Norcorrole as a Delocalized, Antiaromatic System

Changing aromatic properties through stacking: the face-to-face dimer of Ni(II) bis(pentafluorophenyl)norcorrole

Nuclear magnetic resonance (NMR) shielding constants have been calculated for Ni(II) bis(pentafluorophenyl)norcorrole and its face-to-face stacked dimer at the Hartree-Fock (HF), second-order Møller-Plesset perturbation theory (MP2), complete-active-space self-consistent-field (CASSCF) levels as well as at density functional theory (DFT) levels using several functionals. The calculated 1H NMR shielding constants agree rather well with the experimental ones. The shielding constants of N and Ni calculated at DFT, HF, and MP2 levels differ from those obtained in the CASSCF calculations due to near-degeneracy effects at the Ni atom. The calculated magnetically induced current densities show that the monomer is antiaromatic, sustaining a strong global paratropic ring current, and the dimer is aromatic, sustaining a strong diatropic ring current. Qualitatively the same current density is obtained at the employed levels of theory. The most accurate ring-current strengths are probably obtained at the MP2 level. The aromatic dimer has a short intermolecular distance of less than 3 Å. The intermolecular interaction changes the nature of the frontier orbitals leading to a formal double bond between the norcorrole macrocycles.

Tetraquinolines; four linked quinoline units or porphyrinoids

Tetraquinolines (TEQs) have been recently synthesized and proposed to be a new member of the porphyrinoid family with highly distorted, nonplanar, geometries. In this contribution by studying several molecules, closely related to TEQs, we have suggested that the origin of the nonplanarity of TEQs and their counterparts is a combination of steric strain and the propensity of the molecules to avoid antiaromaticity. The tendency of TEQs to coordinate with doubly charged metal ions can be interpreted in terms of their transition from potential antiaromaticity to nonaromaticity. Even metal-coordinated TEQs do not sustain diatropic ring currents. Although full planarization is not possible because of steric strain, doubly oxidized TEQs and their counterparts sustain moderate global diatropic ring currents and partially planarize. The nature of current density in the molecules is studied in the light of Steiner-Fowler selection rules.

Magnetic Antiaromaticity─Paratropicity─Does Not Necessarily Imply Instability

Magnetically induced ring currents are a conventional tool for the characterization of aromaticity. Dia- and paratropic currents are thought to be associated with stabilization (aromaticity) and destabilization (antiaromaticity), respectively. In the present work, I have questioned the validity of the paratropic currents as a measure of antiaromaticity among monocyclic hydrocarbons. I have shown that while reduced/oxidized radical ions of hydrocarbons sustain strong paratropic currents, they often gain extra stabilization via cyclic conjugation compared to their acyclic counterparts.

Multicenter interactions and ligand field effects in platinum(II) tripyrrindione radicals

The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum(II) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert-butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt(II) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand (i.e., aqua vs. isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt(II)-Pt(II) contact in the case of the aqua complex. Overall, the properties of Pt(II) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.

The Effect of Hydrogenation on the Contest between Aromaticity and Antiaromaticity in Norcorrole

Magnetic shielding studies demonstrate that successive hydrogenation of Ni^II norcorrole (NiNc), a stable molecule combining aromatic and antiaromatic features, first weakens and then eliminates the central antiaromatic region, even though the NiNc antiaromatic "core", a 14-membered conjugated cycle with 16 π electrons, is formally preserved throughout the H₂ NiNc-H₈ NiNc series. The differences between aromatic and non-aromatic isotropic shielding distributions and nucleus-independent chemical shift (NICS) values in these hydrogenated porphyrin analogues are highlighted by comparing the results for the members of the H₂ NiNc-H₈ NiNc series to those for the aromatic Ni^II porphyrin complex. The results strongly support the unexpected and counterintuitive conclusion that H₈ NiNc will be nonaromatic, without even a trace of antiaromaticity. Based on these findings, H₈ NiNc is predicted to be the most stable member of the H₂ NiNc-H₈ NiNc series.

