The way I look at it is that Iran is willing to "live" with sanctions. They are totally unwilling to give up their nuclear ambitions. They are totally unwilling to do anything that would slow down the nuclear weapons program.
For Iran, the goal of negotiations is to give them time to develop nuclear weapons. They realize that the nuclear program could be serious slowed down if their was more serious sanctions which would hurt their economy and might cause the Mullahs to lose political control of Iran or military action (a bombing campaign of their facilities).
The US and the interest of most Western countries is to prevent Iran from getting nuclear weapons. Therefore, Iran will keep talking to give the gullible negotiators the illusion that negotiations might achieve that goal. But in reality, they will not accept any such deal. Iran will sign a deal that they can show is a political win for Iran if it also does not slow the nuclear program.
Obama wants a deal with Iran for his "legacy" because right now, he has very little of a legacy. The question for the world is how desperate is Obama to sign a deal? Will he sign a deal that is a clear political win for Iran and that does not slow the Iran nuclear program just so that he has something for his legacy? I think it will be difficult politically for Obama to sign something like that as it has too much opposition not only from Republicans but from Democrats.
So, what will likely happen is some kind of placeholder deal that gives them time. Obama needs time for the public's attention to be turned to something else - and then perhaps he can make another attempt.