Mechanism of magnetisation relaxation in {M III2Dy III2} (M = Cr, Mn, Fe, Al) “Butterfly” complexes: how important are the transition metal ions here?

Abstract

We describe the synthesis, characterisation and magnetic studies of four tetranuclear, isostructural “butterfly” heterometallic complexes: [M^III₂Ln^III₂(μ₃-OH)₂(p-Me-PhCO₂)₆(L)₂] (H₂L = 2,2′-((pyridin-2-ylmethyl)azanediyl)bis(ethan-1-ol), M = Cr, Ln = Dy (1), Y (2), M = Mn, Ln = Dy (3), Y (4)), which extend our previous study on the analogous 5 {Fe₂Dy₂}, 6 {Fe₂Y₂} and 7 {Al₂Dy₂} compounds. We also present data on the yttrium diluted 7 {Al₂Dy₂}: 8 {Al₂Dy_0.18Y_1.82}. Compounds dc and ac magnetic susceptibility data reveal single-molecule magnet (SMM) behaviour for complex 3 {Mn₂Dy₂}, in the absence of an external magnetic field, with an anisotropy barrier U_eff of 19.2 K, while complex 1 {Cr₂Dy₂}, shows no ac signals even under applied dc field, indicating absence of SMM behaviour. The diluted sample 8 {Al₂Dy_0.18Y_1.82} shows field induced SMM behaviour with an anisotropy barrier U_eff of 69.3 K. Furthermore, the theoretical magnetic properties of [M^III₂Ln^III₂(μ₃-OH)₂(p-Me-PhCO₂)₆(L)₂] (M = Cr, 1 or Mn, 3) and their isostructural complexes: [M^III₂Dy^III₂(μ₃-OH)₂(p-Me-PhCO₂)₆(L)₂] (M = Fe, 5 or Al, 7) are discussed and compared. To understand the experimental observations for this family, DFT and ab initio CASSCF + RASSI-SO calculations were performed. The experimental and theoretical calculations suggest that altering the 3d^III ions can affect the single-ion properties and the nature and the magnitude of the 3d^III–3d^III, 3d^III–Dy^III and Dy^III–Dy^III 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^III ion, on slow relaxation of magnetisation within a series of isostructural 3d–4f butterfly compounds.