The 5m is optimistic, with RF quiet environment and large direction receiver, unless this is a perpetual motion machine breaking the laws of physics.
A 2.5cm to 20cm range is more realistic. It doesn't matter how little power the chip uses. Even if it has a BIG capacitor and only transmits in 1:100 duty cycle low data rate bursts.
You can't fit any kind of sensible WiFi aerial on a 2mm x 2mm chip either.
This is no different to an RFID chip, simply sending temperature rather than an ID Tiny range unless less specialist kit to power and talk to it and listen to it. "Smart Building" uses are fantasy.