A simple and efficient in situ generated ruthenium catalyst for chemoselective transfer hydrogenation of nitroarenes: kinetic and mechanistic studies and comparison with iridium systems

Abstract

The catalytic activities of a series of in situ generated homogeneous ruthenium systems based on commercially available [RuCl₂(p-cymene)]₂ and various ligands in transfer hydrogenation of nitroarenes to anilines were investigated. Combination of [RuCl₂(p-cymene)]₂ and tridentate phenanthroline based ligand 2-(6-methoxypyridin-2-yl)-1,10-phenanthroline (phenpy-OMe) exhibited the highest catalytic activity for this reaction using 2-propanol as hydrogen source. This protocol provides a facile route to access aromatic amines under mild conditions in excellent yields. Notably, this system chemoselectively reduced the nitro groups over an array of other reactive functionalities such as ketone, alkene, amide, nitrile, and aryl halide. Operational simplicity, high yields, mild reaction conditions and short reaction times make this an attractive methodology for accessing various functionalized anilines. A series of controlled experiments and careful mechanistic investigation with the possible intermediates suggested that transformation of nitrobenzene to aniline with ruthenium and iridium system proceeded via direct route and condensation route respectively.

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Introduction

Hydrogenation of structurally diverse nitrobenzenes emerged as a highly attractive protocol to synthesize functionalized anilines, which are essential for the synthesis of fine chemicals such as polymers, dyes, pigments, agrochemicals and pharmaceuticals.^1–3 In the last few decades, various homogeneous and heterogeneous systems based on Pt,^4–6 Pd,^7–9 Rh,^10,11 Ru,^12–14 Ni,^15,16 Fe,^17–19 Mo,^20,21 etc. metals were reported for the synthesis of functionalized anilines. Thus far, catalytic hydrogenation is the most frequently operated. However, unfortunately H₂ gas was not sufficient for the conversion of nitroarenes for many of these systems also; which require excess amount of expensive reducing agents such as silanes,^22–24 NaBH₄,²⁵ ammonia borane,^26,27 decaborane,^28,29 hydrazine,^30,31 etc. On the other hand, selective hydrogenation of nitroarenes in presence of other reducible functional groups is challenging with H₂,^32–38 particularly with highly active metal such as Ir, Pt, Pd or Ru based catalysts.^{5–8,12,39–41} To address this issue, other metal (e.g. Au,^42,43 Ag⁴⁴ and Cu,⁴⁵ etc.) based complexes were also investigated, but the catalytic activities of these systems were poor and required harsh reaction condition. This led chemists to develop cleaner, facile, cost-effective and greener methodologies for this chemical transformation. In this regard, transfer hydrogenation (TH) of nitro compounds has emerged as an exceedingly attractive alternative protocols due to simple experimental setups, high reactivities and recyclability of the major side product. Among the various hydrogen source commonly 2-propanol, formic acid and formic acid–amine were successfully employed.^46–53

Considering the demand of environmentally benign processes, TH of nitroarenes using various transition-metal catalysts have been explored by many groups.^17,54–62 Despite significant advancements, these systems still have many limitations and many cases it requires high catalyst loading, expensive phosphine ligands, excess amount of bases, high temperature and longer reaction time. Hence, for the synthesis of functionalized anilines development of a new, more effective and sustainable process is highly desirable.

In situ generated catalytic systems have some advantages over the isolated complexes as it does not require tedious synthesis as well as characterization. In this regard, catalytic system consisting of commercially available metal salts along with easy to synthesize ligands is undoubtedly attractive. Surprisingly, only few such examples are known. Inspired by the reports from Beller and Cai groups for TH of nitroarenes using [RuCl₂(p-cymene)]₂/2,2′:6′,2′′-terpyridine and [Ir(COD)Cl]₂/1,10-phenanthroline respectively;^54,55 we hypothesized that combination of Ru(II) salts with electron rich nitrogen based ligands may have the potential to promote the reduction efficiently (Scheme 1).

Scheme 1 In situ generated catalytic system for the reduction of nitroarenes.

We recently demonstrated bifunctional [RuCl(phenpy-OH)(PPh₃)₂]PF₆ catalyzed TH of a series of ketones and nitriles in 2-propanol. Surprisingly, with this system reduction of neither ketones nor nitriles was observed with substrates having nitro functional groups.⁶³ This encourages us to probe the Ru(II) catalysed TH of nitroarenes in details. To design better performing catalyst elucidation of reaction mechanism is vital, particularly for the reduction of nitroarenes as it is well documented that it follows multistep.^54,55 In addition, with many systems during the reduction of nitroarenes to amines significant amount of side products such as hydroxylamine, hydrazine and azoarenes were also observed.^64,65

We herein report in situ generated highly efficient and selective catalyst based on [RuCl₂(p-cymene)]₂/phenpy-OMe for nitroarene reduction as well as detail mechanistic studies for the transfer hydrogenation of nitrobenzene to aniline using both Ru(II) and Ir(I) complexes.

