Synthesis and Reactivity of 1,2-Bis(di-iso-propylphosphino)benzene

Jul 12, 2018 - The catalytic coupling of CO2 with ethylene to generate acrylates is one such avenue of research. Despite ... 2018 140 (25), pp 7922–...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis and Reactivity of 1,2-Bis(di-iso-propylphosphino)benzene Nickel Complexes: A Study of Catalytic CO2−Ethylene Coupling Melissa N. Hopkins,† Kenichi Shimmei,‡ Katherine B. Uttley,† and Wesley H. Bernskoetter*,† †

The Department of Chemistry, The University of Missouri, Columbia, Missouri 65211, United States Sekisui Chemical Co. Ltd., Osaka 530-8565, Japan

‡

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ABSTRACT: The utilization of renewable carbon dioxide resources in the production of commercial chemicals has been at the center of efforts to improve the sustainability of many widely used consumer products. The catalytic coupling of CO2 with ethylene to generate acrylates is one such avenue of research. Despite decades of investigations, catalytic CO2−ethylene coupling has only recently been experimentally demonstrated. The development of a new nickel-based catalyst, 1,2-bis(di-iso-propylphosphino)benzene nickel(0) 1,5-cyclooctadiene, for acrylate production is detailed here. The stoichiometric reactivity of the catalyst toward the CO2, ethylene, and base reagents, as well as the coordination chemistry of likely catalytic intermediates has been examined. Comparative catalytic experiments were used to identify the influence of commonly employed additives, such as metallic zinc and Lewis acidic salts, on the catalytic CO2−ethylene coupling reaction. In addition, catalytic comparisons of the 1,2-bis(di-isopropylphosphino)benzene nickel(0) 1,5-cyclooctadiene species with a similar previously reported catalyst under newly developed conditions afforded turnover numbers greater than 400, an approximately 4-fold increase over the highest activity reported to date. Significantly, the catalytic trials reveal that small changes in the catalytic conditions and additives can have substantial effects on productivity and that these influences are not uniform with respect to catalyst platforms.

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INTRODUCTION Expanding recognition of the deleterious effects of anthropogenic CO2 production has brought carbon capture, utilization, and storage (CCUS) technology development to the forefront of chemical and engineering research.1 Given the scale of anthropogenic CO2 production, it is doubtful that the CO2 utilization alone will ever directly offset carbon emissions. However, the upconverison of CO2 into value added chemicals could play a significant role in the overall economics of CCUS activities as well as enhance the sustainability of many commercially significant chemicals.2 One avenue toward this goal has been the investigation of transition metal catalyzed “reverse combustion” processes to generate molecular energy sources, such as methane and methanol, from CO2.3 Due to the potentially immense scale for the utilization of such technologies, these studies have been a major focus of CO2 upconversion.4 However, there are also numerous other attractive targets for chemical CO2 utilization which, though smaller in commodity scale, have more favorable thermodynamic energy profiles.5 One such target that has been of interest to our laboratory is the catalytic synthesis of acrylates from coupling CO2 and ethylene. Acrylic acid and its derivatives are primarily produced via propylene oxidation methods for applications in water adsorbing polymers and polyester fibers, as well as other materials.6 The use of the nonrenewable and relatively expensive propylene feedstock makes a CO2−ethylene derived path to acrylates a potentially attractive alternative.7 Ernesto © XXXX American Chemical Society

Carmona was among the pioneers in developing a transition metal mediated route for acrylates via CO2-olefin coupling.8 In 1985, his laboratory reported the first well-defined example of this reaction, using a zero-valent molybdenum ethylene complex to reduce CO2 into a bimetallic acrylate complex.8a

Although subsequent research indicates that group VI metals are challenging targets for catalyst development due strong product inhibition from of the metal−oxygen bond,9 this seminal work opened the field to the first catalytic processes developed nearly three decades later. Modern reports of catalytic acrylate production from CO2− ethylene coupling first appeared in 2012 from the joint laboratories of BASF-University of Heidelburg using a zerovalent nickel source along with bis(ditert-butylphosphino)ethane.10 Shortly thereafter followed several reports of closely related nickel catalyzed coupling reactions using a variety of bidentate diphosphine ligands, in addition to a few less catalytically active systems described with other late transition Special Issue: In Honor of the Career of Ernesto Carmona Received: April 24, 2018

A

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Organometallics metals (Figure 1).11 Acrylate production at nickel has been widely proposed to proceed via an oxidative coupling reaction

acrylate from CO2 and ethylene, in addition to stoichiometric reactions and comparative catalysis trials which highlight the complex role of base and additives in catalytic performance.

