Multifunctional and reactive enantiopure organometallic helicenes: Tuning chiroptical properties by structural variations of mono- and bis(platinahelicene)s

DOI: 10.1002/chem.201101866

Multifunctional and Reactive Enantiopure Organometallic Helicenes: Tuning

Chiroptical Properties by Structural Variations of Mono- and

Bis(platinahelicene)s

Emmanuel Anger,^[a]Mark Rudolph,^[b]Lucie Norel,^[a]Samia Zrig,^[a]Chengshuo Shen,^[a]

Nicolas Vanthuyne,^[c]Loꢀc Toupet,^[a]J. A. Gareth Williams,^[d]Christian Roussel,^[c]

Jochen Autschbach,*^[b]Jeanne Crassous,*^[a]and Rꢁgis Rꢁau*^[a]

Abstract: Acetylacetonato-platina[6]-

and -platina[7]helicenes have been pre-

pared from 2-pyridyl-substituted ben-

zophenanthrene ligands by following a

two-step cycloplatination reaction. The

photophysical properties (UV-visible

absorption and emission behavior) and

chiroptical properties (circular dichro-

ism and molar rotation) of the resolved

enantiomers have been measured.

pared to the parent [6]- and [7]carbo-

helicenes. Furthermore, they are red

phosphors at room temperature and

their large chiroptical properties can be

modulated by oxidation of the metal

center to Pt^IV. Hetero- and homochiral

erochiral

9a¹can isomerize into the homochiral

(P,P)- and (M,M)-bis

(Pt^III–[6]helicene)

(P,M)-bisACTHNGUTERNNUG

(Pt^III–[6]helicene)

AHCTUNGTRENNUNG

9a². Spectral assignments and an analy-

sis of the optical rotation of these sys-

tems were made with the help of time-

dependent density functional theory.

The calculations highlight the contribu-

tions of the metal centers to the chirop-

tical properties. For 9a¹and 9a², s–p

conjugation between the helicenes and

diastereomeric

bis(metallahelicene)s

III

that possess a rare Pt^IIIPt scaffold

bridged by benzoato ligands have also

been prepared. It is shown that the het-

ꢀ

These metallahelicenes constitute

a

ꢀ

novel family of easily accessible heli-

cene derivatives that exhibit large and

tuneable chiroptical properties that can

be rationalized theoretically and com-

the Pt Pt moiety may contribute

strongly to the optical rotation and

electronic circular dichroism.

Keywords: chirality · density func-

tional calculations · helicenes · lu-

minescence · platinum

Introduction

rials due to the variety of efficient methods to build carbon–

metal bonds,^[1a]and due to the intimate electronic interac-

tion between metallic ions and p ligands that gives rise to a

great diversity of appealing properties.^[1b]Furthermore, the

presence of the metal centers allows these properties to be

tuned by using their reactivity (redox behavior, ligand ex-

changes, metal–metal interactions, and so on).^[1c]This ap-

proach has been widely exploited in the last decade with the

use of cyclometalated platinum and iridium complexes that

bear 2-phenylpyridine ligands ((N^C)M, for example, M=

Pt^II, Ir^III) as efficient and robust derivatives for various ap-

plications (for example, dopants for organic light-emitting

diodes,^[1c,2a–e]switches,^[2f]and sensors^[2g]). These applications

rely on the high (electro)phosphorescent efficiencies of such

complexes, which originate from the strong spin–orbit cou-

pling of the heavy-metal ions.^[1b,c,2]Moreover these organo-

metallic species can be used as building blocks for the syn-

thesis of complex systems by means of self-assembly (aggre-

gation) and supramolecular synthesis.^[3]A great variety of

functional 2-phenylpyridines have been devised for the con-

struction of advanced cyclometalated complexes, although it

is noteworthy that this p-conjugated platform has very

rarely been endowed with chiral properties.^[4,5]

Organometallic chemistry is a powerful tool for the con-

struction of multifunctional metal-based p-conjugated mate-

[a] E. Anger, Dr. L. Norel, Dr. S. Zrig, C. Shen, Dr. L. Toupet,

Dr. J. Crassous, Prof. R. Rꢀau

Sciences Chimiques de Rennes, UMR 6226

Institut de Physique de Rennes, UMR 6251

CNRS, Universitꢀ de Rennes 1

Campus de Beaulieu, 35042 Rennes Cedex (France)

Fax : (+33)2-23-23-69-39

E-mail: jeanne.crassous@univ-rennes1.fr

regis.reau@univ-rennes1.fr

[b] M. Rudolph, Prof. J. Autschbach

Department of Chemistry, 312 Natural Sciences Complex

University at Buffalo, State University of New York

Buffalo, NY 14260-3000 (USA)

E-mail: jochena@buffalo.edu

[c] Dr. N. Vanthuyne, Prof. C. Roussel

Chirosciences UMR 6263, ISM2

(Institut de Sciences Molꢀculaires de Marseille)

Universitꢀ Paul Cꢀzanne

13397 Marseille Cedex 20 (France)

[d] Dr. J. A. G. Williams

Department of Chemistry, University of Durham

Durham, DH1 3LE (UK)

[n]Helicenes are composed of n ortho-fused aromatic

rings with helical chirality and a p-conjugated backbone,

which provide them with peculiar optical and electronic

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.201101866.

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Chem. Eur. J. 2011, 17, 14178 – 14198

FULL PAPER

properties.^[6]Due to this unique helical p structure, they dis-

play huge isotropic chiroptical properties (optical rotation,

circular dichroism)^[7]that make them promising building

blocks for the synthesis of functional molecular materials

(for example, nonlinear optical materials,^[8a,b]chiral wave

guides,^[8c]chiroptical switches,^[8d]luminescent materials,^[8e,9a]

and sensors^[8f]). [n]Helicenes have also been used in organo-

catalysis for enantioselective transformations,^[10a–c]as chiral

derivatizing agents,^[10d,e]and as building blocks for the design

of chiral ligands for homogeneous transition-metal cataly-

sis.^[10f]The potential use of helicene derivatives in these ap-

plications depends greatly on the facility of generating dif-

ferent frameworks to modulate and to optimize their elec-

tronic and optical properties.^[11]This requires the develop-

ment of short and efficient strategies to diversify helicene

structures, a task that still remains a challenge to synthetic

chemists.^[6,12]In this context, it is quite surprising to note

that coordination and organometallic chemistry have been

very rarely utilized to create structural diversity in helicene

chemistry. Besides complex A,^[10f]which has been prepared

for catalytic applications,^[10b,c]and B, which may have a non-

linear optical response,^[10g]coordination assemblies C–E of

two aza[6]helicene-based ligands with Ag^I,^[13a]Pd^II, and

Cu^I[13b–d]ions have been recently reported (Scheme 1). The

Herein we describe straightforward access to helicene de-

rivatives that incorporate metals into their ortho-annulated

p-conjugated backbone.^[9]Our strategy towards these metal-

lahelicenes was to exploit the cyclometalation reaction of 4-

(2-pyridyl)benzo[g]phenanthrene

derivatives

5a–e

(Scheme 2) with Pt^IIions. The Pt^IIion was selected because

Scheme 2. Synthesis of acetylacetonato–Pt[n]helicenes 7a–e (n=6, 7).

i) K

₂ACHTUNGNERTN[UNG PtCl₄], ethoxyethanol, H₂O, reflux, 16 h. ii) AcacH, Na₂CO₃, ethoxy-

AHCTUNGTRENNUNG

of its inherent characteristics (for example, efficient elec-

tronic metal–ligand interaction, large spin–orbit coupling,

accessible oxidation potential).^[1b,c,2]The approach fully ex-

ploits the potential of organometallic chemistry since 1) the

helical backbone is completed during a simple ortho-metala-

tion reaction, 2) the presence of the heavy Pt^IIcenter pro-

vides these helicenes with unusual photophysical properties,

and 3) it allows structural engineering to be performed using

the metal reactivity.^[9a]The synthesis of unprecedented het-

erochiral and homochiral bis-assemblies of platinaACHTUNGTRENNUNG(III)-

III

[6]helicenes that incorporate an original Pt^IIIPt scaffold is

ꢀ

described.^[9b]The electronic (UV/Vis, fluorescence, and

phosphorescence spectroscopy) and chiroptical (molar rota-

tion (MR) and circular dichroism (CD)) properties of all

these platinahelicenes have been investigated thoroughly,

and structure–property relationships were established based

on experimental and theoretical studies. These data reveal

that the chiroptical properties of the metallahelicenes can

be fine-tuned by chemical oxidation of the metal center and

Scheme 1. Examples of coordination complexes of helicene ligands and

organometallic helicenes.^{[10b,c,f,g,13,14]}

that s–p conjugation takes place in the bisACTHNUTRGNEUNG

(Pt^III–[6]helicene)

only known family of organometallic helicenes is based on

metallocene moieties.^[14]Within these oligomers, illustrated

with structure F (Scheme 1), the metal centers lie outside

the p-conjugated helicoidal backbone. Note that the recent-

ly prepared, related cobaltocenium complex G^[14a,b]

assemblies. Overall, the preparation of these platinaheli-

cenes with different lengths ([6]- and [7]helicenes), different

metal–ligand interactions (metal-to-ligand charge transfer

(MLCT), s–p conjugation, and so on), and various oxidation

states (Pt^II, Pt^III, Pt^IV), illustrates the potential of organome-

tallic chemistry for the molecular engineering of helicenes

and offers a unique opportunity to investigate how a metal

(Scheme 1) of dibenzo

ACHTUNGTRENN[UNG c,g]fluorenide can also be considered

an organometallic helicene derivative.^[14c,d]

Chem. Eur. J. 2011, 17, 14178 – 14198

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J. Crassous, J. Autschbach, R. Rꢀau et al.

ion can impact the chiroptical properties of this helicoidal

p-conjugated framework.

Results and Discussion

Synthesis and characterization of Pt^II–[n]helicene deriva-

tives (n=6, 7): The target molecules are (acetylaceton-

ACHTUNGTRENNUNGato)Pt–[n]helicenes 7a–e (n=6,7) that bear either pyridyl-

or isoquinolino-substituted 4-benzo[g]phenanthrene ligands

(Scheme 2).^[9a]The series 7a–c offers an opportunity to in-

vestigate the impact of electron-donating (7a) or electron-

withdrawing (7b) substituents on the electronic (absorption,

emission) and chiroptical (CD, optical rotatory dispersion

(ORD)) properties of these novel helicene derivatives. The

fused chiral pinene moiety in 7d (Scheme 2) was selected to

try to obtain a diastereoselective synthesis, and the longer

Pt–[7]helicene 7e to study the influence of the helical length

on the electronic and chiroptical properties.

The synthesis of the target platinahelicenes 7a–e accord-

ing to the classical two-step orthometalation^[1b,c,2]necessi-

tates the preparation of the corresponding 4-(2-pyridyl)ben-

zo[g]phenanthrene ligands 5a–e (Scheme 3).^[9a]These start-

ing materials were synthesized using a Suzuki-coupling/

Wittig/photocyclization reaction sequence (Scheme 3).

Indeed, aldehydes 3a–c were obtained under classical

Suzuki coupling conditions by using Br–pyridine 2X. Deriv-

atives 3d and 3e were obtained from pinene-fused 2-OTf-

Scheme 3. Synthesis of ligands 5a–e. i) [PdACHTUNGTRNEUNG(PPh₃)₄], Na₂CO₃, toluene/

THF, Ar, reflux, 36 h; ii) 2-methylnaphthyltriphenylphosphonium bro-

mide, nBuLi, THF, Ar, RT, 3 h; iii) hn, catalyst I₂, toluene, 18 h.

Single crystals of 5a were grown from a heptane/AcOEt so-

lution at room temperature and were studied by X-ray crys-

tallography (Figure 1, Table S13), which revealed a helical

curvature (hc; the angle between the terminal benzophenan-

threne rings) of 33.48, a value typical for [4]helicenes.^[17]Al-

though the angle between the pyridyl ring and the terminal

phenyl ring is rather high (52.78), this molecule is not config-

urationally stable at room temperature as usually observed

for [4]helicene derivatives.^[17]Indeed, all attempts to resolve

compounds 5a–c by HPLC using diverse chiral stationary

ꢀ

pyridyl (OTf=CF₃SO₃) precursor 2Y^[15]and commercially

available 2-chloroisoquinoline 2Z, respectively. The subse-

quent Wittig reactions afforded mixtures of cis and trans iso-

mers and the photocyclization^[6,16]performed in toluene with

catalytic quantities of iodine afforded benzophenanthrene

derivatives 5a–e with moderate yields (Table S16 in the Sup-

porting Information). Note that

higher yields are obtained when

the C^1’carbon atom is substitut-

ed by a methoxy or a fluoro

substituent (Table S16, 5a–c),

since it prevents photocycliza-

tion from taking place at this

position.^[9a]The photocycliza-

tion process appeared less effi-

cient in the case of 5e, probably

for steric reasons. All these new

derivatives were fully character-

ized by multinuclear NMR

spectroscopy, mass spectrome-

try, and elemental analysis.^[9a]

The NMR spectroscopic data

for 5a–e compare well with

those of related benzo[g]phe-

nanthrene derivatives.^[17]For

example, the protons of the pyr-

idyl (H⁶) or isoquinolino moiet-

ies (H³) have characteristic

Figure 1. Molecular structures of ligand 5a and complexes 7a, 7d^1,2, 7e, and 8b determined by X-ray crystal-

chemical shifts (Table S16). lography.^[12]Except for 7d^1,2, only one stereoisomer is shown.

