Chirality and diastereoselection of Δ/?-configured tetrahedral zinc complexes through enantiopure Schiff base complexes: Combined vibrational circular dichroism, density functional theory, 1H NMR, and X-ray structural studies

Article abstract of DOI:10.1021/ic2009557

The metal-centered Δ/?-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-κ²N,O with X = OCH₃, Cl, Br]. In the solid-state the (R) ligand chirality induces a ?-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 CDCl₃ solution were investigated by ¹H NMR and by vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for the Schiff-base -CH=N- imine proton in ¹H NMR indicates an equilibrium of both Δ- and ?-diastereomers with a diastereomeric ratio of roughly 20:80% for all three ligands. VCD proved to be very sensitive to the metal-centered Δ/?-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.

Full text of DOI:10.1021/ic2009557

ARTICLE
pubs.acs.org/IC
Chirality and Diastereoselection of Δ/Λ-Configured Tetrahedral Zinc
Complexes through Enantiopure Schiff Base Complexes: Combined
Vibrational Circular Dichroism, Density Functional Theory, 1H NMR,
and X-ray Structural Studies
Anne-Christine Chamayou,† Steffen L€udeke,*,‡ Volker Brecht,‡ Teresa B. Freedman,§ Laurence A. Nafie,§
and Christoph Janiak*,†,^
†Institut f€ur Anorganische und Analytische Chemie and ‡Institut f€ur Pharmazeutische Wissenschaften, Universit€at Freiburg,
Albertstrasse 21, D-79104 Freiburg, Germany
§Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100, United States
S Supporting Information
b
ABSTRACT: 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-k2N,O with X = OCH3, 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-
1
Schiff base complexes in CDCl3 solution were investigated by H NMR and by
vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for
the Schiff-base ꢀCHdNꢀ imine proton in 1H 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 CdN
stretch modes. The absolute configuration was assigned on the basis of agreement in sign with theoretical VCD spectra from Density
Functional Theory calculations.
’ INTRODUCTION
for enantioselective recognition and separation,16 and dielectric
materials.17
Since their very first application as catalysts in stereoselective
synthesis1 optically active transition metal complexes,2,3 includ-
ing chiral-at-metal complexes,4 are of constant interest. Classical
textbook examples of metal-centered chirality are pseudo-octa-
hedral six-coordinate tris- or bis-chelate Δ/Λ-metal complexes.
Here, the asymmetry is due to the arrangement of an achiral or
chiral ligand A∧A(*) around a metal M, leading to D3 symmetry
for Δ/Λ-M(A∧A)3 or C2 symmetry for Δ/Λ-M(A∧A)2B2,
respectively.2,5,6 Little attention, however, has been drawn on
metal-centered chirality of four-coordinate metal complexes. It
occurs in nonplanar systems with two asymmetrical chelate rings
A∧B rendering a complex M(A∧B)2 with C2 symmetry. The
metal-centered configuration can be described using the Δ/Λ-
nomenclature originally introduced for trigonal complexes.7 The
chelate ring is interpreted as a segment of a helix or screw along
the C2 rotation axis (Scheme 1).8ꢀ10
If the chelate ligand contains a single, configurationally stable
stereogenic center, the R- or S-chirality of the ligand ((R)-A∧B or
(S)-A∧B) and the Δ- or Λ-configuration at the metal center can
give rise to four different stereoisomers, if enantiomerically pure
ligands are combined with the metal atom: Λ-[M((R)-A∧B)2]
and Δ-[M((R)-A∧B)2] or Λ-[M((S)-A∧B)2] and Δ-[M((S)-
A∧B)2] (Scheme 2).
As a general rule, the diastereomeric ratio depends on the
thermodynamics of stereochemical induction by the ligand.
While this mechanism is well studied for octahedral2,5,18 or five-
coordinated complexes19 relatively little activity has been in-
vested in the diastereoselection of four-coordinate, asymmetric
complexes. Here, the Δ- or Λ-configuration can interconvert
through ligand exchange or rearrangement to a planar geometry.
The kinetics of this process depend on the stability of me-
talꢀligand interaction. Zinc ions are generally supposed to form
weak interactions with ligand atoms. Nevertheless, they are
There are continuous developments of optically active Schiff
base (HSB*) ligands and their transition metal complexes11 for
applications as enantioselective catalysts,12 compounds for sec-
ond harmonic generation,13 chiral magnetic metal clusters,14
chiral fluorescent sensors,15 as homochiral porous lamellar solids
Received: May 6, 2011
Published: October 21, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
Scheme 1. Enantiomeric Absolute Configurations of a
Pseudotetrahedral, Nonplanar Bis-Chelate Complex Viewed
down the C2 Axisa
Scheme 3. Enantiopure (R)- or (S)-N-(1-(4-X-phe-
nyl)ethyl)salicylaldimine Ligands and Major Δ/Λ-bis{(R)- or
(S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-k2N,O}zinc-
(II) Complexesa
a Λ left-handed helicity, Δ right-handed helicity along principal C2 axis
(perpendicular to the paper plane). A and B convey the chelate ring
asymmetry.9
Scheme 2. Enantiomeric and Diastereomeric
Δ/Λ-[M((R/S)-A∧B)2] Pairsa
a The position of the R- or S-stereogenic center on the branched α-
carbon of the B substituent is adapted to the present work (see
Scheme 3) but could also reside in or on the A∧B chelate handle. The
priority for the R- or S-assignment should be B > R1 > R2. In the present
work Λ-metalꢀ(R)-ligand complexes and Δ-metalꢀ(S)-ligand com-
plexes in the white boxes are the major combination. The opposite and
here minor Δ-metalꢀ(R)-ligand complexes and Λ-metalꢀ(S)-ligand
combinations are underlined in gray (see Table 3).
a The minor Δ-ZnꢀR-ligand complexes and Λ-ZnꢀS-ligand combina-
highly abundant in biological systems, being essential for the
functionalities of a number of metalloproteins, expressing both
catalytic and structural roles.20ꢀ22 In the cases of enzymes and
zinc finger proteins, the cation is usually tetrahedrally
coordinated23 leading to a defined asymmetric geometry invol-
ving the zinc atom in their active site. This implies that a defined
tetrahedral asymmetric structure can be established even with
weak metalꢀligand interactions, mediated by strong stereo-
chemical induction by the ligand.
To study this issue one will have to be careful concerning the
choice of method for determining both the absolute configura-
tion and the diastereomeric composition. If a single crystal of
sufficient size and quality is available, a straightforward method
for determination of the metal-centered absolute configuration is
single-crystal X-ray structural analysis.24,25 The solid state struc-
ture, however, does not necessarily represent the geometry which
is the thermodynamically most favorable one in solution or in the
tions are underlined in gray (see Table 3).
gas phase. During crystallization various parameters involving
intermolecular contacts and lattice forces might become impor-
tant. This might severely shift the equilibrium between Δ- and Λ-
geometry at the metal ion, possibly even leading to inversion of
the absolute configuration. Thus, no quantitative information
about stereochemical induction can be drawn from the crystal
structure.
