Next-Generation Water-Soluble Homogeneous Catalysts for

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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Next-Generation Water-Soluble Homogeneous Catalysts for Conversion of Glycerol to Lactic Acid Matthew Finn, J. August Ridenour, Jacob Heltzel, Christopher Cahill, and Adelina Voutchkova-Kostal* Department of Chemistry, The George Washington University, 800 22nd Street NW, Washington, DC 20052, United States S Supporting Information *

ABSTRACT: An attractive method for valorization of glycerol is the catalytic transformation to lactic acid. By overcoming the solubility challenge associated with known homogeneous catalysts for this reaction, we show that thermally robust Ir(I), Ir(III), and Ru(II) N-heterocyclic carbene (NHC) complexes with sulfonate-functionalized wingtips are highly prolific for this process, requiring no cosolvents other than aqueous base. The activity of the catalysts is compared under both conventional heating and microwave conditions. The most active catalyst reaches a TOF of 45 592 h−1 (microwave) and 3477 h−1 (conventional) with 1 equiv of KOH, and proceeds at a constant rate for at least 8 h. Although higher activity is observed with KOH, the catalysts are also highly active with the weaker base, K2CO3 (13 000 h−1 and concurrent formation of formate). The protocol can be modified to achieve quantitative conversion of glycerol in only 3 h. The high activity of these catalysts compared to nonsulfonated analogs is attributed to the stabilization the lactate product in aqueous media. The most active catalyst retains equal activity for crude glycerol. A mechanism is proposed for the most active catalyst precursor involving O−H oxidative addition of glycerol.

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INTRODUCTION The last few decades have seen a rapid rise in the production of biodiesel from crops: In 2011, worldwide production exceeded 1.5 million tons, and continues to grow.1 Glycerol, the major byproduct, amounts for ∼10 wt % of biodiesel produced.1,2 The abundance of glycerol produced and the lack of viable valorization technologies has caused its market price to approach zero.3 This has fueled research into new valorization processes, such as conversion to propane diols,4 glyceric acid,5 cyclic acetals,6 and acroelin.7 One particularly attractive valueadded product of glycerol is lactic acid: a versatile platform chemical with a rich portfolio of applications, including food, cosmetics, pharmaceuticals, fine chemicals,8 and the synthesis of polylactic acid (PLA).9 Use of PLA is growing rapidly due to the shift toward biodegradable and renewable alternatives to petrochemical-derived plastics.10 Furthermore, the traditional fermentation process for producing lactic acid2b,10,11 is difficult to scale, requires complicated purification and workup, and has limited productivity.12 The production of lactic acid from glycerol could offer scalable, atom-economical processes that utilize a low-value feedstock. A summary of the reported homogeneous catalysts for this transformation is presented in Scheme 1. The seminal report of the catalytic conversion of glycerol to lactic acid by Crabtree et al.18 (Scheme 1) proposed that acceptorless dehydrogenation of glycerol forms dihydroxyactone (DHA) or glyceraldehyde, © XXXX American Chemical Society

which then undergo dehydration and intramolecular Cannizaro reaction to afford lactic acid. The most active catalyst identified was an Ir(I) N-heterocyclic carbene (NHC) complex, which afforded 337 turnovers per hour at 115 °C (see Scheme 1).13 Subsequently, Williams et al. reported that a highly robust NHC-pyridine Ir(I) complex reaches 4.5 million turnovers in 32 days, with an average TOF of 6000 h−1 at 145 °C under neat conditions and base.14 A polymeric iridium catalyst reported by Tu et al. allowed recyclability, with an average TOF of 3444 turnovers/h.15 In addition to electron-donating NHC ligands,16 phosphine ligands have also been found useful in designing catalysts for this process: for example, Beller reported that a Ru PNP pincer compound reaches 10 318 turnovers per hour with 67% selectivity for lactic acid at 140 °C in the presence of diglyme as cosolvent.17 A similar PNP ligand was utilized by Hazari et al. to develop an iron catalyst that averages 293 turnovers per hour using NMP as cosolvent.18 Although some of these catalysts are highly active, to develop a homogeneous catalyst that is potentially viable for large-scale conversion of a low-value chemical such as glycerol, exceptional activity must be achieved. Improvements of catalytic activity can be made by addressing the following: (i) the low solubility of these homogeneous catalysts in polar protic media; (ii) poor Received: February 8, 2018

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complexes have high aqueous solubility and are highly charged, enabling them to take advantage of microwave heating effects. All complexes share a motif of at least one NHC ligand functionalized with sulfonates on the N-wingtip. We propose that sulfonate groups are particularly well-suited to enhance solubility in polar protic solvents, while having minimal influence on the electron-donating ability of the NHC ligand and metal coordination.26 This report builds on initial findings by Peris et al. and by our group, which demonstrate the activity of sulfonated NHC complexes for transfer hydrogenation of ketones,22,27 aldehydes,22,27 and imines27 from glycerol, where we observed formation of lactic acid.

Scheme 1. Catalysts Reported for the Synthesis of Lactic Acid from Glycerol via Acceptorless Dehydrogenation

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RESULTS AND DISCUSSION Catalyst Synthesis. Eleven iridium and ruthenium complexes bearing alkyl- and arylsulfonate-functionalized NHC ligands (Charts 1 and 2) were synthesized and

