You are making the assumption that Putin would have responded to ‘negotiations’ with sincerity. Why?
He already spent the last few months before the invasion stringing along the west/world, openly stating there was no planned invasion, having meetings with Macron etc saying they were never going to do anything despite more and more troops organising on the border. First it was for training, then it was for a peace keeping mission.
The Russian government are full of it. Now they are murdering people, destroying a sovereign nation, creating havoc. For what? Due to Putin’s own vanity and megalomania, wanting to carve out a legacy. He will have a legacy alright, but it won’t be the one he hoped for.