To obtain reliable results about the Δ/Λ-equilibrium in
solution in a noninvasive way, that is, avoiding any bias by
molecular interactions with reagents, chromatography columns,
and so forth, it is necessary to use a combination of spectroscopic
methods. The equilibrium between two diastereomers can be
easily studied using nuclear magnetic resonance (NMR). Inte-
gration of characteristic signals with different chemical shifts for
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Inorganic Chemistry
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Figure 1. Thermal ellipsoid plots (50% probability) of the enantiomeric zinc complexes in a perspective, central projection to enhance the view of the Λ,
left-handed helicity (left column) and Δ, right-handed helicity (right column) along the pseudo-C2 axis (perpendicular to the paper plane). The two
chelate rings are highlighted. Note that the thermal ellipsoids in the front appear larger than they really are because of the central projection. See
Supporting Information, Figures S6ꢀS10, for full atom numbering.
the Δ- and the Λ-diastereomer readily delivers the diastereo-
meric ratio. This approach, however, does not allow assignment
of the absolute configuration of the excess (and the deficient)
diastereomer which can be accomplished using chiroptical
spectroscopy, in particular circular dichroism (CD).26 Here,
the difference absorbance of left minus right circularly polarized
light is measured. Thus, the sign of the obtained bands is
characteristic of the absolute configuration of the molecule.
Electronic CD (ECD), which detects the difference absorbance
of visible or UV-light, delivers good results for molecules with a
chromophore with a transition dipole moment being coupled to
a chirality element. For chiral complexes these could be either
ligand transitions or d-d transitions of the metal ion.27 Tetra-
hedrally coordinated zinc, which is only present as d10 zinc(II) in
biological complexes,28 often does not exhibit characteristic
bands in ECD. Thus, this method cannot provide sufficient
information about metalꢀligand bonding. Quasi-enantiomeric
helical ligand chirality in pentacoordinate Zn complexes could,
however, be assessed by the quasi-mirror images of the nfπ*
transition in ECD.29 Vibrational CD (VCD) detects the coupling
of vibrational modes to a chirality element and is therefore not
subject to such limitations. Additionally, VCD samples the
electronic ground state properties of a molecule. As a conse-
quence, the assignment of absolute configuration by comparing
experimental spectra to spectra from quantum-chemical
calculations for VCD is obtainable at comparably low computa-
tional cost. This method is typically very reliable, which is
because spectra are composed of many characteristic infrared
bands corresponding to 3N ꢀ 6 vibrational transitions. VCD is
therefore a widely accepted tool for the determination of chiral
configurations30,31 and has recently also been applied for studies
of the structure of chiral metal complexes.32ꢀ34
We describe here the solid-state structures and solution VCD
and NMR studies of Δ/Λ-bis{(R)- or (S)-N-(1-(4-X-phe-
nyl)ethyl)salicylaldiminato-k2N,O}zinc(II) complexes, synthe-
sized from A∧B asymmetric and enantiopure (R)- or (S)-
salicylaldimine chelate ligands (Scheme 2 and Scheme 3). These
pseudotetrahedral structures involving a chiral zinc ion serve as a
model for studies on the diastereoselection of the Δ/Λ-metal
configuration from the (R/S)-ligand chirality and on the equi-
librium of the Δ/Λ-configuration in solution.
’ RESULTS AND DISCUSSION
By refluxing Zn(NO3)2 6H2O with the enantiopure (R)- or
3
(S)-N-(1-(4-X-phenyl)ethyl)salicylaldimine Schiff bases shown
in Scheme 3 and triethylamine (for deprotonation) in methanol,
the bis{N-(1-(4-X-phenyl)ethyl)salicylaldiminato-k2N,O}zinc-
(II) complexes were obtained in the form of light-yellow crystals
(Supporting Information, Figures S1ꢀS5). The complexes were
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Scheme 4. Assessment of Deviation between Tetrahedral
and Square-Planar Geometry by the Dihedral Angle θ or the
Table 1. Measure of Distortions from Tetrahedral or Square
Planar Metal Geometry in the Bis{N-(1-(4-X-phenyl)-
ethyl)salicylaldiminato-k2N,O}zinc(II) Complexes
τ
tet‑sq Index
θ/dega
τtet‑sq = θ/90ꢀ τ4
37, b
X
compound designation
-OCH3
Λ-Zn-R-1
Δ-Zn-S-1
Λ-Zn-R-2
Δ-Zn-S-2
Λ-Zn-R-3
75.37(8)
75.0(7)
0.84
0.83
0.91
0.91
0.91
0.761
0.760
0.814
0.816
0.821
-Cl
-Br
81.66(7)
81.77(9)
82.31(8)
a Calculated with Diamond.39 b See Table 2 for the two largest angles α
and β.
analyzed and characterized by elemental analysis, specific rota-
tion, IR, NMR, their single-crystal X-ray structures, and
solution VCD.
Solid State Structural Studies. The zinc atoms in the
complexes synthesized here with the ligands 1ꢀ3 have a four-
coordinate structure. The two N,O-bidentate Schiff base ligands
form a distorted N2O2-tetrahedral coordination sphere around
the zinc atom (Figure 1).
through division by 90ꢀ (θ/90ꢀ) to give an index τtet‑sq = 1.00
for tetrahedral and 0.00 for square planar geometry (Scheme 4).
Also, a geometry index τ4 for four-coordinate complexes with
τ4 = [360ꢀ ꢀ (α + β)]/141ꢀ has been proposed,37 inspired by
Addison and Reedijk’s five-coordinate τ5 index,38 with α and β
being the two largest angles in the four-coordinate species. The
values of τ4 will range from 1.00 for a perfect tetrahedral
geometry, since 360 ꢀ 2(109.5) = 141, to zero for a perfect
square planar geometry, since 360 ꢀ 2(180) = 0. Intermediate
structures, including trigonal pyramidal and seesaw, fall within
the range of 0 to 1.00.37 For two chelate ligands, as in the present
bis-bidentate Schiff base complexes, it is, however, better to take
the dihedral angle θ or its normalization index τtet‑sq = θ/90ꢀ.
The geometry index τ4 will not correctly assess a tetrahedral
geometry because of the already imminent distortion induced by
the chelate ring formation. With chelate ligands the two largest
angles will inevitable be larger than 109.5ꢀ; hence, τ4 < 1, even if
the dihedral planes are perfectly perpendicular. Both measures of
distortion are listed in Table 1 and support the notion of a close
to tetrahedral structure for zinc(II) complexes.
The ZnꢀO and ZnꢀN lengths and bond angles in the
bis{N-(1-(4-X-phenyl)ethyl)salicylaldiminato-k2N,O}zinc(II)
complexes are listed in Table 2 and are as expected.8,40ꢀ44
NMR-Determination of Diastereomeric Ratio. From X-ray
studies of selected single crystals we can assume that a certain
configuration at the ligand causes a quantitative induction of a
certain configuration at the metal ion (diastereoselection) during
the crystallization process of Zn-1, Zn-2, and Zn-3, respectively.
Yet to what extent does this induction still hold when the crystal-
linecomplex is dissolved?Todeterminethediastereomeric ratioof
Δ:Λ in solution we performed 1H NMR measurements in CDCl3.
For a quantitative evaluation we chose the imine proton at C-7
represented by a singlet above 8 ppm for each diastereomer. The
assignment was performed by 2D-NMR methods (dataand details
are given in Supporting Information, Figure S11). The corre-
sponding signals and integration results are given in Figure 2.
