Palladium(II) complexes of bis(phosphine)monooxide and bis(phosphine) monosulphide ligands bearing o-N,N-dimethylanilinyl substituents

Article abstract of DOI:10.1139/v05-067

Palladium(II) complexes of bis(phosphine)monooxide and bis(phosphine) monosulphide ligands with the general formula Ar₂P(CH ₂)_nP(E)Ar₂ are described (n = 1 (dmapmE), E = O, S; n = 2 (dmapeE), E = O; Ar = o-N,N-dimethylanilinyl). The precursor, PdCl₂(dmapm), which exists in solution as an equilibrium mixture of the P,P- and P,N-ligated isomers, reacts with peroxides to form PdCl ₂(P,N-dmapmO) and with sulphur to give [PdCl(P,N,S-dmapmS)]Cl, probably by direct oxidation of the "dangling" P atom of the P,N-isomer. PdCl₂(dmape), which exists in solution as a mixture of PdCl₂(P,P-dmape) and [PdCl(P,P,N-dmape)]Cl, reacts with hydroxide in H₂O-CH₂Cl₂ under argon to give the face-to-face Pd(II)₂ complex containing the cation [Pd₂Cl ₂(μ-P,N:O-dmapeO)₂]²⁺, which was isolated and crystallographically characterized as the PF₆ salt. The remarkable structure, consisting of two face-to-face Pd(II) square planes in a head-to-tail orientation, reveals a 12-membered ring containing the two non-bonded metal centres (Pd-Pd = 4.873(2) A). The dmapmS complex shows marginal activity as a catalyst for the hydration of maleic to malic acid.

Full text of DOI:10.1139/v05-067

546
Palladium(II) complexes of bis(phosphine)monooxide
and bis(phosphine)monosulphide ligands bearing
o-N,N-dimethylanilinyl substituents1
Nathan D. Jones and Brian R. James
Abstract: Palladium(II) complexes of bis(phosphine)monooxide and bis(phosphine)monosulphide ligands with the gen-
eral formula Ar2P(CH2)nP(E)Ar2 are described (n = 1 (dmapmE), E = O, S; n = 2 (dmapeE), E = O; Ar = o-N,N-
dimethylanilinyl). The precursor, PdCl2(dmapm), which exists in solution as an equilibrium mixture of the P,P- and
P,N-ligated isomers, reacts with peroxides to form PdCl2(P,N-dmapmO) and with sulphur to give [PdCl(P,N,S-
dmapmS)]Cl, probably by direct oxidation of the “dangling” P atom of the P,N-isomer. PdCl2(dmape), which exists in
solution as a mixture of PdCl2(P,P-dmape) and [PdCl(P,P,N-dmape)]Cl, reacts with hydroxide in H2O–CH2Cl2 under ar-
gon to give the face-to-face Pd(II)2 complex containing the cation [Pd2Cl2(µ-P,N:O-dmapeO)2]2+, which was isolated
and crystallographically characterized as the PF6 salt. The remarkable structure, consisting of two face-to-face Pd(II)
square planes in a head-to-tail orientation, reveals a 12-membered ring containing the two non-bonded metal centres
(Pd—Pd = 4.873(2) Å). The dmapmS complex shows marginal activity as a catalyst for the hydration of maleic to
malic acid.
Key words: phosphine, phosphine oxide, phosphine sulphide, palladium.
Résumé : On décrit les complexes du palladium(II) de ligands bis(phosphine)monooxyde et bis(phosphine)monosulfure
avec des composés de formule générale Ar2P(CH2)nP(E)Ar2 dans lesquels n = 1 (dmapmE), E = O, S; n = 2 (dmapeE),
E = O; Ar = o-N,N-diméthylanilinyle. Le précurseur PdCl2(dmapm), qui existe en solution sous la forme d’un mélange
à l’équilibre des ligands isomères P,P et P,N, réagit avec les peroxydes pour former le PdCl2(P,N-dmapmO) et avec
le soufre pour donner le [PdCl(P,N,S-dmapmS)]Cl, probablement par une oxydation directe de l’atome de P qui se
« balance » dans l’isomère P,N. Le PdCl2(dmape), qui existe en solution sous la forme d’un mélange de PdCl2(P,P-
dmape) et de [PdCl(P,P,N-dmape)]Cl, réagit avec l’ion hydroxyde dans un mélange H2O–CH2Cl2, sous argon, pour
conduire à la formation du complexe face à face Pd(II)2 contenant le cation [Pd2Cl2(µ-P,N:O-dmapeO)2]2+ qui a été
isolé et caractérisé cristallographiquement sous la forme de sel de PF6. La structure remarquable, formée de deux
Pd(II) plans carrés, face à face et en position tête à queue, comporte un cycle à douze chaînons avec deux centres mé-
talliques non liés (Pd—Pd = 4,873(2) Å). Le complexe dmapmS présente une activité marginale comme catalyseur pour
l’hydratation de l’acide maléique en acide malique.
