Theoretical Study on the Stability and Aromaticity of Metallasilapentalynes

Antiaromatic compounds and small cyclic alkynes or carbynes are both challenging for synthetic chemists because of the destabilization caused by their antiaromaticity and highly distorted triple bonds, respectively. These dual destabilizations could be the reason why pentalyne (<b>I</b>), a highly antiaromatic and extremely strained cyclic alkyne, has never been synthesized. Recently, we have successfully synthesized the first metallapentalyne (<b>II</b>), benefiting from the stabilization of a metal fragment by reducing the ring strain and switching the antiaromaticity in pentalyne to the aromaticity in metallapentalyne. An interesting question is raised: can the aromaticity in metallasilapentalyne (<b>III</b>) be retained, considering the fact that the silicon atom is reluctant to participate in π bonding? Here we report a thorough theoretical study on the stability and aromaticity of metallasilapentalynes. The computed energies and magnetic properties reveal the reduced aromatic character of osmasilapentalyne in comparison with osmapentalyne. The effect of the ligands, substituents, and base on the aromaticity and stability of osmasilapentalyne is also discussed, thus providing an important guide to the synthesis of osmasilapentalyne

Interconversion between Ruthenacyclohexadiene and Ruthenabenzene: A Combined Experimental and Theoretical Study

Treatment of ruthenabenzene [(C₉H₆NO)­Ru­{CC­(PPh₃)­CHC­(PPh₃)­CH}­(C₉H₆NO)­(PPh₃)]­Cl₂ (<b>1</b>) with NaBH₄ produces the first ruthenacyclohexa-1,4-diene [(C₉H₆NO)­Ru­{CC­(PPh₃)­CH₂C­(PPh₃)­CH}­(C₉H₆NO)­(PPh₃)]Cl (<b>2</b>), which was fully characterized. Under an oxygen atmosphere, complex <b>2</b> can easily convert to complex <b>1</b>. DFT calculations were carried out to rationalize the high regioselectivity in the reaction of the ruthenabenene <b>1</b> with NaBH₄ and the interconversion between <b>1</b> and <b>2</b>

Synthesis of Five-Membered Osmacycles with Osmium–Vinyl Bonds from Hydrido Alkenylcarbyne Complexes

Treatment of the osmium hydrido butenylcarbyne complex [OsH­{CC­(PPh₃)CH­(Et)}­(PPh₃)₂Cl₂]­BF₄ (<b>1</b>) with excess 2-chloro-4-cyanopyridine in the presence of H₂O₂ generates the fused osmacyclopentadiene <b>2</b>. A detailed mechanism of the conversion has been investigated with the aid of in situ NMR experiments and the isolation of intermediates <b>3</b> and <b>4</b>. In contrast, reaction of <b>1</b> with the propiolic acid ester HCCCOOMe produces the osmafuran <b>5</b>. Analogous reactions of the osmium hydrido phenylethenylcarbyne complex [OsH­{CC­(PPh₃)CH­(Ph)}­(PPh₃)₂Cl₂]­BF₄ (<b>6</b>) with nitriles/CO or HCCCOOMe were also studied, which result in the formation of five-membered osmacycles [Os­{CHC­(PPh₃)­CH­(Ph)­(OH)}­(CH₃CN)­(PPh₃)₂CO]­(BF₄)₂ (<b>7</b>) and [Os­{CHC­(PPh₃)­CH­(Ph)­(OH)}­(PhCN)­(PPh₃)₂CO]­(BF₄)₂ (<b>8</b>). In the presence of NEt₃, <b>6</b> can convert to the osmium hydrido phenylethenylcarbyne complex OsH­{CCCH­(Ph)}­(PPh₃)₂Cl₂ (<b>9</b>) in wet acetonitrile, presumably involving P–C bond cleavage. Similarly, <b>9</b> can react with HCCCOOMe with the aid of HBF₄ to give osmafuran <b>10</b>

Synthesis of Five-Membered Osmacycles with Osmium–Vinyl Bonds from Hydrido Alkenylcarbyne Complexes

