Fine Tuning of the Anisotropy Barrier by Ligand Substitution Observed in Linear {Dy2 Ni2 } Clusters.

Abstract

Three new heterometallic single-molecule magnets (SMMs), [Dy2 Ni2 (bipy)2 (RC6 H4 COO)10 ] [bipy=2,2'-bipyridine, R=H (1), CH3 (2), and NO2 (3)], are synthesized solvothermally with different 3-substituted benzoate ligands (RC6 H4 COO- ), and are characterized both structurally and magnetically. Structural analyses reveal that the three entities are structurally analogous, exhibiting an approximately linear {Dy2 Ni2 } core bridged by ten carboxylate moieties from the RC6 H4 COO- ligands. A noncoordinating substituent group attached on the phenyl ring results in minor geometry distortions of 1-3, but causes a significant decrease in the Mulliken atomic charge on the axially shortest O donor through inductive and/or conjugative effects. Weak intramolecular ferromagnetic (for DyIII ⋅⋅⋅DyIII ) and antiferromagnetic (for DyIII ⋅⋅⋅NiII ) interactions with slightly different coupling strengths are observed in 1-3 at low temperatures, and the effective anisotropy barriers to block the magnetization reversal are 39.9, 25.9, and 2.8 cm-1 , respectively, under zero direct-current field. Ab initio calculations reveal that ligand substitution by the noncoordinating electron-withdrawing/electron-donating group can give rise to good modulation of the energy gap between the two lowest Kramers doublets, as well as the orientation of the local easy axis of the DyIII ion magnetization. The directions of the local easy axis of the DyIII ion can further influence the dipole spin-spin interaction and the molecular anisotropy of the entire molecule, which, together with the energy separation between the ground and first excited ground states, become the significant factors determining the effective anisotropy barrier heights of 1-3. These important results demonstrate that the charge distributions of the ligand-field environments play essential roles in SMM performance, which should be considered seriously and utilized efficiently during the rational design of new, more feasible and practical SMMs.