Pushing Boundaries in Single Molecule Magnets: An Ab Initio Perspective on Harnessing Unusual 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 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 Dy(III) has already reached their maximum potential barrier height for magnetization reversal (Ueff), chemical insights-based developments are hampered. In this connection, using DFT and ab initio CASSCF methods, we have explored the possibility of obtaining lanthanide SMMs in unusual oxidation states such as +4 and +5. We are encouraged by the fact that several such complexes are already reported, with some of them found to exhibit performant SMM characteristics. 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 state in lanthanide SMMs, and this includes (i) the lanthanide-ligand covalency found to increase as high as 45% compared to the LnIII counterpart (ii) yield barrier height for magnetization reversal as high as 8500 cm-1, an unprecedented tuning up to three times larger compared to the best-in-class LnIII counterpart (iii) among various ways to stabilise such high-oxidation state including encapsulation yield several targets with HoO2@SWCNT(4,4) predicted to yield an impressive energy barrier of ~5400 cm−1 (iv) the stronger lanthanide-ligand bonds also found to help in tuning the spin-phonon relaxation as stronger bonds found to offset the vibrations that cause the relaxation, potentially yield larger blocking temperatures - offering a never-before-seen strategy to new class lanthanide SMMs.