[RuCl₂(η⁶-p-cymene)] complexes bearing phosphinous acid ligands: preparation, application in C–H bond functionalization and mechanistic investigations

Abstract

A series of [RuCl₂(η⁶-p-cymene)] complexes bearing phosphinous acid (PA) ligands have been straightforwardly prepared from the dimer [RuCl₂(p-cymene)]₂ and secondary phosphine oxides (SPOs) and fully characterized. The steric parameter quantification of PAs, other L ligands and η⁶-p-cymene allowed a better comprehension of the coordination chemistry of these types of complexes and explained the absence of coordination in the case of bulky SPOs such as Ad₂P(O)H. These complexes were tested in the C–H activation/functionalization of 2-phenylpyridine and a good activity was obtained at 80 °C for the complex exhibiting the highest steric bulk. A study on halide effects, either on the ruthenium complex or for the aryl halide partner, has also been carried out showing drastic differences. Further investigations on halide effects were performed notably by using a cationic ruthenacycle which was found to be an intermediate for the reaction. In order to rationalize the role played by the phosphinous acid, a mechanism involving a concerted metallation deprotonation favored by a phosphinito species has been proposed.

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Introduction

Considered for a long time as a main challenge in organic synthesis, the C–H activation/functionalization has emerged recently as a powerful tool to prepare complex molecules starting from readily available raw materials.¹ In this quest, the use of transition metal catalysis is at the origin of groundbreaking discoveries and now various catalytic systems using palladium,² rhodium,³ ruthenium,⁴ copper,⁵ platinum,⁶ nickel,⁷ cobalt,⁸ f-block elements,⁹etc. are recognized as highly efficient for C–H bond activation. In these systems, major advances were accomplished by clear mechanistic studies which allowed for deeper understanding of the mechanism, notably the role of ligands and/or additives. For instance, Catellani developed a Pd-catalyzed regioselective C–H bond functionalization of arenes using norbornene as a covalent linker.¹⁰ Alternatively, it was demonstrated that C–H activation could occur according to a σ-bond metathesis pathway,¹¹ so called concerted metallation deprotonation (CMD). In such a process, the intermolecular abstraction of the proton by acetato ligands favors the metallacycle formation.¹²

Whereas pioneering studies on catalytic C(sp²)–H activation were carried out using Ru-based complexes,¹³ catalytic systems using Pd or Rh have attracted much attention over the last decade. Nevertheless, interesting Ru-based catalytic systems have been reported in the literature mainly using the dimer [RuCl₂(p-cymene)]₂ in association with carboxylate additives.^4,14 During our research involving secondary phosphine oxides (SPO) and phosphinous acids (PA) as ligands in transition metal catalysis,¹⁵ the catalytic system using [RuCl₂(p-cymene)]₂ and SPO developed by Ackermann in 2005 caught our attention (Scheme 1).¹⁶ A Ru(II) complex, prepared in situ from [RuCl₂(η⁶-p-cymene)]₂ and a sterically hindered diadamantylphosphine oxide, was found to be very efficient for ortho C–H bond activation of the 2-phenylpyridine 1 and coupling with chlorobenzene to yield quantitatively compound 3 without any traces of monoarylated 2. Moreover, this catalytic system showed a high efficiency for various substrates.¹⁷ We wondered about the role played by the SPO or its tautomeric phosphinous acid form in the C–H activation process. Whereas several mechanistic hypothesis have been postulated,^4,17c none of them was further investigated.

Scheme 1 C–H activation/functionalization mediated by [RuCl₂(p-cymene)]₂ and SPO.

Therefore, we wished to investigate the mechanism of this C–H bond activation/functionalization through the synthesis of well-defined [RuCl₂(η⁶-p-cymene)(PA)] complexes and to study their performances in catalysis. Control experiments have been carried out to corroborate the mechanistic proposal. The results of our study are reported herein.

Results and discussion

We started to study the coordination chemistry of SPOs with [RuCl₂(p-cymene)]₂. The synthesis of [RuCl₂(η⁶-arene)(PA)] has been previously reported in the literature,¹⁸ notably two complexes [Ru(η⁶-p-cymene)Cl₂(PA)] 5a and 5l were already described by the groups of Tyler¹⁹ and Leung²⁰ respectively (Fig. 1) and 5a has been successfully applied to nitrile hydratation.^19,21

Fig. 1 [RuCl₂(η⁶-p-cymene)(PA)] complexes described in the literature.

Following the same procedure, [RuCl₂(p-cymene)]₂ was treated with various secondary phosphine oxides 4 in THF at 25 °C to afford the expected complexes 5 (Table 1).²² Of note, in dichloromethane or toluene, the reaction was sluggish compared to THF. Excepted for Cy₂P(O)H 4b which required 24 h of reaction, other SPOs, either symmetrical ones (entries 2–4) or unsymmetrical ones (entries 7–11), were found very reactive. After only 2–3 h, complexes 5 were isolated in almost quantitative yields. Surprisingly, with bulky SPOs such as Ad₂P(O)H 4f or tBu₂P(O)H 4g, we were unable to obtain the corresponding complexes 5 (entries 5 and 6). Despite prolonged reaction times and/or heating to 110 °C, their formation could not be detected by NMR spectroscopy.

Table 1 Preparation of well-defined [RuCl₂(η⁶-p-cymene)(PA)] complexes^a

Entry	R^1	R^2	t (h)	Complex	Yield (%)
a Reaction conditions: [RuCl2(η^6-p-cymene)]2, SPO (2.2 equiv.), THF (0.5 M), 25 °C. b Reaction performed in 1,4-dioxane at 110 °C. NR = no reaction.
1	Cy	Cy	24	5b	89
2	Ph	Ph	2	5c	96
3	p-F-C6H4	p-F-C6H4	6	5d	84
4	3,5-Me2-C6H3	3,5-Me2-C6H3	3	5e	91
5^b	Ad	Ad	24	5f	NR
6^b	tBu	tBu	24	5g	NR
7	Me	Ph	3	5h	74
8	nBu	Ph	3	5i	53
9	Bn	Ph	3	5j	88
10	Cy	Ph	3	5k	98
11	tBu	Ph	3	5l	99

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The well-defined [RuCl₂(η⁶-p-cymene)(PA)] 5 were characterized by ¹H,²³¹³C, and ³¹P NMR spectroscopies, as well as mass spectrometry. The coordination of the phosphinous acid to the metallic center through the phosphorus atom was confirmed by NMR spectroscopy and the absence of ¹J(P,H) coupling. Moreover, ³¹P NMR spectroscopy showed new resonances between 105–130 ppm with a significant shift to a lower field compared to SPOs (Δδ between 68 and 89 ppm, Table 2).

