Inorganic Chemistry Communications 85 (2017) 84–88
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Synthesis, crystal structure of copper(II) complexes comprising 2-(biphenylazo)phenol and 1-(biphenylazo)naphthol ligands and their catalytic activity in nitroaldol reaction Raja Nandhini a, Madhan Ramesh a, Ganesan Prabusankar b, Galmari Venkatachalam a,⁎ a b
PG & Research Department of Chemistry, Government Arts College, Dharmapuri 636 705, Tamilnadu, India Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy TS 502 285, India
a r t i c l e
i n f o
Article history: Received 28 April 2017 Received in revised form 16 June 2017 Accepted 28 June 2017 Available online 1 July 2017 Keywords: Copper Bidentate biphenylazo ligands Crystal structure Nitroaldol reaction
a b s t r a c t New series of copper(II) complexes of the type [Cu(L)2] (L = L1–L5) comprising bidentate 2-(biphenylazo)phenol (HL1–HL4) and 1-(biphenylazo)naphthol (HL5) ligands have been synthesized. The composition of complexes and ligands (HL1–HL4) has been established by elemental analysis and spectral (FT–IR, UV–Vis, 1H NMR and EPR) methods. Molecular structures of copper complexes [Cu(L3)2] (3) and [Cu(L5)2] (5) were established by X-ray crystallography. These Copper(II) biphenylazo complexes exhibit a very good catalytic activity towards nitroaldol reaction of various aldehydes with nitromethane. © 2017 Elsevier B.V. All rights reserved.
Copper complexes are of considerable interest mainly due to their variety in coordination geometry, spectroscopic properties and their biochemical significance [1–3]. Transition metal complexes incorporating azo ligands have displayed several interesting properties related to electron-transfer reactions [4], liquid crystals [5], photochromism [6] and organic transformations [7,8]. The π-acidity and metal binding ability of azo nitrogen have drawn attention to the exploration of the chemistry of metal complexes incorporating azoligands [9–14]. The Henry(nitroaldol) reaction is one of the classical C\\C bond forming reactions in synthetic chemistry. The resulting products of β-nitroalcohols can be conveniently transformed into various valuable building blocks, such 1,2-aminoalcohols, 2-hydroxy carboxylic acids and conjugated nitroalkanes. The copper catalyzed Henry reaction performed at room temperature has captured much focus in recent years [15]. Since the pioneering work of Shibasaki in 1992 [16] the catalytic Henry reaction has received considerable attention and various types of metal-based catalysts have been developed [17,18]. Although many catalytic systems have been reported, to the best of our knowledge, only a very few studies on copper(II) Schiff base complexes in the catalytic nitroaldol reaction [19] and there is no report for these copper(II) biphenylazo complexes to catalyzed nitroaldol Henry reaction.
⁎ Corresponding author. E-mail address:
[email protected] (G. Venkatachalam).
http://dx.doi.org/10.1016/j.inoche.2017.06.027 1387-7003/© 2017 Elsevier B.V. All rights reserved.
Hence, this work is focused on the synthesis and structure of copper(II) complexes comprising 2-(biphenylazo)phenol and 1(biphenylazo)naphthol ligands and their potential to catalyze nitroaldol reaction of various aldehyde. Four 2-(biphenylazo)phenol (HL1 –HL 4 ) [20] and 1(biphenylazo)naphthol (HL5) [21] ligands have been prepared and used in the present study. The brown colored copper(II) biphenylazo complexes of the type [Cu(L) 2] [L = L 1–L5 ] were achieved [22] by reacting 1:2 ratio of Cu(OAc)2·H2 O and biphenylazo ligands HL 1– HL5 (Fig. 1) refluxed in ethanol for 3 h and simple work up procedure by filtration (Scheme 1). These copper(II) complexes were obtained as crystalline solid of yield up to 80%. The 1H NMR spectra of the ligands have been recorded in CDCl3 solution. All the ligands show
Fig. 1. Structure of 2-(biphenylazo)phenol (HL1–HL4) and 1-(biphenylazo)naphthol ligands (HL5).
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Scheme 1. Synthesis of Cu(II) complexes containing 2-(biphenylazo)phenol and 1-(biphenylazo)naphthol ligands.
