Mechanistic insights into catalytic carboxylic ester hydrogenation with cooperative Ru(II)-bis{1,2,3-triazolylidene}pyridine pincer complexes

• Published in: Journal of organometallic chemistry Vol. 845; pp. 30 - 37
• Main Authors: Sluijter, Soraya N., Korstanje, Ties J., van der Vlugt, Jarl Ivar, Elsevier, Cornelis J.
• Format: Journal Article
• Language: English
• Published: LAUSANNE Elsevier B.V 15.09.2017; Elsevier
• Subjects: Carboxylic ester; Catalysis; Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; Hydrogenation; Physical Sciences; Pincer ligand; Ruthenium; Science & Technology; Pincer ligand; Ruthenium; Catalysis; Hydrogenation; Carboxylic ester; N-HETEROCYCLIC CARBENE; DIMETHYL OXALATE; BUILDING-BLOCKS; RUTHENIUM CATALYSTS; H BOND ACTIVATION; HOMOGENEOUS HYDROGENATION; FATTY ALCOHOL; EFFICIENT HYDROGENATION; METAL-COMPLEXES; ACID ESTERS
• ISSN: 0022-328X; 1872-8561
• DOI: 10.1016/j.jorganchem.2017.01.003

Transmetallation of newly designed lutidine-based CNC or CNN ligands L, featuring flanking 1,2,3-triazolylidene (tzNHCs) moieties, from Ag(I) to Ru(II) provided access to well-defined cationic [RuII(CO)(H)(L)(PPh3)]+ complexes 2 and 5. Spectroscopic investigations confirm that, in both complexes, the tridentate ligand binds in a rare facial mode to the metal center. The complexes, that exhibit ligand-based reversible deprotonation/dearomatization reactivity, are active in catalytic ester hydrogenation in the presence of KOtBu (≥20 mol%) as an exogenous base. The beneficial effect of the base on catalytic activity relates to transesterification of substrates to the corresponding tert-butyl ester derivatives, which are hydrogenated considerably faster than methyl esters. The mechanistic findings in this work confirm that this transformation is very complex, with this transesterification, metal-ligand cooperative reactivity, base strength and possibly product inhibition all playing a role. Furthermore, relevant Ru(CNC)(hydride) species have been observed by NMR spectroscopy under near-catalytic conditions. The synthesis of novel bis(triazolyl)pyridine ligands and their coordination chemistry toward Ru(II) are discussed, and these complexes are shown to be active catalysts for the hydrogenation of carboxylic esters. [Display omitted] •Synthesis of a novel bis(triazolyl)pyridine pincer ligand.•Coordination chemistry toward Ru(II).•Efficient hydrogenolysis of carboxylic esters.