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.

[Ag(Sn9-Sn9)]5- and [(η4-Sn9)Ag(η1-Sn9)]7-, as aggregates of spherical aromatic building blocks. Persistence of aromaticity upon cluster gathering

Formation of cluster-based materials requires a fundamental understanding of the resulting cluster aggregation processes. The Sn94- Zintl-ion structure can be viewed as a building block featuring a spherical aromatic species, leading to a cluster gathering upon oxidative coupling and/or mediated by transition metals. Here, we evaluate the spherical aromatic properties of [Sn9-Sn9]6-, [Ag(Sn9-Sn9)]5- and [(η4-Sn9)Ag(η1-Sn9)]7-, as aggregates of two Sn9 building units held together via oxidative coupling and mediated by a Ag(I) transition metal center. Our results from magnetic criteria of aromaticity show that the inherent spherical aromatic characteristics of the parent Sn94- cluster are persistent in the overall aggregate where the enabled shielding cones ascribed to each Sn9 unit are able to interplay between them, leading to an overlap of the shielding regions. Hence, the two approaches for bringing cluster units together are able to retain the inherent spherical aromatic features for each Sn9 unit, leading to a cluster-based dimer where the parent properties remain. Thus, further cluster-based materials can be envisaged from aggregation upon oxidative coupling and/or mediated by transition metals, where the constituent building blocks retain their initial features, useful to guide the formation of more complex cluster-based aggregates.

Magnetically induced ring currents in naphthalene-fused heteroporphyrinoids

The magnetically induced current density of an intriguing naphthalene-fused heteroporphyrin has been studied, using the quantum-chemical, gauge-including magnetically induced currents (GIMIC) method. The ring-current strengths and current-density pathways for the heteroporphyrin, its Pd complex, and the analogous quinoline-fused heteroporphyrin provide detailed information about their aromatic properties. The three porphyrinoids have similar current-density pathways and are almost as aromatic as free-base porphyrin. Notably, we show that the global ring current makes a branch at three specific points. Thus, the global current is composed of a total of eight pathways that include 22 π-electrons, with no contributions from 18-electron pathways.

Synthesis and Characterization of N-Methylporphyrins, Heteroporphyrins, Carbaporphyrins, and Related Systems

MacDonald-type "3 + 1" condensations of an N-methyltripyrrane with a series of dialdehydes afforded a matched set of N-methylporphyrins, N-methylheteroporphyrins, N-methyloxybenziporphyrin, N-methyloxypyriporphyrin, N-methyltropiporphyrin, and a N-methylcarbaporphyrin aldehyde. meso-Unsubstituted heteroporphyrins have been little explored previously, and this strategy was also used to prepare N-unsubstituted 21-oxa-, 21-thia-, and 21-selenaporphyrins. In every case, the N-methylporphyrinoids exhibited weaker, bathochromically shifted UV-Vis absorptions compared to their core unsubstituted congeners. However, proton NMR spectroscopy demonstrated that these derivatives retained strong diamagnetic ring currents and the presence of the internal alkyl substituents had little effect on the global aromatic characteristics. Nevertheless, the UV-Vis spectra of N-methyl-oxybenzi- and N-methyl-oxypyriporphyrins were dramatically altered and gave greatly weakened absorptions. N-Methyl-oxybenzi- and N-methyltropiporphyrins reacted with palladium(II) acetate to give stable palladium(II) complexes, demonstrating that N-alkylation alters the metalation properties for these carbaporphyrinoids. The organometallic derivatives also retained strongly aromatic properties, and the proton NMR spectra showed the N-methyl resonances near -3 ppm. N-Methylcarbaporphyrin-2-carbaldehyde also gave a palladium(II) complex, but this gradually rearranged at higher temperatures to afford a C-methyl complex. The results demonstrate that core alkylation of porphyrinoids greatly alters the reactivity and spectroscopic properties for these systems.

