Metallabenzene versus Cp Complex Formation: A DFT Investigation

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.

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.

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.

Evaluating Transition Metal Barrier Heights with the Latest Density Functional Theory Exchange-Correlation Functionals: The MOBH35 Benchmark Database

A new database of transition metal reaction barrier heights (MOBH35) is presented. Benchmark energies (forward and reverse barriers and reaction energy) are calculated using DLPNO-CCSD(T) extrapolated to the complete basis set limit using a Weizmann-1-like scheme. Using these benchmark energies, the performance of a wide selection of density functional theory (DFT) exchange-correlation functionals, including the latest from the Martin, Truhlar, and Head-Gordon groups, is evaluated. It was found, using the def2-TZVPP basis set, that the ωB97M-V (MAD 1.7 kcal/mol), ωB97M-D3BJ (MAD 1.9 kcal/mol), ωB97X-V (MAD 2.0 kcal/mol), and revTPSS0-D4 (MAD 2.2 kcal/mol) hybrid functionals are recommended. The double-hybrid functionals B2K-PLYP (MAD 1.7 kcal/mol) and revDOD-PBEP86-D4 (MAD 1.8 kcal/mol) also performed well, but this has to be balanced by their increased computational cost.

Constraint of a ruthenium-carbon triple bond to a five-membered ring

The incorporation of a metal-carbon triple bond into a ring system is challenging because of the linear nature of triple bonds. To date, the synthesis of these complexes has been limited to those containing third-row transition metal centers, namely, osmium and rhenium. We report the synthesis and full characterization of the first cyclic metal carbyne complex with a second-row transition metal center, ruthenapentalyne. It shows a bond angle of 130.2(3)° around the sp-hybridized carbyne carbon, which represents the recorded smallest angle of second-row transition metal carbyne complexes, as it deviates nearly 50° from the original angle (180°). Density functional theory calculations suggest that the inherent aromatic nature of these metallacycles with bent Ru≡C-C moieties enhances their stability. Reactivity studies showed striking observations, such as ambiphilic reactivity, a metal-carbon triple bond shift, and a [2 + 2] cycloaddition reaction with alkyne and cascade cyclization reactions with ambident nucleophiles.

The role of Cr, Mo and W in the electronic delocalization and the metal–ring interaction in metallocene complexes

Metal influence over triple-decker, sandwich-like and pyramidal structured benzenes was studied by means of energy decomposition analysis (Morokuma–Ziegler), combined with extended transition state natural orbitals for chemical valence, and Nucleus Independent Chemical Shift descriptors.

Predicting an unconventional facile route to metallaanthracenes

DFT calculations reveal an unconventional facile route to metallaanthracenes caused by stabilisation of phosphonium substituents.

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.

Probing the Origin of Challenge of Realizing Metallaphosphabenzenes: Unfavorable 1,2-Migration in Metallapyridines Becomes Feasible in Metallaphosphabenzenes

Metallabenzenes have attracted considerable interest of both theoretical and experimental chemists. However, metallaphosphabenzene has never been synthesized. Thus, understanding the origin of the challenge of synthesizing metallaphosphabenzene is particularly urgent for experimentalists. Now density functional theory (DFT) calculations have been carried out to examine this issue. Our results reveal that the 1,2-migration in metallapyridines is unfavorable whereas such a 1,2-migration in metallaphosphabenzenes is feasible, which can be rationalized by the reluctance of phosphorus to participate in π bonding. In addition, π-donor ligands and the 5d transition metals can stabilize metallaphosphabenzenes. Compared with hydride and methyl migration, the chloride migration has a relatively lower activation barrier due to the polarization of the M=P bond. CO ligand could further decrease the reaction barrier of the migration due to the reduction of the interaction between the metal centre and the phosphorus atom. All of these findings could help synthetic chemists to realize the first metallaphosphabenzene.

