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

Reactions of a tripodal ligand, <i>N</i>,<i>N</i>′,<i>N</i>″-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⁺ (<b>1</b>), Co²⁺ (<b>2</b>), Zn²⁺ (<b>3</b>), Cd²⁺ (<b>4</b>)] and [Pd₆(TPPA)₈]­Cl₁₂·22H₂O (<b>5</b>). 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 <b>1</b>–<b>4</b>, but it transforms to the anti conformation in cage <b>5</b>. Because of the conformational transformation, the sizes of the windows and the volume of the internal cavity of cage <b>5</b> are increased. ¹H NMR and electrospray mass spectrometric studies show that cage <b>5</b> maintains its structural integrity in solution. Additionally, compounds <b>3</b> and <b>4</b> exhibit strong blue fluorescent emissions, which are 1 order of magnitude higher than that of the free ligand