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

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 (1), H (2), and NO2 (3) (3-X-SalEen is the condensation product of 3-substituted salicylaldehyde and N-ethylethylenediamine). The characteristic spin transition temperature (T1/2) 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 1, 2, and 3 to be 219, 251, and 366 K, respectively. The solution-state magnetic data also support this trend in T1/2 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 t2g and eg orbitals (explaining the lower T1/2 value), while the nitro substituent increases the energy gap between the t2g and eg orbitals and thus increases the T1/2 value.