Unveiling the mechanism of triphos-Ru catalysed C-O bond disconnections in polymers

The study addresses how triphos-Ru catalysts effect selective C-O bond hydrogenolysis relevant to depolymerising lignin and amine-cured epoxy resins. Developing circular strategies for biomass valorisation and plastic recycling is critical to sustainable chemistry, yet the mechanistic basis of triphos-Ru-catalysed C-O bond cleavage remains unclear. Prior work showed effective C(alkyl)-O scission in lignin β-O-4 models and selective depolymerisation of epoxy resins using triphos-Ru-TMM precatalysts, but competing pathways and catalyst deactivation were noted for other ligand systems. This work aims to elucidate, at the molecular level, the activation of the Ru-TMM precatalyst, identify the active Ru species, define the catalytic cycle for dehydrogenation and C-O scission, and rationalise selectivity, thereby enabling improved methods for polymer deconstruction.

Previous studies established that Ru complexes with triphos ligands catalyse hydrogenation/dehydrogenation reactions and enable C-O cleavage in lignin models (e.g., xantphos-Ru and triphos-Ru-TMM systems). Bergman and Ellman reported C-O cleavage of lignin-related substrates with xantphos-Ru, while triphos-Ru-TMM proved highly efficient for β-O-4 linkages, recovering phenols. On more complex lignin models, triphos-Ru catalysts showed competing C–C cleavage and xantphos systems deactivated. Recently, triphos-Ru-TMM enabled selective depolymerisation of epoxy resins/composites. Triphos-Ru catalysts have also been developed for hydrogenation of CO2 to methanol or methyl formate and for alcohol amination chemistry, supporting their broader utility. However, the mechanistic details of the Triphos-Ru-TMM precatalyst activation and the nature of the active species in C–O hydrogenolysis remained unresolved, motivating the present mechanistic investigation.

• Catalytic tests on molecular epoxy and lignin model compounds under standard conditions (toluene-d8, 160 °C, 3 mol% triphos-Ru-TMM, 3 equiv isopropanol) to monitor induction phases, yields, and the effect of additives (phenols such as Me-BPA). Yields quantified by 1H NMR with 1,3,5-trimethoxybenzene internal standard; GC-MS used for confirmation.
• In operando NMR (1H and 31P) in J. Young tubes to observe precatalyst transformation, induction periods (2–4 h), and formation of Ru species during catalysis. Operando detection of volatile products (e.g., isobutene) via 1H/13C NMR.
• Synthesis and isolation of Ru diphenolate complexes by reacting triphos-Ru-TMM with electron-deficient phenols (2-fluorophenol and 2,5-difluorophenol). Structural characterisation by single-crystal X-ray diffraction (CCDC 2289877 for Ru-1) and multinuclear NMR; probing reactivity toward solvents (e.g., THF-d8) and alcohols (iPrOH) to form hydride species.
• Trapping TMM-coordination modes using soft electrophiles: reaction of triphos-Ru-TMM with IPrAuNTf2 to form a diaurated 2-methyleneallyl Ru complex; CO trapping to afford a Ru 2-methylallyl carbonyl complex (CCDC 2303540); regeneration experiments with KI; NMR characterisation.
• Isotope-labelling under transfer hydrogenation conditions (iPrOH-d8/acetone-d6) to probe metal–ligand cooperation and TMM protonation dynamics, assessing D incorporation into TMM (85%).
• Kinetic profiling for deconstruction of model 1 with and without phenol additives (0, 10 mol%, 1 equiv Me-BPA) to quantify effects on induction and rate.
• DFT calculations: M06-D3/def2SVP optimisations with single-point M06-D3/def2TZVP and SMD(toluene) solvation. Model substrates (phenyl surrogates for Me-BPA) used to compute free-energy profiles for (i) precatalyst activation pathways via iPrOH and phenols, (ii) acceptorless dehydrogenation to Ru dihydride, Ru(0)–H2 formation, and (iii) oxidative addition into C–O bonds of alcohol and ketone substrates; analysis of barriers and thermodynamics.
• Independent synthesis of dihydride species 17 using a sterically hindered triphos variant: reduction of triphos-Ru-Cl2 with KCs under H2 to form hydride intermediates and conversion to 17 at rt; alternative generation via β-hydride elimination from alkoxide (potassium 1-phenylethanolate). NMR identification and correlation with computed intermediates.
• Chemoselectivity probe: amine-cured epoxy model 2 to test possible C–N scission pathways under standard catalytic conditions; product analysis by NMR and GC-MS.
• Standard synthetic, spectroscopic, and kinetic procedures were performed under inert atmosphere (glovebox), using purified solvents; NMR assignments by COSY, HSQC, HMBC, and 1H–31P HMBC.

