An Unprecedent Structural Interconversion in Solution of Aggregate Zn(II) Salen Shiff-Base Complexes

This contribution explores the aggregation properties in solution of noncoordinating solvents of a series of amphiphilic Zn(salen) derivatives, through detailed H-1 NMR, DOSY NMR, and optical absorption spectroscopic studies. It is found that these aggregate species are involved in a unique structural interconversion between two defined dimers, A and B, driven by the concentration of water dissolved in chloroform. Dilute CHCl3 solutions are characterized by the presence of dimeric species, A, in which both Zn-II atoms of the Zn(salen) units mutually interact through a Zn center dot center dot center dot O axial coordination, likely adopting a square-base pyramidal structure. Investigations to higher concentrations indicate the existence of a new dimeric species, B, in equilibrium to that observed at lower concentrations, involving a coordination mode interconversion of an intermediate monomer presumably from a square-pyramidal to a trigonal bipyramidal structure. This behavior may be related to the nonconjugated, conformational flexible nature of the bridging diamine of the Schiff base, and is influenced by the solvent polarity. Variable-temperature H-1 NMR studies indicate the existence of a nonequivalent species B' in a fluxional equilibrium with species B.