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

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Abstract

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(LX{H}2)] (1X), in which the pincer ligands coordinate to the CuII center in a tridentate fashion through the central pyridine and two amidato nitrogen atoms. The LX{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 CuII complexes (i.e., 2X, 3X) that contain an aqua ligand hydrogen bonded to the outer-sphere group(s). Whereas complexes with the LX{H}2 ligands are mononuclear, deprotonation gives rise to dimeric complexes (i.e., 4X, 5X) in which one of the pendant heterocycles coordinates to a second CuII center. The complexes were also examined with electronic absorption spectroscopy, cyclic voltammetry, and density functional theory. Our results indicate that the LX{H}2 ligands, despite their neutrality, maintain the strong σ-donating ability of anionic pyridine-amidato ligands.

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