Construction of pincer-type symmetrical ruthenium(II) complexes bearing pyridyl-2,6-pyrazolyl arms: catalytic behavior in transfer hydrogenation of ketones

Abstract

Convenient synthesis of four new distorted octahedral ruthenium(II) complexes (1, 2, 3, 4) having general molecular formula [RuCl₂LPAr₃] (L = pyridine-based tridentate ligands not containing N–H bonds) is described. Their composition and structure were determined by elemental analysis and NMR spectra, and complexes 2 and 4 were also identified by X-ray single-crystal diffraction. All ruthenium(II) complexes exhibited good to excellent catalytic activity in the transfer hydrogenation of ketones. Among them, complex 4 achieved the highest final TOF value of 51 600 h⁻¹ for a high molar ratio of substrate to catalyst (2000 : 1).

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Introduction

Catalytic transfer hydrogenation (TH) of unsaturated organic compounds has become a reliable synthetic protocol beyond conventional reduction reactions,¹ not only because it meets the increasing demand for clean and environmentally benign processes in chemistry, but also because it is considered to be of relatively low cost and easy to carry out. More recently, there has been rapid growth in the various transition metal complexes such as cobalt,² nickel,³ palladium,^4a iron,^4b,c rhodium,^5a and iridium^5b–d employed for the TH of ketones, and particular attention has been devoted to ruthenium-based⁶ complexes.

Varying levels of catalytic efficiency were observed for this transformation in the presence of ruthenium complexes containing Schiff base,⁷ tripodal phosphine [MeC(CH₂PPh₂)₃],⁸ arene,⁹ N-heterocyclic carbene,¹⁰ amine-based ligands,¹¹ and pincer ligands.¹² In particular, pincer-type tridentate pyridine-based frameworks (L₁–Py–L₂) have recently been proven to be convenient and attractive ligands because of their tunable properties and potential application. Thus, much effort has been devoted to the preparation of the tridentate pyridine-based analogues PNP,¹³ PNN¹⁴, and CNN.¹⁵

There has recently been reported a highly active ruthenium(II) NNN phosphine complex¹⁶ for TH of ketones. An acceleration effect is shown by the N–H functionality in its pyrazolyl arms. However, several symmetrical and unsymmetrical Ru(II) catalysts featuring no N–H functionality have also been documented for TH of ketones: complexes (A),^17a (B),^17b (C),^17c and (D)^17d (Scheme 1). Among the two unsymmetrical Ru(II) complexes, (C) exhibited poor catalytic activity in the presence of 0.3 mol% catalyst with final TOF value of 1960 h⁻¹ and (D) reached 5940 h⁻¹ at a substrate-to-catalyst ratio of 200 : 1. Symmetrical (A) (6000 h⁻¹) exhibited higher catalytic activity than (B) (1080 h⁻¹) for a substrate-to-catalyst ratio of 200 : 1.

Scheme 1 Ru(II) complexes (A)–(D).

Previously, our research group reported the synthesis of symmetrical pincer-type tridentate pyridine-bridged framework NNN ligands, not incorporating the NH group (Scheme 2).^18a Based on this result, we intended to synthesize symmetrical ruthenium(II) complexes featuring no N–H functionality which can catalyze the hydrogenation of ketones at a low catalyst loading (Scheme 3).

Scheme 2 Ligands L1 and L2.

Scheme 3 Ru(II) complexes 1, 2, 3 and 4.

Results and discussion

Preparation and characterization of Ru(II) complexes 1–4

Reacting L1 or L2 with an equivalent amount of RuCl₃·3H₂O, PAr₃ (1 equiv.) and triethylamine (1 mL) in ethanol afforded the ruthenium compounds 1–4 in red solid state by a means similar to a literature procedure (Scheme 4).^18b

Scheme 4 Preparation of compounds 1–4.

These target ruthenium complexes 1–4 were characterized by ¹H NMR, ¹³C NMR, ³¹P NMR and elemental analysis (see ESI†). The ³¹P NMR spectra of 1–4 showed a singlet at δ = 39.9, 43.9, 45.8 and 44.1 ppm respectively. In the ¹H NMR spectra of compound 1, 2 and 3, the resonances of –NCH₃ and –CH₂ of pyrazolyl, due to the coordination of L1 with ruthenium metal, have a slight shift compared to those of L1 (1–3: δ (–NCH₃) = 4.17, 4.16, 4.19 ppm, δ (–CH₂) = 6.85, 6.84, 6.86 ppm; L1: δ (–NCH₃) = 3.98 ppm, δ (–CH₂) = 7.14 ppm). There is a similar phenomenon in the ¹H NMR spectrum of compound 4.

