Polyhedron 81 (2014) 619–627

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Polyhedron

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Synthesis, spectral characterization and crystal structure of Ni(II)pyridoxal thiosemicarbazone complexes and their recyclable catalyticapplication in the nitroaldol (Henry) reaction in ionic liquid media

http://dx.doi.org/10.1016/j.poly.2014.07.0180277-5387/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Fax: +91 427 2345124.E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi).

Rajendran Manikandan, Panneerselvam Anitha, Govindan Prakash, Paranthaman Vijayan,Periasamy Viswanathamurthi ⇑Department of Chemistry, Periyar University, Salem 636 011, India

a r t i c l e i n f o

Article history:Received 5 April 2014Accepted 4 July 2014Available online 17 July 2014

Keywords:ONS donor ligandNickel(II) complexesCrystal structureHenry reactionRecyclable

a b s t r a c t

The new square planar nickel(II) complexes 1, 2 and 3 have been synthesized from the reaction of[NiCl2(PPh3)2] with the tridentate Schiff base ligand, pyridoxal thiosemicarbazone (L1), pyridoxalN-methyl thiosemicarbazone (L2) and pyridoxal N-phenyl thiosemicarbazone (L3) respectively in ethanol.These complexes have been characterized by elemental analyses, IR, UV–Vis, 1H NMR, 31P NMR andESI-MS spectroscopy. The molecular structure of the complex [Ni(L2)PPh3] (2) was determined bysingle-crystal X-ray diffraction, which reveals a distorted square planar geometry around the nickel(II)ion. The nitroaldol reaction was studied in detail using the nickel(II) complexes as catalysts in a homo-geneous solution formed by an ionic liquid and methanol. The effect of solvent, ionic liquid, time,temperature, catalyst loading and substituent of the ligand moiety on the reaction was also studied.The b-nitroalcohol products were obtained in good yields of up to 97%. A two step substrate additionmechanism was tentatively proposed based on ESI-MS spectral monitoring of the reaction mass.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Research based on new Schiff bases and their metal complexesrepresents one of the most promising areas of material science,catalysis and chemical research [1–3]. The Schiff bases are easilysynthesized and form complexes with almost all metal ions. Stud-ies on such complexes of transition metals, including nickel, havereceived overwhelming attention in recent times due to theirimportant catalytic properties [4–14]. The synthesis of nickel(II)complexes with mixed O, N and S donor sets is the research inter-est of many groups [15–17]. Furthermore, nickel complexes con-taining triphenylphosphine ligands play an important role insmall molecular catalysts used for the synthesis of fine chemicalsand drug molecules [18–22].

The nitroaldol (Henry) reaction is an attractive synthetic tool inorganic synthesis. The resulting products, b-nitroalkanols, areimportant and versatile intermediates in the synthesis of nitroal-kenes, 2-amino alcohols and a-nitroketones [23]. 2-Amino alcoholsare of particular significance in the synthesis of biologically impor-tant compounds, such as epinephrine [24], anthracycline antibiot-ics [25], while a-nitroketones are valuable intermediates in the

synthesis of several natural products [26]. b-Nitroalkanols aregaining importance due to their fungicidal [27] and antibiotic[28,29] properties, in addition to their synthetic utility, like actingas intermediates in the synthesis of amino sugars [30]. The nitrogroup can be conveniently converted into several other functional-ities by reduction, the Nef reaction, nucleophilic displacement orother means, thus generating a-hydroxy ketones, aldehydes, car-boxylic acids, azides, sulfides and other important bifunctionalcompounds. Moreover, the reaction also turns out to be a key stepin the preparation of some drugs [31,32].

In spite of many applications, the catalytic nitroaldol reaction isrelatively less explored as compared to its aldol counterpart, due tothe non-availability of catalysts. Shibasaki reported the first asym-metric version of the nitroaldol reaction in 1992 [33]. Since thenvarious transition metals such as Cu [34], Zn [35], Co [36], Cr[37], Pd [38], Ru [39] and rare metals [40] have been applied tothe catalytic version of the nitroaldol reaction. In contrast, nickel,an attractive surrogate for precious metals in terms of its abun-dance and low cost, has not been investigated so far in the nitroal-dol reaction. From a catalyst recycling point of view, very fewreports are available [34e–g,41] to date which suggest a recyclablecatalyst for the nitroaldol reaction. To overcome this problem ionicliquids can be used to facilitate the recovery and reuse of catalyst.

620 R. Manikandan et al. / Polyhedron 81 (2014) 619–627

Herein, we report the synthesis and spectral characterization ofthree new nickel(II) complexes containing pyridoxal N(4)-substi-tuted thiosemicarbazone ligands with triphenylphosphine. Thecrystal structure of complex 2 has also been reported, whichsuggests a distorted square planar structure. In addition to our con-tinuous effort in developing efficient transition metal catalysts, thenew nickel(II) complexes were subjected as catalysts in thenitroaldol (Henry) reaction by employing various aldehydes withnitromethane in an ionic liquid medium.