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Enantiopure Organometallic Helicenes

FULL PAPER

phases failed. Likewise, 5d and 5e (Scheme 3), which dis-

play a chiral fused pinene and a 1-isoquinolino moiety, re-

spectively, could not be resolved by chiral HPLC, even

below 58C.

that diasteroselective cyclometalation reaction can take

place in spite of the rather high reaction temperature

(1008C). Overall, orthometalation appears to be a conven-

ient synthetic strategy for the construction of metallaheli-

cene frameworks and readily supplies these organometallic

species in sufficient quantities (0.2–0.5 g scale). This organo-

metallic approach appears one of the most efficient routes

known to date to obtain helicene derivatives.^[12]The precur-

sors are easily available, they are not sensitive materials, the

reaction conditions are not harsh, and the helicenes were

obtained in good yield.

Compounds 5a–e were subsequently subjected to a two-

step cyclometalation reaction^[2](Scheme 2) to afford Pt^II

complexes 7a–e (yields>50%, Table S16 in the Supporting

Information). Since these derivatives are air stable and not

water sensitive, they can be purified in air under ambient

conditions by using all the classic methods of organic

chemistry. They are soluble in usual solvents (THF, CH₂Cl₂,

1

and so on) and easy to handle. The H and ¹³C NMR spec-

Single crystals of complexes 7a, 7d^1,2, and 7e were grown

from CH₂Cl₂/pentane solutions and their structures elucidat-

ed by X-ray crystallography (Tables S13 and S14 in the Sup-

porting Information). The results confirm the proposed plat-

ina[6]- and platina[7]helicene structures (Figure 1). Note

that in all cases, complexes 7a, 7d^1,2, and 7e crystallize as

heterochiral pairs of P and M helices (see the packing in

Figure 2). As expected, the d⁸Pt^IIions display a slightly dis-

torted square-planar geometry, and the metric data of the

Pt^II-coordination sphere (metal–ligand bond lengths and va-

lence angles) are in the range of those reported for other

troscopic data and elemental analyses are consistent with

the proposed (N^C)PtACTHNUTRGNE(NUG acac) (acac=acetylacetonate) struc-

tures. For example, compared to the nonmetalated ligands

1

5a–e, complexes 7a–e display well-resolved H NMR spec-

195

1

6

195

1

2’

ꢀ

tra in which Pt H (pyridine-H ) and Pt H (H ) cou-

1

plings are observed (see Table S16). The H NMR spectro-

scopic signals of the H⁶of the pyridyl group for complexes

7a–e are deshielded, and the CH resonance of the acac

ligand appears at typical d=5.56 ppm (d¹³C=102.8 ppm).

Note that derivative 7d was obtained as a 42:58 diastereo-

meric mixture according to analytical HPLC over a chiral

stationary phase (vide infra). Although this diastereomeric

excess is low (16%), this result is encouraging since it shows

(N^C)PtACHTUNGTRNEUNG

(acac) complexes (Tables S13 and S14).^[2]These

data reveal that the formation of the helicene skeleton indu-

ces little strain within the Pt^II-coordination spheres. The co-

Figure 2. Solid-state organization of a) complex 7a, b) diastereomeric complexes 7d^1,2, and c) complex 7e.

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J. Crassous, J. Autschbach, R. Rꢀau et al.

Table 1. Experimental^[a]and DFT^[18]calculated helicities (hc, angle be-

tween the planes defined by the terminal pyridyl and/or phenyl groups in

8) of complexes 7a, 7d^1,2, 7e, and 8a,b as well as [6]- and [7]carboheli-

cenes.

7a

7d¹

7d²

7e

8a,b

[6]carbo- [7]carbo-

helicene

53.5

helicene

42.3

hc (exptl) 52.3 52.7 38.5 44.5 56.0 (8b)

hc (calcd) 53.6 56.0 53.3 46.2 52.5 (8a)

[a] Determined by X-ray crystallography.

ordinated phenyl and pyridine rings of 7a exhibit a twist

angle of 7.38, and the consecutive twist angles between the

fused aromatic rings are 24.7, 31.0, and 13.48, thereby af-

fording the metalla[6]helicene-like skeleton an hc value of

52.38 (Figure 1, Table 1). This hc value is similar to those en-

countered in organic or heteroatomic [6]helicenes^[6,16b]and

agrees well with the DFT RI-BP/SV(P)^[18]-optimized struc-

ture of 7a (hc 53.68, Table 1). These data show that intro-

ducing a square-planar metal center within a helicoidal skel-

eton does not drastically affect its shape. This information is

of particular interest since the large isotropic chiroptical

properties of helicenes are related to their unique “screw-

shape” p-conjugated structure. The single crystals of deriva-

tives 7d contain a 1:1 mixture of the two possible diastereo-

mers. The two asymmetric carbons of the pinene moiety

have a fixed 6R,8R absolute configuration, the helicene

moiety of 7d¹is P, and that of 7d²is M (Figure 1). It is inter-

esting to note that the hc values of these two diastereoiso-

mers are markedly different (7d¹, 52.78; 7d², 38.58). This dif-

ference may be attributed to packing effects in the solid

state since DFT calculated structures of 7d¹and 7d²give a

similar helicity value ((54.5ꢁ1.5)8), which is close to the ex-

perimental and calculated hc value for 7a (Table 1). The

solid-state helical angle (hc) value for Pt^II–[7]helicene 7e

(only 44.58) is much smaller than for 7a (52.38), as con-

firmed by DFT calculations (46.28). A similar trend is ob-

served theoretically for [6]- and [7]carbohelicenes, with a

calculated decrease from 53.5–42.38.^[18]This tendency can be

partially explained by intramolecular p–p stacking that

takes place between the terminal rings of [7]helicene deriva-

tives that cannot exist in the shorter [6]helicene analogues.

Figure 3. UV/Vis spectra of ligands 5a–e and of the corresponding Pt^II–

[6]- and [7]helicenes 7a–e.

Table 2. Analysis of the first four transitions with large rotatory or oscil-

lator strengths in terms of MO pair contributions for complex (M)-7a.

Excitation E [eV] ([nm])^[a]f^[b]

R^[c]

MO (from) MO (to) [%]

1

4

3.01 (412)

3.87 (321)

13.8

ꢀ21.2

123

122

119

123

120

124

126

124

131

124

127

124

89

47

40

16

13

18

10

51.4 ꢀ475

4.68 ꢀ195

4.94 ꢀ198

10

12

4.37 (283)

4.48 (277)

[a] Excitation energies have been shifted by ꢀ0.25 eV. [b] Oscillator

strengths have been multiplied by 100. [c] Rotatory strengths are in units

of 10^ꢀ40esu²cm^ꢀ2

.

Table 3. Analysis of the first four transitions with large rotatory or oscil-

lator strengths in terms of MO pair contributions and of excitation 8 for

complex (M)-7e.

Excitation E [eV] ([nm])^[a]f^[b]

R^[c]

MO (from) MO (to) [%]

1

2

3

4

2.6 (477)

3.32 (373)

3.44 (360)

3.56 (348)

14.8

ꢀ52.2

136

135

136

134

135

136

133

137

138

139

137

138

139

137

93

74

78

32

26

16

44

19

10

3.02 ꢀ28.1

3.51 ꢀ56

6.2 ꢀ132.7

8

4.03 (308)

11

ꢀ450.8

Photophysical properties of ligands 5a–e and complexes

7a–e: The electronic properties of ligands 5a–e and of their

corresponding platinahelicenes 7a–e were investigated using

UV/Vis spectroscopy. Time-dependent (TD)-DFT^[19]calcula-

tions were conducted for 7a,e (Tables 2, 3, and 4). Electron-

ic absorption spectra of ligands 5a–e essentially show an in-

tense high-energy band (e>4ꢂ10⁴m^ꢀ1cm^ꢀ1) around 290 nm,

together with several shoulders between 290 and 400 nm

(Figure 3). These spectra are typical for helicoidal p-conju-

gated derivatives. The electronic absorption spectra of Pt^II–

[6]helicene complexes 7a–d display several intense absorp-

tion bands between 250 and 370 nm (Figure 3) that are red-

[a] Excitation energies have been shifted by ꢀ0.25 eV. [b] Oscillator

strengths have been multiplied by 100. [c] Rotatory strengths are in units

of 10^ꢀ40esu²cm^ꢀ2

.

talation ring closure (Scheme 2). Two weak, lower-energy

broad bands (ꢂ450 nm) are observed for 7a–d and proba-

bly arise from interactions between the metal and the p li-

gands. The longer Pt^II–[7]helicene 7e displays a redshifted

UV/Vis spectrum relative to Pt^II–[6]helicene 7a (Figure 3,

Table 4). For example, the absorption at the lowest energy

that corresponds to the HOMO–LUMO transition (vide

infra) is bathochromically shifted by around 50 nm.

shifted relative to those of the free ligands 5a–d (l_max

=

290 nm). These bathochromic shifts are consistent with an

elongation of the p-conjugated system through the orthome-

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Chem. Eur. J. 2011, 17, 14178 – 14198

Enantiopure Organometallic Helicenes

FULL PAPER

Table 4. Photophysical data for the ligands 5a–c and of the Pt^II–helicene complexes 7a–c, 7e, 9a², and 10a.^[a]

5a

5b

5c

5e

9a²

absorption

387 (1020), 316 (sh)

383 (320), 327 (sh)

380 (100), 323 (6390),

289 (22600)

385 (2070), 325 (24700), 408 (15600), 387 (16900),

l_max[nm]

A

285 (53100)

330 (55600), 308 (58000),

287 (62400), 239 (106000)

654

(e ACHTUNGTRENNUNG

[m^ꢀ1cm^ꢀ1])

emission

401, 421, 443 (sh)

397, 412, 432 (sh)

406

395, 415

l_max[nm]^[b]

F_fꢂ10^2[b]

t_lum[ns]^[b]

emission

9.0

6.4

3.0

8.2

4.0

9.5

2.4

0.24

17600

391, 413, 437, 464 (fluo),

387, 396, 412, 433 (fluo), 385, 392, 407, 431 (fluo), 390, 409, 434, 460 (fluo), 596, 635

l_max[nm]^[c]

523, 534 (sh), 567, 614 (phos) 522, 533, 566, 610 (phos) 520, 532, 564, 608 (phos) 518, 562, 606, 660 (phos)

20 ns (fluo), 1.1 s (phos)

[c]

t_lum

22 ns (fluo), 1.3 s (phos) 25 ns (fluo), 1.6 s (phos) 2.9 ns (fluo), 1.3 s (phos) 14 ms

7a

7b

7c

7e

10a

absorption

l_max[nm]

452 (7800), 435 (sh)

(7110), 346 (3350), 316

(33500), 294 (34000),

444 (5380), 421 (4770),

317 (34500), 294

450 (4280), 428 (3910),

390 (7280), 317

508 (6830), 485 (sh)

(6250), 356 (17600),

311 (34000), 266 (56200) ACTHNGUTERN(UNG 30400)

438 (3440), 337 (14000),

299 (18800), 250

(e

N

(38300)

(28100), 294 (31000)

255 (61300)

644

emission

640

642

558 (fl), 704 (ph)

666

l_max[nm]^[b]

F_fꢂ10^2[b]

t_lum[ns]^[b]

emission

10

21000

592, 638, 691 (sh)

6.9

12000

586, 630, 688 (sh)

6.5

11000

589, 635, 690 (sh)

0.97

1700 (ph)

670, 722

4.7

20500

615, 650

l_max[nm]^[c]

t_lum[ms]^[c]

45

23

21

1.1

68

[a] In CH₂Cl₂at 298 K; in diethyl ether/isopentane/ethanol (2:2:1 v/v) at 77 K. [b] At 298 K. [c] At 77 K.

An excitation analysis in terms of MO-to-MO contribu-

tions clearly shows that the metal-atom orbitals are involved

in many p MOs for both complexes (selected transitions:

see Tables 2, 3, Figures 4 and 5, and the Supporting Informa-

tion). For example, the excitations at lower energy (calculat-

ed shifted transitions: (M)-7a, 412 nm; (M)-7e, 477 nm) pre-

dominantly correspond to a HOMO-to-LUMO transition

((M)-7a, 89%; (M)-7e, 93%) (Tables 2 and 3, Figure 4 and

5), two MOs that consist of a mixture of Pt- and extended

N^C p orbitals. Note that the shape of the MOs of the two

related complexes 7a,d,e are very similar (Figure 4 and 5

and the Supporting Information) and reveal the presence of

extended p-conjugated systems that span the whole mole-

cule with participation of the metal. The main transitions

are of p–p* type, which are associated with strong dipolar

oscillator strength values. In addition, almost purely metallic

MOs (such as d_z₂-type MO 119 and 132 in 7a and 7e, respec-

tively) are involved in some electronic transitions (see exci-

tation 10 for 7a in Table 2). This theoretical study shows

that the orbitals of the metal centers efficiently interact with

those of the helicoidal p systems. In such organometallic

helicenes, the role of the metal atom is not only to construct

the helicoidal framework by orthometalation (Scheme 2)

but also to markedly impact the electronic properties of the

p-conjugated system.

ether/isopentane/ethanol, 2:2:1 in volume) at 77 K. At room

temperature, they display structured blue fluorescence in

fluo

max

CH₂Cl₂around 400 nm (for example, for 5a: l =400 nm,

f=0.09, t=6.4 ns; for full details, see Table 4). At 77 K,

these fluorescent emissions are accompanied by a long-lived

green phosphorescence around 550 nm (5a, l^p_m^h_a^o_x^s=550 nm,

t=1.1 s), with the same vibrational structure (Dn

ꢂ1400 cm^ꢀ1), which emanate from the triplet state. The

emission properties drastically change when the platinum is

incorporated within the p-conjugated system. Indeed, plati-

nahelicenes 7a–c are efficient (N^C)Pt^IIred phosphors and

emit at l^p_m^h_a^o_x^s=(642ꢁ2) nm (t=11–21 ms), with quantum

yields that are rather high for this region of the spectrum

(7–10%, Table 4). The substituents in 7a–c (OMe, F, H;

Scheme 2) have little influence on the emission energy,

whereas a redshifted emission is observed for the longer Pt–

[7]helicene 7e (l^p_m^h_a^o_x^s=704 nm; t=1.7 ms; Figure 6), with a

lower quantum yield of 1%. The absence of fluorescence in

the platinahelicenes 7a–c,e can be attributed to the effective

spin–orbit coupling associated with the Pt center, which

favors very rapid intersystem crossing from the singlet to

the triplet state. The fact that the phosphorescence of the

complexes is substantially lower in energy than for the free

ligands is testament to the involvement of the metal in the

emitting excited state. The influence of spin–orbit coupling

and the efficient mixing of metal and ligand orbitals is also

manifest in the much shorter lifetimes of triplet emission in

the complexes (vide supra). Overall Pt^II–[6]helicenes 7a–c

and Pt^II–[7]helicenes 7e behave as regular cycloplatinated

emitters with an extended p conjugation.^[1b,c,2]They are the

first helicene derivatives that exhibit strong phosphores-

The study of the emission behavior of ligands 5a–c,e and

of the corresponding Pt^IIcomplexes 7a–c,e confirms the

impact of the metal center on the optical properties of the

helicene moieties. Ligands 5a–c,e exhibit the typical emis-

sion behavior of organic p-conjugated systems.^[25]They are

emissive in solution at RT and in organic glasses (diethyl

Chem. Eur. J. 2011, 17, 14178 – 14198

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14183

J. Crassous, J. Autschbach, R. Rꢀau et al.

Figure 4. Experimental and BHLYP/SV(P) absorption spectra of 7a.

