Complex sentence: Two neptunium chlorides, NpCl 3 and NpCl 4, have been characterized and although several attempts to create NpCl 5 have been made, they have not been successful.
Split into simpler sentences: Two neptunium chlorides, NpCl 3 and NpCl 4, have been characterized. Although several attempts to create NpCl 5 have been made, they have not been successful.