Zhongping Ou

Electrochemistry and catalytic properties for dioxygen reduction using ferrocene-substituted cobalt porphyrins

Cobalt porphyrins having 0-4 meso-substituted ferrocenyl groups were synthesized and examined as to their electrochemical properties in N,N-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte. The examined compounds are represented as (Fc)n(CH3Ph)(4-n)PorCo, where Por is a dianion of the substituted porphyrin, Fc and CH3Ph represent ferrocenyl and/or p-CH3C6H4 groups linked at the four meso-positions of the macrocycle, and n varies from 0 to 4. Each porphyrin undergoes two reversible one-electron reductions and two to six one-electron oxidations in DMF, with the exact number depending upon the number of Fc groups on the compound. The first electron addition is metal-centered to generate a Co(I) porphyrin. The second is porphyrin ring-centered and leads to formation of a Co(I) π-anion radical. The first oxidation of each Co(II) porphyrin is metal-centered to generate a Co(III) derivative under the given solution conditions. Each ferrocenyl substituent can also be oxidized by one electron, and this occurs at more positive potentials. Each compound was investigated as a catalyst for the electoreduction of dioxygen when adsorbed on a graphite electrode in 1.0 M HClO4. The number of electrons transferred (n) during the catalytic reduction was 2.0 for the three ferrocenyl substituted compounds, consistent with only H2O2 being produced as a product of the reaction. Most monomeric cobalt porphyrins exhibit n values between 2.6 and 3.1 under the same solution conditions, giving a mixture of H2O and H2O2 as a reduction product, although some monomeric porphyrins can give an n value of 4.0. Our results in the current study indicate that appending ferrocene groups directly to the meso positions of a porphyrin macrocycle will increase the selectivity of the oxygen reduction, resulting in formation of only H2O2 as a reaction product. This selectivity of the electrocatalytic oxygen reduction reaction is explained on the basis of steric hindrance by the ferrocene substituents which prevent dimerization.

Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical properties and catalytic activity for reduction of molecular oxygen in acid media

Cobalt(III) triarylcorroles containing 0-3 nitro groups on the para-position of the three meso-phenyl rings of the macrocycle were synthesized and characterized by electrochemistry, mass spectrometry, (UV-vis) and (1)H NMR spectroscopy. The examined compounds are represented as (NO2Ph)(n)Ph(3-n)CorCo(PPh3), where n varies from 0 to 3 and Cor represents the core of the corrole. Each compound can undergo two metal-centered one-electron reductions leading to formation of Co(II) and Co(I) derivatives in CH2Cl2 or pyridine containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). A stepwise two electron reduction of each NO2Ph group of the compound is also observed. The first is reversible and occurs in a single overlapping step at the same potential which involves an overall one-, two- or three-electron transfer process for compounds 2-4, respectively. This indicates the lack of an interaction between these redox active sites on the corroles. The second reduction of the NO2Ph groups is irreversible and located at a potential which overlaps the Co(II)/Co(I) process of the compounds. Thin-layer UV-visible spectroelectrochemical measurements in CH2Cl2, 0.1 M TBAP demonstrate the occurrence of an equilibrium between a Co(III) π-anion radical and a Co(II) derivative with an uncharged macrocycle after the first controlled potential reduction of the nitro-substituted corroles. All four cobalt corroles were also examined as catalysts for the electroreduction of O2 when coated on an edge-plane pyrrolytic graphite electrode in 1.0 M HClO4. This study indicates that the larger the number of nitro-substituents on the cobalt corrole, the better the compound acts as a catalyst.

Effect of solvent and protonation/deprotonation on electrochemistry, spectroelectrochemistry and electron-transfer mechanisms of N-confused tetraarylporphyrins in nonaqueous media.

A series of N-confused free-base meso-substituted tetraarylporphyrins was investigated by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1u2009M tetra-n-butylammonium perchlorate (TBAP) and added acid or base. The investigated compounds are represented as (XPh)4 NcpH2 , in which Ncp is the N-confused porphyrin macrocycle and X is a OCH3 , CH3 , H, or Cl substituent on the para position of each meso-phenyl ring of the macrocycle. Two distinct types of UV/Vis spectra are initially observed depending upon solvent, one corresponding to an inner-2H form and the other to an inner-3H form of the porphyrin. Both forms have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the π-system. Each porphyrin undergoes multiple irreversible reductions and oxidations. The first one-electron addition and first one-electron abstraction are located on the porphyrin π-ring system to give π-anion and π-cation radicals with a potential separation of 1.52 to 1.65u2005V between the two processes, but both electrogenerated products are unstable and undergo a rapid chemical reaction to give new electroactive species, which were characterized in the present study. The effect of the solvent and protonation/deprotonation reactions on the UV/Vis spectra, redox potentials and reduction/oxidation mechanisms is discussed with comparisons made to data and mechanisms for the structurally related free-base corroles and porphyrins.

