Design, synthesis, and characterization of binuclear Ni(II) complexes with inherent helical chirality

Article abstract of DOI:10.1021/ja0671215

The first example of design and synthesis of helical molecules using a ridge-tile-like topological mode of nonplanarity is reported. The modular nature of the design of the starting glycine derivatives renders this approach general for the preparation of structurally varied and functionalized derivatives of this type of bended molecules with inherent helical chirality. Copyright

Full text of DOI:10.1021/ja0671215

Published on Web 02/08/2007
Design, Synthesis, and Characterization of Binuclear Ni(II) Complexes with
Inherent Helical Chirality
Vadim A. Soloshonok* and Hisanori Ueki
Department of Chemistry and Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019
Received October 9, 2006; E-mail: vadim@ou.edu
The helix is an old and ever fascinating example of conforma-
tional chirality. Its importance in nature is exemplified by various
biological molecules such as nucleic acids1 and peptides2 in which
the helicity predetermines their higher-order structures and elaborate
biological functions.3 A huge scientific effort has been devoted to
understand4 and mimic5 natural helical chirality. A great variety of
molecules with a helical structure have been prepared and used in
virtually all areas of physical sciences, most notably in catalysis,6
supramolecular chemistry,7 and chiral nanotechnology.8 In particu-
lar, sterically induced out-of-plane deformations in mononuclear9
and polynuclear complexes of d8 transition metals10 were found to
be efficient approaches in making helical molecules of these types.
On the other hand, another type of nonplanar geometry which can
give rise to helical conformation is provided by the bending of a
C2-symmetric molecule at the line perpendicular to its axis of
symmetry, as shown in Figure 1. However, this type of nonplanar
topology is synthetically challenging and remains virtually unex-
plored.11
Figure 1. 1. Two modes of bending (angle θ) of two square planar-
coordinated ligands along the X-X axis.
Scheme 1
Here we describe a general synthetic approach to a novel type
of double bridged binuclear complexes with unusually distorted
square planar coordination geometry around the Ni(II) atoms. These
molecules are inherently helically chiral as a result of their folded,
ridge-tile conformation. These complexes are neutral, well-soluble
in organic solvents, and remarkably chemically and configuration-
ally stable: properties which allow for a systematic study of their
unknown chemical, physical and chiroptical properties.
(up to 6%) by treatment of Ni(II) complex of glycine 3 with NaOH
in DMF.13 Under optimized conditions, using acetonitrile as a
solvent and t-BuOK as a base and running the reaction in open air
at room temperature, a quantitative transformation of the starting
complex 3 into binuclear complex 5 was observed.
Crystallographic analysis of product 5 revealed its desired folded,
ridge-tile-like topology via bending the Ni(II) coordination planes
along the bridging imine nitrogens. The angle of bending θ was
found to be 138.86(17)° thus putting the Ni(II) atoms in close
proximity to each other (2.6672(6) Å) to assume a Ni,Ni bond.14,15
From the stereochemical stand point, the unit cell of the crystals
of compound 5 contains both enantiomers (P)-5 and (M)-5 of right-
and left-handed helicity. It is also interesting to mention that the
distance between the para hydrogens of the picolinic acid moieties
in the enantiomers 5 is very close to 1 nm (1.0071).
Since picolinic acid derived compound 5 had some limited degree
of structural flexibility and potential functionalization, we decided
to explore our recently developed modular approach for preparation
of achiral glycine Ni(II) complexes12b for synthesis of various
molecules with ridge-tile-like topological mode of nonplanarity
and therefore helical chirality. We found that the glycine derivatives
6 derived from the dibutyl-, dibenzyl-, and pipicolinamine modules,
under the standard conditions smoothly produced the corresponding
folded binuclear complexes 7 as racemic mixtures of (P)- amd (M)-
enantiomers (Scheme 2). Interestingly, the bending angles θ and
the deviation from the planarity of the bridging imine nitrogens
were quite similar to that of the sp2 containing picolinic acid derived
complex 5. Thus, bending angles θ for 7a-c were found to be:
To realize the desired folded, ridge-tile-like topological mode
of nonplanarity we envisioned that such conformation could be
possible if two C2-symmetric metal tetra-coordinated planes in a
binuclear complex can be bent along the X,X (Figure 1). However,
from the synthetic standpoint at least three challenging issues of
this type of chemical architecture should be addressed: first, to
control the chemoselectivity of a metal-chelation providing for
preferential formation of C2-symmetric complex 1 over achiral
dimer 2; second, to make the metal-coordinated planes adopt a bent
and potentially more stereochemically congested arrangement versus
planar and therefore achiral structure; third, to make such ridge-
tile deformation to be configurationally stable, thus preventing its
