Palladium(II) Complexation by <i>p-</i>Cyclophane Receptors. A Solution and Solid State Study

The coordination properties of the macrocyclic ligands 1,4,7,16,19,22-hexamethyl-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 studied by means of potentiometric and 1H and 13C measurements in aqueous solution. 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 both mono- and dinuclear complexes in aqueous solution, while L2 gives only mononuclear species. The potentiometric data indicate that in the L1 dinuclear complex each metal is coordinated by a triamine moiety. In the L2 mononuclear complex the Pd(II) ion is coordinated by the N3 unit. The N2O4 moiety does not show any binding ability toward Pd(II), but exhibits a high tendency to protonate. These solution data are confirmed by the crystal structures of [Pd2Cl2L1](ClO4)2·H2O (a) and [PdClL2H2(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 N3 moiety, with a coordination environment almost equal to that found in complex a. The N2O4 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 1H and 13C NMR measurements. The analysis of the NMR data reveals that the dinuclear L1 complex and the L2 mononuclear one show structural features in solution almost equal to those found in the solid state. Solution and solid state data indicate that the Pd(II) complexation gives a marked stiffening of the macrocyclic structures.