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RESULTS AND DISCUSSION Catalysts Synthesis and Stoichiometric Reactivity. One of the leading nickel catalyzed reactions for CO2 functionalization to acrylate reported by Limbach and coworkers utilizes a combination of Ni(COD)2 (COD = 1,5cyclooctadiene) and (R,R)-(+)-1,2-bis(tert-butylmethylphosphino)benzene (BenzP*) (Figure 4).11b While providing an active catalytic system, BenzP* also incorporates P-stereogenic sites which complicate ligand synthesis while providing little benefit to the achiral production of acrylate from CO2− ethylene coupling. Given this factor, our laboratory targeted a more easily prepared and symmetric iPr2P analog of BenzP*, 1,2-bis(di-iso-propylphosphino)benzene (i-PrBenzP, Figure 4), in order to more efficiently investigate the coordination chemistry associated with CO2−ethylene coupling as well as examine the influence of commonly used additives on catalytic acrylate formation. The i-PrBenzP ligand was coordinated to zero-valent nickel by stirring in a tetrahydrofuran solution with Ni(COD)2 at ambient temperature to afford (i-PrBenzP)Ni(COD) (1COD) as a yellow solid upon isolation (Figure 5). The 31P NMR spectrum of 1-COD exhibited a single resonance at 65.89 ppm in benzene-d6, consistent with a symmetric coordination environment about the nickel center. The corresponding 1H NMR spectrum displays a distinctive resonance at 4.43 ppm indicative of nickel coordinated 1,5COD methine protons, along with a set of resonances assigned to the i-PrBenzP ligand. Diphosphine nickel(0) species similar to 1-COD are frequently proposed as the active precatalyst species in CO2−ethylene coupling, motivating closer study of the reactivity of 1-COD toward CO2 and ethylene. Treatment of 1-COD with 2 atm of a 1:1 mixture of CO2 and ethylene in an NMR tube resulted in rapid and complete conversion to a new organometallics species, as judged by NMR spectroscopy. Additionally, treatment of 1-COD with only ethylene produced an identical set of 1H and 31P NMR resonances, indicating the new species, (i-PrBenzP)Ni(C2H4) (1-C2H4), did not arise from a reaction with CO2. 1-C2H4 species proved unstable to the removal of the ethylene atmosphere (reverting quickly back to 1-COD), so it was characterized solely by in situ NMR spectroscopic methods. The nickel(0) ethylene complex exhibits a singlet resonance at 71.70 ppm in the 31P NMR spectrum. Similar nickel(0) ethylene species have been crystallographically characterized and bear only one olefin bound to the nickel center.1016 Due to dynamic exchange between the free and bound ethylene in the solution of 1C2H4, definitive assignment of the stoichiometry of bound ethylene could not be made from the data in hand, so 1-C2H4

Figure 1. General scheme for nickel promoted CO2−ethylene coupling to yield acrylate.

with CO2 and ethylene to generate an undesirably stable intermediate nickel−lactone species (Figure 2).12 Many such nickel−lactone complexes have been isolated and characterized since the first CO2 derived examples described by Heinz Hoberg in 1983.13 In fact, the stubborn stability of these species toward the β-hydride elimination reaction required to form acrylate is a significant obstacle in catalyst development.14 In the nascent catalytic reactions reported to date, this barrier to acrylate formation has been overcome by the use of strong bases and/or Lewis acid additives (Figure 3).10,11 When a strong base, such as NaOt-Bu, is employed, the exogenous base directly removes the mildly acidic β-hydrogen with little or no involvement of the metal. However, the use of strong bases are typically incompatible with efficient one-pot CO2−ethylene coupling catalysis, as undesirable reactions between the base and CO2 compete with the β-hydrogen deprotonation.10 Although a mildly basic environment is required to overcome the slightly unfavorable thermodynamics of acrylate production from CO2−ethylene coupling in the gas phase or most organic solvents,12 the use of a strong base has clear limitations. Lewis acidic additives provide an alternative method to assist in the β-hydride elimination reaction from the nickel−lactone complexes. Prior studies from our laboratory suggest that even weak Lewis acids, such as sodium cation sources, permit transient opening of the five-membered lactone ring, providing the geometric flexibility and the open coordinative site that are necessary for efficient β-hydride elimination.15 Vogt and coworkers have successfully employed this effect to enhance the catalytic performance of (DCPE)/Ni(COD)2 (DCPE = 1,2bis(dicyclohexylphosphino)ethane) through the addition of cocatalytic amounts of LiI salt.11a The recent development of the first catalytic reactions for acrylate synthesis from CO2 and ethylene, as well as the apparent role of Lewis acids in assisting those reactions, has motivated our investigations to new nickel/Lewis acid combinations for this reaction. Herein we described the preparation for a new nickel catalyst of the production of

