Cyano-bridged {Ln2IIIFe2III} 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 {Fe^III₂Ln^III₂} complexes (Ln = Gd, Tb, Dy, Ho, and Er) of square-like topology that crystallize in the triclinic P space group. Solid-state direct-current magnetic susceptibility analysis evidenced weak intramolecular antiferromagnetic Fe^III–Ln^III interactions in 1 (Ln = Gd) together with large local magnetic anisotropies from the Ln^III ion in 2–5 (Ln = Tb, Dy, Ho, and Er). Frequency-dependent 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 Ln^III 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 [−ΔS_max = 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 T_opt = T_max = 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 H_opt = H_max = 1.0 (1), 0.6 (2), 0.4 (3), 0.8 (4), and 0.6 T (5) at T = 2 K].