What is the ruling on the prayer of an Imam who remembered during his Imamate that he was not in a state of purity (wudu'), so he completed the prayer and informed the congregants of the necessity of re-performing it?

It was obligatory for you to interrupt the prayer upon remembering the ritual impurity. You could have appointed a successor, or the congregants could have put forward one of them as an Imam. You must repeat the prayer because you prayed in a state of ritual impurity. As for the congregants, they do not need to repeat the prayer, according to the more preponderant opinion.