Complex sentence: Leopold II (9 April 1835--17 December 1909), reigned as King of the Belgians from 1865 to 1909, became known for the founding and exploitation of the Congo Free State as a private venture.
Split into simpler sentences: Leopold II (9 April 1835--17 December 1909) reigned as King of the Belgians from 1865 to 1909. He became known for the founding and exploitation of the Congo Free State as a private venture.