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µ
eff
(
b
)
µ
eff
(
b
)
3,0
3,0
2,5
2,5
Cu(hfac)
2
L
48
Cu(hfac)
2
L
49
2,0
2,0
Cu(hfac)
2
L
50
Cu(hfac)
2
L
40
T (K)
T, K
0
100
200
300
0
100
200
300
(a)
(b)
Figure13.8
TemperaturedependencesoftheeffectivemagneticmomentforCu(hfac)
2
L
48
andCu(hfac)
2
L
49
(a),
Cu(hfac)
2
L
40
andCu(hfac)
2
L
50
(b).
obtained by varying the conditions of synthesis (solvent, temperature, and reagent ratio). As an extreme
example, 12 different phases were obtained and structurally defined in the reaction of Cu(hfac)
2
with L
48
,
but only two of these exhibited magnetic anomalies on the
eff
(T) curve.
79
Another difficulty was isolation
of single crystals of two or more phases. Sometimes they could be separated mechanically. However, in the
reactions of Cu(hfac)
2
with L
48
, the single crystals of different phases were occasionally indistinguishable
in both color and habit. Therefore, each single crystal had to be checked by X-ray diffraction analysis
before magnetochemical experiment with crystals of the same phase.
Figure 13.8 shows the experimental temperature dependences of
µ
µ
eff
for Cu(hfac)
2
L
n
39, 40,
48 - 50), which differ in the R substituent bonded to the pyrazole fragment. Although the substituents are
remote (Figure 13.7) from the structurally rearranging
(n
=
N-
•
O-Cu(II) clusters,
they have a substantial effect on the character of the temperature dependence. The
N-
•
O-Cu(II)-O
•
-N
>
<
or
>
µ
eff
(T) dependences
N-
•
O-Cu(II)-O
•
-N
can vary greatly because of different combinations of exchange interactions in the
>
<
N-
•
O-Cu(II) units before and after the phase transition.
12
It should be emphasized that in contrast
to the classical spin crossover, spin transitions here are often accompanied by an abrupt increase in the
effective magnetic moment at decreased temperatures (Figure 13.8a). This is explained by the fact that
the phase transition results in a
lengthening
, not shortening, of Cu-O
NO
distances. As a consequence, the
exchange interaction changes from antiferromagnetic to ferromagnetic, and
or
>
µ
eff
ceases to decrease and
starts to increase at decreased temperatures.
In these systems, spin transitions are sensitive not only to variation of the R substituent in the pyra-
zole ring. The temperature of the transition and the form of the magnetic anomaly can be controlled by
synthesizing mixed metal solid solutions M
x
Cu
1
−
x
(hfac)
2
L(M
Mn, Ni, Co)
219
=
or, which occurs much
more rarely (Figure 13.9), by forming Cu(hfac)
2
L
4
x
L
4
1
-
x
solid solutions containing different nitroxides as
organic ligands.
12
Of great interest was the family of heterospin solvates Cu(hfac)
2
L
40
•
0
pentane, hexane,
heptane, octane, octene, butyl chloride, butyl bromide, butyl iodide, amyl chloride, amyl bromide, amyl
iodide, benzene, toluene,
o
-xylene,
m
-xylene,
p
-xylene, ethylbenzene, propylbenzene), which exhibited spin
transition effects and high mechanical stability after the temperature range of the magnetic anomaly. Studies
of these compounds over a wide range of temperatures (28 - 300 K), that is, before and after the structural
transition and the ensuing magnetic phase transition, showed that the solvent molecules, incorporated in
.
5Solv (Solv
=
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