First microwave-assisted synthesis of an electron-rich phosphane and its coordination chemistry with platinum(II) and palladium(II)

Article abstract of DOI:10.1016/j.ica.2011.05.020

The P-O ligand 3-(di(2-methoxyphenyl)phosphanyl)propionic acid (HL) was synthesized by a microwave-assisted reaction of a secondary phosphane. The coordination of HL to Pt^II yielded the neutral mononuclear complex trans-[PtCl(κ²-P,O-

Full text of DOI:10.1016/j.ica.2011.05.020

Inorganica Chimica Acta 375 (2011) 324–328
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Note
First microwave-assisted synthesis of an electron-rich phosphane
and its coordination chemistry with platinum(II) and palladium(II)
Barbara Trettenbrein a, Markus Fessler a, Martin Ruggenthaler a, Stephan Haringer a, Dennis Oberhuber a,
b,
Georg Czermak a, Peter Brüggeller a, , Werner Oberhauser
⇑
⇑
a Institut für Allgemeine, Anorganische und Theoretische Chemie der Universität Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
b Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area di Ricerca di Firenze, via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The P–O ligand 3-(di(2-methoxyphenyl)phosphanyl)propionic acid (HL) was synthesized by a micro-
wave-assisted reaction of a secondary phosphane. The coordination of HL to PtII yielded the neutral
Received 18 February 2011
Received in revised form 12 May 2011
Accepted 20 May 2011
mononuclear complex trans-[PtCl(
(COD = 1,4-cyclooctadiene) with HL in the presence of NEt3 gave the anionic PdII compound of the for-
mula (HNEt3)[PdClMe(
2-P,O-L)] (2). Upon crystallization of the latter compound the neutral chloride-
bridged dimetallic compound cis-[Pd(
j j-P-HL)] (1), while the reaction of PdClMe(g
2-P,O-L)( 4-COD)
Available online 30 May 2011
j
l
-Cl)Me(HL)]2 (3) was obtained. HL, 1 and 3ꢀCH2Cl2 have been char-
Keywords:
Microwave-assisted synthesis
Phosphane
acterized by single crystal X-ray structure analyses.
Ó 2011 Elsevier B.V. All rights reserved.
Palladium
Platinum
X-ray structure
1. Introduction
2. Experimental
The organometallic chemistry of functionalized phosphanes
modified with an additional (non-phosphane) donor group has
revealed a great deal of applications in homogeneous catalysis
[1]. Among these non-phosphane donors, sulfonate [2] and
carboxylate [3] groups have gained particular interest in the
Ni(P-O)-catalyzed olefin oligomerization (SHOP) [4] as well as in
the Pd(P-O)-catalyzed non-strictly CO olefin copolymerization
reaction [2]. In particular, the presence of the 2-methoxyphenyl
moiety in the ligand scaffold has shown to confer high stability
to the ligand against phosphane oxide formation, which is the
most encountered deactivation process occurring in metal-phos-
phane catalyzed reactions [5]. In order to contribute to the field
of synthesis of new phosphanyl-carboxylate ligands, we report
2.1. Methods and materials
All synthetic reactions and manipulations were carried out un-
der an argon atmosphere by using standard Schlenk techniques. Re-
agents were used as received from Aldrich, unless stated otherwise.
Bis-(di(2-methoxy)phenyl)phosphane [6] and [PdClMe(g
4-COD)]
[7] were prepared according to literature methods. The microwave
synthesis was carried out with an Anton Paar Synthos 3000
(1400 W unpulsed microwave) dual magnetron system, with an
operation volume of 60 mL and reaction vessels of PTFE–TFM. Deu-
terated solvents for routine NMR measurements were dried with
activated molecular sieves. 1H, 13C{1H} and 31P{1H} NMR spectra
were obtained with a Bruker Avance DRX-400 spectrometer
acquiring spectra at 400.13, 100.62 and 161.98 MHz, respectively.
