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

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(κ(2)-P,O-L)(κ-P-HL)] (1), while the reaction of PdClMe(η(4)-COD) (COD = 1,4-cyclooctadiene) with HL in the presence of NEt(3) gave the anionic Pd(II) compound of the formula (HNEt(3))[PdClMe(κ(2)-P,O-L)] (2). Upon crystallization of the latter compound the neutral chloride-bridged dimetallic compound cis-[Pd(μ-Cl)Me(HL)](2) (3) was obtained. HL, 1 and 3·CH(2)Cl(2) have been characterized by single crystal X-ray structure analyses.

Introduction

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-phosphane catalyzed reactions [5]. In order to contribute to the field of synthesis of new phosphanyl-carboxylate ligands, we report here a microwave-assisted synthesis of 3-(di(2-methoxyphenyl)phosphanyl)propionic acid (HL) and its coordination chemistry with PtII and PdII.

Experimental

Methods and materials

All synthetic reactions and manipulations were carried out under an argon atmosphere by using standard Schlenk techniques. Reagents were used as received from Aldrich, unless stated otherwise. Bis-(di(2-methoxy)phenyl)phosphane [6] and [PdClMe(η4-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. Deuterated 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 (δ) are reported in ppm relative to TMS (1H and 13C NMR spectra) or 85% H3PO4. IR spectra were acquired on a Nicolet 5700 ATR FT-IR spectrometer. Microanalyses were performed using a Carlo-Erba Model 1106 elemental analyzer. FAB mass spectrometric measurements were carried out on a Finnigan MAT-95 spectrometer, using 3-nitrobenzylalcohol (NOBA) as matrix.

Synthesis of HL

Deaerated ethyl 3-chloropropionate (12 mL, 88.0 mmol) was added to a teflon vessel, which was sealed after bis-(di(2-methoxy)phenyl)phosphane (200.3 mg, 0.813 mmol) had been added. The reaction vessel was heated by microwave irradiation 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 desired 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): δ 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, 21 °C): δ 19.78 (d, 2JPC = 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 (d, 1JPC = 17.1 Hz, ipso-Ar-C), 130.66 (s, Ar-C), 132.42 (d, 2JPC = 5.2 Hz, Ar-C), 161.40 (d, 2JPC = 13.4 Hz, Ar-C), 174.57 (d, 3JPC = 13.9 Hz, CO2H). 31P{1H} NMR (DMSO-d6, 21 °C): δ −33.97 (s). IR (ν, cm−1) 1695 (CO2H). MS (FAB+) m/z: 318.11 (M+).

Synthesis of trans-[PtCl(κ2−P,O-L)(κ-P-HL)] (1)

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 solution of K2PtCl4·4H20 (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): δ 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, 16H, Ar-H). 13C{1H} NMR (DMF-d7, 21 °C): δ 19.78 (s, CH2CO2), 22.64 (d, 2JPC = 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, ). 31P{1H} NMR (DMF-d7, 21 °C): δ 14.74 (d, 2JPP = 467.0 Hz, 1JPtP = 2793.0 Hz, P(CO2H)), 16.86 (d, 2JPP = 467.0 Hz, 1JPtP = 2782.0 Hz, ). 195Pt NMR (DMF-d7, 21 °C): δ −3461.16 (t, 1JPtP = 2799.0 Hz). IR (ν, cm−1) 1695 (CO2H), 1671 . MS (FAB+) m/z: 867.3 (M+H+), 830.4 (M−Cl−).

