Enhancement of Single-Molecule Magnet Properties by Manipulating Intramolecular Dipolar Interactions

A new single-molecule magnet (SMM) complex [K(18-crown-6)][(COT)Er(µ-Cl)₃Er(COT)] (Er₂Cl₃, COT = cyclooctatetraenide dianion) is obtained by the reaction of [(COT)Er(µ-Cl)(THF)]₂ (Er₂Cl₂, THF = tetrahydrofuran) with an equivalent of KCl in the presence of 18-crown-6. The two COT-Er units in the newly formed complex are triply bridged by µ-Cl ligands, leading to the "head-to-tail" alignment of the magnetic easy axes distinctly different from the "staggered" arrangement in the precursor complex. This structural transformation has led to significantly enhanced intramolecular dipolar interactions and a reduced transverse component of the crystal fields, increasing the energy barrier from 150(8) K for Er₂Cl₂ to 264(4) K for Er₂Cl₃ and extending its magnetic relaxation time at 2 K by 2500 times with respect to Er₂Cl₂. More importantly, the blocking temperature increased from lower than 2 K for Er₂Cl₂ to 8 K for Er₂Cl₃, and the magnetic hysteresis loops at 2 K changed from butterfly-shaped for Er₂Cl₂ to open hysteresis loop with coercive force of 7 kOe for Er₂Cl₃. These results suggest that the properties of SMMs can be effectively tuned and improved by rationally arranging magnetic spins via molecular engineering.