Kinetic studies of iodine oxidations of hydroxylamine and hydrazine, and, characterization of trivalent nickel complexes of tetrapeptides

Rong Ming Liu, Purdue University

Abstract

The kinetics for the reactions of $\rm I\sb3\sp{-}/I\sb2$ with excess $\rm NH\sb3OH\sp+/NH\sb2OH$ are studied from pH 2.0 to 6.8. A mechanism is proposed where $\rm I\sb2$ and $\rm NH\sb2OH$ react rapidly to form an $\rm I\sb2NH\sb2OH$ adduct that undergoes general-base (B) assisted deprotonation to give INHOH. At higher pH, hydroxylamine acts as a general base as well as a reductant. Rate constants for various bases follow a Bronsted-Pedersen relationship with a $\beta$ value of 0.58. The rate decreases markedly with increase of $\rm\lbrack H\sp+\rbrack $ and $\rm\lbrack I\sp{-}\rbrack $ due to $\rm NH\sb3OH\sp+$ and $\rm I\sb3\sp{-}$ formation, loss of general-base assistance, and the reverse reaction of $\rm BH\sp+ + I\sp{-} + INHOH$ to reform $\rm I\sb2NH\sb2OH.$ Kinetic evidence is given for $\rm I\sb2NH\sb2OH$ as an intermediate species and for INHOH as a steady-state species that decays to form HNO. Subsequent rapid dehydrative dimerization of HNO gives $\rm N\sb2O$ as the final product. The reactivity and reaction mechanisms of iodine oxidation are compared with those of chlorine and bromine. The halogen oxidation of hydroxylamine proceeds entirely by $\rm X\sp+$-transfer to nitrogen to give XNHOH followed by $\rm X\sp{-}$ loss, as opposed to electron-transfer pathways. Iodine oxidation of hydrazine to dinitrogen (pH 0 to 8.0) shows a reaction mechanism parallel to that of hydroxylamine. $\rm I\sb2$ reacts rapidly with $\rm N\sb2H\sb4/N\sb2H\sb5\sp+$ to form an $\rm I\sb2N\sb2H\sb4$ adduct that decays to $\rm IN\sb2H\sb4\sp+$ but also undergoes general-base assisted deprotonation to give $\rm IN\sb2H\sb3$ with a Bronsted $\beta$ value of 0.48. At high pH, hydrazine acts as a nucleophile and as a general base. At low pH, $\rm I\sb2$ also reacts with $\rm N\sb2H\sb5\sp+$ in addition to the $\rm N\sb2H\sb4$ pathway. The rate-determining steps in the proposed mechanism are attributed to the formation and deprotonation of $\rm IN\sb2H\sb4\sp+$ and the general-base reactions with $\rm I\sb2N\sb2H\sb4.$ Subsequently, $\rm IN\sb2H\sb3$ reacts rapidly with another $\rm I\sb2$ to form $\rm N\sb2$ as the final product. Characterization of $\rm Ni(III)Gly\sb2HisGly$ complex is initiated, where Gly is glycine, and His is L-Hisdine. The $\rm Ni(III)Gly\sb2HisGly$ complex has been prepared by chemical oxidation as well as by flow-through bulk electrolysis. The UV/vis and EPR spectra of $\rm Ni(III)Gly\sb2HisGly$ complex are reported. Decomposition of $\rm Ni(III)Gly\sb2HisGly$ is studied with and without buffer present. The rates of decomposition of $\rm Ni(III)Gly\sb2HisGly$ increase with pH increase. No evidence has been found for the peptide-olefin as a decomposition product.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Chemistry|Analytical chemistry

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