Synthesis, crystal structures and magnetic properties of single and double cyanide-bridged bimetallic Fe2IIICuII zigzag chains

Abstract

The bimetallic complexes [{Fe^III(phen)(CN)₄}₂Cu^II(H₂O)₂]·4H₂O (1), [{Fe^III(phen)(CN)₄}₂Cu^II]·H₂O (2) and [{Fe^III(bipy)(CN)₄}₂Cu^II]·2H₂O (3) and [{Fe^III(bipy)(CN)₄}₂Cu^II(H₂O)₂]·4H₂O (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}₂Cu^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 Fe^III₂Cu^II 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 Fe^III₂Cu^II 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.