Synthesis, X-ray structure and electrochemical oxidation of palladium(II) complexes of ferrocenyldiphenylphosphine

Article abstract of DOI:10.1039/c0dt00016g

Four new complexes, [PdX(κ²-2-C₆R ₄PPh₂)(PPh₂Fc)] [X = Br, R = H (1), R = F (2); X = I, R = H (3), R = F (4)], containing ferrocenyldiphenylphosphine (PPh ₂Fc) have been prepared and fully characterised. The X-ray structures of complexes trans-1, cis-2 and cis-4, and that of a decomposition product of 4, [Pd(κ²-2-C₆F₄PPh₂)(μ-I) (μ-2-C₆F₄PPh₂)PdI(PPh₂Fc)] (5), have been determined. These complexes show a distorted square planar geometry about the metal atom, the bite angles of the chelate ligands being about 69°, as expected. The cis/trans ratio of 1-4 in solution is strongly dependent on solvent. The new complexes and the uncoordinated PPh₂Fc ligand were electrochemically characterised by cyclic and rotating disk voltammetry, UV-visible spectroelectrochemistry, and bulk electrolysis in dichloromethane and acetonitrile. In both cases, oxidation occurs at both the ferrocene and phosphine centres, but the complexes oxidise at more positive potentials than uncoordinated PPh₂Fc; subsequently, the metal-phosphorus bond is cleaved, leading to free PPh₂Fc⁺, which undergoes further chemical and electrochemical reactions. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00016g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, X-ray structure and electrochemical oxidation of palladium(II)
complexes of ferrocenyldiphenylphosphine†
Martin A. Bennett,a Suresh K. Bhargava,*b Alan M. Bond,*c Iko M. Burgar,d Si-Xuan Guo,c Gopa Kar,b
Steven H. Prive´r,b Jo¨rg Wagler,e Anthony C. Willisa and Angel A. J. Torrieroc
Received 23rd February 2010, Accepted 17th June 2010
DOI: 10.1039/c0dt00016g
2
Four new complexes, [PdX(k -2-C6R4PPh2)(PPh2Fc)] [X = Br, R = H (1), R = F (2); X = I, R = H (3),
R = F (4)], containing ferrocenyldiphenylphosphine (PPh2Fc) have been prepared and fully
characterised. The X-ray structures of complexes trans-1, cis-2 and cis-4, and that of a decomposition
2
product of 4, [Pd(k -2-C6F4PPh2)(m-I)(m-2-C6F4PPh2)PdI(PPh2Fc)] (5), have been determined. These
complexes show a distorted square planar geometry about the metal atom, the bite angles of the chelate
ligands being about 69◦, as expected. The cis/trans ratio of 1–4 in solution is strongly dependent on
solvent. The new complexes and the uncoordinated PPh2Fc ligand were electrochemically characterised
by cyclic and rotating disk voltammetry, UV-visible spectroelectrochemistry, and bulk electrolysis in
dichloromethane and acetonitrile. In both cases, oxidation occurs at both the ferrocene and phosphine
centres, but the complexes oxidise at more positive potentials than uncoordinated PPh2Fc;
subsequently, the metal–phosphorus bond is cleaved, leading to free PPh2Fc+, which undergoes further
chemical and electrochemical reactions.
also observed reversible cyclic voltammetry of dppf in CH3CN at
1. Introduction
scan rates from 0.02 to 0.2 V s-1 with peak separations similar to
Research into ferrocene-containing compounds continues apace,
those of ferrocene. According to independent studies, the electro-
largely driven by applications in catalysis and materials science.1,2
One of the best known properties of ferrocenes is their ability
to undergo reversible one-electron oxidation. The presence of a
phosphine substituent in one or both of the ferrocene rings allows
incorporation of the redox-active ferrocenyl moiety into transition
metal complexes, often leading to an increase in reactivity.1
The most widely investigated phosphine-metallocene employed
for this purpose is 1,1¢-bis(diphenylphosphino)ferrocene (dppf).
The redox active ferrocene backbone of dppf provides an area
of specific interest, because it has been proposed to influence
the electron transfer properties of dppf-containing compounds.3
However, the oxidation of dppf, unlike that of ferrocene, is not
always completely reversible.
chemical oxidation of dppf in CH2Cl2 is chemically irreversible.6,9,10
The oxidative electrochemistry of dppf in non-coordinating me-
dia corresponds to essentially reversible one-electron oxidation
followed by a fast chemical reaction,13 possibly associated with
dimerisation of dppf+.11 Although the electrochemically generated
dppf+ undergoes other reactions in addition to dimerisation,11 the
latter was studied over a range of temperatures and concentra-
tions, and activation parameters for the dimerisation step were
reported.
A variety of compounds closely related to dppf, such as 1,1¢-
bis(diisopropylphosphino)ferrocene (dippf),14 1,1¢-bis(diphenyl-
phosphino)ruthenocene (dppr), and 1,1¢-bis(diphenylphosphino)-
osmocene (dppo),7 have also been investigated electrochemically.
The oxidation of dppr is irreversible and occurs at a potential that
is more positive than that of dppf. In FcCH2PR2 complexes (R =
Ph, CH2OH, and CH2CH2CN) electrochemical oxidation of the
ferrocene moiety is closer to reversible.15 The reversible potentials
and the level of chemical reversibility of the oxidation of 1,1¢-
bis(diphenylphosphino)octamethylferrocene (dppomf) and 1,1¢-
bis(diphenylphosphinoindenyl)iron, Fe(dppind)2, are significantly
different from those of dppf.16,17
Dppf complexes of palladium have found application in
organometallic chemistry and the homogeneous catalysis of,
for example, C–C coupling and CO insertion reactions.18–20
While working with palladacyclic complexes containing ferro-
cenyldiphenylphosphine (PPh2Fc), we realised that, although
the chemical behaviour of this ligand has been reported to be
less complex than that of dppf,6,21,22 it has not been studied
as thoroughly, nor has the influence of P-coordination on its
electrochemical behaviour been established. We describe here a
detailed study of the oxidative electrochemistry of PPh2Fc and
The oxidative electrochemistry of dppf has been studied
in acetonitrile,4,5 dichloromethane6–10 and 1,2-dichloroethane.11,12
DuBois et al.4 have demonstrated that, in CH3CN at a glassy
carbon electrode, an irreversible chemical reaction accompanies
oxidation to dppf+. Using a Pt bead electrode, Housecroft et al.5
aResearch School of Chemistry, Australian National University, Canberra,
ACT 0200, Australia
bSchool of Applied Sciences (Applied Chemistry), RMIT Univer-
sity, GPO Box 2476V, Melbourne, Victoria 3001, Australia. E-mail:
suresh.bhargava@rmit.edu.au
cSchool of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
E-mail: alan.bond@sci.monash.edu.au
dCMSE Division, CSIRO, Clayton, 3800, VIC, Australia
eInstitut fu¨r Anorganische Chemie, Technische Universita¨t Bergakademie
Freiberg, D-09596, Freiberg, Germany
† Electronic supplementary information (ESI) available: Cyclic voltammo-
gram at a glassy carbon electrode for a 0.9 mM solution of 3 in CH2Cl2.
