Kinetics and mechanisms of the reactions of aluminium(III) with β-diketones in aqueous solution

The kinetics and mechanisms of the reactions of aluminium(lll) with pentane-2,4-dione (Hpd), 1,1,1-trifluoro pentane-2,4-dione (Htfpd) and heptane-3,5-dione (Hhptd) have been investigated in aqueous solution at 25 degrees C and ionic strength 0.5 mol dm(-3) sodium perchlorate. The kinetic data are consistent with a mechanism in which aluminium(III) reacts with the beta-diketones by two pathways, one of which is acid independent while the second exhibits a second-order inverse-acid dependence. The acid-independent pathway is ascribed to a mechanism in which (Al(H2O)(6))(3+) reacts with the enol tautomers of Hpd, Htfpd, and Hhptd with rate constants of 1.7(+/- 1.3) X 10(-2), 0.79(+/- 0.21), and 7.5(+/- 1.6) X 10(-3) dm(3) mol(-1) s(-1) respectively. The inverse acid pathway is consistent with a mechanism in which (Al(H2O)(5)(OH))(2+) reacts with the enolate ions of Hpd, Htfpd, and Hhptd with rate constants of 4.32(+/-0.18) X 10(6), 5.84(+/- 0.24) X 10(3), and 1.67(+/-0.05) X 10(7) dm(3) mol(-1) s(-1), respectively. An alternative formulation involves a pathway in which (Al(H2O)(4)(OH)(2))(+) reacts with the protonated enol tautomers of the ligands. This gives rate constants of 2.79(+/- 0.12) X 10(4), 3.86(+/- 0.16) X 10(5), and 8.98(+/- 0.25) X 10(3) dm(3) mol(-1) s(-1) for reaction with Hpd, Htfpd, and Hhptd, respectively. Consideration of the kinetic data reported here together with data from the literature, suggest that [Al(H2O)(5)(OH)](2+) reacts by an associative or associative-interchange mechanism. (C) 1998 John Wiley & Sons, Inc.