Chart 1. Catalysts for Acceptorless Dehydrogenation of Glycerola

stabilization of lactate product in aprotic solvents; (iii) the formation of deactivated polyhydride clusters, which have been identified for Ir-NHC catalysts;19 and (iv) mass transfer limitations due to high viscosity of medium. To address these issues, we set out to design catalysts that are (i) soluble in polar media, thus allowing reactions to be carried out under aqueous conditions without added organic solvents, and (ii) active at very low catalyst loadings (high dilution decreases concentration-dependent cluster formation). Microwave heating allows rapid, accurate, and uniform heating to a set temperature,20 which can improve dispersion of the base in aqueous glycerol.21 These advantages stem from the fact that microwave heating uses an alternating electromagnetic field (AEMF) that interacts with polar molecules and ions to cause oscillation and rotation, thus converting the electric energy to kinetic energy, and subsequently to heat.22 Each reaction component has unique dielectric properties (ε′ dielectric constant and ε″ dielectric loss), which determine its ability to capture energy from the AEMF (ε′) and to convert the energy into heat (ε″). The ratio of these two is an energy dissipation factor (or loss tangent, tan δ), which can be used compare the efficiency of conversion of microwave energy to heat by polar molecules (although other factors, such as relaxation times, should also be considered). Generally, the reaction component with the most favorable dielectric properties can undergo selective heating.23 Microwave heating is also particularly well-suited to reactions with polyols, such as glycerol, which have high loss tangents (for glycerol the loss tangent at 2.45 GHz is 0.651, compared to 0.123 for water)24 even though the 2.45 GHz frequency of commercial microwaves is optimized based on the relaxation time of water. Heating efficiency in reactions in glycerol can thus be further enhanced by addition of water and ionic components, such as base and catalyst. As highlighted by Kappe in his seminal study on microwave effects, all of these factors contribute to bulk thermal effects, rather than any “non-thermal microwave effects”.25 Herein we report the design, synthesis, characterization, and application to the conversion of glycerol to lactic acid of a series of iridium and ruthenium homogeneous catalysts. The

a

New compounds designated with asterisk.

characterized with the goal of assessing their activity for the conversion of glycerol to lactic acid. The syntheses of compounds 1 and 3−5 have been previously described,28 while compounds 2 and 6−9 are first reported here. Chart 2. Non-Sulfonated Ir-NHC Catalysts Utilized for Acceptorless Dehydrogenation of Glycerola

a

B

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Organometallics Scheme 2. Synthesis of Compound 8 via Alkoxide Dimer route

2.048(5) Å, respectively, comparable to those reported for analogous Ir-NHC compounds;31 these angles also compare well to those in ([Ir(cod)(EMIm)2]NTf2 (2.067 and 2.059 Å).30 The phenyl wingtips stagger in opposite directions with torsion angles measuring 60.40 and 45.02°. The differences in torsion angle between the two ligands can be attributed to the coordinating sodium ion and closely associated water molecules that act as a bridge between two molecular units (see Figures S2 and S3 for more information). To assess the effect of the sulfonate groups on catalytic activity we also included two non-sulfonated analogs (Chart 2, 10 and 11). Compound 10, a nonsulfonated analogue of 5, was prepared using a procedure adapted from Heinkey et al.,32 while 11 was prepared via transmetalation from the respective Ag-NHC precursors.13,33 Compound 11 was screened by Crabtree et al. in the original report of this reaction13 and is a close non-sulfonated analog of 6. Single crystal X-ray diffraction of 10 shows Ir−C bond lengths of 2.039(5) and 2.026(5) Å for the NHC ligands, Ir−Cl bond length of 2.4157(12) Å, and an average Ir−C bond lengths for Cp* ligand of 2.220 Å (Figure 2). The chelating bond angle of 84.53(19)° between the iridium and two NHC ligands fall into the expected ranges.32 The carbonyl stretching frequencies (νCO) of the iridium carbonyl compounds 8, 9, and 12 (where 12 is the carbonyl analog of 11) were compared in order to assess the potential

Compounds 1−3 and 5 were prepared by silver transmetalation from the corresponding imidazolium salts, while 4 was synthesized by direct metalation using [Ir(cod)OMe]2.27 Compounds 6−9 were prepared from the [Ir(cod)OEt]2 dimer using a protocol analogous to that reported by Hermann and Jimenez (Scheme 2).29 Addition of the imidazolium salt to the dimer at room temperature resulted in clean formation of the bis-NHC Ir(I) complex, whereas Ag transmetalation with [Ir(cod)Cl]2 afforded poor yields. Compounds 1, 3, and 4 were included in light of their previously reported activity for transfer hydrogenation of aldehydes, imines, and ketones from glycerol,22,27 whereas 5 was included in order to evaluate the effect of the pentamethylcyclopentadinyl (Cp*) ligand relative to that of the acetate and iodide ligands in 4. Iridium(I) compounds 6−9 are included in light of previously reported activity of Ir(I) complexes.14,19 Finally, compounds 7 and 9, which feature arylsulfonate wingtips, allow us to compare the effect of the Nsulfonate spacer on catalytic activity and stability, as well as the effect of stronger π-acceptor CO ligands in 9 versus the weaker and more labile cyclooctadiene (COD) ligand in 7. The molecular structure of 7 was determined by single crystal X-ray diffraction (Figure 1). The crystal structure of 7

Figure 1. ORTEP diagram of 7 with ellipsoids shown at 50% probability level (noncoordinating cations, disordered lattice waters, and hydrogen atoms omitted for clarity). Select bond lengths (Å) and angles (deg): C(10)−Ir(1) 2.044(5), C(20)−Ir(1) 2.048(5), and C(10)−Ir(1)−C(20) 87.33(18).

shows a disordered square planar coordination with two phenyl wingtips in a staggered orientation with a closely coordinating sodium ion and several cocrystallized water molecules. The distortion angles between the COD-Ir plane and the carbene carbons (C(20) and C(10) respectively) are 10.00(15) and 12.78(15)°, respectively (Figure S1). These angles suggest greater disorder than that observed in an analogous N-methyl, N′-ethyl analog ([Ir(cod)(EMIm)2]NTf2 (where EMIm = 1ethyl-3-methylimidazole-2-ylidene) (distortion angles of 0.66° and 0.53°).30 The Ir−C bond distances measure 2.044(5) and

Figure 2. ORTEP diagram of 10 with ellipsoids shown at 50% probability level (noncoordinating cations and hydrogen atoms omitted for clarity). Select bond lengths (Å) and angles (deg): Ir(1)-cent 1.8471(4), Ir(1)−C(11) 2.026(5), Ir(1)−C(7) 2.039(5), Ir(1)−Cl(1) 2.4157(12), C(11)−Ir(1)−Cl(1) 91.19(13), C(7)− Ir(1)−Cl(1) 88.62(13), C(7)−Ir(1)−C(11) 84.53(19). C

