New Family of Dinuclear Ni–Ln Complexes from Ternary Coordination by β-Diketonates around the Lanthanoids: Synthesis, Structures, and Magnetic Behavior with Theoretical Supports

A single-pot synthetic endeavor was utilized with a binucleating phenolate-centered unsymmetrical ligand to obtain two new series of Ni–Ln complexes. The method also facilitated the capping of two types of β-diketonate anions around larger lanthanide ions with vacant coordination sites. Using dibenzoylmethane (dbmH) and benzylacetonate (bzaH) as ancillaries, two distinct groups of complexes, [NiLn(HL)2(dbm)2]NO3 (where Ln = Tb3+ (1a), Dy3+ (1b), and Ho3+ (1c)) and [NiLn(HL)2(bza)2]NO3·xMeOH (where Ln = Gd3+ (2a), x = 3; Tb3+ (2b), x = 0; Dy3+ (2c), x = 0; and Er3+ (2d), x = 0) (H2L = 2-methoxy-6-[(E)-2′-hydroxymethyl-phenyliminomethyl]-phenol), were obtained in good yields. For complexes 1a–2d, two HL– units grab NiII ions first in an elongated octahedral N2O4 geometry with apical coordination of two alcohol OH groups. The LnIII centers were next caged in distorted O8 donor-based square antiprism geometry. Both dc and ac magnetic susceptibility measurements demonstrated (i) the involvement of ferromagnetic interaction between the adjacent nickel(II) (S = 1) and other lanthanoids except HoIII and (ii) a lack of slow magnetic relaxation at 2 K in zero applied dc field. The computational study using ab initio methods involving the CASSCF (Complete Active Space Self-Consistent Field), RASSI-SO (Restricted Active Space State Interaction Spin–Orbit), and SINGLE_ANISO programs, along with the POLY_ANISO program, provides detailed insights into the magnetic properties and relaxation mechanisms of single-ion systems, particularly in lanthanide complexes. Theoretical calculations backed the experimental results of the lack of slow relaxation of magnetization for 1a–1c and 2b–2d at zero dc field and lack of prominent maxima for the out-of-phase component of the magnetic susceptibility in the applied dc field. The weak Ni–Ln exchange and the noncollinearity between the anisotropic axes of NiII and LnIII ions led to the significant tunneling gap (Δtun) and the QTM between the ground exchange coupled doublet states in all the complexes that directed the magnetic relaxation even in the ground state.