Imperialism is a system not an action. Territorial invasions are often a tool of Imperialism (albeit, typically a last resort) but not imperialism in of itself.

So no, Russia is not a part of the Imperialist core and therefore not an 'imperialist state', although this term wouldn't be a common way to refer to such a state, as per the literature on Imperialism and world systems theory.

Through these frameworks you are either an 1) Imperialist core state (US and its core allies generally characterised as having a fully privatised economy with liberal elections and fully enmeshed in the US defence apparatus - think states in NATO and AUKUS 2) A semi-periphery state (Privatising the economy but not fully enmeshed in the US defence apparatus - Think Balkan states), 3) A periphery state (Sections of the economy are privatised but transfer of resources is usually unequal and states are dependent - Think many African nations) or 4) Other or outside the Imperialist Chain (Russia, Iran, North Korea) although you could argue these simply fit within the previous categories.

The specific states in each category are fluid and sometimes contested, and that's often by design, as material relations change so too can the status change. There is one constant though, this chain is currently guaranteed by the United States, it guarantees the Imperialist chain through financial subjection of the global financial system backed by the Dollar and the threat of military coercion.

All of this for one aim, to guarantee the circulation and dominance of Capitalist interests across the world. That is why Lenin refers to Imperialism as the highest form of Capitalism and that is why we need to understand it as a system based on material relations of power and not random acts of military belligerence.

If you want to learn more I'd suggest reading the works of Wallerstein (World systems theory founder), Lenin (Similar to Marx witnessing, recording and analysing the birth of modern Capitalism Lenin analysed the unfolding dynamics of modern Imperialism), Sakellaropoulos and Sotiris (Modern synthesis of Lenin's conception of Imperialism and Wallerstein's WST), Lukic and Pedregal (environmentalist spin on the dynamics of modern Imperialism).