Reversible Inter- and Intramolecular Carbon–Hydrogen Activation, Hydrogen Addition, and Catalysis by the Unsaturated Complex Pt(IPr)(SnBu^t₃)(H)

The complex Pt­(IPr)­(SnBu^t₃)­(H) (<b>1</b>) was obtained from the reaction of Pt­(COD)₂ with Bu^t₃SnH and IPr [IPr = <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene]. Complex <b>1</b> undergoes exchange reactions with deuterated solvents (C₆D₆, toluene-<i>d</i>₈, and CD₂Cl₂), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms. Complex <b>1</b> reacts with H₂ gas reversibly at room temperature to yield the complex Pt­(IPr)­(SnBu^t₃)­(H)₃ (<b>2</b>). Complex <b>2</b> also undergoes exchange reactions with deuterated solvents as in <b>1</b> to deuterate the hydride ligands and the methyl hydrogen atoms on the isopropyl group of the IPr ligand. Complex <b>1</b> catalyzes the hydrogenation of styrene to ethylbenzene at room temperature. The reaction of <b>1</b> with 1 equiv of styrene at −20 °C yields the η²-coordinated product Pt­(IPr)­(SnBu^t₃)­(η²-CH₂CHPh)­(H) (<b>3</b>), and with 2 equiv of styrene, it forms Pt­(IPr)­(η²-CH₂CHPh)₂ (<b>4</b>)

Reversible Inter- and Intramolecular Carbon–Hydrogen Activation, Hydrogen Addition, and Catalysis by the Unsaturated Complex Pt(IPr)(SnBu^t₃)(H)

The complex Pt­(IPr)­(SnBu^t₃)­(H) (<b>1</b>) was obtained from the reaction of Pt­(COD)₂ with Bu^t₃SnH and IPr [IPr = <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene]. Complex <b>1</b> undergoes exchange reactions with deuterated solvents (C₆D₆, toluene-<i>d</i>₈, and CD₂Cl₂), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms. Complex <b>1</b> reacts with H₂ gas reversibly at room temperature to yield the complex Pt­(IPr)­(SnBu^t₃)­(H)₃ (<b>2</b>). Complex <b>2</b> also undergoes exchange reactions with deuterated solvents as in <b>1</b> to deuterate the hydride ligands and the methyl hydrogen atoms on the isopropyl group of the IPr ligand. Complex <b>1</b> catalyzes the hydrogenation of styrene to ethylbenzene at room temperature. The reaction of <b>1</b> with 1 equiv of styrene at −20 °C yields the η²-coordinated product Pt­(IPr)­(SnBu^t₃)­(η²-CH₂CHPh)­(H) (<b>3</b>), and with 2 equiv of styrene, it forms Pt­(IPr)­(η²-CH₂CHPh)₂ (<b>4</b>)

Pendant Alkyl and Aryl Groups on Tin Control Complex Geometry and Reactivity with H₂/D₂ in Pt(SnR₃)₂(CNBu^t)₂ (R = Bu^t, Prⁱ, Ph, Mesityl)

The complex Pt­(SnBu^t₃)₂­(CNBu^t)₂­(H)₂, <b>1</b>, was obtained from the reaction of Pt­(COD)₂ and Bu^t₃SnH, followed by addition of CNBu^t. The two hydride ligands in <b>1</b> can be eliminated, both in solution and in the solid state, to yield Pt­(SnBu^t₃)₂­(CNBu^t)₂, <b>2</b>. Addition of hydrogen to <b>2</b> at room temperature in solution and in the solid state regenerates <b>1</b>. Complex <b>2</b> catalyzes H₂−D₂ exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of Bu^t₃SnH from <b>1</b> to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt­(SnMes₃)­(SnBu^t₃)­(CNBu^t)₂, <b>3</b>, when the addition of H₂ to <b>2</b> was carried out in the presence of free ligand Mes₃SnH (Mes = 2,4,6-Me₃C₆H₂). Complex Pt­(SnMes₃)₂­(CNBu^t)₂, <b>5</b>, can be prepared from the reaction of Pt­(COD)₂ with Mes₃SnH and CNBu^t. The exchange reaction of <b>2</b> with Ph₃SnH gave Pt­(SnPh₃)₃(CNBu^t)₂­(H), <b>6</b>, wherein both SnBu^t₃ ligands are replaced by SnPh₃. Complex <b>6</b> decomposes in air to form square planar Pt­(SnPh₃)₂­(CNBu^t)₂, <b>7</b>. The complex Pt­(SnPrⁱ₃)₂­(CNBu^t)₂, <b>8</b>, was also prepared. Out of the four analogous complexes Pt­(SnR₃)₂­(CNBu^t)₂ (R = Bu^t, Mes, Ph, or Prⁱ), only the Bu^t analogue does both H₂ activation and H₂−D₂ exchange. This is due to steric effects imparted by the bulky Bu^t groups that distort the geometry of the complex considerably from planarity. The reaction of Pt­(COD)₂ with Bu^t₃SnH and CO gas afforded <i>trans</i>-Pt­(SnBu^t₃)₂­(CO)₂, <b>9</b>. Compound <b>9</b> can be converted to <b>2</b> by replacement of the CO ligands with CNBu^t via the intermediate Pt­(SnBu^t₃)₂­(CNBu^t)₂­(CO), <b>10</b>

