  Using data taken at 29 center-of-mass energies between 4.16 and 4.70 GeV with
the BESIII detector at the Beijing Electron Positron Collider corresponding to
a total integrated luminosity of approximately 18.8 $\rm fb^{-1}$, the process
$e^+e^- \to p p \bar{p} \bar{n} \pi^{-} + c.c.$ is observed for the first time
with a statistical significance of $11.5\sigma$. The average Born cross
sections in the energy ranges of (4.160, 4.380) GeV, (4.400, 4.600) GeV and
(4.610, 4.700) GeV are measured to be $(21.5\pm5.7\pm1.2)$ fb,
$(46.3\pm10.6\pm2.5)$ fb and $(59.0\pm9.4\pm3.2)$ fb, respectively, where the
first uncertainties are statistical and the second are systematic. The line
shapes of the $\bar{p}\bar{n}$ and $pp\pi^-$ invariant mass spectra are
consistent with phase space distributions, indicating that no hexaquark or
di-baryon state is observed.
