Spontaneous resolution of a Δ/Λ-chiral-at-metal pseudo-tetrahedral Schiff-base zinc complex to a racemic conglomerate with C-H?O organized 41- and 43-helices

Article abstract of DOI:10.1039/c8ce00839f

The Schiff-base ligand N-2-(pyridyl)salicylaldimine (HL) reacts with zinc(ii) acetate or nitrate to obtain the enantiomorphous chiral-at-metal compound Δ/Λ-bis[N-2-(pyridyl)salicylaldiminato-κ²N,O]zinc(ii), [Zn(L)₂] (1), which crysta

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Tiddo J. Mooibroek, Antonio Frontera et al.
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Spontaneous resolution of a ∆/Λ-chiral-at-metal pseudo-tetrahedral
Schiff-base zinc complex to a racemic conglomerate with C−H···O
organized 4
1
- and 4 -helices†
3
a,
b
a
a
Mohammed Enamullah, * Vera Vasylyeva, Mohammad Abdul Quddus, Mohammad Khairul Islam,
b
b,
5
Simon-Patrick Höfert, and Christoph Janiak *
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The Schiff-base ligand, N-2-(pyridyl)salicylaldimine (HL) reacts with zinc(II) acetate or nitrate to give
2
the enantiomorphous chiral-at-metal compound ∆/Λ-bis[N-2-(pyridyl)salicylaldiminato-κ N,O]zinc(II),
1
1
2
0
5
0
[Zn(L)
N,O-chelate ligands form a pseudo-tetrahedral N
atom. The ∆- and Λ-configured molecular complexes in 1 assemble in P (right)- and M (left)-handed 4
and 4 -helical chains in the chiral space groups P4 2 and P4 2, respectively, through weak C−H···O
hydrogen bonding between neighbouring molecules along the chain axis. Only molecules of the same ∆-
or Λ-configuration are combined into a helical chain and only the chains of the same P- or M-handedness
are combined to form homochiral crystals. The supramolecular packing from the analyses of
intermolecular interactions with the Hirshfeld surface features C−H···O bonding as the most apparent
significant contributions. This is a rare example of solely weak C−H···O hydrogen bonding interactions
that leads to spontaneous resolution to a racemic conglomerate. The case also supports the notion of less
repulsive packing interactions between homochiral molecules because of spin polarization. Optimized
structures and excited state properties by DFT/TD-DFT calculations are comparable to the experimental
results.
2
] (1), which crystallizes as a racemic conglomerate via spontaneous resolution. Two deprotonated
2
O
2
-coordination sphere with a ∆/Λ-configured zinc
1
-
3
1
2
1
3 1
2
Introduction
Spontaneous resolution is of continued interests since first
observed by Louis Pasteur in 1848. The interests extend from
2
c
When a racemic compound crystallizes it can form either a
crystalline racemate containing both enantiomers in the same
crystal in equal amounts (heterochiral crystals), or a racemic
conglomerate, that is an equimolar mixture of separate crystals
where each one contains only one of the two enantiomers
4,5,6
3,7,8,9
4
4
5
5
0
5
0
5
organic molecules
to coordination compounds/polymers
2
5
3,7,10
liquid crystals,11
metal-organic frameworks (MOFs),
compounds,8a,e,12
magneto-chiral
self-assembled
mono-
13
layers/fibres and artificial helical structures in supramolecular
3,4,14
chemistry.
The synthesis of chiral metal-complexes from
1
,2,3
(homochiral crystals) (Scheme 1).
The later phenomenon is
achiral ligands via spontaneous resolution touches on the origin
3
0
termed as spontaneous resolution.
15a
of chirality in, for example, biological systems
and is
3
d
characteristically linked to the concepts of crystal engineering.
Spontaneous resolution usually generates
a
racemic
conglomerate, that is, an equimolar mixture of enantiomer-
separated homochiral crystals with the crystals ensemble as a
3
,12a,14c
whole being racemic.
In rare cases enantiopure materials
can be obtained by homochiral seeding if there is also an
7
e,8e,15c
equilibrium between the enantiomeric forms in solution.
Jacques et al. reported that, statistically, only 5 - 10% of all
racemates yield to spontaneous resolution forming a racemic
2
a,16a
conglomerate of homochiral crystals (Scheme 1).
This may
indicate that homochiral interaction is weaker than the
heterochiral one between enantiomers. It may also indicate that
the packing of heterochiral (opposite) enantiomers results in a
60 better space filling and crystal packing index than the packing of
homochiral (pure) enantiomers due to center or plane of
Scheme 1 Schematic presentation of the possible differences in crystal
formation from a racemic solution mixture. R, S are the designations
1
7a
(
devised by Cahn, Ingold and Prelog) of absolute configuration at four-
coordinate and six-coordinate stereogenic centers. Δ (Delta), Λ (Lambda)
are the designations of stereoisomers of tris(bidentate ligand) metal
complexes and other octahedral complexes.