Crystal structures of two unusual, high oxidation state, 16-electron irida-benzenes

Treatment of carbon-yl(1,2-diphenylpenta-1,3-dien-1-yl-5-yl-idene)bis-(tri-phenyl-phosphane)iridium, [IrCO(-C(Ph)=C(Ph)-CH=CH-CH=)(PPh3)2], with either bromine or iodine produced di-bromido-(1,2-diphenylpenta-1,3-dien-1-yl-5-yl-idene)(tri-phenyl-phosphine)iridium(III), [IrBr2{-C(Ph)=C(Ph)-CH=CH-CH=}(PPh3)], (I), and (1,2-diphenylpenta-1,3-dien-1-yl-5-yl-idene)di-iodido-(tri-phenyl-phosphane)iridium(III), [IrI2{-C(Ph)=C(Ph)-CH=CH-CH=}(PPh3)], (II), respectively, which are two rare examples of 16-electron metalla-benzenes. Structural elucidation of (I) and (II) reveals that these isotypic irida-benzenes are unusual, not only in their electron count, but also in their coordination sphere of the Ir(III) atom where they contain an apparent open coordination site. The crystal structures of (I) and (II) confirm that the mol-ecules are complexes containing five-coordinated Ir(III) with only one tri-phenyl-phosphine group bound to the iridium atom, unambiguously proving that the mol-ecules are indeed 16-electron, high-oxidation-state irida-benzenes. The coordination geometry of the Ir(III) atom in both structures can be best described as a distorted square pyramid with the P, two Br (or I) and one C atom in the basal plane and another C atom in the apical position.

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.

Unexpected 1,2-migration in metallasilabenzenes: theoretical evidence for reluctance of silicon to participate in π bonding

Density functional theory (DFT) calculations were carried out to investigate the 1,2-migration in metallasilabenzenes. The results suggested that the chloride migration of metallabenzenes is unfavorable due to the loss of aromaticity in the nonaromatic analogues. In sharp contrast, such a migration in metallasilabenzenes is favorable due to the reluctance of silicon to participate in π bonding. The migration of hydride and methyl group from the metal center to the silicon atom in metallasilabenzenes is computed to be also feasible. In addition, the π donor ligand and the third row transition metal can stabilize metallasilabenzenes. Thus, such a migration becomes less favorable thermodynamically and kinetically. These findings could be very helpful for synthetic chemists to realize the first metallasilabenzene.

Functionalization of metallabenzenes through nucleophilic aromatic substitution of hydrogen

The cationic metallabenzenes [Ir(C(5)H(4){SMe-1})(κ(2)-S(2)CNEt(2))(PPh(3))(2)]PF(6) (1) and [Os(C(5)H(4){SMe-1})(CO)(2)(PPh(3))(2)][CF(3)SO(3)] (2) undergo regioselective nucleophilic aromatic substitution of hydrogen at the metallabenzene ring position γ to the metal in a two-step process that first involves treatment with appropriate nucleophiles and then oxidation. Thus, reaction between compound 1 and NaBH(4), MeLi, or NaOEt gives the corresponding neutral iridacyclohexa-1,4-diene complexes Ir(C(5)H(3){SMe-1}{H-3}{Nu-3})(κ(2)-S(2)CNEt(2))(PPh(3))(2) (Nu = H (3), Me (4), OEt (5)). Similarly, reaction between 2 and NaBH(4) or MeLi gives the corresponding osmacyclohexa-1,4-diene complexes Os(C(5)H(3){SMe-1}{H-3}{Nu-3})(CO)(2)(PPh(3))(2) (Nu = H (8), Me (9)). The metallacyclohexa-1,4-diene rings in all these compounds are rearomatized on treatment with the oxidizing agent O(2), CuCl(2), or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Accordingly, the cationic metallabenzene 1 or 2 is returned after reaction between 3 and DDQ/NEt(4)PF(6) or between 8 and DDQ/NaO(3)SCF(3), respectively. The substituted cationic iridabenzene [Ir(C(5)H(3){SMe-1}{Me-3})(κ(2)-S(2)CNEt(2))(PPh(3))(2)]PF(6) (6) or [Ir(C(5)H(4){SMe-1}{OEt-3})(κ(2)-S(2)CNEt(2))(PPh(3))(2)]PF(6) (7) is produced in a similar manner through reaction between 4 or 5, respectively, and DDQ/NEt(4)PF(6), and the substituted cationic osmabenzene [Os(C(5)H(3){SMe-1}{Me-3})(CO)(2)(PPh(3))(2)]Cl (10) is formed in good yield on treatment of 9 with CuCl(2). The starting cationic iridabenzene 1 is conveniently prepared by treatment of the neutral iridabenzene Ir(C(5)H(4){SMe-1})Cl(2)(PPh(3))(2) with NaS(2)CNEt(2) and NEt(4)PF(6), and the related starting cationic osmabenzene 2 is obtained by treatment of Os(C(5)H(4){S-1})(CO)(PPh(3))(2) with CF(3)SO(3)CH(3) and CO. The stepwise transformations of 1 into 6 or 7 as well as 2 into 10 provide the first examples in metallabenzene chemistry of regioselective nucleophilic aromatic substitutions of hydrogen by external nucleophiles. DFT calculations have been used to rationalize the preferred sites for nucleophilic attack at the metallabenzene rings of 1 and 2. The crystal structures of 1, 3, 6, and 7 have been obtained.