Mots clés : phosphine, oxyde de phosphine, sulfure de phosphine, palladium.
[Traduit par la Rédaction] Jones and James 552
Introduction
soluble metal complexes, with one aim being to develop
transition metal catalysts for olefin hydration, and indeed
marginal activity for hydration of maleic to malic acid was
noted for several of the complexes (4). The activity was
Our group recently reported (1–4) on platinum(II) and
palladium(II) complexes containing bis(phosphine) ligands
bearing two o-N,N-dimethylanilinyl substituents at each P
atom, including 1,1-bis(di(o-N,N-dimethylanilinyl)phosphino)-
methane (dmapm) and 1,2-bis(di(o-N,N-dimethylanilinyl)-
phosphino)ethane (dmape). The ligand structures, for example,
are represented within complexes of formula PdCl2(dmapm)
and PdCl2(dmape) in Schemes 1 and 2, respectively. We had
designed such ligands primarily for the synthesis of water-
better, for example, than that of the bridged-hydroxo com-
2+
plexes of the type [(dppe)Pd(µ-OH)]2
(dppe
=
Ph2P(CH2)2PPh2) reported by Ganguly and Roundhill (5).
This led us to attempts to synthesize bridged-hydroxo com-
plexes of this type but using dmape as the bisphosphine. We
thus reacted PdCl2(dmape) with KOH in aqueous–CH2Cl2,
but (to our surprise) a complex containing the bisphosphine
Received 30 November 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 2 June 2005.
This paper is dedicated to Howard Alper, close friend and research collaborator with one of us (BRJ) and distinguished scholar.
N.D. Jones2 and B.R. James.3 Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.
1This article is part of a Special Issue dedicated to Professor Howard Alper.
2Present address: Department of Chemistry, University of Western Ontario, London, ON N6A 5B7, Canada.
3Corresponding author (e-mail: brj@chem.ubc.ca).
Can. J. Chem. 83: 546–552 (2005)
doi: 10.1139/V05-067
© 2005 NRC Canada

Jones and James
547
Scheme 1. Direct oxidation of one P atom of 1b with peroxide and sulphur.
NMe2
Me2N
NMe2
Me2N
NMe2
P
P
Me2N Pd
Cl
NMe2
P
P
Pd
Cl
Me2N
Cl
Cl
1a
1b
[O]
[S]
Cl
NMe2
Me2N
NMe2
O
P
Me2N Pd
Cl
P
P
NMe2
P
S
NMe2
Me2N Pd
Cl
Me2N
Cl
2
4
Scheme 2. Reaction of 5 with KOH in CH2Cl2–H2O.
Me2N
NMe2
Me2N
NMe2
P
P
Pd
[PF6]2
Cl
Cl
Me2N
NMe2
5a
Me2N
Pd
O
P
CH2Cl2/H2O
P
Cl
Pd
NMe2
NMe2
(i) KOH
(ii) KPF6
Cl
P
Me2N
O
P
Cl
Me2N
NMe2
Me2N
NMe2
Me2N
P
P
N
6
Pd
Cl
Me2
5b
monoxide, dmapeO, was isolated, and because of the current
interest in such ligands, we pursued this line of research, and
the findings are reported here.
allylic alkylation (12), including asymmetric processes (9, 10,
12) Bis(phosphine)monosulphide (BPMS) complexes have
also begun to attract interest and have found application in
asymmetric allylic alkylation (13).