Table 2 ³¹P{¹H} NMR spectral data for SPOs 4 and Ru-complexes 5 ^a

Entry	SPO	^31P{^1H} (ppm)	Complex 5	^31P{^1H} (ppm)
a Measured in CDCl3 at 25 °C.
1	4b	50.0	5b	129.3
2	4c	21.4	5c	107.1
3	4d	18.2	5d	105.6
4	4e	22.8	5e	107.0
5	4h	20.2	5h	109.5
6	4i	28.0	5i	113.1
7	4j	29.6	5j	109.4
8	4k	36.7	5k	114.2
9	4l	47.4	5l	115.5

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To establish unambiguously the structure of [RuCl₂(η⁶-p-cymene)(PA)] complexes 5, suitable crystals for single-crystal X-ray diffraction studies were obtained for most of the compounds. As depicted in Fig. 2, all complexes adopt a distorted octahedral structure with the expected three-legged pseudotetrahedral “piano-stool” geometry around the Ru atom and the η⁶-coordination of the p-cymene. Two chlorine atoms and the phosphorus occupy the other three positions. Bond lengths are similar to those reported for analogous complexes (Table 3); the Ru–Cl bond distances range from 2.3943(10) to 2.4395(7) Å, the Ru–P bond lengths from 2.3009(8) to 2.3680(7) Å and the longest distances have been observed with cyclohexyl and tert-butyl P-substituents (entries 1, 6 and 7). This seems to be the result of a higher steric congestion of PA ligands. The P–O bond lengths (all close to 1.60 Å) are consistent with a P–O single bond. The O⋯Cl distances ranging from 2.961(2) to 3.070(2) Å – except for 5h, 3.175(2) Å, entry 4 – indicate a hydrogen bonding with chlorines. Of note, with 5h, the hydrogen bonding takes place intermolecularly.

Fig. 2 Molecular structures of complexes 5b–d, 5h–i and 5k–l represented at 50% ellipsoid probability. Most of the H atoms have been omitted for clarity.

Table 3 Selected bond lengths (Å) for complexes [RuCl₂(p-cymene)(PA)] 5b–d, 5h–i and 5k–l

Entry	Complex	Ru(1)–Cl(1) (Å)	Ru(1)–Cl(2) (Å)	Ru(1)–P(1) (Å)	P(1)–O(1) (Å)	O⋯Cl (Å)	Ru(1)–Cavg (Å)
1	5b	2.4323(10)	2.3943(10)	2.3376(9)	1.600(3)	2.988(3)	2.203(4)
		2.4320(10)	2.4198(9)	2.3379(9)	1.602(3)	3.005(3)	2.214(4)
		2.4296(10)	2.4072(9)	2.3445(10)	1.612(3)	3.027(3)	2.215(4)
2	5c	2.4171(8)	2.4156(8)	2.3120(8)	1.602(2)	3.070(2)	2.216(3)
3	5d	2.4147(9)	2.3970(8)	2.3009(8)	1.605(2)	3.011(3)	2.212(3)
4	5h	2.4024(6)	2.4197(6)	2.3130(6)	1.6034(19)	3.175(2)	2.210(3)
5	5i	2.4295(11)	2.4032(11)	2.3017(10)	1.604(3)	2.988(3)	2.209(4)
6	5k	2.3966(8)	2.4389(9)	2.3342(8)	1.601(2)	3.009(2)	2.212(3)
7	5l	2.4395(7)	2.4124(7)	2.3680(7)	1.6098(19)	2.961(2)	2.214(3)

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In order to gain more insights into the coordination chemistry of [RuCl₂(p-cymene)(L)] and particularly to address the issue of the absence of reaction with bulky SPOs Ad₂P(O)H 4f or tBu₂P(O)H 4g, we quantified the steric parameter of phosphinous acids and other ligands L (phosphine, phosphite and N-heterocyclic carbene (NHC)) through the percentage buried volume (%V_bur).^24,25 Selected values have been gathered in Table 4.²³ The %V_bur for PA ligands span from 21.9 to 24.6%. These values are significantly lower than those calculated on AuCl(PA) complexes.^15g As expected Ph₂POH is more sterically demanding than Ph₂PH (entries 3 and 9) but less than Ph₂PCH₂OH, Ph₂PnBu, PPh₃ or P(OPh)₃ (entries 9–13). The highest %V_bur calculated in [RuCl₂(p-cymene)(L)] complexes is for the NHC IMes (1,3-dimesityl-imidazol-2-ylidene) (27.9%) which is in the lower range for this ligand (entry 14).²⁴ These low %V_bur values and their narrow range attest probably that the other ligands in the coordination sphere of the Ru are sterically demanding and the L ligand has therefore to minimize its size. The %V_bur of the η⁶-p-cymene has also been quantified in 24 X-ray structures and the size of this ligand was found to be relatively invariant with an average value of 47.5% and a standard deviation of 0.35%.²³ As intuitively expected, the η⁶-p-cymene occupies almost half of the Ru coordination sphere and does not show any structural flexibilities. Therefore, the coordination of sterically hindered ligands, such as Ad₂POH or tBu₂POH, seems unlikely. Indeed, calculations performed on other complexes shown that %V_bur of tBu₂POH and Ad₂POH are around 30.5 and 31.5, respectively.²³

Table 4 Percentage of buried volumes of various ligands L in [RuCl₂(η⁶-p-cymene)(L)] complexes^a

Entry	L	d(Ru–L) (Å)	%Vbur (L)
a Parameters used for SambVca calculations: 3.50 Å for the sphere radius, exact distances between the ligand and the metal were considered, hydrogen atoms were omitted and bond radii scaled by 1.17.
1	Me2POH	2.3078(10)	21.9
2	Cy2POH	2.3376(9)	24.4
3	Ph2POH	2.3120(8)	24.6
4	(p-F-C6H4)2POH	2.3009(8)	24.6
5	MePhPOH	2.3130(6)	22.7
6	nBuPhPOH	2.3017(10)	22.7
7	CyPhPOH	2.3342(8)	24.1
8	tBuPhPOH	2.3680(7)	24.4
9	Ph2PH	2.313(2)	23.2
10	Ph2PCH2OH	2.3516(8)	25.1
11	Ph2PnBu	2.352(2)	25.6
12	PPh3	2.3438(6)	26.8
13	P(OPh)3	2.2642(8)	24.7
14	IMes	2.142(4)	27.9

