Synthesis and chiroptical properties of organometallic complexes of helicenic N-heterocyclic carbenes

Article abstract of DOI:10.1002/chir.23143

Novel [4, 6]helicenes (4a,b) bearing a fused imidazolium unit have been prepared from [4, 6]helicene-2,3-di-n-propyl-amines 3a,b. The in situ formation of N-heterocyclic carbene (NHC) derivatives followed by their complexation to iridium(I) or rhodium(I) gave access to complexes 1a, 1′a, and 1b, containing mono-coordinated helicene-NHC, chloro and COD (COD = 1,5-cyclooctadiene) ligands. Ir and Rh complexes 1a and 1′a were characterized by X-ray crystallography. HPLC and NMR analyses showed that Ir(I) complex 1b existed as a mixture of two diastereomeric complexes corresponding to enantiomeric pairs M-(?)/P-(+)-1b¹ and M-(?)/P-(+)-1b² which differ by the position of COD through space. The chiroptical properties (electronic circular dichroism and optical rotation) of the four stereoisomers were measured. These complexes were also tested as catalysts in a transfer hydrogenation reaction.

Full text of DOI:10.1002/chir.23143

Received: 4 September 2019
DOI: 10.1002/chir.23143
Revised: 7 October 2019
Accepted: 9 October 2019
S P E C I A L I S S U E A R T I C L E
Synthesis and chiroptical properties of organometallic
complexes of helicenic N‐heterocyclic carbenes
Nesrine Hafedh1,2 | Ludovic Favereau1 | Elsa Caytan1 | Thierry Roisnel1 |
Marion Jean3 | Nicolas Vanthuyne3 | Faouzi Aloui2 | Jeanne Crassous1
1 Intitut des Sciences Chimiques de
Abstract
Rennes, ISCR‐UMR 6226, Rennes, France
Novel [4, 6]helicenes (4a,b) bearing a fused imidazolium unit have been pre-
pared from [4, 6]helicene‐2,3‐di‐n‐propyl‐amines 3a,b. The in situ formation
of N‐heterocyclic carbene (NHC) derivatives followed by their complexation
to iridium(I) or rhodium(I) gave access to complexes 1a, 1′a, and 1b, contain-
ing mono‐coordinated helicene‐NHC, chloro and COD (COD = 1,5‐
cyclooctadiene) ligands. Ir and Rh complexes 1a and 1′a were characterized
by X‐ray crystallography. HPLC and NMR analyses showed that Ir(I) complex
1b existed as a mixture of two diastereomeric complexes corresponding to
enantiomeric pairs M‐(−)/P‐(+)‐1b1 and M‐(−)/P‐(+)‐1b2 which differ by the
position of COD through space. The chiroptical properties (electronic circular
dichroism and optical rotation) of the four stereoisomers were measured. These
complexes were also tested as catalysts in a transfer hydrogenation reaction.
2 Laboratory of Asymmetric Organic
Synthesis and Homogenous Catalysis
(UR11ES56), Faculty of Sciences, Avenue
of environment, University of Monastir,
Monastir, Tunisia
3 Aix Marseille Université, Centrale
Marseille, CNRS, iSm2 UMR 7313,
Marseille, France
Correspondence
Jeanne Crassous, Intitut des Sciences
Chimiques de Rennes, ISCR‐UMR 6226,
F‐35000 Rennes, France.
Email: jeanne.crassous@univ‐rennes1.fr
Funding information
Agence Nationale de la Recherche, Grant/
Award Number: 16‐CE07‐0019‐01‐HEL‐
NHC; Campus France; Chirafun CNRS
network
KEYWORDS
chiroptical properties, helicenes, iridium, N‐heterocyclic carbenes, transfer hydrogenation
1 | INTRODUCTION
molecules. During the last two decades, organometallic
complexes based on N‐heterocyclic carbenes (NHCs)
The helical topology of helicenes combined with their
extended π‐conjugation give appealing large‐magnitude
chiroptical properties such as high optical rotation
(OR) values and intense electronic circular dichroism
(ECD) spectra.1 Since several years, our group has been
investigating the preparation of chiral organometallic
complexes bearing helicenic ligands.2,3 These com-
pounds display attractivity for many applications, such
as in optoelectronic devices,4,5 chiroptical switches,6
and asymmetric catalysis.7,8 The interest of these
helicenic organometallic derivatives originates from
the association of the inherent properties of the
helicene with those of the transition metals (controlled
coordination geometry, redox activity, strong spin‐orbit
coupling, strong emission and absorption, reactivity),
giving access to original chiral multifunctional
have experienced a boom due to their important appli-
cations in catalysis,9-11 in medicinal chemistry,12,13 and
in molecular materials.14,15 For these reasons, we have
been recently investigating the development of molecu-
lar architectures associating helicenes, NHCs, and
organometallic chemistry, in order to develop a new
family of chiral complexes, namely, organometallic
NHC‐helicene complexes (see Figure 1, A‐C).16-18 This
strategy enabled us to create novel chiral molecular
materials with unprecedented photophysical properties
and potential applications in enantioselective organo-
metallic catalysis.19,20 In this article, we present novel
[4, 6]helicenic imidazolium salts (4a,b) and their corre-
sponding helicenic NHC ligands mono‐coordinated to
iridium(I) (1a,b) and rhodium(I) (1′a) complexes. We
describe their synthesis and (chir)optical properties.