Figure 2. Proposed reaction steps for the synthesis of acrylate at diphosphine nickel(0) species. B

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Figure 3. Methods for inducing elimination from nickel−lactone complexes.

exchange of the ancillary ligands and isolation of 1-C3H4O2 in good yields following recrystallization (Figure 5).17 The 31P NMR spectrum of 1-C3H4O2 displays a characteristic pair of doublet resonances at 62.92 and 70.53 ppm (J = 9.0 Hz), consistent with two inequivalent P-environments created by the unsymmetric metal−lactone ring. The 13C NMR spectrum exhibited a resonance at 188.50 ppm assigned to the nickel− lactone carbonyl moiety, which was confirmed by 13CO2 isotopic labeling experiments starting from 1-C2H4 as well as by 1H−13C HMBC and HSQC NMR analyses. The 2D NMR experiments also provided definitive assignment of the α- and β-CH2 resonances of the nickel−lactone ring, which were present at 1.10 and 2.84 ppm, respectively. Solutions of 1C3H4O2 exhibited minimal decomposition at ambient temperature over several days, but heating at 60 °C for 2 h in the presence of COD resulted in substantial reversion to 1-COD.18 These observations suggest that the poor yields from the CO2−ethylene coupling synthesis were likely due to both slow rates of reaction under these pressure and temperature conditions and a weak thermodynamic preference for coupling. With the i-PrBenzP substituted nickel−lactone complex in hand, our examination turned toward conversion of the metallacycle to an acrylate complex. Given the absence of CO2 in the isolated samples of 1-C3H4O2, the method of strong base addition (vide supra) was selected for the ring-

Figure 4. Benzene annulated diphosphine ligands for CO2−ethylene coupling.

is depicted with one ethylene ligand on the basis of prior precedent.10,16 Significantly, prolonged exposure of 1-C2H4 to a mixture of ethylene and CO2 at 60 °C did afford a nickel−lactone species, (i-PrBenzP)Ni(κC,κO−CH2CH2COO) (1-C3H4O2), over 2 days. Conversion to the CO2−ethylene coupling product was only ca. 20% complete at this time, and extended reaction time or further heating produced a notable degree of sample degradation with observation of metallic nickel precipitate and free ligand. In order to determine if 1-C3H4O2 formation was simply kinetically slow (relative to decomposition of 1-C2H4) or thermodynamically disfavored, an alternative preparation of the nickel−lactone was targeted. Addition of i-PrBenzP to the p r e v i o u s l y r ep o r t e d ( 2 , 2′ - b ip yr i d i n e) N i (κ C , κ O− CH2CH2COO) nickel−lactone complex resulted in clean

Figure 5. Reactivity of [(i-PrBenzP)Ni] complexes toward ethylene and CO2. C

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Figure 6. Molecule structure of 1-acrylate·NaHMDS at 30% ellipsoids. At the left, the full tetrameric unit is illustrated with hydrogen atoms and iPrr substitutents removed for clarity. Elements labeled by color as Na (yellow), O (red), Ni (green), N (blue), C (gray), Si (tan), and (P) purple. At the right, a single (i-PrBenzP)Ni(η2-CH2CHCO2Na) unit is illustrated.

