Reactions of [RuCl₂(PPh₃)₃] with Nitron and with the “Enders Carbene”: Access to Ruthenium(III) NHC Complexes

The reactions of [RuCl₂(PPh₃)₃] with the “Enders carbene” 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (<b>1</b>) and the “instant carbene” Nitron (<b>2</b>) respectively afforded the Ru^II chelates [RuCl­(<b>3</b>)­(PPh₃)₂] (<b>3</b> = 3,4-diphenyl-1-<i>o</i>-phenylene-1,2,4-triazol-5-ylidene) and [RuCl­(<b>4</b>)­(PPh₃)₂] (<b>4</b> = 4-phenyl-3-phenylamino-1-<i>o</i>-phenylene-1,2,4-triazol-5-ylidene) in a process involving the ortho metalation of the 1-Ph group of the respective carbene ligand. It proved possible to synthesize [RuCl­(<b>3</b>)­(PPh₃)₂] more conveniently in higher yield by using the stable carbene precursor 5-methoxy-1,3,4-triphenyl-4,5-dihydro-1<i>H</i>-1,2,4-triazole (MeO-<b>1</b>-H) instead of the free carbene <b>1</b> in the presence of triethylamine to trap the HCl generated by the ortho metalation. Aerobic oxidation of the Ru^II chelates in the presence of chloride ions furnished [RuCl₂(<b>3</b>)­(PPh₃)₂] and [RuCl₂(<b>4</b>)­(PPh₃)₂], which are rare examples of Ru^III NHC complexes. The crystal structures of all four complexes were determined by single-crystal X-ray diffraction studies. In addition, the crystal structure of the hydrochloride of Nitron was also determined. In the Ru^II chelates, the pentacoordinate metal center is in a distorted-square-pyramidal environment with the carbon atom of the ortho-metalated 1-Ph group occupying the apical position. The coordination sphere of the Ru^III chelates is complemented by a second chlorido ligand, which is positioned <i>trans</i> to this carbon atom

Silanetriols as Powerful Starting Materials for Selective Condensation to Bulky POSS Cages

Controlled condensation reactions of tertiary silanetriols CH₃(CH₂)_<i>n</i>(CH₃)₂CSi­(OH)₃ (<b>1b</b>–<b>f</b>; <i>n</i> = 1–5) in the presence of trifluoroacetic acid and the hydrolysis of CH₃(CH₂)₆(CH₃)₂CSiCl₃ (<b>3</b>) lead to the selective formation of the corresponding disiloxane tetrols [CH₃(CH₂)_<i>n</i>(CH₃)₂CSi­(OH)₂]₂O (<b>2b</b>–<b>g</b>; <i>n</i> = 1–6) in good yields. The TBAF-driven condensation reactions of the silanetriols CH₃(CH₂)_<i>n</i>(CH₃)₂CSi­(OH)₃ (<b>1a</b>–<b>c</b>; <i>n</i> = 0–2) as well as of the disiloxane-1,1,3,3-tetrol <b>2d</b> (<i>n</i> = 3) yield in the selective formation of the first T₈ cages bearing tertiary carbon substituents, CH₃(CH₂)_<i>n</i>(CH₃)₂C (<b>4a</b>–<b>d</b>; <i>n</i> = 0–3), which was not possible via the condensation of their alkoxysilane counterparts so far. The resulting compounds <b>2b</b>–<b>g</b> and <b>4a</b>–<b>d</b> have been characterized by multinuclear NMR, MS, and single-crystal X-ray diffraction

Moving on from Silicon to the Heavier Tetrels: Germyl- and Stannyl-Substituted Phosphole Derivatives

Germyl- and stannyl-substituted phospholes have been prepared and isolated. The increased reactivity of the tetrel carbon bond requires increased effort in purification by initial transformation to the chalcogen derivatives and subsequent reduction to the phosphole after subsequent to chromatographic purification for the germanium derivative. The photophysical properties of the germyl phosphole are comparable to that of its silyl analogue, whereas the stannyl phospholes turned out to be nonluminescent. All isolated compounds have been characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. Furthermore, single-crystal X-ray diffraction and density functional theory (DFT) calculations have been performed on selected compounds

Moving on from Silicon to the Heavier Tetrels: Germyl- and Stannyl-Substituted Phosphole Derivatives

