Structurally characterized intermediates in the stepwise insertion of CO–ethylene or CO–methyl acrylate into the metal–carbon bond of Pd(II) complexes stabilized by (phosphinomethyl)oxazoline ligands

Abstract

The initial CO–ethylene or CO–methyl acrylate insertion steps into the Pd–Me bond of methylpalladium(II) complexes with (phosphinomethyl)oxazoline ligands, leading to metallacycles, have been fully characterized, including by X-ray diffraction.

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The palladium-catalyzed alternating copolymerisation of olefins and carbon monoxide, which leads to the formation of polyketones, has become a major field of research in both academic and industrial laboratories.¹

Although the basic reaction mechanism of CO–olefin copolymerisation, which involves mutually cis sites of square-planar Pd(II) species, has been established,^1,2 detailed investigations on the early stages of the chain-growth process have mostly been carried out with strained alkenes owing to the difficulties often encountered in the isolation of intermediates. The first structural reports of an ethylene–CO coupling product by Green et al., using monodentate PPh₃ and a N,O ligand,^3a then by us using a P,O ligand^3b or a diphosphine-bridged heterodimetallic Fe–Pd complex^3c were followed by only a few examples with P,N,^3d,e N,N^3f and P,P^3gchelating ligands. Furthermore, despite the considerable interest in the copolymerisation of olefins with polar monomers, such as methyl acrylate,^2e,4 only a few CO–methyl acrylate coupling products have been isolated and characterized.^2c,3b,c,5 Using a bidentate phosphine-imine (P,N) ligand, Reddy et al. have reported what appears to be the only structure of a CO–methyl acrylate coupling product.^3e These authors used a large excess of olefin (33 to 67 equiv.) in CH₂Cl₂, with a reaction time between 1–3 h.

Following the synthesis of the ligands 1a,b and of the Pd(II) methyl complexes 2a,b (Scheme 1), we investigated the catalytic activity of Ni(II) complexes with 1b in ethylene oligomerisation,⁶ and that of the Pd(II) complexes 3a,b (Scheme 2) in ethylene–CO copolymerisation.⁷ Starting from 3a,b, we have now isolated the initial intermediates in CO–ethylene or CO–methyl acrylate copolymerisation reactions, without the need to use excess methyl acrylate. The structures of the new insertion products 4b, 6b and 7a,b have been determined by X-ray diffraction as well as those of the known 2a,b and 3a,b for comparative purposes.‡

Scheme 1

Scheme 2 All reactions were performed at room temperature in CH₂Cl₂.

Reactions of 2a,b and 3a,b with CO in CH₂Cl₂ at room temperature were monitored by ³¹P{¹H} and ¹H NMR. CO insertion into their Pd–Me bond produced within a few minutes the acyl complexes 4a,b and 5a,b, respectively (Scheme 2), as evidenced by the large high-field shift of the ³¹P{¹H} NMR resonance (Δδ = –14.7 4a, Δδ = –14.8 4b, Δδ = –15.4 5a, Δδ = –15.1 5b, Table 1).

Table 1 Selected IR and NMR data of the ligands and complexes

	IR		NMR^d
	ν CN	ν CO	^1H	^31P
a In CH2Cl2. b In KBr, cm^–1. d In CDCl3, ppm, J in Hz.
1a	1660^a (s)			–15.8
1b	1660^a (s)			–15.8
2a	1647^a (s)		0.55 Pd–CH3 (d, ^3JPH = 2.7)	33.1
2b	1628^a (s)		0.65 Pd–CH3 (d, ^3JPH = 3.3)	32.8
3a	1633^b (s)		0.60 Pd–CH3 (s)	37.4
3b	1632^b (s)		0.70 Pd–CH3 (s)	37.4
4a	1642^b (s)	1684^b (s)	2.20 PdC(O)CH3 (d, ^4JPH = 1.1)	18.4
4b	1632^b (s)	1685^b (s)	2.17 PdC(O)CH3 (d, ^4JPH = 1.6)	18.0
5a	1644^a (s)	1704^a (s)		22.0
5b	1655^a (s)	1707^a (s)		22.3
	ν CN/CO	ν C(O)OMe
6a	1634^b (s)		2.45 C(O)CH3 (s)	34.4
6b	1629^b (s)		2.49 C(O)CH3 (s)	34.7
7a	1633^b (s)	1683^b (s)	2.52 C(O)CH3 (s)	32.8
7b	1629^b (s)	1684^b (s)	2.55 C(O)CH3 (s)	34.3