The appearance of two signals indicates that, different from the
solid state, the solution in CDCl3 allows for an equilibrium of
both Δ- and Λ-diastereomers with a diastereomeric ratio of
roughly 20:80% for all three Zn-R derivatives. This suggests that
the substitution at the phenyl ring does not have a strong impact
on the thermodynamic equilibrium. The kinetics for the Λ-to-Δ
interconversion, however, seems to depend on the substituent.
This is indicated by the different resolution of the respective
signals, since a rate of about the same magnitude as the time
resolution of the NMR experiment would lead to signal broad-
ening. This is the case for Δ-Zn-R-2 (chlorine, Figure 2a)
and Δ-Zn-R-3 (bromine, Figure 2c), where the substituents
By considering the pseudo-C2 axis passing through the center
of the O1 O2 edge, the metal atom and the center of the
3 3 3
N1 N2 edge, the absolute configuration Δ or Λ is determined
3 3 3
(Scheme 1) and given as part of the compound designation
(Figure 1). For a given (R) or (S) ligand chirality only one Δ or
Λ configuration can be found as based on the absolute structure
or Flack parameter of less than 0.07 (cf. Table 5).25 Such a Flack
parameter close to zero confirms the correct absolute structure
and together with the other refinement parameter and with
normal atom temperature factors and the absence of molecular
disorder rules out that any significant amount of complexes with
the opposite metal chirality, that is, a diastereomeric mixture, is
present within one of the investigated crystals. We note that a
single-crystal X-ray structure (based on only one crystal) would
not rule out the possibility of a diastereomeric mixture of, for
example, Δ-Zn-R-L and Λ-Zn-R-L forms with each Δ and Λ
configuration crystallizing separately in an overall diastereomeric
crystal mixture, akin to spontaneous resolution.35 It can be noted
that for the zinc compounds described here the Λ-Zn config-
uration correlates with the (R) ligand chirality (Λ-Zn-R-L) and
the Δ-Zn configuration correlates with the (S) ligand chirality
(Δ-Zn-S-L, Figure 1). The Δ or Λ configuration is induced
diastereospecifically by the conformational preference of the
chelate rings which results from the steric requirements of the
substituents. The apparently more general preference for the Λ-
M-R,R configuration in bis(salicylaldiminato)metal compounds
is thought to be driven by the minimization of steric hindrance
between the two ligands in the coordination sphere.8,9,13
Visual inspection of the molecular structures in Figure 1
already shows that the four-coordinated bis-chelate zinc com-
plexes described here lie close to a tetrahedral configuration
around the metal.
For a more quantitative assessment, the degree of distortion
from tetrahedral or square planar can be expressed by the
dihedral angle θ between the two planes formed by the donor
atoms with the metal atom, that is, N1-M-O1 and N2-M-O2
(Table 2). This dihedral angle will be 90ꢀ for a tetrahedral
geometry and 0ꢀ for a square planar geometry (not considering
the imminent distortion induced by the chelate ring
formation).36 Furthermore, this angle θ can be normalized
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Table 2. Bond Distances (Å) and Angles (deg) in the Zinc Complexes
Λ-Zn-R-1
Δ-Zn-S-1
Λ-Zn-R-2
Δ-Zn-S-2
Λ-Zn-R-3
ZnꢀO1
ZnꢀO2
ZnꢀN1
ZnꢀN2
1.928(2)
1.934(2)
2.004(2)
2.007(2)
1.906(2)
1.923(2)
2.019(2)
2.036(2)
1.918(2)
1.915(2)
2.018(2)
2.021(2)
1.917(3)
1.920(3)
2.023(3)
2.025(3)
1.924(2)
1.920(3)
2.026(3)
2.028(3)
O1ꢀZnꢀO2
O1ꢀZnꢀN1
O1ꢀZnꢀN2
O2ꢀZnꢀN1
O2ꢀZnꢀN2
N1ꢀZnꢀN2
119.91(10)
96.47(9)
119.45(9)
96.63(8)
119.35(9)
94.60(9)
119.11(11)
94.92(12)
112.72(13)
111.63(13)
94.71(12)
125.83(13)
118.07(12)
94.46(11)
111.94(11)
112.84(12)
95.30(11)
126.14(13)
107.38(10)
107.16(10)
95.54(10)
132.72(11)
106.38(9)
106.95(9)
96.24(9)
111.79(9)
112.77(10)
94.55(9)
133.39(10)
125.94(10)
Figure 3. Signal at 8.4 ppm could be assigned to free ligand. The spike-
in experiment (addition of free ligand R-2 to a sample of Zn-R-2) leads to
increase of the signal at 8.4 ppm (a). Presence of free ligand depends on
presence of water in the solvent. Addition of D2O to a sample of Zn-R-2
leads to rapid increase of the signal at 8.4 ppm because of hydrolysis of
the complex (b).
inverted by Δ/Λ-interconversion through a planar intermedi-
ate. We addressed this question by VCD-based determination
of the absolute configuration at the metal ion of the excess
diastereomer in solution for Zn-1, Zn-2, and Zn-3. The chiral
metal ion should have a strong influence on vibrational modes
corresponding to coordinating ligand atoms. Hence, we ex-
pected VCD to be highly sensitive for the metal-centered
asymmetry of chiral tetrahedral zinc complexes.
Figure 2. NMR signals around 8.1 ppm and 8.0 ppm account for the
ꢀCHdNꢀ imine proton at C-7 of the Δ- and Λ-diastereomer of Zn-R-1,
Zn-R-2, and Zn-R-3, respectively. The diastereomeric ratio (%) is from
integration of the corresponding signals. The additional signal around
8.4 ppm corresponds to C-7 of the free ligand R-1, R-2, or R-3,
respectively.
In view of the fact that contributions from both the Δ- and the
Λ-species have to be taken into account to create a theoretical
spectrum we calculated vibrational frequencies and IR and VCD
intensities for both the Δ- and the Λ-diastereomer based on
modeled geometries for the determination of the absolute
configuration in solution. We chose Zn-R-2 as a model for all
three variants presented here, since the chlorine substituent
appeared to require the lowest computational cost compared
to bromine in Zn-R-3 (more basis functions needed to approx-
imate the atomic orbitals) and methoxy in Zn-R-1 (9-fold
number of conformers). Conformer models were created on
the assumption of a rigid, quasi-tetrahedral core structure around
the metal ion, as suggested from X-ray data (Figure 1) and
represent different rotamers around the single bond between the
nitrogen atom and C-8 of the ligand (Supporting Information,
Figure S12). After a two-step conformational analysis (see the
experimental section for details) three low-energy geometries of
Zn-R-2 (two conformers of Λ and one conformer of Δ) where
chosen for DFT calculation of IR/VCD spectra, relative energies
(ΔE), and relative free energies (ΔG) at the B3LYP level, using
might allow for interconversion near the NMR time scale.
Methoxy (Figure 2c), on the other hand, seems to retard
interconversion between both diastereomers. This results into
well resolved and separated singlets. An additional singlet
around 8.4 ppm indicates presence of free ligand R-1, R-2, or
R-3, respectively. This was proven by a “spike-in” experiment,
where free ligand R-2 was added to a sample of Zn-R-2
(Figure 3a). The amount of free ligand depends on the amount
of water being present in the sample. Presence of water leads to
rapid hydrolysis of the complex as could be shown from a
hydrolysis experiment where D2O was added to a solution of
Zn-R-2 in CDCl3 (Figure 3b).
