Article pubs.acs.org/Organometallics
Efficient Iridium Catalysts for Base-Free Hydrogenation of Levulinic Acid S. Wang,† H. Huang,† V. Dorcet,‡ T. Roisnel,‡ C. Bruneau,† and C. Fischmeister*,† †
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, , Organometallics: Materials and Catalysis, Centre for Catalysis and Green Chemistry, Université de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex, France ‡ Centre de Diffractométrie X Institut des Sciences Chimiques de Rennes UMR 6226 CNRS, Université de Rennes 1, F-35042 Rennes Cedex, France S Supporting Information *
ABSTRACT: The synthesis and characterization of new dicationic ruthenium and iridium complexes bearing a dipyridylamine ligand (dpa) are reported. These complexes display an unusual zwitterionic molecular structure in the solid state. The iridium complex [Cp*Ir(dpa)(OSO3)] (Ir1) was found to be very efficient in base-free hydrogenation of levulinic acid into γ-valerolactone (GVL). TONs as high as 174000 in hydrogenation have been obtained. We have demonstrated that reduction of LA into GVL by transfer hydrogenation with formic acid is in fact operating by hydrogenation fed by preliminary formic acid dehydrogenation. A mechanism based on the characterization and isolation of Ir− H complexes is proposed.
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and pressure of 140 °C and 100 bar, respectively.7 Zhou reported an impressive TON of 71000 obtained with an iridium trihydride pincer catalyst. The reaction could be run at 100 °C, but a high pressure of hydrogen was required (100 bar) and the addition of 1.2 equiv of base was necessary.8 Another iridium catalyst reported by Fu reached a higher TON of 78000 under different reaction conditions (120 °C, 10 bar of H2) without additive.9 Transfer hydrogenation processes have received less attention despite the perspective of running the reaction with formic acid (FA) as a reducing agent, which is a coproduct of LA synthesis from carbohydrate acidic depolymerization.10 The transfer hydrogenation of LA was reported with a TON as high as 2400 by Horváth using the bifunctional Shvo catalyst11 and a TON of 9800 by Fu with an iridium catalyst.9 Both systems operate under additive-free conditions. Recently, following previous results on the synthesis of dipyridylamine-containing ruthenium catalysts,12a we have reported new ruthenium and iridium complexes for the transfer hydrogenation of LA into GVL.12b TONs up to 2760 were obtained, but unlike transformations using Shvo’s catalyst, these catalysts required a base, which limited their potential development. In this paper we present our latest developments in the synthesis, characterization, and catalytic performances of new ruthenium and iridium complexes operating under base-free conditions.
INTRODUCTION Reduction processes play a crucial role in organic syntheses both in academia and industry. Many efficient and selective transformations have been discovered, leading to tremendous developments and implementation of reduction transformations. The growing demand for biosourced compounds is currently fostering the demand for efficient and robust catalysts able to address the transformation of polyfunctional biosourced chemicals into useful chemicals.1 Levulinic acid (LA), one of the biosourced platform chemicals, has attracted strong interest as a precursor of a variety of compounds with a broad range of applications.2 One specific application concerns the reduction of levulinic acid into γ-valerolactone (GVL), a chemical with multiple utilities.3 The synthesis of GVL from LA hence represents an intrinsic interest as a useful biosourced compound, but it is also a challenging transformation for the design and development of reduction catalysts with enhanced performances that could be implemented in other reduction reactions not necessarily involving renewables. Homogeneous and heterogeneous catalysts have been reported for the efficient reduction of LA into GVL. Heterogeneous catalysts are undoubtedly interesting for their easy separation procedures and for the possibility of operating under continuous flow, but their activity is usually low and they require high reaction temperatures, typically higher than 150 °C.4 In 1977, Joó reported the homogeneous hydrogenation of levulinic acid with ruthenium complexes.5 Since then, a number of homogeneous catalysts with improved performances have been reported, of which ruthenium and iridium complexes led to the best results.6 In hydrogenation, Mika reported a TON of 12740 with a [Ru(acac)3]/DPPB catalyst operating under solvent- and additive-free conditions but at the rather high temperature © XXXX American Chemical Society
