LONDON--Senior managers at Google artificial-intelligence unit DeepMind have been negotiating for years with the parent company for more autonomy, seeking an independent legal structure for the sensitive research they do. DeepMind told staff late last month that Google called off those talks, according to people familiar with the matter. The end of the long-running negotiations, which hasn't previously been reported, is the latest example of how Google and other tech giants are trying to strengthen their control over the study and advancement of artificial intelligence. Earlier this month, Google unveiled plans to double the size of its team studying the ethics of artificial intelligence and to consolidate that research. Google Chief Executive Sundar Pichai has called the technology key to the company's future, and parent Alphabet Inc. has invested billions of dollars in AI.