Aromaticity and hyperconjugation are two fundamental concepts in chemistry. Combining them together led to the proposal of the concept of hyperconjugative aromaticity by Mulliken in 1939. Now, it has been attracting considerable attention from both theoretical and experimental chemists. Recently, the concept of hyperconjugative aromaticity has been extended from main-group substituents to transition metal systems including groups 9, 10, and 11 transition metal substituents.
Activation of atmospherically abundant dinitrogen (N2) under mild conditions has been a great challenge in chemistry for decades because of the significantly strong N≡N triple bond. The traditional strategy of N2 activation was mainly limited to metallic species until the ground-breaking achievement of N2 activation by two-coordinated borylenes was achieved experimentally in 2018. On the other hand, carborane derivatives have attracted considerable interest for small-molecule activation. Still, the utilization of carborane derivatives in N2 activation remains elusive.
Activating the C–F bond (the strongest σ bond to carbon) is particularly challenging, let alone in a selective fashion when a weaker C–H bond is present in the same species. Herein, we demonstrate a novel strategy to achieve a thermodynamically and kinetically favorable activation of the C–F bond over the C–H bond dually driven by coordination and aromaticity via density functional theory calculations.
Cyclic molecules with 4n + 2 or 4n electrons are aromatic in the lowest singlet state (S0) or the lowest triplet state (T1) according to Hückel and Baird’s rules. Thus, the design of aromatic species in both the S0 and T1 states (termed as adaptive aromaticity) is particularly challenging. In this work, we demonstrate that metallasilapentalynes show adaptive aromaticity supported by structural, magnetic, and electronic indices, in sharp contrast to metallapentalynes, which exhibit aromaticity in the S0 state only.
Aromaticity and frustrated Lewis pairs (FLP), two important concepts in chemistry, have attracted considerable attention from theoretical and experimental chemists. However, combining these two concepts together for H2 activation is less developed. Herein, we report a density functional theory study on antiaromaticity-promoted H2 splitting. The antiaromatic borole (as Lewis acid) and aromatic pyridine (as Lewis base) were introduced into the cyclooctetraene skeleton. Due to the geometric constraints, such systems can be classified as FLPs.
In comparison with the widely recognized π aromaticity, σ aromaticity is a less developed concept in chemistry, especially for unsaturated systems. Moreover, most studies on σ aromaticity have been mainly limited to the ground state of saturated systems; unsaturated species with σ aromaticity in the excited state have never been reported.
Metallaaromatics have attracted considerable attention in recent years because they can display properties of both organic and organometallic species. However, it remains unclear whether Clar’s rule could be applied to organometallic chemistry despite its proposal in 1950s. Here, we investigate the relative stabilities of 49 organic and organometallic species by density functional theory (DFT) calculations.
Hyperconjugation, an interaction of electrons in a σ orbital or lone pair with an adjacent π or even σ antibonding orbital, can have a strong effect on aromaticity. However, most work on hyperconjugative aromaticity has been limited to main-group substituents. Here, we report a thorough density functional theory study to evaluate the aromaticity in various cyclopentadienes that contain both main-group and transition-metal substituents.
Transition-metal-containing metallaaromatics have attracted considerable interest from both experimental and computational chemists because they can display properties of both organometallic compounds and aromatic organic compounds. In general, the transition metal in a metallabicycle prefers a nonbridged position to the bridgehead one because of the larger ring strain caused by the rigidity in the bridgehead position, as exemplified by metallanaphthalene and metallanaphthalyne.
Aromaticity, one of the most fundamental concepts in chemistry, can be classified as Hückel- and Möbius-type according to the electron count and topology. In comparison with numerous Hückel aromatics containing 4n+2 π-electrons, Möbius aromatics with 4n π-electrons, especially the Craig-type species are particularly limited.