Kinetic study on complexation of boric acid with 4-isopropyltropolone in non-aqueous solvents

Abstract

Rates for the complexation of trigonal boric acid with bidentate 4-isopropyltropolone (Hipt) in 1,1,3,3-tetramethylurea (TMU), N, N-dimethylformamide (DMF) and acetonitrile (AN) to form the tetrahedral 1:1 complex have been followed at various temperatures and pressures by using a high-pressure stopped-flow apparatus with spectrophotometric detection. Rate constants at 25.0 °C and activation parameters in TMU, DMF and AN obtained at I = 0.10 mol kg −1 (NaCF 3SO 3) are as follows: k = 22.8 ± 0.9 mol −1 kg s −1, ΔH ≠ = 43.8 ± 1.3 kJ mol −1, ΔS ≠ = −72±4 J mol −1, ΔV ≠ = −4.7±0.6 cm 3 mol −1 for the reaction in TMU; k = 51.9±3.2 mol −1 kg s −1, ΔH ≠ = 32.0±2.2 kJ mol −1, ΔS ≠ = −104±7 J mol −1 K −1, ΔV ≠ = −4.6±1.3 cm 3 mol −1 for the reation in DMF; k = 171±2 mo −1 kg s −1, ΔH ≠ = 22.3±1.1 kJ mol −1, ΔS ≠ = − 128±4 J mol −1 K −1, ΔV ≠ = −4.6±0.7 cm 3 mol −1 for the reaction in AN. Negative values of ΔS ≠ and ΔV ≠ indicate that the activation mode in the complexation of the bidentate Hipt is associative and that the rate-determining step is the change in the coordination environment of boron from trigonal to tetrahedral.