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Having prepared a series of well-defined [RuCl₂(η⁶-p-cymene)(PA)], we then tested their catalytic performances in C–H activation using 2-phenylpyridine 1 as the benchmark substrate using Ackermann's conditions (Table 5). A control experiment showed that at 120 °C, [RuCl₂(p-cymene)]₂ without addition of SPO was competent for C–H functionalization since 88% of diarylated product 3 was isolated (entry 1). At 100 °C, only minute amounts of products 2 and 3 were obtained with [RuCl₂(p-cymene)]₂ (entry 2) whereas adding either 5 or 10 mol% of Ad₂POH allowed to isolate around 80% of diarylated product 3 (entries 3 and 4). However, lowering the reaction temperature to 80 °C led to the loss of activity of the dimer [RuCl₂(p-cymene)]₂ (entry 5). On one hand, at this temperature, [RuCl₂(p-cymene)]₂ in association with Ad₂P(O)H 4f gave only small amounts of 2 and 3 (entry 6). On the other hand, well-defined [RuCl₂(η⁶-cymene)(PA)] 5a–e and 5i–k were found much more competent and significant quantities of C–H functionalized products were isolated (entries 7–14). Since the reactions did not reach completion, ca. 1 : 1 mixtures of mono- and diarylated products were obtained. The well-defined complexes performed slightly better than in situ-generated complexes (entries 14 and 15) and addition of an extra quantity of SPO reduced the catalytic activity.²³ To our delight, with RuCl₂(η⁶-p-cymene)(tBuPhPOH)] 5l, only compound 3 was formed with an almost quantitative yield (entry 16). Of note, after only 2 h of the reaction, a 1 : 1 mixture of mono- and diarylated products was observed with 5l (entry 17). The electronic properties of phosphinous acids did not influence the catalytic activities of the resulting complexes. For example Ph₂POH-containing 5c performed better than 5b bearing the more electron rich Cy₂POH rather than 5d bearing electron withdrawing ligand (p-F-C₆H₄)₂POH (entries 8–10). In contrast, higher activities have been obtained for more sterically demanding PA ligands, i.e. tBuPhPOH and (3,5-Me₂-C₆H₃)₂POH (entries 16 and 11, respectively). Importantly, complexes bearing phosphine or phosphite exhibited interesting activities upon C–H activation (entries 18–20). In particular, [RuCl₂(η⁶-p-cymene)(PPh₃)] was found to be more selective for mono-functionalization of 2-phenylpyridine 1 (entry 18).

Table 5 Evaluation of catalyst performances in C–H bond activation^a

Entry	Catalyst	Additive	Yield (%)
			2	3
a Reaction conditions: 2-phenylpyridine 1 (142 μL, 1 mmol), chlorobenzene (220 μL, 2.2 mmol, 2.2 equiv.), K2CO3 (415 mg, 3 mmol, 3 equiv.), 5 mol% of Ru complex (2.5 mol% for [RuCl2(p-cymene)]2), NMP (2 mL), 80 °C, and 24 h. b Reaction was performed at 120 °C. c Reaction was performed at 100 °C. d Reaction time: 2 h.
1^b	[RuCl2(p-cymene)]2		0	88
2^c	RuCl2(p-cymene)]2		8	2
3^c	RuCl2(p-cymene)]2	Ad2P(O)H 4f (5 mol%)	0	80
4^c	RuCl2(p-cymene)]2	Ad2P(O)H 4f (10 mol%)	0	83
5	[RuCl2(p-cymene)]2		0	0
6	[RuCl2(p-cymene)]2	Ad2P(O)H 4f (10 mol%)	7	3
7	[RuCl2(η^6-p-cymene)(Me2POH)] 5a		14	15
8	[RuCl2(η^6-p-cymene)(Cy2POH)] 5b		12	11
9	[RuCl2(η^6-p-cymene)(Ph2POH)] 5c		27	22
10	[RuCl2(η^6-p-cymene)((p-F-C6H4)2POH)] 5d		19	11
11	[RuCl2(η^6-p-cymene)((3,5-Me2-C6H3)2POH)] 5e		32	32
12	[RuCl2(η^6-p-cymene)(nBuPhPOH)] 5i		19	19
13	[RuCl2(η^6-p-cymene)(BnPhPOH)] 5j		19	11
14	[RuCl2(η^6-p-cymene)(CyPhPOH)] 5k		22	54
15	[RuCl2(p-cymene)]2	CyPhP(O)H 4k (5 mol%)	21	19
16	[RuCl2(η^6-p-cymene)(tBuPhPOH)] 5l		0	89
17^d	[RuCl2(η^6-p-cymene)(tBuPhPOH)] 5l		14	13
18	[RuCl2(η^6-p-cymene)(PPh3)]		58	6
19	[RuCl2(η^6-p-cymene)(PCy3)]		18	5
20	[RuCl2(η^6-p-cymene)(P(OPh)3)]		19	15

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In Ru-mediated C–H activation, the anionic ligands played an important role, for example carboxylate-containing complexes are extremely efficient due to chelation-assisted C–H activations.^1k,4a Surprisingly, to the best of our knowledge, complexes bearing bromide and iodide ligands have not been investigated so far. Bromide and iodide counterparts bearing tBuPhPOH phosphinous acid were prepared with good yield following the same protocol than for chloride analogues (6l and 7l respectively, Scheme 2). The activity of these catalysts was compared for C–H functionalization of 2-phenylpyridine 1 and as a function of the aryl halide partner (Table 6). Unexpectedly, drastic differences were observed as only low yields of C–H functionalization and with an almost equimolar mixture of mono- and diarylation were obtained (entries 2 and 3). Moreover, whereas aryl bromides are regularly used as coupling partners,²⁶ at 80 °C they exhibited a significantly lower activity than their chlorine counterpart (entries 1, 2 and 4).²⁷ We assume that this is due to an exchange of halogens between the aryl halide to the metal center occurring through the oxidative addition. Halide effects in transition metal catalysis are often difficult to rationalize,²⁸ however, we believe that the iodide atoms increase the congestion around the metal center and decrease its reactivity.²⁹

Scheme 2 Synthesis of bromide- and iodide-containing [RuX₂(η⁶-p-cymene)(PA)] complexes.

