Generally any element heavier than iron releases more energy if you split it than it contains through nuclear binding. U-235 is somewhat unique however in that it releases more energy than is required to split it, and can sustain a chain reaction. If it wasn’t for the unusual properties of U-235, it’s likely we would not have any form of nuclear power today, controlled or uncontrolled. U-235 must be separated from its non-fisionable isotope U-238 (depleted uranium) before it can be usable as fuel. Plutonium is entirely man-made, due to its short half-life, and requires U-235 to make it (as a byproduct), but has similar properties. Nuclear fusion, on the other hand, releases energy when atoms are combined, not split. The nucear binding curve (google it) determines at what atomic weight atoms loose or gain energy as you split/combine them. The element iron happens to be the center point.
Any atom above iron on the periodic table can have a fission energy yielding event.
Has there ever been experiments?
Yes but I am not aware of much documentation on the internet. Thorium has been played with as a possible fuel. Google "Fission experiments" and add your element of choice. Try pretty much any element above mercury and you start seeing interesting results.