Article pubs.acs.org/Organometallics
Half-Sandwich Ruthenium Complexes with Schiff-Base Ligands: Syntheses, Characterization, and Catalytic Activities for the Reduction of Nitroarenes Wei-Guo Jia,* Hui Zhang, Tai Zhang, Dong Xie, Shuo Ling, and En-Hong Sheng College of Chemistry and Materials Science, Center for Nano Science and Technology, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu, 241000, China S Supporting Information *
ABSTRACT: A series of ruthenium(II) p-cymene complexes containing Schiff-base ligands [Ru(p-cymene)LCl] [HL = pyridyl-salicylimine (2a− 2d); HL = thiazol-salicylimine (2e−2h); HL = benzothiazol-salicylimine (2i−2l)] have been synthesized and characterized. All Schiff-base ligands and half-sandwich ruthenium complexes were fully characterized by 1H and 13 C NMR spectra, mass spectrometry, and infrared spectrometry. The molecular structures of ruthenium complexes 2b and 2k were further confirmed by single-crystal X-ray diffraction methods. Furthermore, these half-sandwich ruthenium complexes are active catalysts for the mild hydrogenation of nitroarenes to aromatic anilines in the presence of sodium tetrahydroborate reducing agent in water. The most efficient catalyst, 2b, was found be compatible with nitroarenes of various functional groups.
■
by nanoparticles,47 carbon nanotubes,48 SBA-15, and MCM4149,50 are all good candidate for the hydrogenation of nitroarenes due to high catalytic activity. However, the use of well-defined half-sandwich ruthenium complexes containing Schiff-base ligands as homogeneous catalysts for hydrogenation of nitroarenes has rarely been reported. Herein, we have synthesized the half-sandwich ruthenium complexes with a series of Schiff-base ligands [Ru(p-cymene)LCl] [HL = pyridyl-salicylimine (2a−2d); HL = thiazolsalicylimine (2e−2h); HL = benzothiazol-salicylimine (2i−2l)] (Chart 1) and explored their catalytic activities for the homogeneous reduction of nitroarenes to aromatic anilines. The half-sandwich ruthenium complex catalysts allowed the hydrogenation of nitroarenes to aromatic anilines to proceed in
INTRODUCTION Half-sandwich ruthenium complexes are prolific catalysts in the construction of C−C and C−heteroatom bonds through direct C−H bond activation,1−5 [4+2] and [3+2] cycloaddition of enamides or N-methoxy benzimidoyl halides and alkynes,6−8 addition of arylboronic acids,9−11 C−N bond formation from alcohols and amines,12−14 transfer hydrogenations of ketones,15−20 norbornene ring-opening metathesis polymerization,21−23 and water oxidation.24 Many half-sandwich ruthenium complexes with various N-based ligands such as [N,N],25 [N,O],26−28 [N,C],13,29−32 [N,P],33,34 [N,S], and [N,Se]35,36 have been synthesized and applied to various organic transformations. Schiff-base compounds are an important class of N-based ligands that are readily modifiable to allow fine-tuning of steric and electronic properties of the Ru complex.37 Exploring the reactivity of half-sandwich ruthenium complexes with Schiff-base ligands would therefore be interesting. Aromatic anilines are important intermediates and precursors in the preparation of dyes, agrochemicals, pharmaceuticals, and pigments.38,39 The most commonly used method for the synthesis of aromatic anilines is the reduction of nitroarenes. Catalytic hydrogenation using heterogeneous or homogeneous transition-metal catalysts is a well-established technique and is often employed for the reduction of nitroarenes to aromatic anilines.40−46 There are many reports on the hydrogenation reaction of nitroarenes using transitionmetal catalysts in the presence of a reducing agent such as hydrogen gas, sodium tetrahydroborate, hydrazine, and alcohol as the hydrogen source. Ruthenium-based catalysts supported © XXXX American Chemical Society
Chart 1. Molecular Structures of Half-Sandwich Ruthenium Complexes
Received: November 9, 2015
A
DOI: 10.1021/acs.organomet.5b00933 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 1. Synthesis of Schiff-Base Ligands (1a−1l)
Scheme 2. Synthesis of Half-Sandwich Ruthenium Complexes with Schiff-Base Ligands (2a−2l)
