Structure and aggregation properties of a Schiff-base zinc(ii) complex derived from cis-1,2-diaminocyclohexane

Abstract

This contribution explores the effect of the bridging diamine upon the aggregation properties of a Zn^II Schiff-base complex, 1, both in the solid state and in solution. The X-ray structure of 1, resulting from the harvest of good quality crystals using chloroform and diethyl ether as solvents, shows the presence of a densely packed dimer in the solid state which pentacoordinates two Zn atoms involved in a μ-phenoxo bridge. Detailed studies in solution, through ¹H NMR, DOSY NMR, and optical spectroscopic investigations, indicate the typical aggregation/deaggregation behaviour on switching from non-coordinating to coordinating solvents, in relation to the Lewis acidic character of such Zn^II complexes. Thus, while in DMSO-d₆ both ¹H NMR and DOSY studies suggest the existence of monomeric species, in chloroform solution experimental data support the existence of aggregates. However, unlike our previous studies, ¹H NMR data in chloroform solution indicate the existence of an asymmetric dimer, as observed in the X-ray crystal structure. This further evidences a very rigid backbone of the dimeric aggregate and can be related to the defined stereochemistry of the chelate cis-1,2-diaminocyclohexane bridge.