3
5
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symmetry (i.e., inversion or glide symmetry). In other words, the
chains are interlocked with the corrugated van-der-Waals surface
racemic crystals (heterochiral) generally are more stable and 60 interactions. We report herein the spontaneous resolution of Λ/∆-
2
slightly denser than the corresponding homochiral crystals –
bis[N-2-(pyridyl)salicylaldiminato-κ N^O]zinc(II) to a racemic
conglomerate as a rare example that such spontaneous resolution
can originate via weak intermolecular C-H···O hydrogen bonding
interactions.
which accounts for the greater incidence of racemic crystals over
1
8
5
0
5
0
5
0
5
0
5
0
5
conglomerates. This is termed Wallach's rule. Brock et al.
showed that this tendency is not necessarily thermodynamic in
origin, but rather likely reflects both kinetic factors dealing with
molecular interactions leading to nucleation and crystal growth 65 Results and Discussion
from racemic solution, and greater extent of packing
a
1
1
2
2
3
3
4
4
5
5
arrangements in achiral crystallographic space groups that might
statistically favor racemic crystals over conglomerates.17a
The achiral Schiff base ligand N-2-(pyridyl)salicylaldimine (HL)
reacts with zinc(II) acetate or nitrate to give ∆/Λ-bis[N-2-
A theoretical examination however indicated that homochiral
crystals are the thermodynamically stable phase for 19% of the
examined compounds, indicating that the prevalence of stable
2
(
pyridyl)salicylaldiminato-κ N,O]zinc(II), ∆/Λ-Zn(L)
2
(1) in the
presence of NaHCO
3
under reflux in ethanol (Scheme 2).
–
7
0
Vibrational spectra show very strong bands at 1620 and 1609 cm
1
9
17b
conglomerates is underestimated. MacDonald et al. recently
reported that the conglomerates (homochiral) exhibit slightly
greater thermal stability than the racemic one (heterochiral) (i.e.,
Δ(ΔHfus.) = 4.0 kJ/mol) despite having considerably lower density
1
27,28,29
(νC=N) for the azomethine group.
EI mass spectrum gives
+
the parent ion peak at 458 ([M] ) together with the peak at 199
+
(
[HL+H] ). The spectrum further shows several ions peaks for the
+
+
+
+
[
M–C
5
H
4
N] , [M–C
6
H
4
5 4
(OH)(CHN)] , [M–L] and [C H N]
(Δρcalc. = 3.7%) and less efficient molecular packing in
1
75
species. H NMR spectra (Figure S1 in the ESI†) show singlets at
δ 9.47 (HL) and 9.44 ppm (1) for the imine proton and at δ 13.49
ppm (HL) for the phenolic proton. The aromatic proton adjacent
to the pyridine nitrogen atom is found downfield at δ 8.54 (HL)
and 8.38 ppm (1) due to strong inductive effect of the nitrogen
accordance with Wallach's Rule. The greater stability reflects
stronger hydrogen-bonding interactions via homochiral packing
as compared to heterochiral packing in racemic crystals. Still, the
formation of conglomerates vs. racemates is not yet fully
understood, therefore the formation of racemic conglomerates
3
a,12a-b,20
(spontaneous resolution) cannot be predicted a priori.
80 atom. The spectra further show several aromatic protons peaks at
δ 6.90−7.80 ppm in HL, which shift to relatively upfield upon
coordination to the zinc ion (δ 6.70−7.60 ppm). Cyclic
The racemic solution mixture can spontaneously resolve
during crystallization to form the racemic conglomerate
(enantiomers) due to noncovalent interactions such as strong or
voltammograms demonstrate a weak reduction peak at ca. −0.30
weak hydrogen-bonds (e.g., O−H···O/N/S/Cl, C−H···Cl and N–
−
V due to the [Zn(L)
2
]/[Zn(L)
2
] couple in acetonitrile (Figure S2
3
,4,5d,6b,9,21a
5,15b,16b,22
H···O),
C−H···O and C–H···π interactions,
85
in the ESI†, see the ESI† for details).
π···π stacking,3,6c,9d,15b,21b coordination covalent bonds,9a-
b,11b,14b,23a
and electrostatic and charge transfer bonds (e.g.,
10
N
ion···π/X-H···π, X = O, N, halogen),5a,6a-b,23b. Further, metal-
9
8
11
12
6
7
1
ligand interactions coupled with the interligand steric
5
N
9
c
23a
interactions, interchain coordination bond,
conformational
crystal solvent
play an
2
N
N
4
flexibility and bridging nature of ligands,7
and temperature (e.g., reaction conditions)
e,12b,21
salt
Zn(II)
O
Zn
O
7
e,14e,16b-c
3
OH
HL
NaHCO3
ethanol
24 reflux
h,
N
N
important role in the process. Spontaneous resolution requires an
efficient transfer of stereochemical information between the
neighbours.
∆
/Λ-Zn(L)2
1
( )
It has recently been reported that when molecules interact, the
electronic charges in each of them are redistributed. In chiral
molecules, charge redistributions are accompanied by spin
polarization, which add an enantioselectivity term to the
interaction forces, so that homochiral interaction energies differ
significantly from heterochiral ones. As a result, interactions
between chiral molecules are less repulsive for molecules of alike
chirality (homochiral) than for the molecules of opposite chirality
Scheme 2 Synthetic route to the formation of ∆/Λ-bis[N-2-
2
(
pyridyl)salicylaldiminato-κ N,O]zinc(II) (1).
Solid state structural analyses
Two deprotonated N,O-chelate ligands (L ) form a pseudo-
–
9
0
tetrahedral N -coordination sphere around the zinc atom in
2 2
O
distorted tetrahedral geometry (Fig. 1). The zinc atom sits on the
special position of a 2-fold rotation axis (Fig. S7a in the ESI†) so
that only one chelate ligand is crystallographically unique. The
Zn–O/N bond lengths and angles (Table S2 in the ESI†) are as
expected from reported literatures for the analogous distorted-
2
4
(heterochiral).
