Ruthenium-vinylhelicenes: Remote metal-based enhancement and redox switching of the chiroptical properties of a helicene core

Article abstract of DOI:10.1021/ja304424t

Introducing metal-vinyl ruthenium moieties onto [6]helicene results in a significant enhancement of the chiroptical properties due to strong metal-ligand electronic interactions. The electro-active Ru centers allow the achievement of the first purely helicene-based redox-triggered chiroptical switches. A combination of electrochemical, spectroscopic, and theoretical techniques reveals that the helicene moiety is a noninnocent ligand bearing a significant spin density.

Full text of DOI:10.1021/ja304424t

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Communication
Ruthenium-vinylhelicenes: remote metal-based enhancement and
redox switching of the chiroptical properties of a helicene core.
Emmanuel Anger, Monika Srebro, Nicolas Vanthuyne, Loic Toupet, Stephane
Rigaut, Christian Roussel, Jochen Autschbach, Jeanne Crassous, and Regis Reau
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja304424t • Publication Date (Web): 20 Jun 2012
Downloaded from http://pubs.acs.org on June 24, 2012
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Ruthenium-vinylheliicenes: remote metal-base
d
enhancement
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and redox switching of the chiroptical propertiees of a helicene
core.
£
Emmanuel Anger,† Monika Srebro ‡,
o
, NicolasVanthuyne,§ Loïc Toupet,† Stéphane Rigaut,† Christian
Roussel,§ Jochen Autschbach,*,‡ Jeanne Crassous,*,† and Régis Réau*,†
† Institut des Sciences Chimiques de Rennes, UMR 6226, Institut de Physique de Rennes, UM
RR
6251, Campus de Beaulieu,
CNRS-Université de Rennes 1, 35042 Rennes Cedex, France. ‡ Department of Chemistry, Uni
vversity at Buffalo, State Univer-
sity of New York, Buffalo, NY 14260, USS
A. £ Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland. § Chiros-
ciences, UMR 7313, Stéréochimie Dyna
m
m
ique et Chiralité, Aix-Marseille University, 13397 Ma
r
seille Cedex 20, France
Supporting Information Placeholder
ABSTRACT: Introducing metal-vinyl rut
h
enium moieties onto [6]helicene results in a significan
t
t
enhancement of the chiroptical
properties due to strong metal-ligand elec
t
ronic interactions. The electro-active Ru-centers allow thhe achievement of the first purely
helicene-based redox-triggered chiroptical switches. A combination of electrochemical, spectroscoppic and theoretical techniques re-
veals that the helicene moiety is a non-innocent ligand bearing a significant spin density.
The molecular engineering of [n]helice
ject of intensive research due to their large-magnitude chiropti-
cal properties which are of great interest
tions in chiral material sciences such as n
guides, switches, luminescent materials o
n
e derivatives is a sub-
generated. This redox process significantly impacts the chiropti-
cal properties of the helicene core, affording the first purely
helicene-based redox chiroptical switch.5 These results on
closed- and open-shell complexes are rationalized with the help
of first-principles calculations.
ffor manifold applica-
o
o
nlinear optics, wave-
r
sensors.1 The main
challenge for the development of [n]helicenes towards func-
tional materials is the discovery of efficien synthetic routes and
t
t
simple strategies to improve and tune their chiroptical proper-
ties. In this regard, the most efficient general strategy developed
up to now involves the modification of their ‘screw-shaped’
ortho-fused polycyclic π-framework, since t
hhe unique chiroptical
properties of helicenes are inherently lin
k
ed to this helicoidal
des for example the
ease of the number of
conjugated skeleton. This approach incl
uu
Scheme 1. Synthesis and X-
atoms have been omitted) of enantiopure
HRu(CO)Cl(PiPr3)2, CH2Cl2, r
t., Ar.
r
r
ay crystallographic structures (H
modification of the helical pitch, the incr
e
P-complexes 2a,b. i)
fused aromatic rings, or the incorporation of heteroatoms or
..