CCDC reference numbers 763269–763272. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0dt00016g
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Table 1 31P NMR data for complexes 1–4 in C6D6
Complex
Isomer
dP (multiplicity)a
J
PP/Hz
dP (multiplicity)a
JPP/Hz
2
[PdBr(k -2-C6H4PPh2)(PPh2Fc)] (1)
cis
trans
cis
trans
cis
trans
cis
trans
7.5 (d)
14.2
485
14.8
514c
12.9
484
-81.8 (d)
-88.5 (d)
-75.6 (m)
-84.8 (dm)
-91.5 (d)
-93.0 (d)
-83.7 (m)
-90.0 (dm)
14.4
17.9 (d)
10.5 (dm)
17.3 (dd)
7.3 (d)
19.3 (d)
9.9 (dm)
17.7 (dd)
485
2
b
[PdBr(k -2-C6F4PPh2)(PPh2Fc)] (2)
514
12.9
2
[PdI(k -2-C6H4PPh2)(PPh2Fc)] (3)
484
2
b
[PdI(k -2-C6F4PPh2)(PPh2Fc)] (4)
13.9
512d
512
a d = doublet, m = multiplet, dd = doublet of doublets, dm = doublet of multiplets. b Unresolved JPF
.
c JPF = 2.5 Hz. d JPF = 1.7 Hz.
2
of four derived palladacycles [PdX(k -2-C6R4PPh2)(PPh2Fc)] [X =
Br, R = H (1), R = F (2); X = I, R = H (3), R = F (4)].
of C6D6, showed almost exclusive formation (ca. 90%) of the cis
isomer; its insolubility in CD3CN prevented characterisation in
this solvent. The original spectrum was regenerated in C6D6. In
CD2Cl2, complex 4 exists as an approximately 1 : 0.2 mixture of
trans and cis isomers; in 1 : 1 CD2Cl2/CH3CN, there was some
precipitation of solid, but the ratio of the cis isomer in solution
increased slightly to 1 : 0.3. Similar solvent dependence of cis/trans
ratios has been observed for a number of tertiary phosphine
complexes of palladium, the cis isomer being favoured in more
polar solvents.31,32
2. Results and discussion
2.1. Synthesis of palladacyclic complexes containing
ferrocenyldiphenylphosphine
2
The yellow-orange, mononuclear palladium(II) complex [PdBr(k -
2-C6H4PPh2)(PPh2Fc)] (1) was prepared in 80% yield from
the reaction in CH2Cl2 of the tetranuclear complex [Pd4(m-
Br)4(m-2-C6H4PPh2)4] with PPh2Fc, similarly to its PPh3
The molecular structure of trans-1, determined by X-ray diffrac-
tion of a single crystal grown from CH2Cl2–hexane solution, is
shown in Fig. 1 together with selected bond lengths and angles. The
palladium atom shows approximately square planar coordination,
distorted as a result of the narrow bite angle of the chelate 2-
C6H4PPh2 group, the two phosphorus donor atoms being mutually
trans. The P(1)–Pd(1)–C(112) angle of 69.05(11)◦ is typical of that
usually found for ortho-metallated PPh3 complexes.33 The Pd–
analogue.23,24 The tetrafluoro-substituted counterpart, [PdBr(k -
2
2-C6F4PPh2)(PPh2Fc)] (2), could not be prepared similarly
from [Pd4(m-Br)4(m-2-C6F4PPh2)4],25 but was obtained from the
halide-bridged dimer [Pd2(m-Br)2(k -2-C6F4PPh2)2]26 in 73% yield.
2
2
The corresponding yellow-orange iodo complexes [PdI(k -2-
C6R4PPh2)(PPh2Fc)] [R = H (3), R = F (4)] were obtained by
treating a dichloromethane solution of 1 or 2, respectively, with
a solution of sodium iodide in acetone. The 31P NMR spectra of
complexes 1 and 3 in C6D6 each show a pair of equally intense
doublets at d ca. -85 and +20, split by ca. 500 Hz, consistent
with mutually trans phosphorus nuclei,27 and the highly shielded
resonance is typical of a phosphorus nucleus in a four-membered
chelate ring.28 In the same region, a pair of less intense resonances
due to the cis isomer, split into doublets of ca. 15 Hz, was also
observed (Table 1). The 31P NMR spectra of the fluoro-substituted
complexes 2 and 4 are similar to those of 1 and 3, but show
additional splitting. The resonances at d ca. 20 appear as either
a doublet of doublets (trans isomer) or a doublet of multiplets
(cis isomer), presumably due to coupling to the ortho fluorine
substituent of the C6F4PPh2 chelate groups; similar fluorine
coupling was observed in the 1,2-bis(diphenylphosphino)ethane
(dppe) complexes cis-[M(kC-C6F4PPh2)2(dppe)] (M = Ni, Pd,
Pt).29 As expected, the resonances at d ca. -80 appear as multiplets
(cis isomer) or doublets of multiplets (trans isomer) due to
unresolved fluorine coupling. Under the reaction conditions, the
trans isomer was the main product (85–90%). The Raman spectra
of complexes 1 and 2 show bands at 219 and 214 cm-1, respectively,
typical of n(Pd–Br);30 bands due to n(Pd–I) in complexes 3 and 4
could not be identified.
˚
Br bond length [2.5013(5) A] in trans-1 is significantly shorter
2
than that observed in the closely related complex trans-[PdBr(k -
24
˚
2-C6H4PPh2){PPh2(C6H4Br-2)}] [2.5818(12) A]; in contrast, the
2
Fig. 1 Molecular structure of trans-[PdBr(k -2-C6H4PPh2)(PPh2Fc)]
(trans-1). Ellipsoids show 50% probability levels and hydrogen atoms
The cis/trans ratio in solution of different preparations of 1–4
varies, but in C6D6 the trans isomer always predominates. Over
24 h, a solution of [PdI(k -2-C6F4PPh2)(PPh2Fc)] (4), in CH3CN
˚
have been omitted for clarity. Selected bond lengths (A) and angles
(◦): Pd(1)–Br(1) 2.5013(5), Pd(1)–P(1) 2.2987(10), Pd(1)–P(2) 2.3116(11),
Pd(1)–C(112) 2.043(4), P(1)–Pd(1)–Br(1) 99.58(3), P(1)–Pd(1)–C(112)
69.05(11), P(2)–Pd(1)–Br(1) 94.98(3), P(2)–Pd(1)–C(112) 96.42(11),
P(1)–Pd(1)–P(2) 163.07(4), C(112)–Pd(1)–Br(1) 168.58(11).
