Ruthenium hydroxycyclopentadienyl N-heterocyclic carbene complexes as transfer hydrogenation catalysts

Abstract

A series of novel cationic hydroxycyclopentadienyl and methoxycyclopentadienyl N-heterocyclic carbene ruthenium(II) complexes have been synthesized from the corresponding neutral ruthenium(0) complexes containing both the non-innocent cyclopentadienone ligand and variously functionalized N-heterocyclic carbenes (NHCs). In particular an NHC derivative containing a pyridine group in the side chain has been designed and developed in order to evaluate the influence of a basic, potentially cooperative substituent in the catalytic activity of these complexes. All the prepared complexes were employed as selective catalysts for transfer hydrogenation reactions employing refluxing ⁱPrOH as a hydrogen source and several ketones and aldehydes as substrates. We found that while the presence of oxidizing additives such as CAN and benzoquinone is mandatory to activate the neutral ruthenium(0) complexes, no activation is needed for the cationic Ru(II) catalysts. The catalytic activity of the latter is also influenced by the coordinating ability of the counterion, and indeed the cationic complexes having a pyridine-functionalized NHC ligand and CF₃SO₃⁻ as counterion, present the best conversion (>99%) thus demonstrating the fundamental role played by the basic pyridine in the catalytic activity. With regard to the hydrogenation reaction mechanism, the release of the CO ligand was demonstrated to be the key step and the presence of hydride species has been detected at the end of the reaction.

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Introduction

N-heterocyclic carbenes (NHCs) are among the most popular ancillary ligands due to a combination of unique features, such as the tunability of electronic and steric properties that influence the metal center. Moreover the synthesis of NHC ligand precursors and of the corresponding complexes is rather simple and very versatile¹ allowing the rational design of transition metal catalysts and the improvement of catalytic activity.² Several catalytic reactions such as ruthenium-catalyzed olefin metathesis,³ palladium-catalyzed cross-coupling reaction,⁴ iridium-catalyzed reductions and oxidations,⁵ and gold-catalyzed activation of π-bonds⁶ benefited from the introduction of NHC ligands. Apart from olefin metathesis, Ru–NHC complexes have shown catalytic activity in various redox transformations,⁷ including: transfer hydrogenation,⁸ hydrogenation of olefins⁹ and esters,¹⁰ asymmetric hydrogenation,¹¹ amide synthesis from alcohols and nitriles,¹² dehydrogenation of esters and imines from alcohols,¹³ racemization of chiral alcohols,¹⁴ oxidation of alcohols¹⁵ and water oxidation.¹⁶ Regarding the Ru–NHC complexes, most of the literature features Ru(II) complexes, while low-valent Ru(0)–NHC systems are restricted to a few examples based on either [Ru₃(CO)₁₂] or [Ru(CO)₂(PPh₃)₃] as precursors.¹⁷

Another class of versatile ligands is represented by cyclopentadienones which behave as non-innocent ligands exploiting the cyclopentadienone/hydroxycyclopentadienyl reversible transformation. The most famous example of its cooperativity with ruthenium in bifunctional catalysis is the Shvo catalyst, a well-known system employed in a plethora of catalytic applications.¹⁸ In principle by combining NHC and cyclopentadienone ligands on ruthenium complexes the properties of both ligand should be exploited allowing the design of novel catalysts finely tuning steric and electronic properties, solubility and the insertion of substituents suitable for heterogenization. Furthermore by choosing a proper substituent, NHC ligand itself could cooperate with the metal as a non-innocent species.¹⁹

With this aim in mind, a straightforward approach towards Ru(0)–NHC complexes has been recently communicated by our group,²⁰ based on a dimeric Ru(0) cyclopentadienone dicarbonyl dimer (2). Cleavage of 2 in the presence of a silver carbene precursor provided access to a new class of ruthenium complexes in quantitative yields (some examples in Scheme 1).

Scheme 1 Synthesis of cyclopentadienone imidazolylidene ruthenium(0) complexes 3a–c.

In this paper we first describe an extension of this synthetic methodology to a complex containing a pyridine-functionalized NHC ligand (3d: R₁ = butyl; R₂ = 2-pyridine) then the main body of this work concerns the transformation of the cyclopentadienone imidazolylidene ruthenium(0) complexes (3a–d) into their corresponding cationic hydroxyl- and methoxy-cyclopentadienyl ruthenium(II) derivatives [4a–d[X] (X = Cl, BF₄, NO₃, CF₃SO₃) and 5a,d[CF₃SO₃]], and the use of both Ru(0) and Ru(II) complexes as catalysts for transfer hydrogenation of ketones and aldehydes in refluxing isopropanol. Comparison between monodentate and pyridine-functionalized NHC ligands, has provided some insights into the reaction mechanism.

Results and discussion

Synthesis of imidazolylidene Ru complexes

The novel complex 3d (R₁ = butyl; R₂ = 2-pyridine) was prepared in one pot following the same procedure described in Scheme 1 and fully characterized by spectroscopic methods and elemental analysis. In particular, the ¹³C NMR spectrum shows the downfield shifted resonance of the carbene (δ = 173.20 ppm), whereas in the IR spectrum the CO stretching bands are consistent with those of similar complexes (ν_CO = 2008 cm⁻¹, 1949 cm⁻¹ (3d) vs. ν_CO = 2004 cm⁻¹, 1945 cm⁻¹ (3a)). Further evidences were produced by ESI-MS and X-ray diffraction analyses.

The molecular structure of 3d has been determined by single crystal X-ray crystallography as its 3d·0.5CH₂Cl₂ solvate (Fig. 1).

Fig. 1 ORTEP drawing of 3d. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity. Selected bond lengths (Å): Ru(1)–C(1) 1.845(16), Ru(1)–C(2) 1.883(12), Ru(1)–C(3) 2.511(11), Ru(1)–C(4) 2.279(11), Ru(1)–C(5) 2.199(11), Ru(1)–C(6) 2.212(10), Ru(1)–C(7) 2.280(11), Ru(1)–C(34) 2.139(12), C(3)–O(3) 1.246(13).

Its structure is similar to those previously reported for the analogous complexes 3a–c.²⁰ In particular, the Ru(1)–C(3) distance [2.511(11) Å] is significantly longer than Ru(1)–C(4–7) [2.199(11)–2.280(11) Å, average 2.242(2) Å] and C(3)–O(3) [1.246(13) Å] is essentially a double bond.²¹ The Ru(1)–C(34) contact [2.117(3) Å] is in the typical range for the interaction between Ru(0) and a N-heterocyclic carbene.²²

The reaction of 3a–d with strong acids or a methylating agent such as MeCF₃SO₃, leads to the quantitative formation of the hydroxycyclopentadienyl and methoxycyclopentadienyl cationic complexes 4a–d[X] (X = Cl, BF₄, NO₃, CF₃SO₃) and 5a,d[CF₃SO₃] (Scheme 2), in which Ru(0) is formally oxidized to Ru(II).

Scheme 2 Synthesis of hydroxycyclopentadienyl imidazolylidene ruthenium complexes 4a–d[X] and methoxycyclopentadienyl imidazolylidene ruthenium complexes 5a,d[CF₃SO₃].

Infrared spectroscopy provides a convenient technique for monitoring the progress of the protonation as, in agreement with the reduced back-donation from the metal center to the carbonyl ligands, ν–C≡O stretch vibrations undergo a significative high-energy shift upon formation of the cationic complex [e.g. ν_CO = 2008 cm⁻¹, 1949 cm⁻¹ (3d), and 2041 cm⁻¹, 1991 cm⁻¹ (4d[CF₃SO₃])]. The change in the coordination from η⁴ (cyclopentadienone ligand) to η⁵ (hydroxycyclopentadienyl ligand) is further evidenced by ¹³C-NMR shift of the resonance due to the endocyclic carbon involved in the ketone–alcohol transformation [δ 168.22 ppm (C=O, Cp, in 3d) vs. δ 142.00 ppm (C–OH, Cp, in 4d[CF₃SO₃])]. Ru–carbene signals are also shifted to higher fields; from δ 173.20 ppm (3d) to δ 166.07 ppm (4d[CF₃SO₃]) (see Experimental section for more details).

