Presumably, you are paring your Athlon X4 750K with an FM2+ motherboard -- many of which support DDR3 2400MHz. A kit from the mobo QVL list would be in order.
According to these folks at the
OverClock.Net AMD Athlon 750K/760K/BE Owners Club, clock the UNB to a minimum 2200MHz with DDR3 2133, and if you are not running 4.5+ GHz at 1.5v 55C load, **You Are Doing It Wrong**
A UNB at 2500MHz with DDR3 2400MHz might be a stretch but possible if you can keep your UNB volts down. The 750K is essentially one-half of a 246mm2 SOI Trinity A10-5700 so it will take some volts but the UNB volts tend to drive up heat.