Anodic electrochemistry of ferrocenylphosphine and ruthenocenylphosphine chalcogenide complexes and lewis acid adducts

Article abstract of DOI:10.1021/om050074p

The anodic electrochemistry of a series of bidentate phosphine chalcogenides and bidentate phosphine Lewis acid adducts, both with a metallocene backbone, was examined in dichloromethane containing [NBu ₄][PF₆]. Oxidation of the ferrocene compounds dppfE ₂ and dippfE₂ (E = O, S, BH₃, CH ₃⁺) was reversible on the cyclic voltammetric (CV) time scale, while the oxidation of the dpprE₂ compounds (E = O, S, BH ₃) was irreversible; however, the oxidation of dppfSe₂, dippfSe₂, and dpprSe₂ displayed an irreversible wave and the reduction of a follow-up product.

Full text of DOI:10.1021/om050074p

Organometallics 2005, 24, 2447-2451
2447
Anodic Electrochemistry of Ferrocenylphosphine and
Ruthenocenylphosphine Chalcogenide Complexes and
Lewis Acid Adducts
Brett D. Swartz† and Chip Nataro*
Department of Chemistry, Lafayette College, Easton, Pennsylvania 18042
Received February 2, 2005
The anodic electrochemistry of a series of bidentate phosphine chalcogenides and bidentate
phosphine Lewis acid adducts, both with a metallocene backbone, was examined in
dichloromethane containing [NBu4][PF6]. Oxidation of the ferrocene compounds dppfE2 and
+
dippfE2 (E ) O, S, BH3, CH3 ) was reversible on the cyclic voltammetric (CV) time scale,
while the oxidation of the dpprE2 compounds (E ) O, S, BH3) was irreversible; however, the
oxidation of dppfSe2, dippfSe2, and dpprSe2 displayed an irreversible wave and the reduction
of a follow-up product.
Scheme 1. Electrochemical Dimerization of dppf
Introduction
1,1′-Bis(diphenylphosphino)ferrocene (dppf) has re-
ceived considerable attention in recent years, particu-
larly as a ligand in a variety of catalytic applications.1,2
The main interest in dppf stems from the bite angle of
the chelate phosphine and the redox-active ferrocene
backbone.2 Numerous studies have investigated the
oxidative electrochemistry of dppf, which, unlike that
of ferrocene, is complicated by a follow-up reaction that
has been proposed to be dimerization.3 The dimerization
of dppf+ has been attributed to the presence of the lone
pair of electrons on each phosphorus atom, which can
undergo an intramolecular electron transfer with the
Fe(III) center, forming a phosphorus radical capable of
undergoing a dimerization reaction (Scheme 1).4 To
prevent these reactions, it is necessary to occupy the
lone pair of electrons on each phosphorus atom. Numer-
ous studies have been performed in which dppf is bound
to a metal center, and in many cases, the oxidation of
these complexes is reversible.3-5
1,1′-bis(diisopropylphosphino)ferrocene (dippf) is similar
to that of dppf; the oxidation of dippf is complicated by
a follow-up reaction, but upon coordination, the dippf
oxidation is frequently reversible.6 A second chelate
phosphine in this series that has been examined is 1,1′-
bis(diphenylphosphino)ruthenocene (dppr). Unlike the
iron-centered dppf and dippf, the oxidative electrochem-
istry of dppr and all of the compounds containing a dppr
ligand that have been examined is irreversible.3 While
dippf likely undergoes a dimerization after oxidation,
it is unclear what happens in the case of dppr. Dimer-
ization of dppr+ through phosphorus cannot be excluded;
however, coordination of dppr to a metal would be
anticipated to prevent this type of dimerization. Another
type of dimerization could occur through the ruthenium
In addition to dppf, there are a variety of other
bidentate phosphines with metallocene backbones that
have been examined. The oxidative electrochemistry of
* To whom correspondence should be addressed. E-mail: nataroc@
lafayette.edu.
† Current address: Department of Chemistry, University of Roch-
ester, Rochester, NY 14627.
(1) There are numerous examples in the literature; listed below are
a few examples of the 70+ references in 2004. (a) Cadierno, V.; Diez,
J.; Garcia-Garrido, S. E.; Gimeno, J. Chem. Commun. 2004, 2716. (b)
Bianchini, C.; Meli, A.; Oberhauser, W.; Parisel, S.; Gusev, O. V.;
Kal’sin, A. M.; Vologdin, N. V.; Dolgushin, F. M. J. Mol. Catal. A: Chem.
2004, 224, 35. (c) Yoshida, M.; Morishita, Y.; Fujita, M.; Ihara, M.
