Efficient Solvent-Free Hydrogenation of Levulinic Acid to Î³

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Efficient Solvent-Free Hydrogenation of Levulinic Acid to #-Valerolactone by Pyrazolylphosphite and Pyrazolylphosphinite Ruthenium(II) Complexes Gershon Amenuvor, Banothile Charity Event Makhubela, and James Darkwa ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01281 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016

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ACS Sustainable Chemistry & Engineering

Efficient Solvent-Free Hydrogenation of Levulinic Acid to γ-Valerolactone by Pyrazolylphosphite and Pyrazolylphosphinite Ruthenium(II) Complexes Gershon Amenuvor, Banothile C. E. Makhubela,* and James Darkwa*

Department of Chemistry, University of Johannesburg, PO Box 524, Auckland Park, 2006, South Africa.

Abstract Solvent-free conversion of bio-derived levulinic acid (LA) to γ-valerolactone (GVL) has been achieved by new pyrazolylphosphite and pyrazolylphosphinite ruthenium(II) complexes as catalyst precursors, using both formic acid and molecular hydrogen as hydrogen sources. The reactions were very efficient at moderate temperatures of 100 to 120 °C. With a catalyst loading of 0.1%, 100% LA conversion (with hydrogen gas) was achieved with 100% GVL selectivity at 110 °C and 15 bar. The catalyst was recyclable up to three times without significant loss of activity and selectivity. The catalyst precursors are found to be more efficient when the hydrogen source was molecular hydrogen as compared to formic acid. NMR studies of reactions involving formic acid as a hydrogen source indicate that the initial step in the reaction involves the decomposition of formic acid to CO2 and H2.

Keywords: Solvent-free, ruthenium catalysts, hydrogenation, levulinic acid, γ-valerolactone Email corresponding author: [email protected]

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Introduction Due to the increasing human population, energy demand has also increased.1-4 The limited supply of fossil fuel which is the main source of energy and carbon source currently is a serious concern for the future.5-7 Several key chemicals such as olefins and aromatics obtained through fossil fuel processing are also at risk to gradually declining in supply.6 A shift toward biomass processing for energy and useful chemicals can serve as an alternative for the situation.5,8 The idea of getting biofuel and chemicals from biomass is economically sustainable and has the potential to reduce carbon dioxide emissions while contributing to supply of energy fuels and chemicals demand.9 Biomass is renewable and cheap because nature produces a vast amount of it. It is also nontoxic and biodegradable, thus posing less negative impact on the environment. It has been estimated that about 170 billion tons of biomass is produced every year through photosynthesis,6 and this makes the case for the use of bio-derived feedstock as an alternative to fossil-base feedstock. Biomass already contains almost all the functional groups needed for synthesis of platform chemicals and the conversion of such bio-derived feedstock would require new catalytic technologies.

Levulinic acid (LA) is one of the important biomass-derived chemicals that may serve as a starting material for a number of useful chemicals.10 LA is obtained from hydrolysis of hydroxymethylfurfural (HMF), a chemical derived from certain sugars in the carbohydrate component of biomass.6,8,11 Examples of interesting platform chemicals that can result from LA include 1,4-pentanediol (PDO), 2-methyltetrahydrofuran (M-THF), 4-hydroxyvaleric acid (4HVA), α-angelica lactone and γ-valerolactone (GVL), which are all good solvents and


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intermediate chemicals used to access a wide range of biofuels and commodity chemicals.6,10 The latter has other potential uses such as a biofuel (gasoline blend) and food additive.1 GVL can also serve as a suitable monomer for production of a variety of polyesters6 and copolymers.12 Conversion of LA to GVL with little or no side products and under very economical and environmentally friendly conditions is driven by suitable catalysts. The development of environmentally friendly and cost effective catalytic technology has recently become an area of interest for synthesis of GVL. For example, the use of high temperature and pressure by most heterogeneous catalysts for the synthesis of GVL has been countered by introducing homogeneous catalysts that operate under relatively low temperature and pressure conditions.

Examples of the heterogeneous catalysts that have been in use for hydrogenation of LA to GVL are ruthenium, nickel and palladium supported on carbon and oxides such as Al2O3 and SiO2.6,1214

Some of these systems gave conversions of over 90%, however, most of the catalysts operate

under high H2 pressure and temperature, in the range 50 to 100 bar and 100 to 240 oC respectively. Reports by Al-Shaal et al. show the successful use of ruthenium on various supports, including TiO2 and SiO2, for hydrogenation of LA to GVL at moderate conditions; such as 130 oC and 12 bar hydrogen pressure.15 Despite these moderate reaction conditions required, selectivity to GVL is lower than 100%. Other reports, using palladium nanoparticles and zirconium-based metal–organic frameworks, at moderate reaction conditions produce GVL from ethyl levulinate.16-17 For instance, Ye et al. reported hydrogenation of ethyl levulinate to GVL catalyzed by palladium nanoparticles supported on Nb2O5-doped activated carbon at 100 o

C and 0.5 MPa of hydrogen pressure. A major problem they encountered, though, was GVL

selectivity, observing intermediate products like LA and ethyl 4-hydroxypentanoate.17 Recently, 3 ACS Paragon Plus Environment