Result and discussion

Initially, TH of nitrobenzene was picked as a model reaction to screen the catalytic activities of phenanthroline based NNN-pincer Ru(II) complexes. The reactions were carried out for 5 hours in refluxing 2-propanol employing Ru(II) pre-catalysts (1 mol%) and the results are summarized in Table 1. With complexes 1 and 2 yields as well as selectivity for the aniline were poor (Table 1, entries 1 and 2). Surprisingly, noteworthy improvement in both conversion and selectivity were observed with isolated Ru(II) complexes having chloride counter anion compare with their hexafluorophosphate counter anion analogs probably due to higher solubility in the reaction medium (Table 1, entries 3 and 4). To evaluate the role of the other ligands (e.g. PPh₃) bound to ruthenium centre, complexes [RuCl(phenpy-OMe)(CH₃CN)₂]Cl (5) and [RuCl(phenpy-OH)(CH₃CN)₂]Cl (6) were synthesized. To our delight, catalytic activity significantly increased by substituting strongly bound PPh₃ ligand with relatively weaker acetonitrile. Among all these catalysts, complex 5 bearing phenpy-OMe ligand exhibited the maximum reactivity, showed 100% conversion of nitrobenzene within 5 hours with 98% selectivity for aniline (Table 1, entry 5). Notably, phenpy-OH based Ru(II) complexes showed lower activity compare with the complexes having phenpy-OMe ligand. This suggest that probably metal–ligand cooperativity was not operating due to ionic nature of the nitro group which prefers to bind the metal centre directly. To the best of our knowledge reduction of nitro group using a bifunctional catalyst is not reported yet.

Table 1 Transfer hydrogenation of nitrobenzene in presence of different Ru(II) complexes^a

Entry	Ru(II) cat.	Conversion^b (%)	Aniline^b (%)	Azobenzene^b (%)	Azoxybenzene^b (%)
a Reaction conditions: nitrobenzene (1.0 mmol), Ru(II) cat. (1.0 mol%) and NaO^iPr (40 mol%) at 110 °C for 5 h.b Conversion and yields were determined by GC.
1	1	69	35	5	29
2	2	53	31	3	19
3	3	89	63	7	19
4	4	78	59	3	16
5	5	100	98	1	1
6	6	81	62	4	15

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As the in situ generated catalytic systems have significant advantages over the isolated complexes, we next screened one-pot transfer hydrogenation of nitrobenzene combining [RuCl₂(p-cymene)]₂ with a series of ligands. In a typical experiment, a mixture of nitrobenzene, NaOⁱPr (40 mol%) and Ru(II) (1 mol%) in 2-propanol was heated at 110 °C for 5 h. Initially, several ruthenium salts as catalyst in transfer hydrogenation of nitrobenzene were screened which showed poor to moderate yields of aniline (Table 2, entries 1–6). To boost the catalytic performance in addition to the metal source, a series of ligands were added which exhibited significant improvement indicating strong influence of ligands on the catalytic activity (Table 2, entries 7–19). Monodentate triphenyl phosphine, tri-tert-butylphosphine and triphenyl phosphite ligands produced aniline in 79–44% yields (Table 2, entries 7–9). Under similar condition N-containing ligands such as pyridine, bipyridine and phenanthroline gave 74–79% yields of aniline (Table 2, entries 10–12). There was no significant improvement with tri- and tetradentate nitrogen based ligands except with 2-(6-methoxypyridin-2-yl)-1,10-phenanthroline (phenpy-OMe) which displayed the best performance among all the ligands documented in this study. To our delight, phenpy-OMe (L8) in combination [RuCl₂(p-cymene)]₂ (0.5 mol%) produced aniline in 97% yield within 5 hours. Importantly, in TH of nitrobenzene no significant difference between the isolated (Table 1, entry 5) and in situ generated (Table 2, entry 14) complexes were detected. Nomura et al. reported Ru₃(CO)₁₂ catalysed reduction of nitrobenzene using ethanol/water system under 20 atm. pressure of CO at 150 °C.⁶⁶ Pyridinyl NHC iridium complex promoted TH of nitrobenzene in refluxing 2-propanol was demonstrated by Wang et al. using significantly higher catalyst loading.⁵⁶ Recently, Sarkar et al. screened several ruthenium and iridium complexes for the reduction of nitrobenzene using 2-propanol however, the catalytic activities of these system were poor (5 mol% metal, 24 hours reflux).⁶⁷ Notably, in comparison to the both in situ systems using [RuCl₂(p-cymene)]₂/2,2′:6′,2′′-terpyridine and [Ir(COD)Cl]₂/1,10-phenanthroline reported by Beller and Cai groups respectively, catalyst [RuCl₂(p-cymene)]₂/L8 exhibited much higher activity.^54,55

Table 2 Ru(II) catalysed transfer hydrogenation of nitrobenzene in presence of different ligands^a

Entry	Ligand	Metal salt	Conversion^b (%)	Aniline^b (%)	Azobenzene^b (%)	Azoxybenzene^b (%)
a Reaction conditions: nitrobenzene (1.0 mmol), [Ru(II)] (1.0 mol%), ligand (1.0 mol%) and NaO^iPr (40 mol%) at 110 °C for 5 h.b Conversion and yields were determined by GC.
1	—	RuCl2(PPh3)3	64	39	6	19
2	—	Ru(H)(Cl)(CO)(PPh3)3	63	40	2	21
3	—	Ru(H)2(CO)(PPh3)3	67	47	3	17
4	—	[Ru(COD)Cl2]n	70	41	2	27
5	—	RuCl2(DMSO)4	74	50	3	21
6	—	[RuCl2(p-cymene)]2	71	54	10	7
7	L1	[RuCl2(p-cymene)]2	92	79	4	9
8	L2	[RuCl2(p-cymene)]2	99	74	25	—
9	L3	[RuCl2(p-cymene)]2	47	44	—	3
10	L4	[RuCl2(p-cymene)]2	87	74	4	9
11	L5	[RuCl2(p-cymene)]2	78	69	4	5
12	L6	[RuCl2(p-cymene)]2	89	78	7	4
13	L7	[RuCl2(p-cymene)]2	86	62	3	21
14	L8	[RuCl2(p-cymene)]2	100	97	1	2
15	L9	[RuCl2(p-cymene)]2	84	67	7	10
16	L10	[RuCl2(p-cymene)]2	79	66	2	11
17	L11	[RuCl2(p-cymene)]2	78	64	3	9
18	L12	[RuCl2(p-cymene)]2	98	71	6	21
19	L13	[RuCl2(p-cymene)]2	99	71	28	—

