Syntheses, Spectroscopy, and Structural Analyses of Dinuclear Chiral‐at‐Metal μ‐Aqua‐tetrakis[(R or S)‐N‐1‐(Ar)ethylsalicylaldiminato]di‐Λ‐ or ‐Δ‐nickel(II) Complexes

Enantiopure dinuclear μ-aqua-tetrakis[(R or S)-N-1-(Ar)ethylsalicylaldiminato]di-Λ- or -Δ-nickel(II) [Ar = C6H5 (R-1/S-1), p-MeOC6H4 (R-2/S-2), p-ClC6H4 (R-3/S-3), p-BrC6H4 (R-4)] complexes have been synthesized from the reaction between (R or S)-N-1-(Ar)ethylsalicylaldimine and nickel(II) acetate. Their CD spectra demonstrate chirality transfer from the ligands to the metal ions and a mirror-image relationship for the enantiomeric pair R-3/S-3 in thf. DSC analyses show that the complexes exhibit an irreversible dissociation to the mononuclear species, which in turn undergo decomposition. The 1H NMR spectra reveal the presence of four salicylaldiminates, one aqua ligand, and two molecules of methanol in each dinuclear complex. They also confirm the existence of a dinuclear-bridged aqua bis-octahedral NiII complex in solution. The IR spectra in CH2Cl2 show a very strong band at around 2306 cm–1 due to the νO–H of the bridged aqua ligand. The X-ray structures of R-2, S-2, R-3, and S-3 confirm the formation of dinuclear compounds comprising two nickel ions, four salicylaldiminates, and one aqua ligand with two molecules of methanol (or water in S-2). Each nickel ion is surrounded by two N,O-chelating salicylaldiminates, a bridging salicyl-O atom from the neighboring nickel ion, and a bridging aqua ligand in a distorted octahedral polyhedron. Analyses of the absolute structures reveal a diastereomeric induction of the R or S ligand giving a Λ or Δ configuration at the nickel atoms in R-2/3 or S-2/3 that is independent of the ligand substituents.