posted by Jake .
C6H5NO2+? = C6H5N2Cl
I doubt this conversion can be done in a single step.
Here is a three-step procedure which might work. If one of the other tutors has a better idea, I am sure it would be welcome:
Reduce the nitrobenzene to aniline with Fe/HCl, then treat the aniline with H2SO4/HNO2 to get the phenyl diazonium cation. Finally, add HCl to get the phenyl diazonium chloride salt.