An Acylthiourea Ligated Fe(II) Complex on Silica Nanoparticles for

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Kinetics, Catalysis, and Reaction Engineering

An Acylthiourea Ligated Fe(II) Complex on Silica Nanoparticles for Transfer Hydrogenation of Carbonyl Compounds Dharmalingam Sindhuja, Punitharaj Vasanthakumar, Nattamai Bhuvanesh, and Ramasamy Karvembu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02817 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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Industrial & Engineering Chemistry Research

An Acylthiourea Ligated Fe(II) Complex on Silica Nanoparticles for Transfer Hydrogenation of Carbonyl Compounds Dharmalingam Sindhuja,a Punitharaj Vasanthakumar,a Nattamai S. P. Bhuvanesh,b and Ramasamy Karvembu*,a a

Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India

b

Department of Chemistry, Texas A & M University, College Station, TX 77842, USA

* Corresponding author’s mail id: [email protected] Abstract Transfer hydrogenation of carbonyl compounds using a stable Fe(II) catalyst containing acylthiourea ligand on silica is described. Fe(II) complex supported on silica nanoparticles is prepared by grafting technique, which is a promising candidate in the heterogeneous catalysis. Various characterization methods demonstrate the immobilization of Fe(II)acylthiourea on the surface of silica nanoparticles. All the tested substrates have undergone conversion smoothly in presence of the catalyst, and the products are obtained in moderate to good yields. The heterogeneous catalyst can be easily recovered from the reaction mixture by simple centrifugation method and it can be reused up to eight cycles without the loss of activity. Keywords: Ferrous complex, Silica nanoparticles, Immobilization, Transfer hydrogenation

1. Introduction Catalytic transfer hydrogenation of carbonyl compounds is a ubiquitous reaction in organic synthesis for the production of fine chemicals and pharmaceuticals.1 Reduction of carbonyl compounds is carried out by traditional methods using metal hydride reagents, silanes and molecular hydrogen.2 Transfer hydrogenation using hydrogen donors like isopropanol has attracted a great deal of attention in recent years, because it is easy to handle, cheap and carried out using readily available reagents.3 In general, catalytic transfer hydrogenation is performed by platinum group metals (Ru, Rh, Ir, etc.),4 but, there has been a great interest in the development of iron based heterogeneous as well as homogeneous catalytic systems.1,5 It is obvious that the homogeneous catalytic systems have same phases of catalytic species and reactants, which allow facile interaction between the constituents, result in enhanced activity, higher selectivity and good turnover number (TON).6 However, recovery and separation of precious metal complexes are difficult. In the case of heterogeneous catalytic systems, metal nanoparticles or metal complexes immobilized on a suitable inert support like

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metal oxide,7,8 carbon nanotube,9 graphene,10 polymer,11 etc. have been employed as catalysts. Among the supports, silica nanoparticles have its own place, possess astounding properties like inertness, tuneable pore size, high surface area, rigid framework, chemical and thermal stability, and obviously, being a nanoparticle, it provides better interaction between reactants and the active sites of the catalyst as in the homogeneous catalysis. The new hybrid organicinorganic materials can be prepared by either entrapment or grafting of complexes on the silica surface. Grafting is a feasible technique which enables covalent binding of ligand or complex onto the solid surface. The soluble active complex can be made insoluble by tethering it on an inorganic support, without compromising the efficacy of the complex in catalysis.12 Metal nanoparticles immobilized on silica may undergo agglomeration and hence, there is a need for using stabilizing agent throughout the process. But in the case of complex grafting, the ligands are intact with the metal, which in turn lower the leaching of metal, and the active site is very much available to the reactants for catalytic process. There are plentiful reports on iron complexes as homogeneous catalysts in the literature. Iron complexes containing phosphine-based ligands are widely explored for transfer hydrogenation of amides,13 carbonyl compounds,14 and olefins.15 In most of these cases, the catalytic reaction was performed under inert atmosphere, due to the unstable nature of the complexes. However, immobilized iron complexes are more stable. Therefore, we are interested in synthesizing a new immobilized Fe(II) complex for transfer hydrogenation of carbonyl compounds. The Fe(II) complex was prepared by the immobilization of acylthiourea ligand on silica nanoparticles followed by complexation with FeCl2. The immobilized complex was employed as catalyst, which showed good activity towards transfer hydrogenation of aldehydes and ketones. Moreover, the catalyst can be recycled at least for eight times without any loss in the activity. 2. Experimental methods 2.1 Chemicals The chemicals used were of analytical grade. All the commercial grade solvents were distilled as per the standard procedures and dried over molecular sieves before use. Nano silica and 3-chloropropyltriethoxysilane were purchased from Sigma-Aldrich and used without further purification.