transition metals within the π-skeleton.1,2
Mono- and bis-(ethynyl)carbo[6]helicenes 1a and 1b6 (Scheme
1) were obtained using classic photocyclization reactions1a,e and
resolved (ee > 99%) by chiral HPLC separation (SI). Following
the well-established stereoselective hydroruthenation of al-
kynes,4 1a,b were reacted with HRu(CO)Cl(PiPr3)2 affording the
organometallic derivative 2a,b (Scheme 1) as air stable dark red
Herein, we describe an original strategy for the molecular en-
gineering of helicenes based on introducin
substituents on ‘screw-shaped’ ortho-fused polycyclic π-
framework. The versatility of this origi al approach is illus-
gg lateral organometallic
a
n
trated with the synthesis of mono- and bis(vinyl-
RuII)[6]helicenes 2a,b (Scheme 1). The d6 ruthenium ion was
selected since (i) it displays very efficient electronic coupling
with unsaturated organic ligands,3a,b,4 and (ii) its organometallic
solids (yields: 2a, 90%; 2b, 80
%
%
). Their NMR data fully support
cially the presence of trans-vinyl
e
s and two non-equivalent PiPr3
the proposed structures, esp
ee
complexes are electro-active at fairly low p
synthesis of redox-triggered NLO-active3c a
Remarkably, introducing these remote RuII
ootentials, allowing the
ruthenium-substituted moieti
e
n
n
d optical3d-f switches.
-centers results in a
significant change of the chiroptical properrties of helical π-cores
ligands due to the presence of the helicene cores (SI). The X-ray
diffraction study of RuII-capp
d helicenes 2a,b (Scheme 1, SI),
shows that the helical angles (dihedral angles between the ter-
minal rings of the helix) ar slightly different for these two
u
ee
1a,b. Furthermore, the redox properties of the lateral RuII metal
e
e
centers allow the corresponding radical cations to be easily
complexes (2a, 49.7°; 2b, 60.9°), but in the usual range ob-
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served for [6]helicene derivatives.2j The geometry around the
from HOMO-2(324)-to-LUMO(327) and HOMO-2(324)-to-
LUMO+1(328), respectively (Table S7, Figure 1 bottom).
five-coordinated RuII-centers is square pyramidal, with the two
trans PiPr3, the Cl and the CO ligands forming the basis of the
pyramid and the vinyl-helicene moiety being at the apical posi-
tion. The Ru-C1-C2-C3 bond lengths (2a: 1.980, 1.337 and
1.464 Å) are typical for trans vinyl-Ru complexes,4c and the vinyl-
Ru moiety is almost coplanar with the helicene part (Ru-C1-C2-
C3: 2a, 176.33°; 2b, 168.98°), allowing efficient metal-ligand
electronic interaction through the carbon-carbon double bonds.
This feature was confirmed by UV-vis spectroscopy of com-
plexes 2a,b which displayed large low-energy bands between 380
nm and 460 nm (SI) that do not exist in their organic precur-
sors 1a,b. BHLYP/SV(P) TDDFT calculations following geome-
try optimizations with BP/SV(P) (SI), clearly confirm the pres-
ence of this electronic interaction. For example, the
HOMO(206) of 2a spans over the metal and the vinyl-helicene
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moiety (Figure 1, top)
.
The comparison of the chiroptical properties7 of derivatives
1a,b and their RuII-modified derivatives 2a,b revealed a remark-
able and unexpected feature. Upon introducing the remote
metal centers, the molar rotations (MRs) values are doubled
23
(
in °cm2/dmol: 1a/2a, ±11030/23770; 1b/2b,
[
φ
]
D
±20000/39150)! The difference in the chiroptical properties of
1a,b and 2a,b is also reflected in their corresponding electronic
circular dichroism (ECD) spectra. Note that the shape of the
CD spectra of organic helicenes 1a and 1b are similar, with
higher band intensity for 1b (Figure 2). The experimental ECD
of P-1a and P-2a show two intense ECD bands at 250 nm (nega-
tive) and 335 nm (positive) which are the fingerprint of heli-
cene derivatives (Figure 2a). Indeed, according to the calcula-
tions, the excitation n°6 (325 nm) with the strongest calculated
rotatory strength of 2a corresponds well to the excitation n°3
(312 nm, Figure 1 top) of the organic derivative 1a in the sense
Figure 1. Calculated (BHLYP/SV(P)) CD spectra of
P-1a (red
line) vs. P-2a (black) (top) and -1b (red) vs. P-2b (black) (bottom),
P
and isosurfaces (0.04 au) of selected MOs of 2a (top) and 2b (bot-
tom) involved in the transitions. Compare SI.
that similar MOs centered on the π-conjugated helical platform
are involved (Table S7). The comparison of the experimental
ECD spectra of P-1a and P-2a also shows new low energy bands
of moderate magnitude for 2a (380 nm – 460 nm, Figure 1 top)
that involve metal contribution. For example, P-2a excitations
n°5 and n°3 correspond mainly (58% and 65%) to π-π* transi-
Figure 2. CD spectra in CH2Cl2 at 293 K of a) mono-alkynyl
tions localized in the helicene moiety with visible metal orbital
involvement (HOMO(206)-LUMO+1(208) and HOMO(206)-
LUMO(207), Figure 1). Furthermore, excitation n°2 involves
dominantly MOs centered on the metal fragment (MO(200):
precursors
vinylruthenium
sors -(+)- and
vinylruthenium -(+)- and
P
-(+)- and
-(+)- and
-(-)-1b and their corresponding bis-
-(-)-2b.