Chirality. 2019;1–9.
wileyonlinelibrary.com/journal/chir
© 2019 Wiley Periodicals, Inc.
1

2
HAFEDH ET AL.
FIGURE 1 Chemical structures of formerly reported iridium complexes coordinated to NHCs bearing a (A) [6]helicene,15 (B) [5]helicene,16
and (C) [4]helicene17 unit, respectively. Chemical structures of the novel iridium complexes 1a,b and rhodium complex 1′a reported herein
The catalytic activity of 1b in the transfer hydrogena-
University, on an Agilent Technologies 1260 Infinity with
Igloo‐Cil ovens, using CD‐2095 as circular dichroism
detector. The conditions for HPLC separations are
described in the Supporting Information. The starting
precursors 2a,b were prepared according to literature pro-
cedures.20 Complexes 1a,b and 1′a were prepared accord-
ing to Scheme 1. Single crystals were grown by slow
evaporation of n‐pentane into a dichloromethane solu-
tion of the compounds.
tion reaction21 was also examined.
2 | MATERIALS AND METHODS
2.1 | General
THF was freshly distilled under argon from
sodium/benzophenone. Heptane refers to a mixture of
isomers. All commercially available chemicals were used
without further purification. Column chromatography
purifications were performed over silica gel (Acros,
0.063‐0.200 mm). NMR spectra were recorded at room
temperature on a Bruker AV III 400 MHz equipped with
a tunable BBFO probe and a Bruker Avance I 500 MHz
fitted with a TCI cryoprobe (Biosit platform, Université
2.1.1 | N,N′‐dipropyl[4]helicene‐2,3‐
diamine 3a
In a Schlenk tube under argon, were placed Pd2dba3 (57
mg, 0.02 mmol, 16 mol%) and rac‐BINAP (68 mg, 0.03
mmol, 28 mol%) in dry toluene (5 mL). The solution
was degassed for 10 minutes then heated at 135°C for
15 minutes. After cooling down to room temperature, t‐
BuONa (76 mg, 0.78 mmol, 6 eq.), n‐propylamine (0.13
mL, 1.62 mmol, 12.5 eq.), and 2.3‐dibromo[4]helicene
2a (150 mg, 0.13 mmol, 1 eq.) in toluene (5 mL) were
added. After stirring and heating at 135°C for 48 hours,
the reaction mixture was cool down to room temperature
poured into dichloromethane and filtered over Celite. The
solvent was stripped off and the obtained brown oil was
purified by chromatography over silica gel (heptane: ethyl
acetate 98:2, as the eluent) yielding 3a as a white solid
(119 mg, 69%). Mp = 94‐96°C. 1H NMR (400 MHz,
CDCl3): δ 1.14 (td, J1 = 0.8 Hz, J2 = 7.2 Hz, 6H, Hc,c′);
1.87 (sept, J = 7.2 Hz, 4H, Hb,b′); 3.3 (br s, 4H, Ha,a′);
4.12 (ls, 2H, NH); 7.15 (s, 1H, H4); 7.58‐7.68 (m, 3H);
7.75 (d, J = 8.4 Hz, 1H). 7.79 (s, 2H); 8.00 (dd, J1 = 1.2
1
de Rennes 1). H and 13C chemical shifts are reported in
parts per million (ppm) relative to Me4Si as external stan-
dard. Assignment of proton and carbon signals is based
on standard and band‐selective HSQC and HMBC, COSY
and NOESY (500 ms mixing time). Mass spectrometry
was performed by the CRMPO, University of Rennes 1,
by ESI or by MALDI‐TOF. Melting points were measured
on a melting point apparatus Stuart SMP10. Specific rota-
tions (in deg cm3
g
−1) were measured in a 1‐dm
thermostated quartz cell on a Perkin Elmer‐341 polarim-
eter. Circular dichroism (in M−1 cm−1) was measured on
a Jasco J‐815 Circular Dichroism Spectrometer IFR140
facility (Biosit platform, Université de Rennes 1). UV/vis
spectroscopy was conducted on a Varian Cary 5000 spec-
trometer. Chiral high‐performance liquid chromatogra-
phy (HPLC) was performed by iSm2, Aix Marseille

HAFEDH ET AL.
