Spectroscopic Investigation of a Metal Metal Bonded Fe6 Single Molecule Magnet with an Isolated S 19 2 Giant Spin Ground State

The metal metal bonded molecule [Bu4N][ HL 2Fe6 dmf 2] Fe6 was previously shown to possess a thermally isolated spin S 19 2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of amp; 8764;5 K [J. Am. Chem. Soc.2015, 137, 13949 13956]. Here, we present a comprehensive spectroscopic investigation of this unique single molecule magnet SMM , combining ultrawideband field swept high field electron paramagnetic resonance EPR with frequency domain Fourier transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe6. Of particular importance is the near absence of a 4th order axial zero field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin spin superexchange interactions in traditional polynuclear SMMs such as the celebrated Mn12 acetate. The combined high resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal metal bonds and strong ferromagnetic spin spin exchange interaction