Synthesis and electrochemistry of β-pyrrole nitro-substituted cobalt(ii) porphyrins. The effect of the NO₂ group on redox potentials, the electron transfer mechanism and catalytic reduction of molecular oxygen in acidic media

Abstract

Four cobalt(II) porphyrins, two of which contain a β-pyrrole nitro substituent, were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds are represented as (TRPP)Co and (NO2TRPP)Co, where TRPP is the dianion of a substituted tetraphenylporphyrin and R is a CH3 or OCH3 substituent on the four phenyl rings of the macrocycle. Two reductions and three oxidations are observed for each compound in CH2Cl2 containing 0.10 M tetra-n-butylammonium perchlorate. The first reduction of the compounds without a nitro substituent is metal-centered and leads to formation of a Co(I) porphyrin which then reacts with the CH2Cl2 solvent to generate a carbon σ-bonded Co(III)-R complex. A further reduction then occurs at more negative potentials to generate an unstable Co(II) σ-bonded compound. In contrast to these reactions, the first reduction of the nitro-substituted porphyrins is macrocycle-centered under the same solution conditions and gives a Co(II) porphyrin π-anion radical product. This reversible electron transfer is then followed at more negative potentials by a second reversible one-electron addition to give a Co(II) dianion. Three reversible one-electron oxidations are also seen for each compound. The first is metal-centered and the next two involve the conjugated π-system of the macrocycle. Each neutral Co(II) porphyrin was also examined as to its catalytic activity for electroreduction of molecular oxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO4. The β-pyrrole nitro-substituted derivatives were shown to be better catalysts than the non-nitro substituted compounds under the utilized experimental conditions.

Synthesis and Properties of NitroHPHAC: The First Example of Substitution Reaction on HPHAC

“…1 H and 13 C-NMR spectra were recorded on JEOL (Akishima city, Japan) AL-400 at 400 MHz or Bruker Biospin (Osaka, Japan) AVANCE III at 500 MHz for 1 H and 100 MHz for 13 C with use of tetramethylsilane (0 ppm) or residual solvent for 1 H (7.26 ppm for CDCl 3 ) and 13 C (77.01 ppm for CDCl 3 ) signals, and if not otherwise noted, all spectra were measured in CDCl 3 at 298 K. Cyclic voltammetry (CV) measurements were performed on a CH Instruments-ALS612B electrochemical analyzer using a standard three-electrode cell consisting of Pt working electrodes, a Pt wire counter electrode, and an Ag/AgNO 3 reference electrode under a nitrogen atmosphere. The potentials were calibrated with ferrocene as an external standard.…”

“…Therefore, nitration reaction is one of the first choices for further derivatization of aromatic compounds. Recently, large π-conjugated materials with a nitro group and their derivatives have attracted continuing attention from many research areas [2][3][4][5][6][7][8][9][10][11][12][13][14].…”

A Structural Model of Nitro‐Porphyrin Dyes Based on Spectroscopy and Density Functional Theory

“…These compounds possess enhanced photonic and electrochemical properties that can be finely tuned by grafting a variety of substituents to their peripheral positions, making them very attractive as components in advanced materials and devices. In particular, nitro substituted porphyrinoids have been investigated as redox catalysts, nonlinear optical materials, biomedical theranostics, and artificial solar energy harvesting devices …”

“…Nitro groups drastically affect the electronic properties of the Por when attached to the β‐pyrrole position or even to the meso‐phenyl rings. The strong electron withdrawing nature of the NO 2 moiety polarizes the Por macrocycle, changing its physical and chemical properties . Conversely, nitro groups can also mediate the assembly of supramolecular systems by directly participating in hydrogen bonding with groups such as imidazole or modulating the complexation of the Por with other species in solution .…”

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

“…We wished to find conditionsw hich might stabilizet he singly oxidizeda nd singly reduced forms of the N-confused porphyrins and we also wished to examine the electrochemistry of these compounds where an equilibrium was not observedb etween two tautomeric forms of the porphyrin.O ne such group of compounds are the N-confused porphyrins havinganitro group bound to the inner carbon atom of the central cavity. [18][19][20] It is known that additiono fo ne or more highly electron-withdrawing NO 2 groups to the meso-o rb-pyrrole position of a" regular" porphyrin will significantly modify its physichemical and electrochemical properties, [21][22][23][24] with the type and magnitude of the effect being dependent in large part on the positiono ft he added nitro substituent. However, to the besto fo ur knowledge, nothing was known about how the inner carbon bound nitro group would affect the reduction and oxidation properties or the electron-transfer mechanism for these types of porphyrins.…”