Density Functional Theory (DFT) Calculations and Solu-
tion VCD Experiments. The diastereomeric Δ:Λ- ratio of
about 20:80% for all tested Zn-R complexes implies that the
crystallization process had contributed to full asymmetric
induction from the ligand in the solid phase, as suggested from
single crystal X-ray crystallography. In solution, on the other
hand, asymmetric induction might be decreased or even
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Table 3. Boltzmann Weights Based on Relative Energies
(ΔE) and Relative Free Energies (ΔG), Respectively for
Three Modeled Geometries of Zn-R-2a
Boltzmann
Boltzmann
weights (%) in
respect to ΔG
ΔE
weights (%) in
respect to ΔE (kJ molꢀ1
ΔG
geometry (kJ molꢀ1
)
)
3
3
Λ-Zn-R-2
conformer 1
conformer 2
Δ-Zn-R-2
0.74
4.58
0
39
8
0
45
33
22
0.79
1.83
53
a Supporting Information, Figure S12. Parameters are calculated at the
B3LYP level, using Ahlrichs VTZ basis set for zinc and 6-31+G(d,p) for
carbon, hydrogen, nitrogen, oxygen, and chlorine.
Figure 5. (a) Boltzmann average of theoretical spectra calculated for
Λ-Zn-R-2, conformer 1 (45%), conformer 2 (35%), and for Δ-Zn-R-2
(20%) leads to good overall agreement with the experimental spectrum
(b). Contributions from the absolute configuration of the ligand are
comparably small (c).
conformers of Λ-Zn-R-2 (red, green) than for Δ-Zn-R-2. The
agreement is particularly good for the VCD signal at 1622/
1610 cmꢀ1 representing the in- and out-of-phase vibrations of
the ligand’s CdN stretch mode. The sign seems to coincide with
the configuration at the metal ion: The calculated spectra of the
Λ-conformers and the experimental spectrum of Zn-R-2 both
present a 1622(ꢀ)/1610(+) cmꢀ1 VCD-signal, allowing for the
assignment of Λ-configuration at the metal ion to Zn-R-2 and Δ-
configuration to its enantiomer Zn-S-2. The latter exhibits the
opposite 1622(+)/1610(ꢀ) cmꢀ1 VCD, agreeing with the
corresponding signal in the spectrum calculated for diastereo-
meric Δ-Zn-R-2 (black).
In the spectral region between 1500 and 1000 cmꢀ1 the
experimental spectrum of Zn-R-2 also shows spectral contribu-
tions from the Δ-species. This is in accordance with the results
from NMR spectra in CDCl3 solution and from gas phase
calculations of ΔG, suggesting a proportion of ∼20% Δ-Zn-R-
2 at room temperature. Figure 5a,b shows a theoretical spectrum
representing the spectral contributions of the three modeled
geometries of Zn-R-2, Boltzmann-averaged in respect to ΔG, in
comparison to the experimental spectrum of Zn-R-2. Deviations
between calculated and observed spectra are most obvious for the
band pattern around 1480 cmꢀ1. They might be due to slight
over- or underestimation of the calculated frequencies leading to
different band cancellation effects in the calculated spectrum and
the measured spectrum. Nevertheless, on the basis of good
overall agreement and of agreement in sign of the most promi-
nent VCD signal at 1622/1610 cmꢀ1 it is evident that the excess
diastereomer of Zn-R-2 is Λ-configured at the metal ion. This
means, that ∼80% of dissolved complex maintain the configura-
tion which had been determined for solid Zn-R-2 by single-
crystal X-ray crystallography.
Figure 4. Experimental IR and VCD spectra of Zn-R-2 (blue) and Zn-S-2
(26 mM solution in CDCl3) and calculated spectra corresponding to Λ-
Zn-R-2, conformer 1 (red) and conformer 2 (green), respectively, and to
Δ-Zn-R-2 (black). While the calculated IR spectra are very similar for all
three geometries, the VCD-spectrum of Δ-Zn-R-2 is considerably
different from those corresponding to the two Λ-conformers. The
VCD band at 1622/1610 cmꢀ1 represents the in- and out-of-phase
vibrations of the ligand’s CdN stretch mode and can be used as a marker
for the Δ/Λ-configuration: 1622(ꢀ)/1610(+) cmꢀ1 indicates Λ for
Zn-R-2, the opposite sign indicates Δ for Zn-S-2.
Ahlrichs VTZ basis set on zinc45 and 6-31+G(d,p) on all other
atoms (GAUSSIAN46). The calculated values for ΔE, ΔG, and
the Boltzmann weights both calculated in respect to ΔE and ΔG
are given in Table 3. While the relative energies ΔE suggest about
equal contributions of Δ and Λ, Boltzmann weights calculated in
respect to ΔG (25 ꢀC) lead to a diastereomeric ratio (Δ:Λ) of
22:78% being in very good agreement with NMR data (Figure 2).
Conformer 1 of Λ, the most abundant geometry in respect to
ΔG, is most similar to the geometry from the crystal structure.
We compared the theoretical IR and VCD spectra calculated
for conformers 1 and 2 of Λ-Zn-R-2 and the single conformer of
Δ-Zn-R-2, respectively, to IR and VCD spectra of Zn-R-2 and
Zn-S-2 in solution (Figure 4). The experimental spectra of
enantiomeric Zn-R-2 (blue) and Zn-S-2 (gray) appear as nearly
perfect mirror images. The overall agreement between calculated
spectra and the observed spectrum of Zn-R-2 is better for the two
Interestingly, by evaluating the calculated spectra for Λ-Zn-R-
2 and Δ-Zn-R-2, it becomes obvious that the configuration at the
metal ion accounts for large difference signals in the VCD, while
contributions originating from ligand chirality appear compar-
ably small (Figure 5c). This could be explained by coupled
vibrations due to a chiral arrangement of the vibrational
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Inorganic Chemistry
ARTICLE
Scheme 5. Relative Disposition of CdN Stretch Modes
within the Geometries of Λ-Zn-R-2 (Conformer 1) and
Δ-Zn-R-2a
Figure 7. ECD spectra in CHCl3 solution (concentrations in mol/L: Δ-
Zn-S-1 4.0 ꢁ 10ꢀ4, Λ-Zn-R-1 2.1 ꢁ 10ꢀ4, Δ-Zn-S-2 7.7 ꢁ 10ꢀ4, Λ-Zn-
R-2 1.7 ꢁ 10ꢀ3, Λ-Zn-R-3 1.3 ꢁ 10ꢀ4).
a The double bonded carbon atoms describe a helix around the NꢀN
axis that is right-handed for the Λ-species and left-handed helix for the
Δ-species. Helix radius (r) and pitch (h) can be calculated from atom
distances (dNN), the dihedral angle between the two CdN groups (j),
and the angle spanned between the CdN axis and the NꢀN axis (α)
which were taken from the optimized geometries from B3LYP/Ahlrichs
VTZ, 6-31+G(d,p) calculations.
radius of 1.0 Å and a pitch of 11.7 Å in Δ-Zn-R-2 (Scheme 5).
Therefore, the VCD signal at 1622/1610 cmꢀ1 representing the in-
and out-of-phase vibrations of both ligand’s CdN stretch can be
used as a sensitive probe for Δ/Λ-chirality in the tetrahedral com-
plexes presented in this study.
