If a motherboard doesn't explicitly support a RAM frequency, is still compatible?
The RAM would down-clock automatically to DDR3-2133 speed and it would run fine. This is safe and will only make you lose the speed edge of the faster module.
If there are more than one and they are different: The RAM in your system will 1. all run at the same speed, and 2. run at the lowest supported speed by any one stick of installed RAM. Different sticks of RAM can NOT run at different speeds in the same system. The motherboard will make both RAM modules runs in the lower speed.