Chloride Encapsulation by a Tripodal Tris(4-pyridylurea) Ligand and Effects of Countercations on the Secondary Coordination Sphere

A series of anion complexes of the 4-pyridyl-functionalized tripodal tris­(urea) receptor (L) have been synthesized. Ligand L forms the 1:1 anion complex [Cl⊂L]⁻ with various metal chloride salts, MCl_x (M = Na, K, Mg, Ca, Mn, Co, x = 1 or 2). When M = Na, K, Mg, and Ca, the metal ions are not coordinated by the pyridyl groups of L but are involved in second-sphere coordination to form three-dimensional structures. However, in the complex of Co²⁺, the transition metal ions are directly coordinated by the pyridyl groups. Interestingly, the Mn²⁺ ion forms two complexes with both of the above two types of structure. In all complexes, one chloride ion is “half” encapsulated in the cleft of one ligand by N–H···Cl hydrogen bonds to form the [Cl⊂L] units, which are further linked via intermolecular interactions into three-dimensional structures. Moreover, the fluoride and carbonate complexes of L have also been obtained. The solution anion binding properties of L have been studied by ¹H NMR spectroscopy and electrospray ionization mass spectrometry.