Synthesis and Characterization of a Metallapyridyne Complex

Metallaaromatic Complexes as Candidates for Future Molecular Materials and Electronic Devices: Recent Advancements

In recent years, the field of organometallic chemistry has made a great progress and diverse types of metallaaromatics have successively been reported. In those studies, incorporation of ligated osmium centers into metallaaromatic systems played a prominent role. The reviewed literature documents that certain metallaaromatics with unconventional photophysical properties, redox and electronic transport properties and magnetism, have potential to be widely used in diverse practical applications, with selected examples of amino acid and fluoride anion identification, photothermal effects, functional materials, photodynamic therapy (PDT) in biomedicine, single-molecule junction conductors, and electron-transport layer materials (ETLs) in solar cells.

Preparation, Aromaticity, and Bromination of Spiro Iridafurans

Iridium centers of [Ir(μ-Cl)(C8H14)2]2 (1) activate the Cβ(sp2)-H bond of benzylideneacetone to give [Ir(μ-Cl){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (2), which is the starting point for the preparation of the spiro iridafurans IrCl{κ2-C,O-[C(Ph)CHC(Me)O]}2(PiPr3) (3), [Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2(MeCN)2]BF4 (4), [Ir(μ-OH){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (5), Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-C,N-[C6MeH3-py]} (6), and Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acac]} (7). The five-membered rings are orthogonally arranged with the oxygen atoms in trans in an octahedral environment of the iridium atom. Spiro iridafurans are aromatic. The degree of aromaticity and the negative charge of the CH-carbon of the rings depend on ligand trans to the carbon directly attached to the metal. Aromaticity has been experimentally confirmed by bromination of iridafurans with N-bromosuccinimide (NBS). Reactions are sensitive to the degree of aromaticity of the ring and the negative charge of the attacked CH-carbon. Iridafurans can be selectively brominated, when different ligands lie trans to metalated carbons. Bromination of 3 occurs in the ring with the metalated carbon trans to chloride, whereas the bromination of 6 takes place in the ring with the metalated carbon trans to pyridyl. The first gives IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}(PiPr3) (8), which reacts with more NBS to form IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}2(PiPr3) (9). The second yields Ir{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}{κ2-C,N-[C6MeH3-py]} (10). The origin of the selectivity is kinetic, with the rate-determining step of the reaction being the NBS attack. The activation energy depends on the negative charge of the attacked atom; a higher negative charge allows for a lower activation energy. Accordingly, complex 7 undergoes bromination in the acetylacetonate ligand, giving Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acacBr]} (11).

One-pot Syntheses of Benzo- and Benzofuran-fused Iridaoxabenzenes via CH Bond Activations of Alkyl-bridged Diphenol Derivatives

One-pot syntheses of new π-extended metallaaromatic compounds have been developed by utilizing Ir-mediated CH bond activation of ethylene- or ethylidene-bridged diphenol derivatives. Depending on the bridging alkyl groups, two types of iridaoxabenzenes, both of which are doubly fused with benzo and benzofuran units, have been obtained. Studies on their structures and electronic characters indicate that both complexes have an aromatic character on the iridaoxacycles, and their π-conjugated systems are fully delocalized over the whole molecular skeletons. These novel metallaaromatic complexes exhibited some reactivities which are distinct from those reported for the non-fused metallaaromatic compounds.

Osmathiazole Ring: Extrapolation of an Aromatic Purely Organic System to Organometallic Chemistry

