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

Chiral bipyrimidine-bridged dinuclear Ln complexes of general formula [(μ-bipym){((+)-tfacam)Ln}] and [(μ-bipym){((-)-tfacam)Ln}], have been prepared from the assembly of Ln(AcO)·nHO (Ln = 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 complexes revealed a very weak antiferromagnetic interaction between the Gd ions through the bipyrimidine bridging ligand. CASSCF calculations indicated that the ground Kramers doublet (KD) of both Dy centers is almost purely axial with the anisotropy axis located close to the two tfacamligands at opposite sides of each Dyatom, which create an axial crystal field. In keeping with this, dynamic measurements indicated slow relaxation of the magnetization at zero field with = 55.1 K, a pre-exponential factor of τ = 2.17·10 s and τ = 8 μs. When an optimal dc field of 0.1 T is applied, QTM is quenched and increases to 75.9 K with τ = 6.16 × 10 s. The DyNO coordination spheres and SMM properties of [(μ-bipym){((+)-tfacam)Ln}] and their achiral [(Dy(β-diketonate))(μ-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 compounds, the magnetic exchange interaction between the Dy ions has been calculated to be very weak and, generally, ferromagnetic in nature. Relaxation mechanisms for [(μ-bipym){((+)-tfacam)Ln}] and previously reported analogous have been proposed from 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 ions.