Appraisal of Ruthenium(II)complexes of (4-phenoxyphenylazo)ligands for the synthesis of primary amides by dint of hydroxylamine hydrochloride and aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2019.05.010

A new family of O, N donor-functionalized (4-phenoxyphenylazo)-2-naphthol/4-substituted phenol-based ligands (HL₁-HL₄)has been synthesized. The prepared ligands were successfully utilized for the access of a series of ruthenium(II)carbonyl complexes of the type [Ru(L)Cl(CO)(EPh₃)₃](E = phosphine/arsine), (L = 1-(4-phenoxyphenylazo)-2-naphthol (HL₁), 2-(4-phenoxyphenylazo)-4-chlorophenol (HL₂), 2-(4-phenoxyphenylazo)-4-methylphenol (HL₃)and 2-(4-phenoxyphenylazo)-4-methoxyphenol (HL₄)). All of the ruthenium(II)carbonyl complexes and ligands have been fully characterized by FT-IR, UV–visible, ¹H NMR, ³¹P NMR, mass spectrometry and CHN analysis. The ligands have been analyzed by ¹³C NMR. The UV–visible spectroscopic study reveals that both the ligands and Ru(II)complexes exhibit excellent charge transfer transitions. This is the basic criteria for the oxidative amidation reaction, which is an influential strategy for the transformation of oxygenated organic compounds to the profitable amides. However, this catalytic process makes more impact on the application of new divalent ruthenium(II)azo compounds as catalyst in a single-pot conversion of aldehydes to amides in the presence of NaHCO₃.

Full text of DOI:10.1016/j.jorganchem.2019.05.010

Accepted Manuscript
Appraisal of Ruthenium(II) complexes of (4-phenoxyphenylazo) ligands for the
synthesis of primary amides by dint of hydroxylamine hydrochloride and aldehydes
Govindasamy Vinoth, Sekar Indira, Madheswaran Bharathi, Muniyan Sounthararajan,
Dharmalingam Sakthi, Kuppannan Shanmuga Bharathi
PII:
S0022-328X(19)30191-3
DOI:
https://doi.org/10.1016/j.jorganchem.2019.05.010
JOM 20792
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 20 December 2018
Revised Date: 7 May 2019
Accepted Date: 13 May 2019
Please cite this article as: G. Vinoth, S. Indira, M. Bharathi, M. Sounthararajan, D. Sakthi, K.S. Bharathi,
Appraisal of Ruthenium(II) complexes of (4-phenoxyphenylazo) ligands for the synthesis of primary
amides by dint of hydroxylamine hydrochloride and aldehydes, Journal of Organometallic Chemistry
(2019), doi: https://doi.org/10.1016/j.jorganchem.2019.05.010.
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Appraisal of Ruthenium(II) Complexes of (4-Phenoxyphenylazo) ligands for the Synthesis
of Primary Amides by dint of Hydroxylamine Hydrochloride and Aldehydes
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Govindasamy Vinoth, Sekar Indira, Madheswaran Bharathi, Muniyan Sounthararajan,
Dharmalingam Sakthi and Kuppannan Shanmuga Bharathi*
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Department of Chemistry, School of Physical Science, Periyar University, Periyar Palkalai
Nagar, Salem-636011, Tamil Nadu, India.
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*Corresponding author; e-mail Id: nksbharathi@periyaruniversity.ac.in
Abstract
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A new family of O, N donor-functionalized (4-phenoxyphenylazo)-2-naphthol / 4-substituted
phenol-based ligands (HL1-HL4) has been synthesized. The prepared ligands were successfully
utilized for the access of a series of ruthenium(II) carbonyl complexes of the type
[Ru(L)Cl(CO)(EPh3)3] (E = phosphine / arsine), (L = 1-(4-phenoxyphenylazo)-2-naphthol
(HL1),
2-(4-phenoxyphenylazo)-4-chlorophenol
(HL2),
2-(4-phenoxyphenylazo)-4-
methylphenol (HL3) and 2-(4-phenoxyphenylazo)-4-methoxyphenol (HL4)). All of the
ruthenium(II) carbonyl complexes and ligands have been fully characterized by FT-IR, UV-
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visible, H NMR, 31P NMR, mass spectrometry and CHN analysis. The ligands have been
analyzed by 13C NMR. The UV- visible spectroscopic study reveals that both the ligands and
Ru(II) complexes exhibit excellent charge transfer transitions. This is the basic criteria for the
oxidative amidation reaction, which is an influential strategy for the transformation of
oxygenated organic compounds to the profitable amides. However, this catalytic process makes
more impact on the application of new divalent ruthenium(II) azo compounds as catalyst in a
single-pot conversion of aldehydes to amides in the presence of NaHCO3.
