Concrete floor "too thin" for bolt and sleeve anchor
First - what kind of construction uses a 2" slab for a floor? Trying to get more concrete down there is unlikely without breaking out part of the floor because it needs something to displace in order to do what you want. Even if you got some down there it's very unlikely to hold. Why not go to a 3/8 x 2 1/4 expansion bolt. I know you're rightfully concerned with the 2" slab holding but it should be okay unless that door is a real behemoth. 

Another option is to improvise an extra large (i.e. 6"x 6") block that you can molly-bolt onto the 2" slab. What this does is take advantage of the shear strength of the molly by exerting force laterally when the door strikes the block or stop as opposed to a narrow stop that will want to tilt when it's struck exerting diagonal forces (up and down) on the 2" slab. If you go with an over-sized stop you could also strengthen it by using more than one anchor spreading out the load.