Push and Pull Effect of Methoxy and Nitro Groups Modifies the Spin-State Switching Temperature in Fe(III) Complexes

Abstract

We have explored the impact of electron-donating (methoxy) and electron-withdrawing (nitro) substituents on SalEen ligand based spin crossover (SCO) behavior of Fe(III) complexes. Thus, 3-X-substituted SalEen ligands were employed to prepare [Fe(3-X-SalEen)₂]·NCSe, where X = OMe (<b>1</b>), H (<b>2</b>), and NO₂ (<b>3</b>) (3-X-SalEen is the condensation product of 3-substituted salicylaldehyde and <i>N</i>-ethylethylenediamine). The characteristic spin transition temperature (<i>T</i> _1/2) is shown to shift to a lower temperature when an electron-donating substituent (OMe) is used and to a higher temperature when an electron-withdrawing substituent (NO₂) is used. We used experimental and theoretical methods to determine the reasons for this behavior. The solid-state magnetic data revealed the transition temperatures for complexes <b>1</b>, <b>2</b>, and <b>3</b> to be 219, 251, and 366 K, respectively. The solution-state magnetic data also support this trend in <i>T</i> _1/2 values. UV-vis spectra analysis indicates that there is greater delocalization in the π-manifold of the ligand when the nitro group is the substituent. Theoretical studies through density functional theory methods suggest the methoxy substituent decreases the energy gap between the t_2g and e_g orbitals (explaining the lower <i>T</i> _1/2 value), while the nitro substituent increases the energy gap between the t_2g and e_g orbitals and thus increases the <i>T</i> _1/2 value.