Antiaromatic Compounds Research Articles

Indenone and cyclopentadienone energetics via mass spectrometry and computations: Are these species antiaromatic or “merely” nonaromatic?

The heat of hydrogenation of indenone was measured via two partially independent thermodynamic cycles by carrying out energetic measurements (i.e., electron affinities, proton affinities and ionization potentials) on both negative and positive ions (ΔH°H2=17.8±5.5 and 17.5±5.7kcalmol−1, respectively). High level G3 computations were also carried out to provide the heats of formation of indenone (16.8kcalmol−1) and cyclopentadienone (14.0kcalmol−1). These 4n π electron systems are found to be nonaromatic in contrast to previous views. A recent report on cyclopropenyl anion (J. Org. Chem. 2013, 78, 7370–7372) indicates that this ion is also nonaromatic, and suggests that NMR ring currents and nucleus independent chemical shift (NICS) calculations do not correlate with the energetic criterion for antiaromatic compounds.

Theoretical Study on the Stability and Aromaticity of Metallasilapentalynes

Antiaromatic compounds and small cyclic alkynes or carbynes are both challenging for synthetic chemists because of the destabilization caused by their antiaromaticity and highly distorted triple bonds, respectively. These dual destabilizations could be the reason why pentalyne (I), a highly antiaromatic and extremely strained cyclic alkyne, has never been synthesized. Recently, we have successfully synthesized the first metallapentalyne (II), benefiting from the stabilization of a metal fragment by reducing the ring strain and switching the antiaromaticity in pentalyne to the aromaticity in metallapentalyne. An interesting question is raised: can the aromaticity in metallasilapentalyne (III) be retained, considering the fact that the silicon atom is reluctant to participate in π bonding? Here we report a thorough theoretical study on the stability and aromaticity of metallasilapentalynes. The computed energies and magnetic properties reveal the reduced aromatic character of osmasilapentalyne in comparison...

An Antiaromatic Electrode‐Active Material Enabling High Capacity and Stable Performance of Rechargeable Batteries

Although aromatic compounds occupy a central position in organic chemistry, antiaromatic compounds have demonstrated little practical utility. Herein we report the application of an antiaromatic compound as an electrode-active material in rechargeable batteries. The performance of dimesityl-substituted norcorrole nickel(II) complex (NiNC) as a cathode-active material was examined with a Li metal anode. A maximum discharge capacity of about 207 mAhg(-1) was maintained after 100 charge/discharge cycles. Moreover, the bipolar redox property of NiNC enables the construction of a Li metal free rechargeable battery. The high performance of NiNC batteries demonstrates a prospective feature of stable antiaromatic compounds as electrode-active materials.

An Antiaromatic Electrode‐Active Material Enabling High Capacity and Stable Performance of Rechargeable Batteries

AbstractAlthough aromatic compounds occupy a central position in organic chemistry, antiaromatic compounds have demonstrated little practical utility. Herein we report the application of an antiaromatic compound as an electrode‐active material in rechargeable batteries. The performance of dimesityl‐substituted norcorrole nickel(II) complex (NiNC) as a cathode‐active material was examined with a Li metal anode. A maximum discharge capacity of about 207 mAhg−1 was maintained after 100 charge/discharge cycles. Moreover, the bipolar redox property of NiNC enables the construction of a Li metal free rechargeable battery. The high performance of NiNC batteries demonstrates a prospective feature of stable antiaromatic compounds as electrode‐active materials.

CTED: the new aromaticity index based on corrected total electron density at bond critical points

In this work, the corrected total electron density based on ellipticity (ε) at C–C bond critical points in a given ring and bond length alternations was introduced to estimate π‐electron density distributions in the ring. Then, to evaluate aromaticity of rings with any number of members, the was normalized relative to a system assumed as a full aromatic, which is named as the corrected total electron density (CTED) aromaticity index. For a wide range of aromatic, nonaromatic and antiaromatic compounds, we have compared CTED index with the other commonly used aromatic indices, such as HOMA, PDI, FLU, NICS and recently introduced EL. CTED index was seen to be in agreement with the defined indices, and with general expectations. Hence, as similar to the other indices except from PDI, we have proposed that CTED index could be applied to study the aromaticity of rings without any restriction in the number of members of rings and used to analyze both the local and global aromatic character of rings as a new aromaticity index. Copyright © 2014 John Wiley & Sons, Ltd.

