Curiously, on many terminal emulators the following work:

Ctrl-2 = Ctrl-@ = NUL byte

Ctrl-3 = Ctrl-[ = ESC

Ctrl-4 = Ctrl-\ = default for SIGQUIT

Ctrl-5 = Ctrl-] = jump to definition in vim

Ctrl-6 = Ctrl-^ = mosh escape key

Ctrl-7 = Ctrl-_ = undo in Emacs

I think these probably originate in xterm.