Ligand trans-effect: Using an old concept as a novel approach to bis(dipolar) NLO-phores

Article abstract of DOI:10.1039/b203149c

In plane parallel arrangement and enhancement of NLO-activity are observed upon coordination of heteroditopic dipoles containing a phosphole ring on square-planar d⁸-palladium centre.

Full text of DOI:10.1039/b203149c

Ligand trans-effect: using an old concept as a novel approach to
bis(dipolar) NLO-phores
Claire Fave,a Muriel Hissler,a Katell Sénéchal,a Isabelle Ledoux,b Joseph Zyssb and Régis Réau*a
a Phosphore et Matériaux Moléculaires, UMR 6509 CNRS-Université de Rennes 1, Institut de Chimie,
Campus de Beaulieu, 35042 Rennes Cedex, France. E-mail: regis.reau@univ-rennes1.fr
b Laboratoire de Physique Quantique Moléculaire, ENS Cachan, 61 avenue de Président Wilson, 94235
Cachan, France
Received (in Cambridge, UK) 3rd April 2002, Accepted 24th June 2002
First published as an Advance Article on the web 5th July 2002
In plane parallel arrangement and enhancement of NLO-
activity are observed upon coordination of heteroditopic
dipoles containing a phosphole ring on square-planar d8-
palladium centre.
ligand in the coordination sphere of a Pd(II)-complex.3c–g
Anticipating that this trans-effect could overcome the natural
anti-parallel alignment tendency of 1D-dipolar chromophores at
a molecular level, we envisaged the synthesis of in-plane
bis(dipolar) assemblies B (Scheme 1) by stereoselective
coordination of P,N-chromophores on a d8-square-planar Pd-
centre.
A recent strategy in nonlinear optics (NLO) involves gathering
together identical one dimensional (1D) donor(D)–acceptor(A)
substituted chromophores leading to molecular multi(dipolar)
systems.1 This approach however dictates that noncentrosym-
metric organisation of dipolar NLO-phores is achieved at the
molecular level. For bis(dipolar) derivatives, which are the
simplest multi(chromophoric) systems, noncentrosymmetric
structures have been obtained by coordination of 1D-chromo-
phores to tetrahedral metal ions1e or by connection of two
substituted b-naphthols1f–i (V-shaped molecules A, Scheme 1).
We propose herein a new strategy based on the trans-effect, a
long-standing concept in coordination chemistry.2 The most
useful application of this principle is for the preparation of
specific isomers of d8-square planar complexes.3a–e In ac-
cordance with Pearson’s antisymbiotic effect,2b heteroditopic
P,N-donors can control the orientation of a second chelating
2-(2-Pyridyl)phospholes act as tightly bonded 1,4-chelates
toward Pd(II) centres and, owing to the different electronic
nature of the donor sites, they undergo stereoselective coor-
dination.3f–h We have shown that phospholes possess a highly
polarisable dienic p-system4 and theoretical studies5 have
suggested that this weakly aromatic P-heterocycle is a poten-
tially interesting p-bridge for the engineering of NLO-phores.
The metal-coordinated pyridyl group will act as an electron-
withdrawing substituent,6 thus, in order to obtain the classical
dipolar D–p–A NLO-phore structure,5,6c–e electron-donating
substituents (dibutylaminophenyl, methoxythienyl) were in-
troduced at the C5-carbon atom of the P-ring. The target
2-(2-pyridyl)phospholes 2a,b were prepared via a ‘zircono-
cene’-promoted intramolecular coupling of diynes 1a,b and
subsequent addition of PhPBr2 (Scheme 1).4,7 These com-
pounds were isolated in fairly good yields as air stable solids
after purification by flash column chromatography on basic
alumina (2a, 73% yield; 2b, 55% yield). They exhibit classical
NMR spectroscopic data4,7 (Table 1) and have been charac-
terised by high resolution mass spectrometry and elemental
analyses. The UV/visible spectra of phospholes 2a,b in CH2Cl2
solution show a broad absorption in the visible region attributed
to the p–p* transition of the extended conjugated system (Table
1). The absorption maxima are comparable for both derivatives
and, as expected, are shifted to longer wavelengths (Dlmax
:
20–35 nm) than those recorded for the corresponding deriva-
tives featuring no methoxy3h or dibutylamino electron-donor
end groups. The NLO properties of the donor–acceptor
substituted phospholes 2a,b were determined by the electric-
field-induced second harmonic generation (EFISH, 1.91 mm)
and the nonresonant hyperpolarisabilities were estimated using
the two-level model.8 The EFISH measurements revealed that
phospholes 2a,b exhibit moderate NLO-activities (Table 1),
which are consistent with the weak acceptor character of the
non-coordinated pyridine group.5a,6b Note that the NLO
response of phosphole 2a is slightly superior to that of 2b.
2-(2-Pyridyl)phospholes 2a,b reacted in CH2Cl2 solution
with (CH3CN)4Pd2+, 2 BF42 giving rise, almost quantitatively,
to complexes 3a,b isolated as air stable solids (Scheme 1).
Scheme 1 (i) Cp2ZrCl2, 2 BuLi, THF, 278 to 25 °C; (ii) PhPBr2, THF, 278
to 25 °C.
Table 1 31P{1H} NMR, linear and nonlinear optical data for compounds 2a,b and 3a,b
lmaxa (nm)
e (L mol21 cm21
)
mbb; mb(0) (10248 e.s.u) b (10230 e.s.u)
c
Compound
d 31P{1H}a (ppm)
2a
2b
3a
3b
+11.5
+10.2
+70.2
+68.9
415
417
420 (550)
452 (640)
18000
17800
11900 (3200)
14400 (3100)
170; 130
120; 90
31d
170
180
a Measured in CH2Cl2. b EFISH measurements in CH2Cl2 (1022 mol L21) at 1.91 mm. c HLS measurements in CH2Cl2 at 1.91 mm. d Calculated from the
2
2
experimental HLS value recorded at 1340 nm: b1.91 = b1.34 (R1.34/R1.91); Rw = wo2/(wo 2 w2) (wo 2 4w2).
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This journal is © The Royal Society of Chemistry 2002