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Tay and co-workers18 reported in situ generated ruthenium nanoparticles derived from Ru-NHC complexes and some of these nanoparticles were active at moderately low temperature (130 oC) and H2 pressure (12 bar). The Ru-NHC complexes formed nanoparticles in water as a solvent. However, in organic solvents only those with bidentate ligands remain as homogeneous catalysts. Most of the few homogeneous catalysts that are known for hydrogenation of LA to GVL1,10,19-22 and various carboxylic acids into esters and alcohols22-23 are based on ruthenium phosphine systems, although some examples of iridium and palladium catalysts are known.24-26 Li et al. reported the application of iridium complexes of PNP and PNN pincer type ligands, which were generated in situ, for transformation of LA to GVL in excellent conversions.21 However, high pressures of up to 50 atm were required. Other studies also show that phosphinemodified ruthenium catalysts are also good for LA hydrogenation to GVL.20-21 A study carried out by Chowdhury and co-workers show that ruthenium-triphos-based catalysts generated in situ from [Ru(acac)3] complex and triphos-based ligands are efficient for LA hydrogenation to GVL.20 The homogeneous catalysts were reported to be active but some still require high pressures of 50 to 100 bar and temperatures of 140 to 200 oC and also, only a few of them are known to be active in solvent-free conditions and in aqueous medium.10,19-20 Another major challenge which needs to be addressed is the selectivity of the catalysts toward GVL as the only desirable product. In the process of producing GVL from LA, chemicals such as M-THF and PDO may also be formed as side products.10,27 The formation of these side products are attributed to the catalyst and also to the source of the hydrogen.22 It has been reported that the use of molecular hydrogen also results in the formation of M-THF and PDO as side products.27 Recently, Horvath’s group utilized Shvo catalyst to convert LA to GVL using formic acid with high selectivity toward GVL.27 Furthermore, organic solvents such as methanol, ethanol, 1-


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butanol, 2-butanol, and 1,4-dioxane are usually utilized as the reaction media for GVL production from LA.14,28 However, from economical and green chemistry perspective, it would be desirable to develop a catalytic system that operates effectively under solvent-free conditions. We report in this work a successful greener (solvent-free, low pressure and temperature) approach to the synthesis of GVL, with 100% selectivity from LA with pyrazolylphosphite and pyrazolylphosphinite29 ruthenium(II) pre-catalysts. The pre-catalyst system uses only 15 bar hydrogen gas and trace amounts of 4-hydroxyvaleric acid (4-HVA) intermediates are observed in a few cases when formic acid was used as the hydrogen source. The catalyst precursors work efficiently in the absence of solvent making our approach greener and more economical.

Experimental General information All reactions were performed under a dry, deoxygenated nitrogen atmosphere using standard Schlenk techniques. triethylamine was dried over potassium hydroxide. All solvents were of analytical grade and were dried using MBRAUN SPS-800 solvent drying system and or distilled prior

to

use.

Compounds

chlorodiphenylphosphine

(98%,

Sigma-Aldrich),

diethyl

chlorophosphite (95%, Aldrich), dichloro(p-cymene)ruthenium(II) dimer (Sigma-Aldrich), triethylamine (99%, Sigma-Aldrich), potassium hydroxide (85%, Promark chemicals), Levulinic acid (97%, Sigma-Aldrich) and formic acid (95%, Sigma-Aldrich) were of reagent grades and used as received. The compounds, (3,5-dimethylpyrazol-1H-yl)ethanol,30 L1-L2,29,31 1-229,31 and 2b29 were synthesized as described in literature. 1H NMR,

13

C{1H} NMR and

spectra were recorded on a Bruker Ultrashield 400 (1H NMR 400.17 MHz, 100.62 MHz and

13

31

13

P{1H} NMR

C{1H} NMR

P{1H} NMR 161.99 MHz) in CDCl3 and CD2Cl2 as appropriate at room 5 ACS Paragon Plus Environment


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temperature. Elemental analyses were performed on a Thermo Scientific FLASH 2000 CHNS-O analyzer. MS (ESI) spectra were recorded on a Waters Synapt G2 spectrometer.

All hydrogenation reactions were performed in TAIATSU TECHNO high pressure reactor vessels with 200 °C and 10 MPa capacities fitted into a high pressure autoclave reactor (PPVCTRO1-CE) with an in built heating, cooling and stirring systems. Conversions of the hydrogenation products were determined by Bruker Ultrashield 400 (1H NMR 400.17 MHz).

Synthesis of complexes 1a, 1b and 2a [Ru(p-cymene)Cl(L1)][BArF] (1a) The synthesis of 1a was performed in a similar manner as described for 2b in literature.29 Sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, NaBArF (115 mg, 0.13 mmol) dissolved in 15 mL of dichloromethane was added to complex 1 (74 mg, 0.13 mmol) in 15 mL of dichloromethane and stirred at room temperature for 1 h. The solvent was pumped off after the reaction and the product re-dissolved in a small quantity of dichloromethane and filtered off through a filter membrane to remove the sodium chloride by-product. Pumping off the dichloromethane resulted in orange brown solids. Yield: 0.17 g (96%). 1H NMR (CDCl3): δ 1.20 (d, O-CH2CH3, 6H), 1.23 (d, p-cym-C-(CH3)2, 6H), 2.12 (d, p-cym-CH3, 3H), 2.07 (s, pz-CH3, 3H), 2.16 (s, pz-CH3, 3H), 2.78 (m, p-cym-CH(CH3)2, 1H), 3.85 (m, O-CH2CH3, 2H), 3.99 (m, O-CH2CH3, 2H), 4.07 (t, pz-CH2CH2, 2H), 4.13 (t, pz-CH2CH2, 2H), 5.37 (d, p-cym-CH, 2H), 5.57 (d, p-cym-CH, 2H), 5.80 (s, pz-CH, 1H).

