A series of N-confused free-base meso-substituted tetraarylporphyrins was investigated by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) and added acid or base. The investigated compounds are represented as (XPh)₄NcpH₂, 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 1.65 V 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.

A silver β-octabromo-meso-triarylcorrole has been found to exhibit a strongly saddled geometry, providing the first instance of a strongly saddled corrole complex involving a metal other than copper. The Soret maxima of the Ag octabromocorroles also redshift markedly in response to increasingly electron-donating para substituents on the meso-aryl groups. In both these respects, the Ag octabromocorroles differ from simple Ag triarylcorrole derivatives, which exhibit only mild saddling and substituent-insensitive Soret maxima. These results have been rationalized in terms of an innocent M^III-corrole³⁻ description for the simple Ag corroles and a noninnocent M^II-corrole^.2− description for the Ag octabromocorroles. In contrast, all copper corroles are thought to be noninnocent, while all gold corroles are innocent. Uniquely among metallocorroles, silver corroles thus seem poised on a knife-edge, so to speak, between innocent and noninnocent electronic structures and may tip either way, depending on the exact nature of the corrole ligand.

Invited for the cover of this issue are Abhik Ghosh and his collaborators at UiT (The Arctic University of Norway, Tromsø) and in the United States. The image depicts a silver corrole complex poised on the summit ridge of Mount Sir Alexander, a peak of the Canadian Rockies (image courtesy of Chris Goulet), symbolizing the balance between innocent and noninnocent ligands. Read the full text of the article at 10.1002/chem.201502150.

Two series of copper(I) complexes with diethoxyphosphoryl-substituted 1,10-phenanthroline ligands were synthesized and characterized in the solid state and in solution. The first comprised mixed-ligand Cu^I complexes with phenanthroline and triphenylphosphine. The second series includes bis-chelates with two phenanthroline ligands. According to the X-ray data for the six complexes, the ditopic phenanthroline ligands exhibit bidentate coordination to the copper(I) atom through two nitrogen atoms in both series. Solution equilibria involving different phenanthroline copper(I) species were studied by ¹H and ³¹P NMR spectroscopy, electrochemistry, and spectroelectrochemistry. The solution speciation of these labile complexes is different for these two series and depends on the nature of solvent and the location of the phosphorus substituent on the phenanthroline backbone. Coordinating solvents can replace a bromide, triphenylphosphine, and even a phenanthroline ligand in the inner coordination sphere of the metal center. Copper(I) complexes with α-substituted phenanthrolines easily dissociate even in noncoordinating solvents such as CH₂Cl₂ and CHCl₃. Ligand-exchange reactions leading to less sterically hindered species were observed under the utilized solution conditions. The coordination mode of the phenanthroline chelators does not change under any of the utilized solution conditions, and binding of the phosphoryl group to the metal center was never observed by spectroscopic or spectroelectrochemical methods.

Electrochemical and UV/Vis spectral studies on the quinoxaline compound 2,3-di(2-pyridyl)-6,7-dicyano-1,4-quinoxaline, [(CN)₂Py₂Qx], its metallated derivatives [(CN)₂Py₂QxMCl₂] (M = Pd^II, Pt^II) and their analogues sharing the dicyanopyrazine fragment are presented and discussed. X-ray work on the new quinoxaline complex [(CN)₂Py₂QxPtCl₂] establishes that Pt^II is coordinated to the pyridine N atoms (“py–py” coordination). The spectra of the 1– charged quinoxaline and pyrazine compounds show new intense absorptions in the region of 500–900 nm, and redshifted bands in the 250–400 nm region. By using DFT/time-dependent DFT calculations, the spin density and spectral features of both couples of precursors and Pt^II derivatives were investigated in terms of single-electron excitations between the Kohn–Sham orbitals of the optimized structures in the gas and condensed phases. A detailed comparison is allowed of the experimental and calculated spectral features of the neutral and 1– charged species.

