Aromaticity in Metallabenzenes

Electronic Current Density Induced by Uniform Magnetic Fields in Clarenes

Some planar and non-planar clarenes have been studied using maps of magnetically induced quantum-mechanical current density and tools from differential topology to assess their magnetic response in connection with recent results by Du and Wang. Bond current strengths have been computed to estimate quantitative measures. Isosurfaces of the divergence of induced Lorentz force density have been shown to provide useful additional criteria, especially in the case of non-planar clarenes.

Probing the Limit of the Number of Saturated Atoms for Achieving Hyperconjugative Aromaticity

Aromaticity is a fundamental concept in organic chemistry. Hyperconjugative aromaticity, also known as hyperconjugation-induced aromaticity, has evolved from its origin from main group substituents to transition metal analogues, establishing itself as an important category of aromaticity. Additionally, aromatic compounds comprising two sp3-carbon atoms have recently been reported both experimentally and computationally. However, what is the maximum number of sp3-hybridized atoms needed to maintain hyperconjugative aromaticity? Here, we report that hyperconjugative aromaticity can be achieved in hexa-substituted indoliums and octa-substituted pyrroliums, possessing three-five sp3-hybridized carbon/nitrogen atoms by means of density functional theory (DFT) calculations. The aromaticity was confirmed by using various aromaticity indices, i.e., NICS, MCI, and EDDB. Notably, the strong electron-donating ability and aurophilicity of Au(I) substituents play a pivotal role in maintaining the aromaticity and structural integrity. In addition, increasing the number of hyperconjugative centers will decrease the aromaticity in these five-membered rings. Our findings highlight the significance of transition metal substituents in hyperconjugative aromaticity and offer a novel approach for designing aromatic organometallics.

Allyl-Allyl Coupling Promoted by Catalyst Systems with two Palladium Atoms - A Plethora of Potentially Pericyclic Processes

Recently, Huang and co-workers reported a catalytic reaction that utilizes H2 as the sole reductant for a C-C coupling of allyl groups with yields up to 96 %. Here we use computational quantum chemistry to identify several key features of this reaction that provide clarity on how it proceeds. We propose the involvement of a Pd-Pd bound dimer precatalyst, demonstrate the importance of ligand π-π interactions and counterions, and identify a new, energetically viable, mechanism involving two dimerized, outer-sphere reductive elimination transition structures that determine both the rate and selectivity. Although we rule out the previously proposed transmetalation step on energetic grounds, we show it to have an unusual aromatic transition structure in which two Pd atoms support rearranging electrons. The prevalence of potential metal-supported pericyclic reactions in this system suggests that one should consider such processes regularly, but the results of our calculations also indicate that one should do so with caution.

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.

Metallaaromaticity involving a d0 early transition metal centre: synthesis, structure, and aromaticity of tantallapyridinazirine complexes

Though late transition metal aromatic metallabenzenes and related heteroatom-containing analogues have been well studied, the corresponding aromatic early transition metal complexes remain elusive. Herein, we demonstrate the synthesis of aromatic, planar, and delocalised organotantallapyridinium complexes via a simple one-pot process by sequential treatment of tantalum methyl complex [η5:σ-Me2C(C5H4)(C2B10H10)]TaMe3 with alkynes and isocyanide. Single-crystal X-ray analyses, NMR spectroscopic data and DFT calculations suggest that they are aromatic tantallapyridinium complexes, a class of long-sought-after molecules. This work would shed some light on the preparation of metallaaromatics involving early transition metals.

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.

Aromaticity: Quo Vadis

Aromaticity is one of the most deeply rooted concepts in chemistry. But why, if two-thirds of existing compounds can be classified as aromatic, is there no consensus on what aromaticity is? σ-, π-, δ-, spherical, Möbius, or all-metal aromaticity… why are so many attributes needed to specify a property? Is aromaticity a dubious concept? This perspective aims to reflect where the aromaticity community is and where it is going.