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Next, TH of nitrobenzene was carried out with different ruthenium precursors in presence of ligand L8 and the results are listed in Table 3. Combination of L8 with RuCl₂(PPh₃)₃, Ru(H)(Cl)(CO)(PPh₃)₃, Ru(H)₂(CO)(PPh₃)₃ and [Ru(COD)Cl₂]_n resulted lower yields aniline. However, catalytic efficiency significantly improved in presence of RuCl₂(DMSO)₄ and [RuCl₂(p-cymene)]₂. Among all the ruthenium precursors screened, [RuCl₂(p-cymene)]₂ was found to be the most effective which afforded 97% yield of aniline (Table 3, entry 6). Higher reactivity of this pre-catalyst may be due to easy replacement of weakly coordinated p-cymene ligand from the metal centre which can provide more space for the nitro group to approach the metal centre during catalysis. Under similar reaction condition CH₃CN containing complex 5 also showed significantly higher activity compare to complex 3 having PPh₃ ligand (Table 1, entries 3 and 5). Both this observation further validated that ligand dissociation is an important step in this reaction so that incoming substrate can coordinate to the Ru(II) centre easily. Although, different binding affinity of L8 to the various Ru(II) source to generate the pre-catalysts may also be one of the factor.

Table 3 Transfer hydrogenation of nitrobenzene in presence of different ruthenium precursors^a

Entry	Ruthenium(II) source	Conversion^b (%)	Aniline^b (%)	Azobenzene^b (%)	Azoxybenzene^b (%)
a Reaction conditions: nitrobenzene (1.0 mmol), [Ru(II)] (1.0 mol%), L8 (1.0 mol%) and NaO^iPr (40 mol%) at 110 °C for 5 h.b Conversion and yields were determined by GC.c Only base.
1	RuCl2(PPh3)3	87	62	3	22
2	Ru(H)(Cl)(CO)(PPh3)3	89	63	5	21
3	Ru(H)2(CO)(PPh3)3	82	68	2	12
4	[Ru(COD)Cl2]n	97	61	6	30
5	RuCl2(DMSO)4	99	88	5	6
6	[RuCl2(p-cymene)]2	100	97	1	2
8^c	No Ru(II)	99	8	35	56

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After optimizing ligand (L8) and metal precursor [RuCl₂(p-cymene)]₂, next we evaluated the influence of base. For that purpose TH of nitrobenzene was carried out with a series of bases (Table 4). Carbonate bases were not effective for this reduction. Using NaOⁱPr, NaOH, K^tOBu and KOH as base yields of aniline were reached up to 97%, 84%, 88% and 68% respectively. Based on the superior performance, NaOⁱPr was selected as a base for this catalytic reaction.

Table 4 Transfer hydrogenation of nitrobenzene in presence of different bases^a

Entry	Base	Conversion^b (%)	Aniline^b (%)	Azobenzene^b (%)	Azoxybenzene^b (%)
a Reaction conditions: nitrobenzene (1.0 mmol), [RuCl2(p-cymene)]2 (0.5 mol%), L8 (1.0 mol%) and base (40 mol%) at 110 °C for 5 h.b Conversion and yields were determined by GC.
1	NaO^iPr	100	97	1	2
2	NaOH	99	84	4	11
3	K^tOBu	92	88	1	3
4	KOH	73	68	3	2
5	Na2CO3	21	3	13	5
6	K2CO3	7	7	—	—
7	Cs2CO3	33	33	—	—

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To determine the ideal amount of base required in this reaction, same reaction was carried out with varying amount of NaOⁱPr. Based on the yield of aniline, 40 mol% NaOⁱPr was found to be optimum, beyond that saturation behaviour was observed (Fig. 1).

Fig. 1 Dependence on the amount of NaOⁱPr in TH of nitrobenzene.

Next, to evaluate the scope of the present catalytic system, TH of various nitroarenes were conducted (Table 5). Mono and di-substituted nitroarenes containing electron donating group such as –Me, –OMe and –SMe in ortho, meta and para positions afforded the corresponding anilines in good to excellent yields (Table 5, entries 2–7). Halogenated nitroarenes such as fluoro, chloro and bromo substituted nitroarenes were selectively hydrogenated to the corresponding anilines in moderate to excellent yields without any reductive dehalogenation (Table 5, entries 8–12). Furthermore, 1-nitro naphthalene was reduced to 1-amino naphthalene in excellent yield (99%; Table 5, entry 13).

