'before leaving FOR work' generally means 'before leaving to go to a location of work'. By using 'for' there is a sense of movement of direction, of going ..... towards the location WHERE you will be working. 'before leaving TO work' differs from the above in that it is simply saying you are leaving SO you can work.

It’s better to use “for”, because the emphasis is on the time of your leaving. The time that you get to your workplace is irrelevant to the sentence.

Thank you!

If you use "to" instead of "for" in this sentence, most people will understand you (because it is a small error). But still, it is an error and "for" is correct. You could say, "before going to work".