Tetrahedron Lett. 2004, 45, 1861. (d) Herrera-Alvarez, C.; Gomez-
Benitez, V.; Redon, R.; Garcia, J. J.; Hernandez-Ortega, S.; Toscano,
R. A.; Morales-Morales, D. J. Organomet. Chem. 2004, 689, 2464. (e)
Cadierno, V.; Crochet, P.; Diez, J.; Garcia-Garrido, S. E.; Gimeno, J.
Organometallics 2004, 23, 4836.
(5) There are numerous examples in the literature. Examples
include: (a) Sun, Y.; Chan, H.-S.; Dixneuf, P. H.; Xie, Z. Organome-
tallics 2004, 23, 5864. (b) Ohs, A. C.; Rheingold, A. L.; Shaw, M. J.;
Nataro, C. Organometallics 2004, 23, 4655. (c) O’Connor, A. R.; Nataro,
C.; Rheingold, A. L. J. Organomet. Chem. 2003, 679, 72. (d) Longato,
B.; Coppo, R.; Pilloni, G.; Corvaja, C.; Toffoletti, A.; Bandoli, G. J.
Organomet. Chem. 2001, 637-639, 710. (e) Choi, N.-S.; Lee, S. W. J.
Organomet. Chem. 2001, 627, 226.
(2) Gan, K. S.; Hor, T. S. A. In Ferrocenes; Togni, A., Hayashi, T.,
Eds.; VCH: New York, 1995.
(3) Nataro, C.; Campbell, A. N.; Ferguson, M. A.; Incarvito, C. D.;
Rheingold, A. L. J. Organomet. Chem. 2003, 673, 47 and references
therein.
(4) Pilloni, G.; Longato, B.; Corain, B. J. Organomet. Chem. 1991,
420, 57.
(6) Ong, J. H. L.; Nataro, C.; Golen, J. A.; Rheingold, A. L.
Organometallics 2003, 22, 5027.
10.1021/om050074p CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/05/2005

2448 Organometallics, Vol. 24, No. 10, 2005
Swartz and Nataro
centers of two dppr+, as seen in ruthenocene.7 However,
the use of supporting electrolytes with noncoordinating
anions prevents this type of dimerization7,8 but does not
give a reversible oxidation in the case of dppr.3 Ad-
ditional reactions are also possible; thus, it appears that
there is a combination of factors in the oxidation of dppr
that lead to the instability of the complex upon oxida-
tion.
min, and a precipitate formed. The solution was removed and
the solid dried under vacuum, giving 0.0406 g (56%) of dppfOS
as a brown powder. Anal. Calcd for C34H28FeOP2S‚1/3CH2Cl2:
C, 65.38; H, 4.38. Found: C, 65.02; H, 4.63. 1H NMR (CDCl3):
δ (ppm) 7.52 (m, 20 H, Ph), 4.72 (br s, 2 H, C5H4(OR)), 4.61 (br
s, 2 H, C5H4(SR)), 4.31 (br s, 2 H, C5H4(Sâ)), 4.19 (br s, 2 H,
C5H4(Oâ)). 31P{1H} NMR (CDCl3): δ (ppm) 41.01 (s, PdS),
27.98 (s, PdO).
Preparation of dppfOSe. dppfO (0.0642 g, 0.101 mmol)
was dissolved in CH2Cl2 (2 mL) at room temperature under
argon. Selenium (0.0089 g, 0.113 mmol) was added, and the
mixture was refluxed for 20 h with stirring. The solvent was
reduced to half volume under vacuum, and 5 mL of MeOH
were added to the remaining solution. The solution was cooled
to 0 °C for approximately 30 min, resulting in the formation
of crystals. The solvent was decanted, and the remaining
crystals were dried under vacuum, giving 0.0115 g (16%) of
dppfOSe as red-brown crystals. Anal. Calcd for C34H28FeOP2-
Se: C, 62.89; H, 4.35. Found: C, 63.22; H, 4.55. 1H NMR
(CDCl3): δ (ppm) 7.41 (m, 20 H, Ph), 4.76 (br s, 2 H, C5H4-
(SeR)), 4.62 (br s, 2 H, C5H4(OR)), 4.34 (br s, 2 H, C5H4(Seâ)),
4.20 (br s, 2 H, C5H4(Oâ)). 31P{1H} NMR (CDCl3): δ (ppm) 31.55
(s, JPSe ) 371 Hz, PdSe), 28.10 (s, PdO).