95
Our previous studies along spontaneous resolution demonstrate
4
that
the
racemate
[Rh(η -cod)(R,S-N-phenylglycinato)]
2
8,30,48
tetrahedral Zn(II)-N,O-chelate complexes.
The optimized
spontaneously resolved into the enantiomorphic S- and R-forms,
arranged in homochiral (right)- and (left)-helices,
crystallizing in the opposite chiral space groups P4 and P4
Intermolecular N–H···O hydrogen bonding
structures by DFT show comparable bond lengths and angles to
the experimental values (Fig. S8 and Table S2 in the ESI†). The
P
M
1
3
,
i
°
i
2
5,26
100 O1-Zn1-O1 angle is larger by ca. 11 , while O1 -Zn1-N1 is
respectively.
°
slightly smaller by ca. 4 in the optimized structures. The
interactions connect only the molecules of same S- or R-
configuration into P (right)- or M (left)-helical chains. Two
neighboring homochiral P (right)- or M (left)-handed helical
distortion from tetrahedral geometry and strong deviation from
tetrahedral angles at zinc atom is brought upon by the chelate bite
angle (O−Zn−N = ~ 94°) and additional weak coordination of the
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α
two pyridyl-nitrogen atoms to the zinc atom which extends the
coordination number to 4+2. The Zn-Npyridyl bonds are,
however, substantially longer (~ 2.82 Å) than the Zn-Nimine bonds
O2
N2
47
N1
M
O1
(
~ 2.0 Å) (Table S2 in the ESI†).
β
θ
−
θ/ο
= θ/90o
α
+ β)]/141o
(
τtet-sq
τ
=
4
[360
tetrahedral
square planar
geometry
90
0
1
0
1
0
3
3
0
5
Scheme 3 Assessments of distortions from tetrahedral to square-planar
geometry by θ, τtet-sq and τ , respectively (α and β are the two largest
angles in the four-coordinated species, see Table S2 in the ESI†).
Dihedral angle θ or its normalized index τtet-sq is preferred for the bis-
4
bidentate chelate complexes. Indeed, the index τ fails to assess correctly
4
a tetrahedral geometry because of existing distortion resulting from
chelate ring formation. With chelate ligands, the two largest angles are
already larger than 109.5°, thus τ
4
< 1.0, even if the dihedral planes are
26-29
perfectly perpendicular.
5
O
*
N
N
*
O
M
M
N
O
O
N
Λ
∆
Λ
∆
-M(N^O)
2
-M(N^O)
2
40
Scheme 4 Induced chirality by N^O-chelate ligands at-metal center with
Λ (left)- and Δ (right)-handed enantiomers in C -symmetrical tetrahedral
2
or pseudo-tetrahedral geometry.
The chiral zinc complex 1 crystallizes in the tetragonal, chiral
2,33
4
5
5
6
6
7
5
0
5
0
5
0
space groups P4
1
2
1
2
and P4
3
2
1
2, respectively3
with
spontaneous resolution of the (homochiral) single crystals to a
racemic conglomerate. The crystal structure refinement data for
the absolute structures and determined Flack parameters between
3
4
–
0.005(2) and 0.03(2) (Table 1) indicate that the individual
investigated crystals are enantiopure (homochiral). A Flack
parameter close to zero confirms the correct absolute structure
and excludes the presence of complexes with opposite metal
chirality within the investigated crystal in significant amounts.
The chiral space groups P4 2 2 and P4 2 2 form an
1 1 3 1
It was not possible to visibly
distinguish the crystals with the Δ- and Λ-configurations or P4
Fig. 1 Molecular structures (50% thermal ellipsoids) of the (top) Λ-form
(
data set 1g, 140 K) and (bottom) Δ-configured complexes (data set 1e, 95
K) with the oxygen atoms are facing to the front. Symmetry
transformation (i) y, x, 1–z and y, x, 2–z, respectively.
1
0
enantiomorphous pair.3
2,33
For a more quantitative assessments of the coordination
geometry, along with the DFT optimization, the degree of
distortion from tetrahedral can be expressed by the dihedral angle
θ/° (angle between two planes formed by N,O-chelate with the
1
-
and P4 -helices, respectively (cf. Fig. S11 in the ESI†). So, we
3
performed full crystallographic data set collections on eight
crystals to determine the space group. From the investigated
overall eight crystals, five of them (1a-1c, 1g-1h) were found to
i
i
1
2
2
5
0
5
metal atom, that is, N1−Zn−O1 and N1 -Zn-O1 ), its normalized
value τtet-sq (= θ/90°) or the geometry index τ {= (360° − (α +
4
contain the Λ-configured complex in P4
three of them (1d-1f) contained the Δ-configured complex in
P4 2 (P-helices). We thereby also conclude that the crystals
3 1
2 2 (M-helices) and
β))/141°} (Scheme 3 and Table S3 in the ESI†).27-29,31 The value
of θ is 90° for tetrahedral and 0° for square-planar geometry (not
considering the inherent distortion induced by the chelate ring
1 1
2
present a racemic conglomerate and that no crystalline racemate
was present. The 4-fold screw axes run parallel to the c axis,
bisecting the a and b axes (Fig. S7b in the ESI†). Fig. 2a
illustrates the assembly of the molecular complexes around such
a 4-fold helical axis. The molecules from neighbouring helices fit
into each other with their corrugated van-der-Waals surfaces (Fig.
formation), while τtet-sq and τ
to zero (square-planar geometry). In compound 1 these values are
θ ≈ 87°, τtet-sq ≈ 0.97 and τ ≈ 0.80, which are very close to the
4
values vary from 1.0 (tetrahedral)
4
optimized structures (i.e., 83°, 0.92 and 0.81, respectively, see
Table S3 in the ESI†).
