Stereoselective coordination of ditopic phospholyl-azahelicenes: A novel approach towards structural diversity in chiral π-conjugated assemblies

Article abstract of DOI:10.1039/b714340k

Phosphole-modified aza[4]helicenes act as heteroditopic chelates toward metal ions having different coordination geometries (Pd^II, Cu ^I). The Royal Society of Chemistry.

Full text of DOI:10.1039/b714340k

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Stereoselective coordination of ditopic phospholyl-azahelicenes: a novel
approach towards structural diversity in chiral p-conjugated assembliesw
a
a
a
a
´
Wenting Shen, Sebastien Graule, Jeanne Crassous,* Christophe Lescop,
Heinz Gornitzka and Regis Reau*
b
a
´
´
Received (in Cambridge, UK) 17th September 2007, Accepted 6th December 2007
First published as an Advance Article on the web 18th December 2007
DOI: 10.1039/b714340k
Phosphole-modified aza[4]helicenes act as heteroditopic chelates
toward metal ions having different coordination geometries
(PdII, CuI).
this organometallic route (Fagan–Nugent method)6 offers
straightforward access to these novel phosphole scaffolds
decorated with different helicene-substituents.
The structures of derivatives 2a,b were supported by high-
resolution mass spectrometry and elemental analyses. They
exhibit classical 1H and 13C NMR data, and their 31P{1H}
NMR spectra showed a sharp singlet in the range expected for
P-aryl phospholes4 (2a, +12.2; 2b, +20.7). It is worth noting
that the two stereogenic elements of 2a,b (P-centre, [4]azahe-
licene) undergo rapid inversion in solution at room tempera-
ture.7 Derivative 2a was further characterized by an X-ray
diffraction study (Scheme 1).8 Although the P and N atoms
have a mutual ‘‘anti’’ conformation in the solid state, this
structural feature should not prevent derivative 2a from acting
as a P,N-chelate since the energy barrier to rotation about the
inter-ring C–C bond of (2-pyridyl)phospholes is low (12–16 kJ
molÀ1).9 The twist angles between the phosphole ring and the
p-substituents (azahelicene, 36.51; Ph, 37.71) allow extended
delocalization of the p-system4,9 since the orbital overlap
varies approximately with the cosine of the twist angles. As
expected, the naphtho[1,2-f]quinoline part of 2a displays a
helical geometry with a dihedral angle of 30.11 between the
two terminal aromatic rings (Scheme 1). The UV-Vis absorp-
tion maximum, assigned to p–p* transitions, of 2a is notably
red-shifted (406 nm) compared to that for 5-phenyl-2-pyridyl-
phosphole (390 nm),4b clearly indicating that the [4]azaheli-
cene moiety is conjugated with the phosphole ring. The lmax of
compound 2b is much shorter (380 nm), probably due to steric
interactions between the azahelicene substituent and the fused-
carbocycle of the phosphole ring resulting in a non-planar
structure.
The design and synthesis of molecules endowed with helical
chirality and p-conjugated backbones is of great interest due
to their fascinating optical and electronic properties.1 One
challenge in expanding the potential of these derivatives is to
develop short and efficient synthetic approaches allowing the
generation of different frameworks with diverse properties.
Quite surprisingly, coordination chemistry has rarely been
used for the tailoring of helicoidal p-conjugated systems
(helicenes),2 despite metals being powerful platforms for orga-
nizing organic chromophores into well-defined supramolecu-
lar assemblies.3 The metal ions can furthermore potentially (i)
allow for the generation of structural diversity through varia-
tion in their coordination geometries, and (ii) affect the optical
properties of the coordinated p-conjugated systems. Herein,
we describe the unprecedented controlled self-assembly of
helicenes into sophisticated chiral architectures using supra-
molecular coordination-driven chemistry. Helicenes designed
to act as ditopic P,N-chelates toward metal ions are reported
and, utilizing the power and simplicity of coordination chem-
istry, varied helical p-conjugated assemblies are readily ob-
tained with high stereoselectivity. Moreover, due to intimate
metal–ligand interactions, the optical properties of these
P,N-helicenes are modified upon coordination.
Our strategy was to prepare phosphole-modified azaheli-
cenes having a 1,4-P,N moiety and to generate structurally
diverse assemblies through their coordination onto a metal
having either a square planar (PdII) or a tetrahedral (CuI)
geometry. The (phospholyl)azahelicenes 2a,b (Scheme 1) were
targeted as model compounds since they possess different
structures with either a terminal (2a) or an inner (2b) chelating
P,N-moiety. They were prepared via a ‘‘zirconocene’’-pro-
moted intramolecular coupling of 1,7-diynes4,5 and sub-
sequent addition of PhPBr2 as illustrated by the transforma-
tion 1a - 2a (Scheme 1). They were isolated as air-stable
solids in moderate yields (2a, 63%; 2b, 45%) following
purification by column chromatography. It is noteworthy that
The next step was to use metal centres for organizing these
phosphole-modified azahelicenes 2a,b into well-defined assem-
blies. Square-planar d8 PdII ions were first selected in order to
check whether the coordination of heteroditopic P,N-chelates
a
´
UMR 6226, CNRS-Universite de Rennes 1, 35042 Rennes Cedex,
France. E-mail: regis.reau@univ-rennes1.fr; Fax: +33 223236939
´
UMR-CNRS 5069, Universite Paul Sabatier, 31062 Toulouse Cedex
09, France
b
w Electronic supplementary information (ESI) available: Experimental
procedures and crystallographic data. See DOI: 10.1039/b714340k
Scheme 1 Synthesis of phosphole-based helicenes 2a,b.