M
-(-)-1a and their corresponding mono-
P
M
-(-)-2a, and b) of bis-alkynyl precur-
P
M
P
M
formally a t2g orbital with carbonyl π*-backbonding character;
MO(209): a metal-centered orbital, Figure 1). It is possible that
this particular excitation is somewhat too red-shifted in the
calculations,7b but the overall spectral envelopes agree well with
experiment. These additional ECD bands at long wavelengths
having large contribution from the metal atom are responsible
for the increase of the molar rotation observed in 2a. The most
intense CD-active bands of 2b displaying the highest ꢀε values
were found between 380 and 430 nm and a separate band at
low energy (450 nm) is now clearly observable.
This red shift of the ECD spectrum upon introducing the
two RuII-centres is well-reproduced by theory (Figure 1 bottom,
SI). The P-2b excitation with the strongest rotatory strength
(n°12, calculated at 315 nm) affords two main contribution-
Both contributions involve orbitals predominantly centered
within the helical -system, with negligible metal character. Two
π
other intense excitations (n°5 and n°6) are located at ∼365 nm
and constitute the new positive band of the CD spectra. Their
dominant contributions correspond to, respectively,
HOMO(326)-to-LUMO(327)
LUMO+1(328) products and display a partial charge-transfer
character of * excitations within the helicene ligand en-
and
HOMO(326)-to-
π-π
hanced by the involvement of d orbitals of both metal centers.
Such excitations (n°6 and n°5) in 2b were also found in the case
of 2a (n°5 and n°3) although with a substantially lower inten-
sity. Therefore, their increased intensity can account for the
intensity enhancement of the low-energy tail of the positive CD
band in the P-2b and consequently for the MR enhancement.
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dation from 0.2 to 0.6 V (vs. Fc/Fc+), all these bands (450-600
and 950 nm) disappeared and a new one appeared at 700 nm
(SI).
These results illustrate the power and simplicity of organometal-
lic chemistry to produce new chiral molecular architectures and
to increase of the chiroptical properties of the helicene plat-
forms.
In the spectroelectrochemical IR spectrum of 2a, a blue shift
of the CO stretch from 1913 to 1968 cm-1 (Figure 3) and the
appearance of new C=C bands in the 1640-1520 cm-1 region (SI)
with clean isobestic points were also observed during the forma-
tion of [2a]•+. This slight increase of the energy of the CO
stretch is assigned to a decrease of electron density transferred
from the metal atom to the π* orbitals of the carbonyl ligand
that is weaker than for a purely metal based oxidation.4 Thus,
this fact suggests that the radical cation is largely localized on
the helicene fragment.4c Similarly, the mono-oxidation of 2b,
displaying one CO stretch at 1911 cm-1, results in two CO
stretches at 1913 and 1965 cm-1 of identical intensities (see SI)
that are testimonies of two non equivalent ruthenium centers
on the IR timescale in [2b]•+.4c,d
The remote RuII-metal centres also endow the helicene core
with unprecedented chiroptical redox-triggered switching prop-
erty, due to their electro-active behaviour. Chiroptical switches
are multifunctional materials that may be useful for a variety of
application such as molecular electronics, optical displays or
telecommunication purposes.5,8 It is noteworthy that redox
triggered chiral switches are still quite rare5a and that very few
helicene-based chiroptical switches have been described so far,
almost all being based on photochromic systems.9 In addition,
although examples of electro-active helicene derivatives have
been described in the literature,5b,10 only one was considered as
a potential redox chiral switch.5b Therefore, the electrochemical
behaviour of organometallic species 2a,b was investigated with
the aim to obtain the first purely helicene-based redox chiral
switches. An important and well-established property of (arylvi-
nyl)RuCl(CO)(PiPr3)2 complexes is their multistep reversible
oxidation/reduction processes at fairly low potentials, involving
the ‘non-innocent’ arylvinyl ligands. In fact, the
RuCl(CO)(PiPr3)2 moiety stabilizes the radical cations to about
the same extent as dialkylamino groups, and the charges are
mainly located on the organic arylvinyl fragments.4c Cyclic volt-
ammetry (CV) studies of 2a (CH2Cl2/NBu4PF6, 0.2 M) revealed
