Synthesis and reactivity towards dioxygen of a series of five-coordinated complexes between N,N′-(3,3′-dipropylmethylamine)bis(2-hydroxy-1-naphthylidenamine) and the metal ions Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)

Abstract

The study of metal complexes able to react with dioxygen has aroused great interest in recent years. t001 Oxidation Potential of the [M(NAPRDPT)] Complexes from Cyclic Voltammetry. Complex E° II′/III/V [Mn(NAPHDPT)] −0.31 [MN(NAPMeDPT)] −0.31 [Fe(NAPHDPT)] −0.32 [Fe(NAPMeDPT)] −0.27 [Co(NAPHDPT)] −0.37 [Co(NAPMeDPT)] −0.23 [Ni(NAPHDPT)] +0.35 [Ni(NAPMeDPT)] +0.40 [Cu(NAPHDPT)] +0.75 1 Cu(NAPMeDPT)] +0.86 1 1 Peak potential values at 0.2 V sec −1. Particular attention has been paid to those derivatives which take up dioxygen reversibly because they can mimic biological oxygen carriers [1]. In an attempt to find the parameters able to give some rationalization on the oxygenation reactions we have previously studied the behaviour towards dioxygenation of a series of five-coordinated metal complexes with O 2N 3 donor set (namely [M(SALRDPT)]) by electrochemical and X-ray diffraction techniques [2–5]. We wish to report here on the behaviour of the complexes [M(NAPRDPT)] having the same O 2N 3 donor set. ▪ M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II) R = H, Me As in the case of SAL derivatives, all the NAP compounds undergo in DMSO solution a quasi-reversible one-electron anodic oxidation, except for copper(II) compounds, which are irreversibly oxidized (Table I). The comparison with SAL complexes shows that NAP derivatives are more easily oxidizable. The reaction with dioxygen in DMSO solution was followed by voltammetric techniques. The results indicate that [Co(NAPRDPT)] compounds are able to add dioxygen reversibly. For the manganese(II) and iron(II) complexes irreversible addition occurs, with formation of different oxygenated products. Copper(II) and nickel(II) complexes do not react with dioxygen The equilibrium constant of the reaction: [Co(NAPMeDPT)] + O 2 ⇄ [Co(NAPMeDPT)] · O 2 in DMSO at 20 °C resulted (1.0 ± 0.5) × 10 4 dm 3 mol −1. At least for these types of pentadentate complexes it is possible to state that the location of the oxidation potential allows one to predict whether a compound is able to react with dioxygen but it is not sufficient to predict whether the dioxygenation reaction proceeds reversibly. The manganese(II), iron(II) and cobalt(II) complexes were ESR inactive both as powders or in degassed solutions. By exposure to O 2 of toluene solutions of the [Co(NAPMeDPT)] or [Co(NAPHDPT)] complexes strong ESR signals were recorded, characterized by g-factor values of 2.022 and 2.037 respectively. The [Co(NAPHDPT)] adduct signal exhibited to Co hyperfine interaction, whereas the [Co(NAPMeDPT)] one did not, due to the rotation of the methyl group in this latter system. The ESR spectra have been related to metal species reversibly coordinating O 2 which also showed a different spontaneous decaying process. The IR spectra of solid samples or homogeneous solutions of the different complexes showed significant changes due to O 2 coordination. Hypotheses on the nature of the reversibly and irreversibly coordinating O 2 species have been made on the basis of the frequencies of the IR absorption.