With regard to the methylated complexes 5a[CF₃SO₃] and 5d[CF₃SO₃] IR spectra show a shift of 40 cm⁻¹ from the neutral to the cationic species (e.g. ν_CO = 2004 cm⁻¹, 1945 cm⁻¹ (3a), and 2045 cm⁻¹, 1995 cm⁻¹ (5a[CF₃SO₃])) and both IR and NMR data are similar to those obtained for the hydroxycyclopentadienyl complexes 4a[CF₃SO₃] and 4d[CF₃SO₃] with the methyl group arising at δ 3.19 (¹H NMR) and δ 61.93 (¹³C NMR) ppm.

Moreover, structural evidences were obtained by X-ray diffraction analysis of single crystals of the cationic complexes [4a]⁺ and [4c]⁺ as their [4a][CF₃SO₃]·0.5toluene and [4c][CF₃SO₃]·CHCl₃ solvate salts (Fig. 2). X-ray crystallography confirms that protonation has occurred on the O-atom of the cyclopentadienone ligand. As a consequence, the C(3)–O(3) interaction [1.335(6) and 1.339(9) Å for [4a]⁺ and [4c]⁺, respectively] is considerably elongated compared to the parent neutral complexes [e.g., 1.246(13) Å for 3d] and displays the character of a C(sp₂)–O single bond. In addition the Ru(1)–C(3) distance [2.323(5) and 2.319(7) Å for [4a]⁺ and [4c]⁺, respectively] is shortened in respect to the parent complexes [e.g., 2.511(11) Å for 3d] and comparable to the other Ru(1)–C(4–7) distances [2.227(5)–2.290(5) Å, average 2.253(10) Å for [4a]⁺; 2.271(8)–2.221(7) Å, average 2.248(12) Å for [4c]⁺]. Thus, the protonated ligand of [4a]⁺ and [4c]⁺ is better described as a η⁵-cyclopentadienyl ligand, as previously reported for analogous complexes.^18d,23 The O(3)–H(3) group of [4a]⁺ is involved in an inter-molecular H-bond with the [CF₃SO₃]⁻ anion [O(3)–H(3) 0.885(10) Å, H(3)⋯O(301) 1.80(3) Å, O(3)⋯O(301) 2.626(5) Å, ∠O(3)H(3)O(301) 155(5)°]. Conversely, the O(3)–H(3) group of [4c]⁺ forms an intra-molecular H-bond with the O(6) atom of the side chain of the NHC ligand [O(3)–H(3) 0.84 Å, H(3)⋯O(6) 1.83 Å, O(3)⋯O(6) 2.646(7) Å, ∠O(3)H(3)O(6) 163.7°]. In addition, there is in [4c]⁺ also an inter-molecular H-bond between the O(6)–H(6) group of the complex and the [CF₃SO₃]⁻ [O(6)–H(6) 0.84 Å, H(6)⋯O(301)#1 1.91 Å, O(6)⋯O(301)#1 2.739(9) Å, ∠O(6)H(6)O(301)#1 167.7°; symmetry transformations used to generate equivalent atoms: #1 x + 1, y, z #2 −x + 1, −y + 1, −z + 1]. The carbene interaction Ru(1)–C(34) [2.101(5) and 2.109(7) Å for [4a]⁺ and [4c]⁺, respectively] is almost unchanged compared to the parent neutral complexes [e.g., 2.117(3) Å for 3d].

Fig. 2 Left: ORTEP drawing of [4a]⁺. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity, except H(3). Selected bond lengths (Å): Ru(1)–C(1) 1.882(7), Ru(1)–C(2) 1.881(7), Ru(1)–C(3) 2.323(5), Ru(1)–C(4) 2.290(5), Ru(1)–C(5) 2.238(5), Ru(1)–C(6) 2.227(5), Ru(1)–C(7) 2.256(5), Ru(1)–C(34) 2.101(5), C(3)–O(3) 1.335(6). Right: ORTEP drawing of [4c]⁺. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity, except H(3) and H(6). Selected bond lengths (Å): Ru(1)–C(1) 1.876(8), Ru(1)–C(2) 1.901(9), Ru(1)–C(3) 2.319(7), Ru(1)–C(4) 2.262(7), Ru(1)–C(5) 2.240(7), Ru(1)–C(6) 2.221(7), Ru(1)–C(7) 2.271(8), Ru(1)–C(34) 2.109(7), C(3)–O(3) 1.339(9).

Finally, with the aim of allowing the coordination of the pyridine to the metal centre, complex 4d[CF₃SO₃] has been irradiated with a UV lamp. The photolytic removal of one terminal CO led to the formation of the chelated complex 6d[CF₃SO₃] in good yield (Scheme 3).

Scheme 3 Photolytic removal of terminal CO.

The IR spectrum shows the disappearance of the two absorptions due to the CO stretching of 4d[CF₃SO₃], and the appearance of a unique band at 1967 cm⁻¹ (6d[CF₃SO₃]). Carbene resonance, in ¹³C NMR spectrum, is shifted to lower fields (δ 185.63 ppm (6d[CF₃SO₃]) vs. δ 166.07 ppm (4d[CF₃SO₃])).

The X-ray molecular structure of 6d[CF₃SO₃] further supports the characterization performed in solution (Fig. 3) and consists of a Ru center coordinated to a η⁵-cyclopentadienyl ligand, a terminal CO and a chelating NHC ligand bonded to Ru via the carbene and the N-atom of the pyridine side group. The Ru(1)–Cp distances [2.216(7)–2.302(8) Å, average 2.248(16) Å] are comparable to those reported above for [4a]⁺ and [4c]⁺ and analogous Ru-complexes containing protonated cyclopentadienone ligands.^18d,23 As in the case of [4a]⁺ and [4c]⁺, the C(3)–O(3) interaction [1.338(9) Å] is a single bond, in view of the presence of a H atom on the oxygen, which is involved in H-bond with the triflate anion.

Fig. 3 ORTEP drawing of 6d[CF₃SO₃]. Displacement ellipsoids are at the 30% probability level. H-atoms have been omitted for clarity, except H(3). Selected bond lengths (Å): Ru(1)–C(1) 1.860(9), Ru(1)–C(3) 2.302(8), Ru(1)–C(4) 2.237(7), Ru(1)–C(5) 2.216(7), Ru(1)–C(6) 2.236(7), Ru(1)–C(7) 2.251(7), Ru(1)–C(34) 2.030(7), Ru(1)–N(3) 2.127(10), C(3)–O(3) 1.338(9).

It is worth mentioning that UV irradiation (3 h) of the neutral complexes 3a and 3d leaves the reactant unaltered indicating that the formal oxidation of Ru complex, upon protonation, favor the CO removal. The same result was surprisingly obtained with the cationic complexes 4a[CF₃SO₃] and 5a[CF₃SO₃], for which CO release was expected (16 h of irradiation in ⁱPrOH). Nonetheless irradiating 4a[CF₃SO₃] and 5a[CF₃SO₃], in the presence of a trapping agent such as pyridine, the formation of the complexes 7[CF₃SO₃] and 8[CF₃SO₃] was finally observed (Fig. 4) actually demonstrating that the CO release occurs in the cationic complexes. Indeed neutral complex 3a did not lose CO under UV irradiation even in the presence of pyridine. It is thus likely that, in the case of cationic complexes, without the help of pyridine the removal of CO is an equilibrium in which ruthenium–CO bond is reformed at the end of the reaction leading to the recovery of the precursors 4a[CF₃SO₃] and 5a[CF₃SO₃].

Fig. 4 Molecular structure of 7[CF₃SO₃] and 8[CF₃SO₃].

Formation of 7[CF₃SO₃] and 8[CF₃SO₃] is evidenced by the disappearance of the two carbonyl stretching bands associated to 4a[CF₃SO₃] (2037, 1985 cm⁻¹) and 5a[CF₃SO₃] (2045, 1995 cm⁻¹), with the concomitant appearance of a single band associated with the new complex 7[CF₃SO₃] (1943 cm⁻¹) and 8[CF₃SO₃] (1948 cm⁻¹) (Fig. 4). Complexes 7[CF₃SO₃] and 8[CF₃SO₃] have been found in mixture with other species and their characterization was not complete. However the bathochromic shift observed in IR spectra is in agreement with the replacement of a strong π ligand with a σ N-donor ligand such as pyridine. Furthermore ESI-MS analyses performed on complexes 7[CF₃SO₃] and 8[CF₃SO₃] showed the presence of the molecular ions.

