Improved synthesis of diethyl ferrocenylphosphonate, crystal structure of (FcPOFcPO3Et22)2 ·ZnCl 2, and electrochemistry of ferrocenylphosphonates, FcP(O)(OR)2, FcCH2P(O)(OR)2, 1,10′-fc[P(O)(OR)2]2 and [FcP(O)(OEt) 2]2 ·ZnCl2

Article abstract of DOI:10.1016/j.jorganchem.2004.05.031

An improved synthesis of diethyl ferrocenylphosphonate using the ^tBuLi/^tBuOK system at low temperature is reported and the structure of [FcPO₃Et₂]₂ ·ZnCl₂complex is described. The electrochemical behaviour of FcP(O)(OEt)₂, 1,1′-fc[P(O)(OEt) ₂]₂, FcCH₂P(O)(OEt)₂, and their corresponding acids were compared. Each of them shows a reversible one-electron transfer reaction. Ferrocenylbisphosphonate is more difficult to oxidize than ferrocenylphosphonate due to the presence of two electron-withdrawing substituents. A methylene spacer between the ferrocenyl unit and the phosphonate group renders the compound easier to oxidize. The acids are easier to oxidize than the esters, and their salts, in which the phosphonate group behave as an electron-donating group, are even easier to oxidize than the ferrocene. The ferrocenylphosphonic acid may be, then, considered as a redox-active pH responsive molecule. Elsevier B.V. All rights reserved.

Full text of DOI:10.1016/j.jorganchem.2004.05.031

Journal of Organometallic Chemistry 689 (2004) 2654–2661
www.elsevier.com/locate/jorganchem
Improved synthesis of diethyl ferrocenylphosphonate, crystal
structure of (FcPO3Et2)2 ÆZnCl2, and electrochemistry of
ferrocenylphosphonates, FcP(O)(OR)2, FcCH2P(O)(OR)2,
1,10-fc[P(O)(OR)2]2 and [FcP(O)(OEt)2]2 ÆZnCl2
(Fc=(g5C5H5)Fe(g5C5H4), fc=(g5C5H4)Fe(g5C5H4), R=Et, H)
´ ´ ´ ´
Olivier Oms, Frederic Maurel, Francis Carre, Jean Le Bideau, Andre Vioux,
*
Dominique Leclercq
´ ´ `
Laboratoire de Chimie Moleculaire et Organisation du Solide (CNRS UMR 5637), Universite Montpellier 2, Place Eugene Bataillon,
Case courrier 007, 34095 Montpellier Cedex 05, France
Received 11 March 2004; accepted 25 May 2004
Available online 3 July 2004
Abstract
t
An improved synthesis of diethyl ferrocenylphosphonate using the BuLi/tBuOK system at low temperature is reported and the
structure of [FcPO3Et2]2 ÆZnCl2 complex is described. The electrochemical behaviour of FcP(O)(OEt)2, 1,10-fc[P(O)(OEt)2]2,
FcCH2P(O)(OEt)2, and their corresponding acids were compared. Each of them shows a reversible one-electron transfer reaction.
Ferrocenylbisphosphonate is more difficult to oxidize than ferrocenylphosphonate due to the presence of two electron-withdrawing
substituents. A methylene spacer between the ferrocenyl unit and the phosphonate group renders the compound easier to oxidize.
The acids are easier to oxidize than the esters, and their salts, in which the phosphonate group behave as an electron-donating group,
are even easier to oxidize than the ferrocene. The ferrocenylphosphonic acid may be, then, considered as a redox-active pH respon-
sive molecule.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Diethyl ferrocenylphosphonate; Ferrocenylphosphonic acid; Electrochemistry
1. Introduction
drawing substituents increase the oxidation potential (de-
crease the reactivity towards oxidation), while electron-
donating substituents decrease the oxidation potential
and thus enhance the reactivity of ferrocene [2–9]. This
change of the oxidation potential of the ferrocenyl centre
with the electronic effect of the substituent (chelating li-
gand) has been used to electrochemically sense neutral
or ionic guest molecules allowing their amperometric or
potentiometric titration [1,10–14]. Phosphines substitu-
ents on the ferrocenyl unit allow incorporation of a re-
dox-active moiety into transition metal complexes, often
Ferrocene and ferrocenyl derivatives are well known
for their ability to undergo reversible one-electron oxida-
tion [1]. The redox potential depends on the electronic
effect of the ring substituents on ferrocene. Electron-with-
*
Corresponding author. Tel.: +33-4-67-14-45-28; fax: +33-4-67-14-
38-52.