a coordination geometry and metrical parameters typical of this class of complexes, it does provide definitive characterization for one of the key intermediates in nickel catalyzed CO2− ethylene coupling. Perhaps just as significant as the isolation and characterization of 1-acrylate, its reactivity toward ethylene provided further encouragement regarding the application of this system in catalysis. Treatment of 1-acrylate with even 0.25 atm of ethylene afforded near instantaneous liberation of sodium acrylate and concurrent generation of the nickel(0) ethylene complex, 1-C2H4, as judged by NMR spectroscopy. This conversion establishes the competency of the [(i-PrBenzP)Ni] system to complete all of the necessary steps in a catalytic cycle for CO2−ethylene coupling to acrylate. Using this fundamental insight, our study moved on to an examination of the catalytic behavior of [(i-PrBenzP)Ni] under conditions commonly utilized for other transition metal catalyzed acrylate formation reactions. Catalytic Acrylate Formation. The current state-of-theart nickel catalyzed CO2−ethylene coupling reactions have evolved quickly from the first report by Limbach and coworkers.10 The development of these reactions has primarily been directed by the empirical testing of reaction additives and the judicious selection of bases to drive the thermodynamics of acrylate extrusion. Limbach’s first report employed a bis(ditertbutylphosphino)ethane/Ni(COD)2 system along with NaOtBu as a base to achieve a turnover number (TON) of 10. However, the use of NaOt-Bu proved incompatible with CO2 in a single-stage reaction, requiring the repeated sequential purging of gases and addition of base to make the system function. Later, the same laboratory examined the use of various substituted sodium phenoxide bases and screened a collection of commercially available diphosphine ligands to achieve a marked improvement in the nickel catalyzed reaction.11b The use of less nucleophilic phenoxide bases permitted a single batch reaction process with all reaction components present at the same time. Sodium 2-fluorophenoxide (2-F) was found to be a particularly effective base when paired with the BenzP*/Ni(COD)2 catalyst, producing a TON of 39 under 15 bar total pressure CO2/ethylene (2:1) at 100 °C for 20 h. The system was further improved by increasing the pressure and adding an excess of Zn dust to the reaction mixture to achieve a TON of 107 (Figure 7), the highest productivity reported to date for the CO2−ethylene coupling

opening reactions. Addition of sodium hexamethyldisilazide (NaHMDS) to a tetrahydrofuran solution of 1-C3H4O2 immediately produced a color change from yellow-orange to pale green. The corresponding 31P NMR spectrum displayed two new doublet resonances at 67.69 and 72.97 ppm (J = 45 Hz), consistent with formation of an η2-sodium acrylate nickel complex, (i-PrBenzP)Ni(η2-CH2CHCO2Na) (1-acrylate). Once again, 2D NMR spectroscopy was used to assign the key 1 H and 13C resonances of 1-acrylate. The 1H NMR resonances for the geminal methylene protons appeared at 2.03 and 2.40 ppm, with a corresponding signal at 34.00 ppm for the methylene carbon in the 13C NMR spectrum. The vinylic methine resonances were observed at 3.16 and 49.70 ppm in the 1H and 13C NMR spectra, respectively. Crystals of 1acrylate suitable for X-ray diffraction study were obtained by chilling a pentane solution of the crude synthetic reaction mixture at −35 °C for several days. The complete molecular structure (Figure 6; left) indicates a tetramer of 1-acrylate in the solid state. This is the first crystallographic characterization of an η2-coordinated nickel sodium acrylate complex, though the structure of a closely related nickel acrylic acid complex has been previously reported.10 The aggregate is centered on a core of eight sodium cations and four acrylate anions, along with four [N(SiMe3)2] anions originating from the excess base used in the synthesis. Each acrylate fragment is bound in an η2 fashion to an [(i-PrrBenzP)Ni] moiety. Some disorder is present in the structure, owing to partial occupation of one of the acrylic olefins across two positions. This disorder is also carried to the [(i-PrrBenzP)Ni] unit bound to that acrylate. However, a suitable model was created for this disorder and the data satisfactorily refined. The coordination environments about the nickel centers are quite similar, so a single 1-acrylate unit has been depicted in Figure 6 (right). The geometry about the nickel is best described as square planar with a moderate distortion arising from the constrained angles of the olefin and diphosphine ligands. The olefinic C(19)−C(20) bond distance of 1.43(1) Å is notably longer than that of free acrylic acid (1.29 Å), suggesting substantial bond order reduction due to π-backbonding with the nickel and from deprotonation of the carboxylic acid. The two C−O bond lengths of 1.278(9) and 1.253(6) Å are consistent with delocalization of the C−O double bond, as is the symmetric coordination of the sodium to the carboxylate group. While structure of 1-acrylate exhibits D

