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 NO 2 ( 3 ) (3-X-SalEen is the condensation product of 3-substituted salicylaldehyde and N -ethylethylenediamine). The characteristic spin transition temperature ( T 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 (NO 2 ) 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 T 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 T 1/2 value), while the nitro substituent increases the energy gap between the t 2g and e g orbitals and thus increases the T 1/2 value.