The impact of different substituents at C-13 on the helical
arrangement of the coupled vibrational modes should be negli-
gible. Thus, the absolute configuration of the excess diastereomer
of Zn-R-1 and Zn-R-3 as chloroform solutions can as well be
assigned to the Λ-configuration on the basis of agreement in sign
of the signal at 1622/1610 cmꢀ1 (Figure 6).
While the VCD spectra are dominated by coupled oscillator
signals, in electronic CD ligand-specific transitions, being inde-
pendent from metal-centered chirality, seem to result into similar
intensities as complex-specific signals. The electronic CD spectra
in solution show ligand bands in the region 240ꢀ440 nm
(Figure 7). For d10 Zn(II) there are no d-d transitions. Different
from VCD the bands are significantly affected and, thus, shifted
more by the substituents at the aromatic side chain. The spectra
of enantiomers (Δ-Zn-S-1 vs Λ-Zn-R-1, Δ-Zn-S-2 vs Λ-Zn-R-2)
do not appear as perfect mirror images, which could be explained
by a different degree of hydrolysis as shown in Figure 3. As a
consequence of the similar intensities of complex and ligand CD,
electronic CD might be much more prone to frequency shifts
from different amounts of free ligand than VCD. Nevertheless,
the opposite sign of the transitions reflects the enantiomeric
relationship between the complexes with ligand 1 and ligand 2,
respectively, supporting the results from VCD analysis.
Figure 6. VCD spectrum is not significantly affected by different
substituents at the aromatic side chain: Zn-R-2: -Cl; Zn-R-3: -Br; Zn-
R-1: -OCH3.
transition dipole vectors. The concept of coupled transition
dipoles is well-known as exciton coupling in electronic CD and
is a widely used tool for the assignment of the absolute config-
uration of molecules where two absorbing chromophores parti-
cipate in a chiral system.47ꢀ49 For VCD analogous examples for
coupled vibrational transitions had recently been demonstrated
for tris-(β-diketonato)metal(III) complexes50 and mixed ruthe-
nium 3-trifluoroacetylcamphorato/acetylacetonato complexes,34
where large VCD signals originating from three helically arranged
coordinated CdO stretch modes had been observed. Similarly,
the relative disposition of CdN stretch modes in Λ-Zn-R-2 and
Δ-Zn-R-2 may account for the large VCD in the complex spectra
compared to the ligand spectrum (Figure 5). In the model
structures, the dihedral angle (CꢀNꢀNꢀC) is 155.6ꢀ in Λ-Zn-R-2,
and 162.0ꢀ in Δ-Zn-R-2, respectively. Taking into account the
relative distance between the atoms, the double bonded carbons
describe a right handed helix around the NꢀN axis with a radius of
1.0 Å and a pitch of 12.4 Å in Λ-Zn-R-2, and a left-handed helix with a
’ CONCLUSION
Using an optically active Schiff base (HSB*) ligand we have
established a simple model for strong stereochemical induction
on the metal-centered chirality of presumably weakly bound
zinc(II). All three tested ligand derivates comprising different
substituents (chlorine, bromine, methoxy) at the aromatic
amine-moiety of the HSB* ligand led to identical induction in
the solid state, determined by X-ray structural analysis. Solution
analysis by NMR and VCD-based determination of the absolute
configuration delivered virtually identical results for a diastere-
omeric ratio of ∼20:80 for Δ-Zn-R versus Λ-Zn-R chloroform
solutions at room temperature. These complexes might serve as an
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Inorganic Chemistry
ARTICLE
Table 4. Details of the yield, CHN analysis and characterization for the Δ/Λ-bis{(R)- or
(S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-k2N,O}zinc(II) complexes
compound yield
%C, H, N -
mp
[α]25589 (ꢀ)
X
designation (g) (%) calc. - found (ꢀC) (concentration, mg/mL)
IR (cmꢀ1
)
-OCH3 Λ-Zn-R-1
Δ-Zn-S-1
0.48 66.96, 5.62, 4.88 183
84 66.40, 5.77, 4.76
0.46 66.96, 5.62, 4.88 182
80 66.86, 5.71, 4.79
0.44 61.82, 4.50, 4.81 163
76 60.21, 4.57, 5.02
0.45 61.82, 4.50, 4.81 160
77 58.59, 4.81, 5.54
0.52 53.64, 3.90, 4.17 179
77 53.51, 3.90, 4.09
ꢀ164.0
(4.6)
1611 (vs), 1534 (m), 1514 (m), 1466 (m), 1446 (m), 1404 (m),
1251 (m), 1147. (m), 1022. (m), 840 (m), 758 (m)
+164.2
(8.1)
1610 (vs), 1526 (m), 1459 (s), 1403 (m), 1335 (m), 1299 (m),
1248 (s), 1186 (m), 1143 (m), 1021 (m), 836 (m), 754 (m)
-Cl
Λ-Zn-R-2
Δ-Zn-S-2
Λ-Zn-R-3
ꢀ221.0
(1.0)
1605 (vs), 1534 (s), 1451 (s), 1406 (s), 1313 (m), 1189 (m), 1142 (m),
1092 (s), 825 (s), 749 (vs), 599 (m), 541 (m), 511 (m), 479 (m), 438 (s)
1604 (vs), 1533 (m), 1447 (s), 1404 (m), 1309 (m), 1187 (m), 1143 (m),
1090 (s), 824 (s), 748 (vs), 599 (m), 540 (m), 512 (m), 479 (m), 436 (vs)
1605 (vs), 1533 (s), 1451 (s), 1407 (s), 1312 (m), 1189 (m), 1142 (m),
1097 (m), 1068 (m), 821 (s), 749 (s), 524 (m), 433 (m), 406 (s)
+221.4
(2.6)
-Br
ꢀ228.5
(10.4)
example for a dynamic diastereomeric equilibrium which is consider-
ably shifted on one side by stereochemical induction from the ligand.
Nevertheless, this equilibrium seems to be rather independent from the
substituents at C-13. Further studies will have to address the question of
ligand modifications leading to fine-tuning of this equilibrium.
1098.0 (m), 1067.7 (w), 1024.5 (s), 983.7 (vw), 972.3 (w), 941.4 (vw),
931.2 (vw), 914.0 (w), 884.6 (vw), 865.1 (w), 836.6 (s), 817.2 (m),
784.1 (w), 761.1 (vs), 718.3 (w), 675.1 (vw), 642.3 (w), 632.9 (w),
592.8 (w), 563.6 (w), 553.4 (vw), 536.1 (m), 521.2 (w), 481.1 (w),
435.7 (m). 1H NMR (CD3OD, 200 MHz), δ/ppm (J/Hz) = 8.50 (s, 1H,
H7), 7.35ꢀ7.25 (m, 4H, H3, H5, H11/110), 6.95ꢀ6.80 (m, 4H, H2, H4,
H12/120), 4.59 (q, 1H, H8, J = 6.7), 3.77 (s, 3H, OCH3), 1.59 (d, 3H,
H9, J = 6.9) (see Scheme 3 for the NMR atom numbering). [α]25589 (c =
11.9 mg/mL): +131.1ꢀ.
