I got a leather tunic with mending on it from an abandoned ship. I have a diamond chest plate now and I could not combine the items in an anvil. They are not the same material but i still believe that I should be able to combine them in an anvil. Is there a way I could transfer the mending enchantment on leather to diamond or a way i could isolate the mending enchantment from the leather tunic? I really want mending and I have it on the one thing I do not want it on. Someone please help.
Unfortunately, outside of commands this is not possible. And even with commands, it would be easier to just simply add the enchantment to your Diamond chest piece, and ditch the Leather.
As you mentioned, you cannot combine two items of different materials. You can remove the enchantment, by use of the Grindstone, but that simply removes the enchantment, and refunds you will experience orbs.
The simplest way to obtain the Mending enchantment (outside of commands) is to fish for it, (potentially using an AFK Fish Farm), or by finding a Villager that will trade you for it.