By slow diffusion of diethyl ether into a CH₂Cl₂ solution of complexes, single crystals of 2 and 4 were obtained (Fig. 1 and 2). Attempts to obtain single crystals of 1 and 3 were not successful. The crystal structures of 2 and 4 were consistent with the results of NMR and elemental analysis (see ESI†).

Fig. 1 The molecular structure of complex 2. Selected bond lengths [Å]: Ru1–Cl1 = 2.4558(9), Ru1–Cl2 = 2.4681(10), Ru1–P1 = 2.2958(11), Ru1–N2 = 2.117(3), Ru1–N3 = 1.989(3), Ru1–N4 = 2.081(3); angles [°]: Cl1–Ru1–Cl2 = 87.55(3), P1–Ru1–Cl1 = 87.15(3), P1–Ru1–Cl2 = 171.77(4), N3–Ru1–Cl1 = 178.53(10), N3–Ru1–P1 = 94.31(9), N4–Ru1–N2 = 153.96(12), N3–Ru1–Cl2 = 91.00(9).

Fig. 2 The molecular structure of complex 4. Selected bond lengths [Å]: Ru1–Cl1 = 2.4646(17), Ru1–Cl2 = 2.4550(17), Ru1–P1 = 2.2760(17), Ru1–N1 = 1.999(5), Ru1–N2 = 2.106(5), Ru1–N4 = 2.085(5); angles [°]: Cl1–Ru1–Cl2 = 87.72(6), P1–Ru1–Cl1 = 178.28(6), P1–Ru1–Cl2 = 93.19(6), N1–Ru1–Cl2 = 172.87(16), N1–Ru1–P1 = 93.88(16), N4–Ru1–N2 = 154.9 (2), N1–Ru1–Cl1 = 85.18(16).

The perspective view of 2 is shown in Fig. 1. A distorted octahedral geometry around the ruthenium center was observed, with the two cis Cl atoms and the phosphorus located in the apical position. The bond angles of Cl(1)–Ru–Cl(2), P–Ru–Cl(1), P–Ru–N(3) and N(3)–Ru–Cl(2) are 87.55(3)°, 87.15(3)°, 94.31(9)° and 91.00(9)° and the bond lengths of Ru–Cl(1), Ru–Cl(2), Ru–P, Ru–N(2), Ru–N(3), Ru–N(4) are 2.4558 Å, 2.4681 Å, 2.2958 Å, 2.117 Å, 1.989 Å, 2.081 Å.

The structural assignment of complex 4 is similar to that of complex 2 (Fig. 2). The bond angles of Cl(1)–Ru–Cl(2), P–Ru–Cl(2), P–Ru–N(1) and N(1)–Ru–Cl(1) are 87.72(6)°, 93.19(6)°, 93.88(16)° and 85.18(16)° respectively. The bond angle of P1–Ru1–Cl1 (178.28(6)°) is obviously larger than that (171.77(4)°) of P1–Ru1–Cl2 in compound 2. The Ru–Cl(1) (2.4646 Å) bond length is longer than that of Ru–Cl(2) (2.4550 Å), possibly because the phosphine ligand is in the trans-position of Cl(1). The average bond length (2.063 Å) of Ru–N is a little longer than that (2.028 Å) of [RuCl₂(PPh₃)(Me₄BPPy)] (A)^17a and that (2.053 Å) of the reported analogous complex [RuCl₂(PPh₃)(NNN)] (NNN: tridentate dipyrazolpyridines).¹⁹

Hydrogenation of ketones catalyzed by complexes 1–4

Complexes 1–4 were employed as the catalysts for TH of acetophenone at a low catalyst loading of 0.1 mol%, full details of which are provided in Table 1. From entries 1–3 in Table 1, it can be seen that catalyst 2 containing triphenylphosphine is a judicious choice for this transformation. Over a period of 1 h, conversion of acetophenone reached 64%, 82%, and 59% for 1, 2, and 3 respectively, revealing an order of the catalytic activity for TH of ketones: 2 > 1 > 3, under the precise control of tridentate NNN ligand's electronic and geometric properties. Compared with the L1 ligand, complex 4 bearing L2 ligand was also applied in the catalytic system for the TH reaction of ketones and it exhibited the highest catalytic activity under the same conditions (Table 1, entries 4 vs. 2). So complex 4 was chosen as the catalyst in the following investigation.

