Grafting of borane-protected aliphatic and aromatic aminophosphine ligands to glassy carbon electrodes

Article abstract of DOI:10.1016/j.elecom.2011.05.018

Phosphine-borane moieties with amino-anchoring groups were covalently grafted on glassy carbon substrates, using free radical electrografting methods. The oxidative destruction of phosphine was efficiently quenched thanks to the borane protecting group. T

Full text of DOI:10.1016/j.elecom.2011.05.018

Electrochemistry Communications 13 (2011) 844–847
Contents lists available at ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Grafting of borane-protected aliphatic and aromatic aminophosphine ligands to
glassy carbon electrodes
a,
Quang Thuan Tran a, Jean-François Bergamini a, Claire Mangeney b, Corinne Lagrost a, , Pascal Pellon
⁎
⁎
a
Sciences Chimiques de Rennes (Equipe MaCSE), Université de Rennes 1, CNRS, UMR N°6226, Campus de Beaulieu, 35042 Rennes Cedex, France
b
ITODYS, Université Paris Diderot-Paris 7, CNRS, UMR N°7086, 15 rue Jean de Baïf, 75013 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 April 2011
Received in revised form 12 May 2011
Accepted 12 May 2011
Available online 19 May 2011
Phosphine–borane moieties with amino-anchoring groups were covalently grafted on glassy carbon
substrates, using free radical electrografting methods. The oxidative destruction of phosphine was efficiently
quenched thanks to the borane protecting group. The chelating ability of the attached phosphine ligands was
exemplified with Mn.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Amine oxidation
Diazonium
Surface modification
Phosphine–borane ligand
1. Introduction
2. Experimental
Electrode surfaces modified with organometallic complexes have
been studied for their potential applications in electrocatalysis or in
heterogeneous catalysis [1–5]. Phosphine compounds exhibit excel-
lent chelating ability towards low and high oxidation-state transition
metal derivatives and, therefore, would be particularly attractive to
graft on a surface. However, only a few works report on the grafting of
chelating phosphines on substrates [3,6,7], probably because their
high sensitivity towards oxidation makes them difficult to handle. For
catalytic applications, robust linkage must be built between substrates
and phosphine compounds to prevent from the leaching of the tran-
sition metals. Free radical grafting methods onto carbon surface are
probably the most convenient procedure to covalently introduce
organic moieties on surfaces. The main related strategies consist of the
electrochemical oxidation of amines or reduction of aryldiazonium
salts [8,9]. However, the oxidation potential of phosphines being
lower than that of amines, the phosphorus atom must be protected,
especially when applying the first strategy. As phosphines protecting
group, borane group was successfully reported to impede their
chemical oxidation [10].
2.1. Syntheses of amino-phosphine borane
(3-aminopropyl)diphenylphosphine–borane (1) was prepared
from the reaction of PPh2(Li).BH3 with 7.2 mmol 3-chloropropylamine
(98%, Aldrich) at −80 °C in THF [10]. After recrystallization in diethyl
ether (75% yield) and neutralization with 2 M aqueous NaOH, (1) was
obtained as white powder.
δ (ppm) CDCl3, 1H 300 MHz: 7.70–7.30 (m, 10 H), 2.76 (t, 2 H),
2.26 (m, 2 H), 1.67 (m, 2 H), 1.20 (bs, 2 H), 1.00 (m, 3 H); 31P 121 MHz:
16.15 (m).
(4-aminobenzyl) diphenylphosphine–borane (2) was obtained
from PPh2(Li).BH3 and 7.2 mmol 4-nitrobenzylbromide (Aldrich, 99%).
A subsequent reduction of the nitro moiety with SnCl2 (Aldrich, 99%) in
EtOH gave (2) (14% yield) [11].
δ (ppm) CDCl3, 1H 300 MHz: 7.70–7.30 (m, 10 H); 6.74 (dd,3JHH
=
8.5 Hz, 4JPH =2.1 Hz, 2 H); 6.48 (d,3JHH =8.5, 2 H); 3.58 (bs, 2 H); 3.50
2
(d, JPH =9.2 Hz, 2 H); 31P 121 MHz: 17.15 (m).
2.2. Electrochemistry
Using both strategies, we describe here the electrografting of
aminophosphine–borane ligand. This work aims at demonstrating the
efficiency of borane protection towards electro-oxidation and the
ability of the immobilized ligands for chelating transition metals after
a simple deprotection step.
Electrochemical measurements were performed using an Autolab
electrochemical analyzer (PGSTAT 30, EcoChemie BV). The reference
electrode was SCE, equipped with a salt bridge (and calibrated with
ferrocene), and the counter electrode a Pt foil.
Glassy carbon (GC) disk or plates electrodes were used as sub-
strates in the preparation of electrodes functionalized with the
aminophosphine–borane species. The substrates were polished
with DP-Nap paper 1 μm and Al2O3 slurry 0.3 μm (Struers). Electro-
⁎
Corresponding authors.
E-mail addresses: corinne.lagrost@univ-rennes1.fr (C. Lagrost),
pascal.pellon@univ-rennes1.fr (P. Pellon).
1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2011.05.018