2. Experimental

2.1. Materials and methods

All the chemicals were obtained from commercial sources andused as received. The solvents were purified and dried inaccordance with standard literature methods. The precursor[NiCl2(PPh3)2] was prepared by the literature procedure [42].Microanalyses of carbon, hydrogen, nitrogen and sulfur were car-ried out using a Vario EL III Elemental analyzer at SAIF – Cochin,India. The IR spectra of the ligands and their complexes wererecorded as KBr pellets on a Nicolet Avatar model spectrophotom-eter in the 4000–400 cm�1 range. Electronic spectra of the ligandsand their complexes were obtained in methanol using a ShimadzuUV-1650 PC spectrophotometer in the 800–200 nm range. The 1Hand 31P NMR spectra were measured with a Jeol GSX-300 instru-ment using DMSO-d6 as the solvent. TMS and o-phosphoric acidwere used as an internal standard for 1H and 31P NMR spectrarespectively. The ESI-MS spectra were performed using an LC–MSQ-ToF Micro Analyzer (Shimadzu) in the SAIF, Punjab University,Chandigarh. Melting points were checked on a Technico microheating table and are uncorrected.

2.2. Preparation of the pyridoxal N(4)-substituted thiosemicarbazonehydrochloride ligands

The synthetic route for ligands (L1–3) is presented in Scheme 1.Pyridoxal N(4)-substituted thiosemicarbazone hydrochloride hasbeen prepared according to a modification of the procedure

Scheme 1. Synthesis of the

described in the literature [43,44]. The substituted thiosemicarba-zide (2 mmol) was dissolved in 10 mL of ethanol with constantstirring, and then added to pyridoxal hydrochloride (2 mmol) dis-solved in 10 mL of ethanol. The mixture was stirred at 80 �C for30 min. After the reaction mixture was cooled to room tempera-ture, a yellow solid was isolated by filtration, washed with ethanoland dried in vacuo.

2.2.1. Pyridoxal thiosemicarbazone hydrochloride (L1)L1 was prepared from the reaction of pyridoxal hydrochloride

(0.4072 g; 2 mmol) and thiosemicarbazide (0.1822 g; 2 mmol).Yield: 0.476 g, 86%; Color: yellow solid; M.p.: 285 �C; Anal. Calc.for C9H13ClN4O2S (276.75 g mol�1): C, 39.06; H, 4.73; N, 20.24; S,11.58. Found: C, 39.32; H, 4.53; N, 20.47; S, 11.38%. IR (KBr,cm�1): 3398(s) m(CH2OH), 3306, 3293(m) m(ANHAR), 3190(m)m(OH), 3163(s) m(NH), 2650(s) m(NH+), 1643(s) m(C@N), 843(s)m(C@S). UV–Vis (CH3OH, k/nm (e/M�1 cm�1)): 232 (10925), 346(9074). 1H NMR (300 MHz, DMSO-d6, d): 11.96 (s, 1H, OH), 8.61(s, 1H, NH), 8.53 (s, 1H, CH@N), 8.17 (s, 2H, NH2), 8.04 (s, 1H, Py),5.29 (s, 1H, OH), 4.77 (s, 2H, CH2), 2.62 (s, 3H, CH3).

2.2.2. Pyridoxal-4-methyl-thiosemicarbazone hydrochloride (L2)L2 was prepared from the reaction of pyridoxal hydrochloride

(0.4072 g; 2 mmol) and 4-methyl-thiosemicarbazide (0.2103 g;2 mmol). Yield: 0.4245 g, 73%; Color: yellow solid; M.p.: 226 �C;Anal. Calc. for C10H15ClN4O2S (290.77 g, mol�1): C, 41.30; H, 5.19;N, 19.26; S, 11.02. Found: C, 41.12; H, 5.36; N, 19.57; S, 10.88%.IR (KBr, cm�1): 3380(m) m(CH2OH), 3300(m) m(ANHAR), 3200(s)m(OH), 3152(m) m(NH), 2706(m) m(NH+), 1615(s) m(C@N), 830(s)m(C@S). UV–Vis (CH3OH, k/nm (e/M�1 cm�1)): 230 (11467), 338(11567). 1H NMR (300 MHz, DMSO-d6, d): 12.17 (s, 1H, OH), 8.93(s, 1H, NH); 8.65 (s, 1H, NH-R); 8.52 (s, 1H, CH@N); 8.17 (s, 1H,Py); 5.39 (s, 1H, OH), 4.78 (s, 2H, CH2), 3.02 (s, 3H, CH3), 2.62 (s,3H, CH3).

2.2.3. Pyridoxal-4-phenyl-thiosemicarbazone hydrochloride (L3)L3 was prepared from the reaction of pyridoxal hydrochloride

(0.4072 g; 2 mmol) and 4-phenyl-thiosemicarbazide (0.3344 g;2 mmol). Yield: 0.5645 g, 80%; Color: yellow solid; M.p.: 226 �C;

nickel(II) complexes.

R. Manikandan et al. / Polyhedron 81 (2014) 619–627 621

Anal. Calc. for C15H17ClN4O2S (352.84 g, mol�1): C, 51.06; H, 4.85;N, 15.87; S, 9.08. Found: C, 50.79; H, 4.73; N, 15.58; S, 9.42%. IR(KBr, cm�1): 3388(s) m(CH2OH), 3360(m) m(ANHAR), 3240 m(OH),3157(m) m(NH), 2822(m) m(NH+), 1640(s) m(C@N), 860(m) m(C@S).UV–Vis (CH3OH, k/nm (e/M�1 cm�1)): 233 (31547), 324 (36671),349 (35568). 1H NMR (300 MHz, DMSO-d6, d): 11.58 (s, 1H, OH),9.97 (s, 1H, NH), 8.90 (s, 1H, NH-R), 8.40 (s, 1H, CH@N), 7.52–7.75 (m, 5H, Ph), 6.97 (s, 1H, Py), 5.38 (s, 1H, OH), 4.78 (s, 2H,CH2), 1.56 (s, 3H, CH3).

