Synthesis and redox properties of a phosphine-subsituted para-dioxolene and its bimetallic palladium complex

Article abstract of DOI:10.1139/V08-127

Oxidation of 2,5-bis(diphenylphosphino)-1,4-hydroquinone (8) with iodobenzene diacetate produces the corresponding bis(phosphine) substituted benzoquinone (9), the first phosphine-substitued quinone. Cyclic voltammetry studies reveal that the redox functi

Full text of DOI:10.1139/V08-127

976
Synthesis and redox properties of a phosphine-
subsituted para-dioxolene and its bimetallic
palladium complex
Sharon L. Caldwell, Joe B. Gilroy, Rajsapan Jain, Evan Crawford,
Brian O. Patrick, and Robin G. Hicks
Abstract: Oxidation of 2,5-bis(diphenylphosphino)-1,4-hydroquinone (8) with iodobenzene diacetate produces the cor-
responding bis(phosphine) substituted benzoquinone (9), the first phosphine-substitued quinone. Cyclic voltammetry
studies reveal that the redox functionality of the quinone unit in 9 is retained, and the reduction potentials render this
compound slightly more easily reduced than the parent p-benzoquinone. Reaction of the precursor hydroquinone 8 with
Pd(hfac)2 affords a binuclear complex 10 with the hydroquinonate ligand bridging two Pd(hfac) substrates. The redox
activity of the bridging dioxolene ligand is retained in complex 10, although there are significant changes in the redox
potentials relative to those of the free quinone 9. Chemical oxidation of 10 with AgPF6 yields a persistent cationic
complex 11, which, based on EPR and electronic spectroscopy, can be formulated as containing a bridging
semiquinone ligand.
Key words: p-quinones, phosphines, bridging ligands, redox properties.
Résumé : L’oxydation de la 2,5-bis(diphénylphosphino)-1,4-hydroquinone, 8, par le diacétate d’iodobenzène conduit à
la formation de la benzoquinone substituée par une bis(phosphine), 9, le premier dioxolène substitué par une phos-
phine. Des études de voltampérométrie cyclique révèlent que la fonctionnalité redox de l’unité quinone du produit 9 est
maintenue et que les potentiels de réduction rendent ce composé légèrement plus susceptible d’être réduit que la p-ben-
zoquinone parente. La réaction de l’hydroquinone 8 avec du Pd(hfac)2 conduit à un complexe binucléaire (10), dans le-
quel un ligand hydroquinonate sert de pont à deux substrats Pd(hfac). L’activité redox du ligand dioxolène servant de
pont est maintenue dans le complexe 10, même s’il y a des changements significatifs dans les potentiels redox par rap-
port à ceux de la quinone 9. L’oxydation chimique du composé 10 à l’aide de ONBF4 fournit un complexe cationique
persistant, 11, qui sur la base de la RPE et de la spectroscopie électronique contiendrait un ligand semiquinone servant
de pont.
Mots-clés : p-quinones, phosphines, ligands servant de pont, propriétés redox.
[Traduit par la Rédaction] Caldwell et al. 981
Introduction
esting building blocks for magnetic and multifunctional ma-
terials (2).
