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

Chiral bipyrimidine-bridged dinuclear Ln^III complexes of general formula [(μ-bipym)((+)-tfacam)₃Ln₂] and [(μ-bipym)((-)-tfacam)₃Ln₂], 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 GdIII 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 τ_o = 2.17·10^-6 s and τ_QTM = 8 μs. When an optimal dc field of 0.1 T is applied, QTM is quenched and U_eff increases to 75.9 K with τ_o = 6.16 × 10^-7 s. The DyN₂O₈ coordination spheres and SMM properties of [(μ-bipym)((+)-tfacam)₃Ln₂] and their achiral [(Dy(β-diketonate)₃)₂(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^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 [(μ-bipym)((+)-tfacam)₃Ln₂] 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.