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Organometallics Chart 3. FTIR νCO Stretches of Compounds 8, 9, and 12a

a

Asterisks denote data for compound 12 reported by Crabtree et al.18

effect of N-sulfonate spacers on the electron density of iridium. The symmetric νCO of 8, 9, and 12 differ by 5 cm−1 in total (Chart 3), suggesting electronic differences are negligible. Glycerol Dehydrogenation and Conversion to Lactic Acid. To identify optimal reaction conditions, we examined the activity of catalyst 1 using different temperatures, base concentrations and solvent ratios (Table 1). For ease of optimization these reactions were carried out using microwave heating and fixed to a reaction time of 3 hours.

of the reaction vessels at such temperatures. The reaction showed relatively little sensitivity to the ratio of glycerol to water in the range of 2:1 to 1:2 glycerol/water (v/v) (entries 4, 6, and 7), where the differences observed are likely due to both the changes in glycerol concentration and base equivalents (as a result of keeping the total volume constant). Complete elimination of water by using neat glycerol, however, decreased yields, likely because water reduces the viscosity of the reaction mixture and facilitates better mass transfer of base. A 1:1 ratio of glycerol/water proved optimal, providing a balance between glycerol concentration and viscosity of the reaction mixture. The reaction conditions optimized for catalyst 1 were subsequently extended to catalysts 2−11 under both conventional and microwave heating conditions (Figure 3 and Tables S4 and S5). The TOFs observed under microwave conditions were significantly higher than those obtained by conventional heating due to more efficient heating and improved mass transfer, but the relative trends in activity among the catalysts were almost identical. The three sulfonated ruthenium catalysts (1−3) were less active than the sulfonated iridium catalysts (4− 9). The Ru(bis-NHC) (1) and Ru(pyr-NHC) (2) complexes afforded comparable TOFs, suggesting that the difference in electron-donating ability of the pyridine versus NHC ligand, as well as the hemilabile nature of the NHC-pyridine ligand in 2, do not result in a significant effect on activity. Ru compound 3, which only differs from 2 by a propylsulfonate imidazolium moiety substituted on the pyridine, afforded 2−3 times higher TOF compared to 1 and 2. The higher activity of 3 could be the result of the in situ formation of a more catalytically active CNC Ru complex34 upon displacement of the p-cymene ligand. The higher activity likely for a CNC Ru complex is supported by the report by Beller et al. of an active Ru PNP pincer catalyst for this process.17 Although further optimization of the ligand architecture around the ruthenium species is likely to result in improvements of catalytic activity, in light of the significantly higher activity observed with iridium catalysts, we did not pursue further optimization of ruthenium complexes. We next examine the activity of iridium(III) sulfonated catalysts 4 and 5. The initial screen indicated that 4 is more active than 5, with the difference being more drastic under conventional relative to microwave conditions. To test whether initial substitution of the chloride ligand is responsible for the inferior activity of 5, we compared the activity with and without an excess of NaOAc (100 equiv to catalyst), which is expected to accelerate chloride substitution. The reaction with excess NaOAc doubled the activity under microwave conditions (5743 vs 2850 turnovers/h). As a control, a reaction was also performed with catalyst 4 and excess NaOAc, which resulted in

Table 1. Optimization of Reaction Conditions for Conversion of Glycerol to Lactic Acid Using Catalyst 1

entrya

temp (°C)

KOH (mmol)

KOH (equiv)

1 2 3 4 5 6 7

150 150 150 150 115 150 150

0 1.5 6 42 42 42 42

0 0.04 0.14 1.02 1.02 0.77 1.53

LA Gly/H2O (v/v) (mmol) 1:1 1:1 1:1 1:1 1:1 2:1 1:2

0 1.5 2.6 3.0 0.17 2.1 2.4

TON 0 442 1643 1983 342 1501 1708

a

Conditions: Catalyst 1 (12 ppm relative to glycerol), glycerol, and water (6 mL total in ratio indicated), KOH (quantity indicated), microwave heating in sealed 10 mL vial for 3 h at the indicated temperature; yields determined by NMR using NaOAc as an internal standard.

The quantity of base (KOH) required is stoichiometric relative to glycerol (i.e., 42 mmol), as lower quantities of base and weaker bases (such as carbonates), result in lower yields of lactic acid (LA) with this catalyst, (entries 1−4). As further evidence, in entry 2 we observe the yield of lactic acid (LA) is limited by the molar quantity of base. All reactions with stoichiometric KOH afforded solely lactic acid, with no side products of glycerol oxidation (Figure S4 shows NMR of crude reaction). However, in reactions with substoichiometric KOH traces of 1,2-propane diol (PDO) were observed. Control reactions without base did not produce any lactic acid, and no conversion of glycerol was observed (entry 1). The activity observed with catalyst 1 was sensitive to reaction temperature: decreasing temperature from 150 to 115 °C resulted in a significant decrease in TON (1983 to 342, entries 4 and 5). Temperatures higher than 150 °C were not explored because the high base concentration results in severe corrosion D

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This process would be favored under the highly basic and hightemperature conditions employed and would make 6 less thermally robust.35 We expect that more thermally robust catalysts would have a significant advantage in catalyst lifetime under microwave conditions, where superheating can cause hotspots. A prior report by Crabtree et al. indicated that substituting the cod ligand of nonsulfonated Ir(I) complexes with carbonyls affords an increase in glycerol dehydrogenation activity.13 We thus prepared corresponding dicarbonyl complexes 8 and 9 by passing CO gas through solutions of compounds 6 and 7. While carbonyl derivatives 8 and 9 are significantly more active than the cod precursors (6 and 7) (Figure 4), the relative

Figure 4. Bar graph comparing average turnover frequencies (TOF, turnovers/h) obtained with catalysts 6−9 under microwave conditions.

