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The First C2-Symmetric Chiral Monomethine DyeЧAn Apparent Violation of the Helicity Rule for Optical Rotation.

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isotropicallq. No ;ittempt was made to locate hydrogen atoms. The final cycle
of full-matrix least-squares refinement was converged with R = x(lF,l - l.FLF,i)/
Zlk;l = 0.056 and R = [xir([Fol- ~ F , ~ ) z / ~ ~ r ~ F=,0.063.
~ ' ] i ' 2The first five
residual peaks in the final difference F-map are ghosts of heavy atoms [16]
(121 A. Muller, H Bogge. E. Koniger-Ahlborn. 2. N u f u i ~ f o r ~Bd i1979,
34, 1698.
[13] A. Muller, H. Bogge, E. Koniger-Ahlborn, J C/iem. So(. Cliem. Cononim. 1978,
1141 S.-W. Du. N.-Y. Zhu, P.-C Chen. X:T. Wu, J. ,4401. Slruct. 1993, 291. 167.
[15] MolEN, An Interactive Structure Solution Procedure, Enraf-Nonius, Delft,
The Netherlands (1990).
[16] Further details of the crystal structure investigations may be obtained from the
Fachinformationszentrum Karlsruhe. D-76344 Eggenstein-Leopoldshafen
(Germany), on quoting the depository number CSD-59267.
described by Reichardt et aLE5]Reaction with their respective
hydroximinium salts 4". 61 yielded both enantiomers of 2, which
after recrystallization from ethanol/diethyl ether proved to be
diastereomerically pure['] (Scheme 1).
+ (R)-3
acetlc anhydride
HBFd NaBF4,90 "C
The First C,-Symmetric, Chiral Monomethine
Dye-An Apparent Violation of the Helicity Rule
for Optical Rotation**
20 %
Scheme 1. Syntheses of the monomethine (R.R)-Z.
Lutz Eggers, Volker BUSS," and Gerald Henkel
Dedicated to Professor Fritz P. Schuyer
on thr occasion of'lzis 6Sth birthday
Steric interactions prevent the chromophore in the monomethine cyanine If'] from attaining the planar geometry preferred
by conjugated TC systems: the X-ray structure of the tetrafluoroborate salt of lr2]
reveals a twisted cis,cis conformation with an
angle of 43" between the two indolenine end groups. I n solution
the two oppositely twisted forms equilibrate rapidly: the barrier
to interconversion, determined by dynamic 'H N M R spectros~opy,[~]
is 50.0 kJ mol-
The structure of what turned out to be (R,R)-2 was determined by single-crystal X-ray analysis of the perchlorate
(Fig. I).[*] In the crystal the molecule adopts a twisted cis,cis
configuration along the conjugated chain of five atoms from
N(1) to N(1'). The overall structure is thus very similar to the
one reported for 1.I2]
In an achiraI environment, the two interconverting conformations are enantiomers and cannot be distinguished. Chiral interaction, for example the introduction of a chiral group into the
molecule, should remove the degeneracy, converting the oppositely twisted enantiomers into diastereomers, and render them
susceptible to chiroptical methods. To this end, we have synthesized the chirally substituted symmetrical monomethine cyanines (R,R)-2and ( S , S ) - 2and determined the absolute configuration by X-ray analysis. The observed Cotton effects
apparently contradict established rules for optical rotation.
The key intermediates in the synthesis of 2 are the chirdl
2-methylene indolines 3, which were obtained by dimethylation
of 2-methyl-3-pr0pylindole[~~
and subsequent resolution of the
enantiomers with di-0-benzoyltartaric acid, a method recently
[*] Prof. PhD V. Buss, Dr. L. Eggers
Fdchgebiet Theoretische Chemie der Universitit
Lotharstrasse 1, D-47048 Duisburg (Germany)
F d X : Int. code +(203) 379-2772
e-mail : theobuss@
Prof. Dr. G. Henkel
Fachzebiet Festkorperchemie der Universitdt Duisburg
This work was supported by the Deutsche Forschungsgemeinschaft. For the
opportunity to use N M R facilities. we thank Prof. Dr. W. Grahn. Braunschweig.
VCH Verlagsgesrllschut2frm h H . 0-69451 W(,inhewn. 1996
Fig. I . CrySVdl Structure of (R,R)-2 as perchlorate. Selected bond lengths [pm]:
N(l)-C(2) 138.9. C(2)-C(8) 139.0, N(l)-C(7a) 142.7. C(2)-C(3) 152.9. Selected
dihedral angles ["I: C(2)-C(OC(2') 129.07, C(2)-C(3)-C(4a) 101.5.
