N-H···S hydrogen bonds in a new family of ion-pair complexes between cationic nickel tetraazabicyclononane and anionic metal dithiolates: synthesis characterization and properties of [Ni(C₉H₂₂N₆)][M(mnt)₂](M^II = Cu, Ni, Pd)

Abstract

The ion-pair complexes [Ni(C₉H₂₂N₆)][Cu(mnt)₂] (1), [Ni(C₉H₂₂N₆)][Ni(mnt)₂] (2), and [Ni(C₉H₂₂N₆)][Pd(mnt)₂] (3; mnt^2- = 1,2-dicyanoethylenedithiolate, C₉H₂₂N₆ = 3,7-bis(2-aminoethyl)-1,3,5,7-tetraazabicyclo[3.3.1]nonane) have been synthesized and characterized by elemental analysis, IR, UV/Vis, and ¹H NMR spectroscopy, X-ray powder diffraction, and electrochemical studies, and unambiguously by single-crystal X-ray crystallography. All three complexes are isostructural and crystallize in the monoclinic space group P2₁/n. The N-H···S-type hydrogen-bonding capabilities of the classical sulfur-containing complex anions [M^II(mnt)₂]^2- (A; M = Cu, Ni and Pd) have been explored using amine groups containing the nickel complex cation [Ni(C₉H₂₂N₆)]²⁺ (C) in the ion-pair compounds 1, 2, and 3. In the crystal, the cation interacts with two different anions through three N-H···S hydrogen bonds and, likewise, the anion is hydrogen-bonded to two different cations through three hydrogen bonds, resulting in the formation of a chain-like arrangement. The average N-H···S hydrogen-bond lengths are 3.498(3), 3.505(3), and 3.490(5) Å for compounds 1, 2, and 3, respectively. Another crystallographic feature is the presence of CC-AA-CC-AA-type packing with short S...Ni contacts in the crystal structures. Besides crystallography, the existence of these N-H...S hydrogen bonds between cation [Ni(C₉H₂₂N₆)]²⁺ (C) and anion [M^II(mnt)₂]^2- (A) is strongly supported by the ^IR and ¹H NMR studies. The stability of the [Ni(C₉H₂₂N₆)]²⁺ (C) moiety in compounds 1, 2, and 3 is discussed in light of electrochemical studies performed on these compounds and [Ni(C₉H₂₂N₆)](ClO₄)₂. Complex 1 exhibits a well-resolved ESR signal corresponding to a Cu²⁺ (d⁹) system at liquid-nitrogen temperature.