Stabilization of anti-aromatic and strained five-membered rings with a transition metal

Metallacyclobutadienes: Intramolecular Rearrangement from Kinetic to Thermodynamic Isomers

Metallacyclobutadienes (MCBDs) are key intermediates of alkyne metathesis reactions. There are in principle two isomerization pathway from kinetic to thermodynamic MCBDs, intermolecular and intramolecular. However, systems that simultaneously isolate two kinds of MCBD isomers have not been achieved, thus restricting the mechanistic studies of the isomerization. Here the reactivity of a metallapentalyne that contains an M≡C bond within the aromatic ring, with alkynes to afford a series of MCBD-fused metallapentalenes is studied. In some cases, both kinetic and thermodynamic products are isolated in the same system, which has never been observed in previous MCBD reactions. Furthermore, the isomerization of MCBD-fused metallapentalenes is investigated both experimentally and theoretically, indicating that it is an intramolecular process involving a metallatetrahedrane (MTd) intermediate. This research provides experimental evidence demonstrating that one MCBD can undergo intramolecular rearrangement to transform into another.

Probing σ-Aromaticity-Driven Ring Contraction of Metallabenzocyclobutadiene to Metallabenzocyclopropene

Ring contraction of metallacyclobutadiene to metallacyclopropene is rare because of the increasing strain from a four-membered ring to a three-membered one. Here we demonstrate a new series of reactions of metallabenzocyclobutadiene to metallabenzocyclopropene via density functional theory calculations. The results suggest that these reactions are thermodynamically favorable ranging from -17.4 to -29.4 kcal mol-1, and a low reaction barrier (10.3 kcal mol-1) is achieved when the metal center is Ru and the ligands are one cyanide and one chloride. Further analysis suggests that a strengthened binding energy helps stabilize the transition state in the protonation process. The aromaticity during the reaction was investigated using the electron density of delocalized bonds (EDDB), isomerization stabilization energy, and isodesmic reactions. The EDDB shows that the π-conjugation is disrupted in the intermediate, and then σ-aromaticity is generated and dominant in the products. Our findings could be helpful for experimentalists in developing novel ring contraction reactions driven by aromaticity.

Unraveling the enigma of Craig-type Möbius-aromatic osmium compounds

Nuclear magnetic resonance (NMR) chemical shifts and the magnetically induced current density (MICD) susceptibility of four osmium containing molecules have been calculated at the density functional theory (DFT) level using three relativistic levels of theory. The calculations were performed at the quasi-relativistic level using an effective core potential (ECP) for Os, at the all-electron scalar exact two-component (X2C) relativistic level, and at the relativistic X2C level including spin-orbit coupling (SO-X2C). In earlier studies, the osmapentalene (1) and the osmapentalynes (2 and 3) were considered Craig-type Möbius aromatic and it was suggested that the analogous osmium compound (4) is Craig-type Möbius antiaromatic. Here, the ring-current strengths were obtained with the gauge including magnetically induced currents (GIMIC) method by integrating the MICD susceptibility passing through planes that intersect chemical bonds and by line integration of the induced magnetic field using Ampère-Maxwell's law. The ring-current calculations suggest that 1, 2 and 3 are weakly aromatic and that 4 is nonaromatic. The accuracy of the MICD susceptibility was assessed by comparing calculated NMR chemical shifts to available experimental data. Visualization of the MICD susceptibility shows that the ring current does not pass from one side of the molecular plane to the other, which means that the MICD susceptibility of the studied molecules does not exhibit any Möbius topology as one would expect for Craig-type Möbius aromatic and for Craig-type Möbius antiaromatic molecules. Thus, molecules 1-3 are not Craig-type Möbius aromatic and molecule 4 is not Craig-type Möbius antiaromatic as previously suggested. Calculations of the 1H NMR and 13C NMR chemical shifts of atoms near the Os atom show the importance of including spin-orbit effects. Overall, our study revisits the understanding of the aromaticity of organometallic molecules containing transition metals.

