A new class of anionic phosphinooxazoline ligands in palladium and ruthenium complexes: catalytic properties for the transfer hydrogenation of acetophenone

Abstract

The synthesis of the cationic Pd complex [Pd(dmba)(PCH₂-oxazoline)]Cl [PCH₂-oxazoline = 2-oxazoline-2-ylmethyl)diphenylphosphine] (2) has allowed for the first time the observation of hemilabile behaviour for a phosphinooxazoline ligand. This molecular dynamics is stopped upon removal of the chloride anion, by reaction with either NH₄PF₆, which retains the cationic nature of the complex, or Bu^tOK. The latter reaction leads to the formation of the first example of a Pd complex bearing an anionic phosphinooxazoline ligand, [[upper bond 1 start]Pd(dmba){Ph₂PCH[horiz bar, triple dot above][lower bond 1 start]C([horiz bar, triple dot above]N[upper bond 1 end])CH₂CH₂O[lower bond 1 end]}], abreviated [Pd(dmba)(PCH-oxazoline)] (7), the anionic charge resulting from the monodeprotonation of the PCH₂ group. Following this methodology, the Ru complex [[upper bond 1 start]RuCl(p-cymene){Ph₂PCH[horiz bar, triple dot above][lower bond 1 start]C([horiz bar, triple dot above]N)CH₂CH₂O[lower bond 1 end]}], abreviated [RuCl(η⁶-p-cymene)(PCH-oxazoline)] (8) and containing this four-electron chelating anionic ligand, was prepared and shown to be more reactive for the catalytic transfer hydrogenation of acetophenone in propan-2-ol than the analogous complex bearing the neutral phosphinooxazoline ligand PCH₂-oxazoline. The crystal structure of cis-[[upper bond 1 start]Pd{Ph₂PCH[horiz bar, triple dot above][lower bond 1 start]C([horiz bar, triple dot above]N[upper bond 1 end])CH₂CH₂O[lower bond 1 end]}₂], abbreviated cis-[Pd(PCH-oxazoline)₂] (6), was determined by X-ray diffraction.

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Oxazolines are versatile synthons for ligand design^1,2 and readily available in enantiomerically pure forms, which accounts for their increasing use in asymmetric reactions catalysed by transition metal complexes.^2–5 Nevertheless, reports on anionic ligands incorporating an oxazoline ring are not legion compared to their neutral analogs and such ligands may be classified according to two main families. In class A compounds, the anionic charge is carried by an external heteroatom (C⁻, O⁻, S⁻) and the nitrogen atom of the oxazoline remains a neutral donor. This includes ligands such as bis(oxazolinyl)phenyl,^6–8 (hydroxyphenyl)oxazoline,^9–15 (hydroxyferrocenyl)oxazoline^16,17 and (mercaptoaryl)oxazoline,¹⁷ which upon deprotonation allowed the preparation of complexes with metals such as Ti, Zr, Hf, V, Mn, Ni, Pd, Pt or Cu. An oxazoline moiety was recently combined to a pyrrole for use in copper-catalysed enantioselective cyclopropanation reactions; the NH group is expected to be deprotonated upon in situ reaction with Cu(I) triflate.¹⁸ Some of the isolated complexes were found to be catalysts for reactions such as ethylene polymerisation,⁹ epoxydation,^11b alkyl- and phenylzincation of aldehydes,¹⁶ and conjugate addition to ketones.¹⁷

In the second class of ligands, B, the anionic charge is largely delocalised onto the nitrogen atom of the oxazoline but only one such ligand type has been reported, as illustrated in eqn. (1) . The corresponding complexes were obtained by first deprotonation of the acidic CH₂ of the bis(oxazoline) ligand at low temperature and subsequent quenching with a metal salt. This strategy allowed the development of efficient Ti,^19,20 Cu,^21,22 Rh,²³ Zn^24,25 or Mg²⁶ catalysts for several asymmetric reactions such as reduction of ketones,²⁰ cyclopropanation of olefins,^21,22 allylzincation,^24,25 and hydrocyanation of aldehydes.²⁶ The precatalysts are usually prepared in situ and not isolated (Zn, Mg, Ti) and only three crystal structures have been determined by X-ray diffraction, for the Cu,²⁷ Rh ²³ and Y ²⁸ complexes.

We have previously reported the synthesis of Pd and Ru complexes of type II bearing the neutral P,N-type ligand (oxazolinylmethyl)diphenylphosphine I (abreviated PCH₂-oxazoline) [eqn. (2)] and studied their catalytic properties for ethylene/CO copolymerization and asymmetric transfer hydrogenation of ketones, respectively.^29,30

This provided an extension to work carried out by several groups, including ours, on bifunctional phosphorus ligands and more specifically on phosphorus/oxygen P,O-type ligands III [eqn. (3)].^2,31–37

The often encountered hemilabile character of such neutral ligands in complexes of type IV allows for the temporary pro tection and facile generation of reactive sites in the coordination sphere of their complexes and several of them have been used in homogeneous catalysis. An additional feature of these P,O-type ligands is that the acidity of their PCH₂ protons allows facile deprotonation of this group to form an anionic 4-electron donor phosphino enolate ligand, with loss of the hemilabile behaviour. These strong chelates confer special reactivity to their complexes V, as found, for example, in the reversible CO₂ fixation by palladium complexes,^38,39 the rhodium-catalysed transfer dehydrogenation of cyclooctane⁴⁰ and the highly selective nickel-catalysed conversion of ethylene into linear α-olefins.^41–47

