Probing the Most Aromatic and Antiaromatic Pyrrolium Rings by Maximizing Hyperconjugation and Push-Pull Effect

Probing the Limit of the Number of Saturated Atoms for Achieving Hyperconjugative Aromaticity

Aromaticity is a fundamental concept in organic chemistry. Hyperconjugative aromaticity, also known as hyperconjugation-induced aromaticity, has evolved from its origin from main group substituents to transition metal analogues, establishing itself as an important category of aromaticity. Additionally, aromatic compounds comprising two sp3-carbon atoms have recently been reported both experimentally and computationally. However, what is the maximum number of sp3-hybridized atoms needed to maintain hyperconjugative aromaticity? Here, we report that hyperconjugative aromaticity can be achieved in hexa-substituted indoliums and octa-substituted pyrroliums, possessing three-five sp3-hybridized carbon/nitrogen atoms by means of density functional theory (DFT) calculations. The aromaticity was confirmed by using various aromaticity indices, i.e., NICS, MCI, and EDDB. Notably, the strong electron-donating ability and aurophilicity of Au(I) substituents play a pivotal role in maintaining the aromaticity and structural integrity. In addition, increasing the number of hyperconjugative centers will decrease the aromaticity in these five-membered rings. Our findings highlight the significance of transition metal substituents in hyperconjugative aromaticity and offer a novel approach for designing aromatic organometallics.

Hyperconjugative Aromaticity-Based Circularly Polarized Luminescence Enhancement in Polyaurated Heterocycles

Hyperconjugative aromaticity (HA) frequently appears in metalla-aromatics, but its effect on photophysical properties remains unexplored to date. Herein, we reveal two different HA scenarios in nearly isostructural triaurated indolium and benzofuranylium compounds. The biased HAs show a discernible effect on the spatial arrangement of metal atoms and thus tailor metal parentage in frontier orbitals and the HOMO-LUMO energy gap. Theoretical calculations and structural analyses demonstrate that HA not only influences the degree of electron delocalization over the trimetalated aromatic rings but also affects π-coordination of Au(I) and intercluster aurophilic interaction. Consequently, the triaurated benzofuranylium complex shows better photoluminescence performance (quantum yield up to 49.7%) over the indolium analogue. Furthermore, four pairs of axially chiral bibenzofuran-centered trinuclear and hexanuclear gold clusters were purposefully synthesized to correlate their HA-involved structures with the chiroptical response. The triaurated benzofuranylium complexes exhibit strong circular dichroism (CD) response in solution but CPL silence even in solid film. In contrast, the hexa-aurated homologues display strong CD and intense CPL signals in both aggregated state and solid film (luminescence anisotropy factor glum up to 10-3). Their amplified chiroptical response is finally ascribed to the dominant intermolecular exciton couplings of large assemblies formed through the HA-tailored aggregation of hexanuclear compounds.

Probing Near-infrared Absorbance of E and Z Diazene Isomers via Antiaromaticity

The photoswitching behaviors of heteroaryl azos and azobenzenes have attracted considerable interest due to their applications from material science to pharmacology. However, the use of UV light limits their application, especially in biomedicine and photopharmacology. In this work, using several aromaticity descriptors, including anisotropy of the induced current density analysis and nucleus-independent chemical shifts, we systematically investigate the relationship between anti-aromaticity and the absorption of a series of heterocyclic azos. We have demonstrated that the antiaromatic heterocycles substituted with diazenes enable the significant red shifts of the n → π* and π → π* transition bands of E and Z isomers via density functional theory calculations. Moreover, introducing substituents into heterocycles could further tune the absorption. Finally, the λmax of the first transition bands of the E (ca. 1026 nm) and Z isomers (ca. 1167 nm) of azos is achieved in the near-infrared region.