Treatment of the osmium hydrido butenylcarbyne complex [OsH­{CC­(PPh₃)CH­(Et)}­(PPh₃)₂Cl₂]­BF₄ (<b>1</b>) with excess 2-chloro-4-cyanopyridine in the presence of H₂O₂ generates the fused osmacyclopentadiene <b>2</b>. A detailed mechanism of the conversion has been investigated with the aid of in situ NMR experiments and the isolation of intermediates <b>3</b> and <b>4</b>. In contrast, reaction of <b>1</b> with the propiolic acid ester HCCCOOMe produces the osmafuran <b>5</b>. Analogous reactions of the osmium hydrido phenylethenylcarbyne complex [OsH­{CC­(PPh₃)CH­(Ph)}­(PPh₃)₂Cl₂]­BF₄ (<b>6</b>) with nitriles/CO or HCCCOOMe were also studied, which result in the formation of five-membered osmacycles [Os­{CHC­(PPh₃)­CH­(Ph)­(OH)}­(CH₃CN)­(PPh₃)₂CO]­(BF₄)₂ (<b>7</b>) and [Os­{CHC­(PPh₃)­CH­(Ph)­(OH)}­(PhCN)­(PPh₃)₂CO]­(BF₄)₂ (<b>8</b>). In the presence of NEt₃, <b>6</b> can convert to the osmium hydrido phenylethenylcarbyne complex OsH­{CCCH­(Ph)}­(PPh₃)₂Cl₂ (<b>9</b>) in wet acetonitrile, presumably involving P–C bond cleavage. Similarly, <b>9</b> can react with HCCCOOMe with the aid of HBF₄ to give osmafuran <b>10</b>

C–H Bond Functionalization of Benzoxazoles with Chromium(0) Fischer Carbene Complexes

An efficient C–H bond functionalization of benzoxazoles with chromium(0) Fischer carbene complexes under catalyst-free conditions has been developed. A series of benzoxazoles and chromium(0) carbene complexes are compatible with the reaction. This transformation provides an alternative way for C–H bond alkylation of benzoxazoles. In the reaction mechanism the elimination of the Cr­(CO)₅ fragment seems more favored than the elimination of an alkoxy group, which is in sharp contrast to the previous reports on the reaction of organolithium reagents with chromium(0) Fischer carbene complexes

Synthesis of Imidazopyridinium-Fused Metallacycloallene via One-Pot Reaction of η²‑Alkynol-Coordinated Osmacycle with 2‑Aminopyridine

Metallacycloallenes are metallacyclic derivatives of cyclic allenes, in which a CH₂ (type <b>A</b>) or CH (type <b>B</b>) segment is formally replaced by an isolobal transition-metal fragment. In constrast to the well-developed chemistry of metallacycloallenes of type <b>A</b>, the synthesis of metallacycloallenes with the structural features of type <b>B</b> has met with limited success. In this study, we present the reaction of η²-alkynol-coordinated osmacycle <b>1</b> with 2-aminopyridine in the presence of hypervalent iodine reagent, leading to the formation of imidazopyridinium-fused 4-osmacyclohexa-2,3,5-trienone <b>2</b> and 4-osmacyclohexa-2,5-dienone <b>3</b>. Two key intermediates, η²-ethynyl ketone coordinated osmacycle <b>4</b> and 4-osmacyclohexa-2,5-dienone <b>5</b>, were isolated and fully characterized, which suggest the hypervalent iodine reagent plays an important role in the formation of the fused metallacycloallene <b>2</b>

Conversion of a Hydrido–Butenylcarbyne Complex to η²‑Allene-Coordinated Complexes and Metallabenzenes

