Asymmetrically Crowded “Push–Pull” Octaphenylporphyrins with Modulated Frontier Orbitals: Syntheses, Photophysical, and Electrochemical Redox Properties

A new series of β-substituted octaphenylporphyrins were synthesized and the influence of unsymmetrical substitution on the photophysical and electrochemical properties of the compounds is elucidated. The examined compounds are represented as MOPP­(R)­X₂ where OPP = the dianion of octaphenylporphyrin, R = NO₂, CHO, or CH₂OH, X = Br or CN, and M is Co­(II), Cu­(II), Ni­(II), or Zn­(II). Routes to the trifunctionalized β-octaphenylporphyrins begin with the synthesis of MOPP­(R) (R = NO₂, CHO, and CH₂OH) and the conversion of MOPP­(NO₂) to MOPP­(NO₂)­X₂ (X = Br and CN). These “push–pull” octaphenylporphyrins exhibit high dipole moments, tunable redox properties, and red-shifted electronic spectral features due to asymmetric β-substitution. Photophysical data on the series of MOPP­(R)­X₂ compounds also reflect the nonplanar conformation of these porphyrins. Quantum yield and lifetime data are invariably lower than what has been reported in the literature for related β-substituted porphyrins. The spectroscopic properties and electrochemical redox potentials of the porphyrins are influenced by both the peripheral substituents and nature of the core metal ion. A decrease in the HOMO–LUMO gap and increase in Δ<i>b</i>₁ is observed as the number of electron withdrawing groups on the molecule was increased. In addition, a tuning of the redox potentials could be achieved by introducing both electron donating (CH₂OH) and withdrawing (CN, NO₂, CHO, and Br) substituents into the MOPP skeleton which led to a “cross-polarized push–pull effect” of the β-substituents and a nonplanarity of the molecule. Metal-centered oxidations were exhibited for all of the Co­(II) porphyrins and an M­(II)/M­(III) process was also observed to occur for NiOPP­(R) (R = CH₂OH, H, CHO, and NO₂) and CuOPP­(NO₂)­(CN)₂. These electrode reactions for the latter two series of compounds occur after an initial conversion of the neutral porphyrin to its dicationic form under the electrochemical conditions. Evidence for the site of electron transfer is given in part by comparison with data in the literature for related compounds and in part by theoretical calculations and thin-layer spectroelectrochemical data carried out in the current study

Electrochemistry and Spectroelectrochemistry of Cobalt Porphyrins with π‑Extending and/or Highly Electron-Withdrawing Pyrrole Substituents. In Situ Electrogeneration of σ‑Bonded Complexes

A series of cobalt porphyrins with π-extending or highly electron-withdrawing β-pyrrole substituents were investigated as to their electrochemistry, spectroscopic properties, and reactivity after electroreduction or electroxidation in nonaqueous media. Each porphyrin, represented as PorCo (where Por = TPP­(NO₂)­Y₂ or TPP­(NO₂)­Y₆ and Y = phenyl, phenylethynyl, Br, or CN) was shown to undergo multiple redox reactions involving the conjugated π-ring system or central metal ion which could exist in a Co­(III), Co­(II), or Co­(I) oxidation state under the application of an applied oxidizing or reducing potential. Thermodynamic half-wave potentials for the stepwise conversion between each oxidation state of [PorCo]^<i>n</i> (where <i>n</i> ranged from +3 to −3) were measured by cyclic voltammetry and analyzed as a function of the compound structure and properties of the electrochemical solvent. UV–visible spectra were obtained for each oxidized or reduced porphyrin in up to six different oxidation states ranging from [PorCo]^3– to [PorCo]³⁺ and analyzed as a function of the compound structure and utilized electrochemical solvent. Chemically or electrochemically generated Co­(I) porphyrins are known to be highly reactive in solutions containing alkyl or aryl halides, and this property was utilized to in situ generate a new series of methyl carbon-bonded cobalt­(III) porphyrins with the same π-extending or highly electron-withdrawing substituents as the initial Co­(II) derivatives. The electrosynthesized carbon-bonded Co­(III) porphyrins were then characterized as to their own electrochemical and spectroscopic properties after the addition of one, two, or three electrons in nonaqueous media

Hypercorroles Formed via the Tail that Wagged the Dog: Charge Transfer Interactions from Innocent Corroles to <i>Meso</i>-Nitrophenyl Substituents

