In this paper, we report the synthesis and description of a new family of polynuclear cobalt(II) complexes. Starting from the same initial compounds but varying the reaction time results in the formation of several new clusters, an original structure based on [Co-14][Co-4] clusters was obtained, representing the first one-pot synthesis of a cobalt aggregate salt reported in the literature. The synthesis and magnetic properties of these cobalt compounds are discussed. Three of them display a binuclear molecular structure (1-3) with two encapsulated Co-II ions and show slow relaxation of magnetization at small applied magnetic field (U-eff = 10.7 K for 2 and U-eff = 20.3 K for 3), a characteristic of single-molecule-magnet materials.