N N N N N O– Ni N 2+ I 186 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 186–187 J. Chem. Research (M), 1997, 1216–1236 Kinetics of the Oxidation of Thioglycolic and Thiomalic Acids by a Nickel(III) Oxime–Imine Complex Amitava Dutta, Basudeb Saha, Mahammad Ali and Pradyot Banerjee* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700 032, India The kinetics of the oxidation of thioglycolic and thiomalic acids by [NiIIIL1]2+ (where HL1=15-amino-3-methyl- 4,7,10,13-tetraazapentadec-3-en-2-one oxime) have been investigated at 30.0 °C; an inner-sphere mechanism for both the reactions has been proposed except for the fully deprotonated species of thiomalic acid which follows an outer-sphere route.Studies on the thiol–disulfide interchange reactions are of immense importance in biochemistry.1,2 However this type of transformation by metal complexes has received much less attention. In this investigation we present the kinetic results on the oxidation of thioglycolic acid (H2A) and thiomalic acid (H3A) by the nickel(III) oxime–imine complex I.Spectrophotometric determination at 498 nm (lmax for [NiIII(L1)]2+, hereafter designated as NiIII) revealed a 1:1 stoichiometric ratio for both the reactions leading to the disulfide products [eqn. (1)]. 2NiIII+2RSHh2NiII+RSSR+2H+ (1) RSH and RSSR represent respectively the thiols and the corresponding disulfide products. Generation of free radicals and characterisation of disulfide products were achieved by reported9,24 methods.Under pseudo-first-order conditions with varied concentrations of reductants, a plot of kobs vs. [H2A]T gives a straight line passing through the origin whereas that for H3A is a limiting curve of decreasing slope with nearly zero intercept (Table 1). The corresponding rate laws are given by eqns. (2) and (3) respectively. µd/dt[NiIII] =k[H2A][NiIII] =kobs[NiIII] (2) µd/dt[NiIII] = kQ[H3A]T[NiIII] 1+Q[H3A]T =kobs[NiIII] (3) Q k NiIII+H3AMNiIII.H3AhProducts (3a) Q is the association constant as defined by eqn.(3a) and represents the oxidation of thiomalic acid. The effect of acidity on the reaction rates was investigated in the pH range 2.51–8.05 for H2A and 2.71–8.25 for H3A as shown in Figs 1(a) and (b). Dependence on acidity can best be explained by considering the pK values of H2A and H3A.25 In the experimental pH range, the existing thioglycolic acid species are H2A, HAµ and A2µ.Their reactions can be portrayed as: NiIII+H2AhNiII+radical (4) NiIII+HAµhNiII+radical (5) NiIII+A2hNiII+radical (6) radical+radicalhdisulfide (7) The general rate law derived from the above is: kox= k4[H+]2+k5K1[H+]+k6K1 [H+]2+K1[H+]+K1K2 (8) Considering the pH range 2.51–6.70, the parameters k4, k5 and K1 were evaluated by a non-linear least-squares programme: k4=(22.5�2.8) dm3 molµ1 sµ1, k5=(3.05 �0.10)Å102 dm3 molµ1 sµ1 and K1=(5.36�0.13) *To receive any correspondence.Fig. 1 Variation of kox as a function µlog[H+] for the reduction of [NiIII(L1)]2+ by (a) thioglycolic acid and (b) thiomalic acid at 30.0 °C with [NiIII(L1)]2+=5.0Å10µ5 mol dmµ3, I=0.20 mol dmµ3 (NaClO4) and [buffer] =0.02 mol dmµ3: the solid line represents calculated values, points represent experimental values Table 1 Pseudo-first-order rate constants at various reductant concentrations for the oxidations of thioglycolic acid and thiomalic acid by [NiIII(L1)]2+ with [[NiIII(L1)]2+] =5.0Å10µ5 mol dmµ3, I=0.2 mol dmµ3 (NaClO4), [OAcµ] =0.02 mol dmµ3 and temperature=30.0 °C kobs/sµ1 102[Reductant]/ Reductant mol dmµ3 pH 4.75 pH 5.90 Thioglycolic acid 0.10 0.30 0.50 0.70 0.90 0.10 0.27 0.41 0.56 0.69 Thiomalic acid 0.05 0.10 0.20 0.30 0.50 0.70 1.00 0.75 1.25 1.79 2.33 2.94 3.44 4.11J.CHEM. RESEARCH (S), 1997 187 Å10µ6 mol dmµ3 (pK1=5.23, reported 3.58). Similarly from the pH range 7.0–8.05, the values of k5=(2.86� 0.1)Å102 dm3 molµ1 sµ1 and k6=(5.36�0.13)Å 103 dm3 molµ1 sµ1 were obtained using K2=1.66Å10–10 mol dmµ3.The reacting species of thiomalic acid in the experimental pH range are H3A, H2Aµ, HA2µ and A3µ. Their reactions towards the NiIII complex can be represented by: NiIII+H3AhNiII+radical (12) NiIII+H2AµhNiII+radical (13) NiIII+HA2µhNiII+radical (14) NiIII+A3µhNiII+radical (15) The corresponding rate law is: kox= k12[H+]3+k13K1[H+]2+k141K2[H+]+k15K1K2K3 [H+]3+K1[H+]2+K1K2[H+]+K1K2K3 (16) In suitable pH ranges, the values of the evaluated parameters are: k12=(1.72�0.3)Å102 dm3 molµ1 sµ1, k13=(4.10 �0.2)Å102 dm3 molµ1 sµ1, k14=(1.24�0.5)Å103 dm3 molµ1 sµ1, k15=(2.53�0.09)Å105 dm3 molµ1 sµ1, K1=(4.15�0.03)Å10µ4 mol dmµ3 (pK1=3.38, reported 3.64) and K2=(9.77�0.05)Å10µ6 mol dmµ3 (pK2=5.01, reported 4.64). Putting the corresponding evaluated parameters in eqns.