Is the prayer of the congregants valid behind an imam who remembered in the second rak'ah that he had prayed without wudu, so he completed his prayer and then repeated it alone?

For the Imam to continue the prayer after realizing he is in a state of ritual impurity (hadath) is forbidden, and he must discontinue it. The best course of action is for him to appoint one of the congregants (ma'mumin) to succeed him, or for them to appoint someone, or for them to complete their prayers individually. The Imam must repent, and the prayer of the congregants is valid because they were unaware of the Imam's state of impurity.