Homoleptic Iron(II) Complexes with the Ionogenic Ligand 6,6′-Bis(1<i>H</i>‑tetrazol-5-yl)-2,2′-bipyridine: Spin Crossover Behavior in a Singular 2D Spin Crossover Coordination Polymer

Deprotonation of the ionogenic tetradentate ligand 6,6′-bis­(1<i>H</i>-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 (<b>1</b>) and {Fe­[bipy­(ttr)₂]}<i>_n</i> (<b>2</b>). The anions [Hbipy­(ttr)₂]⁻ and [bipy­(ttr)₂]^2– embrace the Fe^II centers defining discrete molecular units <b>1</b> 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 <b>1</b> shows that the Fe^II is high-spin, and its Mössbauer spectrum is characterized by a relatively large average quadrupole splitting, Δ<i>E</i>_Q = 3.42 mm s^–1. Compound <b>2</b> 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 <i>N</i>-tetrazole atoms of two adjacent {Fe­[bipy­(ttr)₂]}⁰ moieties thereby generating an infinite double-layer sheet. Compound <b>2</b> 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 Δ<i>H</i>^av = 8.27 kJ mol^–1, Δ<i>S</i>^av = 37.5 J K^–1 mol^–1, obtained from calorimetric DSC measurements. Photomagnetic measurements of <b>2</b> 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 <i>T</i>_LIESST = 52 K. The crystal structure of <b>2</b> has been investigated in detail at various temperatures and discussed

New Advances in the One-Dimensional Coordination Polymer Copper(II) Alkanoates Series: Monotropic Polymorphism and Mesomorphism

The polymorphism in the copper­(II) alkanoates, recently discovered for one member, has been thoroughly studied for the whole series, from 3 to 16 C atoms. Three polymorphic phases have been found, all of them sharing the same molecular unit, the <i>paddle-wheel</i>, which grows forming a 1D coordination polymer or <i>catena</i>. The three polymorphs are defined by a different packing of these catenae and a specific arrangement of the alkyl chains. Ten new crystal structures of those compounds have been solved by high resolution powder diffraction and presented in this paper. The polymorphism in this series has been found to be monotropic and is responsible for the complex thermal behavior observed. The most characteristic feature, the endothermic–exothermic effect, has been explained for the first time in these compounds by a combination of data from differential scanning calorimetry (in normal and modulated modes), powder X-ray diffraction and Fourier transform infrared spectroscopy. These techniques, together with small-angle X-ray scattering and optical microscopy, were used to analyze the hexagonal columnar discotic liquid crystal phase of copper­(II) alkanoates. Thus, new information has been found in the packing and stacking of the discs formed by the paddle-wheel units, also maintained in the mesophase