2
deposited an orange solid. This was probably the cis isomer, since
its 31P NMR spectrum in CH2Cl2, containing a small amount
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◦
Table 3 Selected bond lengths (A) and angles (◦) in [Pd(k -2-
C6F4PPh2)(m-I)(m-2-C6F4PPh2)PdI(PPh2Fc)] (5)
2
2
˚
˚
Table 2 Selected bond lengths (A) and angles ( ) in cis-[PdX(k -2-
C6F4PPh2)(PPh2Fc)]
X = Br (cis-2)
X = I (cis-4)
Pd(1)–I(1)
Pd(1)–C(2)
Pd(1)–P(1)
Pd(1)–P(2)
P(1)–Pd(1)–C(2)
P(1)–Pd(1)–P(2)
C(2)–Pd(1)–I(1)
P(2)–Pd(1)–I(1)
P(1)–Pd(1)–I(1)
P(2)–Pd(1)–C(2)
Pd(1)–I(1)–Pd(2)
2.638(4)
2.135(19)
2.315(11)
2.337(10)
69.2(8)
101.1(4)
95.2(7)
Pd(2)–I(1)
Pd(2)–I(2)
Pd(2)–C(32)
Pd(2)–P(3)
P(3)–Pd(2)–I(2)
P(3)–Pd(2)–C(32)
C(32)–Pd(2)–I(1)
I(1)–Pd(2)–I(2)
P(3)–Pd(2)–I(1)
C(32)–Pd(2)–I(2)
2.679(4)
2.652(4)
2.060(15)
2.308(12)
91.4(3)
91.2(7)
84.3(7)
93.91(12)
168.0(3)
174.9(7)
Pd(1)–X(1)
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–C(2)
P(1)–Pd(1)–P(2)
P(1)–Pd(1)–C(2)
X(1)–Pd(1)–P(2)
X(1)–Pd(1)–C(2)
P(1)–Pd(1)–X(1)
P(2)–Pd(1)–C(2)
2.4589(7)
2.2582(13)
2.3332(12)
2.076(4)
102.24(5)
69.41(15)
90.09(3)
97.14(15)
163.39(4)
170.14(14)
2.6192(10)
2.274(3)
2.338(3)
2.034(10)
102.30(10)
69.3(3)
90.71(7)
96.4(3)
161.43(8)
170.2(3)
93.4(3)
164.3(3)
162.2(7)
72.78(11)
38
˚
is 1.810(3) A. In cis-2, there is a significant shortening of the
˚
˚
˚
Pd–C [2.043(4) A] and Pd–P [2.2987(10), 2.3116(11) A] bond
lengths are comparable with those in the latter complex [2.042(9),
P–C(Ph) [1.817(4) and 1.827(5) A] and P–C(Fc) bonds [1.795(4)
˚
A]. In cis-4, the P–C(Ph) bonds are slightly shorter [1.820(10) and
˚
˚
˚
2.285(3) and 2.340(3) A, respectively]. The Pd–P bond lengths
are also similar to those observed in trans-[PdMeCl(PPh2Fc)2]
1.825(10) A] while the P–C(Fc) bond is longer [1.831(12) A] than
those in the free ligand.
34
˚
[2.3328(10) A].
The single crystals of trans-1, cis-2 and cis-4 that were used in
the X-ray analyses discussed above were deposited from CH2Cl2–
hexane solutions of the complexes. Solutions of 1, 2 and 4 in
CH2Cl2 were not indefinitely stable. On one occasion, a solution
of 4 underwent extensive decomposition over several days, and
from the decomposed material a red block crystal was selected,
which gave a weak X-ray diffraction pattern. Although of poor
quality, the data were sufficient to establish the identity of the
The structures of complexes cis-2 and cis-4 were also determined
by single crystal X-ray diffraction analysis and the molecular
structure of cis-2 is shown in Fig. 2; selected bond lengths and
angles are listed in Table 2. Surprisingly, and in contrast to trans-
1, both of these complexes crystallise from CH2Cl2–hexane as the
cis isomer, which is the minor component in solution.
2
red complex as the dinuclear species, [Pd(k -2-C6F4PPh2)(m-I)(m-
2-C6F4PPh2)PdI(PPh2Fc)] (5), derived by dimerisation of 4 and
concomitant loss of PPh2Fc. The molecular structure of 5 is
shown in Fig. 3, and selected bond lengths and angles are listed in
Table 3.
2
Fig. 2 Molecular structure of cis-[PdBr(k -2-C6F4PPh2)(PPh2Fc)] (cis-2).
Ellipsoids show 30% probability levels and hydrogen atoms have been
omitted for clarity.
Complexes cis-2 and cis-4 are isomorphous and show ap-
proximately square planar coordination about the palladium
atom. The bite angles of the chelate ligand are about 69◦, as
2
Fig. 3 Molecular structure of [Pd(k -2-C6F4PPh2)(m-I)(m-2-C6F4PPh2)-
2
expected.33 The palladium–halide bond lengths in cis-[PdX(k -
PdI(PPh2Fc)] (5). Ellipsoids show 30% probability levels and hydrogen
atoms have been omitted for clarity. Because of the poor data set the
carbon atoms were refined isotropically as parts of rigid groups.
˚
2-C6F4PPh2)(PPh2Fc)] [X = Br, 2.4589(7) A; X = I, 2.6192(10)
˚
A] are comparable to, but slightly shorter than, those in other
palladium(II) complexes containing X trans to a tertiary phosphine
ligand, cf. 2.4744(9), 2.4767(9) A in [PdBr2(dppp)]35 and 2.6649(8),
One palladium atom, Pd(1), of complex 5 is coordinated in
an approximately planar array by a chelate 2-C6F4PPh2 ligand,
the phosphorus end of a bridging 2-C6F4PPh2 and a bridging
iodide group. The second palladium atom, Pd(2), is similarly
square planar-coordinated by the carbon end of the bridging
2-C6F4PPh2 ligand, a bridging iodide, a terminal iodide, and a
P-coordinated PPh2Fc ligand; the two iodide groups are mutually
cis as also are the two phosphorus atoms. This structure is
˚
36
˚
2.6446(10) A in [PdI2(dppe)], respectively. There is a shortening
˚
˚
of ca. 0.04 A in the Pd–C and a lengthening of ca. 0.02 A in the
Pd–Pring bond lengths going from X = Br to I, the latter reflecting
the greater trans influence of iodide compared to bromide.37 There
are also associated changes in the P–C bond lengths of the PPh2Fc
ligand upon coordination; in the free ligand, the P–C(Ph) bond
˚
lengths are 1.840(3) and 1.843(3) A and the P–C(Fc) bond length
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Table 4 Electrochemical data for oxidation of PPh2Fc and its metal complexes at a glassy carbon electrode.a
Solvent
CH2Cl2
Compound
E
m (I)/mV
Iox/Ired
E
m (II)/mV
Iox/Ired
E
m (III)/mV
Iox/Ired
0.52
D ¥105/cm2 s-1
PPh2Fc
dppf
649
731
741
708
0
616
705
724
696
0
0.73b
0.36b
0.95
1079
1328
1188
0.48
0.56
0.60
786
0.93
0.89
0.71
0.64
1.44
1.34
1.12
0.83
0.79
1.76
2
[PdI(k -2-C6F4PPh2)(PPh2Fc)] (4)
858
994
0.42
2
[PdBr(k -2-C6H4PPh2)(PPh2Fc)] (1)
1.00
DmFc
PPh2Fc
dppf
0.99
CH3CN
0.76b
0.25b
0.66b
0.80b
0.99
758
0.56
0.52
1247
1064
0.37
0.56
2
[PdI(k -2-C6F4PPh2)(PPh2Fc)] (4)
2
[PdBr(k -2-C6H4PPh2)(PPh2Fc)] (1)
1034
DmFc
a The difference in the mid-point potential (Em) for the Fc0/+ and DmFc0/+ couple is 548 and 509 mV in CH2Cl2 and CH3CN, respectively. Supporting
electrolyte: 0.1 M [Bu4N][PF6]; scan rate: 100 mV s-1; Potentials are mV vs. DmFc0/+ b Values very dependent on switching potential. Magnitudes given
.
are for when potential is switched after process IIox, but ratio approaches unity if the potential switched just after Iox. c Derived from rotating disk electrode
voltammetry and use of the Levich equation.
reminiscent of two others: (1) the heterobimetallic complex
[(OC)3Fe(m-I)(m-dppm)PdI] in which the two metal atoms are
bridged by iodide and bis(diphenylphosphino)methane (dppm)
ligands and a terminal iodide bound to the palladium atom (in
this case, a metal–metal bond is also present);39 and (2) the A-
frame dipalladium complex [Pd2(m-dppm)2(m-I)(CH3)I]BF4, which
contains two bridging dppm ligands and one bridging and one
terminal iodide.40 As in the latter complex, the bridging iodide in
5 is asymmetrically bound to the two palladium atoms [Pd(1)–I(1):
potential is switched just after the appearance of the oxidation
peak (Fig. 4C, process Iox) and the second process at a potential
more positive than 1000 mV vs. DmFc0/+ (partially reversible in
CH2Cl2 and irreversible in CH3CN, Fig. 4C–F, process IIox and
Table 4) prior to the onset of the solvent oxidation. However,
the voltammetry is clearly more complex than that found for the
oxidation of ferrocene (Fig. 4A and B).
