There were no warning leaflets dropped that discussed atomic bombs. I have seen no actual evidence that warning leaflets discussing conventional bombs even mentioned Hiroshima as a target of any sort (despite sometimes being asserted as such online, it was never a city listed on any of the "Le May leaflets"), and I have not seen evidence that warning leaflets of any sort were dropped on Hiroshima (also sometimes asserted online, but without citation and in some cases directly opposed by other sources). There were non-specific bombing leaflets dropped on various Japanese cities, and there were specific "we might bomb these city" leaflets, but again, I don't think any of those covered the atomic-bombed cities.

There were no back-channels whatsoever between the US and Japan (the Japanese tried to use the Soviets as a neutral party for negotiations but it didn't happen). The US did issue the Potsdam Declaration, which was a "surrender or die" sort of thing, but that is not a specific-enough warning to protect civilians.

I've written at some length about this here — if you want the straight dope on the atomic bomb warning leaflets read the report by the guy who ran the project, in which he clearly outlines that no leaflets were dropped prior to the bombing of Hiroshima (they did not even exist), and that the post-Hiroshima leaflet work was beset by various logistical problems so that Nagasaki got no leaflets until after it had been nuked.

One can disagree or agree with the use of the atomic bombs, but nobody should base their assessment of them on the idea that useful warning was given to save lives — it was simply not done.