2
In tetrahedral or pseudo-tetrahedral C -symmetrical metal
2
5,26
2b).
complexes, two asymmetric N^O-chelate ligands will give rise to
metal-centered Λ- and ∆-chirality (i. e., opposite configuration at
We note that it appears, at first sight, somewhat artificial to
speak of "helices formed by the molecules of 1" as there are no
immediately apparent special or stronger supramolecular
interactions along the chain axis. The molecular complexes in 1
2
the metal center), and provide two enantiomers Λ-M(N^O) and
2
6-29
∆
-M(N^O)
2
(Scheme 4).
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have no functional groups for strong hydrogen bonding
interactions. A supramolecular packing analysis by PLATON
does not indicate any π−π interactions between the aromatic 10 adjacent chain (Fig. S9 and Table S4 in the ESI†). Along the
contact with less than 2.7 Å for the (C−)H···ring centroid
3
7
distances and C−H···Cg > 145°, albeit to a molecule from an
ligands. All centroid-centroid contacts of the aromatic rings are
above 4.7 Å, while significant π-stacking should show rather
short centroid-centroid contacts (<3.8 Å) with near parallel ring
helical-chain arranged molecules only two C−H···O hydrogen
5
bonds are found between consecutive molecules (Figs. 2a, S9 and
3
8
Tables S5, S6 in the ESI†).
3
5,36
planes.
PLATON reveals only one significant C−H···π
1
5
Fig. 2 (a) Assembly of five molecules of 1 (Δ-configuration) winding around the 4 (P, right-handed) helix (yellow line). The C−H···O hydrogen bonds
1
are indicated as dashed orange lines (from data set 1e). (b) The interlocking of two neighboring right (∆)-handed helical chains in 1 with the corrugated
van-der-Waals surface through weak interactions; space-filling representation of the molecules, which are differentiated by red and green in their different
helices (from data set 1e).
2
0
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The supramolecular packing in 1 is further supported by the
reported that the interactions between chiral molecules lead to
analyses of intermolecular interactions with Hirshfeld surfaces
spin polarization via charge redistribution that enforce symmetry
3
9
using the program CrystalExplorer, following the methodology 20 constraints on the recognition process between two molecules.
4
0
outlined in references. The 2D fingerprint plot with the
characteristic features due to the C−H···O and C−H···π contacts
is show in Fig. 3a. The plot represents an overlay of all
contributions from close intermolecular contacts. Fig. 3b
These constraints can lead to a selectivity in the interaction
between enantiomers based on their handedness. As a result,
homochiral interaction energies differ from heterochiral ones.
Thus interaction is less repulsive for molecules of alike chirality
5
illustrates the Hirshfeld surface mapped with the dnorm property. 25 (homochiral) than for molecules of different chirality
2
4
The relative contributions to the Hirshfeld surface area due to
close intermolecular contacts are C···C (i.e. π···π) 1.1%, C···H
(enantiomers). As an example, the repulsive interaction energy
for the methyl groups on homochiral molecules is calculated
significantly smaller from that found for the methyl groups on
heterochiral molecules at a C···C distances below 3.0 Å (e.g. by
1
0
(
i.e. C–H···π) 40.7%, C−H···O 11.1% and H···H 38.9% (for
details see Fig. S10 in the ESI†).
It is not straightforward to rationalize the homochiral packing 30 about 0.5 kcal/mol at 2.6 Å). Thereby it is important that the
in 1 from the rather weakly attracting C−H···O and seemingly
insignificant C–H···π interactions. Therefore, we argue that the
H···H contacts are considered less repulsive between homochiral
than between hetarochiral molecules. It has recently been
interacting groups do not have to be chiral. Chiral recognition
occurs because of the spin polarization which is due to the
chirality of the molecule as a whole.
1
3
5
5
2
4
(a)
(b)
Fig. 3 (a) Graphical presentation of Hirshfeld surface with 2D fingerprint plot with the characteristic features due to the C−H···O (red arrows) and
C−H···π (blue arrows) contacts (d and d are the distances from the surface to the nearest atom interior and exterior to the surface, respectively). (b)
Hirshfeld surface mapped with the dnorm property the red spots represent the closest contacts and blue the most distant contacts.
i
e
1 1 1
2 2
16b). We note that many of such homochiral crystal
4
4
5
5
0
5
0
5
A few examples have been reported where the conglomerate
formation appears to be controlled by similar weak C–H···π
interactions (Ag-N′,N′-bis[1-(imidazol-4-yl or pyridin-2-
group P2
packings with weak controlling interactions (as in 1) seem to
5
,9c,15b,21,43
feature simultaneous helical arrangements.
preferential and extended homochiral interactions between the
21), π–π stacking 60 neighbouring helices via noncovalent interactions, the chirality
2,5-diphenyl-3,4-di(3- will extend to a higher dimensionality, which enhances the
pyridyl)cyclopenta-2,4-dien-1-one, forms colonies of homochiral
For
yl)methylidene]benzil-dihydrazone
enantiomorphic space groups P3 21 and P3
([Zn(L)Cl or [Hg(L)Br
crystallizing
in
the
1
4f
1
2
2
]
α
2
]
α
,
L
=
5
d,8e,10a,12,44,45
probability of spontaneous resolution.