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In order to detect the usually elusive palladium acyl, carbonyl complexes of the type [Pd{C(O)Me}(CO)(P,N)]OTf, a CD₂Cl₂ solution of 3b was exposed to an atmosphere of ¹³CO, and ³¹P{¹H}, ¹³C{¹H} and ¹H NMR spectra were recorded at different temperatures. At room temperature, the ³¹P{¹H} NMR resonance for 5b is a doublet centred at δ 21.6 (²J_PC = 9.7 Hz). Its acyl carbon appears in the ¹³C{¹H} spectrum as an intense doublet at δ 222.7 (²J_PC = 9.7 Hz), and the weak signal observed at δ 179.8 corresponds to the coordinated CO of the acyl, carbonyl derivative. Upon decreasing the temperature to –60 °C, the ³¹P{¹H} resonance significantly broadened and shifted to δ 19.1 while the ¹³C{¹H} resonances for the acyl carbon and the coordinated CO also became broader and shifted to δ 222.6 and 175.5, respectively. This is indicative of an equilibrium between CO and OTf coordination which, at –100 °C, is completely shifted towards the acyl, carbonyl species and the ³¹P{¹H} resonance becomes a doublet of doublets centred at δ 18.3 (²J_PC(cis) = 4.1, ²J_PC(trans) = 83.8 Hz) while the acyl carbon and coordinated CO¹³C{¹H} resonances appear as doublets at δ 223.1 and 175.3, respectively (see ESI for details).† These results are in agreement with those for related complexes stabilized by P,P ligands,⁸ but at variance with those with a P,N ligand in which no significant amount of palladium acyl, carbonyl species was detected at –70 °C.^3d

Complexes 2a,b, 3a,b and 4b have slightly distorted square planar coordination geometries with the methyl (2a,b and 3a,b) and acyl (4b) ligands cis to the phosphorus atom (ESI),† in agreement with the donor groups with the largest trans influence avoiding a mutually trans position, as observed in other complexes of the type [Pd(Me)Cl(P,N)].^3e,9 The Pd–N bond distance in 2a (Table 2) is longer than that reported for the analogous PdCl₂ complex [2.058(2) Å],⁷ which reflects the larger trans influence of the methyl group compared with chloride. The Pd–C distance of 1.9701(13) Å in 4b, although slightly shorter than in the analogous Pd–Me complex (2b), is normal for an acetyl–palladium bond. The acyl group adopts an orientation approximately perpendicular to the metal coordination plane, as observed in other complexes of the type [Pd{C(O)Me}Cl(P,N)].^3e,10

Table 2 Selected bond lengths [Å] and angles [°] in complexes 2a,b, 3a,b, 4b, 6b and 7a,b

	2a	2b	3a	3b	4b	6b	7a	7b
Pd1–N1	2.103 (12)	2.171 (3)	2.131 (4)	2.1550 (18)	2.1896 (11)	2.104 (4)	2.082 (3)	2.114 (3)
Pd1–P1	2.212 (4)	2.1879 (10)	2.1699 (12)	2.1771 (12)	2.2549 (13)	2.1878 (14)	2.2051 (10)	2.2091 (14)
Pd1–C17	2.057 (13)	2.049 (4)	2.026 (5)	2.031 (2)	1.9701 (13)	2.030 (5)	2.046 (4)	2.052 (4)
Pd1–O2			2.156 (3)	2.1631 (17)		2.125 (4)	2.112 (2)	2.138 (3)
Pd1–Cl1	2.383 (4)	2.378 (1)			2.3734 (13)
N1–C3	1.291 (18)	1.279 (5)	1.281 (6)	1.275 (2)	1.2757 (15)	1.270 (6)	1.285 (5)	1.273 (6)
C3–C4	1.468 (19)	1.485 (5)	1.476 (6)	1.493 (3)	1.4897 (15)	1.480 (7)	1.496 (5)	1.498 (6)
C4–P1	1.847 (13)	1.835 (4)	1.839 (5)	1.837 (2)	1.8468 (12)	1.841 (5)	1.837 (4)	1.835 (4)
C17–C18					1.506 (2)	1.527 (8)	1.545 (6)	1.528 (7)
C18–C19						1.485 (8)	1.472 (6)	1.486 (7)
C19–O2						1.248 (6)	1.220(5)	1.239 (6)
N1–Pd1–P1	82.2 (3)	82.29 (9)	84.85 (11)	83.03 (6)	80.66 (3)	83.91 (12)	82.13 (9)	81.99 (10)
N1–Pd1–C17	176.4 (5)	171.94 (14)	174.97 (17)	174.30 (8)	173.83 (4)	177.1 (2)	174.17 (15)	175.94 (15)
N1–Pd1–O2			95.14 (14)	94.79 (7)		100.03 (14)	96.10 (12)	100.26 (13)
N1–Pd1–Cl1	93.2 (3)	97.94 (9)			97.09 (3)
P1–Pd1–O2			172.64 (9)	177.32 (4)		174.72 (10)	175.29 (8)	177.54 (9)
P1–Pd1–C17	94.5 (4)	89.70 (13)	90.68 (15)	91.69 (8)	96.35 (4)	93.30 (16)	99.24 (11)	97.25 (14)
P1–Pd1–Cl1	174.66 (15)	179.21 (4)			173.211 (11)
C17–Pd1–O2			89.00 (18)	90.42 (8)		82.80 (18)	82.10 (14)	80.57 (16)
C17–Pd1–Cl1	90.1 (5)	90.09 (13)			86.48 (4)

