Synthesis, crystal structures and magnetic properties of single and double cyanide-bridged bimetallic (Fe2CuII)-Cu-III zigzag chains

The bimetallic complexes [{Fe-III(phen)(CN)(4)}(2)Cu-II(H2O)(2)].4H(2)O (1), [{Fe-III(phen)(CN)(4)}(2)Cu-II].H2O (2) and [{Fe-III(bipy)(CN)(4)}(2)Cu-II].2H(2)O (3) and [{Fe-III(bipy)(CN)(4)}(2)Cu-II(H2O)(2)].4H(2)O (4) (phen=1,10-phenanthroline and bipy=2,2'-bipyridine) have been prepared and the structures of 1-3 determined by X-ray diffraction. The structure of 1 is made up of neutral cyanide-bridged Fe-III-Cu-II zigzag chains of formula [{Fe-III(phen)(CN)(4)}(2)Cu-II(H2O)(2)] and uncoordinated water molecules with the [Fe(phen)(CN)(4)](-) entity acting as a bis-monodentate bridging ligand toward two trans-diaquacopper(II) units through two of its four cyanide groups in cis positions. The structure of 2 can be viewed as the condensation of two chains of 1 connected through single cyanide-bridged Fe-III-Cu-II pairs after removal of the two axially coordinated water molecules of the copper atom. The structure of 3 is like that of 2, the main differences being the occurrence of bipy (phen in 2) and two (one in 2) crystallization water molecules. The crystals of 4 diffract poorly but the analysis of the limited set of diffraction data shows a chain structure like that of 1 the most important difference being the fact that elongation axis at the copper atom is defined by the two trans coordinated water molecules. 1 behaves as a ferromagnetic (Fe2CuII)-Cu-III trinuclear system. A metamagnetic-like behavior is observed for 2 and 3, the value of the critical field (H-c) being ca. 1100 (2) and 900 Oe (3). For H>H-c the ferromagnetic (Fe2CuII)-Cu-III chains exhibit frequency dependence of the out-of-phase ac susceptibility signal at T<4.0 K. The magnetic behavior of 4 corresponds to that of a ferromagnetically coupled chain of low spin iron(III) and copper(II) ions with frequency dependence of the out-of-phase susceptibility at T<3.0 K. Theoretical calculations using methods based on density functional theory (DFT) have been employed to analyze and substantiate the exchange pathways in this family of complexes.