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

A reação de autoxidação dos complexos de Cu(II)/penta e hexaglicina em pH=9 é muito lenta, apresentando um período longo de indução (aproximadamente 4 horas). A presença de S(IV) praticamente não altera a velocidade desta reação. A adição de pequenas quantidades de Ni(II) ou Co(II) aumenta significativamente a velocidade da reação e a eficiência da formação de Cu(III), diminuindo o período de indução para menos de 0,5 s. A constante de velocidade observada para a formação de Cu(III) também depende da concentração de S(IV). A discussão do mecanismo é baseada nas informações disponíveis na literatura e envolve uma cadeia de reações com formação de radicais e um ciclo de oxirredução dos íons metálicos complexados.


Introduction
The oxidation of Cu(II)/tetraglycine complex by dissolved oxygen, at pH=9, is very slow.However, in the presence of sulfite ((1-8) ×10 -5 mol L -1 ) and traces of Ni(II) and Co(II) (10 -5 mol L -1 ), present as unavoidable impurities in the copper salts (Aldrich) and reagents, the Cu(III)/ tetraglycine formation is efficient and extremely fast (the reaction is complete in 5 s). 1 The metal ions in the trivalent oxidation state react with sulfite to initiate a radical chain reaction in which sulfite is oxidized to sulfate and the metal ion is reduced to the bivalent state, with simultaneous consumption of oxygen.
In fact such experiments, at low concentrations, help to understand why there are so many discrepancies on the kinetic data and mechanism proposals on this redox process, since the presence of impurities of trace metal ions are common in commercial salts and used reagents (acids, bases and buffers).
For example Cu(ClO 4 ) 2 salt (from Aldrich) contains Ni(II), Co(II) and Mn(II) as impurities (determined by Further Studies on the Synergistic Effect of Ni(II) and Co(II) Ions J. Braz.Chem.Soc.

ICP OES technique)
. It is also difficult to exclude trace concentration of Fe(III) in highly purified water (10 -9 -10 -8 mol L -1 ). 13 In some cases, synergistic effects were observed when manganese and iron ions (at very low concentrations) in acetate buffer pH 4-6 were present simultaneously. 2,13n the present work we report further studies about the synergistic effect of Co(II) and Ni(II) on the sulfite induced autoxidation of Cu(II)/penta (G 5 ) and hexaglycine (G 6 ) complexes.At pH 8-9 (borate buffer) the autoxidation of these Cu(II) complexes is very slow, the Cu(III) formation is efficient and extremely fast only in the presence of small concentration of nickel and cobalt ions.

Reagents
All reagents were of analytical grade (Merck or Sigma) and were used as received.Solutions were prepared by using deionized water purified with a Milli-Q Plus Water system (Millipore).
Stock solutions of sulfite (1.00 × 10 -2 mol L -1 ) were daily prepared by dissolving Na 2 S 2 O 5 salt in water previously purged with nitrogen.Water was flushed with nitrogen for at least half an hour to remove dissolved oxygen.To prepare diluted solutions of sulfite, small volumes of the stock solutions were properly added to air saturated water.The sulfite content of the stock solution was determined by iodometric titration. 14etal ions as impurities of reagents have shown synergistic effect on the studied reaction, 8 therefore the metal ion stock solutions of Cu(II) and Ni(II) (0.2 mol L -1 ) were prepared from the direct reaction of Cu (wire) (99.99%) or Ni (powder) (99.99%) with double distilled nitric acid followed by standardization with EDTA by a conventional procedure. 15o(II) (0.965 mol L -1 ) stock solution was prepared from the direct reaction of Co(II) carbonate with perchloric acid followed by standardization with EDTA.15 The borate buffer was chosen since the Cu(II) and Cu(III) complexes are stable in this medium.Sodium tetraborate 99.998% (Aldrich) was used. I phosphate or carbonate medium the Cu(III) formation is less effective as the phosphate or carbonate anions may displace the peptide ligand.