Treatment of OsCl₂(PPh₃)₃ with HCCCH­(OH)­Et produces the cyclic complex Os­(PPh₃)₂Cl₂(CHC­(PPh₃)­CH­(OH)­CH₂CH₃) (<b>1</b>), which can undergo dehydration to give the hydrido–alkenylvinylidene complex Os­(PPh₃)₂HCl₂(CC­(PPh₃)­CHCHCH₃) (<b>2</b>). Reaction of <b>2</b> with HBF₄ generates the hydrido–butenylcarbyne complex [OsHCl₂(CC­(PPh₃)CH­(Et))­(PPh₃)₂]­BF₄ (<b>3</b>). The complex <b>3</b> evolves into the unstable metallabenzene [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>4</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) via triple hydrogen eliminations in the presence of excess nitriles in refluxing CHCl₃ in an air atmosphere. The ligand substitution reaction of <b>4</b> with excess CO affords the stable metallabenzene product [(PPh₃)₂(CO)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>5</b>). The key intermediates, η²-allene-coordinated osmium complexes [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCCH₂)]­BF₄ (<b>6</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) can be captured by performing the conversion at room temperature. Remarkably, in the absence of nitriles, reaction of <b>3</b> with excess CO only generates the vinylethenyl complex [(PPh₃)₂(CO)₂ClOs­(CHC­(PPh₃)­CHCHCH₃)]­BF₄ (<b>7</b>). The complexes <b>1</b>–<b>3</b>, <b>5</b>, <b>6a</b>, and <b>7</b> have been structurally characterized by single-crystal X-ray diffraction. Detailed mechanisms of the conversions have been investigated with the aid of density functional theory (DFT) calculations. DFT calculations suggest that the high stablility of the carbonyl coordinated complexes in the conversion inhibits the further transformation to metallabenzene product. However, the transformation is both kinetically and thermodynamically favorable in the presence of the relatively weaker nitrile ligand, which is consistent with the experimental conversion of <b>3</b> to <b>5</b> via unstable metallabenzenes <b>4</b> observed for in situ NMR experiments

Conversion of a Hydrido–Butenylcarbyne Complex to η²‑Allene-Coordinated Complexes and Metallabenzenes

Treatment of OsCl₂(PPh₃)₃ with HCCCH­(OH)­Et produces the cyclic complex Os­(PPh₃)₂Cl₂(CHC­(PPh₃)­CH­(OH)­CH₂CH₃) (<b>1</b>), which can undergo dehydration to give the hydrido–alkenylvinylidene complex Os­(PPh₃)₂HCl₂(CC­(PPh₃)­CHCHCH₃) (<b>2</b>). Reaction of <b>2</b> with HBF₄ generates the hydrido–butenylcarbyne complex [OsHCl₂(CC­(PPh₃)CH­(Et))­(PPh₃)₂]­BF₄ (<b>3</b>). The complex <b>3</b> evolves into the unstable metallabenzene [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>4</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) via triple hydrogen eliminations in the presence of excess nitriles in refluxing CHCl₃ in an air atmosphere. The ligand substitution reaction of <b>4</b> with excess CO affords the stable metallabenzene product [(PPh₃)₂(CO)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>5</b>). The key intermediates, η²-allene-coordinated osmium complexes [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCCH₂)]­BF₄ (<b>6</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) can be captured by performing the conversion at room temperature. Remarkably, in the absence of nitriles, reaction of <b>3</b> with excess CO only generates the vinylethenyl complex [(PPh₃)₂(CO)₂ClOs­(CHC­(PPh₃)­CHCHCH₃)]­BF₄ (<b>7</b>). The complexes <b>1</b>–<b>3</b>, <b>5</b>, <b>6a</b>, and <b>7</b> have been structurally characterized by single-crystal X-ray diffraction. Detailed mechanisms of the conversions have been investigated with the aid of density functional theory (DFT) calculations. DFT calculations suggest that the high stablility of the carbonyl coordinated complexes in the conversion inhibits the further transformation to metallabenzene product. However, the transformation is both kinetically and thermodynamically favorable in the presence of the relatively weaker nitrile ligand, which is consistent with the experimental conversion of <b>3</b> to <b>5</b> via unstable metallabenzenes <b>4</b> observed for in situ NMR experiments

Conversion of a Hydrido–Butenylcarbyne Complex to η²‑Allene-Coordinated Complexes and Metallabenzenes