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2.2 Physical measurements UV-visible (UV-Vis) spectra were recorded using a Shimadzu 2600 spectrophotometer operating in the range of 200-800 nm. Fourier transform infrared (FT-IR) spectra in the range of 4000-600 cm-1 were recorded on a Nicolet iS5 FT-IR spectrophotometer with KBr pellets. Nuclear magnetic resonance (NMR) spectra were recorded using DMSO-d6 as solvent and tetramethylsilane as an internal standard on a Bruker 500/125 MHz spectrometer. The elemental analyses were performed by using Elementar Vario ELIII analyser. X-ray powder diffraction (XRD) patterns were recorded on a PANanalytical X’Pert diffractometer using Cu Kα radiation (k=0.15418 nm). X-ray photoelectron spectroscopic (XPS) analyses were done on a Kratos Axis-Ultra DLD instrument. During the analyses, samples were irradiated with Mg Kα X-ray source. The scanning electron microscopic (SEM) images were obtained by using Bruker microscope. The transmission electron microscopic (TEM) images were seen by using JEOL JEM 2100 microscope. The Brunauer-Emmett-Teller (BET) surface area was analysed by using Micrometrics Gemini V 2380 instrument. Iron content was found out by inductively coupled plasma – optical emission spectroscopic (ICP-OES) analysis using Perkin Elmer optima 5300 DV instrument. Thermal stability was studied by using EXSTAR 6200 thermogravimetry (TG) / differential thermal analysis (DTA). Catalysis experiments were monitored by using Shimadzu GC-2010 gas chromatograph (GC) / gas chromatograph-mass spectrometer (GC-MS) equipped with a 60 m × 0.32 mm Restek Rtx®-5 column. 2.3 Synthesis of N-carbamothioylthiophene-2-carboxamide (L) Ligand (L) was synthesized by following a reported procedure (Scheme 1).16 A solution of thiophene-2 carbonyl chloride (1.069 mL, 0.01 mol) in acetone (15 mL) was added to the suspension of potassium thiocyanate (0.9781 g, 0.01 mol) in acetone (15 mL), and the resulting mixture was refluxed for 1 h to get the isothiocyanate intermediate. After cooling the reaction mixture to room temperature, 5 mL of liquid ammonia in acetone (15 mL) was added and it was stirred for 3 h at 27 °C. The mixture was poured into water, and resultant solid was filtered off, washed with water and recrystallized from acetonitrile. Yield: 73%; m.p.: 143 °C; Anal. Calc. C6H6N2OS2 (%): C, 38.69; H, 3.25; N, 15.04; S, 34.43, Found: C, 38.42; H, 3.19; N, 15.16; S, 34.48; UV-Vis: λmax (ethanol, nm): 255, 294; FT-IR (KBr, cm-1): 3340, 3149 (s; (NH)), 3207 (s; (N–H)), 1654 (s; (C=O)), 1245 (s; (C=S)); 1H NMR (500 MHz, DMSO-d6): δ 11.29 (s, CONH, 1H), 9.74 (s, 1H of NH2), 9.52 (s, 1H of NH2), 8.33 (d, J = 3.6 Hz, 1H), 8.00 (d, J = 4.9 Hz, 1H), 7.22 (t, J = 4.2 Hz, 1H); 13C