The first process is assigned to the oxidation of the ferrocenyl
part of the ligand and is close to reversible if the potential is
switched early enough. However, it is clear that further oxidative
processes may have an important influence on the behaviour of
process Ired if the potential is switched at potentials significantly
more positive than that for process Iox. Indeed, the cyclic voltam-
metry for oxidation of PPh2Fc, when undertaken over a wide
potential range, is strongly dependent on the switching potential
(Fig. 4C and D), number of cycles (Fig. 4E and F) and the scan
rate (not shown). If the potential is switched just after process
Iox, then Ired is detected (Fig. 4C and D) and the Iox/Ired couple
is described in eqn (2) (for clarity PPh2Fc will be written also as
C5H5FeIIC5H4PIIIPh2)
˚
˚
2.638(4) A, Pd(2)–I(1): 2.679(4) A].
3. Electrochemical oxidation of
ferrocenyldiphenylphosphine
In many solvents, ferrocene (Fc)‡ undergoes an ideal reversible,
one-electron oxidation under voltammetric conditions to give the
ferrocenium cation (Fc+) (eqn (1)). However, the presence of a
diphenylphosphino group on one or both cyclopentadienyl rings
usually makes the oxidation of Fc to Fc+ more difficult and hence
often also leads to an increase in the reactivity of the oxidised Fc+
form.5,6,11,41
-
-e
III
III
C5H5FeIIC5H4PIIIPh
C H Fe C H P Ph
5 5 5 4 2
ꢁꢁꢁꢂ
(2)
2 ꢃꢁꢁꢁ
-
Fc ꢀ Fc+ + e−
+e
(1)
with a reversible Em-value of 649 and 616 mV vs. DmFc0/+ in
CH2Cl2 and CH3CN, respectively. These Em values are more
positive by about 100 mV, with respect to the Fc/Fc+ couple, but
not as positive as that found for oxidation of dppf (Table 4). This
positive potential shift relative to Fc is attributed to the electron
acceptor ability of the PPh2 group, which reduces the electron
density of the Fc unit, thus making its oxidation more difficult.
In between processes Iox and IIox, a minor process IIIox is seen, as
is the companion process IIIred. If the switching potential is more
positive than that of process IIox, the couple IIIox/IIIred becomes
much more dominant. In CH3CN, process Ired disappears (Fig. 4D
and F), whereas its intensity decreases in CH2Cl2 (Fig. 4C and E).
Processes analogous to I and III have been observed previ-
ously for the electro-oxidation of (h -C5H5)Fe(h -C5H4CO2H) in
CH3CN.22 As summarised in eqn (2)–(6), the first oxidation process
for phosphine-substituted ferrocenes is oxidation of Fe(II) to
Fe(III) (Fig. 4 process I, eqn (2)). The resulting ferrocenium cation
is postulated to participate in intramolecular electron transfer
from the phosphine group to iron (via cyclopentadienyl, eqn (3))
In the solvents of interest in this study, CH2Cl2 and CH3CN, the
Fc0/+ couple is reversible under conditions of cyclic voltammetry
(Fig. 4A and B). The mid-point potentials (Em) calculated from
the average of the oxidation (Epox) and reduction (Epred) peak
potentials [Em = (Epox + Epred)/2] are 548 and 509 mV, respectively,
vs. the decamethylferrocene/decamethylferrocenium (DmFc0/+
)
couple, used to provide the potential reference scale. Cyclic
voltammograms obtained at a glassy carbon electrode (scan rate =
100 mV s-1, T = 21 ◦C) for oxidation of 1.0 mM solutions of
PPh2Fc in CH2Cl2 (Fig. 4C and E) and CH3CN (Fig. 4D and
F), containing 0.1 M [Bu4N][PF6] as the supporting electrolyte,
exhibit two major oxidation processes (designated Iox and IIox
in Fig. 4) having approximately equal peak current magnitudes.
The first process is observed at ca. 600 mV (reversible) when the
5
5
‡ We use here the standard abbreviations in electrochemistry Fc and DmFc
for ferrocene and decamethylferrocene, respectively. This should not be
confused with the use elsewhere in this paper of Fc as an abbreviation for
Fe(C5H4)(C5H5) in PPh2Fc.
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Fig. 4 Cyclic voltammograms for oxidation of 1.0 mM ferrocene in CH2Cl2 (0.1 M [Bu4N][PF6]) (A) and 1.3 mM ferrocene in CH3CN (0.1 M
[Bu4N][PF6]) (B) and oxidation of 1.0 mM PPh2Fc in CH2Cl2 (0.1 M [Bu4N][PF6]) (C) and (E) and CH3CN (0.1 M [Bu4N][PF6]) (D) and (F) at a glassy
carbon electrode. Scan rate: 100 mV s-1; potentials are vs. DmFc0/+; T: 21 1 ◦C.
and the resulting species is then immediately stabilised by further
chemical one-electron oxidation, with traces of water or oxygen, to
give a phosphine oxide (eqn (4)). The phosphine oxide generated
in this way was reported by Podlaha et al.22 as being responsible
for the redox system III (eqn (6)).
-
-e
III
IV
C5H5FeIIIC5H4PIIIPh
C H Fe C H P Ph
5 5 5 4 2
ꢁꢁꢁꢂ
(5)
2 ꢃꢁꢁꢁ
-
+e
-
-e
III
V
C5H5FeIIC5H4PV (O)Ph
C H Fe C H P (O)Ph
5 5 5 4 2
ꢁꢁꢁꢂ
(6)
2 ꢃꢁꢁꢁ
-
+e
III
III
II
IV
I
V
⎡
⎤
C5H5Fe C5H4P Ph2 ↔ C5H5Fe C5H4P Ph2 ↔ C5H5Fe C5H4P Ph2
⎢
⎥
⎦
⎣
Analysis by rotating disk glassy carbon electrode voltammetry
shows that in CH3CN (0.1 M [Bu4N][PF6]), processes I and II for
PPh2Fc have the same limiting current magnitude. In contrast, in
the case of dppf, the limiting current value for process II is close
to twice that for process I (not shown). Therefore, process II is
assigned to oxidation of the intermediates postulated in eqn (3)
(3)
O
II
IV
II
V
2
O
C5H5Fe C5H4P Ph2 ⎯H⎯⎯→C5H5Fe C5H4P (O)Ph2
(4)
2
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Fig. 5 Cyclic voltammograms for oxidation of 1.0 mM PPh2Fc in CH2Cl2 (0.1 M [Bu4N][PF6]) at a glassy carbon electrode (1 mm diameter) at scan
rates of (A) 500 mV s-1, and (B) 5000 mV s-1. Potentials are vs. DmFc0/+; T: 21 1 ◦C.
(which also could include PIII cation radicals as an alternative
formulation); it is well known that tertiary phosphines can undergo
phosphine-based oxidation at such potentials.42,43
Measurements at a glassy carbon rotating disk electrode with
1.0 mM PPh2Fc solution in CH2Cl2 and CH3CN showed a linear
dependence of the limiting current (for process I) with the square
root of the angular frequency of rotation. The diffusion coefficient
(D) for PPh2Fc, derived from the Levich equation,44 was 0.93 ¥ 10-5
and 1.34 ¥ 10-5 cm2 s-1 in CH2Cl2 and CH3CN, respectively (Table
4), assuming the oxidation to be an overall one-electron process
in both solvents. From Table 4 it is clear that in both solvents the
diffusion coefficient decreases as the size of the molecule increases.
