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Helical Chirality in Pentacoordinate Zinc ComplexesЧSelective Access to Both Pseudoenantiomers with One Ligand Configuration.

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Communications
Stereogenic Metal Centers
Helical Chirality in Pentacoordinate Zinc
Complexes—Selective Access to Both
Pseudoenantiomers with One Ligand
Configuration**
Michael Seitz, Sabine Stempfhuber, Manfred Zabel,
Martin Schtz, and Oliver Reiser*
The controlled construction of complexes with stereogenic
metal centers (“chiral-at-metal complexes”) is an important
task because of the potential impact on various areas of
chemical research, for example, supramolecular chemistry,
asymmetric catalysis, or biological recognition. Since the first
separation of the enantiomers of an inorganic coordination
compound in 1911 by Werner,[1] the area of inorganic
stereochemistry has emerged as a rapidly growing field,
especially over the last years. In particular, the transfer of
chirality from chiral, nonracemic organic ligands to metal
centers with a variety of coordination geometries has
attracted great interest.[2, 3]
Predetermined chirality around a trigonal-bipyramidalcoordinated metal center is rather rare and normally favored
with tripodal ligands.[4] Except for the complexes described
herein, only three other examples with topologically linear
ligands exist.[4a–c] To date, only the employment of enantiomeric (trivial) or diastereomeric[4b] ligands is suitable to
enforce the formation of opposite chiral configurations.
In our case, only two achiral donor atoms are exchanged
in an otherwise isosteric ligand to complete this task. To the
best of our knowledge, this is the first time that this
phenomenon is observed. The closest analogy can be seen
in the cobalt complexes of ligand 1:[5] Whereas 1 a forms an
octahedral complex with D2 configuration upon complexation
[*] Dr. M. Seitz, Prof. Dr. O. Reiser
Institut fr Organische Chemie
Universitt Regensburg
Universittsstrasse 31, 93 053 Regensburg (Germany)
Fax: (+ 49) 941-943-4121
E-mail: oliver.reiser@chemie.uni-regensburg.de
S. Stempfhuber, Dr. M. Zabel
Insititut fr Anorganische Chemie
Universitt Regensburg
Universittsstrasse 31, 93 053 Regensburg (Germany)
Prof Dr. M. Schtz[+]
Institut fr Theoretische Chemie
Universitt Stuttgart (Germany)
Pfaffenwaldring 55, 70 569 Stuttgart (Germany)
[+] Current address: Institut fr Theoretische Chemie
Universitt Regensburg
Universittsstrasse 31, 93 053 Regensburg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(SP 1118, RE-948-5/2 and SCHU 1456/2), the Fonds der Chemischen Industrie, and through generous gifts of chemicals by
Degussa AG.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
242
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460843
Angew. Chem. Int. Ed. 2005, 44, 242 –245
Angewandte
Chemie
with CoIII, ligand 1 b gives rise to a mixture of two complexes,
one of which was identified as the L2-configured species.
We recently reported the novel pentadentate bis(oxazoline) ligand 6 b (Scheme 1), which forms octahedral CoII or
Scheme 2. Reagents and conditions: a) Zn(ClO4)2·6 H2O, THF, room
temperature, 1 h; b) CH3CN, Et2O.
Scheme 1. Synthesis of 6 a and 6 b. Reagents and conditions: a) 2
(1.0 equiv), TsCl (1.1 equiv), NEt3 (2.2 equiv), CHCl3, 0 8C!RT, 20 h,
78 %; b) 2 (2.2 equiv), 4 (1.0 equiv), NaH (2.3 equiv), DMF, 0 8C!RT,
20 h, 88 %; c) 3 (2.1 equiv), 5 (1.0 equiv), NaH (2.1 equiv), DMF,
0 8C!RT, 20 h, 72 %. Ts = p-toluenesulfonyl; DMF = N,N-dimethylformamide.
Figure 1. Crystal structures of the cations of the complexes L2-7 a and
D2-7 b.[16] Hydrogen atoms have been omitted for clarity, except for the
ones at the stereocenters of the oxazolines. a) Front view. b) Top view
(phenyl rings have been omitted for clarity). C gray, H white, N blue,
O red, S yellow, Zn pink.
RuII complexes with exclusive D2 configuration.[6] Herein we
describe trigonal-bipyramidal ZnII complexes of 6 a and 6 b,[7]
which differ only in the heteroatom at the benzylic position of
the pyridine (O versus S).