Electrochemical and spectroelectrochemical properties of building blocks for molecular arrays: reactions of quinoxalino[2,3-b]porphyrins containing metal(II) ions

Quinoxalino[2,3-b]porphyrins are laterally-extended porphyrins with aromatic ring systems fused to the beta,beta -positions of a pyrrolic ring of the macrocycle. They are building blocks for coplanar laterally-extended oligoporphyrins with applications in molecular electronics. The electrochemistry and spectroelectrochemistry of four such quinoxalinoporphyrins containing metal(II) ions and one free-base quinoxalinoporphyrin dissolved in nonaqueous media have been investigated and the data are compared to that seen for the same derivatives of the parent macrocycle lacking the fused quinoxaline ring. The investigated compounds are represented as (P)M and (PQ)M, where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin, PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin and M = 2H, Zn, Cu, Ni or Pd. The complexes all undergo two ring-centered reductions and one or two ring-centered oxidations in PhCN, CH,Cl-2, pyridine or THE Additional redox reactions are also seen for the quinoxaline group in THE The fusion of an electroactive quinoxaline group to the porphyrin macrocycle results in an 80 to 270 mV shift of E-1/2 towards easier reductions and the appearance of a third reduction which is assigned as a quinoxaline-centered redox process. The average HOMO-LUMO gap for the (P)M and (PQ)M derivatives is 2.26 0.09 V and 2.14 0.08 V, respectively. Both values are smaller than the average separation of 2.33 0.13 V for the corresponding derivatives of tetraphenylporphyrin. The electrochemistry and UV-visible spectroelectrochemcal data indicate that moderate communication exists between the quinoxaline unit and the porphyrin pi-ring system. Copyright (c) 2005 Society of Porphyrins & Phthalocyanines.

Synthesis, Characterization, and Electrochemistry of the Manganese(I) Complexes of meso-Substituted [14]Tribenzotriphyrins(2.1.1)

Metalation of 6,13,20,21-tetraaryl-22H-[14]tribenzotriphyrins(2.1.1) (TriP, 1u2009a-d) with [Mn(CO)5 Br] provided Mn(I) tricarbonyl complexes of [14]tribenzotriphyrins(2.1.1) 2u2009a-d in 85-93u2009% yield. The complexes were characterized by mass spectrometry and UV/Vis absorption, IR, and NMR spectroscopy. Single-crystal X-ray analyses revealed that 2u2009b and 2u2009c adopt bowl-shaped conformations. The redox properties of [(TriP)Mn(I) (CO)3 ] (2u2009a-d) were studied by cyclic voltammetry. Each compound undergoes two reversible one-electron reductions to form a porphyrin π anion radical and a dianion in CH2 Cl2 . Two oxidation waves were observed, the first of which corresponds to a metal-centered electron-transfer process. The redox potentials of 2u2009a-d are consistent with the optical spectroscopic data and the relatively narrow HOMO-LUMO gaps that were predicted in DFT calculations. The optical spectra can be assigned by using Michls perimeter model. TDDFT calculations predict the presence of several metal-to-ligand charge-transfer bands in the L-band region between 500 and 700u2005nm.

Electrochemistry of Nitrated N‐Confused Free‐Base Tetraaryl‐Porphyrins in Nonaqueous Media

Four nitrated N-confused free-base tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media. The examined compounds are represented as NO2 (Ar)4 NcpH2 , where NO2 (Ar)4 Ncp is the dianion of a tetraaryl N-confused porphyrin with an inner carbon bound NO2 group and Ar is a p-CH3 OPh, p-CH3 Ph, Ph or p-ClPh substituent on each meso-position of the macrocycle. UV/Vis spectra and NMR spectroscopy data indicate that the same form of the porphyrin exists in CH2 Cl2 and DMF which is unlike the case of non-NO2 N-confused porphyrins. The Soret band of NO2 (Ar)4 NcpH2 exhibits a 30-36u2005nm red-shift in CH2 Cl2 and DMF as compared to the spectrum of the non-NO2 N-confused porphyrins. The first two reductions and first oxidation of NO2 (Ar)4 NcpH2 are reversible in CH2 Cl2 containing 0.1u2009M TBAP. The measured HOMO-LUMO gap averages 1.65u2005V in CH2 Cl2 and 1.53u2005V in DMF, with both values being similar to those of the non-NO2 substituted compounds. The nitro group on the inverted pyrrole is itself not reduced within the negative potential limit of CH2 Cl2 or DMF, but its presence significantly affects both the UV/Vis spectra and redox potentials.