The dihedral angle between the essentially planar indolenine
groups and the central plane formed b y carbon atoms C(2)C(8)-C(2') is 26.8" compared to 21" and 27" in 1. The out-ofplane deformation of the chromophore is caused by the two
N-methyl groups that are bent away from each other but still
have a nearest H-H distance of only 275.6 pm.
The absolute configuration of the two chiral centers C(3) and
C(3') is identical, which means that the integrity of the chiral
center of 3 was conserved throughout the conversion into 2.
There is, of course, another element of chirality present in 2,
namely. the twist of the two indoline rings against each other.
The sense of twist in (R,R)-2 is positive: looking along the axis
C(2)-C(2'), the front group has to be turned clockwise to
eclipse the bonds N(l)-C(2) and N(I')-C(2'). Consequently a
P - h e l i ~ a l conformation
is assigned to this enantiomer in the
solid state. In (S,S)-2,this rotation has to be anticlockwise, so
the conformation is M .
The UV/Vis spectra of (R,R)- and ( S , S ) - 2 are identical, as
expected. Position (438 nm) and intensity (Ig E = 4.57) of the
cyanine band resemble that of I and other monomethine cyanines.''" The CD spectra of (R,R)-and (S,S)-2 (Fig. 2) exhibit
OS70-0833j96j3SO8-C70 $ 1S.O0+ .2SjO
AngrW. Chem. Int. Ed. EngI. 1996. 35. N O . 8
Fig. 2. CD spectra of ( R , R ) - 2 (c = 4 . 4 2 5 ~1 0 - 5 m o l L - i ( . . . . ) ) and ( S , S ) - 2
3 . 9 4 ~l o - ' m o l l - ' (~ --)in ethanolmethanol4il at 23 C.
(c, =
perfect mirror symmetry showing that the optical purity of the
enantiomers, which were synthesized essentially independently
of each other. is very high. The most prominent C D absorption
occurs in the region of the cyanine band, where its maximum
coincides exactly with the maximum of the UV/Vis band. Another C D absorption around 250 nm is probably due to excitations of the indolenine end groups coupled through the cyanine
In solutions of I , rapid interconversion (on the NMR time
of the enantiomers leads to a racemic mixture of the Pand M-helical conformation. Likewise, a fast equilibrium between the M and P form is established in 2, which for this
mixture of diastereomers can, however, never be 1 : 1 . Lower
temperatures should shift the equilibrium towards the more
stable diastereomer, an effect that should be observable in the
C D spectra.
The temperature-dependence of the C D spectra of (R,R)-2
(Fig. 3) shows that this assumption is correct. Lowering the
cient of only 0.85. Whatever the exact numbers, the only plausible interpretation of the spectra is a shift of the equilibrium at
low temperature towards the diastereomer with negative rotatory strength at the long-wavelength cyanine band. It is probable,
but by no means imperative, that this more stable conformation
also corresponds to the structure found in the X-ray analysis for
( R , R ) - 2 , which means that it has a P-helically twisted chromophore.
With their delocalized z-electrons constrained to move on a
helical path, twisted cyanine dyes are paradigmatic of inherently
chiral, open chain z systems, just as the helicenes are for aromatic
For inherently chiral chromophores the sign
of the Cotton effect is determined by the screw sense of the
transition charge density'131--which is definitely positive in the
case of ( R , R ) - 2 .Does the observed negative rotatory strength
for a cyanine with P-helical twist violate this rule'?
CNDOjS calculations for the chromophore (N(1) to N(1')) of
(R,R)-2 with coordinates taken from the X-ray structure show
that this is not the case. The calculated rotational strength is
-174 x
cgs units, the experimental value from the integrated area under the C D band at - 150 "C is - 188 x
units, a more than satisfactory agreement. The apparent discrepancy with what one would expect on the basis of helicai
charge movement is resolved by a component analysis. The
calculated rotatory strength R is the sum of two contributions,
R , , corresponding to the component parallel to the helix axis
(Fig. 4) and R , to the component perpendicular to this axis
Fig. 4 Rotational strength of electronic charge movement w i t h P-helical screw
sense and its dependence o n helix length. The magnetic momenis ifrnare obtained
from the charge rotation and the right-hand-rule. the electric moments 11, from the
charge translation. The rotatory strength R (that is. the scalar product of these
vectors) is positive when the vectors point in the same direction and negative when
they point in opposite directions Top: The positive parallel component of R is more
than cancelled by the negative contribution of the perpendicular component. ledding to an overall negative R and negative Cotton effect. Bottom: helix with one
complete turn. The perpendicular component is approximately iero because of the
(approximately) vanishing perpendicular electric transition moment. leading to a
positive R and positive Cotton effect
I lnm
Fig. 3. Temperature-dependent CD spectra of ( R , R ) - 2 i n ethanolimethanol 4 / l ;
c = 4.425 x
mol L-': temperatures from 23 'C to -150 C.