Comparison of the experimental (dashed black line) and BHLYP/SV(P)

(plain black line) CD spectra of (M)-(ꢀ)-7a. Isosurface plots of the dom-

inant MOs involved in the intense (fourth) calculated excitation in com-

plex 7a and of a nonbonding Pt d_z₂-type MO (no. 119).

Figure 5. Experimental and BHLYP/SV(P) UV/Vis spectra of 7e. Com-

parison of the BHLYP/SV(P) calculated CD spectrum of (M)-7e with

the experimental CD spectrum for complex (ꢀ)-7e. Isosurface plots of

the dominant MOs involved in the intense (fourth) calculated excitation

in complex 7e and of a predominantly nonbonding Pt d_z₂-type MO (no.

132).

cence at room temperature,^[8e,9a,25]a new property induced

by the incorporation of the heavy metal atom within the hel-

icene p-conjugated framework.

of 7d, diastereomers (P)-(6R,8R)-7d¹and (M)-(6R,8R)-7d²

could be obtained with more than 99% enantiomeric ex-

cesses (ee values) by using Chiralpak IB as the chiral sta-

tionary phase and hexane/ethanol/chloroform (85:5:10) as

the mobile phase. The (+) and (ꢀ) enantiomers of the

longer Pt^II–[7]helicene 7e were resolved using Chiralcel OD

as the chiral stationary phase and hexane/2-PrOH (9:1) as

the mobile phase (ee values: 99.5 and 98%, respectively).

The CD spectra have been recorded on solutions of sam-

ples in CH₂Cl₂at a concentration around 5ꢂ10^ꢀ5m. In all

Chiroptical properties (circular dichroism and molar rota-

tion) of Pt^II–[6]helicenes 7a–e: Their chemical stability and

their neutrality make cyclometalated Pt^IIcomplexes that

bear acac ligands purifiable by chromatography or HPLC.

The enantiomeric resolution of complexes 7a,b,d,e, which

are available in good quantities, was efficiently achieved by

HPLC separation over chiral stationary phases. In the case

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Enantiopure Organometallic Helicenes

FULL PAPER

Figure 6. Absorption spectrum (in CH₂Cl₂at 298 K) and emission spectra

(in CH₂Cl₂at 298 K and in EPA at 77 K) of Pt^II–[7]helicene 7e.

Figure 8. CD spectrum of (P)-(+)- and (M)-(ꢀ)-Pt^II-7d^1,2(gray) com-

pared with mirror-image CD spectra of Pt^II-7a (black) (plain lines:

(+)-derivatives; dashed lines: (ꢀ)-derivatives).

cases, they display mirror-image values for the (+) and (ꢀ)

enantiomers within the experimental errors. The intense

mirror-image CD spectra are very similar for both com-

plexes 7a,b (Figure 7), thereby indicating that the electronic

Similarly, the CD of Pt^II–[7]helicene 7e enantiomers are

strongly redshifted relative to that of the Pt^II–[6]helicene 7a

enantiomers (Dl=30–50 nm) and additional bands are ob-

served between 390 and 410 nm (Figure 7). However, con-

trary to what is observed in the carbohelicene series, the CD

bands of 7e display lower intensities (Deꢂ50–100m^ꢀ1cm^ꢀ1)

relative to 7a (Deꢂ100–180m^ꢀ1cm^ꢀ1). In conclusion, where-

as increasing the size of the p system in carbocyclic heli-

cenes (vide infra) results in a redshift of the CD spectra

with only little shape modification, it significantly modifies

the chiroptical properties in these novel organometallic heli-

cenes. Metallahelicenes display peculiar chiroptical behavior

in addition to unprecedented room-temperature phosphor-

escence, thus setting this new family of helicenes apart from

the classic carbohelicene analogues.

To better understand the intriguing chiroptical properties

of platinahelicenes, their CD spectra were calculated by

TD-DFT methods at the BHLYP/SV(P) level of theory.^[19]

For comparison purposes, [6]- and [7]carbohelicenes were

also theoretically studied. The lowest 90 singlet excitations

were calculated. The overlay of the optimized structures of

[6]- and [7]helicenes (Figure 9) shows how the terminating

Figure 7. Mirror-image CD spectrum of Pt^II-7a (dark gray), Pt^II-7b (light

gray), and Pt^II-7e (black) (plain lines: (+)-derivatives; dashed lines: (ꢀ)-

derivatives).

nature of the substituent on the metal-bonded phenyl ring

has little influence on the chiroptical property of platinaheli-

cenes. For the (+) enantiomers, these complexes show an in-

tense negative band at 250 nm and a strong positive band

with several maxima (280–340 nm) tailing down to 410 nm.

The CD spectra of (+)-7d¹and (ꢀ)-7d²display nearly

mirror-image relationship and almost exactly coincide with

the CD spectra of 7a enantiomers (Figure 8). One can

therefore conclude that the CD spectral features originate

only from the p-conjugated platinahelicene framework, and

that which appends a chiral saturated group does not

modify the CD properties. The CD spectrum of (M)-[6]car-

bohelicene was reported by several groups earlier;^[7a,b]it es-

sentially displays a sharp negative band at 325 nm (Deꢂ

ꢀ200m^ꢀ1cm^ꢀ1) decorated with several shoulders at lower

and higher energies, and a sharp positive band at 245 nm

(Deꢂ200m^ꢀ1cm^ꢀ1) with additional smaller bands at higher

energy. The (M)-[7]carbohelicene displays a more intense

and bathochromically shifted CD spectrum: the strong nega-

tive band is redshifted (>350 nm, De=ꢀ250m^ꢀ1cm^ꢀ1), and

the strong positive band at 260–270 nm (De>400m^ꢀ1cm^ꢀ1)

is accompanied by two smaller bands at 290 and 300 nm.^[7a]

ꢀ

C H bonds in [6]helicene are oriented compared to the C

C bonds that take their place in [7]helicene. The same trend

is observed for platinahelicenes 7a,e (Figure 10). As dis-

cussed earlier, the helical angles are in accordance with the

experimental values obtained by X-ray diffraction study.

Increasing the length of the helical p framework of carbo-

helicenes results in a more intense and bathochromically

shifted CD spectrum (Figure 9), a trend seen experimental-

ly.^[7a,b]Note that in contrast to what is observed with platina-

helicenes (Figure 7), the general shape of the CD spectra of

[6]- and [7]helicene derivatives is very similar. The comput-

ed CD spectra of metallahelicenes 7a,e agree reasonably

well with the experimental ones and provide a redshift of

0.25 eV (see Figures 4 and 5), which allows the M-(ꢀ)/P-(+)

absolute configuration assignments for both molecules 7a

and 7e. Note that this M-(ꢀ)/P-(+) assignment is common

in helicenes.^[6,7]

From the MO analysis, one can see that the Pt orbitals

are involved in almost all transitions of 7a, even those of

Chem. Eur. J. 2011, 17, 14178 – 14198

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J. Crassous, J. Autschbach, R. Rꢀau et al.

Figure 10. Top: Overlay of the optimized structures of the 7a and 7e

complexes. Bottom: BHLYP/SV(P) CD spectra comparison for com-

plexes (M)-7a and (M)-7e. No spectral shift has been applied. Circles are

for complex 7a and triangles are for complex 7e.

Figure 9. Top: Overlay of the optimized structures for [6]- and [7]heli-

cene. Bottom: BHLYP/SV(P) CD spectrum for [6]- and [7]helicene. No

spectral shift has been applied. Circles are for [6]helicene and triangles

are for [7]helicene.

This observation, which fits with the experimental results

(Figure 7), illustrates the impact of incorporating a metal

center within the helicoidal frame of helicenes. The differen-

ces in the low-energy regime observed experimentally are

also reproduced theoretically. The lowest seven excitations

of 7e can be described as mainly p-to-p*; however, one of

these transitions is assigned as relatively pure Pt-to-helicene

p* (excitations 5 and 6, MO no. 132, Figure 5). Several exci-

tations include transitions from MO pairs distributed over

the entire molecule as well as in the acac ligand, and not

just the helicene portion. Overall, there are numerous tran-

sitions with rotatory strengths of moderate magnitude in 7e

that are not found in 7a (see the Supporting Information).

This experimental and theoretical study revealed two im-

portant features: 1) different structure–(chir)optical proper-

ty relationships are observed for carbohelicenes and metal-

lahelicenes, and 2) the metal center significantly contributes

to the chiroptical properties of platinahelicenes due the inti-

mate metal–ligand electronic coupling.

high energy (Figure 4, Table 2). For example, the dominant

contributions to the intense CD band at 321 nm (R=475ꢂ

10^ꢀ40cgs) are HOMO to LUMO+2 (47%) and HOMOꢀ1

to LUMO (40%) transitions (Table 2, Figure 4). The MO

contributions can be described as dominantly p-to-p*, with

an involvement of Pt in the p system. Likewise, the CD

band at 283 nm (R=195ꢂ10^ꢀ40cgs) involves two main tran-

sitions (16 and 13%) that involve an occupied d_z₂-like orbi-

tal fully centered on the metal ion (MO no. 119, Figure 4).

Note that the low energy CD band at 412 nm corresponding

to HOMO–LUMO transition is of low rotatory strength

(R=21ꢂ10^ꢀ40cgs). The calculated CD spectra for diastereo-

mers (P)-(6R,8R)-7d¹and (M)-(6R,8R)-7d²closely resemble

those of 7a enantiomers and confirm that the CD signature

originates from the helical ortho-fused p-conjugated system.

As for 7a, the calculated CD for complex 7e displays an

intense CD transition at 4.3 eV (308 nm, shifted value). The

MO contributions for this excitation are also described as

dominantly helicene p-to-p*, with a contribution from Pt to

the p system (see Table 3, Figure 5, and the Supporting In-

formation). The MO pairs involved in this excitation are

quite similar in nature to those of complex 7a (Figure 4,

Table 2). However, the more intense transition in 7e is hyp-

sochromically shifted with respect to 7a (290 nm for 7e

versus 321 nm for 7a; Figure 10), which is the opposite

trend seen between [7]- and [6]carbohelicene (Figure 9).

The specific molar rotations (MR in 8cm²dmol^ꢀ1) of enan-

tioenriched compounds were determined in CH₂Cl₂at con-

centrations of around 0.01 g per 100 mL at 238C using the

sodium D line (Table 5). The (+) and (ꢀ) enantiomers

afford mirror-image values within an error of ꢁ9%. Over-

all, Pt^II–[6]helicenes display rather high MR values ([f]²_D³=

ꢁ6500–8500; Table 5), but lower than that of (P)-[6]carbo-

helicene ([f]²_D³= ꢁ11950).^[7e]Increasing the length of the

platinahelicene (7a!7e) results in an increase of the molar

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Enantiopure Organometallic Helicenes

FULL PAPER

Table 5. Experimental specific and molar rotations and calculated molar rotations of enantioenriched com-

rable to those recorded for the

pounds 7a,b,d²,e and of [6,7]carbohelicenes (M enantiomers).

Pt^IIcomplex 7a (see

ACTHUNTGRENNGTU ables S13

7a

7b

7d²

7e

[6]Carbohelicene

[7]Carbohelicene

and S14) and the hc of the coor-

dinated N^C ligand (568) is un-

changed upon oxidation of the

metal ion. Once again, this hc

data is satisfactorily reproduced

theoretically (Table 1), and an

[a]²_D^3[a,b,c]

[f]²_D^3[a,b,c]

[f]^[d]

ꢀ1360

ꢀ1740

ꢀ7420

–

ꢀ1050

ꢀ7570

ꢀ9900

ꢀ1590

ꢀ3860

ꢀ11950

ꢀ5900

ꢀ23500

ꢀ19152

ꢀ8550

ꢀ10800

ꢀ11445

ꢀ10281

[a] Measured in CH₂Cl₂. [b] In 8cm^ꢀ2dmol^ꢀ1and within an error of 9%. Concentrations: 1–5ꢂ10^ꢀ5m.

