Experimental and theoretical investigations on three Dy III 4 single molecule magnets: structural and magneto-structural correlations.

The work in this report describes the syntheses, crystal structures, dc/ac magnetic behaviour, and theoretical calculations (both ab initio CASSCF and DFT) of three defect dicubane/planar butterfly type tetradysprosium(III) compounds of compositions [Dy III 4 L 4 (μ 3 -OH) 2 (carboxylate) 2 (dmf) 2 ] (carboxylate = formate (1), acetate (2), propionate (3)), where H 2 L = 2-(2-hydroxy-3-ethoxybenzylideneamino)phenol. In the butterfly type structures, two Dy III centres (Dy b ) occupy the body positions while two other (Dy w ) units occupy the wing positions. SHAPE analyses reveal that the coordination geometries of the Dy b and Dy w centres, both octacoordinated, are triangular dodecahedron (TDD) and square antiprism (SAPR), respectively. Variable-temperature magnetic susceptibility measurements give an indication of weak antiferromagnetic interactions and variable-field magnetization measurements reveal strong anisotropy in all the three compounds. The variable-temperature/frequency in-phase/out-of-phase AC susceptibility data reveal that all these three compounds are SMMs with two relaxation channels under zero dc field; slow relaxation (SR) and fast relaxation (FR) processes could be assigned to the SAPR (Dy w ) and TDD (Dy b ) metal centres, respectively. The simulated U eff and τ 0 values are: 49.0 cm -1 and 1.76 × 10 -7 s for 1, 30.3 cm -1 and 1.51 × 10 -8 s for 2 and 23.4 cm -1 and 9.64 × 10 -7 s for 3. Furthermore, ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations reveal that the ground state of Dy III centres are axial in nature with a dominating contribution from m J = |±15/2>. The magnetization relaxation occurs via the first excited KD resulting in the large computed blocking barrier of Dy w (SAPR) centres compared to that of the Dy b (TDD) centres which corroborates the experimental measurements. The exchange parameters obtained from DFT calculations are generally in line with those obtained from the fitting of χ M T vs. T in POLY_ANISO calculations. Interesting structural and magneto-structural correlations have been found, which are the major outcomes of this investigation.