This dramatic change in reactivity observed when going from neutral to anionic P,O-type ligands triggered our interest for new anionic P,N-type ligands derived from I, in which the acidity of the PCH₂ protons should also allow easy deprotonation, particularly after coordination to a metal centre. Here we report the synthesis of such complexes containing a phosphine moiety covalently linked to an oxazoline, whose nitrogen atom formally carries the partly delocalised negative charge, thus resulting in the formation of a covalent rather than dative N–M bond, as depicted in VI. The corresponding ligand, abreviated PCH-oxazoline, is the first representative of a new class of hybrid ligands, which combines some of the features of B and V type systems. In addition to the synthesis of palladium(II) complexes, we present a comparison of the catalytic activity of ruthenium(II) complexes containing PCH₂-oxazoline (type II) or PCH-oxazoline (type VI) ligands for transfer hydrogenation of acetophenone in propan-2-ol. A preliminary communication on related systems where the oxazoline carries two methyl groups on the carbon α to nitrogen has appeared.⁴⁸

Results and discussion

Complexes with the neutral PCH₂-oxazoline ligand I

Mixing 2 equiv. of PCH₂-oxazoline with 1 equiv. of [Pd(NCMe)₄](BF₄)₂ in CH₂Cl₂ [eqn. (4)] led to the formation of [Pd(PCH₂-oxazoline)₂](BF₄)₂ (1) in high yield (95%).

The spectroscopic data (Table 1) are consistent with the coordination of the ligand in a static bidentate fashion.²⁹ The cis arrangement of the P atoms is deduced from the ³¹P{¹H} NMR chemical shift and from the doublet observed in the ¹H NMR spectrum for the PCH₂ protons. Such an arrangement for two atoms of large trans influence is thermodynamically favoured.^49,50 Moreover, the related Ni complex [Ni(phosphinoaryloxazoline)₂]²⁺ has been recently shown to have a cis structure, both in solution and in the solid state.⁵¹

Table 1 Selected NMR and IR data

NMR^a			IR/cm^−1 (CH2Cl2)
Complex	^1H^b	^13C{^1H}^c (^1JPC/Hz)	^31P{^1H}^d	ν(C[double bond, length half m-dash]N)	ν(C[horiz bar, triple dot above]C[horiz bar, triple dot above]N)
a In CD2Cl2, unless otherwise stated.b 300.13 MHz.c 75.4 MHz.d 121.5 MHz.e In CDCl3.
cis-[Pd(PCH2-oxazoline)2](BF4)2, 1	3.75 (d) PCH2^e	—	29.7^e	1633	—
[Pd(dmba)Cl(PCH2-oxazoline)], 2	3.65 (d) PCH2^e	31.5 (25.2)	32.6^e	1660, 1631	—
Pd(dmba)(PCH2-oxazoline)](PF6), 3	3.45 (d) PCH2^e	—	32.8^e	1635	—
[RuCl(η^6-p-cymene)(PCH2-oxazoline)]Cl, 4	3.30 (dd), 3.40 (dd), PCH2	30.5 (34.0)	50.5	1641	—
cis-[Pd(PCH-oxazoline)2], 6	2.60 (s) PCH	41.2 (78.5)	20.5	—	1538
[Pd(dmba)(PCH-oxazoline)], 7	2.82 (s) PCH	40.7 (82.5)	24.4	—	1537
[RuCl(η^6-p-cymene)(PCH-oxazoline)], 8	3.10 (s) PCH	37.0 (78.8)	40.5	—	1546

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Reaction of PCH₂-oxazoline with [Pd(dmba)(μ-Cl)]₂ afforded 2[italic v]ia opening of the chloride bridges [eqn. (5)].

The IR spectrum of 2 in CH₂Cl₂ shows two bands at 1660 and 1631 cm⁻¹, assigned to the C[double bond, length half m-dash]N vibrations of the uncoordinated and coordinated oxazolines, respectively, by analogy with the values for the free ligand (1660 cm⁻¹)²⁹ and for 1. For comparison, we prepared the PF₆ analogue of 2 by carrying out the reaction in the presence of NH₄PF₆ (see Experimental). As expected, its IR spectrum in CH₂Cl₂ exhibits only one band at 1635 cm⁻¹. Therefore, the existence of coordinated and uncoordinated oxazolines in solutions of 1 suggests competition between the oxazoline nitrogen and the chloride for coordination to palladium. Comparison of the ³¹P{¹H} and ¹H NMR spectra with those of the free ligand confirms that PCH₂-oxazoline forms a P,N-chelate, but the broad resonances of the oxazoline protons are again consistent with a hemilabile behaviour of the ligand through reversible oxazoline nitrogen (de)coordination. For the first time, this ligand is shown to display a hemilabile behaviour.²

Reaction of 2 equiv. of PCH₂-oxazoline with 1 equiv. of [Ru(μ-Cl)Cl(η⁶-p-cymene)]₂ in refluxing EtOH [eqn. (6)] led to the isolation of [RuCl(η⁶-p-cymene)(PCH₂-oxazoline)]Cl (4) in good yield (89%).