E-mail address: dleclercq@univ-montp2.fr (D. Leclercq).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.05.031

O. Oms et al. / Journal of Organometallic Chemistry 689 (2004) 2654–2661
2655
O
O
O
CH2
P
P
P
O
P
OR
OR
OR
OR
OR
Fe
Fe
Fe
OR
RO
RO
1,1'-fc[PO3R2]2
FcPO3R2
R = Et, H
FcCH2PO3R2
Scheme 1. The ferrocenylphosphonates.
leading to an increase in the reactivity of the complex [1].
Coordinated ferrocenylphosphine ligands usually exhibit
simple, reversible electrochemistry arising from one-elec-
tron oxidation of the ferrocenyl centre, while free ferroce-
nylphosphine displays complex redox chemistry [9,15]. By
contrast, there are few reports about the electrochemistry
of ferrocenylphosphine oxide [9,16], and ferro-
cenylphosphonates [17–20], despite their potential for
co-ordination chemistry [21–23], for preparation of
redox-active metal phosphonates materials and surface
modification of metals and metal oxides [24].
and diphenyl ferrocenylmethylphosphonate were pre-
pared according to Henderson et al. [19,25].
All experiments were performed under an inert atmo-
sphere.
2.2. Syntheses
2.2.1. Diethyl ferrocenylphosphonate
In a three-necked flask, 100 ml of THF was added
to 5 g of ferrocene (27 mmol) and 0.54 g of potassium
tert-butoxide (5 mmol). The solution was stirred for
20 min at room temperature before being frozen at
ꢀ78 ꢁC. tBuLi (19 mmol) in pentane was added
drop-wise from a dropping funnel. Stirring was main-
tained 30 min before the drop-wise addition of 27 mmol
of chlorodiethylphosphate dissolved in 10 ml of THF.
After 20 min, the mixture was left to return to room
temperature. The black solution was washed with
200 ml of NaOH 1M. The organic phase was extracted
with CH2Cl2 (3·30 ml), dried over MgSO4, filtered
and the solvents were removed under vacuum. The
black residue was dissolved in CH2Cl2 (ca. 2 ml) and
purified with silica column chromatography, using first
CH2Cl2 as eluting solvent to removed excess ferrocene
and the mixture of CH2Cl2/THF (70/30) to obtain 4.9 g
(15 mmol) of FcP(O)(OEt)2 as a brown oil (79% yield).
In this work, we report the electrochemical behaviour
of the ferrocenyl unit linked to:
(i) one phosphonate group, FcPO3R2,
(ii) two phosphonate groups in the 1,10 position, 1,10-
fc[PO3R2]2,
(iii) one phosphonate group through a methylene spac-
er, FcCH2PO3R2 (Scheme 1).
The influence of the change from the ester to the acid
and their salts on the electrochemistry of the ferrocenyl
unit is discussed. The crystal structure and the electro-
chemistry of the complex of diethyl ferrocenylphospho-
nate with zinc chloride are presented.
The preparation of most of these phosphonates deriv-
atives has been already reported [25]. However, the low
yield obtained in the preparation of diethyl ferro-
cenylphosphonate prompted us to search for a more ef-
ficient method for its preparation. The preparation of
diethyl ferrocenylmethylphosphonate, which was not re-
ported by Alley and Henderson [25] is given.
3
1H NMR (CDCl3, d ppm): 1.37 (t, 6H, JHH =7.1 Hz);
4.16 (m, 4H); 4.34 (s, 5H); 4.42 (m, 2H); 4.54 (m, 2H);
31P NMR (CDCl3, d ppm): 27.0; (CD3OD, d ppm)
3
28.5; 13C NMR (CDCl3, d ppm): 16.85 (d, JCP =6.5
Hz); 62.04 (d, JCP =6 Hz); 67.28 (d, JCP =215.3
2
1
2
Hz); 70.24 (s); 71.6 (d, JCP =14.1 Hz); 71.92 (d,
3JCP =15.6 Hz). IR m (cmꢀ1) (P‚O) 1245. Anal. Calc.
for C14H19FePO3: C, 52.17; H, 5.90. Found: C, 52.16;
H, 6.26%.