An osmathiazole skeleton has been generated starting from the cation of the salt [OsH(OH)(≡CPh)(IPr)(PiPr3)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF3SO3) and thioacetamide; its aromaticity degree was compared with that of thiazole, and its aromatic reactivity was confirmed through a reaction with phenylacetylene. Salt 1 reacts with the thioamide to initially afford the synthetic intermediate [OsH{κ2-N,S-[NHC(CH3)S]}(≡CPh)(IPr)(PiPr3)]OTf (2). Thioamidate and alkylidyne ligands of 2 couple in acetonitrile at 70 °C, forming a 1:1 mixture of the salts [OsH{κ2-C,S-[C(Ph)NHC(CH3)S]}(CH3CN)(IPr)(PiPr3)]OTf (3) and [Os{κ2-C,S-[CH(Ph)NHC(CH3)S]}(CH3CN)3(IPr)]OTf (4). Treatment of 3 with potassium tert-butoxide produces the NH-deprotonation of its five-membered ring and gives OsH{κ2-C,S-[C(Ph)NC(CH3)S]}(IPr)(PiPr3) (5). The osmathiazole ring of 5 is slightly less aromatic than the osmathiazolium cycle of 3 and the purely organic thiazole. However, it is more aromatic than related osmaoxazoles and osmaoxazoliums. There are significant differences in behavior between 3 and 5 toward phenylacetylene. In acetonitrile, the cation of 3 loses the phosphine and adds the alkyne to afford [Os{η3-C3,κ1-S-[CH2C(Ph)C(Ph)NHC(CH3)S]}(CH3CN)2(IPr)]OTf (6), bearing a functionalized allyl ligand. In contrast, the osmathiazole ring of 5 undergoes a vicarious nucleophilic substitution of hydride, by acetylide, via the dihydride OsH2(C≡CPh){κ2-C,S-[C(Ph)NC(CH3)S]}(IPr)(PiPr3) (7), which releases H2 to yield Os(C≡CPh){κ2-C,S-[C(Ph)NC(CH3)S]}(IPr)(PiPr3) (8).

Synthesis, structure and aromaticity of metallapyridinium complexes

The first rhena-analogues of pyridinium (cyclopropametalla-2-isoquinolinium complexes) are obtained from o-ethynyl benzonitriles. Structural analysis and DFT calculations confirm their aromatic nature. Compared to rhenapyrylium, rhenapyridinium has a slightly stronger Hückel π-aromaticity, while a chlorine substituent on the rhenapyridinium ring decreases its aromaticity, which is revealed by NICS, EDDB, MCI and ΔBV(ELF_π) analysis.

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

The preparation of aromatic hydride-osmaoxazolium and hydride-oxazole compounds is reported and their reactivity toward phenylacetylene investigated. Complex [OsH(OH)(≡CPh)(IPr)(PⁱPr₃)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF₃SO₃) reacts with acetonitrile and benzonitrile to give [OsH{κ²-C,O-[C(Ph)NHC(R)O]}(NCR)(IPr)(PⁱPr₃)]OTf (R = Me (2), Ph (3)) via amidate intermediates, which are generated by addition of the hydroxide ligand to the nitrile. In agreement with this, the addition of 2-phenylacetamide to acetonitrile solutions of 1 gives [OsH{κ²-C,O-[C(Ph)NHC(CH₂Ph)O]}(NCCH₃)(IPr)(PⁱPr₃)]OTf (4). The deprotonation of the osmaoxazolium ring of 2 and 4 leads to the oxazole derivatives OsH{κ²-C,O-[C(Ph)NC(R)O]}(IPr)(PⁱPr₃) (R = Me (5), CH₂Ph (6)). Complexes 2 and 4 add their Os–H and Os–C bonds to the C–C triple bond of phenylacetylene to afford [Os{η³-C₃,κ¹-O-[CH₂C(Ph)C(Ph)NHC(R)O]}(NCCH₃)₂(IPr)]OTf (R = Me (7), CH₂Ph (8)), bearing a tridentate amide-N-functionalized allyl ligand, while complexes 5 and 6 undergo a vicarious nucleophilic substitution of the hydride at the metal center with the alkyne, via the compressed dihydride adduct intermediates OsH₂(C≡CPh){κ²-C,O-[C(Ph)NC(R)O]}(IPr)(PⁱPr₃) (R = Me (9), CH₂Ph (10)), which reductively eliminate H₂ to yield the acetylide-osmaoxazoles Os(C≡CPh){κ²-C,O-[C(Ph)NC(R)O]}(IPr)(PⁱPr₃) (R = Me (11), CH₂Ph (12)).

