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Inductive effect in the aromatic moieties of [2,2]paracyclophane is theoretically analyzed with the density functional theory. The inclusion of different substituents in one of the moieties seems to affect the behaviour of the other. The nature of activating or deactivating groups as substituents reflect known facts on electrophilic aromatic substitutions derived from the inductive effects. The interesting feature in this case is that the phenomenon is transfered from the substituted deck to the other via transannular effects. The strain suffered by the cyclophane molecule is also analyzed.

Aromatic stabilization energies (ASE) for a set of 29 five-membered heteroaromatic compounds are calculated using molecular descriptors such as magnetic susceptibility exaltation (Λ), nucleus-independent chemical shifts (NICS), and electrotopological indices (EI) via linear, quadratic and cubic fitting polynomials. Theoretical estimations compare fairly well with experimental data when three variables multilinear regression equations are employed.

The equilibrium geometries, energies, harmonic vibrational frequencies, stability, and aromaticities for the , E₄Fe, and [Fe(η⁴ - E₄)₂]^2- (E = N, P, As, Sb, and Bi) species are studied using density functional theory (DFT). The ground states of the E₄Fe and [Fe(η⁴ - E₄)₂]^2- systems are predicted to be C_s and D_4d structures, respectively. Orbital analysis indicates that the orbital interactions between the π orbitals of the ligands and the atomic orbitals of the d⁶ iron center are the main bonding scheme for these [Fe(η⁴ - E₄)₂]^2- (D_4d) complexes. The stability of the [Fe(η⁴ - E₄)₂]^2- complexes exhibits the order P > As > Sb > Bi > N for E. On the basis of comparison with the known ferrocene, the NICS analysis confirms that the [Fe(η⁴ - E₄)₂]^2- (D_4d) as well as ferrocene are aromatic. The dissected NICS reveals that the aromaticities of the [Fe(η⁴ - E₄)₂]^2- (D_4d) are primarily attributed to the effects of their E–E π bonds and Fe lone pairs.

Structure and stability of monocyclic dianions (L = CO, N₂, CS) with 6π-electrons, which are isolobal with cyclobutadiene dianion , have been investigated at the B3LYP and CCSD(T) levels of theory. (CH)₃(BL)^2- have non-planar singlet ground states. have planar singlet ground states, while isomers have folded four-membered central rings. Both and have planar ground states, the ground state of has a six-membered ring with two bridging and one terminal CS. Both and have reduced aromaticity compared to as indicated by the less negative nucleus independent chemical shifts (NICS) values. The mono-, di- and trisubstituted (CH)₃(BL)^2-, , and also have reduced aromaticity, while the 1,3-substituted have positive NICS values due to the localization of the negative charge at the ring carbon centers. In addition, the electrostatic stabilization of Li⁺ in favor of the singlet or triplet states depends on the nature of their structures.

Ab initio calculations show that, for the P_n(CH)_4-nPH with n = 0 - 4, the fully optimized structures have decreasing pyramidality at the tri-coordinated phosphorus atom with n. The interaction between the tri-coordinated phosphorus and the carbons–phosphorus π-system with different n, the differences of the structure, electron properties and energy among the phospholes indicate that the ring strain and the delocalization of the lone pair electron of tri-coordinated phosphorus should be responsible for this phenomena. Various aromatic criteria such as geometry, magnetism, and energy show that these molecules are aromatic, and their aromaticities increase with the number of phosphorus atoms in the nonplanar pentagons. The electron circulation in the aromatic molecules can promote the delocalization of the lone pair. In contrast to the B3LYP prediction, we draw some similar conclusions, but the planarity and aromaticity of molecules increase largely at the MP2 lever due to the consideration of electron correlation.

Optical properties of 60 anthraquinone derivatives in ethanol solution were characterized using QSPR multivariance linear regression (MLR). Application of parameter set comprising molecular descriptors and aromaticity indices allowed for highly accurate prediction of λ_max values of considered anthraquinone dyes. Due to the nature of chromophore the main predictive power comes from hardness what obviously is related to π → π* excitation and consequently the LUMO and HOMO energies. However, due to the fact that substituent effect is much more pronounced on π-electron delocalization in central quinonoid ring rather than in vicinal aromatic ones, the aromaticity indices describing properties of this ring can be effectively used in MLR procedure. The highest predictive power of the final models was found for three parameters regression equation comprising hardness, NICS(1)_zz and normalized Pozharsky geometry based index. The values of standard deviation were equal to 12.6 nm and square of adjusted correlation coefficient reached 0.97. This confirmed validity of the hypothesis that in cases in which molecular orbitals responsible for UV-Vis excitation belong, at least partly, to delocalized ring orbitals the UV-Vis imposed π → π* excitations can be related to the extent of π-electron delocalization expressed in term of geometric and magnetic indices.

