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

Deprotonation of the ionogenic tetradentate ligand 6,6'-bis(1H-tetrazol-5-yl)-2,2'-bipyridine [H(2)bipy(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)]}center dot 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 FeII 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 FeII is high-spin, and its Mossbauer spectrum is characterized by a relatively large average quadrupole splitting, Delta E-Q = 3.42 mm s(-1). Compound 2 defines a strongly distorted octahedral environment for FeII 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 Delta H-av = 8.27 kJ mol(-1), Delta S-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 T-LIESST = 52 K. The crystal structure of 2 has been investigated in detail at various temperatures and discussed.