You can, however, gift any AI empire with money, gem, and items. This may not improve their opinion of you per say (I think it helps delaying war declaration, but can't confirm it), but do have its uses.
To quote Gandalf Parker:
Check the message menu. The AI might not improve its opinion of you (I dont know for sure) but you can send money, gems, items. Its a way to boost the nations you know you can beat. Let them beat the ones they can beat that you cant.
Its a deep and well balanced game.
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-Gateway103