Fig. 2. (i) The ORTEP picture of asymmetric unit of 3 with displacement ellopsolids drawn at 50% probability. (ii) The molecular structure of 3. (iii) The polyhedral view of copper core in 3. Selected bond lengths [Å]: Cu\ \O = 1.904(3), Cu\ \N = 1.972(3), N\ \N = 1.298(3), N\ \C = 1.442(5), N\ \C = 1.362(6) and selected bond angles [°]: O(1)\ \Cu(1) –O(1) = 88.74 (19), N(1)\ \Cu(1)\ \O(1) = 89.88(13), N(1)\ \Cu(1)\ \O(1) = 158.75(12), N(1)\ \Cu(1)\ \N(1) = 98.9(2), C(13)\ \O(1)\ \Cu(1) = 124.4(3), N(2)\ \N(1)\ \Cu(1) = 127.5(3), C(1)\ \N(1)\ \Cu(1) = 119.43(19), C(1)\ \N(1)\ \N(2) = 112.8(3), C(14\ \N(2)\ \N(1) = 121.2(3).
Fig. 3. (i) The ORTEP picture of asymmetric unit of 5 with displacement ellopsolids drawn at 50% probability. (ii) The molecular structure of 5. (iii) The polyhedral view of copper core in 5. Selected bond lengths [Å]: Cu\ \O = 1.904(3), Cu\ \N = 1.972(3), N\ \N = 1.298(3), N\ \C = 1.442(5), N\ \C = 1.362(6) and selected bond angles [°]: O(1)\ \Cu(1)\ \O(1) = 88.74 (19), N(1)\ \Cu(1)\ \O(1) = 89.88(13), N(1)\ \Cu(1)\ \O(1) = 158.75(12), N(1)\ \Cu(1)\ \N(1) = 98.9(2), C(13)\ \O(1)\ \Cu(1) = 124.4(3), N(2)\ \N(1)\ \Cu(1) = 127.5(3), C(1)\ \N(1)\ \Cu(1) = 119.43(19), C(1)\ \N(1)\ \N(2) = 112.8(3), C(14\ \N(2)\ \N(1) = 121.2(3).
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Table 1 Different catalyst loading.a
Entry
Catalyst loading (mol%)
Yield (%)b
TONc
1 2 30.05
0.01 0.02 85
56 68 1700
5600 3400
a Substrate (0.5 mmol), catalyst (0.05 mol%) and nitromethane (2.5 mmol) stirred in CH2Cl2 at room temperature. b Isolated yield after column chromatography. c TON = moles of product per mole of catalyst.
multiplets around 6.7–7.9 ppm for the presence of aromatic protons of biphenylazo ligands. A sharp singlet appeared for OH proton of ligands in the region at 11.7–11.9 ppm. The methoxy protons of the ligand (HL2) appeared as singlet at 3.8 ppm. The presence of the methyl group and t-butyl group for the ligands HL1 and HL3 is appeared as
singlet at 2.3 and 1.4 ppm respectively. Preliminary characterization by elemental analysis of the synthesized complexes supports the expected composition. The structure of the copper complexes shows that the 2-(biphenylazo)phenol and 1-(biphenylazo)naphthol is coordinated to copper atom, via loss of phenolic proton and azonitrogen, an a bidentate O,N-donor forming a six-membered chelate rings. Infrared spectra of all the ligands displayed strong bands around 1490–1450 and 1275–1315 cm− 1 corresponding to ν(N_N) and phenolic ν(C\\O) stretching, respectively. After complexation, ν(N_N) bond is decreases and appears at 1360–1390 cm− 1 indicating the coordination of azo nitrogen of copper metal ion. The coordination through phenolic oxygen is confirmed by the increase of ν(C\\O) bond at higher frequencies in the region 1290–1325 cm− 1 in all the complexes. This is further supported by the disappearance of ν(OH) band in the range 3440–3457 cm− 1 in all the complexes [23–25]. The IR spectra of the complexes are provided in Fig. S1– S13 (Supporting information). Electronic spectra of all the complexes recorded in chloroform solution, show several intense absorptions in the ultraviolet and visible regions. The absorption in the ultraviolet region are attributable to transitions 330–250 nm
Table 2 Nitroaldol reaction of nitromethane with various aldehydes by [Cu(L1–5)2] complexes.a
a
Substrate (0.5 mmol), catalyst (0.05 mol%) and nitromethane (2.5 mmol) stirred in CH2Cl2 at room temperature. Isolated yield after column chromatography. c TON = moles of product per mole of catalyst. b
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Scheme 2. Possible mechanism for [Cu(L)2] catalyzed Henry reaction.