Stable but chimeric antiaromatic 1H-azirines? A threefold reinvestigation

Three different reports on the syntheses of isolable 1H-azirines 6, 15, and 21 were reinvestigated. Instead of the claimed heterocyclic product 6, the isomeric thiazole derivative 7 has been isolated now with nearly identical yield. In the case of the asserted bicyclic 1H-azirine 15, the corrected structure includes the isomeric 3-aminomaleimide moiety of 18. A mechanism to explain the formation of this substance is suggested. The isolation of the antiaromatic compound 21 has also to be rejected. Thus, 1H-azirines keep their classification as very elusive high-energy intermediates.

Stabilization of anti-aromatic and strained five-membered rings with a transition metal

Anti-aromatic compounds, as well as small cyclic alkynes or carbynes, are particularly challenging synthetic goals. The combination of their destabilizing features hinders attempts to prepare molecules such as pentalyne, an 8π-electron anti-aromatic bicycle with extremely high ring strain. Here we describe the facile synthesis of osmapentalyne derivatives that are thermally viable, despite containing the smallest angles observed so far at a carbyne carbon. The compounds are characterized using X-ray crystallography, and their computed energies and magnetic properties reveal aromatic character. Hence, the incorporation of the osmium centre not only reduces the ring strain of the parent pentalyne, but also converts its Hückel anti-aromaticity into Craig-type Möbius aromaticity in the metallapentalynes. The concept of aromaticity is thus extended to five-membered rings containing a metal-carbon triple bond. Moreover, these metal-aromatic compounds exhibit unusual optical effects such as near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and aggregation enhancement.

Theoretical analysis on geometries and electronic structures of antiaromatic pentalene and its N‐substituted derivatives: monomer, oligomers and polymer

The antiaromatic compounds have received a great deal of attention for several decades because of their unusual electronic structures. The electronic structures and properties of antiaromatic pentalene and its six nitrogen heterocyclic derivatives were systematically studied by the density functional theory at the Becke, three‐parameter, Lee–Yang–Parr level with 6‐31G* basis set. The results indicated that all the monomers have stable singlet states and remarkable bond‐length alternations. From the dimer to polymer in those molecules, pentalene(P), cyclopenta[b]pyrrole(CPP), cyclopenta[d]imidazole(CPI), pyrrolo[2,3‐b]pyrrole(PP1) and pyrrolo[3,2‐d]imidazole(PI) are stable diradical structures; pyrrolo[3,2‐b]pyrrole (PP2) and imidazo[4,5‐d]imidazole(II) are stable singlet ground states. The electronic properties including bond length, bond‐length alternation, electron density at bond critical points, Wiberg bond index and nucleus‐independent chemical shift were analyzed. It was found that in diradical molecules the bond‐length alternations are diminished, the charge tends to equilibrate, the π‐electron delocalization and conjugation are strengthened. The electronic properties of singlet ground state molecules have nearly no variations from monomers to polymers. The band structure analysis shows that diradical structure molecules have small band gaps (<1.0 eV), wide bandwidth and small effective masses of holes and electrons which suggest that diradical structure molecules are very good candidates for conductive materials. Copyright © 2011 John Wiley & Sons, Ltd.