13

C{1H} NMR (CDCl3): δ 10.5 (pz-CH3), 13.0

(pz-CH3), 15.7 (O-CH2CH3), 15.8 (O-CH2CH3), 18.2 (p-cym-CCH3), 21.7 (p-cym-C(CH3)2), 30.7, 30.9 (d, 3JP,C = 23.1 Hz, p-cym-C(CH3)2), 47.2, 47.3 (d, 2JP,C = 7.0 Hz, pz-CH2CH2), 64.0,


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64.1 (d, 2JP,C = 5.0 Hz, O-CH2CH3), 64.1, 64.2 (d, 3JP,C = 11.0 Hz, pz-CH2CH2), 88.1, 88.1 (d, JP,C = 5.0 Hz, p-cym-CH), 89.3, 89.4 (d, JP,C = 6.0 Hz, p-cym-CH), 101.2, ( p-cym-Cq), 106.2 (pz-CH), 113.0 (d, p-cym-Cq), 139.4 (pz-C), 148.6 (pz-C). 31P{1H} NMR (CDCl3): δ 115.3 (s, OP-(OCH2CH3)2). Anal calcd: C 45.66, H 3.40, N 2.01%; found: C 45.26, H 3.74, N 1.73%. MS (ESI+): m/z 531, [Ru(p-cymene)Cl(L1)]+; MS (ESI-): m/z 863, BArF-.

[Ru(p-cymene)Cl(L1)][BPh4] (1b) Complex 1b was prepared in a similar manner as described for 2b in literature.29 NaBPh4 (0.12 g, 0.35 mmol) in 5 mL of methanol and 1 (0.20 g, 0.35 mmol) in 25 mL of dichloromethane were used and orange solid product was obtained in good yield. Yield: 0.30 g (100%). 1H NMR (CD2Cl2): δ 1.08 (d, p-cym-C-(CH3)2, 3H), 1.22 (d, p-cym-C-(CH3)2, 3H), 1.25 (m, O-CH2CH3, 6H), 1.54 (d, O-CH2CH3, 6H), 2.12 (s, pz-CH3, 3H), 2.18 (s, pz-CH3, 3H), 2.65 (s, p-cym-CH3, 3H), 2.65 (m, p-cym-CH(CH3)2, 1H), 4.19 (m, O-CH2CH3, 4H), 4.35 (t, pz-CH2CH2, 2H), 4.64 (m, pz-CH2CH2, 2H), 5.48 (d, p-cym-CH, 1H), 5.66 (d, p-cym-CH, 1H), 5.71 (d, p-cym-CH, 1H), 5.98 (d, p-cym-CH, 1H), 76.16 (s, pz-CH, 1H). 13C{1H} NMR (CD2Cl2): δ 12.3 (pz-CH3), 12.3 (pz-CH3), 16.2, 16.3 (d, 3JP,C = 6.2 Hz, O-CH2CH3), 16.5, 16.5 (d, 3JP,C = 5.2 Hz, OCH2CH3), 17.6, 18.4 (p-cym-CCH3), 19.9 (p-cym-C(CH3)2), 21.9 (p-cym-C(CH3)2), 30.9 (pcym-C(CH3)2), 47.5, 47.9 (d, 2JP,C = 8.0 Hz, pz-CH2CH2), 63.9, 63.9 (d, 2JP,C = 7.0 Hz, OCH2CH3), 66.9 (pz-CH2CH2), 69.0, 69.1 (d, 2JP,C = 10.0 Hz, O-CH2CH3), 86.0 (p-cym-CH), 90.1 (p-cym-CH), 95.3 (p-cym-CH), 97.8 (p-cym-CH), 102.8 (p-cym-Cq), 108.2 (p-cym-Cq), 110.5 (pz-CH), 144.1 (pz-C), 156.1 (pz-C).

31

P{1H} NMR (CD2Cl2): δ 112.6 (s, O-P-(OCH2CH3)2).

Anal calcd for 1b.0.25CH2Cl2: C 62.36, H 6.42, N 3.21%; found: C 62.69, H 6.40, N 2.94%. MS (ESI+): m/z 531, [Ru(p-cymene)Cl(L1)]+; MS (ESI-): m/z 319, BPh4-.



