Dramatic anion size effect on structural and magnetic properties within a series of novel antiferromagnetic organometallic radical cation salts:: [Cp2Mo(dmit)+•][X-] (X- = PF6-, AsF6-, SbF6-)

Electrocrystallization of CH2Cl2 solutions of Cp2Mo(dmit), dmit(2-) = 2-thioxo-1,3-dithiole-4,5-dithiolate, in the presence of n-Bu4NX (X- = PF6-, AsF6-, SbF6-) affords the corresponding 1:1 cation radical salts [Cp2Mo(dmit)(+.)][X-]. The three isostructural salts crystallize in the orthorhombic system, space group Cmcm. In the three salts, a second-order transition at a temperature T-1, identified by EPR measurements and low-temperature X-ray Weissenberg photographs reveals also a splitting of some principal reflections and the twinned character of the crystals, whose room temperature structure 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 lower temperature T-2, of first order character, is observed only in the PF6- salt. This triclinic --> triclinic transition leads to a novel superlattice of wave vector q(2) = (0, 1/2, 0) while the q(1)* superlattice is conserved 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 investigated by static susceptibility measurements and Antiferromagnetic Resonance (AFMR) experiments. The detailed analysis of the AFMR rotation patterns confirms the twinned nature and the low temperature triclinic symmetry of the crystals as well as the peculiar behaviour of the PF6- salt, when compared with the two other members of these novel series.