PCIe 2.1 x16 compatibility

are you asking if a PCIe 2.1 x16 is compatible with a PCIe 1.1 x16 slot?

If so, the answer is "usually"

There have been reports of 2.1 spec cards not operating correctly in 1.x spec slots but these are most likely actually pre 1.1 spec slots. If you are looking for a new card, consider getting a 3.0 spec (currently AMD HD7000 series and NVidia 600 series) as these do not have the same issues