Predetermined Ferromagnetic Coupling via Strict Control of M–O–M Angles

An imidazolidine–phenolate
ligand HL yields quadruple-bridged (μ-NCNimidazolidine)2(μ-Ophenolate)2 ferromagnetic
dinuclear nickel and cobalt complexes. Both kinds of bridges contribute
to the ferromagnetic coupling, but the ferromagnetism of these samples
is mainly ascribed to the double μ-Ophenolate links,
on the basis of density functional theory calculations. These studies
demonstrate not only that the short M–O–M angles of
the M2O2 cores favors the parallel alignment
of the electrons but also that these angles are the optimal ones for
maximizing the ferromagnetic contribution in these complexes. And
these acute angles, close to 90°, are predetermined by the geometrical
constrictions imposed by the ligand itself. Thus, HL is an uncommon
polydentate donor that induces ferromagnetism per se in its metal
complexes by strict control of one geometric parameter, the M–O–M
angle.