Scaling molecules on top of arrow with chemfig

Question

i've a problem with chemfig-package. I want to draw some mulecules above an \arrow{-U>}, but i can't scale them in the right way. Does anybody has an idea to work around or do I just made a mistake?

Here's some code for you:

\documentclass[draft=yes,oneside]{scrbook}
\usepackage{chemfig}
\begin{document}

\begin{figure}[h!tbp,width=\textwidth]
\schemedebug{true}
\schemestart
  \chemfig[][scale=.7]{*6((-O_{2}N)-=-(-([::-60]-H)([::+60]=O))=-=)}\arrow{0}[,0]\+\arrow{0}[,0]
  \chemfig[][scale=.7]{[:75]HO-(=[::+60]O)-[::-60]-[::-60](=[::+60]O)-[::-60]HO}
  \arrow{-U>[\chemfig{N*6(-=-=-=)}][\chemfig{N*6(-=-=-=)}]}
\schemestop
\end{figure}
\end{document}

As you see, the molecule does appear above the arrow, but it isn't scaleable with [scale=.2], i.e.

Answer 1

Turning my comment into an answer (just to remove this question from the unanswered ones):

As always when you want to use square brackets in an optional argument you have to enclose the argument in an extra pair of braces. There are quite a number of questions related to this fact:

• LaTeX: optional arguments with square brackets
• ] inside an optional argument
• Bug? LaTeX misparses nested optional arguments

probably the most complete answer being the one by Frank Mittelbach to the second of the linked questions.

Let's see that it works:

\documentclass[draft=yes,oneside]{scrbook}
\usepackage{chemfig}
\begin{document}

\begin{figure}[h!tbp,width=\textwidth]
\schemedebug{true}
\schemestart
  \chemfig[][scale=.7]{*6((-O_{2}N)-=-(-([::-60]-H)([::+60]=O))=-=)}\arrow{0}[,0]\+\arrow{0}[,0]
  \chemfig[][scale=.7]{[:75]HO-(=[::+60]O)-[::-60]-[::-60](=[::+60]O)-[::-60]HO}
  \arrow{-U>[{\chemfig[][scale=.4]{N*6(-=-=-=)}}][{\chemfig[][scale=.4]{N*6(-=-=-=)}}]}
\schemestop
\end{figure}
\end{document}