Metal complexes of ortho-dioxolenes have a number of
interesting physical properties that derive from (i) the ability
of the dioxolene ligand to exist in three different oxidation
states (quinone, semiquinone, catecholate) and (ii) the stabil-
ity of the open-shell (semiquinone) form of the coordinated
dioxolene (1). Features such as strong ligand–metal mag-
netic coupling and “valence tautomerism” — a form of
bistability — have made metal-dioxolene complexes inter-
Less well-studied are metal complexes of corresponding
para-dioxolenes. These ligands offer one significant differ-
ence compared with their ortho counterparts, namely, the
ability to bridge two metals, thereby opening up the possibility
of making binuclear or polymeric materials that may exhibit
cooperative valence tautomeric effects. Bis(pyrazolyl)hy-
droquinone (1) has been used sporadically as a bridging
ligand (3). Derivatives of 2,5-dihydroxy-1,4-benzoquinone
(2) have been extensively studied as structural linkers (4), al-
though the redox activity of some of these has only recently
been developed (5). Nitrogen analogues of 2, e.g., 2,5-
diamino-1,4-benzoquinones (3) and corresponding tetra-
nitrogen species (6) have also appeared. But overall, the
Received 24 April 2008. Accepted 30 June 2008. Published
on the NRC Research Press Web site at canjchem.nrc.ca on
3 September 2008.
S.L. Caldwell, J.B. Gilroy, R. Jain, E. Crawford, and
R.G. Hicks1. Department of Chemistry, University of
Victoria, PO Box 3065 STN CSC, Victoria, BC, Canada,
V8W 3V6.
B.O. Patrick. Crystallography Laboratory, Department of
Chemistry, University of British Columbia, Vancouver, BC,
Canada, V6T 1Z3.
1Corresponding author (e-mail: rhicks@uvic.ca).
Can. J. Chem. 86: 976–981 (2008)
doi:10.1139/V08-127
© 2008 NRC Canada

Caldwell et al.
977
Scheme 1. Synthesis of hydroquinone 8 and quinone 9.
coordination chemistry of p-(hydro)quinones remains rela-
tively underdeveloped.
Kaim and co-workers (7) reported the properties of a
binuclear ruthenium complex (4) containing the bridging
ligand 2,5-bis(diphenylphosphino)-1,4-hydroquinone. This
complex presents an interesting conundrum in that the
trication can be formulated either as a bis-Ru(II) complex
bridged by a semiquinone radical anion [RuII(SQ•–)RuII]3+ or
as a hydroquinonate-bridge mixed-valence species [RuII(Q2–
)RuIII]3+. Spectroscopic data suggest that both forms contrib-
ute significantly to the overall electronic structure of 4, a sit-
uation that arises from substantial mixing of metal- and
ligand-based orbitals.
Scheme 2. Synthesis of palladium complexe 10.
Aside from complex 4, the only other phosphine-
containing dioxolenes are Colbran’s 1,4-hydroquinones con-
taining a single phosphine substituent (8) (though there are a
few examples of phosphonium-substituted [R3P+] (9) or
phosphonate-substituted [X2P(O)] (10) (hydro)quinones).
These systems exhibit ligand-based redox-dependent hemi-
labile behaviour, further adding to the array of interesting
metal-dioxolene chemistry. We are interested in developing
the chemistry of bis(phosphine) dioxolenes because of the
excellent donor qualities of phosphines and the electro-
activity of the dioxolene unit. One of the fundamental as-
pects of redox-active ligand chemistry is how the redox
properties of the ligand are perturbed upon coordination. Be-
cause of the complexities associated with the ruthenium sys-
tem 4, we decided to investigate other complexes with
transition metals that are electronically “inactive”. Herein
we present the synthesis of a dipalladium complex of 2,5-
bis(diphenylphosphino)-1,4-hydroquinone. We also present
the synthesis and characterization of the corresponding free
of 9. The successful synthesis and isolation of 9 appears to
counter the concerns raised by Colbran, who explicitly
raised concerns about the stability of phosphine-containing
quinones (8).
Hydroquinone 8 was reacted with two equivalents of pal-
ladium (II) hexafluoroacetylacetonate to afford bimetallic
palladium complex 10 (Scheme 2). The complex was puri-
fied by extraction into a 50:50 mixture of diethyl ether and
dichloromethane.
The crystal structure of the palladium complex 10 is
shown in Fig. 1. The molecule resides on an inversion cen-
ter. The palladium ion essentially lies within the plane of the
hydroquinonate ligand, while the hfac ligand backbone plane
lies at an angle of 8.63° relative to the plane of the
hydroquinonate ligand. The structural metrics associated
with the P,O chelate to the Pd ion are quite similar to those
found in structures of the related ligand 2-(diphenyl-
phosphino)phenol (11). The bond lengths within the central
2,5-bis(diphenylphosphino)-1,4-benzoquinone,
the
first
phosphine-substituted quinone.
