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₈, and CD₂Cl₂), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms.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₈, and CD₂Cl₂), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms.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>)

Theory and Experiment Demonstrate that Sb(V)-Promoted Methane C–H Activation and Functionalization Outcompete Superacid Protonolysis in Sulfuric Acid

Sb­(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H2, and carbon oligomerization. In contrast to this general assumption, our density functional theory (DFT) calculations revealed an accessible barrier for C–H activation between methane and Sb­(V) in sulfuric acid that could potentially outcompete superacid protonolysis. This prompted us to experimentally examine this reaction in sulfuric acid with oleum, which has never been reported because of presumed superacid reactivity. Reaction of methane at 180 °C for 3 h resulted in very high yields of methyl bisulfate without significant overoxidation. Our DFT calculations show that a C–H activation and Sb-Me bond functionalization mechanism to give methyl bisulfate outcompetes methane protonolysis and many other possible reaction mechanisms, such as electron transfer, proton-coupled electron transfer, and hydride abstraction. Our DFT calculations also explain experimental hydrogen–deuterium exchange studies and the absence of methane carbo-functionalization/oligomerization products. Overall, this work demonstrates that in very strong Brønsted acid solvent, Sb­(V) can induce innersphere reaction mechanisms akin to transition metals and outcompete superacid reactivity

Theory and Experiment Demonstrate that Sb(V)-Promoted Methane C–H Activation and Functionalization Outcompete Superacid Protonolysis in Sulfuric Acid

Sb­(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H2, and carbon oligomerization. In contrast to this general assumption, our density functional theory (DFT) calculations revealed an accessible barrier for C–H activation between methane and Sb­(V) in sulfuric acid that could potentially outcompete superacid protonolysis. This prompted us to experimentally examine this reaction in sulfuric acid with oleum, which has never been reported because of presumed superacid reactivity. Reaction of methane at 180 °C for 3 h resulted in very high yields of methyl bisulfate without significant overoxidation. Our DFT calculations show that a C–H activation and Sb-Me bond functionalization mechanism to give methyl bisulfate outcompetes methane protonolysis and many other possible reaction mechanisms, such as electron transfer, proton-coupled electron transfer, and hydride abstraction. Our DFT calculations also explain experimental hydrogen–deuterium exchange studies and the absence of methane carbo-functionalization/oligomerization products. Overall, this work demonstrates that in very strong Brønsted acid solvent, Sb­(V) can induce innersphere reaction mechanisms akin to transition metals and outcompete superacid reactivity

Experimental Demonstration and Density Functional Theory Mechanistic Analysis of Arene C–H Bond Oxidation and Product Protection by Osmium Tetroxide in a Strongly Basic/Nucleophilic Solvent

Reactions that result in the oxy-functionalization of sp2 C–H bonds to give phenols are relatively rare. Here we report experiments and density functional theory (DFT) calculations that demonstrate selective C–H bond hydroxylation of nitroarenes to their corresponding mono-phenoxide as the exclusive product using OsO4 in a highly basic solvent mixture of water, hydroxide, and pyridine. DFT calculations using a mixed explicit/continuum solvent approach indicate that there is likely a mixture of OsO4-hydroxide/pyridine ground-state structures that have competitive reactivity and that the mechanism involves the nucleophilic addition of an anionic metal-oxo species to the arene followed by a hydride transfer process that is different from the standard [3 + 2] mechanism often invoked for the OsO4 oxidation of σ and π bonds. This work demonstrates the utility of using a strongly basic solvent for C–H bond oxidation reactions as this effectively converts any reactive phenolic product into the corresponding phenoxide, which is protected and essentially inert to further oxidation by the nucleophilic metal-oxo species

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(SnBut3)2­(CNBut)2­(H)2, 1, was obtained from the reaction of Pt­(COD)2 and But3SnH, followed by addition of CNBut. The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt­(SnBut3)2­(CNBut)2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1. Complex 2 catalyzes H2−D2 exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of But3SnH from 1 to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt­(SnMes3)­(SnBut3)­(CNBut)2, 3, when the addition of H2 to 2 was carried out in the presence of free ligand Mes3SnH (Mes = 2,4,6-Me3C6H2). Complex Pt­(SnMes3)2­(CNBut)2, 5, can be prepared from the reaction of Pt­(COD)2 with Mes3SnH and CNBut. The exchange reaction of 2 with Ph3SnH gave Pt­(SnPh3)3(CNBut)2­(H), 6, wherein both SnBut3 ligands are replaced by SnPh3. Complex 6 decomposes in air to form square planar Pt­(SnPh3)2­(CNBut)2, 7. The complex Pt­(SnPri3)2­(CNBut)2, 8, was also prepared. Out of the four analogous complexes Pt­(SnR3)2­(CNBut)2 (R = But, Mes, Ph, or Pri), only the But analogue does both H2 activation and H2−D2 exchange. This is due to steric effects imparted by the bulky But groups that distort the geometry of the complex considerably from planarity. The reaction of Pt­(COD)2 with But3SnH and CO gas afforded trans-Pt­(SnBut3)2­(CO)2, 9. Compound 9 can be converted to 2 by replacement of the CO ligands with CNBut via the intermediate Pt­(SnBut3)2­(CNBut)2­(CO), 10

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(SnBut3)2­(CNBut)2­(H)2, 1, was obtained from the reaction of Pt­(COD)2 and But3SnH, followed by addition of CNBut. The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt­(SnBut3)2­(CNBut)2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1. Complex 2 catalyzes H2−D2 exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of But3SnH from 1 to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt­(SnMes3)­(SnBut3)­(CNBut)2, 3, when the addition of H2 to 2 was carried out in the presence of free ligand Mes3SnH (Mes = 2,4,6-Me3C6H2). Complex Pt­(SnMes3)2­(CNBut)2, 5, can be prepared from the reaction of Pt­(COD)2 with Mes3SnH and CNBut. The exchange reaction of 2 with Ph3SnH gave Pt­(SnPh3)3(CNBut)2­(H), 6, wherein both SnBut3 ligands are replaced by SnPh3. Complex 6 decomposes in air to form square planar Pt­(SnPh3)2­(CNBut)2, 7. The complex Pt­(SnPri3)2­(CNBut)2, 8, was also prepared. Out of the four analogous complexes Pt­(SnR3)2­(CNBut)2 (R = But, Mes, Ph, or Pri), only the But analogue does both H2 activation and H2−D2 exchange. This is due to steric effects imparted by the bulky But groups that distort the geometry of the complex considerably from planarity. The reaction of Pt­(COD)2 with But3SnH and CO gas afforded trans-Pt­(SnBut3)2­(CO)2, 9. Compound 9 can be converted to 2 by replacement of the CO ligands with CNBut via the intermediate Pt­(SnBut3)2­(CNBut)2­(CO), 10

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