Catalytic Hydrogenolysis by Atomically Dispersed Iron Sites Embedded in Chemically and Redox Non-innocent N-Doped Carbon.

Atomically dispersed first-row transition metals embedded in nitrogen-doped carbon materials (M-N-C) show promising performance in catalytic hydrogenation but are less well-studied for reactions with more complex mechanisms, such as hydrogenolysis. Their ability to catalyze selective C-O bond cleavage of oxygenated hydrocarbons such as aryl alcohols and ethers is enhanced with the participation of ligands directly bound to the metal ion as well as longer-range contributions from the support. In this article, we describe how Fe-N-C catalysts with well-defined local structures for the Fe sites catalyze C-O bond hydrogenolysis. The reaction is facilitated by the N-C support. According to spectroscopic analyses, the as-synthesized catalysts contain mostly pentacoordinated FeIII sites, with four in-plane nitrogen donor ligands and one axial hydroxyl ligand. In the presence of 20 bar of H2 at 170-230 °C, the hydroxyl ligand is lost when N4FeIIIOH is reduced to N4FeII, assisted by the H2 chemisorbed on the support. When an alcohol binds to the tetracoordinated FeII sites, homolytic cleavage of the O-H bond is accompanied by reoxidation to FeIII and H atom transfer to the support. The role of the N-C support in catalytic hydrogenolysis is analogous to the behavior of chemically and redox-non-innocent ligands in molecular catalysts based on first-row transition metal ions and enhances the ability of M-N-Cs to achieve the types of multistep activations of strong bonds needed to upgrade renewable and recycled feedstocks.

Transition metal-like carbocatalyst.

Catalytic cleavage of strong bonds including hydrogen-hydrogen, carbon-oxygen, and carbon-hydrogen bonds is a highly desired yet challenging fundamental transformation for the production of chemicals and fuels. Transition metal-containing catalysts are employed, although accompanied with poor selectivity in hydrotreatment. Here we report metal-free nitrogen-assembly carbons (NACs) with closely-placed graphitic nitrogen as active sites, achieving dihydrogen dissociation and subsequent transformation of oxygenates. NACs exhibit high selectivity towards alkylarenes for hydrogenolysis of aryl ethers as model bio-oxygenates without over-hydrogeneration of arenes. Activities originate from cooperating graphitic nitrogen dopants induced by the diamine precursors, as demonstrated in mechanistic and computational studies. We further show that the NAC catalyst is versatile for dehydrogenation of ethylbenzene and tetrahydroquinoline as well as for hydrogenation of common unsaturated functionalities, including ketone, alkene, alkyne, and nitro groups. The discovery of nitrogen assembly as active sites can open up broad opportunities for rational design of new metal-free catalysts for challenging chemical reactions.

CO Displacement in an Oxidative Addition of Primary Silanes to Rhodium(I).

The rhodium dicarbonyl {PhB(OxMe2)2ImMes}Rh(CO)2 (1) and primary silanes react by oxidative addition of a nonpolar Si-H bond and, uniquely, a thermal dissociation of CO. These reactions are reversible, and kinetic measurements model the approach to equilibrium. Thus, 1 and RSiH3 react by oxidative addition at room temperature in the dark, even in CO-saturated solutions. The oxidative addition reaction is first-order in both 1 and RSiH3, with rate constants for oxidative addition of PhSiH3 and PhSiD3 revealing kH/ kD ∼ 1. The reverse reaction, reductive elimination of Si-H from {PhB(OxMe2)2ImMes}RhH(SiH2R)CO (2), is also first-order in [2] and depends on [CO]. The equilibrium concentrations, determined over a 30 °C temperature range, provide Δ H ° = -5.5 ± 0.2 kcal/mol and Δ S ° = -16 ± 1 cal·mol-1K-1 (for 1 ⇄ 2). The rate laws and activation parameters for oxidative addition (Δ H⧧ = 11 ± 1 kcal·mol-1 and Δ S⧧ = -26 ± 3 cal·mol-1·K-1) and reductive elimination (Δ H⧧ = 17 ± 1 kcal·mol-1 and Δ S⧧ = -10 ± 3 cal·mol-1K-1), particularly the negative activation entropy for both forward and reverse reactions, suggest the transition state of the rate-determining step contains {PhB(OxMe2)2ImMes}Rh(CO)2 and RSiH3. Comparison of a series of primary silanes reveals that oxidative addition of arylsilanes is ca. 5× faster than alkylsilanes, whereas reductive elimination of Rh-Si/Rh-H from alkylsilyl and arylsilyl rhodium(III) occurs with similar rate constants. Thus, the equilibrium constant Ke for oxidative addition of arylsilanes is >1, whereas reductive elimination is favored for alkylsilanes.

