Could complex alien life evolve to breathe something other than oxygen?

Oxygen is the sweet spot for energy. Anaerobic respiration works, but it produces much less energy. Because of their low energy metabolisms, anaerobes are limited in their ability to become complex or, at the very least, active.

Fluorine, is the only element more electronegative than oxygen. Is a metabolism based on fluorine conceivable? Yes. Is there anything like that in the universe? Most likely not.

The problem about fluorine is that it is reactive without discrimination. That and the fact that there isn’t much of it in the cosmos. Despite its scarcity, oxygen can persist as a free molecule for a certain amount of time.

Fluorine, not so much. If we could imagine a process that produced free fluorine in the same way that photosynthesis produces free oxygen, you wouldn’t actually end up with a fluorinating atmosphere, because the fluorine would be far too busy setting fires everywhere.

Yes, there are other possible electron acceptors, but they all have the same issue of having less energy available than oxygen. There are a variety of unusual metabolisms found throughout life. For eons, life on our planet endured in the absence of oxygen. But complex life without it could get tricky to attain.