The peripheral nitrogen of N-confused porphyrins (NCP) can serve as a hydrogen bonding donor and acceptor, and a metal coordination site as well. NCP forms versatile dimers with the assistance of metal ions such as Pd^II, Pt^II, Zn^II, Cd^II, Hg^II, Mn^II and Fe^II. The square-planar divalent metal complexes of C₆F₅-substituted NCP with Cu^II, Ni^II and Pd^II act as efficient anion binding receptors. The doubly N-confused porphyrins (N₂CP), possessing both N and NH at the periphery, form self-assembled 1-D hydrogen bonding networks, whose orientations differ in cis (zigzag) and trans (straight) isomers.

meso-aryl expanded porphyrins, that can be regarded as real homologues of meso-tetraaryl porphyrins, were prepared by our synthetic protocol under the modified Rothemund-Lindsey conditions. This new class of molecules exhibits interesting spectral, electrochemical, and coordination properties. These promising features are emphasized, with a particular attention on the annulenic properties.

The use of a high-field NMR instrument (ν(¹H) = 500 MHz) and 2-dimensional NMR techniques (HMQC, HMBC, ROESY) enabled us to fully assign the ¹H and ¹³C chemical shifts of bonellin dimethyl ester. The β-pyrrolic proton of C-3 appeared as a broad singlet at δ = 8.93, whereas that of C-8 gave a quartet with δ = 8.69 and ⁴J_H-H = |1.28| Hz. The C-2¹ methyl protons appeared as a doublet with δ = 3.55 and ⁴J_H-H = |1.07| Hz, while the C-7¹ methyl protons afforded a doublet with δ = 3.51 and ⁴J_H-H = |1.28| Hz. These results suggest that the β-pyrrolic carbons of ring A belong to the aromatic 18 π-electron [18]diazaannulene delocalization pathway, whereas those of ring B remain outside the aromatic pathway. The broadening of the C-3 β-pyrrolic proton signal can be attributed to the allylic 3-CH - 2¹-CH₃ coupling and the 3-CH - 21-NH coupling. At 330 K, the tautomeric exchange 21-NH_a ⇌ 23-NH_b is fast and only one broad signal at δ = -2.49 is seen for these protons. The ROESY spectrum showed clear correlation signals between the 18²-CH₃ and 17¹-CH₂ protons, the 18²-CH₃ and 17⁴-CH₃ protons, as well as between the 18¹-CH₃ and 17-CH protons. These results are compatible with the previous assignment that the absolute configuration at C-17 is S. Application of spin simulation enabled us to determine the chemical shifts and the ³J_H-H coupling constants of the 17-propionate side-chain. The ³J_H-H-values were used to calculate the populations for the 17¹-17 and 17²-17¹ rotamers. A relatively high population value of 0.41 was found for the 17¹-17 g^--rotamer, whose methoxycarbonylmethyl group points to the C-15 methine-bridge. This was interpreted as explaining the high tendency of bonellin to form anhydrobonellin. The rotational freedoms in the 13-propionate side-chain were studied by measuring the ¹H NMR spectra of the side-chain at temperatures between 300 and 195 K. At 300 K, the 13¹- and 13²-CH₂ proton signals appeared as deceptively simple triplets, which at 195 K were split into complex multiplets. At 195 K, the signal arising from the 13¹-CH₂ protons exhibited more splitting, which indicates that these protons have less rotational freedom than the 13²-CH₂ protons.

The nitration reaction of 2,7,12,17-tetraphenylporphycene has been studied. The use of AgNO₃ and a mixture of acetic acid and 1,2-dichloroethane as a mild nitrating system provides an optimized preparation of 9-nitro-2,7,12,17-tetraphenylporphycene and a regioselective synthesis of 9,20-dinitro-2,7,12,17-tetraphenylporphycene. While 25 min of reaction are needed to obtain the mononitrated compound, 4 h are necessary to yield a mixture of 9,20-dinitro and 9,19-dinitro 2,7,12,17- tetraphenylporphycene in a proportion of 3 to 1. From this mixture, the geometric isomers can be isolated by fractional crystallization. 9-Nitro-2,7,12,17-tetraphenylporphycene can be reduced to the corresponding amino derivative, which is the starting material to obtain 9-(glutaric methylesteramide)- 2,7,12,17-tetraphenylporphycene, a versatile derivative useful for conjugation.

