[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]

The new cyano complexes of formulas PPh4[Fe-III(bipy)(CN)(4)].H2O (1), [{Fe-III(bipy)(CN)(4)}(2)M-II(H2O)(4).4H(2)O with M = Mn (2) and Zn (3), and [{Fe-III(bipy)(CN)(4)}(2)Zn-II].2H(2)O (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, tetraphenylphosphonium cations, and water molecules of crystallization. The iron(111) 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 [Fe-III(bipy)(CN)(4)}(2)M-II(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(l) and 5.018(1) A 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(l)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(l)) 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(l) and 5.142(l) Angstrom at Fe(l) and 5.028(l), 5.076(l), and 5.176(l) Angstrom 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(Ill) 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(Ill) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10 Angstrom) 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.