Releasing Antiaromaticity in Metal-Bridgehead Naphthalene

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. Here, we report an efficient aromaticity transformation on aromatic naphthalene through the bridgehead replacement of an osmium fragment, leading to the unprecedented synthesis of metal-bridgehead naphthalene featuring a highly twisted structure as confirmed by X-ray crystallography characterization. Such a twisted conformation works together with its phosphonium substituents to release the antiaromaticity in the planar conformation of the metal-bridgehead naphthalene. Our findings prove the bridgehead involvement of transition metals in unexpected aromaticity modifications and open an avenue for novel metal-bridgehead complexes.

One-pot syntheses of rhena-2-benzopyrylium complexes with a fused metallacyclopropene unit

Facile syntheses of the first cyclopropametalla-2-benzopyrylium complexes containing fused metallapyrylium and metallacyclopropene units.

NIR-responsive metal-containing polymer hydrogel for light-controlled microvalve

NIR-responsive metal-containing polymer hydrogel was prepared via the radical copolymerization of N-isopropylacrylamide and an osmium aromatic complex. It has excellent photothermal property and can be used as a light-controlled microvalve.

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.

Are Hetero-metallapentalenes Aromatic or Not? A DFT Investigation

Aromaticity is one of the most basic concepts in organic chemistry. The planar Möbius aromatic metallapentalynes and metallapentalenes have attracted considerable attention in the past few years. However, the aromaticity of metallapentalenes containing heteroatoms (such as B, N, and O), termed as hetero-metallapentalenes, is rarely studied. Herein, the stability and aromaticity of a series of hetero-metallapentalenes are theoretically investigated. The results reveal lower aromaticity in metallaborapentalene, comparable aromaticity in metallazapentalene, and nonaromaticity in metalloxapentalene relative to that of metallapentalene. Moreover, the effect of Lewis bases on the aromaticity and stability of metallaborapentalene is discussed. These results provide useful information for experimental chemists to realize more hetero-metallapentalenes.

[3+2] cycloaddition reaction of metallacyclopropene with nitrosonium ion: isolation of aromatic metallaisoxazole

The [3+2] cycloaddition of metallacyclopropene with nitrosonium ion gives the first aromatic osmaisoxazole-fused osmapentalene.

Rhodapentalenes: Pincer Complexes with Internal Aromaticity

Pincer complexes are a remarkably versatile family benefited from their stability, diversity, and tunability. Many of them contain aromatic organic rings at the periphery, and aromaticity plays an important role in their stability and properties, whereas their metallacyclic cores are not aromatic. Herein, we report rhodapentalenes, which can be viewed as pincer complexes in which the metallacyclic cores exhibit considerable aromatic character. Rhodapentalenes show good thermal stability, although the rhodium-carbon bonds in such compounds are fragile. Experimental and computational studies suggest that the stabilization of rigid CCC pincer architectures together with an intrinsic aromaticity is vital for these metallacyclic rhodium species. Dearomatization-aromatization reactions, corresponding to metal-ligand cooperation of classical aromatic pincer complexes, were observed in this system. These findings suggest a new concept for pincer chemistry, the internal aromaticity involving metal d-orbitals, which would be useful for exploiting the nature of construction motif and inspire further applications.

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A concept for pincer chemistry: internal aromaticity involving metal d-orbitals

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Stable cyclic rhodium complexes with obvious rhodium carbene character

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Dual stabilization of rigid CCC pincer architectures and intrinsic aromaticity

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Rhodium-containing aromatics with a single metal entity

Successive modification of polydentate complexes gives access to planar carbon- and nitrogen-based ligands

