Europium Triple-Decker Complexes Containing Phthalocyanine and Nitrophenyl–Corrole Macrocycles

A series of europium triple-decker complexes containing phthalocyanine and nitrophenyl–corrole macrocycles were synthesized and characterized by spectroscopic and electrochemical methods in nonaqueous media. The examined compounds are represented as Eu₂[Pc­(OC₄H₉)₈]₂­[Cor­(Ph)_<i>n</i>(NO₂Ph)_3–<i>n</i>], where <i>n</i> varies from 0 to 3, Pc­(OC₄H₉)₈ represents the phthalocyanine macrocycle, and Cor indicates the corrole macrocycle having phenyl (Ph) or nitrophenyl (NO₂Ph) meso substituents. Three different methods were used for syntheses of the target complexes, two of which are reported here for the first time. Each examined compound undergoes five reversible one-electron oxidations and 3–5 one-electron reductions depending upon the number of NO₂Ph substituents. The nitrophenyl groups on the meso positions of the corrole are highly electron-withdrawing, and this leads to a substantial positive shift in potential for the five oxidations and first reduction in CH₂Cl₂, PhCN, or pyridine as compared to the parent triple-decker compound with a triphenylcorrole macrocycle. The measured <i>E</i>_1/2 values are linearly related to the number of NO₂Ph groups on the corrole, and the relative magnitude of the shift in potential for each redox reaction was used in conjunction with the results from thin-layer spectro-electrochemistry to assign the initial site of oxidation or reduction on the molecule. The nitrophenyl substituents are also redox-active, and each is reduced to [C₆H₄NO₂]⁻ in a separate one-electron transfer step at potentials between −1.12 and −1.42 V versus saturated calomel electrode

Europium Triple-Decker Complexes Containing Phthalocyanine and Nitrophenyl–Corrole Macrocycles

A series of europium triple-decker complexes containing phthalocyanine and nitrophenyl–corrole macrocycles were synthesized and characterized by spectroscopic and electrochemical methods in nonaqueous media. The examined compounds are represented as Eu₂[Pc­(OC₄H₉)₈]₂­[Cor­(Ph)_<i>n</i>(NO₂Ph)_3–<i>n</i>], where <i>n</i> varies from 0 to 3, Pc­(OC₄H₉)₈ represents the phthalocyanine macrocycle, and Cor indicates the corrole macrocycle having phenyl (Ph) or nitrophenyl (NO₂Ph) meso substituents. Three different methods were used for syntheses of the target complexes, two of which are reported here for the first time. Each examined compound undergoes five reversible one-electron oxidations and 3–5 one-electron reductions depending upon the number of NO₂Ph substituents. The nitrophenyl groups on the meso positions of the corrole are highly electron-withdrawing, and this leads to a substantial positive shift in potential for the five oxidations and first reduction in CH₂Cl₂, PhCN, or pyridine as compared to the parent triple-decker compound with a triphenylcorrole macrocycle. The measured <i>E</i>_1/2 values are linearly related to the number of NO₂Ph groups on the corrole, and the relative magnitude of the shift in potential for each redox reaction was used in conjunction with the results from thin-layer spectro-electrochemistry to assign the initial site of oxidation or reduction on the molecule. The nitrophenyl substituents are also redox-active, and each is reduced to [C₆H₄NO₂]⁻ in a separate one-electron transfer step at potentials between −1.12 and −1.42 V versus saturated calomel electrode

Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions

Three series of cobalt tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds have the general formula (T<i>p</i>YPP)­Co, butano­(T<i>p</i>YPP)­Co^II, and benzo­(T<i>p</i>YPP)­Co^II, where T<i>p</i>YPP represents the dianion of the meso-substituted porphyrin, Y is a CH₃, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the β- and β′-substituted groups on the four pyrrole rings of the compound. Each porphyrin undergoes one or two reductions depending upon the meso substituent and solvent utilized. Two irreversible reductions are observed for (T<i>p</i>YPP)­Co^II and butano­(T<i>p</i>YPP)­Co^II in CH₂Cl₂ containing 0.1 M tetra-<i>n</i>-butylammonium perchlorate; the first leads to the formation of a highly reactive cobalt­(I) porphyrin, which can then rapidly react with a solvent to give a Co^IIICH₂Cl as the product. Only one reversible reduction is seen for benzo­(T<i>p</i>YPP)­Co^II under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt­(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CH₂Cl₂. The first oxidation is metal-centered for the (T<i>p</i>YPP)Co and benzo­(T<i>p</i>YPP)­Co^II derivatives, leading to generation of a cobalt­(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano­(T<i>p</i>YPP)­Co^II and gives a Co^II π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs) when adsorbed on a graphite electrode in 1.0 M HClO₄. The number of electrons transferred (<i>n</i>) during ORRs is 2.0 for the butano­(T<i>p</i>YPP)­Co^II derivatives, consistent with only H₂O₂ being produced as a product for the reaction with O₂. However, the reduction of O₂ using the cobalt benzoporphyrins as catalysts gave <i>n</i> values between 2.6 and 3.1 under the same solution conditions, thus producing a mixture of H₂O and H₂O₂ as the reduction product. This result indicates that the β and β′ substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media