improvements were not equal. For the N-propylsulfonate analogs (6 and 8) the improvement observed is significantly smaller than that for the N-phenylsulfonate analogs (7 and 9); furthermore, the activity of 9 relative to 7 increases ∼3-fold (Figure 4). We attribute this to the more thermally robust nature of the phenylsulfonate analogs. The TOF afforded by 9 (42 592 h−1) is approximately 4-fold higher than the best reported to-date for pure glycerol (11 055 h−1 observed for glycerol/NMP by Beller et al. at 140 °C in the first hour of reaction).36 To probe the effect of temperature on reaction rate, we performed reactions at 115 °C using microwave and conventional heating. Under these conditions catalyst 9 afforded 2241 h−1 and 416 h−1 using microwave and conventional heating, respectively. To demonstrate the longevity of catalyst 9, the reaction was extended to 8 h (Figure 5). Over that time, the reaction rate remains constant, as evidenced by the linear increase in concentration of product, reaching 293 249 turnovers. The constant reaction rate is likely due to pseudoconstant concentration of substrate and negligible catalyst deactivation under these conditions. Given that very low catalyst loading is employed, after 8 h the total conversion of glycerol was 35%. In order to demonstrate that this catalyst can achieve full conversion of glycerol in 1:1 glycerol/water we tested the activity of 9 at an increased catalyst loading (18 ppm Ir, 0.00215 mol %). To avoid the pressure limitations observed in the initial tests with high catalyst loading, the total volume of

Figure 3. Turnover numbers (labeled, upper horizontal axis) and frequencies (lower horizontal axis) afforded by catalysts 1−11 for conversion of glycerol to lactic acid (based on lactic acid yield) under (a) microwave conditions at 150 °C for 3 h and (b) conventional heating to 150 °C for 24 h (see Tables S4 and S5 for details and yields). Legend: green, nonsulfonated Ir catalysts; blue, sulfonated Ir catalysts; red, sulfonated Ru catalysts.

a significant decrease in activity (553 vs 3122 turnovers/h), as in this case free acetate slows down the Ir-OAc dissociation, which is needed to generate the active catalyst. The Ir(I) catalysts (6−9) were significantly more active than the Ir(III) catalysts (4, 5). In order to avoid exceeding the maximum pressure limit (250 psi) of the microwave vials, the catalyst amount was reduced 10-fold (from 18 to 1.8 ppm Ir). This decreased catalyst loading was sufficient to prevent the pressure from reaching the maximum limit of the reactor. The two catalysts with cyclooctadiene (cod) ligands (6 and 7) achieve 7383 and 13 586 turnovers/h under microwave conditions and 840 and 1635 turnovers/h respectively with conventional heating. The significantly higher TOF and total TONs of 7 relative to 6 is likely due to resistance of the Nphenylsulfonate in 7 to degradation via Hoffman elimination. E

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analogs; the improvement is even more pronounced when ligand degradation can be minimized. Microwave versus Conventional Heating. In all cases reported herein, the microwave protocol afforded significant increase in TOF relative to conventional heating at the same set temperature without sacrificing catalyst lifetime (reactions performed for up to 8 h with catalyst 9 showed no decrease in rate). The average magnitude of this increase was estimated from a plot of average TOFs observed for conventional versus microwave reactions at ∼11-fold. To demonstrate that this increase is merely the result of thermal effects (improved heating efficiency), we carried out the reaction at 150 °C in a silicon carbide vial (designed by Kappe).37 These vials efficiently convert microwave energy to heat but do not allow microwaves to penetrate the vial. The turnover numbers achieved under these reaction conditions were within experimental error of those with the glass vial, confirming the origin of the rate acceleration as thermal efficiency. Toward Milder Base Conditions for Lactic Acid Formation. In an effort to explore the activity of the catalyst under milder base conditions, the reaction with catalyst (9) was attempted with lower base concentration and weaker bases. First, the activity of 9 was tested with potassium carbonate instead of KOH. Due to the lower solubility of K2CO3 in 1:1 glycerol/water, we were only able to achieve a maximum concentration of 2.7 M (0.4 equiv). Interestingly, we observed the production of not only lactic acid, but also 1,2-propanediol, as well as formic acid (Figure S5). Although the turnover numbers were lower than for the reaction with 1 equiv of KOH (TONs: 39 000 LA, 8 300 PDO, 630 FA), this activity is highly promising and offers possibility of generating other products from glycerol. We also explored the effect of lower KOH concentration: Upon lowering the KOH loading to 0.22 equiv (1.7 M), the turnovers and glycerol conversion decreased significantly (27 801 vs 127 777 turnovers; 3 % vs 13% glycerol conversion). This is consistent with prior reports that show the rate dependence on concentration of strong base. Crude Glycerol as Substrate. Given that purification of crude glycerol from biodiesel synthesis is the ultimate application of the catalysts in question, we tested the activity of catalyst 9 for dehydrogenation of crude glycerol (80−85% glycerin purchased from Inkemia). Catalyst 9 affords almost identical activity and selectivity for lactic acid with crude and pure glycerol, suggesting that the impurities do not deactivate the catalyst (Table 2). The slight decrease in TON can be attributed to slightly higher dilution in crude glycerol, as one of the major impurities is water. Effect of Stir Rate for Microwave Reactions. In optimizing the microwave protocol, we found that the catalytic

Figure 5. Extended reaction for the dehydrogenation of glycerol to lactic acid using catalyst 9. Reaction conditions: 2 mL of glycerol (27.4 mmol), 2 mL of aqueous KOH (14 M,), TSP (0.11 mmol, internal std)catalyst 9 (29.5 nmol), microwave heating to 150 °C in a sealed microwave reaction tube. TONs determined by NMR using TSP as an internal standard. TSP: 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt.

reaction was reduced, allowing for increased headspace. Catalyst 9 afforded near-quantitative conversion of glycerol and 91% yield of LA in 3 h (46 400 turnovers). The remaining 9% of glycerol was converted to 1,2-propane diol (PDO), which is formed by hydrogenation of the pyruvaldehyde. The pyruvaldehyde in turn is formed via dehydration of glyceraldehyde (Scheme 3). These results demonstrate the exceptional activity and selectivity for the reduction of glycerol to lactic acid in a short reaction times. Scheme 3. Quantitative Conversion of Glycerol Using 18 ppm (2.15 × 10−3 mol %) Catalyst 9

