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

Abstract

Chromium(III)-lutidinato complexes of general formula [Cr(lutH) n (H 2 O) 6-2n ] 3-n (where lutH -is N,Obonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH) 3 ] 0 leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1-1.0 M HClO 4 and 0.4-1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H ? ] are as follows: k obs1 = k 1 ? k -1 /K 1 [H ? ] and k obs2 = k 2 K 2 [H ? ]/(1 ? K 2 [H ? ]), where k 1 and k 2 are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k -1 is the rate constant for the chelate-ring closure, and K 1 and K 2 are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH -]: k obs1 = k OH(1) ? k O [OH -], where k OH(1) and k O are rate constants of the first ligand liberation from the hydroxo-and oxoforms of the intermediate, respectively, and K 2 is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH) 4 ] 3-were also studied and the values of the pseudo-first-order rate constants are [OH -] independent.

Kinetic studies on chromium-glycinato complexes in acidic and alkaline media

“…Previously, series of complexes with pyridinemonocarboxylato and pyridinedicarboxylato ligands were obtained and kinetic studies on their acid-and base-catalysed aquation were performed [e.g. [6][7][8][9][10]. Additional studies on their effect toward 3T3 fibroblasts revealed that they are not toxic and may be good candidates as biochromium sources [6,10].…”

Mixed chromium(III) complexes with pyridinedicarboxylato and oxalato ligands: kinetic studies in HClO4 solutions

“…3 times) than those determined for the appropriate [Cr(pdaH) 3 ] 0 complexes [14,15,17]. It is peculiar, as the negatively charged species should undergo acid-catalysed aquation easier than molecular ones.…”

“…Our earlier studies on acid-catalysed aquation of other complexes with picolinato-like bonded ligands [14,15,17,19] led to the following conclusions: (i) the first step is much faster than the second one; (ii) the chelate-ring opening takes place at the Cr-N bond; (iii) the pseudo-firstorder rate constants for the first step decrease with an [H ? ] increase; (iv) the formed one-end bonded intermediate is stabilized via the intramolecular hydrogen bond; (v) the pseudo-first-order rate constants for the second step increase with an [H ? ]…”

“…The effect of protolytic dissociation of the other carboxylic group on the UV spectrum depends on this group position: the 3-COOH group causes a distinct red shift of the p ? p* (p 1 ) transition band [14], the 4-COOH group causes a blue shift [17] and the 5-COOH group does not influence the position and intensity of the p 1 band. Determined pK 2 values lie between 2.2 and 2.5.…”

“…Because the bioavailability of the tris-picolinatochromium(III) is very poor, syntheses of novel chromium(III) complexes can be advantageous. Recently [14][15][16][17], a series of chromium(III) complexes with quinolinic, lutidinic and isocinchomeronic acids was obtained and characterized in solution. They are very soluble and practically inert at the physiological pH.…”