EPR, and in the case of the Zn derivative the first spectroelectrochemical characterization is presented.

Electrosynthesis, when combined with classical electrochemical measurements, can often provide unique insights into the mechanism and intermediates involved in formation of the desired product. This is described in the current work which investigates the redox properties of electrosynthesis and electrochemistry of porphyrins with redox active substituents.

The reaction of the ligand 2,3-dicyano-5,6-di(2-pyridyl)-pyrazine, [(CN)(2)dpp], with Pd(OAc)(2), in CH3CN leads to the formation of three novel complexes which were characterized by thermogravimetric analysis, single crystal and powder X-ray measurements, MALDI-TOF mass spectra, IR and UV-visible spectroscopy as well as electrochemistry. As established by crystallographic work, the complex of formula [(CN)(2)dppPd(OAc)(2)].H2O (molar ratio ligand/Pd(II) 1:1; species 3) shows a single Pd(OAc)(2) unit coordinated to each nitrogen of the two pyridyl (py) groups of [(CN)(2)dpp]. i.e. the "py-py " mode of coordination previously established for the analogs [(CN)(2)dppMCl(2)] (M = Pd(II), Pt(II)). The second isolated species is a highly insoluble dinuclear Pd(II) complex of formula [(CN)(2)dpp{Pd(OAc)(2)}(2)].5H(2)O (molar ratio ligand/Pd(II) 1:2; species 1), where the two Pd(OAc)(2) units are each coordinated to one pyrazine nitrogen and one N atom of a pyridyl group on [(CN)2dpp] ( "py-pyz " binding). Based on crystallographic work, the third isolated species is assigned the formula [(CN)(CONH)dppPd (OAc)] (molar ratio ligand/Pd(II) 1:1; species 2), which is formed by hydrolysis of a CN substituent on [(CN)(2)dpp] giving a -CONH- group and characterized by a "py-pyz " type of coordination. Electrochemical and spectroelectrochemical data of complexes 1, 2, and 3 in DMF and DMSO were used to assign the sites of electron transfer.

Zinc tetraphenylporphyrin (TPP)Zn was examined as to its as to its electroreduction properties by cyclic voltammetry and spectroelectrochemistry in six nonaqueous solvents containing tetrabutylammonium perchlorate (TBAP). The porphyrin undergoes an initial one-electron reduction to give a [Formula: see text]-anion radical on the cyclic voltammetry timescale but the radical was not stable on the spectroelectrochemical timescale when utilizting chlorinated alkane solvents containing 0.1 M TBAP (CH2Cl2, 1-2,dichloroethane (C2H4Cl[Formula: see text] where chloride, generated from minute amounts of reduced solvent, formed an axially coordinated complex, (TPP)Zn[Formula: see text](Cl[Formula: see text], prior to reduction of the porphyrin. This spectroelectrochemically characterized product exhibits red-shifted Soret and Q-bands as compared to that of the neutral (TPP)Zn under the same solution conditions and analytical plots obtained from recording the spectrum as a function of potential rule out the possibility of forming the superoxide complex, (TPP)Zn[Formula: see text](O[Formula: see text], in chlorinated alkane solvents.

The electronic structures, and, particularly, the nature of the HOMO in a series of the low-spin PcFeL2, PcFeL′L″, and [PcFeX2]2- iron(II) phthalocyanine complexes were probed by electrochemical, spectroelectrochemical and chemical oxidation approaches and complimented by MCD spectroscopy as well as theoretical (DFT and TDDFT) studies. In general, energies of the metal-centered occupied orbitals in the various six-coordinate iron phthalocyanine complexes correlate well with Lever’s electrochemical parameter, EL, and intercross the phthalocyanine-centered a1u orbital in several compounds with moderate-to-strong p-accepting axial ligands. In these cases, an oxidation of the phthalocyanine macrocycle (Pc(2-)/Pc(1-)) rather than the central metal ion (Fe(II)/Fe(III)) was theoretically predicted and experimentally confirmed. The experimentally derived using MCD spectroscopy or theoretically predicted using TDDFT calculations energy of the MLCT1 transition (dπ→Pc(π*)) on iron(II) phthalocyanines also follows the trend expected for the EL properties of the axial ligands.

A new family of antipodal beta-arylaminodibromoporphyrins was synthesized through a Buchwald-Hartwing C-N coupling reaction and characterized as to its spectroscopic and electrochemical properties. The investigated compounds are represented as (NHR)Br2TPPM where M = 2H, Cu(II) or Zn(II), (NHR)Br2TPP is the dianion of beta-arylaminodibromotetraphenylporphyrin and R is one of the five different aryl groups: Ph, m,m-Me2Ph, NPH = 1-naphthyl, p-CNPh, p-NO2Ph. The yield of desired arylaminoporphyrin depends upon the equivalents of arylamine utilized in the reaction and the electron-donating or electron-withdrawing nature of the R group present on the phenyl ring of the beta-arylamino moiety. The Soret bands of these porphyrins are broader and red-shifted as compared to the related tetraphenylporphyrin derivatives, TPPM, whereas their Q bands are intense due to push-pull effects. The emission bands of the free base and Zn(II) derivatives are weak, broad and red-shifted (Delta lambda(em) = 17-60 nm) as compared to their corresponding TPPM derivatives (M = 2H and Zn) due to the cross-polarized push-pull beta-substituents and the heavy atom effect of the two bromo substituents. The (NHR)Br2TPPM complexes exhibit characteristic split NMR signals due to the unsymmetrical beta-substitution. The DFT optimized structures indicate a moderate nonplanar porphyrin core, with the Cu(II) arylaminoporphyrins having the largest Delta 24 values (average displacement of 24 atoms of macrocyclic core) of 0.595-0.622 angstrom. The redox properties are also influenced by the electronic nature of the arylamino group with the measured half-wave potentials for reduction being positively shifted by 0.09-0.21 V as compared to the parent TPPM complexes whereas marginal shifts were observed for oxidation processes with respect to the TPPM derivatives. This leads to a 0.14-0.20 V decrease in the electrochemically measured HOMO-LUMO gap for the beta-arylaminodibromoporphyrins as compared to the corresponding TPPM analogues. (C) 2021 Elsevier B.V. All rights reserved.