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Ethylene or methyl acrylate insertion into the Pd–acyl bond of 5a and 5b was completed in less than 1 h at room temperature under atmospheric pressure (³¹P NMR monitoring) and afforded 6a,b or 7a,b, respectively (Scheme 2). In all four complexes, coordination of the ketonic oxygen atom to Pd (see ν_CO, Table 1) results in a stabilizing chelate which makes β-hydrogen elimination less likely.¹¹ These complexes are stable at room temperature for several hours in solution and weeks in the solid state, which illustrates the beneficial role of the P–N chelates . The ³¹P{¹H} NMR signals of 6a,b and 7a,b are shifted to low field relative to those of 5a,b (Table 1). In the ¹H NMR spectrum of 6a,b the Pd–CH₂ protons give rise to a triplet of doublets (δ 1.65, 6a and δ 1.67, 6b) whereas the CH₂C=O protons appear as a broad triplet (δ 3.08, 6a and δ 3.12, 6b), indicating a smaller ⁴⁺⁵J_HP coupling.† The CH and CH₂ protons Ha, Hb and Hc of 7a,b were unambiguously identified and resonate at δ 2.46, 2.90 and 3.26 (7a) and δ 2.46, 2.87 and 3.27 (7b) respectively (vicinal and geminal J_HH and J_HP coupling constants are given in the ESI).†

The crystal structures of 6b and 7a,b (see Fig. 1) were determined by X-ray diffraction and the latter two established the regioselective 2,1 insertion of methyl acrylate, which leads to an α-methoxycarbonyl complex. Deviations from idealized square planar geometries are small (Table 2). The similar Pd–C distances in 6b, 7a and 7b are in agreement with that in the only other reported structure of a CO–methyl acrylate complex.^3e The Pd–O bond distances in 6b, 7a and 7b are similar and compare with those in related complexes stabilized by P,N ligands.^3d,e,12 At least in the solid state, there is no interaction between the CO₂Me group of 7a,b and the metal centre, which would have reduced its electrophilicity and increased its steric shielding. It is also interesting to note that related Pd(II) complexes with the P,O chelating ligand Ph₂PNHC(O)Me were generally found to be less reactive than 3a,b or 5a,b, longer reaction times being required.^3b

Fig. 1 Molecular structure of: a) complex 3b, b) the cation in 6b, c) the cation in 7b (CH groups of phenyl rings and H atoms omitted, except inserted olefin). Displacement ellipsoids are drawn at the 50% probability level.

In addition to the structural characterization of CO–ethylene and CO–methyl acrylate insertion products, we have spectroscopically observed a temperature-dependent equilibrium between a triflate, acyl complex, 5b, and a cationic carbonyl, acyl Pd(II) complex. Previous studies have shown that 3a,b catalyse the CO–ethylene copolymerisation at 60–90 °C.⁶ Further investigations are in progress to determine the influence of the ligand bite angle on the reactivity of the chelate ring in 7a,b towards further insertion of small molecules.

We thank Luc Brissieux for preliminary results and the CNRS, the Ministère de la Recherche (Paris) and the Europeen Commission (Palladium Network HPRN-CT-2002-00196 and COST program) for support. We are grateful to Prof. R. Welter and Dr A. DeCian (ULP Strasbourg) for the crystal structure determinations and to Mrs A. Degrémont (LCC) for assistance.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Complete sets of crystallographic parameters for 2a,b, 3a,b, 4b, 6b, 7a,b; experimental procedures and spectroscopic characterizations. See DOI: 10.1039/b613865a

‡ CCDC 622170–622177. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b613865a

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