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.