In contrast to 2, which also has Cl⁻ as a counter ion, the IR spectrum of 4 exhibits only one C[double bond, length half m-dash]N vibration band at 1641 cm⁻¹, which corresponds to a coordinated oxazoline. The ¹H NMR spectrum shows that the protons of the PCH₂-oxazoline ligand appear at six distinct resonances and are thus diastereotopic. This is consistent with the ruthenium centre becoming a stereogenic centre upon chelation of PCH₂-oxazoline. The PCH₂ protons exhibit an ABX spin system with two different ²J_PH coupling constants (8.5 and 13.0 Hz). The analogous complex, [RuCl(η⁶-C₆H₆)(PCH₂-oxazoline)](O₃SCF₃) (5), was prepared for catalytic purposes in order to allow comparison with related complexes containing a tridentate NPN ligand (see below). Its spectroscopic properties are very similar to those of 4 (see Experimental).

These cationic PCH₂-oxazoline complexes were mainly used as precursors for the synthesis of complexes containing the anionic PCH-oxazoline ligand (see below).

Complexes with the anionic PCH-oxazoline ligand

The deprotonation of the PCH₂ group in complexes 1, 2 and 4 in THF was achieved in 78–86% yields, by reaction with Bu^tOK (Scheme 1).⁴⁸ This simple and efficient methodology has been reported before with β-ketophosphines of the type R₂¹PCH₂C(O)R² coordinated to Re,⁵² Fe,⁵³ Ru,^54–56 Rh,^40,57 Pd,⁵⁸ or Pt,⁵⁹ using various bases.

Scheme 1

In all cases, the typical ν(C[double bond, length half m-dash]N) IR absorption band of the coordinated oxazoline in the precursor complex is replaced by a strong absorption in the range 1537–1546 cm⁻¹ (see Table 1). This is consistent with the IR data of a Cu complex described by Lehn et al., which shows an absorption band at 1530 cm⁻¹.²⁷ The shift towards lower energy by about 100 cm⁻¹ on going from PCH₂-oxazoline to anionic PCH-oxazoline is consistent with previous observations made with P,O-ligands when going from IV to V. The PCH carbon appears in the ¹³C{¹H} NMR spectrum as a doublet downfield shifted (6.5–9 ppm) compared to the corresponding PCH₂ and exhibits a larger ¹J_PC coupling constant, 78.5–82.5 Hz [italic v]s. 25.2–34.0 Hz, respectively. This increase is attributed to the change in hybridisation of the carbon atom α to P from sp³ in the neutral chelate to sp² in the anionic one. While the PCH₂ protons of complexes 1–5 exhibit in the ¹H NMR spectrum a doublet, the PCH proton of 6–8 exhibits only a singlet. Complexes 6–8 are air-stable and, in contrast to their precursors, they react with CDCl₃ to give uncharacterised side products. An X-ray diffraction study on single crystals of 6 confirmed that the two phosphorus atoms occupy cis positions (Fig. 1 and Table 2). The coordination around the metal centre approximates a square planar geometry with a slight displacement of the Pd–N(2) vector from the plane defined by the two phosphorus, two nitrogen and palladium atoms [0.150(3) Å]. The non-ideal geometry is also reflected by the angles P(1)–Pd–P(2) and N(1)–Pd(1)–N(2) of 102.16(4)° and 94.40(1)°, respectively. The two planes defining the backbone of the P,N-chelates, Pd(1), P(1), N(1), C(1), C(2) and Pd(1), P(2), N(2), C(17), C(18), form a dihedral angle of 1.9°. However, the angle between the two planes O(1), N(1), C(2), C(3), C(4) and O(2), N(2), C(18), C(19), C(20), corresponding to the two oxazolines, form a dihedral angle of 28.8°, which results in a staggered arrangement for the NCH₂ protons. It is interesting to compare the distances and angles of PCHox in 6 with those of the neutral PCH₂-oxazoline-Me₂ ligand in [PdCl₂(PCH₂-oxazoline-Me₂)] where PCH₂-oxazoline-Me₂ is similar to PCH₂-oxazoline but has two methyl substituents on the carbon α to the nitrogen atom.²⁹ Of special interest are the bond lengths and angles within the five-membered ring chelates. The C–C bond distance decreases on going from the neutral to the anionic ligand [1.480(4) to 1.370(5) Å, respectively] and this latter value is close to the average length of a C[double bond, length half m-dash]C (sp²–sp²) bond (1.34 Å). At the same time, the C–N bond length increases from 1.276(4) to 1.335(4) Å, which is in favour of a single C–N bond [average d(C_sp² –N) = 1.36 Å]. Even though it is likely that electron delocalisation occurs within the chelate ring, we prefer to describe the bonding within the five-membered P,N-chelate as involving a pronounced C[double bond, length half m-dash]C double bond character. The X-ray crystal structure of 6 is significant since there are only three reported X-ray crystal structures of an anionic oxazoline in a class B complex (M = Cu,²⁷ Rh,²³ Y²⁸) although this bonding mode is often invoked, and the only example with a PCH-oxazoline-type ligand.⁴⁸

Fig. 1 ORTEP view of the crystal structure of cis-[Pd(PCH-oxazoline)₂] (6).

