You need a military stalemate for a diplomatic solution.
Diplomacy for the Russkies is just another means of warfare. Diplomacy in this context also implies “Russia gets to keep Crimea and setup a Soviet zone inside Ukraine”, because it seems to mean as if the Russians have a legitimate beef to negotiate, when they are only thieves and murderers. The real solution isn’t diplomacy— but only to play at diplomacy, while arming the Ukrainians to continue their resistance and make the Russians think twice.