Chemistry Reference
In-Depth Information
respectively); both solvated structures contain cofacial
dimers of
80
+
radical cation, and are thus dia-
magnetic; non-coordinated counterions and solvent molecules complete both lattices. However, the solvent
molecules can be removed by applying a gentle vacuum, leading to materials which are paramagnetic and
ferromagnetically order at the relatively high temperatures of 6.7 K (GaCl
4
)
254
π
.
256
The
structure of the desolvated FeCl
4
salt is not known but, based on the paramagnetism of the sample, must
consist of monomeric
80
+
molecules (the high spin iron(III) most likely makes an important contribution
to the bulk magnetic properties as well). A structure of a desolvated GaCl
4
−
salt (prepared by crystal-
lization from a solvent system not containing acetonitrile) indeed contains
monomeric
radicals and not
and 44 K (FeCl
4
)
π
dimers.
255
This material has a T
C
of 7.0 K and, as such, may well be the same structure as the one obtained
by removal of acetonitrile from the solvated structure. The related GaBr
4
−
salt of
80
+
(no solvent) also
consists of monomeric radical cations; however, the solid state packing in this structure is different from
that of the tetrachlorogallate, and the ferromagnetic ordering temperature is lower (0.4 K).
257
Although the bis(1,2,3-dithiazole) radical family
97a-d
was initially designed and pursued for possible
uses as
conducting
materials (Section 9.4.1), their magnetic properties have turned out to be among the
more striking among all molecular radical systems. As can be seen from Table 9.3, several derivatives
have magnetic ordering temperatures above 10 K. Three derivatives (
157
,
189
158
,
189
161
222
) order anti-
ferromagnetically but, like 1,2,3,5-dithiadiazolyl
155
, are spin-canted and, as such, are described as weak
ferromagnets. Genuine ferromagnetic ordering has been established for four of these radicals (
159
,
221,222,224
160
,
222
162
,
224
163
224
); all four have ordering temperatures which rank among the very highest for molec-
ular systems. Interestingly all of the magnetically ordered derivatives contain selenium (the corollary of
which is that the all-sulfur radicals
97a
do not magnetically order). Heavy atom incorporation introduces
significant spin orbit coupling effects, which are believed to contribute to both the spin canting
189
in
157,
0.8
159
0.4
0.0
−
0.4
0.8
−
0.8
160
0.4
0.0
−
0.4
−
0.8
−
3
−
2
−
10
H
(kOe)
1
2
3
Figure9.32
Magnetization vs magnetic field hysteresis loops for
159
and
160
.
222
(Reprinted with permission
from[222].Copyright2008AmericanChemicalSociety.)
Search WWH ::
Custom Search