Germyl- and stannyl-substituted phospholes have been prepared and isolated. The increased reactivity of the tetrel carbon bond requires increased effort in purification by initial transformation to the chalcogen derivatives and subsequent reduction to the phosphole after subsequent to chromatographic purification for the germanium derivative. The photophysical properties of the germyl phosphole are comparable to that of its silyl analogue, whereas the stannyl phospholes turned out to be nonluminescent. All isolated compounds have been characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. Furthermore, single-crystal X-ray diffraction and density functional theory (DFT) calculations have been performed on selected compounds

Activation of Acetyl Ligands through Hydrogen Bonds: A New Way to Platinum(II) Complexes Bearing Protonated Iminoacetyl Ligands

The dinuclear platina-β-diketone [Pt₂{(COMe)₂H}₂(μ-Cl)₂] (<b>1</b>) reacted with 2-pyridyl-functionalized monoximes and with dioximes in the presence of NaOMe to yield oxime–diacetyl platinum­(II) complexes [Pt­(COMe)₂(2-pyCRNOH)] (R = H, <b>4a</b>; Me, <b>4b</b>; Ph, <b>4c</b>) and [Pt­(COMe)₂(HONCR–CRNOH)] (R/R = Me/Me, <b>5a</b>; Ph/Ph, <b>5b</b>; (CH₂)₄, <b>5c</b>; NH₂/NH₂, <b>5d</b>), respectively. The strong intramolecular O–H···O hydrogen bonds in these complexes give rise to an activation of the acetyl ligands for Schiff-base type reactions, thus forming with primary amines iminoacetyl platinum complexes [Pt­(COMe)­(CMeNHR′)­(2-pyCRNO)] (R/R′ = H/Bn, <b>6a</b>; Me/Bn, <b>6b</b>; Ph/Bn, <b>6c</b>; H/CH₂CH₂Ph, <b>6d</b>; H/CH₂CHCH₂, <b>6e</b>; Bn = benzyl) and [{Pt­(CMeNHR′)₂(ONCR–CRNO)}₂] (R/R = Me/Me, <b>7a</b>–<b>d</b>; Ph/Ph, <b>8a</b>–<b>d</b>; (CH₂)₄, <b>9a</b>; R′ = Bn, <b>a</b>; CH₂CH₂Ph, <b>b</b>; CH₂CHCH₂, <b>c</b>; CH₂CH₂OH, <b>d</b>). The intramolecular N–H···O hydrogen bonds in type <b>6</b>–<b>9</b> complexes make clear that protonated iminoacetyl ligands (i.e., aminocarbene ligands) and deprotoanted oxime ligands are present. These complexes could also be obtained in reactions of [Pt­(COMe)₂(NH₂R′)₂] (<b>3</b>) with pyridyl-functionalized monoximes and with dioximes where type <b>4</b>/<b>5</b> complexes were found to be intermediates. In solution, the bis­(iminoacetyl) complexes <b>7</b>–<b>9</b> were found to be present as dimers (as also <b>8a</b> in the solid state) with smaller amounts of monomers. The importance of hydrogen bonds for activation of acetyl ligands was further evidenced by synthesis of complexes [Pt­(COMe)₂(2-pyCHNOMe)] (<b>10</b>) and [Pt­(COMe)₂(HONCMe–CMeNOMe)] (<b>11</b>) bearing <i>O</i>-methylated oxime ligands and their reactivty toward amines. The hydrogen-bond activated acetyl and iminoacetyl ligands in type <b>5</b>, <b>7</b>, and <b>8</b> complexes were found to undergo in CD₃OD solutions facile H/D exchange reactions resulting in complexes bearing C­(CD₃)O/C­(CD₃)NDR′ ligands. The constitution of all complexes was unambiguously confirmed analytically, spectroscopically and in part by single-crystal X-ray diffraction analyses. Structural and NMR parameters as well as DFT calculations gave evidence for relatively strong intramolecular hydrogen bonds

Activation of Acetyl Ligands through Hydrogen Bonds: A New Way to Platinum(II) Complexes Bearing Protonated Iminoacetyl Ligands