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RESULTS AND DISCUSSION Synthesis and Characterization of New Complexes. We previously reported the cationic ruthenium chloride and iridium chloride complexes A and B bearing dipyridylamine Received: July 4, 2017
A
DOI: 10.1021/acs.organomet.7b00503 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
plexes of the general formula [Cp*Ir(bpy)(H2O)]SO4, have been used in various reduction reactions.14,15 Inspired by these chloride-free catalysts bearing a labile aquo ligand, we attempted the synthesis of the dicationic version of complexes A and B (Scheme 1). Having already prepared A and B, we synthesized the new complexes Ru1, Ir1, and Ir2 by abstraction of the chloride ligand with silver sulfate. To our surprise, the complexes were not isolated as dicationic aquo complexes as reported in the literature for Cp*Ir(bipyridine) complexes but as zwitterionic complexes bearing a coordinated sulfato ligand, as evidenced by X-ray analysis of crystals grown in an MeOH/ Et2O mixture (Figure 1). In fact, previously reported bipyridine-containing complexes [Cp*Ir(bpy)(OH2)][SO4] were prepared by reaction of [Cp*Ir(OH2)3][SO4] with bipyridine derivatives and crystals were grown from the aqueous mother liquors. This different procedure may explain the variation in the molecular structures obtained in the solid state. Since the nature of the crystallization solvent may have an influence on the coordination sphere of these complexes, particularly with MeOH, which leads to H bonding with coordinated [OSO3] (Figure 1a), we crystallized the same complex in a CH2Cl2/Et2O mixture. Despite this solvent change, the nature of the complex remained zwitterionic. In this case, intermolecular H bonding between coordinated [OSO3] and N−H was evidenced in the molecular structure (Figure 1b). Ir2 and Ru1 also featured a zwitterionic molecular structure, as depicted in Figure 1c,d, respectively. Unfortu-
ligands (dpa) (Scheme 1). These complexes were efficient in the transfer hydrogenation of LA with FA, provided 2 equiv of Scheme 1. Synthesis of New Complexes Ru1, Ir1, and Ir2
triethylamine was added to the reaction mixture. These complexes were also found to be poor hydrogenation catalysts.12b The latter result could find its origin in the absence of free coordination sites, preventing the formation of a dihydrogen complex intermediate precursor of metal hydride or dihydride species.13 Recently, numerous dicationic metal bipyridine complexes, in particular proton-responsive com-
Figure 1. Molecular structure of Ir1 (a) crystallized in MeOH/Et2O and (b) crystallized in CH2Cl2/Et2O and structures of Ir2 (c) and Ru1 (d). Thermal ellipsoids are plotted at the 50% probability level. Solvent and H are omitted except to highlight H bonding in (a) and (b). B
DOI: 10.1021/acs.organomet.7b00503 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 2. Synthesis of [Cp*Ir(bpy)(OSO3)] (Ir3)
nately, we did not succeed in growing crystals of Ir1 or Ir2 in water. To better understand the origin of these different solid-state molecular structures, the synthesis of the complex [Cp*(bpy)Ir][SO4] (Ir3) was performed as described in the literature (Scheme 2) and crystals were grown in a CH2Cl2/Et2O mixture instead of water. Using this procedure, the complex crystallized in a zwitterionic structure with a sulfato ligand displaying H bonding with a water molecule (Figure 2), demonstrating that
Scheme 3. Transfer Hydrogenation of LA
