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Abstract:
The trinuclear and the tetranuclear complexes
[{iPrtacnCr(CN)(3)}(2){Ni(cyclam)}](NO3)(2) center dot 5H(2)O 1 (
cyclam = 1,4,8,11-tetraazacyclotetradecane, iPrtacn = 1,4,7-
tris-isopropyl-1,4,7-triazacyclononane) and
[{iPrtacnCr(CN)(3)Ni(Me(2)bpy)(2)}(2)](ClO4)(4) center dot 2CH(3)CN 2
(Me(2)bpy = 4,4'-dimethyl-2,2'- bipyridine) were synthesized by
reacting (iPrtacn) Cr(CN)(3) with [Ni(cyclam)](NO3)(2) and
[Ni(Me(2)bpy)(2)(H2O)(2)](ClO4)(2),, respectively. The crystallographic
structure of the two compounds was solved. The molecular structure of
complex 1 consists of a linear Cr - Ni - Cr arrangement with a central
Ni( cyclam) unit surrounded by two Cr( iPrtacn)( CN)(3) molecules
through bridging cyanides. Each peripheral chromium complex has two
pending CN ligands. Complex 2 has a square planar arrangement with the
metal ions occupying the vertices of the square. Each Cr( iPrtacn)( CN)
3 molecule has two bridging and one non-bridging cyanide ligands. The
magnetic properties of the two complexes were investigated by
susceptibility vs. temperature and magnetization vs. field studies. As
expected from the orthogonality of the magnetic orbitals between Cr-III
(t(2g) (3)) and Ni-II (e(g) (2)) metal ions, a ferromagnetic exchange
interaction occurs leading to a spin ground states S = 4 and 5 for 1
and 2, respectively. The magnetization vs. field studies at T = 2, 3
and 4 K showed the presence of a magnetic anisotropy within the ground
spin states leading to zero-field splitting parameters obtained by
fitting the data D-4 = 0.36 cm(-1) and D-5 = 0.19 cm(-1) ( the indices
4 and 5 refer to the ground states of complexes 1 and 2, respectively).
In order to quantify precisely the magnitude of the axial ( D) and the
rhombic ( E) anisotropy parameters, High-field high frequency electron
paramagnetic resonance ( HF-HFEPR) experiments were carried out. The
best simulation of the experimental spectra ( at 190 and 285 GHz) gave
the following parameters for 1: D-4 = 0.312 cm(-1), E-4/D-4 = 0.01,
g(4x) = 2.003, g(4y) = 2.017 and g(4z) = 2.015. For complex 2 two sets
of parameters could be extracted from the EPR spectra because a
doubling of the resonances were observed and assigned to the presence
of complexes with slightly different structures at low temperature: D-5
= 0.154 (0.13) cm(-1), E-5/D-5 = 0.31 (0.31) cm(-1), g(4x) = 2.04
(2.05), g(4y) = 2.05 (2.05) and g(4z) = 2.03 (2.02). The knowledge of
the magnetic anisotropy parameters of the mononuclear
Cr(iPrtacn)(CN)(3), Ni(cyclam)(NCS)(2) and Ni(bpy)(2)(NCS)(2) complexes
by combining HF-HFEPR studies and calculation using a software based on
the angular overlap model (AOM) allowed to determine the orientation of
the local D tensors of the metal ions forming the polynuclear
complexes. We, subsequently, show that the anisotropy parameters of the
polynuclear complexes computed from the projection of the local tensors
are in excellent agreement with the experimental ones extracted from
the EPR experiments.