Dihydrogen Activation by Antiaromatic Pentaarylboroles

Facile metal-free splitting of molecular hydrogen (H(2)) is crucial for the utilization of H(2) without the need for toxic transition-metal-based catalysts. Frustrated Lewis pairs (FLPs) are a new class of hydrogen activators wherein interactions with both a Lewis acid and a Lewis base heterolytically disrupt the hydrogen-hydrogen bond. Here we describe the activation of hydrogen exclusively by a boron-based Lewis acid, perfluoropentaphenylborole. This antiaromatic compound reacts extremely rapidly with H(2) in both solution and the solid state to yield boracyclopent-3-ene products resulting from addition of hydrogen atoms to the carbons alpha to boron in the starting borole. The disruption of antiaromaticity upon reaction of the borole with H(2) provides a significant thermodynamic driving force for this new metal-free hydrogen-splitting reaction.

ChemInform Abstract: Antiaromatic Compounds. Part 31. Cyclobutadienes and Phosphaalkynes - Tools for the Development of Heteroaromatic Compounds and Their Valence Isomers

ChemInform Abstract: Antiaromatic Compounds. Part 31. Cyclobutadienes and Phosphaalkynes - Tools for the Development of Heteroaromatic Compounds and Their Valence Isomers

ChemInform Abstract: News About Acepentalene, Cyclopentadienyl Cations, and Other Elusive Aromatic and Antiaromatic Compounds

Dipotassium acepentalenediide 7-K2 is easily available in quantitative yield from triquinacene 3, it can be trapped with a variety of electrophiles to give 4,7-dihydroacepentalene derivatives 15-R2. The unsubstituted 15-H2 obtained by hydrolysis of 7-K2 rapidly dimerizes and also undergoes other (4˛2) cycloadditions with added dienes. The crystalline dilithium acepentalenediide 7-Li2 has been obtained by metal-metal exchange on 15-(SnMe3)2, char- acterized by a crystal structure analysis and used to prepare the novel metal complexes 19-22. The bis(trimethylstannyl) derivative 15-(SnMe3)2 also led to the first unequivocal detection of neutral acepentalene 5. Computational results indicate that the antiaromatic cyclopentadienyl cation is significantly stabilized in its singlet state by two or more cyclopropyl groups. The readily accessible (tetracyclopropylcyclopentadienone)tricarbonyliron 32 can be alkylated and protonated to yield complexes 40-R and 41-X with a formal tetracyclopropylcyclopentadienyl cation ligand. Tetracyclopropylcyclopentadiene 43, easily prepared in one step from dicyclo- propylacetylene, can be deprotonated, and the resulting anion 46 chlorinated to 45 and transformed to metallocenes 44 and 47. Butadiyne-expanded (n)rotanes 49 and permethylated analogues 50 are considered as potential precursors to Cn carbon clusters. Differential scanning calorimetry with evolved gas analysis has revealed that compounds 49 (n o 5, 6, 8) cleave off ethylene, while the permethylated analogue 50 (n o 6) cleaves off tetramethylethylene. If fragmentation would occur 12 times from exp(12)rotane 49 (n o 12), a C60 carbon cluster would be left over.

Contributions from orbital–orbital interactions to nucleus-independent chemical shifts and their relation with aromaticity or antiaromaticity of conjugated molecules

The out-of-plane components of the nucleus-independent chemical shifts (NICS) for a group of aromatic and antiaromatic [ n]-annulenes have been separated into several contributions on the basis of the gauge-including atomic orbital (GIAO) method. The analysis of the orbital interactions responsible of the NICS(π) zz values shows that the large and positive values found for antiaromatic compounds are due to the predominance of π → π ∗ rotational transitions that overcome the diamagnetic and gauge contributions. However, NICS(π) zz for aromatic compounds are dominated by contributions arising from the gauge transformation and no significant contributions are found from occupied–unoccupied orbital mixing. This analysis has been compared with previous work about ring currents in the same compounds.

