title: Homoleptic Iron(II) Complexes with the Ionogenic Ligand 6,6'-Bis(1H-tetrazol-5-yl)-2,2'-bipyridine: Spin Crossover Behavior in a Singular 2D Spin Crossover Coordination Polymer.
abstract: Deprotonation of the ionogenic tetradentate ligand 6,6'-bis(1H-tetrazol-5-yl)-2,2'-bipyridine [H2bipy(ttr)2] in the presence of Fe(II) in solution has afforded an anionic mononuclear complex and a neutral two-dimensional coordination polymer formulated as, respectively, NEt3H{Fe[bipy(ttr)2][Hbipy(ttr)2]}·3MeOH (1) and {Fe[bipy(ttr)2]}n (2). The anions [Hbipy(ttr)2](-) and [bipy(ttr)2](2-) embrace the Fe(II) centers defining discrete molecular units 1 with the Fe(II) ion lying in a distorted bisdisphenoid dodecahedron, a rare example of octacoordination in the coordination environment of this cation. The magnetic behavior of 1 shows that the Fe(II) is high-spin, and its Mössbauer spectrum is characterized by a relatively large average quadrupole splitting, ΔEQ = 3.42 mm s(-1). Compound 2 defines a strongly distorted octahedral environment for Fe(II) in which one [bipy(ttr)2](-) anion coordinates the equatorial positions of the Fe(II) center, while the axial positions are occupied by peripheral N-tetrazole atoms of two adjacent {Fe[bipy(ttr)2]}(0) moieties thereby generating an infinite double-layer sheet. Compound 2 undergoes an almost complete spin crossover transition between the high-spin and low-spin states centered at about 221 K characterized by an average variation of enthalpy and entropy ΔH(av) = 8.27 kJ mol(-1), ΔS(av) = 37.5 J K(-1) mol(-1), obtained from calorimetric DSC measurements. Photomagnetic measurements of 2 at 10 K show an almost complete light-induced spin state trapping (LIESST) effect which denotes occurrence of antiferromagnetic coupling between the excited high-spin species and TLIESST = 52 K. The crystal structure of 2 has been investigated in detail at various temperatures and discussed.
authors.affiliation: †Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, 46980 Paterna, Valencia, Spain.|‡Department of Physical Chemistry, National Taras Shevchenko University, Volodymyrska Str. 64, Kiev 01601, Ukraine.
authors.forename: Maksym
authors.initials: M
authors.lastname: Seredyuk
authors.affiliation: †Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, 46980 Paterna, Valencia, Spain.
authors.forename: Lucía
authors.initials: L
authors.lastname: Piñeiro-López
authors.affiliation: §Departamento de Física Aplicada, Universitat Politècnica de València, 46022 Valencia, Valencia, Spain.
authors.forename: M Carmen
authors.initials: MC
authors.lastname: Muñoz
authors.affiliation: ∥MAX IV Laboratory, Lund University, Ole Römers väg 1., 223 63 Lund, Sweden.
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Martínez-Casado
authors.forename: Gábor
authors.initials: G
authors.lastname: Molnár
authors.affiliation: #Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de Zaragoza and Institut Laue-Langevin, 38042 Grenoble Cedex 9, France.
authors.forename: José Alberto
authors.initials: JA
authors.lastname: Rodriguez-Velamazán
authors.forename: Azzedine
authors.initials: A
authors.lastname: Bousseksou
authors.affiliation: †Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, 46980 Paterna, Valencia, Spain.
authors.forename: José Antonio
authors.initials: JA
authors.lastname: Real