The chemistry of aromatic osmacycles

Nonplanar Aromaticity of Dinuclear Rare-Earth Metallacycles

While the concept of metalla-aromaticity has well been extended to transition organometallic compounds in diverse geometries, aromatic rare-earth organometallic complexes are rare due to the special (n - 1)d0 configuration and high-lying (n - 1)d orbitals of rare-earth centers. In particular, nonplanar cases of rare-earth complexes have not been reported so far. Here, we disclose the nonplanar aromaticity of dinuclear scandium and samarium metallacycles characterized by various aromaticity indices (nucleus-independent chemical shift, isochemical shielding surface, anisotropy of induced current density, and isomerization stabilization energy). Bonding analyses (Kohn-Sham molecular orbital, adaptive natural density partitioning, multicenter bond indices, and principal interacting orbital) reveal that three delocalized π orbitals, predominantly contributed by the 2-butene tetraanion ligand, result in the formation of six-electron conjugated systems. Guided by these findings, we predicted that the lutetium and gadolinium analogues of dinuclear rare-earth metallacycles should be aromatic, which have been verified by the successful synthesis of real molecules. This work extends the concept of nonplanar aromaticity to the field of rare-earth metallacycles and illuminates the path for designing and synthesizing various rare-earth metalla-aromatics.

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.

Pentacyclic and Hexacyclic Osmaarynes and Their Derivatives

Although osmabenzyne, osmanaphthalyne, osmaphenanthryne, and osmaanthracyne have been previously reported, the synthesis of polycyclic osmaarynes is still a challenge. Herein, we report the successful synthesis of the first pentacyclic osmaarynes (pyreno[b]osmabenzynes 1 a and 2 a) and hexacyclic osmaaryne (peryleno[b]osmabenzyne 3 a). Nucleophilic reaction of osmaarynes was used to obtain the corresponding pyreno[b]osmium complexes (1 and 2) and peryleno[b] osmium complex (3), which exhibited near-infrared luminescence and aggregation-induced emission (AIE) properties. Complexes 2 and 3 are resistant to photodegradation, and complex 2 has better photothermal conversion properties than 3.

Acetylides for the Preparation of Phosphorescent Iridium(III) Complexes: Iridaoxazoles and Their Transformation into Hydroxycarbenes and N,C(sp3),C(sp2),O-Tetradentate Ligands

The preparation of three families of phosphorescent iridium(III) emitters, including iridaoxazole derivatives, hydroxycarbene compounds, and N,C(sp³),C(sp²),O-tetradentate containing complexes, has been performed starting from dimers cis-[Ir(μ²-η²-C≡CR){κ²-C,N-(MeC₆H₃-py)}₂]₂ (R = ^tBu (1a), Ph (1b)). Reactions of 1a with benzamide, acetamide, phenylacetamide, and trifluoroacetamide lead to the iridaoxazole derivatives Ir{κ²-C,O-[C(CH₂^tBu)NC(R)O]}{κ²-C,N-(MeC₆H₃-py)}₂ (R = Ph (2), Me (3), CH₂Ph (4), CF₃ (5)) with a fac disposition of carbons and heteroatoms around the metal center. In 2-methyltetrahydrofuran and dichloromethane, water promotes the C-N rupture of the IrC-N bond of the iridaoxazole ring of 3-5 to form amidate-iridium(III)-hydroxycarbene derivatives Ir{κ¹-N-[NHC(R)O]}{κ²-C,N-(MeC₆H₃-py)}₂{═C(CH₂^tBu)OH} (R = Me (6), CH₂Ph (7), CF₃ (8)). In contrast to 1a, dimer 1b reacts with benzamide and acetamide to give Ir{κ⁴-N,C,C',O-[py-MeC₆H₃-C(CH₂-C₆H₄)NHC(R)O]}{κ²-C,N-(MeC₆H₃-py)}(R = Ph (9), Me (10)), which bear a N,C(sp³),C(sp²),O-tetradentate ligand resulting from a triple coupling (an alkynyl ligand, an amide, and a coordinated aryl group) and a C-H bond activation at the metal coordination sphere. Complexes 2-4 and 6-10 are emissive upon photoexcitation, in orange (2-4), green (6-8), and yellow (9 and 10) regions, with quantum yields between low and moderate (0.01-0.50) and short lifetimes (0.2-9.0 μs).