aminothiazole, and 2-aminobenzothiazole respectively, in moderate to good yields according to literature methods.51−55 The dark red half-sandwich ruthenium complexes (2a−2l) were obtained by reaction of [Ru(p-cymene)(μ-Cl)Cl]2 with 2 equiv of the Schiff-base ligands in the presence of K2CO3 in CH3CN under reflux for 3 h (Scheme 2). Ruthenium complexes were isolated as pure complexes by chromatography on silica gel using EtOAC/petroleum ether as an eluent in yields of 45− 75%. All complexes have been characterized by IR and NMR spectroscopy, as well as mass spectrometry. The half-sandwich ruthenium complexes are air and moisture stable, soluble in chlorohydrocarbon, alcohol, acetonitrile, and DMSO solvents, but slightly soluble in water. The 1H NMR spectra of complexes 2a−2l display a distinct resonance shift of the Schiff-base ligand protons in comparison with the equivalent protons in the free compounds. The 1H
the presence of sodium tetraydroborate reducing agent with high yields in water as solvent. This catalytic system was found to be robust toward the reduction of various nitroarenes with different substituents on the aromatic ring, but could not tolerate the reducable functional groups such as −CHO and COMe. The solid-state structures of complexes 2b and 2k were confirmed by single-crystal X-ray crystallography, and the resolved structures reveal that the complexes adopt the threelegged piano-stool conformation with a six-membered metallocycle formed by coordination of the Schiff-base ligands to the metal centers.
■
RESULTS AND DISCUSSION As outlined in Scheme 1, the Schiff-base ligands (1a−1l) were readily synthesized from the condensation of the corresponding salicyaldehyde and its derivatives with 2-aminopyridine, 2B
DOI: 10.1021/acs.organomet.5b00933 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics NMR spectra of all half-sandwich ruthenium complexes in CDCl3 show a doublet for each of the four p-cymene ring protons and two doublets for the methyl groups of the isopropyl moiety. One of the four proton resonances is attributable to the p-cymene ring, as it is strongly shifted to higher fields, in the range 4.43−4.80 ppm, whereas the other three doublets are in the range 5.14−5.40 ppm, which is typical of half-sandwich ruthenium systems.21,56,57 In the positive ESI mass spectra of 2a−2l peaks due to the cationic fragment [Ru(p-cymene)(L)]+ were observed as the predominant species ([M − Cl]+). X-ray diffraction of single crystals for complexes 2b and 2k was obtained. The crystals were grown by slow diffusion of diethyl ether into a concentrated solution of the complexes in methanol solution. The crystallographic data for half-sandwich ruthenium complexes 2b and 2k are summarized in Table 1. The molecular structures of 2b and 2k are shown in Figures 1 and 2.
Figure 1. Molecular structure of complex 2b with thermal ellipsoids drawn at the 30% level. All hydrogen atoms are omitted for clarity. Selected lengths (Å) and angles (deg): Ru1−O1 2.061(3), Ru1−N1 2.082(3), Ru1−Cl1 2.4162(11), N1−Ru1−O1 87.03(11), N1−Ru1− Cl1 84.96(9), O1−Ru1−Cl1 86.43(9), Ru2−O2 2.057(3), Ru2−N3 2.087(3), Ru2−Cl3 2.4187(12), N3−Ru2−O2 87.75(13), N3−Ru2− Cl3 84.67(10), O2−Ru2−Cl3 85.59(10).
Table 1. Crystallographic Data and Structure Refinement Parameters for Ruthenium Complexes empirical formula fw cryst syst, space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3), Z Dc (mg/m3) μ(Mo Kα) (mm−1) F(000) θ range (deg) limiting indices reflns/unique [R(int)] completeness to θ (deg) data/restraints/params goodness-of-fit on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) a
2b
2k
C22H22Cl2N2ORu 502.39 orthorhombic, Pca21 13.8993(7) 15.3472(8) 20.0927(11) 90.00 90.00 90.00 4286.1(4), 8 1.557 0.996 2032 1.98−27.44 −18, 17; −19, 19; −25, 26 9733/8397 [0.0269] 27.44 (99.8%)
C24H22BrClN2ORuS 602.93 monoclinic, P21/c 8.1235(5) 21.4146(14) 13.0167(9) 90.00 94.2380(10) 90.00 2258.2(3), 4 1.773 2.694 1200 1.83−27.62 −10, 10; −27, 27; −16, 14 5249/4684 [0.0214] 27.62 (99.9%)
9733/1/511 1.016 R1 = 0.0265, wR2 = 0.0599 R1 = 0.0351, wR2 = 0.0648
5249/303/280 1.039 R1 = 0.0261, wR2 = 0.0704 R1 = 0.0307, wR2 = 0.0731
Figure 2. Molecular structure of complex 2k with thermal ellipsoids drawn at the 30% level. All hydrogen atoms are omitted for clarity. Selected lengths (Å) and angles (deg): Ru1−O1 2.0617(15), Ru1−N1 2.1044(18), Ru1−Cl1 2.4160(6), N1−Ru1−O1 87.55(6), N1−Ru1− Cl1 82.39(5), O1−Ru1−Cl1 85.85(5).
