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

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.