Kinetics and mechanism of aquation of chromium(III) complexes with 3-hydroxypicolinic and 2-hydroxynicotinic acids in HClO{4{ solutions

Abstract

Two complexes, 3 ] 0 and [Cr(2-hnic) 3 ] 0 (where 3-hpic = hydroxypicolinic acid and 2-hnic = 2-hydroxynicotinic acid anions), were prepared and characterized in solution. The 3-hpic ligand forms a 5-membered chelate ring via pyridine nitrogen and carboxlate oxygen atoms, whereas the 2-hnic ligand forms a 6-membered chelate ring via carboxylate and phenolate oxygen atoms. The kinetics of the acid-catalyzed aquation were studied spectrophotometrically in the 0.1-1.0 M HClO 4 range, at I ¼ 1:0 M . The rate equations for the first aquation step -the chelate-ring opening -was determined and a mechanism was proposed. In the case of [Cr(3-hpic) 3 ] 0 , the reversible chelate-ring opening at the CrAN bond precedes much slower than the second aquation step -a one-end bonded ligand liberation. The equation rate is of the form: k obs ¼ k 1 þ k À1 =Q 1 [H þ ], where k 1 and k À1 are the rate constants for the forward and the reverse processes in the unprotonated substrate and Q 1 is the protonation constant of the non-bonded pyridine nitrogen atom. In the case of [Cr(2-hnic) 3 ] 0 , the chelate-ring opening at the CrAO (phenolate) bond is the rate-determining step. The observed pseudo-first order rate constant increases as [H þ ] increases:where k 0 and k H are the rate constants of the spontaneous and acid catalyzed processes and Q H is the protonation constant of the coordinated phenolate oxygen atom. The results lead to the conclusion that an aquation mechanism depends on the coordination mode of the ligand.

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO4 and NaOH solutions

“…p* transition of the pyridine ring is the same as that in the noncoordinated ligand but its intensity increases with the increasing number of ligands in the inner coordination sphere. The spectrum of [Cr(lutH) 3 ] 0 , within the visible range, is very similar to that of mer-[Cr(pic) 3 ] 0 [26] and its analogues [27], so we assume the tris-lutidinato complex is also a mer-isomer. In the case of the bis-lutidinato complex as many as five geometrical isomers can exist but based on the obtained results it is not possible to identify a particular one.…”

“…In the case of a series of previously studied the pyridinemonocarboxylato complexes [26,27], it was impossible to separate the k 2 from the k 2 K 2 product. For that reason, the reactivities of these species can be compared with the results for the pyridinedicarboxylato complexes based on the k 2 K 2 values.…”

Chromium(III)-isonicotinate Complexes. Kinetics and Mechanism of Isonicotinate Ligand Liberation in HClO4 Solutions

“…The rate law, for each aquation-stage, derived from Scheme 3 is analogous to those expressed by Equations (2)(3)(4), in which k 0 =0. Thus the meaning of the b parameter in Equation (5) is as follows: Table 3.…”

“…The calculated A a and A b values were practically the same as those obtained experimentally. 3) . Spectral characteristics of chromium(III)-inic complexes are given in Table 1.…”

“…However, because they provide less than 5% of absorbable chromium(III) it is necessary to obtain other chromium-pyridinecarboxylates, which could serve as a better bio-available chromium(III) source. Recently, several new complexes with picolinic [2], 3-hydroxypicolinic [3], 2-hydroxynicotinic [3] and nicotinic acids [4] were obtained and characterized in solution. The results indicate that liberation of picolinate-like and salicylate-like bonded ligands is described by different mechanisms.…”