2. Experimental
2.1. Solvents and reagents
2.2.2. Ferrocenylphosphonic acid
From Me3SiBr: 4.14 g (13 mmol) of diethyl ferro-
cenylphosphonate was dissolved in 25 ml of CH2Cl2 in
a Schlenk tube. 5.1 g (39 mmol) of Me3SiBr was drop-
wise added with stirring at room temperature. After 12
h, the solvent was evaporated to dryness and the oil dis-
solved in 25 ml CH3CN. 4 ml of water was added to pre-
cipitate FcPO3H2, which was filtered and washed
successively with CH2Cl2 and Et2O, leading to 2.82 g
(11 mmol) of ferrocenylphosphonic acid (yield: 85%).
t
Ferrocene (Aldrich) was sublimed before use. BuLi
(Acro˜s) 1.5 M solution in pentane was titrated prior to
use. THF was distilled first over CaH2 and then over
Na/benzophenone. CH2Cl2 was dried over P2O5.tBuOK
(Avocado), diethyl chlorophosphate (Acro˜s), dic-
yclohexylamine (Merck) were used as received. Ethyl
ferrocenylmethylphosphonic acid, ferrocenylmethyl-
phosphonic acid, 1,10-ferrocenylbisphosphonic acid,

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O. Oms et al. / Journal of Organometallic Chemistry 689 (2004) 2654–2661
From Me3SiCl: In a two-necked flask, 6.24 g
About 5 g (23 mmol) of hydroxymethylferrocene pow-
der was added in 1 g portions and the reflux was main-
tained for 3 h. The solution was left to return to room
temperature and 0.95 g (11 mmol) of NaHCO3 in 120
ml of water was added and stirred for 10 min. The or-
ganic and aqueous phases were separated.
(19 mmol) of diethyl ferrocenylphosphonate and 5.79 g
(39 mmol) of NaI were dissolved in 70 ml of CH3CN.
Me3SiCl (5 ml, 39 mmol) was added drop-wise leading
to the precipitation of NaCl. After stirring for 12 h,
20 ml of MeOH was added. After filtration, the solvents
were evaporated under vacuum, leading to a yellow
powder, which was washed, successively by CH2Cl2
and Et2O giving 3.65 g (14 mmol) of ferrocenylphos-
phonic acid (74% yield).
The aqueous phase was washed with 30 ml of ether
and acidified with a few drop of concentrated HCl to
precipitate a yellow powder. 20 ml of CH2Cl2 was added
to dissolve the powder, the two phases were separated
and the aqueous phase was extracted with 2·20 ml of
CH2Cl2. The organic phases were combined, dried over
MgSO4, and concentrated to about 10 ml and a large
amount of acetone was added to precipitate 0.86 g
(3 mmol) of ethyl ferrocenylphosphonic acid FcCH2P-
(O)(OEt)OH (13% yield). 1H NMR (CDCl3, d ppm)
1
M.p. 201 ꢁC (dec.) (198–205 ꢁC dec. [25]); H NMR
(CD3OD, d ppm): 4.33 (s, 5H); 4.44 (m, 2H); 4.51 (m,
2H); 31P NMR (CD3OD): 24.6 ppm; IR (KBr); m
(cmꢀ1) (P‚O) 1201. Anal. Calc. for C10H11FePO3: C,
45.11; H, 4.13; P, 11.65, Fe, 20.97. Found: C, 45.16;
H, 4.09; P, 10.98, Fe, 20.30%.
2
1.28 (t, 3H, 3JHH =6.8 Hz); 2.88 (d, 2H, JHP =20.5
Hz), 3.99 (q, 2H), 4.14 (s, 5H), 4.14 (m, 2H), 4.28 (m,
2H).31P NMR (CDCl3) 29.6 ppm.