Catalytic transfer hydrogenation

The Ru complexes 3a–d, 4a–d[X] [(X = Cl, BF₄, NO₃, CF₃SO₃)] and 5a,d[CF₃SO₃] were evaluated as catalyst precursors under transfer hydrogenation conditions i.e. refluxing ⁱPrOH as hydrogen source employing 4-fluoroacetophenone as model substrate. Catalytic runs were performed to investigate the role of different additives in the activation of 3a–d, to compare the catalytic activity of the ruthenium(0) complexes 3a–d with the corresponding ruthenium(II) cationic complexes 4a–d[X] and 5a,d[CF₃SO₃] and finally to evaluate the influence of a pyridine substituent on the NHC side chain. Results are reported in Table 1: in all cases selectivity is complete and conversion corresponds to yield unless otherwise stated.

Table 1 Catalytic transfer hydrogenation of 4-fluoroacetophenone^a

Entry	[Ru]	Additive	Conversion (%) 8 h	Conversion (%) 24 h
a General conditions: ruthenium complex (5 mol% Ru), ^iPrOH (3 mL), T = 82 °C; conversions determined by ^19F NMR spectroscopy.b CAN 1 mol equiv. per ruthenium center.c Catalyst loading reduced to 1 mol% Ru.d NaIO4, BQ (benzoquinone), KO^tBu and acids: 1 mol equiv. per ruthenium center.e Recycling test.f Yield ∼ 70% the co-product arising from the acid catalyzed etherification of 4-fluoroacetophenol with the solvent 2-propanol.g Yield ∼ 35% the coproduct was not identified.
1	3a	—	0	0
2	3b	—	0	0
3	3c	—	0	0
4	3d	—	0	0
5	3a	CAN^b	25	61
6	3a^c	CAN^b	0	27
7	3b	CAN^b	10	87
8	3c	CAN^b	10	25
9	3d	CAN^b	0	9
10	3a	NaIO4^d	0	0
11	3a	BQ^d	25	55
12	4a[CF3SO3]	—	58	93
13	4a[CF3SO3]^c	—	5	29
14	4a[CF3SO3]^e	—	64	87
15	4a[CF3SO3]	KO^tBu^d	14	35
16	4a[BF4]	—	41	71
17	4a[Cl]	—	16	24
18	3a	CF3SO3H^d	32	>99^f
19	3a	H2SO4^d	45	92
20	3a	HNO3^d	60	65
21	4b[CF3SO3]	—	9	60
22	4c[CF3SO3]	—	35	46^g
23	4d[CF3SO3]	—	84	>99
24	4d[Cl]	—	0	5
25	4d[BF4]	—	37	40
26	3d	CF3SO3H^d	84	>99^f
27	3d	H2SO4^d	76	86
28	3d	HNO3^d	0	36
29	5a[CF3SO3]	—	28	64
30	5d[CF3SO3]	—	10	13
31	6d[CF3SO3]	—	0	5
32	HCF3SO3	—	0	0

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The neutral complexes 3a–d did not present any catalytic activity in the absence of additives (entry 1–4), as well as by adding NaIO₄ (entry 10). However, we found that the addition of one equivalent of CAN (entry 5–9) results in the formation of active intermediates, likely deriving from the release of one CO ligand, favoured by oxidation as discussed in the previous section. In fact the use of benzoquinone, as oxidant additive under the same conditions (entry 11), activates the catalyst even though leading to a slightly lower conversion if compared with CAN.

Reduction of the catalyst loading to 1 mol% (3a) increases the induction time halving the conversion in 24 h (entry 5 vs. 6). In general pre-catalysts 3a–d resemble the behaviour of the triazolylidene ruthenium(0) congener, namely the dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1,3-dimethyl-4-phenyl-1,2,3-triazol-ylidene)ruthenium, previously tested as hydrogen transfer catalyst.²⁴

The oxidation state of the metal center appears to be the key point in order to activate the catalyst and indeed, better conversions are reached with the pre-catalysts 4a–d[X] [(X = Cl, BF₄, NO₃, CF₃SO₃) (entries 12, 13, 16, 17 and 21–25)]. A recycling test has been performed with 4a[CF₃SO₃], the catalyst can be reused although a slight decrease in conversion from 93 to 87% was observed after 24 h (entry 14). Noteworthy also the counterion shows to influence the catalytic activity, and CF₃SO₃⁻ provides the best results: for example we observed complete conversion within 24 h in the case of 4a[CF₃SO₃] and 4d[CF₃SO₃] (entries 12 and 23). Furthermore the latter pyridine-functionalized catalyst 4d[CF₃SO₃] shows a faster activation if compared with 4a[CF₃SO₃] (conversion of 84% and 58% respectively in 8 h). On the other hand, catalytic activity is suppressed in the case of 4a[Cl] and 4d[Cl] (entry 17 and 24), possibly due to the coordinating ability of Cl⁻ counterion; on the contrary 4a[BF₄] and 4d[BF₄] give intermediate results (entry 16 and 25).

The in situ addition of various acids such as HCF₃SO₃, H₂SO₄ and HNO₃ to 3a and 3d (entries 18–20 and 26–28) produces, in the case of triflic acid, similar conversions as for the corresponding preformed catalysts (e.g. entry 23, 4d[CF₃SO₃] vs. entry 26, 3d + CF₃SO₃H in situ). However, a lower selectivity is observed due to the acid catalyzed etherification of 4-fluorobenzylalcohol and isopropanol. Best in situ performances are accomplished by sulfuric acid (entries 19 and 27), which gives much better results than HNO₃ (entry 20 and 28), confirming the detrimental effect of the coordination ability of counterions. A base, such as KO^tBu, has a negative effect on the reaction (entry 15), likely due to deactivation of the pre-catalyst by deprotonation. Finally a blank experiment performed with HCF₃SO₃ in refluxing isopropanol rule out an exclusive role of the acid itself in the catalytic activity (entry 32).

With the aim of evaluating the role of the hydroxyl group in the catalytic reaction methylated complexes 5a[CF₃SO₃] and 5d[CF₃SO₃] have been also tested as catalysts (entry 29 and 30). Quite surprisingly, the methylated complex 5a[CF₃SO₃] shows a remarkable catalytic activity although in the absence of the acidic OH. Worst performances came from the pyridine-functionalized methylated complex 5d[CF₃SO₃], probably due to a deactivation pathway associated with the formation of stable species in the chelated form. Indeed, no conversion is observed when the chelated cationic complex 6d[CF₃SO₃] is employed as pre-catalyst (entry 31).

The substrate scope has been also evaluated extending the reactivity of 3a, 4a[CF₃SO₃] and 4d[CF₃SO₃] as hydrogen transfer catalysts to acetophenone, benzophenone and benzaldehyde. Data are reported in Table 2. All the substrates are converted to the corresponding alcohol by activated 3a and by the cationic complex 4a[CF₃SO₃]. As expected the catalysts are much more active in the case of benzaldehyde (entry 7 and 9), since aldehydes are substrates more prone to reduction than ketones. Additionally, the scale of activity is consistent with the electron withdrawing properties of the ketones examined. Again, 4d[CF₃SO₃] (entry 3, 6 and 9) shows a faster activation. As an example, benzaldehyde reaches complete conversion within 4 h, that becomes 8 h in the case of 4a[CF₃SO₃] (entry 9^c vs. 8).

