According to the Senate report on Russian interference, in 2017-2018 he was working with Russia to carve out an Autonomous Republican of Donbas in Southeastern Ukraine, part of the plan was getting Trump to wink and look the other way.
I posted more about this over in P&E, including a link to an article about how all the Russian oligarchs are hanging out in Dubai these days to avoid sanctions.
When I made that post, I forget to generate the non-paywalled link to that article. Here it is