3
SCHEME 1 Synthesis of iridium(I) complexes 1a,b and of rhodium(I) complex 1′a. (i) n‐propylamine, Pd2dba3/rac‐BINAP, t‐BuONa,
toluene, 135°C, 48 hours; (ii) HCl, (EtO)3CH; (iii) [M(COD)Cl]2 (M = Ir, Rh), t‐BuOK, THF, rt, 13 hours
Hz, J2 = 8 Hz,1H); 8.38 (s, 1H, H1); 9.24 (d, J = 8.4 Hz,
1H, H12). 13C NMR (101 MHz, CDCl3): δ 11.9 (CH3);
11.95 (CH3); 22.65 (2 CH2′); 46.6 (2 CH2); 108.15 (C);
109.5 (C); 125.25 (2 CH); 125.4 (2 CH); 125.6 (2 CH);
127.3 (2 CH); 128.45 (2 CH); 129.7 (2 C); 130.5 (C);
133.4 (2C); 137.3 (C). HRMS (MALDI‐TOF) [M
+H]+(C24H27N2): calcd. 343.2174; found 343.2177.
then heated under air atmosphere at 80°C for 12 hours.
After cooling down to room temperature, the obtained
precipitate was washed with diethyl ether to give 4a as
1
a pale yellow solid (40 mg 73%). Mp = 180‐182°C. H
NMR (400 MHz, CDCl3): δ 1.12 (br s, 6H, Hc,c′); 2.23 (br
s, 4H, Hb,b′); 4.75 (br s, 4H, Ha,a′); 7.67‐7.74 (m, 2H);
7.85‐7.92 (m, 2H); 7.99‐8.10 (m, 3H); 8.30 (s, 1H); 8.93
(d, J = 7.2 Hz, 1H, H13); 9.27 (s, 1H, H1); 11.84 (s, 1H,
H3). 13C NMR (101 MHz, CDCl3): δ11.1 (CH3); 11.2
(CH3); 22.8 (CH2); 22.9 (CH2); 49.1 (CH2); 49.3 (CH2);
111.05 (CH); 111.2 (CH); 126.3 (CH); 126.5 (CH); 126.6
(CH); 127.05 (CH); 127.2 (CH); 128.6 (CH); 128.8 (C);
128.9 (CH); 129.2 (CH); 129.65 (C); 129.8 (C); 130.3 (C);
131.1 (C); 132.1 (C); 133.7 (2C); 146.4 (C3). HRMS (ESI)
[M+] (C25H25N2+): calcd. 353.2012; found 353.2011(1).
2.1.2 | ( )‐N,N′‐dipropyl[6]helicene‐2,3‐
diamine 3b
Racemic compound 3b was prepared according to the
same Buchwald‐Harthwig conditions as for 3a and was
obtained as a yellow solid, with a 50% yield. This aro-
matic diamine showed low stability and was thus used
1
directly to the next step. H NMR (400 MHz, CDCl3):
δ0.83 (t, J = 7.6 Hz, 3H, Hc); 1.06 (t, J = 7.6 Hz, 3H, Hc
′); 1.29‐1.32 (m, 2H, 2Hb); 1.70 (sex, J = 7.2 Hz, 2H, 2Hb
′); 1.87‐1.94 (m, 1H, Ha); 2.09‐2.15 (m, 1H, Ha); 2.97‐3.23
(m, 4H, NH and Ha′); 6.76‐6.82 (m, 2H); 6.94 (s, 1H);
7.24 (d, J = 8Hz, 1H); 7.76‐7.85 (m, 4H); 7.87‐7.90 (m,
2H), 7.94‐8.012 (m, 4H). HRMS (MALDI‐TOF) [M
+H]+(C32H31N2): calcd. 443.2482; found 443.2478.
