Vitalizing nitrogen: N2 fixation by water radical cations

Published in Chemistry
Vitalizing nitrogen: N2 fixation by water radical cations

The discovery of a novel nitrogen fixation approach is the result of a decade of our research on water radical cations. About 10 years ago, we discovered that abundant radical cations of water clusters, especially in the dimer form (H2O)2+•, can be produced at atmospheric pressure. This is achieved by electron stripping from neutral water molecules in a strong electric field – we used the energy-tuneable corona discharge of the pure water vapor. While the experimental evidence was quite compelling, it took quite a few years before our discovery was broadly recognized by the scientific community (CCS Chem. 3, 3559-3566, 2022). Many fellow scientists found it difficult to accept the fact that water dimer radical cations – (H2O)2+• or WDRACs – could be generated at such high intensity as to become one of the major ionic species in water discharge at a given energy. It was also hard to accept that WDRAC was sufficiently stable to survive transportation through the atmospheric interface and vacuum ion guides.

Our discovery was fully acknowledged only in the year 2022. In our follow-up research we have demonstrated the high reactivity of WDRACs toward a wide range of volatile molecules, thus revealing its rich chemistry.

Completely unexpected, in our current project we discovered that WDRAC is also capable of reacting with molecular nitrogen (N2). Nitrogen fixation, i.e. conversion of N2 into nitrogenous compounds, is one of the most important chemical processes for the life on Earth. As N2 molecule is very stable and inert, nitrogen fixation typically requires very harsh conditions, such as high temperatures, pressures and special catalysts. Thus, Haber-Bosch process, which is currently the major industrial process for N2 fixation into ammonia (NH3), requires ≈100 bar and 500 oC. In striking contrast, we found the reaction between N2 and WDRAC to proceed under mild conditions and with no catalyst involved. This route yielded peculiar high-value products: hydroxylamine (NH2OH) and nitroxyl (HNO). Therefore, the discovered reaction presented a novel mechanism of nitrogen fixation with breath-taking potential for further development.

Scheme 1. Our first idea of a possible mechanism for the reaction of N2 with (H2O)2+•.

However, it was too early for us to celebrate. Despite the compelling experimental evidence, the occurrence of this reaction under such mild conditions was very hard to explain from a theoretical point of view. The most apparent problem was that the energy of the products was considerably higher (∆E ≈ +3.8 eV) than the energy of the reacting species. Indeed, such a reaction is not permitted by the basic thermodynamic laws. And yet, all the evidence collected from multiple perspectives – using a variety of analytical methods, including mass spectrometry, infrared spectroscopy, nuclear magnetic resonance and UV-Vis spectroscopy – unanimously indicated toward it.

So, the mechanism of the reaction was puzzling us for more than 3 years. How could such a reaction occur against, under very mild conditions, without a catalyst, without N2 burning, and with such peculiar products? There must be something very unique regarding the interaction between N2 and WDRAC. As Marcel Proust wrote more than one hundred years ago, “[t]he real voyage of discovery…consists not in seeking new landscapes but in having new eyes” (La Prisonnière, 1923). And it took us three big realizations before the answer was found and a realistic mechanism has finally emerged.

Figure 1. Our first realization: this very first sketch was drawn during a lunchbreak in April 2022.

First realization: While N2 disproportionation with WDRACs is thermodynamically not allowed when N2 is present in its ground singlet state, it may occur (∆E ≈ –2.9 eV) when N2 is present in its more active triplet state (N2*). Laser physicists actively utilize this effective transfer to produce nitrogen lasers. They have shown that N2* exhibits exceptional stability: it stays in its active form for more than one second. Importantly, being an electronic transition, N2 activation to N2* occurs on a much shorter timescale compared to chemical atomic rearrangements. Therefore, the event of N2 activation to N2* and the following association could occur within a single collision.

Nevertheless, even though thermodynamically allowed, the mechanism of the reaction between N2* and WDRAC still remained obscure. Earlier research and our own calculations indicate that the global energy minimum for WDRAC ion is the hydrogen-bonded [H3O+•••OH] configuration. However, no matter how hard we tried, we could not find a stable intermediate structure for the binding of N2* with [H3O+•••OH]. And here came our second realization:

 WDRACs co-exist in two configurations. Apart from the hydrogen-bonded [H3O+•••OH] configuration, there is also a distinctly peculiar two-centre-three-electron (2c-3e) configuration [H2O•••OH2]+•, also referred to as hemibonded structure. This form is just 0.3 eV higher on WDRAC energy surface. In contrast to [H3O+•••OH], we could easily locate a stable intermediate structure for the binding of N2* with the [H2O•••OH2]+• configuration. And it has rather simple physics behind it: the association of hemibonded WDRAC with N2* occurs due to their joint stabilization of the positive charge.

Figure 2. The final reaction scheme of our novel nitrogen fixation method.

Having found the reaction intermediate, our last question was awaiting the answer. How do the two protons jump from water moieties to nitrogen to yield the second reaction intermediate? Again, we failed to find a reaction pathway whereby two protons would be successively transferred from the water moieties to nitrogen. And here came our third and final realization:

The semblance and intrinsic symmetry of our intermediate structures might invite excited-state double proton transfer without the symmetry break. Not two one-proton transfers, but one direct transfer of both protons simultaneously! And that hypothesis was confirmed by our calculations. Finally, we had a consistent mechanism – as shown in Figure 2.

Having completed this work, we now look at WDRACs as heaven-sent for N2 fixation. Several factors fall into place quite serendipitously: the symmetry and relative stability of [H2O•••OH2]+• configuration, the excellent stability of excited-state nitrogen (N2*), and the possibility of the excited-state double proton transfer. Once all these factors matched together, a principally novel approach to green N2 fixation under mild conditions became possible. We believe that our discovery should bring about entirely new look and surprisingly appealing possibilities to the problem of nitrogen fixation on an industrial scale.

We have great faith in the future of this project. Furthermore, our current data for somewhat similar molecular systems show that, owing to its unique configuration, WDRAC could be an excellent reagent to functionalize other inert molecules.

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