The aromatic nature of auracycles and diauracycles based on calculated ring-current strengths

We have calculated the magnetically induced current density susceptibility for gold-containing organometallic molecular rings using the gauge-including magnetically induced currents (GIMIC) method. The aromatic nature has been determined by calculating the strength of the magnetically induced ring current susceptibility, which is often called ring current. To our knowledge, we show here for the first time that gold-containing organometallic rings may be aromatic or antiaromatic sustaining ring currents in the presence of an external magnetic field. The calculated aromatic character of the rings agrees with the aromatic nature one expects when using Hückel's aromaticity rules. The studied auracycles and diauracycles with 4n electrons in the conjugated orbitals generally sustain a weak paratropic ring current, whereas those having 4n + 2 electrons in the conjugated orbitals sustain a diatropic ring current that is almost as strong as that of benzene. The number of electrons are obtained by assuming that each C, N and Au atom of the ring contribute one electron, and a H atom connected to a N atom in the ring increases the number of electrons by one. An electron-attracting ligand at Au removes one electron from the ring. Formation of a short Au-Au bonding diauracycles reduces the number of electrons in the ring by two.

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.

Isomerization reactions of metal vinylidene units

Isomerization reactions of unsaturated molecules offer an efficient strategy in atom-economical synthesis. Although isomerization reactions of unsaturated organic and organometallic compounds, such as alkenes, alkynes, and metal carbynes, have been achieved, those of metal vinylidene units that contain cumulated double bonds have never been reported. Herein, we inaugurally discovered isomerization reactions of metal vinylidene units via protonation and deprotonation reactions of metal carbenes. Experimental and theoretical investigations indicate that the electrical characteristics of substituents on the rings play a crucial role in controlling the formation of metal vinylidene units. The isomerization reactions of metal vinylidene units were driven by thermodynamic forces. Moreover, one of the angles at metal vinylidenes was found as 126.9°, representing the smallest angle in metal vinylidenes and the first cyclic 4d transition metal (Ru) vinylidene complex was successfully isolated. These investigations unveil novel structures and reactivity for metal vinylidenes, offering a fresh perspective on the isomerization reactions of unsaturated molecules containing cumulative unsaturated bonds.

Metalla-Carbaporphyrinoids Consisting of an Acyclic N-Confused Tetrapyrrole Analogue Served as Stable Near-Infrared-II Dyes

We present herein the synthesis of novel pseudo-metalla-carbaporphyrinoid species (1M: M=Pd and Pt) achieved through the inner coordination of palladium(II) and platinum(II) with an acyclic N-confused tetrapyrrin analogue. Despite their tetrapyrrole frameworks being small, akin to well-known porphyrins, these species exhibit an unusually narrow HOMO-LUMO gap, resulting in an unprecedentedly low-energy absorption in the second near-infrared (NIR-II) region. Density functional theory (DFT) calculations revealed unique dπ-pπ-conjugated electronic structures involving the metal dπ-ligand pπ hybridized molecular orbitals of 1M. Magnetic circular dichroism (MCD) spectroscopy confirmed distinct electronic structures. Remarkably, the complexes feature an open-metal coordination site in the peripheral NN dipyrrin site, forming hetero-metal complexes (1Pd-BF2 and 1Pt-BF2) through boron difluoride complexation. The resulting hetero metalla-carbaporphyrinoid species displayed further redshifted NIR-II absorption, highly efficient photothermal conversion efficiencies (η; 62-65 %), and exceptional photostability. Despite the challenges associated with the theoretical and experimental assessment of dπ-pπ-conjugated metalla-aromaticity in relatively larger (more than 18π electrons) polycyclic ring systems, these organometallic planar tetrapyrrole systems could serve as potential molecular platforms for aromaticity-relevant NIR-II dyes.

