Structural characterisation, EPR and magnetic properties of f–f and f–d lanthanide(iii) phenolic cryptates

Abstract

The Schiff base axial macrobicyclic ligand L¹ forms 4f–4f and 4f–3d cryptates with formula [Gd₂(L¹ − 3H)(NO₃)₂](NO₃)·1.5H₂O (1), [Tb₂(L¹ − 3H)(NO₃)₂](NO₃)·3EtOH·H₂O (2), [GdCu(L¹ − 3H)(NO₃)](NO₃)·H₂O (3), [LuCu(L¹ − 3H)(NO₃)](NO₃)·H₂O (4) and [GdZn(L¹ − 3H)(NO₃)](NO₃)·H₂O (5). The macrobicyclic receptor L¹ is an azacryptand N[(CH₂)₂NCH–R–CHN–(CH₂)₂]₃N (R =1,3-(2-OH-5-Me–C₆H₂)). The crystal structures of the five compounds have been determined by X-ray crystallography. The ligand is helically wrapped around the two metal ions, leading to pseudo-C₃ symmetries around the metals. In the solid state, the conformation of the cation in 1 and 2 is Λ(δδλ)₅(δδλ)₅ or its enantiomeric form Δ(λλδ)₅(λλδ)₅, while in 3, 4 and 5 it can be described as Λ(δδλ)₅(δ′δ′δ′)₅ (or Δ(λλδ)₅(λ′λ′λ′)₅). In 1, only one enantiomer is found in the crystal lattice, whereas in the other four compounds, both enantiomers are co-crystallised. The magnetic behaviour of the homodinuclear (Gd, Gd) and the heterodinuclear (Gd, Cu) cryptates points to a significant magnetic interaction between the two metal ions. This magnetic interaction is antiferromagnetic in the case of the Gd–Gd cryptate 1 (J = −0.194(6) cm⁻¹), but ferromagnetic for the Gd–Cu one (J = 2.2(1) cm⁻¹). The antiferromagnetic coupling observed for 1 is one of the largest ever reported. Although the ferromagnetic coupling observed for 3 is relatively weak, which is attributed to the strong bending of the bridging network, it is considerably stronger than the one reported for [GdCu(L² − 3H)(DMF)](ClO₄)₂·MeCN. In spite of the similar coordination environment of the Gd(III) ion in compounds 1, 3 and 5 their EPR spectra are different, thereby confirming the magnetic interactions between the Gd(III) ion and the Cu(II) ion in 3 and the other Gd(III) ion in 1.