Boxen bzw klammern mit Reaktionspfeilen

[cgnieder]

Weitere Beispiele zu Deinem Ausgangscode:

\documentclass{article}
\usepackage{chemfig}
\begin{document}

\begin{equation}
  \setatomsep{2em}
  \schemestart
  \chemfig{
    Mg(-[1,,,,dash pattern=on 1pt off 2pt]
       O(-[:350]R^2)(-[:120]R^2))
      (-[3]R^1)
      (-[5]Br?[a])
      (
        -[7,,,,dash pattern=on 1pt off 2pt]
        Br-[5]Mg?[a,,{dash pattern=on 2pt off 2pt}]
        (-[7]R^1)
        (-[5,,,,dash pattern=on 1pt off 2pt]O(-[:300]R^2)(-[:150]R^2))
      )
  }
  \arrow(--.174){<=>}
  \subscheme[-90]{
    \chemup.
      \subscheme{
        2\,RMgX \arrow{<=>} \printatom{R_2Mg} \+ \printatom{MgBr_2}%
      }%
      \chemdown\}
      \arrow{0}[,0]
      Schlenk-Gleichgewicht 
  }
  \arrow(.-19--){<=>}[-90]
  \chemfig{
    Mg(
       -[5,,,,dash pattern=on 1pt off 2pt]
       O(-[:210]R^2)(-[:280]R^2)
      )
      (-[7,,,,dash pattern=on 1pt off 2pt]O(-[:330]R^2)(-[:260]R^2))
      (-[3]Br?[b])
      -[1]Br-[3,,,,dash pattern=on 2pt off 2pt]
      Mg?[b,,{dash pattern=on 2pt off 2pt}]
      (-[3]R^1)
      (-[1]R^1)
  }
  \schemestop
\end{equation}

\begin{equation}
  \setatomsep{2em}
  \schemestart
  \chemfig{
    Mg(-[1,,,,dash pattern=on 1pt off 2pt]
       O(-[:350]R^2)(-[:120]R^2))
      (-[3]R^1)
      (-[5]Br?[a])
      (
        -[7,,,,dash pattern=on 1pt off 2pt]
        Br-[5]Mg?[a,,{dash pattern=on 2pt off 2pt}]
        (-[7]R^1)
        (-[5,,,,dash pattern=on 1pt off 2pt]O(-[:300]R^2)(-[:150]R^2))
      )
  }
  \arrow(--Schlenk[inner sep=3pt,draw=red,fill=red!10]){<=>}
  \subscheme{
    2\,RMgX \arrow{<=>} \printatom{R_2Mg} \+ \printatom{MgBr_2}
  }
  \arrow(.-10--){<=>}[-90]
  \chemfig{
    Mg(
       -[5,,,,dash pattern=on 1pt off 2pt]
       O(-[:210]R^2)(-[:280]R^2)
      )
      (-[7,,,,dash pattern=on 1pt off 2pt]O(-[:330]R^2)(-[:260]R^2))
      (-[3]Br?[b])
      -[1]Br-[3,,,,dash pattern=on 2pt off 2pt]
      Mg?[b,,{dash pattern=on 2pt off 2pt}]
      (-[3]R^1)
      (-[1]R^1)
  }
  \arrow(@Schlenk--){0}[90,.1]
  Schlenk-Gleichgewicht 
  \schemestop
\end{equation}

\end{document}

Grüße