[c] M-(ꢀ) enantiomers. [d] TD-DFT calculated using the M enantiomer.^[18]

rotation values (Table 5). However, this enhancement is

rather weak compared to what is observed in the related

carbon series. Once again, carbo- and metallahelicene deriv-

atives behave differently. Note that the molar rotations of

diastereomers (P)-(6R,8R)-7d¹and (M)-(6R,8R)-7d²are

almost mirror-image and of the same order as the ones for

7a enantiomers.

The calculated linear response values obtained for 7a and

7e are overall in good agreement with the experimental re-

sults, with the computed values being around 10% larger

than the experimental ones (Table 5). The molar rotations

can also be numerically obtained from the rotatory strength

values over a wide range of transitions (see Ref. [7g]). The

fact that extending the number of aromatic cycles from six

to seven is less significant for metalla- than for carboheli-

cenes can be attributed to the hypsochromic effect observed

when comparing the transition of Pt^II–[6]- and Pt^II–[7]heli-

cenes (vide supra). The transitions that involve p conjuga-

tion over the entire molecule and that give rise to smaller

rotatory strengths also have an influence on the final molar

rotation value for 7e. In other words, increasing the size in

Pt–[n]helicenes changes the molecular-orbital scheme to a

greater extent than in the carbohelicene series, which do not

change their chemical structure upon homologation. Note

that the calculated molar rotations of diastereomers (P)-

(6R,8R)-7d¹and (M)-(6R,8R)-7d²do not agree well with

the experimental ones.

Scheme 4. Oxidation of Pt^II–7a,b to Pt^IV–8a,b and reverse reduction pro-

cess. i) I₂, CH₂Cl₂; ii) Zn, AcOH. Synthesis of heterochiral (M,P)-9a¹and

homochiral (P,P)-/ACTHNUTRGNEUNG

(M,M)-9a²benzoato-bridged Pt^III-dimer assemblies

from 6a or 10a. iii) PhCO₂Ag, CHCl₃/THF or acetone, RT, 12 h, Ar,

57%.

overlay of the optimized structures for complexes (M)-7a

and (M)-8a clearly shows that the presence of the iodine li-

gands does not greatly impact the helical structure of the

backbone (Figure 11).

The HPLC analysis of the Pt^IVcomplexes revealed that

the oxidation process affords enantiopure complexes (ee>

99%). This very high stereoselectivity is a crucial feature

since it allows for the investigation of the impact this chemi-

cal transformation has on the chiroptical properties of plati-

nahelicenes. The shape and intensity of the CD spectra of

Pt^IIand Pt^IVmetallahelicene derivatives are indeed differ-

ent. For example, the oxidation of (P)-(+)-7a into (P)-

(+)-8a results in a decrease of the intensity of the CD

bands at high energy (ꢂ230–250 nm) and at lower energy

(340–360 nm), and also in a sign inversion of the CD bands

at 280 nm (Figure 12). The optical rotation values are also

significantly modified upon oxidation. For example, the

molar rotation value drops from ꢀ8550 to ꢀ1500 when

(M)-(ꢀ)-7a is oxidized to (M)-(ꢀ)-8a (Table 6). In fact, ac-

cording to TD-DFT calculations, the CD spectrum of com-

plex 8a deviates from 7a in terms of the number of low-

energy excitations and the corresponding intense broadened

band near 4 eV. As can be seen in Figure 11, the intense

negative transition similar to those seen in 7a and 7e is ac-

companied by two large, positive excitations at a slightly

lower energy. The overlap of these excitations is largely re-

sponsible for the much lower CD intensity compared to

Variation of structure and properties by oxido reduction of

the metal center: One of the most important added features

provided by the metal center is probably its inherent reac-

tivity, which should allow novel original architectures in hel-

icene chemistry to be obtained and with different chiroptical

properties. Our first approach was to use the ability of

(N^C)Pt^IIion to be oxidized into Pt^IV.^[26]The enantiopure

(+)-Pt^II–[6]helicenes 7a,b were therefore reacted with

iodine in CH₂Cl₂to afford the air-stable Pt^IV–helicenes 8a,b

in good yields (60–70%; Scheme 4).^[9a]These organometallic

species give satisfactory elemental analyses and were char-

acterized by ¹H and ¹³C NMR spectroscopy. For example,

compared to 7a,b, all the proton signals of 8a,b are slightly

deshielded, as already observed in N,C-orthometalated Pt^II/

Pt^IV[26]series (Table S16 in the Supporting Information). The

crystal structure of complex 8b^[9a]shows an octahedral ge-

ometry around the Pt^IVcenter with the two iodine atoms oc-

cupying the apical positions (Figure 1). The bond lengths

and valence angles of 8b about the metal center are compa-

Chem. Eur. J. 2011, 17, 14178 – 14198

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J. Crassous, J. Autschbach, R. Rꢀau et al.

Figure 12. CD spectra of (P)-(+)- and (M)-(ꢀ)-Pt^IV-8a compared with

mirror-image CD spectra of Pt^II-7a (plain lines: (+) derivatives; dashed

lines: (ꢀ) derivatives).

Table 6. Experimental specific and molar rotations and calculated molar

rotations of enantiopure bis-assemblies (M,P)-9a /

monoplatinated M helicenes of 7a, 8a, and 10a.

¹(M,M)-9a²relative to

ACHTUNGTRENNUNG

7a 8a

(M,P)-9a¹

G

9a²

10a

ꢀ970

ꢀ6230

[a]²_D^3[a,b,c]

[f]²_D^3[a,b,c]

[f]^[d]

ꢀ1360

ꢀ170

–

ꢀ1950

ꢀ8550

ꢀ1500

ꢀ1228

ꢀ26715

ꢀ26700

ꢀ10281

ꢀ19500

[a] Measured in CH₂Cl₂. [b] In 8cm^ꢀ2dmol^ꢀ1and within an error of 5–

9%. Concentrations: 1–5ꢂ10^ꢀ5m. [c] M- or (M,M)-(ꢀ) enantiomers.

[d] TD-DFT calculated using the M enantiomers except for (M,P)-9a¹.

Table 7. Analysis of transitions 12–15 with large rotatory or oscillator

strengths in terms of MO pair contributions for complex (M)-8a; see also

Tables 8–11.

Excitation E [eV] ([nm])^[a]f^[b]

R^[c]

MO (from) MO (to) [%]

12

13

14

3.86 (321)

3.92 (316)

3.95 (314)

13.8 351.7

79.7 347.8

40.7 ꢀ623.5

117

130

122

130

122

129

130

131

134

131

134

131

132

136

19

14

28

16

26

22

13

45

15

3.99 (311)

14.7 ꢀ134.6

[a] Excitation energies have been shifted by ꢀ0.25 eV. [b] Oscillator

strengths have been multiplied by 100. [c] Rotatory strengths are in units

of 10^ꢀ40esu²cm^ꢀ2

.

with a rotatory strength above 50ꢂ10^ꢀ40cgs (one of the

aforementioned positive excitations), thus leaving eleven

calculated excitations to populate the energy range below

3.8 eV. As a consequence, and unlike complexes 7a, com-

plex 8a has a calculated weak CD band that ranges from ap-

proximately 2 to 3 eV, at which point it changes signs.

Figure 11. Top: BHLYP/SV(P) and experimental CD for complex (M)-

(ꢀ)-8a. Middle: BHLYP/SV(P) CD spectra comparison between com-

plexes (M)-7a and (M)-8a. No spectral shift has been applied. Circles

are for complex 7a and triangles are for complex 8a. Bottom: overlay of

the optimized structures for complexes 7a and 8a.

complex 7a (Figure 11). Although the calculated spectrum

does not agree as well with experiment as for complexes 7a

and 7e, it is important to note the different behavior in the

low-energy regime.

Interesting features arise from the computed CD spec-

trum (Figure 11, Table 7). Excitation 12 is the first excitation

A surprising feature revealed by the theoretical study is

that the iodine groups play an active role in the intense chi-

roptical properties of complex 8a, as can be seen from the

isosurface plots of the dominant MO pairs for the twelfth

through fourteenth excitations (Tables 8–10). The lowest

seven excitations also share one common trait, MO 131,

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Enantiopure Organometallic Helicenes

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Table 8. Isosurface plots of the two dominant MO pairs involved in the

Table 9. Isosurface plots of the two dominant MO pairs involved in the

intense (twelfth) calculated excitation in complex 8a.

intense (thirteenth) calculated excitation in complex 8a.

122

130

131

134

117

130

131

134

which develops along the I-Pt-I fragment (Table 8). Of these

seven, every MO transition pair that contributes at least

10% includes MO 131. The other MOs involved are mostly

described as p with Pt and I character. Excitation numbers

8–10 originate from similar orbitals as transitions 1–7, but

include unoccupied helicene–Pt p* MOs with little to no

contribution from the iodine groups (such as MO 132;

Table 10).

In the event that some of these transitions may be un-

physical charge-transfer excitations caused by approxima-

tions in TD-DFT, the CD spectrum was also calculated

using the CAM-B3LYP^[27]functional in a developer’s version

of the NWChem package.^[28]The range-separated exchange

in this functional should help with such problems. However,

CAM-B3LYP also produced numerous low-energy excita-

tions with similar magnitude and sign patterns. This, com-

bined with experimental evidence, would indicate that these

transitions are physical and play a role in the chiroptical

properties of this complex. Note that here again the comput-

ed molar rotation value (ꢀ1228) for (M)-(ꢀ)-8a satisfactori-

ly reproduces the experimental one (Table 6). Its magnitude

is in agreement with the CD interpretation.

Table 10. Isosurface plots of the three dominant MO pairs involved in

the intense (fourteenth) calculated excitation in complex 8a.

130

122

129

134

131

132

These results show that it is possible to modify the coordi-

nation geometry and the oxidation number of the metal

center while maintaining the helicity and enantiopurity of

metallahelicenes. Overall, the presence of a reactive Pt

center within the helicoidal p-conjugated skeleton allows for

redox-modulated structural engineering that impacts the

high chiroptical properties of this novel type of multifunc-

tional chiral molecules. Note that Pt^IV–8a,b could be re-

duced to Pt^II–7a,b by using zinc powder (purification by

flash column chromatography) without alteration of the chi-

roptical properties. Interestingly, the phosphorescence of

7a,b is completely switched off upon oxidation, thereby pro-

viding these metallahelicenes with multiple functions that

can be switched on/off (emission) or tuned (CD, molar rota-

tion). However, the Pt^II–7a,b/Pt^IV–8a,b systems cannot be

considered chiroptical switches^[29]since they lack efficient

repeatability process due to the purification needed after

each oxidation and reduction step.

Table 11. Isosurface plots of the dominant MO pair involved in the in-

tense (fifteenth) calculated excitation in complex 8a.

130

136

Synthesis of heterochiral and homochiral bis-assemblies

based on a Pt^IIIPt^IIIbonded scaffold: It is well known in or-

ꢀ

ganometallic chemistry that orthometalated group 10 com-

plexes can be transformed into bimetallic species upon reac-

tion with bidentate bridging carboxylate ligands, as, for ex-

ample,

in

bis

[Pd^II–(2-phenylpyridyl)]

and

bis-

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J. Crassous, J. Autschbach, R. Rꢀau et al.

ACHTUNGTRENNUNG ACHTUNGTRENNUNG(benzo[h]quinolinyl)] reported by Gabbai et al. and

[Pd^III–

Ritter et al., respectively.^[30]To build bis-metallahelicene

structures, this organometallic strategy was investigated with

precursor 6a. The m-chloro-bridged dimer 6a (Scheme 4)

was treated with silver benzoate (2 equiv). The reaction was

conducted at room temperature for 12 h under argon in a

mixture of CHCl₃/THF (95:5). After purification by column

chromatography, a mixture of two diastereomeric Pt^III-

dimers 9a¹and 9a²was isolated in a 9:1 molar ratio (57%

yield).^[9b]Note that these complexes are air stable. These or-

ganometallic species were characterized by multinuclear

NMR spectroscopy and X-ray diffraction studies. The ¹H

and ¹³C NMR spectroscopic data reveal that 9a¹has a C₁

symmetry and 9a²has a C₂symmetry (Table S17 in the Sup-

porting Information, Figure 13). Their ¹⁹⁵Pt NMR spectra

Figure 14. Isomerization of heterochiral 9a¹to homochiral 9a²in CDCl₃

followed by the ¹H NMR spectroscopic signals of the OMe substituents:

a) before heating, b) after heating in toluene at 808C for 1 day, c) after

2 days, and d) after 3 days.

(P,P)- and (M,M)-9a²have different helicity values (37.7

and 53.08); this is certainly because of packing, as observed

for the pinene-helicene derivatives 7d^1,2(vide supra). The

two phenylpyridine moieties of 9a²are engaged in intramo-

1

Figure 13. 400 MHz H NMR spectrum in CDCl₃at room temperature of

9a².

lecular p–p interactions (centroid–centroid distance:

III

3.464 ꢃ, Figure 14) due to the short Pt^IIIPt intermetallic

ꢀ

display singlets around d=410 ppm (Na₂PtCl₆as the refer-

III

ence), as observed for other Pt^IIIPt complexes.^[31]Single

distance (2.5788(9) ꢃ). Note that in the crystal structure, p–

p interactions and CH–p interactions that involve the me-

thoxy groups exist between the platinahelicene moieties of

two different molecules of opposite handedness, thus giving

rise to a supramolecular arrangement with heterochiral col-

umns, as depicted in Figure 15 for (P,P)-9a². These solid-

state structures, which fit nicely with the NMR spectroscopic

data recorded in solution, show that complexes 9a^1,2display

a unique helical structure that results from two metal–metal

bonded, superimposed metallohelices with either homochi-

ral or heterochiral configurations.