2.1.4 | Imidazolium chloride 4b
Compound 4b was prepared from 3b, according to the
same procedure as for 4a. The final salt was washed with
ethyl acetate to give 4b as a pale yellow solid (26 mg,
1
67%). Mp = 200°C. H NMR (400 MHz, CDCl3): δ 0.75
(t, J = 7.6 Hz, 3H, Hc); 1.02 (t, J = 7.2 Hz, 3H, Hc′),
1.26‐1.29 (m, 1H, Hb); 1.37‐1.41 (m, 1H, Hb); 2.08‐2.17
(m, 2H, Hb′); 3.70‐3.84 (m, 2H, Ha); 4.54‐4.66 (m, 2H,
Ha′); 6.53 (t, J = 7.6 Hz, 1H); 7.13 (t, J = 7.2Hz, 1H);
7.41 (d, J = 8.4 Hz, 1H); 7.77 (d, J = 8 Hz, 1H); 7,87 (s,
1H); 7.91 (d, J = 8.4 Hz, 1H); 7.97‐8.08 (m, 6H); 8.11 (d,
J = 8 Hz, 2H); 11.71 (s, 1H). 13C NMR (101 MHz, CDCl3):
δ 10.8 (CH3); 11.0 (CH3); 22.0 (CH2); 22.6 (CH2); 48.4
(CH2); 48.95 (CH2); 110.05 (CH); 111.25 (CH); 123.6 (C);
125.0 (CH); 126.3 (CH); 126.8 (C); 126.9 (CH); 127.2 (C);
2.1.3 | Imidazolium chloride 4a
In a round‐bottom flask, were placed N,N′‐dipropylbenzo
[c]phenanhtrene‐2.3‐diamine 3a (50 mg, 0.14 mmol, 1
eq.) and triethyl orthoformate (EtO)3CH (2 mL). Then
an aqueous solution of HCl 12N (28 μL, 0.34 mmol, 2.4
eq.) was added dropwise. The resulting suspension was
stirred under argon at room temperature for 30 minutes

4
HAFEDH ET AL.
127.3 (CH); 127.3 (CH); 127.4 (2CH); 127.7 (CH); 127.9
(CH); 127.9 (CH); 128.5 (2CH); 128.5 (C); 128.9 (C);
129.1 (C); 129.2 (C); 130.6 (C); 131.4 (C); 131.9 (C);
131.9 (C); 133.6 (C); 145.9 (C3). HRMS (MALDI‐TOF)
[M+] (C33H29N2+): calc. 353.2325; found 353.2320.
7.65‐7.78 (m, 2H); 7.80‐7.86 (m, 3H); 8.06 (dd, J1 = 1.2
Hz, J2 = 8 Hz, 1H, H13); 8.97 (s, 1H, H1); 9.08 (d, J =
8.4 Hz, 1H, H13). HRMS (ESI MS) [M+Na]+ (C33H36N2
35-
ClNa193Rh): calcd. 607.1489; found 607.14845.
2.1.7 | Iridium complex of [6]helicene‐
NHC 1b
2.1.5 | Iridium complex of [4]helicene‐
NHC 1a
In a Schlenk tube under argon, were placed successively
[Ir(COD)Cl]2 (48.4 mg, 0.07 mmol, 0.6 eq.), t‐BuOK
(13.5 mg, 0.12 mmol, 1 eq.), and dry THF (5 mL). After
stirring for 10 minutes at room temperature, imidazolium
salt 4b (58.5 mg, 1 eq.) was added, and the solution was
stirred at room temperature for 13 hours. The solvent
was stripped off, and the crude mixture was purified by
flash chromatography over silica gel (diethyl ether as
the eluent) to give complex 1b (62 mg, 65%) as a yellow
In a Schlenk tube under argon, were placed successively
[Ir(COD)Cl]2 (48.4 mg, 0.07 mmol, 0.6 eq.), t‐BuOK
(13.5 mg, 0.12 mmol, 1 eq.), and dry THF (5 mL). After
stirring for 10 minutes at room temperature, imidazolium
salt 4a (50 mg) was added, and the solution was stirred at
room temperature for 13 hours. The solvent was stripped
off, and the crude mixture was purified by flash chroma-
tography over silica gel (diethyl ether as the eluent) to
give complex 1a (63 mg, 76%) as a yellow solid. Mp =
solid. Mp = 269°C. HRMS (ESI) [M+Na]+ (C41H40N2
35-
1
ClNa193Ir): calcd. 811.24016; found 811.2404(0).
