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.

Rational Design to Activate Tetrafluoromethane by Two-Coordinate Borinium

The activation of tetrafluoromethane (CF₄) is quite challenging. The current methods have a high decomposition rate but are expensive, and therefore, their widespread use is limited. Here, inspired by the successful C-F activation within saturated fluorocarbons, we have designed a rational approach based on two-coordinate borinium for activating CF₄ using density functional theory (DFT) calculations. Our calculations predict that this approach is both thermodynamically and kinetically favorable.

Probing a General Strategy to Break the C-C Bond of Benzene by a Cyclic (Alkyl)(Amino)Aluminyl Anion

The oxidative addition of C-C bonds in aromatic hydrocarbons by low valent main group species has attracted considerable attention from both theoretical and experimental chemists due to the big challenge in breaking their aromaticity. Herein, a general strategy to break the C-C bonds in benzene by cyclic (alkyl)(amino)aluminyl anion is demonstrated via density functional theory (DFT) calculations. The results suggest that the activation of the C-C bond of benzene by this anion is both kinetically and thermodynamically unfavorable whereas introducing electron-withdrawing groups makes such C-C bond activation becomes favorable both kinetically and thermodynamically. Such a sharp change on the kinetics and thermodynamics could be rationalized by the frontier molecular orbital theory by decreasing the lowest unoccupied molecular orbitals of the mono- and disubstituted benzenes. Aromaticity is found to stabilize the transition state for the ring open step. All these findings can help develop the chemistry of small-molecule activation.

Theoretical Insight into the Multiple Roles of LiHMDS in Pd-Catalyzed Borylation of Fluorobenzene

Pd-catalyzed borylation of fluorobenzene was theoretically studied. DFT calculations revealed that the reaction occurs through an unprecedented 3 + 6-membered ring transition state, in which one LiHMDS (HMDS = hexamethyldisilazane) acts as a ligand and another LiHMDS is essential to provide Li···N and Li···F interactions, overcoming the large destabilization of the strong phenyl-F bond distortion. The characteristic feature of LiHMDS was elucidated by comparing it with HMDS and NaHMDS analogues.

Mechanistic Insight into the Ni-Catalyzed Kumada Cross-Coupling: Alkylmagnesium Halide Promotes C-F Bond Activation and Electron-Deficient Metal Center Slows Down β-H Elimination

The Ni-catalyzed Kumada-Tamao-Corriu (KTC) cross-coupling between aryl fluorides and alkyl Grignard reagents has been used to achieve a highly selective Csp²-Csp³ bond construction via the carbon-fluorine (C-F) bond activation. However, the detailed mechanism of this groundbreaking KTC reaction remains unclear. Herein, we perform a series of analyses by density functional theory (DFT) calculations in order to understand the reaction mechanisms for the selective activation of a highly inert C-F bond by Ni catalysts with bidentate phosphorus ligands. An alternative mechanism for Ni/Mg bimetallic cooperation C-F bond cleavage instead of a traditional oxidative addition was proposed. The push-pull interaction in the transition state provided by the Ni center and the Lewis acid of the Mg cation smoothly breaks the C-F bond, supported by the significantly decreased activation energy from 30.9 to 4.6 kcal mol^-1 and principal interacting orbital analysis. Owing to the elevated lowest unoccupied molecular orbital energy level and the electron-deficient metal center caused by the bidentate phosphorus ligand, the β-H elimination could be impeded, increasing the selectivity of KTC cross-coupling. Our DFT results rationally explain the experimental observations, which will be helpful for further development of KTC cross-coupling.

Competition of Hydrogen Bonds and Coordinate Bonds Induces a Reversible Crystal Transformation

Achieving reversible molecular crystal transformation between coordinate aggregates and hydrogen bonded assemblies has been a challenging task because coordinate bonds are generally much stronger than hydrogen bonds. Recently, we have reported the incorporation of silver ions into the cyanuric acid-melamine (CAM) network, resulting in the formation of a 1D coordination polymer (crystal 1) through forming the κ₁N-Ag-κ₂N coordination bonds. In this work, we find crystal 1 will undergo reversible transformation to hydrogen bonded coordinate units (crystal 2) through the breaking of coordinate chains and then the addition of CAM hydrogen bonding motifs into the framework. Crystal 2 presents a pseudohexagonal arrangement comprised of the κ₁N-Ag-κ₂N units connected by two sets of the triple hydrogen bonds, which extends two-dimensionally and stacks into a layer-structured crystal. Light was shed on the tautomerization of CA and M ligands associated with the crystal transformations using single crystal X-ray diffraction and infrared spectroscopy by analyzing the bond lengths and vibrations. We also highlight that photoluminescence can be a useful tool to probe the tautomer conversions of conjugated molecules. Furthermore, crystal 1 demonstrates high flexibility and can be bent over 180° and recover to its original shape after stress release. Crystal 2, on the contrary, is brittle and shows distinct mechanical anisotropy along different crystal orientations, as unveiled by nanoindentation measurements. The elastic modulus is well correlated with the chemical bonding strength along each orientation, and it is noteworthy that the contribution of the triple hydrogen bonds is comparable to that of the coordination bonds.

Antiaromaticity-Promoted Radical Anion stability in α-vinyl Heterocyclics

As an electron-rich species, radical anions have a wide range of applications in organic synthesis. In addition, aromaticity is an essential concept in chemistry that has attracted considerable attention from...

Reaction Mechanisms on [3 + 2] Cycloaddition of Azides with Metal Carbyne Complexes: Significant Effects of Aromaticity, Substituent, and Metal Center

Density functional theory calculations were used to investigate the reaction mechanisms on [3 + 2] cycloaddition reactions of azides with metal carbyne complexes. Our results reveal that the formation of a 1,4-metallatriazole regioisomer is a kinetically favorable process in comparison with the formation of 1,5-metallatriazole. Aromaticity plays an important role in stabilizing the products in these reactions. Further analyses show that the electron-donating ligand on metal centers or the electron-withdrawing group on the azide could accelerate the [3 + 2] cycloaddition reaction. All of these findings could be useful for experimental chemists to develop "click reactions" in organometallic chemistry.

Reaction Mechanisms on Unusual 1,2-Migrations of N-Heterocyclic Carbene-Ligated Transition Metal Complexes

Unusual 1,2-migration reactions of N-heterocyclic carbene (NHC) on transition metals were investigated using density functional theory calculations. Our results reveal that the electronic properties, ring strain of the four-membered ring, and aromaticity of NHC play crucial roles in the thermodynamics of such a 1,2-migration. Further studies show that changing the methylene on the metal center in the reactant with a more electronegative group (NH or O) will lead to the formation of products with nitrogen coordinating to the metal center, whereas other groups (BH, CF₂ , and SiH₂ ) will make such a 1,2-migration reverse. In addition, the reversed rearrangement of 1,2-boron, silyl migration could be thermodynamically and kinetically favorable.

Reaction mechanisms of iron(iii) catalyzed carbonyl–olefin metatheses in 2,5- and 3,5-hexadienals: significant substituent and aromaticity effects

Theoretical calculations reveal significant substituent and aromaticity effects on Fe(iii)-catalyzed carbonyl–olefin metatheses of hexadienals.