Synthesis, characterization and electrochemistry of rhodium(III) complexes of meso-substituted [14]tribenzotriphyrin(2.1.1)

A thermal reaction using a series of [14]tribenzotriphyrins(2.1.1) (TriPs, 1a–d) with Rh2(C8H12)Cl2 provides RhIII–TriP complexes (2a–d) in 40−52% yields. The complexes were characterized by mass spectrometry, UV-visible absorption and 1H NMR spectroscopy. Single crystal X-ray analysis reveals that 2b adopts a dome-shaped conformation. The rhodium(III) ion is coordinated by the three pyrrole nitrogen atoms, two chloride ions and the nitrogen atom of an acetonitrile (CH3CN) solvent molecule. The optical spectra can be assigned using Michls perimeter model. The L and B bands of the 2a–d complexes lie at ca. 600 and 500 nm, respectively, and are markedly red shifted relative to those of 1a–d. A reversible one-electron oxidation and two reversible one-electron reductions are observed in the cyclic voltammograms of 2a–d in CH2Cl2. The redox potentials are consistent with the optical data and the relatively narrow HOMO–LUMO gaps that are predicted in DFT calculations. TD-DFT calculations have been used to assign a third intense spectral band at 375 nm to a higher energy π → π* transition of the [14]tribenzotriphyrin(2.1.1) π-system.

meso-Aryl substituted free-base tripyrrins: preparation and electrochemically induced protonation/deprotonation reactions. Single crystal X-ray analysis of (2,6-diFPh)2TriPyH

Three meso-substituted pyrrole-terminated tripyrrins were isolated for the first time as side products in the synthesis of triarylcorroles and characterized by spectroscopic and electrochemical techniques. The examined compounds are represented as (Ar)2TriPyH, where the TriPy is the conjugated tripyrrin monoanion and Ar a 2,6-diFPh, 2,6-diClPh or 2,4-diClPh substituent. A single crystal X-ray structure of (2,6-diFPh)2TriPyH is also presented. This is the first X-ray structure of a meso-aryl substituted tripyrrin. Each tripyrrin undergoes two reductions and three oxidations in CH2Cl2. The first one-electron addition and first one-electron abstraction lead to formation of π–anion and π–cation radicals with a potential separation between the two processes of 1.71 to 1.76 V. However, both electrogenerated products are unstable and undergo a rapid chemical reaction to give new electroactive species which are identified as the deprotonated and protonated compounds, respectively. The reaction products were characterized by spectroelectrochemistry and comparisons are made with spectroscopic data obtained during base and acid titrations in CH2Cl2.

Dimethyl 3,5-diethyl-1H-pyrrole-2,4-dicarboxyl­ate

The title pyrrole derivative, C12H17NO4, consists of a pyrrole ring with two diagonally attached methoxycarbonyl groups and two diagonally attached ethyl groups. The two carbonyl groups are approximately in the same plane as the pyrrole ring, making dihedral angles of 3.50u2005(19) and 6.70u2005(19)°. In the crystal, adjacent molecules are assembled into dimers in a head-to-head mode by pairs of intermolecular N—H⋯O hydrogen bonds.

2-Ethyl 4-methyl 5-ethyl-3-methyl-1H-pyrrole-2,4-dicarboxyl­ate

The title pyrrole derivative compound, C12H17NO4, was synthesized from methyl 3-oxopentanoate by a Knorr-type reaction and contains a pyrrole ring to which two diagonal alkoxycarbonyl groups and two diagonal alkyl substituents are attached. The methylcarbonyl and ethylcarbonyl substituents are approximately co-planar with the pyrrole ring, making dihedral angles of 5.64u2005(2) and 3.44u2005(1)°, respectively. In the crystal, adjacent molecules are assembled by pairs of N—H⋯O hydrogen bonds into dimers in a head-to-head mode.