ꢀ

crystals of pure 9a¹were grown from an iPr₂O/CH₂Cl₂solu-

tion of this 9:1 mixture of 9a^1,2. The X-ray analysis revealed

that 9a¹consisted of two platina[6]helicenes assembled by

two bridging m²-benzoato ligands, with the N atoms having a

mutual trans arrangement (Figure 14) and with each metal

ꢀ

ion being linked to a Cl atom (Pt Cl bond lengths: 2.426(3),

2.428(3) ꢃ). The metal–metal distance (2.5952(6) ꢃ) is con-

III

sistent with a Pt^IIIPt single bond,^[31]and the metals are

ꢀ

formally d⁷Pt^IIIcenters (Figure 14, Table S15). The individu-

al Pt^III–[6]helicenes of dimer 9a¹have opposite P and M

handedness and display helicities ((49ꢁ1)8) that are compa-

rable to those of monomeric Pt^II– and Pt^IV–[6]helicenes

((54ꢁ2)8, vide supra).^[9a]The X-ray diffraction study per-

formed on single crystals shows that 9a²also features a Pt^III-

dimer scaffold supported by two m²-benzoato ligands

(Figure 14), but with the two Pt^III–[6]helices having the same

handedness (Figure 14). The two metallahelicene units in

Pt^IIIdimers are quite rare species,^[31]and the first Pt^III

dimer supported by m²-carboxylate bridging donors (i.e.,

acetato ligands) has been recently obtained by Bruce et al.

by means of ortho metalation of functionalized phenylpyri-

dines by [PtCl₂ACTHNUTRGNEUNG

(dmso)₂] in AcOH heated to reflux.^[31a]In

this case, the sulfoxide was supposed to be the oxidizing

agent.^[32]The formation of complexes 9a^1,2poses two main

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Chem. Eur. J. 2011, 17, 14178 – 14198

Enantiopure Organometallic Helicenes

FULL PAPER

be more stable than 9a¹by 0.4 kcalmol^ꢀ1, whereas computa-

tions with TZVP and the conductor-like screening model

(COSMO)^[34]using the dielectric constant of toluene (e=

2.38) placed the energy difference at 0.5 kcalmol^ꢀ1. Results

that utilize dispersion-corrected DFT (DFT+D) for geome-

try optimizations are discussed in the Supporting Informa-

tion. Given the error limits of DFT, the calculations do not

allow the definitive assignment of one derivative as being

more stable than the other. A comparative analysis of the

solid-state structures of 9a^1,2suggest that steric factors con-

tribute to the energy difference. The heterochiral arrange-

ment in 9a¹is rather compact, thereby resulting in steric

congestion as indicated by the large angle between the two

five-membered platinacycles (46.58) and the nonlinearity of

the Cl-Pt-Pt-Cl fragment (Figure 14). The homochiral as-

sembly 9a²seems to be less constrained than its heterochiral

precursor 9a¹, as indicated by the modest angle between the

two five-membered platinacycles (19.88) and the linearity of

the Cl-Pt-Pt-Cl fragment (Figure 14).^[9b]This is enforced by

Figure 15. Supramolecular arrangement in 9a². Solvent molecules and hy-

drogen atoms have been omitted for clarity.

questions. What is the origin of the Cl ligands (CHCl₃, the

Pt–m-chloro-bridged dimer 6a)? What is the oxidizing agent

(Ag⁺, air, or something else)? Firstly, the reaction of the m-

chloro-bridged dimer 6a with silver benzoate (2 equiv)

could also be conducted in acetone (Scheme 4). In this non-

chlorinated solvent, the formation of the mixture 9a^1,2(9:1

molar ratio) was also observed. Therefore, it is clear that the

Cl^ꢀatoms of 9a^1,2originate from the Pt–m-chloro-bridged

dimer 6a. In other words, the role of the Ag⁺ion is not to

abstract the Cl ligands of 6a (Scheme 4). Secondly, to eluci-

date the nature of the oxidizing agent, several experiments

were conducted. The orthometalation of 2-pyridylbenzophe-

the fact that the metric data of the [Pt₂ACHTNUGTRNE(UGN PhCO₂)₂Cl₂] core of

homochiral precursor 9a²is similar to those of the related

strain-free Pt^IIIdimer reported by Bruce et al.^[31a]The two

complexes 9a^1,2(Figure 14) are chiral. All attempts to

obtain enantioenriched samples of complexes 9a^1,2, either

by HPLC separation or by crystallization methods, failed.

However, enantiopure homochiral (P,P)- and (M,M)-9a²

were obtained according to a synthetic strategy based on the

recent work of Bruce et al.^[31a]The monomeric Pt^IIcomplex

10a that bears a dmso ligand can be easily prepared by

nanthrene 5a (Scheme 3) with [PtCl

N

treating 5a with cis-[Pt^II

ACHTNGURETNNU(G dmso)₂Cl₂] in toluene at 1108C

method of Bruce et al. by replacing acetic acid by benzoic

acid was investigated. With these experimental conditions,

no reaction occurred. In contrast, using [PtCl₂ACTHUNTRGNE(UGN dmso)₂] and

silver benzoate in a CHCl₃/THF mixture under rigorous ex-

clusion of oxygen, the formation of the 9a^1,2mixture was

observed (molar ratio 9:1). These data strongly suggest that

the oxidation of 6a into 9a^1,2(Scheme 4) is due to the Ag⁺

ions, as already previously observed in the case of monome-

tallic species.^[33]

(85% yield, Scheme 4). This derivative was fully character-

ized by multinuclear NMR spectroscopy and X-ray diffrac-

tion studies. Complex 10a (Figure 16) possesses structural

characteristics that are identical to analogous [(N^C)Pt^II-

ꢀ

ACHUTNGRENNUG ACHTUNTGREN(NUGN Pt S)=2.226 ꢃ,

(dmso)Cl] complexes^[35](for example, d

ꢀ

dH

(Pt Cl)=2.239 ꢃ,

d

U

dACHTUNGTRENNUNG(Pt N)=

2.053 ꢃ) and to the other platina(II)[6]helicenes described

in this article (for example, hc=46.28). Complex 10a dis-

plays the same properties as 7a–d, such as good solubility

and air stability, as well as similar photophysical properties

of other platinahelicenes 7a–c,e. Indeed, yellow Pt^II–[6]heli-

cene 10a displays room-temperature phosphorescence

(l^p_m^h_a^o_x^s=666 nm; t=21 ms, F=5%; Table 4). The P and M

enantiomers of 10a were separated by HPLC over Chiral-

pak IA by using hexane/ethanol (70:30) as the mobile

phase. They display molar rotations that are comparable to

the other platina[6]helicene derivatives 7a,b,d^1,2((P)-10a:

ee 93%, (M)-10a ee 90% ([a]²_D³=1100 (ꢁ5%), c=0.01 in

CH₂Cl₂, [f]_D=(7060ꢁ5)%).^[9b]Likewise, their CD spectra

well compare with those of species 7a,b,e with a strong

band at 250 nm, and a broader one centered at 350 nm and

tailing down to 440 nm (Figure 16) with slightly lower De

values (Deꢂ ꢁ75m^ꢀ1cm^ꢀ1at 250 nm and Deꢂ ꢁ55m^ꢀ1cm^ꢀ1

at 350 nm).^[9b]Indeed, the nature of the Pt ligand of platina-

helicenes has a small influence on the chiroptical properties

of these organometallic species.

Very surprisingly, heating the mixture of 9a^1,2(9:1 molar

ratio) in toluene at 808C under inert atmosphere leads to

the complete transformation of 9a¹into 9a²after 3 days.

The course of the reaction can be easily followed by

¹H NMR spectroscopy since the C₁-symmetric complex 9a¹

displays two singlets at d=3.9 and 3.8 ppm for the methoxy

groups, whereas for its C₂-symmetric isomer one signal only

is observed (¹H NMR, d=3.7 ppm; Figure 13 and

Figure 14). This experiment reveals an unexpected isomeri-

zation of a heterochiral M,P assembly of two helicene moi-

eties (9a¹) into its corresponding homochiral (P,P)/

ACTHNUTRGNE(NUG M,M)

isomer (9a²). A process of this type is unprecedented in hel-

icene chemistry, thus illustrating, once again, that metallahe-

licenes allow novel developments in this field.

DFT calculations indicate that the bimetallic derivative

9a²is more stable than its isomer 9a¹by 0.6 kcalmol^ꢀ1at

the RI-BP86/SV(P) level of theory (gas phase). Calculations

with BP86 and the larger TZVP basis set also show 9a²to

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14191

J. Crassous, J. Autschbach, R. Rꢀau et al.

tribution (74%) to the most

redshifted strongest CD band (

ꢂ350 nm,

R=ꢀ538ꢂ

10^ꢀ40esu²cm^ꢀ2, no. 9) is

a

HOMOꢀ2-to-LUMO transition

(Figure 17). In fact, all six of

the labeled transitions involve

an orbital with this s–p conju-

gation. Other transitions in-

clude origins from p systems of

a

single helicene and from

chlorine groups that have some

metal character. The unoccu-

pied orbital of MO pairs fall

into two main categories: heli-

cene p systems not conjugated

ꢀ

through the Pt Pt bond, and

Figure 16. CD spectra in CH₂Cl₂at 293 K of (P,P)-(+)-9a¹and (P)-(+)-10a (plain lines) and of their respective

enantiomers (dashed lines). X-ray crystallography structure of 10a (P isomer shown).

Cl-Pt-Pt-Cl

p

systems. Addi-

Addition of silver benzoate to Pt^IIcomplex 10a in acetone

at room temperature followed by a flash chromatography

over silica gel (AcOEt/heptane, 6:4) afforded the optically

active homochiral M,M and P,P complexes 9a²in 40% yield

Table 12. Selected transitions with large rotatory or oscillator strengths

in terms of MO-pair contributions for complex (M,M)-9a².

Excitation E [eV] ([nm]) f^[a]R^[b]

MO (from) MO (to) [%]

9

18

3.54 (350)

4.07 (305)

69.6 ꢀ538

271

273

272

269

273

272

270

269

271

274

276

280

279

275

279

280

276

275

276

71

25

16

14

26

24

14

12

31

18

16

ACTHNUTRGNEUNG

(Figure 14).^[9b]Red bis(Pt^III–[6]helicene) 9a²displays analo-

18.8 ꢀ951.4

gous emission properties to 7a–c,e (room-temperature phos-

phorescence, l^p_m^h_a^o_x^s=654 nm; t=17 ms; Table 4) but with

lower quantum yield (2.5%). The CD spectra for complexes

(P,P)-(+)- and (M,M)-(ꢀ)-9a²are mirror images

(Figure 16). The (+) enantiomer displays two intense bands,

a negative one at 240 nm and a broad positive one centered

at 350 nm. Two weak bands at low energy (440 and 490 nm)

and three weak bands at higher energy (260, 285, and

305 nm; Figure 16) are also observed. It is noteworthy that

the CD bands of Pt^III-dimer 9a²are very intense, especially

compared to those of their respective monometallic precur-

sors 10a (Figure 16). Likewise, the experimental specific and

molar rotation values for (P,P)-(+)-9a²are very high

([a]²_D³= +2060 (ꢁ5%), [f]²_D³= +28200 (ꢁ5%) (c=0.04 in

CH₂Cl₂)). For comparison, this molar rotation is three times

higher than those of monometallic Pt^II–[6]helicenes 7a,b,d^1,2

21

22

4.17 (297)

4.20 (295)

40.3 1255.8

12.3 ꢀ444.6

[a] Oscillator strengths have been multiplied by 100. [b] Rotatory

strengths are in units of 10^ꢀ40esu²cm^ꢀ2

.

tional MO contributions and isosurface plots are provided

in the Supporting Information. The fact that the two p heli-

ces interact through the s-Pt^III-Pt^IIIscaffold clearly has an

impact on the resulting chiroptical properties of these novel

homochiral superimposed bis-helicene derivatives. This is

also shown in the molar rotation values. The experimental

measurements of molar rotations compare well with the cal-

culated ones. For example, the computed (BHLYP/SV(P))

molar rotation for (M,M)-9a²is [f]_D²³=ꢀ26.7ꢂ10³(exptl

values, [f]²_D³=ꢀ27ꢂ10³, 9%; Table 6).

and 10a ([f]²_D³=6500–8500).^[9a]These data show that the su-

III

perimposition of two metallahelicenes on a Pt^IIIPt scaf-

ꢀ

fold is an efficient approach for enhancing chiroptical prop-

erties of helicene derivatives.

To gain insight into this property, theoretical studies were

conducted. The computed CD spectrum of 9a²(Figure 17)

qualitatively matches the experimental one. The description

of the intense labeled excitations in terms of MO pair con-

tributions is given in Table 12, and the isosurface plots of se-

lected MOs involved are shown in Figure 17. Of great inter-

Complex 9a¹is a (M,P)-bis

ACTHNUTRGNE(NUG helicene) derivative that is

not a meso compound since it displays a C₁symmetry due to

the trans arrangement of the two orthometalated phenylpyr-

idine moieties (Figure 14). Theoretical calculations conduct-

est, the metalla[6]helicene moieties of 9a²appear to be elec-

ed in such (M,P)-bisACTHNGUTERNNU(G helicene) derivative appeared very in-

III

tronically coupled through the Pt^IIIPt s scaffold. The oc-

formative since they revealed that within complex 9a¹, the

ꢀ

cupied MOs involved in the intense transitions in the CD

metalla[6]helicene moieties are also electronically coupled

III

through the Pt^IIIPt s scaffold, with s–p conjugation

ꢀ

spectrum result from a mixing of both metallahelicene p sys-

III

tems and Pt^IIIPt s-scaffold orbitals (see transition nos. 9,

taking place in many MOs. The RI-BP86/SV(P)-calculated

ꢀ

18, 21, and 22 in Table 12). For example, the dominant con-

CD spectrum of 9a¹is displayed in Figure 18. Intense, num-

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Chem. Eur. J. 2011, 17, 14178 – 14198

Enantiopure Organometallic Helicenes

FULL PAPER

Figure 18. Calculated CD spectrum of 9a¹at the BHLYP/SV(P) level of

theory. Labeled excitations are discussed in terms of MO pair contribu-

tions. Isosurfaces of selected MOs of (M,P)-9a¹.