Table 2 Selected bond lengths (Å) and angles (°) for cis-[Pd(PCH-oxazoline)₂], (6)

Pd(1)–P(1)	2.286(1)	N(1)–C(2)	1.335(4)
Pd(1)–P(2)	2.293(1)	C(2)–C(1)	1.370(5)
Pd(1)–N(1)	2.074(3)	C(1)–P(1)	1.737(4)
Pd(1)–N(2)	2.073(3)	C(2)–O(1)	1.380(4)
N(1)–Pd(1)–N(2)	94.40(11)	N(1)–C(2)–C(1)	125.6(3)
N(2)–Pd(1)–P(2)	81.42(8)	C(2)–C(1)–P(1)	112.1(3)
P(2)–Pd(1)–P(1)	102.16(4)	C(1)–P(1)–Pd(1)	102.0(1)
P(1)–Pd(1)–N(1)	82.20(8)	O(1)–C(2)–C(1)	121.3(3)
Pd(1)–N(1)–C(2)	116.2(2)	O(1)–C(2)–N(1)	113.1(3)

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Catalytic study

As part of our interest in the properties of multidentate ligands that contain both phosphine and oxazoline in a chelating array,² we have tested their ruthenium complexes in the catalytic transfer hydrogenation of ketones by propan-2-ol [eqn. (7)].^30,48,60,61 There is considerable current research activity around this reaction in order to both develop new classes of catalysts and to understand the different steps involved in the overall catalytic process.^62–67

We recently described a precursor catalyst for this reaction, [RuCl(η⁶-C₆H₆)(N,P-NPN)](O₃SCF₃) (9) where NPN is a bis(oxazoline)phosphine that forms a bidentate P,N-chelate.⁶⁰ It was shown that the dangling oxazoline arm of this ligand tends to have a detrimental effect on the catalytic activity, possibly owing to the formation of a bis-chelating system of type 10.⁶⁰ This is now further supported by the observation that complex 5 is more active than 9 and 10 (see Table 3).

Table 3 Transfer hydrogenation of acetophenone in propan-2-ol catalyzed by Ru(II) complexes^a

Catalyst	Time/h	Yield^b (%)	Turnover/h^−1
a The reactions were carried out in refluxing propan-2-ol using a 0.1 M substrate concentration and a 0.1 M solution of Pr^iONa as a base and a 1: 200:5 Ru–ketone–base ratio.b Chemical yields determined by GC analysis of the crude reaction mixture at the reported time.c Reaction carried out in propan-2-ol at 30°C.d Taken from ref. 60 with a 1:200:24 Ru–ketone–base ratio.
4	0.25	5	40
1	34	68
3	75	50
6	94	32
5	0.25	23	184
1	61	122
8	0.25	25	200
1	49	98
3	86	58
6	94	32
8^c	8	16	4
9^d	0.25	13.5	108
1	54	108
10^d	0.25	11	88
1	45	90

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In light of our synthetic route to prepare complexes bearing the anionic ligand, it is likely that under catalytic conditions, the PrⁱONa base, which is present in a 5-fold excess to Ru, reacts with the precursor catalyst 4 to form compound 8. To test this hypothesis we used 8 as the catalyst precursor. Whereas the conversion obtained after 6 h at reflux temperature in propan-2-ol was identical to that observed with 4 (94% yield), the turnover frequency (TOF) was higher with [RuCl(η⁶-p-cymene)(PCH-oxazoline)] than with [RuCl(η⁶-p-cymene)(PCH₂-oxazoline]Cl during the first hour (Table 3). This may correspond to an induction period to form the catalyst 8. Although it appears that 4 gives 8 under catalytic conditions, we cannot rule out that both 4 and 8 may be independently active catalysts for transfer hydrogenation. Noyori et al. have recently published⁶⁷ the structure of a preformed catalyst precursor 11, which upon in situ deprotonation of the NH group generates the true catalyst 12 which is to date among the best catalysts for transfer hydrogenation of ketones.^68,69 These results emphasise the spectacular conse quences that ligand deprotonation may have on the catalytic properties of the metal complex.

Compound [RuCl(η⁶-p-cymene)(PCH-oxazoline)] (8) is very similar to 11 with the difference that the PCHox ligand in 8 cannot be further deprotonated to form species analogous to 12.^67a This could account for the higher reactivity of 11 compared to 8. Nevertheless, our results are encouraging and complexes analogous to [RuCl(η⁶-p-cymene)(PCH-oxazoline)] with a chiral version of PCH₂-oxazoline should be synthesised in the future. Furthermore, Noyori et al. have shown that the arene ligand has a strong influence on the reactivity and enantioselectivity of the catalysts, suggesting that complexes containing other arene ligands such as benzene or 1,3,5-trimethylbenzene should also be evaluated.⁷⁰

Conclusion

The versatility of the PCH₂-oxazoline ligand allows its coordination as hemilabile or static neutral P,N- or static anionic P,N-type ligand. It was shown that in the latter case its structural and electronic properties induce higher reactivity in the catalytic transfer hydrogenation of acetophenone in propan-2-ol than its neutral analogue. This finding should lead to further investigations with chiral versions of PCH-oxazoline in order to study their influence on the enantioselectivity of this catalytic reaction. Also as an extension to this work, it would be interesting to evaluate our Ru-phosphinooxazoline complexes in Diels–Alder reactions, since a [Ru(H₂O)(η⁶-p-cymene)-phosphinoaryloxazoline]²⁺ complex has been recently reported to be a good catalyst for such reactions.⁷¹

Experimental

General procedures

All reactions were performed under purified nitrogen. Solvents were purified and dried under nitrogen by conventional methods. The ¹H, ¹³C and ³¹P{¹H} NMR spectra were recorded at 300.13, 75.4 and 121.5 MHz, respectively, on an FT Bruker AC300 instrument, ¹H{³¹P} NMR at 500.13 MHz on an FT Bruker ARX500 instrument, chemical shifts are positive downfield from external TMS for ¹H and ¹³C, and from H₃PO₄ (85% in H₂O) for ³¹P. IR spectra were recorded in the 4000–400 cm⁻¹ range on a Bruker IFS66 FT spectrometer.