Theoretical analysis of expanded porphyrins: Aromaticity, stability, and optoelectronic properties

Expanded porphyrin systems are capable of binding a variety of substrates due to their increased cavity size and aromatic nature, holding important applications as magnetic resonance imaging contrast agents and as sensitizers for photodynamic therapy. It is there of fundamental interest to know the photoelectrical properties of expanded porphyrins using quantum chemistry calculations. In this work, we theoretically designed and screened a series of expanded porphyrins by incorporating terthiophene (TTH) and dithienothiophene (DTT) moieties. Our calculations showed that all the designed molecules exhibit excellent optoelectronic performance than the reference molecule. It is suggested that the porphyrin molecule with TTH moiety has better stability than the one with DTT moiety. Finally, we demonstrated that molecule 2 features with TTH moiety and the inverted selenophene ring outperform other molecules because it exhibits increased HOMO-LUMO gap, planar geometry, and strengthened aromaticity. We expect that this work can provide theoretical guidelines for the design of novel porphyrin materials.

Stabilizing a 20-Electron Metallaazulyne by Aromaticity

The 18-electron rule states that metal complexes with 18 valence electron metal centers are thermodynamically stable because nine valence orbitals of transition metals including one s orbital, three p orbitals, and five d orbitals can collectively accommodate 18 electrons, achieving the same electron configuration as the noble gas in the period. Thus, 20-electron compounds are extremely rare due to a violation of such a rule. Here, we demonstrate a 20-electron metallaazulyne via density functional theory calculations stabilized by aromaticity, which was supported by various aromaticity indices including nucleus-independent chemical shift, anisotropy of the induced current density, the isochemical shielding surface, and electron density of delocalized bonds. Interestingly, when a transition metal fragment is first introduced into the aromatic azulyne molecule, the resulting osmaazulyne becomes antiaromatic, in sharp contrast to the previous transformation from pentalyne to metallapentalyne. More interestingly, when osmaazulyne is reduced by two electrons, the resulting 20e osmaazulyne becomes aromatic. Our findings highlight an important application of aromaticity in stabilizing 20e species, inviting experimental verification.

Computational predictions of adaptive aromaticity for the design of singlet fission materials

The concept of adaptive aromaticity has been demonstrated as an alternative strategy for the design of singlet fission materials.

Synthesis and Characterization of Rhena[10]annulynes

Most of the reported metallacycles were limited to small cyclic complexes that contain six-membered or smaller rings. Larger-membered metallacycles are still rare and mainly focus on the dimetallacycles. Herein, we...

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)).

Adaptive aromaticity in 16-valence-electron metallazapentalenes

According to Hückel's and Baird's rules, cyclic species are generally aromatic only either in the lowest singlet state (S₀) or in the lowest-lying triplet ππ* excited state (T₁). Thus, species with aromaticity both in S₀ and T₁ states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O). Interestingly, metallazapentalenes show adaptive aromaticity whereas metalloxapentalenes display nonaromaticity in the S₀ and T₁ states, which is supported by structural, magnetic, and electronic indices. In addition, a series of metallazapentalenes containing strong σ- or π-donor ligands are predicted to achieve adaptive aromaticity. Our findings expand the family of adaptive aromatics significantly, inviting experimental chemists to realize more hetero-metallapentalenes with adaptive aromaticity.

Transmetalation Reactions of Aromatic Dilithionickelole: Synthesis of Heterobimetallic Complexes Featuring Metalloles as Diene Ligands

The aromatic metallole dianions are important metallaaromatic compounds because of their various reactivities and extensive synthetic applications. Herein we report the reactions of dilithionickelole with MgCl₂ , EtAlCl₂ , Cp*ScCl₂ , Cp*LuCl₂ and Pt(COD)Cl₂ (COD=1,5-cyclooctadiene) affording a series of Ni/M heterobimetallic complexes of the general formula (η⁴ -C₄ R₄ M)Ni(COD), in which the metalloles act as diene ligands, as suggested by single-crystal X-ray, NMR and theoretical analyses. In these reactions, two electrons of the nickelole dianion transferred to Ni, representing different reactivity compared with main-group metallole dianions.