Defining Spectroscopic Features of Heteroannulenic Antiaromatic Porphyrinoids

A new set of spectroscopic tools is proposed that may be used to distinguish antiaromatic compounds from their corresponding aromatic congeners. This prediction is based on a detailed analysis of the optical and photophysical properties of a matched set of expanded porphyrins. In particular, the antiaromatic porphyrinoids having [4n] π-electrons within their conjugation pathway exhibit distinct photophysical features that differ dramatically from what is observed for the corresponding aromatic congeners. The clear diagnostic differences seen between the antiaromatic and aromatic compounds leads us to propose that the spectroscopic methods detailed in this Perspective could emerge as general tools that may be used to characterize the electronic characteristics of complex systems for which a number of potential electronic states can be envisioned on the basis of simple line formulas or analyses of π-electron populations.

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

Cyclooctatetraene (COT), the first 4nπ-electron system to be studied, adopts an inherently nonplanar tub-shaped geometry of D2d symmetry with alternating single and double bonds, and hence behaves as a nonaromatic polyene rather than an antiaromatic compound. Recently, however, considerable 8π-antiaromatic paratropicity has been shown to be generated in planar COT rings even with the bond alternated D4h structure. In this review, we highlight recent theoretical and experimental studies on the antiaromaticity of hypothetical and actual planar COT. In addition, theoretically predicted triplet aromaticity and stacked aromaticity of planar COT are also briefly described.

Aromatic versus Antiaromatic Effect on Photophysical Properties of Conformationally Locked trans-Vinylene-Bridged Hexaphyrins

We have investigated the electronic structures and energy relaxation dynamics of vinylene-bridged hexaphyrins using steady-state and time-resolved spectroscopies along with theoretical calculations in order to reveal their aromaticity-dependent electronic and magnetic properties. Ethynyl-TIPS-substituted planar and rectangular [28]hexaphyrin, regarded as a Huckel antiaromatic compound, tends to adopt a twisted Mobius aromatic topology via structural distortion in order to reduce the total internal energy, in contrast to aromatic [26]hexaphyrin, which maintains a planar conformation in solution. Spectacles-shaped vinylene-bridged [26]- and [28]hexaphyrins represent highly Huckel aromatic and antiaromatic natures, respectively, as revealed by NMR spectroscopy, giving rise to remarkable differences in NICS(0) and HOMA values and shapes of steady-state absorption and emission spectra. In particular, lifetime of the lowest singlet excited state of [28]hexaphyrin (8.6 ps) is 30 times shorter than that of the aromatic congener [26]hexaphyrin (282 ps), as measured by the femtosecond transient absorption technique. Both frontier molecular orbital analyses and vertical excitation energy calculations suggest that vinylene-bridged [28]hexaphyrin has an optically dark lowest singlet state in the NIR region, as observed in the absorption spectrum with a very low oscillator strength, which might act as a ladder state in the excited-state energy relaxation dynamics. Our findings provide further insight into the aromaticity-driven electronic properties of various porphyrinoids as well as of aromatic/antiaromatic hydrocarbon systems.

Perfluoropentaphenylborole

Perfluorination: The fully fluorinated analogue of pentaphenylborole (see structure; B gray, C black, F green) has been prepared using successive transmetallation reactions involving Zr and Sn heterocycles. The highly moisture-sensitive borole is a new member of the perfluoroaryl borane family, a class of antiaromatic compounds of fundamental significance to concepts of aromaticity.

Reversible Oxidation State Change in Germanium(tetraphenylporphyrin) Induced by a Dative Ligand: Aromatic GeII(TPP) and Antiaromatic GeIV(TPP)(pyridine)2

