Pushing Boundaries in Single Molecule Magnets: An Ab Initio Perspective on Harnessing Higher Oxidation States for Unprecedented Lanthanide SMM Performance

The recent breakthrough of attaining blocking temperature near liquid N2 temperature rekindled the interest in lanthanide-based Single Molecule Magnets (SMMs) towards end-user applications. Within this realm, several challenges are present, with a key objective being the further enhancement of the blocking temperature. As the current set of molecules based on DyIII has already reached their maximum potential barrier height for magnetization reversal (Ueff), chemical insights-based developments are hampered. In this connection, using Density Functional Theory (DFT) and ab initio Complete Active Space Self-Consistent Field (CASSCF) methods, we have explored the possibility of obtaining lanthanide SMMs in high-valent oxidation states such as +4 and +5. We begin with various small models of [LnO2]+, [LnO2], and [LnO2]− (Ln varying from Ce to Lu) systems to correlate the nature of the lanthanides to the SMM characteristics. We have also extended our study to include five complexes reported earlier possessing +4 and +5 oxidation states to offer clues to improve the SMM characteristics. Our calculations reveal several advantages in fine-tuning the oxidation states in lanthanide SMMs, and this includes (i) the lanthanide-ligand covalency found to increase as compared to the LnIII counterpart (ii) yield barrier height for magnetization reversal as high as 8424 cm−1, an unprecedented value among any models reported (iii) among various ways to stabilize such high-oxidation states, encapsulation yields several targets, with HoO2@SWCNT(4,4) predicted to yield an impressive energy barrier of ~5400 cm⁻¹ (iv) stronger lanthanide-ligand bonds were also found to quench spin-phonon relaxation, as they offset the vibrations that cause this relaxation. This potentially yields larger blocking temperatures, offering a novel strategy for a new class of lanthanide SMMs.