Complex sentence: The Venetians hoped to secure Ottoman consent to the occupation of Thessalonica, but when Zorzi attempted, probably in February 1424, to carry out is mission to the Sultan's court, not only was he unsuccessful, he was also arrested and imprisoned by Murad.
Split into simpler sentences: The Venetians hoped to secure Ottoman consent to the occupation of Thessalonica. When Zorzi attempted, probably in February 1424, to carry out his diplomatic mission to the Sultan's court, not only was he unsuccessful, he was also arrested and imprisoned by Murad.