Modular Synthesis of Structurally Diverse Azulene-Embedded Polycyclic Aromatic Hydrocarbons by Knoevenagel-Type Condensation

The research interest in azulene-embedded polycyclic aromatic hydrocarbons (PAHs) has significantly increased recently, but the lack of efficient synthetic strategies impedes the investigation of their structure-property relationships and further opto-electronic applications. Here we report a modular synthetic strategy towards diverse azulene-embedded PAHs by a tandem Suzuki coupling and base-promoted Knoevenagel-type condensation with good yields and great structural versatility, including non-alternant thiophene-rich PAHs, butterfly- or Z-shaped PAHs bearing two azulene units, and the first example of a two-azulene-embedded double [5]helicene. The structural topology, aromaticity and photophysical properties were investigated by NMR, X-ray crystallography analysis and UV/Vis absorption spectroscopy assisted by DFT calculations. This strategy provides a new platform for rapidly synthesizing unexplored non-alternant PAHs or even graphene nanoribbons with multiple azulene units.     

Reversible Single Electron Redox Steps Convert Polycycles with a C3P3 Core to a Planar Triphosphinine

Reaction of the imidazolium-stabilized diphosphete-diide IDP with trityl phosphaalkyne affords a mixture which contains the molecules 1 a and 1 b with a central C₃P₃ core, which formally carries a two-fold negative charge. In order to avoid the formation of an antiaromatic 8π electron system within a conjugated dianionic six-membered [C₃P₃]²⁻ ring, 1 a adopts a bicyclic [3.1.0] and 1 b a tricyclic [2.2.0.0] structure which are in a dynamic equilibrium. 1 a , b can be reversibly oxidized to a triphosphinine dication [ 5 ]²⁺ with a central flat aromatic six-membered C₃P₃ ring. This two-electron redox reaction occurs in two single-electron transfer steps via the 7π-radical cation [ 4 ]⋅⁺, which could also be isolated and fully characterized. The profound reversible structural change observed for the two-electron redox couple [ 5 ]²⁺/ 1 a , b is in sharp contrast to the C₆H₆/[C₆H₆]²⁻ couple, which undergoes only a modest structural deformation.     

The First Synthesis of Subporphyrins

Because of the rich coordination chemistry and unique optical and electrochemical properties, porphyrin analogues have been intensively investigated. Among them, subporphyrins have been long-awaited molecules, with only their boron complexes known to date because of the crucial role of the central boron atom as a template in synthesis. The challenges related to the synthesis of a genuine subporphyrin (boron-free) have finally been met and reported in a recent article from Kim, Osuka, and Song. A key strategy of their synthesis lies in the introduction of an exocyclic double bond at the meso-position and subsequent reduction to obtain macrocyclic conjugation. Considering the fact that the development of porphyrin chemistry is ultimately linked to the availability of free base porphyrins, this seminal work will facilitate studies on coordination chemistry and applications in materials science.     

Quantification of the (anti)aromaticity of fulvenes subject to ring size

Tria-, penta-, hepta- and nonafulvenes (1-4) have been studied theoretically at the MP2 ab initio level of theory. For the global minimum structures, the occupation of the bonding π_CC orbital of the exocyclic CC double bond, obtained by NBO analysis, quantitatively proves π-electron delocalization which can reveal partial 2-, 6- and 10-π-electron aromaticity, and 4-, 8- and 12-π-electron antiaromaticity of the ring moieties. Beside the corresponding occupation number, this conjugation was quantified by the length of the exocyclic CC double bond whilst the (anti)aromaticity of the ring moieties of 1-4 was visualized and quantified by through space NMR shielding surfaces (TSNMRS).     

As-5、[As5M]-、[As5MAs5]2- (M=Ti,Zr, Hf)的结构和芳香性

用密度泛函理论(DFT)研究了As-5、[As5M]-和[As5MAs5]2- (M=Ti, Zr, Hf)的结构、频率、能量以及芳香性, 详细讨论了体系中不同类型的键和电子如化学键、孤对电子、核电子等对总的核独立化学位移(NICS)的影响. 结果表明, As-5、[As5M]-和[As5MAs5]2-的基态结构分别具有D5h、C5v和D5h对称性, 而且都具有芳香性. As-5 (D5h)的芳香性主要来源于As—As π键和As—As σ键的作用. [As5M]-(C5v)中各种As—M键的NICS分割值占主要优势, 其次是As—As之间形成的σ键. [As5TiAs5]2-(D5h)中, As—As π键的作用占主要优势. [As5ZrAs5]2-(D5h)中, As—As π键对体系总的NICS贡献相对减小, 而As—Zr键的作用增强. [As5HfAs5]2-(D5h)的芳香性主要来自As—Hf键的作用.     

