Oxidation of the thiosulfate ion by some iron(III) complexes with phenolate - amide-amine coordination: A comparative kinetic study |
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Authors: | Suprava Nayak Anadi C Dash |
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Institution: | (1) Department of Chemistry, Utkal University, Bhubaneswar, 751 004, India |
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Abstract: | The redox reactions of thiosulfate with four iron(III) complexes having phenolate-amide-amine coordination, FeIII(L){L = 1,2-bis(2-hydroxybenzamido)ethane, L1; 1,3-bis(2-hydroxybenzamido)propane, L2; 1,5-bis(2-hydroxybenzamido)3-azapentane, L3; and 1,8-bis(2-hydroxybenzamido)3,6-diazaoctane, L4} have been investigated in 10% v/v MeOH + H2O and I = 0.3 mol dm−3. At constant pH (~ 4.8) and under pseudo-first order conditions of S2O
3
2−
] the reaction obeyed the rate law : − dFeIII(L)]/dt = k
obs FeIII(L)] + k
obs
′
where k
obs
′
denotes the observed rate constant of thiosulfate decomposition; k
obs = aS2O
3
2−
] + bS2O
3
2−
]
T
2
is valid for all the complexes, particularly at pH < 6, while k
obs
′
= H+]S2O
3
2−
]
T
2
is consistent with the rate law for thiosulfate decomposition proposed earlier. The rate data (k
obs) were analysed on the basis of the reactivities of various species of FeIII(L) generated by the equilibrium protonation of the sec-NH of dien and trien spacer units resulting in the ring opening (for FeIII(L3/L4)]), and acid base equilibrium of the aqua ligand bound to the iron(III) centre
(FeIII(L)(OH2)
n
]). The redox activities, both for second and third order paths, show the ligand dependencies : L4<L3<L1<L2 conforming to the fact that the complexes tend to be less susceptible to electron transfer from S2O
3
2−
with (i) the increase of the number of chelate rings, (ii) the decrease of overall charge, and (iii) the decrease of ring
size offered by the amine moiety (from six membered to five membered one as for FeIII(L1/L2)(OH2)2]+. There was no evidence for the formation of inner sphere thiosulfato complex, the possibility of the formation of the outer
sphere ion-pairs, Fe(L/HL)(OH2)n
+/2+, S2O
3
2−
] with low equilibrium constant value may not be excluded. In view of this, the outer sphere electron transfer (ET) mechanism
is the most likely possibility. |
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Keywords: | |
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