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Keywords
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(4-phenoxyphenylazo) ligands; Ru complexes; Spectroscopy; Catalytic conversion; Amide
synthesis
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1. Introduction
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Events associated with organoruthenium(II) complexes containing azo ligands have
significant interest in recent years.[1] These organoruthenium(II) complexes are extensively used
in several industries and biological fields.[2-4] Specifically Ru(II) complexes have found to be
catalytic activity of various organic transformations such as hydroxylation, carboxylation,
hydroformylation, isomerization, epoxidation, dehalogenation, sulfoxidation, dehydrogenation,
polymerization, carbon-carbon bond formation, alcohol and aldehyde reactions.[5-10] The
Synthesized N-arylamide compounds have emerged as different potential drugs (herbicide,
fungicide, insecticide, pesticide, etc.,).[11, 12] So, this method of preparation of amide skeleton has
become one of the most achievable tasks to the organic chemists. The traditional strategies are
used for the preparation of amides via, the coupling of activated carboxylic acid derivatives, viz.,
acid chloride, anhydride, active esters with a variety of amines. Quite a lot of available methods
along with the use of carbonyl compounds (ketones and aldehydes) or carbonyl derivatives
(oximes) have been employed for the synthesis of amides.[13-16] Over the years of centenary,
Beckmann rearrangement is used to transform oxime into the corresponding amides using an
acids or base.[17, 18]
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However, this conversion is quite challenging one due to the formation of undesired
nitriles, aldehydes and carboxylic acids during the course of the reaction. These side products
might affect the yield of the desired amides when the reactions are scaled up. Hence, the
investigation on the synthesis of amides using new catalysts receives more attention. Moreover,
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metal-mediated methodologies for this conversion by dint of hydroxylamine hydrochloride are
sparse.[19] The conversion of aldehydes to amides takes place through the formation of
aldoximes. The rearrangement of aldoximes to amides, has been achieved by the usage of
various transition metal catalysts possess nickel[20], copper[21], zinc[22] palladium[23]
,
gold/silver[24], iridium[25, 26] and rhodium[27] Williams et al. have reported a ruthenium catalyzed
transformation by the results of greater selectivity, appreciable yield and low catalyst loading.[28]
Moreover, Crabtree and his coworkers have used ruthenium terpyridine complexes as catalysts
and achieved the better results without using any acid or base.[29] Ruthenium complexes of
thiazolylazo ligands[30], hydrozone ligands[31], P-donor ligand tris(dimethylamino)phosphine[32]
and phosphine free ligands[33] have also been utilized for the transfer hydrogenation reactions.
Models of ruthenium complexes with different donor sets such as NNN, NNO, SNO, P and N
utilized for the amide conversion are shown in the Fig. 1.
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Fig. 1. Models of ruthenium-complexes for the amide conversion
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In kind, [RuCl2(DMSO)4] complex has been used as potentially active catalyst for the
reactions of aldehydes to primary amides.[34]
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The previous literature results on ruthenium azo chemistry reveal[35] that, it has great
attention owing to its three reasons. At first, the ability of azo functional group in the formation
of metal-carbon bonds or metallacycles due to their π-acidic nature.[36] Moreover, the azo
functional group (-N=N-) can stabilize the ruthenium ion in lower oxidation states because of its
strong π-acidic character, whereas naphtholate / phenolate oxygen being a hard base, which
stabilizes the metal ion in higher oxidation states.[37] Secondly, the naphthol / phenol-based
systems can act as potential bidentate ligands which coordinate to the metal through N and O
donors. Moreover, they have better electron donating activity and provide more steric crowing
compared to the phenyl group. Thirdly, a number of ligands can act as tridentate donor systems
and can able to produce an additional vacancy at the metal center throughout the catalytic
response which facilitates the catalytic cycle. The kind of ligands explored in this work is known
to bind with metal atom in a bidentate fashion with mono anionic O, N donors by forming a five
membered chelate ring.[38]
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Herein, we report the synthesis and characterization of ruthenium(II) carbonyl complexes
containing triphenylphosphine / triphenylarsine incorporated with (phenoxyphenylazo) 2-
naphthol / p-substituted phenols. Furthermore, they have been effectively employed as catalysts
in the transformation of various aldehydes to amides by following hydroxylamine hydrochloride
pathway along with NaHCO3 as an additive.
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2. EXPERIMENTAL SECTION
2.1. Materials and Measurements
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Commercially available RuCl3.3H2O was used as supplied from SRL Pvt. Ltd. All the
reagents used were chemically pure and analar grade. The solvents were freshly purified using
the usual procedures.[39] 4-phenoxyphenylamine, triphenylarsine and triphenylphosphine were
purchased from Aldrich. All substituted phenols like 2-naphthol, 4-chlorophenol, 4-
methylphenol and 4-methoxyphenol have been received from Merck. The starting precursor
[RuHCl(CO)(PPh3)3] and [RuHCl(CO)(AsPh3)3] have been organized through said literature
methods.[40, 41]
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The evaluation of carbon, hydrogen and nitrogen has been completed on VarioEL III
CHNS analyzer. FT-IR spectra of ligands and complexes were accomplished on a Bruker 783
spectrometer for the samples in direct utilization. Electronic spectra of the ligands and complexes
were recorded in CHCl3 solution with a Cary 300 Bio UV–Vis Varian spectrophotometer in the
range 200-800 nm. 1H, 13C and 31P NMR spectra were recorded in CDCl3 with Bruker 300 MHz
tool and the use of TMS as inner reference. Electrospray ionization mass spectra (ESI) have been
recorded on ES-MS Q-TOF mass spectrometer.