2.3. Synthesis of the nickel(II) complexes

All the new metal complexes were prepared according tothe following general procedure. An ethanolic solution of the pyr-idoxal N(4)-substituted thiosemicarbazone hydrochloride ligand(1 mmol) was added to [NiCl2(PPh3)2] (1 mmol) in ethanol. Theresulting red colored solution was refluxed for 5 h. A dark red col-ored crystalline powder was obtained on slow evaporation, whichwas filtered off, washed with ethanol, and dried under vacuo.

2.3.1. Synthesis of [Ni(L1)PPh3] (1)This complex was synthesized from [NiCl2(PPh3)2] (0.654 g;

1 mmol) and L1 (0.2767 g; 1 mmol). Yield: 0.4294 g, 73%; Color:red solid; M.p.: 224 �C; Anal. Calc. for C27H26ClN4NiO2PS (595.71 g,mol�1): C, 54.44; H, 4.40; N, 9.41; S, 5.38. Found: C, 54.23; H, 4.78;N, 9.22; S, 5.13%. IR (KBr, cm�1): 3433(s) m(CH2OH), 3320, 3188(m)m(ANHAR), 2660(m) m(NH+), 1626(s) m(C@N), 1601(s) m(C@N),787(s) m(CAS). UV–Vis (CH3OH, k/nm (e/M�1 cm�1)): 249 (7797),420 (3064). 1H NMR (300 MHz, DMSO-d6, d): 8.51 (s, 1H, CH@N),8.16 (s, 2H, NH2), 8.12 (s, 1H, Py), 7.21–7.64 (m, 15H, PPh3), 5.20(s, 1H, OH), 4.76 (s, 2H, CH2), 2.59 (s, 3H, CH3). 31P NMR (300 MHz,DMSO-d6, d): 28.15. ESI-MS, m/z: 559.2 [M�HCl]+.

2.3.2. Synthesis of [Ni(L2)PPh3] (2)This complex was synthesized from [NiCl2(PPh3)2] (0.654 g;

1 mmol) and L2 (0.2907 g; 1 mmol). Single crystals suitable for X-ray diffraction were grown at room temperature from ethanol.Yield: 0.4207 g, 69%; Color: red solid; M.p.: 264 �C; Anal. Calc. forC28H28ClN4NiO2PS (609.73 g, mol�1): C, 55.16; H, 4.63; N, 9.19; S,5.26. Found: C, 55.42; H, 4.73; N, 9.45; S, 5.56%. IR (KBr, cm�1):3399(s) m(CH2OH), 3308(s) m(ANHAR), 2670(m) m(NH+), 1607(s)m(C@N), 1567(s) m(C@N), 786(s) m(CAS). UV–Vis (CH3OH, k/nm (e/M�1 cm�1)): 252 (10243), 425 (4878). 1H NMR (300 MHz, DMSO-d6, d): 8.62 (s, 1H, NH-R), 8.51 (s, 1H, CH@N), 8.17 (s, 1H, Py),7.44–7.58 (m, 15H, PPh3), 5.30 (s, 1H, OH), 4.77 (s, 2H, CH2), 3.01(s, 3H, CH3), 2.60 (s, 3H, CH3). 31P NMR (300 MHz, DMSO-d6, d):21.50. ESI-MS, m/z: 573.1 [M�HCl]+.

2.3.3. Synthesis of [Ni(L3)PPh3] (3)This complex was synthesized from [NiCl2(PPh3)2] (0.654 g;

1 mmol) and L3 (0.3528 g; 1 mmol). Yield: 0.5508 g, 82%; Color:red solid; M.p.: 260 �C; Anal. Calc. for C33H30ClN4NiO2PS(671.80 g, mol�1): C, 59.00; H, 4.50; N, 8.34; S, 4.77. Found: C,58.63; H, 4.26; N, 8.55; S, 4.48%. IR (KBr, cm�1): 3399(s) m(CH2OH),3267(m) m(ANHAR), 2789(m) m(NH+), 1622(s) m(C@N), 1602(s)m(C@N), 818(s) m(CAS). UV–Vis (CH3OH, k/nm (e/M�1 cm�1)): 270(5679), 396 (2349), 479 (1956). 1H NMR (300 MHz, DMSO-d6, d):8.92 (s, 1H, NH-R), 8.39 (s, 1H, CH@N), 7.22–7.61 (m, 20H, PPh3

and Ph), 7.01 (s, 1H, Py), 5.24 (s, 1H, OH), 4.81 (s, 2H, CH2), 1.57(s, 3H, CH3). 31P NMR (300 MHz, DMSO-d6, d): 25.51. ESI-MS, m/z: 635.3 [M�HCl]+.

2.4. X-ray crystallography

Crystallographic data collections for [Ni(L2)PPh3] (2) were per-formed at 293 K with a Bruker Smart Apex2 CCD diffractometer

using monochromated Mo Ka (k = 0.71073 Å) radiation. The frameintegration and data reduction were performed using Bruker SAINT-

PLUS (Version 7.06a) software [45]. The multi-scan absorption cor-rections were applied to the data using the SADABS (Bruker 1999)program [46]. The sample was stable at room temperature. Thestructure was solved using SIR92 (Altormare et al., 1993) by directmethods [47]. Full matrix least squares refinement on F2 wasperformed using SHELXL-97 (Sheldrick, 1997) programs. The ORTEP

program was employed for drawing ORTEP diagrams of thecomplex (2) and the PLATON program [48] was used for calculationof the intermolecular short contact distances.