NMR (126 MHz, DMSO-d6): δ 182.2 (C=S), 162.1 (C=O), 137.4, 135.4, 132.9, 129.1 (carbons of thiophene ring). 2.4 X-ray structure determination A Bruker Venture X-ray (kappa geometry) diffractometer was employed for crystal screening, unit cell determination and data collection.The goniometer was controlled using the APEX3 software suite.17 Integrated intensity information for each reflection was obtained by reduction of data frames with APEX3.17 SADABS was employed to correct the data for absorption effects.18 A solution was obtained readily using XT/XS in APEX3.19 Hydrogen atoms were placed in idealized positions and were set riding on the respective parent atoms. All the non-hydrogen atoms were refined with anisotropic thermal parameters. Absence of additional symmetry or void was confirmed using PLATON (ADDSYM).20 Olex2 was employed for final data presentation and structure plots.21 2.5 Preparation of functionalized silica nanoparticles Silica nanoparticles were activated by dispersing it in 60 mL of 1:1 mixture of concentrated hydrochloric acid and deionised water, and refluxing for 24 h.22 Activated silica nanoparticles (SNPs-OH) were filtered, washed with water and heated to 80 °C under vacuum for overnight. Owing to the availability of silanol groups on the surface, its modification via immobilization of organic compounds can be done easily. L-SNPs were prepared according to the reported procedure.16 Ligand (L) (0.3998 g, 2.147 mmol) was dissolved in dimethylformamide. 3-Chloropropyl triethoxysilane (CPTES) and triethyl amine were added to the above solution and the resulting mixture was refluxed for 48 h at 110 °C under inert atmosphere (Scheme 2). Then, the solvent was removed under vacuum and the product (L-OEt) was extracted twice with hexane (2×15 mL). L-OEt: UV-Vis: λmax (ethanol, nm): 206, 241; FTIR (KBr, cm-1): 3465 (N–H), 3095 (aromatic C–H), 2923 and 2859 (aliphatic C–H), 1720 (C=O), 1619 (C=C), 1267 (C=S), 1132 and 1068 (Si–OCH2CH3). After solvent evaporation, SNPs-OH (0.5 g) were reacted with L-OEt in dry toluene (15 mL) under nitrogen atmosphere at 100 °C for 48 h. The resulting solid was filtered off, and washed with toluene, diethyl ether and ethanol. The modified silica nanoparticles (L-SNPs) were dried under vacuum at 110 °C for 12 h.


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Fe(II)-L-SNPs were prepared by stirring L-SNPs (0.2 g) with FeCl2 (190 mg, 1.5 mmol) in dry acetone (30 mL) at room temperature for 6 h (Scheme 3). The resulting solid was filtered off, and washed with acetone, ethanol and diethyl ether until the filtrate became colourless. Fe(II)-L-SNPs were dried under vacuum for overnight at 70 C. The tentative structure of the catalyst is shown in scheme 3. 2.6 Transfer hydrogenation of carbonyl compounds To a 10 mL round bottom flask were added carbonyl compound (1 mmol), Fe(II)-LSNPs (2 mg, 0.716 mol%) and KOtBu (112.2 mg, 1 mmol) in isopropanol (3 mL) and the mixture was kept for stirring in an oil bath at 90 C. After completion of the reaction, the reaction mixture was cooled to room temperature and the catalyst was removed by centrifugation. The centrifugate was concentrated and eluted through short silica column using hexane/ethyl acetate eluent. The products were analysed by using GC-MS and the conversions were noted. The catalyst separated from the reaction mixture was washed with diethyl ether, dried in vacuo and used for next cycle. 3. Results and discussion 3.1 Characterization of the ligand Electronic spectrum of the ligand (Figure S1) revealed the presence of ππ* and nπ* transitions. FT-IR spectrum of the ligand showed thioamidic NH2 and amidic NH stretching frequencies at 3340 and 3149 cm-1, and 3207 cm-1 respectively. The frequencies at 1654, 1245 and 1594 cm-1 were attributed to C=O, C=S and CN stretching respectively (Figure S2).23 The 1

H NMR spectrum (Figure S3) showed singlets at 11.29 (amidic NH), and 9.74 and 9.52 ppm

(thioamidic NH2). The protons in the thiophene ring resonated at 7.22-8.33 ppm. The 13C NMR (1H decoupled) spectrum (Figure S4) showed signals at 182.2 and 162.1 ppm, which were assigned to thiocarbonyl (C=S) and carbonyl (C=O) carbons respectively. The signals at 129.1, 132.9, 135.4 and 137.4 ppm were due to carbons in the thiophene ring. The molecular structure of the ligand is shown in figure 1. Suitable crystals for X-ray diffraction study were obtained by slow evaporation of acetonitrile solution of L. Crystal data of L is provided in table S1. Ligand crystallized in monoclinic fashion with a space group P1 21/n1. In the crystal structure of L, intramolecular hydrogen bonding (1.98 Å ) between carbonyl O and NH hydrogen was observed.