More generally, BPMO ligands can be made via metal-
free organic synthesis (for example, by benzylation of a
bis(tertiary phosphine) followed by basic hydrolysis of the
resulting phosphonium salt) (14) or by a Pd-catalyzed pro-
cess involving reaction of 1,2-bromoethane and hydroxide in
a two-phase aqueous system (8). Metal-bound BPMOs may
be synthesized via oxidation of a “dangling” (i.e., nonbonded)
Bisphosphine monooxides (BPMOs) with the mixed P atom
and O atom donors are within a class of chelating agents
that, because of their potential for hemilabile coordination
(6), have garnered attention in homogeneous catalysis. Tran-
sition metal complexes of such ligands have been used in a
variety of catalyzed organic transformations such as hydro-
formylation and other carbonylation processes (7, 8), hydro-
silylation (9), hydrovinylation (10), polymerization (11), and
© 2005 NRC Canada

548
Can. J. Chem. Vol. 83, 2005
P atom in complexes bearing polydentate ligands, exempli-
fied in Co (15, 16), Mo (17), Rh (18), and Pd (9, 19) sys-
tems, but only in a few cases does this result in coordination
of the phosphine oxide (16, 17). Less common is oxidation
of a coordinated bis(phosphine) to give a BPMO in which
both the P(III) atom and the O atom are bound. Examples
that typically involve aerobic oxidation are those of
RhCl(CO)(P,P-BINAP), which generates RhCl(CO)(P,O-
Cumene hydroperoxide
Precursor 1 was prepared in situ by the reaction of dmapm
(140 mg, 0.25 mmol) and trans-PdCl2(PhCN)2 (94 mg,
0.25 mmol) in CH2Cl2 (10 mL), and to the yellow solution
was added cumene hydroperoxide (75 µL, 0.41 mmol). The
solution was stirred overnight and then reduced in vacuo to
~2 mL, when Et2O (20 mL) was added to precipitate the yel-
low product, which was isolated by filtration, washed with
Et2O (3 × 3 mL), and dried under vacuum. Yield: 155 mg
(84%).
BINAPO) (20),
a
Pd(II)-dppm species (dppm
=
Ph2PCH2PPh2) that forms a Pd(II)-P,O-dppmO complex
(21), and RuCl2(BINAP)(N–N) complexes that give
[RuCl(η4-BINAPO)(N–N)]Cl (N–N = bipy, phen), where the
BINAPO ligand is coordinated via the P(III) atom, the O
atom, and two C atoms of the naphthyl ring proximate to the
P(V) atom (22). An excellent introduction to the syntheses
and uses of BPMOs can be found in Cyr’s Ph.D. thesis (23).
This paper describes the oxidation of one P atom of
PdCl2(dmapm) (1) (3) to give PdCl2(P,N-dmapmO) (2) and
the salts [PdCl(P,N,S-dmapmS)]X (X = PF6, 3; Cl, 4) and the
similar oxidation of PdCl2(dmape) (5) (4) to give, after an-
ion exchange, the dipalladium complex [PdCl(µ-P,N:O-
dmapeO)]2[PF6]2 (6). This last reaction constitutes another
example of the oxidation of a coordinated bis(phosphine) to
give a complex in which both the P(III) and O atoms of the
BPMO are coordinated, but it is unique in that the BPMO
serves as a bridging ligand. The topic is particularly appro-
priate for a paper dedicated to Howard Alper because he
must have experienced many times phosphine oxidation con-
comitant with reduction of Pd(II) to Pd(0) species, including
the unwanted metal!
H2O2
To a CH2Cl2 solution (5 mL) containing 1 (28 mg,
0.038 mmol) was added a 3% H2O2 (2 mL) solution. The
two-phase mixture was stirred for 1 h, and the aqueous
phase was removed. The volume of CH2Cl2 was reduced in
vacuo to ~1 mL, and Et2O (10 mL) was added to precipitate
1
the product. Yield: 25 mg (88%). H NMR δ: 1.7–3.3 (br,
12H, NCH3), 2.35 (s, 3H, NCH3), 2.51 (s, 3H, NCH3), 3.47
(s, 3H, NCH3), 3.70 (s, 3H, NCH3), 4.27 (m, 1H, CH2), 4.81
(m, 1H, CH2), 6.8–7.9 (br m, 16H, Ar). 31P{1H} NMR δ:
2
2
26.2 (d, JPP = 7.0), 26.5 (d, JPP = 7.0). ΛM (CH2Cl2): <1.
Anal. calcd. for C33H42ON4Cl2P2Pd: C 52.9, H 5.6, N 7.5;
found: C 53.0, H 6.0, N 7.0.
