Dinickel and dicopper complexes with N,N-linked bis(cyclam) ligands. An ideal system for the investigation of electrostatic effects on the redox behavior of pairs of metal ions

I (R = (CH2)2, (CH2)3, (CH2)4, m-xylyl, p-xylyl) were prepd. by condensation of N,N',N''-tritosylcyclam (2 equiv) with an XRX fragment (X = Br or OTs). M2L4+ (M = Ni, Cu; H4L = I) undergo a 2-electron-oxidn. process, according to 2 reversible 1-electron steps, whose redox potentials are sepd. by the quantity ΔE°. The value of ΔE is related to the comproportionation (or valence scrambling) equil. [MIIIMIIIL]6+ + [MIIMIIL]4+ .dblharw. 2[MIIIMIIL]5+ and results from the combination of a statistical fixed contribution and a term that reflects the electrostatic repulsion between the metal centers of the dinuclear complex. The electrostatic repulsion term decreases when the distance between the metal centers is increased, according to a hyperbolic mode, but does not show any definite dependence upon the macroscopic values of the dielec. const. of the explored media. Moreover, the electrostatic term significantly varies with the stereochem. features of each metal center in its oxidn. state (II or III), which cannot be any longer considered as simple point charges.