252‐254°C. H NMR (400 MHz, CDCl3): δ 1.20‐1.25 (m,
6H, Hc,c′); 1.73‐1.80 (m, 2H, COD); 1.86‐1.95 (m, 2H,
COD); 2.04‐2.17 (m, 2H, 2Hb); 2.28‐2.39 (m, 6H, 4H from
COD and 2Hb,b′); 3.08 (s, 2H, COD); 4.73‐4.93 (m, 6H, 2H
from COD and 4Ha,a′); 7.65‐7.74 (m, 2H); 7,79 (d, J = 8.4
Hz, 1H); 7.84‐7.86 (m, 2H); 7.92‐7.96 (m, 2H); 8.07 (dd, J1
= 1.6 Hz, J2 = 8 Hz, 1H); 8.98 (s, 1H, H1); 9.08 (d, J = 8.4
Hz, 1H, H13). 13C NMR (101 MHz, CDCl3): δ 11.9 (CH3);
11.95 (CH3); 23.0 (CH2), 23.2 (CH2), 29.4 (CH2); 29.5
(CH2); 33.6 (CH2); 33.7 (CH2); 50.3 (CH2); 50.4 (CH2);
52.7 (CH); 52.7 (CH); 87.2 (CH); 87.3 (CH); 107.0 (CH);
107.7 (CH); 125.9 (C); 126.0 (CH); 126.25 (CH); 126.3
(CH); 126.85 (2CH); 127.2 (C); 127.3 (CH); 127.4 (CH);
128.8 (CH); 129.6 (C); 130.2 (C); 130.45 (C); 133.5 (C);
134.0 (C); 134.70 (C); 197.0 (C3). HRMS (ESI MS) [M
+Na]+ (C33H36N235ClNa193Ir): calcd. 711.2089; found
711.2079.
The mixture of four diastereomers was then separated
by HPLC over a chiral stationnary phase (Chiralpak IG,
eluent: hexane/ethanol/dichloromethane (80/10/10), see
Supporting Information) to yield complexes 1b1 and 1b2.
23
First eluted: (P)‐1b1 > 99% ee, ½αꢀD = +2750; second
23
eluted: (M)‐1b1 > 98% ee, ½αꢀD = −2121; third eluted:
23
(P)‐1b2 > 98% ee, ½αꢀD = +2703; fourth eluted: (M)‐1b2
23
> 98% ee, ½αꢀD = −1860 (CH2Cl2, 5 × 10−5 M). Some
isomerization process between 1b1 and 1b2 stereoisomers
was observed (see below and in Supporting Information).
2.2 | Complex 1b1
1H NMR (500 MHz, CD2Cl2): δ 0.73 (t, J = 7.5 Hz, 3H,
H20); 1,09 (t, J = 7.5 Hz, 3H, H23); 1.12‐1.26 (m, 2H);
1.69‐1.77 (m, 2H, H19); 1.79‐1.86 (m, 2H); 1.88‐1.96 (m,
1H, H22); 2.07‐2.15 (m, 1H, H22); 2.17‐2.32 (m, 4H); 2,80
(td, J1 = 2.5 Hz, J2 = 7 Hz, 1H, H2′); 2.96‐2.98 (m, 1H,
H1′); 3.69‐3.75 (m, 1H, H18); 4.05‐4.10 (m, 1H, H18);
4.52‐4.58 (m, 2H, H21, and H5′/6′); 4.59‐4.63 (m, 1H, H5′/
6′); 4.68‐4.74 (m, 1H, H21); 6.50‐6.53 (m, 1H, H16); 7.10
(td, J1 = 1 Hz, J2 = 8 Hz, 1H, H15); 7.46 (s, 1H, H1);
7.48 (d, J = 8.5 Hz, 1H, H17); 7.70 (s, 1H, H5); 7.84 (d, J
= 7.5 Hz, 1H, H14); 7.94 (d, J = 8.5 Hz, 1H); 7.98 (d, J =
8.5 Hz, 1H); 8.01‐8.09 (m, 6H). 13C NMR (126 MHz,
CD2Cl2): δ 11.65 (C20); 12.0 (C23); 22.8 (C19), 23.4 (C22),
29.8 (CH2); 30.0 (CH2); 33.9 (CH2); 34.1 (CH2); 50.15
(C18); 50.55 (C21); 53.,0 (C1′/2′); 53.05 (C1′/2′); 86.9 (C5′/
6′); 87.0 (C5′/6′); 106.8 (C5); 108.3 (C1); 124.3; 125.1;
126.1; 126.2; 126.2; 127.25; 127.3; 127.7; 127.8; 127.8;
127.8; 127.9; 127.9; 128.1; 128.3; 128.4; 128.7; 130.0;
131.5; 132.1; 132.45; 133.8; 134.1; 133.7; 196.6 (C3).