Figure 17. Experimental (CH₂Cl₂) and TD-DFT-calculated CD spectra

for (M,M)-9a². The numbered excitations correspond to those with high

rotatory strength that were analyzed in detail (see Table 12 and the Sup-

porting Information). Isosurfaces of selected MOs of (M,M)-9a².

bered excitations were analyzed in terms of MO pair contri-

butions. Data for these excitations are presented in

Table 13, and isosurface plots of selected MOs are provided

in Figure 18. All six of the labeled transitions include one

common feature: a transition from an orbital that has a p

system on one helicene conjugated to a p system of the

Pt-Cl orbitals. Additional MO contributions and isosurface

plots are provided in the Supporting Information. A com-

parison of the computed CD spectra for (M,P)-9a¹and

(M,M)-9a²is shown in Figure 19. The classification of the

MOs involved in the transitions are the same for 9a¹and

9a²with one exception: for 9a²there are occupied p orbitals

ꢀ

other helicene through the Pt Pt bond. For example, in the

of both helicenes with no conjugation through the Pt Pt

HOMOꢀ2 and HOMO of 9a¹(see Figure 18), there is a

mixing of the p orbitals of the two metallahelicenes with the

s orbitals of the Cl-Pt-Pt-Cl fragment. Other transitions in-

volve the p systems of a single helicene and chlorine with

some metal character. The unoccupied orbitals involved in

the transitions are of same type as those of 9a²: helicene p

bond that strongly contribute to the CD. The computed

(BHLYP/SV(P)) molar rotation for (M,P)-9a¹is also very

high ([f]²_D³=ꢀ19.5ꢂ10³), and the shape of the calculated

CD spectrum displays very intense bands (for example, exci-

tation 9 at 358 nm, R=ꢀ536ꢂ10^ꢀ40cgs). The s–p interaction

might greatly impact the calculated chiroptical properties of

these heterochiral bis(metallahelicene) complexes, since

ꢀ

systems not conjugated through the Pt Pt bond, and Cl-Pt-

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J. Crassous, J. Autschbach, R. Rꢀau et al.

Table 13. Selected transitions with large rotatory or oscillator strengths

Experimental Section

in terms of MO-pair contributions for complex (M,P)-9a¹.

Excitation E [eV] ([nm]) f^[a]R^[b]

MO [from] MO [to] [%]

General: All experiments were performed under an atmosphere of dry

argon using standard Schlenk techniques. Commercially available re-

agents were used as received without further purification. Solvents were

freshly distilled under argon from sodium/benzophenone (tetrahydrofur-

an, diethyl ether) or from phosphorus pentoxide (pentane, dichlorome-

thane). Irradiation reactions were conducted using a Heraeus TQ 150

mercury vapor lamp. Preparative separations were performed by gravity

column chromatography on basic alumina (Aldrich, Type 5016A,

150 mesh, 58 ꢃ) or silica gel (Merck Geduran 60, 0.063–0.200 mm) in

5

3.27 (379)

3.42 (363)

3.46 (358)

4.11 (302)

85.5 ꢀ142.4

22.0 186.4

263

271

272

273

271

263

271

274

275

276

274

276

48

16

36

27

48

19

22

8

9

43.1 ꢀ536.1

58.5 ꢀ547.4

19

[a] Oscillator strengths have been multiplied by 100. [b] Rotatory

1

13

31

3.5–20 cm columns. H, C, and P NMR spectra were recorded using a

strengths are in units of 10^ꢀ40esu²cm^ꢀ2

.

Bruker AM300 instrument. The ¹H NMR spectra show characteristic

platinum satellites (¹⁹⁵Pt, I=¹= , 33.8% natural abundance); J

(Pt,H) con-

3

2

1

stants are observed. ¹⁹⁵Pt, H, and ¹³C NMR spectroscopic chemical shifts

are reported in parts per million (ppm) relative to Na₂PtCl₆or Me₄Si as

respective external standard. Assignment of proton atoms is based on

COSY experiments. Assignment of carbon atoms is based on HMBC,

HMQC, and DEPT-135 experiments. High-resolution mass spectra were

obtained using a Varian MAT 311 or ZabSpec TOF Micromass instru-

ment at CRMPO, University of Rennes 1. Elemental analyses were per-

formed by the CRMPO, University of Rennes 1. Specific rotations (in

8cm^ꢀ2g^ꢀ1) were measured in a 1 dm thermostated quartz cell using a

Jasco-P1010 polarimeter. Circular dichroism (in m^ꢀ1cm^ꢀ1) was measured

using a Jasco J-815 Circular Dichroism Spectrometer (IFR140 facility,

Universitꢀ de Rennes 1).

Triflate 2Y was prepared according to the literature.^[15]Compounds 3a–c

were prepared by Suzuki coupling between 2-bromopyridine and the ap-

propriate boronic acid according to literature procedures.^[36]Complexes

7a–c,^[9a]8a,b,^[9a]and 10a^[9b]were prepared as already described previous-

ly.

Figure 19. Comparison of the computed CD spectra for (M,P)-9a¹and

(M,M)-9a².

2-MethoxyACTHUNRTGNEUNG[(6R,8R)-5,6,7,8-tetrahydro-7,7-dimethyl-6,8-methanoisoquino-

line-3-yl]benzaldehyde (3d): Compound 1a (0.63 g, 3.5 mmol) was dis-

solved in toluene/THF (1:1; 20 mL) and mixed with triflate (ꢀ)-2Y

(1.4 g, 3 mmol), [PdACTHNUTRGNE(UNG PPh₃)₄] (140 mg, 0.04 equiv), and Na₂CO₃(10 mL,

0.5 moll^ꢀ1). The biphasic mixture was heated at reflux for 36 h. After

cooling and adding water (20 mL), the two phases were separated and

the aqueous phase was extracted with dichloromethane. The organic ex-

tracts were combined, dried over MgSO₄, and concentrated under re-

duced pressure. The residue was purified by column chromatography

(silica gel, CH₂Cl₂, then ethyl acetate) to afford 3d as a white solid

(275 mg, 55%). R_f=0.25 (heptane/ethyl acetate 8:2); ¹H NMR

(300 MHz, CDCl₃): d=10.50 (s, 1H; CHO), 8.32 (m, 2H), 8.17 (s, 1H;

H⁶), 7.54 (s, 1H; H³), 7.08 (d, J=8.6 Hz, 1H), 3.97 (s, 3H; OCH₃), 3.01

(d, J=2.5 Hz, 2H), 2.84 (t, J=5.5 Hz, 1H), 2.70 (td, J=9.6, 5.8 Hz, 1H),

2.31 (m, 1H), 1.41 (s, 3H), 1.23 (d, J=9.6 Hz, 1H), 0.65 ppm (s, 3H);

¹³C NMR (75 MHz, CDCl₃): d=189.7, 162.1, 154.0, 145.7, 145.4, 141.2,

134.3, 132.5, 126.5, 124.6, 119.5, 112.1, 55.9, 44.3, 40.1, 39.3, 32.9, 31.9,

26.0, 21.4 ppm; elemental analysis calcd (%) for C₂₀H₂₁NO₂: C 78.15, H

6.89, N, 4.56; found: C 78.05, H 6.81, N, 4.46.

huge molar rotations and CD bands are found in a complex

that has two helicene moieties of opposite handedness!

These features are unexpected and show that organometallic

helicenes exhibit very peculiar electronic properties.

Conclusion

A new family of helicene derivatives—that is, platina[n]heli-

cene derivatives—has been synthesized and their photophys-

ical and their chiroptical properties have been investigated.

These organometallic helicenes display the same general

characteristics as those of other helicene derivatives, such as

huge optical rotations and circular dichroism spectra, but

the presence of the metal center incorporated within the

helical p-conjugated skeleton provides novel properties,

such as phosphorescence at room temperature and versatile

reactivity. Exploiting metal reactivity allows the tuning of

the chiroptical properties of these organanometallic heli-

cenes by modifying the redox state of the metal (7a!8a),

and the building of original bis(metallahelicene)s in which

some of the MOs involved in transitions with intense CD

exhibit s–p conjugation of p-helicene moieties through a

metal–metal bond (9a^1,2). These results show the impact of

organometallic chemistry within helicene chemistry and

open appealing perspectives for future applications in mo-

lecular materials.

2-Methoxy-5-(1-isoquinolino)benzaldehyde (3e): Compound 1a (1 g,

5.55 mmol) was dissolved in toluene/THF (1:1, 15 mL) and mixed with

2c (905 mg, 5.55 mmol), [PdACTHNUTRGNEU(GN PPh₃)₄] (77 mg, 0.013 equiv), and Na₂CO₃

(7 mL, 1 moll^ꢀ1). The biphasic mixture was heated at reflux for 36 h.

After cooling and adding water (20 mL), the two phases were separated,

and the aqueous phase was extracted with dichloromethane. The organic

extracts were combined, dried over MgSO₄, and concentrated under re-

duced pressure. The residue was purified by column chromatography

(silica gel, ethyl acetate 50% in heptane) to afford 3e as a white solid

1

(1.28 g, 87%). R_f=0.27 (heptane/ethyl acetate 1:1); H NMR (400 MHz):

d=10.56 (s, 1H; CHO), 8.60 (d, J=5.7 Hz, 1H), 8.21 (d, J=2.4 Hz, 1H),

8.08 (dd, J=8.6 Hz, 1H), 8.00 (dd, J=8.6, 2.3 Hz, 1H), 7.91 (d, J=

8.1 Hz, 1H), 7.72 (m, 1H), 7.21 (d, J=5.7 Hz, 1H), 7.57 (m, 1H), 7.21 (d,

J=8.6 Hz, 1H), 4.02 ppm (s, 3H); ¹³C NMR (100 MHz, CDCl₃): d=189.4

(CHO), 162.1 (C_quat), 159.0 (C_quat), 142.1 (CH), 137.5 (CH), 136.9 (C_quat),

132.3 (C_quat), 130.2 (CH), 130.1 (CH), 127.5 (CH), 127.1 (CH), 127.1

(CH), 126.5 (C_quat), 124.5 (C_quat), 120.1 (CH), 112.0 (CH), 56.0 ppm

14194

www.chemeurj.org

ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2011, 17, 14178 – 14198

Enantiopure Organometallic Helicenes

FULL PAPER

(OCH₃); elemental analysis calcd (%) for C₁₇H₁₃NO₂: C 77.55, H 4.98, N

5.32; found: C 77.56, H 4.95, N 5.27.

analysis calcd (%) for C₃₁H₂₇NO: C 86.68, H 6.34, N 3.26; found: C

86.63, H 6.59, N 3.20.

1-Methoxy-4-(1-isoquinolino)benzo[g]phenanthrene (5e): The same pro-

cedure was used starting from 4e (180 mg, 0.46 mmol). Two successive

column chromatographies (silica gel, 1) ethyl acetate 50% in heptane,

2) ethyl acetate 20% in heptane) yielded 5e as a beige solid (71 mg,

40%). R_f=0.27 (ethyl acetate/heptanes 1:3); ¹H NMR (500 MHz,

CDCl₃): d=8.76 (d, J=5.7 Hz, 1H), 8.58 (d, J=8.6 Hz, 1H), 8.16 (d, J=

8.0 Hz, 1H), 7.96 (d, J=8.3 Hz, 1H), 7.90 (d, J=8.5 Hz, 1H), 7.71 (d, J=

8.0 Hz, 1H), 7.54 (d, J=8.0 Hz, 1H), 7.33 (m, 3H), 7.30 (d, J=8.0 Hz,

1H), 7.09 (ddd, J=8.0, 7.0, 1.0 Hz, 1H), 7.01 (ddd, J=8.0, 7.0, 1.0 Hz,

1H), 6.87 (ddd, J=8.5, 7.0, 1.0 Hz, 1H), 6.53 (ddd, J=8.5, 7.0, 1.0 Hz,

1H), 6.13 (d, J=8.5 Hz, 1H), 4.20 ppm (s, 3H); ¹³C NMR (125 MHz,

CDCl₃): d=162.1 (C_quat), 156.2 (C_quat), 142.5 (CH), 135.7 (C_quat), 132.1

(CH), 131.4 (C_quat), 131.3 (C_quat), 130.7 (C_quat), 129.8 (C_quat), 129.3 (C_quat),

128.8 (CH), 128.1 (CH), 127.6 (CH), 127.1 (C_quat), 126.0 (CH), 125.9

(CH), 125.8 (C_quat), 125.7 (CH), 125.6 (CH), 125.5 (CH), 125.3 (CH),

124.9 (C_quat), 124.7 (CH), 123.8 (CH), 121.0 (CH), 119.3 (CH), 105.8

(CH), 56.0 ppm (OCH₃); elemental analysis calcd (%) for C₂₈H₁₉NO: C

87.25, H 4.97, N 3.63; found: C 87.18, H 5.01, N 3.52.

