The silsesquioxane ligand (Bu-i)(7)Si7O9(OH)(3) ((POSSH3)-P-iBu) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U-3((POSS)-P-iBu)(3)]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.