If I buy property before my divorce is final does my ex have any rights?

Assets are usually divided as of the date of separation.  However, if you would be using marital assets to buy a house, those marital assets are part of the total pot you will be dividing. If you used marital funds for the down payment this would also be considered in the property division. 

If the funds that you used were separate property, then in that case, if you keep separate property separate it does not become a marital asset.  So if you buy the house alone using only that money and he never lives there or contributes to upkeep, it will likely remain as separate.

Before you do anything, you should consult with an attorney so you can be sure of your rights under applicable state law.