Two Ni/Ce complexes based on bicompartmental ligands with field supported slow magnetic relaxation†

Dinuclear Ni(II)/Ce(III) complexes, namely [Ni(o-van-dap)Ce(H₂O)(NO₃)₃] (1) and [Ni(H₂O)₂(o-van-dmdap)Ce(NO₃)₃] (2), were synthesized and chemically and spectroscopically characterized. Single crystal X-ray analyses revealed that in both 1 and 2 bicompartmental Schiff base-type ligands (o-van-dap)²⁻ and (o-van-dmdap)²⁻ are present, with neighbouring cis-N₂O₂ and O₄ donor sets. The smaller N₂O₂ cavity is occupied by the Ni(II) ions while the larger O₄ cavity contains a Ce(III) ion; both metal ions are linked via a pair of monoatomic O-bridges with the O atoms provided by deprotonated hydroxyl groups. In complex 1 the bridging unit within the Schiff base-type ligand is formed from two carbon atoms; consequently the central Ni(II) atom has a square-planar geometry, and is thus diamagnetic. In complex 2 the bridging unit within the structure of the ligand is composed of three carbon atoms yielding a larger cavity accommodating a paramagnetic, hexacoordinated Ni(II) atom, whose coordination sphere is completed by two axial aqua ligands. The Ce(III) atoms in both 1 and 2 are additionally coordinated by three chelating nitrato co-ligands resulting in coordination number 10 in 2, while another aqua ligand increases the coordination number of the Ce(III) atom in 1 to 11. Variable temperature dc magnetic studies of both 1 and 2 (2–300 K) corroborate the diamagnetism of the Ni(II) atom (S = 0) in 1 and paramagnetism of the Ni(II) atom (S = 1) in 2. The ac magnetic studies showed that both complexes 1 and 2 exhibit field induced slow magnetic relaxation. The obtained values of the energy barriers are Δ/k_B = 24.1 K with τ₀ = 6.96 × 10⁻⁸ s for 1, and Δ/k_B = 9.88 K with τ₀ = 5.36 × 10⁻⁷ s for 2. One-atomic elongation of the bridging unit in the bicompartmental ligand induces change of the magnetic character of the Ni(II) central atoms from S = 0 in 1 to S = 1 in 2, and this change leads to significant differences in the values of the respective Δ/k_B parameters. Theoretical ab initio calculations for complexes 1 and 2 were performed to investigate their magnetic properties in detail.