Theoretical Study on the Stable Structures and Aromaticities for Pnictogen Dianionic Sb4^2-, Bi4^2-, and （SbBi）2^2- Clusters

Through the theoretical calculation of structural optimization, vibrational frequencies and atomization energies with one method of density functional theory （B3LYP） and two post- Hartree-Fock approaches （MP2, CCSD（T））, several stable isomers for new three pnictogen dianionic Sb4^2-, Bi4^2-, and （SbBi）2^2- species were determined. For two homoatomic Sb4^2- and Bi4^2- species, there are three stable isomers： square （D4h）, roof-shaped （C2v-1）, and C2v-2 structure with the square isomer being the ground state. For the heteroatomic dian- ionic （SbBi）2^2- species, there are also three stable isomers： rhombus （D2h）, roof-shaped （C1）, and C2v structures with the rhombic isomer being the ground state. The calculated NICS values show that nucleus-independent chemical shifts （NICS） values of roof-shaped isomers for Sb4^2-, Bi4^2-, and （SbBi）2^2- species are all negative, consequently indicating that these roof-shaped isomers possess aromaticities. NICS values for the planar ring isomers are all positive, suggesting that these three planar ring isomers have antiaromatic characters. The aromaticity for the two stable roof-shaped and square isomers are preliminarily explained and discussed with MO analysis.     

Evidence from current-density mapping for σ-delocalisation in the aromatic hexaiodobenzene cation

Calculation of high σ-delocalisation in the iodine periphery of the hexaiodobenzene cation, accompanied by a four-electron through-space σ ring current, provides direct computational evidence for the attribution to this ion of σ-aromaticity co-existing with the conventional π-aromaticity that it shares with its neutral parent.     

Fullerenocene

The nature of the molecule C₆₀CrC₆₀ was studied from a theoretical point of view. Calculations on this molecule were carried out at the BPW91 level. The results are compared with those from another two molecules, dibenzenechromium that is a known and stable species, and η⁶-fullerene chromium(0) η⁶-benzene, a nonexisting molecule. This analysis has been carried out in order to study the stability of the target compound and provide useful information for its synthesis. The aromaticity of the rings in the three compounds was analyzed by means of HOMA and NICS methods whereas their intrinsic stability was estimated by means of homodesmotic reactions. The electronic structure of the three molecules was also studied. The results show fullerenocene should be a moderately stable species and should display semiconductor behaviour and some of the interesting properties of fullerene itself.     

A Computational View of PATO and its Tautomers

1, 2, 4‐Triazole and 3‐amino‐1, 2, 4‐triazole are useful starting materials for the synthesis of many 1, 2, 4‐triazole‐based explosives. Electronic properties and Kamlet–Jacobs detonation performances of PATO (3‐picrylamino‐1, 2, 4‐triazole) (a relatively inexpensive, insensitive, and thermally stable explosive) and its 1, 3‐ and 1, 5‐ tautomers are investigated computationally using PM3, HF/6‐31G(d, p), and B3LYP/6‐31G(d, p) methods. The relationships between the thermodynamic and kinetic stabilities and the aromaticities are examined for all structures and their relative sensitivities are compared in terms of bond dissociation energies. The use of both PM3 and isodesmic methods in the calculation of heat of formation of PATO is discussed.     

On the correlation between the bifurcation values of the electron localization function and the nuclear independent chemical shift in aromatic and heteroaromatic compounds

In this work, for a representative set of 24 aromatic molecules, that includes hydrocarbons, hetero and charged rings, we explore the correlations between two aromaticity indexes. One of these indexes is based on the bifurcation values of the σ and π electron localization function (ELF), while the other one is the nuclear independent chemical shift (NICS). We observe that the quality of the possible correlations between these two kinds of indexes strongly depends on the kind of rings and on the particular indexes that are considered.