Nah. Chemical weapons filter out of the area too easily. I want the area to remain lifeless for quite some time, not for a short while.
Well, except maybe a nuclear leak would work... Chernobyl proved that keeps most humans away, but doesn't impact animals that much. But building a nuclear reactor from scratch is a lot of work, and people look at you funny when you start buying up a lot of smoke detectors so you have a reactive fuel to use as a base (americium isn't as good as uranium, but it's a lot easier to get your hands on).