Table 6 Halogen effect in C–H activation^a

Entry	X^1 (complex)	Ph-X^2	Yield (%)
			2	3
a Reaction conditions: 2-phenylpyridine 1 (142 μL, 1 mmol), aryl halide (2.2 mmol, 2.2 equiv.), K2CO3 (415 mg, 3 mmol, 3 equiv.), complex (5 mol%), NMP (2 mL), and 24 h.
1	Cl (5l)	Ph-Cl	0	89
2	Br (6l)	Ph-Br	23	18
3	I (7l)	Ph-I	14	8
4	Cl (5l)	Ph-Br	6	21
5	Cl (5l)	Ph-I	3	5
6	Br (6l)	Ph-I	3	7

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From these results, a halide inhibition was suspected; the chlorine dependence was therefore investigated through the use of additives (Table 7). The addition of one equivalent of LiCl in the reaction mixture quenched indeed the C–H activation process (entries 1 and 2). A significant loss of activity was also observed with 1 equiv. of tetrabutylammonium chloride (entry 3). Nevertheless, addition of a stoichiometric amount of silver salt did not boost the catalytic performances of complexes exhibiting a moderate activity such as complexes 5a and 5b (entries 5 and 8). Higher quantities of silver salt led to the inhibition of the C–H functionalization (entries 6 and 9). These results suggest that chlorine atoms play a key role in the C–H activation process.

Table 7 Chlorine dependence in C–H activation^a

Entry	Complex 5	Additives	Yield (%)
			2	3
a Reaction conditions: 2-phenylpyridine 1 (142 μL, 1 mmol), chlorobenzene (220 μL, 2.2 mmol, 2.2 equiv.), K2CO3 (415 mg, 3 mmol, 3 equiv.), complex (5 mol%), NMP (2 mL), and 24 h.
1	5l (R^1, R^2 = Ph, tBu)	None	0	89
2	5l (R^1, R^2 = Ph, tBu)	LiCl (1 equiv.)	0	0
3	5l (R^1, R^2 = Ph, tBu)	nBu4NCl (1 equiv.)	9	6
4	5a (R^1, R^2 = Me, Me)	None	14	15
5	5a (R^1, R^2 = Me, Me)	AgBF4 (5 mol%)	14	16
6	5a (R^1, R^2 = Me, Me)	AgBF4 (1 equiv.)	0	0
7	5b (R^1, R^2 = Cy, Cy)	None	12	11
8	5b (R^1, R^2 = Cy, Cy)	AgBF4 (5 mol%)	9	8
9	5b (R^1, R^2 = Cy, Cy)	AgBF4 (1 equiv.)	2	2

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In order to gain insight into the mechanism, in particular the role of the PA ligand, and as PAs are considered as labile ligands due to a weak M–PA bond,^15d we attempted to determine if the PA remains in the coordination sphere of the ruthenium all along the catalytic cycle. For this purpose, the ruthenium cyclometallated complex 8 was prepared³⁰ and tested in catalysis (Table 8). As anticipated, at 120 °C, this catalyst performed well with 91% of diarylated compound 3 isolated (entry 1). However, at 80 °C, no reaction occurred, even when the ligand tBuPhP(O)H 4l was added (entries 2 and 3). On one hand, the addition of a substoichiometric amount of silver salts such as AgBF₄ and AgSbF₆ allowed the formation small quantities of products 2 and 3 (entries 4 and 5). On the other hand, the combination of the silver salt with OPS tBuPhP(O)H 4l led to restore almost completely the activity of the catalytic system (entries 6 and 7).

Table 8 Control experiments with ruthenacycle 8 ^a

Entry	Additives	T (°C)	Yield (%)
			2	3
a Reaction conditions: 2-phenylpyridine 1 (142 μL, 1 mmol), chlorobenzene (220 μL, 2.2 mmol, 2.2 equiv.), K2CO3 (415 mg, 3 mmol, 3 equiv.), complex 8 (21.3 mg, 5 mol%), NMP (2 mL), and 24 h.
1	None	120	0	91
2	None	80	0	0
3	tBuPhP(O)H 4l (5 mol%)	80	0	0
4	AgBF4 (6 mol%)	80	5	4
5	AgSbF6 (6 mol%)	80	7	8
6	AgBF4 (6 mol%), tBuPhP(O)H 4l (5 mol%)	80	26	52
7	AgSbF6 (6 mol%), tBuPhP(O)H 4l (5 mol%)	80	26	56

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We were also able to prepare quantitatively the cationic ruthenacycle 9c bearing a phosphinous acid ligand by the treatment of complex 8 with OPS 4c in the presence of silver tetrafluoroborate in a stoichiometric amount (Scheme 3). Unfortunately 9c was found to exhibit no activity under our optimal conditions (80 °C). However, when this reaction was performed in the presence of 5 mol% of tetrabutylammonium chloride, 9c displayed moderate activity with the formation of 11% of mono arylated product 2 and 14% of 3. This represents only a slight decrease of the performance compared to catalyst 5c (27% of 2 and 22% of 3, Table 5, entry 9). To further investigate this chlorine effect, complex 9c was treated with 5 equiv. of LiCl in CDCl₃ at room temperature, and rapidly ³¹P NMR spectroscopy showed a new resonance at 108.4 ppm with complete disappearance of the signal at 114.7 ppm after only one hour. Unfortunately, this new compound was found to be unstable and could not be isolated and fully characterized.

Scheme 3 Preparation of the cationic ruthenacycle 9c.

With these results in mind, we propose the following mechanism for C–H activation/functionalization catalyzed by [RuCl₂(η⁶-p-cymene)(PA)] complexes 5 (Scheme 4).³¹ The mechanism initiates by a base-promoted loss of HCl to give the 18e ruthenium κ²-PO-phosphinito species A. A transition to a κ¹ coordination frees a coordination site for the pyridine-containing substrate. At this stage, the CDM mechanism may be triggered by the phosphinito which intercepts the ortho-proton with a concomitant release of the chlorine atom to lead to the cationic ruthenacycle C. Of note, C corresponds to the isolated complex 9c. According to our observations dealing with the reactivity of 9c with LiCl, we believe that the chlorine counterattacks the metallic center and prompts to pyridine-moiety decoordination to afford species D. This step might be at the origin of the observed halide effect; the counterattack is probably less favored with more bulky halides such as bromide and iodide. Finally, the rate determining oxidative addition^4d,32 occurring possibly through a single electron transfer process³¹ and the reductive elimination give the coupling product 2 and release complex 5.

Scheme 4 Proposed mechanism for the C–H activation mediated by [RuCl₂(η⁶-p-cymene)(PA)] complexes.

