Germany is Europe's biggest importer of Russian gas, and has been particularly squeezed as Russia has reduced supplies in recent months.
Chief executive Klaus-Dieter Maubach said the deal would help Uniper's role as "a system-critical energy supplier".
Before Russia invaded Ukraine it supplied Europe with about 40% of its natural gas, and it has responded to Western sanctions by gradually cutting off supplies.
At the start of this month, Russia halted supplies through the Nord Stream 1 pipeline, claiming repairs were needed - but later said flow would not resume until sanctions were lifted.
Uniper, which operates gas, coal, and hydro plants across Europe and is currently controlled by Finnish state-owned energy company Fortum, is the biggest buyer of Russian gas in Germany.
Fortum said Uniper had accumulated close to €8.5bn (£7.4bn) in gas-related losses "and cannot continue to fulfil its role as a critical provider of security of supply as a privately-owned company".