Phosphonate derivatives of pyridine grafted onto oxide nanoparticles

Article abstract of DOI:10.1016/S0040-4039(02)02240-2

The synthesis of phosphonate derived stilbazole 1 by using a Heck reaction is described. The reactivity of the pyridine moiety is studied. After hydrolysis of the ethyl phosphonate groups, the grafting of phosphonic acid derived stilbazole 5 on metal oxides TiO₂ or SnO₂ is reported. The accessibility and reactivity of the pyridine moiety at the surface is demonstrated.

Full text of DOI:10.1016/S0040-4039(02)02240-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9115–9117
Phosphonate derivatives of pyridine grafted onto oxide
nanoparticles
Richard Frantz,* Michel Granier, Jean-Olivier Durand and Ge´rard F. Lanneau
Chimie Mole´culaire et Organisation du Solide UMR 5637, case courrier 007, Universite´ Montpellier 2, place Euge`ne Bataillon,
F-34095 Montpellier cedex 05, France
Received 23 September 2002; revised 8 October 2002; accepted 9 October 2002
Abstract—The synthesis of phosphonate derived stilbazole 1 by using a Heck reaction is described. The reactivity of the pyridine
moiety is studied. After hydrolysis of the ethyl phosphonate groups, the grafting of phosphonic acid derived stilbazole 5 on metal
oxides TiO2 or SnO2 is reported. The accessibility and reactivity of the pyridine moiety at the surface is demonstrated. © 2002
Published by Elsevier Science Ltd.
Stilbazoles derivatives and p-conjugated compounds
with a pyridine unit have been extensively studied for a
wide range of applications, as ligands for transition
metal complexes,1 self-assembled monolayers (SAMs)2
or in the field of organic–inorganic hybrid materials for
Non-linear Optics (NLO).3 Generally SAMs are based
on the interactions of chlorosilanes with OH functions
at the surface of oxides4 or thiolates compounds
adsorbed at the surface of gold.5 In the case of n-alkyl-
silanes monolayers, the reaction is sensitive to traces of
water, homocondensation could occur, with in homo-
geneities on the modified surface. In the case of thiols
the use of gold, which is very expensive, could be a
limiting factor. The formation of monolayers with car-
boxylic acids,6 hydroxamic acids7 and phosphonic
acids8 could be an issue to these problems. So we
present here, the synthesis of a phosphonate derivative
with a p-conjugated pyridine unit and the studies of the
grafting onto oxides nanoparticles.
Diethyl 4-(2-pyridin-4-yl-vinyl)-benzylphosphonate
1
was obtained as described in Scheme 1. Heck reaction,
in standard conditions,9 of 4-vinylpyridine with diethyl
4-bromobenzylphosphonate, synthesized as described in
the literature,10 gave compound 1 in good yield.† We
verified the reactivity of the pyridine moiety of com-
pound 1, which is a potential ligand for transition metal
complexes (Scheme 2).
† Preparative method for diethyl 4-(2-pyridin-4-yl-vinyl)-benzylphos-
phonate 1: In a two necked round-bottomed flask were placed 5 g of
diethyl 4-bromobenzylphosphonate (16.3 mmol), 1.72
g of 4-
vinylpyridine (16.3 mmol), 73.2 mg of palladium diacetate (0.326
mmol), 347 mg of tris-orthotoluylphosphine (1.14 mmol), 6.8 mL of
triethylamine and the mixture was diluted in 25 mL of toluene. The
reaction mixture was heated at 115°C under magnetic stirring. After
12 h the solution was allowed to cool to rt and then was filtered on
Celite and washed three times with toluene. The filtrate was then
concentrated to give a yellow liquid which was purified by chro-
matography column on SiO2, (eluent: ethyl acetate). We obtained
compound 1 after recrystallization on cyclohexane as white needles:
m=4.83 g, yield 89%. Mp=62.2–64.1°C. 1H NMR (200 MHz,
CDCl3, 293 K): l 1.29 (t, 3JHH=7.0, 6H, CH3); 3.20 (d, 2JHP=21.9,
3
3
2H, CH2-P); 4.03 (m, JHH=7.0, 4H, CH2-O); 7.03 (d, Jtrans=16.0,
1H, vinyl); 7.30 (d, 3Jtrans=16.0, 1H, vinyl); 7.35–7.55 (m, 6H,
aromatic); 8.60 (d, 3JHH=6.1, 2H, CH-N). 13C NMR (200 MHz,
CDCl3, 293 K): l 16.8 (d, 3JPC=6, CH3); 34.1 (d, 1JPC=138,
CH2-P); 62.6 (d, 2JPC=6.8, CH2-O); 121.2; 127.6; 130.6; 132.8;
135.2; 144.9 (aromatics); 126.3; 133.1 (C vinyl); 150.6 (C-N). 31P
NMR (200 MHz, CDCl3, 293 K): l 27.2. HRMS (FAB+, GT):
332.1400 (calcd for MH+); 332.1416 (obs.).
Scheme 1. Reagents and conditions: (i) Pd(OAc)2 (4 mol%),
tris-orthotoluylphosphine (15 mol%), 6 equiv. Et3N, toluene,
115°C, 12 h.
* Corresponding author. Fax: +33-4-67-14-38-52; e-mail: frantz@
inorg.chem.ethz.ch
0040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)02240-2

