Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ(2)N,O, diastereoselectively yield Δ/Λ-chiral four-coordinated, non-planar Cu(N^O)2 complexes [Ar = C6H5 R/S-L1, m-C6H4OMe R-L2, p-C6H4OMe R/S-L3, and p-C6H4Br R/S-L4]. Two N,O-chelate ligands coordinate to the copper(II) atom in distorted square-planar mode, and induce metal-centered Δ/Λ-chirality at the copper atom in the C2-symmetric complexes. In the solid state, the R-L1 (or R-L4) ligand chirality diastereoselectively induces a Λ-Cu configuration in Λ-Cu-R-L1 (or Λ-Cu-R-L4), the S-L1 ligand a Δ-Cu configuration in Δ-Cu-S-L1, forming enantiopure crystals upon crystallization. Conversely, the R-L2 ligand combines both Λ/Δ-Cu-R-L2 as a diastereomeric pair in the crystals. In solution, electronic circular dichroism (CD) spectra show full or partial diastereoselectivity towards Λ-Cu for R ligands and towards Δ-Cu for S ligands. The electronic CD spectra measured on all complexes obtained from R ligands (or S ligands), e.g. Cu-R-L1, Cu-R-L2, Cu-R-L3, and Cu-R-L4 (or Cu-S-L1, Cu-S-L3, and Cu-S-L4), show consistent spectral features. TDDFT calculations of the electronic CD spectra for the diastereomers Λ-Cu-R-L1 and Δ-Cu-R-L1 suggest that the CD spectra are largely dominated by the configuration at the metal center (Λ vs. Δ). The experimental CD spectrum of Cu-R-L1 agrees well with the one calculated for the Λ-Cu-R-L1 configuration. Cyclic voltammetry of Cu-R-L1 reveals a quasi-reversible redox wave corresponding to one-electron transfer for the [Cu(II)L2](0)/[Cu(I)L2](-1) couple in acetonitrile. DSC analyses for the complexes show an exothermic peak between 377 and 478 K (ΔH = -12 to -43 kJ mol(-1)), corresponding to a phase transformation from distorted square-planar/tetrahedral to regular tetrahedral geometry on heating.

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The metal-centered Δ/Λ-chirality of four-coordinated, nonplanar Zn(A(^)B)(2) complexes is correlated to the chirality of the bidentate enantiopure (R)-A(^)B or (S)-A(^)B Schiff base building blocks [A(^)B = (R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-κ(2)N,O with X = OCH(3), Cl, Br]. In the solid-state the (R) ligand chirality induces a Λ-M configuration and the (S) ligand chirality quantitatively gives the Δ-M configuration upon crystallization as deduced from X-ray single crystal studies. The diastereoselections of the pseudotetrahedral zinc-Schiff base complexes in CDCl(3) solution were investigated by (1)H NMR and by vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for the Schiff-base -CH═N- imine proton in (1)H NMR indicates an equilibrium of both Δ- and Λ-diastereomers with a diastereomeric ratio of roughly 20:80% for all three ligands. VCD proved to be very sensitive to the metal-centered Δ/Λ-chirality because of a characteristic band representing coupled vibrations of the two ligand's C═N stretch modes. The absolute configuration was assigned on the basis of agreement in sign with theoretical VCD spectra from Density Functional Theory calculations.

Chiroptical broad-range spectral analysis extending from UV to mid-IR was employed to study a family of Co­(II) N-(1-(aryl)­ethyl)­salicylaldiminato Schiff base complexes with pseudotetrahedral geometry associated with chirality-at-metal of the Δ/Λ type. While common chiral organic compounds have well-separated absorption and circular dichroism spectra (CD) in the UV/vis and IR regions, chiral Co­(II) complexes feature an almost unique continuum of absorption and CD bands, which cover in sequence the UV, visible, near-IR (NIR), and IR regions of the electromagnetic spectrum. They can be collected in a single (chiro)­optical superspectrum ranging from the UV (230 nm, 5.4 eV) to the mid-IR (1000 cm–1, 0.12 eV), which offers a fingerprint of the structure and stereochemistry of the metal complexes. Each region of the superspectrum contributes to one piece of information: the NIR-CD region, in combination with TDDFT calculations, allows a reliable assignment of the metal-centered chirality; the UV-CD region facilitates the analysis of the Δ/Λ diastereomeric equilibrium in solution; and the IR-VCD region contains a combination of low-lying metal-centered electronic states (LLES) and ligand-centered vibrations and displays characteristically enhanced and monosignate VCD bands. Circular dichroism in the NIR and IR regions is crucial to reveal the presence of d–d transitions of the Co­(II) core which, due to the electric-dipole forbidden character, would be otherwise overlooked in the corresponding absorption spectra.

