NICKEL(III)-PROMOTED DEPROTONATION OF AN AMINE GROUP OF CYCLAM - CHARACTERIZATION OF THE VIOLET TRANSIENT THROUGH STOPPED-FLOW SPECTROPHOTOMETRIC TECHNIQUES AND DETERMINATION OF THE PK(A) 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 solut ion buffered at a pH = 7 +/- 1 has been characterized by fast data acq uisition spectrophotometry. The violet color originates from a fast de caying 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 deproto nation 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 stu dies at varying pH. A pK(A) value of 7.1 +/- 0.2 has also been obtaine d by the pH dependance of E(1/2)(Ni-III/Ni-II), determined through dif ferential pulse voltammetry investigations. The transient decays accor ding 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 electr on is transferred from the negatively charged nitrogen atom of the vio let species to the Ni-III center of another violet species.