Nickel(III) Promoted Deprotonation of an Amine Group of Cyclam. Characterization of the Violet Transient through Stopped-Flow Spectrophotometric Techniques and Determination of the pKA Value

The violet transient that forms when a solution of [Ni-III(cyclam)](3+) (cyclam = 1,4,8,11 -tetraazacyclotetradecane, 1) is added to a solution buffered at a pH = 7 +/- 1 has been characterized by fast data acquisition spectrophotometry. The violet color originates from a fast decaying absorption band centered at 539 nm (E = (1.33 +/- 0.10) X 10(3) M(-1) cm(-1)), pertinent to a species which forms through the deprotonation of an amine group of the macrocycle in the [Ni-III(cyclam)](3+) complex and whose pK(A) is 7.1 +/- 0.1, as determined by spectral studies at varying pH. A pK(A) value of 7.1 +/- 0.2 has also been obtained by the pH dependance of E(1/2)(Ni-III/Ni-II), determined through differential pulse voltammetry investigations. The transient decays according to a second order pattern (kappa = (5.0 +/- 0.2) x 10(3) M(-1) s(-1)), ascribed to a ''disproportionation'' process, in which an electron is transferred from the negatively charged nitrogen atom of the violet species to the Ni-III center of another violet species.