Q.T. Tran et al. / Electrochemistry Communications 13 (2011) 844–847
845
Scheme 1. Structures of the aminophosphine–borane species and their immobilization onto the electrode surfaces.
oxidative grafting of (1) (5 mM) was performed in MeCN containing
0.1 M Bu4NPF6 (Acros, electrochemical grade), by holding the
potential at 2 V for 5 min while the grafting of (2) (5 mM) was
carried out at 1 V for 5 min in MeOH+0.1 M LiClO4 (Aldrich) in the
presence of 0.25 M lutidine (Acros) [12]. The aromatic aminophos-
phine–borane allowed to generate the corresponding diazonium salt,
following the procedure by Bélanger and co-workers [13]. (4-benzyl)
diphenylphosphine–borane was grafted by the electrochemical
reduction of the corresponding diazonium, in situ generated in
0.5 M aqueous HCl, containing 10 mM NaNO2 (Aldrich, 97%) and
5 mM of (2). The electrografting was achieved by applying a constant
potential at −0.75 V for 5 min (Scheme 1). The electrodes were then
rinsed with ultrapure water and absolute EtOH.
The disk electrodes modified by the phosphine–borane ligands
were treated for 4 h in toluene containing 1,4-diazabicyclo[2.2.2]
octane (DABCO) at 55 °C, for deprotecting the phosphine group [14].
B
A
120
60
40
20
0
80
40
0
2nd
1st
3rd
after 5min-electrolysis
0.0
0.5
1.0
1.5
2.0
0.0
0.5
1.0
1.5
2.0
E (V, vs. SCE)
E (V, vs. SCE)
C
D
120
80
40
0
0
-10
-20
-30
-0.8
-0.4
0.0
0.4
0.8
0.0
0.4
0.8
E (V, vs. SCE)
E (V, vs. SCE)
Fig. 1. (A) CV of 9 mM borane-free analog of 1 in MeCN containing 0.1 M Bu4NPF6 v=200 mV/s. (B–D) CVs demonstrating the grafting of 5 mM phosphine–borane species. (B)
Electrooxidation of 1 in MeCN+0.1 M Bu4NPF6, v=200 mV s−1 (3 scans and after 5 min electrolysis at E=2 V). (C) Electrooxidation of 2 in MeOH+0.1 M LiClO4 +50 eq lutidine,
v=100 mV s−1 (1st scan and after 5 min electrolysis at E=1 V). (D) electroreduction of diazonium salt from 2 in 0.5 M HCl+10 mM NaNO2, v=100 mV s−1
.