Sulfonate Effect. In order to probe the effect of the sulfonate groups on catalytic activity we tested two nonsulfonated analogs in Chart 2 under identical conditions. Compound 10, analog of 5, is approximately 50% less active than 5 (Figure 4 and Table S4 and S5), likely due to its lower solubility. Furthermore, unlike the acceleration observed for 5 upon addition of NaOAc, in the case of 10, acetate addition had no favorable effect (Table S4 entries 13−14), likely due to its the low solubility in the aqueous reaction medium. Similarly, compound 11, a nonsulfonated (methyl) analog of 6 and 7, was significantly less active than 6 and 7 under microwave conditions (6554, 7383, and 13 586 turnovers/h for 11, 6, and 7 respectively), and a similar trend was observed using conventional heating (390, 840, and 1635 turnovers/h for 11, 6, and 7 respectively). Thus, we observe multiple examples where sulfonate functionalization of the NHC wingtips significantly increases catalytic activity relative to nonsulfonated

Table 2. Comparison of Catalyst 9 with Pure and Crude Glycerola

entry

reaction

TON

avg. TOF

1 2

pure glycerol (99.5%) crude glycerol (Inkemia, 80%)

127777 125679

42592 41893

a Conditions: 3 mL of glycerol, 3 mL of H2O, 1 equiv of KOH, 18 ppm catalyst 9, microwave heating in closed 10 mL vial to 150 °C for 3 h; yields determined by NMR using NaOAc as an internal standard.

F

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alkoxide, β-hydride elimination, and finally hydride abstraction using H+ to release H2.38 Given that CO substitution in 9 is not facile,39 we propose the catalytic cycle in Figure 6. First, the 16electron Ir(I) species undergoes O−H oxidative addition with glycerol to form an alkoxy hydride complex; this step has precedents for Ir(I) complexes.40 The resulting Ir(III) alkoxy hydride complex could then eliminate H2 via σ-bond metathesis with an adjacent hydroxyl of glycerol, a reaction with theoretical precedent for Pt-group metals.41 The chelating bis-alkoxy complex can then undergo β-hydride elimination at the primary or secondary position by labializing one of the alkoxy groups, and release the product (glyceraldehyde or dihydroxyacetone) via O−H reductive elimination. Neither of these intermediates has been observed under basic conditions, as both are expected to undergo dehydration and intramolecular Cannizzaro reaction. The difference in activity observed for [(NHC-ph-SO3−)2Ir(cod)]− (7) versus [(NHC-ph-SO3−)2Ir(CO)2]− (9) could be due to (i) more favorable H2 elimination or O−H reductive elimination from Ir center in 9 with stronger π-acceptor (CO) ligands or (ii) the fact that 7 undergoes an entirely different mechanism than 9 (cycooctadiene ligand in 7 could easily be displaced, opening coordination sites, which is not the case for 9).39 Although the presence of carbonyl ligands also opens the possibility for cluster formation,35a,39 in the case of 9 cluster formation is not likely at the low catalyst loadings used, compared to previous reports.13,19 Although a detailed mechanistic investigation will be the subject of a future report, we performed an NMR experiment to identify the Ir−hydride species formed. Upon addition of glycerol and KOH(aq) to 9 at room temperature no change is observed, but heating to 50 °C for 30 min results in a color change from yellow to green, and appearance of two hydride peaks at −10.8 and −11.1 ppm respectively (relative integrations 1:1.3, Figure S6). For comparison, we have previously reported that catalyst 4 (Ir(III) complex) under the same conditions affords a single hydride signal at −12.8 ppm at room temperature.27 The observation of two hydride resonances for the reaction with 9 is consistent with the catalytic cycle proposed (Figure 7).

activity is affected by stir rate, which is often omitted from reported microwave protocols. We hypothesized that changing the stir rate resulted in fluctuations of the internal reaction temperature, which were not captured by the external IR thermometer in the microwave cavity. Thus, we monitored the internal reaction temperature at different stir rates using an in situ fiber optic thermometer (Figure 6).

Figure 6. Effect of stir rate on internal temperature of microwave reactions for glycerol conversion to lactic acid (internal temperature measured using fiber optic thermometer).

Figure 6 shows the relationship between stir rate, TON, and internal reaction temperature. When the stir rate is set to the maximum (1000 rpm), significant splashing can be seen with the internal camera, potentially resulting in inefficient heating and lower TONs. At 1000 rpm stir rate the internal probe measured 143 °C, compared to 150 °C on the wall of the vial. As stated previously, we compared the catalytic activity at 150 °C and 1000 rpm stir rate in a silicon carbide vial. These vials efficiently convert microwave energy to heat, but do not allow microwaves to penetrate the vial. The turnover numbers achieved under these reaction conditions were 49 645, within experimental error of those with the glass vial. When the stir rate is decreased to 300−700 rpm, heating becomes more uniform, and the internal temperature was steady at 147 °C. As expected, no difference in TON was achieved at these two stir rates (700 and 300 rpm). However, when stirring is decreased to 100 rpm the internal temperatures reaches 180 °C, which results in a vast increase in catalytic activity and rapid hydrogen generation. The maximum pressure (30 bar) of the reactor is quickly reached under these conditions, and significant etching of the glass vial is observed. While the use of this microwave vial does not allow us to carry out these reactions at temperatures above 150 °C (due to high base concentration required), this result signals that catalytic activity can further be increased by using a reactor suitable for high temperatures, pressures, and pH. For the existing apparatus, we identified an optimal stir rate of ∼600 rpm. The reaction was repeated in a 30 mL vial; under these conditions, catalyst 9 achieved 124 438 turnovers (vs 127 777 in the 10 mL vial), suggesting that pressure build-up in the 10 mL vials does not affect the productivity of the catalysts under these conditions. Mechanism. The identification of Ir(I) compound 9 as the most active catalyst poses the question of whether its mechanism for acceptorless dehydrogenation significantly differs from that of more extensively studied Ir(III) catalysts. As proposed by Xiao et al, Ir(III) compounds likely undergo ligand substitution by an alcohol, deprotonation to form Ir-