Antiaromaticity-Promoted Radical Stability in α-Methyl Heterocyclics

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. In this work, using various aromaticity indices including anisotropy of the induced current density analysis and nucleus-independent chemical shifts against the radical stabilization energy, we systematically investigated the relationship between aromaticity and the thermodynamic stability of α-methyl heterocyclics. Density functional theory calculations suggest that the stronger the antiaromaticity of the original form heterocyclics, the higher the thermodynamic stability of the corresponding radicals, which is in sharp contrast to the general knowledge that aromaticity brings compounds' thermodynamic stabilities. The principal interacting spin orbital analysis shows that the stronger the π-bond formed between the heterocyclics and the α-methyl carbon, the more spin density the radicals tend to be distributed on the heterocyclics. Thus, the strong π-bonding is one of the factors for improving the thermodynamic stability of radicals.

Electrophilic aromatic substitution reactions of compounds with Craig-Möbius aromaticity

Electrophilic aromatic substitution (EAS) reactions are widely regarded as characteristic reactions of aromatic species, but no comparable reaction has been reported for molecules with Craig-Möbius aromaticity. Here, we demonstrate successful EAS reactions of Craig-Möbius aromatics, osmapentalenes, and fused osmapentalenes. The highly reactive nature of osmapentalene makes it susceptible to electrophilic attack by halogens, thus osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene can undergo typical EAS reactions. In addition, the selective formation of a series of halogen substituted metalla-aromatics via EAS reactions has revealed an unprecedented approach to otherwise elusive compounds such as the unsaturated cyclic chlorirenium ions. Density functional theory calculations were conducted to study the electronic effect on the regioselectivity of the EAS reactions.

Carbolong chemistry: nucleophilic aromatic substitution of a triflate functionalized iridapentalene

The reactivity of the triflate functionalized iridapentalene 1, [Ir{[double bond, length as m-dash]CHC(CH₂C(CO₂Me)₂CH₂)[double bond, length as m-dash]CC[double bond, length as m-dash]CHC(OTf)[double bond, length as m-dash]CH}(CO)(PPh₃)₂]OTf, with C-, N-, O- and S-centered neutral nucleophiles was studied, leading to the isolation of a wide array of irida-carbolong derivatives. As an extension, a polycyclic complex with a rare six-fused-ring structure was constructed. This strategy provides a new route for the construction of functionalized metallaaromatic complexes, and the resulting iridacycles exhibit broad spectral absorption ranges, making them potential photoelectric materials.

Osmaindenes: Synthesis and Reversible Mechanochromism Characteristics

A series of novel osmaindenes 1 - 6 bearing different substituents (CF 3 , H, I, Br, OCH 3 , N(Ph) 2 ) has been synthesized by nucleophilic reaction of water with the corresponding aromatic osmanaphthalyne complexes. All osmaindenes 1 - 6 have been characterized by elemental analysis (EA) and nuclear magnetic resonance (NMR) spectroscopy, although the low solubilities of 3 and 4 precluded the accumulation of their 13 C NMR spectra. Osmaindenes 2 , 3 and 5 have also been characterized by single-crystal X-ray diffraction analysis. Subsequently, through solid-state fluorescence spectroscopy, mechanochromic studies, and powder X-ray diffraction (XRD) analysis, we found that osmaindenes 1 - 6 fluoresce at wavelengths in the range 500-800 nm, while also displaying reversible mechanochromic properties. The solid-state fluorescence emission of 1 after grinding extends into the near-infrared region. This research provides new insight into the design and synthesis of metallic materials with excellent mechanochromic properties.

