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

Abstract A new single‐molecule magnet (SMM) complex [K(18‐crown‐6)][(COT)Er( µ ‐Cl) ₃ Er(COT)] ( Er ₂ Cl ₃ , COT = cyclooctatetraenide dianion) is obtained by the reaction of [(COT)Er( µ ‐Cl)(THF)] ₂ ( Er ₂ Cl ₂ , 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 Er ₂ Cl ₂ to 264(4) K for Er ₂ Cl ₃ and extending its magnetic relaxation time at 2 K by 2500 times with respect to Er ₂ Cl ₂ . More importantly, the blocking temperature increased from lower than 2 K for Er ₂ Cl ₂ to 8 K for Er ₂ Cl ₃ , and the magnetic hysteresis loops at 2 K changed from butterfly‐shaped for Er ₂ Cl ₂ to open hysteresis loop with coercive force of 7 kOe for Er ₂ Cl ₃ . These results suggest that the properties of SMMs can be effectively tuned and improved by rationally arranging magnetic spins via molecular engineering.