Transition-metal nitrides have applications in a range of technological fields. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K elemental copper reacts with nitrogen, forming copper diazenide (CuN2). Through a combination of synchrotron X-ray diffraction and first-principles calculations we have explored the stability and electronic structure of CuN2. We find that the novel compound remains stable down to 25 GPa before decomposing to its constituent elements. Electronic structure calculations show that CuN2 is metallic and exhibits partially filled N2 antibonding orbitals, leading to an ambiguous electronic structure between Cu+/Cu2+. This leads to weak Cu-N bonds and the lowest bulk modulus observed for any transition-metal nitride.