The preparation and full characterization of dichloroferrous complexes of mono-, bis- and tris-α-methyl substituted tris(2-pyridylmethyl)amine (TPA) ligands. Structural bases of stability of the complexes in solution

Abstract

Reaction of FeCl₂ with the mono-, bis- and tris-α-methyl substituted tripods of tris(2-pyridylmethyl)amine (TPA) structure (L_1–3) affords stable complexes that have been fully characterized in solid state and in solution. Whereas L₁ and L₂ coordinate in the tetradentate mode yielding complexes with pseudo-octahedral geometry, L₃ adopts the hypodentate coordination mode with one dangling pyridine, providing a trigonal bipyramidal environment around the metal. Instead of lowering the redox potential, methyl susbstitution increases the Fe^(II)/Fe^(III) value. Being firmly bound in the parent unsubstituted TPA complex, the chloride anions become labile when α-substituted tripods are used as ligands, because of steric constraints at the coordination site. With L₂ and L₃, in acetonitrile and in the presence of supporting electrolyte, exchange reactions rapidly take place yielding cationic species that are more difficult to oxidize. Thus, what could at first be described in electrolytic medium as “stabilization of ferrous state”, corresponds in fact to the formation of different species stabilized by (a) positive charge(s).