Mechanism of magnetisation relaxation in {MIII2DyIII2} (M = Cr, Mn, Fe, Al) "Butterfly" complexes: how important are the transition metal ions here?

We describe the synthesis, characterisation and magnetic studies of four tetranuclear, isostructural "butterfly" heterometallic complexes: [MIII2LnIII2(μ-OH)(-Me-PhCO)(L)] (HL = 2,2'-((pyridin-2-ylmethyl)azanediyl)bis(ethan-1-ol), M = Cr, Ln = Dy (), Y (), M = Mn, Ln = Dy (), Y ()), which extend our previous study on the analogous {FeDy}, {FeY} and {AlDy} compounds. We also present data on the yttrium diluted {AlDy}: {AlDyY}. Compounds dc and ac magnetic susceptibility data reveal single-molecule magnet (SMM) behaviour for complex {MnDy}, in the absence of an external magnetic field, with an anisotropy barrier of 19.2 K, while complex {CrDy}, shows no ac signals even under applied dc field, indicating absence of SMM behaviour. The diluted sample {AlDyY} shows field induced SMM behaviour with an anisotropy barrier of 69.3 K. Furthermore, the theoretical magnetic properties of [MIII2LnIII2(μ-OH)(-Me-PhCO)(L)] (M = Cr, or Mn, ) and their isostructural complexes: [MIII2DyIII2(μ-OH)(-Me-PhCO)(L)] (M = Fe, or Al, ) are discussed and compared. To understand the experimental observations for this family, DFT and CASSCF + RASSI-SO calculations were performed. The experimental and theoretical calculations suggest that altering the 3d ions can affect the single-ion properties and the nature and the magnitude of the 3d-3d, 3d-Dy and Dy-Dy magnetic coupling, thus quenching the quantum tunneling of magnetisation (QTM) significantly, thereby improving the SMM properties within this motif. This is the first systematic study looking at variation and therefore role of trivalent transition metal ions, as well as the diamgnetic Al ion, on slow relaxation of magnetisation within a series of isostructural 3d-4f butterfly compounds.