Results and discussion
The synthesis of 2,5-bis(diphenylphosphino)-1,4-benzo-
quinone (9) is shown in Scheme 1. The immediate precursor
to 9, namely the corresponding hydroquinone 8, was pre-
pared based on methodology described by Kaim (7). 1,4-
Dimethoxy benzene 5 was brominated under acidic condi-
tions to afford 2,5-dibromo-1,4-dimethoxybenzene (6). We
found that the production of 7 through lithiation of 6 at
–78 °C followed by chlorophosphine addition afforded more
reproducible (albeit low) yields and cleaner samples of 7
compared with the Grignard route previously described. The
methoxy groups in 7 were removed with excess aluminum
chloride, affording hydroquinone 8. Oxidation of hydro-
quinone 8 to quinone 9 proved challenging; a number of re-
agents known to convert hydroquinones to quinones (e.g.,
DDQ, FeCl3, NaIO4) were not successful. Iodobenzene
diacetate proved to be a successful oxidant for the synthesis
benzene ring and C–O bonds are typical for
hydroquinon(ate)-type electronic structure (Table 1).
a
The cyclic voltammograms (CVs) of 9 and 10 are shown
in Fig. 2. The CV of quinone 9 has a reversible reduction
wave at a potential of –0.83 V (vs. Fc/Fc+) and a second
quasi-reversible wave at –1.59 V. The reduction potentials of
9
render it slightly more easily reduced than p-
benzoquinone, which under the same experimental condi-
tions has reduction potentials of –0.94 and –1.60 V. The CV
of 10, which exists as the catecholate form of ligand 8 in its
resting state, has two oxidation processes. The first of these
is reversible and occurs at +0.44 V, while the second
oxidation is irreversible with the anodic peak occurring at
+1.04 V. Previous studies on a wide range of Pd (and Pt)
complexes of o-dioxolenes have established that square pla-
© 2008 NRC Canada

978
Can. J. Chem. Vol. 86, 2008
Fig. 1. Molecular structure of 10, (a) top view and (b) side view.
Thermal ellipsoids shown at 50% probability. Hydrogen atoms
are omitted for clarity.
Fig. 2. Cylclic voltammograms of 9 (top) and 10 (bottom) in
CH2Cl2 (0.2 mol/L, Bu4NBr electrolyte, scan rate 0.25 V/s).
Table 1. Selected bond lengths and angles for 10.
Bond lengths (Å)
C11–O3
C12–P1
O3–Pd1
P1–Pd1
O2–Pd1
O1–Pd1
C21–O2
C23–O1
1.342(4)
1.790(3)
1.980(2)
2.1918(9)
2.103(2)
2.016(2)
1.246(4)
1.268(4)
1.393(6)
1.375(6)
Table 2. Electrochemical parameters for 9, 10, and related com-
pounds.
Compound
E1°
E2°
p-benzoquinone
9
10
–0.94
–0.83
1.04 / 0.79a
–1.60
–1.59
0.44
Note: Potentials are reported in V vs. Fc/Fc+. CVs were performed in
CH2Cl2 solution with 0.2 mol/L Bu4NBF4 as electrolyte and scane rate
0.25 V/s.
C21–C22
C22–C23
aIrreversible process; cathodic and anodic potentials given.
Bond angles (°)
O3–C11–C12
C11–C12–P1
C11–O3–Pd1
O3–Pd1–O2
O2–Pd1–O1
O1–Pd1–P1
Pd1–O2–C21
Pd1–O1–C23
O2–C21–C22
O1–C23–C22
122.1(3)
113.0(3)
118.3(2)
89.67(10)
91.91(10)
92.14(7)
121.0(2)
123.1(2)
129.4(4)
129.3(4)
to formation of the semiquinone and quinone forms of the
bridging ligand.