The aromatic nature of porphyrins is commonly attributed to the presence of an [18]annulene substructure. However, this viewpoint has been disputed. Drawing on a range of examples of carbaporphyrinoid systems from the author's own studies, the [18]annulene model is shown to be a self-consistent and insightful approach for considering the aromatic properties of these porphyrin analogs. Benziporphyrins provide a continuum of porphyrinoid structures that range from nonaromatic to highly aromatic species and the presence of 18π electron delocalization pathways provides an excellent explanation for the variations in their properties. The same type of analysis is applied to carbaporphyrins, tropiporphyrins, azuliporphyrins, N-confused porphyrins, pyrazoloporphyrins and dicarbaporphyrins. Nevertheless, these properties might also be explained by a 22π electron delocalization model proposed by Schleyer. The [18]annulene model gains further support from the properties of dideazaporphyrins which cannot take part in this type of 22π electron delocalization but nevertheless retain porphyrin-like aromatic properties. These results support the concept that porphyrins are bridged [18]annulenes.

Vogel discovered a large number of bridged annulene structures, including constitutional porphyrin isomers, heteroporphyrinoid species such as the tetraoxaporphyrin dication, corrole isomers and novel expanded porphyrinoid systems. In this review, the impact of Vogel's work on the author's research program is discussed. Carbaporphyrinoid systems with one or two carbocyclic rings replacing the usual pyrrole subunits have been synthesized and their diatropic characteristics can be related to the presence of 18π electron delocalization pathways within these macrocycles. The preparation of a dideazaporphyrin also provides strong support for the concept, espoused by Vogel, that porphyrins are examples of bridged annulenes. Ring contractions of azuliporphyrins to benzocarbaporphyrins with tert-butyl hydroperoxide in the presence of base have been proposed to involve a Cope rearrangement where a cycloheptatriene unit rearranges to give a norcaradiene-fused carbaporphyrinoid, followed elimination of tert-butyl alcohol, and Vogel's investigations into these types of valence tautomerization processes provides insights into these unusual transformations. In addition, a new class of porphyrin isomers has been synthesized and these porphyrin analogs extend the observations previously made for N-confused porphyrins and Vogel's constitutional isomers.

Iridium metalation of [26]hexaphyrin 1 and [36]octaphyrin 2 gave Ir(III) complex of [26]hexaphyrin 3 and Ir(I) complex of [36]octaphyrin 4, respectively. The complex 3 is an aromatic molecule displaying considerably electron-rich nature while the complex 4 shows weak antiaromatic nature with a small HOMO–LUMO gap of 0.82 eV.

Palladium(II) complexes of aromatic hemiporphyrazines were prepared by the reaction of bis(dibenzylideneacetone)palladium(0) with the corresponding metal-free macrocycles. Single crystal X-ray analysis revealed that a palladium(II) ion was coordinated inside the macrocyclic cavity to form two Pd–C bonds. Electronic properties of the metalloorganic compounds were characterized by NMR, UV-vis-NIR, and magnetic circular dichroism (MCD) spectroscopy.

Synthesis of bona fideN-confused phlorin derivatives through simple chemical reduction of N-confused porphyrin precursors using sodium borohydride, p-toluenesulfonyl hydrazide, etc. is described. Spectroscopic, X-ray diffraction analyses and DFT-assisted calculations of these species support the nonaromatic phlorin electronic structure.

Using the sterically hindered base, 1,8-diazabicyclo[5.4.0]undec-7-ene, for enolization and tert-butyldimethylsilyl-trifluoromethanesulfonate for silylation, chlorophyll (Chl) $a$ produced after 15 min at 0 $°$C in deaerated pyridine solution under argon, after work-up and chromatographic purification on a sucrose column, tert-butyldimethylsilyl-enol ether of Chl $a$ in a yield of 77%. The 13¹-deoxo-13¹,13²-didehydro-chlorophyll $a$, was obtained in a yield of 23% through a reaction sequence, where Chl $a$ was first reduced with sodium borohydride to 13${}^1(R,S)$-hydroxy-Chl $a$, which via demetalation yielded 13${}^1(R,S)$-hydroxypheophytin $a$. In the presence of the sterically hindered base, 1,8-bis(dimethylamino)naphthalene, trifluoroacetylimidazole dehydrated 13${}^1(R,S)$-hydroxypheophytin $a$ to 13¹-deoxo-13¹,13²-didehydro-pheophytin $a$, which after metalation yielded 13¹-deoxo-13¹,13²-didehydro-Chl $a$. Using 1,8-bis(dimethylamino)naphthalene and trifluoroacetylimidazole, the straight conversion of 13${}^1(R,S)$-hydroxy-Chl $a$ to 13¹-deoxo-13¹,13²-didehydro-Chl $a$ was found unsuccessful. The major products were characterized by electronic absorption spectra (UV-vis) and practically completely assigned ¹H and ${}^{13}$C NMR spectra. Some intermediates of the syntheses were also characterized by ESI-TOF mass spectra. Compared with Chl $a$, the macrocyclic ring-current in the synthesized Chl $a$ enol derivatives was found weakened by the expansion of the $\pi$-system to include the isocyclic ring E. Nevertheless, these enol derivatives were still considered to be diamagnetic and aromatic. The possibility of the functional role of the enol derivatives of chlorophyll in photosynthesis is discussed.

