Synthesis, magnetic and photomagnetic study of new iron(ii) spin-crossover complexes with N4O2 coordination sphere

A new family of neutral mononuclear iron(II) spin crossover (SCO) compounds, Fe(L¹⁻⁶)₂ (L¹⁻⁶ = N'-((pyridin-2-yl)methylene)benzohydrazide (HL¹), N'-(1-(pyridin-2-yl)ethylidene)-benzohydrazide (HL²), N'-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL³), 2-hydroxy-N'-((pyridin-2-yl)methylene)benzohydrazide (HL⁴), 2-hydroxy-N'-(1-(pyridin-2-yl)ethylidene)benzohydrazide (HL⁵), 2-hydroxy-N'-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL⁶)) with N₄O₂ donor sets have been synthesized from series tridentate Schiff base ligands with N,N,O donor sets. The investigation of magnetic properties of these compounds reveal that in the measured temperature range, compound 1 is in the high-spin (HS) state, and compound 3 and 6 are mainly in the low-spin (LS) state, whereas the other compounds exhibit various SCO properties: compound 2 undergoes a gradual incomplete SCO with characteristic temperature T(1/2) higher than 350 K; compound 4 exhibits a special stepwise thermally induced SCO occurring at ~150 K (smooth) and 200 K (two-steps, with T(S1↑/↓) = 204/202 K and T(S2↑/↓) = 227/219 K) with a mixture of the HS and LS states yielded below 100 K; compound 5 shows a gradual and complete LS↔HS SCO with characteristic temperature T(1/2) = 273 K. All the three SCO compounds show the LIESST (light induced exited spin state trapping) effect with different levels of photoconversion. To thoroughly analyze these behaviours, Mössbauer spectra and DSC of 4 and 5, crystal structures of all the compounds at 290 K and 5 in the LS state at 110 K were carried out, which confirmed the structural changes accompanying the spin transition. In addition, alkyl substitution effect on the ligand field was suggested for this system.