1-(2-Naphthyl)-2-{2-methoxy-5-[(6R,8R)-5,6,7,8-tetrahydro-7,7-dimethyl-

6,8-methanoisoquinoline-3-yl]phenyl}ethylene (4d): 2-Naphthylmethyltri-

phenylphosphonium bromide (1.45 g, 3.0 mmol) was suspended in dry

THF (40 mL) under argon and cooled to ꢀ788C. After addition of n-bu-

tyllithium (1.6n) in hexanes (2.3 mL, 3.7 mmol), the obtained red mix-

ture was stirred for 5 min at ꢀ788C and then for 30 min at room temper-

ature. A solution of 3d (1.02 g, 3.33 mmol) in dry THF (20 mL) was

added dropwise to the suspension and cooled again to ꢀ788C. The

orange heterogeneous mixture was stirred at ꢀ788C for 5 min, then at

room temperature for 2 h. After filtration over a Celite pad, the colorless

filtrate was evaporated under reduced pressure to yield a white solid. Pu-

rification by column chromatography (silica gel, 20% ethyl acetate in

heptane) afforded 4d as a cis and trans isomeric mixture (675 mg, 52%).

¹H NMR (300 MHz, CDCl₃): d=8.28 (d, J=18.7 Hz, 1H), 8.09 (s, 1H),

8.00–7.34 (m, 16H), 7.06–6.95 (m, 1.6H), 6.87 (s, 2H), 6.80 (s, 1H), 3.99

(s, 1.5H), 3.87 (s, 3.5H), 3.77 (s, 1.8H), 3.07 (s, 1H), 2.88 (t, J=4.9 Hz,

0.5H), 2.77 (t, J=4.9 Hz, 1.5H), 2.66 (s, 3H), 2.36 (m, 0.5H), 2.21 (m,

0.5H), 1.87 (m, 1.5H), 1.82–1.73 (m, 0.5H), 1.46 (s, 1.5H), 1.38 (s, 3H),

1.29 (d, J=9.2 Hz, 0.6H), 1.14 (d, J=9.5 Hz, 1H), 0.72 (s, 1.5H),

0.58 ppm (s, 3H); ¹³C NMR (75 MHz, CDCl₃): d=157.6, 155.2, 145.7,

145.5, 145.2, 145.0, 140.7, 140.4, 135.4, 133.8, 133.5, 133.0, 132.6, 132.0,

130.5, 129.6, 128.5, 128.2, 128.0, 127.9, 127.7, 127.5, 127.4, 127.1, 126.9,

126.6, 126.5, 126.2, 126.0, 125.9, 125.75, 124.9, 123.8, 123.7, 119.5, 111.0,

67.0, 55.6, 44.4, 44.2, 40.1, 40.00, 39.40, 39.3, 33.0, 32.5, 32.0, 31.8, 26.1,

26.0, 25.7, 22.4, 21.5, 21.4, 14.1 ppm; elemental analysis calcd (%) for

C₃₁H₂₉NO: C 86.27, H 6.77, N 3.25; found: C 85.67, H 6.03, N 3.31.

(M)- and (P)-[3-{4’-(benzo[g]phenanthrenyl)}-{(6R,8R)-5,6,7,8-tetrahy-

dro-7,7-dimethyl-6,8-methanoisoquinolineato-N,C^3’}]platinum acetylacet-

onate (7d^1,2): K

₂ACHTUNTRGNE[NUG PtCl₄] (50 mg, 0.12 mmol) was added to a solution of

ligand 5d (50 mg, 0.11 mmol) in ethoxyethanol (9 mL) and water (3 mL).

The suspension was gently warmed with stirring until all the platinum

salt dissolved. It was then heated at reflux for 16 h to yield a dark green

suspension. Water (10 mL) was then added to the cooled suspension. The

green solid was filtered and dried in air. It was then dissolved in dichloro-

methane, dried on MgSO₄, and the solution was concentrated under re-

1-(2-Naphthyl)-2-[2-methoxy-5-(1-isoquinolino)phenyl]ethylene (4e): 2-

Naphthylmethyltriphenylphosphonium bromide (583 mg, 1.33 mmol) was

suspended in dry THF (30 mL) under argon and cooled to ꢀ788C. After

addition of n-butyllithium (1.6n) in hexanes (1 mL, 1.59 mmol), the ob-

tained red mixture was stirred for 5 min at ꢀ788C, and then for 30 min at

room temperature. A solution of 3e (350 mg, 1.33 mmol) in dry THF

(10 mL) was added dropwise to the suspension and cooled again to

ꢀ788C. The orange heterogeneous mixture was stirred at ꢀ788C for

5 min then at room temperature for 2 h. After filtration over a Celite

pad, the colorless filtrate was evaporated under reduced pressure to yield

a white solid. Purification by column chromatography (silica gel, 20%

ethyl acetate in heptane) afforded 4e as a cis and trans isomeric mixture

(388 mg, 76%). ¹H NMR (300 MHz, CDCl₃): d=8.67 (d, J=4.8 Hz,

0.05H; diast. A), 8.51 (d, J=5.6 Hz, 1H; diast. B), 8.25 (d, J=8.7 Hz,

0.05H; diast. A), 8.08 (d, J=2.4 Hz, 0.05H; diast. A), 7.84–7.67 (m,

6.75H), 7.49–7.37 (m, 7.5H), 7.16 (d, J=8.5 Hz, 1H; diast. B), 6.44 (m,

1H; diast. B), 4.04 (s, 0.15H; OCH₃, diast. A), 3.98 ppm (s, 3H; OCH₃,

diast. B); ¹³C NMR (75 MHz, CDCl₃): d=diast. A was not observed,

159.9, 157.9, 142.0, 136.7, 135.1, 133.6, 132.5, 132.1, 132.0, 130.8, 130.8,

129.7, 128.1, 128.1, 127.6, 127.4, 127.2, 126.8, 126.6, 126.4, 126.1, 125.9,

125.9, 119.4, 111.2, 55.8 ppm; elemental analysis calcd (%) for C₂₈H₂₁NO:

C 86.79, H 5.46, N 3.61; found: C 85.87, H 5.31, N 3.48.

duced pressure to yield the crude m-chloro-bridged dimer [(C^N)Pt

ACHTUNGTRENNUNG(m-

Cl)₂Pt(C^N)] as yellow solid. 2,4-Pentadione (35 mL, 0.35 mmol,

A

a

3 equiv) and Na₂CO₃(105 mg, 1 mmol, 9 equiv) were added to a solution

of crude dimer in ethoxyethanol (5 mL). The reaction mixture was

heated at reflux for 2 h and then concentrated under reduced pressure.

Purification by flash chromatography over silica gel using dichlorome-

thane as the eluant yielded 7d^1,2as a yellow solid (52 mg, 65%). Elemen-

tal analysis calcd (%) for C₃₆H₃₃NO₃Pt: C 59.83, H 4.60, N 1.94; found: C

59.75, H 4.50, N 1.97. Pure diastereomers (P)-(6R,8R)-7d¹and (M)-

(6R,8R)-7d²could be obtained with more than 99% enantiomeric excess-

es (ee values) by using Chiralpak IB as the chiral stationary phase and

hexane/ethanol/chloroform (85:5:10) as the mobile phase.

Complex (P)-(6R,8R)-7d¹: ee: 99%; [a]²_D³= +908 (ꢁ5%) (c=0.01 in

1

CH₂Cl₂), [f]_D= +6560 (ꢁ5%); H NMR (300 MHz, CDCl₃): d=8.47 (d,

J=8.5 Hz, 1H), 8.39 (d, J=8.7 Hz, 1H), 8.37 (s, 1H; ¹⁹⁵Pt satellites: ³J-

AHCTUNGTRENNUNG

(Pt,H)=40 Hz, H⁶), 7.87 (m, 2H), 7.75 (d, J=8.5 Hz, 1H), 7.46 (s, 1H;

ACTHNUTRGNEUNG

¹⁹⁵Pt satellites: ³J(Pt,H)=42 Hz, H^2’), 7.37 (t, J=7.4 Hz, 1H), 7.11 (t, J=

7.5 Hz, 1H), 6.15 (s, 1H), 5.57 (s, 1H; H_acac), 4.20 (s, 3H; OCH₃), 2.77 (t,

J=5.2 Hz, 1H), 2.67–2.49 (m, 1H), 2.29 (d, J=16.6 Hz, 1H), 2.08 (m,

6H), 1.98 (m, 2H), 1.38–1.15 (m, 4H), 0.98 (d, J=9.5 Hz, 1H), 0.42 ppm

(s, 3H); ¹³C NMR (75 MHz, CDCl₃): 185.9 (C_quat), 184.2 (C_quat), 166.4

(C_quat), 154.7 (C_quat), 145.2 (C_quat), 143.6 (C_quat), 141.2 (CH), 138.3 (C_quat),

134.3 (C_quat), 132.2 (C_quat), 131.5 (C_quat), 131.2 (C_quat), 128.6 (CH), 127.6

(CH), 127.3 (CH), 126.43 (CH), 126.1 (C_quat), 125.14 (CH), 123.8 (C_quat),

123.4 (CH), 121.9 (CH), 121.8 (CH), 107.9 (CH), 102.7 (CH), 55.8

(OCH₃), 44.2 (CH), 39.5 (C_quat), 39.2 (CH), 32.6 (CH₂), 31.7 (CH₂), 28.3

(CH₃), 27.3 (CH₃), 25.8 (CH₃), 21.1 ppm (CH₃).

1-Methoxy-4-[(6R,8R)-5,6,7,8-tetrahydro-7,7-dimethyl-6,8-methanoiso-

quinoline-3-yl]benzo[g]phenanthrene (5d): The mixture of cis- and trans-

4d (420 mg, 0.97 mmol) was dissolved in toluene (700 mL) and I₂(25–

35 mg) was added. The solution was irradiated for one night using a Her-

aeus TQ 150 mercury vapor lamp. The solvent was evaporated under re-

duced pressure and the residue was purified by column chromatography

(silica gel, heptane/ethyl acetate 80:20) to afford 5d as a yellow oil

(272 mg, 65%). R_f=0.23 (heptane/ethyl acetate 8:2); ¹H NMR

(300 MHz, CDCl₃): d=8.32–8.21 (m, 1H), 8.01 (d, J=8.2 Hz, 1H), 7.81

(d, J=8.41 Hz, 2H), 7.64 (m, 3H), 7.48 (m, 1H), 7.21–7.13 (m, 1H),

7.02–6.86 (m, 2H), 6.85–6.65 (m, 1H), 5.93–5.39 (m, 1H), 3.94 (s, 3H),

2.48 (s, 1H), 2.30 (s, 1H), 2.20–1.94 (m, 1H), 1.93–1.51 (m, 3H), 1.11 (s,

5H), 0.68 (m, 2H), 0.26 (s, 1H), 0.00 ppm (s, 2H); ¹³C NMR (75 MHz,

CDCl₃): d=155.7, 155.6, 145.5, 139.6, 132.6, 131.7 (ꢂ2), 131.5, 130.6,

130.4, 130.2, 130.1, 129.5, 129.1, 127.8, 127.7, 126.3, 126.2, 126.0, 125.9, (ꢂ

2), 125.8, 125.5, 124.9, 124.5, 124.4, 123.8, 129.9 (ꢂ2), 105.4, 56.0, 44.3,

44.1, 39.8, 39.2, 38.9, 32.2, 31.7, 26.0, 22.3, 21.7, 21.3, 14.1 ppm; elemental

Complex (M)-(6R,8R)-7d²: ee: 99%; [a]²_D³=ꢀ1047 (ꢁ5%) (c=0.01 in

CH₂Cl₂), [f]_D=ꢀ7570 (ꢁ5%); ¹H NMR (300 MHz, CDCl₃): d=8.47 (d,

J=8.5 Hz, 1H), 8.39 (d, J=7.2 Hz, 1H), 8.38 (s, 1H; ¹⁹⁵Pt satellites: ³J-

AHCTUNGTRENNUNG

(Pt,H)=40 Hz, H⁶), 7.87 (d, J=5.6 Hz, 3H), 7.75 (d, J=8.5 Hz, 1H), 7.46

(s, 1H; ¹⁹⁵Pt satellites: ³J

7.28 (s, 1H), 7.10 (dd, J=11.2, 4.18 Hz, 1H), 6.14 (s, 1H), 5.57 (s, 1H;

_acac), 4.20 (s, 3H; OCH₃), 2.76 (t, J=5.27 Hz, 1H), 2.60–2.46 (m, 1H),

(Pt,H)=46 Hz, H^2’), 7.33 (t, J=7.41 Hz, 1H),

ACHTUNGTRENNUNG

H

2.35–2.20 (m, 1H), 2.13–1.95 (m, 6H), 1.49–1.08 (m, 4H), 0.95 (d, J=

9.71 Hz, 1H), 0.59 ppm (s, 3H); ¹³C NMR (75 MHz, CDCl₃): d=185.99

(C_quat), 184.22 (C_quat), 166.54 (C_quat), 154.63 (C_quat), 145.29 (C_quat), 143.2

(CH), 141.27 (C_quat), 138.31 (C_quat), 134.34 (C_quat), 132.21 (C_quat), 131.42

(C_quat), 131.3 (C_quat), 128.66 (CH), 127.6 (CH), 127.4 (CH), 126.4 (CH),

Chem. Eur. J. 2011, 17, 14178 – 14198

ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

14195

J. Crassous, J. Autschbach, R. Rꢀau et al.

126.1 (C_quat), 125.4 (CH), 125.2 (CH), 123.8 (C_quat), 123.7 (C_quat), 123.43

(CH), 122.13 (CH), 121.93 (CH), 107.8 (CH), 102.7 (CH), 55.8 (OCH₃),

44.2 (CH), 39.6 (CH), 39.2 (C_quat), 32.58 (CH₂), 31.7 (CH₂), 28.3 (CH₃),

27.3 (CH₃), 25.9 (CH₃), 21.3 ppm (CH₃).

