Abstract:
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 [PF 6 - (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 (dxz,dyz)4(dxy)1 ground state. Interesting features werefound 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 ΔH = 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(dxz,dyz)3 ground state is stabilized. The conversion between the electron (dxz,d yz)4(dxy)1/(dxy) 2(dxz,dyz)3 configurations was found to be temperature dependent and was detected in the same material for the first time in iron complexes. We synthesized a novel compound, namely a liquid crystalline iron(III) Schiff base complex with the asymmetric ligand [Fe(L)X(H2O)2]+X-, where X = PF 6 - is the counterion. This compound has a labilelow-spin electron configuration that switches between the (dxz,d yz)4(dxy)1/(dxy) 2(dxz,dyz)3 ground states and is temperature-dependent. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
