Intramolecular Hydrogen Bonding in CuII Complexes with 2,6-Pyridinedicarboxamide Ligands: Synthesis, Structural Characterization, and Physical Properties

The significance of second-sphere interactions in synthetic catalysts and metalloenzyme active sites has encouraged the development of ligand frameworks capable of supporting intramolecular hydrogen-bonding networks. With this approach in mind, we prepared a series of 2,6-pyridinedicarboxamide ligands (H2LX) that feature outer-sphere heterocyclic groups (pyridine, pyrimidine, or quinoline rings). Reaction with CuCl2 yields complexes with the general formula [CuCl2(L-X{H}(2))] (1(X)), in which the pincer ligands coordinate to the Cu-II center in a tridentate fashion through the central pyridine and two amidato nitrogen atoms. The L-X{H}(2) ligands have no net charge, however, as the pendant heterocycles are both protonated. X-ray crystallographic studies revealed that the two outer-sphere N-H groups form intramolecular hydrogen bonds with the chlorido ligands. Modification of the syntheses provided Cu-II complexes (i.e., 2(X), 3(X)) that contain an aqua ligand hydrogen bonded to the outer-sphere group(s). Whereas complexes with the L-X{H}(2) ligands are mononuclear, deprotonation gives rise to dimeric complexes (i.e., 4(X), 5(X)) in which one of the pendant heterocycles coordinates to a second Cu-II center. The complexes were also examined with electronic absorption spectroscopy, cyclic voltammetry, and density functional theory. Our results indicate that the L-X{H}(2) ligands, despite their neutrality, maintain the strong sigma-donating ability of anionic pyridine-amidato ligands.