Chemical shifts (d) are reported in ppm relative to TMS (1H and
13C NMR spectra) or 85% H3PO4. IR spectra were acquired on a Nico-
let 5700 ATR FT-IR spectrometer. Microanalyses were performed
using a Carlo-Erba Model 1106 elemental analyzer. FAB mass spec-
trometric measurements were carried out on a Finnigan MAT-95
spectrometer, using 3-nitrobenzylalcohol (NOBA) as matrix.
here
a
microwave-assisted synthesis of 3-(di(2-methoxy-
phenyl)phosphanyl)propionic acid (HL) and its coordination
chemistry with PtII and PdII.
2.2. Synthesis of HL
⇑
Corresponding authors. Tel.: +43 512 5075115; fax: +43 512 5072934 (Peter
Brueggeller); tel.: +39 055 5225284; fax: +39 055 5225203 (Werner Oberhauser).
E-mail addresses: Peter.Brueggeller@uibk.ac.at (P. Brüggeller), werner.oberhauser@
iccom.cnr.it (W. Oberhauser).
Deaerated ethyl 3-chloropropionate (12 mL, 88.0 mmol)
was added to
a
teflon vessel, which was sealed after
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.05.020

B. Trettenbrein et al. / Inorganica Chimica Acta 375 (2011) 324–328
325
bis-(di(2-methoxy)phenyl)phosphane (200.3 mg, 0.813 mmol) had
been added. The reaction vessel was heated by microwave irradia-
tion at 100 °C for 2 h. Afterwards, deaerated water (50 mL) was
added to the reaction mixture. The water phase was separated,
concentrated to dryness and the viscous residue was dissolved in
deaerated EtOH (20 mL). Then NaOCH3 (7.0 g, 130.1 mmol) was
added to the reaction solution, which was stirred at 80 °C for 4 h.
The reaction solvent was then completely removed and the crude
residue was dissolved in deaerated water (20 mL) and HCl was
added at room temperature, causing the precipitation of the de-
sired product as an off-white product, which was then separated
from solution by filtration and dried by vacuum. Yield 106.2 mg
(41%). Mp 145–148 °C. Anal. Calc. for C17H19O4P (318.29): C,
64.15; H, 6.02. Found: C, 63.99; H, 6.04%. 1H NMR (DMSO-d6,
21 °C): d 2.21–2.22 (s, 4H, CH2CH2COO), 3.73 (s, 6H, OCH3), 6.91–
7.37 (m, 8H, Ar-H), 12.17 (s, 1H, COOH). 13C{1H} NMR (DMSO-d6,
16H, Ar-H). 13C{1H} NMR (DMF-d7, 21 °C): d 19.78 (s, CH2CO2),
2
22.64 (d, JPC = 28.3 Hz, CH2P), 55.49 (s, OCH3), 55.80 (s, OCH3),
111.22–161.46 (Ar-C), 174.39 (s, CO2H), 176.71 (s, CO2ꢁ). 31P{1H}
2
1
NMR (DMF-d7, 21 °C): d 14.74 (d, JPP = 467.0 Hz, JPtP = 2793.0 Hz,
P(CO2H)), 16.86 (d, 2JPP = 467.0 Hz, 1JPtP = 2782.0 Hz, PðCO ꢁÞ). 195Pt
NMR (DMF-d7, 21 °C): d ꢁ3461.16 (t, 1JPtP = 2799.0 Hz). IR2(m, cmꢁ1
)
1695 (CO2H), 1671 ðCO2ꢁÞ. MS (FAB+) m/z: 867.3 (M+H+), 830.4
(MꢁClꢁ).
2.4. Synthesis of (HNEt3) [PdClMe(j
2-P,O-L)] (2)
In a Schlenk tube HL (50.0 mg, 0.157 mmol) was dissolved in
deaerated CH2Cl2 (10 mL). Afterwards, NEt3 (5.0 mL) was added
to the solution and allowed to stir for 1 h. Then [PdClMe(g4
-
COD)] (30.1 mg, 0.157 mmol) was added to the latter solution,
which turned slightly yellow. After a reaction time of half an hour
the solvent was removed completely and the resulting brownish
solid was washed with n-hexane and dried under vacuum.
