Kinetics and mechanism of tris-quinolinatochromium(III) aquation in HClO4 media

Abstract

The chromium(III)-quinolinato complexes, [Cr(quinH) 3 ] 0 , [Cr(quinH) 2 (H 2 O) 2 ] + and [Cr(quinH)(H 2 O) 4 ] 2+ (where quinH = N,O-bonded quinolinic acid anion), were obtained and characterized in solution. The tris-quinolinato complex undergoes acid-catalyzed aquation to give the diaqua-product, whereas subsequent ligand liberation processes are exceptionally slow. Kinetics of the aquation were studied spectrophotometrically over the 0.1-1.0 M HClO 4 range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr-N bond, is much faster than the second one. The following rate laws were established: k obs = k 1 + k )1 /Q 1 [H + ] and k obs = k 2 Q 2 [H + ]/(1 + Q 2 [H + ]), where k 1 and k 2 are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k )1 is the rate constant of the chelate-ring closure, Q 1 and Q 2 are the protonation constants of the pyridine nitrogen and 3-carboxylate group in the one-end bonded intermediate, respectively. Kinetic parameters have been determined and the mechanism has been discussed.

Synthesis and kinetic studies in aqueous solution on chromium(III) complexes with isocinchomeronic acid—potential new biochromium sources

“…The presented kinetic data are very close to those determined for the [Cr(quinH)] 0 aquation [20]. The most substantial difference is between the activation parameters calculated for the chelate ring opening (DH = = 56 kJ mol -1 , DS = = -95 Jk -1 mol -1 ), what can be interpreted in terms of a more associative mode of the activation process.…”

“…These expressions are of the same form as those determined for [Cr(quinH) 3 ] 0 aquation, where quinH 2 is 2,3-pyridinedicarboxylic acid [20]. Moreover, the values of the a and b parameters are very similar.…”

“…The carboxylic proton dissociation barely affects the electronic spectrum, contrary to that in the isomeric complex-[Cr(quinH) 3 ], where the 3-COOH group dissociation is associated with a very distinctive spectral changes [20]. Therefore, it was not possible to determine pK a values using spectrophotometrical method (the biggest absorbance changes were ca.…”

“…Moreover, the Cr-O bond breaking in those complexes is a much faster process than that found for the Cr-O-icaH intermediate. These differences can be rationalised, assuming formation of the intramolecular hydrogen bond between the pyridine nitrogen atom and carboxylic oxygen atoms, which protects the oxygen atom against proton attack (Scheme 2, P 1 + species complex [20]. It is understandable because, the proton on 3-carboxylic group can be stabilized by an intramolecular hydrogen bond formed between the hydrogen and the carbonyl oxygen atoms.…”

“…Because chromium(III) pyridinecarboxylates are used as biochromium sources, we have decided to obtain a series of analogous complexes, which could serve more efficiently for these purposes. In our previous work [20], chromium(III)-qinolinate complexes were obtained and characterized. The aim of this work is to compare the effect of the nonbonded carboxylic group position on the chromium pyridinedicarboxylates reactivity and to recommend a better biologically active chromium(III) source.…”