Osmabenzenes from osmacycles containing an eta2-coordinated olefin

Treatment of HC[triple bond]CC(CH3)(OH)CH=CH2 with [OsCl2(PPh3)3] in dichloromethane yielded the eta2-olefin-coordinated osmacycle [Os{CH=C(PPh3)C(=CH2)-eta2-CH=CH2}Cl2(PPh3)2] (9). Transformations of osmacycle 9 by treatment with benzonitrile under various conditions have been investigated. Reaction of 9 with excess benzonitrile at room temperature afforded the dicationic osmacycle [Os{CH=C(PPh3)C(=CH2)-eta2-CH=CH2}(PhCN)2(PPh3)2]Cl2 (11) by ligand substitution, which reacted further to the intramolecularly coordinated eta2-allene complex [Os{CH=C(PPh3)C(CH3)=(eta2-C=CH2)}(PhCN)2(PPh3)2]Cl2 (12). In contrast, heating a chloroform solution of 9 to the reflux temperature in the presence of excess benzonitrile generated osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}(PhCN)2(PPh3)2]Cl2 (14). Complexes 11, 12 and 14 are in fact isomers. In the absence of excess benzonitrile, the isolated dicationic 12 and 14 readily dissociate the benzonitrile ligands in solution to produce the neutral complex [Os{CH=C(PPh3)C(CH3)=(eta2-C=CH2)}Cl2(PPh3)2] (13) and the monocationic osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}Cl(PhCN)(PPh3)2]BPh4 (15), respectively. Mechanisms for the formation of osmabenzene 14 from 11 and 12 are proposed based on DFT calculations.

Aromaticity in Metallabenzenes

The electronic structure and bonding situation in 21 metallabenzenes (metal=Os, Ru, Ir, Rh, Pt, and Pd) were investigated at the DFT level (BP86/TZ2P) by using an energy decomposition analysis (EDA) of the interaction energy between various fragments. The aim of the work is to estimate the strength of the pi bonding and the aromatic character of the metallacyclic compounds. Analysis of the electronic structure shows that the metallacyclic moiety has five occupied pi orbitals, two with b1 symmetry and three with a2 symmetry, which describe the pi-bonding interactions. The metallabenzenes are thus 10 pi-electron systems. This holds for 16-electron and for 18-electron complexes. The pi bonding in the metallabenzenes results mainly from the b1 contribution, but the a2 contribution is not negligible. Comparison of the pi-bonding strength in the metallacyclic compounds with acylic reference molecules indicates that metallabenzenes should be considered as aromatic compounds whose extra stabilization due to aromatic conjugation is weaker than in benzene. The calculated aromatic stabilization energies (ASEs) are between 8.7 kcal mol(-1) for 13 and 37.6 kcal mol(-1) for 16 which is nearly as aromatic as benzene (ASE=42.5 kcal mol(-1)). The classical metallabenzene model compounds 1 and 4 exhibit intermediate aromaticity with ASE values of 33.4 and 17.6 kcal mol(-1). The greater stability of the 5d complexes compared with the 4d species appears not to be related to the strength of pi conjugation. From the data reported here there is no apparent trend or pattern which indicates a correlation between aromatic stabilization and particular ligands, metals, coordination numbers or charge. The lower metal-C5H5 binding energy of the 4d complexes correlates rather with weaker sigma-orbital interactions.

Recent Advances in Metallabenzene Chemistry

Research into aromatic metallacycles, though discussed in the literature over the last quarter century, has undergone a major expansion since 2000. A wide variety of new metallabenzenes, encompassing new synthetic methods and new metal centers, is now available. New aromatic metallacycle topologies (iridanaphthalene, osmabenzynes) have been isolated and characterized. The first metallabenzene valence isomers (iridabenzvalenes, rhodabenzvalenes) and constitutional isomers (isoosmabenzenes) are now known. This review discusses the synthesis, chemistry, and physical properties of these intriguing aromatic compounds.