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Q.T. Tran et al. / Electrochemistry Communications 13 (2011) 844–847
Table 1
P/C atomic ratio calculated from XPS analyses and surface concentrations for the modified GC surfaces.
P/C
ΓXPS mol cm−2
Introduction of Mn(I)
complexes E°(V)
ΓECHEM-Mn mol cm−2
ΓthCPML-Mn mol cm−2
1 hydrochloride
Electro-oxidation of 1
Electro-oxidation of 2
Diazonium generation of 2 and electro-reduction
0.072
0.029
0.022
0.013
–
–
–
2.1·10−10
1.6·10−10
0.94·10−10
1.52
1.43
1.38
1.35–2.6·10−10
1.3–2.6·10−10
0.6–1.2·10−10
1.02·10−10
0.8·10−10
0.8·10−10
Then the electrodes were refluxing in CH2Cl2 containing Mn(CO)5Br
for 4 h, yielding the corresponding organometallic complexes immo-
bilized onto the GC surfaces.
(Fig. 1B and C). These results indicate the formation of a blocking film
on the surface.
The electrochemical reduction of in situ generated aryldiazonium
salt is another efficient procedure to strongly attach organic moiety
onto carbon surface via the formation of C―C bonds [13], contrariwise
to the former method that produces C―N bonds. Cyclic voltammo-
grams of 2 in aqueous HCl containing NaNO2 exhibit a large
irreversible reduction peak located at 0.06 V (Fig. 1D). This peak is
due to the reduction of the generated diazonium cation. It produces an
aryl radical species that grafts onto the carbon surface. As a result, the
peaks disappear on subsequent potential scans.
In order to further confirm the grafting of the phosphine–borane
ligands onto the carbon surface, XPS analyses were performed on 3
samples of modified GC plates. Low-intensity signals due to phospho-
rous atoms in a BH3–PPh2 environment were observed at 133 (P2p) and
190 (P2s) eV. It is worth outlining that the signal at 190 eV contains two
distinct contributions: P2s and B1s. After subtracting the P2s contribu-
tion (estimated from the P2s/P2p intensity ratio measured on
triphenylphosphine as reference sample), an atomic P/B ratio close to
1 was calculated. These analyses unambiguously demonstrate the
presence of phosphine–borane ligands at the carbon surface. Assuming
a carbon atom surface density equal to that of basal plane graphite, the
surface concentration (ΓXPS) of the phosphine–borane ligands could be
roughly estimated from the atomic P/C ratio [18]. Note that this
calculation provides underestimated values as only one monolayer of
the carbon atoms of the substrates is considered. Because the sampling
depth for XPS is ~10 nm, more than one monolayer of carbon atoms are
obviously probed.
2.3. XPS
XPS photoelectron spectra were collected from modified GC plates
on a Thermo VG Scientific Escalab 250 system fitted with a mono-
chromatic Al Kα X-ray source (hν=1486.6 eV, spot size=650 μm,
power 15 kV×200 W). The pass energy was set at 150 and 40 eV for
the survey and the narrow regions, respectively. Spectral calibration
was determined by setting the main C1s component at 285 eV. The
surface composition was estimated using the integrated peak areas
and the corresponding Scofield sensitivity factors corrected for the
analyzer transmission function.
3. Results and discussion
The electrochemical oxidation of 1 and 2 was investigated in
MeCN+0.1 M Bu4NPF6 and compared to that of their borane-free
analogs. In the absence of borane moiety, an irreversible oxidation
peak corresponding to the oxidation of the phosphine group is
observed at potential less positive than that corresponding to the
amine group. This precludes any electrografting of the phosphine-
ligand using the electro-oxidation of the amino group [8,15]. Addition
of borane substituent onto the phosphine group allows the selective
oxidation of amine group for 1 and 2, keeping intact the phosphine
group and its chelating ability. For instance, the cyclic voltammetry of
the borane-free analog of 1 displays an irreversible oxidation peak at
1.05 V (Fig. 1A) contrariwise to 1 that does not exhibit this peak, only
allowing the oxidation of amine at 1.7 V (Fig. 1B). The borane moiety
is an efficient protecting group towards the electrochemical oxidation,
even for highly positive potentials.
It is further interesting to evaluate the chelating ability of the
immobilized phosphine group towards coordination compounds.
After a deprotection step, the modified electrodes were used as an
active platform for introducing metallic Mn(I) complexes using Mn
(CO)5Br as a model system. Cyclic voltammograms of the Mn (I)-
modified electrodes in CH2Cl2 +0.2 M Bu4NPF6 display reversible
signal centered at 1.52, 1.43 and 1.38 V, respectively (see Table 1)
corresponding to the Mn(II)/Mn(I) couple (Fig. 2). The peak currents
The grafting route based on the reduction of diazonium salt does
not require protecting phosphine from its oxidation. However, phos-
phine groups are very sensitive to air oxidation. Then an additional
interest to borane protection is the possibility to store and manipulate
easily the phosphine-modified surfaces under ambient atmosphere.
60
40
30
20
40
The grafting of primary amines to carbon surface by electro-
oxidation is nowadays a well-documented procedure, namely
concerning the aliphatic amines [8,15]. Mechanistic studies have
demonstrated that the radical cation initially formed upon electron
transfer deprotonates to give an aminyl radical that covalently binds
to the carbon surface [15]. More recently, several works successfully
applied this strategy to aromatic amines, leading to the binding of
the corresponding aromatic aminyl radical to the carbon surface
[12,16,17]. Cyclic voltammetry recorded for the oxidation of 1 and 2
is consistent with the grafting of a film onto the electrode surface.
Irreversible peaks are observed at Epa=1.7 V (Fig. 1B) and 0.85 V
(Fig. 1C) for 1 and 2, respectively and assigned to the amine
oxidation. On successive scans, the peak current decreases and
disappears after 5 scans. Similarly, after 5 min-electrolysis at 2 V and
1 V for 1 and 2, respectively, no oxidation peak could be further
detected in the voltammograms recorded in the modifier solution
10
0
0.0 0.1 0.2 0.3 0.4
-1
20
0
v (V.s
)
-20
0.0
0.4
0.8
1.2
1.6
E (V, vs. SCE)
Fig. 2. CV in CH2Cl2 +0.2 M Bu4NPF6 of surface-modified electrode after electrooxida-
tion of 2 and formation of the corresponding Mn complex. v are 0.05, 0.1, 0.2, 0.3 and
0.4 V s−1
.