Conclusions

In summary, a series of [RuCl₂(η⁶-p-cymene)] complexes bearing phosphinous acid (PA) ligands has been straightforwardly prepared starting from the dimer [RuCl₂(p-cymene)]₂ and secondary phosphine oxides. These complexes were fully characterized, notably by single-crystal X-ray diffraction which allowed us to calculate the percentage buried volumes of PAs and other phosphorus-based ligands. This quantification of the steric parameter led to a better comprehension of the coordination chemistry for these types of complexes and explained the absence of coordination in the case of bulky SPOs such as Ad₂P(O)H. The performances of these complexes were evaluated in C–H activation/functionalization of 2-phenylpyridine. At 80 °C, an efficient complex bearing the bulkier PA was identified. A thorough comparison of aryl halides as coupling partners revealed a sharp halide effect. Further investigations allowed us to establish a halide dependence: large quantities of chlorine inhibited the C–H activation/functionalization but stoichiometric amounts were also necessary. On the basis of these results and the isolation of a reaction intermediate, a mechanism involving successively the formation of κ²-PO-phosphinito species, a concerted metallation deprotonation favored by the phosphinito, a chlorine counterattack and single electron transfer oxidative addition were proposed. Since the hindered Ad₂P(O)H does not coordinate to the metal center even though it improves the C–H activation, we believe that it participates in the CDM as an outer-sphere base. An enantioselective version of C–H activation/functionalization taking advantage of the P-stereogenic center of PA ligands is currently under investigation in our laboratory.

Experimental

All reagents were obtained from commercial sources and used as received. Secondary phosphine oxides 4c and 4e–g were obtained from chemical suppliers. Other SPOs were prepared according to literature procedures: 4b,³³4d,³³4h,³⁴4i,³⁴4j,³⁵4k^15a and 4l.³⁴ THF was purified and dried over the Braun solvent purification system (MB-SPS-800). Analytical Thin Layer Chromatography (TLC) was carried out on a Merck silica gel 60 F₂₅₄. The products were revealed by ultraviolet light (254 or 366 nm) and stained with dyeing reagents solutions. Flash chromatography was performed on a Combiflash® Companion or with Merck silica gel 60 (230–400 mesh). ¹H, ¹³C, ³¹P and ¹⁹F NMR spectra were recorded in CDCl₃ at ambient temperature on Bruker Avance III 300 or 400 spectrometers operating at 300 and 400 MHz respectively for ¹H. ¹³C, ³¹P and ¹⁹F nuclei were observed with ¹H decoupling. Solvent residual signals were used as internal standards.³⁶ Chemical shifts (δ) and coupling constants (J) are given in ppm and Hz respectively. HRMS were recorded on a SYNAPT G2 HDMS (Waters) or on a QStar Elite (Applied Biosystems SGIEX) equipped with an Atmospheric Pressure Ionization (API) source. Mass spectra were obtained using a Time Of Flight (TOF) analyser. X-ray diffraction: intensity data were collected on a Bruker-Nonius KappaCCD diffractometer using MoKα radiation (0.71073 Å) at 293(2) K. Data reduction was performed using the HKL-2000 software package. The structure was resolved using the software SIR92³⁷ by direct methods and refined using SHELXL-97.³⁸ For compound 5d, intensity data were collected on an Agilent SuperNova AtlasS2 diffractometer using MoKα radiation (0.71073 Å) at 293(2) K. Data reduction was performed using the CrysAlisPro software package (version 1.171.37.31). The structure was resolved using the software SHELXS-97 by direct methods and refined using SHELXL-2013-4. The CIF files of compounds 5b–d, 5h, 5i, 5k and 5l have been deposited with CCDC numbers 1434226–1434232, respectively.

General procedure for the synthesis of complexes [RuCl₂(η⁶-arene)(PA)] 5

In a Schlenk flask, a solution of ruthenium dimer [RuCl₂(η⁶-p-cymene)]₂ (61.2 mg, 0.1 mmol, 2 equiv. of ruthenium) and secondary phosphine oxide (0.22 mmol, 2.2 equiv.) in THF (2 mL) was stirred at room temperature for 3 h. The reaction mixture was half-concentrated and n-hexane (10 mL) was added to initiate precipitation. The red precipitate was filtered off and washed with n-hexane. Recrystallisation from DCM/n-hexane gave crystals of the desired product.

[RuCl₂(η⁶-p-cymene)(Cy₂POH)] 5b. According to the general procedure, complex 5b was obtained after 24 h of reaction as a red solid (92 mg, 89%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 5.59 (d, J(H,H) = 6.0 Hz, 2H, H^Ar), 5.56 (d, J(H,H) = 6.0 Hz, 2H, H^Ar), 5.13 (br. s, 1H, PO–H), 2.81 (sept, J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 2.40–1.20 (m, 22H, CH₂ and CH), 2.10 (s, 3H, C–CH₃), 1.28 (d, J(H,H) = 7.0 Hz, 6H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 107.8 (C^Ar), 94.1 (C^Ar), 89.0 (C^Ar–H), 84.5 (C^Ar–H), 42.9 (CH, C–P), 30.9 (CH), 27.8–25.8 (10 CH₂), 22.3 (CH₃), 18.3 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 129.3 (s). HRMS (ESI+): m/z: calcd for C₂₂H₃₇Cl₂NaOPRu: 543.0895 [M + Na]⁺; found: 543.0898.

[RuCl₂(η⁶-p-cymene)(Ph₂POH)] 5c. According to the general procedure, complex 5c was obtained as a red solid (97 mg, 96%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.75–7.65 (m, 4H, H^Ar), 7.55–7.40 (m, 6H, H^Ar), 5.40 (d, J(H,H) = 5.9 Hz, 2H, H^Ar), 5.26 (d, J(H,H) = 6.0 Hz, 2H, H^Ar), 2.51 (sept, J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 2.01 (s, 3H, C–CH₃), 0.98 (d, J(H,H) = 7.0 Hz, 6H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 137.5 (C^Ar–P), 131.7 (C^Ar–H), 131.3 (C^Ar–H), 128.4 (C^Ar–H), 108.8 (C^Ar), 96.8 (C^Ar), 89.7 (C^Ar–H), 87.0 (C^Ar–H), 30.4 (CH), 21.9 (CH₃), 17.9 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 107.0 (s). HRMS (ESI+): m/z: calcd for C₂₂H₂₅Cl₂NaOPRu: 530.9956 [M + Na]⁺; found: 530.9957.