Syntheses and reactivities of strained fused-ring metallaaromatics containing planar eleven-carbon chains

Carbolong complexes are one of the primary types of metallaaromatics, and they include metallapentalynes and metallapentalenes. A series of 7C-10C and 12C-carbolong complexes with planar ligand skeletons respectively containing 7-10 and 12 carbon atoms in their backbones, have been previously reported. Herein, two classes of strained substances, metallabenzyne-fused metallapentalenes and metallabenzene-fused metallapentalynes, were prepared, both representing 11C-carbolong complexes with a planar carbon-chain ligand. Furthermore, the former type is also the carbolong derivatives containing a metallabenzyne skeleton, another primary metallaaromatic framework. Metallabenzyne-fused metallapentalenes show versatile reactivities, and the most interesting one is the metal carbyne bond shift from a 6-membered to a more strained 5-membered ring, affording the above-mentioned metallabenzene-fused metallapentalyne. This work makes carbolong chemistry more complete, and provides a method to achieve metallabenzynes, which is anticipated to concurrently advance the development of these two types of metallaaromatics.

Quest for Double Möbius Aromaticity

Neutral four-membered rings with 4n electrons are generally Hückel antiaromatic. On the other hand, 4n electron system can also be Möbius aromatic, although identification has escaped so far. A recent study of double Möbius aromaticity has been put forwarded in the D2h symmetric singlet ground state of four member Pa2B2 ring. Although interesting, but the synthesis possesses a significant challenge as Pa is rare, highly radioactive and toxic. Herein, a synthetically viable four membered Rh2B2 cluster is proposed which contains double Möbius aromaticity. Interestingly, the three membered RhB2 - cluster also possess Möbius aromaticity and is the smallest ring to show such phenomenon.

Metallaantiaromaticity of 10-Platinacorrole Complexes

The aromaticity of cyclic π-conjugated organometallic compounds is known as metallaaromaticity. π-Conjugated metallacycles, such as metallabenzenes and metallapentalenes, have been investigated in order to understand the involvement of the d electrons from the metal center in the π-conjugated systems of the organic ligands. Here, we report the synthesis of Pd(II) 10-platinacorrole complexes with cyclooctadiene (COD) and norbornadiene (NBD) ligands. While the Pd(II) 10-platinacorrole COD complex adopts a distorted structure without showing appreciable antiaromaticity, the corresponding NBD complex exhibits a distinct antiaromatic character due to its highly planar conformation. Detailed density functional theory (DFT) calculations revealed that two d orbitals are involved in macrocyclic π-conjugation. We furthermore demonstrated that Craig-Möbius antiaromaticity is not present in the studied system. The synthesis of 10-platinacorroles enables a systematic comparison of the antiaromaticity and aromaticity of closely related porphyrin analogues, providing a better understanding of π-conjugation that involves d orbitals.

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.

Experimental and theoretical evidences for the formation of transition metal complexes with five coplanar metal-carbon σ bonds

The σ bond is an important concept in chemistry, and the metal-carbon (M-C) σ bond in particular is a central feature in organometallic chemistry. Synthesis of stable complexes with five coplanar M-C σ bonds is challenging. Here, we describe the synthesis of two different types of stable complexes with five coplanar M-C σ bonds, and examine the stability of such complexes which use rigid conjugated carbon chains to chelate with the metal center. Density functional theory (DFT) calculations show that the M-C σ bonds in these complexes have primarily a covalent character. Besides the σ nature, there are also a π conjugation component among the metal center and carbons, which causes delocalization. This work expanded the coplanar M-C σ bonds to five.

A Triplet Iron Carbyne Complex

Tuning the spin state of metal carbynes, which have broad applications in organic synthesis and material science, presents a formidable challenge for modern chemists as the strong field nature of carbyne ligands dictates low-spin ground spin states (S = 0 or 1/2) for known metal carbynes. Through the oxidative addition reaction of a low-coordinate iron(0) N-heterocyclic carbene complex with the C-S bond of a thioazole-2-ylidene, we synthesized the first triplet (S = 1) metal terminal carbyne, an iron cyclic carbyne complex. Different from the classical metal carbynes, the triplet complex features an LXZ-type carbyne ligand and a weak Fe≡C triple bond, which endow it with the unique reactivity pattern of facile carbyne coupling, weak affinity toward nucleophiles, and facial addition reactions with electrophiles.

