Studies of Δ,Λ-[M(1,10-phen)2(S-ala)]qX diastereoisomeric systems of nickel and zinc with different counter-anions

Abstract

Δ,Λ-[M(phen)2(S-ala)]+(Xq-)1/q diastereoisomeric systems, in which M = nickel or zinc, phen = 1,10-phenanthroline, S-ala- = S-alaninate (q = 1 and 2), with a series of counteranions have been prepared as methanolic solutions and characterized by spectral (CD, ORD, UV-vis) and conductance data. In both nickel and zinc systems the CD spectra in the β′ band region of the diimine ligand exhibit exciton splittings with sign patterns corresponding to a predominance of the Λ isomers. Analysis of the ORD spectra by means of many-term Drude equations suggests the order Zn > Ni for the absolute values of excitation resonance energy. Differences between nickel and zinc systems have been observed concerning the effects of the counter-anion nature upon the formation equilibria. In the nickel systems these effects would consist mainly of outer-sphere interactions affecting the position of the [Λ]/[Δ] equilibrium, whereas in the zinc ones ligand/counter-anion exchange equilibria would also occur to a significant degree. By using the molar-ratio method the stability orders [Zn(phen)(S-ala)2] ≈ [Zn(phen)2(S-ala)]+ > [Zn(phen)3]2+ and [Ni(phen)3]2+ > [Ni(phen)2(S-ala)]+ have been established. The ion-pairing and the third stepwise stability constants for [Zn(phen)2(S-ala)]Cl have also been estimated. The counteranion dependence of the apparent stability constants for the zinc systems, is discussed in terms of the counter-anion basicity and the possible “discriminating qualities” of the [Zn(phen)(S-ala)]+ species.