Synthesis of (HNEt3) [PdClMe(κ2-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(η4-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 C24H37ClNO4Pd (576.40): C, 52.88; H, 6.79. Found: C, 52.73; H, 6.59%. 1H NMR (CD2Cl2, 21 °C): δ 0.08 (d, 3JPH = 2.8 Hz, 3H, PdCH3), 1.25 (t, 3JHH = 7.2 Hz, 9H, CH3CH2), 2.42 (dm, 2JPC = 29.6 Hz, 2H, CH2CO2), 2.70 (m, 2H, CH2P), 3.21 (q, 3JHH = 7.2 Hz, 6H, CH3CH2), 3.86 (s, 6H, OCH3), 7.00–7.73 (m, 8H, Ar-H). 13C{1H} NMR (CD2Cl2, 21 °C): δ −6.43 (s, PdCH3), 8.44 (s, CH3CH2), 22.27 (d, 1JPC = 32.14 Hz, CH2P), 33.27 (s, CH2CO2), 45.07 (s, CH3CH2), 55.51 (s, OCH3), 111.05 (d, 3JPC = 4.1 Hz, Ar-C), 118.10 (d, 1JPC = 17.2 Hz, ipso-Ar-C), 120.34 (d, 2JPC = 11.9 Hz, Ar-C), 132.61 (s, Ar-C), 160.38 (s, Ar-C), 179.26 (s, ). 31P{1H} NMR (CD2Cl2, 21 °C): δ 37.66 (s). IR (ν, cm−1) 1585 . MS (FAB+) m/z: 439.0 (M+–HNEt3–Cl).

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 solution of HL on air, while single crystals of 1 were obtained from a corresponding water-acetic acid-acetone solution. Single crystals of 3·CH2Cl2, were obtained by a reverse gas-phase diffusion, where 3, dissolved in CH2Cl2, was exposed to n-hexane at room temperature 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 diffractometer using φ–ω-scans and graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Cell refinement, data reduction and empirical absorption correction were carried out with the Denzo and Scalepack programs [8a]. All structure determination calculations were performed with shelxtl nt V6.1 including shelxs-97 and 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 3·CH2Cl2, which were refined isotropically with fixed U.

Table 1

Crystallographic data and structure refinement details for compounds HL, 1 and 3·CH2Cl2.a

	HL	1	3·CH2Cl2
Empirical formula	C17H19O4P	C34H37ClO8P2Pt	C37H44Cl4O8P2Pd2
Formula weight	318.29	866.11	1033.26
a (Å)	12.9634(4)	12.4658(2)	19.1988(3)
b (Å)	8.1575(6)	16.1336(2)	13.9774(2)
c (Å)	15.3186(8)	17.3074(3)	17.2253(2)
α (°)
β (°)	98.790(3)	94.6333(8)	93.7793(7)
γ (°)
V (Å3)	1600.90(15)	3469.46(9)	4612.35(11)
Z	4	4	4
Dcalc (Mg/m3)	1.321	1.658	1.488
Absorption coefficient (mm−1)	0.187	4.263	1.124
F(0 0 0)	672	1720	2080
Θ Range for data collection (°)	1.59–27.50	1.64–27.47	1.80–27.48
Limiting indices	−16 ≤ h ≤ 16	−15 ≤ h ≤ 16	−24 ≤ h ≤ 24
	−10 ≤ k ≤ 10	−20 ≤ k ≤ 20	−18 ≤ k ≤ 18
	−19 ≤ l ≤ 19	−22 ≤ l ≤ 22	−22 ≤ l ≤ 22
Reflections collected	9060	30 073	39 158
Independent reflections (Rint)	3672 (0.0376)	7954 (0.0230)	10 569 (0.0380)
Data/restraints/parameters	3638/0/203	7912/0/420	10 459/17/499
Goodness-of-fit on F2	1.033	1.030	1.026
Final R indices [I > 2σ(I)] R1, wR2	0.0372, 0.0950	0.0253, 0.0609	0.0371, 0.0881
R indices (all data) R1, wR2	0.0460, 0.1130	0.0321, 0.0718	0.0465, 0.1052
Largest difference in peak and hole (e Å−3)	0.200 and −0.210	1.260 and −0.910	0.974 and −0.570

Temperature, 243(2) K; crystal shape, prism; crystal size, 0.25 × 0.15 × 0.10 (HL, 1), 0.45 × 0.20 × 0.03 (3·CH2Cl2); crystal colour, colourless; crystal system, monoclinic; space group, P21/c; absorption corrections, multi-scan; refinement method, full-matrix least-squares on F2.

