Synthesis and structures of dinickel(II) complexes with a binucleating ligand, N,N′-bis[3-(N,N-dimethyl)aminopropyl]-N,N′-dimethyl-1,3-diamino-2-hydroxypropane (HL1), and investigation of carboxylic acid capture assisted by intramolecular hydrogen bonding

New carboxylic-acid-coordinated dinickel(II) complexes with binucleating ligands—HL1, which contains dimethylaminopropyl arms, and its analog HL2, which contains one dimethylaminopropyl arm and one bis(methoxyethyl) arm—both of which contain a 2-hydroxytrimethylene bridge between the two chelating sites, were synthesized. The structures of acetic acid coordinated complexes, [Ni2L1(μ-CH3COO)2(CH3COOH)]BPh4 (1), and [Ni2L2(μ-CH3COO)2(CH3COOH)]BPh4 (6), a propionic acid coordinated complex, [Ni2L1(μ-C2H5COO)2(C2H5COOH)]BPh4 (3), and a benzoic acid coordinated complex, [Ni2L1(μ-PhCOO)2(PhCOOH)]BPh4 (4) were revealed by X-ray crystallography, as well as those of the parent dinickel(II) complexes [Ni2L1(μ-CH3COO)2]BPh4 (2), and [Ni2L2(μ-CH3COO)2]BPh4 (5). In complexes 1, 3, 4, and 6, one carboxylic acid molecule is coordinated to one of the dinuclear nickel(II) ions, forming an intramolecular hydrogen bond with a μ-alkoxo bridge. Spectrophotometric investigation revealed that complex 1 underwent partial dissociation of the acetic acid in acetonitrile to give an equilibrium between 1 and 2, and that 2 reversibly associated with an acetic acid molecule in acetonitrile to form 1. Intramolecular hydrogen bonding plays an important role in the stabilization of carboxylic acid coordination to the dinickel(II) complexes with binucleating ligands.