A Dihydrodisilene Transition Metal Complex from an N‑Heterocyclic Carbene-Stabilized Silylene Monohydride

Through the use of an N-heterocyclic carbene (NHC) and the super-silyl group (<i>t</i>Bu₃Si), the novel silylene hydride <b>2</b> could be synthesized and isolated in 41% yield. The reaction of <b>2</b> with bis­(1,5-cyclooctadiene)­nickel(0) afforded complex <b>3</b>, which represents the first example of a dihydrodisilene transition metal complex. Compounds <b>2</b> and <b>3</b> were fully characterized, including single-crystal X-ray diffraction analysis. The reaction mechanism for the formation of <b>3</b> from <b>2</b> was investigated by density functional theory calculations, which showed that migration of the NHC from silicon to nickel takes place in this reaction

Highly Electron-Rich Pincer-Type Iron Complexes Bearing Innocent Bis(metallylene)pyridine Ligands: Syntheses, Structures, and Catalytic Activity

The first neutral bis­(metallylene)­pyridine pincer-type [<b>ENE</b>] ligands (E = Si^II, Ge^II) were synthesized, and their coordination chemistry and reactivity toward iron was studied. First, the unprecedented four-coordinate complexes <b>κ</b>^<b>2</b><i><b>E,E</b></i>′-<b>[ENE]­FeCl</b>_<b>2</b> were isolated. Unexpectedly and in contrast to other related pyridine-based pincer-type Fe­(II) complexes, the N atom of pyridine is reluctant to coordinate to the Fe­(II) site due to the enhanced σ-donor strength of the E atoms, which disfavors this coordination mode. Subsequent reduction of <b>κ</b>^<b>2</b><i><b>Si,Si</b></i>′<b>-[SiNSi]­FeCl</b>_<b>2</b> with KC₈ in the presence of PMe₃ or direct reaction of the [<b>ENE</b>] ligands using Fe­(PMe₃)₄ produced the highly electron-rich iron(0) complexes <b>[ENE]­Fe­(PMe</b>_<b>3</b><b>)</b>_<b>2</b>. The reduction of the iron center substantially changes its coordination features, as shown by the results of a single-crystal X-ray diffraction analysis of <b>[SiNSi]­Fe­(PMe</b>_<b>3</b><b>)</b>_<b>2</b>. The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination environment, with a coordinative (pyridine)­N→Fe bond, and a trimethylphosphine ligand occupying the apical position. This geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature ¹H and ³¹P NMR spectra of the <b>[ENE]­Fe­(PMe</b>_<b>3</b><b>)</b>_<b>2</b> complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0) complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of <b>[ENE]­Fe­(PMe</b>_<b>3</b><b>)</b>_<b>2</b> with CO resulted in the isolation of <b>[ENE]­Fe­(CO)</b>_<b>2</b> and the hitherto unknown <b>κ</b>^<b>2</b><i><b>E,E</b></i>′<b>-[ENE]­Fe­(CO)</b>_<b>2</b><b>L</b> (L = CO, PMe₃) complexes. All complexes were fully characterized (NMR, MS, XRD, IR, and ⁵⁷Fe Mössbauer spectroscopy), showing the highest electron density on the iron center for pincer-type complexes reported to date. DFT calculations and ⁵⁷Fe Mössbauer spectroscopy confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active precatalysts for the hydrosilylation of ketones

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

The silicon analogues of an acylium ion, namely, sila-acylium ions <b>2a</b> and <b>2b</b> [RSi­(O)­(NHC)₂]Cl stabilized by two <i>N</i>-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO₂ using the donor stabilized silyliumylidene cations <b>1a</b> and <b>1b</b> [RSi­(NHC)₂]­Cl (<b>1a</b>, <b>2a</b>; R = <i>m</i>-Ter = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂ and <b>1b</b>, <b>2b</b>; R <b>=</b> Tipp = 2,4,6-<i>i</i>Pr₃C₆H₂). Structurally, compound <b>2a</b> features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions <b>2a</b> and <b>2b</b> with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of <b>2a</b> to H₂O afforded a stable silicon analogue of carboxylate anion as a dimer form, [<i>m</i>-TerSi­(O)­O]₂^2–·2­[NHC–H]⁺ (<b>3</b>), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion <b>2b</b> afforded cyclotetrasiloxanediol dianion [{TippSi­(O)}₄{(O)­OH}₂]^2–·2­[NHC–H]⁺ (<b>4</b>). Metric and DFT (Density Functional Theory) evidence support that <b>2a</b> and <b>2b</b> possess strong SiO double bond character, while <b>3</b> and <b>4</b> contain more ionic terminal Si–O bonds. Mechanistic details of the formation of different (SiO)_<i>n</i> (<i>n</i> = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry