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 protonation reaction of the resulting sodium salt with HCl as shown in Scheme 1.

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 protonation 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 presence of the carboxylic acid functionality and a 13C NMR doublet at 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).

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.

The reaction of KL with K2PtCl4·4H2O in a 2:1 stoichiometric ratio gave 1 with 72% yield (Scheme 2).

Scheme 2

The trans stereochemistry of the phosphorus atoms in 1 and the concomitant presence of the κ2-P,O and κ-P coordination mode of L− and HL, respectively, is unambiguously confirmed by a single crystal X-ray structure analysis (Fig. 2). Selected bond distances and angles are compiled in Table 2.

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 3·CH2Cl2.

	1	3·CH2Cl2
Pt(1)–P(1)	2.3319(10)
Pt(1)–P(2)	2.2892(10)
Pt(1)–Cl(1)	2.2830(11)
Pt(1)–O(8)	2.044(3)
Pd(1)–Cl(1)		2.4131(10)
Pd(1)–Cl(2)		2.4672(10)
Pd(2)–Cl(1)		2.4114(11)
Pd(2)–Cl(2)		2.4513(11)
Pd(1)–P(1)		2.2208(10)
Pd(2)–P(2)		2.2182(11)
Pd(1)–C(10)		2.044(4)
Pd(2)–C(20)		2.038(4)
Cl(1)–Pt(1)–O(8)	177.93(8)
P(1)–Pt(1)–P(2)	175.03(3)
P(2)–Pt(1)–O(8)	86.21(8)
Cl(1)–Pd(1)–Cl(2)		84.44(4)
Cl(1)–Pd(2)–Cl(2)		84.82(4)
Pd(1)–Cl(1)–Pd(2)		87.81(4)
Pd(1)–Cl(2)–Pd(2)		85.72(3)
P(1)–Pd(1)–C(10)		87.87(12)
P(2)–Pd(2)–C(20)		85.69(12)
P(1)–Pd(1)–Cl(1)		175.68(4)
P(2)–Pd(2)–Cl(1)		174.48(4)

The coordination of L− to PtII led to the formation of a six-membered heteroatom ring, showing a bite angle of 86.21(8)°, which is comparable to that found for bis(2-methoxyphenyl)phosphanylethylsulphonate 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 solid state, is maintained also in solution. Accordingly, both phosphorus nuclei are part of an AB spin system characterized by 2JPP 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.

The reaction of [PdClMe(η4-COD)] with [(HNEt)L] in an 1:1 stoichiometric ratio gave the anionic PdII-compound 2 with 51% yield (Scheme 2). The cis coordination of the methyl group to palladium with respect to the phosphorus donor atom is proved by a characteristic 3JPH of 2.8 Hz [2,12] for PdCH3, while the κ2-P,O-coordination of L− to palladium is confirmed by the 13C NMR singlet at 179.26 ppm. Upon crystallization of 2, in the presence of traces of water, 3·CH2Cl2 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 3·CH2Cl2 is shown in Fig. 3 and selected bond distances and angles are reported in Table 2.

Fig. 3

ORTEP-plot of 3·CH2Cl2. 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.

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 3·CH2Cl2. The regioselectivity of this latter dimerization reaction is object of a further study. Both HL ligands in 3·CH2Cl2 are cis-coordinated with respect to the central Pd2(μ-Cl)2 core, which shows a folding angle of 136.50(4)°.

Conclusions

The new P–O ligand HL was synthesized by means of a microwave-assisted reaction involving for the first time a secondary phosphane. The coordination of HL to PtII and PdII has been confirmed by multinuclear NMR-, IR-, MS-spectroscopy and single crystal X-ray structure analysis. Compound 1 is a rare example of a PtII compound showing both metal coordination modes (i.e. κ-P and κ2-P,O) of a P–O ligand.