A Chiral Bipyrimidine-Bridged Dy₂ SMM: A Comparative Experimental and Theoretical Study of the Correlation Between the Distortion of the DyO6N2 Coordination Sphere and the Anisotropy Barrier.

ABSTRACT: Chiral bipyrimidine-bridged dinuclear Ln^III complexes of general formula [(?-bipym){((+)-tfacam)₃Ln}₂] and [(?-bipym){((-)-tfacam)₃Ln}₂], have been prepared from the assembly of Ln(AcO)₃·nH₂O (Ln^III = Dy, Gd), (+)/(-)-3-(trifluoroacetyl)camphor enantiopure ligands ((+)/(-)-Htfacam) and bipyrimidine (bipym). The structure and chirality of these complexes have been supported by single-crystal X-Ray diffraction and circular dichroism. The study of the magnetic properties of the Gd^III complexes revealed a very weak antiferromagnetic interaction between the Gd^III ions through the bipyrimidine bridging ligand. Ab initio CASSCF calculations indicated that the ground Kramers doublet (KD) of both Dy^III centers is almost purely axial with the anisotropy axis located close to the two tfacam^-ligands at opposite sides of each Dy^IIIatom, which create an axial crystal field. In keeping with this, ac dynamic measurements indicated slow relaxation of the magnetization at zero field with U _eff = 55.1 K, a pre-exponential factor of ?_o = 2.17·10^-6 s and ?_QTM = 8 ?s. When an optimal dc field of 0.1 T is applied, QTM is quenched and U _eff increases to 75.9 K with ?_o = 6.16 × 10^-7 s. The DyN₂O₈ coordination spheres and SMM properties of [(?-bipym){((+)-tfacam)₃Ln}₂] and their achiral [(Dy(?-diketonate)₃)₂(?-bpym)]analogous have been compared and a magneto-structural correlation has been established, which has been supported by theoretical calculations. In contrast to the Gd^III compounds, the magnetic exchange interaction between the Dy^III ions has been calculated to be very weak and, generally, ferromagnetic in nature. Relaxation mechanisms for [(?-bipym){((+)-tfacam)₃Ln}₂] and previously reported analogous have been proposed from ab initio calculations. As the magnetic exchange interaction found to be very weak, the observed magnetization blockade in these systems are primarily dictated by the single ion anisotropy of Dy^III ions.

SUBMITTER: Diaz-Ortega IF 

PROVIDER: S-EPMC6236069 | biostudies-literature | 2018

REPOSITORIES: biostudies-literature