Complex sentence: When Ramon Berenguer IV married Petronila of Aragon, he declined the title of "Count of Barcelona" for his heirs in favour of that the "King of Aragon" designating himself "Princeps" (from Latin: the first one in dignity).
Split into simpler sentences: When Ramon Berenguer IV married Petronila of Aragon, he declined the title of "Count of Barcelona" for his heirs in favour of that the "King of Aragon". Because of that, he denominated himself "Princeps" (from Latin: the first one in dignity).