Complex sentence: The building was the first to receive the U.S. Green Building Council's LEED Platinum rating, making it the most environmentally friendly building in the country at the time when it was built.
Split into simpler sentences: The building was the first to receive the Leadership in Energy and Environmental Design (LEED) "Platinum" rating from the United States Green Building Council. This recognition confirmed that the center was the most environmentally friendly building in the country at the time when it was built.