Treatment of GeCl2(dioxane) with Li2(TPP)(OEt2)2 (TPP = tetraphenylporphyrin) in THF yields Ge(TPP), the first free Ge(II) porphyrin complex. In pyridine Ge(TPP) is converted to Ge(TPP)(py)2, an antiaromatic Ge(IV) complex, whereas in benzene the reaction is reversed, and pyridine dissociates from Ge(TPP)(py)2 to form Ge(TPP). That reversible reaction represents an unusual, if not unique, example of an oxidation-state change in a metal induced by coordination of a dative ligand. UV-vis and 1H NMR spectroscopy show that Ge(TPP) is an aromatic Ge(II) porphyrin complex, while the 1H NMR spectrum of Ge(TPP)(py)2 clearly indicates the presence of a strong paratropic ring current, characteristic of an antiaromatic compound. Both Ge(TPP) and Ge(TPP)(py)2 have been crystallographically characterized, and the antiaromaticity of Ge(TPP)(py)2 leads to alternating short and long C-C bonds along the 20-carbon periphery of its porphine ring system. Coordination of pyridine to Ge(TPP) greatly increases its reducing ability: the Ge(TPP)0/2+ redox potential is about +0.2 V, while the Ge(TPP)(py)2(0/+) redox potential is -1.24 V (both vs. ferrocene). The equilibrium constant of the reaction Ge(TPP) + 2 py = Ge(TPP)(py)2 in C6D6 is 22 M-2. The germanium complex of the more electron-withdrawing tetrakis[3,5-bis(trifluoromethyl)phenyl]porphyrin, Ge(TArFP), and its pyridine adduct Ge(TArFP)(py)2 were synthesized. The equilibrium constant of the reaction Ge(TArFP) + 2 py = Ge(TArFP)(py)2 in C6F6/C6D6 is 2.3 x 10(4) M-2. Density functional theory calculations are consistent with the experimental observation that M(TPP)(py)2 formation from M(TPP) and pyridine is most favorable for M=Si, borderline for Ge, and unfavorable for Sn.

Cyclobutadiene: The Antiaromatic Paradigm?

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Direct estimate of conjugation and aromaticity in cyclic compounds with the EDA method

The nature of the interatomic interactions in cyclic conjugated molecules has been investigated with the Energy Decomposition Analysis (EDA). The focus of this work is on the strength of the pi bonding and pi conjugation in carbocyclic and heterocyclic molecules. The calculated deltaE(pi) values suggest that the EDA may be used directly for estimating the strength of pi conjugation in aromatic, homoaromatic, homoantiaromatic and antiaromatic compounds. The theoretical data show a pattern which agrees with the 4n + 2 rule. The extra aromatic stabilization energy has been obtained by comparing cyclic conjugation with the strength of pi conjugation in acyclic reference compounds. The deltaE(pi) values indicate that benzene is significantly stabilized by aromatic stabilization, but the total pi bonding contribution to the carbon-carbon bonding becomes even more stabilizing when the geometry is distorted toward a D(3h) form which has three long and three short C-C bonds. The D(6h) equilibrium structure is enforced by the sigma orbital interactions and by the quasiclassical electrostatic attraction. The VRE and ASE values suggest that pyridine is more strongly stabilized by aromatic conjugation than benzene. The EDA data of the six-membered cyclic 6pi species C5H5E (E = CH, N-Bi) and (HB = NH)3 predicts strength of aromatic stabilization in the order N > CH > P > As > Bi >> borazine. The ASE values of the five-membered heterocyclic systems C4H4E (E = NH, O, S) are smaller than for the benzene analogues showing the order NH approximately S > O. The ASE value for Cp is very small which probably comes from the choice of the reference system. The VRE results indicate that Cp is strongly stabilized by cyclic conjugation. The ASE values for the cyclic singlet carbenes indicate weak aromatic stabilization in the 2pi system cyclopropenylidene which is much stronger in the 6pi compound cycloheptatrienylidene while the 4pi molecule cylopentadienylidene is clearly antiaromatic. The triplet states of all three cyclic carbenes are antiaromatic. The strength of the pi conjugation in the homoconjugated cyclic compounds suggests weak aromatic stabilization in the 6pi compounds 1,3-cyclobutene and 1,3-cyclopentadiene but weak antiaromatic destabilization in the 2pi compound cyclopropene and in the 8pi molecules 1,3-cyclohexadiene and 1,3,5-cycloheptatriene. The 4n + 2 rule thus holds also for homoconjugated systems. A large negative value for the ASE is calculated for 1,3-cyclobutadiene.

Cyclobutadiene: The Antiaromatic Paradigm?

Cyclobutadiene: The Antiaromatic Paradigm?