Syntheses

The compounds [Pd(dmba)(μ-Cl)]₂ ,⁷² [Ru(μ-Cl)Cl(η⁶-p-cymene)]₂,⁷³ and [Pd(NCMe)₄](BF₄)₂⁷⁴ were prepared according to literature procedures.

cis-[Pd(PCH₂-oxazoline)₂](BF₄)₂ (1). In a 100 mL Schlenk flask were placed together the ligand PCH₂-oxazoline (0.715 g, 2.660 mmol) and [Pd(NCMe)₄](BF₄)₂ (0.590 g, 1.330 mmol) in CH₂Cl₂ (40 mL). The pale yellow solution was stirred for 12 h and the volume reduced under vacuum to about 5 mL. Addition of pentane afforded a white solid, which was further washed with Et₂O (2 × 10 mL) and dried in [italic v]acuo. The product was obtained as a white solid (1.035 g, yield 95%). ¹H NMR (CDCl₃ , 300.13 MHz): δ 3.75 (d, 4 H, ²J_PH  = 11.9 Hz, PCH₂), 4.40 (t, 4 H, ³J_HH  = 9.5 Hz, NCH₂), 4.80 (t, 4 H, ³J_HH  = 9.5 Hz, OCH₂), 7.30–7.65 (m, 20 H, aryl). ³¹P{¹H} NMR (CDCl₃, 121.5 MHz): δ 29.7. Anal. calc. for C₃₂H₃₂B₂F₈N₂O₂P₂Pd: C, 46.95; H, 3.94; Found: C, 47.22; H, 3.68%.

[Pd(dmba)(PCH₂-oxazoline)]Cl (2). In a 100 mL Schlenk flask were placed together the phosphine oxazoline ligand PCH₂-oxazoline (1.095 g, 4.070 mmol) and [Pd(dmba)(μ-Cl)]₂ (1.120 g, 2.03 mmol) in CH₂Cl₂ (30 mL). The pale yellow solution was stirred for 1 h after which the volume was reduced under vacuum to about 3 mL. Addition of hexane led to the precipitation of a colourless oil, which was triturated with Et₂O (2 × 5 mL); the solid thus obtained was washed with hexane (2 × 30 mL) and dried under vacuum. The product was obtained as a white powder (1.995 g, 90%). ¹H NMR (CDCl₃, 300.13 MHz): δ 2.90 [s, 6 H, N(CH₃)₂], 3.65 (d, 2 H, ²J_PH  = 8.4 Hz, PCH₂), 4.05 [overlapping s and m, 4 H, (CH₃)₂NCH₂ and oxazoline NCH₂], 4.45 (t, 2 H, ³J_HH  = 9.4 Hz, OCH₂), 6.30–7.00 (m, 4 H, aryl from dmba), 7.40–7.90 (m, 10 H, aryl). ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 31.5 (d, ¹J_PC  = 25.2 Hz, PCH₂), 51.0 [s, N(CH₃)₂], 54.7 (s, NCH₂), 69.5 [s, (CH₃)₂NCH₂], 73.0 (s, OCH₂), 123.2–149.5 (m, aryl), 167.7 (d, ²J_PC  = 10.9 Hz, C[double bond, length half m-dash]N). ³¹P{¹H} NMR (CDCl₃ , 121.5 MHz): δ 32.6. Anal. calc. for C₂₅H₂₈ClN₂OPPd: C, 55.06; H, 5.18; N, 5.14. Found: C, 55.20; H, 5.17; N, 5.04%.

[Pd(dmba)(PCH₂-oxazoline)](PF₆) (3). In a 100 mL Schlenk flask were placed together [Pd(dmba)(PCH₂-oxazoline)]Cl (2) (0.400 g, 0.737 mmol) and NH₄PF₆ (0.122, 0.740 mmol) in CH₂Cl₂ (30 mL). The pale yellow solution was stirred for 2 h after which the solution was filtered through Celite and its volume reduced under vacuum to about 2 mL. Addition of pentane led to the precipitation of a pale yellow solid, which was further washed with Et₂O (2 × 10 mL) and dried in [italic v]acuo. The product was obtained as a pale yellow solid (0.438 g, 91%). ¹H NMR (CDCl₃, 300.13 MHz): δ 2.90 [s, 6 H, N(CH₃)₂], 3.45 (d, 2 H, ²J_PH  = 10.5 Hz, PCH₂), 4.00 [s, 2 H, (CH₃)₂NCH₂], 4.05 (t, 2 H, ³J_HH  = 11.1 Hz, oxazoline NCH₂), 4.60 (t, 2 H, ³J_HH  = 11.1 Hz, OCH₂), 6.40–7.00 (m, 4 H, aryl from dmba), 7.40–7.90 (m, 10 H, aryl). ³¹P{¹H} NMR (CDCl₃, 121.5 MHz): δ 32.8. Anal. calc. for C₂₅H₂₈N₂OP₂Pd: C, 45.85; H, 4.31. Found: C, 45.61; H, 4.22%.