The electrochemical data for 9 and 10 are summarized in
Table 2. The data are presented to permit direct comparisons
of the two electron-transfer processes available to dioxo-
o
lenes; thus, E1 corresponds to the quinone/semiquinone
o
redox couple, while E2 is the semiquinone/hydroquinonate
couple. The redox potentials for free ligand 9 and
dipalladium complex 10 differ by approximately two volts, a
difference that can be attributed to the presence of two posi-
tively charged metal ions in the latter. Another interesting
o
contrast between 9 and 10 is the difference between E1 and
nar Pd(II) is redox inert and that redox processes are invari-
ably ligand-based, corresponding to interconversion between
catecholate, semiquinone, and quinone forms of the (coordi-
nated) dioxolene (12, 13). These studies, in conjunction with
the spectroscopic data for the one-electron oxidized form of
10 (see below), suggest that the redox processes observed in
10 are ligand based, i.e., the two oxidation steps correspond
E2o (∆E) for each species. For free quinone 9, this difference
is +0.74 V, a fairly typical value for p-quinones (14). The ir
-
reversibility of the second oxidation process in 10 makes it
o
difficult to accurately determine E2 (and hence ∆E), but us-
ing the midpoint of anodic and cathodic peak potentials, an
o
effective E2 of +0.92 V can be estimated, which leads to a
© 2008 NRC Canada

Caldwell et al.
979
Fig. 3. Electronic spectra of 10 (red line) and 11 (black line) in
Scheme 3. Synthesis of palladium semiquinone complex 11.
CH2Cl2.
∆E of 0.48 V, a substantially lower value compared with the
uncoordinated dioxolene.
The reversibility of the first oxidation of 10 prompted at-
tempts to prepare a cation of this species. Palladium
hyroquinonate complex 10 was treated with one equivalent
of silver hexafluorophosphate to afford semiquinone com-
plex 11 (Scheme 3). This complex is air and moisture sensi-
tive and also appears to slowly degrade in solution even
under anaerobic conditions, but freshly prepared samples
were sufficiently long-lived to permit solution characteriza-
tion. Solutions of 11 could be re-reduced with Zn powder to
regenerate 10 in ~ 70%–75% yield, demonstrating the per-
sistence of 11 and ruling out the possibility that the spectro-
scopic data on 11 are from a decomposition product.
The electronic spectra of 11 and its precursor 10 are
shown in Fig. 3. Complex 10 has absorption maxima at 358
and 442 nm, while the electronic spectrum of 11 has absorp-
tion maxima at 308 and 361 nm as well as low energy max-
ima at 464 and 582 nm. The broad maximum centered at
582 nm is characteristic of semiquinone radical anions (15).
The EPR spectrum of 11 recorded in degassed dichloro-
methane in a capillary tube at room temperature (RT) is
shown in Fig. 4. The g-factor of 2.0084 is slightly higher
than the free electron value (2.0023), indicating that the
paramagnetic species has most of its spin density located on
an organic moiety, as has been shown for Pd and Pt com-
plexes of o-semiquinone-type radical anion ligands (12). The
spectrum has a dominant five-line pattern with approximate
1:4:6:4:1 intensities; also evident are less intense satellite
peaks on either side of this pentet. The overall pattern is sug-
gestive of hyperfine coupling to four (approximately) equiv-
alent S = 1/2 nuclei and additional coupling to two
palladium ions (105Pd, I = 5/2, 22.2% abundant). Unfortu-
nately, the linewidths allow only for estimates of the
hyperfine coupling to be made; the experimental spectrum
could be simulated by coupling to two equivalent phospho-
rus nuclei and two equivalent protons with similar hyperfine
coupling constants (a(P) ~ a(H) ~ 3 G) and additional cou-
pling to the two palladium nuclei (a(Pd) = 2.1 G). The pro-
ton hyperfine coupling constants are consistent with other
(free) semiquinone radical anions (16), supporting the notion
that 11 is appropriately described as containing a bridging p-
semiquinone ligand.
