Structural, Magnetic and Dynamic Characterization of Liquid Crystalline Iron(III) Schiff Base Complexes with Asymmetric Ligands

The iron(III) complexes that were formed by coordination of the Fe-III ion with the asymmetric tridentate liquid crystalline Schiff base ligand (L), the water molecules and the different counterions [PF6- (1), NO3- (2), and Cl- (3)] were studied by electron paramagnetic resonance (EPR) spectroscopy. EPR spectroscopy demonstrated that each of the complexes investigated consists of two types of iron centers: S = 1/2 low-spin (LS) and S = 5/2 high-spin (HS). LS iron complexes 2, 3 and LS complex 1 in the temperature range 4.2-250 K have a (d(xz), d(yz))(4)(d(xy))(1) ground state. Interesting features were found for the monocationic FeIII complex 1, [Fe(L)X(H2O)(2)]X-+(-), with X = PF6- as the counterion. The LS and HS iron centers of 1 are coupled together antiferromagnetically and form a dimer structure by means of the water molecules and the PF6- counterion. The second-type of LS and HS centers that are visible by means of EPR spectroscopy were best observed in the liquid crystalline (387-405 K) phase. The monitoring and the simulation of the EPR spectra enabled us to trace the dynamics of changing the number of the second-type of LS centers with respect to the first-type of LS centers. The observed dynamic process is characterized by the enthalpy value Delta H = 27.9 kJ/mol, which was caused by reorientation of the PF6- counterion. Calculation of the observed g values for the second-type of LS complex 1 indicated that, in this case, the (d(xy))(2)(d(xz),d(yz))(3) ground state is stabilized. The conversion between the electron (d(xz),d(yz))(4)(d(xy))(1)/(d(xy))(2)(d(xz),d(yz))(3) configurations was found to be temperature dependent and was detected in the same material for the first time in iron complexes.