2.5. Catalytic nitroaldol (Henry) reactions with the nickel(II)complexes in ionic liquid media

The catalytic reactions were performed in air in a reaction flaskequipped with a magnetic stirrer at room temperature. A mixtureof catalyst (3 lmol) and ionic liquid (0.10 mmol) in 2 mL of meth-anol was stirred. To this solution, the aldehyde (1 mmol) wasadded and the mixture was stirred for a further 15–20 min. Nitro-methane (5.0 mmol) was slowly added to the resulting solutionover an interval of 1 h using a syringe. The progress of the reactionwas checked by TLC. After 12 h the product was separated usingrepetitive washing of the reaction mixture (five times) with a hex-ane ethyl acetate 4:1 mixture. After complete washing, the ionicliquid containing the nickel(II) complex was dried under vacuumand stored in a desiccator for its use in subsequent catalytic runs.The product was then purified using silica gel column chromatog-raphy, giving the b-hydroxy nitroalkanes in high isolated yields.The products were confirmed by 1H NMR spectra.

2-Nitro-1-phenyl-ethanol: 1H NMR (300 MHz, DMSO-d6, d):7.44–7.62 (m, 5H, ArH), 5.57 (m, 1H, CH), 4.33–4.41 (m, 2H, CH2),3.22 (s, 1H, OH).

1-(4-Chloro-phenyl)-2-nitro-ethanol: 1H NMR (300 MHz, DMSO-d6, d): 7.42–7.49 (m, 4H, ArH), 5.41–5.43 (m, 1H, CH), 4.49–4.51(m, 2H, CH2), 2.90 (s, 1H, OH).

1-(2-Fluoro-phenyl)-2-nitro-ethanol: 1H NMR (300 MHz, DMSO-d6, d): 7.21–7.47 (m, 4H, ArH), 5.32–5.34 (m, 1H, CH), 4.42–4.46(m, 2H, CH2), 2.85 (s, 1H, OH).

2-Nitro-1-(4-nitro-phenyl)-ethanol: 1H NMR (300 MHz, DMSO-d6, d): 7.28–8.31 (m, 4H, ArH), 5.52–5.53 (m, 1H, CH), 4.58–4.62(m, 2H, CH2), 3.12 (s, 1H, OH).

2-(1-Hydroxy-2-nitro-ethyl)-phenol: 1H NMR (300 MHz, DMSO-d6, d): 9.92 (s, 1H, OH) 7.11–7.21 (m, 4H, ArH), 5.62–5.63 (m, 1H,CH), 3.93–3.99 (m, 2H, CH2), 3.25 (s, 1H, OH).

2-Nitro-1-p-tolyl-ethanol: 1H NMR (300 MHz, DMSO-d6, d):7.22–7.34 (m, 4H, ArH), 5.32–5.33 (m, 1H, CH), 4.53–4.58 (m, 2H,CH2), 2.73 (s, 1H, OH), 2.35 (s, 3H, CH3).

1-(4-Methoxy-phenyl)-2-nitro-ethanol: 1H NMR (300 MHz,DMSO-d6, d): 6.98–7.01 (m, 4H, ArH), 5.27–5.29 (m, 1H, CH),4.45–4.48 (m, 2H, CH2), 3.88 (s, 3H, OCH3), 3.25 (s, 1H, OH).

1-Naphthalen-1-yl-2-nitro-ethanol: 1H NMR (300 MHz, DMSO-d6, d): 7.46–7.85 (m, 7H, ArH), 5.56 (m, 1H, CH), 4.58–4.60 (m,2H, CH2), 2.82 (s, 1H, OH).

1-Nitro-4-phenyl-but-3-en-2-ol: 1H NMR (300 MHz, DMSO-d6,d): 7.30–7.56 (m, 5H, ArH), 6.66–6.74 (dd, 1H, CH), 6.39–6.44 (d,1H, CH), 5.12–5.15 (m, 1H, CH), 4.41–4.49 (m, 2H, CH2), 2.52 (s,1H, OH).

2-Nitro-1-pyridin-2-yl-ethanol: 1H NMR (300 MHz, DMSO-d6, d):7.44–8.10 (m, 4H, pyH), 5.24–5.26 (m, 1H, CH), 4.39–4.42 (m, 2H,CH2), 2.80 (s, 1H, OH).

1-Furan-2-yl-2-nitro-ethanol: 1H NMR (300 MHz, DMSO-d6, d):7.31–7.42 (m, 1H, fuH), 6.63–6.65 (d, 1H, fuH), 6.37–6.38 (d, 1H,fuH), 5.43–5.44 (m, 1H, CH), 4.35–4.37 (m, 2H, CH2), 2.89 (s, 1H,OH).

622 R. Manikandan et al. / Polyhedron 81 (2014) 619–627

2-Nitro-1-thiophen-2-yl-ethanol: 1H NMR (300 MHz, DMSO-d6,d): 7.09–7.28 (m, 3H, tpnH), 5.13–5.15 (m, 1H, CH), 4.14–4.18 (m,2H, CH2), 3.01 (s, 1H, OH).