Table 1 Screening the catalytic activity of various complexes^a

Entry	Catalyst	Base	Conv.^b (%)
			10 min	60 min	120 min	240 min
a Reaction conditions: acetophenone (3.2 mmol), catalysts 1–4 (0.1 mol%) and base (2 mol%) dissolved in iPrOH (4 mL).b Conversion determined by GC.
1	1	iPrOK	43	64	69	75
2	2	iPrOK	55	82	89	90
3	3	iPrOK	18	59	76	80
4	4	iPrOK	76	88	90	91

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After examining the effects of various bases in the presence of catalyst 4, we found only a trace amount of desired product was detected in the absence of base, indicating that the presence of base is essential for the reaction to proceed efficiently (Table 2, entry 10). Strong bases showed a better promotional role than weak ones (Table 2, entries 1–9). KOH was selected as the reaction promoter, although iPrOK, iPrONa, and iPrOLi also worked well in the reaction.

Table 2 Optimizing condition of base for TH of ketones^a

Entry	Base	Conv.^b (%)
		10 min	60 min	120 min	240 min
a Reaction conditions: acetophenone (3.2 mmol), catalyst 4 (0.1 mol%) and base (2 mol%) dissolved in iPrOH (4 mL).b Conversion determined by GC.
1	tBuOK	61	87	88	90
2	iPrOK	76	88	90	91
3	iPrONa	75	89	90	90
4	iPrOLi	75	89	90	91
5	KOH	85	90	90	91
6	K3PO4	67	83	84	85
7	K2CO3	35	50	58	68
8	NaHCO3	56	76	80	85
9	CH3COOK	—	—	—	—
10	—	—	—	—	—

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After attempts to optimize the control of the reaction conditions, we found that the transformation was smoothly performed in the presence of KOH with a substrate-to-catalyst ratio of 2000 : 1, using iPrOH as hydrogen source. Next, the catalytic behavior of complex 4 was explored with aryl-substituted acetophenone derivatives and additional ketones as substrates (Table 3).

Table 3 Transfer hydrogenation of various ketones^a

Entry	Ketones	Time (min)	Conv.^b (%)	TOF^c (h^−1)
a Reaction conditions: acetophenone (3.2 mmol), catalyst 4 (0.05 mol%), KOH (2.0 mol%), iPrOH (4 mL).b Conversion determined by GC. Data in parentheses are the GC yields after 2 min.c TOF in 2 min.d 0.1 mol% 4 was used.e 0.2 mol% 4 was used.
1		10	97 (73)	43 800
2		30	98 (50)	30 000
3		5	98 (83)	49 800
4		5	96 (82)	49 200
5		60	96 (40)	24 000
6		10	94 (76)	45 600
7		10	98 (86)	51 600
8		900	93^d (35)	10 500
9		30	91 (76)	45 600
10		60	92 (27)	16 200
11		120	93 (28)	16 800
12		900	71^d (18)	5400
13		60	92 (38)	22 800
14		30	98 (42)	25 200
15		120	97^d (48)	14 400
16		30	93 (61)	36 600
17		1200	53^e (27)	4050
18		120	90 (26)	15 600
19		30	99 (63)	37 800
20		60	92 (46)	27 600
21		60	90 (30)	18 000

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As seen in Table 3, most of the substrates could be converted in good to excellent yields (≥90%) and with a TOF value of 12 000–51 600 h⁻¹ in initial reaction. Favorable yields were expected for the weaker electron-deficient substrates (Table 3, entries 2–6, 14) as the strong electron-withdrawing CF₃ group in the substrate reduced the electron density in the ketone group and diminished the catalytic efficiency (Table 3, entry 15). It should be noted that the electron-donating substituents, that is, methyl and methoxyl, made the ketone substrates more electron-rich and thus reacted less efficiently than their analogues bearing electron-deficient moieties (Table 3, entries 3 vs. 8, 11 and 4, 6 vs. 9, 12 and 2, 5 vs. 10) except 2′-chloroacetophenone (Table 3, entry 7).

