Structure-Property Relationships in Tricyanoferrate(III) Building Blocks and Trinuclear Cyanide-Bridged Complexes

The preparation, structures, and magnetic properties of two tricyanoferrates, [NEt4](2)[(Tp*(Me))Fe-II(CN)(3)]center dot MeCN (1) and [NEt4][(Tp*(Me))Fe-III(CN)(3)]center dot H2O (2), and three trinuclear derivatives, {[(Tp*(Me))Fe-III(CN)(3)](2)[Ni-II(L)(2)]}center dot n(solvent) (L = bpy, 3; tren, 4; DETA, 5) are described. Magnetic measurements show that 2 is an S = (1)/(2) complex [g = 2.65(1)], while 3-5 display S-T = 2 spin ground states, owing to cyanide-mediated ferromagnetic exchange between Fe-LS(III) (S = (1)/(2)) and Ni-II (S = 1) ions. The chi T vs. T data for 3-5 were modelled using an isotropic Heisenberg model and give J/k(B) and g(avg) values of +11.1(1) K and 2.48(1) for 3, 9.1(1) K and 2.6(1) for 4, and +11.2(1) K and 2.41(5) for 5 (with zJ' = -0.28 K). High-field/frequency EPR data indicates significant g-anisotropy in 2 (g(z) = 3.6, g(y) = 2.2, g(x) = 2.0) and [NEt4][(Tp*)Fe-III(CN)(3)]center dot H2O (6) (g(z) = 3.5, g(y) = 2.0, g(x) = 1.3), while 5 shows D = -2.09 cm(-1), E = 0.08 cm(-1), and B-4(0) = -2.3x10(-3) cm(-1) with g(z) = 2.4 and g(x) = g(y) = 1.95. Despite significant magnetic anisotropy in 3-5, frequency-dependent behavior is only seen for 5 above 1.8 K, suggesting that rapid quantum tunnelling of the magnetization is operative.