9116
R. Frantz et al. / Tetrahedron Letters 43 (2002) 9115–9117
Scheme 2. Reagents and conditions: (i) 1 equiv. of BF3·Et2O,
THF, rt, 4 h; (ii) 0.25 equiv. of RuCl3·xH2O, H2O:EtOH
(1:1), 100°C, 5 h; (iii) 1.5 equiv. of CH3I, CH3CN, rt,
overnight.
Scheme 3. Grafting of precursor 5 and reactivity at the
surfaces. Reagents and conditions: (i) a. 3 equiv. of Me3SiBr,
CH2Cl2, rt, 12 h, b. 15 equiv. H2O, rt; (ii) H2O, 100°C, 3 days;
(iii) excess of BF3·Et2O, THF, rt, 24 h.
Compound 1 treated in THF solution at room temper-
ature with BF3·Et2O gave the corresponding Lewis
acidic borane adduct 2.14 No complexation by the
phosphonate function occurred. In the same way, based
on the literature,11 adding 4 equiv. of 1 to a solution of
RuCl3·xH2O in a mixture of EtOH:H2O (1:1) at reflux
led to the corresponding ruthenium complex
[Ru(L)4Cl2] (with L=compound 1). Finally, a solution
of compound 1 treated with methyliodide in acetonitrile
at room temperature gave the corresponding N-
methylpyridinium salt 4. These three reactions con-
firmed the good ability of the pyridine moiety to form
adducts or metal transition complexes. 31P NMR con-
firmed the stability of the phosphonate group, no mod-
ification of the chemical shift (27 ppm on average) was
observed. The grafting of compound 5 on different
oxide particles was next examined (Scheme 3).
Figure 1. hpdec MAS 31P NMR of solids 5, Ti1, Sn1.
that the phosphonic group has reacted at the surface of
the oxide with formation of PꢀOꢀmetal bonds. The
phosphorus atom has different environments on the
surface of the oxide because of different coordination
modes, one two or three PꢀOꢀmetal bonds are likely to
be involved in the anchoring at the surface (Scheme 3
corresponds to two PꢀOꢀM bonds). Elemental analysis
showed a low ratio P/MO2 between 1.5 and 2% charac-
teristic of a monolayer coverage of 5 at the surface of
the oxide particles.12–14 In order to verify the reactivity
of the pyridine functions at the surface, particles Ti1
and Sn1, in suspension in THF, were treated with
BF3·Et2O for 24 h. Solids Ti1B and Sn1B (Scheme 3)
were characterized by solid-state 31P and 11B NMR
spectroscopy. No modification of the 31P NMR signals
were observed, which confirmed that there was no
modification of the phosphorus part anchored on the
surface. Compounds Ti1B and Sn1B gave sharp solid-
state 11B NMR signals at l=−1.8 and −1.9 ppm,
respectively, characteristic of borane adducts (Fig. 2).
Treatment of compound 1 with an excess of bro-
motrimethylsilane (3 equiv.) in dichloromethane at
room temperature led to the corresponding phosphonic
acid 5 after hydrolysis with excess of water (15 equiv.).
We then performed the grafting reactions of acid 5
(Scheme 3) with two types of particle oxides, TiO2 P25
Degussa (particle size of 21 nm) (Ti1), SnO2 Merck
(particle size 20 nm) (Sn1). The surface of TiO2 and
SnO2 were modified by refluxing the particles for 3 days
with phosphonic acid 5 dissolved in H2O at 100°C, this
solvent was chosen because of the low solubility of the
acid 5. A large excess of acid was used to ensure total
coverage of the particle surfaces. The solids obtained
were analyzed using elemental analysis and solid-state
31P NMR spectroscopy. Solid-state 31P hpdec mas
NMR (Fig. 1) of acid 5 gave a symmetrical signal at
l=21.1 ppm, characteristic of a homogeneous com-
pound. As benzylic phosphonic acids are usually shifted
to 27–30 ppm,13 compound 5 is in a zwitterionic form.
The signals of Ti1 and Sn1 are shifted below 20 ppm,
The unsymmetrical peaks width at half maximum have
been enlarged to l=15 ppm. These spectra indicate
Elemental analysis showed ratios B/N of 0.95 and 0.9
for Ti1B and Sn1B, respectively, which confirmed that

R. Frantz et al. / Tetrahedron Letters 43 (2002) 9115–9117
9117
Acknowledgements
We thank the Degussa society for a gift of TiO2 P25.
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