’ EXPERIMENTAL SECTION
Commercially available solvents, triethylamine, Zn(NO3)2 6H2O,
3
(R)-N-(1-(4-methoxyphenyl)ethyl)salicylaldimine, R-1. mp
and the enantiopure amines (from BASF AG, Ludwigshafen, Germany)
(R)-1-(4-methoxyphenyl)ethylamine, (S)-1-(4-methoxyphenyl)ethylamine,
(R)-1-(4-chlorophenyl)ethylamine, (S)-1-(4-chlorophenyl)ethylamine,
(R)-1-(4-bromophenyl)ethylamine were used as received. Elemental
analyses were performed on a VarioEL from Elementaranalysensysteme
GmbH. Infrared spectra were recorded in the range 4000ꢀ400 cmꢀ1 on
a Nicolet Magna 760 Spectrometer using a diamond orbit ATR unit.
Polarimetric measurements were carried out with a Perkin-Elmer 341
polarimeter in CHCl3 at 25 ꢀC, and the specific optical rotation values of
68 ꢀC. Yield: 5.22 g, 98%. C16H17NO2 (255.32 g molꢀ1): calculated C
3
75.27, H 6.71, N 5.49; found C 75.01, H 6.71, N 5.31%. IR
(cmꢀ1):3046.2 (vw), 2981.2 (vw), 2934.7 (vw), 2863.5 (vw), 2837.3
(w), 2362.5 (vw), 2338.2 (vw), 2058.0 (vw), 1966.7 (vw), 1021.2 (vw),
1896.2 (vw), 1622.8 (s), 1606.4 (s), 1579.9 (m), 1501.3 (s), 1458.4 (w),
1439.1 (w), 1373.1 (m), 1337.2 (vw), 1316.8 (vw), 1278.5 (vs), 1242.1
(vs), 1204.7 (w), 1180.4 (vs), 1149.0 (w), 1114.2 (w), 1096.9 (m),
1066.9 (w), 1023.2 (vs), 986.1 (vw), 971.9 (m), 913.0 (w), 881.6 (vw),
864.2 (w), 836.8 (vs), 817.8 (vw), 783.9 (w), 760.9 (vs), 717.3 (m),
639.5 (w), 591.7 (m), 561.8 (w), 535.1 (m), 521.2 (vw), 480.6 (w),
435.7 (m). 1H NMR (CD3OD, 200 MHz), δ/ppm (J/Hz) = 8.49 (s, 1H,
H7), 7.35ꢀ7.25 (m, 4H, H3, H5, H11/110), 6.95ꢀ6.80 (m, 4H, H2, H4,
H12/120), 4.58 (q, 1H, H8, J = 6.7), 3.78 (s, 3H, OCH3), 1.59 (d, 3H,
[α]25
were determined according to the literature.51 Electronic
589
circular dichroism (ECD) spectra were recorded on an Applied Bio-
physics Chirascan spectrometer with a 1 cm cell in CHCl3.
1H NMR, 13C NMR, and 2D spectra in CDCl3 were recorded at 400
MHz on a Bruker DRX 400. 1H NMR spectra in CD3OD were recorded
at 200 MHz on a Bruker Advance DPX 200. Chemical shifts in CDCl3
refer to the solvent signal at 7.26 ppm for 1H NMR and 77.00 ppm for
13C NMR, respectively. Chemical shifts in CD3OD refer to the solvent
signal at 3.31 ppm.
H9, J = 6.9). (see Scheme 3 for the NMR numbering notation). [α]25
589
(c = 10.6 mg/mL): ꢀ131.3ꢀ.
(S)-N-(1-(4-chlorophenyl)ethyl)salicylaldimine, S-2. mp 46 ꢀC.
Yield: 5.77 g, 90%. C15H14NOCl (259.74 g molꢀ1): calculated C 69.36, H
3
5.43, N 5.39; found C 68.77, H 5.36, N 5.41%. IR (cmꢀ1):3050.19 (w),
2972.11 (m), 2927.02 (m), 2885.77 (m), 1948.29 (w), 1901.37 (w),
1630.28 (vs), 1577.49 (s), 1491.51 (s), 1404.87 (s), 1278.32 (s), 1095.27
Syntheses
(R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldimines (1ꢀ3).52
A solution of salicylaldehyde (20 mmol, 2.11 mL) in methanol (10 mL)
was stirred with two drops of conc. H2SO4 at room temperature for 10
min. After an equimolar amount of (R)- or (S)-N-(1-(4-X-phenyl)-
ethylamine (20 mmol, 3 mL) was added the color changed to yellow,
and the solution was refluxed for 5ꢀ6 h at 80 ꢀC to give a yellow solution.
The solvent was evaporated to half its volume in vacuum; then, the
solution was allowed to evaporate slowly at room temperature. After 2ꢀ3
days, yellow bright crystals were obtained. See ref 52 for further details and
characterization of R-3.
1
(s), 825.61 (vs), 758.19 (vs). H NMR (CD3OD, 200 MHz), δ/ppm
(J/Hz) = 8.54 (s, 1H, H7), 7.38ꢀ7.26 (m, 2H, H3, H5 and 4H, H11/110,
H12/120), 6.92ꢀ6.83 (m, 2H, H2, H4), 4.61 (q, 1H, H8, J = 6.6), 1.59 (d,
3H, H9, J = 6.4). (see Scheme 3 for NMR atom numbering). [α]25589 (c =
11.8 mg/mL): +140.9ꢀ.
(R)-N-(1-(4-chlorophenyl)ethyl)salicylaldimine, R-2. mp
38 ꢀC. Yield: 11.51 g, 90%. C15H14NOCl (259.74 g molꢀ1): calculated
3
C 69.36, H 5.43, N 5.39; found C 69.21, H 5.58, N 5.38%. IR
(cmꢀ1):2978.0 (vw), 2882.8 (vw), 1623.2 (s), 1575.9 (w), 1489.2
(m), 1451.6 (w), 1409.6 (w), 1376.6 (w), 1317.6 (vw), 1276.3 (m),
1204.0 (w), 1125.4 (w), 1086.3 (s), 1008.5 (w), 974.6 (w), 920.8 (w),
849.0 (vw), 824.6 (m), 749.7 (vs), 648.0 (w), 541.4 (m), 483.4 (w),
434.2 (m). 1H NMR (CDCl3, 400 MHz), δ/ppm (J/Hz) = 13.35 (s, 1H,
OH), 8.41 (s, 1H, H7), 7.35ꢀ7.28 (m, 1H (7.32, dd, H3) and 4H (H11/
110, H12/120)), 7.25 (d, 1H, H5, J = 7.50), 6.97 (d, 1H, H2, J = 8.32),
6.89 (dd, 1H, H4, J = 7.46, 7.46), 4.52 (q, 1H, H8, J = 6.71), 1.61 (d, 3H,
(S)-N-(1-(4-methoxyphenyl)ethyl)salicylaldimine, S-1. mp
70 ꢀC. Yield: 5.19 g, 98.4%. C16H17NO2 (255.32 g molꢀ1): calculated C
3
75.27, H 6.71, N 5.49; found C 74.03, H 6.73, N 5.42%. IR (cmꢀ1): 3076
(vw), 3047 (vw), 2994 (vw), 2977 (vw), 2960 (vw), 2935 (w), 2902
(vw), 2863 (vw), 2837 (w), 2719 (vw), 2656 (vw), 2536 (vw), 1623 (s),