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2.2. Synthesis of 4-phenoxyphenylazo ligands
2.2.1. Synthesis of 1-(4-phenoxyphenylazo)-2-naphthol (HL1)
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4-phenoxyphenylamine (1.85g; 10 mmol) was mixed with 3 mL concentrated sulphuric
acid and diazotized at 0-5 oC with sodium nitrite (0.685g; 10 mmol). The diazonium chloride that
formed was coupled with an alcoholic 10% sodium hydroxide solution (50 mL) of 2-naphthol
(1.08g; 10 mmol) at the same reaction temperature. The reaction was completed just after 30
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minutes. A dark red precipitate was obtained and dried in vacuum. The product was
recrystallized from methanol. Yield: 84%; Anal. calcd (%) for C22H16N2O2: C, 77.63; H, 4.74; N,
8.23. Found: C, 77.59; H, 4.71; N, 8.19. FT-IR (cm-1): 3418 (νO-H), 1449 (νN=N); 1124 (ν(C-O));
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UV-Vis {Chloroform, λmax, nm}: 316, 418; H NMR (300 MHz, CDCl3): δ 9.28 (s, ArOH, 1H);
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8.19-6.90 (m, ArH, 15H); C NMR (75 MHz, CDCl3): 156.37, 155.50, 154.05, 136.38, 131.86,
127.17, 126.70, 125.23, 124.72, 123.88, 121.06, 120.50, 119.18, 118.18, 116.88, 107.62. ESI-
MS, m/z: 340 [MH]+. The phenoxyphenylazo ligands, (HL2-4), were synthesized by following
the above procedure.
2.2.2. Synthesis of 2-(4-phenoxyphenylazo)-4-chlorophenol (HL2)
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Yield: 67%; Colour: Yellow solid; Anal. calcd (%) for C18H13ClN2O2: C, 66.57; H, 4.03;
N, 8.63. Found: C, 66.50; H, 4.00; N, 8.54. FT-IR (cm-1): 3435 (νO-H), 1412 (νN=N); 1189 (ν(C-O));
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UV-Vis {Chloroform, λmax, nm}: 308, 358; H NMR (300 MHz, CDCl3): δ 9.14 (s, ArOH, 1H);
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7.63-6.73 (m, ArH, 12H); C NMR (75 MHz, CDCl3): 159.21, 157.08, 150.30, 145.86, 133.53,
128.48, 126.51, 125.67, 124.72, 122.79, 121.83, 120.70, 118.92, 118.88. ESI-MS, m/z: 324
[MH]+.
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2.2.3. Synthesis of 2-(4-phenoxyphenylazo)-4-methylphenol (HL3)
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Yield: 81%; Colour: Brown solid; Anal. calcd (%) for C19H16N2O2: C, 74.98; H, 5.30; N,
9.20. Found: C, 74.94; H, 5.28; N, 9.17. FT-IR (cm-1): 3403 (νO-H), 1432 (νN=N); 1138 (ν(C-O));
UV-Vis {Chloroform, λmax, nm}: 312, 364; 1H NMR (300 MHz, CDCl3): δ 12.64 (s, ArOH, 1H);
7.87-6.79 (m, ArH, 12H); 2.38 (s, ArCH3, 3H); 13C NMR (75 MHz, CDCl3): 158.82, 155.19,
149.31, 144.97, 135.74, 132.62, 129.62, 128.73, 127.72, 122.81, 119.29, 118.73, 117.49, 115.80,
19.02. ESI-MS, m/z: 304 [MH]+.
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2.2.4. Synthesis of 2-(4-phenoxyphenylazo)-4-methoxyphenol (HL4)
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Yield: 82%; Colour: Orange solid; Anal. calcd (%) for C19H16N2O3: C, 71.24; H, 5.03; N,
8.74. Found: C, 71.20; H, 5.00; N, 8.71. FT-IR (cm-1): 3436 (νO-H), 1431 (νN=N); 1155 (ν(C-O));
UV-Vis {Chloroform, λmax, nm}: 312, 362; 1H NMR (300 MHz, CDCl3): δ 12.39 (s, ArOH, 1H);
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7.87-6.69 (m, ArH, 12H); 3.86 (s, ArOCH3, 3H); C NMR (75 MHz, CDCl3): 158.86, 157.14,
156.22, 152.08, 145.73, 141.27, 136.18, 129.31, 127.35, 123.38, 122.01, 120.36, 119.57, 112.63,
