How can I vent a portable A/C unit through bars on a window?
A picture might be helpful here, but generally speaking, any HVAC installer should be able to create an "adapter" out of sheet metal to fit between the bars and connect to your hose. It's not critical that it maintain the same shape, so you can convert from a round hose to a square vent as long as it supports the same or larger volume of airflow. 

Just as an example, my stoves exhaust fan has a round duct, but I have a rectangular vent on the outside of my house.