On Approaching the Limit of Molecular Magnetic Anisotropy: A Near‐Perfect Pentagonal Bipyramidal Dysprosium(III) Single‐Molecule Magnet

Abstract We report a monometallic dysprosium complex, [Dy(O t Bu) 2 (py) 5 ][BPh 4 ] ( 5 ), that shows the largest effective energy barrier to magnetic relaxation of U eff =1815(1) K. The massive magnetic anisotropy is due to bis‐trans‐disposed tert‐butoxide ligands with weak equatorial pyridine donors, approaching proposed schemes for high‐temperature single‐molecule magnets (SMMs). The blocking temperature, T B  , is 14 K, defined by zero‐field‐cooled magnetization experiments, and is the largest for any monometallic complex and equal with the current record for [Tb 2 N 2 N(SiMe 3 ) 2 4 (THF) 2 ].