Lanthanide-Radical Magnetic Coupling in [LnPc2]0: Competing Exchange Mechanisms Captured via Ab Initio Multi-Reference Calculations

We present a computational investigation of the intramolecular exchange coupling in [LnPc2]0 (Ln = Tb, Dy, Ho, and Er) between the Ln3+ 4f electrons and the spin-1/2 radical on the phthalocyanine ligands. A series of ab initio multi-configurational/multi-reference Complete/Restricted Active Space Self-Consistent-Field calculations (CASSCF/RASSCF), including non-perturbative spin–orbit coupling, were performed on [LnPc2]0 and on the smaller model compound [LnPz2]0. We find that the exchange coupling mechanisms are restricted by symmetry, but also dependent on the spin polarization effect triggered by the Pc2 ligands π–π∗ excitations. The calculated exchange splittings are small, amounting to at most a few cm–1, in disagreement with previous literature reports of strong antiferromagnetic coupling, but in good agreement with recent EPR experiments on [TbPc2]0. Furthermore, the coupling strength is found to decrease from [TbPc2]0 to [ErPc2]0, with decreasing number of unpaired electron spins in the lanthanide ground (Hund’s rule) Russell–Saunders term.