He said that in a secret letter. He did not expect that secret letter to Russia to appear in public.
He does not want to piss-off his own party even more, but he's managing to do it. They hate his withdrawal time-table from Iraq, because it is the same as the one the Bush administration negotiated with Iraq, and now he's pissing-off democrats who care about our national security.
I do agree with the other answers here though, he was doing it to get cooperation with the Russians over the Iran issue. It is just wierd that he attempted to do something so large in secret.