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.

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The dimeric complex acetato(η4‐cycloocta‐1,5‐diene)rhodium(I), [Rh(O2CMe)(η4‐cod)]2 (cod = cycloocta‐1,5‐diene), reacts with amino acids [HAA = L‐alanine, (S)‐2‐amino‐2‐phenylacetic acid (L‐phenylglycine), N‐methylglycine, and N‐phenylglycine] and with the amino alcohol (S)‐2‐amino‐2‐phenylethanol to afford the aminocarboxylato(η4‐cycloocta‐1,5‐diene)rhodium(I) complexes [Rh(AA)(η4‐cod)] (AA = deprotonated amino acid = aminocarboxylato ligand) and [(S)‐2‐amino‐2‐phenylethanol](η4‐cycloocta‐1,5‐diene)rhodium(I) acetate, [Rh{(S)‐HOCH2–CH(Ph)‐NH2}(η4‐cod)](O2CMe) (V). The complexes are characterized by IR, UV/Vis, 1H/13C NMR and mass spectroscopy. The achiral N‐phenylglycine ligand gives a chiral N‐phenylglycinato complex [Rh(O2C–CH2–NHPh)(η4‐cod)] (IV) with the amine nitrogen atom becoming the stereogenic center upon metal coordination. Complex IV crystallizes in the tetragonal, chiral space group P43 and the crystal structure reveals twofold spontaneous resolution of a racemic mixture into homochiral helix‐enantiomers. The investigated crystal contained only one type of helix, namely (left‐handed or M‐) 43‐helical chains. This is traced first to an intermolecular N–H···O hydrogen bonding from the stereogenic amino group to a neighboring unligated carboxyl oxygen atom that connects only molecules of the same (R)‐configuration into (left‐handed or M‐) 43‐helical chains. This intrachain homochirality is supplemented, secondly, by the interlocking of adjacent chains with their corrugated van der Waals surface to allow for an interchain transmission of the sense of helicity, building the single crystal from the same homochiral helix‐enantiomer. The enantiomeric amino alcohol complex V crystallizes in the monoclinic, noncentrosymmetric (Sohncke) space group P21. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

A series of 1,1 0 -binaphthalene-2,2 0 -diyl phosphate (BNPPA À ) salts have been synthesized. Their crystal packings show a separation of the hydrophobic naphthyl and hydrophilic (RO) 2 PO 2 À phosphate/cation/solvate regions. Hydrogen bonding in the latter is the driving force for ''inverse bilayer'' formation, with a hydrophilic interior exposing the hydrophobic binaphthyl groups to the exterior. Stacking of the inverse bilayers occurs less through p-p and more through CHÁ Á Áp interactions between the naphthyl groups, which correlates with the formation of thin crystal plates along the stacking direction. Cations used with R-or rac-BNPPA À are protonated isonicotin-1-ium amide (1), isonicotin-1-ium acid (2), guanidinium (3), the metal complexes transtetraammine-dimethanol-copper(II) (4), trans-diaqua-tetramethanol-copper(II) (5) and cis-diaquabis(ethylene diamine)-nickel(II) (6). Crystallization occurs with inclusion of water and methanol solvent molecules, except in 2. Starting from R-BNPPA, inversion takes place with calcium acetate to give 1 as the racemate. 2 is crystallized as the R-BNPPA salt. The inversionsymmetrical complex trans-[Cu(H 2 O) 2 (CH 3 OH) 4 ] 21 in 5 has Cu-OH 2 bond lengths of 1.937(4) A ˚, and Cu-O(methanol) of 2.112(4) and 2.167(4) A ˚, corresponding to a compressed tetragonal geometry.Scheme 1 Molecules with a polar head (J) and a non-polar tail (Q) can spontaneously form (a) monolayers, (b) micelles, (c) bilayers and (d) liposome vesicles.