Dramatic anion size effect on structural and magnetic properties within a series of novel antiferromagnetic organometallic radical cation salts: [Cp2Mo(dmit)(+center dot)][X-] (X- = PF6-, AsF6-, SbF6-)
R. Clerac et al., Dramatic anion size effect on structural and magnetic properties within a series of novel antiferromagnetic organometallic radical cation salts: [Cp2Mo(dmit)(+center dot)][X-] (X- = PF6-, AsF6-, SbF6-), EUR PHY J B, 9(3), 1999, pp. 431-443
Electrocrystallization of CH2Cl2 solutions of Cp2Mo(dmit), dmit(2-) = 2-thi
oxo-1,3-dithiole-4,5-dithiolate, in the presence of n-Bu4NX (X- = PF6-, AsF
6-, SbF6-) affords the corresponding 1:1 cation radical salts [Cp2Mo(dmit)(
+.)][X-]. The three isostructural salts crystallize in the orthorhombic sys
tem, space group Cmcm. In the three salts, a second-order transition at a t
emperature T-1, identified by EPR measurements and low-temperature X-ray We
issenberg photographs reveals also a splitting of some principal reflection
s and the twinned character of the crystals, whose room temperature structu
re may also be described in the monoclinic P2(1)/m space group. The second
order monoclinic --> triclinic transition is associated with the apparition
of a superlattice characterized by the critical wave vector q(1)* = (0, 1/
2, 1/2) when expressed in the monoclinic system. A second transition at a l
ower temperature T-2, of first order character, is observed only in the PF6
- salt. This triclinic --> triclinic transition leads to a novel superlatti
ce of wave vector q(2) = (0, 1/2, 0) while the q(1)* superlattice is conser
ved down to 10 K in the AsF6- and SbF6- salts. Finally, a third transition
to an antiferromagnetic ground state is observed for the three salts and in
vestigated by static susceptibility measurements and Antiferromagnetic Reso
nance (AFMR) experiments. The detailed analysis of the AFMR rotation patter
ns confirms the twinned nature and the low temperature triclinic symmetry o
f the crystals as well as the peculiar behaviour of the PF6- salt, when com
pared with the two other members of these novel series.