2.2.3. Mono dicyclohexylammonium salt of ferrocenyl-
phosphonic acid, FcPO3Hꢀ(C6H11)2NH2
+
To 0.532 g (2 mmol) of ferrocenylphosphonic acid
dissolved in 20 ml of methanol was added 0.724 g (4
mmol) of dicyclohexylamine with stirring. After 4 h,
the solution was filtered, leading to 0.85 g of the dicyclo-
hexylammonium salt (95% yield), m.p.=208–209 ꢁC, 1H
NMR (200 MHz, CD3OD) d (ppm) 4.45 (2H, m), 4.28
(5H, s), 4.25 (2H, m), 3.21 (2H, m), 1.89 (10Heq, m),
1.37 (10Hax, m); 31P NMR (81 MHz, CD3OD) 17.7
ppm; FABꢀ m/z 265 (FcPO3Hꢀ). Anal. Calc. for
C22H34NFePO3 (446.85): C, 59.08; H, 7.61; N, 3.13;
Fe, 12.50; P, 6.94. Found: C, 58.89; H, 7.75; N, 3.27;
Fe, 12.22; P, 7.10%.
The organic phase was washed with 30 ml water and
dried over MgSO4. The solvent was evaporated under
vacuum and the product purified by silica column chro-
matography using first CH2Cl2/hexane (80/20) followed
by CH2Cl2/THF (75/25) as eluant, leading to 4.42 g (13
mmol) of diethyl ferrocenylmethylphosphonate as a
brown oil (56.5% yield). 1H NMR (CDCl3, d ppm)
1.28 (t, 6H, JHH =7 Hz); 2.95 (d, 2H, JHP =18.5 Hz);
4.02 (q, 4H); 4.15 (s, 5H); 4.14 (m, 2H); 4.26 (m, 2H);
31P NMR (CDCl3) 26.5 ppm.
3
2
2.2.6. Ferrocenylmethylphosphonic acid
In a Schlenk tube containing 0.86 g (2.6 mmol) of di-
ethyl ferrocenylmethylphosphonate was added 0.52 g (5
mmol) of triethylamine dissolved in 20 ml of CH2Cl2.
0.78 g (5 mmol) of trimethylbromosilane was added
drop-wise and the stirring was maintained for 12 h. 30
ml of a saturated solution of NaHCO3 was added. After
3 h of stirring the two phases were separated and the
aqueous phase washed with 20 ml of CH2Cl2, and acid-
ified with concentrated HCl to precipitate a yellow pow-
der which was washed successively with 2·20 ml of
water and 20 ml of ether leading to 0.6 g (2.1 mmol)
of ferrocenylmethylphosphonic acid (83% yield). 1H
2.2.4. Sodium salts of ferrocenylphosphonic acid
190 mg (0.71 mmol) of ferrocenylphosphonic acid
was added to 28.5 mg (0.71 mmol) of NaOH dissolved
in 10 ml of water. After stirring for 2 h, the water was
evaporated in vacuum leading to 179 mg (0.62 mmol)
of the monosodium salt (87% yield): 1H NMR (200
MHz, CD3OD, d ppm) 4.47 (2H, m), 4.30 (5H, s),
4.27 (2H, m); 31P NMR (81 MHz, CD3OD) 17.9 ppm.
Reaction of 200 mg of ferrocenylphosphonic acid
with 30 mg of NaOH dissolved in 20 ml water gave
227 mg (97% yield) of the di sodium salt: 1H NMR
(200 MHz, CD3OD, d ppm) 4.43 (2H, m), 4.27 (5H,
s), 4.13 (2H, m); 31P NMR (81 MHz, CD3OD) 16.1
ppm (15.3 ppm, D2O [25]).
2
NMR (DMSO D6, d ppm) 2.69 (d, 2H, JHP =19.5
Hz), 4.05 (m, 2H), 4.13 (s, 5H), 4.19 (m, 2H). 31P
NMR (DMSO D6) 21.8 ppm (23.1 ppm [25]).13C
1
NMR (CDCl3, d ppm) 30.6 (d, JCP =134.2 Hz), 67.8
(s), 69.5 (s), 70.1 (d, JCP =3.6 Hz), 81.2 (d, JCP =3.5
3
2
2.2.5. Diethyl ferrocenylmethylphosphonate
The procedure of Alley and Henderson [25] for the
preparation of FcCH2P(O)(OEt)(OH) was followed. In
a three-necked flask 6.39 g (46 mmol) of diethylphosph-
ite H(O)P(OEt)2 were added drop-wise to a suspension
of 1.105 g (46 mmol) of sodium hydride in 90 ml of tol-
uene at 0 ꢁC under stirring. After stirring for 45 min, the
solution was allowed to return to room temperature and
then refluxed until the solution became uncoloured.