Fig. 4. X-band EPR spectrum (top) and simulation (bottom) of
11 in degassed CH2Cl2 at 293 K.
this case, coordination to two Pd ions produces dramatic
changes in the ligand redox potentials as well as in the dif-
ference between the sequential redox events inherent in
dioxolene compounds. These findings suggest that future ef-
forts to design metal-p-dioxolene complexes with interesting
redox characteristics (e.g., valence tautomerizm) will require
careful consideration of how the redox properties of the
bridging ligand are altered when bound to metals.
Experimental
General considerations
All reagents were purchased from either Lancaster or
Sigma-Aldrich and were used as received. NMR spectra
were obtained on a 300 MHz instrument. IR spectra were re-
corded as KBr pellets on a PerkinElmer Spectrum One FT-
IR spectrometer. UV–vis spectra were taken on a Cary 50
instrument. Mass spectra were recorded on a Kratos Concept
II instrument using electron impact (EI) or liquid secondary
ion (LSIMS) source. Elemental analyses were performed by
Canadian Microanalytical Services Ltd., Vancouver, BC,
Canada. Cyclic voltammetry experiments were run on a
Bioanalytical Systems CV50 voltammetric analyzer using an
electrochemical cell containing the analyte (1 mmol/L),
Bu4NBF4 (0.2 mol/L) as electrolyte, CH2Cl2 solvent, a silver
wire as the reference electrode, and ferrocene/ferrocenium as
an internal reference. The CV of p-benzoquinone (Table 2)
was performed to enable direct comparisons with 9 run un-
Summary
We have prepared the first example of a quinone with tri-
valent phosphorus as a substituent. The redox properties of
phosphine-substituted quinone 9 are consistent with the p-
dioxolene functionality. Coordination to a diamagnetic and
redox-inactive metal allowed for the unambiguous elucida-
tion of the redox properties of the coordinated dioxolene. In
© 2008 NRC Canada

980
Can. J. Chem. Vol. 86, 2008
Table 3. Crystallographic data for 10.
fractions were combined and washed with distilled water
(3 × 100 mL) before being dried with magnesium sulfate
and taken to dryness in vacuo. Trituration of the residue
with methanol afforded 7 as an off-white solid, yield 1.0 g
(9.9%). NMR spectra were consistent with that the litera-
ture-reported compound (7).
Formula
C40H24O6F12P2Pd2·2CH2Cl2
FW
1273.18
Dimensions (mm)
a (Å)
b (Å)
0.05 × 0.32 × 0.45
15.1186(17)
9.6386(9)
16.4640(18)
90.0
97.109(4)
90.0
2380.7(4)
1.776
c (Å)
Synthesis of 2,5-bis(diphenylphosphino)-1,4-
benzoquinone (9)
α (°)
β (°)
Saturated solutions of 2,5-diphenylphosphino-1,4-
hydroquinone 8 (100 mg, 0.21 mmol) and iodobenzene
bisacetate (101 mg, 0.31 mmol) were obtained by heating
the reagents in ethanol (20 mL and 5 mL respectively). The
iodobenzene bisacetate solution was then added dropwise to
the solution of 8, causing an immediate colour change to
dark red. The solution was stirred for 10 min with gentle
heating before it was allowed to cool on ice for 30 min. The
quinone was isolated by filtration and washed with methanol
(2 × 5 mL) followed by diethyl ether (2 × 5 mL), affording 9
as a dark red microcrystalline solid; yield 46.8 mg, 47.0%.
