A new decomposition mechanism for metal complexes under water-oxidation conditions

Herein, water-oxidation reaction by cobalt(II) phthalocyanine, N,N′-bis (salicylidene) ethylenediamino cobalt(II), nickel(II) Schiff base (N,N′-bis (salicylidene)ethylenediamino nickel(II), nickel(II)) phthalocyanine-tetrasulfonate tetrasodium, manganese(II) phthalocyanine, 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride, manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) was investigated using electrochemistry, UV-vis spectroscopy and spectroelectrochemistry. According to our results, a new decomposition pathway for these metal complexes under water-oxidation conditions was proposed. The produced metal oxide obtained by decomposition of metal complex under water -oxidation conditions not only catalyzes water-oxidation reaction but this metal oxide also accelerates decomposition of the corresponding complex to form higher amounts of the metal oxide. We hypothesize that such a mechanism could be investigated for many metal complexes under different oxidation or reduction reactions.


Results
Decomposition reactions of metal complexes (Fig. S1) in the presence of a fluorine-doped tin oxide electrode (FTO) under water-oxidation conditions were carried out. As shown in Fig. 1, Mn, Co and Ni complexes (Fig. S1) were slowly decomposed under water-oxidation reaction, and as previously reported, the corresponded metal oxides were formed [17][18][19] . Such decomposition can significantly degrade the complex ( Fig. 1; the reference electrode in the paper is Ag/AgCl/KCl sat ).
In the next step, metal complexes were investigated in the presence of the corresponding metal oxides covered on FTO in the absence of potential. The aim of this step was to track the adsorption of the metal complexes on the surface of the metal oxide covered on FTO in the absence of water-oxidation reaction. As shown in Fig. S4, the adsorption of metal complexes on the surface of the metal oxide covered on FTO is low.
Then, decomposition of the metal complexes in the presence and absence of the covered metal oxide on FTO was investigated (for the structure of complexes and set up see Fig. S1 and S2, respectively). Under water-oxidation reaction, as shown in Fig. 2, decomposition of these metal complexes is high in the presence of the corresponded metal oxide. A decrease of concentration of metal complex in solution corresponds not to the adsorption of metal complex on metal oxide (Fig. S4), but is related to the decomposition of metal complex on the surface of metal oxide. The current is also higher in the presence of metal oxides (Table S1 and Fig. S6). Indeed, in the presence of metal oxides, the relative rates for the decomposition reaction are up to seven times greater than the decomposition reaction in the absence of metal oxides (Table S2).
Based on obtained experimental data, metal oxides could accelerate the decomposition reaction of metal complexes under water-oxidation conditions. It is hypothesized that under water-oxidation reaction, metal complexes would be decomposed into metal oxides that not only could oxidize water but also have the potential of being a desired catalyst for the decomposition of the corresponded metal complexes.
The precipitated oxides on the surface of FTO could be detected by spectroelectrochemistry (Fig. S2). Such cases for Co, Mn and Ni complexes are shown in Fig. 1e-h. These oxides are nanosized particles with different shape and morphology depends on the ligand and the conditions of reaction ( Fig. 1e-h). The effect of the produced Mn oxide on the decomposition of the Mn phet was also investigated in another experiment. As shown in Fig. 3, manganese oxide covered on FTO was obtained by the decomposition of the Mn phet under water-oxidation conditions; the decomposition of Mn phet in the presence of this FTO was significantly more than a fresh FTO. This experiment showed that Mn oxide accelerates Mn phet decomposition. A similar character was observed for Ni phet (Fig. 3).

Discussion
Under water-oxidation conditions, the decomposition of metal complexes (Fig. S1) in the presence of higher load of the metal oxides showed a significant increase. For instance, Ni phet showed highly decomposition in the presence of an Ni foam. As shown in Fig. 4a, for the degradation of Ni phet by Ni foam or a bare FTO a time-delay was observed, but such a time-delay for NiO/FTO electrode is not displayed (Fig. 4b). It is hypothesized that Ni oxide www.nature.com/scientificreports www.nature.com/scientificreports/ formation on a bare FTO or Ni foam, which accelerates the degradation of metal complex needs time to form and, thus, a time-delay was observed. In contrast, an obtained NiO/FTO electrode immediately decomposed Ni phet.
On the other hand, the higher amounts of Ni oxides on the Ni foam at 1.6 V or Mn chips even at 0.8 V significantly increase the decomposition reaction (Fig. 4). Interestingly, at 1.6 V Mn is oxidized to MnO 4 − as a strong  Table S4, see also Figs S3 and S5.
www.nature.com/scientificreports www.nature.com/scientificreports/ oxidant. The oxidant decomposed the complex immediately (Fig. 4e). This experiment clearly showed that a high-valent metal oxide-based compound (e.g., MnO 4 − in this case), which was produced under water-oxidation reaction, had an important role in decomposition of the metal complex.
If one of the reaction products is a catalyst for the same reaction, it is called a self-catalysis reaction. In this case, the reaction of the decomposition of a complex could be called a self-decomposition reaction because metal oxides obtained by decomposition of metal complex are also a catalyst for the decomposed reaction.
Our experiments showed that formed metal oxides during water-oxidation reaction not only accelerate water-oxidation reaction but also drive the decomposition of metal complexes. This self-decomposition reaction (Najafpour-Feizi reaction; Fig. 5; see also supplementary animation) is critical and could be investigated for other oxidation reactions because the reaction could indicate the details of the decomposition of the metal complexes in the presence of the metal oxide under the oxidation reaction.

Conclusions
To find out the details of the effect of the metal oxide on the metal complexes under water-oxidation reaction, the effect of metal oxides on the decomposition of a wide variety of metal complexes under water-oxidation reaction was studied. By the results, we propose a new decomposition pathway (Najafpour-Feizi reaction) for some metal complexes under water-oxidation reaction. Under these conditions, the role of the metal oxide is not only simplify water-oxidation reactions on the surface of fluorine-doped tin oxide electrode but also accelerates the decomposition of the complex to form higher amounts of the metal oxide. We showed that such a phenomenon could occur for Mn, Co and Ni complexes under water-oxidation reaction. This phenomenon is important because it shows a few amounts of formed metal oxide under water-oxidation reaction could result in the high decomposition of the metal complex.
As a further outlook, we suggest that such a reaction may occur for other oxidation reactions such as epoxidation, alcohol or sulfide oxidation and even corresponded photochemical reactions. Equivalently, we hypothesize that for reduction reactions in the presence of metal complexes, the role of the formed metallic nanoparticles on the decomposition of metal complexes during the reaction should be investigated.  Table S5.