racemization via rapid inversion. To address the first issue, we
assumed that X, B, and A elements might be covalently bonded
comprising a whole tridentate ligand. Next, the required bending
can be provided by repulsive steric interactions between a sub-
stituent on X of one ligand with the A element of another ligand.
Finally, to provide for the configurational stability of 1, the bridging
elements X should have a planar geometry like in sp2-hybridized
nitrogen atom. On top of these structural features, further require-
ments for models 1 to be chemically stable and neutral were also
incorporated in our design.
In connection with our project on asymmetric synthesis of
R-amino acids,12 we found that tridentate ligands 4 (Scheme 1)
possessing the desired structural features can be generated in situ
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10.1021/ja0671215 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
bended molecules with inherent helical chirality. Moreover, we
demonstrated that the introduction of elements of central chirality
in the design allowed preparation and optical separation of the
helical stereoisomers but contributed little to their chiroptical
properties. Chemical stability as well as the rigid and stable ridge-
tile-like structure of these helical molecules was also demonstrated.
Taking into account these structural features, one can expect that
these novel type of helical molecules might be useful structural
elements in macromolecular design or can be used as chiral
nanoscale building blocks in the field of chiral nanotechnology.
Scheme 3
Acknowledgment. The authors are thankful to Dr Takada
(JASCO corp.) for the CD spectra. This work was supported by
the Department of Chemistry and Biochemistry, University of
Oklahoma. The authors gratefully acknowledge generous financial
support from Central Glass Company (Tokyo, Japan) and Ajino-
moto Company (Tokyo, Japan).
132.25(8)° (a), 137.21(15)° (b), and 130.83(8)° (c), respectively.
The deviation of the bridging imine nitrogens was ranging from
0.322 to 0.882 Å and the Ni,Ni distances were 2.6427(4) (a),
2.6952(9) (b), 2.6156(3) Å (c).
Supporting Information Available: Experimental procedures,
characterizations of new compounds, and crystallographic data. This
material is available free of charge via the Internet at http://pubs.acs.org.
Finally to prepare this new type of helical molecules in
enantiomerically pure form and taking advantage of the modular
nature of our design, we synthesized chiral complex 8 and subjected
it to the standard dimerization procedure. The application of chiral
complex 8 resulted in formation of two diastereomers (M,S,S)-9
and (P,S,S)-9, isolated in quantitative yield and in a ratio of 21:79,
respectively (Scheme 3). The diastereomers (P,S,S)-9 and (M,S,S)-9
were easily separated by regular column chromatography on silica
gel and fully characterized including X-ray crystallographic studies.
As it follows from the structure of the diastereomer (M,S,S)-9, the
corresponding N-benzyl groups are in close proximity to each other
rendering (M,S,S)-9 relatively unstable as compared to major
diastereomer (P,S,S)-9 in which the N-benzyl groups point away
from each other. This different mode of stereochemical interactions
in the diastereomers (P,S,S)-9 and (M,S,S)-9 is probably responsible
for some geometric differences between them. Thus, both diaster-
eomers exhibited significant folded, ridge-tile structures, however
the torsion angles θ between two square planar faces were 132.80-
(8)° for (P,S,S)-9 and 136.02(7)°, slightly larger, for (M,S,S)-9.
Moreover, as a result of different torsion angles θ, short Ni-Ni
distances were found to be also different, measuring 2.6341(3) Å
for (P,S,S)-9 and 2.6813(4) Å for (M,S,S)-9. Examination of the
chiroptical properties, such as CD spectra and optical rotation,
revealed that diastereomers (P,S,S)-9 and (M,S,S)-9 behave rather
as helical pseudo-enantiomers. Thus, extraordinary large [R]D values
of -1022 for (M,S,S)-9 and +1222 for (P,S,S)-9 as well as strong
and opposite Cotton effects in the CD spectra indicated that helical
chirality virtually completely overwhelmed the two stereogenic
centers in the compounds (P,S,S)-9 and (M,S,S)-9.16
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Finally, to demonstrate thermal stability of these ridge-tile
1
shaped compounds, we took H NMR spectra of complex 7a in
toluene. Thus, heating compound 7a gradually from ambient
temperature to 100 °C did not result in any changes or broadening
of peaks, indicating the absence of any transformations.
In summary, we report the first example of design and synthesis
of helical molecules using ridge-tile-like topological mode of
nonplanarity. The modular nature of the design of starting glycine
derivatives renders this approach general for the preparation of
structurally varied and functionalized derivatives of this type of
(14) On the nature of the Ni(II)-Ni(II) bond, see: Banks, C. V.; Anderson,
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Å. See, for example: Peng, S.-M.; Goedken, V. L. J. Am. Chem. Soc.
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(16) See the Supporting Information.
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