6
c
M-helix – space group C2), C–H···π and C–H···N interactions
Thermal analysis
(
Cu-bi(pyridyl)triazolate crystallizing in 4
1
-helical structure –
4
1
space group P4
3
2
1
2), C–H···Cl interactions (homochiral 1D-
The thermodynamic parameters of racemic crystals (heterochiral)
1
7b
helical metal–organic framework in α,α'-bis(pyrazolyl)-m- 65 will differ from racemic conglomerates (homochiral).
To
2
1
xylylene zinc(II) chloride – space group P2
1
), and C–H···O
further support the notion that the crystal batch presents solely a
racemic conglomerate and that no crystalline racemate was
present we carried out a dynamic scanning calorimetry (DSC)
measurement. If both types of crystals would be present DSC
and/or C–H···π interactions (kinetically controlled helical
supramolecular structure of myo-Inositol hexabenzoate – space
4
2
group P6
1
and thermodynamically controlled helical structure
of 2,3:6,7-dibenzobicyclo[3.3.1]nona-2,6-diene-4,8-dione – space 70 may show a binary phase by two different phase transitions for
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the melting or decomposition of homochiral and heterochiral
strong (classical) hydrogen bonding or π−π stacking. Instead, the
resolution and helical chain orientation seems be controlled solely
through C−H···O hydrogen bonding to neighbouring molecules
2
8,46,47
crystals.
The DSC heating curve (Fig. S3 in the ESI†)
exhibits a single-phase transformation at ca. 260 °C (∆H = −36.5
kJ/mol) which we see as evidence for having only one type, name 60 along the chain axis. C−H···O hydrogen bonding is normally
5
the homochiral crystals in the racemic conglomerate in
accordance with the X-ray analyses (where only homochiral
crystals are found, vide supra).
considered one of the weakest supramolecular (noncovalent)
interactions. The results reflect apparently a higher stability for
the racemic conglomerate (homochiral crystals) in the absence of
any strong interactions. This may also correspond to a more
efficient packing in the homochiral crystals, opposite to Wallach's
rule.
Further, we invoke a less repulsive interactions between
homochiral than between heterochiral molecules because of spin
polarization and see it as remarkable that such weak
Electronic and computed spectra.
6
5
The electronic spectra of the free ligand (HL) and complex (1) in
chloroform (Fig. S4 and Table S1 in Supp. Info.) feature several
identical bands/shoulders below 390 nm for intra-ligand n→π∗
and π→π* transitions (LL), respectively. An additional broad
1
1
2
2
3
3
4
4
0
5
0
5
0
5
0
5
band above 390 nm with absorption maxima at 416 nm is found 70 supramolecular interactions can drive spontaneous resolution.
2
8,47,48
in 1, due to metal-ligand (ML) charge transfer transitions.
Time dependent electronic spectra (Fig. S5 in the ESI†) show
decomposition of the compound upon dissolution in chloroform
Experimental section
All the reagents and solvents were obtained from commercial
sources and used without further purification. FT-IR-spectra were
recorded on a Nicolet iS10 spectrometer as KBr discs at ambient
temperature. Electronic spectra were obtained with a Shimadzu
UV 1800 spectrophotometer in cyclohexane at 25 °C. Elemental
(ca. 10% within 5 min. or 100% within 2 hours), a common
feature of the labile distorted-tetrahedral Zn(II)-N,O-chelate
2
8,47-48
complexes.
7
8
8
9
9
5
0
5
0
5
To gain further insight on the ∆/Λ-chirality at-metal (i.e.,
spontaneous resolution) we attempted to measure electronic
circular dichroism (ECD) spectra. Because the crystals of 1 are a
racemic conglomerate the overall crystal mixture behaves like a
racemate. Thus, measuring an ECD spectrum of the crystal
mixture in the solid-state or in solution would show no
enantiomer signal. Only if we measure the ECD spectrum of a
single (tiny) crystal in the solid state or as a very dilute solution
we would possibly be able to see the ECD pattern of the single
enantiomer. It was not feasible to run ECD for a single crystal in
the solid state with the instrument available to us. In solution, we
conducted ECD-measurements for a single crystal (four attempts,
each with a single crystal) or several crystals (two attempts, each
analyses were done on
a Vario EL instrument from
Elementaranalysensysteme GmbH. Thermal analysis was
performed on a SHIMADZU DSC-60 differential scanning
calorimeter (DSC) under nitrogen gas at 40−260 °C (just before
–
1
the decomposition temperature) and heating rate of 10 K min .
TM
An Epsilon Instruments (BASi) electrochemical analyzer was
used for cyclic voltammetry (CV) experiments in acetonitrile
containing tetra-N-butyl-ammonium-hexaflorophosphate (TBAP)
as supporting electrolyte. The three-electrode measurement was
carried out at 298 K with a platinum disc working electrode, a
platinum wire auxiliary electrode and an Ag/AgCl reference
electrode. The solution containing the complex and TBAP was
3
with 3-4 crystals for higher concentration) in CHCl . In all
1
deoxygenated for 10 minutes with nitrogen gas. H NMR spectra
attempts the ECD spectra look rather similar and the solutions
appear ECD-inactive (Figure S12 in the ESI†). This may indicate
either a too low concentration of the enantiomeric species or
racemization due the absence or loss of the weak supramolecular
interactions which had led to spontaneous resolution in the solid
state. In earlier work we have already shown the helicity
inversion of similar four-coordinated nonplanar Zn(II) and Cu(II)
Schiff-base complexes in solution,49,50 which can occur through
the planar conformation.
were recorded on a Bruker Avance DPX 300 spectrometer at 300
MHz in CDCl at 20 °C. ECD spectra of single crystals or a few
3
crystals were run with an Olis RSM100 Spectropolarimeter in
chloroform (0.1 mL, sufficient color) at 20 °C. The EI mass
spectrum was obtained on a Finnigan Trace GC Ultra. Isotopic
6
4/66/68
distributions patterns for
visible in the mass spectrum.