3.2 Characterization of L-OEt Electronic spectrum (Figure S5) of L-OEt showed two absorption bands at 206 and 241 nm, which can be attributed to ππ* and nπ* transitions respectively. FT-IR spectrum is shown in figure S6. A broad band at 3465 cm-1 was attributed to merged stretching vibrations of NH and OH. Aromatic (CH) and aliphatic (CH) were observed at 3095 and 2923 cm-1 respectively. Similarly, a band at 1269 cm-1 corresponded to (C=S). (SiOCH2CH3) was observed as doublet at 1068 and 1132 cm-1.24 3.3 Characterization of L-SNPs and Fe(II)-L-SNPs FT-IR spectrum of silica nanoparticles showed a broad band at 3435 cm-1, which was due to (OH) of surface silanol groups. Similarly, (SiOSi) was observed at 1107 (stretching) and 827 cm-1 (bending). L-SNPs and Fe(II)-L-SNPs showed (NH)+(OH), (CH) and (SiOSi) at 3424-3430, 2918-2925 and 1103-1107 cm-1 respectively in their FT-IR spectra (Figure S7). In addition, bands were observed at 1633 and 1466 cm-1 corresponding to (C=O) and (CN) respectively. Hence, successful immobilization of the ligand was clearly evidenced by FT-IR spectral analysis. Coordination of the ligand with Fe was expected through S donor atom; but in the spectrum, band due to (CS) was hidden by the broad SiOSi peak.22 The X-ray diffraction (XRD) pattern of SNPs, L-SNPs and Fe(II)-L-SNPs are presented in figure S8. The XRD pattern of SNPs showed a broad peak at 2 = 22, which confirmed amorphous or low crystallinity of the material. L-SNPs and Fe(II)-L-SNPs also showed a broad peak at the same angle, which confirmed low crystallinity of the material even after ligand grafting and complexation.25 The presence of Fe(II) complex on the surface of SNPs was confirmed by XPS. Figure 2 shows the XPS spectrum of Fe(II)-L-SNPs, in which the sharp photoelectron peaks were observed at 102, 283 and 531 eV, which corresponded to Si 2p, C 1s and O 1s respectively. The binding energies corresponding to +2 oxidation state of iron were observed at 709 (Fe 2p3/2) and 723 (Fe 2p1/2), which were in agreement with the literature reports.26 As there was no binding energy reliable to Fe(III) (711 and 718 eV) detected, +2 oxidation state of Fe was confirmed. The morphology of Fe(II)-L-SNPs was analysed by using SEM and TEM. The SEM images (Figure 3a) confirmed spherical shape of the silica nanoparticles. The energy dispersive X-ray analysis (EDX) revealed elemental composition (Si, O, Fe, Cl, N and S) of the catalyst (Figure 3b). TEM images of Fe(II)-L-SNPs (Figure 4) established the uniform dispersion of


