Cationic Half-Sandwich Ruthenium(II) Complexes of 2-Picolyl-Based N-Arylguanidines as Catalysts for the Aldehyde–Water Shift Reaction

The bridge-splitting reaction (bsr) of [η⁶-(p-cymene)Ru(μ-Cl)Cl]₂ with two equiv of guanidines 1–3 and 5–9 in toluene at RT afforded cationic complexes, [η⁶-(p-cymene)Ru(NN)Cl]Cl (11–18) in 80%–93% yields and the analogous reaction involving [η⁶-(p-cymene)Ru(μ-Cl)Cl]₂ and two equiv of guanidines 4 and 10 and the reaction involving [(η⁶-C₆Me₆)Ru(μ-Cl)Cl]₂ and two equiv of guanidines 3 and 9 afforded cationic complexes [η⁶-(p-cymene)Ru(NN)Cl]Cl (19 and 20) and [(η⁶-C₆Me₆)Ru(NN)Cl]Cl (21 and 22) in 83%–89% yields. The bsr reaction of [η⁶-(p-cymene)Ru(μ-Cl)Cl]₂ with guanidine 9 followed by an anion exchange reaction with NH₄PF₆ in MeOH at RT afforded [η⁶-(p-cymene)Ru(NN)Cl]PF₆ (23) in 90% yield. Molecular structures of two guanidines and seven complexes were determined by SCXRD. Thirteen new complexes were screened as catalysts for the Aldehyde–Water Shift (AWS) reaction of 4-anisaldehyde at 0.1 mol % catalyst loading, and from the screening experiments, 18 turned out to be the best catalyst. The scope of a variety of substrates was explored and a plausible mechanism of AWS reaction is proposed. Some key reactions were carried out to strengthen the proposed mechanism of AWS reaction. This work represents the first practical, sustainable, and, in the case of cinnamaldehyde, a chemo-selective procedure for aryl carboxylic acids obtained through AWS reaction. The catalytic activity of 18 surpasses the catalytic activities of the known complexes reported in the literature.