3. Results and discussion

The ligands L1–L3, synthesized from the condensation ofpyridoxal hydrochloride with the appropriate N(4)-substitutedthiosemicarbazide, were reacted with [NiCl2(PPh3)2] and formedtheir corresponding nickel complexes (Scheme 1). The complexesobtained are red in color, air-stable, non-hygroscopic in natureand soluble in common organic solvents such as dichloromethane,chloroform, benzene, acetonitrile, ethanol, methanol, dimethyl-formamide and dimethylsulfoxide. All the ligands and theircomplexes were structurally characterized by elemental analyses,IR, electronic, 1H NMR, 31P NMR and ESI-MS spectra. For furtherconfirmation, the structure of complex (2) was elucidated byX-ray crystal analysis.

3.1. IR spectra

The IR spectra of the complexes were compared with those ofthe ligands in order to identify the tautomeric as well as the ionicform of pyridoxal-thiosemicarbazone in the solid state, in additionto fix the mode of coordination. The formation of the ligands as HCladducts has been evidenced by a band appearing around 2650–2822 cm�1, which could be addressed as the m(NAH+ Cl�) stretch-ing vibration, confirming that the nitrogen atom of the pyridinering bears a proton [49]. There is no peak observed around2500 cm�1 assignable to the SH group; absorptions at 830–860 cm�1, which could be assigned to the m(C@S) vibration of thefree ligand, indicate that it is present in the thione form. This bandcompletely disappeared in the complexes, and two new bandsappeared at 1567–1602 and 786–818 cm�1, corresponding to pos-sible m(C@NAN@C) and m(CAS) stretching vibrations respectively, indi-cating coordination of the thiolate sulfur atom after enolizationfollowed by deprotonation. In the high-energy range, a strongand broad band appears with a maximum at 3380–3398 cm�1

(ligand) and 3399–3433 (complexes) that can be unambiguouslyassigned to the m(OH) (hydroxymethyl) vibration [49]. Further,the stretching vibrations of the m(NHR) (aminic) group appear inthe 3188–3360 cm�1 region; while the mNH (hydrazinic) band,occurring at 3152–3163 cm�1 in the free ligands, disappears inthe complexes, suggesting deprotonation of the ANH group [50].Coordination of the phenolate group, observed for the free ligandsat 3190–3240 cm�1, was indicated by the disappearance of the OHstretching vibration in the IR spectra of the complexes [43]. Thefree ligands showed a very strong absorption around 1615–1643 cm�1, characteristic of the azomethine m(>C@N) group. Inall the complexes, the band due to azomethine was observed at alower region, around 1607–1626 cm�1, indicating the coordinationof the azomethine nitrogen atom to nickel center. The IR spectraldata suggested an ONS tridentate ligand behavior for all thesecomplexes, in agreement with the X-ray structural analysis.

3.2. Electronic spectra

Electronic spectra of the ligands and the nickel(II) complexes,recorded in methanol solution, showed two to three bands in theregion 230–479 nm. The bands appearing in the region 230–349 nm have been assigned to intra-ligand transitions, the bandat 396 nm has been assigned to a ligand-to-metal charge transfertransition [51]. The electronic spectra of the nickel(II) complexesalso showed bands attributable to d–d spin allowed transitions inthe region 420–479 nm [52].

3.3. 1H NMR and 31P NMR spectra

The nature of the coordination of the ligand was also estab-lished by a comparison of the 1H NMR chemical shift values ofthe complex with the related ligand. All the ligands show broadsignals at d 11.58–12.17 and 8.61–9.97 due to the phenolic OHand NANHA protons, indicating their existence in the hydroxy-thione tautomeric form, possessing a ANHA(C@S)ANHR moiety.These signals were absent in the spectra of the complexes, suggest-ing the bideprotonation of the ligand, coordination through thephenolic oxygen atom and thioenolisation [53]. A singlet observedat d 8.40–8.53 in the spectra of the free ligands, assigned to theazomethine proton, undergoes a shift to d 8.39–8.51 in the com-plexes, indicating the coordination of the azomethine nitrogenatom to the metal. The signals corresponding to the protons ofthe aromatic moieties of the ligands and their complexes wereobserved as multiplets in the range d 7.21–7.75. Further, the signalappearing at d 6.97–8.17 has been assigned to the aromaticpyridine protons [54]. The peak appearing at d 1.56–3.02 has beenassigned to the methyl group. The methylene protons and methy-lene OH protons (CH2OH) appeared in the regions d 4.76–4.81 and5.20–5.39, respectively for all the ligands and complexes. Inaddition, the terminal ANHAR proton appeared in the region d8.17–8.92 [54]. The 31P NMR spectra of the complexes showed asharp singlet in the region d 21.50–28.15, which is characteristicof coordinated triphenylphosphine in nickel(II) complexes [55].

3.4. Mass spectra

ESI-mass spectral analyses of the new complexes were studiedin order to confirm the molecular mass of the complexes. The m/zvalues of the molecular ion peaks for the complexes [Ni(L1)PPh3](1), [Ni(L2)PPh3] (2) and [Ni(L3)PPh3] (3) were obtained at 559.2,573.1 and 635.3 [M�HCl]+ respectively. The calculated molecularmasses corresponding to these complexes are 595.71, 609.73 and671.80. The obtained molecular masses are in good agreementwith the calculated molecular masses.

