I've not seen any suggestion of a link between the two; I'm not sure what the link would be. I would have expected any useful findings from Unit 731 to go into the United States' general chemical & biological weapons programme.

The general aim of MKUltra was into "mind control", but with a very strong focus on use of drugs, particularly newly developed psychedelics, for interrogations. This doesn't seem to fit with the aims or methods of Unit 731.

The initial stimulus for MKUltra seems to have come primarily from two sources. First, the apparent 'brainwashing' of American captives in the Korean war, the desire to defend against this, and to make use of it. Second, the development of psychedelic drugs and their perceived potential for manipulating or breaking down people's perceptions and consciousness.

There was also a separate investigation by the US military into the use of psychedelics in warfare. I don't know so much about this, but it's hard to see any potential link with Unit 731 - in fact, the point was to make a more humane alternative to the US' WMD arsenal, a chemical weapon which would incapacitate the enemy without causing serious harm.