Structure, Magnetic Behavior, and Anisotropy of Homoleptic Trinuclear Lanthanoid 8‑Quinolinolate Complexes

Three
complexes of the form [LnIII3(OQ)9] (Ln = Gd, Tb, Dy; OQ = 8-quinolinolate) have been synthesized and
their magnetic properties studied. The trinuclear complexes adopt
V-shaped geometries with three bridging 8-quinolinolate oxygen atoms
between the central and peripheral eight-coordinate metal atoms. The
magnetic properties of these three complexes differ greatly. Variable-temperature
direct-current (dc) magnetic susceptibility measurements reveal that
the gadolinium and terbium complexes display weak antiferromagnetic
nearest-neighbor magnetic exchange interactions. This was quantified
in the isotropic gadolinium case with an exchangecoupling parameter
of J = −0.068(2) cm–1. The
dysprosium compound displays weak ferromagnetic exchange. Variable-frequency
and -temperature alternating-current magnetic susceptibility measurements
on the anisotropic cases reveal that the dysprosium complex displays
single-molecule-magnet behavior, in zero dc field, with two distinct
relaxation modes of differing time scales within the same molecule.
Analysis of the data revealed anisotropy barriers of Ueff = 92 and 48 K for the two processes. The terbium complex,
on the other hand, displays no such behavior in zero dc field, but
upon application of a static dc field, slow magnetic relaxation can
be observed. Ab initio and electrostatic calculations were used in
an attempt to explain the origin of the experimentally observed slow
relaxation of the magnetization for the dysprosium complex.