Does a divorce take place if a man says to his wife: "I swear by Allah I will divorce you if you do something," and then she does it, knowing that his intention was only to threaten?

What appears is that this is a formula of threat and warning of divorce, so no divorce occurs by it, as this is a promise and not an execution.

If the wife does what she was forbidden from, no divorce befalls her, but the husband is obligated to perform an expiation for breaking an oath (kaffarat yameen), which is: feeding ten poor persons, or clothing them; and if he finds not, then fasting for three days.

It is advisable to beware of using divorce pronouncements for threatening purposes.