Chiral Heterometallic Cu8Ln4 Complexes with Enantiopure Schiff Base Ligands: Synthesis, Structural, Spectroscopic and Magnetic Studies

Abstract

The enantiomerically pure Schiff base ligands H₂L-S and H₂L-R yield chiral heterometallic dodecanuclear complexes of the form [Cu₈Ln₄(OH)₈(OMe)₄(O₂CBu^t)₈(L-S or L-R)₄(H₂O)₄] where Ln^III=Gd (1S), Tb (2S), Dy (3S, 3R), Ho (4S, 4R), Er (5S) or Y (6S, 6R) and H₂L=(S or R)-2-{[(1-hydroxypropan-2-yl)imino]methyl}-6-methoxyphenol. The complexes are isomorphous and crystallize in the non-centrosymmetric polar space group C2 in enantiomeric conformation. The chirality of the Schiff base ligands originates from the respective S- or R- enantiomer of 2-aminopropan-1-ol, is imparted to the complexes and to the crystals that belong to non-centrosymmetric space group. The chirality and enantiomeric conformation of all complexes are retained in dmso solutions as confirmed by Circular Dichroism spectra which consist of mirror images, expected for enantiomeric pairs. All complexes consist of four distorted cubane-like subunits [Cu₂Ln₂(μ₃-OH)₂(μ₃-OMe)(μ₃-OR)], which share the Ln^III ions and result in a cyclic distorted tetragonal arrangement; each edge of the {Ln^III ₄} quadrilateral is occupied by two μ-OH^- ions that further bridge to a Cu^II ion. Magnetic susceptibility measurements revealed ferromagnetic interactions for 3S with Ln^III=Dy and antiferromagnetic interactions for all other complexes. AC susceptibility data of 3S under 1 kOe external dc field indicate slow magnetic relaxation phenomena below 2 K.

Keywords: chirality; crystal structures; enantiomers; heterometallic complexes; magnetic studies.