A new family of trinuclear nickel (II) complexes as single-molecule magnets

Abstract

Three new trinuclear nickel (II) complexes with the general composition [Ni₃L₃(OH)(X)](ClO₄) have been prepared in which X = Cl⁻ (1), OCN⁻ (2) or N₃⁻ (3) and HL is the tridentate N,N,O donor Schiff base ligand 2-[(3-dimethylaminopropylimino)methyl]phenol. Single-crystal structural analyses revealed that all three complexes have a similar Ni₃ core motif with three different types of bridging, namely phenoxido (μ₂ and μ₃), hydroxido (μ₃) and μ₂-Cl (1), μ_1,1-NCO (2) or μ_1,1-N₃ (3). The nickel (II) ions adopt a compressed octahedron geometry. Single-crystal magnetization measurements on complex 1 revealed that the pseudo-three-fold axis of Ni₃ corresponds to a magnetic easy axis, being consistent with the magnetic anisotropy expected from the coordination structure of each nickel ion. Temperature-dependent magnetic measurements indicated ferromagnetic coupling leading to an S = 3 ground state with 2J/k = 17, 17 and 28 K for 1, 2, and 3, respectively, with the nickel atoms in an approximate equilateral triangle. The high-frequency EPR spectra in combination with spin Hamiltonian simulations that include zero-field splitting parameters D_Ni/k = −5, −4 and −4 K for 1, 2, and 3, respectively, reproduced the EPR spectra well after a anisotropic exchange term was introduced. Anisotropic exchange was identified as D_i,j/k = −0.9, −0.8 and −0.8 K for 1, 2 and 3, respectively, whereas no evidence of single-ion rhombic anisotropy was observed spectroscopically. Slow relaxation of the magnetization at low temperatures is evident from the frequency-dependence of the out-of-phase ac susceptibilities. Pulsed-field magnetization recorded at 0.5 K shows clear steps in the hysteresis loop at 0.5–1 T, which has been assigned to quantum tunneling, and is characteristic of single-molecule magnets.