Mononuclear Dysprosium(III) Complexes with Triphenylphosphine Oxide Ligands: Controlling the Coordination Environment and Magnetic Anisotropy

We report the synthesis, structural and magnetic characterization of five mononuclear Dy-III ion complexes using triphenylphosphine oxide as a monodentate ligand. They have formulae [Dy-III(OPPh3)(3)(NO3)(3)] (1), [Dy-III(OPPh3)(4)(NO3)(2)](NO3) (2), [Dy-III(OPPh3)(3)Cl-3] (3), [Dy-III(OPPh3)(4)Cl-2]Cl (4) and [Dy-III(OPPh3)(4)Cl-2](FeCl4) (5). These complexes are characterized using single crystal X-ray diffraction, which revealed that each complex has a unique coordination environment around the Dy-III ion, which results in varying dynamic magnetic behavior. Ab initio calculations are performed to rationalize the observed magnetic behavior and to understand the effect that the ligand and coordination geometry around the Dy-III ion has on the single-molecule magnet (SMM) behavior. In recent years, seven coordinate Dy-III complexes possessing pseudo similar to D-5h symmetry are found to yield attractive blocking temperatures for the development of new SMM complexes. However, here we show that the strength of the donor ligand plays a critical role in determining the effective energy barrier and is not simply dependent on the geometry and the symmetry around the Dy-III ion. Seven coordinate molecules possessing pseudo D-5h symmetry with strong equatorial ligation and weak axial ligation are found to be inferior, exhibiting no SMM characteristics under zero-field conditions. Thus, this comprehensive study offers insight on improving the blocking temperature of mononuclear SMMs.