2.1.6 | Rhodium complex of [4]helicene‐
NHC 1′a
In a Schlenk tube under argon, were placed successively
[Rh(COD)Cl]2 (38.9 mg, 0.08 mmol, 0.6 eq.), t‐BuOK
(14.75 mg, 0.13 mmol, 1eq.), and dry THF(5 mL). After
stirring for 10 minutes at room temperature, imidazolium
salt 3a (51 mg) was added, and the solution was stirred at
room temperature for 13 hours. The solvent was stripped
off, and the crude mixture was purified by flash chroma-
tography over silica gel (diethyl ether as the eluent) to
give complex 1′a (40 mg, 51%) as a yellow solid. Mp =
1
240‐243°C. H NMR (400 MHz, CDCl3): δ1.25‐1.30 (m,
6H, Hc,c′); 2.07‐2.19 (m, 6H, 2H from COD and 2Hb);
2.37‐2.57 (m, 6H, 2H from COD and 2Hb′); 3.48 (s, 2H,
COD); 4.88‐5.06 (m, 4H, Ha,a′); 4.73‐5.25 (s, 2H, COD);

HAFEDH ET AL.
5
2.3 | Complex 1b2
proton of the precarbenic CH. Both types of methylenes
and methyls of the n‐propyl chain were found to be iso-
chronous and appeared as large singlets. The 13C NMR
also confirmed the imidazolium formation with the ter-
tiary pre‐carbenic carbon C3 resonating at 196.9 ppm.
The chemical structure of 4a was finally confirmed by
X‐ray crystallography (P‐1 space group), which showed
(a) the helical structure with a small helicity (dihedral
angle between terminal rings) of 29.94° (a typical value
for [4]helicene units),1 (b) the imidazolium cycle fused
to the helical core, (c) the heterochiral assembly in a
head‐to‐tail fashion, and (d) the n‐propyl chains. Interest-
ingly, the chloride ions form channels all along the z axis
(see Figure 2).
Upon reaction of 4a with [Ir (COD)Cl]2 in the pres-
ence of t‐BuOK as the base, the iridium(I) complex 1a
bearing COD, chloro ligands, and monodentate [4]
helicenic carbene was obtained with 76% yield. This com-
plex was fully characterized by NMR spectroscopy, mass
spectrometry, and X‐ray crystallography. The X‐ray struc-
ture of racemic 1a (P‐1 centro‐symmetric space group,
1H NMR (500 MHz, CD2Cl2): δ 0.70‐0.76 (m, 1H, H19);
0.79 (t, J = 6.5 Hz, 3H, H20); 0.85‐0.89 (m, 1H); 1.11 (t, J
= 7.5 Hz, 3H, H23); 1.26‐1.31 (m, 1H); 1.33‐1.42 (m, 1H,
H19); 1.73‐1.78 (m, 2H); 1.93‐2.00 (m, 1H, H22); 2.08‐2.22
(m, 5H, H22 + 4HCOD); 2.64‐2.67 (m, 1H, H5′); 2.77‐2.79
(m, 1H, H6′); 3.70‐3.76 (m, 1H, H18); 4.06‐4.12 (m, 1H,
H18); 4.56‐4.62 (m, 3H, H21 + 2HCOD); 4.63‐4.70 (m, 1H,
H21); 6.64‐6.68 (m, 1H, H16); 7.23 (t, J = 7.5 Hz, 1H,
H15); 7.56 (s, 1H, H1); 7.61 (d, J = 8.5 Hz, 1H, H17); 7.72
(s, 1H, H5); 7.83 (d, J = 8 Hz, 1H, H14); 7.93 (d, J = 9
Hz, 1H); 7.96 (d, J = 8.5 Hz, 1H); 8.02‐8.06 (m, 6H). 13C
NMR (126 MHz, CD2Cl2): δ11.28 (C20); 11.45 (C23); 23.0
(C19), 23.8 (C22), 29.3 (CH2); 29.3 (CH2); 33.3 (CH2); 33.4
(CH2); 59.5 (C18); 50.05 (C21); 52.4 (C6′); 52.6 (C5′); 86.4
(CH); 86.45 (CH); 106.5 (C5); 107.3 (C1); 123.7; 125.2;
125.6; 125.95; 126.1; 126.4; 126.7; 127.0; 127.1; 127.3;
127.35; 127.45; 127.7; 127.9; 128.0; 128.1; 128.1; 129.2;
131.0; 131.6; 131.9; 133.2; 133.6; 133.7; 196.2 (C3).