Hz). IR (KBr) m (cmꢀ1) (P‚O) 1225.
2.2.7. 1,10-Ferrocenylbis(diethyl phosphonate)
The procedure of Alley and Henderson [25] was fol-
lowed, but we failed to obtain an as good yield as the
1
88% reported and only a 24% yield was obtained. H
3
NMR (CDCl3, d ppm) 1.33 (t, 12H, JHH =6.9 Hz),
4.11 (q, 8H), 4.59 (m, 8H); 31P NMR (CDCl3, d

O. Oms et al. / Journal of Organometallic Chemistry 689 (2004) 2654–2661
2657
ppm) 25.9; 13C NMR (CDCl3,
d ppm) 16.9
(d, JCP =6.6 Hz), 62.3 (d, JCP =6.6 Hz), 68.8 (d,
2.4. Structure determinations
3
2
3
1JCP =214 Hz), 73.4 (d, JCP =15.6 Hz), 74.3 (d,
Single-crystals of bis(diethyl ferrocenylphospho-
nate)zinc chloride complex were obtained by slow
evaporation of a diethyl ether solution at ambient tem-
perature. A suitable crystal was mounted on a glass fi-
bre at 173 K and X-ray data collected on a CAD 4
Nonius diffractometer, using Mo Ka radiation (k
2JCP =13.6 Hz).
2.2.8. 1,10 Ferrocenylbisphosphonic acid
It was obtained in 67% yield following Alley and
Henderson [25].
1H NMR (CD3OD, d ppm) 4.61 (4.5–4.7, Na salt,
D2O [25]); 31P NMR (CD3OD, d ppm) 23.6 (21.7, Na
salt, D2O [25]); IR (KBr) m (cmꢀ1) (P‚O) 1197.
˚
0.71069 A). The structure solution was carried out with
the SHELXS-86 program [26]. Hydrogen atoms posi-
tions were calculated (SHELXL) [27]. The H atoms were
not refined but taken in account in the last cycle of the
refinement. The refinement (with F2s) converged to
the final R1 and wR2 values of 0.0428 and 0.1140,
respectively.
2.2.9. [FcP(O)(OEt)2]2 ÆZnCl2 complex
To a solution of 1.08 g (3.3 mmol) of diethyl ferro-
cenylphosphonate dissolved in 5 ml of MeOH was add-
ed drop-wise, at 0 ꢁC 0.23 g (1.7 mmol) of ZnCl2 in 5 ml
of MeOH. After stirring overnight at room temperature,
the solvent was evaporated to dryness to give 1.19 g (1.5
mmol) of an orange brown powder, which crystallized
on slow evaporation of an ether solution; m.p.=66.6–
72.7 ꢁC. Anal. Calc. for C28H38Fe2P2O6ZnCl2: C,
43.05; H, 4.87; P, 7.94; Fe, 14.30; Zn, 8.38; Cl, 9.10.
Found: C, 43.17; H, 4.73; P, 7.57; Fe, 13.87; Zn, 8.16;
Cl, 8.90%. 1H NMR (CDCl3, d ppm) 1.40 (t, 6H),
3JHH =6.7 Hz), 4.31 (m, 4H), 4.40 (s, 5H), 4.47 (m,
2H), 4.67 (m, 2H); 31P NMR (CD3OD, d ppm) 29.2;
IR (KBr) m (cmꢀ1) (P‚O) 1207.
3. Results and discussion
3.1. Synthesis
Diethyl ferrocenylphosphonate FcP(O)(OEt)2 has
been prepared in 33% yield by reaction of mono lithio-
ferrocene with diethylchlorophosphate (EtO)2P(O)Cl at
0 ꢁC [25].
To improve the reactivity of the system, we used a
‘‘super base’’ as proposed by Sanders and Mueller-
t
Westerhoff [28]. BuOK activated, at low temperature,
t
both BuLi for the mono-lithiation of ferrocene and
2.3. Physical measurements
the lithiated ferrocene for the substitution of diethyl
chlorophosphate. The addition of 0.26 equivalent of
tBuOK and an excess of ferrocene and diethylchloro-
phosphate versus tBuLi gave diethyl ferrocenylphospho-
nate in 79% yield after purification by silica column
chromatography (Scheme 2).
