Self-Assembly of Discrete M6L8 Coordination Cages Based on a Conformationally Flexible Tripodal Phosphoric Triamide Ligand

Reactions of a tripodal ligand, N,N′,N″-tris­(3-pyridinyl)­phosphoric triamide (TPPA), and a series of transition-metal ions result in the assembly of five discrete M6L8 coordination cages [M6(TPPA)8(H2O)12]­(ClO4)12·57H2O [M = Ni2+ (1), Co2+ (2), Zn2+ (3), Cd2+ (4)] and [Pd6(TPPA)8]­Cl12·22H2O (5). X-ray structural analyses reveal that the cages have large internal cavities and flexible windows. The flexible ligand TPPA adopts the syn conformation in cages 1–4, but it transforms to the anti conformation in cage 5. Because of the conformational transformation, the sizes of the windows and the volume of the internal cavity of cage 5 are increased. 1H NMR and electrospray mass spectrometric studies show that cage 5 maintains its structural integrity in solution. Additionally, compounds 3 and 4 exhibit strong blue fluorescent emissions, which are 1 order of magnitude higher than that of the free ligand.