Origin of the Rarely Reported High Performance of Mn-doped Carbon-based Oxygen Reduction Catalysts

Recent efforts to develop durable high-performance platinum-group metal (PGM)-free oxygen reduction reaction (ORR) electrocatalysts have focused on Fe- and Co-based molecular and pyrolyzed catalysts. While Mn-based catalysts have advantages of lower toxicity and higher durability, their activity has been generally poor. Nevertheless, several examples of high-performance Mn-based catalysts have been reported. Thus, it is necessary to understand why Mn-based materials much more rarely show high catalytic ORR performance and to determine the factors that can lead to the achievement of such high performance in these rare cases. We have studied the effects of the changes in the macrocycle structure, axial ligand, distance between the active sites, interactions with the dopant N atoms and the presence of an extended carbon network on the ORR catalysis of various Mn-, Fe-, and Co-based systems through the comparison of the adsorption energies of the ORR intermediates. We find that the sensitivity to the local environment changes is the largest for Mn and is the smallest for Co, with Fe between Mn and Co. Our results showed that the strong binding of OH by Mn and the strong sensitivity of the Mn to the modification of its environment necessitate a precise combination of local environment changes to achieve a high onset potential (V_onset ) in Mn-based catalysts. By contrast, the weaker binding of OH by Fe and Co and their weaker sensitivity to local environment changes lead to a wide variety of local environments with favorable catalytic activity (V_onset >0.7 V) for Co- and Fe-based systems. This explains the scarcity of reported Mn-based pyrolyzed catalysts and suggests that precise material synthesis and engineering of the active site can achieve high-performance Mn-based ORR electrocatalysts with high activity and durability.

Fine-Tuning of Nonlinear Optical Contrasts of Hexaphyrin-Based Molecular Switches Using Inverse Design

In the search for new nonlinear optical (NLO) switching devices, expanded porphyrins have emerged as ideal candidates thanks to their tunable chemical and photophysical properties. Introducing meso-substituents to these macrocycles is a successful strategy to enhance the NLO contrasts. Despite its potential, the influence of meso-substitution on their structural and geometrical properties has been scarcely investigated. In this work, we pursue to grasp the underlying pivotal concepts for the fine-tuning of the NLO contrasts of hexaphyrin-based molecular switches, with a particular focus on the first hyperpolarizability related to the hyper-Rayleigh scattering (β_HRS). Building further on these concepts, we also aim to develop a rational design protocol. Starting from the (un)substituted hexaphyrins with various π-conjugation topologies and redox states, structure-property relationships are established linking aromaticity, photophysical properties and β_HRS responses. Ultimately, inverse molecular design using the best-first search algorithm is applied on the most favorable switches with the aim to further explore the combinatorial chemical compound space of meso-substituted hexaphyrins in search of high-contrast NLO switches. Two definitions of the figure-of-merit of the switch performance were used as target objectives in the optimization problem. Several meso-substitution patterns and their underlying characteristics are identified, uncovering molecular symmetry and the electronic nature of the substituents as the key players for fine-tuning the β_HRS values and NLO contrasts of hexaphyrin-based switches.

Disclosure of Ground-State Zimmerman-Möbius Aromaticity in the Radical Anion of [6]Helicene and Evidence for 4π Periodic Aromatic Ring Currents in a Molecular "Metallic" Möbius Strip

In 1966, Zimmerman proposed a type of Möbius aromaticity that involves through-space electron delocalization; it has since been widely applied to explain reactivity in pericyclic reactions, but is considered to be limited to transition-state structures. Although the easily accessible hexahelicene radical anion has been known for more than half a century, it was overlooked that it exhibits a ground-state minimum and robust Zimmerman-Möbius aromaticity in its central noose-like opening, becoming, hence, the oldest existing Möbius aromatic system and with smallest Möbius cycle known. Despite its overall aromatic stabilization energy of 13.6 kcal mol-1 (at B3LYP/6-311+G**), the radical also features a strong, globally induced paramagnetic ring current along its outer edge. Exclusive global paramagnetic currents can also be found in other fully delocalized radical anions of 4N+2 π-electron aromatic polycyclic benzenoid hydrocarbons (PAH), thus questioning the established magnetic criterion of antiaromaticity. As an example of a PAH with nontrivial topology, we studied a novel Möbius[16]cyclacene that has a non-orientable surface manifold and a stable closed-shell singlet ground state at several density functional theory levels. Its metallic monoanion radical (0.0095 eV band gap at HSE06/6-31G* level) is also wave-function stable and displays an unusual 4π-periodic, magnetically induced ring current (reminiscent of the transformation behaviour of spinors under spatial rotation), thus indicating the existence of a new, Hückel-rule-evading type of aromaticity.

PorphyStruct: A Digital Tool for the Quantitative Assignment of Non-Planar Distortion Modes in Four-Membered Porphyrinoids

PorphyStruct, a new digital tool for the analysis of non‐planar distortion modes of different porphyrinoids, and its application to corrole structures is reported. The program makes use of the normal‐coordinate structure decomposition technique (NSD) and employs sets of normal modes equivalent to those established for porphyrins in order to describe the out‐of‐plane dislocation pattern of perimeter atoms from corroles, norcorroles, porphycenes and other porphyrinoids quantitatively and in analogy to the established terminology. A comparative study of 17 porphyrin structures shows very similar results to the original NSD analysis and no systematic error. Application to corroles is successful and reveals the necessity to implement an extended basis of normal modes for a large share of experimental structures. The results frequently show the concomitant occurence of several modes but remain interpretable. For group XI metal corroles the phenomenon of supersaddling was unravelled, allowing for more in‐depths discussions of structure‐function correlations.

