Structural and theoretical investigation on two dinuclear Fe(III) complexes of tridentate NNO-donor Schiff base ligands

Two new Fe(III) complexes of formulas [FeL1(N3)(OMe)]2 (1) and [FeL2(N3)(OH)]2•H2O (2) have been synthesized by using two tridentate NNO-donor Schiff base ligands HL1{(2-[(3-methylaminoethylimino)-methyl]-phenol)} and HL2 {(2-[1-(2-dimethylaminoethylimino)methyl]-phenol)}, respectively. Substitution of the –CH3 group on the secondary amine group of the N,N-dimethyldiamine fragment of the Schiff base by a H-atom leads to a structural change from a methoxido-bridged dimer (1) to a single hydroxido-bridged dimer (2). Both complexes have been characterized by single-crystal X-ray analyses, IR and UV–vis spectroscopy and DFT studies. Both dimers are centrosymmetric containing two FeIII atoms which are bridged by two methoxido ions in compound 1 and by two hydroxido ions in compound 2. The chelating tridentate Schiff base and a terminal azido group complete the distorted octahedral environment around each iron center in both complexes. To explain the preferential bridging ability of methoxido and hydroxido groups in complexes 1 and 2, we have carried out theoretical calculations based on DFT to compare their formation energies and dimerization energies. In the case of complex 2, it is noticed that the close proximity of the azido group from the iron centers allow the hydroxido bridged complex to gain extra stability through H-bonds, whereas due to the steric crowding of the azido group and the methyl moiety of adjacent molecules, complex 1 disfavors the formation of hydrogen bonds as well as hydroxido bridging