which ligand orbitals and those in the visible region 400–475 nm are probably due to ligand-to-metal charge transfer transitions [26]. Representation spectra are given in Fig. S14–S18 (see Supporting information). The solution EPR spectrum of the complex (5) in the liquid nitrogen temperature and the ‘g’ value is around g = 2.106. These values indicate that the unpaired electron lies predominantly in the dx2-y2 orbital of Cu(II) and the spectral features are characteristics of axial symmetry. The representation EPR spectrum is shown in Fig. S19. Suitable X-ray quality crystals of complexes [Cu(L 3) 2 ] (3) and [Cu(L5)2] (5) was grown by slow evaporation of CH2Cl2 and EtOH solution. The molecular structure and important structure parameters of 3 and 5 are depicted in Figs 2 and 3. Crystal data and structure refinement for molecule 3 and 5 is shown (Table S20 see supporting information). The molecule 3 and 5 crystalized in triclinic space group, Pῑ monoclinic space group C2/c, respectively. 3 and 5 is a mononuclear Cu(II) complex. The copper center in (5) is tetra coordinated. The geometry of copper can be described as distorted square planar geometry. The sum of bond angles around copper is 360°. The coordination sphere of copper is satisfied by two phenolic oxygen and two azo nitrogen atoms. 2-(biphenylazo)naphthol act as a bidentate and mono anionic ligand, which results in a neutral copper complexes. As shown in Figs 2 & 3(iii), the central copper atom and the ligands form two six-membered rings, which increase the stability of the complex. These two rings are not in the same plane. The Cu\\O bond distances are comparable. Similarly the Cu\\N bond distances are comparable. The Cu\\O and Cu\\N bond distances found in 3 and 5 is comparable with that of copper(II) Schiff base square planar complexes (1.902 Å and 1.964 Å) [27]. As shown in molecular packing, week interactions between molecules are absent in Fig. S21(a) and S21(b) (see Supporting information). Benzaldehyde has been chosen an a model substrate to explore the catalytic performance of these copper(II) complexes in Henry nitroaldol reaction. This may be due to the poor Lewis acidity of the catalytic system, which is insufficient for the formation of nitronate ion. The best results observed when substrates were stirred for 3 h
in presence of 0.0025 mmol (0.05 mol%) of 1–5 at room temperature. The reaction uses relatively mild reaction conditions and catalyst loading. In the absence of catalyst no product was observed. Higher catalyst loading did help to drive the reaction to minimum yield when 0.05 mol% catalyst was used (Table 1). As expected, desired reaction did not proceeds at all in the absence of the catalyst. Reactions were performed with various solvents like CHCl 3 , CH2Cl2, EtOH etc. Among the solvents studied, CH2Cl2 was found to be the choice for these reactions. Under the optimized reaction (CH2Cl2 , room temperature) condition, different aldehydes were tested and results are summarized in Table 2. It was found that these catalysts worked well for various aromatic aldehydes with generally good yields. The catalytic reaction was performed using Cu(OAc)2·H2O as catalyst for nitroaldol reaction and obtained only 30% yield. The catalyst design required a weakly acidic metal complex bearing moderately basic charged ligands that would facilitate deprotonation of nitroalkanes and these copper(II) complexes were found to fulfill the requirements best. The product nitroalcohol was formed after a work up procedure [28] characterization of representative isolated products is given in Fig. S22–S24 (see Supporting information) and these copper(II) complexes have been proved to be a good catalytic system for the Henry reaction. It was possible to re-use the catalyst, which was recovered by the addition of ethanol to cool the reaction mixture followed by filtration to isolate the catalyst. The isolated catalyst (1) was used for the nitroaldol reaction of benzaldehyde under reaction conditions similar to that of fresh catalyst and gave same conversion (85%). The possible mechanism for the Henry reaction catalyzed by [Cu(L)2] complex is explained by the mechanistic pathway proposed by J.R. Pedro et al. [29]. The reaction of copper(II) complexes with nitromethane (a) and aldehyde can generate an intermediate complex (c) via intramolecular addition of nitronate to aldehyde (b) followed by reaction with fresh nitroalkane. The intermediate (c) can complete the catalytic cycle by reaction with fresh aldehyde (Scheme 2). The procedure nitroalcohol (d) was formed after work up procedure.
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In conclusion, the present study shows that the biphenylazo ligands derived from different para-substituted phenols and β-naphthol with 2aminobiphenyl can readily coordinate with copper as bi-dentate O, N donor mode. These new copper(II) complexes were characterized by UV–Vis, ESR and X-ray crystallography. The copper(II) complexes were used as catalysts for nitroaldol reaction of various aldehydes with nitromethane and proved as potential catalysts. Acknowledgment The authors are thankful to Prof. G. Prabusankar IIT-Hyderabad for allowing us to use single crystal X-ray diffraction facility established through an outstanding investigator award. Appendix A. Supplementary material Single crystal X-ray study, crystal data and structure refinement parameters for the molecule 3 and 5, FT–IR, UV–Vis, and EPR spectra of the complexes and FT–IR and 1H NMR spectra of the ligand are associated with this article. CCDC 1553019 (3) and 1544955 (5) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam. ac.uk/data_request/cif or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223336, 033; or e-mail:
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