A chiral molecular square with metallo-corners for enantioselective sensing

Article abstract of DOI:10.1021/ja0256257

A family of chiral molecular squares based on fac-Re(CO)₃Cl metallocorners and enantiopure atropisomeric bipyridine bridging ligands (L_1-4) have been synthesized in high yields by refluxing ClRe(CO)₅ and L_1-4 in 1:1 molar ratio. These novel chiral metallocycles have been characterized by ¹H and ¹³C{1H} NMR, UV-vis, luminescence, and circular dichroism (CD) spectroscopies, FAB mass spectrometry, and microanalysis. Molecular square 4 which contains four 1,1′-bi-2-naphthol functionalities exhibits interesting enantioselective luminescence quenching by 2-amino-1-propanol. This research illustrates the potential of generating novel functional materials based on supramolecular chemistry. Copyright

Full text of DOI:10.1021/ja0256257

Published on Web 04/09/2002
A Chiral Molecular Square with Metallo-Corners for Enantioselective Sensing
Suk Joong Lee and Wenbin Lin*
Department of Chemistry, CB # 3290, UniVersity of North Carolina, Chapel Hill, North Carolina 27599
Received January 17, 2002
Scheme 1
Metal-directed self-assembly has been widely used to construct
supramolecular systems such as grids, helicates, boxes, and cylin-
ders.1 Compared to covalently bonded organic counterparts, metal-
organic supramolecules can be assembled with much ease and
higher efficiency.2 The incorporation of metal centers can also im-
part novel functionalities such as host-guest recognition,3 inclu-
sion,4 catalysis,5 and fluorescence sensing.6 Among many metal-
organic supramolecular systems, the construction of molecular
squares based on metallo-corners with ∼90 ° angles and linear
bridging ligands has proven to be the most reliable strategy, as ex-
emplified by successful design of numerous molecular squares based
on the cis-[M(phosphine)2]2+ corners (M ) Pd or Pt) by Stang et
Scheme 2
al.,1d the cis-[M(en)]2+ corners (M ) Pd and Pt, en is ethylenedi-
amine) by Fujita et al.,1e and the fac-(CO)3ReX corners (X ) halide)
by Hupp et al.,1g,1h and the M2(carboxylate)2 corners by Cotton et
al.1i Of particular interest to us are elegant studies by Hupp et al.
that demonstrated the feasibility of constructing microporous molec-
ular materials based on neutral Re-based molecular squares.1g We
envision that the incorporation of axially chiral bridging ligands
into such Re-based molecular squares could lead to enzyme-like
chiral supramolecular systems exploitable for enantioselective
recognition, sensing, separation, and catalysis.7-9 Herein we wish
to report the synthesis and characterization of a family of novel
chiral molecular squares [Cl(CO)3Re(L1-4)]4 (where L1-4 is enan-
tiopure 4,4′-bis(pyridyl)-1,1′-binaphthyl) and the first observation
1
of enantioselective luminescence sensing by a chiral metallocycle.
Enantiopure atropisomeric 6,6′-dichloro-2,2′-diethoxy-1,1′-binaph-
thyl-4,4′-bipyridine L1 was synthesized in five steps starting from
readily available 1,1′-bi-2-naphthol (BINOL) in 63.6% overall yield
(Scheme 1). The bis(tert-butyldimethylsilyl) and bis(benzyl) ana-
logues L2 and L3 were similarly prepared, while the bis(hydroxy)
ligand L4 was obtained quantitatively by treating L2 with tetra(n-
butyl)ammonium fluoride (TBAF) in THF. All these ligands have
been characterized by 1H and 13C{1H} NMR, UV-vis, and circular
dichroism (CD) spectroscopies and high-resolution mass spec-
trometry.
The H NMR spectra of 1-3 all exhibit well-resolved signals
for a single ligand environment, indicative of diastereoselectivity
in the assembly of chiral molecular squares based on L1-L3. On
the basis of their single ligand environment, we believe that one
single enantiomer has formed during the assembly of 1-3, which
possesses an approximate D4 symmetry (Scheme 2). Available
spectroscopic data cannot pinpoint the position of Cl atoms on the
fac-Re(CO)3Cl corners, and this ambiguity has been fairly well-
established among achiral cycles based on fac-Re(CO)3Cl metal-
locorners.8 The 1H signals of 4 appear very broad, probably a
consequence of facile torsion motion of the atropisomeric L4 ligand.
Numerous attempts have failed to produce X-ray diffraction-quality
single crystals of 1-4.
The electronic spectra of L1-4 are characterized by three π f
π* transitions at ∼240 nm, ∼300 nm, and ∼355 nm in the UV
region. Metallocycles 1-4 exhibit all these three π f π* transitions
with slight bathochromic shifts (∼5 nm) for the absorptions near
360 nm. In addition, there are new bands around 325 nm for 1-4,
which are probably due to the MLCT excitations. CD spectra of
ligands L1-4 exhibit three bisignate bands corresponding to the three