3.5. X-ray crystallography

Crystals of complex 2 suitable for X-ray crystal structure deter-mination were grown from ethanol. An ORTEP view of the molec-ular structure and the atom labeling scheme of complex 2 is shownin Fig. 1. The crystallographic data and selected bond distances andangles are listed in Tables 1 and 2, respectively. Analysis of theX-ray crystallography data reveals that the complex displays asquare planar geometry, in which the nickel core is coordinatedby the tridentate ligand and one triphenylphosphine ligand. Theligand L2 is coordinated to the metal by losing two protons fromits tautomeric thiol form and phenolic AOH. It acts as a dibasic tri-dentate ligand via the phenolic-oxygen, imine-nitrogen and thiol-sulfur atoms. The fourth coordination site is occupied by the tri-phenylphosphine molecule. The CAS bond of 1.746(2) Å indicatesthe thiol form [54]. Two chelate rings are formed by complexationof the ligand to Ni, five- (Ni, S1, C7, N3, N2) and six-membered (Ni,N2, C6, C5, C1, O1) chelate rings with a bite angle of 87.52(5)�(S(1)ANiAN(2)) and 94.69(6)� (O(1)ANiAN(2)). In this complex,the S(1)ANiAO(1) and P(1)ANiAN(2) bond angles [177.40(5)�,174.65(5)�] deviate slightly from the ideal angle of 180�. Thevariation in the bonding parameters indicates that there is consid-erable distortion in the NiSNOP core around the nickel atom. TheNiAS, NiAP and NiAO bond lengths are 2.13, 2.20 and 1.85 Å,which are quite normal. The observed NiAN bond distance forthe complex is slightly shorter due to a strong p interactionbetween the imine nitrogen and nickel atoms. All other observed

Fig. 1. ORTEP view of complex 2.

Table 1Crystal data and structure refinement for complex 2.

Complex 2

Empirical formula C28H28ClN4NiO2PSFormula weight 609.73Color redCrystal dimensions (mm) 0.30 � 0.30 � 0.25Temperature (K) 293(2)Wavelength (Å) 0.71073Crystal system monoclinicSpace group P21/cUnit cell dimensionsa (Å) 12.1371(5)b (Å) 14.6157(6)c (Å) 15.9640(7)a (�) 90b (�) 102.280(2)c (�) 90Volume (Å3) 2767.1(2)Z 4Calculated density (g cm�3) 1.464Absorption coefficient (mm�1) 0.965F(000) 1264Theta range for data collection (�) 2.21–29.86Absorption correction Lorentz-polarizationRefinement method full-matrix least squares on F2

No. of reflections measured total: 7951, unique: 5775Data/restraints/parameters 7951/2/354Goodness-of-fit (GOF) on F2 1.028R indices R1/wR2[I > 2r(I)] 0.0351/0.0835R1/wR2 (all data) 0.0621/0.0991Dqmax, Dqmin (e Å3) 0.0431, �0.299

Table 2Selected bond lengths (Å) and angles (�) for complex 2.

Ni(1)AS(1) 2.1326(5) S(1)ANi(1)AP(1) 90.41(2)Ni(1)AP(1) 2.2076(5) S(1)ANi(1)AO(1) 177.40(5)Ni(1)AN(2) 1.8887(16) S(1)ANi(1)AN(2) 87.52(5)Ni(1)AO(1) 1.8465(14) P(1)ANi(1)AO(1) 87.28(4)S(1)AC(7) 1.746(2) P(1)ANi(1)AN(2) 174.65(5)P(1)AC(11) 1.8165(19) O(1)ANi(1)AN(2) 94.69(6)P(1)AC(23) 1.818(2) Ni(1)AS(1)AC(7) 95.87(7)P(1)AC(17) 1.815(2) Ni(1)AP(1)AC(11) 116.79(6)O(1)AC(1) 1.298(2) Ni(1)AP(1)AC(17) 108.79(7)O(2)AC(10) 1.417(3) Ni(1)AP(1)AC(23) 113.14(7)N(2)AN(3) 1.389(2) Ni(1)AO(1)AC(1) 127.60(12)N(2)AC(6) 1.294(2) Ni(1)AN(2)AN(3) 121.44(12)N(3)AC(7) 1.307(3) Ni(1)AN(2)AC(6) 125.95(13N(4)AC(7) 1.338(2) C(11)AP(1)AC(17) 105.91(9)N(4)AC(8) 1.439(3) C(11)AP(1)AC(23) 104.20(9)N(1)AC(2) 1.327(2) C(23)AP(1)AC(17) 107.40(9)N(1)AC(3) 1.351(3)

R. Manikandan et al. / Polyhedron 81 (2014) 619–627 623

bond lengths are within the range of those already reported inrelated complexes [56–61].

3.6. Catalytic studies of the nitroaldol (Henry) reaction

We have investigated the catalytic activity of nickel(II)complexes in the nitroaldol (Henry) reactions between differentaldehydes and nitromethane, with an overall yield of the corre-sponding b-nitroalcohols of up to 75–97%.

