Ruthenium complexes bearing an unsymmetrical pincer ligand with a 2-hydroxypyridylmethylene fragment: active catalysts for transfer hydrogenation of ketones

Abstract

Five ruthenium(ii) complexes were synthesized, including (HO-C5H3N-CH2-C5H3N-C5H4N)Ru(PPh3)Cl2 (3), [(HO-C5H3N-CH2-C5H3N-C5H4N)Ru(PPh3)2Cl][PF6] (4) and [(HO-C5H3N-CH2-C5H3N-C5H4N)Ru(PPh3)2OH][PF6] (5) bearing an unsymmetrical pincer NNN ligand with a 2-hydroxypyridylmethylene fragment, and [(CH3O-C5H3N-CH2-C5H3N-C5H4N)2Ru][Cl]2 (6) and [(CH3O-C5H3N-CH2-C5H3N-C5H4N)2Ru][PF6]2 (7) containing 2-methoxypyridylmethylene moieties. 4 reacts with H2O at room temperature to give 5 whose crystal structure reveals the existence of intramolecular hydrogen-bonding between its two -OH groups. 3 exhibits high catalytic activity for transfer hydrogenation of ketones.

Ruthenium NNN complexes with a 2‐hydroxypyridylmethylene fragment for transfer hydrogenation of ketones

“…When L 2 was treated with an equimolar amount of RuCl 2 (PPh 3 ) 3 in refluxing methanol, the orange product 1 was obtained (Scheme ). The 31 P NMR spectrum of 1 in CD 3 OD exhibits one singlet for the PPh 3 group at 56.5 ppm, which is comparable to similar NNN ruthenium complexes reported by us . The 1 H NMR spectrum shows one singlet at 6.38 ppm for the CH proton in the pyrazolyl ring, and two singlets at 2.96 and 1.66 ppm for the two methyl groups.…”

“…X‐ray diffraction analysis shows 4 is a dicationic complex with the ruthenium center coordinating to two tridentate ligands L 3 in a distorted octahedral geometry (Figure ). Both NNN ligands are coordinated to the Ru(II) center in a meridional fashion, which is analogous to the reported complex [(CH 3 OC 5 H 3 NCH 2 C 5 H 3 NC 5 H 4 N) 2 Ru][PF 6 ] 2 …”

Optimum bifunctionality in a 2-(2-pyridyl-2-ol)-1,10-phenanthroline based ruthenium complex for transfer hydrogenation of ketones and nitriles: impact of the number of 2-hydroxypyridine fragments

“…41,42 We attributed this difference to the steric environment around the metal center. 41,42 We attributed this difference to the steric environment around the metal center.…”

“…Under the optimized reaction condition, chemoselectively 5-hexen-2-one was hydrogenated to afford 5-hexen-2-ol as the major product along with a small amount of 2-hexanol ( Table 2, entry 23). 42 The chemoselective hydrogenation of α,β-unsaturated ketones to the corresponding allylic alcohols is an important step in the industrial synthesis of fine chemicals. Based on this observation, it was tempting to envision the possibility that by introducing more steric bulk near the olefin group we could enhance the chemoselectivity in TH further towards the ketonic moiety.…”

“…Among them, Kelson demonstrated that a ruthenium pyridonato complex catalyzed TH of ketones (Scheme 2, A). 42 This complex probably followed an inner-sphere mechanism and showed moderate activity. reduction of acetophenone (TOF = 408 h −1 ), although the mechanism of this reaction is not clear yet.…”