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

Treatment of [NEt4][(Tp∗Me)FeIII(CN)3]·H2O with nickel(II) trifluoromethanesulfonate affords {[(Tp∗Me)FeIII(CN)3]2[NiII(DMF)4]2[OTf]2}·2DMF (1) while {[(Tp∗Me)FeIII(CN)3]2[NiII(bpy)2]2[ClO4]2}·3MeCN·2H2O·MeOH (2) is obtained from Ni(ClO4)2·6H2O and 2,2′-bipyridine mixtures. In the frame of an isotropic Heisenberg model, the experimental χT versus T data were modeled well with the following best set of parameters: J/kB=+9.0(4) and +8.5(4)K and gav=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 16.8K above the ST=3 ground state. Analysis of the ac susceptibility suggests that 1 exhibits fast quantum tunneling of the magnetization above ca. 1.8K while 2 displays slow relaxation in the range seen for many SMMs; at Hdc=2.2kOe an SMM energy barrier of Δeff=15.7K is estimated for 2. Upon prolonged standing in air, 1 is readily transformed into a new system that exhibits a respectable energy barrier (Δeff=20.4K) suggesting that the desolvation is able to dramatically alter the dynamics and the quantum properties of the square-shaped {FeIII2(μ-CN)4NiII2}SMM.