temperature to - 150 C increases the amplitudes of all bands by
about one half. Analysis of the data in terms of an equilibrium
between the P and M diastereomers["I fails to converge with
meaningful results. Additional data, preferably at elevated temperatures, would probably improve the analysis. However, if as
an additional irestraint, the two components are assumed to
have COmpardbk. but opposite strengths, the model yields a free
energy difference of 4 kJ mol- I , but with a correlation coeffiAngiw C l w i i .
E d Engl 1996. 35. N o . 8
(which is not the C, symmetry axis!). These components are
obtained, respectively, as the products of the parallel and perpendicular components of the electric and magnetic transition
moments. The calculated values are + 3 for R , , and
- 177 x
cgs units for R I. Because of the much larger
contribution of the perpendicular component, the total rotatory
strength is negative.[l4I
It is evident that the sign and magnitude of the rotatory
strength are strongly dependent on the particular structural
parameters of the helix. Increasing the twist of the chromophore, which corresponds to a greater turn on the helical
path, will lead to a larger parallel component of the electric
moment and thus to an increase of the positive contribution to
R . Likewise, increasing the length of the twisted chromophore
will result in a smaller perpendicular component of the electric
VCH ~~rlugs~e.sell.cclia/t
mhH. 0-69451 Weinhein?.1996
B 15.00 + 2 5 / I
transition moment and a decrease of the negative contribution
to R . Figure4 illustrates what happens after one turn of the
helix is complete. Because of the very small perpendicular electric transition moment the rotatory strength is now derived exclusively from the parallel component of R , which for a P helix
is, of course, positive.
We conclude that the negative rotatory strength of the P-helical cyanine dye ( R , R ) - 2is not a violation, but rather a confirmation of the rules that connect the screw sense of helical charge
movement with the sign of the observed Cotton effect.
Received: September 18, 1995
Revised version: November 22, 1995 [Z 8408 IE]
German version: Angew. Chem. 1996, 108, 913-915
Keywords: chirality . circular dichroism
dyes . helices .
Formation of a Novel p-Nonasulfido Ligand and
Its Degradation into a p-Disulfido Ligand at a
Diiridium Center**
Masayuki Nishio, Hiroyuki Matsuzaka,
Yasushi Mizobe, and Masanobu Hidai*
The reactivity of complexes containing two or more metals in
close proximity is now attracting significant attention, since activation and reactions of substrates at the multimetallic center in
these complexes may lead to novel reactions that are not possible at the monometallic center. We have recently shown that
thiolato-bridged diruthenium complexes can undergo unique
transformations of various substrates such as terminal alkynes
and hydrazines."] These studies have subsequently been extended to the preparation of the related thiolato-bridged dirhodium
and diiridium complexes. Thereby we have found that the Ir"
complex lrZ1
(Cp* = q5-C5Me,) reacts with S, to give the dinuclear Ir"' complex 2 that contains a novel p-S, ligand. Herein we
report on the synthesis and characterization of 2 as well as on its
reaction with NaBPh, that leads to the paramagnetic diiridium
p - S , complex 3.
[I] R. Kuhn, A. Winterstein. G. Baker. Ber. D t d 7 . Chem. Ge5. 1930. 63. 31763184.
[2] R. Allmann. T. Debaerdemaeker. C~jsr.Sr~ucr Connnun. 1976. 5. 21 1-214.
[3] W. Grahn. H:H. Johannes. J. Rheinheimer. B. Knieriem. E. U. Wurthwein.
Liebigs Ann. 1995. 1003-1009.
[4] M. P. Prochazka, L. Eklund, R. Carlson. Actu ChCHl. Scnnd. 1990, 44, 610613.
[Cp*lr(p-SiPr)21rCp*] I
[5] C. Reichardt. U. Budnik, K. Harms, G. Schifer, J. Stein, Liebig,$ Ann. 1995.
329- 340.
[6] W. Grahn. Liebigs Ann. Chem. 1981, 107-121.
[7] Based on the sufficient chemical shift difference between the (equivalent) 3- and
[Cp*Ir(p-SiPr),(pS,)IrCp*][BPh,] 3
3'-methyl protons in CS,S)-and (R,R)-2 and the (enantiotopic) methyl protons
in the meso-product (R,S)-Z.