Yield 45.8 mg (51%). Mp 109 °C (decomposition). Anal. Calc. for
2
21 °C): d 19.78 (d, JPC = 12.6 Hz, CH2CO2H), 31.31 (1JPC = 18.6 Hz,
CH2P), 55.96 (s, OCH3), 111.30 (s, Ar-C), 121.30 (s, Ar-C), 125.03
1
2
(d, JPC = 17.1 Hz, ipso-Ar-C), 130.66 (s, Ar-C), 132.42 (d, JPC
=
=
2
3
5.2 Hz, Ar-C), 161.40 (d, JPC = 13.4 Hz, Ar-C), 174.57 (d, JPC
C
24H37ClNO4Pd (576.40): C, 52.88; H, 6.79. Found: C, 52.73; H,
13.9 Hz, CO2H). 31P{1H} NMR (DMSO-d6, 21 °C): d ꢁ33.97 (s). IR
6.59%. 1H NMR (CD2Cl2, 21 °C): d 0.08 (d, 3JPH = 2.8 Hz, 3H, PdCH3),
(m
, cmꢁ1) 1695 (CO2H). MS (FAB+) m/z: 318.11 (M+).
3
2
1.25 (t, JHH = 7.2 Hz, 9H, CH3CH2), 2.42 (dm, JPC = 29.6 Hz, 2H,
3
CH2CO2), 2.70 (m, 2H, CH2P), 3.21 (q, JHH = 7.2 Hz, 6H, CH3CH2),
3.86 (s, 6H, OCH3), 7.00–7.73 (m, 8H, Ar-H). 13C{1H} NMR (CD2Cl2,
2.3. Synthesis of trans-[PtCl(j2ꢁP,O-L)(
j-P-HL)] (1)
1
21 °C): d ꢁ6.43 (s, PdCH3), 8.44 (s, CH3CH2), 22.27 (d, JPC
=
32.14 Hz, CH2P), 33.27 (s, CH2CO2), 45.07 (s, CH3CH2), 55.51 (s,
In a Schlenk tube HL (100.0 mg, 0.314 mmol) was suspended in
water (20 mL) and on addition of KOH (17.6 mg, 0.314 mmol), the
suspension became a clear solution. To this latter solution a solu-
tion of K2PtCl4ꢀ4H20 (76.5 mg, 0.157 mmol) in water (20 mL) was
added under stirring, which was continued at room temperature
for 2 h. Afterwards, the solvent was completely removed obtaining
the crude off-white product, which was suspended in a small
amount of MeOH (2 mL), filtered off and then dried under vacuum.
Yield 98.4 mg (72%). Mp 204 °C. Anal. Calc. for C34H37ClO8P2Pt
(866.11): C, 47.15; H, 4.31. Found: C, 47.01; H, 4.14%. 1H NMR
(DMF-d7, 21 °C): d 2.25 (m, 2H, CH2), 2.53 (m, 2H, CH2), 2.93 (m,
4H, CH2), 3.85 (s, 6H, OCH3), 3.92 (s, 6H, OCH3), 6.99–7.91 (m,
3
1
OCH3), 111.05 (d, JPC = 4.1 Hz, Ar-C), 118.10 (d, JPC = 17.2 Hz,
2
ipso-Ar-C), 120.34 (d, JPC = 11.9 Hz, Ar-C), 132.61 (s, Ar-C), 160.38
(s, Ar-C), 179.26 (s, CO2ꢁ). 31P{1H} NMR (CD2Cl2, 21 °C): d 37.66
(s). IR (
m
, cmꢁ1) 1585 ðCO2ꢁÞ. MS (FAB+) m/z: 439.0 (M+–HNEt3–Cl).