Table 2 Catalytic transfer hydrogenation^a

Entry	Substrate	[Ru]	Additive^b	Conv. (%) 8 h	Conv. (%) 24 h
a Reaction conditions: [Ru] = 5 mol%, ^iPrOH (3 mL), T = 82 °C. Conversions determined by GC.b CAN 1 mol equiv. per ruthenium center.c Complete conversion is reached within 4 h.
1		3a	CAN	10	77
2		4a[CF3SO3]	—	15	68
3		4d[CF3SO3]	—	45	48
4		3a	CAN	0	42
5		4a[CF3SO3]	—	0	36
6		4d[CF3SO3]	—	42	43
7		3a	CAN	31	39
8		4a[CF3SO3]	—	>99
9		4d[CF3SO3]	—	>99^c

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Mechanistic insight

On the basis of the results described above, we can draw some considerations regarding the active catalytic species. First of all, the trapping experiments with pyridine (see cyclization of 4d[CF₃SO₃] to 6d[CF₃SO₃], Scheme 3 and complexes 7[CF₃SO₃] and 8[CF₃SO₃], Fig. 4) demonstrates that CO release is involved in the activation, leaving unaltered the Ru–carbene bond. Indeed, all the crude products of catalytic reactions have been analyzed by IR and NMR spectroscopy and only in a couple of cases traces of Shvo catalyst, which would results from NHC ligand release, were visible in NMR spectra, accompanied by a very bright yellow color of the solution typical of Shvo complex.¹⁸ Furthermore, an outer sphere mechanism is likely to be followed,^18b in that the complex [CpRu(CO)₂][CF₃SO₃], obtained by reaction between [Cp₂Ru₂(CO)₄] with AgOTf, did not present any catalytic activity, supporting the role of hydroxycyclopentadienyl complex as a bifunctional catalyst. Noteworthy, in the case of 4d[CF₃SO₃] the pyridine itself can be involved in the catalytic cycle as a cooperative ligand through the formation of the active catalyst A in which an hydride and two different acid hydrogens should be available for the hydrogen transfer (Fig. 5).

Fig. 5 Proposed activation of the pre-catalyst 4d[CF₃SO₃].

Indeed, 4d[CF₃SO₃] results to be the best pre-catalyst. Furthermore from the crude reaction mixture at the end of the reaction some resonances in the ¹H-NMR spectrum are detected in the hydride region: a singlet at δ −10.18 ppm is presumably associated with monomeric Ru–H species, while the signal at δ −14.95 ppm falls in the region of bridging hydrides (Ru–H–Ru). Even though we were not able to isolate these hydride species, in analogy with what observed in the synthesis of Shvo catalyst (δ RuHRu −18.80 ppm for the dimeric specie and δ −9.37 ppm for the monomeric one),^18d,23 the formation of active hydride complexes containing the NHC ligand in place of a terminal CO reasonably occurs.

Labeling experiments on the reduction of 4-fluoroacetophenone with 4a[CF₃SO₃] and 4d[CF₃SO₃] in isopropanol-d₈ (see ESI for reaction conditions and spectra†) lead to the formation of the corresponding deuterated alcohol further confirming the role of isopropanol as hydrogen source as well as the action of the catalysts in its activation.

With regard to 4c[CF₃SO₃], which in our experiments leads to the worst conversion after 24 h and lower selectivity (Table 1, entry 22), its molecular structure (Fig. 2, right side) evidences an hydrogen bond between the hydroxycyclopentadienyl ligand and the –OH group in the NHC side chain. Furthermore 4c[CF₃SO₃], upon CO release, would be prone to an intramolecular dehydrogenation, probably leading to the formation of an inactive catalyst. By comparing the behavior of 4c[CF₃SO₃] with that of an iridium complex containing the NHC hydroxymethyl ligand which lead to the formation of an complex upon oxidation and cyclometallation²⁵ we can propose a similar reactivity for our ruthenium complex in agreement with the ν(CO) and ν(C=O) bands found at 1950 cm⁻¹ and 1723 cm⁻¹.

Concerning monodentate NHC complexes 4a–c[X] (X = Cl, BF₄, NO₃, CF₃SO₃) due to the absence of a N-containing side group able to be protonated, the active catalytic species has to be different from the proposed intermediate A. Once a vacant coordination site is formed by CO removal, proton abstraction from ⁱPrOH should be performed by the counteranion CF₃SO₃⁻ as external base, or by the oxygen of the –OH group (or –OMe group) on the catalyst itself, but at present we do not have any evidence to support such hypothesis.

Experimental

General

Solvents: dichloromethane (CH₂Cl₂), tetrahydrofuran (THF), diethyl ether (Et₂O), petroleum ether referring to a fraction of bp 60–80 °C, acetonitrile (CH₃CN) were dried and distilled prior to use. Acetone has been degassed and stored under inert atmosphere on molecular sieves. Other solvents such as ethylacetate (EtOAc), chloroform, ethanol (EtOH), methanol (MeOH), heptane, hexane, 2-propanol, CDCl₃, D₂O, CD₃CN (Sigma Aldrich) have been employed without further purification. Reagents: triruthenium-dodecacarbonil (Ru₃(CO)₁₂) (Strem), methyl iodide, silver oxide, 1-methylimidazole, 1-chlorobutane, 1-butylimidazole, 1-chloroethanol, 1,3-diphenylacetone, 4,4′-dimethoxybenzil (Alfa Aesar), 2-bromopyridine, 4-fluoroacetophenone, acetophenone, benzophenone, benzaldehyde, dodecane, trifluoromethanesulfonic acid, tetrafluoroboric acid diethyl ether complex (Sigma Aldrich), chloridric, nitric and sulfuric acid, methyl trifluoromethanesulfonate, pyridine, cerium ammonium nitrate (CAN), benzoquinone, sodium periodate have been employed as purchased. 1,3-Dimethylimidazolium iodide (1a),²⁶ 1-methyl-3-butyl-imidazolium chloride (1b), 1-methyl-3-(2-hydroxyethyl)imidazolium chloride (1c),²⁷ 1-butyl-3-(2-pyridinyl)-imidazolium bromide (1d),²⁸ 3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone,²⁹ dicarbonyl(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)ruthenium dimer,³⁰ dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone-1,3(dimethyl)-imidazol-2-ylidene)ruthenium (3a), dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone-1-methyl-3-butyl-imidazol-2-ylidene)ruthenium (3b), dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone-1-methyl-3-butyl-imidazol-2-ylidene)ruthenium (3c),²⁰ have been prepared as previously reported. The prepared derivatives were characterized by spectroscopic methods. The NMR spectra were recorded using Varian Inova 300 (¹H, 300.1; ¹³C, 75.5 MHz), Varian Mercury Plus VX 400 (¹H, 399.9; ¹³C, 100.6 MHz), Varian Inova 600 (¹H, 599.7; ¹³C, 150.8 MHz) spectrometers at 298 K; chemical shifts were referenced internally to residual solvent peaks. Full ¹H- and ¹³C-NMR assignments were done, when necessary, by gHSQC and gHMBC NMR experiments using standard Varian pulse sequences. Infrared spectra were recorded at 298 K on a Perkin-Elmer Spectrum 2000 FT-IR spectrophotometer. ESI-MS spectra were recorded on Waters Micromass ZQ 4000 with samples dissolved in MeOH or CH₃CN. Elemental analyses were performed on a Thermo-Quest Flash 1112 Series EA instrument. The conversions were monitored by GC analysis (Agilent Technologies 7890A GC system) and ¹⁹F-NMR. UV irradiation was performed by using a commercial Hg lamp (365 nm, 125 W).

Synthesis of dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)-1-(butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium (3d)