• Precatalyst activation requires isopropanol; phenols significantly accelerate activation and catalysis. A 2–4 h induction is observed without additives, with full conversion at ~12 h. Adding 10 mol% Me-BPA slightly accelerates, reaching full conversion by ~8 h. With 1 equiv Me-BPA, 68% product after 2 h and full conversion by 6 h.
• Model reactivity: Ketone III (proposed intermediate) in iPrOH is reduced to alcohol III without Me-BPA and shows no C–O cleavage; adding 1 equiv Me-BPA gives clean deconstruction to Me-BPA over 16 h, implicating a phenol-mediated pathway.
• Yields on models: Phenol product up to 96% observed (Fig. 2), and >90% overall deconstruction under optimal conditions.
• TMM ligand reactivity: Soft Lewis acids (IPrAu+) form a diaurated 2-methyleneallyl Ru complex; decomplexation with KI regenerates triphos-Ru-TMM, indicating dynamic Au2–C bonding. Under CO, a Ru 2-methylallyl carbonyl complex is trapped and structurally confirmed (CCDC 2303540). Isotope-labelling shows 85% D incorporation into TMM under transfer hydrogenation, supporting reversible TMM protonation (metal–ligand cooperation).
• Formation of Ru diphenolates: Electron-deficient phenols rapidly convert triphos-Ru-TMM into Ru diphenolate complexes (Ru-1 from 2-fluorophenol; Ru-2 from 2,5-difluorophenol). X-ray confirms Ru-1. Isobutene formation is observed in operando, consistent with double protonation of TMM and release of isobutene. Ru-1 reacts with THF-d8 to a Ru propionate.
• Catalytic competency of phenolates: Ru-1 is an active precatalyst (48% Me-BPA in 16 h on model 1), whereas Ru-2 is inactive. Ru-1 with 20 equiv iPrOH forms ~40% acetone (by 1H NMR), suggesting dynamic exchange to isopropanolate and β-hydride elimination. Converting Ru-2 to a hydride complex (Ru-H) with potassium 1-phenylethanolate enables C–O cleavage of model 1, supporting phenolate entry into the catalytic cycle via β-hydride elimination.
• DFT mechanistic insights for activation: Double protonation of TMM by iPrOH to a diisopropanolate (AC5) is prohibitive (second protonation barrier 49.3 kcal/mol). First protonation barrier is 32.2 kcal/mol; subsequent β-hydride elimination to a hydride (AC7) and reductive elimination of isobutene has a barrier of 29.0 kcal/mol, explaining sluggish induction. Protonation by model S phenol has ΔG = 38.5 kcal/mol, aligning with the experimental need for iPrOH to initiate activation before phenols accumulate.
• Dehydrogenation and C–O scission cycle (computed): Phenolate complex II exchanges phenolate for iPrO– (ΔG +13.3; barrier 19.2 kcal/mol) to form 12; β-H elimination to acetone gives hydride 13 (exergonic by −12.2 kcal/mol). Further β-H elimination yields dihydride 17. Dihydride 17 forms Ru(0)–H2 complex 18 (+14.7 kcal/mol), which coordinates substrates. Oxidative addition into o-phenoxyketone (ketone S) proceeds with a 23.8 kcal/mol barrier (TS4), while into o-phenoxyalcohol is unfeasible (60.1 kcal/mol). Thus, selectivity is governed by substrate coordination/backbonding and oxidative addition barriers, not solely C–O bond strengths (alcohol vs ketone homolytic BDE difference ~10.4 kcal/mol). Protonation of the Ru–C bond after oxidative addition regenerates II over a small barrier (6.5 kcal/mol), releasing phenol product.
• Experimental corroboration of intermediates: Dihydride 17 was generated from triphos-Ru-Cl2 by reduction with KCs under H2 and via alkoxide β-hydride elimination, matching computed intermediates. A Ru(0)/Ru(II) oxidative addition pathway is implicated, with Ru(0)–H2 as the entry point to C–O activation.
• Chemoselectivity: In an amine-cured epoxy model (2), no C–N scission was detected; observed amines arise from reductive amination pathways of the ketone intermediate, suggesting C–O cleavage is preferred under these conditions.

