Nanosecond Laser Photolysis of Ni(II) Tetraphenylporphyrin. Kinetic Studies on Association and Dissociation of Aliphatic Amines at the Axial Positions

Abstract

Nanosecond laser photolysis studies of nickel(II) tetraphenylporphyrin, NiIITPP, in toluene solutions containing primary, secondary, and tertiary aliphatic amines were carried out for elucidation of the association and dissociation mechanisms of the axial amines. From the absorption spectroscopic studies, the primary amines were found to react with NiIITPP to give the six coordinate species. The secondary and the tertiary amines, however, give no coordinated species. The laser photolysis of NiIITPP in toluene containing primary amines revealed that the four- and six-coordinate species undergo mutual exchange upon laser excitation. In secondary amine−toluene solutions, the four coordinated species undergoes light-induced transformation to yield the six-coordinate species. However, no formation of the six coordinate species are observed for tertiary amine−toluene solutions of NiIITPP by photolysis. The six and four coordinate species produced by laser photolysis of the amine−toluene solutions decay according to the pseudo-first-order kinetics. The plot of the rate constant, kobsd, for the decay of these transient species vs the primary amine concentration at 300 K is concave upward: the value kobsd initially decreases toward a minimum and then increases with an increase in the amine concentration. Because the laser photolysis of NiIITPP in primary amine−toluene solutions results in the nonequilibrium state among four and six-coordinate species, kobsd is the rate constant to reach the equilibrium state. It is assumed that the four and six coordinate species in the nonequilibrium state achieve the equilibrium via the formation of a five coordinate NiIITPP as an intermediate. The rate constants, kobsd, represented as a function of the primary amine concentration are well explained with the use of the steady-state approximation with regard to the five coordinate species. On the basis of the laser photolysis in the temperature range 200−300 K, the reaction mechanisms for the equilibrium reactions between NiIITPP and various amines are discussed in detail.