Consequences of incremental steric crowding at the Fe^IIN₆(S=2) co-ordination sphere. Synthesis, spectra and electrochemistry

Abstract

The effect of methyl substituents adjacent to donor atoms on the complexing properties of a novel family of tridentate (N₃) heterocyclic ligands has been explored. In the course of this investigation a new group of high-spin iron(II) complexes has been synthesised. The characterization in solution of Fe^IIN₆ co-ordination has been done on the basis of paramagnetically shifted ¹H NMR spectroscopy. Increased steric crowding progressively decreases the values of 10 Dq for these ligands with iron(II) and nickel(II) in the grossly octahedral complexes. The iron(II) complexes also display metal-to-ligand charge-transfer (m.l.c.t.) transition at ca. 360 nm. Cyclic voltammetric studies (MeCN solutions) reveal reversible one-electron Fe^III-Fe^II redox couples at high potentials [1.06-1.22 V vs. saturated calomel electrode (SCE)]. The electrochemical experiments further corroborate the fact that in the present system steric effects due to 3-Me substituent(s) predominate over the electronic contributions of the 5-Me substituent.