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. 2025 Jun 20;15(13):11530-11543.
doi: 10.1021/acscatal.5c02021. eCollection 2025 Jul 4.

Oxidation of Alcohols to Carboxylates with N2O Catalyzed by Ruthenium(II)-CNC Complexes

José Bermejo  1 , Laura L Santos  1 , Eleuterio Álvarez  2 , Joaquín López-Serrano  1 , Andrés Suárez  1
Affiliations

Oxidation of Alcohols to Carboxylates with N2O Catalyzed by Ruthenium(II)-CNC Complexes

José Bermejo et al. ACS Catal. .

Abstract

Air-stable ruthenium-(II) complexes based on a picoline-derived CNC pincer ligand, [RuH-(CNC)-(CO)L]X (L = PPh3, X = Br; L = CO, X = BF4), were found to catalyze under basic conditions the oxidation with N2O of a series of alcohols to carboxylates. Both [RuH-(CNC)-(CO)L]X complexes react readily with strong bases (tBuOK or KHMDS), giving rise to a Ru-(II) complex containing a deprotonated CNC* ligand (when L = PPh3) or a Ru(0)-CNC derivative (for L = CO). Furthermore, the mechanism of the catalytic reaction has been elucidated through density functional theory (DFT) calculations. The catalytic cycle has been shown to proceed through an outer-sphere mechanism comprising four key transformations, which involve Ru-(II) intermediates based on the deprotonated CNC* ligand: (i) alkoxide dehydrogenation to yield a Ru-(II) hydride complex and an aldehyde molecule, (ii) N2O insertion into the ruthenium-hydride bond to yield a hydroxy ruthenium species and N2, (iii) nucleophilic attack of the hydroxo ligand in the Ru-OH complex to the intermediate aldehyde, and (iv) dehydrogenation of the formed alcoholate to regenerate the catalytically active Ru-(II) hydride and produce the carboxylate product.

Keywords: alcohols; carboxylates; nitrous oxide; ruthenium complexes; transfer hydrogenation.

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Figures

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1. Synthesis of the Bis-imidazolium Salt (2), and Silver (3) and Ruthenium (4 and 5) Complexes
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1
ORTEP perspective (50% ellipsoid probability) of the cationic fragment 4 + . Counteranion is a 9:1 mixture of Cl– and Br–, respectively. Hydrogen atoms, with exception of the hydrido ligand and the methylene hydrogens, have been omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ru(1)–N(3) 2.1401(19), Ru(1)–C(1) 2.086(2), Ru(1)–C(19) 2.046(2), Ru(1)–C(49) 1.848(3), Ru(1)–P(1) 2.4502(6), C(1)–Ru(1)–C(19) 156.04(9), C(1)–Ru(1)–N(3) 88.26(8), C(19)–Ru(1)–N(3) 76.75(8), C(49)–Ru(1)–N(3) 172.24(9), C(1)–Ru(1)–P(1) 103.58(7), C(19)–Ru(1)–P(1) 94.89(7), C(1)–Ru(1)–C(49) 95.59(10), C(19)–Ru(1)–C(49) 97.36(10), C(49)–Ru(1)–P(1) 96.47­(8), N(3)–Ru(1)–P(1) 89.14(5).
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2. Reactions of Ru­(II)-CNC Complexes 4 and 5 with Strong Bases
2
2
ORTEP perspective (50% ellipsoid probability) of complex 6. Hydrogen atoms, with exception of the hydrido ligand and the methine hydrogen, have been omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ru(1)–N(3) 2.133(3), Ru(1)–C(1) 2.073(3), Ru(1)–C(19) 2.050(3), Ru(1)–C(49) 1.841(3), Ru(1)–P(1) 2.4602(8), C(1)–Ru(1)–C(19) 162.35(13), C(1)–Ru(1)–N(3) 88.60(13), C(19)–Ru(1)–N(3) 78.31(13), C(49)–Ru(1)–N(3) 170.76(13), C(1)–Ru(1)–P(1) 97.44(9), C(19)–Ru(1)–P(1) 94.20(9), C(1)–Ru(1)–C(49) 95.71(14), C(19)–Ru(1)–C(49) 95.69(14), C(49)–Ru(1)–P(1) 98.26(10), N(3)–Ru(1)–P(1) 89.25(8).
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3
DFT calculated stabilities of the Ru­(II)-CNC* (6 and 7) and Ru(0)-CNC (6-iso and 7-iso) complexes, and interconversion energy barriers (ΔG ‡). In parentheses, energies (ΔG in THF) in kcal/mol.
4
4
Reaction steps (1a)–(1c) and the overall reaction (2) of the oxidation of alcohols with N2O under basic conditions.
5
5
Catalytic control experiments: (i) acceptorless dehydrogenation of 1-hexanol, (ii) oxidation of 1-hexanol with N2O in the presence of 4 Å molecular sieves, (iii) hydrogenation of N2O, and (iv) oxidation of benzaldehyde with H2O.
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3. Catalytic Cycle Calculated by DFT for the Oxidation of Ethanol with N2O Catalyzed by Complex 6
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6
Free energy (ΔG in toluene, kcal/mol) profile calculated by DFT for the PPh3 dissociation from 6 and alcohol dehydrogenation by A (the origin of energies is 6 + EtO– + 2 N2O).
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7
Geometries optimized by DFT of the TS A→B , TS B→A , TS C→D , TS′ C→D , TS D→A and TS′ A→B transition states.
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Free energy (ΔG in toluene, kcal/mol) profile calculated by DFT for N2O reduction by complex B (the origin of energies is 6 + EtO– + 2 N2O).
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Free energy (ΔG in toluene, kcal/mol) profile calculated by DFT for the nucleophilic attack of Ru–OH (D) to acetaldehyde, 1-hydroxyethanolate dehydrogenation and acetic acid deprotonation by D (the origin of energies is 6 + EtO– + 2 N2O).
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Labeling for NMR signal assignment of complex 6.
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