A mixture of 1-butyl-3-(2-pyridinyl)-1H-imidazolium bromide (1d) 0.094 g (0.33 mmol), Ag₂O 0.092 g (0.40 mmol) and dicarbonyl(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)ruthenium dimer (2) 0.200 g (0.16 mmol) in dry CH₂Cl₂ were stirred in the dark under inert atmosphere at room temperature for 1 h. Upon filtration on a celite pad and removal of the solvent the quantitative formation of the dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)-1-(butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium (3d) was verified by ¹H-NMR, ¹³C-NMR, ESI-MS and X-ray crystal structure. Suitable crystals were obtained by slow diffusion from toluene/hexane double layer ¹H-NMR (599.7 MHz, CDCl₃) δ (ppm): 8.36 (dd, 1H, CH_py), 7.60 (m, 4H, CH_aryl), 7.04–6.97 (m, 10H, CH_aryl + 1H, CH_py + 2H, CH_NHC), 6.60–6.55 (m, 4H, CH_aryl + 1H, CH_py), 3.67 (s, 6H, –OCH₃), 3.54 (t, 2H, NCH₂), 1.46 (m, 2H, –CH₂CH₂–), 1.04 (m, 2H, –CH₂CH₃), 0.75 (t, 3H, –CH₃); ¹³C–{¹H}NMR (150.8 MHz, CDCl₃) δ (ppm): 201.77 (CO), 173.20 (C_carbene), 168.22 (C₁=O, Cp), 158.41 (–COCH₃), 152.16 (C_ipso), 148.11 (CH_py), 138.40 (CH_py), 135.17 (Cq_aryl), 133.45 (CH_aryl), 129.49 (CH_aryl), 127.43 (CH_aryl), 125.28 (Cq_aryl), 124.85 (CH_aryl), 124.54 (CH_NHC), 123.89 (CH_py), 122.43 (CH_NHC), 121.82 (CH_py), 112.76 (CH_aryl), 103.10 (C_2,5, Cp), 79.69 (C_3,4, Cp), 54.98 (–OCH₃), 50.72 (NCH₂), 33.08, 19.41 (–CH₂CH₂), 14.11 (–CH₃); IR (CH₂Cl₂) ν(CO): 2008 cm⁻¹, 1949 cm⁻¹, ν(C=O) 1583 cm⁻¹, ν(C=C) 1609 cm⁻¹, 1518 cm⁻¹. ESI-MS (m/z) (+) = 804 [M + H]⁺; 826 [M + Na]⁺. Anal. calcd (%) for C₄₅H₃₉N₃O₅Ru: C, 67.23; H, 4.89; N, 5.23. Found: C, 67.21; H, 4.87; N, 5.24.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1,3-dimethylimidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (4a[CF₃SO₃])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1,3-dimethylimidazol-2-ylidene)ruthenium complex (3a) 0.100 g (0.143 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. 1.2 equivalent of HCF₃SO₃ (solution at 0.98% in CH₂Cl₂) 1.55 mL (0.172 mmol) were subsequently added. The reaction mixture was stirred for 10 minutes at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid obtained was identified as 4a[CF₃SO₃] by IR, ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ESI-MS. The yield was quantitative and suitable crystals for X-ray diffraction analysis were prepared by toluene/hexane double layer. ¹H-NMR (599.7 MHz, CDCl₃) δ (ppm): 7.43 (m, 4H, CH_aryl), 7.27–7.01 (m, 10H, CH_aryl), 6.97 (s, 2H, CH_NHC), 6.65 (m, 4H, CH_aryl), 3.71 (s, 6H, –OCH₃), 3.27 (s, 6H, NCH_3,NHC); ¹³C–{¹H}NMR (150.8 MHz, CDCl₃) δ (ppm): 199.64 (CO), 161.35 (C_carbene), 159.88 (–COCH₃), 142.77 (C–OH, Cp), 133.86 (CH_aryl), 130.93 (CH_aryl), 129.07 (CH_aryl), 125.27 (Cq_aryl), 128.38 (Cq_aryl), 125.97 (CH_aryl), 121.40 (CH_NHC), 113.81 (CH_aryl), 104.85 (C_2,5, Cp), 87.90 (C_3,4, Cp), 55.55 (–OCH₃), 39.23 (CH_3,NHC). ¹⁹F-NMR (282.4 MHz, CDCl₃) δ (ppm): –78.34 (CF₃SO₃). IR (CH₂Cl₂) ν(CO): 2037 cm⁻¹, 1985 cm⁻¹, ν(C=C) 1611 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) = 699 [M]⁺; 149 [M]⁻. Anal. calcd (%) for C₃₉H₃₃F₃N₂O₈RuS: C, 55.18; H, 3.92; N, 3.30. Found: C, 55.14; H, 3.90; N, 3.32.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1,3-dimethylimidazol-2-ylidene)ruthenium]chloride (4a[Cl])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1,3-dimethylimidazol-2-ylidene)ruthenium complex (3a) 0.023 g (0.0330 mmol) was dissolved in 5 mL of CH₂Cl₂ under inert atmosphere. Two equivalent of HCl (aqueous solution at 37%, 0.005 mL, 0.0660 mmol) were subsequently added. The reaction mixture was stirred for 2 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 4a[Cl] by IR, ¹H-NMR, ¹³C-NMR, ESI-MS obtained in quantitative yield. ¹H-NMR (599.7 MHz, CDCl₃) δ (ppm): 7.57 (m, 4H, CH_aryl), 7.23–7.03 (m, 10H, CH_aryl), 6.93 (s, 2H, CH_NHC), 6.65 (m, 4H, CH_aryl), 3.71 (s, 6H, –OCH₃), 3.03 (s, 6H, NCH_3,NHC); ¹³C–{¹H}NMR (150.8 MHz, CDCl₃) δ (ppm): 199.79 (CO), 162.42 (C_carbene), 159.24 (–COCH₃), 144.52 (C–OH, Cp), 133.47 (CH_aryl), 130.95 (CH_aryl), 128.60 (CH_aryl), 128.18 (Cq_aryl), 125.35 (CH_aryl), 121.60 (CH_NHC), 113.24 (CH_aryl), 104.55 (C_2,5, Cp), 86.61 (C_3,4, Cp), 55.09 (–OCH₃), 39.22 (CH_3,NHC). IR (CH₂Cl₂) ν(CO): 2033 cm⁻¹, 1981 cm⁻¹, ν(C=C) 1611 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) = 699 [M]⁺. Anal. calcd (%) for C₃₈H₃₃ClN₂O₅Ru: C, 62.11; H, 4.53; N, 3.81. Found: C, 62.13; H, 4.55; N, 3.79.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1,3-dimethylimidazol-2-ylidene)ruthenium]tetrafluoroborato (4a[BF₄])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1,3-dimethylimidazol-2-ylidene)ruthenium complex (3a) 0.050 g (0.072 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. 1.2 equivalent of tetrafluoroboric acid diethyl ether complex (0.012 mL, 0.086 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 4a[BF₄] by IR, ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR was obtained in quantitative yield. ¹H-NMR (599.7 MHz, CDCl₃) δ (ppm): 7.42 (m, 4H, CH_aryl), 7.31–7.03 (m, 10H, CH_aryl), 7.05 (s, 2H, CH_NHC), 6.68 (m, 4H, CH_aryl), 3.74 (s, 6H, –OCH₃), 3.29 (s, 6H, NCH_3,NHC); ¹³C–{¹H}NMR (150.8 MHz, CDCl₃) δ (ppm): 199.06 (CO), 160.82 (C_carbene), 159.57 (–COCH₃), 141.82 (C–OH, Cp), 133.43 (CH_aryl), 130.40 (CH_aryl), 128.80 (CH_aryl), 128.78 (C_aryl), 127.89 (C_aryl), 125.74 (CH_aryl), 120.80 (CH_NHC), 113.47 (CH_aryl), 104.31 (C_2,5, Cp), 87.51 (C_3,4, Cp), 55.15 (–OCH₃), 38.75 (CH_3,NHC); ¹⁹F-NMR (282.4 MHz, CDCl₃) δ (ppm): –152.01 (BF₄). IR (CH₂Cl₂) ν(CO): 2038 cm⁻¹, 1987 cm⁻¹, ν(C=C) 1611 cm⁻¹, 1520 cm⁻¹. Anal. calcd (%) for C₃₈H₃₃BF₄N₂O₅Ru: C, 58.09; H, 4.20; N, 3.57. Found: C, 58.11; H, 4.17; N, 3.58.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1-methyl-3-butyl-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (4b[CF₃SO₃])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1-methyl-3-butyl-imidazol-2-ylidene)ruthenium complex (3b) 0.100 g (0.135 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. 1.2 equivalent of HCF₃SO₃ (solution at 0.98% in CH₂Cl₂ 1.39 mL, 0.157 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 4b[CF₃SO₃] by IR, ¹H-NMR, ¹³C-NMR and ¹⁹F-NMR was obtained in quantitative yield. ¹H-NMR (599.7 MHz, CDCl₃) δ (ppm): 7.44 (m, 4H, CH_aryl), 7.28–7.02 (m, 10H, CH_aryl), 7.05 (2H, CH_NHC), 6.66 (m, 4H, CH_aryl), 3.72 (s, 6H, –OCH₃), 3.70 (m, 2H, –NCH₂), 3.32 (s, 3H, NCH_3,NHC), 1.49 (m, 2H, –CH₂CH₂–), 0.97 (m, 2H, –CH₂CH₃), 0.77 (m, 3H, –CH₃); ¹³C–{¹H}NMR (150.8 MHz, CDCl₃) δ (ppm): 199.27 (CO), 160.57 (C_carbene), 159.47 (–COCH₃), 142.83 (C–OH, Cp), 133.43 (CH_aryl), 130.55 (CH_aryl), 128.60 (CH_aryl), 128.09 (C_aryl), 126.24 (C_aryl), 123.02 (CH_aryl), 121.01 (CH_NHC), 113.39 (CH_aryl), 104.42 (C_2,5, Cp), 87.45 (C₃,₄, Cp), 55.13 (–OCH₃), 50.81 (–NCH₂), 38.91 (–NCH₃), 32.97 (–CH₂CH₂–), 19.48 (–CH₂CH₃), 13.65 (–CH₃); ¹⁹F-NMR (282.4 MHz, CDCl₃) δ (ppm): –78.37. IR (CH₂Cl₂) ν(CO): 2035 cm⁻¹, 1984 cm⁻¹, ν(C=C) 1610 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) = 741 [M]⁺, 149 [M]⁻. Anal. calcd (%) for C₄₂H₃₉F₃N₂O₈RuS: C, 56.62; H, 4.38; N, 3.15. Found: C, 56.59; H, 4.37; N, 3.16.

