C O M M U N I C A T I O N S
Scheme 1
(R,S,S)-11 and (R,R,S)-12 by NMR was not possible due to fast
interconversion between them. On the other hand, as it follows from
the X-ray studies, only enantiomerically pure diastereomer (R,S,S)-
11 was observed in the solid state. Assignments of the absolute
configuration in 11 were made as follows: stereogenic carbon (R),
stereogenic nitrogen (S), and stereogenic axis (S).
In summary, we proposed here a new approach to the design of
organic molecules with switchable central chirality based on the
cleavage and formation of metal coordination bonds. Feasibility
of this approach was demonstrated by preparation of achiral
pentadentate ligand 3 and its coordination with d8 metal (Ni, Pd)
to produce the corresponding tetracoordinated complexes possessing
elements of central and axial chirality. The formation of the
corresponding diastereomers by coordination of the different arms
of the ligand was confirmed by crystallographic analyses of
complexes (S,S)-8a and (S,R)-9b. Moreover, we demonstrated
virtually faultless chiral communication between the central chirality
of ligand (R)-10 and the central and axial chirality of complexes
of this type, allowing preparation of enantiomerically pure (R,S,S)-
11 in the solid state.
Scheme 2
Taking into account synthetically simple and straightforward
preparation of pentadentate ligands 3 and (R)-10, as well as virtually
unlimited structural flexibility provided by the modular nature of
our design, various ligands of this type can be prepared and fine-
tuned to control stability and rate of interconversion of the
corresponding diastereomeric complexes and make them more
selective to an external stimulus, leading to the development of a
new generation of organic chiroptical molecular switches.
axis is directed along with the noncoordinated C-N bond of the
o-(amino)benzophenone residue.
Acknowledgment. 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 Ajinomoto Company
(Tokyo, Japan).
The complexes 8b and 9b, containing Pd(II), are also neutral
and very well soluble in polar organic solvents and have charac-
teristic yellow-orange color. The 1H NMR spectroscopic properties
of compounds 8b and 9b are very similar to those described for
diastereomers 8a and 9a. Fortunately, X-ray analysis revealed that
Pd-containing complexes crystallized in a single (S,R)-9b diaster-
eomeric form, existing in the unit cell with its (R,S) enantiomer.
The nature of the stereogenic center and axis in 9b is the same as
discussed for diastereomer (S,S)-8a, except that the absolute
configuration of the stereogenic nitrogen is opposite, due to the
coordination of different oxygen of the o-(amino)benzophenone
residue to the metal. Thus, the crystallographic analysis of Ni(II)-
and Pd(II)-containing complexes (S,S)-8a and (S,R)-9b undoubtedly
confirmed that coordination of the achiral C2-symmetric pentaden-
tate ligand 3 with d8 metal (Ni, Pd) produced the corresponding
tetracoordinated complexes possessing elements of central and axial
chirality.
Finally, to explore a possibility of preparation of this type of
complexes in enantiomerically pure form, we prepared chiral ligand
(R)-10 (Scheme 2). Synthesis of the ligand (R)-10 was conducted
according to Scheme 1, except that (R)-R-(phenyl)ethylamine was
used in place of benzylamine, as an “Amine” module. Heating an
acetonitrile solution of (R)-10 in the presence of Pd(II) resulted in
formation of yellow-orange colored diastereomeric products (R,S,S)-
11 and (R,R,S)-12. These products were carefully purified by
column chromatography on silica gel. Only one colored spot was
observed during the purification step as well as on TLC using
various eluents, suggesting that the stereochemical information of
the stereogenic center in starting ligand 10 was quantitatively
transferred to the newly created stereogenic nitrogen as well as the
axis of chirality. Unfortunately, investigation of the diastereomers
Supporting Information Available: Experimental procedures,
characterizations of new compounds, and crystallographic data. This
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