Building 1D lanthanide chains and non-symmetrical [Ln2] “triple-decker” clusters using salen-type ligands: magnetic cooling and relaxation phenomena

Abstract

A solvothermal reaction between Ln(NO₃)₃·6H₂O (Ln: Gd, Tb and Dy), 2-hydroxy-1-naphthaldehyde, 2-OH-naphth, and ethylenediamine, en, in MeOH in the presence of a base, NEt₃, led to the formation of the 1D coordination polymers [Ln(L)(MeO)(MeOH)_0.5]_n·MeOH (Ln = Gd (1·MeOH), Tb(2), Dy (3·MeOH); H₂L = 1,1′-((1E,1′E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(naphthalen-2-ol), the Schiff-base ligand derived from the condensation of 2-OH-naphth and en), while a similar reaction in an excess of NaN₃ yielded 1D coordination polymers [Ln(L)(N₃)_0.75(MeO)_0.25(MeOH)]_n (Ln = Gd (4), Tb (5), Dy (6)). Finally, upon replacing ethylenediamine with o-phenylenediamine, o-phen, we managed to isolate the discrete dimers [Dy₂(L′)₃(MeOH)]·2MeOH (7·2MeOH) and [Gd₂(L′)₃(MeOH)]·2MeOH (8·2MeOH) (H₂L′ = 1,1′-((1E,1′E)-(1,2-phenylenebis(azanylylidene))bis(methanylylidene))bis (naphthalen-2-ol), the Schiff-base ligand from the condensation of 2-OH-naphth and o-phen). Polymers 1–3 describe one-dimensional chains, containing alternating seven- and eight-coordinate Ln^III metal centers, polymers 4–6 contain eight-coordinate lanthanide ions, while in both 7 and 8 the two Ln^III centers are eight- and seven-coordinate, adopting square antiprismatic and “piano-stool” geometry, respectively. The magnetocaloric properties of the three Gd^III analogues were determined from magnetic measurements, yielding the magnetic entropy change –ΔS_m = 21.8, 23.0 and 16.0 J kg⁻¹ K⁻¹ at T = 3.0 K on demagnetization of 7 T to 0, for 1, 4 and 8, respectively. The study of the magnetic properties also revealed that all three Dy^III analogues (3, 6 and 7) display out-of-phase signals, therefore suggesting slow magnetic relaxation, while such behaviour was not established in the Tb^III analogues.