Table 5 Transfer hydrogenation of various nitro compounds using ruthenium catalyst^a

Entry	Substrate	Product	Time (h)	Yield^b (%)
a Reaction conditions: nitrobenzene (1.0 mmol), [RuCl2(p-cymene)]2 (0.5 mol%), L8 (1.0 mol%) and NaO^iPr (40 mol%) at 110 °C for 5–10 h.b Yields were determined by GC.c Yields were determined by ^1H NMR.
1			5	98
2			5 (8)	72 (91)
3			5	99
4			5 (8)	75 (97)
5			5 (10)	67 (99)
6			5	98
7			5 (10)	62 (98)
8			5	73
9			5 (10)	55 (91)
10			5	64
11			5	93
12			5	94
13			5	99^c
14			5	99
15			5	77
16			5	74
17			5	95
18			5	99
19			5	91^c
20			5	98^c
21			5	91
22			5	97
23			5	86^c
24			5	81^c
25			10	54
26			5	71^c
27			5	93
28			5	96

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Afterward, we moved to multifunctional substrates for transfer hydrogenation under the same catalytic condition. Chemoselective TH of highly challenging substrates bearing other reducible groups such as keto, alkene, nitrile and amide were carried out which produced the respective amines in good to excellent yields (74–99%; Table 5, entries 14–20). Selective reduction of p-nitro benzyl alcohol was also achieved, formed the desired amine in excellent yield without affecting the –OH group at the benzylic position (91%; Table 5, entry 21). This catalytic system efficiently hydrogenated the di-nitroarenes to the corresponding nitroanilines (81–86%; Table 5, entries 23 and 24). In addition, reduction of heterocyclic nitroarenes such as 3-nitro pyridine and 6-nitroquinoline also furnished the respective amines in moderate yields (54–71%; Table 5, entries 25 and 26). Applying the same protocol, nitrosoarenes and azobenzene were also converted to amines smoothly (93–96% yield; Table 5, entries 27 and 28).

Mechanistic investigation

A probable mechanism for the TH of nitroarenes using the present catalytic system is shown in Scheme 2. Formation of metal hydride from metal halide in presents of NaOⁱPr in 2-propanol is well documented in literature.^68–71 Next, insertion of nitrobenzene into metal dihydride followed by elimination of water in presence of 2-propanol leads to formation of nitrosobenzene. Subsequently, following the direct route hydrogenation of nitrosobenzene afforded aniline. Based on the previously reported proposed catalytic cycle for the TH of nitrobenzene, we hypothesized that probably more reactive ruthenium dihydride was the active intermediate for this reaction.^54,67,68 However, possibility of ruthenium monohydride cannot be ruled out. To isolate this proposed intermediate, we reacted complexes [RuCl(phenpy-OMe)(CH₃CN)₂]Cl (5) and [RuCl(phenpy-OMe)(CH₃CN)₂]PF₆ (7) with various bases such as NaOⁱPr, KO^tBu, KOH, and Cs₂CO₃ in 2-propanol in presence and absence of nitrobenzene. However, all these attempts to isolate the intermediate were unsuccessful.

Scheme 2 Proposed mechanism for the transfer hydrogenation of nitrobenzene.

TH of nitroarenes to aniline commonly follows two different pathways: (1) direct route involving reduction of nitrosoarene to hydroxyl amine followed by hydrogenation to amine and (2) condensation route involving coupling of nitrosoarene with hydroxyl amine to afford azoxy compound which finally reduce to aniline via azo and hydrazo intermediates.^55,67 In order to determine which route our catalytic system was following, a time dependent product distribution for the TH of nitrobenzene was conducted. As shown in Fig. 2, yield of aniline steadily increased with time at the expense of nitrobenzene concentration. However, concentration of the intermediate products such as azoxybenzene and azobenzene were remained significantly low throughout the reaction. This result indicate that during the TH of nitrobenzene to aniline [RuCl₂(p-cymene)]₂/L8 system preferentially followed the direct route which is consistent with prior reports with Ru(II) catalysts.⁶⁷

Fig. 2 Time dependent product distribution during TH of nitrobenzene.

To gain more information about this mechanism, reduction kinetics of nitrosobenzene was studied. Within 10 minutes, nitrosobenzene was converted to azoxybenzene which was generated from condensation between nitrosobenzene and hydroxylamine. Then, gradually azoxybenzene was converted to aniline via azobenzene. Results from this experiment undoubtedly advocate that for TH of nitrosobenzene, condensation route was dominating and azoxybenzene and azobenzene were the intermediates in this process (Fig. 3).

Fig. 3 Time dependent product distribution during TH of nitrosobenzene.

Next, kinetics of reduction of azobenzene to aniline was probed under same catalytic condition (Fig. 4). In this case, reduction was very fast and within 30 minutes 61% conversion of azobenzene to aniline was achieved; however after 5 hours of heating 92% yield of aniline was observed. Surprisingly, the reaction became extremely slower after 30 minutes and similar retardation of reaction rate was noticed during TH of nitrobenzene after 2 hours.

Fig. 4 Kinetics of reduction of azobenzene.

We postulated that excess of aniline generated in this reaction may decelerated the catalytic activity by possible binding to the active metal centre as this system followed inner-sphere mechanism. To prove this hypothesis, two control reactions were carried out under same catalytic condition in the presence and absence of excess of amines (Scheme 3). Rate of TH of nitrobenzene was affected when 1 eq. 4-methoxyaniline was added to the reaction mixture. After 5 h, in absence of additional 4-methoxyaniline yield of aniline was 97%. However, under similar reaction condition with 1 eq. of 4-methoxyaniline yield of aniline was reduced to 75%.

Scheme 3 Catalytic reduction of nitrobenzene in absence and presence of externally added amine.

To investigate the influence of base in the catalytic condition, a time dependent product distribution for the reduction of nitrobenzene was carried out with only NaOⁱPr in absence of any Ru(II) complex (Fig. 5). As expected reaction was significantly sluggish, nitrobenzene was slowly converted to azoxybenzene in first 2 hours. After 8 hours of heating 78% yield of azobenzene was achieved. Notably, only 10% and 12% yields of aniline was detected after 8 hours and 16 hours respectively. This data indicates that base can promote the conversion of nitrobenzene to other intermediate products following the condensation route, however to produce aniline other catalyst is require.

