Metal -bis(hel icene) assemblies incorporating ?conjugated phosphole-azahelicene ligands: Impacting chiroptical properties by metal variation

Full text of DOI:10.1021/ja809396f

Published on Web 02/12/2009
Metal-Bis(helicene) Assemblies Incorporating π-Conjugated
Phosphole-Azahelicene Ligands: Impacting Chiroptical Properties by Metal
Variation
Se´bastien Graule,† Mark Rudolph,‡ Nicolas Vanthuyne,§ Jochen Autschbach,*,‡ Christian Roussel,§
Jeanne Crassous,*,† and Re´gis Re´au*,†
Sciences Chimiques de Rennes, Campus de Beaulieu, UMR 6226, CNRS-UniVersite´ de Rennes 1, 35042 Rennes
Cedex, France, Department of Chemistry, 312 Natural Sciences Complex, UniVersity at Buffalo, State UniVersity of
New York, Buffalo, New York 14260-3000, and Chirosciences UMR 6263, Institut de Sciences Mole´culaires de
Marseille (ISM2) Chirotechnologies, UniVersite´ Paul Ce´zanne, 13397 Marseille Cedex 20, France
Received December 8, 2008; E-mail: jochena@buffalo.edu; regis.reau@univ-rennes1.fr
Scheme 1. Synthesis, Solid-State Structures, and Coordination of
2a and 2ba
Helicenes possess a unique screw-shaped π-conjugated structure
that provides them with huge optical rotation values.1 Therefore,
there is a growing interest in using helicenes or analogous
derivatives as building blocks for the design of chiral ligands for
use in asymmetric catalysis,2a,b nonlinear optical materials,2c and
waveguides.2d In this context, the development of simple strategies
to tune the chiroptical properties of helicene-based derivatives by
varying their structures is of great interest.3 Little is known about
the coordination chemistry of helicene derivatives,2a,3b,4 although
metal ions are versatile templates for assembling π-conjugated
ligands into supramolecular architectures.5 We have therefore
investigated an unprecedented strategy based on the synthesis of
chiral metal-bis(helicene) complexes via stereoselective coordina-
tion of aza[6]helicenes bearing a phosphole moiety. The novel
diastereomeric ligands 2a and 2b (Scheme 1) were designed on
the basis of two key properties of phosphole-based π-conjugated
systems. First, 2-(2-pyridyl)phospholes are 1,4-P,N chelates that
undergo highly stereoselective coordination to metallic ions with
different coordination geometries as a result of their heteroditopic
nature (trans effect6a) and the specific properties of the phosphole
ring (ease of inversion at P, steric hindrance provided by the P
substituent).6b,c Second, in these complexes, the phosphole and
pyridyne moieties are conjugated, resulting in an intimate electronic
interaction between the metal and the π-conjugated P,N-chelates
via metal-ligand charge transfer.6b,c Herein, we describe the
synthesis and chiroptical properties of the phosphole-modified
azahelicenes 2a and 2b and their PdII and CuI complexes. The
interest in modifying azahelicenes by a phosphole moiety and the
crucial role played by the metal centers in tuning the chiroptical
properties of these novel chiral π-conjugated assemblies are shown
and discussed on the basis of first-principles theoretical calculations.
Derivatives 2a and 2b were obtained from the newly prepared
aza[6]helicene diyne 1 [see the Supporting Information (SI)]
according to the Fagan-Nugent route6,7 (Scheme 1). The starting
material 1 was readily resolved into its (+)-1 (ee 99%) and (-)-1
(ee 97%) enantiomers using HPLC over a Chiralcel OD-H stationary
phase. The intense electronic circular dichroism (CD) band at ∼330
nm (CH2Cl2) enabled their absolute configurations to be established.3,8
It is noteworthy that optically pure functionalized azahelicenes are
relatively rare1,3 and that 1 is the first diyne of this type. P-(+)-1
and M-(-)-1 afforded the corresponding target aza[6]helicene
phospholes 2a and 2b in ∼50% yield (Scheme 1). It should be
noted that the Zr/P exchange required a reaction temperature (40
a (i) Cp2ZrCl2, 2BuLi, THF, 40°C, then PhPBr2, 24 h, rt. X-ray structure
j
of (P*,SP*)-2a (space group P1).