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CONCLUSIONS Here we report the design, synthesis, and application of highly active catalysts for the conversion of glycerol to lactic acid via acceptorless dehydrogenation of glycerol in aqueous base under microwave and conventional heating conditions. The catalysts consist of Ir(I), Ir(III) and Ru(II) complexes bearing sulfonatefunctionalized N-heterocyclic carbene ligands (NHCs). The sulfonate functionalization renders the catalyst very soluble in the aqueous reaction medium, which results in significant improvement in activity compared to nonsulfonated analogs. Further improvements result from the use of N-substituents on the NHC that disfavor Hoffman elimination, thus enhancing thermal stability. The significant improvement in apparent activity of the catalysts under microwave conditions (11-fold on average) are likely due to more efficient heating and improved dispersion of base in viscous glycerol. The most active catalyst is [(NHC-ph-SO3−)2Ir(CO)2]− (9), which affords 45 592 turnovers/h under microwave conditions and was observed to afford up to 293 249 turnovers in 8 hours. Under these conditions, 9 achieves average turnover frequencies approximately 4-fold higher than the best reported catalyst to-date for pure glycerol. Although higher activity is observed with strong G

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modeled. In complex 7, PART, SIMU, and RIGU commands were used to model two-part disorder in both of the sulfonate groups and two of the five water molecules (OW1A/OW1B, OW3A/OW3B), and PART and ISOR commands were used to model disorder in a third water molecule (OW5A/OW5B). Additionally, the OMIT 1 0 0 command was used in complex 10 to remove a reflection affected by the beamstop. Structures were checked for additional symmetry using PLATON.49 Thermal ellipsoid figures were prepared with ORTEP III.50 Data collection and refinement details for complexes 7 and 10 are included in Table S1, and select bond lengths can be found in tables S2 and S3. Pyr-NHC Ligand (2a). 2-(Imidazol-1-yl)pyridine (0.4734 g, 3.26 mmol) and 1,2-propane sultone (0.5924 g, 0.485 mmol) were measured into a pyrex pressure tube with MeCN (7 mL). The tube was sealed and heated at 100 °C for 16 h at which point a white precipitate is observed. After cooling to room temperature, the solid was collected by filtration and washed with 3 × 10 mL of CH2Cl2 and dried in vacuo at room temperature. Ligand 2a was recovered as a white solid (828 mg, 3.1 mmol, 94% yield). 1H NMR (400 MHz, D2O) δ 9.65 (t, J = 1.7 Hz, 1H), 8.61 (ddd, J = 4.9, 1.8, 0.8 Hz, 1H), 8.20 (dd, J = 2.2, 1.7 Hz, 1H), 8.17 (ddd, J = 8.2, 7.6, 1.8 Hz, 1H), 7.85−7.79 (m, 2H), 7.65 (ddd, J = 7.6, 5.0, 0.9 Hz, 1H), 4.55 (t, J = 7.2 Hz, 2H), 3.07−2.99 (m, 2H), 2.50−2.39 (m, 2H). 13C NMR (101 MHz, D2O) δ 194.92, 149.18, 146.20, 140.89, 134.48, 125.48, 123.23, 120.00, 114.94, 48.47, 47.15, 24.92. HRMS(ESI/Q-TOF) m/z: [M + H]+ Calcd for C11H14N3O3S 268.0756. Found 268.0759. Ru(Pyr-NHC-iPrSO3)(p-cymene)Cl (2). Ligand 2a (75 mg, 0.28 mmol) was dissolved in a degassed solution of 16 mL of MeOH and 3 mL of H2O. The flask was wrapped in foil, and Ag2O (65 mg, 0.28 mmol) was added in darkness and the solution stirred at 50 °C for 90 min. A solution of NaCl (35 mg, 0.60 mmol) in 1 mL of H2O was added and the solution stirred for another 15 min. The silver solution was then filtered and transferred to a solution of [Ru(p-cymene)Cl2]2 (100 mg, 0.16 mmol) in 15 mL of H2O and stirred at room temperature overnight. The solution was filtered and the filtrate dried in vacuo at 50 °C. Then, 50 mL of MeCN was added to the crude product, stirred for 1h at room temperature and filtered over Celite. The clear yellow filtrate was dried, dissolved in a minimum amount of MeOH, and transferred to a silica column. Elution with 800 mL of acetone/MeOH (80/20) and 400 mL of acetone/MeOH (50/50) afforded a bright orange band. Removal of the solvent in vacuo afforded 2 as a highly hygroscopic orange solid (45 mg, 0.0837 mmol, 30% yield). 1 H NMR (400 MHz, CD3OD) δ 9.32 (ddd, J = 5.8, 1.5, 0.7 Hz, 1H), 8.24−8.17 (m, 2H), 8.02 (ddd, J = 8.3, 1.2, 0.7 Hz, 1H), 7.78 (d, J = 2.3 Hz, 1H), 7.52 (ddd, J = 7.5, 5.8, 1.3 Hz, 1H), 6.47 (dd, J = 6.1, 1.3 Hz, 1H), 6.31 (dd, J = 6.3, 1.3 Hz, 1H), 6.13 (dd, J = 6.4, 1.3 Hz, 1H), 5.88 (dd, J = 6.1, 1.4 Hz, 1H), 4.85−4.78 (m, 1H), 4.72 (ddd, J = 13.8, 10.6, 5.8 Hz, 1H), 3.06 (dt, J = 7.5, 5.1 Hz, 2H), 2.65−2.54 (m, 1H), 2.43 (p, J = 7.0 Hz, 2H), 2.27 (s, 3H), 0.98 (dd, J = 6.9, 3.8 Hz, 6H). 13C NMR (101 MHz, CD3OD) δ 185.30, 156.75, 153.38, 142.66, 126.19, 124.25, 118.06, 113.51, 92.80, 92.40, 88.33, 82.60, 51.35, 49.29, 32.48, 27.77, 22.90, 22.52, 19.32. HRMS(ESI/Q-TOF) m/z: [M + H]+ Calcd for C21H27N3O3SRuCl 538.0505. Found 538.0504 General Procedure for Synthesis of Ir(I) Catalysts. Under a nitrogen atmosphere, [Ir(cod)Cl]2 (66 mg, 0.098 mmol) was dissolved in 8 mL of degassed EtOH (200 proof). A solution of NaH (25 mg, 60 wt % in mineral oil) in 2 mL of EtOH was added dropwise; the solution quickly from orange to yellow and was stirred for 30 min at 25 °C. A suspension of the imidazolium ligand (0.37 mmol) in 3 mL of EtOH and 0.5 mL of H2O was slowly added to the iridium solution via syringe and stirred at 25 °C for 72 h. The solvent was then removed under reduced pressure and the crude product redissolved in a minimum amount of MeOH (3 mL). Addition of Et2O afforded a precipitate, which was collected, washed 3 times more with Et2O and dried under vacuum. Na[Ir(NHC-nPrSO3)2(cod)] (6). Following the general procedure for Ir(I) catalysts, complex 6 was recovered as a hygroscopic red solid (78 mg, 0.106 mmol, 57% yield). 1H NMR (400 MHz, CD3OD) δ 7.26 (dd, J = 14.0, 2.0 Hz, 2H), 7.17 (dd, J = 10.3, 2.0 Hz, 2H), 4.57−4.45

