I can't see any mentions of 1100 series in docs for 2100 wlc, does it really mean, that 1100 series can't connect to 2100 series?
Q. What access points are supported by the Cisco 2100 Series Wireless LAN Controller?
A. The Cisco 2100 Series Wireless LAN Controller supports Cisco Aironet access points running Lightweight Access Point Protocol (LWAPP) only. This includes any LWAPP-enabled Cisco Aironet 1130, 1230, 1240, 1250, and 1500 Series access points.
In question about 'what ap's can run lwapp' answer also don't specify 1100 series, although there is LAP model for 1121.
Q. Which AP models can run Lightweight AP Protocol (LWAPP)?
A. These Cisco Aironet AP platforms are able to run LWAPP:
Aironet 1500 Series
Aironet 1240 AG Series
Aironet 1230 AG Series
Aironet 1200 Series
Aironet 1130 AG Series
Aironet 1000 Series