A Metal-Bridged Tricyclic Aromatic System: Synthesis of Osmium Polycyclic Aromatic Complexes

Nucleophilic Reactions of Osmanaphthalynes with PMe3 and H2 O

Members of a new class of complexes, 2(CF₃ ), 2(H), 2(Br), 2(I), and 2(OCH₃ ), have been synthesized in a one-pot method involving the treatment of osmanaphthalynes bearing corresponding substituents (1(CF₃ ), 1(H), 1(Br), 1(I), and 1(OCH₃ )) with trimethylphosphine (PMe₃ ) and water. The main reaction process involves two steps, namely a ligand-exchange with trimethylphosphine and nucleophilic addition of water to the Os≡C bond of the osmanaphthalyne. The substituents have a significant influence on the rate of the reaction, as befits a nucleophilic addition. Fortunately, the key intermediate [1(OCH₃ )]' could be successfully captured, and the detailed reaction mechanism has been explored with the aid of density functional theory (DFT) calculations, which were in excellent agreement with the experimental findings. All of the target complexes have been fully characterized by ¹ H, ³¹ P{¹ H}, and ¹³ C{¹ H} NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis.

Are Hetero-metallapentalenes Aromatic or Not? A DFT Investigation

Aromaticity is one of the most basic concepts in organic chemistry. The planar Möbius aromatic metallapentalynes and metallapentalenes have attracted considerable attention in the past few years. However, the aromaticity of metallapentalenes containing heteroatoms (such as B, N, and O), termed as hetero-metallapentalenes, is rarely studied. Herein, the stability and aromaticity of a series of hetero-metallapentalenes are theoretically investigated. The results reveal lower aromaticity in metallaborapentalene, comparable aromaticity in metallazapentalene, and nonaromaticity in metalloxapentalene relative to that of metallapentalene. Moreover, the effect of Lewis bases on the aromaticity and stability of metallaborapentalene is discussed. These results provide useful information for experimental chemists to realize more hetero-metallapentalenes.

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

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

σ-Aromaticity in a Fully Unsaturated Ring

Aromaticity is one of the most fundamental and fascinating chemical topics, attracting both experimental and theoretical chemists owing to its many manifestations. Both σ- and π-aromaticity can be classified depending on the character of the cyclic electron delocalization. In general, σ-aromaticity stabilizes fully saturated rings with σ-electron delocalization whereas the traditional π-aromaticity describes the π-conjugation in fully unsaturated rings. Here, we demonstrate a strong correlation between nucleus-independent chemical shift (NICS) values and extra cyclic resonance energies (ECREs), which are used to evaluate the σ-aromaticity in an unsaturated three-membered ring (3MR) of cyclopropene, which were computed by molecular orbital (MO) theory and valence bond (VB) theory, respectively. Further study shows that the fully unsaturated ring in methylenecyclopropene and its metallic analogy is σ-aromatic. Our findings revolutionize the fundamental knowledge of the concept of σ-aromaticity, thus opening an avenue to design σ-aromaticity in other fully unsaturated systems, which are traditionally reserved as the domain of π-aromaticity.

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.

Aromatic Osmacyclopropenefuran Bicycles and Their Relevance for the Metal-Mediated Hydration of Functionalized Allenes

The aromatic osmacyclopropenefuran bicycles [OsTp{κ³ -C¹ ,C² ,O-(C¹ H₂ C² CHC(OEt)O)}(Pⁱ Pr₃ )]BF₄ (Tp=hydridotris(1-pyrazolyl)borate) and [OsH{κ³ -C¹ ,C² ,O-(C¹ H₂ C² CHC(OEt)O)}(CO)(Pⁱ Pr₃ )₂ ]BF₄ , with the metal fragment in a common vertex between the fused three- and five-membered rings, have been prepared via the π-allene intermediates [OsTp(η² -CH₂ =CCHCO₂ Et)(OCMe₂ )(Pⁱ Pr₃ )]BF₄ and [OsH(η² -CH₂ =CCHCO₂ Et)(CO)(OH₂ )(Pⁱ Pr₃ )₂ ]BF₄ , and their aromaticity analyzed by DFT calculations. The bicycle containing the [OsH(CO)(Pⁱ Pr₃ )₂ ]⁺ metal fragment is a key intermediate in the [OsH(CO)(OH₂ )₂ (Pⁱ Pr₃ )₂ ]BF₄ -catalyzed regioselective anti-Markovnikov hydration of ethyl buta-2,3-dienoate to ethyl 4-hydroxycrotonate.

Synthesis and Characterization of Dirhenadehydro[12]annulenes

The 12-membered-ring metallacycles [mer-Re{≡CCH=C(R)C≡C-}Cl(PMe2 Ph)3 )]2 (R=CMe3 , 1-adamantyl), which are organometallic analogues of antiaromatic octadehydro[12]annulene, are prepared by heating the methyl carbyne complexes mer-Re{≡CCH=C(R)C≡CH}(CH3 )Cl(PMe2 Ph)3 . An intermolecular σ-bond metathesis between the Re-CH3 bond and the acetylenic C-H bond is proposed for their formation.

Synthesis of aromatic ruthenabenzothiophenes via C–H activation of thiophenes

The first ruthenabenzothiophenes have been achievedviathe C–H activation of thiophene.

Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects

Aromaticity, one of the most important concepts in organic chemistry, has attracted considerable interest from both experimentalists and theoreticians. It remains unclear which NICS index is best to evaluate the triplet-state aromaticity. Here, we carry out thorough density functional theory (DFT) calculations to examine this issue. Our results indicate that among the various computationally available NICS indices, NICS(1)zz is the best for the triplet state. The correlations can be improved from 0.840 to 0.938 when only neutral species are considered, demonstrating the significant effect of the charge on the triplet-state aromaticity. In addition, calculations suggest that five-membered cyclic species with "hyperconjugative" aromaticity (and antiaromaticity) in the S0 state will become antiaromatic (and aromatic) in the T1 state, indicating an important role of hyperconjugation. Finally, a moderate correlation (r(2) =0.708) is identified between the NICS(1)zz values and spin distributions.

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.

Five-membered cyclic metal carbyne: synthesis of osmapentalynes by the reactions of osmapentalene with allene, alkyne, and alkene

The synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five-membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five-membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient.

Stabilizing Two Classical Antiaromatic Frameworks: Demonstration of Photoacoustic Imaging and the Photothermal Effect in Metalla-aromatics

Antiaromatic species are substantially less thermodynamically stable than aromatic moieties. Herein, we report the stabilization of two classical antiaromatic frameworks, cyclobutadiene and pentalene, by introducing one metal fragment through the first [2+2] cycloaddition reaction of a late-transition-metal carbyne with alkynes. Experimental observations and theoretical calculations reveal that the metal fragment decreases the antiaromaticity in cyclobutadiene and pentalene simultaneously, leading to air- and moisture-stable products. These molecules show broad absorption from the UV to the near-IR region, resulting in photoacoustic and photothermal effects for metalla-aromatic compounds for the first time. These results will encourage further efforts into the exploration of organometallic compounds for photoacoustic-imaging-guided photothermal therapy.