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[Ru(p-cymene)Cl(L2)][BArF] (2a) Complex 2a was prepared in a similar manner as described for 1a in literature. NaBArF (115 mg, 0.13 mmol) in 15 mL of dichloromethane and 2 (82 mg, 0.13 mmol) in 15 mL of dichloromethane were used and orange brown solid product was obtained in good yield. Yield: 0.16 g (83%). 1H NMR (CDCl3): δ 0.73 (d, p-cym-CH3, 6 H), 1.79 (s, pz-CH3, 3H), 1.92 (s, pzCH3, 3 H), 2.21 (s, p-cym-CH3, 3H), 2.36 (m, p-cym- CH-(CH3)2, 1H), 3.40 (q, Hz, CH2, 2H), 3.95 (t, CH2, 2H), 5.17 (d, p-cym-CH, 2H), 5.29 (d, p-cym-CH, 2H), 5.90 (pz-CH, 1H). 13C{1H} NMR (CDCl3): δ 10.4 (pz-CH3), 13.4 (pz-CH3), 17.0 (p-cym-CH3), 20.8 (p-cym-CH(CH3)2), 30.4 (p-cym-CH(CH3)2), 47.7 (pz-CH2CH2), 66.1, 66.2 (d, 3JP,C = 14.1 Hz, pz-CH2CH2), 87.0 ( p-cym-CH), 87.1 ( p-cym-CH), 92.1 ( p-cym-CH), 92.2 ( p-cym-CH), 95.9 (p-cym-Cq), 106.6 (pz-CH), 110.3 (p-cym-Cq), 140.0 (pz-C), 149.3 (pz-C). 31P{1H} NMR (CDCl3): δ 124.7 (s, O-P(C6H5)2). Anal calcd: C 50.24, H 3.25, N 1.92%; found: C 51.19, H 3.15, N 1.84%. MS (ESI+): m/z 595, [Ru(p-cymene)Cl(L1)]+; MS (ESI-): m/z 863, BArF-.

Molecular structure determination Single-crystals suitable for X-ray diffraction analysis for compounds 1a, and 1b were grown and used to determine the molecular structures for the respective compounds. Complex 1a (ca. 4 mg) was dissolved in a mixture of dichloromethane and diethylether in a 3:2 ratio; total volume 10 mL. The mixture was transferred into a vial with a needle hole through the lid and kept at -4 oC to allow slow evaporation. After a week, shiny brown diamond-shaped crystals suitable for X-


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ray diffraction analysis were obtained. Crystallization of 1b was performed by dissolving ca. 4 mg of 1b in a mixture of dichloromethane and dimethyl sulfoxide solution in a 7:3 ratio; total volume 10 mL, and kept at room temperature. Slow evaporation of the dichloromethane from the mixture in a vial left behind shiny orange diamond-shaped crystals. The procedure for compound 1a is used to describe the general experimental method adopted in X-ray structural determination of all the compounds.

Crystal data was collected on a Bruker APEXII diffractometer with Mo Kα (λ = 0.71073 Å) radiation and diffractometer to crystal distance of 4.00 cm. The initial cell matrix was obtained from three series of scans at different starting angles. Each series consisted of 12 frames collected at intervals of 0.5º in a 6º range about with an exposure time of 10 s per frame. The reflections were successfully indexed by an automated indexing routine built in the APEXII program suite. The data were collected using the full sphere data collection routine to survey the reciprocal space to the extent of a full sphere to a resolution of 0.75 Å. Data were harvested by collecting 2982 frames at intervals of 0.5º scans in ω and φ with exposure times of 10 s per frame.32 A successful solution by the direct methods of SHELXS 2013 provided all nonhydrogen atoms from the E-map. All non-hydrogen atoms were refined with anisotropic displacement coefficients. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighbouring atoms with relative isotropic displacement coefficients.33

General procedures for the hydrogenation reactions



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Hydrogenation with formic acid An autoclave reactor was charged with levulinic acid (20 mmol) catalyst (0.02 mmol/0.1 mol%), formic acid (20 mmol) and KOH (20 mmol). The reactor was purged several times with nitrogen gas and the mixture was stirred at 120 °C for 16 h. After the reaction time the mixture was cooled to room temperature and the gas generated in the course of the reaction was released. A sample of the reaction mixture was taken and analyzed by 1H NMR spectroscopy.

Hydrogenation with molecular hydrogen An autoclave reactor was charged with levulinic acid (20 mmol) catalyst (0.02 mmol/0.1 mol%), hydrogen gas (20 bar) and KOH (1 mmol/5 mol%). The reactor was purged several times with nitrogen gas and the mixture was stirred at 120 °C for 16 h. After the reaction time the mixture was cooled to room temperature and the gas generated in the course of the reaction was released. A sample of the reaction mixture was taken and analyzed by 1H NMR spectroscopy.

Results and discussion Synthesis of ruthenium complexes Compounds L1,31 L229,34 were prepared according to literature as illustrated in Scheme 1. Complexes 1,31 229 and 2b29 were also synthesized according to literature as illustrated in Scheme 2. Treatment of dichloromethane solutions of complexes 1 and 2 with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF) in one molar equivalents for 1 h gave the corresponding cationic complexes 1a and 2a as orange brown solids in high yields of 8396% yields. Complex 1b was synthesized in a similar manner as reported for 2b,29 by treating 1 with one molar equivalent of sodium tetraphenylborate (NaBPh4) in dichloromethane for 20 h 10 ACS Paragon Plus Environment

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and was obtained as an orange solid in 100% yield. The complexes were characterized by 1H, 13

C{1H} and

31

P{1H} NMR spectroscopy, ESI-MS and in selected cases single crystal X-ray

crystallography. As expected, there were significant shift in the 1H, 13C{1H} and 31P{1H} NMR spectra for all the cationic complexes relative to their neutral analogues as reported in literature.29,31 Mass spectroscopic data show positive molecular ions of the cationic species and negative molecular ions for the anionic counter ions for all the complexes.

a

(C₆H₅)₂PCl (12 h)/(C2H₅O)₂PCl (2 h), toluene/THF, room temperature

Scheme 1: Synthesis of pyrazolylphosphinite and pyrazolylphosphite compounds.



