Probing the Lignin Disassembly Pathways with Modified Catalysts Based on Cu-Doped Porous Metal Oxides

Described are the selectivities observed for reactions of lignin model compounds with modifications of the copper-doped porous metal oxide (CuPMO) system previously shown to be a catalyst for lignin disassembly in supercritical methanol (Matson et al., <i>J. Amer. Chem. Soc</i>. 2011, 133, 14090–14097). The models studied are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether, biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics of the α-O-4, β-O-4, 4-O-5, 5-5, and β-5 linkages characteristic of lignin. Also, briefly investigated as a substrate is poplar organosolv lignin. The catalyst modifications included added samarium­(III) (both homogeneous and heterogeneous) or formic acid. The highest activity for the hydrogenolysis of aryl ether linkages was noted for catalysts with Sm­(III) incorporated into the solid matrix of the PMO structure. In contrast, simply adding Sm³⁺ salts to the solution suppressed the hydrogenolysis activity. Added formic acid suppressed aryl ether hydrogenolysis, presumably by neutralizing base sites on the PMO surface but at the same time improved the selectivity toward aromatic products. Acetic acid induced similar reactivity changes. While these materials were variously successful in catalyzing the hydrogenolysis of the different ethers, there was very little activity toward the cleavage of the 5-5 and β-5 C-C bonds that represent a small, but significant, percentage of the linkages between monolignol units in lignins

Unexpected NO Transfer Reaction between <i>trans</i>-[Ru^II(NO⁺)(NH₃)₄(L)]³⁺ and Fe(III) Species: Observation of a Heterobimetallic NO-Bridged Intermediate

The reaction between <i>trans</i>-[Ru^II(NO⁺)­(NH₃)₄(L)]³⁺, L = ImN, IsN, Nic, P­(OMe)₃, P­(OEt)₃, and P­(OH)­(OEt)₂, and the Fe­(III) species [Fe^III(TPPS)], metmyoglobin, and hemoglobin was monitored by UV–vis, EPR, and electrochemical techniques (DPV, CV). No reaction was observed when L = ImN, IsN, Nic, and P­(OH)­(OEt)₂. However, when L = P­(OMe)₃ and P­(OEt)₃, the reaction was quantitative and the products were <i>trans</i>-[Ru^III(H₂O)­(NH₃)₄(P­(OR)₃)]³⁺ and [Fe^II(NO⁺)] species. Reaction kinetics data and DFT calculations suggest a two-step reaction mechanism with the initial formation of a bridged [Ru–(μNO)–Fe] intermediate, which was confirmed through electrochemical techniques (<i>E</i>⁰′ = −0.47 V vs NHE). The calculated specific rate constant values for the reaction were in the ranges <i>k</i>₁ = 1.1 to 7.7 L mol^–1 s^–1 and <i>k</i>₂ = 2.4 × 10^–3 to 11.4 × 10^–3 s^–1 for L = P­(OMe)₃ and P­(OEt)₃. The oxidation of the ruthenium center (Ru­(II) to Ru­(III)) containing the nitrosonium ligand suggests that NO can act as an electron transfer bridge between the two metal centers