Ligands 6 were readily assembled from the known
oxazoline 2[8] in a modular fashion. After deprotonation of 2
with NaH, the resulting anion was trapped with the pyridine
dielectrophile 4[9] to give 6 a in 88 % yield. Analogously,
tosylation of 2 resulted in 3,[6] which was treated with the
pyridine dinucleophile 5[6, 10] to yield 6 b in 72 % yield.
The zinc complexes[11] were prepared by mixing equimolar
amounts of zinc(ii) perchlorate hexahydrate and the corresponding ligand 6 in THF at ambient temperature
(Scheme 2). The precipitated complexes 7 were recrystallized
from acetonitrile/diethyl ether to give colorless crystals that
were suitable for X-ray crystallographic analysis.
The crystal structures of 7 a and 7 b show two pentacoordinate zinc complexes with distorted trigonal-bipyramidal
geometry (Figure 1, Table 1). The complexes each have a
single but opposite configuration at the metal center: L2 for
7 a and D2 for 7 b. This remarkable reversal in helical folding
around the metal center is not easy to understand. The Npy
Zn and NoxazZn distances are approximately equal in both
complexes, and the ZnO (2.22 and 2.27 ) and the ZnS
(2.53 ) distances are within normal limits. The most
pronounced difference is the stronger distortion of the
oxygen (O-Zn-O = 1538) atoms towards the pyridine moiety
relative to that of the sulfur atoms (S-Zn-S = 1698), which is
dictated by the fact that the benzylic CO bond (1.44 ) is
shorter than the corresponding CS (1.81 ) bond.
To assess the energy difference between the particular
helical folding of ligands 6 a and 6 b in the zinc complexes,
ab initio calculations at the second-order Møller–Plesset
perturbation theory (MP2) level were performed. Geometry
optimizations were carried out for the two complex cations
L2-7 a and D2-7 b and the corresponding pseudoenantiomers
D2-7 a and L2-7 b.[12] The geometry optimizations reflect well
the geometries found in the X-ray crystal structure of L2-7 a
Angew. Chem. Int. Ed. 2005, 44, 242 –245
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
243
Communications
Table 1: Selected bond lengths [], angles [8], and dihedral angles [8] of
the cations of L2-7 a and D2-7 b.[a]
Npy–Zn []
Noxaz 1–Zn []
Noxaz 2–Zn []
X1–Zn []
X2–Zn []
Npy-Zn-Noxaz 1 [8]
Npy-Zn-Noxaz 2 [8]
Npy-Zn-X1 [8]
Npy-Zn-X2 [8]
Noxaz 1-Zn-Noxaz 2 [8]
Noxaz 1-Zn-X1 [8]
Noxaz 1-Zn-X2 [8]
Noxaz 2-Zn-X1 [8]
Noxaz 2-Zn-X2 [8]
X1-Zn-X2 [8]
-CH2-X1-CH2- [8]
-CH2-X2-CH2- [8]
Npy-CH-CH2-X1 [8]
Npy-CH-CH2-X2 [8]
L2-7 a (X = O)
D2-7 b (X = S)
2.031
1.961
1.953
2.220
2.275
113.74
116.44
76.93
76.07
129.62
115.31
80.43
80.64
107.30
152.56
112.87
113.51
24.92
22.88
2.096
1.981
1.981
2.530
2.530
110.12
110.12
84.43
84.43
139.76
83.84
100.02
100.02
83.84
168.86
104.19
104.19
3.77
3.77
[a] py = pyridine, oxaz = oxazoline.
and D2-7 b; the bond lengths and angles found for the
pseudoenantiomers D2-7 a and L2-7 b are all in the normal
range and are similar to their counterparts.[11]
Two different steric interactions can be identified which
work in opposite directions for the L2 and D2 configuration of
each complex. In the L2 configuration, the phenyl substituents
of the oxazoline rings point toward the pyridine backbone, as
manifested in the especially close distance between the ortho
carbon atom of the phenyl ring and the heteroatom X
(Figure 2 a). This interaction is especially severe for L2-7 b
whereas in the corresponding L2 configuration the phenyl
rings are far away from each other. This interaction seems to
be particularly unfavorable in D2-7 a (sum of van der Waals
radii of two hydrogen atoms is 2.40 ), resulting in the
exclusive formation of L2-7 a. In agreement with this analysis,
the calculations predict a decrease in energy on going from
the pseudoenantiomers D2-7 a and L2-7 b to the experimentally confirmed structures L2-7 a and D2-7 b of 2–4 kcal mol1
(X = O) and 6–8 kcal mol1 (X = S), respectively.