Figure 7. Proposed catalytic cycle for glycerol dehydrogenation by catalyst 9.

bases, 9 is also highly active with the weaker base, K2CO3 (13 000 h−1 and concurrent formation of formate). Activity is also retained for crude glycerol. The low catalyst loadings used disfavor previously reported catalyst deactivation via cluster formation. We propose a mechanism for 9 that involves O−H oxidative addition of glycerol, σ-bond metathesis to eliminate hydrogen, β-hydride elimination and O−H reductive elimination to expel the dehydrogenation products. The results presented here highlight the importance of enhanced catalyst solubility for reactions in highly polar media, and the substantial enhancement that can be afforded for by microwave heating for viscous and highly base-concentrated reactions.

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EXPERIMENTAL SECTION

General Considerations. The syntheses of the catalysts were carried out under nitrogen using standard Schlenk technique, unless otherwise stated. Commercial chemicals were used without further purification. [RuCl2(p-cymene)]2 and IrCl3 were purchased from Acros-Organics. Solvents were dried using a solvent purification system (SPS MBraun) or 4 Å molecular sieves. Glycerol (>99%, Alfa Aesar) was dried over activated 4 Å molecular sieves. Noncommercial reagents: ligands, catalyst precursors including, [Ir(cod)Cl]2 and [IrCp*Cl2]2, and catalysts 1, 3−5, and 11 were synthesized via previously reported procedures; for synthetic details, please refer to the Supporting Information. NMR spectra were recorded on an Agilent NMR spectrometer operating at 400 MHz. Single Crystal X-ray Diffraction. Single crystals from the bulk samples of compounds 7 and 10 were isolated and mounted on MiTeGen micromounts. Reflection data were collected at 100(2)K with 0.5° ω scans on a Bruker SMART diffractometer equipped with an APEX II CCD detector using Mo Kα (λ = 0.71073 Å) radiation. The data were integrated using the SAINT program42 within the APEX II software suite,43 and absorption corrections were applied using SADABS.44 Complex 7 was solved via direct methods using Superflip,45 and complex 10 was solved via direct methods using SIR 92.46 Both complexes 7 and 10 were refined using SHELXL-201447 in the WinGX software suite.48 In each structure, all non-hydrogen atoms were located in difference Fourier maps and were refined anisotropically. Aromatic hydrogen atoms were placed in idealized positions by utilizing the HFIX43 command and allowed to ride on the coordinates of the parent atom with isotropic thermal parameters (Uiso) fixed at 1.2Ueq. The hydrogen atoms on all the water molecules in complex 7 could not be located in the difference Fourier map and were thus not H

DOI: 10.1021/acs.organomet.8b00081 Organometallics XXXX, XXX, XXX−XXX

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33.25, 30.93, 20.22, 13.80, 9.40. HRMS(ESI/Q-TOF) m/z: [M − PF6]+ Calcd for C25H39ClN4Ir 623.2492. Found 623.2483. General Procedure for Glycerol Dehydrogenation. A 10 mL microwave tube was charged with glycerol (3 mL, 41 mmol), aqueous KOH (3 mL, 14 M, 42 mmol), catalyst (0.5−0.7 μmol), and a magnetic stir bar. The reaction mixture was thoroughly stirred to ensure adequate mixing before being placed in the cavity of a CEM Discover or Anton Parr Monowave microwave reactor. Heating with 150 W power and a maximum pressure of 250 PSI allowed the desired temperature (150 °C) to be reached in 1 min on average. After cooling to room temperature, the reaction was opened, and aqueous solution of standard was added (NaOAc or TSP, approximately 0.1 M solution in D2O). Lactic acid formation was determined via 1H NMR spectroscopy using D2O as solvent and NaOAc or TSP as an internal standard. For reactions performed using conventional heating, the same procedure was followed, but using 20 mL reaction tubes that were placed in a Heidolph 12 Carousel and heated to 150 °C for 24 h.