Metal–Ligand Synergistic Effects in the Complex Ni(η²‑TEMPO)₂: Synthesis, Structures, and Reactivity

In the current investigation, reactions of the “bow-tie” Ni­(η²-TEMPO)₂ complex with an assortment of donor ligands have been characterized experimentally and computationally. While the Ni­(η²-TEMPO)₂ complex has <i>trans</i>-disposed TEMPO ligands, proton transfer from the C–H bond of alkyne substrates (phenylacetylene, acetylene, trimethylsilyl acetylene, and 1,4-diethynylbenzene) produce <i>cis</i>-disposed ligands of the form Ni­(η²-TEMPO)­(κ¹-TEMPOH)­(κ¹-R). In the case of 1,4-diethynylbenzene, a two-stage reaction occurs. The initial product Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-<i>C</i>C­(C₆H₄)­CCH] is formed first but can react further with another equivalent of Ni­(η²-TEMPO)₂ to form the bridged complex Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-κ¹-<i>C</i>C­(C₆H₄)­C<i>C</i>]­Ni­(η²-TEMPO)­(κ¹-TEMPOH). The corresponding reaction with acetylene, which could conceivably also yield a bridging complex, does not occur. Via density functional theory (DFT), addition mechanisms are proposed in order to rationalize thermodynamic and kinetic selectivity. Computations have also been used to probe the relative thermodynamic stabilities of the <i>cis</i> and <i>trans</i> addition products and are in accord with experimental results. Based upon the computational results and the geometry of the experimentally observed product, a <i>trans</i>–<i>cis</i> isomerization must occur

Metal–Ligand Synergistic Effects in the Complex Ni(η²‑TEMPO)₂: Synthesis, Structures, and Reactivity

In the current investigation, reactions of the “bow-tie” Ni­(η²-TEMPO)₂ complex with an assortment of donor ligands have been characterized experimentally and computationally. While the Ni­(η²-TEMPO)₂ complex has <i>trans</i>-disposed TEMPO ligands, proton transfer from the C–H bond of alkyne substrates (phenylacetylene, acetylene, trimethylsilyl acetylene, and 1,4-diethynylbenzene) produce <i>cis</i>-disposed ligands of the form Ni­(η²-TEMPO)­(κ¹-TEMPOH)­(κ¹-R). In the case of 1,4-diethynylbenzene, a two-stage reaction occurs. The initial product Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-<i>C</i>C­(C₆H₄)­CCH] is formed first but can react further with another equivalent of Ni­(η²-TEMPO)₂ to form the bridged complex Ni­(η²-TEMPO)­(κ¹-TEMPOH)­[κ¹-κ¹-<i>C</i>C­(C₆H₄)­C<i>C</i>]­Ni­(η²-TEMPO)­(κ¹-TEMPOH). The corresponding reaction with acetylene, which could conceivably also yield a bridging complex, does not occur. Via density functional theory (DFT), addition mechanisms are proposed in order to rationalize thermodynamic and kinetic selectivity. Computations have also been used to probe the relative thermodynamic stabilities of the <i>cis</i> and <i>trans</i> addition products and are in accord with experimental results. Based upon the computational results and the geometry of the experimentally observed product, a <i>trans</i>–<i>cis</i> isomerization must occur

Synthesis of [Pt(SnBu^t₃)(IBu^t)(μ-H)]₂, a Coordinatively Unsaturated Dinuclear Compound which Fragments upon Addition of Small Molecules to Form Mononuclear Pt–Sn Complexes