Understanding the structures and aromaticity of heteroporphyrins with computations

Heteroporphyrins are porphyrin derivatives with replacement of the pyrrolic NH moiety by other heteroatom-containing groups, such as PH, AsH, SiH2, O, S, etc. For all studied heteroporphyrins, the macrocycle structure is distorted due to the presence of large heteroatoms. The HOMO-LUMO gap of heteroporphyrins is generally decreased compared to regular porphyrins. Both nucleus independent chemical shifts values and visualized anisotropy of induced current density were computed to describe the aromaticity of heteroporphyrins. The plots of anisotropy of induced current density suggest that the ring current diverged into an outer and an inner pathway at each ring. The current mainly passes through the outer path at the pyrrolic rings with inner hydrogen and through the inner path at the pyrrolic rings without inner hydrogen. In both regular porphyrin and O-substituted heteroporphyrins, the aromatic pathway is mainly contributed by the 22π-electron aromatic route model. Heteroatoms such as PH, AsH, S, Se and Te have little contribution to the aromaticity of heteroporphyrins. In addition, the π conjugation is also interrupted at the CH2 and SiH2 moiety, and the ring current mainly passes through the outer path of the heteroporphyrins with CH2 and SiH2 replacing the pyrrolic NH moiety. Therefore the 18π-[18]annulene model is dominated in PH-, AsH-, S-, Se-, Te-, CH2- and SiH2-substituted heteroporphyrins. These computational studies shed new light on the aromatic characters of heteroporphyrins, and will facilitate the further development of various novel heteroporphyrins.

Alkylation, metalation and ring contraction of tropiporphyrin

Tropiporphyrins are an intriguing class of carbaporphyrinoids that incorporate a cycloheptatriene subunit. Previous investigations have shown that this system acts as a trianionic ligand that can form stable silver(III) complexes. In this work, tropiporphyrin was reacted with excess methyl iodide and potassium carbonate in refluxing acetone to give an internally alkylated derivative. The reaction occurred regioselectively to introduce a methyl group onto the 24-position. Reaction with palladium(II) acetate afforded a palladium(II) complex that retained the [Formula: see text]-methyl substituent. In contrast, reaction of [Formula: see text]-unsubstituted tropiporphyrin with palladium(II) acetate gave two palladium(II) benziporphyrins, one of which possessed a formyl substituent. Although these organometallic derivatives were obtained in relatively low yields, the observation of unexpected ring contraction products demonstrates that the reactivity of the tropiporphyrin system deserves to be further investigated.

Switching between Hückel and Möbius aromaticity: a density functional theory and information-theoretic approach study

Benziporphyrins are versatile macrocycles exhibiting aromaticity switching behaviors. The existence of both Hückel and Möbius (anti)aromaticity has been reported in these systems, whose validity is respectively governed by the [4n + 2] and [4n] π-electron rule on the macrocyclic pathway. Despite the experimental evidence on the floppiness of benziporphyrins, the switching mechanism between Hückel and Möbius structures is still not clear, as well as the factors influencing the stability of the different π-conjugation topologies. For these reasons, we performed a systematic study on A,D-di-p-benzihexaphyrins(1.1.1.1.1.1) with two redox states corresponding to [28] and [30] π-electron conjugation pathways. Whereas benzi[28]hexaphyrin obeys Möbius aromaticity, benzi[30]hexaphyrin follows Hückel aromaticity. The dynamic interconversion between Möbius and Hückel aromaticity is investigated through the rotation of a phenylene ring, which acts as the topology selector. Further analyses of the energy profiles using energy decomposition and information-theoretic approaches provide new insights into conformational stability, aromaticity and antiaromaticity for these species. Strong and opposite cross correlations between aromaticity indexes and information-theoretic quantities were found for the two macrocyclic systems with opposite global aromaticity and antiaromaticity behaviors. These results indicate that Hückel and Möbius aromaticity and antiaromaticity, though qualitatively different, are closely related and can be interchanged, and information-theoretic quantities provide a novel understanding about their relevance. Our present results should provide in-depth insights to appreciate the nature and origin about Möbius (anti)aromaticity and its close relationship with Hückel (anti)aromaticity.

The effect of anion complexation on the aromatic properties of aromatic and antiaromatic porphyrinoids

The effect of anion complexation on magnetically induced current densities and excitation energies of antiaromatic molecular rings has been investigated by calculations on expanded antiaromatic porphyrinoids including orangarin, rosarin, amethyrin and on a theoretically predicted strongly antiaromatic hydrocarbon ring.