1607 (s), 1579 (m), 1536 (w), 1511 (m), 1501 (m), 1461 (w), 1441
(m), 1415 (w), 1375 (m), 1338 (vw), 1317 (vw), 1303 (w), 1279 (s),
1244 (s), 1202 (w), 1180.9 (s), 1149.6 (m), 1126.0 (vw), 1114.7 (w),
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Inorganic Chemistry
ARTICLE
Table 5. Crystal Data and Structure Refinement for the Zinc Complexes
Λ-Zn-R-1
Δ-Zn-S-1
Λ-Zn-R-2
Δ-Zn-S-2
Λ-Zn-R-3
empirical formula
C
32H32ZnN2O4
C32H32ZnN2O4
573.97
C30H26Cl2ZnN2O2
582.80
C30H26Cl2ZnN2O2
582.80
C30H26Br2ZnN2O2
671.72
M/g molꢀ1
573.97
3
crystal size/mm
2θ range/deg
h; k; l; range
crystal system
space group
a/Å
0.5 ꢁ 0.48 ꢁ 0.33
6.806 ꢀ 46.952
(10; ( 12; ( 21
monoclinic
P21
0.28 ꢁ 0.06 ꢁ 0.02
2.40 ꢀ 54.00
(10; ( 12; ( 21
monoclinic
P21
0.31 ꢁ 0.08 ꢁ 0.03
4.10 ꢀ 54.00
ꢀ9, 12; ꢀ12, 8; ꢀ14, 32
orthorhombic
P212121
0.19 ꢁ 0.10 ꢁ 0.04
4.20 ꢀ 54.00
ꢀ12, 11; ( 13; ꢀ23. 33
orthorhombic
P212121
0.46 ꢁ 0.05 ꢁ 0.03
3.10 ꢀ 54.00
(12; ꢀ13, 11; ꢀ29, 33
orthorhombic
P212121
8.244(5)
8.1814(2)
9.9745(2)
17.0287(4)
90.810(2)
1389.49(5)
2
9.8239(14)
10.4309(16)
26.433(4)
90
9.859(2)
10.064(2)
10.522(2)
26.340(5)
90
b/Å
10.010(6)
17.064(10)
90.974(10)
1408.0(15)
2
10.464(2)
26.470(5)
90
c/Å
β/deg
V/Å3
2708.6(7)
4
2730.7(10)
4
2789.4(10)
4
Z
Dcalc/g cmꢀ3
1.354
1.372
1.429
1.418
1.600
F (000)
600
600
1200
1200
1344
μ/mmꢀ1
max/min transmission
0.912
0.924
1.134
1.125
3.775
0.7530/0.6571
11494 (0.0452)
5635
0.9863/0.7840
34711 (0.0798)
6077
0.9657/0.7200
12402 (0.0427)
5277
0.9574/0.8163
12066 (0.0609)
5936
0.9115/0.2779
16969 (0.0482)
5995
reflect. collected (Rint
)
independent reflections
obs. reflect. [I > 2σ(I)]
parameters refined
3766
4647
4389
4265
4480
352
352
334
334
328
max./min. ΔFa/e Åꢀ3
R1/wR2 [I > 2σ(I)] b
R1/wR2 (all reflect.) b
goodness-of-fit on F2 c
weight. scheme w; a/b d
Flack parameter e
0.236/ꢀ0.298
0.0315/0.0566
0.0470/0.0587
0.722
0.220/ꢀ0.220
0.0372/0.0650
0.0648/0.0725
1.041
0.467/ꢀ0.492
0.0383/0.0719
0.0510/0.0754
1.027
0.486/ꢀ0.354
0.0492/0.0851
0.0810/0.0952
0.973
0.637/ꢀ0.531
0.0396/0.0646
0.0676/0.0705
1.001
0.0197/0.0000
0.0253/0.0000
0.0234/0.0000
0.0203/0.0000
0.0141/0.0000
0.068(8)
0.017(9)
ꢀ0.001(10)
0.007(13)
0.008(9)
2
a Largest difference peak and hole. b R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|; wR2 = [∑w(Fo ꢀ Fc2)2/∑w(Fo2)2]1/2. c Goodness-of-fit = [∑[w(Fo ꢀ Fc2)2]/(n ꢀ p)]1/2
.
2
2
d w = 1/[σ 2(Fo ) + (aP)2 + bP] where P = (max(Fo2 or 0)+2Fc2)/3. e Absolute structure parameter.25
H9, J = 6.72). 1H NMR (CD3OD, 200 MHz), δ/ppm (J/Hz) = 8.54 (s,
1H, H7), 7.42ꢀ7.26 (m, 2H (H3, H5) and 4H (H11/110, H12/120),
6.91ꢀ6.83 (m, 2H, H2, H4), 4.61 (q, 1H, H8, J = 6.5), 1.58 (d, 3H, H9,
J = 7.0). 13C NMR (CDCl3, 400 MHz), δ/ppm =163.7, 160.9, 142.3,
133.0, 132.4, 131.4, 129.5 (C7, C1, C10, C13, C5, C3, C6), 128.8 (C12,
C120), 127.7 (C11, C110), 118.7, 117.0, 67.9, 24.9 (C4, C2, C8, C9).
(see Scheme 3 for NMR atom numbering). [α]25589 (c = 11.8 mg/mL):
ꢀ140.7ꢀ.
added triethylamin (140 μL). The mixture was refluxed for 1 h and filtered.
Slow solvent evaporation from the filtrate gave colorless to light-yellow
crystals after a few days. Analytical results are summarized in Table 4.
The IR spectra of the metal complexes show the characteristic
absorption attributed to the ν-(CdN) vibration around 1615 cmꢀ1 as
described in similar complexes.53
Δ/Λ-bis{(R)-N-(1-(4-methoxyphenyl)ethyl)salicylaldiminato-
2
1
k N,O}zinc(II), Δ/Λ-Zn-R-1. H NMR (CDCl3, 400 MHz), δ/ppm
(J/Hz) of Λ-species = 7.96 (s, 1H, H7), 7.30 (dd, 1H, H3, J = 8.88, 7.75),
7.02 (d, 1H, H5, J = 7.75), 6.85 (d, 2H, H 12/120, J = 8.36), 6.82 (d, 1H,
H2, J = 7.8), 6.58 (dd, 1H, H4, J = 9.0, 7.66), 6.57 (d, 2H, H 11/110, J =
8.45), 4.23 (q, 1H, H8, J = 6.79), 3.73 (s, 3H, OCH3); 1.45 (d, 3H, H9,
J = 6.81); δ/ppm of Δ-species = 8.12 (s, 1H, H7), 7.18 (2H), 6.77 (3H),
4.55 (q, 1H, H8), 3.75 (s, 3H, OCH3). (see Scheme 3 for atom numbering).
Δ/Λ-bis{(R)-N-(1-(4-chlorophenyl)ethyl)salicylaldiminato-
k2N,O}zinc(II), Δ/Λ-Zn-R-2. 1H NMR (CDCl3, 400 MHz), δ/ppm
(J/Hz) of Λ-species = 8.03 (s, 1H, H7), 7.34 (dd, 1H, H3, J = 8.75, 7.62),
7.09 (d, 1H, H5, J = 7.58), 6.97 (d, 2H, H 12/120, J = 8.08), 6.84 (d, 1H,
H2, J = 7.56), 6.81 (d, 2H, H 11/110, J = 8.14), 6.64 (dd, 1H, H4, J =
8.75, 7.43), 4.21 (q, 1H, H8, J = 6.86), 1.51 (d, 3H, H9, J = 6.86); δ/ppm
of Δ-species = 8.15 (s, 1H, H7), 7.22 (m, 4H), 7.03 (d, 1H), 6.58 (dd,
1H), 4.57 (q, 1H, H8). 13C NMR (CDCl3, 400 MHz), δ/ppm = 170.3,
168.9, 138.7, 135.8, 135.3, 133.6 (C1, C7, C10, C5, C3, C13), 129.0
(C11, C110), 128.7 (C12, C120), 123.2, 118.1, 114.6, 69.1, 22.1 (C2, C6,
C4, C8, C9) (see Scheme 3 for atom numbering).