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the complex on spherical shaped silica. This morphology of SNPs was unaltered even after its surface modification (Figure S9). Hence, it was established that there was no significant impact for immobilization on the morphology of silica nanoparticles. ICP-OES study established the amount of iron (3.58 mmol/g) in the catalyst, which is in agreement with EDX data. Surface area and pore size distribution of Fe(II)-L-SNPs were analysed by using BET and BJH (Barett-Joyner-Halenda) methods respectively. The isotherm showed type IV curves, with hysteresis loop characteristic of mesoporous material (Figure S10). The BET surface area of the catalyst was compared with SNPs. Interestingly, considerable reduction in surface area of SNPs was observed. The surface area and pore volume of SNPs were 207 m2/g and 0.255 cm3/g respectively, whereas in the case of Fe(II)-L-SNPs surface area and pore volume were found to be 167.77 m2/g and 0.34 cm3/g respectively. TGA curve of the complex was recorded from the ambient temperature to 800 C (Figure S11). It showed that the initial loss of 1.6% (150 C) on heating was due to the removal of adsorbed water molecules from the surface. The weight loss of 7.3% between 150 and 500 C might be due to the removal of anchored complex from silica. 3.4 Transfer hydrogenation of carbonyl compounds 3.4.1 Optimization of reaction conditions Initially, reaction parameters such as hydrogen donor, base, amount of base, amount of catalyst and temperature were optimized by using acetophenone (1a) as a model substrate. 2propanol was chosen as solvent as well as hydrogen donor, as the conversion of acetophenone was not proceeded well in water, ethanol and formic acid/triethylamine mixture. At the outset, optimal amount of the catalyst was found to be 0.716 mol% of Fe on the silica nanoparticles (Table S2, entry 8). Five different bases (NaOH, KOH, K2CO3, Na2CO3 and KOtBu) were tested and among the five bases, KOtBu gave excellent conversion with 100% selectivity (Table S2, entry 5). Amount of KOtBu (1 mmol) was optimized. The optimal time was found to be 16 h, below which the conversion was incomplete. Similarly, hydrogenation of acetophenone was carried out at different temperatures, 40, 60 and 80 °C (Table S2, entries 1214). The reaction proceeded well at 80 °C. Transfer hydrogenation was also carried out without the catalyst. The conversion observed in this case was only 28%, which did not show any improvement even after extended time.



3.4.2 Scope of the catalytic system With the optimized reaction conditions, scope of the catalytic system was extended to a wide range of aryl ketones and aryl aldehydes (Tables 1 and 2). It was found that the present system exhibited high yield with good selectivity compared to previous reports. Catalytic hydrogenation of acetophenone yielded >99% of 1-phenylethanol (2a), whereas, nanoFe@SiO2Ru (100 mg for 1 mmol of acetophenone) system yielded only 86% of alcohol product at 100 C under microwave condition.27 The conversion of ortho and para-methyl acetophenone to corresponding alcohol was 45 and 76% respectively with 100% selectivity (Table 1, 2b and 2c). Further, 4-methoxy acetophenone gave the corresponding alcohol with the selectivity of 62% after 16 h (Table 1, 2d), but 0.2 mol% ruthenium(II) picolyl-NHC complex was utilised for 4-methoxy acetophenone (2 mmol) to achieve the same product with 64% selectivity in 24 h.28 In the hydrogenation of sterically hindered 2,4-dimethyl acetophenone, quantitative conversion was observed with high selectivity (Table 1, 2e). Interestingly, 3-nitro acetophenone was fully converted to 3-amino acetophenone with 100% selectivity (Table 1, 2f). Our present catalytic system is chemoselectively reducing nitro in the presence of keto group. 3-Amino acetophenone was not reduced further to furnish the corresponding amino alcohol. Furthermore, halo (F, Cl, Br and I) substituted ketones were studied, in which Cl, Br and I containing substrates were reduced selectively (Table 1, 2g-2j). Selectivity (37%) was low in the case of 4-fluoro acetophenone. 4-Chloro acetophenone and 4-bromo acetophenone gave expected alcohol with good conversion and selectivity, but nickel nanoparticles (50 mg for 1 mmol of ketone) afforded only debrominated product (1-phenyl ethanol) from 4-bromo acetophenone at 100 C under microwave conditions (Table 1, 2h and 2i).29 In the case of 4-iodo acetophenone (Table 1, 2j), ketone was satisfactorily converted to the alcohol. Notably, 2-chloro acetophenone gave good conversion (100%) with high selectivity (100%) (Table 1, 2k). Likewise, benzophenone gave diphenyl methanol with high selectivity (100%) (Table 1, 2l), but 2.0 mol % of Fe(II) pincer complex with 9 bar hydrogen gas was required for the hydrogenation of benzophenone, and the yield of diphenyl methanol was only 15%.30 In the same way, transfer hydrogenation of aldehydes was carried out by using Fe(II)-L-SNPs. Benzaldehyde was completely converted to benzyl alcohol (Table 2, 4a). Similarly, methyl, halo or nitro substituted benzaldehyde gave the expected product in excellent conversion with good selectivity (Table 2, 4b-4h). Conversion of 2nitrobenzaldehyde by Fe(II)-L-SNPs was 99% (Table 2, 4f) while 0.0038 mmol of iron phthalocyanine heterogenized catalyst showed only 25% conversion after 4 h.31 4-Nitro