Reply to the 'Comment on "The oxidation state in low-valent beryllium and magnesium compounds"' by S. Pan and G. Frenking, Chem. Sci., 2022, , DOI: 10.1039/D2SC04231B

A recent article by Pan and Frenking challenges our assignment of the oxidation state of low valent group 2 compounds. With this reply, we show that our assignment of Be(+2) and Mg(+2) oxidation states in Be(cAACDip)2 and Mg(cAACDip)2 is fully consistent with our data. Some of the arguments exposed by Pan and Frenking were based on visual inspection of our figures, rather than a thorough numerical analysis. We discuss with numerical proof that some of the statements made by the authors concerning our reported data are erroneous. In addition, we provide further evidence that the criterion of the lowest orbital interaction energy in the energy decomposition analysis (EDA) method is unsuitable as a general tool to assess the valence state of the fragments. Other indicators based on natural orbitals for chemical valence (NOCV) deliver a more reliable bonding picture. We also emphasize the importance of using stable wavefunctions for any kind of analysis, including EDA.

A QCT View of the Interplay between Hydrogen Bonds and Aromaticity in Small CHON Derivatives

The somewhat elusive concept of aromaticity plays an undeniable role in the chemical narrative, often being considered the principal cause of the unusual properties and stability exhibited by certain π skeletons. More recently, the concept of aromaticity has also been utilised to explain the modulation of the strength of non-covalent interactions (NCIs), such as hydrogen bonding (HB), paving the way towards the in silico prediction and design of tailor-made interacting systems. In this work, we try to shed light on this area by exploiting real space techniques, such as the Quantum Theory of Atoms in Molecules (QTAIM), the Interacting Quantum Atoms (IQA) approaches along with the electron delocalisation indicators Aromatic Fluctuation (FLU) and Multicenter (MCI) indices. The QTAIM and IQA methods have been proven capable of providing an unbiased and rigorous picture of NCIs in a wide variety of scenarios, whereas the FLU and MCI descriptors have been successfully exploited in the study of diverse aromatic and antiaromatic systems. We used a collection of simple archetypal examples of aromatic, non-aromatic and antiaromatic moieties within organic molecules to examine the changes in π delocalisation and aromaticity induced by the Aromaticity and Antiaromaticity Modulated Hydrogen Bonds (AMHB). We observed fundamental differences in the behaviour of systems containing the HB acceptor within and outside the ring, e.g., a destabilisation of the rings in the former as opposed to a stabilisation of the latter upon the formation of the corresponding molecular clusters. The results of this work provide a physically sound basis to rationalise the strengthening and weakening of AMHBs with respect to suitable non-cyclic non-aromatic references. We also found significant differences in the chemical bonding scenarios of aromatic and antiaromatic systems in the formation of AMHB. Altogether, our investigation provide novel, valuable insights about the complex mutual influence between hydrogen bonds and π systems.

η6-Coordinated ruthenabenzenes from three-component assembly on a diruthenium μ-allenyl scaffold

The room temperature reactions with internal alkynes, RCCR, of the μ-allenyl acetonitrile complex [Ru₂Cp₂(CO)₂(NCMe){μ-η¹:η²-C¹HC²C³Me₂}]BF₄ (1-NCMe), freshly prepared from the tricarbonyl precursor [Ru₂Cp₂(CO)₃{μ-η¹:η²-C¹HC²C³Me₂}]BF₄, 1, proceeded with alkyne insertion into ruthenium-allenyl bond and allenyl-CO coupling, affording compounds [Ru₂Cp₂(CO)₂{μ-η²:η⁵-C(R)C(R)C¹HC²(C³MeCH₂)C(OH)}]BF₄ (R = Ph, 2; R = CO₂Me, 3; R = CO₂Et, 4) in 83-94% yields. Deprotonation of 2-4 by triethylamine gave [Ru₂Cp₂(CO)₂{μ-η²:η⁵-C(R)C(R)CHC(CMeCH₂)C(O)}] (R = Ph, 5; R = CO₂Me, 6; R = CO₂Et, 7) in 75-88% yields, and 2-4 could be recovered upon HBF₄·Et₂O addition to 5-7. All the products, 2-7, were fully characterized by elemental analysis, IR and multinuclear NMR spectroscopy. The structure of 2 was ascertained by single crystal X-ray diffraction and investigated by DFT calculations, revealing a six-membered ruthenacycle with Shannon aromaticity index in line with related compounds. The formation of ruthenium-coordinated ruthenabenzenes from a preexistent diruthenium scaffold is a versatile but underdeveloped approach exploiting cooperative effects typical of a dimetallic core.