The findings resolve the long-standing question of how triphos-Ru-TMM precatalysts effect C–O hydrogenolysis in lignin and epoxy motifs. Catalyst activation proceeds through TMM protonation and β-hydride elimination sequences to generate hydride and ultimately Ru(0)–H2 species, with phenolate complexes serving as crucial resting and entering points in the catalytic cycle. Phenols produced in the first scission event feed back to accelerate activation by forming Ru diphenolates, explaining the observed induction period and its suppression by phenol additives. Computations clarify that the decisive factor in selectivity is the ability of an electron-rich Ru(0) species to coordinate and undergo oxidative addition into the C–O bond of phenoxyketones via π(η2) interactions with the carbonyl; the corresponding alcohols are disfavoured by much higher barriers. This mechanistic picture accounts for differences between lignin and epoxy models, the role of isopropanol, and the observed kinetics. The combined experimental trapping/isolation (Ru-1, Ru-2, Ru-H, 17; isobutene detection; Au-trapped allyl species) and DFT energetics provide a coherent cycle that can inform the optimisation of depolymerisation conditions and the design of tailored Ru catalysts for polymer recycling and biomass valorisation.

This work unveils a comprehensive mechanism for triphos-Ru-catalysed C–O bond hydrogenolysis relevant to lignin valorisation and epoxy deconstruction. Key advances include: identification of Ru phenolate complexes as central species for both activation and catalysis; demonstration of a Ru(0)/Ru(II) oxidative addition pathway initiated by a Ru(0)–H2 complex; experimental observation and synthesis of critical intermediates (isobutene release, Ru-allyl/Au complexes, Ru diphenolates, Ru hydrides, and dihydride 17); and DFT quantification of activation barriers and thermodynamics that match operando kinetics. The results explain induction phenomena and substrate selectivity and suggest that managing phenol concentrations and promoting phenolate equilibria can accelerate deconstruction. Future work could explore ligand and counterion effects to modulate Ru(0) formation and oxidative addition barriers, expand substrate scope to complex polymer matrices, develop conditions that shorten induction without excess alcohol, and translate insights to C–N scission or other challenging bond activations in crosslinked polymers.

• Several reactive intermediates (e.g., isopropanolate adducts) could not be directly observed at room temperature, consistent with dynamic equilibria; identification relies on indirect evidence and DFT.
• DFT models replaced Me-BPA moieties with phenyl groups for computational tractability, which may slightly alter energetics relative to full systems.
• Electron-withdrawing phenolate precatalysts exhibited varying activity (Ru-2 inactive), indicating sensitivity to phenolate electronics and limiting generality.
• The activation barriers for TMM protonation via iPrOH are high, consistent with observed induction; precise rates under varied conditions were not exhaustively mapped.
• No C–N scission was achieved under tested conditions; broader conditions or catalysts might be needed for amine-cured networks.
• Isolation of some in-operando observed Ru species (e.g., the 49.6 ppm 31P signal) was not achieved, limiting full structural assignment.