A Computational Foray into the Formation and Reactivity of Metallabenzenes

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.

In Silico Analysis of the Aromaticity of Some Carbo-Metallabenzenes and Carbo-Dimetallabenzenes (Carbo-mers Proposed from Metallabenzenes)

In the current work, we introduce a novel class of molecules termed carbo-metallabenzenes, and their aromaticity has been comprehensively analyzed. The molecules were strategically designed with the insertion of acetylene (C≡C or C2) units in some selected metallabenzenes. Furthermore, if a second metallic unit is inserted (replacing a sp2 carbon) in the carbo-metallabenzenes rings, a new family of carbo-mers is generated, and this second group has been named as carbo-dimetallabenzenes. The primary objective of this work is to ascertain, through various methodologies, whether these newly proposed molecules retain the aromatic characteristics observed in carbo-benzene. The methodologies employed for bond analysis and aromaticity exploration include the analysis of the molecular orbitals, energy decomposition analysis, electron density of delocalized bonds, magnetically induced current density, and the induced magnetic field (Bind). This study sheds light on that the insertion of the metallic centers reduces the electronic delocalization and their aromaticity is, in some cases, comparable with the electronic delocalization of the inorganic iminobora-borazine and also provides valuable insights into their electronic structure through a multifaceted analysis.

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.

Atom Swapping on Aromatic Rings: Conversion from Phosphinine Pincer Metal Complexes to Metallabenzenes Triggered by O2 Oxidation

Herein, we report a novel method for the synthesis of metallabenzenes by swapping the phosphorus atom in an aromatic phosphinine ring with transition metal fragments. The oxidation of a phosphine-phosphinine-phosphine pincer iridium complex by O₂ triggered the replacement of the phosphorus atom of the phosphinine ring by an iridium fragment to afford iridabenzene. Dianionic rhodabenzene was also synthesized from a phosphinine rhodium complex by oxidation of the phosphorus atom, followed by subsequent reduction using metallic potassium. The aromaticity of the newly synthesized irida- and rhoda-benzenes was evaluated both experimentally and theoretically.

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.

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.

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.

Substituent effects on the structure and properties of (para-C5H4X)Ir(PH3)3 complexes in the ground state (S0) and first singlet excited state (S1): DFT and TD-DFT investigations

The ground and lowest singlet excited state geometries of selected ( para-C5H4X)Ir(PH3)3 iridabenzene complexes ( para-substituent = NH2, OMe, Me, H, F, Cl, CCl3, CF3, NO2) are optimized using the MPW1PW91 procedure employing the LanL2DZ(Ir) and 6-311G(d, p) (C, H, N, O, P, F, Cl, P) basis sets. The excited state is generated using the time-dependent density function method. The effects of electron-donating groups and electron-withdrawing groups on the energy, atomization energy, rotational constants, and frontier orbital energies in the first singlet excited state (S1) of iridabenzene are investigated and compared to those of the ground state (S0). The Ir–C and Ir–P bonds in the studied molecules are analyzed by electron localization function and localized-orbital locator methods. The correlations between the Ir-C and Ir–P bond distances, electron localization function, and localized-orbital locator values Hammett constants (σp) and dual parameters (σI and σR) are given for the two studied states. The para-delocalization index is used for investigation of the aromaticity of the studied complexes.

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.

Rhodafuran from a methoxy(alkenyl)carbene by the rhoda-1,3,5-hexatriene route

A methoxy(alkenyl)carbenerhodium complex [RhCp*Cl{[double bond, length as m-dash]C(OMe)CH[double bond, length as m-dash]CPh2}(PMe3)]PF6 (2) has been synthesized and used as the starting material for the study of the effect of the metal center (Rh vs. Ir) in the formation of new rhodacycle complexes. While η3 and η5 indenylrhodium complexes have been achieved by the C-H bond activation of a phenyl ring, insertion of terminal alkynes into the rhodium-carbene bond led to the first example of the synthesis of rhodafuran complexes through rhoda-1,3,5-hexatriene intermediates. This new method represents an efficient process to obtain metallafuran complexes.

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.

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

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

Halogenation of carbyne complexes: isolation of unsaturated metallaiodirenium ion and metallabromirenium ion

The halogenation of metallapentalyne led to the formation of metallapentalenes. Experimental and computational studies indicate that four resonance structures, including the first examples of metallaiodirenium and metallabromirenium ions, contribute to the overall structure of the complexes.

The halogenation of metallapentalyne led to the formation of metallapentalenes, which were fully characterized and studied by DFT calculations. The experimental and computational studies indicate that four resonance forms contribute to the overall structure of the metallapentalenes, and one resonance form could be viewed as the first examples of metallaiodirenium and metallabromirenium ions.

σ Aromaticity Dominates in the Unsaturated Three-Membered Ring of Cyclopropametallapentalenes from Groups 7-9: A DFT Study

Aromaticity, an old but still fantastic topic, has long attracted considerable interest of chemists. Generally, π aromaticity is described by π-electron delocalization in closed circuits of unsaturated compounds whereas σ-electron delocalization in saturated rings leads to σ aromaticity. Interestingly, our recent study shows that σ aromaticity can be dominating in an unsaturated three-membered ring (3MR) of cyclopropaosmapentalene. An interesting question is raised: Can the σ aromaticity, which is dominant in the unsaturated 3MR, be extended to other cyclopropametallapentalenes? If so, how could the metal centers, ligands, and substituents affect the σ aromaticity? Here, we report a thorough theoretical study on these issues. The nucleus-independent chemical shift calculations and the anisotropy of the current-induced density plots reveal the dominant σ aromaticity in these unsaturated 3MRs. In addition, our calculations show that substituents on the 3MRs have significant effects on the σ aromaticity, whereas the ligand effect is particularly small.