2.5. X-ray crystal structure determinations
Single crystals of HL, suitable for an X-ray structure analysis,
were obtained by slow evaporation of a 1:1 (v/v) water–EtOH solu-
tion of HL on air, while single crystals of 1 were obtained from a
corresponding water-acetic acid-acetone solution. Single crystals
Table 1
Crystallographic data and structure refinement details for compounds HL, 1 and 3ꢀCH2Cl2.a
HL
1
3ꢀCH2Cl2
Empirical formula
Formula weight
a (Å)
b (Å)
c (Å)
C17H19O4P
318.29
12.9634(4)
8.1575(6)
15.3186(8)
C34H37ClO8P2Pt
866.11
12.4658(2)
16.1336(2)
17.3074(3)
C37H44Cl4O8P2Pd2
1033.26
19.1988(3)
13.9774(2)
17.2253(2)
a
(°)
b (°)
98.790(3)
94.6333(8)
93.7793(7)
c
(°)
V (Å3)
1600.90(15)
4
1.321
0.187
672
3469.46(9)
4
1.658
4.263
1720
4612.35(11)
4
1.488
1.124
2080
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(0 0 0)
)
H
Range for data collection (°)
1.59–27.50
ꢁ16 6 h 6 16
ꢁ10 6 k 6 10
ꢁ19 6 l 6 19
9060
1.64–27.47
ꢁ15 6 h 6 16
ꢁ20 6 k 6 20
ꢁ22 6 l 6 22
30 073
1.80–27.48
ꢁ24 6 h 6 24
ꢁ18 6 k 6 18
ꢁ22 6 l 6 22
39 158
Limiting indices
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
3672 (0.0376)
3638/0/203
1.033
7954 (0.0230)
7912/0/420
1.030
10 569 (0.0380)
10 459/17/499
1.026
Final R indices [I > 2
r
(I)] R1, wR2
0.0372, 0.0950
0.0460, 0.1130
0.200 and ꢁ0.210
0.0253, 0.0609
0.0321, 0.0718
1.260 and ꢁ0.910
0.0371, 0.0881
0.0465, 0.1052
0.974 and ꢁ0.570
R indices (all data) R1, wR2
Largest difference in peak and hole (e Åꢁ3
)
a
Temperature, 243(2) K; crystal shape, prism; crystal size, 0.25 ꢂ 0.15 ꢂ 0.10 (HL, 1), 0.45 ꢂ 0.20 ꢂ 0.03 (3ꢀCH2Cl2); crystal colour, col-
ourless; crystal system, monoclinic; space group, P21/c; absorption corrections, multi-scan; refinement method, full-matrix least-squares on
F2.

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B. Trettenbrein et al. / Inorganica Chimica Acta 375 (2011) 324–328
Me
Cl
Me
O
O
O
O
Me
O
1) NaOMe
EtOH
2) HCl/H2O
Cl(CH2)2CO2CH2CH3
P
P
P
microwave
100°C, 2h
O
OH
H
O
O
O
Me
H
Me
Me
Me
HL
Scheme 1.
SHELXL-97 [8b]. Final refinements on F2 were carried out applying
anisotropic thermal parameters for all non-hydrogen atoms, while
all hydrogen atoms were included in the refinement using a riding
model with isotropic U values depending on the Ueq of the adjacent
non-hydrogen atoms, except for the carboxylic acid hydrogen
atoms in 3ꢀCH2Cl2, which were refined isotropically with fixed U.
3. Results and discussion
The new P–O ligand HL was obtained by a microwave-assisted
reaction of bis(di(2-methoxy)phenyl)phosphane in neat ethyl-
3-chloropropionate, followed by a hydrolysis step and proton-
ation reaction of the resulting sodium salt with HCl as shown in
Scheme 1.