In CH2Cl2 (0.1 M [Bu4N][PF6]) both processes I and II are
electrochemically reversible and process III is less evident than
in CH3CN. If the switching potential is more positive than that
for process IIox, the reversible couple IIIox/IIIred becomes more
dominant (Fig. 4C and E), while the process Ired is more prominent
at lower scan rates (e.g. 100 mV s-1) and disappears when the
scan rate is increased up to 5000 mV s-1 (Fig. 5). Upon repetitive
cycling of the potential at low scan rate, the voltammetric pattern
remains unchanged, demonstrating that the parent complex is
regenerated during the electrochemical turnover. Meanwhile, at
increasing scan rates, a new reversible process, V (Fig. 5), appears
with an Em-value that is slightly more negative than that of process
II. In addition, processes II and III decrease in intensity and IIIred
becomes more positive. All these observations indicate that one or
more rapid and reversible homogeneous reactions follow the first
electron transfer process, probably through a dimeric intermediate,
as has been suggested for dppf.7,11 Another small process at ca.
1.28 V in CH2Cl2 (process IVox in Fig. 4E) probably arises from an
oxidation product derived from process IIox. The significant level
of chemical reversibility of process II in CH2Cl2 demonstrates a
solvent dependence in the electrochemical behaviour of PPh2Fc;
probably the more polar and coordinating CH3CN causes the
irreversibility of process II by reactions with the oxidised product.
3.1. UV-visible spectroelectrochemistry
The voltammetric data discussed above were obtained with a
glassy carbon working electrode. Data derived from a platinum
disk working electrode are similar, allowing spectroelectrochem-
ical studies at this metal electrode to be assumed to be directly
relevant to the voltammetry reported at glassy carbon. During the
course of oxidative electrolysis of a 0.2 mM PPh2Fc solution in
CH2Cl2 at 750 mV vs. DmFc0/+ using a platinum gauze working
electrode in a thin layer configuration, the colour changed from
pale yellow to light green. As shown in Fig. 6, this change was
associated with a decrease in intensity of the bands at 214 and
248 nm and a shift of the maximum wavelength in this region
to 224 and 257 nm, respectively. New bands at 293 and 350 nm
(Fig. 6A) were formed as the oxidation proceeded. Furthermore,
two isosbestic points (ip) at 234 and 285 nm were observed in the
Fig. 6 (A) UV-visible spectra obtained from 0.2 mM PPh2Fc solution in CH2Cl2 (0.1 M [Bu4N][PF6]) during the course of oxidative electrolysis at
750 mV vs. DmFc0/+. (B) Comparison of spectra for a 0.2 mM PPh2Fc solution in CH2Cl2 (—) before and (
) after electrolysis.
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early stages of this experiment. The oxidised solution does not
show any significant changes in the visible region between 600
and 710 nm; in particular, no bands around 617 nm were observed
characteristic of ferrocenium species.45 The observed spectroscopic
changes were not reversible upon attempted re-reduction at 0.20 V
vs. DmFc0/+, demonstrating that on the spectroelectrochemical
time-scale (minutes vs. seconds for cyclic voltammetry), a chemical
reaction is coupled to ferrocene oxidation.
of 715 mV (minor) and 855 mV (major) vs. DmFc0/+ were detected.
The latter may be associated with process III discussed above. The
31P NMR spectrum of PPh2Fc shows a singlet at d -17, which
progressively disappears during the course of bulk electrolysis and
is replaced by an intense resonance at d 50, the source of which we
have not identified. There are also two much weaker resonances
of equal intensity at d 36 and 44, the chemical shifts of which
are in the range expected for tertiary phosphine oxide species [cf.,
P(O)Ph2Fc in CDCl3, d 30.3],47 but neither corresponds to that
expected from simple oxidation of PPh2Fc. This result is consistent
with the observations of Pilloni et al.,11 who have identified a range
of oxidation products resulting from the electrochemical oxidation
of dppf. NMR analysis as soon as possible after completion of bulk
electrolysis was important because further irreversible changes in
solution composition occurred with time. These data confirm the
complexity of the electrochemical oxidation of PPh2Fc in organic
solvents.
3.2. Controlled-potential bulk electrolysis
In order to obtain more detailed information about the first
oxidation process, the apparent number of electrons exchanged
(napp) during controlled potential electrolytic oxidation of PPh2Fc
in CH2Cl2 was determined. Changes in the 31P NMR spectra ac-
companying bulk electrolysis also were investigated. For the NMR
measurements, the supporting electrolyte, 0.1 M [Bu4N][PF6],
was replaced by 0.1 M [Bu4N][BF4], which did not affect the
voltammetric behaviour of PPh2Fc. To obtain napp, the current
was recorded as a function of time until it had decayed to about
2% of the initial value. Under identical experimental conditions, a
blank experiment was performed and this current was subtracted
from the total current in order to obtain the Faradaic electrolysis
current. The plot of ln(I) vs. t was non-linear over the period of
electrolysis, indicating that the electron transfer was accompanied
by relatively slow and complex subsequent reactions.46 The total
integrated current gave an napp value of 0.62 0.10 per molecule. A
value of n = 1 is obtained by analysis of the shape of rotating disk
electrode voltammograms, indicating that additional reactions
occur on the much longer time scale of bulk electrolysis. An napp
< 1 value indicates that a product formed at relatively long times
reacts with the starting material.
4. Electrochemical oxidation of palladium(II)
complexes of PPh2Fc
Coordination of the phosphorus atom of PPh2Fc to a metal
centre effectively introduces a substituent effect into the PPh2Fc
electrochemistry. Thus, major processes Iox and IIox are still
expected to be encountered, but at different potentials. In the
palladium(II) complexes 1 and 4, which were chosen for detailed
study, Em-values for couple Iox/Ired associated with oxidation of the
ferrocene component of the PPh2Fc ligand are shifted to even more
positive potentials by 92 mV in CH2Cl2 and 108 mV in CH3CN
for compound 4 and 59 mV in CH2Cl2 and 80 mV in CH3CN
for compound 1, respectively (Table 4). This additional shift is
attributed to a decrease in electron density at the electroactive
moiety of the ligand due to s-donation of the lone pairs of
the phosphorus to palladium. The larger shifts observed for
complex 4 relative to those of 1 are indicative of a stronger s-
donation effect, presumably induced by the more electronegative
C6F4 group. Resolved processes associated with the cis- and trans-
isomers detected by NMR spectroscopy (see above) were not
observed, implying that the Em-values are essentially isomer-
independent.