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R1

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R1 N

R2

N

Cl

P O

1

N

b N

Ru P

R2

O

R

Cl R2

R2

R1

1: R1 = Me, R2 = ethoxy 2: R1 = Me, R2 = Ph c

+

R1

Cl

X-

R2

Ru

N

P

N

O

1a: R1 = Me, R2 = ethoxy, X = BArF 1b: R1 = Me, R2 = ethoxy, X = BPh4 2a: R1 = Me, R2 = Ph, X = BArF 2b: R1 = Me, R2 = Ph, BPh4

R2

R1 b

[Ru(p-cymene)Cl2]2, DCM, room temperature, 16 h. cNaBArF (1 h)/NaBPh4 (20 h), room temperature.

Scheme 2 Synthesis of pyrazolylphosphinite and pyrazolylphosphite ruthenium complexes.

Molecular structures of complexes 1a and 1b The solid state structures of complexes 1a and 1b were analyzed using single crystal X-ray crystallography. The crystal data and structure refinement parameters for these complexes are presented in Table 1. The crystals for complex 1a were grown in a dichloromethane/diethylether mixture at -4 oC for a week and were obtained as shiny brown crystals while 1b was obtained from a mixture of dichloromethane and dimethyl sulfoxide at room temperature. Complexes 1a and 1b crystalized in P21/c and P21 space groups respectively. The crystal structures obtained for 1a and 1b as shown in Figures 1 and 2 confirmed the successful abstraction of one chloride from 12 ACS Paragon Plus Environment

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the ruthenium center that allowed the coordination of the pyrazole nitrogen atom to the ruthenium metal. As expected, these complexes assumed a piano-stool structure about the ruthenium center with the aromatic ring of the p-cymene showing η6-bonded to the ruthenium.35 The seven-membered heterocyclic rings formed as a result of the bonding of the nitrogen atom to the ruthenium center are non-planar as expected for non-aromatic rings. Selected bond distances and angles of 1a and 1b are represented as captions under each molecular structure. These data are in accordance with those observed for complexes 2 and 2b.29 The Ru–Cl bond length (2.3946(14) Å) and Ru–P (2.2719(16) Å) bond length recorded for 1a are relatively longer compared to those of 1b (i.e. 2.3842(18) Å and 2.2592(12) Å respectively). The Ru–N bond lengths for 1a and 1b were found to be 2.134(4) Å and 2.136(3) Å respectively and these are consistent with Ru–N (N from pyrazole) bond lengths ranging from 2.106 to 2.141 Å reported for similar complexes.29,36,37



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Table 1: Crystallographic data for complexes 1a and 1b.

Empirical formula Formula weight Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° Volume/Å3 Z ρcalcg/cm3 Radiation Theta range for data collection/° Reflections collected Independent reflections Data/restraints/param eters Goodness-of-fit on F2 Final R indexes [I > = 2σ (I)] Final R indexes [all data]

1a C53H47BClF24N2O3PRu 1394.65 293(2) monoclinic P21/c 25.081(2) 18.1195(15) 25.559(2) 90 93.531(2) 90 11593.3(17) 8 1.596 MoKα (λ = 0.71073) 1.626 to 43.468

1b C45H55BClN2O3PRu 850.20 99.96 monoclinic P21 11.291(6) 17.801(10) 11.563(9) 90 118.637(9) 90 2040(2) 2 1.383 MoKα (λ = 0.71073)

104547 13701 [Rint = 0.0375, Rsigma = 0.0212] 13701/0/1607

28356 9931 [Rint = 0.0482, Rsigma = 0.0531]

1.031 R1 = 0.0518, wR2 = 0.1138 R1 = 0.0632, wR2 = 0.1236

1.021

4.11 to 57.272°

9931/13/519

R1 = 0.0287, wR2 = 0.0635 R1 = 0.0311, wR2 = 0.0649


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Page 15 of 33

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Figure 1: Molecular structure for complex 1a. The BArF- ion is omitted for clarity. Selected bond lengths (Å) and bond angles (deg.): Ru(1)–Cl(1), 2.3946(14); Ru(1)–P(1), 2.2719(16); Ru(1)–N(2), 2.134(4), Ru(1)–C(13), 2.229(5); Ru(1)–C(14), 2.282(6); Ru(1)–C(15), 2.273(6); Ru(1) C(16), 2.234(6); Ru(1)–C(17), 2.194(5); P(1)–Ru(1)–Cl(1), 83.52(5); N(2)–Ru(1)–Cl(1), 85.37(11); N(2)–Ru(1)–P(1), 94.10(12); O(1)–P(1)–Ru(1) 119.62(16), N(1)–N(2)–Ru(1), 127.5(3); O(3)–P(1)–Ru(1), 117.28(17);

O(2)–P(1)–Ru(1),

109.1(12).