[PdCl(P,N,S-dmapmS)]PF6 (3)
To a mixture of 1 (44 mg, 0.060 mmol) and S8 (13 mg,
0.42 mmol of S) was added 1,2-dichloroethane (5 mL), and
the resulting yellow solution was refluxed for 3.5 h. The sol-
vent was removed in vacuo, and the residue was dissolved in
warm H2O (20 mL). After filtration of the mixture through
Celite 545 to remove excess S8 and unreacted 1, aq. KPF6
solution (5 mL, 90 mg, 0.49 mmol) was added; this immedi-
ately precipitated a yellow-orange solid that was isolated by
filtration and washed with H2O (10 mL). Yield: 30 mg
Experimental
Unless otherwise noted, synthetic procedures were per-
formed using standard Schlenk techniques under an atmo-
sphere of dry Ar or N2. The precursors, trans-PdCl2(PhCN)2
(24), PdCl2(dmapm) (1) (3), and PdCl2(dmape) (5) (4), and
the dmapm and dmape ligands (25) were made according to
literature procedures. All other reagents were purchased
from commercial sources and used as supplied. Solvents
were dried over the appropriate agents and distilled under N2
prior to use. NMR spectra were recorded in CDCl3 solution
on a Varian AV300 (121 MHz for 31P) or AV400 (162 MHz
for 31P) spectrometer at 300 K. Residual solvent proton (1H,
relative to external SiMe4 δ 0.00) and external P(OMe)3
(31P{1H}, δ 141.00 vs. external 85% aq. H3PO4) were used
as references. Downfield shifts were taken as positive; s =
singlet, d = doublet, m = multiplet, br = broad, spt = septet,
and all J-values are given in Hz. Conductivity measurements
were obtained using a Thomas Serfass conductance bridge
model RCM151B1 (Arthur H. Thomas Co. Ltd.) connected
to a 3404 cell (Yellow Springs Instrument Co.); measure-
ments were made at 25 °C using ~10–3 mol L–1 solutions of
the complexes and are reported as ΛM in Ω–1 mol–1 cm2. Ele-
mental analyses were conducted in the UBC Chemistry De-
partment by Mr. Peter Borda using a Carlo Erba 1108 analyzer.
1
(56%). H NMR δ: 2.15 (s, 12 H, NCH3), 2.86 (s, 12 H,
NCH3), 4.50 (br m, 2H, CH2), 7.30 (m, 4H, Ar), 7.46 (m,
4H, Ar), 7.57 (m, 4H, Ar), 7.87 (m, 2H, Ar), 8.50 (dd, 2H,
2
4
Ar, JHH = 7.39, JHP = 15.3). 31P{1H} NMR δ: 37.0 (d,
2JPP = 33.6), 48.6 (d, JPP = 33.6), –145 (spt, JPF = 710,
2
1
–
PF6 ). Anal. calcd. for C33H42N4ClF6P3PdS: C 45.3, H 4.8,
N 6.4; found: C 45.2, H 5.0, N 6.2.
[PdCl(P,N,S-dmapmS)]Cl (4)
This compound was made in the same manner as outlined
for 3, except that, after the reaction of 1 (130 mg,
0.18 mmol) and S8 (45 mg, 1.4 mmol), the reaction mixture
was cooled to room temperature and then filtered through
Celite 545. The filtrate volume was reduced in vacuo to
~1 mL, and Et2O (20 mL) was added to precipitate the yel-
low product. Yield: 100 mg (71%). The NMR data for 4, ex-
–
cept for the absence of the PF6 resonance, are the same as
for 3. ΛM (H2O): 99.
[PdCl(␮-P,N:O-dmapeO)]2[PF6]2 (6)
To a CH2Cl2 (10 mL) solution of 5 (30 mg, 0.041 mmol)
was added aq. KOH solution (5 mL, 1 mol L–1). This caused
an immediate colour change in the organic layer from yel-
low to orange-red. The two phase mixture was stirred for 1 h
at room temperature, and then the aqueous layer was re-
moved. H2O (5 mL) was then added to the organic layer, fol-
PdCl2(P,N-dmapmO) (2)
This complex was prepared at room temperature either in
a single phase using cumene hydroperoxide as oxidant or in
a two-phase mixture using aqueous H2O2.