Figure
2
and Supporting Information) shows
a
2.4 | General procedure for catalytic
transfer hydrogenation
pseudotetrahedral geometry around the iridium center,
with the two double bonds of COD, the chlorine and
the carbenic C forming the tetrahedron. Complex 1a dis-
plays a typical C3–Ir bond length of 2.011 Å. These metric
data are similar to corresponding values for other
reported carbenic cycloiridiated complexes.23 The [4]
helicenic part shows a helicity of 27.36° in the solid state,
but it is not configurationally stable in solution.
The analogous rhodium(I) complex 1′a was also pre-
pared with 51% yield according to the same procedure
as 1a but using [Rh (COD)Cl]2 as the Rh(I) organometal-
lic precursor. Its structure was ascertained by X‐ray crys-
tallography. Very similar features as for 1a were found for
1′a which also crystallized in the P‐1 space group (C3‐Rh
bond length of 2.011 Å, helicity 27.20°).
Acetophenone (1.0 mmol), t‐BuOK (0.1 mmol), and cata-
lyst (0.01 or 0.02 mmol) were introduced into an oven‐
dried Schlenk tube. Dry i‐PrOH (3.0 mL) was then added
and the mixture was refluxed, for the specified time,
under argon. The efficiencies of the reactions were deter-
1
mined by H NMR and the enantioselectivities by chiral
HPLC.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis
The synthesis of iridium(I) complex 1a bearing a [4]
helicenic NHC ligand was first investigated. It started
with the preparation of 2,3‐dibromo‐carbo[4]helicene 2a
according to a known procedure,22 which was then trans-
formed into 2,3‐di‐n‐propylamino‐carbo[4]helicene 3a, by
a Buchwald‐Hartwig coupling using classical conditions
(69% yield, see Scheme 1). Upon reaction with
triethylorthoformate and HCl, diamine 3a then afforded
helical imidazolium chloride salt 4a with 73% yield. Com-
pounds 3a and 4a were characterized by NMR spectros-
Similarly to 4a, [6]helicenic imidazolium salt 4b was
prepared in two steps from 2,3‐dibromo‐carbo[6]helicene
2b,22 according to Scheme 1, ie, through a Buchwald‐
Hartwig coupling giving 2,3‐dipropylamino‐carbo[6]
helicene 3b followed by a cyclization forming the
imidazolium unit. Compounds 3b and 4b were also fully
characterized by NMR spectroscopy and mass spectrome-
1
try. For example, the H NMR spectrum of diamine 3b
displayed a set of six distinguished signals for shielded
and deshielded protons Ha,b,c and Ha′,b′,c′, respectively,
of the n‐propyl chains due to their different positions rel-
ative to the helicene inner core.
Compound 3b crystallized into single crystals and its
X‐ray crystallographic structure is depicted in Figure 3A
which shows the heterochiral (P and M) pair (P‐1 space
group). Compound 3b arranges along the y axis into
1
copy and mass spectrometry. For example, the H NMR
spectrum of salt 4a displayed characteristic signals com-
ing from the helicenic unit and the imidazolium part.
Indeed, a singlet at 9.27 ppm and a doublet 8.93 ppm cor-
respond to H1 and H13 protons, respectively, while the
highly deshielded proton at 11.84 ppm corresponds to

6
HAFEDH ET AL.
FIGURE 2 X‐ray crystallographic structures of imidazolium salt 4a (A) and its views along the z (B) and y (C) axes, of iridium complex 1a
(D) and rhodium complex 1′a (E)
homochiral columns in a head‐to‐tail fashion (Figure 3B).
The hexahelicenic diamine is now configurationally sta-
ble and displays a helicity of 58.65°.
which showed the typical highly deshielded signal at
11.7 ppm for H3 and at 145.9 for C3.