Synthesis and Characterization of Rare Earth Corrole–Phthalocyanine Heteroleptic Triple-Decker Complexes

We recently reported the first example of a europium triple-decker tetrapyrrole with mixed corrole and phthalocyanine macrocycles and have now extended the synthetic method to prepare a series of rare earth corrole–phthalocyanine heteroleptic triple-decker complexes, which are characterized by spectroscopic and electrochemical methods. The examined complexes are represented as M₂[Pc­(OC₄H₉)₈]₂[Cor­(ClPh)₃], where Pc = phthalocyanine, Cor = corrole, and M is Pr­(III), Nd­(III), Sm­(III), Eu­(III), Gd­(III), or Tb­(III). The Y­(III) derivative with OC₄H₉ Pc substituents was obtained in too low a yield to characterize, but for the purpose of comparison, Y₂[Pc­(OC₅H₁₁)₈]₂­[Cor­(ClPh)₃] was synthesized and characterized in a similar manner. The molecular structure of Eu₂[Pc­(OC₄H₉)₈]₂­[Cor­(ClPh)₃] was determined by single-crystal X-ray diffraction and showed the corrole to be the central macrocycle of the triple-decker unit with a phthalocyanine on each end. Each triple-decker complex undergoes up to eight reversible or quasireversible one-electron oxidations and reductions with <i>E</i>_1/2 values being linearly related to the ionic radius of the central ions. The energy (<i>E</i>) of the main Q-band is also linearly related to the radius of the metal. Comparisons are made between the physicochemical properties of the newly synthesized mixed corrole–phthalocyanine complexes and previously characterized double- and triple-decker derivatives with phthalocyanine and/or porphyrin macrocycles

Synthesis and Characterization of Rare Earth Corrole–Phthalocyanine Heteroleptic Triple-Decker Complexes

We recently reported the first example of a europium triple-decker tetrapyrrole with mixed corrole and phthalocyanine macrocycles and have now extended the synthetic method to prepare a series of rare earth corrole–phthalocyanine heteroleptic triple-decker complexes, which are characterized by spectroscopic and electrochemical methods. The examined complexes are represented as M₂[Pc­(OC₄H₉)₈]₂[Cor­(ClPh)₃], where Pc = phthalocyanine, Cor = corrole, and M is Pr­(III), Nd­(III), Sm­(III), Eu­(III), Gd­(III), or Tb­(III). The Y­(III) derivative with OC₄H₉ Pc substituents was obtained in too low a yield to characterize, but for the purpose of comparison, Y₂[Pc­(OC₅H₁₁)₈]₂­[Cor­(ClPh)₃] was synthesized and characterized in a similar manner. The molecular structure of Eu₂[Pc­(OC₄H₉)₈]₂­[Cor­(ClPh)₃] was determined by single-crystal X-ray diffraction and showed the corrole to be the central macrocycle of the triple-decker unit with a phthalocyanine on each end. Each triple-decker complex undergoes up to eight reversible or quasireversible one-electron oxidations and reductions with <i>E</i>_1/2 values being linearly related to the ionic radius of the central ions. The energy (<i>E</i>) of the main Q-band is also linearly related to the radius of the metal. Comparisons are made between the physicochemical properties of the newly synthesized mixed corrole–phthalocyanine complexes and previously characterized double- and triple-decker derivatives with phthalocyanine and/or porphyrin macrocycles

Synthesis and Characterization of Palladium(II) Complexes of <i>meso</i>-Substituted [14]Tribenzotriphyrin(2.1.1)

Metalation of 6,13,20,21-tetrakis-aryl-22<i>H</i>-[14]­tribenzotriphyrin­(2.1.1) (TriPs) with PdCl₂ provides Pd^II–TriP complexes in 45–56% yields. The complexes were characterized by mass spectrometry, and UV–visible absorption, magnetic circular dichroism, and ¹H NMR spectroscopy. A single crystal X-ray analysis reveals that the Pd^II–TriPs adopts a deeply saddled conformation. The palladium­(II) ion is coordinated by two pyrrole nitrogen atoms and two chloride ions to form the square-planar coordination environment. The redox properties of the Pd^II–TriPs were studied by cyclic voltammetry. Each compound undergoes one irreversible and two reversible one-electron reductions. There is a marked red-shift of the main spectral bands, relative to those of the free-base TriP ligand, due to a marked relative stabilization of the LUMO upon coordination by PdCl₂

Synthesis, Characterization, Protonation Reactions, and Electrochemistry of Substituted Open-Chain Pentapyrroles and Sapphyrins in Nonaqueous Media

