Observation of *

Abstract Using data taken at 29 center-of-mass energies between 4.16 and 4.70 GeV with the BESIII detector Beijing Electron Positron Collider corresponding to a total integrated luminosity of approximately 18.8 , process e^+e^- \to p \bar{p} \bar{n} \pi^{-} + c.c. is observed for first time statistical significance 11.5\sigma . The average Born cross sections in energy ranges (4.160, 4.380) GeV, (4.400, 4.600) (4.610, 4.700) are measured be (21.5\pm5.7\pm1.2) fb, (46.3\pm10.6\pm2.5) fb (59.0\pm9.4\pm3.2) respectively, where uncertainties second systematic. line shapes \bar{p}\bar{n} pp\pi^- invariant mass spectra consistent phase space distributions, indicating that no hexaquark or di-baryon state observed.