How to put a box around atoms in ChemFig?

How does one put a box around a group of atoms?

For example, I'm looking to put a box around the hydroxyl group with its bond (--OH) in the following molecule and can't seem to get any luck with the documentation or fbox. Any ideas?

\chemfig{C(-[2]H)(-[4]H)(-[6]H)-C(-[2]H)(-[6]H)-OH}


TikZ masters surely can do this better but a basic idea is this: use chemfig's @{node name} syntax for marking nodes and draw the rectangle afterwards using the specified nodes:

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

\chemfig{C(-[2]H)(-[4]H)(-[6]H)-C(-[2]H)(-[6]H)-[@{b1,0}]O@{H}H}
\chemmove{
\draw[-,red]
(b1) -- ++(0,.25) -| (H.east)
(b1) -- ++(0,-.25) -| (H.east) ;
}

\end{document}