Bis-bidentate vs. bis-tridentate imino-heterocycle ligands in the formation of dinuclear helical complexes of Fe(ii)

Abstract

By reaction of the bis-bidentate imino-quinoline ligand 1 with the octahedral cation Fe²⁺ in a 3 ∶ 2 metal/ligand molar ratio, a partial hydrolysis of the ligands is observed instead of the formation of the expected triple helicate, due to the excessive crowding of the trans-1,2-cyclohexyl spacers. The monomeric [Fe^II(2)₂]²⁺ complex is obtained, whose crystal and molecular structure have been determined, in which the amino-imino-quinoline tridentate ligands 2, originating from 1 by loss of one quinoline aldehyde, coordinate Fe²⁺ with a mer disposition. Double helical complexes can instead be obtained by using the bis-tridentate ligand 3, featuring the same spacer of ligand 1, i.e.trans-1,2-cyclohexyl, and two imino-phenanthroline donor sets. The spectral and electrochemical properties of the low-spin double helical [Fe^II₂(3)₂]⁴⁺ complex have been studied and compared with those of the low spin [Fe^II(4)]²⁺ complex, which can be considered the monomeric half of the helicate. Significantly enhanced kinetic inertness and resistance to oxidation were found for the double helical complex.