(86 MHz, CD₂Cl₂): d=410.80 ppm (large); UV/Vis (CH₂Cl₂, c=4.55ꢂ

10^ꢀ5m): l (e)=304 (31500), 329 (29000), 385 (9900), 404 (9600), 425

(7700), 520 nm (370m^ꢀ1cm^ꢀ1); elemental analysis calcd (%) for

C₆₂H₄₂Cl₂N₂O₆Pt₂: C 54.27, H 3.09, N 2.04; found : C 54.04, H 3.01 N

1.98.

[1-ACHTUNGTRENNUNG{4’-ACHTUNGTRENNUNG

(Benzo[g]phenanthrenyl)}isoquinolinato-N,C^3’]platinum acetylacet-

Complex (ꢁ)-(P,P)- and -(M,M)-9a²: Complex (P,M)-9a¹(0.100 g,

0.057 mmol) was stirred in toluene (20 mL) at 808C under argon for 3 d.

The solvent was then removed by evaporation and column chromatogra-

phy over silica gel (heptane/EtOAc 6:4, R_f=0.7) yielded 9a²(0.080 g,

90%) as a red-orange solid. ¹H NMR (400 MHz, CDCl₃): d=8.67 (dd,

J=8.8, 1.1 Hz, 4H), 8.12 (d, J=8.5 Hz, 2H), 8.04 (dd, J=5.90, 1.2 Hz,

onate ((ꢁ)-7e): Using the same procedure as 7d, 7e was obtained as a

1

red solid. Yield (two steps): 58%; H NMR (500 MHz, CDCl₃): (cycles A

and B are (5’–8’) and (5–8), respectively; see Scheme 3) d=8.80 (d, J=

3

6.4 Hz, 1H; ¹⁹⁵Pt satellites: J

(Pt,H)=33 Hz, H³), 8.35 (d, J=8.2 Hz, 1H;

H^12’), 8.26 (d, J=8.5 Hz, 1H; cycle A), 7.79 (d, J=8.5 Hz, 1H; H^11’), 7.75

(d, J=8.5 Hz, 1H; H^{9’ or 10’}), 7.60 (d, J=8.5 Hz, 1H; H^{9’ or 10’}), 7.50 (s, 1H;

2H; ¹⁹⁵Pt satellites: ³J

(m, 6H), 7.18–7.13 (m, 2H), 6.83 (ddd, J=8.4, 6.9, 1.3 Hz, 2H), 6.66 (s,

2H; ¹⁹⁵Pt satellites: ³J(Pt,H)=37 Hz, H^2’), 6.02 (ddd, J=7.4, 5.9, 1.5 Hz,

(Pt,H)=33 Hz, H⁶), 7.75–7.62 (m, 10H), 7.62–7.52

ACHTUNGTRENNUNG

¹⁹⁵Pt satellites: ³J(Pt,H)=39 Hz, H^2’), 7.38 (d, J=7.6 Hz, 1H; cycle A),

7.28 (d, 1H; cycle B), 7.17 (d, J=6.4 Hz, 1H; H⁴), 7.07 (m, 1H; cycle B),

6.93 (m, 1H; cycle A), 6.81 (d, J=8.5 Hz, 1H; cycle B), 6.78 (m, 1H;

AHCTUNGTRENNUNG

ꢀ

2H; H⁵), 5.76–5.69 (ddd, J=8.8, 7.3, 1.5 Hz, 2H; H⁴), 5.28 (dd, J=8.4,

1.0 Hz, 2H; H³), 3.67 ppm (s, 6H; OCH₃); ¹³C NMR (101 MHz, CDCl₃):

d=183.4 (C), 163.4 (C), 156.5 (C), 147.0 (CH), 136.5 (C), 134.2 (CH),

133.8 (CH), 132.0 (C), 131.3 (CHꢂ2), 131.1 (C), 129.8 (C), 128.8 (C),

128.6 (CH), 128.5 (CHꢂ2), 127.1 (CH), 125.9 (CH), 125.8 (CHꢂ2), 125.6

(CH), 124.8 (CH), 124.0 (C), 123.4 (C), 121.9 (CH), 120.6 (CH), 118.4

(CH), 118.3 (C), 105.3 (CH), 102.2 (C), 55.7 ppm (CH); ¹⁹⁵Pt NMR

(86 MHz, CD₂Cl₂): d=410.04 ppm; UV/Vis (CH₂Cl₂, c=4.85ꢂ10^ꢀ5m): l

(e)=314 (36500), 334 (35000), 388 (11700), 411 (10600), 523 nm

(450m^ꢀ1cm^ꢀ1); elemental analysis calcd (%) for C₆₂H₄₂Cl₂N₂O₆Pt₂: C

54.27, H 3.09, N 2.04; found: C 53.98, H 2.98, N 2.03.

cycle A), 6.40 (m, 1H; cycle B), 5.58 (s, 1H; H C acac), 4.22 (s, 3H;

OCH₃), 2.12 (s, 3H; CH₃acac), 2.10 ppm (s, 3H; CH₃acac); ¹³C NMR

(125 MHz, CDCl₃): d=186.2 (C=O), 184.3 (C=O), 171.5 (C_quat), 155.9

(C_quat), 144.6 (C_quat), 138.4 (CH³), 136.2 (C_quat), 134.3 (C_quat), 131.9 (C_quat),

131.3 (C_quat), 131.0 (C_quat), 130.3 (CH^B), 128.6 (C_quat), 127.5 (CH^{9’ 10’}),

or

126.8 (CH^A), 126.6 (C_quat), 126.3 (CH^11’), 125.7 (CH), 125.6 (C_quat), 125.5

(CH^B), 125.0 (CH^B), 125.0 (C_quat), 124.8 (CH^A), 124.4 (CH^A), 123.4 (CH^9’

^10’), 123.3 (C_quat), 121.4 (CH^12’), 117.4 (CH⁴), 107.0 (CH^2’), 102.8 (CH

or

acac), 56.0 (OCH₃), 28.4 and 27.3 ppm (CH₃acac); elemental analysis

calcd (%) for C₃₂H₂₅NO₃Pt: C 58.40, H 3.71, N 2.06; found: C 57.81, H

3.62, N 1.99.

Complexes (P,P)-(+)-9a²and (M,M)-(ꢀ)-9a²: Silver benzoate (10 mg,

(P)-(+)- and (M)-(ꢀ)-7e were resolved using Chiralcel OD as the chiral

stationary phase and hexane/2-PrOH (9:1) as the mobile phase (ee

values: 99.5 and 98%, respectively). (P)-(+)-7e: ee: 99.5%; [a]²_D³= +

1320 (ꢁ9%) (c=0.01 in CH₂Cl₂), [f]_D=8940 (ꢁ9%). (M)-(ꢀ)-7e: ee:

98%; [a]²_D³=ꢀ1590 (ꢁ9%) (c=0.01 in CH₂Cl₂), [f]_D=ꢀ10800 (ꢁ9%).

0.046 mmol) was added to

a solution of complex 10a (30 mg,

0.046 mmol) in a degassed mixture of acetone (30 mL). After stirring at

room temperature under argon for 16 h, the solvent was removed by

evaporation. Column chromatography over silica gel (heptane/EtOAc

6:4, R_f=0.7) yielded 9a²(12 mg, 39%) as a red-orange solid that con-

Reduction of Pt^VI-8a to Pt^II-7a: Zn dust (2.2 mg, 2 equiv) was added to a

solution of (ꢁ)-8a (15 mg, 17 mmol) in acetic acid (2 mL). After stirring

for one night, the crude mixture was poured into water and extracted

with CH₂Cl₂, then dried over MgSO₄. Purification by flash chromatogra-

phy over silica gel using dichloromethane as the eluent yielded (ꢁ)-7a as

an orange solid (10 mg, 90%).

1

tained 10% of 2b (as evidenced by H NMR spectroscopy).

The same procedure starting from complex (P)-(+)-10a (93% ee)

(20 mg, 0.031 mmol) using chromatography over silica gel (heptane/

EtOAc 8:2) yielded (P,P)-(+)-9a²(9 mg, 40%), [a]_D²³= +2060ꢁ9%,

[f]_D= +28200 (ꢁ9%) (c=0.04 in CH₂Cl₂).

The same procedure starting from complex (M)-(ꢀ)-10a (ee 90%)

(20 mg, 0.031 mmol) using chromatography over silica gel (heptane/

EtOAc 8:2) yielded (M,M)-(ꢀ)-9a²(8 mg, 36%), [a]_D²³=ꢀ1950ꢁ9%,

[f]_D=ꢀ26715ꢁ9% (c=0.04 in CH₂Cl₂).

Mixture of 9a¹and 9a²

ACHTUNGTRENNUNG(90:10): Silver benzoate (0.030 g, 0.50 mmol) was

added to a solution of m-chloro-bridged complex 6a (0.100 g, 0.10 mmol)

in a degassed mixture of CHCl₃/THF (30:1) (31 mL). After stirring at

room temperature under argon for 16 h, the solvent was removed by

evaporation. Column chromatography over silica gel (heptane/EtOAc

6:4, R_f=0.7) yielded 9a¹(0.090 g, 57%) as a red-orange solid that con-

tained 10% of 9a¹(as evidenced by H NMR spectroscopy).

1

Complex (P,M)-9a¹: Single crystals of pure 9a¹were grown by slow evap-

Acknowledgements

oration of an iPr₂O/CH₂Cl₂solution. ¹H NMR (400 MHz, CDCl₃): d=

ACTHNUTRGNEUNG

8.79 (dd, J=5.9, 1.3 Hz, 1H; ¹⁹⁵Pt satellites: ³J(Pt,H)=30 Hz, H⁶⁽¹⁾), 8.73

This work is supported by the Ministꢄre de lꢅEducation Nationale, de la

Recherche et de la Technologie, the Centre National de la Recherche

Scientifique (CNRS), the Rꢀgion Bretagne, Rennes Mꢀtropole, and the

National Science Foundation (CHE 0447321).

(d, J=8.4 Hz, 1H), 8.68 (ddd, J=14, 8.3, 1.3 Hz, 4H), 8.40 (d, J=

8.55 Hz, 1H), 8.29 (d, J=8.6 Hz, 1H), 8.20 (dd, J=6, 1.3 Hz, 1H; ¹⁹⁵Pt

ACHTUNGTRENNUNG

satellites: ³J(Pt,H)=30 Hz, H⁶⁽²⁾) 7.94 (d, J=8.6 Hz, 1H), 7.81 (d, J=

8.6 Hz, 1H), 7.75 (d, J=8.2 Hz, 1H), 7.73–7.64 (m, 6H), 7.62 (dd, J=8.1,

0.9 Hz, 1H; H^8’(2)), 7.60–7.52 (m, 4H), 7.24 (ddd, J=8.4, 6.9, 1.6 Hz, 1H),

7.18–7.12 (ddd, J=8.3, 6.8, 1.5 Hz, 1H), 6.92 (s, 1H; ¹⁹⁵Pt satellites: ³J-

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ACHTUNGTRENNUNG ACHTUGNTREN(NUGN Pt,H)=30 Hz,

(Pt,H)=30 Hz, H^2’(1)), 6.90 (s, 1H; ¹⁹⁵Pt satellites: ³J

H^2’(2)), 6.84–6.76 (m, 2H; H^7’(2)and H⁵⁽¹⁾), 6.65 (ddd, J=8.6, 7.5, 1.5 Hz,

1H; H⁴⁽¹⁾), 6.29 (d, J=8.50 Hz, 1H; H^5’(2)), 5.97 (ddd, J=7.5, 6, 1.6 Hz,

1H; H⁵⁽²⁾), 5.91–5.79 (m, 3H; H⁴⁽²⁾, H³⁽¹⁾, and H³⁽²⁾), 4.33 (ddd, J=8.3,

(2)

7.1, 1.4 Hz, 1H; H^6’(2)), 3.94 (s, 3H; OCH₃⁽¹⁾), 3.80 ppm (s, 3H; OCH₃

)

((1) and (2) denote the two helices respectively deshielded and shielded

for partial assignment); ¹³C NMR (101 MHz, CDCl₃): d=183.4 (C_quat),

183.0 (C_quat), 182.8 (C_quat), 164.7 (C_quat), 156.8 (C_quat), 155.7 (C_quat), 147.3

(CH), 146.8 (CH), 136.6 (CH), 135.5 (CH), 133.9 (CH), 133.7 (CH),

133.1 (C_quat), 132.5 (C_quat), 131.4 (CH), 131.3 (C_quat), 131.2 (3ꢂCH), 130.8

(C_quat), 130.4 (C_quat), 129.4 (CH), 129.3 (CH), 129.0 (C_quat), 128.9 (CH),

128.6 (C_quat), 128.4 (3ꢂCH), 127.6 (CH), 127.1 (CH), 127.0 (CH), 126.9

(C_quat), 126.8 (CH), 126.4 (CH), 126.2 (CH), 125.8 (CH), 125.5 (CH),

125.4 (CH), 125.2 (CH), 124.8 (C_quat), 124.5 (C_quat), 124.3 (C_quat), 123.9

(CH), 123.5 (CH), 123.2 (CH), 121.8 (CH), 12.1.5 (CH), 119.7 (CH),

118.7 (CH), 106.3 (CH), 55.98 (OCH₃), 55.54 ppm (OCH₃); ¹⁹⁵Pt NMR

14196

www.chemeurj.org

ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2011, 17, 14178 – 14198

Enantiopure Organometallic Helicenes

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Received: June 18, 2011

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Published online: November 3, 2011

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Article Doi

Multifunctional and reactive enantiopure organometallic helicenes: Tuning chiroptical properties by structural variations of mono- and bis(platinahelicene)s

DOI: 10.1002/chem.201101866

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