Fig. 5 Time dependent product distribution during TH of nitrobenzene using only NaOⁱPr.

In literature, several iridium complexes were also reported to be highly effective for the TH of nitroarenes to anilines and they followed condensation route.^55,67 To investigate it further with our ligand system, TH of nitrobenzene with [Ir(COD)Cl]₂/L8 was carried out. Based on the time dependent product distribution during TH of nitrobenzene, it was evident that [Ir(COD)Cl]₂/L8 also followed the condensation route (Fig. 6). Initially, under the optimized condition rate of aniline formation with the ruthenium and iridium complexes were almost similar although they followed different pathways however, at the end Ru(II) system became little faster (Fig. 7).

Fig. 6 Time dependent product distribution during TH of nitrobenzene with [Ir(COD)Cl]₂/L8 system.

Fig. 7 Performance comparison between ruthenium and iridium complexes.

To demonstrates the practical aspect of the [RuCl₂(p-cymene)]₂/L8 system, gram scale reaction of 1-nitro-3-vinylbenzene was performed under the optimized condition which afforded 83% yield of 3-vinylaniline (Scheme 4).

Scheme 4 Synthetic application: gram scale reaction of m-nitro styrene.

Conclusion

In summary, a convenient and highly efficient protocol for the reduction of structurally diverse nitroarenes to the corresponding anilines under homogeneous condition is developed. In situ generated homogeneous ruthenium catalyst derived from commercially available [RuCl₂(p-cymene)]₂ and tridentate phenanthroline based Phenpy-OMe (L8) ligand displayed excellent activity in chemoselective TH of nitroarenes using 2-propanol as a hydrogen source. Challenging substrates with other reducible functional groups such as keto, alkene, nitrile and amide were unaffected during the reduction. Halide substituted nitroarenes were also reduced selectively to the corresponding anilines without any reductive dehalogenation. Mechanistic studies and a series of controlled experiments in TH of nitro, nitroso and azobenzene revealed that this catalytic system preferentially followed direct route. The present methodology provides a potential strategy for accessing aminearenes in high yields under mild reaction conditions by combining commercially available simple Ru(II) precursors and nitrogen based tridentate ligands.

Experimental section

General procedure and materials

All the manipulation were carried out under argon atmosphere using standard Schlenk line technique. Glasswares were oven dried and immediately prior to use. Solvent were dried according to literature methods. Distilled under argon atmosphere and deoxygenated prior to use. PdCl₂ (60% Pd on metal basis) and RuCl₃·nH₂O (39% Ru on metal basis) were purchased from Arora-Matthey, India. All the commercial reagents were purchased from Sigma-Aldrich, Alfa-Aesar, Spectrochem, Avra and SD-fine chemical. 2-Bromo-1,10-phenanthroline,⁷² 2-tributylstannyl pyridine,⁷³ 6-methyl-2-tributylstannylpyridine,⁷⁴ 6-methoxy-2-tributylstannylpyridine,⁷⁵ 6-bromo-2-tributylstannylpyridine⁷⁶ and 2,9-di-(pyrid-2-yl)-1,10-phenanthroline^77,78 (L13) were synthesized according to literature procedure. 2-(6-Methoxypyridin-2-yl)-1,10-phenanthroline (L8), 6-(1,10-phenanthrolin-2-yl)pyridin-2-ol (L9), complex 1 and complex 2 were reported from our group.⁶³ Ligands L1–L7 were commercially available. Nitrosobenzene was prepared according to literature procedure.^{79 1}H, ¹³C and ³¹P NMR spectra were recorded on JEOL 400 MHz, and 500 MHz Spectrometer using CDCl₃, CD₃CN, CD₂Cl₂ and DMSO-D₆. Chemical shift (δ) was reported in ppm, coupling constant (J) was reported in hertz. The complexes were grinded thoroughly, washed several time with dry diethyl ether and dried under vacuum prior to use elemental analysis. All the elemental analysis were carried out on a Thermoquest EA1110 CHNS/O analyser. ESI-MS were recorded on a Waters Micromass Quattro Micro triple-quadrupole mass spectrometer. All the GC analysis were performed using Perkin-Elmer Clarus 600 Gas Chromatograph and GC-MS were taken using Agilent 7890A Gas Chromatograph equipped with Agilent 5890 triple-quadrupole mass system.

Synthesis of 2-(6-methylpyridin-2-yl)-1,10-phenanthroline (L10). A mixture of 2-bromo-1,10-phenanthroline (0.7 g, 2.7 mmol), 6-methyl-2-tributylstannylpyridine (2.06 g, 5.4 mmol) and Pd(PPh₃)₄ (0.311 g, 0.2695 mmol) in 65 mL of toluene were heated at the reflux temperature for 5 days. The reaction mixture was allowed to cool to room temperature and the solvent was removed in vacuum. The product was purified by neutral alumina column chromatography using hexane–ethyl acetate. Final product was obtained as off white solid. Yield: 639 mg (87%). ¹H NMR (400 MHz, CDCl₃): δ = 9.21 (dd, J_H,H = 4.12 Hz, J_H,H = 2.28 Hz, 1H), 8.78 (d, J_H,H = 8.24 Hz, 1H), 8.72 (d, J_H,H = 7.76 Hz, 1H), 8.33 (d, J_H,H = 8.24 Hz, 1H), 8.24 (dd, J_H,H = 8.02 Hz, J_H,H = 6.40 Hz, 1H), 7.82–7.75 (m, 3H), 7.63 (dd, J_H,H = 8.24 Hz, J_H,H = 3.64 Hz, 1H), 7.21 (d, J_H,H = 7.32 Hz, 1H), 2.67 (s, 3H). ¹³C{¹H} NMR (125 MHz, CDCl₃): 157.93, 156.62, 155.54, 150.41, 146.48, 145.49, 137.27, 136.95, 136.24, 129.15, 128.72, 126.65, 123.86, 122.99, 121.08, 119.86, 24.74. ESI-MS: m/z = 273.1182 (100%, MH⁺).