Procedure
In the experiments freshly prepared solutions were employed.Cu(II)/G 5 or Cu(II)/G 6 solutions were prepared by dissolving pentaglycine or hexaglycine in 0.1 mol L -1 borate buffer solution (pH = 9.0) followed by the addition of Cu(II) solution (solutions were prepared to have 25% excess of ligand to restrain the Cu(OH) 2 precipitation).Aliquots of Co(II) or Ni(II) solutions were added to Cu(II)/ G n (G n = G 5 or G 6 ) solution in order to study the synergistic effect.The solutions of metal ion complex consisted of [Cu(II)] = 2.0x10 -3 mol L -1 , 2.0 ×10 -6 ≤ [Ni(II)] or [Co(II)] ≤ 4.0 × 10 -5 mol L -1 , and pH=9.0.The ionic strength, I, was adjusted in 0.2 mol L -1 with NaClO 4 .
An equal volume of sulfite solution (2.0 × 10 -5 ≤ [S(IV)] ≤ 14.0 × 10 -5 mol L -1 ) was mixed with the metal ion complex solution (in borate buffer).The final concentrations after the mixture are indicated in all figures.
The kinetics were followed at 365 nm, the wavelength of maximum absorption of the Cu(III)/G n complexes, by using a HP8453A diode array spectrophotometer coupled to a Pro-K.2000Stopped-Flow Mixing Accessory (Applied Photophysics).The optical path length was 0.998 cm.
In all experiments, air saturated solutions were employed for which the oxygen concentration can be considered 2.5 × 10 -4 mol L -1 . 16A pHmeter Metrohm model 713 with a glass electrode (filled with sat.NaCl) was used in the pH measurements.The final pH was adjusted with 1.0 mol L 1 NaOH or 1.0 mol L -1 HClO 4 solutions.The temperature was kept at 25.0 °C.

Results and Discussion
The UV-visible spectrum of Cu(III)/G n in aqueous solution at pH=9.0 consists of two intense bands at 365 and 250 nm. 17 The observed synergistic effect depends on the type and the concentration of the second metal ion.For instance, for a same concentration level of Ni(II) or Co(II) such as 1.0 × 10 -5 mol L -1 (Figure 1), the Cu(III) formation is faster in the presence of Co(II).However, in the presence of Ni(II), a higher limit absorbance was reached (compare Figures 1A and 1B), showing that Cu(III) formation was more efficient.
The first order Cu(III) formation was calculated neglecting the induction period.It was obtained by the initial slope of ln (Absorbance t -Absorbance i ) vs. time plots.According to the references 18 and 19, the later part of the kinetic trace (after the induction period) can exhibit the maximum rate of Cu(III) formation.In this case, any calculated rate can be subject to large error due to the interference of the autocatalytic behaviour.
Figure 1C shows that the Cu(III) formation exhibits dependence on Ni(II) and Co(II) concentrations and the slope reaches constant values at high concentrations of Ni(II) and Co(II) ions.The observed saturation can be interpreted in terms of complex formation between Ni(II) or Co(II) and sulfite, 2,20,21 although there is no evidence from the spectrophotometric data.
The effect of sulfite concentration on the autoxidation of Cu(II)/G n in the presence of Ni(II) or Co(II) was also investigated.Figure 2 shows typical absorbance vs. time traces for the Cu(III)/G n formation after addition of different sulfite concentrations.The reaction is significantly faster and the induction period decreases on the increase of the Ni(II) or Co(II) (Figures 1A and 1B) and sulfite (Figures 2A and 2B) concentration.The slope is not linear with sulfite (Figure 2C), as it was obtained for Cu(II)/tetraglycine. 1 Data for sulfite concentrations smaller than 1.0 × 10 -5 mol L -1 would better define the intercept (in absence of sulfite) in the Figure 2C.However at such low sulfite concentration the absorbance changes were smaller with large error.
Similar experiments were carried out using G 6 as a ligand (Figure 3).It was observed that the limit absorbance at 20 s was about the same (using G 5 or G 6 ) for the same concentration of Ni(II) or Co(II).In the presence of Co(II), the Cu(III) formation is slightly faster in G 5 medium than G 6 .(compare Figures 1 and 3), however, the opposite was observed for the synergistic effect of Ni(II).
Figures 2A and 2B shows that for a fixed concentration of Cu(II) and Ni(II) (in G 6 or G 5 medium) the limit absorbance (after 20 s) is slightly smaller and the Cu(III) formation is faster in G 6 medium for the same S(IV) concentration.