109.0(15);

O(2A)–P(1)–Ru(1),


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 2: Molecular structure for complex 1b. Selected bond lengths (Å) and bond angles (deg.): Ru(1)–Cl(1), 2.3842(18); Ru(1)–P(1), 2.2592(12); Ru(1)–N(2), 2.136(3); Ru(1)–C(17), 2.327(3); Ru(1)–C(13), 2.235(3); Ru(1)–C(18), 2.284(3); Ru(1)–C(14), 2.175(3); Ru(1)–C(15), 2.187(3); P(1)–Ru(1)–Cl(1), 84.60(4); N(2)–Ru(1)–Cl(1), 86.10(8); N(2)–Ru(1)–P(1), 93.05(9); N(1)– N(2)–Ru(1), 125.7(2) ; C(2)–N(2)–Ru(1), 128.2(2); O(3)–P(1)–Ru(1), 118.05(10); O(2)–P(1)– Ru(1), 110.90(9) ; O(1)–P(1)–Ru(1), 117.44(10).

Hydrogenation of LA with formic acid The hydrogenation reactions were first carried out with formic acid as a hydrogen source in 1:1 ratio with respect to LA. Contrary to the traditional method whereby the reaction occurs in a solvent medium6,10,18-21 our reactions were performed in a solvent-free medium in all cases. The reactions were conducted at relatively low temperature of 120 °C and at catalyst loading of 0.1%. 16 ACS Paragon Plus Environment

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In the absence of catalyst precursor no significant reaction was observed (9% conversion). In the presence of the catalyst precursors there was at least 50% conversions of the LA to GVL and traces of 4-HVA (Figure S1) as intermediate and in some cases only GVL was produced (Table 2; entries 1 and 4). After 16 h there was 79% conversion of LA observed with neutral catalyst precursor 2. However, 96% conversion was observed with the neutral catalyst precursor 1, showing the effect of the substituents on the phosphorus atom. The reactions were more selective for formation of the desired product (GVL). As can be seen in Table 2, catalyst precursors 1 and 1b gave 100% selectivity while the rest of the catalyst precursors gave relatively low amounts (10-14%) of the intermediate product (4-HVA) in addition to GVL. In general, the neutral catalyst precursors exhibit higher activity as compared to their cationic counterparts. The displacement of the chloride ions by the pyrazolyl nitrogen in forming the cationic complexes might have rendered the complexes more stable to dissociate as a result of chelate ring formation, thereby preventing facile formation of the active species.

In the course of the catalytic reactions, a pressure of about 15 bar was generated in the autoclave reactors. The pressure increase is expected to be due to initial decomposition of the formic acid to CO2 and H2 catalyzed by the ruthenium complexes. Similar observation was made by Deng et al. where they performed their reaction with an in situ generation of the active catalytic species from RuCl3.3H2O and PPh3.12 We studied the catalytic decomposition of the formic acid by monitoring a small scale reaction in a J Young NMR tube. The reaction was first performed in the absence of LA. The pre-catalyst 2a, KOH and formic acid were loaded into a J Young NMR tube and heated in an oil bath at 120 °C. After 0.33 h, we observed formation of hydrogen gas, which was analyzed by 1H NMR spectroscopy. The chemical shift of the H2 appeared as a singlet



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at 4.56 ppm (Figure S2). To the best of our knowledge, beside the recent report by Cervantes and co-workers25 using a palladium catalyst, this is the first example of H2 formation from formic acid catalyzed by ruthenium. When the reaction was continued for 2 h, the intensity of the H2 peak increased gradually, indicating further decomposition of the formic acid to produce more H2. In the course of the decomposition of the formic acid, there was a cleavage of the p-cymene auxiliary ligand from the ruthenium center and this was observed as a free molecule in the 1H NMR spectrum (Figure S2). The 31P{1H} NMR also shows a chemical shift change from 124.7 ppm to 94.8 ppm after the p-cymene ligand was cleaved off (Figure S3). The cleavage of the pcymene from the ruthenium center could be the initial stage which allows partial coordination of the formate to the ruthenium center followed by subsequent decomposition. A similar observation was made when LA was included in the reaction mixture.

The role of the base was to deprotonate the formic acid (FA) to the formate ion, which can then coordinate to the metal center in the 1st step of FA decomposition to H2 and CO2.25 Performing hydrogenation reactions with a base is also known to enhance catalytic activity by accelerating H2 heterolysis.38


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Table 2: Hydrogenation of LA using formic acid

h

Amounts of Products detected (%) GVL 4-HVA

Entry

Catalyst

Conversion (%)

1

1

96

100

0

2

2

79

86

14

3

1a

37

87

13

4

1b

51

100

0

5

2a

68

90

10

6

2b

63

86

14

Conditions: LA 20 mmol; catalyst precursor 0.02 mmol (0.1 mol%); formic acid 20 mmol; KOH 20 mmol; 120 °C; 16 h. Conversions were estimated from 1H NMR spectra.

