Sidious offered the power "he knew" to stop Padme from dying, Anakin falls for it thinking that Sidious has this power. When he pledges to the Sith, Sidious doesn't know . By lying to Anakin about this power got him to turn. But I think Sidious did this because he wanted to get rid of the one person who might've been able to turn Anakin back: Padme. With the Jedi gone and Padme dead, Anakin/Vader was his. (Until ROTJ) What do you think?