Is an employee who attempted to expose his boss's transgressions of corruption and bribery at work absolved of responsibility, even though his attempts were unsuccessful and led to harassment that might result in the termination of his contract?

You must advise your superior and forbid him from evil, and remind him of Allah and the Last Day, unless you fear harm to yourself. If he does not respond, then refer his matter to the competent authorities with evidence, unless you fear harm to yourself. Since you have advised him, informed your colleagues, and referred his matter to his superior, and that has not been effective, then you have discharged your responsibility, and you will have a great reward for your effort.