Adaptive σ aromaticity in the rhenacyclopropene rings

Species generally exhibit one-state aromaticity either in the lowest singlet state (S0 ) or the lowest triplet state (T1 ) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T1 state of these species is formed by the ππ* excitation, the σ-aromaticity of these three-membered rings in the S0 state could hold in the T1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry.

The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

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.

Synthesis of metalla-dual-azulenes with fluoride ion recognition properties

Azulene-based conjugated systems are of great interests due to their unusual structures and photophysical properties. Incorporation of a transition metal into azulene skeleton presents an intriguing opportunity to combine the dπ-pπ and pπ-pπ conjugated properties. No such metallaazulene skeleton however has been reported to date. Here, we describe our development of an efficient [5 + 2] annulation reaction to rapid construction of a unique metal-containing [5-5-7] scaffold, termed metalla-dual-azulene (MDA), which includes a metallaazulene and a metal-free organic azulene intertwined by sharing the tropylium motif. The two azulene motifs in MDA exhibit distinct reactivities. The azulene motif readily undergoes nucleophilic addition, leading to N-, O- and S-substituted cycloheptanetrienyl species. Demetalation of the metallaazulene moiety occurs when it reacts with nBu4NF, which enables highly selective recognition of fluoride anion and a noticeable color change. The practical [5 + 2] annulation methodology, facile functional-group modification, high and selective fluoride detection make this new π-conjugated polycyclic system very suitable for potential applications in photoelectric and sensing materials.

Adaptive Aromaticity in 18e Metallapentalenes

Complexes with aromaticity in both the lowest singlet state (S0) and the lowest triplet state (T1) (denoted as adaptive aromaticity) are rare because according to Hückel's and Baird's rules, a species could be aromatic in either the S0 or T1 state in most cases. Thus, it is particularly challenging to design species with adaptive aromaticity. Previous reports on adaptive aromaticity were mainly focused on 16e metallapentalenes. Here, we demonstrate that 18e metallapentalenes could possess adaptive aromaticity supported by a set of aromaticity indices when the nitrido and imido ligands are introduced via density functional theory calculations. Further investigation suggests that the metal-carbon bond strength plays an important role in the S0 state aromaticity and the T1 state aromaticity could be attributed to spin electron localization. All these findings could be useful for the development of metallaaromatic chemistry.

Mechanisms in the Catalytic Reduction of N2O by CO over the M13@Cu42 Clusters of Aromatic-like Inorganic and Metal Compounds

Metal aromatic substances play a unique and important role in both experimental and theoretical aspects, and they have made tremendous progress in the past few decades. The new aromaticity system has posed a significant challenge and expansion to the concept of aromaticity. From this perspective, based on spin-polarized density functional theory (DFT) calculations, we systematically investigated the doping effects on the reduction reactions of N₂O catalyzed by CO for M₁₃@Cu₄₂ (M = Cu, Co, Ni, Zn, Ru, Rh, Pd, Pt) core-shell clusters from aromatic-like inorganic and metal compounds. It was found that compared with the pure Cu₅₅ cluster, the strong M-Cu bonds provide more structural stability for M₁₃@Cu₄₂ clusters. Electrons that transferred from the M₁₃@Cu₄₂ to N₂O promoted the activation and dissociation of the N-O bond. Two possible reaction modes of co-adsorption (L-H) and stepwise adsorption (E-R) mechanisms over M₁₃@Cu₄₂ clusters were thoroughly discovered. The results showed that the exothermic phenomenon was accompanied with the decomposition process of N₂O via L-H mechanisms for all of the considered M₁₃@Cu₄₂ clusters and via E-R mechanisms for most of the M₁₃@Cu₄₂ clusters. Furthermore, the rate-limiting step of the whole reactions for the M₁₃@Cu₄₂ clusters were examined as the CO oxidation process. Our numerical calculations suggested that the Ni₁₃@Cu₄₂ cluster and Co₁₃@Cu₄₂ clusters exhibited superior potential in the reduction reactions of N₂O by CO; especially, Ni₁₃@Cu₄₂ clusters are highly active, with very low free energy barriers of 9.68 kcal/mol under the L-H mechanism. This work demonstrates that the transition metal core encapsulated M₁₃@Cu₄₂ clusters can present superior catalytic activities towards N₂O reduction by CO.