[RuCl₂(η⁶-p-cymene)((p-F-C₆H₅)₂POH)] 5d. According to the general procedure, complex 5d was obtained after 6 h of reaction as a red solid (92 mg, 84%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) = 7.75–7.65 (m, 4H, H^Ar), 7.20–7.10 (m, 4H, H^Ar), 5.38 (d, J(H,H) = 5.7 Hz, 2H, H^Ar), 5.27 (d, J(H,H) = 5.8 Hz, 2H, H^Ar), 2.54 (sept, J(H,H) = 7.0 Hz, 1H, CH(CH₃)₂), 2.00 (s, 3H, C–CH₃), 1.01 (d, J(H,H) = 6.9 Hz, 6H, CH(CH₃)₂). ¹³C NMR (75 MHz, CDCl₃): δ (ppm) = 164.1 (C–F), 134.1 (C^Ar–H), 133.2 (C^Ar–P), 115.7 (C^Ar–H), 109.3 (C^Ar), 97.1 (C^Ar), 88.7 (C^Ar–H), 87.1 (C^Ar–H), 30.6 (CH), 21.7 (CH₃), 18.0 (CH₃). ¹⁹F{¹H} NMR (377 MHz, CDCl₃): δ (ppm) = −107.6 (s). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ (ppm) = 105.7 (s). HRMS (ESI−): m/z: calcd for C₂₂H₂₂Cl₂OPRu: 542.9803 [M − H]⁻; found: 542.9792.

[RuCl₂(η⁶-p-cymene)((3,5-Me₂-C₆H₃)₂POH)] 5e. According to the general procedure, complex 5e was obtained as a red solid (103 mg, 91%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) = 7.30 (d, J(H,P) = 11.4 Hz, 4H, H^Ar), 7.10 (s, 2H, H^Ar), 6.51 (br. s, 1H, PO–H), 5.40 (d, J(H,H) = 5.8 Hz, 2H, H^Ar), 5.18 (d, J(H,H) = 5.8 Hz, 2H, H^Ar), 2.56 (sept, J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 2.35 (s, 12H, C–CH₃), 2.06 (s, 3H, C–CH₃), 0.98 (d, J(H,H) = 7.0 Hz, 6H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 137.8 (C^Ar), 137.7 (C^Ar), 137.3 (C^Ar), 136.7 (C^Ar), 132.82 (C^Ar–H), 132.80 (C^Ar–H), 129.1 (C^Ar–H), 129.0 (C^Ar–H), 107.9 (C^Ar), 95.3 (C^Ar), 90.21 (C^Ar–H), 90.16 (C^Ar–H), 87.72 (C^Ar–H), 88.66 (C^Ar–H), 30.3 (CH), 21.6 (CH₃), 21.4 (CH₃), 17.7 (CH₃). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ (ppm) = 105.7 (s). HRMS (ESI−): m/z: calcd for C₂₆H₃₂Cl₂NaOPRu: 563.0618 [M − H]⁺; found: 563.0620.

[RuCl₂(η⁶-p-cymene)(MePhPOH)] 5h. According to the general procedure, complex 5h was obtained as a red solid (63 mg, 71%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.80–7.70 (m, 2H, H^Ar), 7.60–7.50 (m, 3H, H^Ar), 6.15 (br. s, 1H, PO–H), 5.24 (d, J(H,H) = 6.3 Hz, 1H, H^Ar), 5.21 (d, J(H,H) = 5.8 Hz, 1H, H^Ar), 5.08 (d, J(H,H) = 5.8 Hz, 1H, H^Ar), 5.04 (d, J(H,H) = 6.1 Hz, 1H, H^Ar), 2.65 (sept, J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 2.12 (d, J(H,P) = 10.0 Hz, 3H, P–CH₃), 1.94 (s, 3H, C–CH₃), 1.13 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 1.04 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 140.9 (C^Ar–P), 131.1 (C^Ar–H), 128.9 (C^Ar–H), 128.8 (C^Ar–H), 107.6 (C^Ar), 96.1 (C^Ar), 91.3 (C^Ar–H), 89.3 (C^Ar–H), 87.6 (C^Ar–H), 86.1 (C^Ar–H), 30.5 (CH), 22.2 (CH₃), 21.9 (CH₃), 18.7 (CH₃), 18.3 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 109.5 (s). HRMS (ESI+): m/z: calcd for C₁₇H₂₃Cl₂NaOPRu: 468.9798 [M + Na]⁺; found: 468.9806.

[RuCl₂(η⁶-p-cymene)(nBuPhPOH)] 5i. According to the general procedure, complex 5i was obtained as a red solid (51 mg, 53%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.80–7.70 (m, 2H, H^Ar), 7.60–7.45 (m, 3H, H^Ar), 5.26 (d, J(H,H) = 6.0 Hz, 1H, H^Ar), 5.18 (d, J(H,H) = 5.8 Hz, 1H, H^Ar), 5.14 (d, J(H,H) = 6.0 Hz, 1H, H^Ar), 5.05 (d, J(H,H) = 5.8 Hz, 1H, H^Ar), 2.80–2.65 (m, 1H, CH₂), 2.61 (sept, J(H,H) = 7.0 Hz, 1H, CH(CH₃)₂), 2.25–2.10 (m, 1H, CH₂), 1.91 (s, 3H, C–CH₃), 1.65–1.15 (m, 4H, CH₂), 1.11 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 0.97 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 0.97 (t, J(H,H) = 7.0 Hz, 3H, CH₂–CH₃). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 138.5 (C^Ar–P), 130.8 (C^Ar–H), 129.2 (C^Ar–H), 128.9 (C^Ar–H), 107.4 (C^Ar), 95.7 (C^Ar), 92.2 (C^Ar–H), 88.8 (C^Ar–H), 87.0 (C^Ar–H), 86.8 (C^Ar–H), 31.4 (CH₂), 30.4 (CH), 24.6 (CH₂), 24.0 (CH₂), 22.2 (CH₃), 21.6 (CH₃), 18.2 (CH₃), 13.8 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 113.1 (s). HRMS (ESI+): m/z: calcd for C₂₀H₂₉Cl₂NaOPRu: 511.0268 [M + Na]⁺; found: 511.0270.