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our own hands using (BenzP*)Ni(COD) under these conditions. However, the moderate activity of the [(iPrBenzP)Ni] system did indicate that it would make a reasonable platform for assessing the influence of reaction additives. Additionally, use of the nickel lactone complex, 1C3H4O2, afforded a turnover of 55, comparable to that obtained with 1-COD, consistent with the two species generating acrylate via a common pathway. Given our prior research into the role of Lewis acidic salts in the ring-opening of stable nickel−lactone intermediates in acrylate formation reactions, we were keen to test the effect of adding alkali metal salts to the catalytic mixture.15 Computational studies by Vogt and co-workers suggest that Li+ should be considerably more effective than harder salts such as Na+ in mediating β-H elimination reactions from nickel−lactone intermediates.11a Lithium iodide was selected for initial experiments, given its relatively high solubility in THF and prior precedent in enhancing CO2 −ethylene coupling catalysis.11a Unfortunately, addition of 1 mmol (100 equiv per Ni) of LiI to the catalytic mixture reduced the TON to 14. It has been suggested by other reports that while the presence of iodide may aid some steps of the proposed catalytic cycle it could also act as a deactivating agent by trapping the catalyst as nickel(II) species.19 In an effort to reactivate any nickel(II) species, 2 mmol (200 equiv per Ni) of Zn dust was added as a potential reducing agent; however, the resulting TON of 30 showed only partial recovery of the prior catalytic activity. Replacing the LiI with NaI provided a TON of 35. A similar influence using NaI as also observed with (BenzP*)Ni(COD) as catalyst. However, conducting catalysis with only Zn dust as the additive produced a TON = 84, a substantial improvement from the unadulterated reaction and the Lewis acid/Zn combinations. A trial using (BenzP*)Ni(COD) also showed a modest enhancement to a TON = 109. The origin of this improved productivity is not entirely clear with the limited data in hand, but the enhancement is qualitatively similar to that observed with the (BenzP*)/Ni(COD)2 system.11b Prior studies of CO2−ethylene coupling to acrylate have noted the great importance of base selection in balancing the need to provide a thermodynamic driving force for the reaction and avoiding excessive side reactions with CO2 and other reaction components.11 With this factor in mind, we explored the use of a slightly less basic and more sterically encumbered phenoxide base, sodium 2-chlorophenoxide (2-Cl). Our initial experiments employed 2-Cl under the conditions which had afforded the highest TON with 3-F, including 2 mmol of Zn dust. This produced a rather disappointing TON of 30. Removal of the Zn additive from 1-COD also a limited TON of 21, suggesting that for the [(i-PrBenzP)Ni], 2-Cl holds little to no advantage over 3-F base. However, catalytic trials using (BenzP*)Ni(COD) with the Zn and 2-Cl afforded a notably active TON of 135, suggesting that the influence of the base is not uniform across catalysts bearing even quite similar ancillary ligands. Likewise, inclusion of cocatalytic amounts of NaI and Zn did little to improve the activity of 1-COD with 2-Cl base (TON = 30), but this additive combination proved quite potent for (BenzP*)Ni(COD) (TON = 404). The productivity of (BenzP*)Ni(COD) under these conditions is the highest TON for CO2−ethylene coupling reported to date, though the [(BenzP*)Ni] system was already a leading catalyst platform. Intriguingly, there appears to be a vastly different influence of the reductant and Lewis acid additives between the two similar (diphosphino)benzene nickel(0) catalysts, with Lewis acids

Figure 7. [BenzP*Ni] catalyzed sodium acrylate synthesis reported by Limbach and co-workers.

reaction.11b The use of Zn dust to enhance catalytic acrylate production was first described by Vogt and co-workers, though the role of Zn in these reactions has not been fully elucidated.11a One possibility is that Zn acts as a reducing agent to reactivate stable nickel(II) salts formed during catalysis. This hypothesis is partially supported by the observation that only zero-valent nickel complexes have thus far been reported to mediate CO2−ethylene coupling.12 Our investigations into [(i-PrBenzP)Ni] catalyzed CO2− ethylene coupling began with the use of 1-COD in conjunction with a base (sodium 3-fluorophenoxide, 3-F), which has previously been described for screening ancillary ligand effects.11b The use of isolated samples of 1-COD for catalysis was selected over the more common in situ catalyst generation technique of mixing ligand/Ni(COD)2 due to prior observations that such mixing can also produce (ligand)2Ni complexes, which appear catalytically inactive. A tetrahydrofuran solution of catalyst 1-COD was treated with excess 3-F under 30 bar of CO2/ethylene (1:2) for 20 h at 110 °C and produced a TON of 45 (Table 1). This is less than the TON of 82 achieved in Table 1. Nickel Catalyzed CO2−Ethylene Coupling to Acrylate

