Spectroscopic and electrochemical characterization of solutions and films of a new redox couple: Co(II)/Co(III) N-confused porphyrin

Designing N-Confused Metalloporphyrin-Based Covalent Organic Frameworks for Enhanced Electrocatalytic Carbon Dioxide Reduction

Electrochemical conversion of carbon dioxide (CO2 ) into value-added products is promising to alleviate greenhouse gas emission and energy demands. Metalloporphyrin-based covalent organic frameworks (MN4 -Por-COFs) provide a platform for rational design of electrocatalyst for CO2 reduction reaction (CO2 RR). Herein, through systematic quantum-chemical studies, the N-confused metallo-Por-COFs are reported as novel catalysts for CO2 RR. For MN4 -Por-COFs, among the ten 3d metals, M = Co/Cr stands out in catalyzing CO2 RR to CO or HCOOH; hence, N-confused Por-COFs with Co/CrN3 C1 and Co/CrN2 C2 centers are designed. Calculations indicate CoNx Cy -Por-COFs exhibit lower limiting potential (-0.76 and -0.60 V) for CO2 -to-CO reduction than its parent CoN4 -Por-COFs (-0.89 V) and make it feasible to yield deep-reduction degree C1 products CH3 OH and CH4 . Electronic structure analysis reveals that substituting CoN4 to CoN3 C1 /CoN2 C2 increases the electron density on Co-atom and raises the d-band center, thus stabilizing the key intermediates of the potential determining step and lowering the limiting potential. For similar reason, changing the core from CrN4 to CrN3 C1 /CrN2 C2 lowers the limiting potential for CO2 -to-HCOOH reduction. This work predicts N-confused Co/CrNx Cy -Por-COFs to be high-performance CO2 RR catalyst candidates. Inspiringly, as a proof-of-concept study, it provides an alternative strategy for coordination regulation and theoretical guidelines for rational design of catalysts.

Organometallic chemistry confined within a porphyrin-like framework

The world of modified porphyrins changed forever when an N-confused porphyrin (NCP), a porphyrin isomer, was first published in 1994. The replacement of one inner nitrogen with a carbon atom revolutionised the chemistry that one is able to perform within the coordination cavity. One could explore new pathways in the organometallic chemistry of porphyrins by forcing a carbon fragment from the ring or an inner substituent to sit close to an inserted metal ion. Since the NCP discovery, a series of modifications became available to tune the coordination properties of the cavity, introducing a fascinating realm of carbaporphyrins. The review surveys all possible carbatetraphyrins(1.1.1.1) and their spectacular coordination and organometallic chemistry.

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

Electrochemistry of N-confused inner amino-substituted free-base tetraarylporphyrins in nonaqueous media

Four N-confused free-base tetraarylporphyrins containing an amino group on the inner carbon of the inverted pyrrole were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in dichloromethane and N,N-dimethylformamide containing 0.1 M tetra-[Formula: see text]-butylammonium perchlorate as supporting electrolyte. The examined compounds are represented as (NH[Formula: see text](Ar)[Formula: see text]NcpH[Formula: see text], where “Ncp” is the N-confused porphyrin macrocycle and Ar is a [Formula: see text]-CH[Formula: see text]OPh, [Formula: see text]-CH[Formula: see text]Ph, Ph or [Formula: see text]-ClPh substituent on each meso-position of the macrocycle. Each porphyrin undergoes multiple irreversible reductions and oxidations. The first one-electron addition and first one-electron abstraction are located on the porphyrin [Formula: see text]-ring system to give [Formula: see text]-anion and [Formula: see text]-cation radicals with a HOMO–LUMO gap ranging from 1.21 to 1.34 V in CH[Formula: see text]Cl[Formula: see text] and 1.06 to 1.12 V in DMF. Both electrogenerated products are unstable and undergo a rapid chemical reaction to give new electroactive species which can be further reduced at more negative potentials or further oxidized at more positive potentials. The effects of the inner amino-substituent on the UV-vis spectra and electrochemical properties of the porphyrin are discussed, and comparisons made to data in the literature for both structurally related N-confused porphyrins containing an inner carbon NO[Formula: see text] group and derivatives without the inner substituent.

