A soft phosphorus atom to "harden" an erbium(iii) single-ion magnet.

Beyond the lanthanide organometallic single-ion magnet (SIM) (Cp*)Er(COT)1 ([Cp*]- = pentamethylcyclopentadienide; COT2- = cyclooctatetraenide) that has a good performance, we managed to replace one coordinated carbon atom on the [Cp*]- ring by a soft phosphorus atom and obtained (Dsp)Er(COT) (CCDC No. ; 1835955; Dsp- = 3,4-dimethyl-2,5-bis(trimethylsilyl)phospholyl) whose sandwich structure is reported here for the first time. This substitution results in a remarkable change of magnetic dynamics. It exhibits slow magnetic relaxation under a zero applied direct current (DC) magnetic field with an energy barrier (Δ/k B) of 358 K and magnetic hysteresis up to 9 K, both of which are higher than those of (Cp*)Er(COT). With the descended local symmetry of (Dsp)Er(COT), the energy barrier and blocking temperature (T B) both improve unexpectedly and are among the highest ones in Er(iii)-based single-molecule magnets (SMMs).