55.11. ESI-MS, m/z: 320 [MH]+.
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2.3. Synthesis of ruthenium(II) phenoxyphenylazo complexes
2.3.1. Synthesis of [Ru(L1)Cl(CO)(PPh3)2] (1)
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To a solution of [RuHCl(CO)(EPh3)3] (E = P & As) (0.1 g, 1.0 mmol) benzene (20 mL),
HL1 (0.04 g, 1.0 mmol) was added and the reaction mixture was refluxed for 8 h under N2
atmosphere to yield green solution. The solvent was then removed under reduced pressure. The
residue was checked by thin layer chromatography and purified by column chromatography.
Chloroform have been used as eluent in the column chromatography and the green coloured band
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was collected and dried. Yield: 78%; Colour: green solid; m.p.: 102 C; Anal. calcd (%) for
C59H45ClN2O3P2Ru: C, 68.90; H, 4.41; N, 2.72. Found: C, 68.87; H, 4.38; N, 2.69. FT-IR (cm-1):
1924 (νCO), 1434 (νN=N), 1230 (νAr(C-O)); UV-Vis {Chloroform, λmax, nm}: 260, 338, 430, 537; 1H
NMR (300 MHz, CDCl3): δ 7.71-7.08 (m, ArH, 45H). 31P NMR (162 MHz, CDCl3): δ 24.35 (s,
2P). ESI-MS, m/z: 1028 [MH]+. The Ru(II) complexes 2-8 were prepared by following the above
procedure using the phenoxyphenylazo ligands (HL2-4).
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2.3.2. Synthesis of [Ru(L2)Cl(CO)(PPh3)2] (2)
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Yield: 62%; Colour: green solid; m.p.: 80 C; Anal. calcd (%) for C55H42Cl2N2O3P2Ru:
C, 65.22; H, 4.18; N, 2.77. Found: C, 65.19; H, 4.15; N, 2.74. FT-IR (cm-1): 1943 (νCO), 1433
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(νN=N), 1236 (νAr(C-O)); UV-Vis UV-Vis {Chloroform, λmax, nm}: 303, 370, 411, 525; H NMR
(300 MHz, CDCl3): δ 8.35-6.57 (m, ArH, 42H). 31P NMR (162 MHz, CDCl3): δ 29.13 (s, 2P).
ESI-MS, m/z: 1012 [MH]+.
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2.3.3. Synthesis of [Ru(L3)Cl(CO)(PPh3)2] (3)
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Yield: 75%; Colour: green solid; m.p.: 84 oC; Anal. calcd (%) for C56H45ClN2O3P2Ru: C,
67.77; H, 4.57; N, 2.82. Found: C, 67.74; H, 4.55; N, 2.78. FT-IR (cm-1): 1929 (νCO), 1433
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(νN=N), 1241 (νAr(C-O)); UV-Vis UV-Vis {Chloroform, λmax, nm}: 307, 333, 448, 534; H NMR
(300 MHz, CDCl3): δ 7.87-6.79 (m, ArH, 42H); 2.38 (s, ArCH3, 3H). 31P NMR (162 MHz,
CDCl3): δ 25.61 (s, 2P). ESI-MS, m/z: 992 [MH]+.
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2.3.4. Synthesis of [Ru(L4)Cl(CO)(PPh3)2] (4)
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Yield: 80%; Colour: green solid; m.p.: 82 oC; Anal. calcd (%) for C56H45ClN2O4P2Ru: C,
66.70; H, 4.50; N, 2.78. Found: C, 66.67; H, 4.48; N, 2.72. FT-IR (cm-1): 1926 (νCO), 1434
(νN=N), 1239 (νAr(C-O)); UV-Vis {Chloroform, λmax, nm}: 302, 369, 443, 665; 1H NMR (300 MHz,
CDCl3): δ 7.97-6.84 (m, ArH, 42H); 3.85 (s, ArOCH3, 3H). 31P NMR (162 MHz, CDCl3): δ
24.98 (s, 2P). ESI-MS, m/z: 1008 [MH]+.
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2.3.5. Synthesis of [Ru(L1)Cl(CO)(AsPh3)2] (5)
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Yield: 73%; Colour: green solid; m.p.: 118 oC; Anal. calcd (%) for C59H45As2ClN2O3Ru:
C, 68.90; H, 4.41; N, 2.72. Found: C, 68.87; H, 4.38; N, 2.69. FT-IR (cm-1): 1943 (νCO), 1432
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(νN=N), 1232 (νAr(C-O)); UV-Vis {Chloroform, λmax, nm}: 260, 341, 435, 534; 1H NMR (300 MHz,
CDCl3): δ 7.81-7.10 (m, ArH, 45H). ESI-MS, m/z: 1116 [MH]+.