°C) that was low enough to prevent inversion of the aza[6]helicene
moiety (inversion barrier ∼36 kcal mol-1).1 Derivatives 2a and 2b
possess a novel stereogenic element: the P atom of the phosphole
ring (Scheme 1). One key property of phospholes is that their
inversion barrier at P (∼15-16 kcal mol-1) is much lower than
that of regular phosphanes (∼35-36 kcal mol-1) because of the
highly aromatic character of planar phospholes.9 Therefore, the
P-(+)-diyne 1 afforded a mixture of diastereomers (P,RP)-2a and
(P,SP)-2b, whereas its M-(-)-enantiomer gave their (M,SP)-2a and
(M,RP)-2b mirror images (Scheme 1). Indeed, the 31P NMR
spectrum of the reaction mixture displays two singlets of equal
1
intensity at 14.0 and 14.5 ppm. Likewise, the H and 13C NMR
spectra showed expected signals for the aza[6]helicene and the
phosphole moieties for each diastereomer. It is worth noting that
variable-temperature 31P NMR spectroscopy confirmed that deriva-
tives 2a and 2b interconvert with a barrier of ∼16 kcal mol-1
.
Slow crystallization of the diastereomeric mixture of phospholes
2a and 2b at room temperature afforded single crystals of 2a only
(Scheme 1).10 The metric and geometrical data for the azahelicene
and phosphole moieties of (P*,SP*)-2a are fully consistent with
those for related derivatives6b and fit nicely with the BP/SV(P)-
and BP/TZVP-optimized structures.11 For example, the helical
curvature of the aza[6]helicene fragment is classic, with an angle
of 45.8° between the pyridine ring and the terminal phenyl ring. It
is noteworthy that the twist angle between the phosphole ring and
the aza[6]helicene substituent is relatively small (26.3°), in principle
allowing an electronic interaction between the two π systems. This
is confirmed by the fact that the lower-energy excitation observed
by UV-visible spectroscopy appears at 430 nm (see the SI), which
† Universite´ de Rennes 1.
‡ State University of New York at Buffalo.
§ Universite´ Paul Ce´zanne.
9
10.1021/ja809396f CCC: $40.75
2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 3183–3185 3183

C O M M U N I C A T I O N S
Figure 2. Optimized11 structure of complex Pd(2a)2 (3′).
2+
properties of phosphole-modified azahelicenes and (ii) it is more
efficient to organize these heteroditopic ligands around a square-
planar PdII metal center than around a tetrahedral CuI center.12
In order to gain more insight into the role of the PdII ion, complex
3 was investigated using theoretical calculations. The first issue
was to confirm the absolute configuration of the P centers of
enantiomerically pure assembly 3. Calculations at the BP/SV(P)
level of theory for the ligand having a P-helix revealed that the
Pd(2a)22+ assembly is 81.6 kJ/mol more stable than its diastereoi-
somer Pd(2b)22+. This large value is in agreement with the ex-
perimental observation that the coordination of interconverting 2a
and 2b to PdII is highly stereoselective (Scheme 1).
Figure 1. CD spectra of (+)-(P,P)-3 and (+)-(P,P)-4 (solid lines) and
their respective enantiomers (dashed lines) in CH2Cl2 at 293 K.
is red-shifted compared with 2-pyridyl-1,5-diphenylphosphole (390
nm).9b,c It should be noted that the calculated11 UV-visible spec-
trum well reproduces the experimental one after a red shift of 0.25
eV.