In the reaction of ferrocenylmethanol with sodium di-
ethylphosphite, FcCH2P(O)(OEt)OH was extracted
from the aqueous phase according to Alley and Hender-
son [25], but in lower yield (13% instead of 65%). The
low yield in FcCH2P(O)(OEt)(OH) prompted us to look
at the organic phase, from which diethyl ferro-
cenylmethylphosphonate FcCH2P(O)(OEt)2 was isolat-
ed in 56% yield.
The NMR spectra were performed on a Bruker
Avance DPX 200, chemical shifts for 1H and 13C–
{1H} are referenced to SiMe4 and deuterated solvents
and for 31P to H3PO4 (85%) and deuterated solvent. In-
frared spectra were taken on a Thermo Nicolet Avatar
320 FT-IR spectrophotometer as KBr pellets. Cyclic
voltammograms and potential square wave voltammo-
grams were obtained with a Voltalab 10 in a three-elec-
trode cell. A 0.20 cm diameter platinum disc working
electrode, a platinum wire auxiliary electrode and a sat-
urated calomel reference electrode were used to record
voltammograms. The electrolyte was nBu4NPF6(0.1 M
in methanol). The concentration of electro-active species
was 0.001 M. Pulse amplitude of 25 mV and a scan rate
of 5 mVsꢀ1 were used to record the potential square
wave voltammograms.
Ferrocenylphosphonic acids were prepared by con-
version of the corresponding esters under mild condi-
tions using Me3SiBr and water (Scheme 3) [29].
O
tBuLi/ tBuOK
Li
P
(EtO)2P(O)Cl
OEt
Fe
Fe
Fe
OEt
THF, -78ºC
-78ºC
79%
Scheme 2. Synthesis of diethyl ferrocenylphosphonate.

2658
O. Oms et al. / Journal of Organometallic Chemistry 689 (2004) 2654–2661
O
O
O
H2O
Me3SiBr
CH2Cl2
R
P
R
P
R
P
OH
OH
OEt
OEt
OSiMe3
OSiMe3
Scheme 3. Synthesis of ferrocenylphosphonic acid from its ester.
3.2. Complexation
The complexation property of the diethyl ferro-
cenylphosphonate has been studied by its reaction with
zinc chloride. Addition of zinc chloride to diethyl ferro-
cenylphosphonate in methanol gave, after evaporation
of the solvent, an orange brown solid (Scheme 4).
Suitable crystals for X-ray crystallography were
grown by slow evaporation of a diethyl ether solution.
The crystallographic data are given in Table 1.
Fig. 1. Ortep view of the environment of Zn atom. The H atoms have
been omitted for clarity.
˚
The P–C (Cp) distance (1.770(6) A) is comparable with
those reported for fc(P(O)Ph2)2 and fc(P(O)(iPr)2)2
˚
(1.785–1.780 A) [30]. The P–OZn distance (1.467 A) is
˚
˚
shorter than the P–OR distances (1.555–1.568 A).
A view of the molecule is presented in Fig. 1, with se-
lected bond distances and angles listed in Table 2.
The crystal structure consists of discrete ZnCl2(O-
P(OEt)2Fc)2 complex units without any solvent mole-
cule. Two chlorine atoms and two oxygen atoms from
the phosphoryl groups define a distorted tetrahedral ar-
rangement around the zinc centre. The Fe–C distances,
The complexation of metal by alkyl phosphonates
has been reported to be equilibrated in solution [31].
The 31P NMR chemical shift of diethyl ferro-
cenylphosphonate in deuterated methanol was 28.5
ppm while it was 29.2 ppm for the complex. Addition
of 5 equivalents of ZnCl2 to the phosphonate solution
shifted the chemical shift to 29.9 ppm, indicating that
an equilibrium really took place.
˚
which lie in the range 2.002(6)–2.071(6) A (mean 2.038
˚
A), are similar to those of other ferrocene compounds.
3.3. Electrochemistry
MeOH
Cyclic voltammetry and potential square wave exper-
iments were carried out in methanol containing 0.1 M
tetrabutylammonium hexafluorophosphate as support-
ing electrolyte. Fig. 2 presents, as example, cyclic vol-
tammograms (CV) of FcP(O)(OEt)2 (1 mM) at a scan
[FcP(O)(OEt)2]2ZnCl2
2 FcPO3Et2 + ZnCl2
Scheme 4. Complexation of zinc dichloride by diethyl ferro-
cenylphosphonate.
rate of 100 mVsꢀ1
.