Reshaping aromatic frameworks: expansion of aromatic system drives metallabenzenoids to metallapentalenes

Reshaping an aromatic framework to generate other skeletons is a challenging issue due to the stabilization energy of aromaticity. Such reconfigurations of aromatics commonly generate non-aromatic products and hardly ever reshape to a different aromatic framework. Herein, we present the transformation of metallaindenols to metallapentalenes and metallaindenes in divergent pathways, converting one aromatic framework to another with an extension of the conjugation framework. The mechanistic study of this transformation shows that phosphorus ligands play different roles in the divergent processes. Further theoretical studies indicate that the expansion of the aromatic system is the driving force promoting this skeletal rearrangement. Our findings offer a new concept and strategy to reshape and construct aromatic compounds.

Stereoelectronic Modulation of a Single-Molecule Junction through a Tunable Metal-Carbon dπ-pπ Hyperconjugation

Conjugated molecules play a critical role in the construction of single-molecule devices. However, most conventional conjugated molecules, such as hydrocarbons, involve only a pπ-pπ conjugation of light elements. While the metal d-orbitals can introduce abundant electronic effects to achieve novel electronic properties, it is very scarce for the charge transport study of dπ-pπ conjugated pathways with a metal involved. Here, we employed the single-molecule break junction technique to investigate the charge transport through dπ-pπ conjugated backbones with metal-carbon multiple bonds integrated into the alternative conjugated pathways. The involved dπ-pπ conjugation not only supports high conductivity comparable to that of conjugated hydrocarbons but also significantly enhances the tunable diversity in electronic properties through the metal-induced secondary interaction. Specifically, the introduction of the metal brings an unconventionally stereoelectronic effect triggered by metal-carbon dπ-pπ hyperconjugation, which can be tuned by protonation taking place on the metal-carbon multiple bonds, collectively modulating the single-molecule rectification feature and transmission mechanism. This work demonstrates the promise of utilizing the diverse electronic effect of metals to design molecular devices.

Planar Four-Membered Diboron Actinide Compound with Double Möbius Aromaticity

The Möbius rule predicts that a planar four-membered metallacycle can be aromatic with four mobile electrons, but such a simple ring has escaped recognition because it usually favors Hückel anti-aromaticity. Here, we report that a quasi-square four-membered actinide compound (Pa₂B₂) is doubly Möbius aromatic. Chemical bonding analyses reveal that this diboron protactinium molecule has four delocalized π electrons in addition to four delocalized σ electrons, satisfying the 4n Möbius rule for both σ and π components. Energetically, the block-localized wavefunction method, which is the simplest variant of ab initio valence bond theory, shows that the delocalization energy for the π and σ electrons reaches up to 65.0 and 72.3 kcal/mol, respectively, while the extra cyclic resonance energy (ECRE) amounts to 45 kcal/mol. The large positive ECRE values strongly confirm the unprecedented double Möbius aromaticity in Pa₂B₂. We anticipate that this new type of aromatic molecule can enrich the concept of Möbius aromaticity and open a new avenue for actinide compounds.