[RuCl(η⁶-p-cymene)(PCH₂-oxazoline)]Cl (4). In a 100 mL Schlenk flask fitted with a reflux condenser were placed together the ligand PCH₂-oxazoline (0.138 g, 0.513 mmol) and [Ru(μ-Cl)Cl(η⁶-p-cymene)]₂ (0.157 g, 0.256 mmol) in EtOH (15 mL). The yellow solution was heated to reflux temperature for 1.5 h, then cooled to room temperature and the solvent was removed under vacuum. The solid was partially dissolved in THF (2 mL) and precipitated by addition of pentane (15 mL). This procedure was repeated twice and afforded the product as a yellow powder (0.260 g, yield: 89%). ¹H NMR (CD₂Cl₂ , 300.13 MHz): δ 1.15 [d, 3 H, ²J_HH  = 6.8 Hz, CH(CH₃)(CH₃)], 1.25 [d, 3 H, ²J_HH  = 6.8 Hz, CH(CH₃)(CH₃)], 1.50 (s, 3 H, p-CH₃), 2.50 [sept, 1 H, ²J_HH  = 6.8 Hz, CH(CH₃)₂], ABX spin system δ_A 3.30 (dd, 1 H, ²J_HH  = 17 Hz, ²J_PH  = 8.5 Hz, PCH_AH), δ_B 3.40 (dd, 1 H, ²J_HH  = 17 Hz, ²J_PH  = 13.0 Hz, PCHH_B), 4.10 (dd, 1 H, ²J_HH  = 21.0 Hz, ³J_HH  = 10.4 Hz, OCHH), 4.75 (dd, 1 H, ²J_HH  = 14.2 Hz, ³J_HH  = 7.5 Hz, NCHH), 5.10 (dd, 1 H, ²J_HH  = 21.0 Hz, ³J_HH  = 10.4 Hz, OCHH), 5.20 (m, 1 H, NCHH), 5.60 (d, 1 H, ³J_HH  = 5.8 Hz, p-cymene), 5.70 (d, 1 H, ³J_HH  = 5.8 Hz, p-cymene), 6.15 (d, 1 H, ³J_HH  = 6.0 Hz, p-cymene), 6.25 (d, 1 H, ³J_HH  = 6.0 Hz, p-cymene), 7.30 (m, 2 H, aryl), 7.40–7.70 (m, 8 H, aryl H). ¹³C{¹H} NMR (CD₂Cl₂, 75.4 MHz): δ 17.4 (s, p-CH₃), 21.8 [s, CH(CH₃)(CH₃)], 22.8 [s, CH(CH₃)(CH₃)], 30.5 (d, ¹J_PC  = 34 Hz, PCH₂), 31.2 [s, CH(CH₃)₂], 58.6 (s, NCH₂), 73.5 (s, OCH₂), 84.7 (s, aromatic p-cymene), 87.3 (d, J_PC  = 6.6 Hz, aromatic p-cymene), 89.3 (d, ²J_PC  = 6.9 Hz, aromatic p-cymene), 101.4 (s, aromatic p-cymene), 117.1 (d, J_PC  = 4.7 Hz, aromatic p-cymene), 126.2–135.5 (m, aryl), 175.6 (d, ²J_PC  = 16.9 Hz, C[double bond, length half m-dash]N). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 50.5. Anal. calc. for C₂₆H₃₀NOPCl₂Ru: C, 54.20; H, 5.21; N, 2.43. Found: C, 54.17; H, 5.31; N, 2.27%.

[RuCl(η⁶-benzene)(PCH₂-oxazoline)](O₃SCF₃) (5). In a 100 mL Schlenk flask were placed together the ligand PCH₂-oxazoline (0.210 g, 0.78 mmol) and [Ru(μ-Cl)Cl(η⁶-benzene)]₂ (0.195 g, 0.39 mmol) in CH₂Cl₂ (15 mL). The dark orange solution obtained was stirred for 1 h at room temperature and then filtered through a canula fitted with glass fiber paper. The resulting orange solution was evaporated to about 1 mL and an orange precipitate was obtained by addition of hexane. The orange solid was further washed with 2 × 10 mL of hexane. After drying under vacuum for 2 h, solid Ag(O₃SCF₃) (0.200 g, 0.78 mmol) and 20 mL of CH₂Cl₂ were added. After a few minutes, a pale yellow suspension appeared and the reaction mixture was stirred for 1 h. The suspension was filtered over Celite and the volume of the orange filtrate was reduced under vacuum to ca. 2 mL. Addition of Et₂O afforded a yellow solid, which was further washed with 10 mL of hexane and Et₂O. Pure 5 was obtained by crystallisation from 1:3 CH₂Cl₂–hexane (0.375 g, 75%). ¹H NMR (CDCl₃, 300.13 MHz): ABX spin system δ_A 3.25 (dd, 1 H, ²J_HH  = 17.5 Hz, ²J_PH  = 8.5 Hz, PCH_AH), δ_B 3.40 (dd, 1 H, ²J_HH  = 17.5 Hz, ²J_PH  = 12.5 Hz, PCHH_B), 4.15 (m with appearance of overlapping dt, 1 H, J_HH  = 10.2 Hz, J_HH  = 10.5 Hz, OCHH), 4.75 (m, 1 H, NCHH), 4.90 (m, NCHH), 5.20 (m with appearance of overlapping dt, 1 H, J_HH  = 8.5 Hz, J_HH  = 9.6 Hz, OCHH), 5.80 (s, 6 H, η⁶-benzene), 7.30 (m, 2 H, aryl), 7.50–7.80 (m, 8 H, aryl H). ³¹P{¹H} NMR (CDCl₃, 121.5 MHz): δ 48.4. Anal. calc. for C₂₄H₂₆ClF₃NO₄PSRu: C, 44.38; H, 4.04; N, 2.16. Found: C, 44.49; H, 3.95; N, 2.27%.

