Crystal and molecular structure ofN-[1-(2-phenylethyl)-4-piperidinylium]-N-phenylpropanamide (fentanyl) citrate-toluene solvate

The crystal structure of fentanyl has been determined as part of a structure-activity relationship study on a congeneric series of 4-anilinopiperidines. The compound crystallizes as a citrate toluene solvate, C₂₂H₂₈N₂O · C₆H₈O₇ · C₇H₈, in the monoclinic space groupP2₁/c witha = 18.292(6),b = 9.825(4),c = 19.479(4) Å, = 107.43(3) °, andZ = 4. The structure was solved by direct methods using the program Simpel, and refined by block-diagonal least squares to anR value of 0.092. The torsion angle that describes the conformation of theN-phenyl ring relative to the piperidine ring is 123 ° and differs from that found in other 4-substituted anilinopiperidines (172–177 °).