© 2005 NRC Canada

Jones and James
549
Table 1. Selected crystallographic data for the “half molecule”
lowed by KPF6 (83 mg, 0.45 mmol), and stirring was
continued for 0.5 h. The aqueous layer was removed, and
the CH2Cl2 phase was washed with H2O (3 × 10 mL) before
being filtered through Celite 545. The filtrate was reduced to
dryness in vacuo, and the residue taken up in CDCl3 for
analysis by 31P{1H} NMR spectroscopy. About 10% of the
dioxide, dmapeO2 (δP 31.5), was apparent in the residue. A
satisfactory elemental analysis was not obtained for 6, but
X-ray diffraction quality, pale yellow, irregular-shaped crys-
tals of 6·4CDCl3 (~60% yield) were formed by slow evapo-
ration of the CDCl3 solution. 31P{1H} NMR δ: 28.1 (d,
of 6·4CDCl3.
6·4CDCl3
Empirical formula
C36H46Cl7F6N4OP3Pd
Formula mass
Crystal size (mm3)
Crystal system
Space group
a (Å)
1 112.23
0.20 × 0.10 × 0.10
Triclinic
P1 (No. 2)
12.8710(3)
14.0672(3)
15.4779(4)
72.938(1)
68.222(1)
73.376(1)
2 437.8(1)
2
b (Å)
c (Å)
–
2JPP = 62), 29.3 (d, 2JPP = 62), –145 (spt, PF6 , 1JPF = 710).
α (°)
β (°)
Catalytic hydration of maleic acid
Complex 4 was tested as a catalyst (at 10–3 mol L–1) for
hydration of maleic acid (0.1 mol L–1) in aqueous solution at
100 °C. The experimental procedure has recently been de-
γ (°)
Volume (Å3)
Z
1
scribed (4), including the determination, by H NMR spec-
Dcalcd. (Mg m–3)
1.515
Absorption coefficient (mm–1)
F(000)
0.919
1 124
troscopy, of the conversion of the maleic acid (cis-
CH(CO2H)=CH(CO2H)) to the hydration (malic acid) and
isomerization (fumaric acid) products.
θ range for data collection
Reflections collected
Independent reflections
Observed reflections
No. variables
1.45–25.00
16 763
8 168 (Rint = 0.0639)
5 134 (I > 2σ(I))
531
X-ray crystallographic analysis of complex 6
Data (see Table 1) were collected on a Siemens SMART
system at 173(2) K. An initial set of cell constants was cal-
culated from reflections harvested from three sets of 20
frames, oriented such that orthogonal wedges of reciprocal
space were surveyed. This produced orientation matrices de-
termined from 20 reflections. Final cell constants were cal-
culated from a set of 7876 strong reflections from the actual
data collection. The so-called hemisphere data collection
technique, where a randomly oriented region of reciprocal
space is surveyed to the extent of 1.3 hemispheres to a reso-
lution of 0.84 Å, was used. Three major swaths of frames
were collected with 0.30° steps in ω and, in case the lattice
was found to be triclinic (as for 6), additional sets of frames
were collected to better model the absorption correction. The
space group was determined based on systematic absences
and intensity statistics (26), and a direct-methods solution
provided the location of most of the non-H atoms, with sev-
eral full-matrix least-squares – difference Fourier cycles lo-
cating the remaining non-H atoms. All non-H atoms were
refined anistropically, and H atoms were included as riding
atoms with relative isotropic displacement parameters. The
structure is dimeric, with half being in the asymmetric unit
Final R indices (I > 2σ(I))a
R1 = 0.071, wR2 = 0.1563
R indices (all data)b
Goodness-of-fit on F2
R1 = 0.1179, wR2 = 0.1768
0.96
Largest diff. peak and hole (e Å–3) 1.328 and –0.90
aR1 = Σ||FO| – |FC||/Σ|FO| (observed data).
bwR2 = (Σ(FO – FC2)2/Σw(FO2)2)1/2 (all data).
2
in solution as an equilibrium mixture of the neutral P,P- (1a)
and P,N-ligated (1b) isomers (3), as shown in Scheme 1. Net
O atom or S atom addition to 1 results in formation of
BPMO and BPMS complexes: room temperature reactions
with cumene hydroperoxide in CH2Cl2 or with H2O2 in
CH2Cl2–H2O and with S8 in refluxing 1,2-dichloroethane
cleanly give PdCl2(P,N-dmapmO) (2) and [PdCl(P,N,S-
dmapmS)]Cl (4), respectively (Scheme 1). Presumably, the
oxidations occur at the free P atom of isomer 1b. Complex 2
is a non-electrolyte in CH2Cl2 solution (ΛM < 1) and does
not dissolve in H2O, while 4 dissolves in H2O as a 1:1 elec-
trolyte (ΛM = 99), and the Cl– counterion is easily exchanged
–
that also contains one PF6 anion and two CDCl3 molecules.
