Pushing boundaries in single molecule magnets: an ab initio perspective on harnessing higher oxidation states for unprecedented lanthanide SMM performance

Abstract

The recent breakthrough of attaining a blocking temperature near liquid N₂ temperature rekindled the interest in lanthanide-based single molecule magnets (SMMs) for 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 Dy^III have already reached their maximum potential barrier height for magnetisation reversal (U_eff), chemical insight-based developments are hampered. To address these challenges, 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 eight 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, including the following: (i) increased lanthanide–ligand covalency compared to the Ln^III counterpart, (ii) a magnetisation reversal barrier height as high as 8424 cm⁻¹, an unprecedented value compared to any models reported, (iii) among various ways to stabilise such high-oxidation states, encapsulation yielding several targets, with HoO₂@SWCNT(4,4) predicted to yield an impressive energy barrier of ∼5400 cm⁻¹ and (iv) stronger lanthanide–ligand bonds that were also found to quench spin–phonon relaxation, as they offset the vibrations that cause this relaxation. These potentially yield higher blocking temperatures, offering a novel strategy for developing a new class of lanthanide SMMs.