(m, 2H), 4.24−4.12 (m, 2H), 4.07 (m, 1H), 4.05 (s, 6H), 3.89 (m, 1H), 3.73−3.65 (m, 2H), 2.99−2.89 (m, 4H), 2.50−2.41 (m, 2H), 2.35−2.24 (m, 2H), 2.24−2.14 (m, 4H), 2.05−1.95 (p, J = 7.5 Hz, 2H), 1.91−1.71 (m, 2H). 13C NMR (101 MHz, CD3OD) δ 176.60, 161.59, 123.12, 120.66, 109.99, 77.36, 74.54, 49.10, 37.40, 32.01, 29.46, 26.15. HRMS(ESI/Q-TOF) m/z: [M + H]+ Calcd for C22H35N4S2O6IrNa 731.1530. Found 731.1535 Na[Ir(NHC-PhSO3)2(cod)] (7). Following the general procedure for Ir(I) catalysts, complex 7 was recovered as a bright red solid (69 mg, 0.086 mmol, 53% yield). X-ray quality crystals were grown via vapor diffusion of acetone into a concentrated solution of 7 in water. 1H NMR (400 MHz, CD3OD) δ 8.02−7.97 (m, 4H), 7.37−7.32 (m, 4H), 7.24 (d, J = 2.0 Hz, 2H), 7.14 (d, J = 2.0 Hz, 2H), 4.78−4.70 (m, 2H), 3.61 (q, J = 7.7 Hz, 2H), 3.22 (s, 6H), 2.51−2.38 (m, 2H), 2.28 (dd, J = 15.2, 7.8 Hz, 2H), 2.17−2.05 (m, 2H), 1.69 (td, J = 14.4, 7.3 Hz, 2H). 13C NMR (101 MHz, CD3OD) δ 177.58, 147.47, 142.61, 128.28, 127.14, 124.30, 124.25, 80.73, 73.91, 38.00, 35.94, 28.26. HRMS(ESI/ Q-TOF) m/z: [M − Na]− Calcd for C28H30N4S2O6Ir 775.1236. Found 775.1227. Ir(NHC-nPrSO3)(CO)2 (8). Complex 6 (54 mg, 0.073 mmol) was dissolved in 10 mL of degassed MeOH, and the system was flushed with N2 for 10 min. CO(g) was bubbled through the solution at room temperature for 90 min, and the solution changed from orange to yellow in color. The solvent was reduced to about 2 mL in vacuo at room temperature, at which point addition of Et2O afforded the formation of a yellow precipitate. The solid was washed 3 × 5 mL further with Et2O and dried in vacuo at room temperature to yield complex 8 as a yellow solid (35 mg, 0.051 mmol, 69% yield). 1H NMR (400 MHz, CD3OD) δ 7.45 (d, J = 2.0 Hz, 2H), 7.41 (d, J = 2.0 Hz, 2H), 4.06 (t, J = 7.6 Hz, 4H), 3.98 (s, 6H), 2.78 (t, J = 7.2 Hz, 4H), 2.13 (m, 4H). 13C NMR (101 MHz, CD3OD) δ 179.88, 166.83, 124.26, 122.46, 49.33, 47.71, 37.74, 26.28. HRMS(ESI/Q-TOF) m/z: [M − Na]+ Calcd for C16H22N4S2O8Ir 655.0508. Found 655.0505 Ir(NHC-PhSO3)2(CO)2 (9). Complex 7 (69 mg, 0.086 mmol) was dissolved in 10 mL of degassed MeOH, and the system was flushed with N2 for 10 min. CO(g) was bubbled through the solution at room temperature for 90 min, and a solution changed from red/orange to an orange/yellow color. The solvent was reduced to about 2 mL in vacuo, after which Et2O was added until a yellow precipitate was formed. The solid was washed 3 times with Et2O and dried in vacuo to yield complex 9 as a yellow solid (36 mg, 0.047 mmol, 54% yield). 1H NMR (400 MHz, D2O) δ 7.90−7.85 (m, 4H), 7.50−7.46 (m, 4H), 7.24 (d, J = 2.0 Hz, 2H), 7.00 (d, J = 2.0 Hz, 2H), 3.44 (s, 6H). 13C NMR (101 MHz, D2O) δ 179.49, 168.73, 163.16, 142.84, 140.92, 126.64, 125.27, 123.95, 122.39, 38.31. HRMS(ESI/Q-TOF) m/z: [M + H]+ Calcd for C22H19N4S2O8IrNa 747.0177. Found 747.0155. [Ir(bis-NHC)Cp*Cl]PF6 (10). [IrCp*Cl2]2 (30.6 mg, 0.038 mmol) and 1,1′-methylenebis(3-butyl-1H-imidazol-3-ium) dichloride (31.8 mg, 0.081 mmol) were added to a Schlenk flask under a nitrogen atmosphere. Et3N (0.1 mL, 0.717 mmol) was added via syringe, followed by 30 mL of dry and degassed MeCN. The mixture was refluxed under a nitrogen atmosphere overnight. After cooling to room temperature, the solvent was removed under vacuum. The yellow residue was washed with 3 × 5 mL of acetone and filtered over Celite. A small amount of silica was added to the filtrate, and solvent was removed under vacuum. The yellow silica cake was loaded onto a silica column conditioned with acetone, elution of a yellow band was achieved with acetone/KPF6. The yellow band was collected and solvent was removed under vacuum, leaving a yellow residue with excess KPF6. Washing with CH2Cl2 and filtration over Celite leaves 10 in solution, which can be dried under vacuum to a yellow powder (54 mg, 0.07 mmol, 92%). Single crystals suitable for X-ray analysis were grown via vapor diffusion of Et2O into a concentrated solution of 10 in CH2Cl2 at room temperature. 1H NMR (400 MHz, CDCl3) δ 7.45 (dd, J = 2.1, 0.8 Hz, 2H), 7.09−7.07 (m, 2H), 6.24 (d, J = 13.6 Hz, 1H), 5.58 (d, J = 13.6 Hz, 1H), 4.26 (td, J = 11.9, 5.6 Hz, 2H), 3.83 (ddd, J = 12.5, 10.9, 5.7 Hz, 2H), 1.84 (dd, J = 17.1, 7.1 Hz, 4H), 1.79 (d, J = 0.7 Hz, 15H), 1.44 (h, J = 7.5 Hz, 4H), 1.02−0.93 (m, 6H). 13C NMR (101 MHz, CDCl3) δ 165.62, 122.19, 120.79, 92.91, 49.94,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00081. Synthetic details for previously reported ligands and catalysts, NMR mechanistic studies, crystallographic parameters, NMR spectra for new catalysts (PDF) Accession Codes

CCDC 1586716−1586717 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Matthew Finn: 0000-0001-9474-7022 Christopher Cahill: 0000-0002-2015-3595 Adelina Voutchkova-Kostal: 0000-0002-7016-5244 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSF for funding through NSF CAREER award (no. 1554963).

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REFERENCES

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