Zn(II)-containing ions are clearly
Synthesis of the Schiff base ligand (HL)
The computed electronic spectra by TD-DFT (Fig. S4 in the
Salicyldehyde (1.730 g, 14.18 mmol) was dissolved in 10 mL of
methanol, 2-3 drops of concentrated H SO were added and the
2 4
ESI†) for both P4
1
(∆)- and P4 (Λ)-enantiomers of the compound
3
are identical and show the best fit to the experimental one (see the
ESI† for details on computed spectra).
1
00 solution stirred for 10 min. An equimolar amount of 2-amino-
pyridyl (1.332 g, 14.17 mmol) was added into this solution. The
reaction mixture was then refluxed for 6 h, and the colour turned
to bright yellow. The solvent was evaporated to 50% in vacuo,
and the solution was left standing for crystallisation at room
Conclusions
The enantiomorphous complex Λ/∆-bis[N-2-(pyridyl)salicylaldi-
2
minato-κ N,O]zinc(II) (1) crystallizes to a racemic conglomerate 105 temperature. The crystals were filtered off and washed three
5
0
via spontaneous resolution during crystallization. The ∆-
configured molecular complexes in 1 assemble in right (P)- and
times with methanol (5 mL in each). The bright yellow crystals of
N-2-(pyridyl)salicylaldimine (HL) were dried in air for 2 d.
N-2-(pyridyl)salicylaldimine (HL). Yield: 2.35 g (84%). IR
the Λ-configured ones into left (M)-handed 4
chain, respectively. Only molecules of the same (∆ or Λ)-
configuration are combined into a chain and subsequently only 110 1574s (C=C). MS (EI, 70 eV): m/z (%) = 199 (100) [M+H] . H
chains of the same 4 - or 4 -handedness are found in a NMR (CDCl , 300 MHz): δ = 6.98 (dt, JHH = 7.5 Hz, JHH = 0.9
Hz, 1H, H ), 7.06 (d, JHH = 8.1 Hz, 1H, H ), 7.25 (ddd, JHH = 6.0
1 3
- and 4 -helical
–
1
(KBr, cm ): ν = 3051, 2975w (C-H), 1608, 1588vs (C=N), and
+
1
5
5
1
3
3
homochiral crystal. This resolution occurs in the absence of
5
3
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Page 8 of 11
Hz, JHH = 0.9 Hz, 1H, H
4
), 7.36 (d, JHH = 7.8 Hz, 1H, H
6
), 7.43
(CCDC numbers 1844266-1844273).
(
=
ddd, JHH = 7.2, 6.9 Hz, JHH = 1.8, 1.2 Hz, 1H, H11), 7.53 (dd, JHH
7.5, 7.8 Hz, JHH = 1.5, 1.8 Hz, 1H, H ), 7.80 (dt, JHH = 7.8, 7.5
9
Computational section
Hz, JHH = 2.1, 1.8 Hz, 1H, H10), 8.54 (dd, JHH = 4.5, 4.8 Hz, JHH
1.2, 1.5 Hz, 1H, H12), 9.47 (s, 1H, H ), and 13.49 (s, 1H, OH)
see Scheme 1 for atom numbering).
=
60 All calculations were performed with the Gaussian 09 software
package The initial geometries for computation were generated
from the cif-files having opposite chiral space groups with
5
8
5
7
(
P4
3
(Λ) (from data set 1a)- and P4
1
(∆) (from data set 1d)-
Synthesis of compound 1
enantiomers, respectively. The optimizations were performed by
Two equivalents of N-2-(pyridyl)salicylaldimine (HL) (396 mg, 65 DFT with the B3LYP/6-31g(d) for both enantiomers,
2
.00 mmol) were dissolved in 10 mL of ethanol and added to a
respectively. Both optimized structures (Fig. S8 in the ESI†)
show the same free energy minima as expected for the equi-stable
1
1
2
2
3
3
0
5
0
5
0
5
solution of zinc(II) acetate or zinc(II) nitrate (1.0 mmol) in 10 mL
of methanol or ethanol. Into this solution two equivalents of
enantiomers (e.g., −3073.783631 au for P4
−3073.783627 au for P4 -isomer). For excited state properties,
mixture refluxed for 24 h. After the solvent volume was reduced 70 Time Dependent Density Functional Theory (TD-DFT) was
1
-isomer and
NaHCO
3
(dissolved in 5 mL of methanol) was added and the
3
to ca. 50% in vacuo, the solution was left standing for
crystallization by slow solvent evaporation at room temperature.