Skeletally recombined $N$-fused pentaphyrin bromide underwent aromatic nucleophilic substitution-like reactions when treated with several amine reagents. As a result, an irregular bond such as a vinylene or phenylene linkage was formed between the inside of the macrocycle and the amine nitrogen. This linkage extended the conjugation network and created a “switchback $\pi$-plane” in the product, achieving a new kind of three-dimensional zigzag-like delocalization of $\pi$-electrons.

An alternative route to benziporphyrins has been developed. Reaction of an $\alpha$-unsubstituted pyrrole ethyl ester with isophthaloyl chloride in the presence of aluminum chloride afforded a diketone that underwent selective reduction with diborane to give a benzitripyrrane. Cleavage of the ethyl esters with sodium hydroxide in refluxing ethylene glycol, followed by acid catalyzed condensation with a pyrrole dialdehyde and oxidation with DDQ, generated the targeted benziporphyrin product. Spectrophotometric titration of the benziporphyrin with trifluoroacetic acid (TFA) demonstrated the sequential formation of mono- and dicationic species. At lower dilutions, the free base benziporphyrin showed unusually strong ${}^{\mathrm{\text{5}}}{J}_{\mathrm{\text{HH}}}$ coupling between two adjacent methyl substituents, indicating that the connecting unit has substantial olefinic characteristics.

Even though oxypyriporphyrin, a pyridone-containing porphyrinoid system, has the equivalent coordination core to true porphyrins, its coordination chemistry has been little explored. In this study, the first examples of palladium(II), platinum(II) and silver(II) oxypyriporphyrins have been synthesized. Conventional conditions failed to result in the formation of a platinum(II) complex, but moderate yields were obtained when oxypyriporphyrin was reacted with platinum(II) chloride in refluxing mixtures of DMF and acetic acid containing potassium acetate. The palladium(II) and platinum(II) derivatives were characterized by X-ray crystallography. The structurally analogous carbaporphyrinoid system oxybenziporphyrin was also shown to react with platinum(II) chloride under the same conditions to give a platinum(II) hydroxybenziporphyrin complex. Unlike oxybenziporphyrin, this complex is only weakly diatropic. In the presence of base, the diatropic character was reasserted to afford an aromatic anionic species.

The synthesis and optical, electrochemical, and magnetic properties of nickel(II) complexes of 5,10,20-triaryl-5,15-diazaporphyrin (TriADAP) are reported. Metal-templated cyclization of unsymmetrically substituted nickel(II)–bis(1-amino-9-chloro-5-mesityldipyrrin; mesityl = 2,4,6-trimethylphenyl) complexes afforded the corresponding TriADAPs or 5-aryl-15-benzyl-10,20-dimesityl-5,15-diazaporphyrin, depending on the combination of base and solvent. The latter macrocycle was converted to TriADAP by deprotection of the $N$-benzyl group through Pd/C-promoted hydrogenation. TriADAPs were isolated in both 18$\pi$ (cation) and 19$\pi$ (neutral) forms. The interconversion between these two oxidation states resulted in a distinct change in the optical properties of the DAP $\pi$-system. NMR spectroscopy of the 18$\pi$ TriADAP cations showed that they had aromatic character, whereas EPR spectroscopy of the 19$\pi$ TriADAP showed a highly delocalized electron spin of the $\pi$-radical. The para substituents of the $N$-aryl groups of TriADAPs had a small but distinct impact on their HOMO and LUMO energies. The change in the net charge of one electron was directly reflected in the redox properties of the DAP ring; TriADAP was more easily reduced and less easily oxidized than DAP. The difference in the net charge was also reflected by the shielding of the pyrrolic $\beta$-protons observed in the ${}^{\mathrm{\text{1}}}$H NMR spectra. The present results confirm that TriADAP is a highly promising framework for constructing a new class of azaporphyrin-based materials with 18$\pi$–19$\pi$ redox-switchable optical and magnetic properties.