Anisotropy of the vorticity tensor as a magnetic indicator of aromaticity

The vorticity vector of the current density J^B, induced in the electron cloud of a molecule by a magnetic field B, is defined by V^B = ∇ × J^B.

Computational study of aromaticity, 1H NMR spectra and intermolecular interactions of twisted thia-norhexaphyrin and its multiply annulated polypyrrolic derivatives

The recently synthesized twisted thia-norhexaphyrin and its multiply annulated polypyrrolic derivatives have been studied computationally. Gauge-including magnetically induced current calculations predict a global nonaromatic character of the initial thia-norhexaphyrin due to the highly-twisted conformation of the macrocycle. Upon the oxidation of the thia-norhexaphyrin four multiply annulated polypyrrolic aromatic macrocycles are formed for which the global aromatic character is confirmed in agreement with experimentally measured ¹H NMR spectra. The calculation of the proton chemical shifts for the studied compounds by direct comparison with the tetramethylsilane standard leads to a significant mean absolute error. At the same time a linear regression procedure for the two selected groups of protons (CH and NH protons) provides much better values of calculated chemical shifts and tight correlation with experiment. The separate consideration of NH protons is motivated by the numerous intermolecular hydrogen bonds in which the protons are involved, which induce considerable upfield shifts, leading to a significant underestimation of the corresponding chemical shifts. Such a selected correlation can be used for accurate estimation of proton chemical shifts of the related porphyrinoids. Bader's theory of Atoms in Molecules has been applied for the studied twisted thia-norhexaphyrin and its multiply annulated polypyrrolic derivatives to characterize intramolecular H-bonds and other non-covalent interactions.

Frequency-dependent current density tensors as density functions of dynamic polarizabilities

Relationships accounting for contributions to the first-order charge density ρ⁽¹⁾ and current density J⁽¹⁾, induced in the electrons of a molecule by a monochromatic plane wave, have been obtained via time-dependent quantum mechanical perturbation theory. Their gauge invariance and invariance in passive translations of the coordinate system have been demonstrated (i) within the (long wavelengths) electric dipole approximation, in which only the electric field contributions to these densities are required, and (ii) within the electric quadrupole approximation, in which nonseparable terms provided by the magnetic field and by the electric field gradient, assumed uniform over the molecular domain, are needed. It is shown that the physical meaning of current density tensors depending on the frequency ω of the monochromatic wave shone on the molecule, and corresponding to derivatives of the current density J⁽¹⁾(r, ω) with respect to components of the perturbing fields, is that of property density. Therefore, frequency-dependent current density tensors can be interpreted as the integrand function in 3d-space integrals defining dynamic molecular response tensors, e.g., electric dipole, mixed electric dipole-magnetic dipole, and electric dipole-quadrupole polarizabilites. Plots of current density tensors are expected to provide important information on the molecular domains giving dominant contributions to these properties.

Local versus global aromaticity in azuliporphyrin and benziporphyrin derivatives

Carbaporphyrinoids afford fascinating examples of competition between local and global aromaticity in conjugated, polycyclic systems. Thus, whereas density functional theory calculations reveal only a modest effect of metal complexation on the current density profiles of true carbaporphyrins and azuliporphyrins, the impact is much greater for benziporphyrins, underscoring a strong competition between local and global aromaticity in the latter system. Furthermore, the calculations shed light on the remarkable efficacy of suitably placed electron-donating substituents on the benzene ring in boosting the global diatropic currents in a metallobenziporphyrin.

Gauge invariance and origin independence of electronic charge density and current density induced by optical fields

Expressions for the first-order polarization charge density ρ ⁽¹⁾ and current density J ⁽¹⁾ induced in a molecule by a monochromatic plane wave, obtained by time-dependent quantum mechanical perturbation theory, have been investigated to assess their gauge invariance and independence of the coordinate system in passive and active translations. The conditions arrived at show that, within the (long wavelengths) dipole approximation, only the electric contributions to these densities are needed to rationalize the phenomenology. To the next higher quadrupole approximation, assuming that the magnetic field and the electric field gradient are uniform over the molecular dimensions, corresponding contributions to ρ ⁽¹⁾ and J ⁽¹⁾ are considered. It has been found that total densities are independent of the origin, whereas the contributions from electric and magnetic fields are not separately invariant. A magnetic contribution to J ⁽¹⁾, which is by itself origin independent, can be defined by means of an approach based on continuous translation of the origin of the coordinate system.