Table 1
Crystallographic data
The separation of the anodic and cathodic potential
is nearly constant at 67 mV as found for ferrocene under
the same condition and the ratio Ipa/Ipc is almost unity.
The peak currents vary linearly with v1/2 indicating a dif-
fusion-controlled process. These results are in good
agreement with a reversible one-electron transfer reac-
tion. The value of DE higher than 59 mV, also observed
for ferrocene (FcH), is possibly due to uncompensated
solution resistance between the working and reference
electrodes. The half wave potentials of the different com-
pounds prepared are displayed in Table 3, along with
the direct comparison with ferrocene.
Formula
C28H38Cl2Fe2O6P2Zn
780.46
Formula weight
Colour, habit
Crystal size (mm)
Temperature (K)
System
Orange brown plate
0.55·0.30·0.11
173
Monoclinic
C2/c
Space Group
˚
a (A)
˚
22.935(4)
7.708(2)
18.920(4)
104.73(2)
3235.0(14)
4
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
˚
As observed before, the presence of phosphonate
group directly linked to the ferrocenyl ring renders the
oxidation more difficult (DE1/2 =261 mV), consistent
with the electron-withdrawing effect of the phosphonate
substituent (entry1) [9,30]. This effect is enhanced by the
presence of two phosphonate units (entry 11) and be-
comes twice as great as for the mono-substituted com-
pound (DE1/2 =485 mV). The presence of the
methylene spacer between the cyclopentadienyl ring
and the P atom shields and/or counter-balances this
k (A)
0.71069
19.24
1.603
l (cmꢀ1
)
D (gcmꢀ3
T (K)
Total number of Reflections (1)
)
173
1909
Total number of Reflections I>2r(I)(2)
RF2 (1)
1524
0.0575
RF2 (2)
wRF2 (1)
0.0428
0.1161
wRF2 (2)
Residual density maximum/minimum
0.1140
0.490/ꢀ0.870

O. Oms et al. / Journal of Organometallic Chemistry 689 (2004) 2654–2661
2659
Table 2
Selected bond lengths ( A) and angles (ꢁ) for complex [FcP(O)(OEt)2]2 ÆZnCl2
˚
Zn–Cl
2.223(2)
1.978(4)
1.467(4)
1.555(4)
1.568(4)
1.770(6)
1.453(8)
O(3)–C(3)
C(1)–C(2)
1.470(7)
Zn–O(1)
P–O(1)
P–O(2)
P–O(3)
P–C(11)
O(2)–C(1)
1.432(10)
1.386(8)
1.435(8)
1.432(8)
1.407(9)
1.389(9)
C(11)–C(12)
C(11)–C(15)
C(12)–C(13)
C(13)–C(14)
C(16)–C(17)
Cl–Zn–Cl0
123.26(10)
98.0(3)
O(1)–P–O(2)
O(1)–P–O(3)
O(2)–P–O(3)
O(1)–P–C(11)
O(2)–P–C(11)
O(3)–P–C(11)
P–C(11)–C(12)
P–C(11)–C(15)
116.7(3)
111.4(3)
102.0(2)
110.6(2)
104.8(2)
111.0(3)
130.1(4)
121.7(4)
O(1)–Zn–O(10)
O(1)–Zn–Cl
O(1)–Zn–Cl0
P–O(1)–Zn
106.52(13)
109.81(14)
154.0(3)
121.8(4)
119.9(4)
108.2(5)
P–O(2)–C(1)
P–O(3)–C(3)
C(12)–C(11)–C(15)
effect due to its electron donating effect (DE1/2 =24 mV,
entry 7). Note that the diphenyl ferrocenylphosphonate
is slightly more difficult to oxidize (DE1/2 =57 mV,
entry 6).
1,2x10-5
8,0x10-6
8,0x10-6
6,0x10-6
4,0x10-6
The phosphonic acids are easier to oxidize than the
esters. The variation of the half wave potential from di-
ethyl ferrocenylphosphonate (entry 1) to ferrocenyl-
phosphonic acid (entry 2) is +104 mV, and doubles on
going from fc(PO3Et2)2 to fc(PO3H2)2 (180 mV). The
methylene spacer renders this variation from the ethyl
ester (entry 10) to acid (entry 11) less marked: 42 mV.