[RuCl₂(η⁶-p-cymene)(BnPhPOH)] 5j. According to the general procedure, complex 5j was obtained as a deep-red solid (92 mg, 88%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) = 7.70–7.60 (m, 2H, H^Ar), 7.50–7.40 (m, 3H, H^Ar), 7.10–7.00 (m, 3H, H^Ar), 6.85–6.75 (m, 2H, H^Ar), 5.30–5.15 (m, 3H, H^Ar), 5.10 (d, ³J(H,H) = 5.9 Hz, 1H, H^Ar), 4.01 (m, 1H, CH₂–P), 3.54 (d, ²J(H,P) = 15.2 Hz, 1H, CH₂–P), 2.70 (sept, ³J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 1.97 (s, 3H, C–CH₃), 1.16 (d, ³J(H,H) = 6.9 Hz, 3H, CH(CH₃)₂), 1.07 (d, ³J(H,H) = 6.9 Hz, 3H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 136.4 (C^Ar–P), 133.7 (C^Ar), 131.0 (C^Ar–H), 130.5 (C^Ar–H), 129.7 (C^Ar–H), 128.5 (C^Ar–H), 128.0 (C^Ar–H), 126.4 (C^Ar–H), 108.7 (C^Ar), 97.4 (C^Ar), 91.3 (C^Ar–H), 88.3 (C^Ar–H), 87.5 (C^Ar–H), 87.1 (C^Ar–H), 38.9 (CH₂), 30.4 (CH), 22.3 (CH₃), 21.7 (CH₃), 18.3 (CH₃). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ (ppm) = 109.3 (s). HRMS (ESI+): m/z: calcd for C₂₃H₂₇Cl₂NaOPRu: 545.0113 [M + Na]⁺; found: 545.0101.

[RuCl₂(η⁶-p-cymene)(CyPhPOH)] 5k. According to the general procedure, complex 5k was obtained as a red solid (100 mg, 98%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.75–7.65 (m, 2H, H^Ar), 7.55–7.45 (m, 3H, H^Ar), 5.52 (d, J(H,H) = 6.5 Hz, 1H, H^Ar), 5.48 (d, J(H,H) = 6.5 Hz, 1H, H^Ar), 5.19 (d, J(H,H) = 5.5 Hz, 1H, H^Ar), 4.90 (d, J(H,H) = 5.5 Hz, 1H, H^Ar), 2.58 (sept, J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 2.50–2.20 (m, 2H, CH and CH₂), 1.99 (s, 3H, C–CH₃), 1.90–1.10 (m, 9H, CH₂), 1.10 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 0.75 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 137.5 (C^Ar–P), 129.4 (C^Ar–H), 128.4 (C^Ar–H), 127.3 (C^Ar–H), 105.6 (C^Ar), 94.5 (C^Ar), 93.4 (C^Ar–H), 89.3 (C^Ar–H), 84.3 (C^Ar–H), 82.7 (C^Ar–H), 41.2 (CH–P), 29.2 (CH), 26.0–25.0 (5 CH₂), 21.9 (CH₃), 19.2 (CH₃), 17.0 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 114.2 (s). HRMS (ESI+): m/z: calcd for C₂₂H₃₁Cl₂NaOPRu: 537.0426 [M + Na]⁺; found: 537.0421.

[RuCl₂(η⁶-p-cymene)(tBuPhPOH)] 5l. According to the general procedure, complex 5l was obtained as a deep-red solid (97 mg, 99%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 7.80–7.70 (m, 2H, H^Ar), 7.55–7.45 (m, 3H, H^Ar), 6.20 (br. s, 1H, PO–H), 5.48 (d, J(H,H) = 5.9 Hz, 1H, H^Ar), 5.39 (d, J(H,H) = 6.3 Hz, 1H, H^Ar), 5.35 (d, J(H,H) = 6.3 Hz, 1H, H^Ar), 5.15 (d, J(H,H) = 6.0 Hz, 1H, H^Ar), 2.78 (sept, J(H,H) = 7.0 Hz, 1H, CH(CH₃)₂), 2.01 (s, 3H, C–CH₃), 1.21 (d, J(H,P) = 15.0 Hz, 9H, C(CH₃)₃), 1.18 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 1.01 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 137.1 (C^Ar–P), 130.4 (C^Ar–H), 130.0 (C^Ar–H), 128.0 (C^Ar–H), 107.6 (C^Ar), 94.2 (C^Ar), 92.3 (C^Ar–H), 90.5 (C^Ar–H), 86.6 (C^Ar–H), 86.1 (C^Ar–H), 39.9 (C), 30.4 (CH), 26.3 (CH₃), 22.0 (CH₃), 21.7 (CH₃), 18.0 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 115.1 (s). HRMS (ESI+): m/z: calcd for C₂₀H₂₉Cl₂NaOPRu: 511.0268 [M + Na]⁺; found: 511.0272.

[RuBr₂(η⁶-p-cymene)(tBuPhPOH)] 6l. According to the general procedure using the dimer [RuBr₂(η⁶-p-cymene)]₂, complex 6l was obtained as a deep-red solid (94 mg, 81%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) = 7.80–7.60 (m, 2H, H^Ar), 7.55–7.40 (m, 3H, H^Ar), 5.55 (d, J(H,H) = 6.1 Hz, 1H, H^Ar), 5.52 (br. s, 1H, PO–H), 5.41 (d, J(H,H) = 6.1 Hz, 1H, H^Ar), 5.37 (d, J(H,H) = 6.2 Hz, 1H, H^Ar), 5.27 (d, J(H,H) = 6.2 Hz, 1H, H^Ar), 2.93 (sept, J(H,H) = 7.0 Hz, 1H, CH(CH₃)₂), 2.08 (s, 3H, C–CH₃), 1.23 (d, J(H,P) = 15.0 Hz, 9H, C(CH₃)₃), 1.20 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 1.04 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂). ¹³C NMR (75 MHz, CDCl₃): δ (ppm) = 137.6 (C^Ar–P), 130.3 (3 C^Ar–H), 127.8 (2 C^Ar–H), 109.6 (C^Ar), 94.3 (C^Ar), 91.8 (C^Ar–H), 91.2 (C^Ar–H), 86.6 (C^Ar–H), 85.0 (C^Ar–H), 40.7 (C), 31.1 (CH), 26.6 (CH₃), 22.2 (CH₃), 22.1 (CH₃), 18.7 (CH₃). ³¹P{¹H} NMR (121 MHz, CDCl₃): δ (ppm) = 113.3 (s). HRMS (ESI+): m/z: calcd for C₂₀H₂₉Br₂NaOPRu: 600.9246 [M + Na]⁺; found: 600.9247.

