It is true that <D- maps to command key. You can see it by :help <D-.
However, this will not work in your vim+terminal. It only works in MacVim. See here
The best shot for your intention is to map the combination as hex code, sending to terminal.
Still I will not recommend doing this. command is too important for mac os environment. For example, anyone uses an input method can’t map it to command+space.\
Please consider using the usual method here: <leader>.