As anticipated, increasing the steric hindrance of substrates retarded the reaction. But attempts to prolong reaction time or increase catalyst loading provided satisfactory yields (Table 3, entries 16–19). Surprisingly, although acetophenones bearing an ortho-Cl or Br substituent reacted slower than those bearing meta or para substituents, a contradictory result was observed for the ortho-methyl, methoxyl substrates (Table 3, entries 2 vs. 3, 4 and 5 vs. 6 and 7 vs. 8, 9 and 10 vs. 11, 12).

According to the related literature,^16b–d,20 we were encouraged to propose a plausible mechanism as shown in Scheme 5, even though the Ru–H active species of complex 4 was not isolated successfully. Ru(II) species (E) forms Ru(II)-alkoxide (F) in the presence of KOH and iPrOH; then the subsequent β-H elimination from (F) generates Ru(II)–H intermediate (G) with release of acetone. Coordination to the metal center and insertion to the Ru–H bond in (G) by the carbonyl of a ketone substrate yields Ru(II)-alkoxide (I), which undergoes alcohol metathesis to give the desired product and complete the catalytic cycle.

Scheme 5 A plausible catalytic cycle.

Experimental section

General considerations

Unless otherwise noted, all the starting materials were purchased from commercial sources and used as received. The solvents were dried using standard procedures prior to use. All the preparations and purifications of air- and/or moisture-sensitive compounds were carried out under a nitrogen atmosphere using standard Schlenk techniques. Ligands L1 and L2 were prepared by means of a literature procedure.^{18a 1}H NMR, ¹³C NMR and ³¹P NMR spectra were recorded using a Bruker Advance II 400 MHz spectrometer. The chemical shifts of ³¹P{¹H} NMR were relative to 85% H₃PO₄ as external standard, and those of ¹H NMR relative to TMS as internal standard. Elemental analysis was carried out with a Carlo Erba 1106 (Italy) elemental analyzer. Mass spectrometry (MS) was performed with an Agilent LCMS-IT-TOF Premier mass spectrometer. Single crystals of 2 and 4 were measured with a Xcalibur Eos diffractometer using graphite monochromated Mo Kα radiation (λ = 0.7107 Å) at 142.95(10) K and 140.00(10) K, respectively. Using Olex2, the structure was solved with the Superflip structure solution program using charge flipping and refined with the ShelXL-2012 refinement package using least squares minimization. The chromatographic analyses (GC) were performed with an Agilent 6890N instrument equipped with an FID detector and an EC-WAX capillary column (30 m × 0.25 mm, 0.25 μm film) to detect the reaction products.

Synthesis of Ru(II) complexes 1–4

Under nitrogen atmosphere, a mixture of RuCl₃·3H₂O (143 mg, 0.55 mmol) and L1 or L2 (215/146 mg, 0.55 mmol) in EtOH (30 mL) was refluxed for 5 h. The color of the solution changed from black-brown to red-brown slowly and further generated red-brown precipitates. After being cooled to room temperature, the precipitates were filtered, washed with EtOH and Et₂O, and dried under vacuum. Without purification, this compound was slurried in EtOH solution (15 mL) of MOTPP/TPP/TFTPP (194/144/256 mg, 0.55 mmol) and was treated with excess Et₃N (1 mL). The solid substance slowly dissolved and the color of the solution changed to red-orange. After refluxing for 6 h, the red-orange solution was cooled to room temperature and the solvent was removed under vacuum to give a solid red substance.

