Synthesis, characterisation and electrochemistry of eight Fe coordination compounds containing substituted 2-(1-(4-R-phenyl-1H-1,2,3-triazol-4-yl) pyridine ligands, R = CH3, OCH3, COOH, F, Cl, CN, H and CF3

Eight different Dichloro(bis{2-[1-(4-R-phenyl)-1H 1,2,3 triazol-4-yl-kappa N-3]pyridine-kappa N})iron(II) compounds, 2-9, have been synthesised and characterised, where group R = CH3 (L-2), OCH3 (L-3), COOH (L-4), F (L-5), Cl (L-6), CN (L-7), H (L-8) and CF3 (L-9). The single crystal X-ray structure was determined for the L-3 which was complemented with Density Functional Theory calculations for all complexes. The structure exhibits a distorted octahedral geometry, with the two triazole ligands coordinated to the iron centre positioned in the equatorial plane and the two chloro atoms in the axial positions. The values of the Fe-II/III redox couple, observed at ca. - 0.3 V versus Fc/Fc(+) for complexes 2-9, varied over a very small potential range of 0.05 V. The observation that the different R substituents have virtually no effect on the values of the Fe-II/III redox couple for all eight complexes 2-9, is explained by the character of the highest molecular orbitals of complexes 2-9, which do not show any communication of electron density between the various ligands and the metal Fe. However, the HOMOs of the free ligands L-2- L-9, display extended pi-character over the entire ligand, explaining the sensitivity of the H-1 NMR C-H-triazole peak, which is dependent on the electron donating/withdrawing power of the R substituent attached to the 2-[1-(4-R-phenyl)-1H-1,2,3-triazol-4-yl] pyridine ligands.