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

Abstract

Conventional and stopped-flow spectrophotometry was used to to study the kinetics of ligand substitution in a number 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, CH(3), and NO(2) in the 5-position of the salicylaldehyde ring and N-alkyl groups R = n-propyl, isopropyl, phenyl, and neopentyl. Under excess conditions ([Htfpd](0) >> [VO(R-X-sal)(2)](0)), substitution by Htfpd occurs in two observable steps, as characterized by pseudo-first-order rate constants k(obsd(1)) and k(obsd(2)). Both rate constants increase linearly with [Htfpd](0) according to k(obsd(1)) = k(s(1)) + k(1)[Htfpd](0) and k(obsd(2)) = k(s(2)) + k(2)[Htfpd](0), with k(s(1)) and k(s(2)) describing small contributions of solvent-initiated pathways. Depending on the nature of R and X, second-order rate constants k(1) and k(2) lie in the range 0.098-0.87 M(-1) s(-1) (k(1)) and 0.022-0.41 M(-1) s(-1) (k(2)) at 298 K. For ligand substitution in the system VO(n-propyl-sal)(2)/Htfpd, the activation parameters DeltaH++ = 35.8 +/- 2.8 kJ mol(-1) and DeltaS++ = -146 +/- 23 J K(-1) mol(-1) (k(1)) and DeltaH++ = 40.2 +/- 1.3 kJ mol(-1) and DeltaS++ = -142 +/- 11 J K(-1) mol(-1) (k(2)) were obtained. The Lewis acidity of the complexes VO(n-propyl-X-sal)(2) with X = H, Cl, Br, CH(3), and NO(2) was quantified spectrophotometrically by determination of equilibrium constant K(py), describing the formation of the adduct VO(n-propyl-X-sal)(2).pyridine. The adduct VO(tfpd)(2).n-propyl-salH, formed as product in the system VO(n-propyl-sal)(2)/Htfpd, was characterized by its dissociation constant, K(D) = (3.30 +/- 0.10) x 10(-3) 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).

Vanadium(IV) Complexes with Methyl-Substituted 8-Hydroxyquinolines: Catalytic Potential in the Oxidation of Hydrocarbons and Alcohols with Peroxides and Biological Activity

“…The bite angle of the chelating ligand is ~80 • (Tables S1-S3, Supplementary Materials), and the dihedral angle between the least-squares planes formed by the organic ligands is equal to 60.38(4) • for 1 and 50.32(3) • for 2. This kind of coordination geometry seems to be typical for [VO(N∩O) 2 ] bearing two N,O-donor bidentate ligands, contrary to [VO(O∩N∩N∩O)] of tetradentate Schiff base ligands in which fivecoordinated vanadium(IV) generally displays a distorted square-pyramidal environment with a basal square plane constituted by the two iminic nitrogen and two phenoxide oxygen atoms [4,6,64,[66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82] (Table S7, Supplementary Materials). The V = O [1.590 (2) S1 and S2) are in good agreement with those reported for five-coordinated vanadyl compounds (Tables S3 and S7, Supplementary Materials).…”

“…contrary to [VO(O∩N∩N∩O)] of tetradentate Schiff base ligands in which five-coordinated vanadium(IV) generally displays a distorted square-pyramidal environment with a basal square plane constituted by the two iminic nitrogen and two phenoxide oxygen atoms [4,6,64,[66][67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82] (Table S7, Supplementary Materials ). The V=O [1.590(2) and 1.595(3) Å], V-O [1.921(1) and 1.924(2) Å] and V-N [2.122(2) and 2.125(2) Å] bond lengths in 1 and 2 (Tables S1 and S2) are in good agreement with those reported for five-coordinated vanadyl compounds (Table S3 and S7, Supplementary Materials).…”

27 Mechanisms of reactions in solution

“…189 A range of substitution reactions of bis(N-alkylsalicylaldimato)oxovanadium(III) [VO(R-X-sal) 2 ] (R ~alkyl; X ~H, Cl, Br, Me, NO 2 -5) complexes with Htfpd have been investigated. 190 A twostep mechanism is proposed involving the formation of [VO(R-X-sal) 2 ]. in the first step and [VO(R-X-sal)(tfpd)]?Htfpd in the second.…”

12 Vanadium, niobium and tantalum

“…The kinetics and mechanism of ligand substitution in five co-ordinate, bis-chelate oxovanadium(IV) compounds was investigated using conventional and stopped-flow spectrophotometry in order to correlate ligand substitution patterns with Lewis acidity and ligand basicity. 52 Speciation studies of vanadium in various oxidation states with a variety of biologically-relevant ligands have been undertaken, including a-hydroxycarboxylic acids, oxydiacetates and L-ascorbic acid, L-alanine, L-aspartic acid, pyridinedicarboxylic acids, and peptide analogues. 53 New vanadyl insulin-mimetic compounds have been reported.…”