the presence of one chemically and electrochemically reversible
oxidation at E1° = +0.173 V vs. the ferrocene/ferrocenium
(Fc/Fc+) standard, followed by a second irreversible oxidation at
Epa= +0.781 V (100 mV s-1) (SI). A full reversibility of the proc-
ess was observed upon several oxidation/reduction cycles be-
tween 0 V and 0.4 V (SI). Interestingly, the one-electron oxida-
tion at E1° = +0.173 V affording [2a]•+PF6-,4c takes place at a
significantly lower potential than that observed for thiophene-
based helicenes (+1.3 V),10d rendering the helicene core easy to
oxidize. The bis(RuII)-[6]-helicene 2b (Scheme 1) was also inves-
tigated since it bears two redox active moieties and is therefore
a unique platform to investigate the impact of multi-oxidation
processes on the electronic property of the prototype [6]-
helicene fragment. Two consecutive reversible one-electron
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Figure 3. IR region of the
2a. BHLYP/SV(P) electron spin density, ꢀρ
Numbers listed are fractions of the total integrated spin density
obtained from Mulliken decompositions of ꢀρ
v(CO) stretch upon mono-oxidation of
•
+
=
ρα ρβ, in [2a] .
–
.
Upon the second oxidation, these two bands disappear and a
new
(CO) at 1968 cm-1 grows up. The ‘non-innocent’ charac-
ν
ter of the vinylhelicene ligand in [2a]•+ and [2b]•+ was confirmed
by EPR measurements, with average g values close to ge (see SI).
Finally, theoretical analyses (B3LYP/SV(P)) of the electronic
structure of the radical cations [2a]•+, [2b]•+ and [2b]2+ (in its
singlet and triplet state) were performed. The plots of the elec-
tron spin density clearly show that the unpaired charge density
is not only localized on the metal atoms but also spread out
over the helicene π-ligand (Figure 3 for [2a]•+ and SI). Thus, in
the case of [2b]•+, these data cannot be straightforwardly ana-
lysed according to the concept of mixed-valence and Robin and
Day classification.3a,b,4c,d Note that i) the electron spin density
and the Singly Occupied Molecular Orbitals (SOMOs) exhibit a
general picture revealing a similar spatial distribution of the
electron-hole pair, and ii) the SOMOs of [2a]•+/[2b]•+ display
the same characteristics as the HOMOs of the neutral systems
2a/2b, indicating that oxidation results in little reorganization
of the organometallic helicene (SI).
°
oxidation waves were observed at E1 = +0.146 and E2° = +0.285
V (vs. Fc/Fc+) accompanied by a third chemically irreversible
wave at ca. Epa = +0.83 V (v 100 mV s-1) (SI).The splitting of the
first two waves (∼130 mV) suggests a stepwise oxidation of the
redox-active bis(RuII)-[6]-helicene 2b to [2b]•+ and [2b]2+, with
substantial comproportionation (Kc∼234,i.e. ∼90% of the
monoradical cation upon full one electron oxidation).3g
These redox processes were monitored by UV/vis/NIR spec-
troelectrochemical spectroscopy in a transparent thin-layer
(OTTLE) cell in dichloroethane (DCE) solutions (see SI). The
formation of the radical cation [2a]•+ results in a slight decrease
of high energy bands (< 480 nm), the appearance of sets of
structured absorptions at ∼500 nm, and two lower-energy ab-
sorption bands at ∼630 and ∼1000 nm (see SI). The formation
of [2b]•+ and [2b]2+ was also monitored and showed three sets of
new bands appearing during the first oxidation stage, the first
one between 450-600 nm, a second one at 650 nm, and a large
extended band centered at 950 nm (see SI). Upon further oxi-
The fact that the helicene moiety bears a significant spin den-
sity in the oxidized organometallic species, due to the peculiar
property of the Ru(CO)Cl(PiPr3)2 fragment, leads to the expecta-
tion that the reversible redox processes could impact the chi-
roptical behaviour. Indeed, the ECD spectra of P-2a and P-
[2a]•+PF6- in DCE have noteworthy different features (Figure
4a). The oxidation of P-2a results in a significant decrease of the
ECD-active bands at 340 nm (
ꢀ(
ꢀε) = -45 M-1 cm-1), and the
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appearance of new broad ECD-active bands (of positive sign for
the P-stereoisomer) ranging from 430 nm to 580 nm ( ꢀε
=+17 M-1 cm-1 at 500 nm) and in the NIR-region (
ꢀε) = +8.7
ACKNOWLEDGMENT
ꢀ(
)
We thank the Ministère de l’Education Nationale, de la Re-
cherche et de la Technologie, and the Centre National de la
Recherche Scientifique (CNRS). L. Norel, Y.-M. Hervault, G.
