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

Abstract

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