Hard to say, I am not an expert to this, but I think it's somewhere like Layer 1 and 1/4
Eg. WPA2 is defined in IEEE 802.11 extension with letter
i. Because Layer 1 usualy defines physical properties of signals and encoding(+encryption), I suppose it definately belongs here.
On the other side, there is some more communication which looks like L2. Every wireless client has it's own MAC-like address (BSSID) and they are exchanging keys for encryption so there are some elements pointing to Layer 2, but it's much closer to Layer 1.
ISO/OSI Model is just theoretical and many practicaly used protocols cannot be divided strinctly into one layer, so my opinion is Layer 1.25
EDIT: It definately cannot be higher than L2, because we can do eg. ARP request over wlan which is connecting L2 and L3.