There are a number of misconceptions in your question.
Neutronium does not exist. Its name was coined in 1926 to designate a hypothetical element number zero (no protons in the nucleus). The core of a neutron star is sometimes said to consist of neutronium, but mostly in popular literature. In reality, a neutron star core is effectively one gigantic atomic nucleus.
Neutron stars do not explode. However, they can implode into a black hole. That does not create any lighter elements. If there was an "element zero" then no lighter element could exist anyway.
Iron is the element with the highest mass defect. As more protons and neutrons are added to light nuclei, energy is released in the merge (nuclear fusion) - up until iron is reached. Beyond iron, iron is released as the nucleus is split (nuclear fission).