Synthesis of 2-(6-bromopyridin-2-yl)-1,10-phenanthroline (L11). A mixture of 2-bromo 1,10-phenanthroline (0.7 g, 2.7 mmol), 6-bromo-2-tributylstannyl pyridine (2.42 g, 5.4 mmol) and Pd(PPh₃)₄ (0.311 g, 0.2695 mmol) in 65 mL toluene were heated at reflux temperature for 5 days. The reaction mixture was allowed to cool to room temperature and the solvent was removed in vacuum. The product was purified by neutral alumina column chromatography using hexane–ethyl acetate. Final product was obtained as white solid. Yield: 819 mg (90%). ¹H NMR (400 MHz, CDCl₃): δ = 9.22 (dd, J_H,H = 4.22 Hz, J_H,H = 1.40 Hz, 1H), 8.98 (dd, J_H,H = 7.80 Hz, J_H,H = 0.92 Hz, 1H), 8.79 (d, J_H,H = 8.71 Hz, 1H), 8.37 (d, J_H,H = 8.70 Hz, 1H), 8.27 (dd, J_H,H = 8.30 Hz, J_H,H = 1.82 Hz, 1H), 7.86–7.74 (m, 3H), 7.66 (m, 1H), 7.54 (d, J_H,H = 7.8 Hz). ¹³C{¹H} NMR (125 MHz, CDCl₃): 157.42, 154.64, 150.57, 146.44, 145.85, 141.53, 139.45, 137.23, 136.48, 129.29, 128.54, 127.27, 126.68, 123.16, 121.57, 121.14. ESI-MS: m/z = 336.0138 (100%, MH⁺).

Synthesis of 2-(pyridin-2-yl)-1,10-phenanthroline (L12). A mixture of 2-bromo-1,10-phenanthroline (0.7 g, 2.7 mmol), 2-tributylstannylpyridine (1.99 g, 5.4 mmol) and Pd(PPh₃)₄ (0.311 g, 0.2695 mmol) in 60 mL of toluene were heated at the reflux temperature for 4 days. The mixture was allowed to cool to room temperature and the solvent was removed in vacuum. The product was purified by column chromatography using neutral alumina with hexane–ethyl acetate as eluent. Final product was obtained as off white solid. Yield: 619 mg (89%). ¹H NMR (400 MHz, CDCl₃): δ = 9.22 (dd, J_H,H = 4.58 Hz, J_H,H = 3.08 Hz, 1H), 8.98 (d, J_H,H = 7.96 Hz, 1H), 8.79 (d, J_H,H = 8.56 Hz, 1H), 8.72 (d, J_H,H = 4.92 Hz, 1H), 8.36 (d, J_H,H = 8.56 Hz, 1H), 8.26 (dd, J_H,H = 8.24 Hz, J_H,H = 6.72 Hz, 1H), 7.90 (dt, J_H,H = 7.94 Hz, J_H,H = 1.84 Hz, 1H), 7.78 (d, J_H,H = 8.56 Hz, 1H), 7.64 (dd, J_H,H = 7.94 Hz, J_H,H = 3.68 Hz, 1H), 7.36 (dt, J_H,H = 5.04 Hz, J_H,H = 1.24 Hz, 1H). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 156.26, 156.15, 150.48, 149.14, 146.32, 145.76, 137.15, 137.08, 136.39, 129.16, 128.85, 126.87, 126.65, 124.26, 123.08, 122.85, 120.96. ESI-MS: m/z = 258.1030 (100%, MH⁺).

Synthesis of [RuCl(phenpy-OMe)(PPh₃)₂]Cl (3). In a Schlenk flask 2-(6-methoxypyridin-2-yl)-1,10-phenanthroline (0.1 g; 0.348 mmol) and RuCl₂(PPh₃)₃ (0.334 g; 0.348 mmol) were taken and 60 mL dry methanol was added with stirring. The mixture was refluxed for 24 h under argon atmosphere. It was allowed to cool to room temperature. The solvent was evaporated in vacuum, washed with dry diethyl ether and hexane, finally dried under vacuum to provide the title compound as a red powder. Yield: 295 mg (86%). ¹H NMR (500 MHz, CD₂Cl₂): δ = 9.12 (d, J_H,H = 5.2 Hz, 1H), 8.11 (d, J_H,H = 8.1 Hz, 1H), 7.98–7.93 (m, 3H), 7.82 (t, J_H,H = 7.9 Hz, 1H), 7.52 (d, J_H,H = 8.65 Hz, 1H), 7.38 (d, J_H,H = 7.65 Hz, 1H), 7.27–7.25 (m, 1H), 7.14–7.11 (m, 6H), 7.02–6.99 (m, 12H), 6.93–6.90 (m, 12H), 6.84 (d, J_H,H = 8.3 Hz, 1H), 4.11 (s, 3H). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ = 168.79, 158.20, 156.91, 156.10, 149.63, 147.56, 140.71, 134.39, 132.86, 132.82, 132.78, 130.46, 130.40, 129.96, 129.93, 128.68, 127.86, 127.82, 127.79, 126.76, 122.45, 116.60, 108.68, 56.12. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ = 20.21 (s, PPh₃). ESI-MS: m/z = 948.1616 ([M − Cl]⁺). Anal. calculated (C₅₄H₄₃Cl₂N₃OP₂Ru) (found): C, 65.92 (65.73); H, 4.41 (4.23); N, 4.27 (4.11).

