Iron Coordination Chemistry with New Ligands Containing Triazole and Pyridine Moieties. Comparison of the Coordination Ability of the N‑Donors

We report the synthesis, characterization, and solution
chemistry of a series of new FeII complexes based on the
tetradentate ligand N-methyl-N,N′-bis­(2-pyridyl-methyl)-1,2-diaminoethane or the pentadentate
ones N,N′,N′-tris­(2-pyridyl-methyl)-1,2-diaminoethane and N,N′,N′-tris­(2-pyridyl-methyl)-1,3-diaminopropane,
modified by propynyl or methoxyphenyltriazolyl groups on the amino
functions. Six of these complexes are characterized by X-ray crystallography.
In particular, two of them exhibit an hexadentate coordination environment
around FeII with two amino, three pyridyl, and one triazolyl
groups. UV–visible and cyclic voltammetry experiments of acetonitrile
solutions of the complexes allow to deduce accurately the structure
of all FeII species in equilibrium. The stability of the
complexes could be ranked as follows: [L5FeII-py]2+ > [L5FeII–Cl]+ > [L5FeII-triazolyl]2+ > [L5FeII-(NCMe)]2+, where L5 designates a pentadentate coordination sphere composed of
the two amines of ethanediamine and three pyridines. For complexes
based on propanediamine, the hierarchy determined is [L5FeII–Cl]+ > [L5FeII(OTf)]+ > [L5FeII-(NCMe)]2+, and no ligand exchange could be evidenced for [L5FeII-triazolyl]2+. Reactivity of the [L5FeII-triazolyl]2+ complexes with hydrogen
peroxide and PhIO is similar to the one of the parent complexes that
lack this peculiar group, that is, generation of FeIII(OOH)
and FeIV(O), respectively. Accordingly, the ability of
these complexes at catalyzing the oxidation of small organic molecules
by these oxidants follows the tendencies of their previously reported
counterparts. Noteworthy is the remarkable cyclooctene epoxidation
activity by these complexes in the presence of PhIO.