A rare case of a zero-field single-ion magnet in a cerium(III) pseudo-icosahedral complex

The synthesis and structural characterisation of [Ln(Tp^2-Fu)₂]I (1-Ln; Ln = La, Ce, Pr, Nd) (Tp^2-Fu = hydrotris(3-(2′-furyl)-pyrazol-1-yl)borate) have been reported as an isomorphous series adopting pseudo-icosahedral ligand field geometries. Continuous shape measurement (CShM) analyses on the crystal field environments of 1-Ln show the smallest values yet reported for complexes employing two hexadentate ligands (e.g. bis-scorpionate environments), with the smallest belonging to 1-La. Single-ion magnetism for 1-Ce, 1-Pr and 1-Nd was probed with ac magnetic susceptibility studies revealing slow magnetic relaxation for 1-Nd in applied magnetic fields and in zero-applied field for 1-Ce, which is a rare observation for Ce(III)-based single-ion magnets. The energy barrier to magnetic relaxation for 1-Ce was experimentally determined to be U_eff = 30(5) cm⁻¹, which is comparable to that of other cerium-based single-molecule magnets in the literature, where these systems stablise the m_J = ±5/2 state and possess large energy gaps between the ground and first excited state that do not agree with the experimentally determined barrier.