Recent Advances in the Reactions of Cyclic Carbynes

The acyclic organic alkynes and carbyne bonds exhibit linear shapes. Metallabenzynes and metallapentalynes are six- or five-membered metallacycles containing carbynes, whose carbine-carbon bond angles are less than 180°. Such distortion results in considerable ring strain, resulting in the unprecedented reactivity compared with acyclic carbynes. Meanwhile, the aromaticity of these metallacycles would stabilize the ring system. The fascinating combination of ring strain and aromaticity would lead to interesting reactivities. This mini review summarized recent findings on the reactivity of the metal-carbon triple bonds and the aromatic ring system. In the case of metallabenzynes, aromaticity would prevail over ring strain. The reactions are similar to those of organic aromatics, especially in electrophilic reactions. Meanwhile, fragmentation of metallacarbynes might be observed via migratory insertion if the aromaticity of metallacarbynes is strongly affected. In the case of metallapentalynes, the extremely small bond angle would result in high reactivity of the carbyne moiety, which would undergo typical reactions for organic alkynes, including interaction with coinage metal complexes, electrophilic reactions, nucleophilic reactions and cycloaddition reactions, whereas the strong aromaticity ensured the integrity of the bicyclic framework of metallapentalynes throughout all reported reaction conditions.

Metallaaromatic Chemistry: History and Development

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Constraint of a ruthenium-carbon triple bond to a five-membered ring

The incorporation of a metal-carbon triple bond into a ring system is challenging because of the linear nature of triple bonds. To date, the synthesis of these complexes has been limited to those containing third-row transition metal centers, namely, osmium and rhenium. We report the synthesis and full characterization of the first cyclic metal carbyne complex with a second-row transition metal center, ruthenapentalyne. It shows a bond angle of 130.2(3)° around the sp-hybridized carbyne carbon, which represents the recorded smallest angle of second-row transition metal carbyne complexes, as it deviates nearly 50° from the original angle (180°). Density functional theory calculations suggest that the inherent aromatic nature of these metallacycles with bent Ru≡C-C moieties enhances their stability. Reactivity studies showed striking observations, such as ambiphilic reactivity, a metal-carbon triple bond shift, and a [2 + 2] cycloaddition reaction with alkyne and cascade cyclization reactions with ambident nucleophiles.

Recent Advances in Metallaaromatic Chemistry

Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Ring contraction of metallabenzooxirene to metal carbonyl complexes - a comparative study with the Wolff rearrangement of oxirene and benzooxirene

A detailed quantum mechanical study on the role of transition metal fragments towards the epoxidation reaction of metallabenzynes (1M, M = Fe, Ru, Os) to give metallabenzyne epoxide or metallabenzooxirene (2M), followed by the Wolff type 1,2-rearrangement to give metal carbonyls (4M), has been carried out at the M06/def2-TZVPP//BP86/def2-SVP level of theory. The epoxide or oxirene product (2A) of linear alkynes like acetylene is unstable and converts to a ketene (4A) by highly exothermic (-78.7 kcal mol^-1) Wolff rearrangement via an oxocarbene (3A) intermediate. The epoxide product of cyclic alkynes like benzyne (2C) is comparatively more stable than benzooxocarbene (3C), yet it undergoes rearrangement to cyclopentadienylketene (4C) through Wolff type ring contraction reactions (-65.3 kcal mol^-1). The replacement of one of the carbyne carbons in benzyne by 14 VE metal fragment M(PH₃)₂Cl₂, M = Fe, Ru and Os enhances the stability of the 18 VE metallabenzyne epoxide product (2M, -88.1 kcal mol^-1 for 2Fe, -87.2 kcal mol^-1 for 2Ru and -82.8 kcal mol^-1 for 2Os) as compared to the 16 VE metallacyclohexadienone (3M). The ring contracted 18 VE product of 2M is a carbonyl bridged metallacyclopentadienyl complex (4M, M = Fe, Ru and Os). Even though this interconversion is thermodynamically favourable, it involves a high-energy barrier (19.3, 28.1 and 27.9 kcal mol^-1 for 2Fe, 2Ru and 2Os, respectively). However, the hepta-coordinated (5M) terminal metallacyclopentadienyl carbonyl analogues of ketene (4A/4C) are not minima on the potential energy surface.