additives (equiv/Ni) catalyst

base

(i-PrBenzP)Ni(COD) (1-COD) (BenzP*)Ni(COD) 1-C3H4O2 1-COD 1-COD 1-COD (BenzP*)Ni(COD) 1-COD (BenzP*)Ni(COD) 1-COD 1-COD (BenzP*)Ni(COD) 1-COD (BenzP*)Ni(COD)

3-F 3-F 3-F 3-F 3-F 3-F 3-F 3-F 3-F 2-Cl 2-Cl 2-Cl 2-Cl 2-Cl

LiI

NaI

100 100 100 100

100 100

Zn dust

TONa

200 200 200 200 200 200

45 82 55 14 30 35 64 84 109 30 21 135 30 404

200 200 200

a

TON values reported per Ni equivalent and are the average of two or more trials. E

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Synthesis of (i-PrBenzP)Ni(COD) (1-COD). A 20 mL scintillation vial was filled with 141 mg (0.45 mmol) of Ni(COD)2 and dissolved in THF. Then, 109 mg (0.40 mmol) of i-PrBenzP was dissolved in THF and added dropwise to the nickel solution over a period of 10 min producing a dark brown solution. The reaction was stirred for 24 h at ambient temperature. The THF solvent was then removed under vacuum, and the dark residue was extracted with pentane, filtered through a Pasteur pipet, and dried under reduced pressure. The resulting solid was recrystallized in a minimal amount of pentane at −35 °C to afford 95 mg (50%) of 1-COD as yellow crystals. Anal. Found (calcd) for C25H44P2Ni: C, 64.18 (64.54); H, 9.13 (9.53). 1H NMR (C6D6): δ 0.90 (dd, J = 6.1, 12 Hz, 12H, isopropyl CH3), 0.97 (dd, J = 6.1, 12 Hz, 12H, isopropyl CH3), 2.34 (m, 4H, isopropyl CH), 2.49 (m, 4H, COD CH2), 2.65 (m, 4H, COD CH2), 4.43 (s, 4H, COD CH), 7.09 (m, 2H, C6H4), 7.40 (m, 2H, C6H4). 31P {1H} NMR (C6D6): δ 65.89 ppm (s). 13C {1H} NMR (C6D6): δ 18.66 (isopropyl CH3), 19.07 (isopropyl CH3), 26.31 (isopropyl CH), 32.81 (COD CH2), 80.38 (COD CH), 128.45 (Ar), 130.10 (Ar), 146.91 (Ar). Generation and Spectral Data for (i-PrBenzP)Ni(C2H4) (1C2H4). A J. Young NMR tube was charged with 10 mg of 1-COD and approximately 500 μL of C6D6. The tube was cooled to −196 °C, degassed, and 1 atm of ethylene was admitted via calibrated gas bulb. Upon thawing, 1-C2H4 forms immediately (as judged by NMR spectroscopy), although the sample’s visual appearance remains unchanged. The product was not stable in the absence of ethylene, and quickly reverted to 1-COD upon remove of the volatiles. NMR spectra indicate essentially complete conversion to1-C2H4 under these conditions. 1H NMR: δ 0.82 (m, 12H, isopropyl CH3), 1.14 (m, 12H, isopropyl CH3), 2.20 (m, 4H, isopropyl CH), 7.12 (m, 2H, Ar), 7.43 (m, 2H, Ar). The resonance for ethylene was not located due to rapid exchange with free ethylene. 31P {1H} NMR (C6D6): δ 71.70 (s). Synthesis of (i-PrBenzP)Ni(κC,κO−CH2CH2COO) (1-C3H4O2). A 20 mL scintillation vial was charged with 100 mg (0.64 mmol) of 2,2′-bipyridine, 35 mg (0.35 mmol) of succinic anhydride, and approximately 5 mL of THF. This mixture was then added dropwise to a solution of 176 mg (0.64 mmol) of Ni(COD)2 and 5 mL of THF in a second 20 mL scintillation vial. The reaction solution was stirred at ambient temperature for 4 h and then filtered through a fine frit. The resulting red powder (bipyNi-lactone) was collected and washed with pentane and diethyl ether. Then, 113 mg (0.40 mmol) of the red powder was combined with 122 mg (0.40 mmol) of i-PrBenzP and dissolved in approximately 10 mL of methylene chloride. The reaction was stirred at ambient temperature for 24 h, the solvent removed under vacuum, and the residue extracted with diethyl ether and filtered through Celite. The filtrate solution was dried under reduced pressure, and the resulting orange solid material was recrystallized in minimal diethyl ether at −35 °C to afford 130 mg (75%) of 1C3H4O2 as an orange precipitate. 1H NMR δ 0.78 (dd, J = 6.4, 12 Hz, 6H, isopropyl CH3), 0.86 (dd, J = 6.4, 12 Hz 6H, isopropyl CH3), 0.97 (dd, J = 6.4, 12 Hz, 6H, isopropyl CH3), 1.10 (m, 2H, α-CH2), 1.32 (dd, J = 6.4, 12 Hz, 6H, isopropyl CH3), 1.92 (m, 2H, P−CH), 2.18 (m, 2H, P−CH), 2.84 (m, 2H, β-CH2), 6.96 (m, 2H, Ar), 7.01 (m, 1H, Ar), 7.07 (m, 1H, Ar). 13C {1H} NMR (C6D6): δ 12.10 (αCH2), 18.32 (isopropyl CH3), 18.71 (isopropyl CH3), 19.14 (isopropyl CH3), 25.30 (isopropyl CH), 26.19 (isopropyl CH), 37.11 (β-CH2), 129.99, 130.40, 131.31 (Ar), 188.50 (lactone carbonyl). 31P {1H} NMR (C6D6): δ 62.92 (d, J = 9.0 Hz), 70.53 (d, J = 9.0 Hz). IR (KBr): νC−O = 1629 cm−1. Synthesis of (i-PrBenzP)Ni(η2-CH2CHCO2Na) (1-acrylate). A 20 mL scintillation vial was charged with 45 mg (0.10 mmol) of 1C3H4O2, 37 mg (0.20 mmol) of sodium hexamethyldisilazane, and approximately 10 mL of THF. The reaction was stirred for 30 min at ambient temperature and the volatiles removed under vacuum. The residue was washed with pentane followed by extraction with diethyl ether. Concentration of the diethyl ether solution and cooling to −35 °C for several days afforded 42 mg of 1-acrylate as a yellow-green solid. X-ray diffraction and NMR analysis indicated some contamination with sodium hexamethyldisilazane, which cocrystallized with 1-