Isolation, Reactivity, and Tunable Properties of a Strained Antiaromatic Osmacycle

Strain and antiaromaticity in compounds are recognized as two substantial destabilizing features, and consequently, realization of dual destabilizing features in a single molecule is challenging and far more difficult in a single ring. Moreover, transformation of an antiaromatic framework to different antiaromatic or aromatic species is a significant subject in antiaromatic chemistry and has attracted increasing interest. In this work, we isolated a highly strained antiaromatic metallacycle in which a cyclic metal vinylidene unit is embedded. Computational studies revealed its ring strain energies and antiaromatic character and showed that the metal incorporation and the phosphonium substituents play a crucial role in its stabilization. The mechanism of its formation has been illustrated by density functional theory (DFT) calculations and the isolation of a key intermediate. We further discovered diverse reactivities and structural reshuffling of this unusual strained antiaromatic complex according to its two destabilizing characters. We obtained two isomers of metallaindenes fused with oxiranes from the direct oxidation of the metal vinylidene or by nucleophilic addition to an isolated metallacyclocumulene formed by the reaction of metal vinylidene with hydroxide ion, achieving a reconfiguration of the antiaromatic framework. Transformations of the antiaromatic metallacycle by electrophiles to various aromatic metallaindynes have been achieved, and that a condensed Fukui function was employed to confirm the regioselectivity of the electrophilic additions, and the acid/base-induced aromaticity switch along with tunable photophysical properties were investigated. These interesting transformations not only enrich the chemistry of metal vinylidenes and antiaromatics and could also perform potentially as switchable optical materials.

Hetero-carbolong chemistry: experimental and theoretical studies of diaza-metallapentalenes

Four diaza-osmapentalenes were prepared by two-step reactions, through the treatment of an alkyne-coordinated osmium complex with azo compounds, followed by the addition of AgSbF₆/CO. Their aromaticity was confirmed by crystal parameters, NMR spectra and theoretical calculations. These complexes are the first diaza-metallapentalenes representing a new class of metallaaromatics.

Unique Properties and Emerging Applications of Carbolong Metallaaromatics

ConspectusAromatic compounds are important in synthetic chemistry, biomedicines, and materials science. As a special type of aromatic complex, transition-metal-based metallaaromatics contain at least one transition metal in an aromatic framework. The chemistry of metallaaromatics has seen much progress in computational studies and synthetic methods, but their properties and applications are still emerging. In recent years, we have disclosed a series of metal-centered conjugated polycyclic metallacycles in which a carbon chain is chelated to a metal center through at least three metal-carbon bonds. These are termed carbolong complexes and exhibit good stability to water, oxygen, light, and heat on account of their polydentate chelation and aromaticity, making them easy to handle. Carbolong complexes are not only special π-conjugated aromatics but also organometallics; therefore, they have the properties of both species. In this Account, we showcase the recent advances in their applications based on their different properties.First, carbolong complexes are a special kind of π-conjugated aromatic, with the ability to transmit electrons, allowing them to function as single-molecule conductors and candidates for electron transporting layer materials (ETLs) in solar cells. A series of carbolong complexes have been proved to be useful as achievable ETLs which enhance device performance in both organic solar cells and perovskite solar cells.Second, due to the involvement of d orbitals in the conjugation, carbolong complexes normally exhibit strong and broad absorption, even in some cases extending to the near-infrared region (NIR). The absorbed optical energy can be converted into light, heat, and ultrasound; consequently, carbolong compounds can be used as core moieties in smart materials. For example, 7C carbolong complexes were found to exhibit aggregation-enhanced near-infrared emission (AIEE). Some 12C carbolong complexes have been designed into the core moieties of NIR-responsive polymers, such as cylindrical NIR-responsive materials, self-healing materials, and shape memory materials. In contrast to the stereotypically toxic osmium compounds such as the highly toxic OsO₄, some osmium carbolong complexes exhibit low cell cytotoxicity and good biocompatibility; consequently, they also have potential applications in the biomedical area. For example, benefiting from broad absorption in the NIR, 9C and 12C carbolong complexes have been used in photoacoustic imaging and photothermal therapy, respectively. In addition, photodynamic therapeutic applications which take advantage of a carbolong peroxo complex are discussed.Third, as special transition-metal complexes chelated by carbon-based ligands, a carbolong peroxo complex has displayed catalytic activity in the dehydrogenation of alcohols and a bimetallic carbolong complex has been used to catalyze difunctionalization reactions of unactivated alkenes.Overall, aromatic carbolong complexes have been applied to photovoltaics, smart materials, phototherapy, and catalytic reactions. Moving forward, we hope that this Account will shed light on future studies and theoretical research and encourage more discoveries of the properties of other metallaaromatics.