Preparation of dpprO2. dppr (0.0497 g, 0.083 mmol) was
dissolved in THF (5 mL) at room temperature. The mixture
was cooled to 0 °C, and 0.020 mL (0.59 mmol) of H2O2 (30%)
were added. The mixture was warmed to room temperature
for approximately 30 min, filtered, and the solvent was
removed under vacuum, leaving 0.0153 g (29%) of dpprO2 as
a gray solid. Anal. Calcd for C34H28O2P2Ru‚1/2THF: C, 64.76;
H, 4.83. Found: C, 64.38; H, 5.02. 1H NMR (CDCl3): δ (ppm)
7.35 (m, 20 H, Ph), 4.80 (br s, 4 H, C5H4(OR)), 4.59 (br s, 4 H,
C5H4(Oâ)). 31P{1H} NMR (CDCl3): δ (ppm) 27.57 (s).
To further explore the oxidative electrochemistry of
dppf, dippf, and dppr, a series of compounds in which
the lone pair of the phosphorus atoms is bonded to
nonmetallic elements was examined. The first class of
compounds contain two P(V) atoms with chalcogenide
atoms bonded to each phosphorus. Although a number
of these compounds have been prepared, only dppfO2
has been studied electrochemically.4 Those studies have
shown dppfO2 to undergo a reversible single-electron
oxidation, indicating that the occupation of the lone pair
of electrons on each phosphorus atom prevents dimer-
ization. The second class of compounds is prepared by
reacting Lewis acids with the phosphorus lone pair. The
compounds formed by the reaction of chalcogenides and
Lewis acids with bidentate phosphines having metal-
locene backbones will provide a better understanding
of the oxidative electrochemistry of bidentate phos-
phines containing metallocene backbones.
Experimental Section
General Procedures. All reactions were carried out using
standard Schlenk techniques under argon. The compounds
9
10
dppfE2 and dippfE2 (E ) O, S, Se), dppfO,11 dppf(BH3)2,12
and dppr13 were prepared according to literature methods.
Ferrocene, decamethylferrocene, dppf, and dippf were supplied
by Strem Chemicals, Inc. BH3‚thf (1 M in THF) and [Me3O]-
[BF4] were purchased from Aldrich. Dichloromethane (CH2-
Cl2) and hexanes were dried over CaH2 and distilled under
nitrogen. Ether and THF were dried over potassium benzophe-
none ketyl and distilled under nitrogen. HPLC grade dichlo-
romethane used for electrochemistry was dried over CaH2 and
distilled under argon. 31P{1H} and 1H NMR spectra were
recorded using a JEOL Eclipse 400 FT-NMR spectrometer.
Elemental analysis was performed by Quantitative Technolo-
gies, Inc.
Preparation of dpprS2. dppr (0.0508 g, 0.085 mmol) was
dissolved in CHCl3 (5 mL) at room temperature. Sulfur (0.0052
g, 0.162 mmol) was then added, and the mixture was refluxed
for 20 h with stirring. The solution was filtered and the solvent
was removed under vacuum, leaving 0.0253 g (47%) of dpprS2
as a beige solid. Anal. Calcd for C34H28P2RuS2‚1/3CHCl3: C,
58.62; H, 4.06. Found: C, 58.28; H, 4.06. 1H NMR (CDCl3): δ
(ppm) 7.62 (m, 8 H, Ph), 7.35 (m, 12 H, Ph), 4.81 (br s, 4 H,
C5H4(SR)), 4.62 (br s, 4 H, C5H4(SR)). 31P{1H} NMR (CDCl3): δ
(ppm) 40.56 (s).
Preparation of dpprSe2. dppr (0.0501 g, 0.083 mmol) was
dissolved in CHCl3 (5 mL) at room temperature. Selenium
(0.0135 g, 0.171 mmol) was then added, and the mixture was
refluxed for 20 h with stirring. The solution was filtered and
the solvent was removed under vacuum, leaving 0.0328 g (52%)
of dpprSe2 as a gray solid. Anal. Calcd for C34H28P2RuS2‚1/2-
1H NMR of dppfO2. Although this compound had been
previously prepared, the 1H NMR spectrum was not reported.4
1H NMR (CDCl3): δ (ppm) 7.57 (m, 20 H, Ph), 4.70 (s, 4 H,
(C5H4(OR)), 4.25 (s, 4 H, (C5H4(Oâ)).
Preparation of dppfOS. dppfO (0.0681 g, 0.119 mmol) was
dissolved in CH2Cl2 (2 mL) at room temperature under argon.