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

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

Reactions of osmapyridinium with terminal alkynes

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

Aromaticity of metallabenzenes and related compounds

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

Impacts of terminal modification of [Ru(phen)2dppz]2+on the luminescence properties: a theoretical study

Lowest singlet transitions were found to be related to the intriguing luminescence properties of three different dppz-like ruthenium(ii) complexes through a theoretical study.

A computational study on 3-azonia-, 3-phosphonia-, and 3-arsoniaspiro[2.2]pentanes and related three-membered heterocycles

A theoretical study at the ab initio MP2/6-311++G(d,p) level of theory is carried out to characterize several heterocyclic spiro[2.2]pentane cations with N, P, and As as spiro atoms. The strain and relative stability of the spiropentanes are obtained through isodesmic reactions. Nucleus-independent chemical shifts (NICS) and 3D NICS isosurfaces show σ-aromatic characteristics, similar to those found in cyclopropane. The interaction with the Cl(-) anion, which results in four different stationary structures, is studied and characterized by means of the atoms in molecules methodology, and Cl⋅⋅⋅pnicogen, Cl⋅⋅⋅H, and Cl⋅⋅⋅C interactions are found. The most stable structure in all cases corresponds to opening of one of the three-membered rings, due to the attack of the Cl atom, and C-Cl bond formation. Furthermore, the reaction with the 3-boranuidaspiro[2.2]pentane anion results in the formation of a new compound through cleavage of one ring of both reactants.

The mechanism of catalytic methylation of 2-phenylpyridine using di-tert-butyl peroxide

The mechanism of palladium chloride-catalyzed direct methylation of arenes with peroxides is elucidated by using the energetics computed at the M06 density functional theory. The introduction of a methyl group by tert-butyl peroxides at the ortho-position of a prototypical 2-phenyl pyridine, a commonly used substrate in directed C-H functionalization reactions, is examined in detail by identifying the key intermediates and transition states involved in the reaction sequence. Different possibilities that differ in terms of the site of catalyst coordination with the substrate and the ensuing mechanism are presented. The important mechanistic events involved are (a) an oxidative or a homolytic cleavage of the peroxide O-O bond, (b) C-H bond activation, (c) C-C bond activation, and (d) reductive elimination involving methyl transfer to the aromatic ring. We have examined both radical and non-radical pathways. In the non-radical pathway, the lowest energy pathway involves C-H bond activation prior to the coordination of the peroxide to palladium, which is subsequently followed by the O-O bond cleavage of the peroxide and the C-C bond activation. Reductive elimination in the resulting intermediate leads to the vital C-C bond formation between methyl and aryl carbon atoms. In the non-radical pathway, the C-C bond activation is higher in energy and has been identified as the rate-limiting step of this reaction. In the radical pathway, however, the activation barrier for the C-C bond cleavage is lower than for the peroxide O-O bond cleavage. A combination of a radical pathway up to the formation of a palladium methyl intermediate and a subsequent non-radical pathway has been identified as the most favored pathway for the title reaction. The predicted mechanism is in good agreement with the experimental observations on PdCl2 catalyzed methylation of 2-phenyl pyridine using tert-butyl peroxide.

m-Metallaphenol: synthesis and reactivity studies

Treatment of the osmium complex [Os{CHC-(PPh3 )CH(OH)-η(2) -C≡CH}(PPh3 )2 (NCS)2 ] (1) with excess triethylamine produces the first m-metallaphenol complex [Os{CHC(PPh3 )CHC(OH)CH}(PPh3 )2 (NCS)2 ] (2). The NMR spectroscopic and structural data as well as the nucleus-independent chemical-shift (NICS) values suggest that osmaphenol 2 has aromatic character. The reactivity studies demonstrate that 2 can react with different isocyanates to form the annulation reaction products [Os{CHC(PPh3 )CHC(OCONR)C}(PPh3 )2 (NCS)2 ] (R=Ph (3), iPr (7), Bn (8)) via the carbamate intermediates [Os{CHC(PPh3 )CHC(O-CONHR)CH}(PPh3 )2 (NCS)2 ] (R=Ph (4), iPr (5), Bn (6)). In addition, the similar annulation reactions can be extended to other unsaturated compounds containing N-C multiple bonds, for example, isothiocyanates, pyridine, and sodium thiocyanate, which can produce the corresponding fused osmabenzene complexes. In contrast, the reactions of 2 with common electrophiles, such as NOBF4 , NO2 BF4 , N-bromosuccinimide, and N-chlorosuccinimide only led to the decomposition of the metallaphenol ring. The experimental results suggest that 2 is very electrophilic and readily reacts with nucleophiles, which is mainly due to the metal center and the strong electron-withdrawing phosphonium group.

Quantum chemical predictions of structural, bonding and spectroscopic properties of ruthenanaphthalenes and ring-fused B–N ruthenabenzenes

In this work, the electronic structure and properties of the ruthenanaphthalenes, and ring-fused B – N ruthenabenzenes isomers have been explored using the hybrid density functional mpw1pw91 theory. These calculations show more stability in isomers with Ru – N bond. This is compatible with principles of minimum energy and minimum polarizability. Molecular orbital analysis shows increase of hardness in isomers with Ru – B bonds rather than Ru – N isomers. Also, calculations show that magnitude of the first hyperpolarizability tensor of all molecules is rather moderate. Nucleus-independent chemical shift analysis (NICS) has been used for studying of aromaticity in these species.