Revealing the Reasons for Degeneration of Resonance-Assisted Hydrogen Bond on the Aromatic Platform: Calculations of Ortho-, Meta-, Para-Disubstituted Benzenes, and (Z)-(E)-Olefins

The energies of the O-H∙∙∙O=C intramolecular hydrogen bonds were compared quantitatively for the series of ortho-disubstituted benzenes and Z-isomers of olefins via a molecular tailoring approach. It was established that the hydrogen bond energy in the former series is significantly less than that in the latter one. The reason for lowering the hydrogen bond energy in the ortho-disubstituted benzenes compared to the Z-isomers of olefins is the decrease in the π-contribution to the total energy of the complex interaction, in which the hydrogen bond per se is enhanced by the resonance effect. By the example of the para- and meta-disubstituted benzenes, as well as E-isomers of olefins, it was explicitly shown that the aromatic ring is a much poorer conductor of the resonance effect compared to the double bond. The hydrogen bond in the ortho-disubstituted benzenes has a lower energy than a typical resonance-assisted hydrogen bond because the aromatic moiety cannot properly assist the hydrogen bond with a resonance effect. Thus, a hydrogen bond on an aromatic platform should fall into a special category, namely an aromaticity-assisted hydrogen bond, which is closer by nature to a simple hydrogen bond rather than to a resonance-assisted one.

A new strategy for hyperconjugative antiaromatic compounds utilizing negative charges: a dibenzo[b,f]silepinyl dianion

Herein we propose a new strategy for hyperconjugative antiaromatic compounds utilizing negative charges and design the 5,5-diphenyldibenzo[b,f]silepinyl dianion (pseudo 16π-electron system) in which negative hyperconjugation occurs between the anionic π-cloud and the σ*(Si-Ph) orbital. Essentially, reduction of the dibenzo[b,f]silepin with lithium readily generated a dilithium salt of the dibenzosilepinyl dianion, and its hyperconjugative antiaromaticity has been evidenced by the upfield shifts of ¹H NMR signals and theoretical calculations, including large NICS_zz values and ACID plots.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Tuning the hyperconjugative aromaticity in Au(III)-substituted indoliums

As a fundamental concept in chemistry, aromaticity has been extended from traditional organics to organometallics. Similarly, hyperconjugative aromaticity (HCA) has also been developed from main group to transition metal systems through the hyperconjugation of the substituents. However, it remains unclear that how the oxidation state of transition metal in the substituents affects the HCA. Herein, we demonstrate via density functional theory calculations that HCA could disappear in indoliums when the Au(i) substituents are changed to the Au(iii) ones. By tuning the ligand or cis-trans isomerization, HCA could be regained or enhanced in indoliums containing Au(iii) substitutents.

Probing hyperconjugative aromaticity in 2H-pyrrolium and cyclopentadiene containing group 9 transition metal substituents: bridged carbonyl ligands can enhance aromaticity

Bridged carbonyls can enhance hyperconjugative aromaticity of group 9 transition metal disubstituted 2H-pyrrolium and cyclopentadiene.

Hyperconjugative aromaticity and protodeauration reactivity of polyaurated indoliums

Aromaticity generally describes a cyclic structure composed of sp²-hybridized carbon or hetero atoms with remarkable stability and unique reactivity. The doping of even one sp³-hybridized atom often damages the aromaticity due to the interrupted electron conjugation. Here we demonstrate the occurrence of an extended hyperconjugative aromaticity (EHA) in a metalated indole ring which contains two gem-diaurated tetrahedral carbon atoms. The EHA-involved penta-aurated indolium shows extended electron conjugation because of dual hyperconjugation. Furthermore, the EHA-induced low electron density on the indolyl nitrogen atom enables a facile protodeauration reaction for the labile Au-N bond. In contrast, the degraded tetra-aurated indolium with a single gem-dimetalated carbon atom exhibits poor bond averaging and inertness in the protodeauration reaction. The aromaticity difference in such two polyaurated indoliums is discussed in the geometrical and electronic perspectives. This work highlights the significant effect of metalation on the aromaticity of polymetalated species.

Hyperconjugative aromaticity combines the concepts of hyperconjugation and aromaticity and explains cyclopentadiene stability. Here, the authors demonstrate extended hyperconjugative aromaticity in a metallated indole ring, which shows extended electron conjugation due to the dual hyperconjugation.

