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 Delta/kB = 24.1 K with tau 0 = 6.96 x 10-8 s for 1, and Delta/kB = 9.88 K with tau 0 = 5.36 x 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 Delta/kB parameters. Theoretical ab initio calculations for complexes 1 and 2 were performed to investigate their magnetic properties in detail. Dinuclear Ni(ii)/Ce(iii) complexes [Ni(o-van-dap)Ce(H2O)(NO3)3] (1) and [Ni(H2O)2(o-van-dmdap)Ce(NO3)3] (2): examples of tuning magnetic properties via a small structural change within the ligands used.