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

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

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

Porphyrins as Photoredox Catalysts: Experimental and Theoretical Studies

Metalloporphyrins not only are vital in biological systems but also are valuable catalysts in organic synthesis. On the other hand, catalytic properties of free base porphyrins have been less explored. They are mostly known as efficient photosensitizers for the generation of singlet oxygen via photoinduced energy transfer processes, but under light irradiation, they can also participate in electron transfer processes. Indeed, we have found that free base tetraphenylporphyrin (H₂TPP) is an efficient photoredox catalyst for the reaction of aldehydes with diazo compounds leading to α-alkylated derivatives. The performance of a porphyrin catalyst can be optimized by tailoring various substituents at the periphery of the macrocycle at both the β and <i>meso</i> positions. This allows for the fine tuning of their optical and electrochemical properties and hence their catalytic activity

Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media

The first examples for the facile, reversible, and stepwise electrogeneration of triply ring-reduced porphyrin macrocycles are presented. The investigated compounds are represented as MTPP­(NO₂)­(PE)₆, MTTP­(PE)₈, NiTPP­(NO₂)­(Ph)₄, and MTPP­(CN)₄, where TTP and TPP are the dianions of tetratolylporphyrin and tetraphenylporphyrin, respectively, NO₂, phenylethynyl (PE), and CN are substituents at the β-pyrrole positions of the macrocycle, and M = Cu^II, Ni^II, Zn^II, Co^II, or 2H. Each porphyrin undergoes three or four reductions within the negative potential limit of the electrochemical solvent. The UV–visible spectra of the first three reduction products were characterized by means of thin-layer UV–vis spectroelectrochemistry, and the generation of multianionic porphyrins is interpreted in terms of extensive stabilization of the LUMOs due to the electron-withdrawing and/or extended π-conjugation of the β-substituents

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

Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 12. New Heteropentanuclear Complexes Carrying Four Exocyclic Cis-platin-like Functionalities as Potential Bimodal (PDT/Cis-platin) Anticancer Agents

Heteropentanuclear porphyrazines having the formula [(PtCl₂)₄LM] where L = tetrakis-2,3-[5,6-di­(2-pyridyl)­pyrazino]­porphyrazinato dianion and M = Zn^II, Mg^II(H₂O), Pd^II, Cu^II or Co^II were characterized by elemental analyses, IR–UV–visible spectroscopy and electrochemistry and the data compared to new and previously published results for the corresponding homopentanuclear compound [(PtCl₂)₄LPt]. This latter species has four external N_2(py)PtCl₂ coordination sites which closely resemble cis-platin, (NH₃)₂PtCl₂, the potent chemotherapeutic anticancer drug, and is able to act as a photosensitizer for the generation of ¹O₂, the cytotoxic agent in photodynamic therapy (PDT). UV–visible spectra and half wave potentials for reduction of [(PtCl₂)₄LM], [(PtCl₂)₄LPt], the parallel series of mononuclear [LM] compounds and the pentanuclear [(PdCl₂)₄LM] compounds were examined in the nonaqueous solvents dimethyl sulfoxide, pyridine, and dimethylformamide. The complete set of available data indicate that external coordination of the PtCl₂ and PdCl₂ units significantly increases the level of the electron-deficiency of the entire molecular framework despite the fact that these groups are far away from the central porphyrazine π-ring system and have coordination sites nearly orthogonal to the plane of the macrocycle. The pentanuclear species [(M′Cl₂)₄LM] (M′ = Pt^II, Pd^II) undergo multiple one-electron transfers and exhibit an easier reducibility as compared to related electrode reactions of the parent compounds [LM] having the same central metal. Aggregation phenomena and reducibility of the porphyrazines to their monoanionic form (prevalently in DMF) are observed for some of the examined compounds and were analyzed and accurately taken into account. Quantum yields of ¹O₂ (Φ_Δ), of interest in PDT, were measured for [(PtCl₂)₄LM] with M = Zn^II, Mg^II(H₂O), or Pd^II and the related macrocycles [(PdCl₂)₄LM] and [LM] in dimethylformamide (DMF) and/or DMF preacidified with HCl (DMF/HCl, [HCl]: 1–2 × 10^–4 M). Excellent Φ_Δ values (0.5–0.6) which qualify the compounds as potent photosensitizers in PDT were obtained for the pentanuclear species having Zn^II or Pd^II as central metal ions. The [(PtCl₂)₄LZn] and [(PtCl₂)₄LPd] complexes are of special interest as potential bimodal anticancer agents because of the incorporated four cis-platin-like functionalities