As depicted in Table 1, iridium-based catalysts were active under base-free conditions, as expected from removal of the chloride ligand in these complexes in comparison to the catalytic activity of B1 and B2.12b A preliminary screening under neat conditions revealed the best performances obtained with Ir1 vs Ir2, whereas Ru1 was inactive under similar conditions (Table 1, entries 1−6). Ir1 was thus selected for further studies in water, where the best results were obtained at 110 °C with 1 and 0.5 M concentrations of levulinic acid (Table 1, entries 7−10). The catalyst loading could be further decreased to 0.01 mol %, but the reaction required longer times to reach full conversion and a high yield of γ-valerolactone (Table 1, entry 13). Under these conditions, a TON of 9000 was reached, which is close to the highest TON reported so far for the transfer hydrogenation of LA (9800).9 Further attempts to increase the reaction productivity were not successful (Table 1, entries 14 and 15). No conversion was observed using isopropyl alcohol instead of formic acid as a hydrogen donor with all three catalysts (0.05 mol % of catalysts, 110 °C, 16 h, iPrOH (2 mL)). In 2009, Fu and Guo observed a rapid increase of pressure in a reactor where a ruthenium-catalyzed transfer hydrogenation of LA was supposed to take place.10b They postulated the fast dehydrogenation of formic acid into H2 and CO2 and concluded that GVL was obtained by hydrogenation of LA rather than transfer hydrogenation. The possible contribution of hydrogenation during the transfer hydrogenation of LA catalyzed by Ir1 was considered, since this catalyst may also be an efficient FA dehydrogenation catalyst, as is the case for many cationic Cp*Ir(bipyridine) and related complexes.17 In order to fully address this issue, a series of experiments was conducted, bearing in mind that the potential hydrogenation of LA during a transfer hydrogenation experiment cannot be strictly compared to a hydrogenation reaction due to different metal hydride formation pathways at the onset of the reaction. In contrast to conventional hydrogenation requiring activation of H2 into M-H or M(H2), hydrogenation occurring in a transfer reaction process with formic acid will be initiated by the formation of M-H from formic acid dehydrogenation. Ir1 was first evaluated in the dehydrogenation of FA under experimental conditions identical with those used for TH of levulinic acid (FA, 4 mmol; Ir1, 0.05 mol % vs FA; 110 °C) using a eudiometer system to measure the volume of released gas. Ir1 was indeed found to be an efficient formic acid dehydrogenation catalyst leading to complete dehydrogenation of FA in 1 h (see Figure S2 in the Supporting Information). The dehydrogenation was also measured under GVL synthesis conditions (LA, 2 mmol; FA, 4 mmol; Ir1, 0.1 mol % vs LA;
Figure 2. Molecular structure of [Cp*Ir(bpy)OSO3] (Ir3). H atoms are omitted except in the H-bonded water molecule. Thermal ellipsoids are plotted at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ir1−N11, 2.094; Ir1−N22, 2.091; Ir1− O1, 2.080; Ir1−Cp* centroid, 1.783; N11−Ir1−N22, 77.07; N11− Ir1−O1, 86.17; N22−Ir1−O1, 79.68.
these solid-state structures are highly dependent on the crystallization solvents.16 However, it is very likely that these complexes are present as ion pairs when they are dissolved in water. Catalysis. Transfer Hydrogenation. The three complexes Ru1, Ir1, and Ir2 were evaluated in the reduction of levulinic acid into γ-valerolactone. In the literature, transfer hydrogenation and H2 hydrogenation processes are often opposed against each other. Transfer hydrogenation is rightly considered as a safer transformation than hydrogenation, the latter being however a more atom efficient reaction. In order to get a full overview of the potential of our new catalysts, we investigated their efficiency in both base-free transfer hydrogenation and direct hydrogenation with hydrogen. The transfer hydrogenation of LA with our catalysts was first investigated by varying temperature, catalyst loading, and solvents (Scheme 3). C
DOI: 10.1021/acs.organomet.7b00503 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Transfer Hydrogenation of Levulinic Acida
a
entry
cat. (amt (mol %))
t (h)
T (°C)
1 2 3 4 5 6c 7 8 9 10 11 12 13 14 15
Ru1 (0.1) Ir1 (0.1) Ir2 (0.1) Ir1 (0.1) Ir1 (0.1) Ir1 (0.1) Ir1 (0.1) Ir1 (0.1) Ir1 (0.1) Ir1 (0.1) Ir1 (0.05) Ir1 (0.05) Ir1 (0.01) Ir1 (0.001) Ir1 (0.001)
16 16 16 16 16 16 16 16 16 16 16 24 72 16 16
110 110 110 120 140 120 110 110 110 110 110 110 110 110 130
solvent neat neat neat neat neat neat H2O, H2O, H2O, H2O, H2O, H2O, H2O, H2O, H2O,
conversn (%)b
85 [LA] [LA] [LA] [LA] [LA] [LA] [LA] [LA] [LA]
= = = = = = = = =
2 mol L−1 1 mol L−1 0.5 mol L−1 0.33 mol L−1 1 mol L−1 1 mol L−1 1 mol L−1 1 mol L−1 1 mol L−1
100
100 92