The change of the first ethyl group (entries 7, 8) gives
rise to a greater variation (32 mV) than the second (en-
try 9) (10 mV). It is noteworthy that this variation de-
creases with the length of the spacer even for
phosphonates containing p conjugated ferrocenyl units
4,0x10-6
0,0
0,1
0,2
0,3
0,4
0,5
v1/2 (V1/2.s-1/2
)
-4,0x10-6
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
Potential / V vs SCE
Fig. 2. Cyclic voltammogram of FcP(O)(OEt)2 and I=f(v1/2).
Table 3
Oxidation of ferrocenyl phosphorus derivatives by voltammetry, in methanol, 0.1 M Bu4NPF6, scan rate=100 mVsꢀ1, Pt working electrode, SCE
reference, 22 2 ꢁC
DEa (mV)
DE1/2 (mV)/FcH
b
E1/2 (mV)/SCE
c
Entry
Compounds
Epa (mV)/SCE
Epc (mV)/SCE
0
1
FcH
FcPO3Et2
390
648
548
380
372
206
441
408
381
367
316
581
471
312
311
140
379
346
310
303
74
67
77
68
61
66
62
62
71
64
353
614
510
344
341
173
410
377
345
335
838
658
603
0
+261
+57
ꢀ9
2
FcPO3H2
3
FcPO3HꢀDCHAH+
FcPO3Hꢀ Na+
FcPO32ꢀ2 Na+
FcCH2PO3Ph2
FcCH2PO3Et2
FcCH2PO3EtH
FcCH2PO3H2
4
ꢀ12
ꢀ180
+57
+24
ꢀ8
5
6
7
8
9
ꢀ18
+485
+305
+250
d
10
11
12
fc[PO3Et2]2
d
fc[PO3H2]2
d
[FcPO3Et2]2 ÆZnCl2
a
DE=Epa ꢀEpc
.
b
E1/2 =(1/2)(Epa +Epc).
DE1/2 =E1/2/SCEꢀE1/2(FcH+/FcH)/ SCE.
c
d
Determined by PSWV.

2660
O. Oms et al. / Journal of Organometallic Chemistry 689 (2004) 2654–2661
group, which may be modulated by the nature of the
3,0x10-6
substituents around the phosphorus centre. The pres-
ence of only one peak and the low variation of E1/2 from
diethyl ferrocenylphosphonate to the zinc complex show
that this kind of complexation cannot be useful for elec-
trochemically recognise metal ions. However, the large
variation of the half wave potential of the ferrocenyl-
phosphonic acid and its di-sodium salt indicates that it
is a redox-active pH responsive molecule, which may
be considered as a new potential electrochemical sensor.
FcPO3Et2
[FcP(O)(OEt)2]2ZnCl2
2,0x10-6
1,0x10-6
5. Supplementary material
0,0
0,2
0,4
0,6
0,8
1,0
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Da-
ta Centre, CCDC No. 231570. Copies of this information
may be obtained free of charge from the Director, CCDC,
12Union Road, Cambridge, CB2 1EZ, UK [fax: +44 1223
336033, or e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk].
Potential / V
Fig. 3. PSWV curves of de FcPO3Et2 and of the zinc complex
[FcP(O)(OEt)2]2 ÆZnCl2.
[20], and does not exist for ferrocenyl carboxylate [32]. A
huge effect is observed during the neutralisation of the
ferrocenylphosphonic acid, the salts being easier to oxi-
dize than ferrocene (entries 3–5). An identical variation
of the half wave potential of ꢀ169 mV is observed for
the conversion of the acid to the mono-salt and to the
di-salt. The nature of the cation, sodium or cyclohexyl
ammonium, has little effect on the half wave potential.
The negatively charged phosphonate group is no longer
an electro-withdrawing group and renders the ferrocenyl
unit easier to oxidize than ferrocene. The differences of
ꢀ169 and ꢀ337 mV between the half wave potentials
of the ferrocenylphosphonic acid and of its mono and
di-sodium salt show that the ferrocenylphosphonic acid
can act as a redox-active pH responsive molecule. This
aspect is currently under investigation.
Acknowledgements
Financial support from Centre National de la Re-
cherche Scientifique, CNRS, and from the French Min-
ister of Research is gratefully acknowledged. We thank
Dr R. Astier for Crystallographic data measurements.
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