Electronic and magnetic properties of the [Ni(salophen)]: An experimental and DFT study

The effect of the coordination of a Ni(II) ion on the electronic and magnetic properties of the ligand salophen were experimentally and theoretically evaluated. The complex [Ni(salophen)] was synthesized and characterized through FTIR and an elemental analysis. Spectral data obtained using DMSO as a solvent showed that the ligand absorption profile was significantly disturbed after the coordination of the metal atom. In addition to a redshift of the salophen ligand absorption bands, mainly composed by π → π^∗electronic transitions, additional bands of around 470 nm were observed, resulting in a partial metal-to-ligand charge transfer. Furthermore, a significant increment of its band intensities was observed, favoring a more intense absorption in a broader range of the visible spectrum, which is a desired characteristic for applications in the field of organic electronics. This finding is related to an increment of the planarity and consequent electron delocalization of the macrocycle in the complex, which was estimated by the calculation of the current strengths at the PBE0/cc-pVTZ (Dyall.v3z for Ni(II)) level.

Rational Synthesis of Antiaromatic 5,15-Dioxaporphyrin and Oxidation into β,β-Linked Dimers

5,15-Dioxaporphyrin was synthesized for the first time by a nucleophilic aromatic substitution reaction of a nickel bis(α,α'-dibromodipyrrin) complex with benzaldoxime, followed by an intramolecular annulation of the α-hydroxy-substituted intermediate. This unprecedented molecule is a 20π-electron antiaromatic system, in terms of Hückel's rule of aromaticity, because lone pair electrons of oxygen atoms are incorporated into the 18π-electron conjugated system of the porphyrin. A theoretical analysis based on the gauge-including magnetically induced current method confirmed its antiaromaticity and a dominant inner ring pathway for the ring current. The unique reactivity of 5,15-dioxaporphyrin forming a β,β-linked dimer upon oxidation was also revealed.

Calculations of current densities and aromatic pathways in cyclic porphyrin and isoporphyrin arrays

Magnetically induced current density susceptibilities have been studied for a number of cyclic ethyne- and butadiyne-bridged porphyrin and isoporphyrin arrays. The current density susceptibilities have been calculated using the gauge-including magnetically induced current (GIMIC) method, which is interfaced to the TURBOMOLE quantum chemistry code. Aromatic properties and current pathways have been analyzed and discussed by numerical integration of the current density susceptibilities passing selected chemical bonds yielding current strength susceptibilities. Despite the interrupted π-framework, zinc(ii) isoporphyrin sustains a ring current of ca. 10 nA T^-1. Porphyrin and isoporphyrin dimers sustain a significant current strength at the linker, whereas the larger porphyrinoid arrays sustain mainly local ring currents. Isoporphyrin dimers with saturated meso carbons have strong net diatropic ring-current strengths of 20 nA T^-1 fulfilling Hückels aromaticity rule. Porphyrin trimers and tetramers exhibit almost no current strength at the linker. The porphyrin moieties maintain their strong net diatropic ring current.

The aromaticity of verdazyl radicals and their closed-shell charged species

The aromaticity of fourteen 3-oxo-verdazyl (1–8) and Kuhn verdazyl (9–14) radicals with different substituents has been investigated computationally using the gauge-including magnetically induced current-density (GIMIC) method.

New electron delocalization tools to describe the aromaticity in porphyrinoids

There are several possible pathways in the macrocycle of large porphyrinoids and, among aromaticity indices, only AV_minis capable of recognizing the most aromatic one.

Optical and magnetic properties of antiaromatic porphyrinoids

Magnetic and spectroscopic properties of a number of formally antiaromatic carbaporphyrins, carbathiaporphyrins and isophlorins with 4n π electrons have been investigated at density functional theory and ab initio levels of theory.