Complexes 3a,b were characterised by high-resolution mass
spectrometry and gave satisfactory elemental analyses. The
31P{1H} NMR spectra of the crude reaction mixtures contained
only one sharp resonance, indicating the formation of only one
geometric isomer. The large 31P NMR coordination downfield
chemical shifts ( > 50 ppm, Table 1) are consistent with the
formation of five-membered P,N-palladacycles.3f,g Only one set
of 1H and 13C{1H} NMR signals are recorded for the
2-pyridylphosphole ligands indicating that complexes 3a,b
possess a highly symmetric structure. The NMR data of the
2-pyridylphosphole moieties of 3a and 3b are comparable9 and
very similar to those of a related cis-(2-pyridyl-5-thienylphos-
phole)2Pd2+ complex, recently characterised by an X-ray
diffraction study.3g
molecular assembly. The elucidation of the origin of the
dramatic increase of the NLO-activity observed upon coordina-
tion and the non-centrosymmetric macroscopic organisation of
these new NLO-phores are under active investigation.
We thank the CNRS, the Ministère de la Recherche et de
lAEducation Nationale, the Conseil Régional de Bretagne (PRIR
n° 99CC10) and the Institut Universitaire de France for
financial support.
Notes and references
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The ionic nature of complexes 3a,b precludes EFISH
experiments and hence their first molecular hyperpolarisibilities
were measured by means of harmonic light scattering (HLS)
experiments. A fundamental wavelength of 1.91 mm was used in
order to circumvent problems associated with enhancement of b
by two-photon absorption fluorescence since 3a,b exhibit low-
energy UV-vis absorptions (Fig. 1). Complexes 3a,b exhibit
fairly high nonlinear optical activities with b values reaching
170–180 3 10230 e.s.u. These large values clearly indicate that
the trans-effect has imposed a parallel organisation of P,N-
dipoles 2a,b in the Pd-coordination sphere. As expected, the
square-planar metal centre acts as a template imposing a
noncentrosymmetric assembly of identical 1D-chromophores
2a,b. Furthermore, the metal plays a puzzling role since a
considerable enhancement of the NLO-activities is observed
upon complexation (Table 1). The b value of derivative 2b at
1.91 mm (313 10230 e.s.u) was deduced from the experimental
HLS at 1.34 µm (35 3 10230 e.s.u.) using the two-level
dispersion approximation.10 The molecular hyperpolarisibility
of complex 3b (180 3 10230 e.s.u.) is much higher than the sum
over the contribution of two sub-chromophores 2b. In a first
approach, this effect could be related to an increase of the
acceptor character of the pyridine groups and/or to a modifica-
tion of the phosphole dienic p-system polarisability4 upon
coordination. However, it is very likely that the origin of this
large b enhancement is due to the appearance of new
contributions to the second-order molecular hyperpolarisability.
This assumption is supported by the UV-vis spectra of
complexes 3a,b that show two maxima (Fig. 1). Phospholes can
be regarded as classical phosphines7b,c acting predominantly as
s-donors whereas the p-acceptor ability of pyridine is well-
kwown.6 It is thus very probable that the low energy UV-vis
absorptions are due to charge transfers from the metal or the
phosphorus-metal fragments to the pyridine ligands.11 A simple
vector model shows that these metal-to-ligand (MLCT) or
ligand-to-metal-to-ligand charge transfers (LMLCT) will co-
herently contribute to the second harmonic generation (mole-
cule B, Scheme 1).
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In conclusion, we have described the first NLO-phores based
on phosphole rings and we have shown that coordination
chemistry offers a simple synthetic methodology for controlling
the in-plane parallel arrangement of 1D-P,N-dipoles in a
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8 According to the two-state model, the nonresonant hyperpolarisability
2
2
can be estimated as b0 = bw (wo 2 w2) (wo 2 4w2)/wo2 where bw is the
value measured at 2w with the fundamental laser at w.6c–e
.
9 Selected 13C{1H} NMR data for complexes 3a,b (75.469 MHz, CDCl3).
3a: d 157.4 (m, C2, py), 153.3 (m, PCCphos), 152.6 (s, C6 py), 151.1 (d,
J = 18.1 Hz, PCC phos), 141.1 (s, C4 py), 125.3 (s, C5 py), 124.2 (s, C3
py); 3b: d 153.6 (m, C2 py), 152.8 (m, PCC phos), 151.7 (s, C6 py);
151.1 (d, J = 17.3 Hz, PCC phos), 140.9 (s, C4 py), 134.9 (d, J = 54.9
Hz, PC phos), 132.5 (d, J = 52.9 Hz, PC phos), 124.8 (s, C5 py), 123.7
(m, C3, py).
10 Note that care must be taken when comparing HLS b values obtained
under different experimental conditions: P. Kaatz and D. P. Shelton, J.
Phys. Chem., 1996, 100, 8157 and ref. 6a.
11 Theoretical calculations in order to elucidate the origin of these
transitions are in progress. Note that a negative solvatochromism was
observed for these transitions.
Fig. 1 UV-visible spectra of complexes 3a and 3b.
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