A series of planar and nonplanar free-base β-pyrrole substituted meso-tetraarylporphyrins were characterized by electrochemistry, spectroelectrochemistry, and protonation or deprotonation reactions in neutral, acidic, and basic solutions of CH₂Cl₂. The neutral compounds are represented as H₂(P), in which P represents a porphyrin dianion with one of several different sets of electron-withdrawing or -donating substituents at the messo and/or β-pyrrole positions of the macrocycle. The conversion of H₂(P) to [H₄(P)]²⁺ in CH₂Cl₂ was accomplished by titration of the neutral porphyrin with trifluoroacetic acid (TFA) while the progress of the protonation was monitored by UV/Vis spectroscopy, which was also used to calculate logβ₂ for proton addition to the core nitrogen atoms of the macrocycle. Cyclic voltammetry was performed after each addition of TFA or TBAOH to CH₂Cl₂ solutions of the porphyrin and half-wave potentials for reduction were evaluated as a function of the added acid or base concentration. Thin-layer spectroelectrochemistry was used to obtain UV/Vis spectra of the neutral and protonated or deprotonated porphyrins under the application of an applied reducing potential. The magnitude of the protonation constants, the positions of λ_max in the UV/Vis spectra and the half-wave or peak potentials for reduction are then related to the electronic properties of the porphyrin and the data evaluated as a function of the planarity or nonplanarity of the porphyrin macrocycle. Surprisingly, the electroreduction of the diprotonated nonplanar porphyrins in acid media leads to H₂(P), whereas the nonplanar H₂(P) derivatives are reduced to [(P)]²⁻ in CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). Thus, in both cases an electrochemically initiated deprotonation is observed.

A two-electron oxidation of the Cu^II (9) and Zn^II (12) complexes of tetraphenyltetrabenzoporphyrin (TPTBP) results in the formation of stable antiaromatic [(TPTBP)Cu^II(H₂O)]²⁺⋅2 [SbF₆]⁻ (10) and [(TPTBP)Zn^II(H₂O)₂]²⁺⋅2 [SbF₆]⁻ (13) with 16π electrons on the inner ligand perimeter. X-ray structures of the parent TPTBP complexes, the dications, and singly oxidized species [(TPTBP)Cu^II]^⋅+[SbF₆]⁻ (11) reveal that the use of TPTBP rather than a porphyrin ligand reduces the degree of nonplanarity in the 16π-electron species relative to the parent 18π complex. Significant high-field shifts of the ¹H NMR signals of the outer ring protons and large positive values in calculations of nucleus-independent chemical shifts on the central cavity of the porphyrin ring provide unambiguous evidence for the antiaromatic character of the 16π Zn^II species. A combination of magnetic circular dichroism spectroscopic studies and TD-DFT calculations on both the Zn^II and Cu^II species demonstrates that the main electronic bands of the dicationic species can be readily assigned by using Michl’s 4N perimeter model. Femtosecond transient absorption studies clearly demonstrated that the number of π electrons on the inner ligand perimeter and the configuration of the central metal ion play a critical role in the excited-state relaxation dynamics. Redox potentials for conversion between the 16π, 17π, and 18π systems were measured by cyclic voltammetry in dichloromethane and benzonitrile, and UV/Vis spectra of each oxidation/reduction product were monitored by thin-layer spectroelectrochemistry.

The site of electron-transfer reduction of AuPQ⁺ (PQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quino-xalino[2, 3−b′]porphyrin) and AuQPQ⁺ (QPQ=5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b′:12,13-b′′]porphyrin) is changed from the Au^III center to the quinoxaline part of the PQ macrocycle in the presence of Sc³⁺ in benzonitrile because of strong binding of Sc³⁺ to the two nitrogen atoms of the quinoxaline moiety. Strong binding of Sc³⁺ to the corresponding nitrogen atoms on the quinoxaline unit of ZnPQ also occurs for the neutral form. The effects of Sc³⁺ on the photodynamics of an electron donor–acceptor compound containing a linked Zn^II and Au^III porphyrin ([ZnPQ–AuPQ]PF₆) have been examined by femto- and nanosecond laser flash photolysis measurements. The observed transient absorption bands at 630 and 670 nm after laser pulse irradiation in the absence of Sc³⁺ in benzonitrile are assigned to the charge-shifted (CS) state (ZnPQ^.+–AuPQ). The CS state decays through back electron transfer (BET) to the ground state rather than to the triplet excited state. The BET rate was determined from the disappearance of the absorption band due to the CS state. The decay of the CS state obeys first-order kinetics. The CS lifetime was determined to be 250 ps in benzonitrile. Addition of Sc³⁺ to a solution of ZnPQ–AuPQ⁺ in benzonitrile caused a drastic lengthening of the CS lifetime that was determined to be 430 ns, a value 1700 times longer than the 250 ps lifetime measured in the absence of Sc³⁺. Such remarkable prolongation of the CS lifetime in the presence of Sc³⁺ results from a change in the site of electron transfer from the Au^III center to the quinoxaline part of the PQ macrocycle when Sc³⁺ binds to the quinoxaline moiety, which decelerate BET due to a large reorganization energy of electron transfer. The change in the site of electron transfer was confirmed by ESR measurements, redox potentials, and UV/Vis spectra of the singly reduced products.