Sulfur (0.0044 g, 0.137 mmol) was then added, and the mixture
was refluxed for 20 h with stirring. The solvent was reduced
to half volume under vacuum, and 10 mL of hexanes were
added. The solution was cooled to 0 °C for approximately 30
1
CHCl3: C, 50.71; H, 3.52. Found: C, 50.80; H, 3.53. H NMR
(CDCl3): δ (ppm) 7.62 (m, 8 H, Ph), 7.36 (m, 12 H, Ph), 4.86
(br s, 4 H, C5H4(SeR)), 4.64 (br s, 4 H, C5H4(SeR)). 31P{1H} NMR
(CDCl3): δ (ppm) 31.36 (s, JP-Se ) 369 Hz).
Preparation of dippf(BH3)2. dippf (0.0508 g, 0.121 mmol)
was dissolved in THF (5 mL), and then 0.25 mL (0.25 mmol)
of BH3‚thf was added. The mixture was stirred at room
temperature for 1 h. The solvent was removed under vacuum,
and the residue was washed with ether (10 mL), giving 0.0496
g (92%) of dippf(BH3)2 as an orange solid. Anal. Calcd for
C22H42B2FeP2‚Et2O: C, 60.04; H, 10.08. Found: C, 60.07; H,
(7) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991,
30, 4687.
(8) (a) Ramachandran, B. M.; Trupia, S. M.; Geiger, W. E.; Carrol,
P. J.; Sneddon, L. G. Organometallics 2002, 21, 5078. (b) Trupia, S.;
Nafaday, A.; Geiger, W. E. Inorg. Chem. 2003, 42, 5480.
(9) (a) Oxide: ref 4. (b) Sulfide: Bishop, J. J.; Davison, A.; Katcher,
M. L.; Lichtenberg, D. W.; Merrill, R. E.; Smart, J. C. J. Organomet.
Chem. 1971, 27, 241. (c) Selenide: Pilloni, G.; Longato, B.; Bandoli,
G.; Corain, B. J. Chem. Soc., Dalton Trans. 1997, 819.
(10) Necas, M.; Beran, M.; Woollins, J. M.; Novosad, J. Polyhedron
2001, 20, 741.
1
10.26. H NMR (CDCl3): δ (ppm) 4.69 (br s, 4 H, C5H4(BR)),
4.39 (br s, 4 H, C5H4(Bâ)), 2.14 (m, 4 H, CH), 1.63 (m, 30 H,
CH3 and BH3). 31P{1H} NMR (CDCl3): δ (ppm) 32.73 (br s).
Preparation of dppr(BH3)2. The preparation of this
compound was carried out as per the synthesis of dippf(BH3)2,
except 0.0511 g (0.085 mmol) of dppr and 0.20 mL (0.2 mmol)
of BH3‚thf were used. This gave 0.0509 g (95%) of dppr(BH3)2
as a beige solid. Anal. Calcd for C34H34B2P2Ru: C, 65.10; H,
5.46. Found: C, 64.80; H, 5.76. 1H NMR (CDCl3): δ (ppm) 7.41
(11) Grushin, V. V. Organometallics 2001, 20, 3950.
(12) Donaghy, K. J.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 1997,
36, 547.
(13) Li, S.; Wei, B.; Low, P. M. N.; Lee, H. K.; Hor, T. S. A.; Xue, F.;
Mak, T. C. W. J. Chem. Soc., Dalton Trans. 1997, 1289.

Ferrocenyl- and Ruthenocenylphosphine Chalcogenides
Organometallics, Vol. 24, No. 10, 2005 2449
Table 1. Formal Potentials (V vs Fc0/+) for
Chalcogenides and Lewis Acid Adducts of
1,1′-Diphosphinometallocenes and Potential
Difference (∆E in V vs Corresponding Phosphine)
from the Starting Phosphinea
(m, 20 H, Ph), 4.75 (br s, 4 H, C5H4(BR)), 4.59 (br s, 4 H, C5H4-
(Bâ)), 1.35 (br s, 6H, BH3). 31P{1H} NMR (CDCl3): δ (ppm)
16.53 (s).
Preparation of [dppf(CH3)2][BF4]2. dppf (0.0511 g, 0.092
mmol) was dissolved in hexanes (5 mL), and then [(CH3)3O]-
[BF4] (0.0274 g, 0.185 mmol) was added. The mixture was
stirred for 1 h at room temperature. The solvent was removed
under vacuum, and the solid was washed with 5 mL of ether.