Synthesis of [dicarbonyl-η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1-methyl-3-(2-hydroxyethyl-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (4c[CF₃SO₃])

Dicarbonyl-η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1-methyl-3-(2-hydroxyethyl-imidazol-2-ylidene)ruthenium complex (3c) 0.050 g (0.069 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. 1.2 equivalent of HCF₃SO₃ (solution at 0.98% in CH₂Cl₂ 0.73 mL, 0.082 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 4c[CF₃SO₃] by IR, ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ESI-MS was obtained in quantitative yield. Suitable crystals for X-ray diffraction analysis were prepared by slow diffusion from CH₂Cl₂/Et₂O double layer. ¹H-NMR (399.9 MHz, CDCl₃) δ (ppm): 7.46 (m, 4H, CH_aryl), 7.30–7.03 (m, 10H, CH_aryl), 7.40 (s, 1H, CH_NHC), 7.01 (s, 1H, CH_NHC), 6.68 (m, 4H, CH_aryl), CH₂ not detected, (m, 2H, –NCH₂), 3.73 (s, 6H, –OCH₃), 3.24 (s, 3H, –NCH₃); ¹³C-NMR (100.6 MHz, CDCl₃) δ (ppm): 206.97 (CO), 162.98 (C_carbene), 159.58 (–COCH₃), 142.23 (C–OH, Cp), 133.41 (CH_aryl), 130.49 (CH_aryl), 128.84 (CH_aryl) 128.62 (C_aryl), 126.08 (C_aryl), 123.02 (CH_aryl), 120.75 (CH_NHC), 113.48 (CH_aryl), C_aryl, Cp not detected, 58.43 (–NCH₂), 55.14 (–OCH₃), 51.82 (–CH₂OH), 38.45 (–NCH₃); ¹⁹F-NMR (282.4 MHz, CDCl₃) δ (ppm): –78.37. IR (CH₂Cl₂) ν(CO): 2038 cm⁻¹, 1988 cm⁻¹, ν(C=C) 1610 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) = 729 [M]⁺, 149 [M]⁻. Anal. calcd (%) for C₄₀H₃₅F₃N₂O₉RuS: C, 54.67; H, 3.85; N, 3.19. Found: C, 54.68; H, 3.83; N, 3.22.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (4d[CF₃SO₃])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium complex (3d) 0.100 g (0.124 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. 1.2 equivalent of HCF₃SO₃ (solution at 0.98% in CH₂Cl₂ 1.35 mL, 0.149 mmol) were subsequently added. The reaction mixture was stirred for 15 minutes at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly orange solid identified as 4d[CF₃SO₃] by IR, ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ESI-MS was obtained in quantitative yield. ¹H-NMR (599.7 MHz, CD₃CN) δ (ppm): 7.88 (m, 1H, CH_py), 7.38 (m, 1H, CH_py), 7.34 (m, 1H, CH_py), 7.28 (d, 1H, CH_NHC), 7.25 (d, 1H, CH_NHC), 7.23 (m, 1H, CH_py), 7.17–6.99 (m, 10H, CH_aryl), 6.78 (m, 4H, CH_aryl), 6.48 (m, 4H, CH_aryl), 3.70 (t, 2H, NCH₂), 3.68 (s, 6H, –OCH₃), 1.72 (m, 2H, –CH₂CH₂–), 1.19 (m, 2H, –CH₂CH₃), 0.88 (t, 3H, –CH₃); ¹³C–{¹H}NMR (150.8 MHz, CD₃CN) δ (ppm): 199.59 (CO), 166.73 (C_carbene), 160.80 (–COCH₃), 149.27 (C_ipso), 147.43 (CH_py), 144.94 (CH_py), 136.56 (C₁–OH, Cp), 134.52–114.29 (C_aryl), 127.93 (CH_NHC), 125.71 (CH_py), 125.13 (CH_py), 121.95 (CH_NHC), 104.10 (C_2,5, Cp), 94.05 (C_3,4, Cp), 55.98 (–OCH₃), 52.87 (–NCH₂), 33.72 (–CH₂CH₂), 20.42 (–CH₂CH₃), 13.93 (–CH₃); ¹⁹F-NMR (282.4 MHz, CD₃CN) δ (ppm): –79.33 (CF₃SO₃). IR (CH₂Cl₂) ν(CO): 2041 cm⁻¹, 1991 cm⁻¹, ν(C=C) 1610 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) = 804 [M]⁺, 149 [M]⁻. Anal. calcd (%) for C₄₆H₄₀F₃N₃O₈RuS: C, 57.91; H, 4.23; N, 4.41. Found: C, 57.89; H, 4.22; N, 4.38.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium]chloride (4d[Cl])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium complex (3d) 0.025 g (0.031 mmol) was dissolved in 5 mL of CH₂Cl₂ under inert atmosphere. Two equivalent of HCl (aqueous solution at 37%, 0.005 mL, 0.061 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 4d[Cl] by IR, ¹H-NMR, ¹³C-NMR obtained in quantitative yield. ¹H-NMR (599.7 MHz, CD₃CN) δ (ppm): 8.10 (m, 1H, CH_py), 7.69 (m, 1H, CH_py), 7.48 (d, 2H, CH_NHC), 7.45 (d, 2H, CH_NHC), 7.41–7.19 (m, 10H, CH_aryl + 2H, CH_py), 6.98 (m, 4H, CH_aryl), 6.65 (m, 4H, CH_aryl), 3.68 (t, 2H, –NCH₂), 3.69 (s, 6H, –OCH₃), 1.70 (m, 2H, –CH₂CH₂–), 1.14 (m, 2H, –CH₂CH₃), 0.84 (t, 3H, –CH₃); ¹³C–{¹H}NMR (150.8 MHz, CD₃CN) δ (ppm): 199.73 (CO), 166.09 (C_carbene), 160.67 (–COCH₃), 150.34 (C_ipso), 147.87 (CH_py), 143.52 (CH_py), 136.47 (C₁–OH, Cp), 134.62–114.23 (C_aryl + CH_py), 124.71 (CH_NHC), 122.13 (CH_NHC), 102.43 (C_2,5, Cp), 87.44 (C_3,4, Cp), 55.95 (–OCH₃), 52.65 (–NCH₂), 33.24 (–CH₂CH₂), 20.40 (–CH₂CH₃), 13.93 (–CH₃); IR (CH₂Cl₂) ν(CO): 2041 cm⁻¹, 1992 cm⁻¹, ν(C=C) 1609 cm⁻¹, 1521 cm⁻¹. Anal. calcd (%) for C₄₅H₃₉ClN₃O₅Ru: C, 64.43; H, 4.69; N, 5.01. Found: C, 64.40; H, 4.66; N, 5.02.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium]tetrafluoroborate (4d[BF₄])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium complex (3d) 0.050 g (0.062 mmol) was dissolved in 5 mL of CH₂Cl₂ under inert atmosphere. 1.2 equivalent of tetrafluoroboric acid diethyl ether complex (0.012 mL, 0.086 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 4d[BF₄] by IR, ¹H-NMR, ¹⁹F-NMR, ¹³C-NMR, ESI-MS was obtained in quantitative yield. ¹H-NMR (599.7 MHz, CD₃CN) δ (ppm): 7.95 (m, 1H, CH_py), 7.49–7.16 (m, 10H, CH_aryl + 3H, CH_py + 2H, CH_NHC), 6.98 (m, 4H, CH_aryl), 6.69 (m, 4H, CH_aryl), 3.69 (s, 6H, –OCH₃), 3.67 (t, 2H, NCH₂), 1.75 (m, 2H, –CH₂CH₂–), 1.25 (m, 2H, –CH₂CH₃), 0.79 (t, 3H, -CH₃); ¹³C–{¹H}NMR (150.8 MHz, CD₃CN) δ (ppm): 199.65 (CO), 166.30 (C_carbene), 160.78 (–COCH₃), 151.52 (C_ipso), 149.07 (CH_py), 142.05 (CH_py), 134.63 (C₁–OH, Cp), 134.06–114.30 (C_aryl), 126.82 (CH_NHC), 124.74 (CH_py), 124.25 (CH_py), 122.20 (CH_NHC), 104.16 (C_2,5, Cp), 87.35 (C_3,4, Cp), 56.00 (–OCH₃), 52.84 (NCH₂), 33.21 (–CH₂CH₂–), 20.55 (–CH₂CH₃), 13.91 (–CH₃); ¹⁹F-NMR (282.4 MHz, CD₃CN) δ (ppm): −151.63 (BF₄). IR (CH₂Cl₂) ν(CO): 2041 cm⁻¹, 1992 cm⁻¹, ν(C=C) 1609 cm⁻¹, 1521 cm⁻¹. Anal. calcd (%) for C₄₅H₃₉BF₄N₃O₅Ru: C, 60.66; H, 4.41; N, 4.71. Found: C, 60.63; H, 4.40; N, 4.69.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylmethoxycyclopentadienyl)(1,3-dimethyl-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (5a[CF₃SO₃])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1,3-dimethylimidazol-2-ylidene)ruthenium complex (3a) 0.100 g (0.143 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. Three equivalent of MeCF₃SO₃ (0.048 mL, 0.429 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 5a[CF₃SO₃] by IR, ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ESI-MS was obtained in quantitative yield. ¹H-NMR (599.7 MHz, CD₃CN) δ (ppm): 7.40–7.31 (m, 10H, CH_aryl), 7.13 (s, 2H, CH_NHC), 7.04 (m, 4H, CH_aryl), 6.66 (m, 4H, CH_aryl), 3.68 (s, 6H, –OCH₃), 3.43 (s, 6H, NCH₃), 2.99 (s, 3H, –OCH₃); ¹³C-NMR (150.8 MHz, CD₃CN) δ (ppm): 199.52 (CO), 161.90 (C_carbene), 160.59 (–COCH₃), 141.45 (C–OCH₃, Cp), 134.53 (CH_aryl), 132.34 (CH_aryl), 129.66 (CH_aryl), 129.28 (C_aryl), 128.24 (C_aryl), 126.22 (CH_aryl), 121.71 (CH_NHC), 114.04 (CH_aryl), 104.86 (C_2,5, Cp), 93.24 (C_3,4, Cp), 62.47 (–OCH₃, Cp), 55.87 (–OCH₃), 39.71 (–NCH₃); ¹⁹F-NMR (282.4 MHz, CD₃CN) δ (ppm): −78.23. IR (CH₂Cl₂) ν(CO): 2045 cm⁻¹, 1995 cm⁻¹; ν(C=C) 1610 cm⁻¹, 1521 cm⁻¹. ESI-MS (m/z) (+) = 713 [M]⁺; 149 [M]⁻. Anal. calcd (%) for C₄₀H₃₅F₃N₂O₈RuS: C, 55.68; H, 4.09; N 3.25. Found: C, 55.65; H, 4.07; N 3.27.