Cyclic voltammograms obtained after exhaustive oxidative
electrolysis showed that process Iox/Ired had disappeared com-
pletely (Fig. 7), consistent with the irreversibility encountered
in the oxidation-reduction sequence employed in the UV-visible
spectroelectrochemical experiment. Two processes with Em values
Cyclic voltammograms for complex 4 in CH2Cl2 and CH3CN
(Fig. 8) over a wide potential range exhibit the expected two
major redox processes. The first, involving oxidation of the
ferrocene centre, is reversible (Em = 741 mV vs. DmFc0/+) in
red
CH2Cl2, with Ipox/Ip ratios for oxidation (Ipox) and reduction
(Ipred) in the range 0.95–1.0 over the scan rate range of 100 to
1000 mV s-1 when the potential is switched just after Iox. The
second process, associated with oxidation of the phosphorus atom,
occurs at more positive potentials (Em = 1188 and 1064 mV
vs. DmFc0/+ in CH2Cl2 and CH3CN, respectively). These two
principal oxidation processes, together with the smaller one at
858 mV vs. DmFc0/+ in CH2Cl2, parallel Iox, IIox and IIIox observed
for uncoordinated PPh2Fc (compare Fig. 4 and 8). The rate of the
chemical reaction coupled to the oxidised ferrocenium cation from
the Pd complex is much slower in CH2Cl2 (Iox/Ired ª 1) than it is
for the corresponding process in free PPh2Fc; however, the rates in
CH3CN are similar for the two cases. Nevertheless, the appearance
of minor peaks after the first oxidation process indicates that
Fig. 7 Cyclic voltammograms obtained before (—) and after (
)
exhaustive controlled potential bulk electrolysis at 700 mV vs. DmFc0/+
for 3.3 mM PPh2Fc in CH2Cl2 (0.1 M [Bu4N][BF4]). Scan rate: 100 mV s-1;
T: 21
1
◦C.
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(7)
withdrawing character of the ferrocenyl substituents.48 Conse-
quently, the instability of 4+ may be due to a weakening of
the phosphorus–palladium bond, which is purely s in character,
by the positive charge on the PPh2Fc ligand. In this case, the
subsequent cleavage of the phosphorus–metal bond would lead
to free PPh2Fc+ (eqn (7)), which is susceptible to the series of
reactions described above (eqn (2)–eqn (6)).
Cyclic voltammograms for oxidation of 1 (Fig. 9) include the
well defined Iox/Ired couple, and at more positive potentials exhibit
two poorly defined oxidation peaks at approximately 1050 and
1350 mV. Similar behaviour was observed for its iodo analogue 3
(Fig. S1, ESI†). No voltammetric oxidation of the closely related
2
ferrocenyl-free analogue [PdBr(k -2-C6F4PPh2)(PPh3)] is found
at potentials up to the solvent oxidation limit. All data imply
that the oxidation processes are associated with PPh2Fc and not
palladium.
4.1. UV-visible spectroelectrochemistry
In situ spectroelectrochemical studies on the oxidation of 4 at a
platinum gauze electrode at 0.80 V vs. DmFc0/+ are consistent
with the reaction scheme involving oxidation to 4+ followed by
breaking of the Pd–P bond, as summarised in eqn 7. The electronic
spectrum of the oxidised solution (Fig. 10A) shows absorption
bands at 293 and 350 nm, which closely match those obtained
by electrochemical oxidation of PPh2Fc (Fig. 10B). Furthermore,
the wavelengths of the two isosbestic points at 255 and 310 nm
observed at early stages of the spectroelectrochemical study are
similar to those found during the first minutes of bulk electrolysis
of PPh2Fc. Again, the isosbestic points disappear at longer times,
confirming that the initial oxidation process for 4 occurs on
PPh2Fc and that palladium redox chemistry is not involved.
Moreover, the application of a potential of 400 mV vs. DmFc0/+
to the oxidised solution does not lead to recovery of the initial
spectrum for 4, as is to be expected if 4+ is unstable on the
spectroelectrochemical time scale.
4.2. Controlled-potential bulk electrolysis
Fig. 8 Cyclic voltammograms at a glassy carbon electrode of (A) 3.8 mM
of 4 in CH2Cl2 (0.1 M [Bu4N][PF6]) and (B) and (C) 0.7 mM of 4 in CH3CN
(0.1 M [Bu4N][PF6]). Scan rate: 100 mV s-1; potentials vs. DmFc0/+; T: 21
The voltammogram obtained after oxidative bulk electrolysis of 4
(chosen as a representative complex) at a glassy carbon electrode at
a potential of 820 mV vs. DmFc0/+ in CH2Cl2 (0.1 M [Bu4N][BF4])
(Fig. 11) is similar to that of free PPh2Fc ligand (Fig. 7). However,
1
◦C.
on this occasion, the total integrated current corresponded to napp
=
2
[PdI(k -2-C6F4PPh2)(PPh2Fc)]+ (4+) is not indefinitely stable,
consistent with the instability reported for oxidised forms of
complexes containing the dppf ligand.13 Ferrocenylphosphines are
poorer ligands than triphenylphosphine, owing to the electron
1.0 0.1 electrons per molecule and the colour of the solution
changed from orange to dark green. The 31P NMR spectrum after
bulk electrolysis showed that the initially present pairs of equally
intense doublets at d ca. -85 and +20, corresponding to the trans
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5. Conclusions
The electrochemical oxidation of ferrocenyldiphenylphosphine,
PPh2Fc, in dichloromethane and acetonitrile has been investigated
in detail by cyclic and rotating disk voltammetry, UV-visible
spectroelectrochemistry and bulk electrolysis. The electrochemical
behaviour of PPh2Fc can be summarised as: (a) process Iox is
reversible in CH2Cl2 and CH3CN when the potential is switched
after the appearance of the oxidation peak. This is assigned to the
oxidation of the ferrocenyl part of the ligand (eqn (2)). Further
oxidative processes may have an influence on Ired if the potential
is switched at more positive potentials; (b) processes IIox/IIred are
observed at potentials more positive than 1000 mV. Process IIox is
partially reversible in CH2Cl2 and irreversible in CH3CN. This can
be assigned to the oxidation of intermediates postulated in eqn (3);
(c) process IIIox is observed between process Iox and IIox. Process
IIIred becomes dominant in CH3CN, but is less evident in CH2Cl2.
This corresponds to the oxidation of the tertiary phosphine oxide
described in eqn (6); (d) process IV is observed in CH2Cl2 and
probably arises from an oxidation product derived from process
IIox; (e) processes Vox/Vred are reversible, which appear in CH2Cl2 at
scan rates faster than 300 mV s-1. As a consequence, the behaviour
of PPh2Fc appears to parallel that of dppf; the initially formed
cation PPh2Fc+ rapidly undergoes subsequent chemical reactions
to unidentified products.
2
The palladacyclic complexes [PdX(k -2-C6R4PPh2)(PPh2Fc)]
[X = Br, R = H (1), R = F (2); X = I, R = H (3), R = F
(4)] are oxidised at more positive potentials than PPh2Fc with
subsequent cleavage of the metal–phosphorus bond and formation
of PPh2Fc+. The increase in abundance of the cis isomers in
acetonitrile relative to that of dichloromethane does not seem to
affect the electrochemical behaviour of complexes 1–4; presumably
the electrochemical behaviour of the isomers is very similar.
The complexes trans-1, cis-2 and cis-4 have been structurally
characterised by X-ray crystallography. In dichloromethane, the
trans isomers are strongly favoured, whereas the cis isomers
predominate in acetonitrile. A single crystal X-ray structural
2
analysis has identified a binuclear complex, [Pd(k -2-C6F4PPh2)(m-
I)(m-2-C6F4PPh2)PdI(PPh2Fc)] (5), as a decomposition product of
complex 4 in dichloromethane.
6. Experimental
Fig. 9 Cyclic voltammograms at a glassy carbon electrode for 1.0 mM
of 1 in (A) and (B) CH2Cl2 (0.1 M [Bu4N][PF6]) and (C) CH3CN (0.1 M
[Bu4N][PF6]). Scan rate: 100 mV s-1; potentials vs. DmFc0/+; T: 21 1 ◦C.