cis-[Pd(PCH-oxazoline)₂] (6). In a 100 mL Schlenk flask were placed together cis-[Pd(PCH₂-oxazoline)₂](BF₄)₂ (1.005 g, 1.228 mmol) and Bu^tOK (0.305 g, 2.700 mmol) in THF (30 mL). The colourless suspension turned red–orange within 15 min with formation of a white precipitate; the reaction mixture was then stirred for 12 h. The solvent was removed under reduced pressure and CH₂Cl₂ (30 mL) was added. The white precipitate was filtered off by means of a canula fitted with glass fiber filter paper. The volume of the solution was reduced under vacuum to about 4 mL; addition of Et₂O afforded an orange solid, which was further washed with Et₂O (2 × 20 ml) and dried in [italic v]acuo. The product was obtained as an orange powder (0.616 g, yield 78%). ¹H NMR (CD₂Cl₂, 300.13 MHz): δ 2.60 (s, 1 H, PCH), 3.90 (t, 2 H, ³J_HH  = 7.9 Hz, NCH₂), 4.35 (t, 2 H, ³J_HH  = 7.9 Hz, OCH₂), 7.00–7.40 (m, 20 H, aryl). ¹³C{¹H} NMR (CD₂Cl₂ , 75.4 MHz): δ 41.2 (d, ¹J_PC  = 78.5 Hz, PCH), 52.7 (s, NCH₂), 70.1 (s, OCH₂), 126.5–135.0 (m, aryl), 180.5 (d, C[double bond, length half m-dash]N). ³¹P{¹H} NMR (CD₂Cl₂, 121.5 MHz): δ 20.5. Anal. calc. for C₃₂H₃₀N₂O₂P₂Pd: C, 59.78; H, 4.70; N, 4.36. Found: C, 60.04; H, 4.62; N, 4.18%.

[Pd(dmba)(PCH-oxazoline)] (7). In a 100 mL Schlenk flask were placed together complex [Pd(dmba)Cl(PCH₂-oxazoline)] (1.020 g, 1.875 mmol) and Bu^tOK (0.220 g, 1.970 mmol) in THF (20 mL). The colourless suspension turned yellow within 5 min with formation of a white precipitate; the reaction mixture was stirred then for 12 h. The solvent was removed under reduced pressure and CH₂Cl₂ (30 mL) was added. The white precipitate was filtered off by means of a canula fitted with glass fiber filter paper. The volume of the solution was reduced under vacuum to about 4 mL; addition of pentane afforded a yellow solid, which was further washed with hexane (2 × 20 mL) and dried in [italic v]acuo. The product was obtained as a yellow powder (0.800 g, yield 84%). ¹H NMR (CD₂Cl₂, 300.13 MHz): δ 2.80 [s, 6 H, N(CH₃)₂], 2.82 (s, 1 H, PCH), 3.70 (t, 2 H, ³J_HH  = 7.9 Hz, oxazoline NCH₂), 3.95 [s, 2 H, (CH₃)₂NCH₂], 4.30 (t, 2 H, ³J_HH  = 7.9 Hz, OCH₂), 6.55–7.00 (m, 4 H, aryl from dmba), 7.40–7.70 (m, 10 H, aryl). ¹³C{¹H} (CD₂Cl₂, 75.4 MHz): δ 40.7 (d, ¹J_PC  = 82.5 Hz, PCH), 50.9 [s, N(CH₃)₂], 52.2 (s, oxazoline NCH₂), 69.3 [s, (CH₃)₂NCH₂], 73.3 (s, OCH₂), 122.8–149.5 (m, aryl). ³¹P{¹H} NMR (CDCl₃, 121.5 MHz): δ 24.4. Anal. calc. for C₂₅H₂₇N₂OPPd: C, 59.01; H, 5.35; N, 5.50. Found: C, 58.79; H, 5.22; N, 5.46%.

