Pyrazole-pyridine-pyrazole (NNN) ruthenium() complex catalyzed acceptorless dehydrogenation of alcohols to aldehydes

A new series of cationic ruthenium( ii ) complexes, [RuCl(PPh 3 ) 2 (κ 3 -NNN- L1 )]Cl ( 1 ), [RuH(PPh 3 ) 2 (κ 3 -NNN- L2 )]Cl ( 2 ), and [RuH(PPh 3 ) 2 (κ 3 -NNN- L3 )]Cl ( 3 ), bearing a 2,6-bis (1-R-5-methyl-pyrazol-3-yl) pyridine ligand ( L1 : R = H, L2 : R = C 6 H 4 - p -CF 3 , L3 : R = C 6 H 4 - p -OCH 3 ), were synthesized and characterized with NMR, HRMS, and single-crystal X-ray diffraction. Their catalytic application was investigated in the acceptorless dehydrogenation (AD) of primary alcohols. Complex 1 outperformed 2 and 3 in terms of the selectivity towards the aldehydes, and provided the aldehydes with a yield of up to 99%, with good functional group tolerance under mild conditions. In addition, the only by-product of the reaction was dihydrogen, which can be collected as clean energy, and the reaction meets the requirements of environment-friendly chemistry. Complex 1 also proved to be a promising catalyst in the sequential AD/condensation reaction between the primary and secondary alcohols, affording α,β-unsaturated ketones in moderate to good selectivity. Complex 1 shows good catalytic activity (up to 99% yield) and excellent selectivity (up to 100%) for the AD of alcohols to aldehydes.