Key factors in the synthesis of polycyclic iridaaromatics via the methoxyalkenylcarbene pathway

Polycyclic iridaaromatic compounds are of great interest not only because of the contributions made in "aromatic chemistry", but also because of the possibility of improving the results of the applications of the corresponding organic analogues in different fields. Therefore, understanding the requirements necessary to build on demand this type of compound with specific properties is of great importance. In this work, the keys to successfully synthesize iridaaromatic complexes via methoxyalkenylcarbenes are established. Experimental and theoretical results show (i) that bearing two aromatic substituents on the gamma carbon of the methoxyalkylcarbene promotes the C-H bond activation; (ii) the need for large steric hindrance of the second substituent for a selective synthesis and, (iii) the selectivity in the C-H bond activation towards the less sterically hindered system.

Iridaaromatics via Methoxy(alkenyl)carbeneiridium Complexes

This review describes the development of a versatile methodology to synthesize polycyclic metallaaromatic hydrocarbons based on iridium, as well as the studies that helped us to determine and understand what is required in order to broaden the scope and the selectivity of the methodology and stabilize the complexes obtained. This methodology aims to open the door to new materials based on graphene fragments.

Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex

An unusual 1,3-hydrogen shift from the metal center to the C_β atom of the C₃-chain of the allenylidene ligand in a hydride-osmium(II)-allenylidene complex is the beginning of several interesting transformations in the cumulene. The hydride-osmium(II)-allenylidene complex was prepared in two steps, starting from the tetrahydride dimer [(Os(H···H){κ³-P,O,P-[xant(PⁱPr₂)₂]})₂(μ-Cl)₂][BF₄]₂ (1). Complex 1 reacts with 1,1-diphenyl-2-propyn-1-ol to give the hydride-osmium(II)-alkenylcarbyne [OsHCl(≡CCH=CPh₂){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ (2), which yields OsHCl(=C=C=CPh₂){κ³-P,O,P-[xant(PⁱPr₂)₂]} (3) by selective abstraction of the C_β–H hydrogen atom of the alkenylcarbyne ligand with K^tBuO. Complex 3 is metastable. According to results of DFT calculations, the migration of the hydride ligand to the C_β atom of the cumulene has an activation energy too high to occur in a concerted manner. However, the migration can be catalyzed by water, alcohols, and aldehydes. The resulting alkenylcarbyne-osmium(0) intermediate is unstable and evolves into a 7:3 mixture of the hydride-osmium(II)-indenylidene OsHCl(=C_IndPh){κ³-P,O,P-[xant(PⁱPr₂)₂]} (4) and the osmanaphthalene OsCl(C₉H₆Ph){κ³-P,O,P-[xant(PⁱPr₂)₂]} (5). Protonation of 4 with HBF₄ leads to the elongated dihydrogen complex [OsCl(η²-H₂)(=C_IndPh){κ³-P,O,P-[xant(PⁱPr₂)₂]}]BF₄ (6), while the protonation of 5 regenerates 2. In contrast to 4, complex 6 evolves to a half-sandwich indenyl derivative, [Os(η⁵-IndPh)H{κ³-P,O,P-[xant(PⁱPr₂)₂]}][BF₄]Cl (7). Phenylacetylene also provokes the 1,3-hydrogen shift in 3. However, it does not participate in the migration. In contrast to water, alcohols, and aldehydes, it stabilizes the resulting alkenylcarbyne to afford [Os(≡CCH=CPh₂)(η²-HC≡CPh){κ³-P,O,P-[xant(PⁱPr₂)₂]}]Cl (8).

Ruthenabenzene: A Robust Precatalyst

Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.

