1 research outputs found
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 [H<sub>2</sub>bipy(ttr)<sub>2</sub>] in the presence of Fe<sup>II</sup> in solution has afforded
an anionic mononuclear complex and a neutral two-dimensional coordination
polymer formulated as, respectively, NEt<sub>3</sub>H{Fe[bipy(ttr)<sub>2</sub>][Hbipy(ttr)<sub>2</sub>]}·3MeOH (<b>1</b>) and
{Fe[bipy(ttr)<sub>2</sub>]}<i><sub>n</sub></i> (<b>2</b>). The anions [Hbipy(ttr)<sub>2</sub>]<sup>−</sup> and [bipy(ttr)<sub>2</sub>]<sup>2–</sup> embrace the Fe<sup>II</sup> centers
defining discrete molecular units <b>1</b> with the Fe<sup>II</sup> 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<sup>II</sup> is high-spin, and its Mössbauer spectrum is characterized
by a relatively large average quadrupole splitting, Δ<i>E</i><sub>Q</sub> = 3.42 mm s<sup>–1</sup>. Compound <b>2</b> defines a strongly distorted octahedral environment for
Fe<sup>II</sup> in which one [bipy(ttr)<sub>2</sub>]<sup>−</sup> anion coordinates the equatorial positions of the Fe<sup>II</sup> center, while the axial positions are occupied by peripheral <i>N</i>-tetrazole atoms of two adjacent {Fe[bipy(ttr)<sub>2</sub>]}<sup>0</sup> 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><sup>av</sup> = 8.27 kJ mol<sup>–1</sup>, Δ<i>S</i><sup>av</sup> = 37.5 J K<sup>–1</sup> mol<sup>–1</sup>, 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><sub>LIESST</sub> = 52 K. The crystal
structure of <b>2</b> has been investigated in detail at various
temperatures and discussed