[Fe(bipy)(CN)₄]^- as a Versatile Building Block for the Design of Heterometallic Systems:  Synthesis, Crystal Structure, and Magnetic Properties of PPh₄[Fe^III(bipy)(CN)₄]·H₂O, [{Fe^III(bipy)(CN)₄}₂M^II(H₂O)₄]·4H₂O, and [{Fe^III(bipy)(CN)₄}₂Zn^II]·2H₂O [bipy = 2,2‘-Bipyridine; M = Mn and Zn]

The new cyano complexes of formulas PPh₄[Fe^III(bipy)(CN)₄]·H₂O (<b>1</b>), [{Fe^III(bipy)(CN)₄}₂M^II(H₂O)₄]·4H₂O with M = Mn (<b>2</b>) and Zn (<b>3</b>), and [{Fe^III(bipy)(CN)₄}₂Zn^II]·2H₂O (<b>4</b>) [bipy = 2,2‘-bipyridine and PPh₄ = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex <b>1</b> is made up of mononuclear [Fe(bipy)(CN)₄]^- anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes <b>2</b> and <b>3</b> consists of neutral centrosymmetric [{Fe^III(bipy)(CN)₄}₂M^II(H₂O)₄] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)₄]^- entity of <b>1</b> is present in <b>2</b> and <b>3</b> acting as a monodentate ligand toward M(H₂O)₄ units [M = Mn(II) (<b>2</b>) and Zn(II) (<b>3</b>)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)₄]^- units in trans positions build a distorted octahedron surrounding Mn(II) (<b>2</b>) and Zn(II) (<b>3</b>). The structure of the [Fe(phen)(CN)₄]^- complex ligand in <b>2</b> and <b>3</b> is close to that of the one in <b>1</b>. The intramolecular Fe−M distances are 5.126(1) and 5.018(1) Å in <b>2</b> and <b>3</b>, respectively. <b>4</b> exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)₄]^- units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two <i>cis</i>-cyanide (Fe(1)) and three <i>fac</i>-cyanide (Fe(2)) groups. The environment of the iron atoms in <b>4</b> is distorted octahedral as in <b>1</b><b>−</b><b>3</b>, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron−zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) Å at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) Å at Fe(2). The magnetic properties of <b>1</b><b>−</b><b>3</b> have been investigated in the temperature range 2.0−300 K. <b>1</b> is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of <b>3</b> correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the −CN−Zn−NC− bridging skeleton (iron−iron separation larger than 10 Å) being very weak. More interestingly, <b>2</b> exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.