Treatment of OsCl₂(PPh₃)₃ with HCCCH­(OH)­Et produces the cyclic complex Os­(PPh₃)₂Cl₂(CHC­(PPh₃)­CH­(OH)­CH₂CH₃) (<b>1</b>), which can undergo dehydration to give the hydrido–alkenylvinylidene complex Os­(PPh₃)₂HCl₂(CC­(PPh₃)­CHCHCH₃) (<b>2</b>). Reaction of <b>2</b> with HBF₄ generates the hydrido–butenylcarbyne complex [OsHCl₂(CC­(PPh₃)CH­(Et))­(PPh₃)₂]­BF₄ (<b>3</b>). The complex <b>3</b> evolves into the unstable metallabenzene [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>4</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) via triple hydrogen eliminations in the presence of excess nitriles in refluxing CHCl₃ in an air atmosphere. The ligand substitution reaction of <b>4</b> with excess CO affords the stable metallabenzene product [(PPh₃)₂(CO)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>5</b>). The key intermediates, η²-allene-coordinated osmium complexes [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCCH₂)]­BF₄ (<b>6</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) can be captured by performing the conversion at room temperature. Remarkably, in the absence of nitriles, reaction of <b>3</b> with excess CO only generates the vinylethenyl complex [(PPh₃)₂(CO)₂ClOs­(CHC­(PPh₃)­CHCHCH₃)]­BF₄ (<b>7</b>). The complexes <b>1</b>–<b>3</b>, <b>5</b>, <b>6a</b>, and <b>7</b> have been structurally characterized by single-crystal X-ray diffraction. Detailed mechanisms of the conversions have been investigated with the aid of density functional theory (DFT) calculations. DFT calculations suggest that the high stablility of the carbonyl coordinated complexes in the conversion inhibits the further transformation to metallabenzene product. However, the transformation is both kinetically and thermodynamically favorable in the presence of the relatively weaker nitrile ligand, which is consistent with the experimental conversion of <b>3</b> to <b>5</b> via unstable metallabenzenes <b>4</b> observed for in situ NMR experiments

Conversion of a Hydrido–Butenylcarbyne Complex to η²‑Allene-Coordinated Complexes and Metallabenzenes

Treatment of OsCl₂(PPh₃)₃ with HCCCH­(OH)­Et produces the cyclic complex Os­(PPh₃)₂Cl₂(CHC­(PPh₃)­CH­(OH)­CH₂CH₃) (<b>1</b>), which can undergo dehydration to give the hydrido–alkenylvinylidene complex Os­(PPh₃)₂HCl₂(CC­(PPh₃)­CHCHCH₃) (<b>2</b>). Reaction of <b>2</b> with HBF₄ generates the hydrido–butenylcarbyne complex [OsHCl₂(CC­(PPh₃)CH­(Et))­(PPh₃)₂]­BF₄ (<b>3</b>). The complex <b>3</b> evolves into the unstable metallabenzene [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>4</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) via triple hydrogen eliminations in the presence of excess nitriles in refluxing CHCl₃ in an air atmosphere. The ligand substitution reaction of <b>4</b> with excess CO affords the stable metallabenzene product [(PPh₃)₂(CO)­ClOs­(CHC­(PPh₃)­CHCHCH)]­BF₄ (<b>5</b>). The key intermediates, η²-allene-coordinated osmium complexes [(PPh₃)₂(RCN)­ClOs­(CHC­(PPh₃)­CHCCH₂)]­BF₄ (<b>6</b>; RCN = benzonitrile, 2-cyanobenzaldehyde, 3-methoxyacrylonitrile, 2-cyanoacetamide) can be captured by performing the conversion at room temperature. Remarkably, in the absence of nitriles, reaction of <b>3</b> with excess CO only generates the vinylethenyl complex [(PPh₃)₂(CO)₂ClOs­(CHC­(PPh₃)­CHCHCH₃)]­BF₄ (<b>7</b>). The complexes <b>1</b>–<b>3</b>, <b>5</b>, <b>6a</b>, and <b>7</b> have been structurally characterized by single-crystal X-ray diffraction. Detailed mechanisms of the conversions have been investigated with the aid of density functional theory (DFT) calculations. DFT calculations suggest that the high stablility of the carbonyl coordinated complexes in the conversion inhibits the further transformation to metallabenzene product. However, the transformation is both kinetically and thermodynamically favorable in the presence of the relatively weaker nitrile ligand, which is consistent with the experimental conversion of <b>3</b> to <b>5</b> via unstable metallabenzenes <b>4</b> observed for in situ NMR experiments