The dinuclear platina-β-diketone [Pt₂{(COMe)₂H}₂(μ-Cl)₂] (<b>1</b>) reacted with 2-pyridyl-functionalized monoximes and with dioximes in the presence of NaOMe to yield oxime–diacetyl platinum­(II) complexes [Pt­(COMe)₂(2-pyCRNOH)] (R = H, <b>4a</b>; Me, <b>4b</b>; Ph, <b>4c</b>) and [Pt­(COMe)₂(HONCR–CRNOH)] (R/R = Me/Me, <b>5a</b>; Ph/Ph, <b>5b</b>; (CH₂)₄, <b>5c</b>; NH₂/NH₂, <b>5d</b>), respectively. The strong intramolecular O–H···O hydrogen bonds in these complexes give rise to an activation of the acetyl ligands for Schiff-base type reactions, thus forming with primary amines iminoacetyl platinum complexes [Pt­(COMe)­(CMeNHR′)­(2-pyCRNO)] (R/R′ = H/Bn, <b>6a</b>; Me/Bn, <b>6b</b>; Ph/Bn, <b>6c</b>; H/CH₂CH₂Ph, <b>6d</b>; H/CH₂CHCH₂, <b>6e</b>; Bn = benzyl) and [{Pt­(CMeNHR′)₂(ONCR–CRNO)}₂] (R/R = Me/Me, <b>7a</b>–<b>d</b>; Ph/Ph, <b>8a</b>–<b>d</b>; (CH₂)₄, <b>9a</b>; R′ = Bn, <b>a</b>; CH₂CH₂Ph, <b>b</b>; CH₂CHCH₂, <b>c</b>; CH₂CH₂OH, <b>d</b>). The intramolecular N–H···O hydrogen bonds in type <b>6</b>–<b>9</b> complexes make clear that protonated iminoacetyl ligands (i.e., aminocarbene ligands) and deprotoanted oxime ligands are present. These complexes could also be obtained in reactions of [Pt­(COMe)₂(NH₂R′)₂] (<b>3</b>) with pyridyl-functionalized monoximes and with dioximes where type <b>4</b>/<b>5</b> complexes were found to be intermediates. In solution, the bis­(iminoacetyl) complexes <b>7</b>–<b>9</b> were found to be present as dimers (as also <b>8a</b> in the solid state) with smaller amounts of monomers. The importance of hydrogen bonds for activation of acetyl ligands was further evidenced by synthesis of complexes [Pt­(COMe)₂(2-pyCHNOMe)] (<b>10</b>) and [Pt­(COMe)₂(HONCMe–CMeNOMe)] (<b>11</b>) bearing <i>O</i>-methylated oxime ligands and their reactivty toward amines. The hydrogen-bond activated acetyl and iminoacetyl ligands in type <b>5</b>, <b>7</b>, and <b>8</b> complexes were found to undergo in CD₃OD solutions facile H/D exchange reactions resulting in complexes bearing C­(CD₃)O/C­(CD₃)NDR′ ligands. The constitution of all complexes was unambiguously confirmed analytically, spectroscopically and in part by single-crystal X-ray diffraction analyses. Structural and NMR parameters as well as DFT calculations gave evidence for relatively strong intramolecular hydrogen bonds

A Stable Planar-Chiral <i>N</i>‑Heterocyclic Carbene with a 1,1′-Ferrocenediyl Backbone

This paper focuses on the stable, ferrocene-based <i>N</i>-heterocyclic carbene (NHC) <i>rac</i>-[Fe­{(η⁵-<i>t-</i>BuC₅H₃)­NpN}₂C:] (<b>A′-Np</b>, Np = neopentyl), which is planar-chiral due to the two <i>tert</i>-butyl substituents in 3,3′-positions. <b>A′-Np</b> was synthesized in nine steps starting from 1,1′-di-<i>tert</i>-butylferrocene (<b>1</b>), the first step being its 3,3′-dilithiation to afford <i>rac</i>-[Fe­(η⁵-<i>t</i>-BuC₅H₃Li)₂] (<i>rac</i>-fc′Li₂, <b>2</b>). The structures of <i>rac</i>-fc′(SiMe₃)₂ (<b>3</b>), <i>rac</i>-fc′Br₂ (<b>4</b>), <i>rac</i>-fc′(N₃)₂ (<b>5</b>), and the immediate carbene precursor [<b>A′-Np</b>H]­BF₄ were determined by single-crystal X-ray diffraction (XRD). The chemical properties of <b>A′-Np</b> were found to be very similar to those of its <i>tert</i>-butyl-free congener <b>A-Np</b>, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. −4.0 eV) and low singlet–triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm^–1 was derived from IR data of <i>cis</i>-[RhCl­(<b>A′-Np</b>)­(CO)₂], indicating the high donicity of <b>A′-Np</b> as a ligand. Consistent with its ambiphilic nature, <b>A′-Np</b> was found to react readily with carbon monoxide, affording the betainic enolate (<b>A′-Np</b>)₂CO as four stereoisomers, viz. (<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>⁺), (<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>⁺), (<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>⁺), and (<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>⁺). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. <b>A′-Np</b> was found to react swiftly with dichloromethane, affording the addition product <b>A′-Np</b>H–CHCl₂ in a reaction that is unprecedented for diaminocarbenes. <b>A-Np</b>H–CHCl₂ was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of <b>A′-Np</b> by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of −0.310 vs ferrocene/ferrocenium (THF/NBu₄PF₆). The electrochemical data previously published for <b>A-Np</b> were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording <b>A-Np</b>(H₂O). The hydrolysis products of <b>A-Np</b> and <b>A′-Np</b> were found to be reversibly oxidized at half-wave potentials of −0.418 and −0.437 V, respectively