The solution was filtered, and the solid was dried under
vacuum, giving 0.054 g (71%) of [dppf(CH3)2][BF4]2 as an
orange solid. Anal. Calcd for C36H34B2F8FeP2: C, 57.04; H,
4.52. Found: C, 57.05; H, 4.55. 1H NMR (CDCl3): δ (ppm) 7.67
(m, 20 H, Ph), 4.87 (br s, 4 H, C5H4(CR)), 4.67 (br s, 4 H, C5H4-
(Câ)), 2.69 (d, J ) 13.2 Hz, 6 H, CH3). 31P{1H} NMR (CDCl3):
δ (ppm) 22.77 (s).
dppf
E°
dippf
E°
dppr
∆E
∆E
E°
∆E
O2
0.45
0.48
0.22 0.37
0.25
0.32 0.81b
0.37
OS
OSe
S2
0.45,b 0.19c 0.22
0.48
0.25 0.38
0.09 0.21,b
-0.24c
0.26 0.39
0.76 0.96
0.33 0.51b
0.07
0.12
Se2
0.32,b
0.16 0.32,b
-0.14c
-0.16c
(BH3)2
0.49
0.34 0.91b
0.91
0.47
[(CH3)2]2+ 0.99
Preparation of [dppr(CH3)2][BF4]2. The preparation of
this compound was carried out as per the synthesis of [dppf-
(CH3)2][BF4]2, except 0.0524 g (0.087 mmol) of dppr and 0.0266
g (0.18 mmol) of [(CH3)3O][BF4] were used. This gave 0.0193
g (35%) of [dppr(CH3)2][BF4]2 as a yellow solid. Anal. Calcd
for C36H34B2F8P2Ru: C, 53.83; H, 4.27. Found: C, 54.14; H,
4.00. 1H NMR (CDCl3): δ (ppm) 7.64 (m, 20 H, Ph), 5.03 (br s,
4 H, C5H4(CR)), 4.96 (br s, 4 H, C5H4(Câ)), 2.67 (d, J ) 13.2 Hz,
6 H, CH3). 31P{1H} NMR (CDCl3): δ (ppm) 22.13 (s).
Preparation of [dippf(CH3)2][BF4]2. dippf (0.0481 g, 0.115
mmol) was dissolved in THF (5 mL), and then [(CH3)3O][BF4]
(0.0344 g, 0.233 mmol) was added. The mixture was stirred
for 1 h at room temperature. The solvent was removed, and 5
mL of ether was added. The solution was filtered, and the
remaining solid was dried under vacuum, giving 0.042 g (81%)
of [dippf(CH3)2][BF4]2 as an orange powder. Anal. Calcd for
C24H42B2F8FeP2‚3/4Et2O: C, 47.86; H, 7.36. Found: C, 47.83;
H, 7.15. 1H NMR (CDCl3): δ (ppm) 5.10 (m, 8 H, (C5H4), 2.16
(m, 4 H, CH), 1.33 (m, 24 H, CH3). 31P{1H} NMR (CDCl3): δ
(ppm) 43.08 (s).
Electrochemical Procedures. Standard electrochemical
techniques were employed, and all scans were performed under
an argon atmosphere. The supporting electrolyte was 0.1 M
[Bu4N][PF6]. Glassy carbon was used as the working electrode,
and Pt was used as the counter electrode. The glassy-carbon
electrode was polished with two different diamond pastes, 1.0
µm and 0.25 µm, and rinsed with CH2Cl2 prior to use. The
experimental reference electrode was Ag/AgCl, separated from
the solution by a frit, and either ferrocene or decamethylfer-
rocene were used as an internal standard. The potential was
referenced to the NHE by adding 0.66 V.14 Electrochemical
experiments were conducted at ambient temperature (22 ( 1
°C) using a PAR Model 263A potentiostat/galvanostat, and
voltammograms were recorded using PowerSuite software.
Cyclic voltammograms were performed at scan rates of 50
mV/s and in increments of 100 mV/s from 100 to 1000 mV/s.
The formal oxidation potentials and the difference between
the formal potentials of each compound and the corresponding
phosphine (∆E) are presented in Table 1.
a A scan rate of 100 mV/s was used for all data. b Irreversible
wave. c Potential for the follow-up cathodic wave.
Figure 1. Proton labeling scheme.
This is very similar to the peak in dppfO2; therefore, it
is attributed to the PdO. The peaks further downfield
have chemical shifts similar to those of the correspond-
ing dppfE2 (E ) S, Se), and in the case of dppfOSe,
coupling to 77Se is observed. The 31P-77Se coupling is
9c
significantly less than that for dppfSe2 but is in the
typical range for other PdSe compounds.10 The 1H NMR
spectra show the C5H4 protons to be represented by four
broad signals, which are assigned as C5H4(ER/â), where
R/â represents the position with respect to the chalco-
genide (Figure 1). The assignments of the C5H4 protons
in the mixed-chalcogenide compounds were based upon
comparison to the parent dppfE2 compounds. For
dppfOS, the C5H4(OR) protons were assigned to the peak
furthest downfield. The chemical shifts of the protons
in the positions R/â to the P(S)Ph2 group are similar to
the shifts for the C5H4(Sâ) protons (4.64 ppm) and the
C5H4(SR) protons (4.29 ppm) of dppfS2. The final peak
in the C5H4 region is assigned as the C5H4(Oâ) protons.