Tautomerism-Induced Cis-Trans Isomerization of Pyridylethenyl N-Confused Porphyrin

Pyridylethenyl-substituted N-confused porphyrins (NCPs) were synthesized, and their cis-trans isomerization was studied. Among four possible isomers, trans-3H and cis-2H types of structures, of which aromaticity and absorption/emission properties differ largely, were isolated. The cis-isomer was largely stabilized by the intramolecular hydrogen bonding between the pyrrolic-NH and the pyridinic-N in the vicinity. The thermal cis-trans isomerization proceeded even at 30 °C, which was significantly accelerated by the pyridine added to the system. The kinetic studies revealed that the isomerization reaction was second-order and the activation energy of the thermal isomerization from cis to trans isomer was ΔG₀^⧧_cis→trans = 35.7 kcal/mol at 298 K, which is significantly smaller than that of Ni complex (42.3 kcal/mol). An intermolecular proton transfer induced cis-trans isomerization mechanism was proposed.

N-confusedmeso-tetraaryl-substituted free-base porphyrins: determination of protonation and deprotonation constants in nonaqueous media

The protonation and deprotonation reactions for a series N-confused meso-tetraaryl-substituted free-base porphyrins containing electron-donating or electron-withdrawing substituents was monitored in CHCl3and DMF by UV-visible spectroscopy during titrations with trifluoroacetic acid or tetra-n-butylammonium hydroxide. The spectroscopic data was also used to calculate equilibrium constants for these reactions. The examined compounds are represented as (XPh)4NCPH2, where "NCP" represents the N-confused porphyrin π-conjugated macrocycle and X is a CH3O , CH3, H or Cl para-substituent on the four meso-phenyl rings (Ph) of the compound. The porphyrins can exist in two tautomeric forms depending upon the solvent and each tautomer undergoes two stepwise protonation reactions leading to formation of the mono- and bis-protonated porphyrins, [(XPh)4NCPH3]+and [(XPh)4NCPH4]2+. A single step deprotonation is observed for the same compounds in DMF and the product is assigned as [(XPh)4NCPH]-. Comparisons are made between UV-visible spectra of the protonated, neutral and deprotonated forms of the porphyrin and the effect of the porphyrin ring substituents and tautomeric form of the neutral porphyrin on the UV-visible spectra and protonation constants is discussed along with data from DFT calculations.

Monomeric and polymeric copper and zinc tripyrrins

Neutral transition metal complexes of different alpha,omega-dimethyltripyrrins TrpyMX with M = Cu(II) and Zn(II) have been prepared with a variety of anionic halogeno and pseudohalogeno ligands X, and have been studied with respect to coordination modes and structural distortion. Only four- and five-coordinate species have been observed throughout the series. All four-coordinate species display unstrained, but distorted tetrahedral or strained and distorted square-planar coordination environments for zinc(II) and copper(II) species, respectively, thus following the expectations from simple ligand field arguments. Five-coordinate species do not form easily and were observed either in donor solvents or in the solid as 1D coordination polymers with distorted trigonal-bipyramidal coordination and different topologies.

Cyanide-Promoted Demetalation of TrpyNiNCO: A Route to Sensitive Metallotripyrrins of CoII, FeII, and MnII

Etheral solutions of free base tripyrrins (HTrpy) were prepared by treatment of nickel isocyanate complexes (TrpyNiNCO) with excess cyanide. From these solutions sensitive metallotripyrrins with cobalt(II), iron(II), and manganese(II) ions (TrpyMX) and with a choice of external ligands X could be obtained in pure, crystalline form. Four cobalt and one iron chelate were characterized by X-ray crystallography. Tetracoordinate cobalt(II) species with X = I, NCO, and NCS displayed unstrained tetrahedral coordination geometries, whereas the pentacoordinate TrpyCoNO3 with the O,O-nitrato ligand narrows a trigonal bipyramidal coordination. TrpyFeNCO undergoes a redox-transformation to (TrpyFeNCO)2O upon crystallization and was structurally characterized as this with an almost linear Fe-O-Fe subunit. Donor association was studied by UV-vis spectroscopy employing different solvents and showed that TrpyMnX and TrpyFeX species are very prone to the formation of pentacoordinate species, whereas TrpyCoX compounds have an intermediate tendency to do so. Nevertheless, complex fragments of all three metal ions form 1D coordination polymers with dicyanamido ligands, which were investigated by means of IR and SQUID measurements.