Benzene and Borazine, so Different, yet so Similar: Insight from Experimental Charge Density Analysis

Although benzene and borazine are isoelectronic and isostructural, they have very different electronic structures, mainly due to the polar nature of the B-N bond. Herein, we present an experimental study of the charge density distribution obtained from the multipole model formalism and Hirshfeld atom refinement (HAR) based on high-resolution X-ray diffraction data of borazine B₃N₃H₆ (1) and B,B',B″-trichloroborazine (2) crystals. These data are compared to those obtained from HAR for benzene (4) and 1,3,5-trichlorobenzene (5) and further compared with values obtained from density functional theory calculations in the gas phase, where N,N',N″-trichloroborazine (3) was also included. The results confirm that, unlike benzene, borazines are only weakly aromatic with an island-like electronic delocalization within the B₃N₃ ring involving only the nitrogen atoms. Furthermore, delocalization indices and interacting quantum atom energy for bonded and non-bonded atoms were found to be highly suitable indicators capable of describing the origin of the discrepancies observed when the degree of aromaticity in 2 and 3 is evaluated using common aromaticity indices. Additionally, analysis of intermolecular interactions in the crystals brings further evidence of a weakly aromatic character of the borazines as it reveals surprising similarities between the crystal packing of borazine and benzene and also between B,B',B″-trichloroborazine and 1,3,5-trichlorobenzene.

Energetic and Geometric Characteristics of Substituents, Part 3: The Case of NO2 and NH2 Groups in Their Mono-Substituted Derivatives of Six-Membered Heterocycles

Substituted heterocyclic arenes play important roles in biochemistry, catalysis, and in the design of functional materials. Exemplary six-membered heteroaromatic molecules, that differ from benzene by inclusion of one heteroatom, are pyridine, phosphorine, arsabenzene, and borabenzene. This theoretical study concerns the influence of the heteroatom present in these molecules on the properties of substituents of two types: electron-donating (ED) NH2 group and electron-accepting (EA) NO2 group, attached at the 2-, 3-, or 4-position. The effect is evaluated by the energy of interaction (Erel) between the substituent and the substituted system and electronic properties of the substituents described by the charge of the substituent active region (cSAR) index. In addition, several geometric descriptors of the substituent and heteroaromatic ring, as well as changes in the aromaticity, are considered. The latter are assessed using the Electron Density of Delocalized Bonds (EDDBs) property of delocalized π electrons. The obtained results show that the electronegativity (EN) of the heteroatom has a profound effect on the EA/ED properties of the substituents. This effect is also reflected in the geometry of studied molecules. The Erel parameter indicates that the relative stability of the molecules is highly related to the electronic interactions between the substituent and the heteroarene. This especially applies to the enhancement or weakening of π-resonance due to the EN of the heteroatom. Additionally, in the 2-heteroarene derivatives, specific through-space ortho interactions contribute to the heteroatom effects.

The spin–orbit effects on platinabenzene: a ring current and electron delocalization approach

The aromaticity of platinabenzene was studied using the electronic structure and magnetic descriptors with relativistic corrections. It presents aromatic character following the Hückel's rule, negative NICS values, and diatropic current densities.

Occurrence of Double Bond in π-Aromatic Rings: An Easy Way to Design Doubly Aromatic Carbon-Metal Structures

A chemical bonding of several metallabenzenes and metallabenzynes was studied via an adaptive natural density partitioning (AdNDP) algorithm and the induced magnetic field analysis. A unique chemical bonding pattern was discovered where the M=C (M: Os, Re) double bond coexists with the delocalized 6c-2e π-bonding elements responsible for aromatic properties of the investigated complexes. In opposition to the previous description where 8 delocalized π-electrons were reported in metallabenzenes and metallabenzynes, we showed that only six delocalized π-electrons are present in those molecules. Thus, there is no deviation from Hückel's aromaticity rule for metallabenzynes/metallabenzenes complexes. Based on the discovered bonding pattern, we propose two thermodynamically stable novel molecules that possess not only π-delocalization but also retain six σ-delocalized electrons, rendering them as doubly aromatic species. As a result, our investigation gives a new direction for the search for carbon-metal doubly aromatic molecules.