A Stable Neutral Compound with an Aluminum–Aluminum Double Bond

Homodinuclear multiple-bonded neutral Al compounds, aluminum analogues of alkenes, have been a notoriously difficult synthetic target over the past several decades. Herein, we report the isolation of a stable neutral compound featuring an AlAl double bond stabilized by N-heterocyclic carbenes. X-ray crystallographic and spectroscopic analyses demonstrate that the dialuminum entity possesses <i>trans</i>-planar geometry and an Al–Al bond length of 2.3943(16) Å, which is the shortest distance reported for a molecular dialuminum species. This new species reacts with ethylene and phenyl acetylene to give the [2+2] cycloaddition products. The structure and bonding were also investigated by detailed density functional theory calculations. These results clearly demonstrate the presence of an AlAl double bond in this molecule

A Stable Neutral Compound with an Aluminum–Aluminum Double Bond

Homodinuclear multiple-bonded neutral Al compounds, aluminum analogues of alkenes, have been a notoriously difficult synthetic target over the past several decades. Herein, we report the isolation of a stable neutral compound featuring an AlAl double bond stabilized by N-heterocyclic carbenes. X-ray crystallographic and spectroscopic analyses demonstrate that the dialuminum entity possesses <i>trans</i>-planar geometry and an Al–Al bond length of 2.3943(16) Å, which is the shortest distance reported for a molecular dialuminum species. This new species reacts with ethylene and phenyl acetylene to give the [2+2] cycloaddition products. The structure and bonding were also investigated by detailed density functional theory calculations. These results clearly demonstrate the presence of an AlAl double bond in this molecule

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

The silicon analogues of an acylium ion, namely, sila-acylium ions <b>2a</b> and <b>2b</b> [RSi­(O)­(NHC)₂]Cl stabilized by two <i>N</i>-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO₂ using the donor stabilized silyliumylidene cations <b>1a</b> and <b>1b</b> [RSi­(NHC)₂]­Cl (<b>1a</b>, <b>2a</b>; R = <i>m</i>-Ter = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂ and <b>1b</b>, <b>2b</b>; R <b>=</b> Tipp = 2,4,6-<i>i</i>Pr₃C₆H₂). Structurally, compound <b>2a</b> features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions <b>2a</b> and <b>2b</b> with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of <b>2a</b> to H₂O afforded a stable silicon analogue of carboxylate anion as a dimer form, [<i>m</i>-TerSi­(O)­O]₂^2–·2­[NHC–H]⁺ (<b>3</b>), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion <b>2b</b> afforded cyclotetrasiloxanediol dianion [{TippSi­(O)}₄{(O)­OH}₂]^2–·2­[NHC–H]⁺ (<b>4</b>). Metric and DFT (Density Functional Theory) evidence support that <b>2a</b> and <b>2b</b> possess strong SiO double bond character, while <b>3</b> and <b>4</b> contain more ionic terminal Si–O bonds. Mechanistic details of the formation of different (SiO)_<i>n</i> (<i>n</i> = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

The silicon analogues of an acylium ion, namely, sila-acylium ions <b>2a</b> and <b>2b</b> [RSi­(O)­(NHC)₂]Cl stabilized by two <i>N</i>-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO₂ using the donor stabilized silyliumylidene cations <b>1a</b> and <b>1b</b> [RSi­(NHC)₂]­Cl (<b>1a</b>, <b>2a</b>; R = <i>m</i>-Ter = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂ and <b>1b</b>, <b>2b</b>; R <b>=</b> Tipp = 2,4,6-<i>i</i>Pr₃C₆H₂). Structurally, compound <b>2a</b> features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions <b>2a</b> and <b>2b</b> with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of <b>2a</b> to H₂O afforded a stable silicon analogue of carboxylate anion as a dimer form, [<i>m</i>-TerSi­(O)­O]₂^2–·2­[NHC–H]⁺ (<b>3</b>), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion <b>2b</b> afforded cyclotetrasiloxanediol dianion [{TippSi­(O)}₄{(O)­OH}₂]^2–·2­[NHC–H]⁺ (<b>4</b>). Metric and DFT (Density Functional Theory) evidence support that <b>2a</b> and <b>2b</b> possess strong SiO double bond character, while <b>3</b> and <b>4</b> contain more ionic terminal Si–O bonds. Mechanistic details of the formation of different (SiO)_<i>n</i> (<i>n</i> = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry

A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene

By employing the chelate dicarbene <b>1</b>, the new chloro­germylium­ylidene complex <b>2</b> could be synthesized and isolated in 95% yield. Dechlorination of <b>2</b> with sodium naphthalenide furnishes the unique cyclic germadicarbene <b>3</b> which could be isolated in 45% yield. Compound <b>3</b> is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of <b>3</b> with two lone pairs of electrons at the Ge atom

Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (−SitBu3) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C–C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC6 rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC5 rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C–C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented

A Cyclic Germadicarbene (“Germylone”) from Germyliumylidene

By employing the chelate dicarbene <b>1</b>, the new chloro­germylium­ylidene complex <b>2</b> could be synthesized and isolated in 95% yield. Dechlorination of <b>2</b> with sodium naphthalenide furnishes the unique cyclic germadicarbene <b>3</b> which could be isolated in 45% yield. Compound <b>3</b> is the first isolable Ge(0) complex with a single germanium atom stabilized by a dicarbene. Its molecular structure is in accordance with DFT calculations which underline the peculiar electronic structure of <b>3</b> with two lone pairs of electrons at the Ge atom