Aromaticity is a fundamental and important concept in chemistry, and usually, the enhancement of aromaticity brings additional thermodynamic stability to a compound. Moreover, since radicals can act as intermediates in chemical reactions, they have attracted considerable attention from both experimental and theoretical chemists for a long time. However, it remains unclear whether there is a relationship between the thermodynamic stability of cyclic planar radicals and their aromaticity.
As a fundamental chemical property, aromaticity guides the synthesis of novel structures and materials. Replacing the carbon moieties of aromatic hydrocarbons with transition metal fragments is a promising strategy to synthesize intriguing organometallic counterparts with a similar aromaticity to their organic parents. However, since antiaromaticity will endow compound instability, it is a great challenge to obtain an antiaromatic organometallic counterpart based on such transition metal replacement in aromatic hydrocarbons.
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.
Benzene, the prototype of aromatics, has six equivalent C‐C bonds (1.397 Å), which are intermediate between a C‐C double bond and a C‐C single bond. For over 80 years, chemists have spent much effort on freezing a localized structure to obtain a distorted bond‐length alternating benzene ring in the ground state, leading to various localized trisannelated benzene rings. However, most of the central benzene rings are still aromatic or nonaromatic. Here we report an antiaromatic benzene ring caused by hyperconjugation.
Density functional theory (DFT) calculations were carried out to investigate the stability and aromaticity of metallapentalocyclobutadienes. The results reveal unexpected higher stabilisation achieved with a 3d ruthenium fragment compared to the 4d osmium counterpart. Moreover, direct 1–3 metal–carbon bonding in the metallabutadiene unit of these two complexes is negligible.
Antiaromatic species are substantially less thermodynamically stable than aromatic moieties. Herein, we report the stabilization of two classical antiaromatic frameworks, cyclobutadiene and pentalene, by introducing one metal fragment through the first [2+2] cycloaddition reaction of a late-transition-metal carbyne with alkynes. Experimental observations and theoretical calculations reveal that the metal fragment decreases the antiaromaticity in cyclobutadiene and pentalene simultaneously, leading to air- and moisture-stable products.
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.