proving far more enhancing for the chiral BenzP* system. There is insufficient data at present to draw any meaningfully hypothesis as to the origin(s) of these variations, but it certainly gives pause to the generalization of findings from one Ni catalyst system to the next.

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CONCLUDING REMARKS

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EXPERIMENTAL SECTION

The utilization of the CO2 toward the synthesis of acrylates is an endeavor which is just beginning to gain promise with the aid of transition metal catalyst development. The reactivity of the newly prepared catalyst, 1,2-bis(di-iso-propylphosphino)benzene nickel(0) 1,5-cyclooctadiene, indicates that the widely proposed stepwise process for CO2−ethylene (Figure 2) does appear to be a viable mechanism. However, most catalyst productivity improvements to date have relied on screening of ancillary ligands, optimization of reaction conditions, and the empirical use of cocatalytic additives. While these are undoubtedly effective techniques, our findings suggest that the influence of the base/reductant/Lewis acid mixtures in acrylate syntheses may not translate intuitively across even very similar catalyst structures. From an optimistic perspective, it is clear that relatively minor changes in the base or additives can result in significant improvements to catalytic performance. However, comparative catalysis experiments with 1-COD and (BenzP*)Ni(COD) indicated the application of an optimized formula for one catalyst architecture to other systems maybe misleading. Likely, there exist multiple interactions between the Zn reductant, Lewis acidic salts, phenoxide bases, and nickel species employed in these catalytic systems. Creating a more rational based approach to catalyst system development will require a much better understanding of the chemical intricacies of these complex reaction mixtures. Our laboratory is currently endeavoring to use the easily prepared 1-COD catalyst to investigate the role of each additive in the reaction and elucidate how these additives and bases interact to influence catalytic acrylate production.