Metalla-aromatics: Planar, Nonplanar, and Spiro

ConspectusThe concept of aromaticity is one of the most fundamental principles in chemistry. It is generally accepted that planarity is a prerequisite for aromaticity, and typically the more planar the geometry of an aromatic compound is, the stronger aromatic it is. However, it is not always the case, particularly when transition metals are involved in conjugation and electron delocalization of aromatic systems, i.e., metalla-aromatics. Because of the intrinsic nature of transition-metal orbitals, besides planar geometries, the most stable molecular structures of metalla-aromatic compounds could take nonplanar and even spiro geometries. In this Account, we outline several unprecedented types of metalla-aromatics developed recently in our research group.Around seven years ago, we found that 1,4-dilithio-1,3-butadienes, dilithio reagents with π-conjugation, could function as non-innocent ligands and react with low-valent transition-metal complexes, generating monocyclic metalla-aromatic compounds. Later on, by taking advantage of the unique behavior of dilithio reagents and the intrinsic nature of different transition metals, we have synthesized a series of metalla-aromatic compounds, of which four types are discussed here, and each of them represents the first of its kind. First, nearly planar aromatic dicupra[10]annulenes, a 10 π-electron aromatic system with two bridging Cu atoms participating in the orbital conjugation and electron delocalization, are synthesized by annulating two dilithio reagents with two Cu(I) complexes.Second, four kinds of spiro metalla-aromatics, featuring planar (with Pd, Pt, or Rh as the spiro atom) geometry with a whole 10π aromatic system, octahedral (tris-spiro metalla-aromatics with V as the spiro atom) geometry with an entire 40π Craig-Möbius aromatic system, tetrahedral (with Mn as the spiro atom) geometry having two independent and perpendicular 6π planar aromatic rings, and tetrahedral (with Mn as the spiro atom) geometry with one planar and one nonplanar 6π aromatic rings, respectively, are generated. In sharp contrast to spiroaromaticity with carbon acting as the spiro atom described in Organic Chemistry, the metal spiro atom herein takes part in orbital conjugation and electron delocalization.Third, nonplanar aromatic butadienyl diiron complexes are realized. Different from planar aromatic systems featuring delocalized π-type overlap, this nonplanar metalla-aromaticity is achieved by the novel σ-type overlap between the two Fe 3d_xz orbitals and the butadienyl π orbital, forming a 6π aromatic system. Fourth, dinickelaferrocene, a ferrocene analogue with two aromatic nickeloles, is synthesized from our monocyclic aromatic dilithionickelole and FeBr₂. The aromaticity of dinickelaferrocene and its nickelole ligands is realized by electron back-donation from the Fe 3d orbital to the π* orbital of nickeloles, which also deepens our understanding of the origin of aromaticity.The search for unprecedented and exciting aromatic systems, particularly with transition metals being involved, will continue to drive this intriguing research field forward. Given the synthetic strategies and various types of metalla-aromatics developed and described, diversified metalla-aromatics of interesting structures and reaction chemistry, novel chemical bonding modes, and useful functions can be expected.

In Silico Analysis of the Electronic Delocalization in Some Double Fused-Ring Metallabenzenes

In the current work, some metallabenzenes with one and several fused rings were analyzed in terms of their electronic delocalization. These fused-ring metallabenzenes are known as metallabenzenoids, and their aromatic character is not free of controversy. The systems of the current work were designed from crystallographic data of some synthesized molecules, and their electronic delocalization (aromaticity) was computationally examined in terms of the molecular orbital analysis (Hückel's rule), the induced magnetic field, and ring currents. The computational evidence allows us to understand if these molecules are or are not aromatic compounds.

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.

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.

Incorporation of Boron into Uranium Metallacycles: Synthesis, Structure, and Reactivity of Boron-Containing Uranacycles Derived from Bis(alkynyl)boranes

The reaction of uranacyclopropene complex (C₅ Me₅ )₂ U[η² -1,2-C₂ (SiMe₃ )₂ ] with B-aryl bis(alkynyl)borane PhB(C≡CPh)₂ led to the first six-membered uranium metallaboracycle, while the reaction with B-amino bis(alkynyl)borane (Me₃ Si)₂ NB(C≡CPh)₂ afforded an unexpected uranaborabicyclo[2.2.0] complex via [2+2] cycloaddition. The reaction with CuCl revealed the non-innocent property of the rearranged bis(alkynyl)boron species towards oxidant. The reactions with isocyanide DippNC: (Dipp=2,6-iPr₂ -C₆ H₃ ) and isocyanate tBuNCO afforded the novel uranaborabicyclo[3.2.0] complexes. All new complexes have been structurally characterized. DFT calculations were performed to provide more insights into the electronic structures and the reaction mechanism.

