| Abstract
| - Conventional and stopped-flow spectrophotometry was used to to study the kinetics of ligand substitution in anumber of bis(N-alkylsalicylaldiminato)oxovanadium(IV) complexes (=VO(R-X-sal)2) by 1,1,1- trifluoropentane-2,4-dione (=Htfpd) in acetone, according to the following reaction: VO(R-X-sal)2 + 2Htfpd → VO(tfpd)2 + 2R-X-salH.The acronym R-X-salH refers to N-alkylsalicylaldimines with substituents X = H, Cl, Br, CH3, and NO2 in the5-position of the salicylaldehyde ring and N-alkyl groups R = n-propyl, isopropyl, phenyl, and neopentyl. Underexcess conditions ([Htfpd]0 ≫ [VO(R-X-sal)2]0), substitution by Htfpd occurs in two observable steps, as characterizedby pseudo-first-order rate constants kobsd(1) and kobsd(2). Both rate constants increase linearly with [Htfpd]0 accordingto kobsd(1) = ks(1) + k1[Htfpd]0 and kobsd(2) = ks(2) + k2[Htfpd]0, with ks(1) and ks(2) describing small contributions ofsolvent-initiated pathways. Depending on the nature of R and X, second-order rate constants k1 and k2 lie in therange 0.098−0.87 M-1 s-1 (k1) and 0.022−0.41 M-1 s-1 (k2) at 298 K. For ligand substitution in the systemVO(n-propyl-sal)2/Htfpd, the activation parameters ΔH⧧ = 35.8 ± 2.8 kJ mol-1 and ΔS⧧ = −146 ± 23 J K-1 mol-1(k1) and ΔH⧧ = 40.2 ± 1.3 kJ mol-1 and ΔS⧧ = −142 ± 11 J K-1 mol-1 (k2) were obtained. The Lewis acidityof the complexes VO(n-propyl-X-sal)2 with X = H, Cl, Br, CH3, and NO2 was quantified spectrophotometrically bydetermination of equilibrium constant Kpy, describing the formation of the adduct VO(n-propyl-X-sal)2·pyridine. Theadduct VO(tfpd)2·n-propyl-salH, formed as product in the system VO(n-propyl-sal)2/Htfpd, was characterized by itsdissociation constant, KD = (3.30 ± 0.10) × 10-3 M. The mechanism suggested for the two-step substitutionprocess is based on initial formation of the adducts VO(R-X-sal)2·Htfpd (step 1) and VO(R-X-sal)(tfpd)·Htfpd (step2).
- The kinetics of ligand substitution in salicylaldiminato complexes VO(R−N∩O)2 by β-diketones HO∩O was studied in acetone. Substitution occurs in two observable steps with the size of second-order rate constants k1 and k2 depending on the pattern of substituents on VO(R−N∩O)2 and on the acidity of HO∩O. The Lewis acidity of the vanadium center, as quantified by equilibrium constant Kpy for the addition of pyridine to complexes VO(R−N∩O)2, is an essential factor in the mechanistic course of the overall substitution process.
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