Magnetic, EPR, and Computational Insights into ∼D6h Tb(III) and Gd(III) Complexes.

Lanthanide complexes continue to gain significant interest in molecular quantum science due to their rich electronic structures, which can be engineered to support protected spin states relevant for quantum technologies. Carefully tailored ligand fields can give rise to avoided level crossings between crystal-field perturbed states, which may reduce sensitivity to magnetic field fluctuations. Motivated by this, we investigate an axially compressed pseudo-D6h symmetry air-stable macrocyclic Tb(III) complex [TbIII(LN6)(Ph3SiO)2]+ using ab initio calculations, magnetic and EPR measurements. The axially compressed, pseudo-D6h symmetry imposes a ground state that is predominantly composed of mJ = ±6 for the oblate J = 6 Tb(III) ion, with small admixtures from lower |mJ⟩ components. We investigate the hexagonal vs nonhexagonal transverse crystal field contributions to the tunneling gap, calculating different models by selecting from the ab initio crystal field parameters. Our calculations predict a small tunneling gap, ≈1 GHz, and we test this prediction experimentally using multifrequency EPR on La(III)-diluted powder samples. Our EPR results are consistent with the possibility of a low-energy (several GHz) tunneling gap. The Gd(III) analog [GdIII(LN6)(Ph3SiO)2]+ provides an orbital-quenched comparison, and its magnetic data and EPR spectra are well described using the axial zero-field splitting parameter D, demonstrating experimentally the largely axial ligand field in this pseudo-D6h environment.