Unconventional Aromaticity in Organometallics: The Power of Transition Metals

Aromaticity, one of the most fundamental concepts in chemistry, has attracted considerable attention from both theoreticians and experimentalists. Much effort on aromaticity in organometallics has been devoted to metallabenzene and derivatives. In comparison, aromaticity in other organometallics is less developed. This Account describes how our group has performed quantum chemical calculations to examine aromaticity in recently synthesized novel organometallic complexes. By collaborations with experimentalists, we have extended several aromaticity concepts into organometallics to highlight the power of transition metals. In general, the transition metal could participate in delocalization either out of rings or in the rings. We examined the former by probing the possibility of transition metal substituents in hyperconjugative aromaticity, where the metal is out of the rings. Calculations on tetraaurated heteroaryl complexes reveal that incorporation of the aurated substituents at the nitrogen atom can convert nonaromaticity in the parent indolium into aromaticity in the aurated one due to hyperconjugation, thus extending the concept of hyperconjugative aromaticity to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. Recently, we also probed the strongest aromatic cyclopentadiene and pyrrolium rings by hyperconjugation of transition metal substituents. Moreover, theoretical calculations suggest that one electropositive substituent is able to induce aromaticity; whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. We also probed the possibility of Craig-type Möbius aromaticity in organometallic chemistry, where the position of the transition metals is in the rings. According to the electron count and topology, aromaticity can be classified as Hückel-type and Möbius-type. In comparison with numerous Hückel aromatics containing 4 n+2 π-electrons, Möbius aromatics with 4 n π-electrons, especially the Craig-type species, are particularly limited. We first examined aromaticity in osmapentalynes. Theoretical calculations reveal that incorporation of the osmium center not only reduces the ring strain of the parent pentalyne, but also converts Hückel antiaromaticity in the parent pentalyne into Craig-type Möbius aromaticity in metallapentalynes. Further studies show that the transition metal fragments can also make both 16e and 18e osmapentalenes aromatic, indicating that the Craig-type Möbius aromaticity in osmapentalyne is rooted in osmapentalenes. In addition, Möbius aromaticity is also possible in dimetalla[10]annulenes, where the lithium atoms are not spectator cations but play an important role due to their bonding interaction with the diene moieties. We then examined the possibility of σ-aromaticity in an unsaturated ring. Traditional π-aromaticity is used to describe the π-conjugation in fully unsaturated rings; whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. We found that the unsaturated three-membered ring in cyclopropaosmapentalene is σ-aromatic. Very recently, we extended σ-aromaticity into in a fully unsaturated ring. The concepts and examples presented here show the importance of interplay and union between experiment and theory in developing novel aromatic systems and, especially, the indispensable role of computational study in rationalization of unconventional aromaticity. All these findings highlight the strong power of transition metals originating from participation of d orbitals in aromaticity, opening an avenue to the design of unique metalla-aromatics.

Bonded to Carbon or Nitrogen? This is a Question on the Regioselectivity in Hyperconjugative Aromaticity

In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. Although it has been extensively investigated in various reactions, the regioselectivity of hyperconjugative aromaticity on either main group systems or transition metal ones remains elusive due to the challenge of synthesizing the target products. Here we report a joint theoretical and experimental study on this issue. Theoretical calculations predicted that electron-withdrawing groups prefer an attachment to the sp³-hybridized carbon atom rather than the nitrogen atom in indoliums. For the electron-donating groups, the two isomers bonded to the sp³-hybridized carbon or nitrogen atom are almost isoenergetic. When both sp²- and sp³-hybridized carbon and nitrogen atoms in the five-membered ring of indoliums are considered, the isomer with the polyaurated substituents bonded to the sp³-hybridized carbon atom is thermodynamically more stable than that with the polyaurated substituents bonded to the sp³-hybridized nitrogen atom. This prediction is reasonably verified by experimental observation. Bond dissociation energy is found to be more important than aromaticity in rationalizing such a preference. Our findings could help experimentalists to design and realize more novel hyperconjugative aromatics.