UV–vis (CH2Cl2) λmax: 530 nm (ε = 752), 410 nm (ε =
2756), 230 nm (ε = 25660), 210 nm (ε = 7242). FT-IR
(KBr, cm–1): 3421 (w, br), 2972 (w), 1644 (vs), 1584 (m),
1556 (m), 1434 (m), 1322 (w), 1200 (m), 1067 (w), 1025
γ (°)
V (Å3)
Dcalc (g cm–3)
System
Monoclinic
Space group
P 21/c (#14)
Z
2
µ (mm–1)
T (K)
0.114
173.15
50.2°
2θmax (°)
Unique reflections
4193, Rint = 0.024
0.042
a
R1
b
wR2
0.088
aR1 = Σ (|F0| – |Fc|)/Σ|F0|.
bwR2 = [Σw(|F0| – |Fc|)2/Σw(|F0|)2]1/2
.
1
(w), 911 (w), 746 (m), 696 (m), 498 (w). H NMR (CD2Cl2)
δ: 7.39 (m, 20H), 6.18 ppm (dd, 2H, J = 1.5 Hz, 4 Hz). 13C
NMR (CD2Cl2, ppm) δ: (186.4, d, J = 16 Hz), (153.5, d, J =
27 Hz), (140.6, s (v.br)), (134.8, d, J = 22 Hz), (133.6, d, J =
9 Hz), (130.4, s), (129.6, d, J = 8 Hz). 31P NMR (CD2Cl2,
ppm) δ: –15.98. MS (EI) m/z: 475 (M+, 100%). Anal. calcd.
for C30H22O2P2: C 75.63, H 4.65, P 13.00; found: C 74.87,
H 4.56, P 12.86.
der identical experimental conditions. EPR spectra were
obtained using a Bruker EMX spectrometer with diluted
samples (~10–5 mol/L) in capillary tubes. A capillary tube
was employed to minimize the absorption of microwave ra-
diation by the sample. Spectra were simulated using
WinEPR SimFonia.
Synthesis of ␮-(2,5-diphenylphosphino-1,4-hydro-
X-ray structure determination
quinone)-bis-palladiumhexafluoroacetylacetonate (10)
2,5-Diphenyl phosphine-1,4-hydroquinone 8 (250 mg,
0.52 mmol) and palladium (II) hexafluoroacetylacetonate
(544 mg, 1.01 mmol) were dissolved in a minimum amount
of toluene (15 mL and 5 mL respectively) with gentle heat-
ing. The hydroquinone solution was added dropwise to the
solution containing bis(hexafluoroacetylacetonate)palladium.
Immediately a dark brown precipitate formed. The reaction
mixture was stirred for 5 min and filtered to isolate the pre-
cipitate. Purification of the isolated solid was achieved by
dissolving the crude product in 50:50 CH2Cl2/Et2O and fil-
tering out insoluble matter. Palladium complex 10 was iso-
lated as a golden brown solid; yield 204 mg, 35.4%. UV–vis
(CH2Cl2) λmax: 440 nm (ε = 5000), 360 nm (ε = 9900), 245
(ε = 79300), 215 nm (ε = 43600). FT-IR (KBr, cm–1): 3446
(s, br), 2917 (w), 1629 (m), 1465 (w), 1439 (w), 1380 (w),
1259 (s), 1209 (m), 1152 (vs), 1098 (m), 566 (w), 532 (w).
