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

Abstract

], where k 0 and k 1 are rate constants for the chelate-ring opening via spontaneous and acid-catalysed reaction paths, respectively, and K p1 is the protonation constant. The proposed mechanism assumes formation of the reactive intermediate as a result of proton addition to the coordinated carboxylate group of the didentate ligand. Some kinetic studies on the second reaction stage, the one-end bonded glycine liberation, were also done. The obtained results were analogous to those for stage I. In this case, the proposed reactive species are intermediates, protonated at the carboxylate group of the monodentate glycine.

Kinetic studies on H+-catalysed aquation of some chromium(III)-oxalato-amino acid complexes

“…Obviously, the proton can be also attached to the carboxylate group of coordinated ox, but the other possibility seems to be more likely and is consistent with our earlier results obtained for the acid-catalysed [Cr(gly) 3 ] 0 [9], where no other option of proton attachment is possible. No difference between the initial spectra of the substrate in neutral and acidic medium indicates that the protolytic equilibrium K p1 is shifted left.…”

“…There are two groups of simple compounds, namely [Cr(Aa) 3 ] and [Cr(ox) 2 (Aa)] 2-, which could be suitable for biochromium sources. Several complexes, namely [Cr(Gly) 3 ], [Cr(ox) 2 (Aa)] 2-(Aa = alanine, valine or cysteine) and their aqua-derivatives have been recently prepared and characterized in aqueous solution [9,10]. In acidic media, the complexes undergo Aa ligand substitution leading to an appropriate diaqua derivative.…”

“…This time, complexes with serine and aspartic acid are investigated. The chosen amino acids possess three donor groups characterized by the following pK a values: 2.09 (a-COOH), 3.86 (b-COOH), 9.82 (a-NH 3 ) and 2.13 (a-COOH), 9.05 (a-NH 3 ), ca. 13.6 (b-OH), respectively [11].…”

Kinetic studies on acid-catalysed aquation of some [Cr(ox)2(amino acid)]2− complexes

“…Our results can be also compared with those for acid-catalysed aquation of [Cr(ox) 2 (gly)] 2-[18] and [Cr(gly) 3 ] [19], where analogous rate laws were established. In the case of [Cr(ox) 2 (gly)] 2-, the determined rate constant k H at 298 K is very similar to those presented in Table 4, suggesting the same mechanism for glycine dissociation.…”

“…It is not obvious whether the proton is attached to the oxygen atom of the amino acid or oxalato ligand. Here, we presume protonation of the amino acid, similar to the aquation of [Cr(gly) 3 ], where two stages of acid-catalysed glycine dissociation were observed [19]. In that case, only one possibility of proton attachment (to the coordinated carboxylate group) exists.…”