General Considerations. All manipulations were carried out using standard vacuum, Schlenk, cannula, or glovebox techniques. Carbon dioxide (laser-grade) and ethylene (dry) were purchased from Airgas and used as received. i-PrBenzP was prepared as previously described, with the exception that a commercially available Grignard reagent was employed.20 BenzP* was obtained as racemic mixture as previously described.21 All other chemicals were purchased from Aldrich, Fisher, VWR, Strem, or Cambridge Isotope Laboratories. Phenoxide bases were prepared from the sodium hydride deprotonation of commercially available phenols. All other nonvolatile solids were dried under vacuum at 50 °C overnight. Solvents were dried and deoxygenated using literature procedures.22 1H, 13C, and 31P NMR spectra were recorded on Bruker 300 MHz DRX, 500 MHz DRX, or 600 MHz spectrometers at ambient temperature, unless otherwise noted. 1H and 13C chemical shifts are referenced to residual solvent signals; 31P chemical shifts are referenced to an external standard of H3PO4. Probe temperatures were calibrated using ethylene glycol and methanol as previously described.23 X-ray crystallographic data were collected on a Bruker CMOS diffractometer with Mo Kα radiation. Samples were collected in inert oil and quickly transferred to a cold gas stream. The structures were solved from direct methods and Fourier syntheses and refined by full-matrix least-squares procedures with anisotropic thermal parameters for all non-hydrogen atoms. Crystallographic calculations were carried out using SHELX programs. High-pressure catalytic hydrogenation reactions were performed using a Parr 5500 series compact reactor with glass insert. F

DOI: 10.1021/acs.organomet.8b00260 Organometallics XXXX, XXX, XXX−XXX

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Organometallics acrylate. 1H NMR: δ 0.81 (m, 6H, isopropyl CH3), 1.00 (m, 6H, isopropyl CH3), 1.28 (m, 6H, isopropyl CH3), 1.38 (m, 6H, isopropyl CH3), 1.93 (m, 2H, isopropyl CH), 2.03 (s, 1H, acrylate CH2), 2.40 (s, 1H, acrylate CH2), 3.16 (1H, acrylate CH) 2.51 (m, 2H, isopropyl CH), 6.82 (m, 1H, Ar), 7.09 (m, 1H, Ar), 7.35 (m, 1H, Ar), 8.93 (m, 1H, Ar).13C {1H} NMR (C6D6): δ 19.42 (isopropyl CH3), 19.61 (isopropyl CH3), 24.53 (isopropyl CH), 26.28 (isopropyl CH) 34.00 (O2CCHCH2), 49.70 (O2CCHCH2) 118.14, 127.77 (Ar) quaternary carbonyl carbon not located. 31P {1H} NMR (C6D6): δ 67.69 (d, J = 45 Hz), 72.97 (d, J = 45 Hz). General Procedure for Catalytic Acrylate Production Experiments. In a typical experiment, a stainless-steel autoclave reactor fitted with glass insert was charged with catalyst (0.01 mmol), sodium phenoxide base (8 mmol), zinc dust (2 mmol), Lewis acid salt (1 mmol), and 25 mL of THF. The vessel was sealed under an inert atmosphere and removed from the glovebox. The reactor was pressurized with 20 bar of ethylene and 10 bar of carbon dioxide, which were added sequentially. The reactor was then heated with stirring to 110 °C for 20 h. Following the reaction time period, the vessel was removed from the heating element, cooled in an ice water bath, and the atmosphere slowly vented. The reaction residue was extracted with D2O, and a standard of sodium sorbate was added as an internal standard. The organic soluble species were removed with diethyl ether washing. Quantitation of the acrylate salt was then determined by integration of the 1H NMR spectrum.

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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.8b00260. Selected NMR Spectra (PDF) Accession Codes

CCDC 1838682 contains 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

Wesley H. Bernskoetter: 0000-0003-0738-5946 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Curators of the University of Missouri are acknowledged for financial support of this work. M.N.H thanks the National Science Foundation (CHE-1350047) for summer support. K.S. was supported by the Sekisui Chemical Co., Ltd. W.H.B is a fellow of the Alfred P. Sloan Foundation.

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DOI: 10.1021/acs.organomet.8b00260 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00260 Organometallics XXXX, XXX, XXX−XXX