Synthesis, Protonation and CuII Complexes of Two Novel Isomeric Pentaazacyclophane Ligands: Potentiometric, DFT, Kinetic and AMP Recognition Studies

The synthesis and coordination chemistry of two novel ligands, 2,6,9,12,16-pentaaza[17]metacyclophane (L) and 2,6,9,12,16-pentaaza[17]paracyclophane (L-2), is described. Potentiometric studies indicate that L-1 and L-2 form a variety of mononuclear complexes the stability constants of which reveal a change in the denticity of the ligand when moving from L-1 to L-2, a behaviour that can be qualitatively explained by the inability of the paracyclophanes to simultaneously use both benzylic nitrogen atoms for coordination to a single metal centre. In contrast, the formation of dinuclear hydroxylated complexes is more favoured for the paro L-2 ligand. DFT calculations have been carried out to compare the geometries and relative energies of isomeric forms of the [CuL](2+) complexes of L-1 and L-2 in which the cyclophane acts either as tri- or tetradentate. The results indicate that the energy cost associated with a change in the coordination mode of the cyclophane from tri- to tetradentate is moderate for both ligands so that the actual coordination mode can be determined not only by the characteristics of the first coordination sphere but also by the specific interactions with additional nearby water molecules. The kinetics of the acid promoted decomposition of the mono- and dinuclear Cu-II complexes of both cyclophanes have also been studied. For both ligands, dinuclear complexes convert rapidly to mononuclear species upon addition of excess acid, the release of the first metal ion occurring within the mixing time of the stopped-flow instrument. Decomposition of the mononuclear [CuL2](2+) and [CuHL2](3+) species occurs with the same kinetics, thus showing that protonation of [CuL2](2+) occurs at an uncoordinated amine group. In contrast, the [CuL1](2+) and [CuHL1](3+) species show different decomposition kinetics indicating the existence of significant structural reorganisation upon protonation of the [CuL1](2+) species. The interaction of AMP with the protonated forms of the cyclophanes and the formation of mixed complexes in the systems Cu-L-1-AMP, Cu-L-2-AMP, and Cu-L-3-AMP, where L-3 is the related pyridinophane containing the same polyamine chain and 2,6-dimethylpyridine as a spacer, is also reported. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)