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  • Final Report Summary - NEWOXMET (Key insights into oxidation chemistry through synthetic systems: N2O activation with first-row transition-metals and O2 activation in heterobimetallic Fe-Ni systems)

Final Report Summary - NEWOXMET (Key insights into oxidation chemistry through synthetic systems: N2O activation with first-row transition-metals and O2 activation in heterobimetallic Fe-Ni systems)

Nowadays oxidation reactions are one of the most important chemical transformations applied both in bulk and fine chemistry. Despite the fact that this field of research has tremendously advanced in the last two decades, key issues remain to be successfully explored both from the catalysis point of view and also from the fundamental understanding of key reaction intermediates. This research project entitled “Key insights into oxidation chemistry through synthetic systems: N2O activation with first-row transition-metals and O2 activation in heterobimetallic Fe-Ni systems” consists in two well-defined subprojects. Results and advances in these two subprojects are detailed below.

- Subproject A. N2O activation with first-row transition-metals

Nitrous oxide constitutes an environmentally friendly alternative to the traditional oxidants used nowadays as it has a high active-oxygen content and only generates unproblematic N2 as waste. Although N2O is a minor component of the earth atmosphere, it is ~300 times more potent as a greenhouse gas than CO2 and it is involved in the depletion of stratospheric ozone. Thus, active research is aimed towards the design of new methods that allow its decomposition into innocuous N2 and O2. However, a much more interesting approach (applied in subproject A) that would have a higher impact would be its consumption as an oxidation reagent in useful chemical syntheses, thus achieving two aims: decreasing the amount of this greenhouse gas in the atmosphere and performance of practical chemical reactions. N2O has been long considered an inert molecule because, despite being thermodynamically a potent oxidant, it is kinetically very stable. These kinetic barriers can be overcome through binding and activation by suitable metal ions, which react with N2O under mild conditions leading to the formation of metal-oxo species that can further act as oxidants in the oxidation of organic substrates

With the main aim of achieving of N2O activation under mild conditions and applying this gas as oxidant, in this subproject, a series of CuII, CoII and NiII complexes with dianionic highly donating ligands (Py2CON2) have been prepared. Importantly, three of the prepared compounds (based on cobalt(II)) proved to be reactive towards N2O. The reaction was extremely fast and it occurred in a few minutes even at -80ºC. Unfortunately, no reaction intermediates could be spectroscopically trapped. Remarkably, it was observed that the combination of the designed cobalt(II) complexes with N2O afforded a catalytic system for the oxidation of aldehydes. This work has been recently published (Dalton Trans 2016, 45, 14530-14533).

- Subproject B. Heterobimetallic Fe-Ni systems involved in O2 activation
Mixed metal or heterobimetallic oxygen intermediates are less common than homometallic species both in biological systems and in synthetic models. Yet they are of great interest due to possible “synergistic” effect of two different metal ions acting together. Indeed, synthetic heterometallic systems have been shown to be useful for catalytic reactions. These studies support the notion that two different metals might work together to activate O2 in such a way that the resulting M-O2-M’ species (where M and M’ are two different metals) exhibit reactivity different from that observed using any of the two metals individually. Thus, these heterometallic species are of significant interest due to their potentially unique properties and their possible involvement in various catalytic processes.
Thus, in subproject B we envisioned the preparation of heterometallic Ni-O2-Fe species by reaction of a stable nickel-superoxo compound [NiII(O2)(b-diketiminato)] and an iron(II) complex containing tetradentate or pentadentate aminopyridine ligands. Following the tasks described in the working plan, we synthesized and characterized the nickel-superoxo compound according to the reported procedures and we could also prepare and fully characterize the starting iron(II) complexes. Unfortunately, no interaction between the nickel-superoxo species and the iron(II) complexes was observed. A possible reason for this lack of reactivity could be the steric bulk around the nickel-superoxo core imposed by the bulky b-diketiminato ligand. For this reason, the working plan was reevaluated and it was planned the preparation of other mononuclear nickel-oxygen species that could serve as starting synthons for the synthesis of the heterobimetallic Ni-Fe species. In order to do so, the above mentioned Py2CON2 ligand was used to synthesize the corresponding nickel(II) complex and its reaction with different oxidants was studied. In the case of meta-chlorperbenzoic acid (mCPBA) and NaOCl, the formation of high-valent mononuclear nickel-oxygen species was observed. These species were characterized by several techniques including UV-vis, resonance Raman, EPR, XAS and MS and their structure was supported by theoretical calculations. Both compounds were found to be highly reactive towards different substrates such as alkanes, alkenes or sulfides with reaction rates much higher that those previously reported for mononuclear nickel-oxygen species. These results suggested that these compounds might be relevant in nickel-catalyzed oxidation reactions. From these studies, two papers have been published in journals with high impact factor (Chem. Eur. J. 2015, 21, 15029; J. Am. Chem. Soc. 2016, 138, 12987). Thanks to our contribution in this field the fellow was invited to write a review on this topic in Chem. Eur. J. In 2016 (Chem. Eur. J. 2016, 22, 13422).
Overall, the results obtained demonstrate that with a suitable ligand scaffold, metal complexes capable of activating different oxidants, including N2O, can be prepared. The new oxidation methodolgies developed in this project set the basis for the performance of synthetically useful transformations under mild and environmentally friendly conditions. This may have an impact in the chemical industy for the production of bulk chemicals, fine chemicals and drugs.
Over the course of this CIG project, the fellow has been fully integrated in the host group by participating in management, teaching and supervision tasks. Moreover, the fellow has good perspectives to obtain a permanent position at the host university in the near future.

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