Hydrogenation of LA with molecular hydrogen We have also demonstrated solvent-free hydrogenation of LA to GVL using molecular hydrogen. The reactions were first conducted at 120 °C using 20 mmol of LA, 0.02 mmol (0.1 mol%), 5 mol% of KOH and 20 bar of hydrogen gas for 16 h (Scheme 3). At these reaction conditions, all the six catalysts performed very well with excellent conversions (Figure 3). With the exception of 1, which gave 73% conversion, over 80% conversion was obtained with the rest of the catalyst precursors. Catalyst precursor 2a performs exceptionally well giving the highest conversion of 100%. Generally, there was much improvement in catalytic activity of the catalyst precursors with the high pressure reaction (using hydrogen gas) as compared to the reactions performed with formic acid. One interesting trend in performance observed is the high activity 19 ACS Paragon Plus Environment


exhibited by 1a and 2a compared to 1b and 2b. This could be attributed to the high solubility of 1a and 2a since they have bulky organic fluorine-containing moieties on the counter ion. The tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ion is more polar and hence more soluble in polar LA phase than the tetraphenylborate counter ion of 1b and 2b.

Scheme 3: Hydrogenation of LA to GVL using molecular hydrogen.

120 100 Conversion/%

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100

94 84

80

90

84 73

60 40 20 0 1

2

1a

1b

2a

2b

Catalyst Figure 3: Hydrogenation of LA to GVL using molecular hydrogen. LA 20 mmol; catalyst precursor 0.02 mmol (0.1 mol%); H2 20 bar; KOH 1 mmol (5 mol%) ; 120 °C; 16 h. Conversions were estimated from 1H NMR spectra. 100% selectivity.

In addition to the excellent performance observed for the reactions conducted with molecular hydrogen, high selectivity towards the formation of GVL was also achieved. Contrary to the 20 ACS Paragon Plus Environment

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reports on the poor selectivity associated with the use of molecular hydrogen for LA hydrogenation,27 we observed 100% selectivity towards GVL in all our reactions in 12 h. Typically, as illustrated in Figure 4, in the time dependence study carried out using 2a (monitored by 1H NMR spectroscopy) the reaction proceeded directly to form GVL, and no amounts of 4-HVA intermediate were observed by NMR spectroscopy.

Figure 4:

1

H NMR spectra of reactions involving 2a. No traces of 4-HVA observed. LA 20

mmol; catalyst precursor 0.02 mmol (0.1 mol%); H2 20 bar; KOH 1 mmol (5 mol%) ; 120 °C; 16 h. Conversions were estimated from 1H NMR spectra.



Detailed time dependence studies conducted on catalysts 1a and 2a are shown in Figure 5. The reaction was followed closely at an interval of 4 h. Catalyst precursor 2a shows excellent activity and hence gives a shorter induction period compared to 1a. After 4 h we observed 60% conversion with 2a and the reaction then proceeded gradually to give 100% conversion in 12 h. Catalyst precursor 1a showed a longer induction period and lower reaction activity resulting in only 17% conversion after 4 h. The reaction became faster after 4 h to give 79% conversion in 12 h.

120 100

100 Conversion/%

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90 84

79

80 66 61

60

1a

2a

40 17

20 0 0 0

5

10 Time/ h

15

20

Figure 5: Time dependent study of LA hydrogenation with molecular hydrogen. LA 20 mmol; catalyst precursor 0.02 mmol (0.1 mol%); H2 20 bar; KOH 1 mmol (5 mol%) ; 120 °C; 16 h. Conversions were estimated from 1H NMR spectra.

Effects of Temperature, Hydrogen Pressure and Substrate Catalyst Ratio


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A primary concern about reduction of LA to GVL is the high temperatures or pressures required in many homogeneously and heterogeneously catalyzed systems. As one is concerned about selectivity of the reaction toward GVL, parameters such as temperature must be modulated in order to achieve product specificity. Chalid and co-workers12 demonstrated that reactions performed at lower temperatures result in the formation of 4-HVA selectively whereas at high temperatures GVL is selectively formed. In view of this, we decided to decrease the temperature in order to establish effect of temperature on the selectivity and activity of the catalysts. Interestingly, we noticed that in decreasing the temperature from 120 oC to 100 oC using catalyst precursor 2a, selectivity toward GVL was maintained at 100%. However, the conversion decreases slightly to 89% at 100 oC while there was 100% conversion at 110 oC (Table 3; entries 2 and 3). Thus, the catalysts can perform well with high selectivity even at low temperatures. We have also studied the hydrogen pressure effect on the catalytic activity and selectivity of the reaction using 2a. The initial hydrogen pressure used to obtain complete conversion with 100% selectivity was 20 bar. When the pressure was reduced to 10 bar at 110 oC, there was only a slight decrease in conversion (Table 3; entry 5, 96% conversion). However, decreasing the hydrogen pressure further to 5 bar resulted in a drastic reduction of the conversion of the LA (Table 3; entry 6, 32%) while 100% selectivity was maintained. The temperature and pressure trend observed in our work was in accordance with earlier reports.14

The catalyst loading has an influence on the conversion of LA to some extent (Table 3; entries 7 to 9). As discussed earlier, with substrate/catalyst ratio of 1000 at 15 bar and 110 oC, we observed complete conversion of LA with 100% selectivity for GVL. There was no significant decrease in LA conversion when the substrate/catalyst ratio was increased to 1600. However,