A similar technique was employed for the assignment
of the protons in the dppfOSe spectrum. Since it was
difficult to determine the assignments of SeR and OR
1
1
using just H NMR, H-1H COSY was employed. The
resulting spectrum allowed for the assignment of the
SeR protons and the OR protons.
Reactions of dppf, dippf, and dppr with BH3‚thf and
[(CH3)3O][BF4] yielded Lewis acid adducts of the start-
ing phosphines, as determined by NMR spectroscopy.
The synthesis and spectroscopic data for dppf(BH3)2
have been reported.12 The dppr and dippf analogues
were prepared in a similar manner. The 31P signal for
the BH3 adducts shifts upfield by approximately 30 ppm
as compared to that for the phosphines; this is similar
to the difference in shift for PPh3 and H3B‚PPh3.15 The
Results and Discussion
Synthesis. The dppr chalcogenide compounds were
prepared using methods similar to those used in pre-
paring the dppf analogues.9 The compounds were pre-
pared in reasonable yield and characterized by NMR.
Numerous attempts were made to prepare the ditellu-
rides of dppf, dippf, and dppr, but none were successful.
Reaction of dppfO with sulfur or selenium yields the
new mixed chalcogenide products dppfOS and dppfOSe.
The 31P{1H} NMR spectrum shows two singlets for each
product, each having a peak at approximately 28 ppm.
2+
preparation of dppf(CH3)2 from dppf and CH3I has
been reported, but the characterization was limited to
IR and elemental analysis.9b Complications in the
(14) Lu, S.; Strelets, V. V.; Ryan, M. F.; Pietro, W. J.; Lever, A. B.
P. Inorg. Chem. 1996, 35, 1013.
(15) Durand, M.; Jouany, C.; Jugie, G.; Elegant, L.; Gal, J. F. J.
Chem. Soc., Dalton Trans. 1977, 57.

2450 Organometallics, Vol. 24, No. 10, 2005
Swartz and Nataro
Figure 2. Cyclic voltammetry scan of the oxidation of 1.0
mM dppfS2 in CH2Cl2/0.10 M Bu4NPF6 at 100 mV/s.
Figure 3. Cyclic voltammetry scan of the oxidation of 1.0
mM dippfSe2 in CH2Cl2/0.10 M Bu4NPF6 at 100 mV/s.
oxidation of H3C-PPh2Fc+I- were attributed to the
iodide; therefore, a less reactive anion was desired.16
The reaction between the phosphines and [(CH3)3O]-
[BF4] gives the desired diphosphonium tetrafluoroborate
salts in reasonable yield. Similar to the BH3 adducts
and the phosphine oxides, the 31P chemical shift for the
CH3+ adducts is approximately 30 ppm downfield of the
phosphine.
which the oxidation occurs for the sulfide is similar to
that for the analogous oxide compounds and is ap-
proximately 0.3 V more positive than for the phosphine.
The potential at which dpprS2 oxidation occurs is
significantly closer to dppr than to dpprO2. As all of the
waves for the dppr-based oxidations are irreversible, the
E1/2 values cannot be accurately determined;18 thus, no
conclusions can be drawn from the dppr data.