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2.3.6. Synthesis of [Ru(L2)Cl(CO)(AsPh3)2] (6)
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Yield: 67%; Colour: green solid; Anal. calcd (%) for C55H42As2Cl2N2O3Ru: C, 60.01; H,
3.85; N, 2.54. Found: C, 59.98; H, 3.82; N, 2.51. FT-IR (cm-1): 1920 (νCO), 1431 (νN=N), 1221
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(νAr(C-O)); UV-Vis {Chloroform, λmax, nm}: 257, 353, 405, 531; H NMR (300 MHz, CDCl3): δ
8.32-6.60 (m, ArH, 42H). ESI-MS, m/z: 1101 [MH]+.
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2.3.7. Synthesis of [Ru(L3)Cl(CO)(AsPh3)2] (7)
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Yield: 87%; Colour: green solid; m.p.: 126 oC; Anal. calcd (%) for C56H45As2ClN2O3Ru:
C, 62.26; H, 4.20; N, 2.59. Found: C, 62.23; H, 4.18; N, 2.54. FT-IR (cm-1): 1919 (νCO), 1432
(νN=N), 1218 (νAr(C-O)); UV-Vis {Chloroform, λmax, nm}: 259, 354, 414, 530; 1H NMR (300 MHz,
CDCl3): δ 7.86-6.80 (m, ArH, 42H); 2.38 (s, ArCH3, 3H). ESI-MS, m/z: 1080 [MH]+.
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2.3.8. Synthesis of [Ru(L4)Cl(CO)(AsPh3)2] (8)
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Yield: 79%; Colour: green solid; m.p.: 112 oC; Anal. calcd (%) for C56H45As2ClN2O4Ru:
C, 61.35; H, 4.14; N, 2.56. Found: C, 61.32; H, 4.12; N, 2.52. FT-IR (cm-1): 1919 (νCO), 1433
(νN=N), 1227 (νAr(C-O)); UV-Vis {Chloroform, λmax, nm}: 254, 350, 412, 561; 1H NMR (300 MHz,
CDCl3): δ 7.96-6.69 (m, ArH, 42H); 3.85 (s, ArOCH3, 3H). ESI-MS, m/z: 1096 [MH]+.
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2.4. Representative procedure for the rearrangement of aldehydes to amides
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Conversion of aldehydes to amides was carried out using the procedure described in the
literature.[34] Under nitrogen atmosphere, the corresponding aldehyde (1 mmol), NH2OH·HCl (1
mmol), NaHCO3 (1 mmol), toluene (3 mL) and ruthenium(II) catalyst 1 (0.01 mmol) were
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introduced into a RB flask, and the reaction mixture was stirred at 120 oC in an oil bath for 12 h.
After the completion of reaction, 2-3 mL of MeOH was added to the reaction mixture. The
catalyst and NaHCO3 which settled down the flask was removed by filtration through celite. The
filtrate has been dried and the crude product was purified by the use of column chromatography
(MeOH/CH2Cl2). Finally, the isolated amide was characterized by the aid of 1H NMR.
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3. RESULTS AND DISCUSSION
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Phenoxyphenylazo ligands have been prepared in good yields by diazotization of 2-
naphthol / 4-substituted phenol with 4-phenoxyphenylamine at 0-5 oC (Scheme 1).
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Scheme 1. Synthesis of phenoxyphenylazo ligands
On the complexation of Ru(II) precursor, [RuHCl(CO)(EPh3)3] (E = Phosphorous and
Arsine), with azo ligands in benzene led to the synthesis of ruthenium(II) phenoxyphenylazo
complexes (1-8) of general formula [Ru(L)Cl(CO)(EPh3)2] (Scheme 2).
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EPh3
OH
N
O
CO
Cl
[RuH(CO)Cl(EPh3)3]
Ru
Benzene / 8 h
N
Et3N
N
N
EPh3
O
O
E = P, As
HL1
EPh3
CO
Cl
OH
O
[RuH(CO)Cl(EPh3)3]
Ru
Benzene / 8 h
R
N
R
N
Et3N
N
N
O
EPh3
O
R = Cl (HL2), CH3 (HL3), OCH3 (HL4)
E = P, As
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Scheme 2. Synthesis of ruthenium(II) 1-(4-phenoxyphenylazo)-2-naphtholate / 2-(4-
phenoxyphenylazo)-4-phenolate complexes.