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there was a dramatic decrease in LA conversion when the substrate/catalyst ratio was increased to 2200 and 2800 resulting in 68% and 63% respectively. It should be noted here that despite the reduction in conversion of LA as a result of decreasing catalyst loadings the GVL selectivity was not affected. Reports by Li and co-workers show that the presence of strong base such as KOH, NaOH and LiOH is necessary to obtain GVL from LA in high yields.21 A similar trend was observed in our work. As shown in Table 3 (entry 10), in the absence of a base conversion was only 50%. Also apart from apart from the base accelerating H2 heterolysis, and thus enhancing catalytic activity;38 the base also helps in neutralizing HCl formed in the catalytic process.39

Table 3: Molecular hydrogenation of LA to GVL at varying conditions using 2a Entry

Temperature/ oC

H2 Pressure/ bar

n(LA)/n(Catalyst)

Conversion/%

1

120

20

1000

100

2

110

20

1000

100

3

100

20

1000

89

4

110

15

1000

100

5

110

10

1000

96

6

110

5

1000

32

7

110

15

1600

94

8

110

15

2200

68

9

110

15

2800

63

10a

110

15

1000

50

11b

110

15

1000

45

Conditions: 12 h, KOH 0.001 mol, aNo base added, bIn the presence of mercury

Mercury drop test 24 ACS Paragon Plus Environment

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In order to establish the nature of the active species responsible for the catalysis (i.e. either homogeneous or heterogeneous or both), a mercury drop test or poisoning experiments were conducted. The reactions were performed using pre-catalyst 2a and one molar equivalent of elemental mercury and molecular hydrogen. In the presence of mercury, the catalyst can only give 45% conversion of LA (Table 3; entry 11). This is an indication that both homogeneous and in situ generated heterogeneous species, which has been referred to as “cocktail” of catalysts,40 are responsible for the catalytic activity. Some of the pre-catalysts might have decomposed to form ruthenium nanoparticles in the course of the reaction, which could also catalyze the transformation. The added mercury scavenges the ruthenium nanoparticles and inhibits catalytic activity, while the residual molecular catalytic species continue to catalyze the reaction, albeit to a minimal extent as shown by the reduced conversion.

Catalyst Recyclability The recyclability of pre-catalyst 2a was performed using 20 mmol of LA, 0.02 mmol of 2a, and 5 mol% of KOH relative to LA at 110 oC and 15 bar for 12 h. This resulted in 100% conversion of LA after 12 h. The crude mixture was dissolved in ethanol and transferred into a Schlenk tube. The volatiles (ethanol, GVL and water) were removed under vacuum at 85 oC leaving the catalyst behind.27 The catalyst was carefully washed back into the reactor using 5 mL of ethanol and was dried in a vacuum oven at 40 oC. After complete removal of the solvent the reactor was recharged with 20 mmol of LA and 15 bar hydrogen gas and the mixture was heated at 110 oC for 12 h. This procedure was repeated until the fourth recycling. The second and third recycling resulted in 94% and 91% conversion respectively of LA (Figure 6). The fourth cycle yielded only 49% conversion of LA. This means deactivation of the catalysts occurs significantly after



the third cycle. It is important to note here that the selectivity of the catalyst for GVL remains 100% till the fourth recycling.

120 100 Conversion/%

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Page 26 of 33

100

94

91

80 60

49

40 20 0 Run 1

Run 2

Run 3

Run 4

Figure 6: Catalyst recyclability studies for 2a. LA 20 mmol; catalyst 0.02 mmol (0.1 mol%); H2 15 bar; KOH 1 mmol (5 mol%); 110 °C. Conversions were estimated from 1H NMR spectra.

Conclusions We have demonstrated here that ruthenium(II) complexes anchored on pyrazolylphosphite and pyrazolylphosphinite ligands are good catalysts for neat hydrogenation of LA to GVL. All the reaction conditions used are ecofriendly, which is the ideal way of contributing to the sustainability. The catalysts are very selective toward the formation of GVL; especially with hydrogen gas. The LA conversion is 100% with hydrogen gas and selectivity towards forming GVL is 100% at 110 °C and 15 bar. Trace amounts of 4-HVA were observed only in reactions where formic acid was used as the hydrogen source. We also found out that increasing the


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catalyst loading decreases the conversion of the LA. Furthermore, the activity and selectivity of the catalysts is maintained up to the third recycling.

Acknowledgements We gratefully acknowledge funding for this project from the National Research Foundation (South Africa) and the University of Johannesburg. Our profound gratitude also goes to Mr. Lee Madeley, Dr Charmaine Arderne and Dr Collins Obuah for their help with the single crystal Xray analysis.

Supporting information Electronic supporting information (ESI): This material is available free of charge. Crystallographic data has been deposited with the Cambridge Crystallographic Data Centre with CCDC 1412964 (1a) and 1412965 (1b). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223 336063; [email protected] or http://www.ccdc.cam.ac.uk).

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Efficient Solvent-Free Hydrogenation of Levulinic Acid to γ-Valerolactone by Pyrazolylphosphite and Pyrazolylphosphinite Ruthenium(II) Complexes Gershon Amenuvor, Banothile C. E. Makhubela,* and James Darkwa*

Highlights: Pyrazolylphosphite and pyrazolylphosphinite ruthenium(II) complexes are highly efficient pre catalysts for γ-valerolactone (GVL) formation from levulinic acid (LA) through a solvent-free process.

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Efficient Solvent-Free Hydrogenation of Levulinic Acid to Î³

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