Are Metallacyclopentadienes Always Non-Aromatic?

Even though metallacyclopentadienes (MCPs) are among the most common metallacycles, their electron delocalization (aromaticity) has received far less attention than other metallacycles, such as metallabenzenes. We systematically studied the aromaticity of MCPs with energetic (isomerization stabilization energy), density (delocalization index) and magnetic (current density) aromaticity indices. The indices agree that metallacyclopentadienes are, in general, weakly aromatic at most. The 18e− complexes showed the expected weak aromaticity, and only the d8 molecules are somewhat anti-aromatic. However, the theoretical account of the aromaticity of the 16e− MCPs is more convoluted. We find that the aromatic criteria for a 16e−d4 ruthenacyclopentadiene disagree. The lack of agreement shows that significant electron delocalization is not always related to great stability or to strong diatropic currents.

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.

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.

Theoretical study on the stability and aromaticity in silapentafulvenes towards triplet ground state species

Pentafulvenes are dipolar hydrocarbons since they shift their π-electrons to achieve Hückel aromaticity and thus the electron donating groups at the exocyclic position can enhance their aromaticity. Silapentafulvenes are analogues of pentafulvene formed by the replacement of the carbon atoms at the exocyclic C[double bond, length as m-dash]C double bond with a silicon atom in pentafulvene. It remains unclear how the aromaticity of 5-silapentafulvenes and 6-silapentafulvenes can be changed due to the polarization of the C[double bond, length as m-dash]Si double bond. Here we perform density functional theory calculations and reveal the increased aromatic character in 6-silapentafulvenes and the reduced aromaticity of 5-silapentafulvenes in the ground state. In addition, the origin of the relative thermodynamic stability of the silapentafulvene isomers can be attributed to the bond dissociation energy (BDE) of the exocyclic bond. More interestingly, some triplet ground state 5-silapentafulvene species are predicted by introducing amino groups on the ring, which is supported by the coupled cluster calculations. Our findings could be useful for experimentalists to realize silaaromatics.

Relativity or aromaticity? A first-principles perspective of chemical shifts in osmabenzene and osmapentalene derivatives

The topology of the magnetically induced current density in osmabenzene suggests that the molecule is a novel type of Craig–Möbius aromatic system.

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.

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.

An isolable catenane consisting of two Möbius conjugated nanohoops

Besides its mathematical importance, the Möbius topology (twisted, single-sided strip) is intriguing at the molecular level, as it features structural elegance and distinct properties; however, it carries synthetic challenges. Although some Möbius-type molecules have been isolated by synthetic chemists accompanied by extensive computational studies, the design, preparation, and characterization of stable Möbius-conjugated molecules remain a nontrivial task to date, let alone that of molecular Möbius strips assembling into more complex topologies. Here we report the efficient synthesis, crystal structure, and theoretical study of a catenane consisting of two fully conjugated nanohoops exhibiting Möbius topology in the solid state. This work highlights that oligoparaphenylene-derived nanohoops, a family of highly warped and synthetically challenging conjugated macrocycles, can not only serve as building blocks for interlocked supermolecular structures, but also represent a new class of compounds with isolable Möbius conformations stabilized by non-covalent interactions.

Molecules exhibiting Möbius topology are fascinating but challenging synthetic targets. Here, the authors report the elegant synthesis and crystal structure of a catenane formed from two fully conjugated, interlocked Möbius nanohoops, and use theoretical calculations to understand its conformational stability and aromaticity.

Strict Correlation of HOMO Topology and Magnetic Aromaticity Indices in d-Block Metalloaromatics

Magnetic aromaticity and antiaromaticity of closed shell metalloaromatics with 4d transition metals (Nb, Tc, Rh) is strictly correlated with the orbital topology (Möbius or Hückel) of their π-HOMO, investigated computationally with DFT methods. A surprisingly simple rule emerged: the metallacycle is aromatic (antiaromatic) when the number of π MOs is even and the π-HOMO is of Möbius (Hückel) topology-and vice versa when the number of π MOs is odd.

Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo-Mo δ Bond

We have synthesized and characterized the mixed-ligand dimolybdenum paddlewheel complex Na[(DAniF)₃Mo₂(C₃S₅)] (Na[1]; DAniF = N,N'-di-p-anisylformamidinate, dmit = 1,3-dithiole-2-thione-4,5-dithiolate), which has a six-membered chelating [Mo₂S₂C₂] ring created by equatorial coordination of the dmit (C₃S₅) ligand to the Mo₂ unit. One-electron oxidation of Na[1] using Cp₂FePF₆ yields the neutral complex [(DAniF)₃Mo₂(C₃S₅)] ([1]), and removal of two electrons from Na[1] using AgBPh₄ gives [(DAniF)₃Mo₂(C₃S₅)]BPh₄ ([1]BPh₄). In the crystal structures, [1]^- and [1] present dihedral angles of 118.9 and 142.3° between the plane defined by the Mo-Mo bond vector and the dmit ligand, respectively, while DFT calculations show that in [1]⁺ the Mo-Mo bond and the dmit ligand are coplanar. Complex [1] is paramagnetic with a g value of 1.961 in the EPR spectrum and has a Mo-Mo bond distance of 2.133(1) Å, increased from 2.0963(9) Å for [1]^-. Consistently, a broad absorption band is observed for [1] in the near-IR region, which arises from charge transfer from the dmit ligand to the cationic Mo₂⁵⁺ centers. Interestingly, complex [1]⁺ has an aromatic [Mo₂S₂C₂] core, as evidenced by a large diamagnetic anisotropy, in addition to the coplanarity of the core structure, which shifts downfield the ¹H NMR signal of the horizontal methine proton (ArN-(CH)-NAr) but upfield those of the vertical protons, relative to the methine proton resonances for the precursor ([1]^-). The magnetic anisotropy (Δχ = χ_⊥ - χ_∥) for the [Mo₂S₂C₂] ring in [1]⁺ is -105.5 ppm cgs, calculated from the McConnell equation, which is about 2-fold larger than that for benzene. The aromaticity of the [Mo₂S₂C₂] ring is supported by theoretical studies, including single-point calculations and gauge-including atomic orbital (GIAO) NMR spectroscopic calculations at the density functional theory (DFT) level. DFT calculations also show that the [Mo₂S₂C₂] core in [1]⁺ possesses a set of three highest occupied and three lowest unoccupied molecular orbitals in π character, corresponding to those of benzene in symmetry, and six π electrons that conform to the Hückel 4n + 2 rule for aromaticity. Therefore, this study shows that an aromatic [Mo₂S₂C₂] core is formed by coupling the δ orbital of the Mo≣Mo bond with the π orbital of the C═C bond through the bridging atoms (S), thus validating the equivalency in bonding functionality between δ and π orbitals.

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.

The aromaticity of dicupra[10]annulenes

The aromaticity of dicupra[10]annulenes closely resembles that of naphthalene.

A Metallaanthracene and Derived Metallaanthraquinone

Metalla-analogues of archetypal aromatic molecules are attracting ever increasing interest. Although metallabenzenes (which fall within this class) have been well studied, fused-ring metallabenzenes are rare and of the linear polycyclic metallaaromatic hydrocarbons, only metallanaphthalene is known. Herein we report the first metallaanthracene, [Ir(C₁₃ H₈ {CH₂ CO₂ Me-5})Cl(PPh₃ )₂ ]O₃ SCF₃ (5), which represents the next member of this series of polycyclic compounds. Structurally, 5 has a number of features in common with anthracene including fused-ring planarity and bond-length alternation. In analogues of classic reactions of anthracene, 5 forms a Diels-Alder adduct with maleic anhydride and on oxidation the unprecedented fused-ring metallaanthraquinone, [Ir(C₁₅ H₆ O{Br-6}{OMe-7}{=O-8}{=O-15})Br(PPh₃ )₂ ], is obtained.

Planar vs. three-dimensional X62−, X2Y42−, and X3Y32− (X, Y = B, Al, Ga) metal clusters: an analysis of their relative energies through the turn-upside-down approach

Despite the fact that B and Al belong to the same group 13 elements, the B₆²⁻ cluster prefers the planar D_2h geometry, whereas Al₆²⁻ favours the O_h structure, which is caused by orbital interactions.