Remarkably, the D2 configurations exhibit perfect C2
point-group symmetry, whereas this symmetry is distorted to
C1 for the L2 conformers. The ab initio calculations and the
measured X-ray structures both agree with this distortion. As
a consequence, the L2 conformers exist in two isometric
versions on their higher symmetric potential-energy surfaces,
which in the present case correspond to the molecular
symmetry group C2(M).[13] The L2 configurations of C2
point-group symmetry are likely to be saddle points (transition structures) on the interconversion pathways connecting
the two related C1 versions. Hence, the vibrational ground
state splits for the L2 conformers.
The solution behavior of 7 a and 7 b was investigated by
means of NMR and CD spectroscopic analysis of the bulk
material obtained in the complexation experiments. The 1H
and the 13C NMR spectra[11] in CD3CN show a single species
for each complex. As expected, the signals for the complexed
ligands are shifted downfield with respect to the free ligands.
In particular, the 13C NMR spectra clearly show the C2 symmetry of the complexes in both cases, as well as the
pentacoordination of the ligand, indicated by the strong
downfield shift of the signals for the quaternary carbon atom
of the oxazolines (6 a!7 a: 164.9 to 175.4 ppm; 6 b!7 b: 164.6
to 172.7 ppm). Therefore, both species must be either D2 or
L2 .
CD spectroscopic analysis of the complexes in CH3CN
solution showed a quasi-mirror image of the n!p* transition
(Figure 3), which indicates a quasi-enantiomeric relationship
between the two zinc complexes. Application of the sector
rules for C2-symmetric pyridines put forward by Palmer and
co-workers[14] predicts the configuration around the zinc atom
Figure 2. Calculated structures of D2-7 a and L2-7 b. C gray, H white,
N blue, O red, S yellow, Zn gray ball.
(C–S distance 3.13 and 3.26 , sum of van der Waals radii for
C and S = 3.50 ), clearly favoring D2-7 b (C–S distance
3.43 ). The same trend is seen for 7 a (L2-7 a: C–O distance
3.01 and 3.19 ; D2-7 a: C–O distance 3.77 ), but still
acceptable in L2-7 a owing to the smaller van der Waals
radius of oxygen (C+O = 3.22 ).
The second steric interaction that must be considered is
the distance between the two phenyl rings (Figure 2 b). In the
L2 configuration, these phenyl rings are especially close to
each other as manifested in the distance between two of the
ortho-hydrogen atoms (D2-7 a: 2.43 ; D2-7 b: 2.46 ),
244
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. CD spectra of the complexes L2-7 a and D2-7 b in CH3CN.
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 242 –245
Angewandte
Chemie
as L2 for 7 a and D2 for 7 b, in complete agreement with the
crystal structures.
In summary, a new ligand system is introduced that allows
the completely stereoselective transfer of chirality from the
pentadentate bis(oxazolines) to a trigonal-bipyramidal coordinated central metal atom in the solid state as well as in
solution. For the first time, two opposite, pseudoenantiomeric
configurations were observed with a single ligand design, that
is, 6 a and 6 b differ only by two single atoms and are otherwise
stereochemically identical.
Received: June 1, 2004
Revised: September 15, 2004
[13]
[14]
.
[15]
Keywords: bisoxazoline ligands · coordination chemistry ·
helical chirality · stereogenic metal centers · zinc
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[11] See Supporting Information.
[12] The ab initio energy-minimization calculations were performed
with the new analytic local MP2 gradient based on density fitting
(DF-LMP2).[15a] The SVP AO basis set by Schfer et al.[15b]
Angew. Chem. Int. Ed. 2005, 44, 242 –245
[16]
www.angewandte.org
together with the SVP/MP2FIT and TZVP/MP2FIT fitting
basis sets[15c] (the former for the LMP2, the latter for the
Hartree–Fock specific parts of the gradient[15a]) was used. For
zinc, a quasi-relativistic energy-adjusted pseudopotential based
on the Ne-like Zn20+ core together with the related 6s5p3d AO
basis set[15d] was employed. At the minimum-energy geometries
of L2-7 a and D2-7 a, single-point energy calculations were also
performed with the TZVP basis set by Schfer et al.[15e] in
conjunction with the TZVP-MP2FIT and TZVPPMP2FIT fitting
basis sets,[15c] and an additional set of f functions with exponent
3.22 on the Zn center.
P. R. Bunker, P. Jensen, Molecular Symmetry and Spectroscopy,
NRC Research, Ottawa, 1998.
R. B. Dyer, R. A. Palmer, R. G. Ghirardelli, J. S. Bradshaw, B. A.
Jones, J. Am. Chem. Soc. 1987, 109, 4780 – 4786.
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Schfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829.
CCDC-236 253 and 236 254 contain the supplementary crystallographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336-033; or deposit@
ccdc.cam.ac.uk).
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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