R1 = ∑∥Fo| − |Fc∥/∑|Fo|; wR2 = [∑w(|Fo2| − |Fc2|)2/∑w|Fo2|2]1/2.
to those of a half-sandwich ruthenium complex containing [N,O] anionic bidentate ligands.26−28,67−69 The reactivity of the half-sandwich ruthenium complexes for the reduction of nitroarenes was first studied using 1-chloro-4nitrobenzene as model substrate. As shown in Table 2, the halfsandwich ruthenium complex 2b was found to be very active toward the reduction of nitro compounds. The complex 2b was chosen as the optimal catalyst to screen the various solvents (Table 3, entries 1−7). A high yield of 92% of the desired products was achieved in water solvent. The yields improved slightly with a shortened reaction time when the reaction temperature was raised (Table 3, entries 8 and 9). A lower catalyst loading (5 mol % and 1 mol % 2b) contributed to longer reaction times and yields (Table 3, entries 10 and 11). A control experiment without the ruthenium catalyst showed no reaction (Table 3, entry 12).
The crystal packing was solved with orthorhombic Pca21 (2b) and monoclinic P21/c (2k) space groups. As shown in Figure 1 and Figure 2, each Ru is surrounded by one chlorine atom, one nitrogen atom, and one oxygen atom from the Schiffbase ligand and one of the p-cymene rings. Both the ruthenium centers have six-coordinate geometry assuming that the pcymene ring serve as a three-coordinated ligand, which is common for half-sandwich “piano-stool” structures.58−66 In addition, crystal 2b crystallized in the non-centrosymmetric space group Pca21, the structural unit of 2b contains two independent molecules, and they are a pair of racemic isomers (Figure 1). The Ru−O distances (2.061(3) and 2.057(3) Å in 2b and 2.0617(15) Å in 2k) and Ru−N distances (2.082(3) and 2.087(3) Å in 2b and 2.1044(18) Å in 2k) are comparable C
DOI: 10.1021/acs.organomet.5b00933 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Under the same reaction conditions, the −COOH and −NHCOMe groups were unaffected. Noteworthy is that the highly sterically hindered 2,6-dimethylnitrobenzene was reduced in 82% yield (Table 4, entry 24). To further evaluate the practical utility of the half-sandwich ruthenium catalyst system, the model reaction was carried out on a gram scale in the presence of 20 equiv of NaBH4 and 0.5 mol % 2b catalyst with H2O/EtOH = 1:1 as mixed solvents, and the desired product was obtained in 94% yield in 72 h (Scheme 3). A plausible reaction mechanism for the reduction of nitroarenes is shown in Scheme 4. The reaction of the complex 2b with an excess (4 equiv) of NaBH4 produces the halfsandwich ruthenium hydride active species complex [(pcymene)(L1b)RuH] in almost quantitative yield. The halfsandwich ruthenium hydride active species complex can be confirmed through 1H NMR (δ −10.08 and −13.90 ppm), as was reported.70,71 The imine CN double bond has been reduced to give an amine-coordinated ruthenium hydride complex, [(p-cymene)RuH(OArCH2NHPy)] (intermediate A). Then, the reduction of the nitroarene may be proposed to occur via the outer-sphere mechanism (intermediate B), which gives the hydroxyl amine first and then the final arylamine.72
Table 2. Screening of Half-Sandwich Ruthenium Complex Catalystes for 1-Chloro-4-nitrobenzene Reductiona
entry
catalyst
time (h)
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12
2a 2b 2c 2d 2e 2f 2g 2h 2i 2j 2k 2l
24 20 24 24 24 24 24 24 24 24 24 24
68 92 83 73 72 81 75 70 61 71 70 65
a
Reaction conditions: 0.3 mmol of nitrobenzene, 1.2 mmol of sodium tetrahydroborate, Ru catalysts (10 mol %), water (2 mL), room temperature. bIsolated yield.
