posted on 2020-12-17, 18:41authored bySvyatoslav Tolstikov, Irina Golomolzina, Sergey V. Fokin, Artem Bogomyakov, Vitaly Morozov, Sergey Tumanov, Olga Minakova, Sergey Veber, Matvey V. Fedin, Sergey A. Gromilov, Galina V. Romanenko, Victor Ovcharenko
The Cu(hfac)2 complex
with a paramagnetic ligand 2-(1-ethyl-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L5Et) was synthesized
in the form of two polymorphs: α-[Cu(hfac)2L5Et] (bluish green crystals) and β-[Cu(hfac)2L5Et] (brown crystals). Despite the structural similarity
of the two modifications, whose solid phases are formed by polymer
chains with cis coordination of the hfac ligands,
their magnetic properties proved quite different. For β-[Cu(hfac)2L5Et], the repeated cooling–heating cycles
at temperatures of 260–180 K lead to a reversible spin transition
β-[Cu(hfac)2L5Et]-HT↔β-[Cu(hfac)2L5Et]-LT, resulting from a considerable change
in the Cu–ONO distances in the Cu–O•–N< heterospin exchange clusters from 2.312(6) Å (HT
form) to 2.011(2) Å (LT form), which leads to a transition of
ferromagnetic to antiferromagnetic exchange in them. For α-[Cu(hfac)2L5Et], the first cooling also leads to a decrease
in μeff from 2.6 μB at 200 K to
0.35 μB at 100 K on the curve of the μeff(T) dependence. However, this is not a
spin transition in the starting α-[Cu(hfac)2L5Et], but, rather, the consequence of the irreversible phase
transformation of α-[Cu(hfac)2L5Et] into
β-[Cu(hfac)2L5Et] induced by the cooling
of the metastable α modification to T <
200 K. When the cooling–heating cycles are repeated further,
the reversible spin transition inherent in β-[Cu(hfac)2L5Et] is reproduced. For this reason, all the μeff values recorded in the range of 200–100 K for the
cooled α-[Cu(hfac)2L5Et] sample are higher
than those for β-[Cu(hfac)2L5Et] up to T ∼ 75 K until the whole starting α modification
transforms into the β form. This effect has never been observed
earlier for transition metal complexes with nitroxides. The experimental
μeff(T) dependence observed during
the first cooling of α-[Cu(hfac)2L5Et]
was called a spin transition in the nascent phase to distinguish it
from the μeff(T) curve for β-[Cu(hfac)2L5Et] corresponding to the typical reversible spin
transition for heterospin complexes of transition metals with organic
radicals. The results of thermomagnetic measurements correlate with
FTIR and EPR data.