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

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Abstract
The new cyano complexes of formulas PPh4[FeIII(bipy)(CN)4]·H2O (1), [{FeIII(bipy)(CN)4}2MII(H2O)4]·4H2O with M = Mn (2) and Zn (3), and [{FeIII(bipy)(CN)4}2ZnII]·2H2O (4) [bipy = 2,2‘-bipyridine and PPh4 = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 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 2 and 3 consists of neutral centrosymmetric [{FeIII(bipy)(CN)4}2MII(H2O)4] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)4]- entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H2O)4 units [M = Mn(II) (2) and Zn(II) (3)] 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) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)4]- complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe−M distances are 5.126(1) and 5.018(1) Å in 2 and 3, respectively. 4 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 cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1 − 3, 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 1 − 3 have been investigated in the temperature range 2.0−300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 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, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.

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