Dysprosium-based complexes with a flat pentadentate donor: a magnetic and ab initio study

Abstract

The reactivity of the well-known pentadentate N₃O₂ Schiff base H₂L (2,6-bis(2-hydroxyphenyliminomethyl)pyridine) towards a lanthanoid metal, in this case Dy^III, has been investigated for the first time. This reactivity markedly depends on the pH of the medium and, accordingly, two different complexes, [Dy(HL)(NO₃)₂]·H₂O (1·H₂O) and [Dy(L)(NO₃)(EtOH)(H₂O)]·2H₂O (2·2H₂O), could be isolated from dysprosium(III) nitrate and H₂L. In addition, reaction of H₂L with dysprosium(III) chloride in methanol yields [Dy(HL′)₂][Dy(L)(Cl₂)] (3), where H₂L′ ((6-(2-hydroxyphenyliminomethyl)-2-methoxyhydroxymethyl)pyridine) is an N₂O₂ hemiacetal donor derived from the partial hydrolysis of the H₂L ligand, and subsequent addition of the methanol solvent to the carbonyl group. This latter reaction has been firstly observed for a lanthanoid metal. Single crystal X-ray diffraction studies of 1·1.15Py·0.3CH₃C₆H₅, 2·2H₂O and 3 show that the Schiff base is acting as a nearly flat pentadentate donor in all the cases, this behaviour being independent of the deprotonation degree of the phenolic oxygen atoms, both mono- or bisdeprotonated. Complexes 1·1.15Py·0.3CH₃C₆H₅ and 2·2H₂O show DyN₃O₆ cores, with distorted geometries closer to spherical tricapped trigonal prism or spherical capped square antiprism for 1·1.15Py·0.3CH₃C₆H₅ and 2·2H₂O, respectively. In the case of 3, the [Dy(HL′)₂]⁺ cation shows a dysprosium ion in an N₄O₄ triangular dodecahedron environment, while the [Dy(L)(Cl₂)]⁻ anion displays a DyN₃O₂Cl₂ core with distorted pentagonal bipyramidal geometry. Moreover, attempts to dilute 1·H₂O with yttrium yielded single crystals of (Et₃NH)[Dy_0.09Y_0.91(L)(NO₃)₂] (4), where the Schiff base shows a similar pentadentate coordination mode. Dynamic magnetic studies of 1·H₂O, 2·2H₂O and 3 show that 2·2H₂O and 3 present field-induced slow relaxation of the magnetisation, with U_eff barriers of 46.1(9) and 31.0(7) K for 2·2H₂O and 3, respectively, while 1·H₂O does not exhibit frequency-dependent peaks of the out of phase susceptibility, even in the presence of an external dc magnetic field. By contrast, the dilute sample 4 behaves as a SIM at zero dc field, with an energy barrier of ca. 49 K. Ab initio calculations using CASSCF methods including spin–orbit effects qualitatively support the obtained magnetic results, indicating that axiality is not the only factor that should be taken into account in order to increase effective energy barriers.