Regulation of Ligand Substitution in Mononuclear Dy/Gd Complexes: Slow Magnetic Relaxation and Magnetocaloric Effects

Four mononuclear complexes [Ln(pmbp)3dmbipy]·CH3CH2OH (1-Dy and 1-Gd, Hpmbp = 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone, dmbipy = 4,4′-dimethyl-2,2′-bipyridine) and [Ln(pmbp)3dmobipy] (2-Dy and 2-Gd, dmobipy = 4,4′- dimethoxy-2,2′-bipyridine) have been synthesized and structurally and magnetically characterized. Magnetic susceptibility studies of two dysprosium complexes indicate that 2-Dy is a zero-field single-molecule magnet (SMM), whereas 1-Dy is a field-induced SMM. As can be seen from the CASSCF calculation results, the weight of the axial crystal-field (CF) parameter B (2, 0) is 14.43% larger than the nonaxial ones for 2-Dy, while the weight is not dominant for 1-Dy. This is in agreement with the experimental results. Unlike two dysprosium complexes, the two gadolinium complexes are almost indistinguishable in their magnetocaloric effects.