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The ligands and complexes have been characterized by IR, UV-visible, H NMR, 13C
NMR, 31P NMR, mass spectral analysis as well as elemental analysis. The complexes were
formed by the coordination of ruthenium ion to the azo nitrogen and phenolic oxygen in the
monoanionic bidentate donor set of our ligands (HL1-HL4), which leads to the formation of five
membered chelate ring due to its strong π-acidic character.[42-44] All the ruthenium(II) complexes
(1-8) are air and moisture stable. And the complexes are all completely soluble in common
organic solvents like benzene, dichloromethane, chloroform, dimethyl sulfoxide,
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dimethylformamide, toluene, acetonitrile and methanol. These complexes were utilized as
catalysts in the catalytic activity for the conversion of aldehydes to primary amides in a single-
pot reaction. Then, the reaction was carried out by taking the substrates, aldehyde and
NH2OH·HCl, along with the base NaHCO3 in the presence of ruthenium catalyst 1. The catalytic
activity of ruthenium(II) complexes (1-8) have been optimized under different conditions.
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3.1. Infrared spectra
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Infrared spectra of the ligands and ruthenium(II) complexes exhibit many characteristic
features within 4000-400 cm-1. Sharp stretching vibrations around 3436-3403 cm-1, 1449-1431
cm-1 and 1189-1124 cm-1 in the infrared spectra of the free ligands, HL1-HL4, are assigned to
the naphtholic / phenolic (νO-H), phenoxyphenylazo (νN=N) and phenolic (νC-O) groups
respectively.[45] The disappearance of naphtholic, phenolic O-H band and increase in the C-O
stretching frequencies in the region 1241-1230 cm-1 indicates the coordination of the metal
through naphtholic / phenolic oxygen in the complexes. The bands appeared around 1940 cm-1
and 1433 cm-1 indicate the existence of metal carbonyls and azo groups respectively in the
complexes.[46-48] All the ruthenium(II) complexes are shown strong vibrational bands
corresponding to M-P / M-As, M-O and M-N in the region 694-613 cm-1 / 689-607 cm-1, 530-
515 cm-1 and 453-410 cm-1 respectively.[47, 49, 50]
3.2. Electronic spectra
The electronic spectra of the phenoxyphenylazo ligands, HL1-HL4, and
organoruthenium(II) complexes (1-8) have been carried out in chloroform solvent. The ligands
display intense peaks due to the π-π* and n-π* transitions at 316-308 nm and 418-358 nm
respectively. Ruthenium(II) complexes display a high-intensity π-π* transition at 307-254 nm
and low-intensity n-π* transition at 370-333 nm. All the complexes are shown metal to ligand,
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(Ru(dπ)-to-(Lπ*)), charge transfer transition in the region 448-405 nm[51] and d-d transitions
are observed at around 665-525 nm. Furthermore, ruthenium(II) complexes are affirmed by
octahedral geometry[52] and representative electronic spectra of the Ru(II) complexes (5-8) are
exhibited in Fig. 2.
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Fig. 2. Electronic spectra of ruthenium(II) complexes (5-8)
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3.3. 1H and 31P NMR spectra
NMR spectra of the ligands and complexes are further supported to their assigned
structures. The proton NMR spectra of the free ligands, HL1-HL4, show peaks which appeared
as single resonance around 12.64-9.14 ppm in the naphtholic / phenolic protons screened as a
singlet (Ar-OH) and aromatic protons (ArH) appeared as multiplets at 8.19-6.69 ppm. The
methoxy protons (Ar-OCH3) in HL4 and methyl protons (Ar-CH3) in HL3 ligands are exhibited
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at 3.86 ppm and 2.38 ppm respectively. The absence of the peak due to free naphtholic /
phenolic (Ar-OH) protons in the ruthenium(II) complexes (1-8) indicates the coordination of
the naphtholate / phenolate oxygen atom to the ruthenium(II) ion by the proton displacement of
OH group. The aromatic protons are appeared as multiplets in the region 8.35-6.57 ppm in the
spectra of all the complexes. The methoxy protons in the complexes (4, 8) and the methyl
protons in the complexes (3, 7) are appeared as singlets at 3.85 ppm and 2.38 ppm respectively.
1
The ruthenium(II) complexes (1-8) are diamagnetic (S = 0) in nature. The selected H NMR
spectra of the Ru(II) complexes are displayed in Fig. 3.
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Fig. 3. 1H NMR spectrum of ruthenium(II) carbonyl complex (3)
31P-NMR spectra of the complexes (1-4), show a sharp singlet at 29.13-24.35 ppm
which confirms the coordination of two triphenylphosphine groups to ruthenium present in the
complexes.
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3.4. 13C NMR and Mass spectra
The 13C NMR spectra of the phenoxyphenylazo ligands, HL1-HL4, show the downfield
shift of azo naphtholic / phenolic carbons (145.73-153.67 ppm) and upfield shift of Ar-OCH3
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(55.11 ppm), Ar-CH3 (19.02 ppm) carbons and are further support the assigned structure of the
ligands. The mass spectral peaks of ligands and complexes are shown fine agreement with their
experimental molecular ion peaks and are further support the assigned structure. The mass
spectrum of complex 7 is shown in Fig. 4.
