You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
What is the molecular formula of the molecule identified by 2-[(2-chloro-5-nitrophenyl)methyl]-1-phenyl-1,3,4,5-tetrahydropyrrolo[1,2-a][1,4]diazepine ?
user
C21H20ClN3O2
assistant