Electrochemistry of Chalcogenides. The oxidative
electrochemistry of dppfO2 has been examined in dichlo-
roethane with tetrabutylammonium perchlorate as the
supporting electrolyte; the reversible wave occurs 0.21
V positive of dppf.4 Similar results were obtained for
the oxidation of dppfO2 and dippfO2 in CH2Cl2 (Table
1). While the oxidation of dppf and dippf yields a product
The oxidative electrochemistry of the diselenide com-
plexes in this study was similar; there was an irrevers-
ible oxidation wave and a cathodic wave due to a follow-
up product (Figure 3). For all three diselenide compounds,
the cathodic wave is approximately 0.5 V less positive
than the irreversible oxidation wave, suggesting that
the follow-up products are similar. The oxidative elec-
trochemistry of PFc3 and SedPFc3 has been examined
and, although slightly more complicated due to the three
iron centers, there is an irreversible oxidation with a
follow-up cathodic wave that is less positive by 0.5 V.17
The follow-up product is proposed to result from a
reaction that occurs following an intramolecular electron
transfer from the Se to the iron(III) center.17 However,
this explanation is based upon the faulty premise that
selenium is more electronegative than phosphorus. The
polarizability of PdE bonds increases on going from E
) O to E ) Se.20 DFT calculations suggest that the
π-bond order in EdPMe3 decreases going down the
chalcogenides.21 While an intramolecular electron trans-
fer may play a role in the reactivity of the oxidized
selenides, the weaker PdSe bond could greatly influence
the reactivity. This difference has been noted in the
reactivity of EdPR3 compounds; selenium is readily
transferred between different phosphines at room tem-
perature, while the transfer of sulfur requires elevated
temperatures.22
+
that is unstable with respect to dimerization, dppfO2
and dippfO2+ are stable on the time scales used in this
study. The dimerization of dppf+ and dippf+ is proposed
to occur through the nonbonding electrons on a phos-
phorus atom (Scheme 1). In dppfO2 and dippfO2, the
phosphorus atoms are not able to participate in the
dimerization mechanism; therefore, the dimerization is
not possible and the oxidation is reversible. Geiger has
recently examined the oxidative electrochemistry of the
related phosphine PFc2Ph and phosphine oxide OdPFc2-
Ph.17 Similar to the dppf system, the oxidation of PFc2-
Ph is complicated by a follow-up reaction, while the
oxidation of OdPFc2Ph is not.
Like the iron systems, the oxidation of dpprO2 occurs
approximately 0.4 V more positive than dppr; however,
unlike the iron compounds, the oxidation is irreversible
at all scan rates. This is not surprising as the parent,
dppr, exhibits an irreversible oxidation under similar
conditions. The irreversibility of the dpprO2 suggests
that the decomposition of dppr+ does not have to involve
the nonbonding electrons on phosphorus, but it does not
preclude their involvement.
The oxidative electrochemistry of the phosphine sul-
fide compounds is analogous to that of the phosphine
oxides; the oxidation of the compounds with the iron
center, dppfS2 (Figure 2) and dippfS2, is reversible, while
the oxidation of the ruthenium-centered dpprS2 is
irreversible. For the iron compounds, the potential at
The oxidative electrochemistry of the mixed oxide-
chalcogenide species, dppfOS and dppfOSe, showed
features of both parent dichalcogenide compounds. The
oxidation of dppfOS is reversible, similar to dppfO2 and
(18) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Funda-
mentals and Applications, 2nd ed.; Wiley: New York, 2001; Chapter
6.
(19) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(20) Shagidullin, R. R.; Vandyukova, I. I.; Nuretdinov, I. A. Izv.
Akad. Nauk SSSR, Ser. Khim. 1978, 1407.
(21) Sandblom, N.; Ziegler, T.; Chivers, T. Can. J. Chem. 1996, 74,
2363.
(22) Brown, D. H.; Cross, R. J.; Keat, R. J. Chem. Soc., Dalton Trans.
1980, 871.
(16) Kotz, J. C.; Mivert, C. L.; Lieber, J. M.; Reed, R. C. J.
Organomet. Chem. 1975, 91, 87.
(17) Barriere, F.; Kirss, R. U.; Geiger, W. E. Organometallics 2004,
24, 48.

Ferrocenyl- and Ruthenocenylphosphine Chalcogenides
Organometallics, Vol. 24, No. 10, 2005 2451
Table 2. EL and σp Values for Various Phosphine
dppfS2, while the potential at which oxidation occurs is
identical with that of dppfS2, although not substantially
different from the potential at which dppfO2 occurs. The
potential at which the oxidation of dppfOSe occurs is
the same as for dppfO2. However, a cathodic follow-up
product is seen, just as in the oxidation of dppfSe2. It
seems that the chalcogenide does play a slight role in
the oxidation potential of the iron center, but the
presence of even one phosphine selenide provides a
route of decomposition for the oxidation product.