Mechanism
In the Scheme 1 the main reactions are represented.As reported before 1 the metal ion in the 3+ oxidation state is the initiator to form SO 3 •-(equations ( 5)-( 7)).If the initiator is not initially added, it is generated by the oxidation of the metal ion complex by oxygen (equations ( 1), ( 3) and ( 4)).Initiation by Cu(III)/G n as result of the spontaneous oxidation of Cu(II)/G n by oxygen (equation ( 1)) or by disproportionation of Cu(II)/G n (equation ( 2) suggested by Margerum's group), 25,26 is less probable since the reactions are very slow.2) and ( 4)) This synergistic effect was better understood by comparative studies of the reactions of Ni(II) and Co(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.The Co(III)/G n complexes seem to be stable for 20 h (the monitored time), since the absorbance at 265 nm did not decrease.In comparison, at the same pH, the autoxidation of Cu(II)/G n in the presence of S(IV) does not occur in the first 4 h.

Initiation in the absence of added Ni(II) or
The synergistic effect of Ni(II) and Co(II), can be explained by the faster oxidation of Ni(II)/G n or Co(II)/G n  -and SO 4 •-(equations ( 9)-( 13)).In an autocatalytic process the Cu(III)/G n then can be reduced by SO 3 2-(equation ( 7)).
In our previous work on the systems Ni(II)/cyclam 28 and Mn(II)/acetate, 2 the oxygen consumption were followed, showing that the redox cycling depends on the balance of sulfite and oxygen concentrations in solution.This balance could be demonstrated in the case of Fe(II)/ (III)/H 2 O, 11,29 Co(II) and Mn(II) in azide medium 30 and Ni(II) in hydroxide medium. 31t is not always possible to elucidate the nature of the oxidizing metal ion species.In the case of complexes of Ni(II)/(III) and Co(II)/(III) with G 5 and G 6 , there is no information in the literature.However, there are also the probability of mixed complex formation, as suggested for Ni(II)/G 4 complexes 20 (equations ( 14)-( 18)).

[Ni
where H -x G n is the peptide with x deprotonated peptide nitrogen coordinated to nickel ion.The complexity of the system does not allow a definitive assignment of the species involved.Lepentsiotis et.al. 32 suppose the formation of Ni III L (SO 4 •-) (L=lysylglycylhistidine carboxiamide), where the SO 4 •-radical may coordinate to Ni III L complex.Green et al. 33 considered the formation of reactive dimmer species of Ni(II) and Ni(III)Gly 2 HisGly complexes in the oxidative self-decomposition of Ni(III) complex.
Important aspects of Co(II)/Co(III) complex in the presence of oxygen need to be considered as some possible step in the oxidation.Studies 34 with Co(II) peptides (glygly, gly-ala, ala-gly and ala-ala) showed the formation of dimeric complexes with μ-superoxo bridges.In the case of Co II aspargine the formation of dioxygen complex occurs prior to the oxidation of the metal center. 35he pH dependence of Co II G n oxidation by oxygen (in the presence or absence of sulfite), 27 may be explained by the possible formation of oxygen adduct or μ-superoxo bridge.This property of Co(II)/(III) complexes may explain the different behaviour of Co II G n compared to Ni II G n in the presence of oxygen and sulfite.

Conclusions
The coordination chemistry, using proper ligands, allows the elucidation of the mechanism of S(IV) oxidation, since it is possible to follow the changes in the At 365 nm the Cu(III)/G n formation can be conveniently monitorated, since neither Ni(II)/Ni(III)/G n nor Co(II)/Co(III)/G n complexes interfere in the absorbance value due the low used concentrations ([Ni(II)] or [Co(II)] = 10 -6 -10 -5 mol L -1 ).Figures 1A and 1B show the absorbance changes at 365 nm during the autoxidation of Cu(II)/G 5 in the presence of 5.0 × 10 -5 mol L -1 S(IV) at pH=9.0.In the absence of Ni(II) or Co(II) ([M(II)] = zero) the reaction occurs very inefficient and slowly with induction period of 4 h.Introduction of trace amounts of Ni(II) or Co(II) affect the kinetic significantly.The Cu(III)/G n formation is strongly accelerated and the induction period decreases gradually (to less than 0.5 s) as the Ni(II) or Co(II) concentration increases.

CuScheme 1 .
Scheme 1. Mechanism of the sulfite induced autoxidation of Cu(II)/G n .Synergistic effect of Ni(II) or Co(II).