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benzaldehyde was fully converted with 62% selectivity (Table 2, 4h). Fe(II)-L-SNPs can be used for selectively reducing aldehyde in presence of nitro group. 3.4.3 Heterogeneous nature of the catalyst The two crucial factors which attest the heterogeneous catalytic system as favourable in industries are life time and reusability level of the catalyst. Acetophenone was chosen to examine the heterogeneity of the catalyst. The samples were collected at regular intervals and analysed in GC. Once the reaction was half done, catalyst was separated from the reaction mixture by centrifugation. The reaction was proceeded without the catalyst for requisite time. Even after extending the time, yield of the product (43%) was unaltered. Hence, heterogeneous nature of the catalyst was confirmed (Figure 5). To support the above, the centrifugate was analysed for Fe content, it was found to be negligible. 3.4.4 Reusable property of the catalyst For testing recyclability of the catalyst, acetophenone was used and the reaction was allowed for 16 h. After completion of the reaction, catalyst was separated by centrifugation, washed with diethyl ether and dried in vacuo. Recovered catalyst was used in the next run, the results are shown in figure 6. Fresh catalyst gave 100% yield of alcohol. Remarkably, there was no meaningful change in the yield till eight consecutive runs. A small decrease in the yield after eighth run was observed. To our satisfaction, Fe(II)-L-SNPs could be used up to eight cycles. TEM images of the reused catalyst is shown in figure S12. It was found that there was no significant change in the morphology. 3.4.5 Proposed mechanism The mechanism proposed for the transfer hydrogenation of acetophenone is shown in figure 7.32,33 In the first step, base reacts with isopropyl alcohol in presence of the catalyst to generate Fe-alkoxide intermediate (I) which forms FeH species (II) through migration of H from α-carbon of isopropoxide with the release of acetone. When the substrate is interacting with II via FeH and NH, a cyclic transition state (III) is formed according to Noyori’s outer sphere mechanism. Finally, the hydride transfer from Fe to substrate yields 1-phenylethanol. 4. Conclusion New heterogeneous Fe(II) complex of acylthiourea ligand was synthesized and characterized. Complex was uniformly distributed onto the surface of silica nanoparticles. The catalytic activity of the complex was studied towards transfer hydrogenation of carbonyl



compounds. Various reaction conditions were optimized, and the scope of the work was extended to different carbonyl compounds. The selectivity order of hydrogenation by Fe(II)L-SNPs catalyst was aldehyde > nitro > ketone. The recyclability of the used catalyst was also effective, and it can be used up to eight cycles without any loss in activity. Overall, the present study showed the advantages of iron complexes as well as the silica nanoparticles support in catalysis. Supporting information Characterization data of the catalyst and GC-MS chromatogram of all the entries are provided. Acknowledgement We thank BRNS (34/14/52/2014-BRNS/2069) for financial support. References (1) (a) Zell, T.; Ben-David, Y.; Milstein, D. Highly efficient, general hydrogenation of aldehydes catalyzed by PNP iron pincer complexes. Catal. Sci. Technol. 2015, 5, 822. (b) Canivet, J.; Suss-Fink, G. Water-soluble arene ruthenium catalysts containing sulfonated diamine ligands for asymmetric transfer hydrogenation of αaryl ketones and imines in aqueous solution. Green Chem. 2007, 9, 391. (c) Ahlford, K.; Lind, J.; Maler, L.; Adolfsson, H. Rhodium-catalyzed asymmetric transfer hydrogenation of alkyl and aryl ketones in aqueous media. Green Chem. 2008, 10, 832. (2) Wang, D.; Astruc, D. The golden age of transfer hydrogenation. Chem. Rev. 2015, 115, 6621. (3) Mary Sheeba, M.; Preethi, S.; Nijamudheen, A.; Muthu Tamizh, M.; Datta, A.; Farrugia, L. J.; Karvembu, R. Half-sandwich Ru(η6-C6H6) complexes with chiral aroylthioureas for enhanced asymmetric transfer hydrogenation of ketones – Experimental and theoretical studies. Catal. Sci. Technol. 2015, 5, 4790. (4) (a) Hayes, J. M.; Deydier, E.; Ujaque, G.; Lledós, A.; Malacea-Kabbara, R.; Manoury, E.; Vincendeau, S.; Poli, R. Ketone hydrogenation with iridium complexes with “non N-H” ligands: The key role of the strong base. ACS Catal. 2015, 5, 4368. (b) Sudakar, P.; Gunasekar, G. H.; Baek, I.; Yoon, S. Recyclable and efficient heterogenized Rh and Ir catalysts for the transfer hydrogenation of carbonyl compounds in aqueous medium. Green Chem. 2016, 18, 6456. (c) Kanchanadevi, A.; Ramesh, R.; Semeril, D. Efficient and recyclable Ru(II) arene


