A Novel Series of Heterotrinuclear Complexes Involving Imidazolate Bridging Schiff's Base Ligands. Synthesis, Crystal Structure, and Magnetic Properties

Mononuclear copper(II) complexes with an unsymmetrical
tridentate Schiff's base involving two imidazole
moieties,
[Cu(HL)(H2O)](ClO4)2
(1) and [Cu(L)(hfac)] (3), have been
prepared and characterized, where HL stands for the
1:1 condensation product of 2-imidazolecarboxaldehyde and histamine.
Under basic conditions, 1 easily undergoes
a self-assembly process affording an insoluble imidazolate-bridged
polymeric species
{[Cu(L)(H2O)](ClO4)}n
(2). By using 1 and 3 as
“ligand complexes” toward M(hfac)2 (hfac =
hexafluoroacetylacetonate) a series of
imidazolate-bridged heterometal trinuclear complexes with the formula
[Cu(L)(hfac)M(hfac)2Cu(hfac)(L)]
(M =
ZnII, CuII, NiII, MnII)
(4−7) can be prepared. The structures of
these complexes have been solved by X-ray
crystallographic methods. All the trinuclear complexes are
isostructural and consist of trinuclear molecules with
a V-shaped conformation, which are formed by two CuL(hfac)
“ligands” bonded through imidazolate bridges to
the central metal in cis-position. The ligand environment about
the central metal atom can be described as a
distorted octahedron, with the nitrogen atoms from the imidazolate
groups and two oxygen atoms of the hfac
ligands in the equatorial plane and the remaining two oxygen atoms from
the hfac ligands filling the axial positions
at longer distances. The coordination geometry of the
copper(II) ion is very close to a square pyramid with
one
oxygen atom from the hfac in a more distant axial position. The
magnetic susceptibility data (2−300 K) revealed
antiferromagnetic interactions between copper(II) ions and the
central metal. The magnetic susceptibility data
were quantitatively analyzed using the theoretical expressions deduced
from the spin Hamiltonian for a symmetrical
three-spin system H =
−JCuM(SCu1·SM
+ SM·SCu2) to give
the coupling parameters JCuCu = −60.6
cm-1, JCuNi
=
−21.6 cm-1, and
JCuMn = −3.2 cm-1.
These magnetic behaviors are discussed on the basis of the
localized-orbital model of exchange interactions.