–
for PF6 by reaction with KPF6 to give [PdCl(P,N,S-
Several SAME and DELU restraints were added to assist in
the refinement of the solvent molecules, one of which had
considerable translational motion that affected the apparent
bond lengths. Complete crystallographic information is
available.4
dmapmS)]PF6 (3). That the NMR data for 4 in CH2Cl2 are
the same as those for the cation of 3 (see below) argues
strongly that it is not the dissolution of 4 in water that pro-
motes dissociation of a chloride and that 4 is ionic, unlike
the neutral 2. The softer S atom of the P=S fragment (vs. the
harder O atom of P=O fragment) is apparently a sufficiently
good ligand to displace a coordinated chloride in these
Pd(II) systems.
Results and discussion
The P(III)-containing precursor PdCl2(dmapm) (1) exists
We recently observed a Pt analogue of 2, PtI2(P,N-
4 Supplementary data for this article are available on the Web site or may be purchased from the Depository of Unpublished Data, Document
Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2, Canada. DUD 3660. For more information on obtaining mate-
rial refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 262359 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 Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

550
Can. J. Chem. Vol. 83, 2005
Fig. 1. ORTEP representation (50% ellipsoids) of the molecular structure of the cation of 6. H atoms are omitted for clarity.
dmapmO), that formed as an aerobic decomposition product
of the heterobimetallic catalyst precursor PtPdCl4(µ-
N,P:P′,N′-dmapm) during the Heck coupling of PhI and sty-
rene in DMF–H2O with K2CO3 as base (1).
Table 2. Selected bond lengths (Å) and angles (°) for 6·4CDCl3
(esds in parentheses).
Bond distances (Å)
Pd(1)—Cl(1)
Pd(1)—P(1)
P(2)—O(1)
2.296(2)
2.182(2)
1.515(4)
Pd(1)—N(1)
Pd(1)—O(1*)
Pd(1)—Pd(1*)
2.114(5)
2.124(4)
4.873(2)
Unlike 1, PdCl2(dmape) (5) exists in CH2Cl2 as an equi-
librium mixture of the neutral PdCl2(P,P-dmape) (5a) and
ionic [PdCl(P,P,N-dmape)]Cl (5b) isomers (4), as shown in
Scheme 2. Reaction of this mixture with KOH in a biphasic
H2O–CH2Cl2 medium at room temperature, followed by
Bond angles (°)
P(1)-Pd(1)-N(1)
P(1)-Pd(1)-O(1*)
Pd(1)-P(1)-Cl(1)
Pd(1)-P(1)-C(17)
C(17)-C(18)-P(2)
P(2)-O(1)-Pd(1*)
87.17(14) Cl(1)-Pd(1)-O(1*)
174.99(13) Cl(1)-Pd(1)-N(1)
95.71(13)
176.45(14)
87.8(2)
113.8(5)
108.7(3)
–
Cl––PF6 anion exchange, gives the remarkable dipalladium
complex [PdCl(µ-P,N:O-dmapeO)]2[PF6]2 (6, Scheme 2). An
ORTEP of the structure of the cation is shown in Fig. 1, and
selected bond distances and angles appear in Table 2.
The structure of 6 consists of two face-to-face, square pla-
nar Pd(II) moieties in a head-to-tail orientation (i.e., they are
related by an inversion centre that lies at the mid-point of
the Pd–Pd axis). The metal centres are separated by a dis-
tance of 4.873 Å, and each is P,N-chelated by the dmape(O)
and P,O-bridged to the other, thus forming a 12-membered
ring containing the metal centres. The ring is flanked on ei-
ther side by two CDCl3 molecules (not shown in Fig. 1). The
P-C-C angles in the dmape(O) ligand “backbone” (113.8°
and 111.2°) are very similar to that of 110.7° found
for dmape (25), indicating that the dmape(O) ligand is un-
stressed, with the Pd centres being too far removed from one
another to interact. The Pd—P and Pd—N bond lengths
(2.182 and 2.114 Å), as well as the P-Pd-N angle (87.17°),
are similar to those found for 1b (2.180 Å, 2.132 Å, and
86.10°, respectively) (4).