General comments
Dichloromethane (HPLC grade, Merck) was dried over basic
alumina (electrochemistry) or refluxed over calcium hydride
and distilled under nitrogen (synthesis) before use. Hexane was
dried over sodium/benzophenone and distilled prior to use. The
supporting electrolytes [Bu4N][PF6] and [Bu4N][BF4] (GFS Chem-
icals) were recrystallised twice from ethanol.49 The compounds
PPh2Fc50 and [Pd4(m-Br)4(m-2-C6H4PPh2)4]23 were prepared by
isomer (see above), and the pair of less intense resonances due
to the cis-isomer had disappeared and were replaced by a small
resonance at d 5.6, a broad signal with multiple resonances from d
16 to 34, and an intense resonance at d ca. 50. The d 50 resonance
presumably arises from the same species that is obtained from
exhaustive electrolysis of free PPh2Fc, thus lending support to the
reaction sequence proposed in eqn (7). The disappearance of the
doublets at d ca. -85 and the absence of any new resonances in
the region indicates that the four-membered chelate ring has been
opened; the complex resonances at d 16 to 34 may be due to the
resulting oligomeric species.
2
literature methods; the complex [Pd2(m-Br)2(k -2-C6F4PPh2)2] was
made by the reaction of [PdBr2(COD)] and 2-Me3SnC6F4PPh2,
which will be described elsewhere.26 All other chemicals were
commercially available and used as received.
1H (300 MHz), 19F (282 MHz) and 31P (121 MHz) NMR
spectra obtained from synthesised samples were measured on a
Bruker Avance 300 spectrometer at room temperature. 31P NMR
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Fig. 10 (A) UV-Visible spectra of 0.2 mM of 4 in CH2Cl2 before (—) and after (
) oxidative electrolysis at 820 mV vs. DmFc0/+. (B) Spectral comparison
of 0.2 mM PPh2Fc (
of data.
) and 0.2 mM 4 (—) in CH2Cl2 after exhaustive bulk electrolysis. Absorbance normalisation was applied to facilitate comparison
the computer programs Denzo and Scalepack53 and the crystal
structures were solved by direct methods.54
Electrochemistry
Electrochemical experiments were carried out in a standard three
electrode arrangement with a BAS Model 100B electrochemical
workstation (Bioanalytical Systems, West Lafayette, IN, USA). A
platinum wire auxiliary electrode and a Ag/AgCl (CH2Cl2, 0.1 M
[Bu4N][PF6]) single junction quasi-reference electrode were used
for voltammetric studies in CH2Cl2, although the potentials are
quoted relative to the DmFc0/+ potential scale. The same reference
electrode was used with the other techniques in CH2Cl2, but a
Ag/Ag+ (0.01 M AgNO3, 0.1 M [Bu4N][PF6]) double junction
reference electrode was used for studies in CH3CN. A glassy
Fig. 11 Cyclic voltammograms obtained before (—) and after (
)
carbon (GC) working electrode with a diameter of 0.1 cm and
an effective area of 0.0072 cm2 was used for cyclic voltammetric
experiments; the GC working electrode used for rotating disc
experiments had a diameter of 0.3 cm and an area of 0.071 cm2,
and was used with a BAS RDE-2 accessory. The areas or radii of
the voltammetric working electrodes were determined from cyclic
voltammograms using the peak current derived from the oxidation
of a 1.0 mM solution of ferrocene in CH3CN (0.1 M [Bu4N][PF6])
(diffusion coefficient of 2.3 ¥ 10-5 cm2 s-1) degassed with N2 and
with application of the relevant equations, i.e., the Randles-Sevcik
equation for the transient voltammetry with macrodisk electrodes,
and the Levich equation for the rotating disk electrode (RDE)
voltammetry.44 For spectroelectrochemical studies, a Pt gauze
working electrode and a Pt wire auxiliary electrode separated from
the Pt gauze by a fine porosity frit were used with a rectangular
quartz cuvette (1.0 mm optical path length) as the electrochemical
cell. In the case of bulk electrolysis experiments, a GC tube working
electrode and a Pt gauze auxiliary electrode separated from the test
solution by a fine porosity glass frit were used. In order to avoid the
[Bu4N][PF6] signal in the 31P NMR, this supporting electrolyte was
replaced by 0.1 M [Bu4N][BF4] in bulk electrolysis experiments.
Prior to each voltammetric experiment, the working electrode
was polished with 0.3 mm alumina (Buehler, Lake Bluff, IL) on
a clean polishing cloth (Buehler), sequentially rinsed with distilled
water and acetone, and then dried with lint-free tissue paper. The
solutions used for voltammetric, spectroelectrochemical and bulk
controlled potential bulk electrolysis of 3.8 mM 4 in CH2Cl2 (0.1 M
[Bu4N][BF4]) at 820 mV vs. DmFc0/+. Scan rate: 100 mV s-1; T: 21 1 ◦C.
spectra of solutions after bulk electrolysis were measured on a
300 MHz Varian Unity-Plus spectrometer in CD2Cl2. Chemical
shifts (d) are given in ppm, internally referenced to residual
solvent signals (1H), internal CFCl3 (19F) or external 85% H3PO4
(31P). 31P NMR data for complexes 1–4 are listed in Table 1.
Raman spectra were obtained in open glass capillaries on a Perkin
Elmer Raman Station 400F spectrometer and mass spectra on a
Bruker Apex 3 instrument. Elemental analyses were performed
by the Microanalytical Unit at the Research School of Chemistry,
Australian National University.
X-Ray crystallography†
Crystals of trans-1, cis-2 and cis-4 were obtained by layering a
CH2Cl2 solution of the compound with hexane; a suitable crystal
of 5 was selected from a decomposed solution of 4. The crystals
were coated in viscous oil and mounted on fine-drawn capillaries.
Data were collected at 130 K (trans-1), 150 K (cis-2) or 200 K (cis-4
and 5) on a Nonius-Kappa CCD diffractometer using graphite-
˚
monochromated Mo-Ka radiation (l = 0.71073 A). Crystal data
and details of data collection are given in Table 5. Data were
measured by use of COLLECT51 or SHELXL-97.52 The intensities
of reflections were extracted, the data were reduced by use of
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2
2
Table 5 Crystal and refinement data for trans-[PdBr(k -2-C6H4PPh2)(PPh2Fc)] (trans-1), cis-[PdX(k -2-C6F4PPh2)(PPh2Fc)] [X = Br (cis-2), I (cis-4)]
2
and [Pd(k -2-C6F4PPh2)(m-I)(m-2-C6F4PPh2)PdI(PPh2Fc)] (5)
trans-1
cis-2
cis-4
5
Formula
Fw
C40H33BrFeP2Pd
817.76
Orthorhombic
Pbca
Orange, block
10.5519(5)
17.1919(9)
36.2298(18)
90
6572.3(6)
8
1.653
0.25 ¥ 0.15 ¥ 0.10
2.331
5789 (0.041)
5062
406
0.038
0.088
C40H29BrF4FeP2Pd
889.73
Monoclinic
P21/c
Orange, plate
14.9344(5)
17.4528(9)
14.7656(5)
115.348(2)
3478.1(3)
4
1.699
0.10 ¥ 0.07 ¥ 0.02
2.226
6742 (0.077)
4136
442
0.046
0.071
C40H29F4FeIP2Pd
936.76
Monoclinic
P21/a
Yellow, needle
14.9111(7)
17.6801(8)
15.0706(6)
115.165(3)
3596.0(3)
4
1.730
0.40 ¥ 0.18 ¥ 0.04
1.900
6366 (0.123)
3747
443
0.060
0.154
C58H39F8FeI2P3Pd2
1503.31
Monoclinic
P21/c
Red, block
14.185(2)
19.347(2)
20.540(2)
99.952(5)
5552.1(11)
4
1.798
0.30 ¥ 0.15 ¥ 0.11
2.16
7626 (0.21)
2157
263
0.105
0.093
Crystal system
Space group
Crystal colour, habit
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/g cm-3
Crystal dimensions
m/mm-1
No. indep reflns (Rint
)
No. obs reflns [I > 2s(I)]
No. params refined
Ra
a
Rw
GOF
1.102
0.721/-0.532
0.907
0.561/-0.620
0.912
1.56/-1.36
1.089
1.82/-1.08
-3
˚
r
max/rmin/e A
a Refined on F2 of the observed reflections.
electrolysis measurements were purged with a solvent-saturated
nitrogen gas before each experiment and were blanketed with
nitrogen gas during the experiment.
