Electronic and magnetic properties of iron (III) dinuclear complexes with carboxylate bridges

A series of dinuclear iron (III) complexes with carboxylate bridges, of the type [Fe2L2(H2O)(4)](NO3)(2)(H2O)(m) (m = 1-3), where L = Schiff base derived from L-alpha-amino acids and salicylaldehyde have been prepared and characterised by different spectroscopic techniques, magnetic susceptibility, conductivity, and electrochemical measurements. The dimeric iron complexes contain hexa-coordinated iron (III), with the metal ion surrounded by water molecules, the salicylidenimine ligand, and the bridging carboxylate groups. The inequivalence of the iron atoms is detected by Mossbauer spectroscopy and this is reflected by the presence of two overlapping quadrupole doublets (delta(1) similar to 0.52-0.67 mm s(-1), delta(2) similar to 0.64-0.80 mm s(-1), DeltaE(Q1) similar to 0.38-0.80 mm s(-1), and DeltaE(Q2) (similar to 0.68-1.01 mm s(-1)), with an approximate intensity ratio of 1:1 for all complexes. The magnetic susceptibilities of the complexes were deter-mined over the temperature interval 5-300 K and revealed a decrease in the effective magnetic moment with decreasing temperature. The plots of the reciprocal of the molar susceptibility versus temperature indicate that the two iron (III) ions are very weakly antiferromagnetically coupled (-J similar to 0.09-0.21 cm(-1)). Electrochemical studies show that the first two peaks can be assigned to two consecutive one-electron reduction process, affording (FeFeIII)-Fe-II and (FeFeII)-Fe-II. The comproportionation constant K-c at 298 K is in the 5.3 x 10(3)-2.5 x 10(4) range. The values obtained are in agreement with an intermediate electron coupling between redox sites. (C) 2004 Elsevier Ltd. All rights reserved.