The reaction of Pt­(COD)₂ with one equivalent of tri-<i>tert</i>-butylstannane, Bu^t₃SnH, at room temperature yields Pt­(SnBu^t₃)­(COD)­(H)­(<b>3</b>) in quantitative yield. In the presence of excess Bu^t₃SnH, the reaction goes further, yielding the dinuclear bridging stannylene complex [Pt­(SnBu^t₃)­(μ-SnBu^t₂)­(H)₂]₂ (<b>4</b>). The dinuclear complex <b>4</b> reacts rapidly and reversibly with CO to furnish [Pt­(SnBu^t₃)­(μ-SnBu^t₂)­(CO)­(H)₂]₂ (<b>5</b>). Complex <b>3</b> reacts with <i>N</i>,<i>N</i>′-di-<i>tert</i>-butylimidazol-2-ylidene, IBu^t, at room temperature to give the dinuclear bridging hydride complex [Pt­(SnBu^t₃)­(IBu^t)­(μ-H)]₂ (<b>6</b>). Complex <b>6</b> reacts with CO, C₂H₄, and H₂ to give the corresponding mononuclear Pt complexes Pt­(SnBu^t₃)­(IBu^t)­(CO)­(H)­(<b>7</b>), Pt­(SnBu^t₃)­(IBu^t)­(C₂H₄)­(H)­(<b>8</b>), and Pt­(SnBu^t₃)­(IBu^t)­(H)₃ (<b>9</b>), respectively. The reaction of IBu^t with the complex Pt­(SnBu^t₃)₂(CO)₂ (<b>10</b>) yielded an abnormal Pt-carbene complex Pt­(SnBu^t₃)₂(<i>a</i>IBu^t)­(CO) (<b>11</b>). DFT computational studies of the dimeric complexes [Pt­(SnR₃)­(NHC)­(μ-H)]₂, the potentially more reactive monomeric complexes Pt­(SnR₃)­(NHC)­(H) and the trihydride species Pt­(SnBu^t₃)­(IBu^t)­(H)₃ have been performed, for NHC = IMe and R = Me and for NHC = IBu^t and R = Bu^t. The structures of complexes <b>3</b>–<b>8</b> and <b>11</b> have been determined by X-ray crystallography and are reported

Thermodynamic, Kinetic, Structural, and Computational Studies of the Ph₃Sn–H, Ph₃Sn–SnPh₃, and Ph₃Sn–Cr(CO)₃C₅Me₅ Bond Dissociation Enthalpies

The kinetics of the reaction of Ph₃SnH with excess •Cr­(CO)₃C₅Me₅ = •<b>Cr</b>, producing H<b>Cr</b> and Ph₃Sn–<b>Cr</b>, was studied in toluene solution under 2–3 atm CO pressure in the temperature range of 17–43.5 °C. It was found to obey the rate equation <i>d</i>[Ph₃Sn–<b>Cr</b>]/<i>d</i>t = <i>k</i>[Ph₃SnH]­[•<b>Cr</b>] and exhibit a normal kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 1.12 ± 0.04). Variable-temperature studies yielded Δ<i>H</i>^‡ = 15.7 ± 1.5 kcal/mol and Δ<i>S</i>^‡ = −11 ± 5 cal/(mol·K) for the reaction. These data are interpreted in terms of a two-step mechanism involving a thermodynamically uphill hydrogen atom transfer (HAT) producing Ph₃Sn• and H<b>Cr</b>, followed by rapid trapping of Ph₃Sn• by excess •<b>Cr</b> to produce Ph₃Sn–<b>Cr</b>. Assuming an overbarrier of 2 ± 1 kcal/mol in the HAT step leads to a derived value of 76.0 ± 3.0 kcal/mol for the Ph₃Sn–H bond dissociation enthalpy (BDE) in toluene solution. The reaction enthalpy of Ph₃SnH with excess •<b>Cr</b> was measured by reaction calorimetry in toluene solution, and a value of the Sn–Cr BDE in Ph₃Sn-<b>Cr</b> of 50.4 ± 3.5 kcal/mol was derived. Qualitative studies of the reactions of other R₃SnH compounds with •<b>Cr</b> are described for R = ⁿBu, ^tBu, and Cy. The dehydrogenation reaction of 2Ph₃SnH → H₂ + Ph₃SnSnPh₃ was found to be rapid and quantitative in the presence of catalytic amounts of the complex Pd­(IPr)­(P­(<i>p</i>-tolyl)₃). The thermochemistry of this process was also studied in toluene solution using varying amounts of the Pd(0) catalyst. The value of Δ<i>H</i> = −15.8 ± 2.2 kcal/mol yields a value of the Sn–Sn BDE in Ph₃SnSnPh₃ of 63.8 ± 3.7 kcal/mol. Computational studies of the Sn–H, Sn–Sn, and Sn–Cr BDEs are in good agreement with experimental data and provide additional insight into factors controlling reactivity in these systems. The structures of Ph₃Sn–<b>Cr</b> and Cy₃Sn–<b>Cr</b> were determined by X-ray crystallography and are reported. Mechanistic aspects of oxidative addition reactions in this system are discussed