Volume 59, Issue 331, July 2023

Reactions of the 2,5-pyridine dicarboxylic acid (dicarpy)-bridged iron (III) dimer, [Fe((saloph))₂-μ-dicarpy], with β-mercaptoacetic acid and β-mercaptoethylamine in aqueous perchloric acid

Pius O Ukoha¹, Clara O Anidobu², Uchechukwu R Obeta³, Uchechukwu S Oruma^1♦

¹Coordination Chemistry and Inorganic Pharmaceuticals Unit, Department of Pure and Industrial Chemistry, University of Nigeria, Nsukka-410001, Enugu State, Nigeria
²Department of Chemical Sciences, Federal University Wukari, Nigeria
³Department of Chemistry, Michael Okpara University of Agriculture, Umudike-440109, Abia State, Nigeria

^♦Corresponding author
Coordination Chemistry and Inorganic Pharmaceuticals Unit, Department of Pure and Industrial Chemistry, University of Nigeria, Nsukka-410001, Enugu State, Nigeria

ABSTRACT

Kinetics of the reduction of [(Fe(saloph))₂-μ-dicarpy] by ̀β-mercaptoacetic acid and β -mercaptoethylamine was followed spectrophotometrically at 424 nm. The reactions were conducted at pseudo-first order conditions of a large excess (≥ 20-fold) of the reductants over the oxidant at 29°C and ionic strength of 0.001 moldm^-3 (NaClO₄). Under these conditions, k_obs increased with concentration of reductants and pseudo-first order plots were linear for at least 85% extent of reaction. Second order rate constants were fairly constant at 0.112 ± 0.007 dm³mol^-1S^-1 (β-mercaptoacetic acid) and 0.59 ± 0.016 dm³mol^-1S^-1 (β-mercaptoethylamine) indicating first order dependence of rate on concentration of the reductant. Plots of log k_obs versus Log[reductant] gave slopes close to unity for both reactions, implying first order dependence of rate on concentration of the reductants respectively. Within the acid concentration 1x 10^-4 ≤ [H+] ≥80 X10^-4mol dm^-3 rates of the reactions decreased as acid concentration increased. This has been interpreted in terms of possible protonation or deprotonation of the redox partners before the electron transfer step. Both reactions showed positive primary salt effect, indicating involvement of like charges at the rate determining step. There was insignificant retardation of the reactions on addition of catalytic amounts of CH₃COO-, Cl- and NO₃-. This is indicative of likely operation of parallel inner and outer-sphere paths for the reactions. The reactions have been rationalized on the basis of parallel inner-sphere and outer-sphere paths mediated by protonated reductants and oxidant species at low pH for β -mercaptoethylamine reaction and dissociated β-mercaptoacetic acid molecule for the [(Fe(saloph))₂-μ-dicarpy)]- mercaptoacetic acid reaction.

Discovery, 2023, 59, e88d1284

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Published: July 2023