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(shortest inter-ring distance, 4.1 A) indicates that no intra-
molecular p–p interaction takes place, the two azahelicenes
moieties wind with the same helical sense (Fig. 1). Two
features directly arising from the coordination of P,N-heli-
cenes on PdII are particularly noteworthy. The first is the
unique structure of complex 3b featuring three helicoidal
moieties: two organic (the azahelicenes) and one inorganic
(the PdP2N2 core) (Fig. 1, left). This nicely illustrates that
metallohelicenes can provide novel topologies in chiral
p-conjugated derivative chemistry. The second is the highly
stereoselective assembly of P,N-helicenes 2a,b on to PdII. It is
remarkable that amongst the numerous possible stereoisomers
(26), only one pair of enantiomers is obtained. This highly
diastereoselective assembly is due to a combination of electro-
nic (trans-effect, . . .) and steric (repulsion of (i) the P–Ph
substituents, (ii) the azahelicenes, . . .) factors in the metal
coordination sphere. For example, the anti-configuration of
the P–Ph groups (Fig. 1, right) is clearly sterically favoured
over a possible syn-arrangement. Indeed, phosphole-modified
azahelicenes appear to be well designed to self-assemble on to
metal ions in a highly stereoselective way to afford quantita-
tively original chiral p-conjugated metallohelicenes.
Having demonstrated that helicenes 2a,b with different
structures act as P,N-chelates, the next step was to illustrate
the possibility of generating other topologies using metals with
other coordination geometries. CuI, a tetrahedral d10 centre,
was thus selected. The reaction of derivatives 2a,b with a CuI
source (2 : 1 molar ratio) was conducted in CH2Cl2 solution at
room temperature. Upon exposure to pentane vapours, air-
stable single crystals of complexes 4a,b (Scheme 2) grew from
the homogeneous solutions (yields 477%). Their solution
state 31P{1H} NMR spectra exhibited a broad singlet at d
5–6 ppm (n1/2 = 85 Hz), a typical shift for (2-pyridyl)phosp-
hole-CuI complexes.4e,f Their structures were confirmed by
X-ray diffraction studies (Fig. 2), both complexes crystallise in
centrosymmetric space groups. The chiral CuI centres have a
distorted tetrahedral geometry with the two azahelicenes hav-
ing an almost perpendicular arrangement (angle between the
two N–Cu–P planes: 4a, 83.71; 4b, 86.41, Fig. 2). The novel
assemblies 4a,b have an unprecedented topology, in which two
helicenes are connected by a ‘‘spiro’’ centre (i.e. the CuI atom).
Scheme 2 Synthesis of PdII-complexes 3a,b and CuI-complexes 4a,b.
2a,b is stereoselective, as could be expected from the different
trans-effects10 of their donor sites. Derivatives 2a,b reacted in
CH2Cl2 solution with [Pd(CH3CN)4][SbF6]2 (2 : 1 molar ratio)
giving rise almost quantitatively to air-stable complexes 3a,b
(Scheme 2). The simplicity of their multinuclear NMR spectra
clearly shows that these complexes are obtained as single
diastereoisomers. Their high frequency 31P NMR chemical
shifts (3a, +76.6 ppm; 3b, +81.6 ppm) are typical for PdII-
complexes bearing (2-pyridyl)phosphole ligands in a mutually
cis configuration.4a–c The stereoselective coordination of het-
eroditopic P,N-helicenes 2a,b on PdII was confirmed by an
X-ray diffraction study performed on single crystals of 3b. Due
to the ‘‘trans effect’’, the square-planar metal imposes a
noncentrosymmetric assembly of 2a,b with the azahelicene
fragments having a mutual syn-arrangement (Fig. 1). This
result clearly shows that, in spite of their unusual structures
and the presence of sterically demanding azahelicene substi-
tuents, the heteroditopic P,N-moieties of derivatives 2a,b
dictate their coordination behaviour. This point is very im-
portant in view of the extension of the helical backbone to
configurationally more stable helicenes (aza[6]helicenes and
higher derivatives).
Complex 3b has an approximate C2-symmetry. The PdII
centre shows a highly distorted square-planar geometry with
an angle between the two N–Pd–P planes of 19.51 (Fig. 1, left).