(8) and (16), kox values at different experimental [H+] were obtained and these showed an excellent agreement with the experimental values for both thiols.The low spin (t2g 6eg 1) nickel(III) complexes having an unpaired electron in the dz2 orbital are generally substitutionally inert, although there is no experimental estimate of the ligand exchange rate for [NiIII(L1)]2+ and thereby reactions involving this complex are likely to follow an outersphere route. However, the formation of a hydrogen bonded intermediate in many of the electron transfer reactions of NiIII/IV oxime–imine complexes17,20–22 has been proposed.The initial rapid increase in the absorbance in the 20–30 ms timescale of thioglycolic acid oxidation points to inner-sphere coordination of the thiol molecule to the metal centre, probably by the partial release of one of the nitrogen atoms of the coordinated N6-oxime–imine frame. In the oxidation of thiomalic acid and other carboxylic acids (glycolic and malic acid), no such initial rise in absorbance was noted but the former oxidation was found to proceed through rate saturation. The higher reactivity of H2A towards [NiIII(L1)]2+ (E°=0.49 V, kex=102 dm3 molµ1 sµ1) is higher than that towards [MnIII(cdta)]µ (E°=0.81 V, kex=4.4Å108 dm3 molµ1 sµ1).This can be explained by considering the hydrogen bond formation between the carboxylato hydrogen atom and the oximato oxygen atom (�N·Oµ), which may substantially increase the lability of the metal centre through the sulfur atom and thereby provide a lower energy pathway for the electron transfer process.The lack of a carboxylato proton in HAµ and A2µ species explains their lower reactivity towards [NiIII(L1)]2+ than [MnIII(cdta)]µ. For the oxidation of thiomalic acid, a higher formation constant obtained from kinetics indicates that the reaction proceeds either through the initial formation of a hydrogen bonded adduct or through the coordination of the ligand to the metal centre by the SH group. In the oxidation of A3µ, no hydrogen bonding effect is possible and so an outer-sphere mechanism is more likely.We acknowledge the CSIR (New Delhi) for financial assistance. Techniques used: UV–VIS, elemental analysis, pH-metry References: 29 Table 2: Pseudo-first-order rate constants at various concentrations of glycolic acid and malic acid Table 3: Decomposition rate of [NiIII(L1)]2+ as a function of pH at 40.0 °C Fig. 2: kox vs. µlog[H+] for malic acid oxidation by [NiIII(L1)]2+ Schemes: 2 Received, 10th January 1997; Accepted, 21st February 1997 Paper E/7/00249A References cited in this synopsis 1 H. F. Gilbert, Adv. Enzymol., 1995, 63, 69. 2 R. Singh and G. M. Whitesides, The Chemistry of Sulfur Containing Functional Groups, ed. S. Patai and Z. Rappoport, Wiley, London, 1993. 9 S. Gangopadhyay, M. Ali, A. Dutta and P. Banerjee, J. Chem. Soc., Dalton Trans., 1994, 841. 17 A. McAuley, C. J. d P. R. West, J. Chem. Soc., Dalton Trans., 1988, 2279. 20 S. Bhattacharya, M. Ali, S. Gangopadhyay and P. Banerjee, J. Chem. Soc., Dalton Trans., 1994, 3733. 21 B. Saha, S. Gangopadhyay, M. Ali and P. Banerjee, Proc. Acad. Sci. (Chem. Sci.), 1995, 107, 393. 22 S. Bhattacharya, M. Ali, S. Gangopadhyay and P. Banerjee, J. Chem. Soc., Dalton Trans., 1996, 2645. 24 W. R. Cullen, B. C. Mcbride and J. R. Reglinski, J. Inorg. Biochem., 1984, 21, 45. 25 Stability Constants of Metal Ion Complexes, ed. L. G. Sillen and A. E. Martell, The Chemical Society, London, Special Publ. No. 17, 1964, pp. 376, 423, 367 and 411.