Autoxidation of Ni(II) and Co(II) tetra, penta and hexaglycine complexes accelerated by oxy sulfur radicals

Abstract

A autoxidação dos complexos de Ni(II) e Co(II) com tetra, penta e hexaglicina, em meio de tampão borato, é acelerada por espécies de enxofre(IV) (H 2 SO 3 , HSO 3 -and SO 3 2-). A formação dos complexos de Ni(III) e Co(III) foi acompanhada espectrofotometricamente em 325 e 265 nm, respectivamente. Técnicas eletroquímicas também foram empregadas para caracterizar a geração destes complexos. A velocidade da reação de autoxidação aumenta com a concentração de S(IV) e é máxima em pH ≅ 8,5. O processo é autocatalítico com Ni(III) ou Co(III) atuando como iniciadores, formados pela oxidação espontânea de Ni(II) ou Co(II) pelo oxigênio molecular. A dependência da constante de velocidade de pseudo-primeira ordem com a concentração de sulfito evidenciou possíveis reações paralelas com formação de um complexo com ligantes mistos antes da etapa da oxidação.The autoxidation of Ni(II) and Co(II) complexes with tetra, penta and hexaglycine, in borate medium, is accelerated by sulfur(IV) species (H 2 SO 3 , HSO 3 -and SO 3 2-). The formation of Ni(III) and Co(III) complexes was followed spectrophotometrically at 325 and 265 nm, respectively. Electrochemical techniques were also employed to characterize the generation of these complexes. The autoxidation rate increases with S(IV) concentration and is maximum at pH ≅ 8.5. The process is autocatalytic with Ni(III) or Co(III) acting as initiators, formed by spontaneous oxidation by oxygen. The dependence of the pseudo first order rate constant with sulfite concentration showed evidences of back or parallel reactions with formation of mixed ligand complex prior to the oxidation step.

Reaction mechanisms in solution

“…The N-dealkylation does not proceed to completion, suggesting another competing oxidation process, probably involving m-peroxo dicobalt complexes. Kinetics of autoxidation of Ni II and Co II complexes of tetrapenta-and hexa-glycine have been reported, 38 expanding on previous reports by the same authors. The decomposition of hexaaquacobalt(III) in aqueous perchloric acid has been reported.…”

Further studies on the synergistic effect of Ni(II) and Co(II) ions on the sulfite induced autoxidation of Cu(II) penta and hexaglycine complexes

“…Our previous work 27 carried out in solutions containing each metal ion complex (Ni(II)/G n or Co(II)/G n ) in the presence of S(IV) and oxygen (at pH=9) showed that Ni(III)/G n formation is fast reaching a limit absorbance after 1s, followed by its decomposition via ligand oxidation. At the same experimental conditions, Co(III)/G n formation is much slower than Ni(III)/G n formation, after S(IV) consumption the Co(III)/G n formation is slow due to the oxidation of Co(II)/G n by the oxygen still remaining in solution.…”

Rotating ring-disk voltammetric investigations on the degradation rate of the nickel(III)-glycylglycyl-L-histidine complex

“…In the case of Ni(II), several works have been reported in the literature (11,13,(16)(17)(18)(19). Currently, increasing interest in the formation of Ni(III) peptide complexes has been noticed because of the probable participation of these species in DNA damage processes (7).…”