Adaptive σ-Aromaticity in an Unsaturated Three-Membered Ring

Adaptive σ aromaticity in the rhenacyclopropene rings

Species generally exhibit one-state aromaticity either in the lowest singlet state (S0 ) or the lowest triplet state (T1 ) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T1 state of these species is formed by the ππ* excitation, the σ-aromaticity of these three-membered rings in the S0 state could hold in the T1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry.

Actinide-Actinide Bonding: Electron Delocalisation and σ-Aromaticity in the Tri-Thorium Cluster [{Th(η8 -C8 H8 )(μ-Cl)2 }3 K2 ]

The tri-thorium cluster [{Th(η8 -C8 H8 )(μ3 -Cl)2 }3 {K(THF)2 }2 ]∞ (Nature 2021, 598, 72-75) was reported to feature intriguing σ-aromatic bonding between the thorium atoms, a mode of metal-metal bonding unique in the actinide series. However, the presence of this bonding motif has since been challenged by others. Here, we computationally explore electron delocalisation in a molecular cluster fragment of [{Th(η8 -C8 H8 )(μ3 -Cl)2 }3 {K(THF)2 }2 ]∞ and examine its responses to an applied magnetic field using a variety of methods. We also discuss the importance of the choice of basis set for the Th atoms and issues regarding locating QTAIM bond critical points. When taken together, the computed data consistently suggest the presence of delocalised Th-Th bonding and Th3 σ-aromaticity.

Fused metallacyclopropenes from alkynylphenols

Metallacycloprop-1-enes can be constructed from coordinated alkynes. In this work, we report a convenient synthesis of fused osmacyclopropene complexes 1 and 3 from OsCl2(PPh3)3 and alkynylphenols. Paramagnetic alkyne-coordinated phenolate complexes 2 were obtained as the key intermediates for the formation of osmacyclopropenes, demonstrating selective CC activation under different conditions.

Adaptive Aromaticity in 18e Metallapentalenes

Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hückel's and Baird's rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes. Here, we demonstrate that 18e metallapentalenes could possess adaptive aromaticity supported by a set of aromaticity indices when the nitrido and imido ligands are introduced via density functional theory calculations. Further investigation suggests that the metal-carbon bond strength plays an important role in the S0 state aromaticity and the T1 state aromaticity could be attributed to spin electron localization. All these findings could be useful for the development of metallaaromatic chemistry.

Synthesis and characterization of bi(metallacycloprop-1-ene) complexes

In all previously reported metallacycloprop-1-ene or η²-vinyl complexes, the metal center bears only one vinyl moiety. We have now successfully synthesized and structurally characterized the first complexes bearing two η²-vinyl moieties or spiro bi(metallacycloprop-1-ene) complexes from reactions of alkynes with rhenium phosphine complexes. Computational studies indicate that the metallacycloprop-1-ene rings are aromatic and the complexes represent a rare σ-type spirometalla-aromatic system.

Planar Inorganic Five-Membered Heterocycles with σ+π Dual Aromaticity in Both S0 and T1 States

Cyclic species being aromatic in both the lowest singlet and triplet electronic states (so-called adaptive aromaticity) are scarce. To date, the reported systems are mostly organometallic heterocycles with the aromaticities...

Computational predictions of adaptive aromaticity for the design of singlet fission materials

The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.

Adaptive aromaticity in 16-valence-electron metallazapentalenes

According to Hückel's and Baird's rules, cyclic species are generally aromatic only either in the lowest singlet state (S₀) or in the lowest-lying triplet ππ* excited state (T₁). Thus, species with aromaticity both in S₀ and T₁ states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O). Interestingly, metallazapentalenes show adaptive aromaticity whereas metalloxapentalenes display nonaromaticity in the S₀ and T₁ states, which is supported by structural, magnetic, and electronic indices. In addition, a series of metallazapentalenes containing strong σ- or π-donor ligands are predicted to achieve adaptive aromaticity. Our findings expand the family of adaptive aromatics significantly, inviting experimental chemists to realize more hetero-metallapentalenes with adaptive aromaticity.

[Bi6Mo3(CO)9]4-: a multiple local σ-aromatic cluster containing a distorted Bi6 triangular prism

The first Zintl cluster containing a distorted Bi₆ triangular prism, [Bi₆Mo₃(CO)₉]^4-, has been synthesized and structurally characterized. Quantum chemical calculations indicated that the distorted cage cluster features multiple local σ-aromaticity.