Two mononuclear Dy III crown ether complexes [Dy(15C5)(H 2 O) 4 ](ClO 4 ) 3 Á(15C5)ÁH 2 O (1) and [Dy(12C4)(H 2 O) 5 ](ClO 4 ) 3 ÁH 2 O (2) have been prepared and characterized. X-ray diffraction studies show that both compounds crystallize as half sandwich type structures with muffin and pseudo-capped square antiprismatic geometries respectively. Despite the comparable local environments of the Dy III ions they display remarkably different dynamic magnetic properties with only 1 displaying SMM properties in zero field. The solid state emission spectra for both 1 and 2 display sharp bands associated with f-f transitions.From the fine structure of the 4 F 9/2 -6 H 15/2 band, the Stark splitting of the 6 H 15/2 ground state permitted the energy difference between the ground and first excited state to be determined. For 1 this value (DE = 58.0 AE 3.0 cm À1 ) is in excellent agreement with ab initio calculations and the experimentally observed SMM behaviour. For 2, the photoluminescence data and theoretical calculations support a less well isolated ground state (DE = 30 AE 3.0 cm À1 ) in which a rapid relaxation process affords no SMM behaviour in zero-field.

The introduction of the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH) in 4f-metal chemistry has afforded a new dinuclear complex, [Dy(NO)(sacbH)(HO)(MeCN)] (1), with the metal ions adopting a rare spherical tricapped trigonal prismatic coordination geometry. The deprotonated phenoxido O atoms of the organic chelate occupy the axial triangular faces of the prism and were found to be very close to the main anisotropy axes of the two Dy ions. As a result, the {Dy} compound exhibits frequency- and temperature-dependent out-of-phase ac signals below ∼25 K in the absence of a static dc field, yielding an energy barrier of 109.3(1) K for the reversal of magnetization. Fast and efficient quantum tunneling of magnetization, attributed to the strong tails of signals below ∼15 K, was suppressed through the application of a small dc field, yielding entirely visible χ″ signals below 27 K. Single-crystal magnetic hysteresis studies confirmed the single-molecule magnet (SMM) behavior of 1; the hysteresis loops appear at temperatures below ∼5 K, which is one of the highest blocking temperatures in the field of 4f-SMMs to date. This joint magneto-structural and ab initio study demonstrates the ability of more common coordination numbers (i.e., 9), but with rare coordination geometries (i.e., spherical tricapped trigonal prismatic), to promote axiality that enhances the molecular anisotropy and subsequently the magnetization dynamics of the system.

The first supramolecular cage formed by three benzo-15-crown-5 macrocycles encapsulating a [Dy(OH2)8](3+) guest cation is reported, with the Dy(iii) centre exhibiting local pseudo square antiprismatic D4d symmetry. The anisotropy barrier extracted from ac susceptibility studies, emission spectroscopy and ab initio calculations reveals that the second excited state Kramers doublet plays a key role in the magnetization dynamics due to the Ising character and near coparallel nature of the ground and first excited Kramers doublets.

A binuclear DyIII complex exhibits slow relaxation of magnetization under an applied dc field, while its tetranuclear counterpart exhibits “true” zero-field SMM behaviour.