Table 3. Screening of Solvents for 1-Chloro-4-nitrobenzene Reduction Catalyzed by Ruthenium Complex 2ba
■
Reaction conditions: 0.3 mmol of nitrobenzene, 1.2 mmol of sodium tetrahydroborate, 2b (14.0 mg, 10 mol %), solvent (2 mL), room temperature. bIsolated yield. c50 oC. d85 oC. e2b 5 mol %. f2b 1 mol %. gWithout catalyst.
CONCLUSION In summary, we have synthesized and characterized a series of novel half-sandwich ruthenium complexes with Schiff-base ligands and evaluated their ability as catalysts in hydrogenation of nitroarenes using sodium tetrahydroborate as hydride source in water solvent. The hydrogenation reaction offers varied functional group compatibility and broad substrate scope and provides aromatic anilines in good to excellent yields on a gram-scale basis. The advantages of these homogeneous halfsandwich ruthenium catalyst systems include the following: (1) the catalysts were easily synthesized, (2) the catalysts showed high catalytic activities toward nitroarene reduction in the presence of NaBH4 in water, and (3) the catalytic mechanisms of nitroarene reduction were explored. We tentatively propose a half-sandwich ruthenium hydride complex as the active species in catalyzing the nitroarene reduction, and further mechanistic studies are in progress.
With the optimal reaction conditions in hand, we started to expand the scope and efficiency of this methodology. A series of functionalized anilines were obtained in good to excellent yields (Table 4). As shown in Table 4, the catalyst 2b was found to be very active toward the reduction of various nitro compounds. Electron-withdrawing and electron-donating substituents on the aromatic backbone were able to provide the desired products in high yields. However, reduction of nitrotoluene generally takes a longer time for an electron-donating group such as −CH3 or −OCH3 (Table 4, entries 10−13). Next, we wanted to investigate if the half-sandwich ruthenium system promotes chemoselective nitro hydrogenation even in the presence of other functional groups that are easily reduced. As such, the chemoselective reduction reaction of the nitro group in the presence of other substituents such as −CHO, −CH2OH, −COMe, −COOH, and −NHCOMe was performed (Table 4, entries 14−20). In the case of 4-nitroacetophenone and 3- and 4-nitrobenzaldehyde, the −CHO and −COMe groups were reduced togther with the nitro group.
Materials and Measurements. All the operations were carried out under a pure nitrogen atmosphere using standard Schlenk techniques. All solvents were purified and degassed by standard procedures. The Schiff-base ligands were synthesized according to procedures described in the previous literature.51−55 1H and 13C NMR were recorded on a 300 or 500 MHz NMR spectrometer at room temperature. Chemical shifts (δ) are given in ppm relative to internal TMS and are internally referenced to residual 1H and 13C solvent resonances. IR spectra were recorded on a Niclolet AVATAR-360IR spectrometer. Elemental analyses were performed on a PerkinElmer 2400 CHN analyzer. Mass spectrometry was performed on a Bruker BIFLEX III MALDI-TOF-MS instrument. Synthesis of Schiff-Base Compounds 1a−1l. All Schiff base ligands were synthesized by dissolving the corresponding functionalized salicyldehyde (1.0 mmol) in ethanol (10 mL), followed by addition of 2-aminopyridine (1.2 mmol), 2-aminothiazole (1.2 mmol), and 2-aminobenzothiazole (1.2 mmol), respectively. The reaction mixture was then refluxed for 6 h, and the color turned to yellow. The solvent was evaporated to 50% in vacuo and cooled to room temperature. The resulting solid was filtered off and washed three times with ethanol (3 mL each).
entry
solvent
time (h)
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12
H2O MeOH MeCN DMF DCM toluene THF H2O H2O H2O H2O H2O
20 24 25 24 24 25 25 20 16 32 75 24
92 80 55 75 70 32