Synthesis of [RuCl(phenpy-OH)(PPh₃)₂]Cl (4). In a Schlenk flask 6-(1,10-phenanthrolin-2-yl)pyridin-2-ol (0.05 g; 0.183 mmol) and RuCl₂(PPh₃)₃ (0.175 g; 0.183 mmol) were taken and 35 mL dry methanol was added with stirring. The mixture was refluxed for 24 h under argon atmosphere. It was allowed to cool to room temperature. The solvent was removed in vacuum, washed with dry diethyl ether and hexane, finally dried under vacuum to provide the title compound as a brick red powder. Yield: 148 mg (83%). ¹H NMR (500 MHz, CD₂Cl₂): δ = 11.34 (s, 1H), 9.39–9.35 (m, 1H), 8.22–8.13 (m, 1H), 7.85–7.72 (m, 2H), 7.65–7.64 (m, 1H), 7.63–7.62 (m, 2H), 7.61–7.60 (m, 1H), 7.56–7.52 (m, 2H), 7.47–7.43 (m, 6H), 7.34–7.05 (m, 12H), 6.97–6.83 (m, 12H). ¹³C{¹H} NMR (125 MHz, CD₂Cl₂): δ = 156.21, 154.79, 147.63, 144.07, 142.76, 138.60, 137.42, 137.33, 133.78, 133.62, 132.60, 132.51, 132.43, 131.99, 131.91, 131.88, 131.86, 128.75, 128.55, 128.46, 128.01, 127.67, 127.60, 120.31, 114.18, 112.67, 112.61. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ = 21.39 (s, PPh₃). ESI-MS: m/z = 934.1452 ([M − Cl]⁺). Anal. calculated (C₅₃H₄₁Cl₂N₃OP₂Ru) (found): C, 65.64 (65.78); H, 4.26 (4.21); N, 4.33 (4.38).

Synthesis of [RuCl(phenpy-OMe)(CH₃CN)₂]Cl (5). In a Schlenk flask 2-(6-methoxypyridin-2-yl)-1,10-phenanthroline (0.1 g; 0.348 mmol) and [RuCl₂(p-cymene)]₂ (0.107 g; 0.174 mmol) were taken and 40 mL dry CH₃CN was added. The mixture was stirred for 24 h under argon atmosphere. The solvent was removed in vacuum, washed with dry Et₂O and dried under vacuum to provide the title compound as a purple black powder. Yield: 159 mg (85%). ¹H NMR (500 MHz, CD₃CN): δ = 9.20 (dd, J_H,H = 5.1 Hz, J_H,H = 1.05 Hz, 1H), 8.53–8.48 (m, 2H), 8.42 (d, J_H,H = 8.6 Hz, 1H), 8.17–8.13 (m, 2H), 8.1–8.05 (m, 2H), 7.88 (q, J_H,H = 5.1 Hz, 1H), 7.23 (dd, J_H,H = 7.0 Hz, J_H,H = 2.3 Hz, 1H), 4.17 (s, 3H), 2.11 (s, 3H), 1.91 (s, 3H). ¹³C{¹H} NMR (125 MHz, CD₃CN): δ = 154.70, 144.81, 142.19, 141.10, 135.42, 132.43, 130.46, 128.64, 126.91, 126.25, 123.87, 123.24, 123.18, 121.43, 121.06, 116.86, 109.64, 57.19, 2.66, 1.82. ESI-MS: m/z = 506.0345 ([M − Cl]⁺). Anal. calculated (C₂₂H₁₉Cl₂N₅ORu) (found): C, 48.81 (48.57); H, 3.54 (3.46); N, 12.94 (12.77).

Synthesis of [RuCl(phenpy-OH)(CH₃CN)₂]Cl (6). In a Schlenk flask 6-(1,10-phenanthrolin-2-yl)pyridin-2-ol (0.05 g; 0.183 mmol) and [RuCl₂(p-cymene)]₂ (0.056 g; 0.092 mmol) were taken and 35 mL dry CH₃CN was added. The mixture was stirred for 36 h under argon atmosphere. The precipitate was filtered, washed with dry Et₂O and dried under vacuum to provide the title compound as a purple black powder. Yield: 52 mg (54%). ¹H NMR (500 MHz, CD₃CN): δ = 11.79 (s, 1H), 9.38 (d, J_H,H = 4.00 Hz, 1H), 9.19 (d, J_H,H = 4.40 Hz, 1H), 8.48–7.37 (m, 2H), 8.21–8.10 (m, 2H), 7.95–7.84 (m, 2H), 7.59 (d, J_H,H = 7.15 Hz, 1H), 7.09 (d, J_H,H = 9.60 Hz, 1H), 3.09 (s, 3H), 1.93 (s, 3H). ¹³C{¹H} NMR (125 MHz, CD₃CN): δ = 168.97, 166.52, 154.30, 153.06, 152.04, 147.91, 139.65, 139.56, 135.97, 134.41, 133.78, 133.34, 128.19, 126.86, 123.81, 122.65, 121.19, 3.15, 2.69. ESI-MS: m/z = 492.0173 ([M − Cl]⁺). Anal. calculated (C₂₁H₁₇Cl₂N₅ORu) (found): C, 47.83 (47.41); H, 3.25 (3.18); N, 13.28 (12.97).

