Solvation-Induced Helicity Inversion of Pseudotetrahedral Chiral Copper(II) Complexes

Abstract

The helicity of four-coordinated nonplanar complexes is strongly correlated to the chirality of the ligand. However, the stereochemical induction of either the Δ- or the Λ-configuration at the metal ion is also modulated by environmental factors that change the conformational distribution of ligand rotamers. Calculation of the potential energy surface of bis{(R)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-κ(2)N,O}copper(II) with X = Cl at the density functional theory level showed a clear dependence of the helicity-determining angle θ between the two coordination planes on the relative population of different ligand conformers. The influence of different substituents (X = H, Cl, Br, and OCH3) on complex helicity was studied by determination of the absolute configuration at the metal ion in complexes with either (R)- or (S)-configured ligands. X-ray single-crystal analysis showed that (R)-configured ligands with H, Cl, Br induce Δ, while OCH3-substituted (R)-configured ligands induce Λ in the solid state. According to vibrational circular dichroism and electronic circular dichroism studies in solution, however, all tested complexes with (R)-ligands exhibited a propensity for Δ, with high diastereomeric ratio for X = Cl and X = Br and moderate diastereomeric ratio for X = H and X = OCH3 substituted ligands. Therefore, solvation of copper complexes with X = OCH3 goes along with helicity inversion. This solid-state versus solution study demonstrates that it is not sufficient to determine the chiral-at-metal configuration of a compound by X-ray crystallography alone, because the solution structure can be different. This is particularly important for the use of chiral-at-metal complexes as catalysts in stereoselective synthesis.

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

“…39,40 The chance for Z′ > 1 is higher in non-centrosymmetric space groups with (enantiopure) chiral molecules which have difficulties in packing efficiently in the absence of centrosymmetry. 41 Different from our previous studies on four-coordinated Cu(II) with asymmetric (N^O) and chiral Schiff base ligands 12,42 the distortion from square-planar geometry, as assessed by the dihedral angle θ (cf. Scheme 1b) at the Ni atoms in 1R and 1S is very small.…”

Synthesis and Characterization of Bis[(R or S)-N-1-(X-C6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O]-Λ/Δ-cobalt(II) (X = H, p-CH3O, p-Br) with Symmetry- and Distance-Dependent Vibrational Circular Dichroism Enhancement and Sign Inversion

“…The observed Δ-Co- S or Λ-Co- R configuration in the solid state is supposed to be induced diastereospecifically by the conformational preference of the coordinated N,O-chelate rings, resulting from minimum steric and/or chelation requirements (leading to thermodynamic stability) by the ligand chirality and/or substituents on the aryl ring. These results are in parallel to the preferred solid-state formation of Λ-M- R or Δ-M- S (unique or major diastereomers) in the homoleptic bis[( R or S )- N -1-(X-C 6 H 4 )ethyl-2-oxo-1-naphthaldiminato-κ 2 N , O ]-Λ- or Δ-M(II) ( Table 1 ) 20 , 21 , 26 and the analogous bis[( R or S )- N -1-( p -X-C 6 H 4 )ethylsalicylaldiminato-κ 2 N , O ]-Λ- or Δ-M(II) (M = Co, Cu, Ni, Zn; X = H, p -CH 3 O, p -Cl/Br; Table S4 ), 19 , 22 − 24 as evidenced by X-ray structural analyses. On the contrary, copper(II) complexes with ( R or S )- N -1-( p -X-C 6 H 4 )ethylsalicylaldimine (X = H, p -Cl/Br) exhibit the oppositely configured Δ-M- R or Λ-M- S diastereomers in the solid state.…”

“…On the contrary, copper(II) complexes with ( R or S )- N -1-( p -X-C 6 H 4 )ethylsalicylaldimine (X = H, p -Cl/Br) exhibit the oppositely configured Δ-M- R or Λ-M- S diastereomers in the solid state. 22 With this exception, the phenomena of diastereoselection and chirality induction-at-metal in the solid state are, in general, solely controlled by the ligand S or R chirality and are independent of the ligand substituents or modification and/or metal-ion selection. Solution studies also support this notion with diastereomeric excess of Δ-Co- S or Λ-Co- R in the S - or R -ligated complexes ( Tables 1 and S4 ).…”

Review for chiral-at-metal complexes and metal-organic framework enantiomorphs

“…Nuclear magnetic resonance (NMR) spectroscopy provided the alternative strategy to study the identification of chiral MOFs to substrates [65]. The interaction of chiral agent, 1-phenyl-2,2,2trifluoroethanol (TFPE), and chiral UMCM-1 was revealed with solid-state 13 C NMR technique. The signal of TFPE was observed with adsorption-induced change of mobility after loaded into achiral UMCM-1.…”

“…Similarly, various salts of the coordination complex, named as Hexol, were also obtained [10]. Some chiral metal complexes with chiral ligands have been reported [11][12][13][14], a little different to the chiral-at-metal complexes discussed hereafter. The examples in Figs.…”