Influence of Ligand Rigidity and Ring Substitution on the Structural and Electronic Behavior of Trivalent Iron and Gallium Complexes with Asymmetric Tridentate Ligands

Species 1−6 are [MIII(L)2]ClO4 complexes formed with the PhO-−CHN−CH2−Py imines, (LI)- and (LtBuI)-, and
PhO-−CH2−NH−CH2−Py amines, (LA)- and (LtBuA)-, in which PhO- is a phenolate ring and Py is a pyridine ring
and the prefix tBu indicates the presence of tertiary butyl groups occupying the positions 4 and 6 of the phenol
ring. Monometallic species with d5 high-spin iron (1, 2, 3, 4) and d10 gallium (5, 6) were synthesized and characterized
to assess the influence of the ligand rigidity and the presence of tertiary butyl-substituted phenol rings on their
steric, electronic, and redox behavior. Characterization by elemental analysis, mass spectrometry, IR, UV−visible,
and EPR spectroscopies, and electrochemistry has been performed, and complexes [FeIII(LtBuI)2]ClO4 (2), [FeIII(LtBuA)2]ClO4 (4), and [GaIII(LtBuI)2]ClO4 (5) have been characterized by X-ray crystallography. The crystal structures
show the imine ligands meridionally coordinated to the metal centers, whereas the amine ligands are coordinate
in a facial mode. Cyclic voltammetry shows that the complexes with the ligands (LtBuI)- and (LtBuA)- were able to
generate ligand-based phenoxyl radicals, whereas unsubstituted ligands displayed ill-defined redox processes. EPR
spectroscopy supports high-spin configurations for the iron complexes. UV−visible spectra are dominated by charge-transfer phenomena, and imine compounds exhibit dramatic hyperchromism when compared to equivalent amines.
The tertiary butyl groups on the phenolate ring enhance this trend. Detailed B3LYP/6-31G(d)-level calculations
have been used to account for the results observed.