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 Fe^II 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 Fe^II 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 Fe^II species in equilibrium. The stability of the complexes could be ranked as follows: [L₅Fe^II-py]²⁺ > [L₅Fe^II–Cl]⁺ > [L₅Fe^II-triazolyl]²⁺ > [L₅Fe^II-(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 [L₅Fe^II–Cl]⁺ > [L₅Fe^II(OTf)]⁺ > [L₅Fe^II-(NCMe)]²⁺, and no ligand exchange could be evidenced for [L₅Fe^II-triazolyl]²⁺. Reactivity of the [L₅Fe^II-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 Fe^III(OOH) and Fe^IV(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.