Chiral Crystallization of a Heterodinuclear Ni-Ln Series: Comprehensive Analysis of the Magnetic Properties.

Four heterodinuclear (H2O)2NiL-Ln­(NO3)3 complexes (Ln = Tb, Dy, Er, Yb) with a double
phenoxo bridge coming from the dideprotonated Schiff-base ligand are
synthesized and characterized by crystal and powder X-ray diffraction
studies. This series of compounds devoid of any chiral center, crystallizes
in a noncentrosymmetric space group P21, as the previously described (H2O)2NiL-Gd­(NO3)3 equivalent. All four complexes are ferromagnetically
coupled. If this behavior is clearly shown by the χMT increase at low temperature in the case of the
Ni–Tb and Ni–Dy complexes, it necessitates the preparation
of the Zn–Er and Zn–Yb equivalent entities to be evidenced
in the case of the Ni–Er and Ni–Yb complexes. Out-of-phase
susceptibility signals are found in the four cases, but the SMM behavior
is neither confirmed, nor completely studied because of the presence
of fast quantum tunnelling at zero field. Thorough ab initio multiconfiguration
calculations are carried out, achieving a realistic account of ligand
field effects, exchange coupling and magnetic anisotropy in the discussed
systems. The calculations reveal the ferromagnetic intercenter exchange
coupling, the interplay with spin–orbit effects leading to
a Ising-like scheme of the lowest levels. The ab initio simulation
of the magnetic susceptibility is in semiquantitative agreement with
experimental data, certifying the reasonableness of the theoretical
treatments in obtaining valuable information for the interacting mechanisms.
The anisotropy is accounted for by drawing polar diagrams of state-specific
magnetization functions, obtained by handling of the data resulting
from ab initio calculations including the spin–orbit effects.
Supplementary, Density Functional Theory (DFT) calculations are carried
out, presenting new methodological clues and assessments. The DFT
is not perfectly adequate for lanthanide systems because of orbital
pseudodegeneracy issues. However, we show that in particular circumstances
the DFT can be partly used, succeeding here in mimicking different
orbital configurations of the Ni–Tb system. The DFT seems to
offer reasonable estimations of exchange coupling parameters, while
it remains problematic in the complete account of Ligand Field splitting.
The Paper presents unprecedented methodological advances and correlations
with phenomenological and heuristic interpretation of experimental
data, taking into focus relevant d-f systems constructed with a prototypical
binucleating ligand.