A Bulky Pd(II) α‑Diimine Catalyst Supported on Sulfated Zirconia for the Polymerization of Ethylene and Copolymerization of Ethylene and Methyl Acrylate

The reaction of (N∧N)­PdMe₂ (N∧N is Ar–NCMeMeCN–Ar; Ar = 2,6-<i>bis</i>(diphenylmethyl)-4-methylbenzene) and sulfated zirconia (<b>SZO</b>) in diethyl ether forms organometallic Pd-sites that polymerize ethylene and copolymerize ethylene and methyl acrylate. The Pd-sites bind CO and were studied by infrared and solid-state NMR spectroscopies. Analysis of the reaction mixture shows that more methane than expected evolves during the grafting reaction, suggesting that some Pd-sites do not contain a Pd-Me group. Consistent with this observation, deuterium labeling experiments show that ∼9% of palladium sites are active in polymerization reactions. (N∧N)­PdMe₂/SZO polymerizes ethylene with activity as high as 1342 kg_PE/(mol_active Pd*h) and incorporated up to 0.46% methyl acrylate in copolymerization reactions

A Well-Defined Ni(II) α‑Diimine Catalyst Supported on Sulfated Zirconia for Polymerization Catalysis

The reaction of (α-diimine)­NiMe₂ (α-diimine = (2,6-ⁱPr₂-C₆H₃)­NCMeMeCN­(2,6-ⁱPr₂-C₆H₃)) with partially dehydroxylated sulfated zirconia (<b>SZO</b>_<b>300</b>) in MeCN results in the formation of [(α-diimine)­NiMe­(NCMe)]­[<b>SZO</b>_<b>300</b>] ([<b>1</b>]­[<b>SZO</b>_<b>300</b>]) and methane. Reactions in Et₂O resulted in mixtures of [(α-diimine)­NiMe­(OEt₂)]­[<b>SZO</b>_<b>300</b>] ([<b>2</b>]­[<b>SZO</b>_<b>300</b>]) and [(α-diimine)­NiMe­(OEt₂)]­[Me<b>SZO</b>_<b>300</b>] ([<b>2</b>]­[Me<b>SZO</b>_<b>300</b>]), which were characterized by solid-state NMR spectroscopy. Contacting these solids with ethylene and monitoring the reaction by solid-state NMR showed that Ni–Me sites insert ethylene. [<b>1</b>]­[<b>SZO</b>_<b>300</b>] and [<b>2</b>]­[<b>SZO</b>_<b>300</b>]/[<b>2</b>]­[Me<b>SZO</b>_<b>300</b>] are active ethylene polymerization catalysts and show properties similar to those of closely related homogeneous catalysts. [<b>2</b>]­[<b>SZO</b>_<b>300</b>]/[<b>2</b>]­[Me<b>SZO</b>_<b>300</b>] copolymerizes ethylene and methyl 10-undecenoate to form copolymers with up to 0.4% incorporation of the polar monomer

Differentiation between Chelate Ring Inversion and Aryl Rotation in a CF₃‑Substituted Phosphine-Sulfonate Palladium Methyl Complex

