How to connect with those distant senior colleagues who are in their last PhD year? I have some senior colleagues in their final year of PhD. busy most of their time and in quite stress (as much any last year PhD student). we share a lot of history together. and also have some sort of common academic fields interests. It has been a really long time since we got together. 

I am interested in partnering with them doing some projects together. What is the proper way of connecting to them ? what the best way of explaining what i would like to do with them. I am afraid that an extra profession email would be silly but also a too friendly one would be understood as unserious. 

If i happen to talk to them what kind of topics that should not be brought up? for a PhD student in his last year of program.
If they are in their last year, collaboration might not be practical. Last year students are typically writing their thesis, applying for jobs, etc. Further, they may leave academia after graduation. 

With this in mind, you can treat them like you would anyone else. An informal e-mail should do fine, or you can request a phone/Skype connection.