π f π* transitions. CD spectra of metallocycles 1-4 exhibit three
major bands similar to those of atropisomeric bipyridine ligands,
but with much higher intensities (Figure 1). The enhanced CD
signals for 1-4 are consistent with the presence of multi-ligands
in each metallocycle, while the similarity between the CD spectra
of metallocycles 1-4 and atropisomeric bipyridine ligands L1-4
Chiral molecular squares [Cl(CO)3Re(L1-4)]4 (1-4) were pre-
pared in very high yields by refluxing ClRe(CO)5 and L1-4 in 1:1
1
molar ratio (Scheme 2). H and 13C{1H} NMR spectra of 1-4
showed a single L1-4 ligand environment, suggesting the formation
of cyclic species. FAB-MS showed the presence of molecular ions
due to tetranuclear species for 1-3,10 and thus unambiguously
established their cyclic tetrameric nature. The highest M/Z peak is
due to the [M - Cl]+ species in the FAB-MS of 4. The formulations
of 1-4 are supported by microanalysis results. Interestingly, 4 can
also be prepared in high yields by treating 2 with TBAF, which
provides further evidence for the tetrameric structure of 4. The IR
spectra of metallocycles 1-4 exhibit three carbonyl stretches,
consistent with the formation of the fac-[Cl(CO)3Re] metallocorners
that have local Cs symmetry.
* To whom correspondence should be addressed. E-mail: wlin@unc.edu.
9
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J. AM. CHEM. SOC. 2002, 124, 4554-4555
10.1021/ja0256257 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
plex is responsible for the luminescence quenching of phenylene-
acetylene dendrimers with BINOL core by amino alcohols.14,15 It
is interesting to note that no enantioselectivity was observed for
the luminescence quenching of 4 by 1-amino-2-propanol, which
supports the involvement of amino groups in the formation of a
ground-state hydrogen-bonded complex and an excited-state proton-
transfer complex.16
In summary, a family of novel chiral molecular squares have
been readily assembled using enantiopure atropisomeric bipyridyl
bridging ligands and fac-Re(CO)3Cl corners. Metallocycle 4 exhibits
interesting enantioselective luminescence quenching by chiral amino
alcohols. Higher enantioselectivity of 4 versus L4 is probably a
consequence of a better-defined chiral environment in the metal-
locycle. Exploration of chiral metallocycles for applications in
asymmetric catalysis is currently underway.
Figure 1. Circular dichroism spectra of (S)-1-4 and (R)-4 in acetonitrile
at concentrations of 1.3-1.9 × 10-5 M.
Acknowledgment. We thank NSF (DMR-9875544) for financial
support, Mr. X. Linghu for experimental help, and Prof. H. Thorp
for helpful discussions. W. L. is an A.P. Sloan Fellow, a Beckman
Young Investigator, a Cottrell Scholar of Research Corp, and a
Camille Dreyfus Teacher-Scholar.
Supporting Information Available: Experimental procedures,
analytical data, one table, and 10 figures (PDF). This material is
available free of charge via the Internet at http://pubs.acs.
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Figure 2. Stern-Vo¨lmer plots of (R)-4 in the presence of (S)- and (R)-2-
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We have studied the luminescence quenching of (R)- and (S)-4
in the presence of chiral amino alcohols. The luminescence signal
of enantiopure 4 at 412 nm can be quenched by both enantiomers
of 2-amino-1-propanol, but at significantly different rates. Figure
2 shows the Stern-Vo¨lmer plots of (R)-4 (2.2 × 10-6 M) in the
presence of (R)- and (S)-2-amino-1-propanol in THF.9 It is evident
from Figure 2 that luminescence quenching of chiral metallocycle
4 by 2-amino-1-propanol is enantioselective. For (R)-4, the Stern-
Vo¨lmer quenching constant Ksv is 7.35 M-1 in the presence of (S)-
2-amino-1-propanol, and 6.02 M-1 in the presence of (R)-2-amino-
1-propanol. (R)-4 has an enantioselectivity factor ksv(R - S)/ksv(R
- R) of 1.22 for luminescence quenching in favor of (S)-2-amino-
1-propanol. The opposite trend in enantioselectivity was observed
for the quenching of (S)-4 by 2-amino-1-propanol, which lends
further support to a chirality-based luminescence-quenching selec-
tivity. This magnitude of enantioselectivity for 4 is significantly
higher that of free ligand L4 (1.04), suggesting a better-defined
chiral environment conferred by metallocycle 4.14 Pu et al. has
proposed that the formation of a nonemissive hydrogen-bonded
complex and a poorly emissive excited-state proton-transfer com-
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(12) Free ligand L4 emits at 401 nm with ∼1/3 of the intensity of 4.
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