Pair iMac with iphone

Bluetooth pairing an iPhone and a Mac is not supported. 

If you are attempting to use Handoff/Continuity or AirDrop, that does involve Bluetooth but the devices are NOT paired; Bluetooth just needs to be on and the devices in range of each other. The following may help with Handoff/Continuity issues: Use Continuity to connect your Mac, iPhone, iPad, and Apple Watch - Apple Support

Also see Share files between your Mac and Bluetooth devices - Apple Support

And see the information in: Bluetooth: Why can't I pair my iPhone or … - Apple Community

If you are referring to pairing via Bluetooth, there is no protocol to do such pairing, nor is there a requirement. While there are many activities that might require Bluetooth to be turned on both devices, those activities do not require pairing in the traditional sense.

What are you hoping to accomplish?