Cyano-bridged {Ln2 IIIFe2 III} molecular squares (Ln = Gd, Tb, Dy, Ho, and Er) :tuning the slow magnetic relaxation and magnetocaloric effects in zero-dimensional lanthanide Prussian blue analogues

Abstract

An isostructural series of neutral cyano-bridged tetranuclear iron(III)–lanthanide(III) complexes of general formula {[Fe(htpzb)(CN)(μ-CN)2]2[Ln(dmbpy)(NO3)2(H2O)]2}·2CH3CN·2H2O [Ln = Gd (1), Tb (2), Dy (3), Ho (4), and Er (5); htpzb = hydrotris(pyrazolyl)borate and dmbpy = 4,4’-dimethyl-2,2’-bipyridine] was synthesized and structurally and magnetically characterized. Single-crystal X-ray analysis of 1–5 revealed the formation of neutral cyano-bridged {FeIII 2 LnIII 2 } complexes (Ln = Gd, Tb, Dy, Ho, and Er) of square-like topology that crystallize in the triclinic P1¯ space group. Solid-state direct-current magnetic susceptibility analysis evidenced weak intramolecular antiferromagnetic FeIII–LnIII interactions in 1 (Ln = Gd) together with large local magnetic anisotropies from the LnIII ion in 2–5 (Ln = Tb, Dy, Ho, and Er). Frequencydependent alternating current magnetic susceptibility signals occurred for 1–5 under an applied dc magnetic field of H = 1.0 (1) or 0.5 T (2–5), indicating field-induced slow magnetic relaxation effects typical of single-molecule magnets. Depending on the non-Kramer (Tb, Ho) or Kramer (Gd, Dy, Er) nature of the LnIII ion, a single magnetic relaxation process via Orbach or Raman mechanism (2 and 4) or a multiple magnetic relaxation process that combines Orbach or Raman plus quantum tunneling of magnetization and/or direct (1, 3, and 5) mechanisms occurred along this series. 1–5 showed large magnetocaloric effects with a high to moderate maximum value of the magnetic entropy change at optimum working temperatures just above He liquefaction [−ΔSmax = 16.51 (1), 5.42 (2), 6.02 (3), 4.56 (4), and 5.86 J kg−1 K−1 (5) for H = 5 T at Topt = Tmax = 2 (1), 4 (2, 3 and 5), and 6 K (4)], as well as a high to moderate magnetocaloric index at rather low optimum working fields [MCI = 6.4 (1), 3.3 (2), 4.7 (3), 0.9 (4), and 3.6 J kg−1 K−1 T−1 (5) for Hopt = Hmax = 1.0 (1), 0.6 (2), 0.4 (3), 0.8 (4), and 0.6 T (5) at T = 2 K].