The solution conformations and dynamic properties of the CF₃-sbustituted (<i>ortho</i>-phosphino­arenesulfonate)­Pd complexes (PO-CF₃)­PdMe­(L) ([PO-CF₃]⁻ = 2-{(<i>o</i>-CF₃-Ph)₂P}-4-Me-benzenesulfonate, L = 2,6-lutidine (<b>3</b>), pyridine (<b>4</b>)) were studied by NMR spectroscopy, taking particular advantage of ³¹P–¹⁹F through-space couplings and ¹H–¹H and ¹H–¹⁹F nuclear Overhauser effects. In CD₂Cl₂ solution in the temperature range of −80 to 20 °C, <b>3</b> adopts an <i>exo</i>₂ conformation. One <i>o</i>-CF₃-Ph ring is positioned such that the CF₃ group points toward Pd (<i>exo</i>) and exhibits through-space ⁴<i>J</i>_PF coupling. The other <i>o</i>-CF₃-Ph ring is positioned such that the CF₃ group points away from Pd (<i>endo</i>) and does not exhibit through-space ⁴<i>J</i>_PF coupling, and the <i>o</i>-H lies in the deshielding region near an axial site of the Pd square plane and exhibits a low-field chemical shift (δ > 9). Complex <b>4</b> exists as a 2:1 mixture of <i>exo</i>₂ and <i>exo</i>₃ isomers in CD₂Cl₂ solution at −90 °C. In <i>exo</i>₂-<b>4</b>, one CF₃ group is <i>exo</i> and exhibits through-space ⁴<i>J</i>_PF coupling, while the other CF₃ group is <i>endo</i> and does not exhibit through-space ⁴<i>J</i>_PF coupling. In <i>exo</i>₃-<b>4</b>, both CF₃ groups are <i>exo</i> and exhibit through-space ⁴<i>J</i>_PF couplings. Complex <b>4</b> undergoes two dynamic processes: rotation of the axial <i>o</i>-CF₃-Ph ring (A_aR), which interconverts <i>exo</i>₂-<b>4</b> and <i>exo</i>₃-<b>4</b> (Δ<i>G</i>^⧧ = 9.9(5) kcal/mol), and chelate ring inversion (RI), which permutes the axial and equatorial <i>o</i>-CF₃-Ph rings (Δ<i>G</i>^⧧ = 21(1) kcal/mol)

Heterolytic Activation of C–H Bonds on Cr^III–O Surface Sites Is a Key Step in Catalytic Polymerization of Ethylene and Dehydrogenation of Propane

We describe the reactivity of well-defined chromium silicates toward ethylene and propane. The initial motivation for this study was to obtain a molecular understanding of the Phillips polymerization catalyst. The Phillips catalyst contains reduced chromium sites on silica and catalyzes the polymerization of ethylene without activators or a preformed Cr–C bond. Cr^II sites are commonly proposed active sites in this catalyst. We synthesized and characterized well-defined chromium­(II) silicates and found that these materials, slightly contaminated with a minor amount of Cr^III sites, have poor polymerization activity and few active sites. In contrast, chromium­(III) silicates have 1 order of magnitude higher activity. The chromium­(III) silicates initiate polymerization by the activation of a C–H bond of ethylene. Density functional theory analysis of this process showed that the C–H bond activation step is heterolytic and corresponds to a σ-bond metathesis type process. The same well-defined chromium­(III) silicate catalyzes the dehydrogenation of propane at elevated temperatures with activities similar to those of a related industrial chromium-based catalyst. This reaction also involves a key heterolytic C–H bond activation step similar to that described for ethylene but with a significantly higher energy barrier. The higher energy barrier is consistent with the higher p<i>K</i>_a of the C–H bond in propane compared to the C–H bond in ethylene. In both cases, the rate-determining step is the heterolytic C–H bond activation

The impact of Metal–Ligand Cooperation in Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium PNP Pincer

The metal–ligand cooperative activation of CO₂ with pyridine-based ruthenium PNP pincer catalysts leads to pronounced inhibition of the activity in the catalytic CO₂ hydrogenation to formic acid. The addition of water restores catalytic performance by activating alternative reaction pathways and leads to unprecedented Ru-catalyzed CO₂ hydrogenation activity. The mechanism of the underlying chemical transformations is proposed on the basis of DFT calculations, kinetic experiments, and NMR reactivity studies

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activitie

Solid-Phase Polarization Matrixes for Dynamic Nuclear Polarization from Homogeneously Distributed Radicals in Mesostructured Hybrid Silica Materials

Mesoporous hybrid silica–organic materials containing homogeneously distributed stable mono- or dinitroxide radicals covalently bound to the silica surface were developed as polarization matrixes for solid-state dynamic nuclear polarization (DNP) NMR experiments. For TEMPO-containing materials impregnated with water or 1,1,2,2-tetrachloroethane, enhancement factors of up to 36 were obtained at ∼100 K and 9.4 T without the need for a glass-forming additive. We show that the homogeneous radical distribution and the subtle balance between the concentration of radical in the material and the fraction of radicals at a sufficient inter-radical distance to promote the cross-effect are the main determinants for the DNP enhancements we obtain. The material, as well as an analogue containing the poorly soluble biradical bTUrea, is used as a polarizing matrix for DNP NMR experiments of solutions containing alanine and pyruvic acid. The analyte is separated from the polarization matrix by simple filtration