The microwave-assisted conversion of bis(di(2-methoxy)-
phenyl)phosphane (i.e. secondary phosphane) into a most likely
phosphonium intermediate, characterized by a 31P{1H} NMR singlet
centred at 28.71 ppm in EtOH, is so far unique [9]. Upon hydrolysis
of the phosphonium intermediate, giving NaL and successive pro-
tonation of the latter compound with HCl in water gave HL after
work up with 41% yield. Alternative synthetic protocols such as a
Michael reaction between bis(di(2-methoxy)phenyl)phosphane
and ethyl acrylate [10] did not give the desired product, while the
thermal reaction between 3-chloropropionic acid and the in situ
formed Na-salt of bis(di(2-methoxy)phenyl)phosphane in a THF/
DMSO solvent mixture gave HL with only 8% yield. A broad 1H
NMR singlet centered at 12.17 ppm (DMSO-d6) is proof of the pres-
ence of the carboxylic acid functionality and a 13C NMR doublet at
Fig. 1. ORTEP-plot of HL. Hydrogen atoms, except for the carboxylic group H(1), are
omitted for clarity and thermal ellipsoids are shown at the 30% probability level.
of 3ꢀCH2Cl2, were obtained by a reverse gas-phase diffusion, where
3, dissolved in CH2Cl2, was exposed to n-hexane at room tempera-
ture under light exclusion. Crystallographic data and structure
refinement details for all three compounds are summarized in
Table 1. Diffraction data were collected on a Nonius Kappa CCD dif-
174.57 ppm (3JPC = 13.9 Hz) is in agreement with
a P(1)C(1)
C(2)C(3) dihedral angle of 70.4(2)° [10], as found in the crystal
structure of HL (Fig. 1).
fractometer using
u–x-scans and graphite-monochromated Mo-
The reaction of KL with K2PtCl4ꢀ4H2O in a 2:1 stoichiometric
Ka
radiation (k = 0.71073 Å). Cell refinement, data reduction and
ratio gave 1 with 72% yield (Scheme 2).
empirical absorption correction were carried out with the Denzo
and Scalepack programs [8a]. All structure determination calcula-
tions were performed with SHELXTL NT V6.1 including SHELXS-97 and
The trans stereochemistry of the phosphorus atoms in 1 and the
concomitant presence of the j j-P coordination mode of
2-P,O and
Lꢁ and HL, respectively, is unambiguously confirmed by a single
O
(HNEt3)
O
O
O
Pt
Ar'2P
PAr'2
O
1) NEt3
1) KOH
OH
HL
Pd
2) K2PtCl4
2) PdClMe(COD)
P
Me
Cl
Ar'2
Cl
2
1
O
O
Cl
Ar'2P
Me
PAr'2
Me
OH
Pd
Pd
HO
Ar' = 2-Methoxyphenyl
Cl
3
Scheme 2.

B. Trettenbrein et al. / Inorganica Chimica Acta 375 (2011) 324–328
327
Fig. 3. ORTEP-plot of 3ꢀCH2Cl2. The solvent molecule and hydrogen atoms, except
for the carboxylic acid hydrogen atoms H(1) and H(2), are omitted for clarity and
thermal ellipsoids are shown at the 30% probability level.
characteristic 3JPH of 2.8 Hz [2,12] for PdCH3, while the 2-P,O-coor-
j
dination of Lꢁ to palladium is confirmed by the 13C NMR singlet at
179.26 ppm. Upon crystallization of 2, in the presence of traces of
water, 3ꢀCH2Cl2 was obtained in only a small amount (i.e. 10%). The
latter compound proved to be insoluble in all commonly used
organic solvents. An ORTEP-plot of 3ꢀCH2Cl2 is shown in Fig. 3
and selected bond distances and angles are reported in Table 2.
The partial conversion of 2 into 3 may be rationalized by a
water mediated protonation of the coordinating carboxylic oxygen
atom in 2, yielding NEt3 and the mononuclear T-shaped PdII species
[PdClMe(HL)] [13], which subsequently dimerizes and crystallizes
yielding 3ꢀCH2Cl2. The regioselectivity of this latter dimerization
reaction is object of a further study. Both HL ligands in 3ꢀCH2Cl2
Fig. 2. ORTEP-plot of 1. Hydrogen atoms, except for the carboxylic acid hydrogen
atom H(1), are omitted for clarity and thermal ellipsoids are shown at the 30%
probability level.