Electrochemistry of Lewis Acid Adducts. The
addition of a Lewis acid to phosphinometallocenes has
been shown to affect the electrochemistry. For example,
there is an irreversible wave in the oxidation of PPh2-
Fc, but in the BH3 adduct there is a reversible wave.16
The oxidation of the BH3 adducts of dppf, dippf, and
dppr give results similar to those for the oxides; the dppf
and dippf adducts display one reversible oxidation wave,
while the dppr adduct has an irreversible oxidation. The
potentials at which oxidation occurs for the borane
adducts are more positive than those of the starting
phosphines and are similar to the oxidation potentials
for the chalcogenide compounds. This is similar to the
trend found for the oxidation potential of H3B‚PPh2Fc,
which is 0.2 V more positive than that of PPh2Fc.16
The electrochemistry of a second type of Lewis acid
adduct was also examined. In this type, the phosphorus
atoms were alkylated, yielding diphosphonium com-
pounds. As with all of the previous iron compounds,
when there is not a lone pair of electrons on either
phosphorus atom, the oxidation of the compound is
reversible. The potential at which the oxidation occurs
for dppf(CH3)22+ and dippf(CH3)22+ is significantly more
positive than that of the phosphines, and this is likely
attributable to the +2 charges of the compounds that
are a result of the addition of two CH3+ groups. This is
similar to the case for H3C-PPh2Fc+I-, which has a
reversible oxidation that is approximately 0.45 V more
positive than that of PPh2Fc.16 While this difference is
significantly less than what is observed for the dppf and
dippf compounds, it is not surprising, since the dppf and
dippf compounds are dications, while the PPh2Fc adduct
is a monocation. No wave is observed for the oxidation
of dppr(CH3)22+; it is presumed to be outside of the
solvent window.
Substituents
substituent
C5H4PPh2
C5H4PO(Ph)2
C5H4PS(Ph)2
C5H4PSe(Ph)2
C5H4PPh2BH3
C5H4PPh2Me+
C5H4P(iPr)2
C5H4PO(iPr)2
C5H4PS(iPr)2
C5H4PSe(iPr)2
C5H4P(iPr)2BH3+
C5H4P(iPr)2CH3
EL
σp
0.45a
0.55
0.57
0.49
0.57
0.83
0.36b
0.51
0.52
0.43
0.53
0.81
0.19c
0.53c
0.47c
0.58d
0.62d
1.18c
0.06c
0.41c
0.59d
0.55d
0.60d
0.72d
a Reference 3. b Reference 6. c Reference 19. d Calculated using
eq 1.
of EL versus the Hammett substituent constant, σp,
gives an excellent correlation.14 To ensure that this
correlation is applicable to 1,1′-diphosphinoferrocenes,
a plot of EL versus σp was constructed for compounds
in which both the EL and σp values are known. The slope
and intercept obtained for the 1,1′-diphosphinofer-
rocenes are in excellent agreement with the data for the
Fe(C5H4R)2 compounds.14 From this relationship, the σp
for any substituent can be estimated from the EL value
using eq 1.14 The σp values for all of the substituents in
σp ) 0.45EL + 0.36
(1)
this study are presented in Table 2, including those
estimated using this method. For the asymmetric
compounds dppfOS and dppfOSe, the E° value versus
the NHE can be estimated by summing the individual
EL values. For dppfOS the E° value is 1.14 V, and the
estimate from the EL values of -PO(Ph)2 and -PS(Ph)2
is 1.12 V. Similarly, the E° value for dppfOSe is 1.11 V
and the estimate is 1.04 V.
Conclusion
The lone pair of electrons on the phosphorus atoms
of dppf and dippf is involved in a side reaction, likely
dimerization, following oxidation of the compounds.
When the lone pairs of electrons are occupied with
oxygen, sulfur or a Lewis acid, the follow-up reaction of
the oxidation product can be avoided. However, occupy-
ing the lone pair on phosphorus with selenium gives rise
to a different follow-up reaction upon oxidation. The
decomposition of dppr+ does not appear to involve the
lone electron pairs of the phosphorus atoms, as the
occupation of the electrons with chalcogenides and
Lewis acids does not lead to reversibility of the oxida-
tion. In addition, there is an excellent correlation
between the potential at which oxidation occurs for 1,1′-
bisphosphinoferrocenes and the Hammett parameters
for the phosphine groups.
For Lewis acid adducts of the iron compounds, the
Lewis acid occupies the lone pair on phosphorus, and
this prevents the oxidized species from displaying any
reactivity on the cyclic voltammetric time scale. This
compliments what is seen for the oxides, sulfides, and
many metal complexes with dppf or dippf ligands. The
irreversible nature of the oxidative electrochemistry of
dppr and compounds containing dppr remains un-
explained. Thus far, we can conclude that the support-
ing electrolyte does not appear to influence the revers-
ibility of the oxidation. In addition, the availability of
the phosphorus lone pair does not seem to be essential
for the decomposition of the oxidation product. Ad-
ditional studies will be required to explain this phe-
nomenon.
Acknowledgment. We thank the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, for partial support of this re-
search and the Kresge Foundation for the purchase of
the JEOL Eclipse 400 MHz NMR instrument.
Electrochemical Parametrization. The electro-
chemical parameter, EL, for various C5H4R ligands can
be estimated from the 1/2E° vs NHE values for 1,1′-
disubstituted ferrocene compounds, Fe(C5H4R)2.14 A plot
OM050074P