The inversion at the phosphole P atom is a clue for obtaining
highly stereoselective coordination of (2-pyridyl)phosphole ligands
on metallic centers, such as square-planar PdII or tetrahedral CuI
ions, since the P atom can adapt its configuration to minimize steric
repulsion.6b Indeed, a diastereomeric mixture of the interconverting
aza[6]helicene phospholes (P,SP)-2a and (P,RP)-2b reacted with
Pd(CH3CN)4,2SbF6 (2/1 molar ratio) in CH2Cl2 to afford complex
3 (78% yield) as a single stereoisomer (Scheme 1). Its elemental
analysis is consistent with a [Pd(azahelicene phosphole)2 ·2SbF6]
formula. The 31P{1H} NMR spectrum of 3 displays a single peak
at 75.0 ppm, and its 1H and 13C NMR spectra show only one set of
signals assignable to the aza[6]helicene phosphole ligands. These
multinuclear NMR data compare well with those of related
dicationic PdII(2-pyridylphosphole)2 complexes having a distorted
square-planar coordination sphere in which the P atoms have a
mutual syn arrangement, in accordance with the trans effect.6b,c
The simplicity of these NMR spectra clearly shows that the
coordination of aza[6]helicene phospholes 2a and 2b to PdII is
highly stereoselective. Therefore, complex 3 was obtained as a
single enantiomer {[R]D23 ) +1275 ((2%), (c 0.01, CH2Cl2)},
whereas its mirror image {[R]2D3 ) -1250 ((2%), (c 0.01, CH2Cl2)}
was prepared using the mixture of (M,RP)-2a and (M,SP)-2b ligands.
The same approach using Cu(CH3CN)4,PF6 (2/1 ratio) afforded
complex 4 {86% yield, Scheme 1, [R]2D3 ) +910 and -900 ((2%),
(c 0.01, CH2Cl2)}, which exhibits a broad 31P NMR signal (δ 5-6
ppm) in the expected range for dicationic tetrahedral CuI(2-
pyridylphosphole)2 complexes.6b This complex was also character-
ized by high-resolution mass spectrometry and elemental analysis.
The syntheses of complexes 3 and 4 provide a unique opportunity
to investigate the impact of the nature of the metal on the chiroptical
properties of these chiral metal-bis(helicene) assemblies. The
specific molar rotation of the PdII complex 3 measured in CH2Cl2
([Φ]D ) 23 100 ( 2%) is much larger than that of its CuI analogue
4 ([Φ]D ) 13 100 ( 2%). Moreover, their CD spectra are very
different. For example, the P-helicene-containing complexes 3 and
4, namely, (+)-(P,P)-3 and (+)-(P,P)-4 (Figure 1), both display
two intense CD bands at 270 nm (negative) and ∼330 nm (positive).
However, the magnitude of the CD spectrum of the PdII complex
3 is much larger than that of the CuI complex 4 (Figure 1).
Furthermore, the CD spectrum of the PdII assembly 3 displays
intense bands at ∼370 nm as well as weak bands at lower-energy
wavelengths (410-450 nm) that are not observed for the CuI
complex 4 (Figure 1). These results clearly show that (i) it is
possible to perform a coordination-driven tuning of chiroptical
The difference in the energies of these two diastereomeric
complexes can be attributed to steric factors, as suggested by the
fact that the square-planar PdII coordination sphere of Pd(2b)22+ is
2+
much more distorted than that of Pd(2a)2 (twist angles between
the N-Pd-P planes: Pd(2b)22+, 54.8°; Pd(2a)22+, 24.7°).13 In fact,
the metric data for the PdN2P2 core of Pd(2a)22+ (3′) (Figure 2) fit
well with those of other dicationic Pd(2-pyridylphosphole)2 com-
plexes established by X-ray diffraction studies.6b,14 Significant
parameters are the twist angle between the N-Pd-P planes (3′,
24.7°; X-ray, 13.7-19.5°), the lengths of the Pd-P (3′, 2.319 and
2.302 Å; X-ray, 2.25-2.26 Å) and Pd-N (3′, 2.200 and 2.190 Å;
X-ray, 2.12-2.16 Å) bonds, and the P,N bite angles (3′, 79.9 and
79.7°; X-ray, 81.0-82.8°). The curvature of the helices is retained
in the coordination sphere of the metal (2a, 45.8°; 3′, 49.5°), and
the two twist angles between the coordinated phosphole and the
pyridine rings are 19.4 and 23.4°, allowing an electronic interaction
between the phosphole and the azahelicene π systems to take place.