Similar to 1a, [6]helicene‐NHC‐Ir complex 1b was pre-
pared from 4b upon reaction of [Ir (COD)Cl]2 in the pres-
ence of t‐BuOK and was obtained with 65% yield. This
compound showed a peak at 811.2404(0) in the ESI‐
The obtention of [6]helicene‐benzimidazolium chlo-
ride 4b was confirmed by 1H and 13C NMR analyses
1
HRMS corresponding to [M+Na]+ and its H NMR spec-
trum corresponded to a mixture of two sets of signals.
3.2 | Separation of stereoisomers
Complex 1b was then subjected to chiral HPLC separa-
tion (see experimental part and Supporting Information).
A set of four signals was obtained, corresponding to enan-
tiomeric pairs (M)‐(−)/(P)‐(+)‐1b1 and (M)‐(−)/(P)‐(+)‐
1b2. The final structures of 1b1 and 1b2 were clarified
by NOESY experiments, which revealed that the COD
ligand could adopt two positions, one directed towards
the helical moiety (1b1, Scheme 2) and one on the oppo-
site direction (1b2). Indeed, complex 1b1 shows spatial
proximity between 14 and 15 protons from helicene ter-
minal aromatic ring and one of the COD double bound,
whereas compound 1b2 only shows spatial proximity of
COD ligand with protons from the alkyl chains on the
opposite side of the helicene. Moreover, NOESY correla-
tions reveal that orientation of alkyl chains from the
imidazole moiety relative to helicene changes slightly
according to the position of the COD ligand (see
Supporting Information). The interconversion barrier
between 1b1 and 1b2 was estimated to 104 kJ/mol at 25°
C (see Supporting Information), so that exchange takes
only a few hours at ambient temperature.
FIGURE
3
X‐ray crystallographic structure of [6]helicenic
diamine 3b. (A) Heterochiral pair; (B) head‐to‐tail organization
into homochiral columns

one negative at 279 nm (Δε = −52 M−1 cm−1) and one
positive at 341 nm (Δε = +29 M−1 cm−1), with some
unresolved vibronic progression. We interpret this pecu-
liar behavior by the presence of quinoidal forms that
may coexist in such a compound. Oxidized impurities
cannot be excluded due to the poor stability of such aro-
matic diamine derivative. On the contrary, the pure enan-
tiomers (P)‐(+) and (M)‐(−)‐1b1 display very intense UV‐
vis and ECD spectra, with UV‐vis spectra showing one
intense band at 265 nm (ε > 70 × 103 M−1 cm−1) and
other intense ones (16 × 103 M−1 cm−1 < ε < 35 × 103
3.3 | Chiroptical properties
The chiroptical properties (OR and ECD) of pure stereo-
isomers (P)‐(+) and (M)‐(−)‐1b1/1b2 were then investi-
gated. For comparison, pure enantiomers (P)‐(+) and
(M)‐(−)‐3b (ee′s > 99.5% and 98%) were also prepared
by chiral HPLC over Chiralpak IA stationary phase (hep-
tane/ethanol/dichloromethane (80/10/10) as the eluent,
see Supporting Information). (P)‐(+) and (M)‐(−)‐3b dis-
23
play specific rotations values ½αꢀD of +760 and −733°
cm2 g−1 ( 5%, CH2Cl2, 5 × 10−5 M), which are moderate
values for hexahelicenic structures.1 Similarly, their UV‐
vis and ECD spectra show moderate intensities, as
depicted on Figure 4. Indeed, the UV‐vis spectrum of 3b
shows one intense band at 270 nm (ε = 14 × 103 M−1
cm−1) and moderate ones (4 × 103 M−1 cm−1 < ε < 8 ×
103 M−1 cm−1) at 295, 320, 341, and 350 nm, while the
ECD spectrum of (P)‐(+)‐3b displays two main bands,
M
−1 cm−1) at 310, 325, 341, 361, and 375. Two other weak
bands at 395 and 420 nm are also present. The ECD spec-
trum of (P)‐(+)‐1b1 displays one strong negative band at
261 nm (Δε = −288 M−1 cm−1) and two main strong pos-
itive bands at 341 nm (Δε = +219 M−1 cm−1) and 361 nm
(Δε = +148 M−1 cm−1), with some vibronic progression,
accompanied by two weak ECD bands at 395 and 420