tmux only binds "single" characters (8-bit) plus a few special keys which can be found in the terminal database.
The manual is a little vague:
tmux allows a command to be bound to most keys, with or without a prefix key. When specifying keys, most represent themselves (for example ‘A’ to ‘Z’). Ctrl keys may be prefixed with ‘C-’ or ‘^’, and Alt (meta) with ‘M-’. In addition, the following special key names are accepted: Up, Down, Left, Right, BSpace, BTab, DC (Delete), End, Enter, Escape, F1 to F12, Home, IC (Insert), NPage/PageDown/PgDn, PPage/PageUp/PgUp, Space, and Tab.
However, this has been discussed before (along with the ctrl and alt modifiers):