3.6.1. OptimizationOptimization of various factors, such as reaction medium, cata-

lyst loading, reaction time and temperature of the catalytic systemwere carried out using a representative catalyst, [Ni(L2)PPh3] (2)for the nitroaldol reaction of benzaldehyde with nitromethane.The reaction was carried out at room temperature for 5 h and theyield was observed to be about 52% (Table 3, entry 1). However,an ionic liquid had to be added with the solvent medium in orderto improve the separation of the catalyst. So, the above reactionwas carried out with the readily available 1-ethyl 3-methylimida-zolium bromide ([emim][Br]), and it was found that the addition ofthe ionic liquid to the medium slightly improved the yield from52% to 64% (Table 3, entry 2). While conducting the reaction inthe absence of the solvent ethanol, a decrease in yield (up to40%) (Table 3, entry 3) was observed, which showed the vital roleof the solvent in the catalytic system. Hence, we checked the influ-ence of various solvents like THF, dichloromethane, acetonitrile,chloroform, toluene and methanol (Table 3). The catalytic effi-ciency of the various solvents was observed as methanol > etha-nol > THF > acetonitrile > chloroform > dichloromethane > toluene,which confirmed that the polar solvents showed much greaterinfluence on the catalytic system, with high yields, in contrast tonon-polar solvents. Similarly, the effect of various ionic liquids,such as [emim]Br, [emim]BF4 and [emim]OH, were studied on arepresentative reaction and [emim]BF4 was found to give the bestperformance (Table 3, entry 11). It should be noted here that thenitroaldol reaction requires a slight balance between acidity andbasicity in the catalytic system in order to stimulate the startingmaterials, likely aldehyde and nitromethane respectively. There-fore, high basicity (as in the case of the OH� counterion) of theionic liquid might cause a lowering of the product yield. On theother hand, moisture-resistant neutral counter ions like BF4 maynot alter the acidity or basicity of the reaction media, but couldhelp in the polarization of nitromethane due to the highly electro-negative nature of the fluorine atom. This in turn would increasethe activity of the reactant, as established by the higher productyield in the same reaction time. Hence, from the above preliminaryoptimization studies, [emim]BF4 along with methanol asco-solvent was concluded as the best reaction medium for ourcatalytic system.

In order to find the optimum reaction time, the representativereaction was carried out for different time intervals, viz 5–16 h.The maximum yield was observed in 12 h, which was fixed asthe optimum time for other reactions (Table 4, entry 4). Further,to study the effect of catalyst concentration on the reaction, thesubstrate to catalyst ratio was varied by changing the catalystamount from 1.0 lmol to 5.0 lmol (Table 5). A moderate yield

Table 3Screening of solvents and ionic liquids for the nitroaldol (Henry) reaction.a

Entry Ionic liquid Solvent Yield (%)b

1 – Ethanol 522 [emim]Br Ethanol 643 [emim]Br – 404 [emim]Br Methanol 725 [emim]Br THF 606 [emim]Br Acetonitrile 587 [emim]Br Chloroform 538 [emim]Br Dichloromethane 509 [emim]Br Toluene 48

10 [emim]OH Methanol 5911 [emim]BF4 Methanol 7712 [emim]PF6 Methanol 68

a Reaction conditions: benzaldehyde (1 mmol), nitromethane (5 mmol), ionic liquid (0.10 mmol) and complex 2 (5.0 lmol) in 2 mL ofsolvent were stirred for 5 h at room temperature.

b Isolated yields by column chromatography.

Table 4Optimization of the reaction temperature and time for the nitroaldol (Henry)reaction.a

Entry Time (h) Yield (%)b

1 6 802 8 843 10 884 12 915 14 916 16 91

a Reaction conditions: benzaldehyde (1 mmol), nitromethane (5 mmol),[emim]BF4 (0.10 mmol) and complex 2 (5.0 lmol) in 2 mL of methanol were stirredat room temperature.

b Isolated yields by column chromatography.

Table 5Effect of catalyst loading in the nitroaldol (Henry) reaction.a

Entry Catalyst (lm) Yield(%)b

1 1.0 302 2.0 353 2.5 654 3.0 925 4.0 906 5.0 91

a Reaction conditions: benzaldehyde (1 mmol), nitromethane (5 mmol),[emim]BF4 (0.10 mmol) and complex 2 in 2 mL of methanol were stirred for 12 h atroom temperature.

b Isolated yields by column chromatography.Table 6Selection of favorable catalysts for the nitroaldol (Henry) reaction.a

Entry Complex Yield(%)b

1 1 882 2 923 3 85

a Reaction conditions: aldehyde (1 mmol), nitromethane (5 mmol), [emim]BF4

(0.10 mmol) and complex (1–3) (3.0 lmol) in 2 mL of methanol were stirred for12 h at room temperature.

b Isolated yields by column chromatography.

624 R. Manikandan et al. / Polyhedron 81 (2014) 619–627

was observed even at alow catalyst concentration (2.5 lmol). Theyield increase with increasing concentration of the catalyst andreaches a higher value of 92% at 3.0 lmol concentration (Table 5,entry 4). Since a further increase of the catalyst concentrationresults in lowering yield, 3.0 lmol was taken as the optimum cat-alyst concentration.

3.6.2. Selection of favorable catalystIn order to choose the best catalyst among the synthesized com-

plexes, the nitro aldol reaction of benzaldehyde with nitromethanewas carried out using the catalysts 1, 2 and 3 under the above opti-mized conditions (Table 6). From the results, complex 2 was foundto be the best catalyst and showed a yield of 92%, which may due tothe N(4) substituent on the thiosemicarbazone fragment of theligand in complex 2. The order of reactivity of the complexes withrespect to the different substitutions on ligands is given byCH3 > H > C6H5.

