Abstract
Conventional and stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in a number of mono g -diketone salen iron(III) complexes, [Fe(salen)(O 7 O)], by 8-hydroxyquinoline (=HO 7 N) in alcohol media (O 7 O m =anion of the g -diketones pentane-2,4-dione, 2,6-dimethylheptane-3,5-dione, 2,2,6,6-tetramethylheptane-3,5-dione, 1-phenylbutane-1,3-dione, 1,3-diphenylpropane-1,3-dione, and 1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione; salen=anion of N , N '-disalicylideneethylenediamine). As shown by spectrophotometry and conductometry, the solutions of complexes [Fe(salen)(O 7 O)] in alcohols ROH are subject to solvolytic dissociation, leading to neutral and charged solvento species [Fe(salen)S 2 ] (S=RO m and ROH, respectively). The dissociation constants K D for the systems [Fe(salen)(O 7 O)]/ROH, as determined by concentration-jump experiments as well as by dilution experiments, range from 3 2 10 m 5 M to 1 2 10 m 3 M. The reaction of complexes [Fe(salen)(O 7 O)] with HO 7 N in alcohols is biphasic, with first-order rate constant k 1 describing the initial solvolytic dissociation of [Fe(salen)(O 7 O)] (solvent assisted D-mechanism) and second-order rate constant k 2 , the consecutive reaction of the solvento species [Fe(salen)S 2 ] with HO 7 N to form [Fe(salen)(O 7 N)]. Depending on the nature of the coordinated g -diketone and solvent ROH, k 1 ranges from 0.12 to 31 s m 1 and k 2 from 1 2 10 3 to 26 2 10 3 M m 1 s m 1 at 298 K. Both the initial solvolysis step and the consecutive anation step involve a change in the coordination of the Salen ligand from non-planar to planar and vice versa .