Tuning the spin-transition properties of pyrene-decorated 2,6-bispyrazolylpyridine based Fe(ii) complexes

Abstract

Two 2,6-bispyrazolylpyridine ligands (bpp) were functionalized with pyrene moieties through linkers of different lengths. In the ligand 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1) the pyrene group is directly connected to the bpp moiety via a C-C single bond, while in the ligand 4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-(pyren-1-yl)butanoate (L2) it is separated by a benzyl ester group involving a flexible butanoic chain. Subsequent complexation of Fe(II) salts revealed dramatic the influence of the nature of the pyrene substitution on the spin-transition behaviour of the resulting complexes. Thus, compound [Fe(L1)(2)](ClO(4))(2) (1) is blocked in its high spin state due to constraints caused by a strong intermolecular π-π stacking in its structure. On the other hand, the flexible chain of ligand L2 in compounds [Fe(L2)(2)](ClO(4))(2) (2) and [Fe(L2)(2)](BF(4))(2)·CH(3)CN·H(2)O (3) prevents structural constraints allowing for reversible spin transitions. Temperature-dependent studies of the photophysical properties of compound 3 do not reveal any obvious correlation between the fluorescence of the pyrene group and the spin state of the spin transition core.

Tuning the Spin States of Two Apical Iron(II) Ions in a Pentanuclear Iron(II) Cluster Helicate through the Choice of Anions

“…In consequence, this molecular response leads to a highly distorted coordination octahedron as described above, favoring compound 2 to remain in the HS state over the whole temperature range. A similar effect was also observed recently in another iron(II) complex with the pyrene groups of the ligands and showing steadily the HS state [47]. There is no obvious π-π stacking in 3.…”

Iron(II) Complexes of Tridentate Indazolylpyridine Ligands: Enhanced Spin-Crossover Hysteresis and Ligand-Based Fluorescence

“…This observation has consequences for the design of fluorescent spin-crossover compounds, which have been pursued by several groups with mixed success. [40][41][42][43][44][45][46] The strongest coupling between spin-crossover and emission has been achieved using remote fluorophores tethered to iron complex centers, either in individual molecules 40,41 or more complex nanostructures. 42,43 This antenna effect may be a more promising approach towards multifunctional fluorescent switches, than compounds where the emissive group is directly ligated to the iron center as in this work.…”

Spin state switching in iron coordination compounds

“…In general, fluorescence quenching was observed in one spin state, as found for the heterodinuclear triple helicate iron(II) complex including a luminescent ion (Eu) [437] or for the 1D-1,2,4-triazole iron(II) chain including pyrene [438]. When this later fluorophore was decorated on a ligand of an Fe(II) mononuclear complex [439], temperature-dependent studies of the photophysical properties did not reveal any obvious correlation between the fluorescence of the pyrene group and the spin state of the Fe(II) ion.…”