The given sequence appears to represent a combination of chemical elements. Breaking it down:

C2N4 - Carbon $C$ and Nitrogen $N$ C2N2 - Carbon $C$ and Nitrogen $N$ CN3CNO - Carbon $C$, Nitrogen $N$, and Oxygen $O$ C2N5ON - Carbon $C$, Nitrogen $N$, and Oxygen $O$

It seems like the sequence is showing different combinations of Carbon, Nitrogen, and Oxygen atoms.