Optical and electrochemical properties of spirobifluorene iridanaphthalene complexes

Three new spirobifluorene iridaaromatic compounds bearing electron-withdrawing or electron-donor substituents or another iridanaphthalene moiety have been synthesized and structurally characterized. Thorough experimental and theoretical evaluation revealed that these novel systems present a high thermal, air and electrochemical stability as well as low optical and electronic energy gap values with a significant redshift of the absorption maximum in the UV-Vis spectra and predicted remarkably higher first hyperpolarizabilities compared to their organic counterparts. Therefore, the combination of a metallaaromatic system with a spirobifluorene moiety leads to the design and development of new spirobifluorene derivatives. These new systems have shown interesting optical and electronic properties making them of interest for future applications in optoelectronics.

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.

Conjugated polymers based on metalla-aromatic building blocks

Conjugated polymers usually require strategies to expand the range of wavelengths absorbed and increase solubility. Developing effective strategies to enhance both properties remains challenging. Herein, we report syntheses of conjugated polymers based on a family of metalla-aromatic building blocks via a polymerization method involving consecutive carbyne shuttling processes. The involvement of metal d orbitals in aromatic systems efficiently reduces band gaps and enriches the electron transition pathways of the chromogenic repeat unit. These enable metalla-aromatic conjugated polymers to exhibit broad and strong ultraviolet-visible (UV-Vis) absorption bands. Bulky ligands on the metal suppress π-π stacking of polymer chains and thus increase solubility. These conjugated polymers show robust stability toward light, heat, water, and air. Kinetic studies using NMR experiments and UV-Vis spectroscopy, coupled with the isolation of well-defined model oligomers, revealed the polymerization mechanism.

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.

Structure, bonding and adaptive aromaticity in rhenium-oxo complexes: a DFT study

The concept of adaptive aromaticity has been extended to a rhenium-oxo complex, introducing a new member into this novel family.

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...

Novel Third-Order Nonlinear Optical Materials with Craig-Möbius Aromaticity

Electron delocalization in aromatic materials significantly impacts their third-order nonlinear optics (NLO). Despite organometallic complexes with Craig-Möbius aromaticity attracting great attention for their unusual physicochemical properties, their third-order NLO have been little studied to date. Herein, 12 Craig-Möbius aromatic organometallics with a stable structure similar to osmapentalyne, namely, carbolong complexes, are screened by DFT. They exhibit high third-order NLO responses because of the d and p electron delocalization in the organometallic ring. Furthermore, electron-hole distribution analyses draw a conclusion that extending the conjugated plane will increase the π-conjugation system to enhance the local excitation in the plane, and the introduction of typical aromatic ligands can result in the organometallic ring-to-ligand charge transfer (RLCT), which are effective methods to improve the third-order NLO response. This study opens a new window in the application of Craig-Möbius aromatic complexes and provides a new approach for third-order NLO materials design.

Metalla-phenalene complexes: synthesis, structure and aromaticity

Metallaaromatics show a diversity of aromaticity. In this work, we report the synthesis and structural characterization of the first rhena-phenalene complexes. In addition to the Hückel aromaticity and σ-aromaticity, pseudo π anti-aromaticity is observed. DFT computations show that this anti-aromaticity (paramagnetic properties) is induced by the fused aromatic naphthyl ring.

An Air- and Water-Stable B4 N4 -Heteropentalene Serving as a Host Material for a Phosphorescent OLED

Replacement of the carbon-carbon bonds of antiaromatic compounds with polar boron-nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B₄ N₄ -heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B₄ N₄ -heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

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.

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.