Bright yellow to light orange coloured crystals (see Fig. S11 in
the ESI† for crystals data sets 1a-1e), suitable for X-ray
measurement, were obtained within 4-5 days. The crystals were
employed with the B3LYP/TZVP on both optimized structures,
incorporating PCM (Polarization Continuum Model) using
chloroform as a solvent, respectively. For calculations, 72 excited
states were considered (Table S7 in the ESI†).
filtered off, washed two times with ethanol (2 ml), and dried in 75 Acknowedgements
vacuo at 30 °C. Alternatively, identical light orange crystals
We acknowledge the Wazed Miah Science Research Centre
(
crystal data set 1g) were obtained via slow diffusion of methanol
into concentrated solution of the dried compound in
dichloromethane.
Bis[N-2-(pyridyl)salicylaldiminato-κ N,O]zinc(II), ∆/Λ-Zn(L)
(
WMSRC) at Jahangirnagar University, Dhaka, Bangladesh for
a
1
obtaining elemental, H NMR and CV data. Our sincere thanks to
Professor Andrew Woolley and Mr. Ryan Woloschuk,
80 Department of Chemistry, University of Toronto, Canada, for
running the ECD spectra. ME thanks the Alexander-von-
Humboldt (AvH) Foundation, Bonn for its continuous support.
2
2
–
1
(1). Yield: 0.25 g (74%). IR (KBr, cm ): ν = 3080, 3050, 3014m
1
(H-C), 1620, 1609vs, (C=N), and 1585vs (C=C). H NMR
(
J
H
H
H
CDCl
3
, 300 MHz): δ = 6.73 (t, JHH = 7.0 Hz, 1H, H
HH = 8.6 Hz, 1H, H ), 7.12 (dd, JHH = 7.1, 6.8, JHH = 1.8 Hz, 1H,
), 7.27 (d, JHH = 7.4 Hz, 1H, H ), 7.42 (d, JHH = 8.0 Hz, 2H,
9,11), 7.62 (t, JHH = 7.2 Hz, 1H, H10), 8.38 (d, JHH = 3.6 Hz, 1H,
12), and 9.44 (s, 1H, H ) (see Scheme 1 for atom numbering).
5
), 6.96 (d,
3
4
6
8
5
7
+
+
MS (EI, 70 eV): m/z (%) = 458 (80) [M] , 380 (10) [M–C
3
[
C H N O
24 18 4 2
6
5
H
4
N] ,
+
+
38 (40) [M–C
6
H
4
(OH)(CHN)] , 261 (100) [M–L1] , 198 (20)
+
+
+
HL1] , 197 (25) [HL1−H] , and 78 (55) [C
5
H
4
N] .
Zn (459.81): calcd C 62.69, H 3.95, N 12.18; found C
3.11, H 3.71, N 12.02.
90
X-ray Crystallography
Suitable single crystals (crystal data sets 1a – 1h) were carefully
selected under a polarizing microscope. Data collection: Bruker
APEX2 CCD diffractometer (with microfocus tube); sources:
Mo–Kα radiation (0.71073 Å) for 1a, 1e, 1f, 1g, 1h and Cu–Kα
radiation (1.54178 Å) for 1b, 1c, 1d; multilayer mirror, ω- and φ-
95
4
4
5
5
0
5
0
5
5
1
scan; data collection with Apex2, cell refinement and data
5
2
reduction with SAINT (Bruker),
experimental absorption 100
5
3
correction with SADABS. Structure Analysis and Refinement:
The structures were solved by direct methods using SHELXS-
5
4
2
9
7; refinement was done by full-matrix least squares on F
5
4
using the SHELXL-97 program suite. All non-hydrogen
positions were refined with anisotropic displacement parameters. 105
Hydrogen atom positions were found and refined in data sets
1
a−1f, 1h and positioned geometrically using a riding model in
data set 1g. Crystal data and details on the structure refinement
are given in Table 1. Graphics were drawn with the
5
5
DIAMOND. Computation on the supramolecular C−H···O 110
5
6
interactions were carried out using the PLATON for Windows
5
7
and Mercury 3.0. The structural data for this paper have been
deposited with the Cambridge Crystallographic Data Center
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PAPER
Table 1. Crystal data for compound 1 (crystal data sets 1a−1h).
Crystal No.
1a
1b
1c
1d
CCDC no.
1844266
1844268
1844270
1844267
Empirical formula
C
24
H
459.81
18Zn
1
N
4
O
2
C
24
H
459.81
18Zn
1
N
4
O
2
C
24
H
459.81
18Zn
1
N
4
O
2
24 1 4 2
C H18Zn N O
–
1
M / g mol
459.81
0.23 × 0.21 × 0.19
296(2)
3
Crystal size / mm
Temperature / K
0.30 × 0.30 × 0.20
296(2)
0.37 × 0.28 × 0.24
296(2)
0.27 × 0.25 × 0.16
296(2)
θ range / ° (complet.)