UV-visible spectra (Varian Cary 5 NIR/Vis/UV spectropho-
tometer) over the wavelength range from 200 to 800 nm were
recorded continuously in spectroelectrochemical experiments at
a scan rate of 600 nm min-1 until the electrolysis was complete.
All electrochemical studies were carried out at room temperature
(21 1 ◦C).
d 4.08 (m, 2H, ferrocenyl), 4.37 (s, 5H, ferrocenyl), 4.44 (m,
2H, ferrocenyl) (trans-isomer); 4.05 (m, 2H, ferrocenyl), 4.20 (s,
5H, ferrocenyl), 4.75 (m, 2H, ferrocenyl) (cis-isomer); 6.7–7.5 (m,
13H, aromatics), 7.79 (m, 4H, aromatics), 8.23 (m, 3H, aromatics).
19F NMR (C6D6): d -117.7 (m), -134.1 (m), -147.8 (m), -157.5
(m) (trans-isomer); -121.7 (m), -138.1 (m), -147.6 (m), -157.8
(m) (cis-isomer). Raman (cm-1): 214 (Pd–Br str). ESI-MS (m/z):
809 [M–Br]+. Found: C 54.38; H 3.85; Br 9.52; F 8.21. Calcd for
C40H29BrFeF4P2Pd: C 54.00; H 3.29; Br 8.98; F 8.54.
Syntheses: [PdBr(j2-2-C6H4PPh2)(PPh2Fc)] (1)
[PdI(j2-2-C6H4PPh2)(PPh2Fc)] (3)
To a yellow suspension of [Pd4(m-Br)4(m-2-C6H4PPh2)4] (192 mg,
0.107 mmol) in CH2Cl2 (45 mL) was added solid PPh2Fc (160 mg,
0.43 mmol). The mixture was stirred for 72 h, during which time the
solid dissolved and a clear dark orange solution was obtained. The
solvent was removed in vacuo and the residue was recrystallised
from CH2Cl2–hexane to give a yellow-orange solid (0.28 g, 80%).
Cis/trans ratio: 5 : 95. 1H NMR (C6D6): d 4.03 (m, 2H, ferrocenyl),
4.42 (s, 5H, ferrocenyl), 4.53 (m, 2H, ferrocenyl) (trans-isomer);
4.06 (m, 2H, ferrocenyl), 4.28 (s, 5H, ferrocenyl), 4.79 (m, 2H,
ferrocenyl) (cis-isomer); 6.5–7.6 (m, 17H, aromatics), 7.80 (m, 4H,
aromatics), 8.25 (m, 3H, aromatics). Raman (cm-1): 219 (Pd–Br
str). ESI-MS (m/z): 737 [M–Br]+. Found: C 58.80; H 4.08; Br 9.93.
Calcd for C40H33BrFeP2Pd: C 58.75; H 4.07; Br 9.77.
To an orange solution of 1 (180 mg, 0.2 mmol) in CH2Cl2 (15 mL)
was added a solution of NaI (83 mg, 0.5 mmol) in acetone (15 mL)
and the mixture was stirred for 30 min. The solvent was removed
in vacuo, the residue was dissolved in CH2Cl2 and the suspension
was filtered through Celite. Hexane was added to the filtrate and
solution was evaporated under reduced pressure. The precipitated
yellow-orange solid was isolated by filtration, washed with hexane
1
and dried in vacuo (160 mg, 84%). Cis/trans ratio: 10 : 90. H
NMR (C6D6): d 4.02 (m, 2H, ferrocenyl), 4.37 (s, 5H, ferrocenyl),
4.54 (m, 2H, ferrocenyl) (trans-isomer); 4.04 (m, 2H, ferrocenyl),
4.29 (s, 5H, ferrocenyl), 4.78 (m, 2H, ferrocenyl) (cis-isomer); 6.6–
7.5 (m, 17H, aromatics), 7.80 (m, 4H, aromatics), 8.23 (m, 3H,
aromatics). ESI-MS (m/z): 737 [M–I]+. Found: C 56.04; H 3.73; I
14.67. Calcd for C40H33FeIP2Pd: C 55.56; H 3.85; I 14.67.
[PdBr(j2-2-C6F4PPh2)(PPh2Fc)] (2)
[PdI(j2-2-C6F4PPh2)(PPh2Fc)] (4)
To a yellow solution of [Pd2(m-Br)2(m-2-C6F4PPh2)2] (200 mg,
0.19 mmol) in CH2Cl2 (15 mL) was added solid PPh2Fc (143 mg,
0.39 mmol). The colour immediately darkened to orange-red,
and after stirring the mixture for 30 min, hexane was added.
The volume was reduced in vacuo to precipitate a yellow-orange
solid, which was isolated by filtration, washed with hexane and
To an orange solution of 2 (180 mg, 0.2 mmol) in CH2Cl2 (15
mL) was added a solution of NaI (75 mg, 0.5 mmol) in acetone
(15 mL) and the mixture was stirred for 30 min. The solvent was
removed in vacuo, the residue was dissolved in CH2Cl2 and the
suspension was filtered through Celite. Hexane was added to the
dark orange filtrate and solution was evaporated under reduced
1
dried (250 mg, 73%). Cis/trans ratio: 10 : 90. H NMR (C6D6):
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pressure. The precipitated yellow-orange solid was isolated by
21 J. C. Kotz, C. L. Nivert, J. M. Lieber and R. C. Reed, J. Organomet.
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filtration, washed with hexane and dried in vacuo (150 mg,
1
79%). Cis/trans ratio: 15 : 85. H NMR (C6D6): d 4.09 (m, 2H,
ferrocenyl), 4.33 (s, 5H, ferrocenyl), 4.45 (m, 2H, ferrocenyl) (trans-
isomer); 4.05 (m, 2H, ferrocenyl), 4.22 (s, 5H, ferrocenyl), 4.71 (m,
2H, ferrocenyl) (cis-isomer); 6.7–7.4 (m, 13H, aromatics), 7.80
(m, 4H, aromatics), 8.18 (m, 3H, aromatics). 19F NMR (C6D6): d
-117.2 (m), -134.6 (m), -148.2 (m), -157.6 (m) (trans-isomer);
-115.8 (m), -138.1 (m), -147.5 (m), -157.5 (m) (cis-isomer).
ESI-MS (m/z): 809 [M–I]+. Found: C 51.78; H 3.05; F 7.36;
I 13.76. Calcd for C40H29FeF4IP2Pd: C 51.29; H 3.12; F 8.11;
I 13.55.
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Acknowledgements
The Monash University ARC Special Research Centre for Green
Chemistry and CSIRO are gratefully acknowledged for provision
of facilities. J. W. thanks the Deutscher Akademischer Austausch-
dienst (DAAD) for the award of a Postdoctoral Fellowship.
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9090 | Dalton Trans., 2010, 39, 9079–9090
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©