[RuI₂(η⁶-p-cymene)(tBuPhPOH)] 7l. According to the general procedure using the dimer [RuI₂(η⁶-p-cymene)]₂, complex 7l was obtained as a deep-red solid (121 mg, 90%). ¹H NMR (300 MHz, CDCl₃): δ (ppm) = 7.70–7.60 (m, 2H, H^Ar), 7.50–7.40 (m, 3H, H^Ar), 5.71 (d, J(H,H) = 6.2 Hz, 1H, H^Ar), 5.54 (d, J(H,H) = 6.1 Hz, 1H, H^Ar), 5.43 (d, J(H,H) = 6.1 Hz, 1H, H^Ar), 5.39 (d, J(H,H) = 6.3 Hz, 1H, H^Ar), 4.72 (br. s, 1H, PO–H), 3.21 (sept, J(H,H) = 7.0 Hz, 1H, CH(CH₃)₂), 2.22 (s, 3H, C–CH₃), 1.24 (d, J(H,P) = 14.8 Hz, 9H, C(CH₃)₃), 1.23 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂), 1.08 (d, J(H,H) = 7.0 Hz, 3H, CH(CH₃)₂). ¹³C NMR (75 MHz, CDCl₃): δ (ppm) = 138.9 (C^Ar–P), 130.6 (C^Ar–H), 130.1 (C^Ar–H), 127.5 (C^Ar–H), 112.9 (C^Ar), 95.5 (C^Ar), 92.8 (C^Ar–H), 89.8 (C^Ar–H), 86.3 (C^Ar–H), 84.0 (C^Ar–H), 41.3 (C), 32.4 (CH), 27.2 (CH₃), 23.1 (CH₃), 22.2 (CH₃), 19.9 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 113.9 (s). HRMS (ESI+): m/z: calcd for C₂₀H₂₉I₂KOPRu: 710.8726 [M + K]⁺; found: 710.8737.

[Ru(η⁶-p-cymene)(Ph₂POH)(2-phenylpyridine-κ²-NC)BF₄] 9c. In a round-bottom flask, were introduced in turn AgBF₄ (56 mg, 0.29 mmol), ruthenacycle 8 (122 mg, 0.29 mmol), Ph₂PHO (58 mg, 0.29 mmol) and 5 mL of dry DCM. The reaction mixture was stirred at room temperature for 1 h and filtered on Celite®. The volatiles were removed to afford complex 9c as a brown-grey solid (194 mg, 99%). ¹H NMR (400 MHz, CDCl₃): δ (ppm) = 9.27 (d, J(H,H) = 5.4 Hz, 1H, H^Ar), 8.09 (d, J(H,H) = 7.7 Hz, 1H, H^Ar), 7.60–7.55 (m, 2H, H^Ar), 7.50 (td, J(H,H) = 7.7 and 1.1 Hz, 1H, H^Ar), 7.36–7.29 (m, 3H, H^Ar), 7.26–7.20 (m, 3H, H^Ar), 7.16 (d, J(H,H) = 8.2 Hz, 1H, H^Ar), 7.06 (t, J(H,H) = 7.7 Hz, 1H, H^Ar), 7.03–6.99 (m, 1H, H^Ar), 6.78 (dt, J(H,H) = 8.0 and 2.6 Hz, 2H, H^Ar), 6.50 (dd, J(H,H) = 8.4 and 1.2 Hz, 1H, H^Ar), 6.47 (dd, J(H,H) = 8.2 and 1.0 Hz, 1H, H^Ar), 6.40 (d, J(H,H) = 6.4 Hz, 1H, H^Ar), 6.05 (dd, J(H,H) = 6.4 and 1.4 Hz, 1H, H^Ar), 5.86 (dd, J(H,H) = 6.1 and 1.2 Hz, 1H, H^Ar), 5.33 (d, J(H,H) = 6.0 Hz, 1H, H^Ar), 2.24 (sept, J(H,H) = 6.9 Hz, 1H, CH(CH₃)₂), 1.88 (s, 3H, C–CH₃), 0.80 (d, J(H,H) = 6.9 Hz, 3H, CH(CH₃)₂), 0.72 (d, J(H,H) = 6.9 Hz, 3H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃): δ (ppm) = 171.5 (C^Ar), 171.3 (C^Ar), 165.0 (C^Ar), 156.49 (C^Ar–H), 156.47 (C^Ar–H), 144.9 (C^Ar), 141.54 (C^Ar–H), 141.51 (C^Ar–H), 137.0 (C^Ar–H), 136.6 (C^Ar), 136.0 (C^Ar), 132.5 (C^Ar), 134.7 (C^Ar), 130.3 (C^Ar–H), 130.2 (C^Ar–H), 130.1 (C^Ar–H), 130.0 (C^Ar–H), 129.37 (C^Ar–H), 129.35 (C^Ar–H), 129.2 (C^Ar–H), 129.1 (C^Ar–H), 129.0 (C^Ar–H), 127.8 (C^Ar–H), 127.7 (C^Ar–H), 127.5 (C^Ar–H), 127.3 (C^Ar–H), 125.0 (C^Ar–H), 123.1 (C^Ar–H), 122.5 (C^Ar–H), 119.3 (C^Ar–H), 113.5 (C^Ar), 113.4 (C^Ar), 108.5 (C^Ar), 96.61 (C^Ar–H), 96.59 (C^Ar–H), 96.56 (C^Ar–H), 96.4 (C^Ar–H), 85.3 (C^Ar–H), 31.0 (CH), 22.2 (CH₃), 22.2 (CH₃), 18.8 (CH₃). ³¹P{¹H} NMR (162 MHz, CDCl₃): δ (ppm) = 114.7 (s). ¹⁹F NMR (376 MHz, CDCl₃): δ (ppm) = −150.5 (s). HRMS (ESI+): m/z: calcd for C₃₃H₃₃NOPRu: 592.1347 [M − BF₄]⁺; found: 592.1345.

Acknowledgements

This work was supported by the Ministère de l'enseignement supérieur et de la recherche (L. G. Ph.D. grant), the CNRS, AMU, and Centrale Marseille. Dr Valérie Monnier and Christophe Chendo (Spectropole, Fédération des Sciences Chimiques de Marseille) are gratefully acknowledged for MS analyses. We thank Matthieu Ortega for the preparation of SPOs, Dr Alphonse Tenaglia for fruitful discussions and Dr Julie Broggi for useful comments on the manuscript. We are indebted to both referees for relevant comments and suggestions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and spectral data. CCDC 1434226–1434232. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt04683a

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