A Stable Planar-Chiral <i>N</i>‑Heterocyclic Carbene with a 1,1′-Ferrocenediyl Backbone

This paper focuses on the stable, ferrocene-based <i>N</i>-heterocyclic carbene (NHC) <i>rac</i>-[Fe­{(η⁵-<i>t-</i>BuC₅H₃)­NpN}₂C:] (<b>A′-Np</b>, Np = neopentyl), which is planar-chiral due to the two <i>tert</i>-butyl substituents in 3,3′-positions. <b>A′-Np</b> was synthesized in nine steps starting from 1,1′-di-<i>tert</i>-butylferrocene (<b>1</b>), the first step being its 3,3′-dilithiation to afford <i>rac</i>-[Fe­(η⁵-<i>t</i>-BuC₅H₃Li)₂] (<i>rac</i>-fc′Li₂, <b>2</b>). The structures of <i>rac</i>-fc′(SiMe₃)₂ (<b>3</b>), <i>rac</i>-fc′Br₂ (<b>4</b>), <i>rac</i>-fc′(N₃)₂ (<b>5</b>), and the immediate carbene precursor [<b>A′-Np</b>H]­BF₄ were determined by single-crystal X-ray diffraction (XRD). The chemical properties of <b>A′-Np</b> were found to be very similar to those of its <i>tert</i>-butyl-free congener <b>A-Np</b>, both being ambiphilic NHCs with rather high calculated HOMO energies (ca. −4.0 eV) and low singlet–triplet gaps (ca. 35 kcal/mol). A Tolman electronic parameter value of 2050 cm^–1 was derived from IR data of <i>cis</i>-[RhCl­(<b>A′-Np</b>)­(CO)₂], indicating the high donicity of <b>A′-Np</b> as a ligand. Consistent with its ambiphilic nature, <b>A′-Np</b> was found to react readily with carbon monoxide, affording the betainic enolate (<b>A′-Np</b>)₂CO as four stereoisomers, viz. (<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>⁺), (<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>⁺), (<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>⁺), and (<i>S</i>_p<i>S</i>_p-<b>A′-Np</b>)C­(O^–)­(<i>R</i>_p<i>R</i>_p-<b>A′-Np</b>⁺). The former two isomers were structurally characterized as a racemic compound by single-crystal XRD. <b>A′-Np</b> was found to react swiftly with dichloromethane, affording the addition product <b>A′-Np</b>H–CHCl₂ in a reaction that is unprecedented for diaminocarbenes. <b>A-Np</b>H–CHCl₂ was obtained analogously. Both compounds were structurally characterized by single-crystal XRD. An electrochemical investigation of <b>A′-Np</b> by cyclic and square wave voltammetry revealed a reversible oxidation of the carbene at a half-wave potential of −0.310 vs ferrocene/ferrocenium (THF/NBu₄PF₆). The electrochemical data previously published for <b>A-Np</b> were identified to be incorrect, since unnoticed hydrolysis of the NHC had taken place, affording <b>A-Np</b>(H₂O). The hydrolysis products of <b>A-Np</b> and <b>A′-Np</b> were found to be reversibly oxidized at half-wave potentials of −0.418 and −0.437 V, respectively