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Fig. 4. Mass spectrum of ruthenium(II) carbonyl complex (7)
3.5. Catalytic studies for the synthesis of primary amides via aldehydes with
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hydroxylamine hydrochloride
We have utilized our synthesized complexes as catalysts to realize a few valuable
chemical reactions. Due to the rapid growth of using Ru(II) complexes in a variety of catalytic
transformations, we are intrigued to assess the catalytic activity of ruthenium catalysts in amide
synthesis reaction from the most interesting sustainable materials, aldehyde and hydroxylamine
hydrochloride. Subsequently, we have achieved in finding powerful catalysts with high yield
both from economic and environmental perspectives. Moreover, the catalytic activity of the
ruthenium complexes will be enhanced by the presence of a ligand with a bulky group. We
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have well accomplished the screening of the specifications such as solvent, base and catalyst
loading. The present framework has accompanying the huge preferences: 1) appropriateness to
different kinds of substrates, 2) the need of only one equivalent of hydroxylamine
hydrochloride, 3) the utilization of toluene as solvent, 4) a basic workup strategy, to be specific
catalyst / product separation by filtration, 5) the utilization of an easily synthesized ruthenium
azo complex 1 that is easy to handle and 6) water as a sole by-product.
3.6. Effect of solvent, base and low catalyst loading
We have examined the catalytic reactions using a range of solvents and bases in Table
1 and 2. We started from the reaction of benzaldehyde with NH2OH.HCl in presence of base
(NaHCO3) and catalyst 1 using both polar and non-polar solvents like methanol, acetonitrile,
dichloromethane, dimethyl sulfoxide, dimethylforamide, benzene and toluene. The reaction in
toluene has shown excellent conversion (98%) whereas in acetonitrile has shown good yield
(86%).[34] Methanol medium has given moderate yield (63%) while the other solvents benzene,
DCM, DMSO and DMF have provided poor yields (40-18%). From this, we have concluded
that toluene is the best shot as a medium for the conversion of amides. Then we have carried
out the reaction in toluene using the catalyst 1 without any base and are found with poor yield
(9%). Hence, we have tended to optimize the base. By varying the bases in the catalytic
reactions, we have concluded that NaHCO3 (98% yield) is the most suitable base than the other
ones (86-12% yield).
Moreover, we have performed the catalyst loading trials using benzaldehyde as a test
substrate in the optimized solvent, toluene, and base, sodium bicarbonate. A sequence of
reaction conditions has been opted for the amide conversion using different catalyst:substrate
(C:S) ratios (1:50, 1:100, 1:150, 1:200 and 1:250.) and the results are given in Table 3. While
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increasing the C/S ratio from 1:50 to 1:100, the conversion has also been increased (entry 1 and
2). However, on further increase in the ratio from 1:150 to 1:200 followed by 1:250, the
conversion has been decreased gradually (entry 3-5). Thus, we have accomplished that the
catalyst:substrate ratio of about 1:100 is the best compromise with 98% yield and 9800 TON.
3.7. Substrate scope of the amidation reaction from aldehydes and hydroxylamine
hydrochloride
We have exploited the reaction of a series of aliphatic, aryl and heterocyclic
aldehydes with hydroxylamine hydrochloride under the above optimized conditions.
Initially, we have subjected all the complexes (1-8) as catalysts for this conversion. From
these results, we have optimized that the complex 1 showed a better catalytic activity than
the rest and the data are shown in Table 4. In general, the conversion of amide products
has shown good to excellent yields. The reaction of benzaldehyde was well-tolerated in
this procedure and has given an excellent conversion 98% (entry 1) which may be due to
resonance stabilization. When comparing the conversion of aldehydes with electron
donating p-substituents to electron withdrawing p-substituents, the conversion of later
cases are greater than the former ones. The conversion of aldehydes with electron
withdrawing p-substituents, i.e., -NO2, Cl & Br has shown higher yields 95%, 93% and
92% (entry 5, 6 & 7) respectively.
This protocol is also successful for the utilization of aryl aldehydes with electron
donating p-substituents, i.e., -OCH3, -CH3 and –OH has shown good yields 86%, 82% &
79% (entry 2, 3 and 4) respectively. The conversion against the conjugated aldehyde,
cinnamaldehyde (entry 8) 84% yield) is less superior to that of benzaldehyde, because of
the presence of extended conjugated double bond in the former one. In addition
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heterocyclic amide compounds were also synthesized in this Beckmann rearrangement
utilizing catalyst 1 with appreciable yield. The heterocyclic aldehydes with N and S atoms
viz.,
pyrrole-2-carboxaldehyde,
2-pyridinecarboxaldehyde
and
thiophene-2-
carboxaldehyde (entry 9, 10 & 11) have shown good yields 93%, 91% and 89%
respectively. We have also carried out the amide conversion of aliphatic aldehydes, yields
are lesser than aromatic aldehydes. The conversion of ethanal, propanal, 3-methylbutanal
and n-butanal (entry 12-15) are 78%, 74%, 73% and 68% respectively.
