Sciencemadness Discussion Board - thoughts on possible practical synthesis of benzene nitride (2D-C3N)

thoughts on possible practical synthesis of benzene nitride (2D-C3N)

clearly_not_atara - 28-8-2023 at 16:44

In 2017, a research team from China (and one Israeli) announced the first successful "macroscale" (milligrams) preparation of two-dimensional C₃N, a novel carbon nitride with a graphite-like structure (Yang et al 2017, attached):

As can be seen in the structure diagram, the material can be described formally as a 2D "salt" of (Ph⁶⁺)(N^3-)₂, hence my suggested trivial name "benzene nitride".

It arises from the thermal dehydrogenative condensation of 2,3-diaminophenazine, prepared from an 0.8 millimolar solution in water at 250 C in an autoclave. This reaction releases three equivalents of hydrogen for each mole of 2,3-DAP, which poses an issue for the use of ordinary pressure vessels. Nonetheless, it is a remarkably simple reaction for the synthesis of an interesting exotic material.

2,3-diaminophenazine can be produced by the oxidation of ortho-phenylenediamine with CuCl2 in refluxing methanol (Mei et al 2003, attached); the product precipitates as crystals. The authors bought their 2,3-DAP, but it seems to be somewhat expensive, while o-phenylenediamine is cheap.

Assuming that the vessel (lined with inert poly-p-phenylene) is not itself a catalyst, and water plays no role beyond hydrogen bonding, identifying a suitable protic solvent that is stable and liquid at the reaction temperature could allow the reaction to be performed at atmospheric pressure and higher concentration. I am not sure if a nitrogen atmosphere is called for; the solubility of O2 should be quite low at this temperature.

One such possibility is monopotassium phosphate, which melts at 252-254 C; the sodium equivalent and most other dihydrogen phosphates decompose (by dehydration to a pyrophosphate) well below this temperature. This would require a reasonable way to add the 2,3-DAP to the molten salt so that it does not incoherently decompose too quickly. Identifying a suitable vessel for molten KH2PO4 would be important -- glass would probably be a poor choice. The pKaH+ of aniline is 4.6, while the pKa2 of phosphoric acid is 6.4, so the substrate should be mostly neutral.

Another possibility is PEG-400, which begins to decompose in the range 250-300 C, but holding the reaction mixture just below this temperature should be safe. PEG-600 is stable to somewhat higher temperatures, but it is less common. For an organic solvent, an inert atmosphere is probably a good precaution. The flash point of PEG-400 is given variously as 170 - 250 C.

But maybe ScienceMadness can think of a better solvent?

Attachment: c3n_yang2017.pdf (1.9MB)
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Texium - 29-8-2023 at 06:50

Quote: Originally posted by clearly_not_atara

One such possibility is monopotassium phosphate, which melts at 252-254 C; the sodium equivalent and most other dihydrogen phosphates decompose (by dehydration to a pyrophosphate) well below this temperature. This would require a reasonable way to add the 2,3-DAP to the molten salt so that it does not incoherently decompose too quickly.

Sorry if this is a naïve question, but would it not be possible to simply mix the solids together prior to heating the mixture above 250 °C?

I like the molten salt idea, though I’d also be concerned that the organic solute would just oil out on the surface rather than actually dissolving into the salt.

clearly_not_atara - 29-8-2023 at 11:21

The problem with a "mixture" is that it leaves the monomer exposed to air. Since the polymerization happens by eliminating hydrogen, the diaminophenazine is probably susceptible to oxidation. Most protic solvents at high temperatures will effectively exclude O2 — the maximum number density of O2 molecules in water at STP is about 4% of the density in air, and the solubility decreases with temperature and solvent polarity.

But looking back at the SI, the reaction takes 48 hours at 250 C under the original conditions. So if the solvent is stirred and the substrate added slowly it should not react too quickly.