Ligand-Centered Triplet Diradical Supported by a Binuclear Palladium(II) Dipyrrindione

Oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. Although the stabilization of radicals within oligopyrrolic π systems is more common for macrocyclic ligands, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition-metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged binuclear complex in the presence of water or triethylamine. Electrochemical, spectroelectrochemical, and computational analyses of the binuclear complex indicate the accessibility of two quasi-reversible ligand-centered reduction processes. The product of a two-electron chemical reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltocenium salt features a folded dianionic complex that maintains the hydroxo bridges between the divalent palladium centers. X-band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a diradical with spin density almost entirely located on the two dipyrrindione ligands. As established from the EPR temperature dependence, the associated exchange coupling is weak and antiferromagnetic (J ≈ −2.5 K), which results in a predominantly triplet state at the temperatures at which the measurements have been performed.

The coordination and redox chemistry of the dipyrrindione scaffold, which is found in several heme metabolites, is investigated in heteroleptic palladium(II) complexes. The bis(aqua) complex undergoes a deprotonation-driven dimerization to form a hydroxo-bridged binuclear species. Crystallographic, electrochemical, and spectroscopic data, as well as computational analysis, demonstrate that a two-electron reduction of the binuclear complex leads to a diradical dianion with spin density delocalized over the two dipyrrindione ligands.

Synthesis and Properties of an 18π Aromatic Norcorrole P(V) Complex

[16]Norcorrole is an antiaromatic ring-contracted porphyrinoid. Here, we show that the phosphorus insertion of the free-base [16]norcorrole switches its antiaromaticity to aromaticity. The treatment of a free-base meso-dimesityl[16]norcorrole with phosphorus tribromide in pyridine afforded an [18]norcorrole P(V) complex, which exhibited porphyrin-like absorption and fluorescence spectra. The phosphorus center adopted a distorted tetragonal pyramidal coordination geometry. The distinct aromatic nature of the [18]norcorrole P(V) complex was corroborated both experimentally and theoretically.

A Volumetric Analysis of the 1H NMR Chemical Shielding in Supramolecular Systems

The liquid state NMR chemical shift of protons is a parameter frequently used to characterize host-guest complexes. Its theoretical counterpart, that is, the 1H NMR chemical shielding affected by the solvent (1H CS), may provide important insights into spatial arrangements of supramolecular systems, and it can also be reliably obtained for challenging cases of an aggregation of aromatic and antiaromatic molecules in solution. This computational analysis is performed for the complex of coronene and an antiaromatic model compound in acetonitrile by employing the GIAO-B3LYP-PCM approach combined with a saturated basis set. Predicted 1H CS values are used to generate volumetric data, whose properties are thoroughly investigated. The 1H CS isosurface, corresponding to a value of the proton chemical shift taken from a previous experimental study, is described. The presence of the 1H CS isosurface should be taken into account in deriving structural information about supramolecular hosts and their encapsulation of small molecules.

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

The nucleophilic substitution of aromatic moieties (SNAr) has been known for over 150 years and found wide use for the functionalization of (hetero)aromatic systems. Currently, several "types" of SNAr reactions have been established and notably the area of porphyrinoid macrocycles has seen many uses thereof. Herein, we detail the SNAr reactions of seven types of porphyrinoids with differing number and type of pyrrole units: subporphyrins, norcorroles, corroles, porphyrins, azuliporphyrins, N-confused porphyrins, and phthalocyanines. For each we analyze the substitution dependent upon: a) the type of nucleophile and b) the site of substitution (α, β, or meso). Along with this we evaluate this route as a synthetic strategy for the generation of unsymmetrical porphyrinoids. Distinct trends can be identified for each type of porphyrinoid discussed, regardless of nucleophile. The use of nucleophilic substitution on porphyrinoids is found to often be a cost-effective procedure with the ability to yield complex substituent patterns, which can be conducted in non-anhydrous solvents with easily accessible simple porphyrinoids.

Friedel–Crafts acylation of antiaromatic norcorroles: electronic and steric modulation of the paratropic current

The first ever Friedel–Crafts acylation of an antiaromatic macrocycle was performed for norcorrolatonickel(ii) reacting with aromatic or aliphatic carboxylic acid chlorides in the presence of AlCl₃.

