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

Abstract

Enantiopure bis[{(R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ(2)N,O}]nickel(ii) complexes {Ar = C6H5 ( or ), p-OMeC6H4 ( or ), and p-BrC6H4 ( or )} 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 / and / 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 and 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.

From Mesocates to Helicates: Structural, Magnetic and Chiro‐Optical Studies on Nickel(II) Supramolecular Assemblies Derived from Tetradentate Schiff Bases

“…[52] As an example, we summarize the assignment of the two transitions contributing most to the two observed nega- Figure 12) and 370 nm (#17), respectively.T he former band is am ixingo fs everale xcitations, the dominant ones being those from the p C=N , p py C=N , and d xz orbitals to am ixed p N3 * + d yz orbital( the z axis is along the Ni-Ni direction). Weh ave already observed this behavior before forh igh-spin Ni complexesw ith chromophoric ligands.…”

“…It is apparent from Figure1 1t hat the two pairs of spectra are almostp erfectly superimposable in the longer wavelength region,4 00-900 nm, whereas shorter wavelength transitions (above 350 nm) maintain the same shape and sign in the two mediab ut with different relative intensity;ahigher intensity is observedi ns olution than in the solid state. [51,52] In the current case, the situation is furtherc omplicated by the presence of four chromophores, each with several transitions. Furthermore, intermoleculari nteractions that may occur in the microcrystalline solid-state samples are more effective for ligand-centered transitions, because of their stronger electric-dipole allowed character.…”

Synthesis, X-ray, and Spectroscopic Study of Dissymmetric Tetrahedral Zinc(II) Complexes from Chiral Schiff Base Naphthaldiminate Ligands with Apparent Exception to the ECD Exciton Chirality

“…Overall, the combination of ligand chirality, chirality at metal, and ligand folding gave rise to discrete metal helicates of preferred helicity in a selective way. 24 Here, in the Zn(II) structures, the conformational difference between the symmetry-independent molecules is nearly insignificant. The two symmetry-independent molecules are quite superimposable ( Figure 3).…”

“…With chelate ligands, α and β are greater than 109.5°; thus, τ 4 < 1, even if the dihedral planes are perfectly perpendicular. 20,24 Inorganic Chemistry ECD spectra were also recorded in the solid state 45 on microcrystalline samples in mixtures with an inert salt (KCl). 46a,b Solid-state spectra (Supporting Information, Figure S5) are comparable with solution ones in the accessible range (>250 nm).…”

On the zinc(II) and mercury(II) compounds with Schiff-base N-(p-anisoyl)-2-oxo-1-naphthylideneamine. Tautomerism of the ligand and polymerization of mercury compound via Hg⋯I contact

“…Metal complexes with 2-hydroxy-1-naphthaldimines still attract attention of the chemists worldwide due to their various properties, e.g. structural characteristics in the metal co-ordination sphere [4], biological activities [5][6], colori-/fluorimetric properties, etc. [7][8].…”