Cyanide-Bridged Iron−Copper Molecular Squares with Doublet and Quintet Spin Ground States

Reactions of [FeII(CN)2(bpy)2] and [FeIII(CN)2(bpy)2](PF6) with [CuII(bpy)(CH3OH)2](PF6)2 in methanol yielded
cyanide-bridged cyclic tetranuclear complexes of [FeII2CuII2(μ-CN)4(bpy)6](PF6)4·2H2O·4CHCl3 (1) and [FeIII2CuII2(μ-CN)4(bpy)6](PF6)6·4CH3CN·2CHCl3 (2), respectively. In the squares of 1 and 2, the Fe2+/3+ (low-spin)
and Cu2+ ions are alternately bridged by the cyanide ions, the carbon atoms of which coordinate to the iron ions.
Variable-temperature magnetic susceptibility studies of complexes revealed that the Cu2+ ions in 1 are magnetically
isolated. In the square of 2, the adjacent Fe3+ and Cu2+ ions were ferromagnetically coupled through the cyanide
bridges with a J1 value of +6.3(1) cm-1 and the weaker antiferromagnetic interactions between the orthogonal
Fe3+···Fe3+ and Cu2+···Cu2+ pairs are operative (J2 = −3.1(1) cm-1). The propagation of the ferromagnetic
interaction in 2 can be understood by the orthogonal magnetic orbitals of the low-spin Fe3+ (dπ) and Cu2+ (dσ)
ions. Cyclic voltammograms of the squares have been recorded and discussed. Crystal data for 1:  triclinic space
group P1̄, a = 14.491(4) Å, b = 15.312(4) Å, c = 12.959(3) Å, α = 91.38(2)°, β = 109.56(2)°, γ = 65.41(2)°,
Z = 1. Crystal data for 2:  triclinic space group P1̄, a = 14.472(9) Å, b = 15.310(6) Å, c = 12.523(5) Å, α =
102.58(3)°, β = 107.19(4)°, γ = 75.08(4)°, Z = 1.