Does divorce occur after the wife menstruates and becomes pure, based on the husband’s oath to that effect, and what is the ruling on the period preceding menstruation?

Your statement to your wife: "It is over" is among the allusions to divorce. The matter depends on your intention. What you intended was a promise of divorce and a determination to carry it out in the future after she becomes pure from menstruation. This does not effect a divorce. Since you swore an oath to that, if your wife becomes pure, do not divorce her, and you must pay an expiation for breaking an oath (kaffarat yamin).