Synthesis of [dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylmethoxycyclopentadienyl1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (5d[CF₃SO₃])

Dicarbonyl-(η⁴-3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopenta-2,4-dienone)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium complex (3d) 0.050 g (0.072 mmol) was dissolved in 10 mL of CH₂Cl₂ under inert atmosphere. Three equivalent of MeCF₃SO₃ (0.023 mL, 0.216 mmol) were subsequently added. The reaction mixture was stirred for 1 h at room temperature, then the solvent was removed under vacuum and the crude washed twice with 10 mL of Et₂O. The slightly brown solid identified as 5d[CF₃SO₃] by IR and ESI-MS was obtained in quantitative yield. ¹H-NMR (599.7 MHz, CD₃CN) δ (ppm): 8.47 (m, 1H, CH_py), 7.87 (m, 1H, CH_py), 7.71 (m, 1H, CH_py), 7.65 (d, 1H, CH_NHC), 7.56 (d, 1H, CH_NHC), 7.46–6.59 (m, 18H, CH_aryl + 1H, CH_py), 3.69 (s, 6H, –OCH₃), 3.63 (t, 2H, NCH₂), 3.15 (s, 6H, –OCH₃, Cp), 1.52 (m, 2H, –CH₂CH₂–), 1.11 (m, 2H, –CH₂CH₃), 0.77 (t, 3H, –CH₃); ¹³C–{¹H}NMR (150.8 MHz, CD₃CN) δ (ppm): 199.85 (CO), 167.40 (C_carbene), 160.61 (–COCH₃), 150.52 (C_ipso), 149.56 (CH_py), 145.70 (CH_py), 140.66 (C₁–OCH₃, Cp), 134.60–114.14 (C_aryl), 127.56 (CH_NHC), 124.19 (CH_py), 121.64 (CH_py), 120.73 (CH_NHC), C_aryl, Cp not detected, 63.36 (–OCH₃, Cp), 55.93 (–OCH₃), 52.14 (–NCH₂), 33.04 (–CH₂CH₂), 20.15 (–CH₂CH₃), 14.01 (–CH₃); ¹⁹F-NMR (282.4 MHz, CD₃CN) δ (ppm): –79.32 (CF₃SO₃). IR (CH₂Cl₂) ν(CO): 2045 cm⁻¹, 1996 cm⁻¹, ν(C=C) 1611 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) = 818 [M]⁺; 149 [M]⁻. Anal. calcd (%) for C₄₇H₄₂F₃N₃O₈RuS: C, 58.32; H, 4.34; N, 4.34. Found: C, 58.30; H, 4.32; N, 4.35.

Synthesis of [carbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylmethoxycyclopentadienyl)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (6d[CF₃SO₃])

[Dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1-butyl-3-(2-pyridinyl)-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (4d[CF₃SO₃]) was dissolved in 25 mL of ⁱPrOH. The reaction mixture was irradiated under stirring for 3 h, then the solvent was removed under vacuum and the crude washed twice with 20 mL of hexane. The brown solid was identified as 6 and obtained with an yield of 78%. 6d[CF₃SO₃] has been completely characterized by IR, ¹H-NMR, ¹³C-NMR, ¹⁹F-NMR, ESI-MS. Suitable crystals for X-ray diffraction analysis were prepared by slow diffusion from CH₂Cl₂/hexane double layer. ¹H-NMR (599.7 MHz, CDCl₃) δ (ppm): 8.33 (d, 1H, CH_py), 8.27 (m, 1H, CH_py), 8.09 (m, 1H, CH_py), 7.04 (m, 4H, CH_aryl), 7.30–6.66 (m, 2H, CH_NHC + 10H, CHaryl), 3.86 (m, 2H, NCH₂), 3.79 (s, 3H, –OCH₃), 3.74 (s, 3H, –OCH₃), 1.47 (m, 2H, –CH₂CH₂), 0.97 (m, 2H, –CH₂CH₃), 0.72 (t, 3H, –CH₃); ¹³C–{¹H}NMR (150.8 MHz, CDCl₃) δ (ppm): 199.57 (CO), 185.63 (C_carbene), 159.30, 159.08 (–COCH₃), 154.60 (C_ipso), 153.34 (CH_py), 141.53 (C₁–OH, Cp), 138.08 (CH_py), 133.31–112.74 (C_aryl + CH_NHC), 97.38 (C_2,5, Cp), 84.34 (C_3,4, Cp), 55.21 (–OCH₃), 50.67 (NCH₂), 32.66, 19.35 (–CH₂CH₂), 13.56 (–CH₃); ¹⁹F-NMR (282.4 MHz, CD₃CN) δ (ppm): −78.19 (CF₃SO₃); IR (CH₂Cl₂) ν(CO): 1967 cm⁻¹, ν(C=C) 1609 cm⁻¹, 1521 cm⁻¹; ESI-MS (m/z) (+) = 776 [M]⁺, 149 [M]⁻. Anal. calcd (%) for C₄₅H₄₀F₃N₃O₇RuS: C, 58.37; H, 4.36; N, 4.54. Found: C, 58.35; H, 4.35; N, 4.51.