89.29(7)
111.2(2)
111.2(5)
137.1(3)
N(1)-Pd(1)-O(1*)
P(1)-C(17)-C(18)
C(18)-P(2)-O(1)
other examples of crystallographically characterized Pd(II)
complexes bearing P,O-bound BPMO ligands (12, 21, 27–
31), where the average Pd—O, Pd—P, and P=O distances
are 2.134, 2.241, and 1.507 Å, respectively, and the average
P=O-Pd angle is 125°. There is no obvious correlation be-
tween these parameters, but it is interesting to note that in all
cases, including 6, the Pd—O distance is shorter than the
Pd—P distance. The Pd-O=P angle in these complexes
shows a wide variation between 108° and 144°. There are
also very similar crystallographic data for Pd(II) complexes
P,O-bonded to heterofunctional BPMOs of the type
R2PN(H)P(O)R2 (32).
Complex 6 shows some similarities to a Pd(II)2 compound
made recently by Leung and co-workers via a fundamentally
different route (30). Their complex was synthesized by an
initial [4+2] cycloaddition reaction between a Pd-coordinated
The bond lengths of Pd—O (2.124 Å), Pd—P (2.182 Å),
and P=O (1.515 Å), and the Pd-O=P angle (137.1(3)°) in 6,
are comparable to literature data that we have found for nine
© 2005 NRC Canada

Jones and James
551
phosphole and vinyl diphenylphosphine oxide, which gener-
ated a P,O-coordinated BPMO at a Pd(II) centre. Subse-
quent treatment with HCl to remove an ancillary
cyclometallated benzylamine ligand afforded the Pd(II)2
complex containing (like 6) two bridging BPMO ligands, the
cation of which can be formulated as [Pd2Cl2(µ-P:O-
BMPO)2(µ-Cl)]+. This species contains a bridging chloride
that serves to complete the square planar coordination at
each Pd, and thus, instead of the 12-membered ring seen in
6, the complex contains two fused eight-membered rings
(30). We are not aware of any crystallographically character-
ized complexes other than 6 that contain two BPMO and no
other bridging ligands. The observed P=O, Pd—P, and Pd—
O bond lengths and the Pd-O=P angle in 6 are very similar
to those found for Leung’s compound.
ducing a metal centre (see eq. [1]). Such chemistry is of key
importance in the reduction of Pd(II) to Pd(0) species, par-
ticularly in a range of Pd-catalyzed coupling reactions (e.g.,
Heck (34), Stille (35), and Suzuki (36) reactions), amina-
tions (37), and formation of diaryl ethers (38), where the
supposed Pd(0) active catalyst is generated in situ from
Pd(II) precursors. In the Heck reaction, for example, the ac-
tive catalyst is often derived from a mixture of Pd(OAc)2
and a phosphine, and formation of Pd(0) from this combina-
tion has been studied by Amatore et al. (39). During a study
of the Pd-catalyzed carbonylation of aryl halides, Grushin
and Alper (40) showed by use of a chiral phosphine that
Pd(II)–PR3 complexes are reduced by OH– to Pd(0) species
(eq. [1]) via a cis-hydroxo(phosphine)–Pd(II) intermediate,
and not via attack at the phosphine by external hydroxide:
The 31P{1H} data (δ and J values) for complexes 2 and 6
and for the cation within 3 and 4 reveal the expected doublet
of doublet pattern for species containing two inequivalent P
atoms, but there are insufficient data to assign the respective
resonances of the P(III) and P(V) centres, even with knowl-
edge of the 31P{1H} data for the precursor complexes 1 and
5 in both their isomeric forms (see Schemes 1 and 2) (3, 4).
The two doublet resonances for 2 and 6 differ by only 0.3
and 1.2 ppm, respectively, and even though there is a larger
difference of 11.6 ppm for the cation, assignments still can-
not be made. The lower field resonances seen for 3/4 (δ 37.0,
48.6) vs. those seen for 2 (δ 26.2, 26.5) are, however, consis-
tent with the usual trend of deshielding with formation of
five-membered ring systems of metal bisphosphine chelate
complexes (33).