Synthesis of [RuCl(phenpy-OMe)(CH₃CN)₂]PF₆ (7). In a Schlenk flask 2-(6-methoxypyridin-2-yl)-1,10-phenanthroline (0.1 g; 0.348 mmol) and [RuCl₂(p-cymene)]₂ (0.107 g; 0.174 mmol) were taken and 60 mL dry CH₃CN was added with stirring. The mixture was stirred for 24 h under argon atmosphere. Solid ammonium hexafluorophosphate (0.850 g; 5.22 mmol) was added to the solution. The solution was stirred at room temperature for overnight and solvent was removed in vacuum. Precipitate was dissolved in DCM and filtered. The filtrate was evaporated, washed with dry diethyl ether and hexane, and finally dried under vacuum to provide the title compound as a dark violet powder. Yield: 167 mg (74%). ¹H NMR (500 MHz, CD₃CN): δ = 9.15 (d, J_H,H = 4.75 Hz, 1H), 8.53 (t, J_H,H = 7.8 Hz, 2H), 8.46 (d, J_H,H = 7.55 Hz, 1H), 8.2 (br-s, 2H), 8.09–8.04 (m, 2H), 7.85 (t, J_H,H = 7.45 Hz, 1H), 7.22 (d, J_H,H = 7.8 Hz, 1H), 4.14 (s, 3H), 2.78 (s, 3H), 1.94 (s, 3H). ¹³C{¹H} NMR (125 MHz, CD₃CN): δ = 159.78, 158.81, 154.73, 149.76, 141.23, 135.65, 132.75, 130.42, 128.99, 128.93, 128.58, 127.30, 126.36, 121.37, 110.79, 109.73, 57.27, 3.06, 2.51. ³¹P{¹H} NMR (202 MHz, CD₃CN): δ = −144.11 (sept, J = 705.7, PF₆⁻). ESI-MS: m/z = 506.0396 ([M − PF₆]⁺). Anal. calculated (C₂₂H₁₉ClF₆N₅OPRu) (found): C, 40.59 (40.32); H, 2.94 (2.85); N, 10.76 (10.59).

General procedure for TH of nitrobenzene using Ru-catalyst

In a screw cap tube methanolic solution of L8 (2.87 mg, 0.01 mmol) and [RuCl₂(p-cymene)]₂ (3.06 mg, 0.005 mmol) were taken and solvent was evaporated. Then 6 mL dry 2-propanol was added to it under argon atmosphere and heated at 60 °C for 15 min to generate pre-catalyst in situ fashion. After that nitrobenzene (123 mg, 1 mmol) or nitrosobenzene (107 mg, 1 mmol) or azobenzene (182 mg, 1 mmol), NaOⁱPr (32.8 mg, 0.4 mmol), toluene (as internal standard) and 6 mL 2-propanol were added to the catalyst solution. The mixture was heated for specified time at 110 °C (oil bath temperature). It was allow to cool at room temperature, filtered through a small plug of neutral alumina and directly subjected for GC analysis to determined conversion as well as product selectivity of the reaction. All the reactions were carried out three times for reproducibility. The desired amine products were isolated and purified by column chromatography (silica) using hexane–ethyl acetate as eluent.

General procedure for TH of nitrobenzene using Ir-catalyst

In a screw cap tube methanolic solution of L8 (2.87 mg, 0.01 mmol) and [Ir(COD)Cl]₂ (3.36 mg, 0.005 mmol) were taken and solvent was evaporated. Then 6 mL dry 2-propanol was added to it and heated at 60 °C for 15 min under argon atmosphere to generate in situ pre-catalyst. After that nitrobenzene (123 mg, 1 mmol), NaOⁱPr (32.8 mg, 0.4 mmol), toluene (as internal standard) and 6 mL 2-propanol were added to the catalyst solution. The mixture was refluxed at 110 °C (oil bath temperature) for the specified time. The conversion and product selectivity were determined by gas chromatography. The reactions was repeated three times and an average data were plotted as conversion (%) vs. time (minute).

General procedure for time dependent TH experiment with Ru-catalyst

In a screw cap tube nitrobenzene (123 mg, 1 mmol) or nitrosobenzene (107 mg, 1 mmol) or azobenzene (182 mg, 1 mmol) was added to in situ generated pre-catalyst (0.01 mmol) solution. Subsequently, NaOⁱPr (32.8 mg, 0.4 mmol), toluene (as internal standard) and 6 mL dry 2-propanol were added under argon atmosphere. The mixture was refluxed at 110 °C (oil bath temperature) and progress was monitored at different time interval. The conversion and product selectivity were determined by gas chromatography. All of this reactions were repeated three times and an average data were plotted as conversion (%) vs. time (minute).

Conflict of interest

The authors declare no competing financial interests.

Acknowledgements

We are grateful to DST-INSPIRE, Science and Engineering Research Board, India and Council of Scientific & Industrial Research (CSIR), India for financial support. B. P., K. C. thanks UGC India and S. S., M. M., A. M. thanks CSIR India, for fellowships.

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Footnote

† Electronic supplementary information (ESI) available: General procedures for time dependent transfer hydrogenation studies, control experiments procedure, characterization data and NMR spectra of the products are available. See DOI: 10.1039/c6ra22221h

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