Kinetics and Mechanism of Ligand Substitution in Bis(N-alkylsalicylaldiminato)oxovanadium(IV) Complexes

Conventional and stopped-flow spectrophotometry was used to to study the kinetics of ligand substitution in a number of bis(A/-alkylsalicylaldiminato)oxovanadium(IV) complexes (=VO(R-X-sal)2) by 1,1,1- trifluoropentane-2,4dione (=Htfpd) in acetone, according to the following reaction: VO(R-X-sal)2 + 2Htfpd — V0(tfpd)2 + 2R-X-salH. The acronym R-X-salH refers to A/-alkylsaticylaldimines with substituents X = H, Cl, Br, CHs, and N02 in the 5-position of the salicylaldehyde ring and N-alkyl groups R = -propyl, isopropyl, phenyl, and neopentyl. Under excess conditions ([Htfpd]o [VO(R-X-sal)2]o), substitution by Htfpd occurs in two observable steps, as characterized by pseudo-first-order rate constants Aobsd(i) and Aobsd(2). Both rate constants increase linearly with [Htfpd] according to Aobsd(i) = (i) + [Htfpd]o and Aobsd(2) = ks(2) + [Htfpdjo, with (i) and (2) describing small contributions of solvent-initiated pathways. Depending on the nature of R and X, second-order rate constants k) and lie in the range 0.098-0.87 M-" s (k)) and 0.022-0.41 M s~ (kz) at 298 K. For ligand substitution in the system VO(n-propyl-sal)2/Htfpd, the activation parameters A = 35.8 ± 2.8 kJ mol" and A = -146 ± 23 J K mol~ (k)) and ... 40.2 ± 1.3 kJ mol and A = -142 ± 11 J K mol- ( were obtained. The Lewis acidity of the complexes VO(-propyl-X-sal)2 with X = H, Cl, Br, CHs, and N02 was quantified spectrophotometrically by determination of equilibrium constant /(py, describing the formation of the adduct VO(n-propyl-X-sal)2*pyridine. The adduct VO(tfpd)2propyl-salH, formed as product in the system VO(-propyl-sal)2/Htfpd, was characterized by its dissociation constant, Ko = (3.30 ± 0.10) x 10 M. The mechanism suggested for the two-step substitution process is based on initial formation of the adducts VO(R-X-sal)2*Htfpd (step 1) and VO(R-X-sal)(tfpd)*Htfpd (step 2).