Grelot and O. Cador are warmly thanked for their kind help in
spectroelectrochemical and EPR measurements. The theoretical
component of this work has received financial support by the
National Science Foundation (CHE 0952253). MS and JA
acknowledge the Center for Computational Research (CCR) at
the University at Buffalo for providing computational re-
sources. MS is grateful for financial support from the Founda-
tion for Polish Science (‘START’ scholarship) as well as from
Polish Ministry of Science and Higher Education (‘Mobility
Plus’ program).
ꢀ(
M-1 cm-1 at 900 nm, Figure 4b). Exploiting these differences,
along with the reversibility of the redox processes, the first
electrochemical chiral switch based on a pure helicene moiety
was achieved. More specifically, stepping potentials between -
0.4 and +0.4 V of a DCE solution of 2a (0.2 M, n-Bu4PF6) in an
electrochemical cell leads to a fully reversible modulation of the
CD signals both at 340 and 500 nm (Figure 4c) over several
cycles. Remarkably, this redox chiroptical switch can be used
both at a high-energy wavelength, which belongs to the classic
CD-active fingerprint of helicene derivatives, and at a low-
energy wavelength, which is due to the presence of the Ru-
center (vide supra). Likewise, the bis(vinylRu)-system P-2b/P-
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•
+
[2b] also behaves as a reversible electrochemical chiroptical
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ꢀ(
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Figure 4. a) CD spectra of
P-(+) / M-(-) enantiomers of 2a and of
7 (a) Furche, F.; Ahlrichs, R.; Wachsmann, C.; Weber, E.; Sobanski, A.; Vogtle, F.;
their oxidized species in DCE at room temperature. b) NIR-CD
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Adriaenssens L. et al. Chem. Eur. J. 2009, 15, 1072-1076. (c) Liberko, C. A.; Miller, L.
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2000
,122, 1717-1724. (b) Rudolph, M.; Ziegler, T.;
•
+
,
391, 92-100. (c) Newman, M. S.; Lednicer, D. J. Am.
region spectra of
chiroptical switching
P
-(-)-2a (blue) and
P-(-)-[2a] (red). c) Redox
-(+)2a]•+ observed by CD spec-
,
P
-(+)-2a <-> [
P
troscopy at 340 and 500 nm.
.
2
ASSOCIATED CONTENT
,
,
Supporting Information. Experimental details, CIF files, electrochemical and spec-
troscopic data of the products. Computational details for theoretical calculations,
additional calculated data. This material is available free of charge via the Internet at
http://pubs.acs.org.
10
J
9
M.; Rajca, S.; Lapkowski, M.; Rajca, A. J. Am. Chem. Soc. 2010, 132, 3246-3247. (e)
Gilbert, A. M.; Katz, T. J.; Geiger, W. E.; Robben, M. P.; Rheingold, A. L. J. Am.
AUTHOR INFORMATION
Chem. Soc. 1993
Bossi, A.; Licandro, E.; Mussini, P. R.
,
115, 3199-3211. (f) Rose-Munch, F.; Li, M.; Rose, E.; Daran, J. -C.;
rganometallics 2012, 31, 92-104. (f) J. E. Field,
O
Corresponding Author. * jeanne.crassous@univ-rennes1.fr,
regis.reau@univ-rennes1.fr, jochena@buffalo.edu
T. J. Hill, D. Venkataraman, J. Org. Chem. 2003, 68, 6071-6078. (g) M. Spassova, I.
Asselberghs, T. Verbiest, K. Clays, E. Botek, B. Champagne, Chem. Phys. Let. 2007,
439, 213-218.
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Table of Content
PiPr3
PiPr3
Ru
Cl
Cl
110
100
90
Ru
CO
CO
PiPr3
PiPr3
.+
Ox
80
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Red
70
60
0
2
4
6
8
number of cycles
5
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