This distortion, which is due to the overlapping of the two
coordinated azahelicene moieties, results in a chiral helicene-
type coordination sphere around the PdII centre.10b It is likely
that the configuration at the metal is also rigid in solution due
to the overlapping of the helices (Fig. 1, right) which prevents
a ‘‘flopping’’ of the ligands. It is important to note that the
structure of the [4]azahelicene moieties is not perturbed by the
coordination on to the metal centre. For example, the dihedral
angles between the two terminal aromatic rings in complex 3b
(30 and 35.11) are comparable to those of 2a (30.11). Finally,
although the rather large distance between the two helicenes
Fig. 1 Views of dicationic complex 3b (H atoms, counteranions and
solvent molecules have been omitted for clarity). One of the two
symmetrically independent complexes is shown.
Fig. 2 Cationic complexes 4a and 4b (one stereoisomer, H atoms,
counteranions and solvent molecules have been omitted for clarity).
Simplified view showing the packing of 4b along the b-axes.
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We thank the Ministere de la Recherche et de l’Enseigne-
ment Supe
CNRS, the Re
137104).
´
rieur, the Institut Universitaire de France, the
´
gion Bretagne and the ANR (PHOSHELIX-
Notes and references
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2 (a) For metal complexes of phosphorus-containing helicenes, see:
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inetti, J. Organomet. Chem., 2007, 692, 1156; (b) for other com-
plexes containing helicenes, see: T. J. Katz, A. Sudhakar, M. F.
Teasley, A. M. Gilbert, W. E. Geiger, M. P. Robben, M. Wuensch,
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Chem. Soc., 1993, 115, 3182; (c) Y. Dai and T. J. Katz, J. Org.
Chem., 1997, 62, 1274.
Fig. 3 UV-visible spectra of free helicene 2b, and of its PdII- (3b) and
CuI- (4b) complexes in CH2Cl2.
As observed for the square planar PdII complexes, (i) the
coordination of heteroditopic helicenes 2a,b on tetrahedral
CuI centres does not affect their structure, and (ii) the co-
ordination is highly stereoselective. Among the numerous
theoretically possible stereoisomers, only two (complex 4a)
and one (complex 4b) diastereoisomers were found in the solid
state. Note that the diastereoisomers of 4a differ only by the
conformation of the flexible –(CH2)4– moiety grafted on to the
phosphole rings. This result demonstrates the versatility of
supramolecular coordination-driven chemistry in generating
structural diversity in the field of helicoidal p-conjugated
systems. In fact, starting from one programmed heteroditopic
P,N-helicene such as 2a,b, air-stable chiral p-conjugated as-
semblies can be readily obtained in an highly stereoselective
way upon coordination on metal ions having different coordi-
nation geometries.
3 (a) O. Maury and H. Le Bozec, Acc. Chem. Res., 2005, 38, 691; (b)
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Finally, two important features are worth noting that
illustrate the potential functionality of these metallohelicenes.
First, the p–p* transition of phosphole-modified helicenes 2a,b
are hardly perturbed upon coordination on the metal centres,
but new transitions of low intensity are observed at higher
wavelength (lmax 4450 nm, Fig. 3). These low energy UV-Vis
absorptions, which are due to charge transfer involving the
metal and the 2-pyridylphosphole ligand,4a–d reveal that the
coordinated p-conjugated helicenes are electronically coupled
with the metal centres. It is interesting to note that these long
wavelengths absorptions are more red-shifted for the PdII-
complexes than for their CuI-analogues (Fig. 3), showing that
the optical properties of P,N-helicenes 2a,b can be tuned by
coordination. Secondly, helicene complexes 3b and 4b aggre-
gate into infinite columns in the solid state due to intermole-
cular p-stacking of the helicene moieties (intermolecular
distances, 3.5–3.6 A) (Fig. 2). This type of supramolecular
organization, is crucial in obtaining helicene-based materials
with appealing properties (NLO-phores, circularly polarized
materials, . . .).11
´
´
´
5 (a) See ESI.w For the synthesis of azahelicenes, see: D. C. Har-
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7 The inversion barrier of phospholes is ca. 15 kcal molÀ1, see: L. D.
Quin and G. S. Quin, in Phosphorus–Carbon Heterocyclic Chem-
istry: The Rise of a New Domain, ed. F. Mathey, Elsevier Science
Ltd, Oxford, 2001.
8 CCDC 651119–651122 for 2a, 4b, 4a and 3b, respectively. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b714340k.
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In conclusion, we have described a rational and simple
synthetic approach to a variety of nano-scale chiral p-con-
jugated molecules via stereoselective coordination of hetero-
ditopic phosphole-modified azahelicenes on metal centres. The
high stereoselectivity of this method, giving rise to a reduced
number of enantiomeric pairs, should facilitate the isolation of
enantiomerically pure substances. Extension of this versatile
synthetic methodology to phosphole-modified azahelicenes
with longer conjugated systems is under active investigation.
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This journal is The Royal Society of Chemistry 2008
852 | Chem. Commun., 2008, 850–852