3.6.3. Catalytic nitroaldol (Henry) reaction of different substratesThe ultimate aim of our catalytic system is to apply a wide

range of aromatic, substituted, conjugated and heterocyclic aro-matic aldehydes in the nitroaldol reactions. Hence, the catalyticnitroaldol reaction of different aldehydes was carried out withnitromethane under the optimized reaction conditions using com-plex 2 as the catalyst (Table 7). Benzaldehyde with substituents ofvarying electronic properties were all smoothly converted into thecorresponding b-nitroalcohols in excellent yields. Electron donat-ing substituents (methyl and methoxy) on the benzaldehyde gaveslightly higher yields (95% and 97%) as compared to the unsubsti-tuted benzaldehyde (92%), whereas electron withdrawing substit-uents (chloro, fluro, nitro, hydroxy) on the benzaldehyde gaveslightly lower yields (82%, 88%, 78% and 75% respectively). The

Table 7Nitroaldol (Henry) reaction of nitromethane with various aldehydes.a

Entry Aldehyde Yield (%)b

1 Y = H 922 Y = Cl 823 Y = F 884 Y = NO2 785 Y = OH 756 Y = CH3 957 Y = OCH3 97

8 90

9 92

10 87

11 91

12 84

a Reaction conditions: aldehyde (1 mmol), nitromethane (5 mmol), [emim]BF4 (0.10 mmol) and complex 2 (3.0 lmol) in 2 mL ofmethanol were stirred for 12 h at room temperature.

b Isolated yields by column chromatography.

1 2 3 4 50

20

40

60

80

Yie

ld %

No. of cycles

Fig. 2. Recyclability study of the catalytic system using benzaldehyde andnitromethane as model substrates and complex 2 as the catalyst in the presenceof the ionic liquid [emim]BF4.

R. Manikandan et al. / Polyhedron 81 (2014) 619–627 625

substrate 1-naphthaldehyde exhibited a yield of 90% with this cat-alytic system. The conversion also proceeded easily in the presenceof heteroatoms such as O, N, and S in the substrates, and a range ofheterocyclic aromatic b-nitroalcohols were obtained in good yield.Further, the complex efficiently catalyzed the nitroaldol reaction ofcinnamaldehyde (a conjugated aldehyde) with 92% yield.

3.6.4. Catalyst recyclingRecovery and reusability of catalysts is an important theme in

catalysis. This means the catalysts can be used in many catalyticcycles, and thereby is more commercial for industrial catalysis.The catalyst was recovered and recycled after the nitroaldol reac-tion as follows. After completion of the catalytic nitroaldol reac-tion, the solvent was completely evaporated under reducedpressure. The products were extracted with the solvent mixturen-hexane:ethylacetate (4:1). The extract was concentrated andpurified by column chromatography. Then the isolated catalystwas washed four times with hexane and dried for 2–3 h under vac-uum. The recovered catalyst (2) was used straightaway (without

further addition of complex) in subsequent catalytic cycles in thenitroaldol reaction of benzaldehyde. The performance of the

Fig. 3. ESI-MS spectrum of the catalytic reaction medium using benzaldehyde and nitromethane as model substrates and complex 1 as the catalyst.

Scheme 2. A probable mechanism for the catalytic nitroaldol (Henry) reaction by Ni(II) complex 1.

626 R. Manikandan et al. / Polyhedron 81 (2014) 619–627

catalyst remained stable over five catalytic cycles (Fig. 2). In com-parison to other reported metal catalysts [35–41] for the Henryreaction, our catalytic system seems to be the best in terms ofactivity, cost and recyclability. Moreover, the catalysts are easierto prepare and recover than other systems, e.g. catalysts basedon metals such as Cu, Zn, Pd, Ru and La [35,36,39–41].

3.6.5. Possible mechanismThe exact mechanism of the nitroaldol reaction is still under

investigation. However, we monitored the reaction using ESI-MSspectra in order to investigate the intermediate formed during

the course of the reaction. The reaction of complex 1 with benzal-dehyde and nitromethane was observed by the gradual replace-ment of the main peak associated with 1, from m/z = 559 (due to[Ni(L2)PPh3]), to new peaks at m/z = 665, (assigned to [Ni(L2)(-PPh3)(benzaldehyde)]+) (I) and m/z = 724, (assigned to [Ni(L2)(-PPh3)(benzaldehyde)(nitromethane)]+) (II) (Fig. 3). These peakdisplacements support the coordination of substrates to the metaland the formation two intermediates, as given in Scheme 2. Thecarbonyl carbon possesses enhanced electrophilicity and hencefavors nucleophilic attack by a neighboring electron rich nitrome-thine group.

R. Manikandan et al. / Polyhedron 81 (2014) 619–627 627

4. Conclusion

New nickel(II) pyridoxal N(4) substituted thiosemicarbazonecomplexes incorporating triphenylphosphine have been synthe-sized and characterized. An X-ray diffraction study of complex 2confirms the ONS coordination mode of the thiosemicarbazoneligand and reveals a distorted square planar geometry around thenickel(II) ion. The new complexes act as excellent recyclable cata-lysts for the nitroaldol reaction in ionic liquid medium with goodto excellent isolated yields. All these complexes were screened fortheir catalytic activity in the nitroaldol reaction, which clearlyshowed the effect of N-substitution on the ligands, and complex 2has been found to be the most active catalyst. This method offersseveral advantages, including short reaction time, excellent yields,simple work-up, ease of separation and recyclability of the catalyst,as well as the ability to tolerate a wide variety of substitutents onthe aldehydes and environmentally-benign reaction conditions.The reusability of this catalyst was high and it can be reused fivetimes without any significant decrease from its initial activity. Themechanism of the catalytic reactions was studied in detail and aplausible mechanism was proposed based on an ESI-MS spectrum.

Acknowledgments

We are thankful to the IISC Bangalore, STIC Cochin, SAIF PunjabUniversity, Chandigarh for spectral studies and the Department ofBio Physics, University of Madras, for single-crystal XRD studies.

Appendix A. Supplementary data

CCDC 943845 contains the supplementary crystallographic datafor complex 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CambridgeCrystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk,.

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