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.