Complex sentence: There was no formal act on the part of the Holy See recognizing Dol as a new metropolitan church; it never had control over Rennes or Nantes, and Dol mainly exercised ascendancy over the new Sees of St. Brieuc and Tréguier.
Split into simpler sentences: There was no formal act on the part of the Holy See recognizing Dol as a new metropolitan church. Dol never had control over Rennes or Nantes, and mainly exercised ascendancy over the new Sees of St. Brieuc and Tréguier.