For example, the shape of the LUMO+1 and HOMO-4 molecular
orbitals (MOs) clearly reflects the conjugation between the coor-
dinated P- and N-heterocycles (see the SI). It is noteworthy that
the orbitals of both the metal and the π-conjugated P,N ligands are
involved in many of the MOs (LUMO, LUMO+2, LUMO+4,...;
see the SI), showing the intimate electronic interaction between
these two fragments.
The computed11 CD spectrum of 3′ agrees very well with the
experimental one for complex 3 after a red shift of 0.25 eV (Figure
3). According to these calculations, the most intense CD bands
involve π-π* transitions of the extended phosphole-azahelix π
systems (see Figure 3 and Tables S4 and S5 in the SI). The over-
and underestimation of the CD intensities of the bands at 330 and
250 nm, respectively, are similar to those for free hexahelicene.8a,c
The PdII center is involved in many transitions and contributes in
particular to the low-energy tail of the first CD band through partial
metal-ligand charge transfer (see the SI). For example, the
dominant contribution (14%) to excitation 9 in Figure 3 is
essentially a transition from a metal-centered MO (MO308, Figure
4) to a metal-ligand MO (MO333, Figure 4). It should be noted
that these low-energy CD bands involving the metal center are not
observed for the CuI complex 4 (Figure 1), clearly showing the
impact of the metal ions on the CD properties. The long-wavelength
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C O M M U N I C A T I O N S
lographic data and a CIF file for 2a, and computational details for 3′.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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In conclusion, because of their peculiar properties (ease of
inversion at P, polarizable π systems, etc.),9 phospholes are unique
building blocks for the tailoring of azahelicene derivatives that can
be assembled on metal centers. The nature of the metal center has
a profound impact on the chiroptical properties of the assemblies,
opening a novel and potent means of tuning this key property of
chiral screw-shaped π-conjugated structures. The theoretical analysis
confirmed the process of stereoselective coordination of phosphole-
modified azahelicenes to PdII and revealed the intimate metal-helix
electronic interactions that impact the chiroptical properties of the
metal-bis(helicene) assembly.
(10) It should be noted that the crystallization was performed with a mixture of
(P*,SP*)-2a and (P*,RP*)-2b and afforded single crystals of racemic
(P*,SP*)-2a.
(11) The computations used DFT/TD-DFT with the BP and BHLYP functionals
and were performed with the Turbomole program (see the SI for details
and references).
Acknowledgment. We thank the Ministe`re de l’Education
Nationale de la Recherche et de la Technologie, the Centre National
de la Recherche Scientifique (CNRS), the Conseil Re´gional de
Bretagne, the Agence Nationale de la Recherche (ANR) (Project
PHOSHELIX-137104), and the National Science Foundation (CHE
0447321).
(12) The BP/SV(P)-optimized structures of 3 show that the coordination sphere
is distorted, as observed for related model complexes.6b
(13) It is noteworthy that the overlapping of two P-helices induces a distorted
square-planar geometry with a ∆ configuration around the Pd center (Figure
2) while M-helices induce a Λ configuration.
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F.; Toupet, L.; Re´au, R. Organometallics 2002, 21, 1591–1602.
Supporting Information Available: Experimental procedures and
spectroscopic data (UV-vis and CD spectra) for 1-4, X-ray crystal-
JA809396F
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