Carbolong chemistry: nucleophilic aromatic substitution of a triflate functionalized iridapentalene

Preparation, Aromaticity, and Bromination of Spiro Iridafurans

Iridium centers of [Ir(μ-Cl)(C8H14)2]2 (1) activate the Cβ(sp2)-H bond of benzylideneacetone to give [Ir(μ-Cl){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (2), which is the starting point for the preparation of the spiro iridafurans IrCl{κ2-C,O-[C(Ph)CHC(Me)O]}2(PiPr3) (3), [Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2(MeCN)2]BF4 (4), [Ir(μ-OH){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (5), Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-C,N-[C6MeH3-py]} (6), and Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acac]} (7). The five-membered rings are orthogonally arranged with the oxygen atoms in trans in an octahedral environment of the iridium atom. Spiro iridafurans are aromatic. The degree of aromaticity and the negative charge of the CH-carbon of the rings depend on ligand trans to the carbon directly attached to the metal. Aromaticity has been experimentally confirmed by bromination of iridafurans with N-bromosuccinimide (NBS). Reactions are sensitive to the degree of aromaticity of the ring and the negative charge of the attacked CH-carbon. Iridafurans can be selectively brominated, when different ligands lie trans to metalated carbons. Bromination of 3 occurs in the ring with the metalated carbon trans to chloride, whereas the bromination of 6 takes place in the ring with the metalated carbon trans to pyridyl. The first gives IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}(PiPr3) (8), which reacts with more NBS to form IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}2(PiPr3) (9). The second yields Ir{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}{κ2-C,N-[C6MeH3-py]} (10). The origin of the selectivity is kinetic, with the rate-determining step of the reaction being the NBS attack. The activation energy depends on the negative charge of the attacked atom; a higher negative charge allows for a lower activation energy. Accordingly, complex 7 undergoes bromination in the acetylacetonate ligand, giving Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acacBr]} (11).

Stabilizing a 20-Electron Metallaazulyne by Aromaticity

The 18-electron rule states that metal complexes with 18 valence electron metal centers are thermodynamically stable because nine valence orbitals of transition metals including one s orbital, three p orbitals, and five d orbitals can collectively accommodate 18 electrons, achieving the same electron configuration as the noble gas in the period. Thus, 20-electron compounds are extremely rare due to a violation of such a rule. Here, we demonstrate a 20-electron metallaazulyne via density functional theory calculations stabilized by aromaticity, which was supported by various aromaticity indices including nucleus-independent chemical shift, anisotropy of the induced current density, the isochemical shielding surface, and electron density of delocalized bonds. Interestingly, when a transition metal fragment is first introduced into the aromatic azulyne molecule, the resulting osmaazulyne becomes antiaromatic, in sharp contrast to the previous transformation from pentalyne to metallapentalyne. More interestingly, when osmaazulyne is reduced by two electrons, the resulting 20e osmaazulyne becomes aromatic. Our findings highlight an important application of aromaticity in stabilizing 20e species, inviting experimental verification.

An aromatic dimetallapolycyclic complex with two rhenapyrylium rings

Extension of the polycyclic benzo-rhenapyrylium structure by a fused metallaaromatic ring and a benzene unit are reported. The dirhena-aromatic complex 4 shows strong absorption in the visible region and a significant absorption in the near-infrared (NIR) region (λ_max = 842 nm). DFT calculations are performed to understand its aromatic nature and electronic behavior.

Synthesis, structure and aromaticity of metallapyridinium complexes

The first rhena-analogues of pyridinium (cyclopropametalla-2-isoquinolinium complexes) are obtained from o-ethynyl benzonitriles. Structural analysis and DFT calculations confirm their aromatic nature. Compared to rhenapyrylium, rhenapyridinium has a slightly stronger Hückel π-aromaticity, while a chlorine substituent on the rhenapyridinium ring decreases its aromaticity, which is revealed by NICS, EDDB, MCI and ΔBV(ELF_π) analysis.

Synthesis and Characterization of Rhena[10]annulynes

Most of the reported metallacycles were limited to small cyclic complexes that contain six-membered or smaller rings. Larger-membered metallacycles are still rare and mainly focus on the dimetallacycles. Herein, we...

Adaptive aromaticity in 16-valence-electron metallazapentalenes

According to Hückel's and Baird's rules, cyclic species are generally aromatic only either in the lowest singlet state (S₀) or in the lowest-lying triplet ππ* excited state (T₁). Thus, species with aromaticity both in S₀ and T₁ states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O). Interestingly, metallazapentalenes show adaptive aromaticity whereas metalloxapentalenes display nonaromaticity in the S₀ and T₁ states, which is supported by structural, magnetic, and electronic indices. In addition, a series of metallazapentalenes containing strong σ- or π-donor ligands are predicted to achieve adaptive aromaticity. Our findings expand the family of adaptive aromatics significantly, inviting experimental chemists to realize more hetero-metallapentalenes with adaptive aromaticity.