Table 2
Selected bond distances [Å] and angles [°] for 1 and 3ꢀCH2Cl2.
1
3ꢀCH2Cl2
Pt(1)–P(1)
Pt(1)–P(2)
Pt(1)–Cl(1)
Pt(1)–O(8)
2.3319(10)
2.2892(10)
2.2830(11)
2.044(3)
are cis-coordinated with respect to the central Pd2(
l-Cl)2 core,
which shows a folding angle of 136.50(4)°.
Pd(1)–Cl(1)
Pd(1)–Cl(2)
Pd(2)–Cl(1)
Pd(2)–Cl(2)
Pd(1)–P(1)
Pd(2)–P(2)
Pd(1)–C(10)
2.4131(10)
2.4672(10)
2.4114(11)
2.4513(11)
2.2208(10)
2.2182(11)
2.044(4)
4. Conclusions
The new P–O ligand HL was synthesized by means of a micro-
wave-assisted reaction involving for the first time a secondary
phosphane. The coordination of HL to PtII and PdII has been con-
firmed by multinuclear NMR-, IR-, MS-spectroscopy and single
crystal X-ray structure analysis. Compound 1 is a rare example of
Pd(2)–C(20)
2.038(4)
Cl(1)–Pt(1)–O(8)
P(1)–Pt(1)–P(2)
P(2)–Pt(1)–O(8)
Cl(1)–Pd(1)–Cl(2)
Cl(1)–Pd(2)–Cl(2)
Pd(1)–Cl(1)–Pd(2)
Pd(1)–Cl(2)–Pd(2)
P(1)–Pd(1)–C(10)
P(2)–Pd(2)–C(20)
P(1)–Pd(1)–Cl(1)
P(2)–Pd(2)–Cl(1)
177.93(8)
175.03(3)
86.21(8)
a PtII compound showing both metal coordination modes (i.e.
j-P
84.44(4)
84.82(4)
87.81(4)
85.72(3)
87.87(12)
85.69(12)
175.68(4)
174.48(4)
and
j
2-P,O) of a P–O ligand.
Acknowledgements
This research was financially supported by the Fonds zur Förde-
rung der wissenschaftlichen Forschung (FWF) and the For-
schungsförderungsgesellschaft (FFG), Vienna, Austria, the Tiroler
Wissenschaftsfonds (TWF), Innsbruck, Austria, and the companies
Verbund AG and D. Swarowski & Co.
crystal X-ray structure analysis (Fig. 2). Selected bond distances
and angles are compiled in Table 2.
The coordination of Lꢁ to PtII led to the formation of a six-mem-
bered heteroatom ring, showing a bite angle of 86.21(8)°, which is
comparable to that found for bis(2-methoxyphenyl)phos-
phanylethylsulphonate ðP-SO3ꢁÞ in [Pd(P-SO3)2] of 85.30(12) and
84.44(13)° [2]. The 31P{1H} NMR spectrum of 1, acquired in dry
DMF-d7, proved that the relative stereochemistry, found for the so-
lid state, is maintained also in solution. Accordingly, both phospho-
Appendix A. Supplementary material
CCDC 811119, 811120 and 811121 contain the supplementary
crystallographic data for HL, 1 and 3ꢀCH2Cl2, respectively. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.05.020.
2
rus nuclei are part of an AB spin system characterized by JPP of
476.0 Hz [11]. 13C{1H} NMR and IR data are in agreement with
the concomitant presence of a coordinating carboxylate moiety
and a free carboxylic acid functionality.
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yield (Scheme 2). The cis coordination of the methyl group to pal-
ladium with respect to the phosphorus donor atom is proved by a
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