Relationship between pKa of 8-quinolinol derivatives and a π-donor ability of the 8-quinolinolato oxygen in linear nitrosylruthenium(II) complexes

Abstract The relationship between the p K a of 8-quinolinol derivatives {8-quinolinol (Hqn), 2-methyl- (H2-Meqn), 2,4-dimethyl- (H2,4-diMeqn), 5-chloro- (H5-Clqn) and 5,7-dichloro-8-quinolinols (H5,7-diClqn)} and a π-donor ability of the 8-quinolinolato oxygens has been investigated by the identification of the structures of the major products, [RuCl(QN)(QN′)NO] (HQN=8-quinolinol derivative; HQN′=different 8-quinolinol derivatives), obtained by the reaction of [RuCl 3 (QN or QN′)NO] − with HQN′ or HQN. The results obtained clearly showed that the oxygen of the 8-quinolinol derivative that has a higher p K a predominantly coordinates in the trans position to the NO ligand and is a better π-electron donor. The order of the π-electron donor ability for the oxygen of the 8-quinolinol derivatives is as follows: H2-Meqn≥H2,4-diMeqn>Hqn≥H5-Clqn>H5,7-diClqn, almost agreeing with the magnitude of the p K a values of the corresponding 8-quinolinols. The structures of cis- 1 [RuCl(5,7-diClqn) 2 NO] and cis- 1 [RuCl(5,7-diClqn)(2-Meqn)NO] were determined by X-ray diffraction.