Polydentate complexes containing combinations of nitrogen and carbon (N and C) ligating atoms are among the most fundamental and ubiquitous molecules in coordination chemistry, yet the formation of such complexes with planar high-coordinate N/C sites remains challenging. Herein, we demonstrate an efficient route to access related complexes with tetradentate CCCN and pentadentate CCCCN and NCCCN cores by successive modification of the coordinating atoms in complexes with a CCCC core. Combined experimental and computational studies reveal that the rich reactivity of metal-carbon bonds and the inherent aromaticity of the metallacyclic skeletons play key roles in these transformations. This strategy addresses the paucity of synthetic approaches to mixed N/C planar pentadentate chelating species and provides valuable insights into the synthesis of carbon-based high-coordinate complexes. Furthermore, the resulting complexes are the examples of organometallic species with combined photoacoustic, photothermal, and sonodynamic properties, which makes them promising for application in related areas.

Coordination complexes based on polydentate ligands that contain both nitrogen and carbon ligating atoms are ubiquitous, but design of those with planar cores remains challenging. Here the authors show that complexes with planar CCCN, CCCCN and NCCCN cores can be accessed by modification of the coordinating atoms in CCCC core complexes.

Access to Metal-Bridged Osmathiazine Derivatives by a Formal [4+2] Cyclization

Treatment of osmacyclopentadiene derivatives 1 with phenyl or isopropyl isothiocyanate gave the fused five and six-membered osmacycles 2-5 by a formal [4+2] cyclization. The facile protonation of the newly generated exocyclic imine in complexes 2-5 afforded conjugation-extended osmacycle derivatives 6-9. Compounds 2-9 each contain two main-group heteroatoms (N and S) in the fused six-membered ring located at the ortho (for S) and para (for N) positions relative to the osmium centre; these species can be regarded as rare osma-1,3-thiazine derivatives and represent the first fused metallathiazine derivatives. In contrast to the non-planar organic 6H-1,3-thiazine, nearly coplanar metallathiazines 8 and 9 can be achieved by tuning the groups on the two nitrogen atoms. These unique metal-bridged osma-1,3-thiazine derivatives exhibit remarkable stabilities, broad spectral absorptions spanning the visible spectra, and considerable photothermal properties, which suggests their potential applications in material science.

Carbolong Chemistry: A Story of Carbon Chain Ligands and Transition Metals

The construction of metal-carbon bonds is one of the most important issues of organometallic chemistry. However, the chelation of polydentate ligands to a metal via several metal-carbon bonds is rare. Metallapentalyne, which can be viewed as a 7-carbon (7C) chain coordinated to a metal via three metal-carbon bonds, was first reported in 2013. Although metallapentalyne contains a metal-carbon triple bond in a five-membered ring (5MR) and the bond angle around the carbyne carbon is only 129.5°, metallapentalyne exhibits excellent stability to air, moisture, and heat. Metallapentalyne possesses the rare planar Möbius aromaticity, which is in sharp contrast to the Hückel antiaromaticity in pentalyne. The metal fragment not only relieves the large ring strain present in pentalyne but also results in the transformation of the antiaromaticity in pentalyne to aromaticity in metallapentalyne. With the extension of the carbon chain from 7 to 12 carbon atoms, a series of novel polycyclic frameworks were constructed via the formation of several metal-carbon bonds. Some interesting phenomena were observed for these complexes. For instance, (1) the carbyne carbon of the 7C framework could react with both nucleophilic and electrophilic reagents, leading to the formation of 16- and 18-electron metallapentalenes; (2) σ aromaticity was first observed in an unsaturated system in the 8C framework; (3) two classical antiaromatic frameworks, cyclobutadiene and pentalene, were simultaneously stabilized in the 9C framework for the first time; (4) three fused 5MRs bridged by a metal are coplanar in the 10C framework; (5) the first [2 + 2 + 2] cycloaddition of a late transition metal carbyne complex with alkynes was realized during the construction of an 11C framework; (6) the largest number of carbon atoms coordinated to a metal atom in the equatorial plane was observed in the 12C framework; and (7) sharing of the transition metal by multiple aromatic units has seldom been observed in the metalla-aromatics. Therefore, the term carbolong chemistry has been used to describe the chemistry of these novel frameworks. More interestingly, carbolong complexes exhibit diverse properties, which could lead to potential future applications. As the discovery and creation of molecular fragments lead to advancements in chemistry, medical science, and materials chemistry, these novel polydentate carbon chain chelates might have important influences in these fields due to their facile synthesis, high stability, and unique properties.