Open-chain pentapyrroles were isolated as side-products from the synthesis of triaryl-corroles and then converted to the corresponding sapphyrins by catalytic oxidation in acidic media. The investigated compounds were characterized by UV–vis and ¹H NMR spectroscopy, mass spectrometry, electrochemistry, and spectroelectrochemistry and are represented as (Ar)₄PPyH₃ and (Ar)₄SH₃, where Ar is a F^– or Cl^– substituted phenyl group, PPy is a trianion of the open-chain pentapyrrole, and S is a trianion of the sapphyrin. Cyclic voltammetry and thin-layer UV–vis spectroelectrochemistry measurements were carried out in PhCN and CH₂Cl₂ containing 0.1 M tetra-<i>n</i>-butylammonium perchlorate. The open-chain pentapyrroles undergo two reversible one-electron reductions and two reversible one-electron oxidations to generate [(Ar)­PPyH₃]⁻, [(Ar)­PPyH₃]^2–, [(Ar)­PPyH₃]⁺, and [(Ar)­PPyH₃]²⁺ which were spectroscopically characterized. The corresponding sapphyrins exhibit two or three reversible one-electron oxidations in PhCN, but the reductions of these compounds are irreversible because of coupled chemical reactions following electron transfer. Comparisons are made between redox potentials and spectral properties of the open-chain pentapyrroles, sapphyrins, and structurally related corroles. Protonation of the open-chain pentapyrroles and sapphyrins was also carried out in CH₂Cl₂, and equilibrium constants were calculated by monitoring the spectral changes during titrations with trifluoroacetic acid. The pentapyrroles undergo a simultaneous two-proton addition to generate [(Ar)₄PPyH₅]²⁺ while the sapphyrins undergo two stepwise single proton additions to give [(Ar)₄SH₄]⁺ and [(Ar)₄SH₅]²⁺, respectively

Molecular Oxygen Reduction Electrocatalyzed by <i>meso</i>-Substituted Cobalt Corroles Coated on Edge-Plane Pyrolytic Graphite Electrodes in Acidic Media

Five <i>meso</i>-substituted cobalt­(III) corroles were examined as to their catalytic activity for the electoreduction of O₂ when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO₄. The investigated compounds are represented as (T<i>p</i>RPCor)­Co­(PPh₃), where T<i>p</i>RPCor is the trianion of a <i>para</i>-substituted triphenylcorrole and R = OMe, Me, H, F, or Cl. Three electrochemical techniques, cyclic voltammetry, linear sweep voltammetry with a rotating disk electrode (RDE), and voltammetry at a rotating ring disk electrode (RRDE), were utilized to evaluate the catalytic activity of the corroles in the reduction of O₂. Cobalt corroles containing electron-withdrawing substituents were shown to be better catalysts than those having electron-donating groups on the three <i>meso</i>-phenyl rings of the triarylcorroles

Gold(III) Porphyrins Containing Two, Three, or Four β,β′-Fused Quinoxalines. Synthesis, Electrochemistry, and Effect of Structure and Acidity on Electroreduction Mechanism

Gold­(III) porphyrins containing two, three, or four β,β′-fused quinoxalines were synthesized and examined as to their electrochemical properties in tetrahydrofuran (THF), pyridine, CH₂Cl₂, and CH₂Cl₂ containing added acid in the form of trifluoroacetic acid (TFA). The investigated porphyrins are represented as Au­(PQ₂)­PF₆, Au­(PQ₃)­PF₆, and Au­(PQ₄)­PF₆, where P is the dianion of the 5,10,15,20-tetrakis­(3,5-di-<i>tert</i>-butylphenyl)­porphyrin and Q is a quinoxaline group fused to a β,β′-pyrrolic position of the porphyrin macrocycle. In the absence of added acid, all three gold­(III) porphyrins undergo a reversible one-electron oxidation and several reductions. The first reduction is characterized as a Au^III/Au^II process which is followed by additional porphyrin- and quinoxaline-centered redox reactions at more negative potentials. However, when 3–5 equivalents of acid are added to the CH₂Cl₂ solution, the initial Au^III/Au^II process is followed by a series of internal electron transfers and protonations, leading ultimately to triply reduced and doubly protonated Au^II(PQ₂H₂) in the case of Au^III(PQ₂)⁺, quadruply reduced and triply protonated Au^II(PQ₃H₃) in the case of Au^III(PQ₃)⁺, and Au^II(PQ₄H₄) after addition of five electrons and four protons in the case of Au^III(PQ₄)⁺. Under these solution conditions, the initial Au­(PQ₂)­PF₆ compound is shown to undergo a total of three Au^III/Au^II processes while Au­(PQ₃)­PF₆ and Au­(PQ₄)­PF₆ exhibit four and five metal-centered one-electron reductions, respectively, prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for reduction and UV–visible spectra of the electroreduced species are analyzed using linear free energy relationships

Synthesis of a Neo-Confused Octaphyrin and the Formation of Its Mononuclear Complexes

Novel neo-confused octaphyrin(1.1.1.1.1.1.1.0) (<b>1</b>) was synthesized by oxidative ring closure of an octapyrrane bearing two terminal “confused” pyrroles. Crystal structures of its Zn­(II) and Cu­(II) complexes (<b>2</b> and <b>3</b>) show a figure-of-eight conformation with unique mononuclear coordination structures. Photophysical data and theoretical calculations suggest that the neo-confused octaphyrin <b>1</b> is a 34π electron conjugated species showing nonaromaticity. Coordination of copper and zinc ions results in the further narrowing of the HOMO–LUMO gaps