1H NMR (CD2Cl2) δ: 7.81 (m, 8H), 7.64 (m, 4H), 7.51 (m,
8H), 6.71 (dd, 2H, J = 6 Hz, 13 Hz), 6.17 ppm (s, 2H). 13C
NMR (CD2Cl2, ppm) δ: (176.8, q, J = 35 Hz), (175.0, q, J =
35 Hz), (168.8, t, J = 13 Hz), (133.4, d, J = 12 Hz),
X-ray diffraction data were collected on a Bruker X8
APEX II diffractometer with graphite-monochromatized Mo
Kα radiation (λ = 0.710 73 Å). The structure was solved by
direct methods (SHELXTL); see Table 3 for crystallographic
data.2
Synthesis of 2,5-bis(diphenyl phosphino)-1,4-
dimethoxybenzene (7)
n-BuLi (27.0 mL of a 1.6 mol/L hexanes solution,
43.2 mmol) was added dropwise to a –78 °C solution of 2,5-
dibromo-1,4-dimethoxybenzene 6 (6.0 g, 20.3 mmol) in
freshly distilled diethyl ether (100 mL) and tetrahydrofuran
(50 mL) under argon. Upon addition, the solution was al-
lowed to stir for 30 min at –78 °C before it was warmed to
RT for an additional 4 h of stirring. The resulting slurry was
once again cooled to –78 °C before diphenylchloro-
phosphine (8.5 mL, 47.0 mmol) was added dropwise. The
reaction mixture was allowed to warm slowly to RT, stirring
overnight to yield a pale yellow solution. The reaction mix-
ture was then concentrated in vacuo, and the resulting resi-
due was extracted into diethyl ether (3 × 100 mL). The ether
2 Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of Un-
published Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 3809. For more in-
formation on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/cms/unpub_e.shtml. CCDC 694246 contains the crystallographic data for
this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2008 NRC Canada

Caldwell et al.
981
McGarvey, and J.G. Vos. Inorg. Chem. 37, 5925 (1998);
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(133.2, s), (129.9, d, J = 13 Hz), (127.9, d, J = 59 Hz),
(122.4, d, J = 55 Hz), (118.6, d, J = 19 Hz), (116.7, d, J =
16 Hz), (91.7, s). 31P NMR (CD2Cl2, ppm) δ: +41.95. MS
(+ LSIMS) m/z: 1103.8 (M+, 100%). Anal. calcd. for
C40H24F12O6P2Pd2: C 43.54, H 2.19; found: C 43.69, H 2.35.
Synthesis of ␮-(2,5-diphenylphosphino-1,4-semiquinone)-
bis-palladiumhexafluoroacetylacetonate (11)
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Palladium complex 10 (130 mg, 0.12 mmol) was dis-
solved in freshly distilled dichloromethane (25 mL) under
argon and allowed to stir for 10 min. In one portion, under a
flow of argon, silver hexafluorophosphate (30 mg,
0.12 mmol) was added to the solution causing a colour
change from orange to dark purple. The volume of the solu-
tion was reduced in vacuo (~10 mL), and dry hexane
(~10 mL) was added causing the precipitation of a dark pur-
ple microcrystalline solid. The solution was cooled on ice
for 10 min before it was filtered under inert atmosphere. The
solid was washed with dry hexanes (2 × 10 mL) and dried in
vacuo for 30 min before it was isolated in an inert atmo-
sphere glove box. Semiquinone complex 11 was isolated as
a microcrystalline dark purple solid; yield 125 mg, 85.0%.
This complex is highly reactive in solution and in the solid
state, precluding satisfactory elemental analysis. Character-
ization of 11 was completed using freshly prepared samples
7. S. Ernst, P. Hanel, J. Jordanov, W. Kaim, V. Kasack, and E.
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handled under an inert atmosphere. UV–vis (CH2Cl2) λmax
:
582 nm (ε = 6250), 464 (ε = 4000), 361 nm (ε = 8000),
308 nm (ε = 27500). FT-IR (KBr, cm–1): 1630 (s), 1611 (m),
1449 (s), 1259 (s), 1225 (s), 1155 (s), 1103 (s), 833 (s), 691
(m), 557 (m), 530 (m). Satisfactory elemental analysis could
not be obtained for this compound.
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Acknowledgements
We thank the University of Victoria and the Natural Sci-
ences and Engineering Research Council of Canada for fi-
nancial support.
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pounds. Vol. 1. Edited by S. Patai. John Wiley and Sons Inc.,
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© 2008 NRC Canada

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