Synthesis of [carbonyl-pyridin-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1,3-dimethylimidazol-2-ylidene)ruthenium] trifluoromethanesulfonate (7[CF₃SO₃])

[Dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylhydroxycyclopentadienyl)(1,3-dimethylimidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (4a[CF₃SO₃]) 0.060 g (0.071 mmol) and pyridine 0.023 mL (0.283 mmol) were dissolved in ⁱPrOH (25 mL). The reaction mixture was irradiated under stirring for 6 h and 7[CF₃SO₃] identified from the reaction mixture by IR and ESI-MS. IR (CH₂Cl₂) ν(CO): 1943 cm⁻¹, ν(C=C) 1610 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) (CH₃CN) = 671 [M − py]⁺, 643 [M − py − CO]⁺.

Synthesis of [carbonyl-pyridin-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylmethoxycyclopentadienyl)(1,3-dimethylimidazol-2-ylidene)ruthenium] trifluoromethanesulfonate (8[CF₃SO₃])

[Dicarbonyl-(η⁵-3,4-bis(4-methoxyphenyl)-2,5-diphenylmethoxycyclopentadienyl)(1,3-dimethyl-imidazol-2-ylidene)ruthenium]trifluoromethanesulfonate (5a[CF₃SO₃]) 0.050 g (0.058 mmol) and pyridine 0.023 mL (0.232 mmol) were dissolved in toluene (25 mL). The reaction mixture was irradiated under stirring for 18 h and 8[CF₃SO₃] identified from the reaction mixture by IR and ESI-MS. IR (CH₂Cl₂) ν(CO): 1948 cm⁻¹, ν(C=C) 1610 cm⁻¹, 1520 cm⁻¹. ESI-MS (m/z) (+) (MeOH) = 764 [M]⁺; 149 [M]⁻.

General procedure for transfer hydrogenation

Complex (15 μmol, 5% mol), additive (1 eq. when needed) and ⁱPrOH (3 mL) were stirred at reflux for 15 min. Then 4-fluoroacetophenone (36 μL, 300 μmol) was added and samples were taken at regular intervals. Aliquots (ca. 0.05 mL) were diluted with CDCl₃ (0.5 mL) and conversions were determined by ¹⁹F-NMR spectroscopy and GC analysis.

X-ray crystallography

Crystal data and collection details for 3d·0.5CH₂Cl₂, [4a][CF₃SO₃]·0.5toluene, [4c][CF₃SO₃]·CHCl₃ and [6d][CF₃SO₃] are reported in Table S1.† The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a CCD detector using Mo-Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).³¹ Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F².³² All hydrogen atoms were fixed at calculated positions and refined by a riding model, unless otherwise stated. All non-hydrogen atoms were refined with anisotropic displacement parameters.

3d·0.5CH₂Cl₂. The asymmetric unit of the unit cell contains one 3d complex (on a general position) and half of a CH₂Cl₂ molecule disordered over two symmetry related (by 2) positions. The disordered solvent molecule has been refined isotropically. The crystals are non-merohedrally twinned. The TwinRotMat routine of PLATON³³ was used to determine the twinning matrix and to write the reflection data file (.hkl) containing the two twin components. Refinement was performed using the instruction HKLF 5 in SHELXL and one BASF parameter (0.291(8) after refinement). The Bu-group of the 3d complex is disordered over two positions and has been refined isotropically using one occupancy factor per disordered group. Similar U restraints (SIMU line in SHELXL; s.u. 0.01) have been applied to C, N and O atoms. Restraints to bond distances were applied as follow (s.u. 0.02): 1.53 Å for C–C in the disordered Bu-group of 3d; 1.75 Å for C–Cl in CH₂Cl₂. The Ph-rings have been constrained to fit regular hexagons (AFIX 66 line in SHELXL).

[4a][CF₃SO₃]·0.5toluene. The asymmetric unit of the unit cell contains two cations, two anions and one toluene molecule (all located in general positions). The H-atoms bonded to the O-atoms have been located in the Fourier map and refined using the 1.5 fold U_iso value of the parent atoms. The O–H distances have been restrained to 0.89 Å (s.u. 0.01). Similar U restraints (SIMU line in SHELXL; s.u. 0.01) have been applied to C and F atoms. The F-atoms have been restrained to isotropic behaviour (ISOR line in SHELXL, s.u. 0.01).

[4c][CF₃SO₃]·CHCl₃. The asymmetric unit of the unit cell contains two cations, two anions and two CHCl₃ molecules (all located in general positions). One CHCl₃ molecule is disordered over two positions and has been refined using one occupancy parameter per disordered group. The H-atoms bonded to the O-atoms have been initially located in the Fourier map and, then, refined by a riding model. The F-atoms have been restrained to isotropic behaviour (ISOR line in SHELXL, s.u. 0.01).

[6d][CF₃SO₃]. The asymmetric unit of the unit cell contains one cation and one anion (all located in general positions). The F and O atoms of the CF₃SO₃⁻ anion are disordered and, thus, have been split and refined using one occupancy parameter per disordered group. The chelating NHC ligand in the cation is disordered and, thus, it has been split into two positions and refined using one occupancy parameter per disordered group. The H-atoms bonded to the O-atoms have been initially located in the Fourier map and, then, refined by a riding model. Similar U restraints (SIMU line in SHELXL; s.u. 0.01) have been applied to C, N, F and O atoms. The C, N, O and F atoms of the disordered groups have been restrained to isotropic behaviour (ISOR line in SHELXL, s.u. 0.01). The Ph-rings have been constrained to fit regular hexagons (AFIX 66 line in SHELXL). Restraints to bond distances were applied as follow (s.u. 0.02): 1.53 Å for C–C in the Bu-group; 1.43 Å for C(sp³)–O; 1.34 Å for C(sp²)–O.

Conclusions

A series of cationic hydroxy- or methoxy-cyclopentadienyl ruthenium(II) N-heterocyclic carbene complexes have been prepared in quantitative yield upon protonation with strong acids or methylation with MeCF₃SO₃ of neutral ruthenium(0) complexes containing both the non-innocent cyclopentadienone ligand and variously functionalized NHCs. These novel complexes have been successfully employed as selective catalysts in transfer hydrogenation. The reaction is general and several substrates such as various ketones and aldehydes can be selectively reduced to the corresponding alcohol. The catalytic activity can thus be tuned by the proper choice of both substituents on NHC ligand and counterions. Neutral Ru(0) complexes 3a–d need an oxidizing agent such as CAN or benzoquinone in order to be activated, while the corresponding cationic complexes 4a–d[X] (X = Cl, BF₄, NO₃, CF₃SO₃) and 5a,d[CF₃SO₃], formally containing a Ru(II) center are more prone to release a CO ligand and do not need any additives to be activated. In the cationic complexes the counterion plays a non-innocent role and CF₃SO₃⁻ showed the best outcome due to its coordinating ability.

Noteworthy the insertion of a pyridine substituent on the NHC side chain further improves the catalytic activity due to the presence of a second cooperative ligand containing a basic nitrogen.

With regard to the reaction mechanism CO release and its key role in the catalyst activation was demonstrated by trapping experiment with pyridine as well as by the UV-mediated formation of the chelated complex 6d[CF₃SO₃]. Hydride species has been identified in the reaction mixture of the pyridine functionalized complex 4d[CF₃SO₃] and the cooperative function of nitrogen- (pyridine in 4d[X] and 5d[CF₃SO₃]) and oxygen-containing (CF₃SO₃⁻ or –OH and –OMe in 4a–c[X] and 5a[CF₃SO₃]) moieties with the vacancy on ruthenium prone to accept an hydride species has been drawn as consideration in order to explain ⁱPrOH activation and subsequent active catalyst formation.

The versatility of NHC ligands which can be variously functionalized, together with the presence of the non-innocent cyclopentadienone open the way to the rational design of novel metal-ligand bifunctional catalysts liable to heterogenization and tunable solubility.

Acknowledgements

Authors wish to thank the University of Bologna and the FARB Project “Catalytic transformation of biomass-derived materials into high added-value chemicals”, 2014–2015.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra of novel complexes; selected ESI-MS; crystal data and experimental details; catalytic time–conversion profiles. CCDC 1421543–1421546. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18345f

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