Norcorrole: Aromaticity and Antiaromaticity in Contest

Magnetic shielding studies demonstrate that nickel norcorrole (NiNc) and norcorrole (H₂Nc) provide unusual examples of stable molecules with high antagonistic levels of antiaromaticity and aromaticity: Both incorporate an antiaromatic "core", a 14-membered cyclic conjugated subsystem with 16 π electrons, surrounded by an aromatic "halo" in the form of a ring of either 14 atoms and 14 π electrons with a new type of homoconjugation (NiNc), or 18 atoms with 18 π electrons (H₂Nc).

Dipyrrin based metal complexes: reactivity and catalysis

Sometimes named half-porphyrins, bis-pyrrolic dipyrrin ligands endow their metal complexes with unique properties such as the potential to functionalize the heterocyclic backbone or the meso position and the ability to catalyze interesting chemical transformations. Thus, strategies towards the derivatization of or at the meso group and the use of dipyrrin metal complexes for the formation of a broad range of polypyrrolic derivatives such as 2,2'-bis-dipyrrins, nor-/hetero-corroles and porphynoids have been elaborated. Furthermore, the chelating ability of dipyrrins and the possibility of modifying their steric and electronic characteristics by functionalization can be exploited for the development of numerous complexes featuring appealing properties. Hence, C-H activation/amination, polymerization or oxidation reactions can be catalyzed by dipyrrin metal complexes and classical reagents such as Grignard species, Rh-based or Suzuki-Miyaura catalysts have been revisited by incorporation of dipyrrins in the coordination sphere of the metal cations. This contribution aims to review and illustrate all these aspects, highlighting the potential of these complexes for the design and synthesis of valuable organic compounds and metallo-organic architectures.

Meso-1,4-phenylene bridged nickel norcorrole dimer

Nickel norcorrole dimer 3 linked with a phenylene bridge was prepared by a nickel-mediated homolytic cross-coupling of meso-phenylene bisdipyrrin nickel(II) dimer 2b. The structures of 2b and 3 were elucidated by X-ray diffraction analysis. Each nickel norcorrole fraction of the nickel norcorrole dimer 3 exposed a bowl-like structure, which conserved anti-conformation. Structural distinctions between the monomeric and dimeric nickel norcorroles were established by vibrational spectroscopic analysis and assessed with density functional theory calculations. Enhanced flexibility of the norcorrole planes in solution states of monomeric nickel norcorrole was compromised to that of bowl-shaped norcorroles in the solid states of nickel norcorrole dimer 3, where C[Formula: see text] was the formulated conformation. The deformation of the planarity derived from the objection of laser pulses effectuated vibration shifts of specific Raman-active motions toward a higher-frequency region, as associating with a paratropic nature of the [Formula: see text]-electron delocalization circuit of norcorrole. Computational simulation exposed a reliable drift of the Raman frequencies.

Relativity or aromaticity? A first-principles perspective of chemical shifts in osmabenzene and osmapentalene derivatives

The topology of the magnetically induced current density in osmabenzene suggests that the molecule is a novel type of Craig–Möbius aromatic system.

Main-Group-Element Isophlorin Complexes Revisited: The Question of a Subvalent Central Atom

A careful density functional theory reexamination of the geometric and electronic structures of reduced main-group porphyrin complexes E(Por)L₂ (E = Si or Ge; L = pyridine or tetrahydrofuran), B₂(Por), and C₂(Por) has confirmed these as pure isophlorin derivatives with normal-valent coordinated central atoms. Only axially unligated Ge(Por) and the dications [B₂(Por)]²⁺ and [C₂(Por)]²⁺ feature aromatic porphyrin ligands. The calculations faithfully reproduce the strong bond-length alternation along the outer rim of the macrocycle in the reduced complexes, consistent with antiaromatic character, as well as much stronger ruffling in the reduced group 14 complexes relative to their nonreduced counterparts such as E(Por)X₂ (E = Si or Ge; X = F or Cl). The latter is thought to reflect the lower barrier to nonplanar deformation for the antiaromatic systems. In addition, unlike B₂(Por) and its dication, which are planar, C₂(Por) and its dication are predicted to be strongly ruffled, reflecting the smaller size of the central C₂ unit. The calculations also predict characteristically low ionization potentials and singlet-triplet gaps for the antiaromatic complexes. A brief exploratory study of the as-yet-unknown group 15 complexes E(TPP)(Ph)(py), where E = P and As, also indicated an antiaromatic isophlorin macrocycle coordinated to a pentavalent group 15 center.