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.

Syntheses of Amino-Substituted Iridabenzofurans and Subsequent Selective N-Functionalisation

The first examples of amino-substituted fused-ring metallabenzenes, the cationic iridabenzofuran [Ir(C₇ H₄ O{NH₂ -2}{OMe-7})(CO)(PPh₃ )₂ ][O₃ SCF₃ ] (5) and neutral analogue Ir(C₇ H₄ {NH₂ -2}{OMe-7})Cl(PPh₃ )₂ (6), can be prepared by reduction of the corresponding nitro-substituted iridabenzofurans with zinc and concentrated hydrochloric acid. N-functionalised derivatives of 5 and 6 are formed through alkylation, sulfonylation or acylation. Thus, consecutive treatments with methyl triflate and base gives the corresponding trimethylammonium-substituted iridabenzofurans while sulfonamide derivatives are formed with p-toluenesulfonyl chloride. N-Acylation of 5 or 6 with acid chlorides, however, selectively form either amide or imide products depending on the charge on the metal and the steric size of the acid chloride. Cationic 5 gives amide substituted products regardless of the conditions whereas neutral 6 rapidly undergoes di-N-acylation with excess benzoyl chloride under mild conditions to give the imide-substituted product Ir(C₇ H₄ O{N[C(O)Ph]₂ -2}{OMe-7})Cl(PPh₃ )₂ (13). Selective mono-acylation of 6 can be achieved with one equivalent of benzoyl chloride or with excess of the sterically congested pivaloyl chloride.

Amphipathic metal-containing macromolecules with photothermal properties

Metal-containing macromolecules with extraordinary properties have attracted a great deal of interest owing to their potential application in the development of functional materials.

Syntheses, structural characterisation and electronic structures of ferrocenyl-osmafuran heterobinuclear organometallic complexes

The electron transfer through the backbone of metallafurans was studied by spectroelectrochemistry and theoretical calculation.

Synthesis of aromatic aza-metallapentalenes from metallabenzene via sequential ring contraction/annulation

The concept of aromaticity has long played an important role in chemistry and continues to fascinate both experimentalists and theoreticians. Among the archetypal aromatic compounds, heteroaromatics are particularly attractive. Recently, substitution of a transition-metal fragment for a carbon atom in the anti-aromatic hydrocarbon pentalene has led to the new heteroaromatic osmapentalenes. However, construction of the aza-homolog of osmapentalenes cannot be accomplished by a similar synthetic manipulation. Here, we report the synthesis of aza-osmapentalenes by sequential ring contraction/annulation reactions of osmabenzenes via osmapentafulvenes. Nuclear magnetic resonance spectra, X-ray crystallographic analysis, and DFT calculations all suggest that these aza-osmapentalenes exhibit aromatic character. Thus, the stepwise transformation of metallabenzenes to metallapentafulvenes and then aza-metallapentalenes provides an efficient and facile synthetic route to these bicyclic heteroaromatics.

Mechanisms of large Stokes shift and aggregation-enhanced emission of osmapentalyne cations in solution: combined MD simulations and QM/MM calculations

The large Stokes shift and aggregation-enhanced emission of OCs are caused by the photoexcitation-induced chromophore's symmetrical variation and molecular aggregation in solutions, respectively.

Reactions of osmapyridinium with terminal alkynes

The reactions of osmapyridinium with terminal alkynes are presented, which lead to the formation of ten-membered osmacycles and η⁴-coordinated cyclopentadiene complexes.

Aromaticity of metallabenzenes and related compounds

In this review, we focus on the aromaticity of a particular family of organometallic compounds known as metallabenzenes, which are characterized by the formal replacement of a CH group in benzene by an isolobal transition metal fragment.