h; k; l range
Crystal system
Space group
a = b /Å
2.26-25.08 (100%)
±11; ±11; –25, 24
Tetragonal
4.92-65.89 (99.6)
±11; ±11; -24, 23
Tetragonal
4.92-64.80 (96.8)
-11, 10; -11, 10; –24, 19
Tetragonal
4.92-66.00 (98.8)
±11; ±11; -24, 25
Tetragonal
P4
3
2
1
2
P4
3
2
1
2
P4
3
2
1
2
1 1
P4 2 2
9.9154(8)
21.343(2)
2098.3(4)
4
1.455
1.199
9.9041(3)
21.3545(7)
2094.69(11)
4
9.9138(3)
21.3276(8)
2096.15(12)
4
9.9077(12)
21.330(3)
2093.90(4)
4
1.459
1.864
c /Å3
V /Å
Z
−
3
D
calc/g cm
µ /mm
F(000)
1.458
1.864
944
1.457
1.862
944
−
1
944
944
Max./min. transmission
Reflect. collected
0.795 / 0.715
12332
0.605 / 0.523
21092
0.605 / 0.523
15506
0. 753 / 0. 620
16993
Indep. reflect. (Rint
)
1870 (0.030)
1870 / 0 / 177
0.117 / –0.162
0.0191 / 0.0485
0.0217 / 0.0505
1.092
1816 (0.034)
1816 / 0 / 177
0.130 / –0.219
0.0197 / 0.0507
0.0197 / 0.0507
1.134
1712 (0.027)
1712 / 0 / 177
0.135 / –0.235
0.0203 / 0.0538
0.0204 / 0.0539
1.143
1799 (0.034)
1799 / 0 / 177
0.108 / –0.183
0.0193 / 0.0502
0.0197 / 0.0509
1.127
Data/restraints/parameters
–
3 a
Max./min. ∆ρ / e.Å
b
R1/wR2 [I > 2σ(I)]
b
R1/wR2 (all data)
2
c
Good.-of-fit on F
d
Flack parameter
–0.01(1)
0.02(2)
0.03(2)
0.03(2)
Crystal No.
CCDC no.
1e
1844269
1f
1g
1844273
1h
1844272
1844271
Empirical formula
C
24
H
459.81
18Zn
1
N
4
O
2
C
24
H
459.81
18Zn
1
N
4
O
2
C
24
H
459.81
18Zn
1
N
4
O
2
24 1 4 2
C H18Zn N O
459.79
0.08 × 0.10 × 0.15
150(2)
–
1
M / g mol
3
Crystal size / mm
Temperature / K
0.23 × 0.22 × 0.16
95(2)
0.26 × 0.14 × 0.13
95(2)
0.25 × 0.25 × 0.25
140(2)
θ range / ° (complet.)
h; k; l range
Crystal system
Space group
a = b /Å
2.29-25.13 (99.5)
±11; ±11; -25, 23
Tetragonal
2.29-27.58 (99.6)
-9, 12; -12, 11; -16, 27
Tetragonal
2.280-27.673 (99.5)
±12; ±12; ±27
Tetragonal
2.278-27.998 (99.9)
±13; ±13; ±27
Tetragonal
P4
1
2
1
2
P4
1
2
1
2
P4
3
2
1
2
3 1
P4 2 2
9.8258(19)
21.005(5)
2027.90(7)
4
1.506
1.241
9.8329(12)
21.061(2)
2036.3(4)
4
1.500
1.236
9.8592(4)
21.1220(10)
2053.14(19)
4
9.8663(6)
21.1146(17)
2055.4(3)
4
c /Å
V /Å
3
Z
−
3
D
calc/g cm
1.487
1.226
944
1.486
1.224
944
−
1
µ (Mo Kα) /mm
F(000)
944
944
Max./min. transmission
Reflect. collected
0.745 / 0.524
11665
0.860 / 0.740
7224
0.746 / 0.669
98896
0.747 / 0.697
37194
Indep. reflect. (Rint
)
1807 (0.034)
1807 / 0 / 177
0.163 / –0.162
0.0156 / 0.0405
0.0164 / 0.0408
1.080
2344 (0.026)
2344 / 0 / 177
0.277 / –0.321
0.0235 / 0.0565
2388 (0.024)
2388 / 0 / 141
0.234 / –0.148
0.0161/ 0.0456
0.0163/ 0.0457
1.113
2477 (0.029)
2477 / 0 / 141
0.231 / –0.146
0.0178/ 0.0473
0.0187/ 0.0477
1.137
Data/restraints/parameters
–
3 a
Max./min. ∆ρ / e.Å
b
R1/wR2 [I > 2σ(I)]
b
R1/wR2 (all data)
0.0266 / 0.0575
1.040
2
c
Good.-of-fit on F
d
Flack parameter
0.01(1)
0.00(1)
-0.005(2)
-0.012(3)
a
b
2
2
2
2
2
1/2
c
2
2 2
Largest difference peak and hole. –
R = [Σ(║F
1
o
│–│F
c
║)/Σ│F
o
│]; wR
2
= [Σ [w(F
o
–F
c
) ]/Σ[w(F
o
o c
) ]] . – Goodness-of-fit = [Σ [w(F –F ) ] /(n–
1/2
d
34
p)] . – Absolute structure parameter.
5
This journal is © The Royal Society of Chemistry [year]
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Page 10 of 11
Notes and references
(
4
2
c) X. Chen, M. Du and T. C. W. Mak, Chem. Commun., 2005,
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000, 2401–2408.
a
Department of Chemistry, Jahangirnagar University, Dhaka-
342, Bangladesh. Email: enamullah@juniv.edu
Institut für Anorganische Chemie und Strukturchemie, Heinrich-
1
b
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Heine-Universität Düsseldorf, Universitätsstr. 1, D-40225
Düsseldorf, Germany. Email: janiak@uni-duesseldorf.de
†
Electronic Supplementary Information (ESI) available: Figures
1
and analyses of UV-Vis., H NMR, DSC and CV. Tables of
excitation properties and selected bond lengths and angles,
packing analyses, Hirshfeld surface plots. For ESI† and
crystallographic data in CIF or other electronic format see DOI: --
1
1 (a) D. M. Walba, E. Körblova, R. Shao, J. E. Maclennan, D. R. Link,
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1
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