The plausible mechanism for the transformation of aldehyde to amide in the
presence of ruthenium catalyst (Scheme 3) is followed by the reported metal catalyzed
reactions[53]. In first step, oxidative addition of aldehyde with OH group to form oxime.[29]
It get coordinated to the ruthenium(II) catalyst (1) and to form species I followed by the
elimination of HCl. In second step, the elimination of nitrile from species (I) and gives the
intermediate (II).[28] At the concurrent nucleophilic attack of nitrile over the coordinated
hydroxide resulted a ruthenium iminolate species (III) in third step.[53a, b] Finally, the
hydrolysis of ruthenium iminolate species (III) leads to revitalize the catalyst with the
formation of the product.
Though there are similar catalytic systems involving octahedral Ru(II) complexes
containing polydentate N-, O- and/or S-donor ligands with azo group[30, 31], we have
utilized 4-phenoxyaniline, instead of aniline, with 4-substituted phenol/2-naphthol for the
synthesis of our azo system. Among our four catalysts, the catalyst that contains 2-
naphthol moiety (1) shows better results than others, bearing phenolic moiety (2-4). As
the coordination mode and environment of all these four ligands are similar, the better
efficiency of the optimized ligand may be due to the influence of bulkiness of the former
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one when compared to the later. Moreover, we have compared the efficiency of our
optimized catalyst with other literature reports including the one with similar kind of 2-
naphthol moiety and found that our catalyst has shown comparable or better than the
rest (see Table 5). It is interesting to compare the activity of our catalyst with the one
reported by Ramesh et al., based on the ligand moiety.[48a] The later one has three units of
p-substituted (-CH3, -OCH3, Br, NO2) (phenylazo)naphthol moiety with six coordinated
ruthenium(III) ion, whereas our catalyst has p-(phenoxyphenylazo)naphthol moiety along
with (PPh3)2, CO and Cl with six coordinated Ru(II) ion. It reveals that our catalyst has
shown greater efficiency than the other, that may be due to influence of 2-naphthol
moiety along with mixed ligands in our systems.
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Scheme 3. Plausible mechanism for the ruthenium(II) catalytic (1) conversion of aldehydes
to amides
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4. CONCLUSION
We report here, the synthesis of eight new ruthenium(II) carbonyl complexes that
contains bidentate oxygen and nitrogen chelating phenoxyphenylazo ligands. All the ligands
and complexes were well characterized by spectral and analytical methods. The studies
confirmed the coordination of naphtholic/phenolic oxygen and azo nitrogen of the ligand to
the ruthenium(II) ion which is present in an octahedral environment. The thorough
investigation in the present work shows, ruthenium complex 1 is the best catalyst for the
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conversion of aldehydes to primary amides. Catalytic activity for an expansive scope of
substrates was assessed utilizing 1 mol % of catalyst 1 for 12 h. our catalyst is not only well
suited for different classes of aldehydes viz., aliphatic aldehydes, aryl aldehydes with electron
donating or electron withdrawing substituents, conjugated aldehydes and heterocyclic
aldehydes, but also it has shown better activity against all of them. These results reveal that
the bulkiness of the ligand as well as the mixed ligand system on these kind of catalytic
amidation reactions might lead to better efficiency.
Acknowledgement
The authors are grateful for the financial support from the University Grants
Commission, (UGC-BSR, No. F. 30-319/2016) Government of India, New Delhi, India. The first
author acknowledges Periyar University, Salem, Tamil Nadu for providing University Research
Fellowship (URF).
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Supporting Information
Additional supporting information may be found online in the supporting information
section at the end of the article.
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Table 1. Screening of different solvents for the synthesis of primary amides catalyzed by
ruthenium complex 1.a
513
Entry
Solvents
Conversionb %
Methanol
Benzene
63
40
98
86
32
34
18
1
2
3
4
5
6
7
Toluene
Acetonitrile
Dichloromethane
DMSO
DMF
514
515
a. Reaction conditions: Benzaldehyde (1 mmol), NH2OH.HCl (1 mmol), NaHCO3 (1 mmol),
Catalyst 1 (1 mol %) and Solvent (3 mL); b. Isolated yield
516
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524
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526
Table 2. Screening of different bases for the preparation of primary amides catalyzed by
ruthenium complex 1.a
527
Entry
Base
Conversionb %
Et3N
12
86
58
98
09
12
63
1
KHCO3
K2CO3
NaHCO3
-
2
3
4
5[c]
6[d]
7
NaHCO3
Na2CO3
528
529
530
a. Reaction conditions: Benzaldehyde (1 mmol), NH2OH.HCl (1 mmol), bases (1 mmol),
Catalyst 1 (1 mol %) and toluene (3 mL); b. Isolated yield; c. Absence of base; d. Absence of
catalyst
531
532
533
534
535
536
29