[1]
[LnPd(II)-PR3]2+ + OH– → LnPd(0)
+ O=PR3 + H+
where L = a general ligand. The intermediate then decom-
poses to Pd(0) by deprotonation and reductive elimination of
phosphine oxide. Formation of 6 almost certainly occurs via
such an initial reduction, following substitution of hydroxide
for coordinated chloride or amine of 5a or 5b to give, after
generation of HCl, a species such as “Pd(0)(dmapeO)”. Sub-
sequent oxidation to 6 is thought to occur via reaction with
the CH2Cl2 solvent. Some indirect evidence for this is that 6
was not detected spectroscopically in the organic phase when
the reaction of 5 with hydroxide was conducted in the ab-
sence of chlorinated solvent (for example, in H2O–CH3NO2).
Oxidative addition reactions of M(0)(phosphine) complexes
(M = Pd, Pt) with chlorinated solvents to generate M(II)-
chloro species are not uncommon (41), but further work is
needed to substantiate the suggestion here.
1
The H NMR data for 2 and 3/4 show the resonances of
the CH2 protons as two separate multiplets for the former
complex (δ 4.27, 4.81) and one broad multiplet for the latter
(δ 4.50); the signal is seen at δ 2.23 as a triplet (2JHP = 4.2)
for free dmapm (3). Peaks for all 24 NMe protons (δ 2.68 s
for free dmapm) are also seen; for 2, there is a very broad
“background” signal (δ 1.7–3.3) and four other singlets of
3H each at δ 2.35, 2.51, 3.47, and 3.70. Complex 2 is clearly
Complex 4 was tested as a catalyst precursor for hydration
of maleic to malic acid in aqueous solution at 100 °C, for
comparison with corresponding, recently reported data using
PdCl2(dmape) (5) and the related [PdX(P,P,N-dmapcp]X
complexes as catalyst precursors (4); X = Cl or I,
and dmapcp is defined above. The activity of 4 is very simi-
lar to that found for [PdCl(P,P,N-dmapcp]Cl (4), as follows:
after 24 h, 15% of the maleic acid has been converted to 7%
malic acid and 8% fumaric acid, these percentages corre-
sponding to the turn-over numbers, which are three times
greater than those reported for hydration of diethylmaleate
in THF–H2O at 120 °C using [{Ph2P(CH2)2PPh2}Pd(µ-
1
fluxional, but without low temperature H NMR studies, like
those done for the 1a ↔ 1b system (Scheme 1) (3), it is dif-
ficult to assign the NMe resonances. However, based on the
δ values of 3.46 and 3.79 for the methyl protons of the coor-
dinated NMe2 group of 1b (3), the singlets at δ 3.47 and 3.70
likely refer to the coordinated NMe2 group of complex 2.
Fluxionality originating from exchange of free and coordi-
nated anilinyl groups could arise via reversible formation of
a five-coordinate PdCl2(P,N,O-dmapeO) intermediate, analo-
gous to the associative process envisioned for such exchange
within [PtCl(P,P,N-dmapcp]+ via a [PtCl(P,P,N,N-dmapcp]+
intermediate (4); dmapcp is similar to dmape, but in place of
the -(CH2)2- backbone the ligand has the cyclopentane
2+
OH)]2 as the catalyst precursor (5). The activity remains
marginal, and indeed is comparable to that of dilute HCl for
the hydration. However, HCl gives about twice as much
isomerization product than do our Pd-P,P,N and Pd-P,P,S
complexes (4).
1
-CH(CH2)3CH- moiety. The H NMR data for 6, consisting
of broad, overlapping signals, were uninformative.
The mechanism of formation of complex 6 under an an-
aerobic atmosphere deserves comment, as we are unaware of
other systems in which a metal–BMPO complex is isolated
from a metal–bisphosphine precursor in the absence of an
oxidant such as oxygen or some other O atom donor. It is
well known that tertiary phosphines in the presence of base
act as two-electron reductants, being themselves converted
to the phosphine oxide, with the two electrons commonly re-
Conclusions
We have demonstrated the formation of bisphosphine
monoxide (BPMO) complexes of Pd(II), both by direct oxi-
dation of a “dangling” P(III) atom and by reaction with
aqueous hydroxide in a mixed solvent system, where depro-
tonation and reductive elimination of phosphine oxide from
a cis-hydroxophosphine-Pd(II) intermediate is favoured. In
© 2005 NRC Canada

552
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Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support of this
work, the University of British Columbia for a University
Graduate Fellowship (NDJ), and Dr. Victor Young (X-Ray
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