174368-07-1Relevant academic research and scientific papers
Kinetics and Mechanism of the Formation and Acid Dissociation of Cobalt(II), Nickel(II), and Copper(II) Complexes with the Highly Enolized β-Diketone 3-(N-Acetylamido)pentane-2,4-dione (=Hamac) in Aqueous Solution
Hirsch, Jürgen,Paulus, Helmut,Elias, Horst
, p. 2343 - 2351 (1996)
The β-diketone Hamac = 3-(N-acetylamido)pentane-2,4-dione was characterized by potentiometric, spectrophotometric, and kinetic methods. In water, Hamac is very soluble (2.45 M) and strongly enolized, with [enol]/ [ketone] = 2.4 ± 0.1. The pKa of Hamac is 7.01 ± 0.07, and the rate constants for enolization, ke, and ketonization, kk, at 298 K are 0.0172 ± 0.0004 s-1 and 0.0074 ± 0.0015 s-1, respectively. An X-ray structure analysis of the copper(II) complex Cu(amac)2·toluene (=C21H28CuN2C6; monoclinic, C2/c; a = 20.434(6), b = 11.674(4), c = 19.278(6) A? β = 100.75(1)° Z = 8; Rw = 0.0596) was carried out. The bidentate anions amac- coordinate the copper via the two diketo oxygen atoms to form a slightly distorted planar CuO4 coordination core. Rapid-scan stopped-flow spectrophotometry was used to study the kinetics of the reaction of divalent metal ions M2+ (M = Ni,Co,Cu) with Hamac in buffered aqueous solution at variable pH and 1 = 0.5 M (NaClO4) under pseudo-first-order conditions ([M2+]0 ? [Hamac]0) to form the mono complex M(amac)+. For all three metals the reaction is biphasic. The absorbance/time data can be fitted to the sum of two exponentials, which leads to first-order rate constants kf (fast initial step) and ks (slower second step). The temperature dependence of kf and ks was measured. It follows from the kinetic data that (i) the keto tautomer of Hamac, HK, does not react with the metal ions M2+, (ii) the rate constant kf increases linearly with [M2+]0 according to kf = k0 + k2[M2+]0, and (iii) the rate constant ks does not depend on [M2+]0 and describes the enolization of the unreactive keto tautomer HK. The pH dependence of the second-order rate constant k2 reveals that both the enol tautomer of Hamac, HE, and the enolate, E-, react with M2+ in a second-order reaction to form the species M(amac)+. At 298 K rate constants kHE are 18 ± 6 (Ni), 180 ± 350 (Co), and (9 ± 5) × 104 (Cu) M-1 s-1 and rate constants kE are 924 ± 6 (Ni), (7.4 ± 0.6) ± 104 (Co), and (8.4 ± 0.2) × 108 (Cu) M-1 s-1. The acid dissociation of the species M(amac)+ is triphasic. Very rapid protonation (first step) leads to M(Hamac)2+, which is followed by dissociation of M(Hamac)2+ and M(amac)+, respectively (second step). The liberated enol Hamac ketonizes (third step). The mechanistic implications of the metal dependence of rate constants kHE, kE, k-HE, and k-E are discussed.