Q.T. Tran et al. / Electrochemistry Communications 13 (2011) 844–847
847
vary linearly as a function of the scan rates in the range 0.05–0.5 V s−1
as expected for surface-immobilized redox species (Fig. 2) [19].
compounds are excellent ligands towards transition metals, this
work provides a simple and versatile method for immobilizing such
organometallic complexes onto carbon surfaces, allowing the
possible electrochemical tuning of their oxidation state. Further
work is currently in progress to use these phosphine-modified
electrodes in catalytic reactions.
Surface concentration of the immobilized Mn complexes (ΓECHEM-Mn
)
for the three experiments could be derived from the Faraday's law by
taking account the large roughness of our GC electrodes
ΓECHEMꢀMn ¼ Q=nFAρ
where Q is the charge obtained from integration under the oxidation
peak (baseline corrected), n=1 the number of electrons exchanged, F
the Faraday constant (96,500 C mol−1), A the geometric area of the
electrode (0.076 cm2) and ρ is the roughness of the electrode, lying
between 4 and 8 [20]. Molecular mechanics optimization (MM2) allows
to roughly estimate the molecular surfaces of Mn-1 and Mn-2
complexes to be 150 and 190 Å2, respectively. The experimental surface
coverage determined from electrochemical data was then found to
correspond to 1–3 close-packed monolayers (Table 1).
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to carbon surfaces using free radical grafting methods, namely the
electrooxidation of aliphatic or aromatic amines and the electro-
reduction of aryldiazonium salt in situ generated from its corre-
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