Characterization of nickel(II) dipeptide complexes of alpha-aminoisobutyric acid and kinetics of oxidation of tetracyanonickelate(II) by aqueous chlorine

Mark William Beach, Purdue University

Abstract

Potentiometric studies of the pH dependent formation of nickel(II) complexes with Aib$\sb2$ (the dipeptide of $\alpha$-aminoisobutyric acid) indicate that the complexes formed are very similar to other Ni(II) dipeptide complexes. The stability of these complexes is influenced by both steric and inductive effects that are related to the $\alpha$-carbon methyl groups. A low-spin six-coordinate complex, Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2)\sb2\sp{2-}$, is the predominant species at pH $\geq$ 11. This low-spin behavior is unusual for six-coordinate nickel(II) complexes, which typically form high-spin structures. An increase in solution temperature also favors the formation of the low-spin Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2)\sb2\sp{2-}$ complex. Temperature dependent UV-Vis spectra suggest that the Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2)\sb2\sp{2-}$ complex may also exist in a high-spin form. Oxidation of Ni$\sp{\rm II}$(CN)$\sb4\sp{2-}$ by aqueous chlorine proceeds by three parallel pathways via Cl$\sb2$O, Cl$\sb2$, and HOCl to yield trans-Ni$\sp{\rm III}$(CN)$\sb4$(H$\sb2$O)$\sb2\sp{-}$. The pathway that predominates depends on the total active chlorine concentration, the Cl$\sp{-}$ concentration, the solution acidity, and the type of buffer present as well as its concentration. The values of the second-order rate constants (M$\sp{-1}$ s$\sp{-1}$, 25.0$\sp\circ$C, $\mu$ = 0.10) for oxidations by Cl$\sb2$O, Cl$\sb2$, and HOCl are 5.3 $\times$ 10$\sp7$, 3.2 $\times$ 10$\sp5$, and 0.5, respectively. The proposed mechanism involves the formation of a nickel(IV) intermediate by Cl$\sp{+}$ transfer to nickel in the rate-determining step. This is followed by a rapid electron transfer between the nickel(IV) intermediate and Ni$\sp{\rm II}$(CN)$\sb4\sp{2-}$ to give the final nickel(III) product. When the Cl$\sb2$O path predominates and higher concentrations of Ni$\sp{\rm II}$(CN)$\sb4\sp{2-}$ are used, the formation of Cl$\sb2$O (catalyzed by acetic acid) becomes rate limiting. The second-order rate constant for Cl$\sb2$O formation from HOCl is 0.12 M$\sp{-1}$ s$\sp{-1}$ and the third-order rate constant for the acetic acid-catalyzed pathway is 280 M$\sp{-2}$ s$\sp{-1}$. Under these conditions the direct HOCl oxidation of Ni$\sp{\rm II}$(CN)$\sb4\sp{2-}$ also contributes to the appearance of nickel(III).

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

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