Enhancing the Sensitivity of Solid-State NMR Experiments with Very Low Gyromagnetic Ratio Nuclei with Fast Magic Angle Spinning and Proton Detection.

Many transition metals commonly encountered in inorganic materials and organometallic compounds possess NMR-active nuclei with very low gyromagnetic ratios (γ) such as 89Y, 103Rh, 109Ag, and 183W. A low-γ leads to poor NMR sensitivity and other experimental challenges. Consequently, nuclei with low-γ are often impossible to study with conventional solid-state NMR methods. Here, we combine fast magic angle spinning (MAS) and proton detection to enhance the sensitivity of solid-state NMR experiments with very low-γ nuclei by 1-2 orders of magnitude. Coherence transfer between 1H and low-γ nuclei was performed with low-power double quantum (DQ) or zero quantum (ZQ) cross-polarization (CP) or dipolar refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT). Comparison of the absolute sensitivity of CP NMR experiments performed with proton detection with 1.3 mm rotors and direct detection with 4 mm rotors shows that proton detection with a 1.3 mm rotor provides a significant boost in absolute sensitivity, while requiring approximately 1/40th of the material required to fill a 4 mm rotor. Fast MAS and proton detection were applied to obtain 89Y and 103Rh solid-state NMR spectra of organometallic complexes. These results demonstrate that proton detection and fast MAS represents a general approach to enable and accelerate solid-state NMR experiments with very low-γ nuclei.

Mild partial deoxygenation of esters catalyzed by an oxazolinylborate-coordinated rhodium silylene.

An electrophilic, coordinatively unsaturated rhodium complex supported by borate-linked oxazoline, oxazoline-coordinated silylene, and N-heterocyclic carbene donors [{κ(3)-N,Si,C-PhB(Ox(Me2))(Ox(Me2)SiHPh)Im(Mes)}Rh(H)CO][HB(C6F5)3] (, Ox(Me2) = 4,4-dimethyl-2-oxazoline; Im(Mes) = 1-mesitylimidazole) is synthesized from the neutral rhodium silyl {PhB(Ox(Me2))2Im(Mes)}RhH(SiH2Ph)CO () and B(C6F5)3. The unusual oxazoline-coordinated silylene structure in is proposed to form by rearrangement of an unobserved isomeric cationic rhodium silylene species [{PhB(Ox(Me2))2Im(Mes)}RhH(SiHPh)CO][HB(C6F5)3] generated by H abstraction. Complex catalyzes reductions of organic carbonyl compounds with silanes to give hydrosilylation products or deoxygenation products. The pathway to these reactions is primarily influenced by the degree of substitution of the organosilane. Reactions with primary silanes give deoxygenation of esters to ethers, amides to amines, and ketones and aldehydes to hydrocarbons, whereas tertiary silanes react to give 1,2-hydrosilylation of the carbonyl functionality. In contrast, the strong Lewis acid B(C6F5)3 catalyzes the complete deoxygenation of carbonyl compounds to hydrocarbons with PhSiH3 as the reducing agent.