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complexes

as

catalysts

in

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Synthesis,

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Scheme 1 Synthesis of L

Scheme 2 Synthesis of L-SNPs

Scheme 3 Synthesis of Fe(II)-L-SNPs



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Figure 1 The molecular structure of L

(a)

(c)

(b)

(d)

Figure 2 XPS spectra of Fe(II)-L-SNPs, (a) C 1s, (b) O 1s (c) Fe 2p (d) wide spectrum


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cps/eV

(a)

14

(b)

12

10

Figure 8 (a) SEM image and (b) EDAX spectrum of Fe(II)-L-SNPs 8 Cl S N Fe C O 6

Si

S Cl

Fe

4

2

0 1

2

3

4 keV

5

6

7

Figure 3 (a) SEM image and (b) EDAX spectrum of Fe(II)-L-SNPs

(a)

Figure 4 TEM images of Fe(II)-L-SNPs (a) 20 nm (b) 50 nm


(b)

8


Figure 5 Heterogeneity test of the catalyst

Figure 6 Reusability of the catalyst


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Figure 7 Proposed mechanism for transfer hydrogenation of carbonyl compounds by Fe(II)-L-SNPs



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Table 1 Transfer hydrogenation of ketones catalysed by Fe(II)-L-SNPsa

2a, 99b, 100%c, 100%d, 139e

2b, 45%b, 100%c, 45%d, 63e

2c, 76%b, 100%c, 75%d, 105e

2d, 62%b, 99%c, 62%d, 87e

2e, 11%b, 100%c, 11%d, 15e

2f, 99%b, 100%c, 100%d, 140e

2g, 86%b, 37%c, 32%d, 45e

2h, 97%b, 100%c, 97%d, 135e

2i, 98%b,100%c, 98%d, 137e

2j, 99%b, 90%c, 89%d, 124e

2k, 99%b, 100%c 100%d, 88e

2l, 79%b, 100%c, 79%d, 110e

a

Reaction conditions: 1 (1 mmol), Fe(II)-L-SNPs (0.716 mol%), KOtBu (1 mmol), iPrOH (3 mL) b GC conversion, c selectivity, d GC yield, e TON


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Table 2 Transfer hydrogenation of aldehydes catalysed by Fe(II)-L-SNPs a

4a, 99%b,100%c, 100%d, 138e

4b, 99%b, 96%c, 96%d, 134e

4c, 99%b, 64%c, 64%d, 89e

4d, 99%b,100%c, 99%d, 138e

4e, 99%b, 99%c, 99%d, 138e

4f, 99%b, 69%c, 68%d, 95e

4g, 99%b, 73%c, 72%d, 101e

4h, 99%b, 62%c, 62%d, 87e

Reaction conditions: 3 (1 mmol), Fe(II)-L-SNPs (0.716 mol%), KOtBu (1 mmol), iPrOH (3 mL) b GC conversion, c selectivity, d GC yield, e TON

a



For Table of Contents only


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An Acylthiourea Ligated Fe(II) Complex on Silica Nanoparticles for

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