Palladium(II) complexation by p-cyclophane receptors. A solution and solidstate study

The coordination properties of the macrocyclic ligands 1,4,7,16,19,22-hexam ethyl-1,4,7,16,19,22-hexaaza[9.9]-p-cyclophane (L1) and 1,4,7-trimethyl-19, 22,28,31-tetraoxa-1,4,7,14,23-pentaaza[9.25]-p-cyclophane (L2) have been st udied by means of potentiometric and H-1 and C-13 measurements in aqueous s olution. L1 is composed of two equal triamine binding units connected by p- phenylene spacers. L2 presents a similar molecular architecture, a triamine moiety of L1 being replaced by a cyclic N2O4 binding unit. L1 can form bot h mono- and dinuclear complexes in aqueous solution, while L2 gives only mo nonuclear species. The potentiometric data indicate that in the L1 dinuclea r complex each metal is coordinated by a triamine moiety. In the L2 mononuc lear complex the Pd(LT) ion is coordinated by the N-3 unit. The N2O4 moiety does not show any binding ability toward Pd(II), but exhibits a high tende ncy to protonate. These solution data are confirmed by the crystal structur es of [Pd(2)Cl(2)L1](ClO4)(2). H2O (a) and [PdClL2H(2)(H2O)](ClO4)(3) (b). In complex a, each Pd(II) ion is four coordinated by the three amine groups of the triaza moiety and a chloride anion, in a square planar geometry. In the mononuclear complex b, the metal is coordinated by the N-3 moiety, wit h a coordination environment almost equal to that found in complex a. The N 2O4 moiety is diprotonated and encapsulates in its cavity a water molecule, held by a hydrogen-bond network. The solution structures of the L1 and L2 complexes have been studied by means of H-1 and C-13 NMR measurements. The analysis of the NMR data reveals that the dinuclear L1 complex and the L2 m ononuclear one show structural features in solution almost equal to those f ound in the solid state. Solution and solid state data indicate that the Pd (II) complexation gives a marked stiffening of the macrocyclic structures.