[RuCl(η⁶-p-cymene)(PCH-oxazoline)] (8). In a 100 mL Schlenk flask were placed together [RuCl(η⁶-p-cymene)-(PCH₂-oxazoline)]Cl (0.440 g, 0.765 mmol) and Bu^tOK (0.090 g, 0.800 mmol) in THF (20 mL). The yellow solution turned orange within 5 min with formation of a white precipitate; the reaction mixture was then stirred for 12 h. The solvent was removed under reduced pressure and CH₂Cl₂ (30 mL) was added. The white precipitate was filtered off by means of a canula fitted with glass fiber filter paper. The volume of the deep orange solution was reduced under vacuum to about 4 mL; addition of Et₂O afforded a brown–orange solid, which was further washed with Et₂O (2 × 20 ml) and dried in [italic v]acuo. The product was isolated as a brown–orange powder (0.355 g, yield: 86%). ¹H NMR (CD₂Cl₂, 500.13 MHz): δ 0.95 [d, 3 H, ²J_HH  = 6.9 Hz, CH(CH₃)(CH₃)], 1.10 [d, 3 H, ²J_HH  = 6.9 Hz, CH(CH₃)(CH₃)], 2.10 (s, 3 H, p-CH₃), 2.20 [sept, 1 H, ²J_HH  = 6.9 Hz, CH(CH₃)₂], 3.10 (s, 1 H, PCH), 3.70 (m with appearance of t of d, ²J_HH  = 9.0 Hz, ³J_HHcis  = 9.0 Hz, ³J_HHtrans  = 3.6 Hz, OCHH), 3.85 (m with appearance of overlapping dt, 1 H, J_HH  = 10.2 Hz, J_HH  = 9.0 Hz, NCHH), 4.25 (m with appearance of overlapping dt, 1 H, J_HH  = 10.2 Hz, J_HH  = 7.8 Hz, OCHH), 4.25 (overlapping m, 1 H, J_HH  = 1.2 Hz, p-cymene), 4.35 (m with appearance of t of d of d, ²J_HH  = 7.8 Hz, ³J_HHcis  = 7.8 Hz, ³J_HHtrans  = 3.6 Hz, ⁵J_HH  = 1.2 Hz, NCHH), 5.20 (d, 1 H, ³J_HH  = 6.2 Hz, p-cymene), 5.40 (d, 1 H, ³J_HH  = 5.7 Hz, p-cymene), 5.70 (d, 1 H, ³J_HH  = 6.2 Hz, p-cymene), 7.30–7.40 (m, 6 H, aryl), 7.50–7.70 (m, 4 H, aryl H). ¹³C{¹H} NMR (CD₂Cl₂ , 75.4 MHz): δ 18.4 (s, CH₃), 22.0 [s, CH(CH₃)(CH₃)], 23.0 [s, CH(CH₃)(CH₃)], 31.4 [s, CH(CH₃)₂], 37.0 (d, ¹J_PC  = 78.8 Hz, PCH), 55.4 (s, NCH₂), 69.3 (s, OCH₂), 83.5 (s, Prⁱ–C···CH from p-cymene), 84.5 (s, Me–C···CH from p-cymene), 88.6 (d, J_PC  = 5.9 Hz, Prⁱ–C···CH from p-cymene), 93.3 (d, J_PC  = 5.9 Hz, Me–C···CH from p-cymene), 97.4 (s, quat C p-cymene), 103.1 (s, quat C p-cymene), 126.8–142.2 (m, aryl), 179.3 (d, ²J_PC  = 43.4 Hz, C[double bond, length half m-dash]N). ³¹P{¹H} NMR (CD₂Cl₂ , 121.5 MHz): δ 40.5. Anal. calc. for C₂₆H₂₉NOPRuCl: C, 57.88; H, 5.38; N, 2.59. Found: C, 57.65; H, 5.29; N, 2.56%.

X-Ray structural analysis⁷⁵

The intensity data were collected on a Rigaku/ADSC CCD diffractometer at 180(1) K in 0.50° oscillations with 35.0 s exposures. The structure was solved by heavy-atom Patterson methods and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were fixed in calculated positions with C–H = 0.98 Å. Selected data are given in Table 4.

Table 4 Selected crystallographic data for complex cis-[Pd(PCH-oxazoline)₂], 6

a R  =  Σhkl(‖Fobs∣ − ∣Fcalc‖)/Σhkl∣Fobs ∣; Rw  =  [Σhklw(∣Fobs∣ − ∣Fcalc∣)^2/ΣhklwFobs^2]^1/2, w  = 1/σ^2(Fobs).
Formula	C32H30N2O2P2Pd
FW	642.95
Crystal system	Monoclinic
Space group	P21/n (no. 14)
T/K	180(1)
a/Å	17.293(2)
b/Å	9.7528(13)
c/Å	17.6784(6)
β/°	115.2840
U/Å^3	2695.9(4)
Z	4
μ/mm^−1	8.42
No. of reflections collected	24477
No. of unique reflections	6934
No. of variable parameters	352
No. of reflections [I>3σ(I)]	4082
Residuals (R, Rw)^a	0.037, 0.031

---

CCDC reference number 440/211. See http://www.rsc.org/suppdata/nj/b0/b004786o/ for crystallographic files in cif format.

Acknowledgements

We are grateful to Prof. M. D. Fryzuk (Vancouver) for his hospitality toward F.N. and to the Centre National de la Recherche Scientifique (Paris) and the Ministère de l'Enseignement Supérieur de la Recherche et de la Technologie (Paris) for financial support and a PhD grant to F.N.

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Footnotes

† Part of the Doctoral Thesis of F. N.

‡ Deceased, October 27th 1998.

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