Theoretical Insight into the Structural Nonplanarity in Aromatic Fused-Ring Metallabenzenes

Metalla-aromatics have attracted considerable attention due to their fascinating structural and reactive properties as well as their potential as prospective functional materials. Metallabenzenes and their fused-ring counterparts are significant members of metalla-aromatics, while their crystal structures often display seemly counterintuitive nonplanar geometry. The geometric bending of metallabenzenes has been attributed to the unfavorable antibonding interactions in the σ-space orbitals rather than the general opinion regarding the π-space orbitals of an aromatic compound. However, the origin of the geometric bending in fused-ring metallabenzenes remains elusive. In this work, we elucidated that such a "σ-control mechanism" still holds for fused-ring metallabenzenes by performing systematic calculations for a plethora of metallabenzenes and fused-ring metallabenzenes. Furthermore, we found that a more bent geometry can be achieved for fused-ring metallabenzenes than their corresponding metallabenzenes by fusing the aromatic rings at the ortho-position of a metal center to induce extra repulsion. The more significant bending in fused-ring metallabenzenes also favors the aromaticity enhancement. These findings not only provide mechanistic insight into the unexpected geometric distortion in both metallabenzenes and fused-ring metallabenzenes but also pave the way to design and develop bent metalla-aromatics with enhanced metalla-aromaticity, which hold great potential as aromatic functional materials.

Butadienyl Diiron Complexes: Nonplanar Metalla-Aromatics Involving σ-Type Orbital Overlap

A new class of nonplanar metalla-aromatics, diiron complexes bridged by a 1,3-butadienyl dianionic ligand, were synthesized in high yields from dilithio reagents and two equivalents of FeBr₂ . The complexes consist of two antiferromagnetically coupled high-spin Fe^II centers, as revealed by magnetometry, Mössbauer spectroscopy, and DFT calculations. Furthermore, experimental (X-ray structural analysis) and theoretical analyses (NICS, ICSS, AICD, MOs) suggest that the complexes are aromatic. Remarkably, this nonplanar metalla-aromaticity is achieved by an uncommon σ-type overlap between the ligand p and metal d orbitals, in sharp contrast to the intensively studied planar aromatic systems featuring delocalized π-type bonding. Specifically, the σ-type interaction between the two Fe 3d_xz orbitals and the butadienyl π orbital results in the formation of a six-electron conjugated system and hence enables the aromatic character.

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.

One-pot syntheses of irida-polycyclic aromatic hydrocarbons

Metalla-analogues of polycyclic aromatic hydrocarbons (PAHs) have captivated chemists with their fascinating structures and unique electronic properties. To date, metallabenzene, metallanaphthalene and metallaanthracene have been reported. Metalla-analogues with more complicated fused rings have rarely been reported. Herein, we have successfully synthesized a series of new iridafluoranthenes and fused-ring iridafluoranthenes ranging from pentacyclic to heptacyclic metallaaromatic hydrocarbons in high yields under mild reaction conditions for the first time. Their photophysical and redox properties were also explored using UV-vis spectroscopy and electrochemistry combined with TD-DFT calculations. The present work may offer an important guideline for the design and construction of new polycyclic metallaaromatic hydrocarbons and metalla-nanographenes.

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.

Tetralithio Metalla-aromatics with Two Independent Perpendicular Dilithio Aromatic Rings Spiro-fused by One Manganese Atom

Herein, we present the realization of a class of unprecedented aromatic structures 2: metalla-aromatics with two independent and perpendicular aromatic rings spiro-fused by a transition-metal spiro atom, of which their corresponding organic analogues are impossible. Tetralithio spiro manganacycles 2 are readily synthesized from 1,4-dilithio-1,3-butadienes 1 and MnCl₂ in the presence of lithium. The aromaticity of 2 is supported by experimental measurements (X-ray structural analysis, NMR) and theoretical analyses (NICS, ACID, MOs). The spiro atom Mn in 2 uses its 3d_xz and 3d_xy orbitals to form the two perpendicular manganacycles, which are two independent 6π aromatic systems. Theoretical analyses reveal that the Li cations play an indispensable role in governing their geometric and electronic structures and hence their aromaticity. Therefore, this work contributes not only to enrich the concept of aromaticity, but also to deepen the understanding of the fundamental chemical bonding.

