Kinetic and mechanistic studies: I. Direct determination of self-exchange electron-transfer rate constants of nickel(III,II) complexes by (61)nickel EPR line broadening. II. Kinetics of the hydrolysis of 1-cyanoformamide, oxamide, and oxamic acid in hydrochloric acid solutions

Jean-Fu Wang, Purdue University

Abstract

A new method for the direct determination of electron-transfer self-exhange rate constants of nickel(III,II) complexes has been developed. This novel method is based on the influence of a nickel isotope ($\sp{61}$Ni, nuclear spin quantum number I = 3/2) on the line width of the room-temperature electron paramagnetic resonance (EPR) signal of Ni(III) complexes in solution. An electron-transfer self-exchange rate constant k$\sb{11}$ between the +3 and +2 oxidation states of the nickel complex of tri-$\alpha$-aminoisobutyric acid (Aib$\sb3$) has been determined to be 450 $\pm$ 50 M$\sp{-1}$ s$\sp{-1}$ at 24$\sp\circ$C by this method. This rate constant reflects an "inner-sphere enhancement" due to bridging by an axial water molecule. The effect of an axially coordinated, nonbridging ligand such as pyridine on the exchange rate of Ni$\sp{\rm III,II}$(H$\sb{-2}$Aib$\sb3$)$\sp{0,-}$ has also been studied. The self-exchange electron-transfer rate constant between Ni$\sp{\rm III}$(H$\sb{-2}$Aib$\sb3$)(py)(H$\sb2$O) and Ni$\sp{\rm II}$(H$\sb{-2}$Aib$\sb3$)-is 103 $\pm$ 4 M$\sp{-1}$s$\sp{-1}$ at 23$\sp\circ$C. The hydrolysis of 1-cyanoformamide has been studied in concentrated hydrochloric acid solutions over the concentration range 6.44 to 11.1 M. For the system of consecutive, irreversible, three step first-order reactions, 1-cyanoformamide ${\buildrel{\rm k\sb{AB}}\over\longrightarrow}$ oxamide ${\buildrel{\rm k\sb{BC}}\over\longrightarrow}$ oxamic acid ${\buildrel{\rm k\sb{CD}}\over\longrightarrow}$ oxalic acid, k$\sb{\rm AB}$: k$\sb{\rm BC}$: k$\sb{\rm CD}$ = 23.2: 0.61: 1.00 at 11.1 M and k$\sb{\rm AB}$: k$\sb{\rm BC}$: k$\sb{\rm CD}$ = 0.52: 0.84: 1.00 at 6.44 M. The ratio of k$\sb{\rm AB}$ to k$\sb{\rm BC}$ drops rapidly when the HC1 concentration decreases from 11.1 M to 6.44 M. For all acid concentrations studied, k$\sb{\rm BC}$ and k$\sb{\rm CD}$ are of the same order. It is shown that in a consecutive reaction, when the rate constants of each step are close, computer analysis is necessary. For the hydrolysis of 1-cyanoformamide in concentrated HCl, the maximum rates of the first and third steps were observed at 11.1 M HCl and the maximum rate of the second step was at about 9.8 M HCl. In concentrated hydrochloric acid solutions, both the activity of hydrochloric acid and the activity of water, rather than concentration of hydrochloric acid, should be used in the rate expression. The unusual stability of 1-cyanoformamide in 1 M HCl solution relative to other alkyl amides is also discussed.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

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