Structure-property trends in cyanido-bridged tetranuclear FeIII/NiII single-molecule magnets

Treatment of [NEt4][(Tp*(Me))Fe-III(CN)(3)]center dot H2O with nickel(II) trifluoromethanesulfonate affords {[(TP*(Me))Fe-III(CN)(3)](2)[Ni-II(DMF)(4)](2)[OTf](2)}center dot 2DMF (1) while {[(TP*(Me))Fe-III(CN)(3)](2)[Ni-II(bPY)(2)](2)[ClO4](2)}center dot 3MeCN center dot 2H(2)O center dot MeOH (2) is obtained from Ni(ClO4)(2)center dot 6H(2)O and 2,2'-bipyridine mixtures. In the frame of an isotropic Heisenberg model, the experimental chi T versus T data were modeled well with the following best set of parameters: J/k(B) = +9.0(4) and +8.5(4) K and g(av) = 2.4(1) and 2.5(1) for 1 and 2, respectively; the first excited state (S = 2) for 1 and 2 are ca. 18 and 16.8 K above the S-T = 3 ground state. Analysis of the ac susceptibility suggests that 1 exhibits fast quantum tunneling of the magnetization above ca. 1.8 K while 2 displays slow relaxation in the range seen for many SMMs; at H-dc = 2.2 kOe an SMM energy barrier of Delta(eff) = 15.7 K is estimated for 2. Upon prolonged standing in air, 1 is readily transformed into a new system that exhibits a respectable energy barrier (Delta(eff) = 20.4 K) suggesting that the desolvation is able to dramatically alter the dynamics and the quantum properties of the square-shaped {Fe-2(III)(mu-CN)(4)Ni-2(II)}SMM. (C) 2012 Elsevier Ltd. All rights reserved.