(R)-N-(1-(4-bromophenyl)ethyl)salicylaldimine, R-3. mp
74 ꢀC. Yield: 6.77 g, 90%. C15H14NOBr (304.19 g molꢀ1): calculated
3
C 59.23, H 4.64, N 4.60; found C 59.41, H 4.34, N 4.47%. IR
(cmꢀ1):3050.66 (w), 2972.86 (w), 2888.47 (w), 1900.74 (w),
1630.47 (s), 1577.11 (m), 1489.33 (m), 1452.00 (m), 1416.23 (w),
1401.97 (m), 1379.82 (m), 1315.23 (w), 1278.05 (s), 1220.65 (w),
1204.70 (m), 1149.68 (m), 1129.32 (m), 1109.48 (s), 1008.87 (s),
975.93 (s), 922.79 (s), 846.18 (s), 822.52 (s), 798.22 (m), 757.29 (s),
716.47 (m), 645.87 (m), 621.64 (w), 523.38 (s), 474.17 (m), 455.76
(w). 1H NMR (CD3OD, 200 MHz), δ/ppm (J/Hz) = 8.54 (s, 1H, H7),
7.50 (“d”, 2H, H12/120, J = 8.6), 7.38ꢀ7.25 (m, 2H, H3, H5), 7.32 (“d”,
2H, H11/110, J = 8.4), 6.92ꢀ6.84 (m, 2H, H2, H4), 4.59 (q, 1H, H8, J =
6.7), 1.58 (d, 3H, H9, J = 6.2) (see Scheme 3 for NMR atom
numbering). [α]25589 (c = 11.1 mg/mL): ꢀ122.0ꢀ.
Δ/Λ-bis{(R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-
k2N,O}zinc(II). General Procedure. A methanolic solution (10 mL) of
(R)-N-(1-(4-X-phenyl)ethyl)salicylaldimine [2-((R)-(1-(4-X)ethylimino)-
methyl)phenol] (0.26 g, 1.0 mmol) was added to a methanolic solution
(5 mL) of zinc(II) nitrate hexahydrate (0.15 g, 0.5 mmol) and to this was
Λ-bis{(R)-N-(1-(4-bromophenyl)ethyl)salicylaldiminato-
k N,O}zinc(II), Λ-Zn-R-3. H NMR (CDCl3, 400 MHz), δ/ppm
2
1
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Inorganic Chemistry
ARTICLE
(J/Hz) of Λ-species = 8.03 (s, 1H, H7), 7.34 (dd, 1H, H3, J =
8.81, 8.45), 7.12 (d, 2H, H 12/120, J = 8.34), 7.09 (d, 1H, H5, J = 7.0),
6.84 (d, 1H, H2, J = 8.47), 6.75 (d, 2H, H 11/110, J = 8.25), 6.64
(dd, 1H, H4, J = 8.82, 7.31), 4.20 (q, 1H, H8, J = 6.69), 1.51 (d, 3H,
H9, J = 6.80); δ/ppm of Δ-species = 8.15 (s, 1H, H7), 7.16 (4H),
7.04 (1H), 6.58 (1H), 4.54 (q, 1H, H8). (see Scheme 3 for atom
numbering).
’ ASSOCIATED CONTENT
S
Supporting Information. Crystal photographs, additional
b
molecular structure figures, details of 2D-NMR studies, geome-
tries of modeled conformers, CIF files reported in this paper.
This material is available free of charge via the Internet at http://
pubs.acs.org.
X-ray Crystallography. Single-crystal X-ray structural analysis is a
direct method to determine the absolute configuration of a chiral molecule
using diffraction intensity differences due to anomalous X-ray scattering or
dispersion in the presence of a “heavy atom” with the Bijvoet or related
methods.24 The distinction of inversion-related models of a noncentrosym-
metric crystal structure is measured by the Flack parameter.25 Suitable single
crystals were carefully selected under a polarizing microscope. Data
Collection: Bruker AXS with APEXII CCD area-detector, MoꢀKαradiation
(λ = 0.71073 Å), temperature 203(2) K, graphite monochromator, double-
pass method ω-scan, Data collection and cell refinement with APEX2,54
respectively, cell refinement and data reduction with SAINT,54 experimental
absorption correction with SADABS.55 Structure Analysis and Refinement:
The structure was solved by direct methods (SHELXS-97);56 refinement
was done by full-matrix least-squares on F2 using the SHELXL-97 program
suite.56 All non-hydrogen positions were refined with anisotropic tempera-
ture factors. Hydrogen atoms on carbon were positioned geometrically
(CꢀH = 0.99 Å for aliphatic CH, CꢀH = 0.97 Å for CH3, 0.94 Å for
aromatic CH) and refined using a riding model (AFIX 13, 33, 43,
respectively), with Uiso(H) = 1.2Ueq(CH) and 1.5Ueq(CH3).
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: steffen.luedeke@pharmazie.uni-freiburg.de (S.L.), janiak@
uni-duesseldorf.de (C.J.).
Present Addresses
^Institut f€ur Anorganische Chemie und Strukturchemie, Uni-
versit€at D€usseldorf, Universit€atsstrasse 1, D-40225 D€usseldorf,
Germany.
’ ACKNOWLEDGMENT
We thank Dr. Klaus Ditrich (ChiPros) at BASF AG, Ludwig-
shafen, for providing the enantiopure (R)- or (S)-N-(4-X-phe-
nyl)ethylamines. We acknowledge the financial support DFG-
grant Ja466/14-1. Mr. Robin Br€uckner is thanked for his help in
some of the synthetic work. We would like to acknowledge the
use of the computing resources provided by the Black Forest
Grid Initiative.
Graphics were obtained with DIAMOND.39 Crystal data and details
on the structure refinement are given in Table 5. The structural data has
been deposited with the Cambridge Crystallographic Data Center
(CCDC-numbers 841950ꢀ841954).
’ DEDICATION
Dedicated to Prof. Henri Brunner on the occasion of his 75th
birthday.
VCD Measurements. CDCl3 solution samples were placed in a
100 μm path length BaF2 cell. IR and VCD spectra of 26 mM solutions of
Zn-R-2, Zn-S-2 and Zn-R-3 were recorded on a ChiraIR Dual-PEM
VCD spectrometer from BioTools with 4 cmꢀ1 resolution. The spectra
represent the average of a 4-h measurement. For 4 cmꢀ1 resolution VCD
spectra of Zn-R-1 (26 mM) and R-2 (66 mM) we employed a Bruker
Tensor 27 FTIR spectrometer equipped with the Bruker PMA 50 VCD
sidebench module, using a data collection time of 6 h. All spectra were
corrected for background effects by solvent subtraction. For VCD measure-
ments between 1800 and 800 cmꢀ1, the optimum retardation value of the
photoelastic modulator (PEM) used for the high-frequency circular
polarization modulation of the FT-IR beam was set near the middle of
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