Complex 1. Red solid (306 mg, 61%). ¹H NMR (400 MHz, CD₂Cl₂, TMS, 25 °C, ppm): δ 7.53 (t, ³J(H,H) = 7.2 Hz, 4H, phenyl), 7.48 (dd, ³J(H,H) = 11.3, 7.4 Hz, 3H, 4-pyridyl, phenyl), 7.44 (d, ³J(H,H) = 7.1 Hz, 4H, phenyl), 7.33 (d, ³J(H,H) = 7.8 Hz, 2H, 3,5-pyridyl), 7.07 (t, ³J(H,H) = 9.0 Hz, 6H, phenyl), 6.85 (s, 2H, pyrazolyl), 6.67 (d, ³J(H,H) = 8.0 Hz, 6H, phenyl), 4.17 (s, 6H, NCH₃), 3.75 (s, 9H, OCH₃). ¹³C NMR (101 MHz, CDCl₃, TMS, 25 °C, ppm): δ 160.0 (phenyl C), 155.9 (pyridyl C), 152.1 (pyrazolyl C), 147.8 (pyridyl CH), 134.7 (phenyl C), 131.7 (pyrazolyl C), 129.3 (phenyl C), 129.2 (phenyl CH), 128.9 (phenyl CH), 125.7 (phenyl CH), 125.2 (pyridyl CH), 116.7 (phenyl CH), 113.1 (phenyl CH), 104.6 (pyrazolyl CH), 55.2 (s, OCH₃), 39.7 (s, NCH₃). ³¹P NMR (162 MHz, CDCl₃, 25 °C, ppm): δ 39.9. Anal. calcd for C₄₆H₄₂Cl₂N₅O₃PRu (915.14): C, 60.33; H, 4.62; N, 7.65. Found: C, 60.36; H, 4.58; N, 7.62. HRMS (ESI) m/z: calcd for C₄₆H₄₂Cl₂N₅O₃PRuNa [M + Na]⁺: 938.1344; found: 938.1338. C₄₆H₄₂Cl₂N₅O₃PRu (915.1446).

Complex 2. Red solid (308 mg, 68%). ¹H NMR (400 MHz, CD₂Cl₂, TMS, 25 °C, ppm): δ 7.54 (d, ³J(H,H) = 6.6 Hz, 3H, phenyl), 7.50 (d, ³J(H,H) = 3.9 Hz, 3H, 4-pyridyl, phenyl), 7.48 (s, 1H, phenyl), 7.45–7.41 (m, 6H, phenyl), 7.30 (d, ³J(H,H) = 7.6 Hz, 4H, phenyl), 7.23 (d, ³J(H,H) = 7.3 Hz, 2H, 3,5-pyridyl), 7.17 (dd, ³J(H,H) = 7.5, 3.3 Hz, 9H, phenyl), 6.84 (s, 2H, pyrazolyl), 4.16 (s, 6H, NCH₃). ¹³C NMR (101 MHz, CDCl₃, TMS, 25 °C, ppm): δ 155.7 (pyridyl C), 152.1 (pyrazolyl C), 147.6 (pyridyl CH), 137.9 (phenyl C), 133.9 (pyrazolyl C), 133.3 (phenyl CH), 132.0 (phenyl CH), 129.3 (phenyl C), 128.9 (phenyl CH), 128.2 (phenyl CH), 127.5 (phenyl CH), 125.3 (pyridyl CH), 116.9 (phenyl CH), 104.8 (pyrazolyl CH), 39.7 (s, NCH₃). ³¹P NMR (162 MHz, CDCl₃, 25 °C, ppm): δ 43.9. Anal. calcd for C₄₃H₃₆Cl₂N₅PRu (825.11): C, 62.55; H, 4.39; N, 8.48. Found: C, 62.47; H, 4.35; N 8.41. HRMS (ESI) m/z: calcd for C₄₃H₃₆Cl₂N₅PRuNa [M + Na]⁺: 848.1027; found: 848.1021. C₄₃H₃₆Cl₂N₅PRu (825.1129).

