Chirality at metal and helical ligand folding in optical isomers of chiral bis(naphthaldiminato)nickel(II) complexes

Enantiopure bis[(R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]nickel(ii) complexes Ar = C6H5 (1R or 1S), p-OMeC6H4 (2R or 2S), and p-BrC6H4 (3R or 3S) are synthesized from the reactions between (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldimine and nickel(ii) acetate. Circular-dichroism spectra and their density-functional theoretical simulation reveal the expected mirror image relationship between the enantiomeric pairs 1R/1S and 3R/3S in solution. CD spectra are dominated by the metal-centered Λ- or Δ-chirality of non-planar four-coordinated nickel, this latter being in turn dictated by the ligand chirality. Single crystal structure determination for 1R and 1S shows that there are two symmetry-independent molecules (A and B) in each asymmetric unit that give a Z′ = 2 structure. Two asymmetric and chiral bidentate N^O-chelate Schiff base ligands coordinate to the nickel atom in a distorted square planar N2O2-coordination sphere. The conformational difference between the symmetry-independent molecules arises from the "up-or-down" folding of the naphthaldiminato ligand with respect to the coordination plane, which creates right- (P) or left-handed (M) helical conformations. Overall, the combination of ligand chirality, chirality at the metal and ligand folding gives rise to discrete metal helicates of preferred helicity in a selective way. Cyclic voltammograms (CV) show an oxidation wave at ca. 1.30 V for the [Ni(L)2]/[Ni(L)2]+ couple, and a reduction wave at ca. -0.35 V for the [Ni(L)2]/[Ni(L)2]- couple in acetonitrile

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

Enantiopure dinuclear -aqua-tetrakis[(R or S)-N-1-(Ar)ethylsalicylaldiminato]di-- or --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 H-1 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 Ni-II complex in solution. The IR spectra in CH2Cl2 show a very strong band at around 2306 cm(-1) due to the O-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 or configuration at the nickel atoms in R-2/3 or S-2/3 that is independent of the ligand substituents