A tetranuclear CuII complex, Cu4(dafone)4(N3)8 (1, dafone = 4,5-diazafluoren-9-one) is described. The propensity of dafone to chelate through one short and one long bond is readily accommodated by the tendency of CuII to undergo Jahn–Teller elongation in its six-coordinate complexes. The crystal structure of 1 consists of two distortion isomers of the tetranuclear complex molecule. In each isomer, there are four μ1,1-azido bridges and two μ1,1,1-azido bridges as well as two terminally coordinated azide ions. The isomers, both of which are centrosymmetric, differ in the way the Jahn–Teller axes of the different CuII sites are aligned. Magnetic susceptibility measurements show an overall ferromagnetic interaction, which could be fitted with a two parameter model (J1 = 22.8 cm–1, J3 = –8.2 cm–1). The magnetic interaction was modelled by using DFT calculations, which showed that the two isomers are magnetically quite different; one is a ferromagnetically coupled tetramer, and the other may be viewed as two ferromagnetic dimers. The X-band EPR spectra are in broad agreement with this model.