Check the motherboard manufacturer's webpage for the motherboard you are wanting to buy, it will tell you if it is compatible. However, with the way Ryzen works, you are losing 5% or more performance by not using DDR4-3200 or faster.
Really? It's a simple process of looking at the memory compatibility lists for a motherboard. The ASUS Crosshair VII Hero, for example, lists 5 models of DDR4-2400 made by Crucial.