Complex 3. Red solid (413 mg, 73%). ¹H NMR (400 MHz, CD₂Cl₂, TMS, 25 °C, ppm): δ 7.54 (d, ³J(H,H) = 6.6 Hz, 2H, phenyl), 7.50 (dd, ³J(H,H) = 14.1, 8.7 Hz, 5H, 4-pyridyl, phenyl), 7.46 (d, ³J(H,H) = 7.3 Hz, 6H, phenyl), 7.43 (t, ³J(H,H) = 8.7 Hz, 6H, phenyl), 7.37 (d, ³J(H,H) = 6.7 Hz, 4H, phenyl), 7.32 (d, ³J(H,H) = 7.8 Hz, 2H, 3,5-pyridyl), 6.86 (s, 2H, pyrazolyl), 4.19 (s, 6H, NCH₃). ¹³C NMR (101 MHz, CDCl₃, TMS, 25 °C, ppm): δ 155.3 (pyridyl C), 151.9 (pyrazolyl C), 148.6 (pyridyl CH), 137.7 (pyrazolyl C), 137.3 (phenyl C), 133.4 (phenyl CH), 132.8 (phenyl CH), 131.7 (phenyl C), 131.4 (phenyl C), 129.7 (phenyl CH), 129.1 (phenyl CH), 128.7 (phenyl CH), 124.7 (s, CF₃), 117.3 (pyridyl CH), 105.1 (pyrazolyl CH), 39.9 (s, NCH₃). ³¹P NMR (162 MHz, CDCl₃, 25 °C, ppm): δ 45.8. Anal. calcd for C₄₆H₃₃Cl₂N₅F₉PRu (1029.08): C, 53.65; H, 3.23; N, 6.80. Found: C, 53.89; H, 3.29; N 6.81. HRMS (ESI) m/z: calcd for C₄₆H₃₃Cl₂N₅F₉PRuNa [M + Na]⁺: 1052.0648; found: 1052.0641. C₄₆H₃₃Cl₂N₅F₉PRu (1029.0750).

Complex 4. Red solid (246 mg, 64%). ¹H NMR (400 MHz, CD₂Cl₂, TMS, 25 °C, ppm): δ 7.36 (t, ³J(H,H) = 7.8 Hz, 1H, 4-pyridyl), 7.26–7.22 (m, 3H, phenyl), 7.17 (d, ³J(H,H) = 7.8 Hz, 2H, 3,5-pyridyl), 7.09 (dd, ³J(H,H) = 7.9, 2.8 Hz 12H, phenyl), 6.55 (s, 2H, pyrazolyl), 4.01 (s, 6H, NCH₃), 2.33 (s, 6H, CCH₃). ¹³C NMR (101 MHz, CDCl₃, TMS, 25 °C, ppm): δ 155.9 (pyridyl C), 151.2 (pyrazolyl C), 142.4 (pyridyl CH), 134.2 (pyrazolyl C), 133.4 (phenyl C), 131.9 (phenyl CH), 128.8 (phenyl CH), 127.4 (phenyl CH), 116.3 (pyridyl CH), 104.5 (pyrazolyl CH), 37.3 (s, NCH₃), 12.3 (s, CH₃). ³¹P NMR (162 MHz, CDCl₃, 25 °C, ppm): δ 44.1. Anal. calcd for C₃₃H₃₂Cl₂N₅PRu (701.08): C, 56.49; H, 4.60; N, 9.98. Found: C, 56.38; H, 4.67; N 9.69. HRMS (ESI) m/z: calcd for C₃₃H₃₂Cl₂N₅PRuNa [M + Na]⁺: 724.0714; found: 724.0709. C₃₃H₃₂Cl₂N₅PRu (701.0816).

Typical procedure for the catalytic transfer hydrogenation of ketones

Under nitrogen atmosphere, ketone (3.2 mmol), catalyst (1.6 μmol) and 2-propanol (3 mL) were introduced into a Schlenk tube. The solution was stirred at 82 °C for 10 min. Then 1 mL of 0.064 mmol KOH in 2-propanol solution was introduced to initiate the transfer hydrogenation. At certain times, 0.1 mL of the reaction mixture was sampled and diluted with 1 mL of 2-propanol pre-cooled at −10 °C for immediate GC analysis. The conversions were determined by averaging the results of two runs of each catalytic reaction. After the reaction was finished, the mixture was purified by flash silica gel column chromatography (petroleum ether (60–90 °C)/EtOAc = 50 : 1) to afford the corresponding alcohol product, which was identified by comparison with an authentic sample through NMR and GC analysis.

Conclusions

Versatile pincer-type tridentate pyridine-bridged frameworks, not incorporating NH groups, and their Ru(II) complexes were successfully synthesized and characterized. These ruthenium complexes, featuring no NH functionality, still exhibited very high catalytic activity in the TH reaction of ketones with a high substrate-to-catalyst ratio (2000 : 1).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of the X-ray crystallographic data and refinement of complexes 2 and 4. CCDC 999183 and 999184. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra07524b

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