Synthesis and Characterization of Cyclopropaosmanaphthalenes Containing a Fused σ-Aromatic Metallacyclopropene Unit

Metalla-aromatics are important complexes that show unique properties owing to their highly conjugated systems, which show Hückel or Möbius aromaticity. Recently, several metalla-aromatics showing spiro-aromaticity or σ-aromaticity have been reported. Herein, we report the isolation of the first cyclopropametallanaphthalenes, in which the metallacyclopropene ring shows σ-aromaticity and weak hyperconjugative aromaticity. The reaction of OsCl₂ (PPh₃ )₃ with o-ethynylphenyl alkynes in the presence of PPh₃ followed by protonation with HCl yielded the first cyclopropametallanaphthalenes. The reaction mechanism and the aromaticity were also investigated by density functional theory studies.

Unconventional Aromaticity in Organometallics: The Power of Transition Metals

Aromaticity, one of the most fundamental concepts in chemistry, has attracted considerable attention from both theoreticians and experimentalists. Much effort on aromaticity in organometallics has been devoted to metallabenzene and derivatives. In comparison, aromaticity in other organometallics is less developed. This Account describes how our group has performed quantum chemical calculations to examine aromaticity in recently synthesized novel organometallic complexes. By collaborations with experimentalists, we have extended several aromaticity concepts into organometallics to highlight the power of transition metals. In general, the transition metal could participate in delocalization either out of rings or in the rings. We examined the former by probing the possibility of transition metal substituents in hyperconjugative aromaticity, where the metal is out of the rings. Calculations on tetraaurated heteroaryl complexes reveal that incorporation of the aurated substituents at the nitrogen atom can convert nonaromaticity in the parent indolium into aromaticity in the aurated one due to hyperconjugation, thus extending the concept of hyperconjugative aromaticity to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. Recently, we also probed the strongest aromatic cyclopentadiene and pyrrolium rings by hyperconjugation of transition metal substituents. Moreover, theoretical calculations suggest that one electropositive substituent is able to induce aromaticity; whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. We also probed the possibility of Craig-type Möbius aromaticity in organometallic chemistry, where the position of the transition metals is in the rings. According to the electron count and topology, aromaticity can be classified as Hückel-type and Möbius-type. In comparison with numerous Hückel aromatics containing 4 n+2 π-electrons, Möbius aromatics with 4 n π-electrons, especially the Craig-type species, are particularly limited. We first examined aromaticity in osmapentalynes. Theoretical calculations reveal that incorporation of the osmium center not only reduces the ring strain of the parent pentalyne, but also converts Hückel antiaromaticity in the parent pentalyne into Craig-type Möbius aromaticity in metallapentalynes. Further studies show that the transition metal fragments can also make both 16e and 18e osmapentalenes aromatic, indicating that the Craig-type Möbius aromaticity in osmapentalyne is rooted in osmapentalenes. In addition, Möbius aromaticity is also possible in dimetalla[10]annulenes, where the lithium atoms are not spectator cations but play an important role due to their bonding interaction with the diene moieties. We then examined the possibility of σ-aromaticity in an unsaturated ring. Traditional π-aromaticity is used to describe the π-conjugation in fully unsaturated rings; whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. We found that the unsaturated three-membered ring in cyclopropaosmapentalene is σ-aromatic. Very recently, we extended σ-aromaticity into in a fully unsaturated ring. The concepts and examples presented here show the importance of interplay and union between experiment and theory in developing novel aromatic systems and, especially, the indispensable role of computational study in rationalization of unconventional aromaticity. All these findings highlight the strong power of transition metals originating from participation of d orbitals in aromaticity, opening an avenue to the design of unique metalla-aromatics.