Treatment of I with excess S, (S/Ir 5 ) in toluene at room
[8] X-ray structure determination for (R,R)-Z:Siemens P4RA four-circle diffractemperature gave an orange precipitate of 2, which was crystaltometer. Mo,, radiation (i = 0.71073 A). graphite monochromator, rotating
lized from CHClJhexane and isolated as an orange crystalline
anode. scintillation counter, 150 K. empirical absorption correction,
SHELXTL-PLUS programs, direct methods, full-matrix least-squares refinesolid in 38 % yield (Scheme 1 ) . The structure of 2 was determent; C,,H,,N,O,CI, formula weight 487.02. tetragonal, space group P4,2,2,
u =7.998(2), c = 40.070(20) A, V = 2563.20 A', 2 = 4, p = 1.262 g ~ m - ~ ,
p(MoK,) = 1.47 mm- ', transmission range 0.934-0.871 ; crystal dimensions
0.47 x 0.18 x 0.16 mm. (0 scan. 20,,, = 54". 4440 reflections measured
( + h , + k , I ; Flack parameter -0.05(13) for the correct space group). 1740
independent reflections used for final refinement, R(R,) = 0.0598 (0.0573) for
s Pr prs,
S S-lpr
1302 observed reflections ( I > 2 u ( I ) ) ,183 variables. all non-hydrogen atoms
s: S
anisotropic, H atoms at idealized positions (one common isotropic tempera1
ture factor), one scaling parameter, one isotropic extinction parameter. Further details of the Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the Cambridge CrysRrS Slpr
tallographic Data Centre as supplementary publication no. CCDC-I 79-6.
cp', ,r./\.,r/cP*
Copies of the data can be obtained free of charge on application to The DirecBPh;
tor, CCDC. 12 Union Road, Cambridge CB2 1EZ. UK (fax: Int. code
+(1223) 336-033; e-mail: teched(n
[9] V. Prelog, G. Helmchen, Angew. Chem. 1982. 94,614-631; Angeir. Chem. i n / .
Ed. Engl 1982, 21. 567-583.
Scheme 1. Reagents and conditions; a) S,. toluene, room temperature; b) NaBPh,,
1101 J. Fabian. H. Hartmann, Light Absorptron of Orgunic Coloranrs, Springer.
CH,CIJTHF, room temperature.
Berlin, 1980, p. 172 ff.
[ I t ] a) W. W. Wood, W. Fickett. J. G Kirkw0od.J. Chen?.Phw. 1952.20.561-568;
b) A. Moscowitz. K. M. Wellman. C. Djerassi. J. A m . Chen7. Soc. 1963. 85,
351 5-3516.
mined by X-ray analysis (Fig. l).f3'The most remarkable fea[12] V Buss. K. Kolster, Chem. P/7),5., in press.
ture of 2 is a unique bridging nonasulfido chain, which is bond[I 31 a) 0. E. Weigang in Fundumentul Aspects und Recent Developments in ORD und
ed at each end to an Ir atom.
CD (Eds.: F. Ciardelli, P. Salvadori), Heyden, London, 1973, p. 61 ff.; b) E.
Although polysulfido ligands are ubiquitous in various metal
Charney, The Mokciilur Basis of Opricul Activrrv. Wiley, New York, 1979,
p. 223 ff.; c) for the optical rolation of a free electron constrained to move on
c0mplexes,[~1the nonasulfido ligand is, to our knowledge, that
a helix, see I. Tinoco, Jr., R. W. Woody, J. Chon. PIzys. 1964, 40, 160-165.
with the longest sulfur chain, and the known examples are lim[14] Positive and negative components also contribute to the twisted ci.7-diene chiited to the monometallic complexes [MSJ (M = Ag,['] Aut6]).
rality, as discussed by the late G Snatzke in C/iiruhr?.--Froln Weuk Bosones t o
The present reaction differs significantly from that of the
the x-Heli.y (Ed: R. Janoschek), Springer, Berlin, 1992, p. 59ff.
\\dr A
related Ru complexes [Cp*Ru(p-SR),RuCp*] with S,, in which
b> VCH Verlug~gesellschu//nihH. 0-69451 Weatheim, 1996
Prof. M. Hidai. M. Nishio, Dr. H. Matsuzaka. Dr. Y. Mizobe
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
Hongo. Tokyo 113 (Japan)
Fax: Int. code +(3) 5800-6945
This work was supported by the Ministry of Education, Science and Culture of
Japan. M. N. acknowledges the JSPS Research Fellowships for Young Scientists.
0570-0833i96i3508-0872$ 15.00 + ,2510
Angew. Chent. Inr. Ed. Engl. 1996. 3S,No. 8
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