Synthesis and Characterization of Pincer-Molybdenum Precatalysts for

Article pubs.acs.org/Organometallics

Synthesis and Characterization of Pincer-Molybdenum Precatalysts for CO2 Hydrogenation Yuanyuan Zhang,†,‡ Paul G. Williard,‡ and Wesley H. Bernskoetter*,†,‡ †

Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States

‡

S Supporting Information *

ABSTRACT: A family of low-valent molybdenum complexes supported by the pincer ligand PNMeP (PNMeP = MeN(CH2CH2PPh2)2) was prepared and characterized, including (PNMeP)Mo(C2H4)2, which contains an agostic interaction between the metal and the N-methyl substituent. This β-agostic C−H bond was cleaved by molybdenum and produced a cyclometalated molybdenum formate complex, (κ4-PNMeP)Mo(C2H4)(κ2-O2CH), upon exposure to CO2. This species serves as a promotor of CO2 hydrogenation to formate under basic conditions, a rare transformation for group VI metals. The performance of the precatalyst was enhanced with the addition of Lewis acid salts.

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INTRODUCTION The price volatility associated with fossil fuel sourced commodity chemicals has motivated considerable research into the utilization of cheap, nontoxic, and sustainable carbon feedstocks.1 Carbon dioxide has emerged as a leading target among alternative carbon sources owing to its ready availability and low cost. While the direct utilization of CO2 in industry is unlikely to mitigate emissions from anthropogenic activities, its potential remains largely untapped. Currently, CO2 is used in the production of only a few chemicals, including urea, salicylic acid, and polycarbonates.2 This limited usage has spurred considerable interest in fundamental studies to establish useful methodologies for converting this ubiquitous resource into valuable chemicals.3 Our laboratory and others have found that molybdenum complexes are excellent mediators of reductive CO2 functionalization reactions, often by coupling CO2 with other reactive small molecules.4 Recently, Berke and co-workers reported a bifunctional PNP pincer supported molybdenum species, (N(CH2CH2PiPr2)2)Mo(NO)(CO), which readily couples carbon dioxide and dihydrogen to produce a molybdenum formate complex (Figure 1).5 This species may be stoichiometrically deprotonated with sodium amides to

liberate formate, though catalytic turnover was obviated by competing side reactions such as the deactivating cycloaddition of CO2 across a molybdenum−amide bond. Our own laboratory has employed a related pincer ligand, Triphos (Triphos = PhP(CH2CH2PPh2)2), on molybdenum to promote several CO2 functionalizations, including coupling with ethylene to form acrylates and stoichiometric hydrogenation to formate (Figure 2).4 Despite these advances, and the recent development of molybdenum-based catalysts for the dehydrogenation of formic acid,6 catalytic hydrogenation of CO2 using well-defined complexes based on this low-cost metal has remained elusive.7 The development of CO2 to formate hydrogenation processes has been an area of considerable activity in recent research, driven by formate’s use in numerous agrochemicals and its potential as a chemical hydrogen storage (CHS) material for renewable energy applications.8 While the development of CO2 hydrogenation using precious metals, such as Ru,9 Rh,10 and Ir,11 has made significant progress, in some cases reaching turnover numbers (TONs) of more than 3 million,11g,12 these species bear significant cost and toxicity concerns. The use of low-cost or earth-abundant metal catalysts could enhance the sustainability and economic viability of these transformations.13 Recently, a highly active iron complex supported by a tertiary amine pincer ligand, (MeN(CH2CH2PiPr2)2)Fe(H)BH4, was reported for the hydrogenation of CO2 to formate, with TONs approaching 60000.15 Though this conversion remains far below those achieved by precious metals, its performance is significantly higher than that of other earth-abundant catalysts which

Figure 1. CO2 insertion into a Mo−H bond as reported by Berke and co-workers. © XXXX American Chemical Society

Received: November 17, 2015

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DOI: 10.1021/acs.organomet.5b00955 Organometallics XXXX, XXX, XXX−XXX

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Figure 2. Formate and acrylate formation from CO2 by (Triphos)Mo(PPh3)(N2)2.

Figure 3. Reduction of 1-Cl3 in the presence of dinitrogen and subsequent formation of 1-PPh3.

Figure 4. Synthesis of 1-C2H4 and 2-C2H4 via reduction of 1-Cl3 in the presence of ethylene.

preceded it.13 Notably, use of the tertiary amine PNP ligand yielded much higher activity on iron than the corresponding bifunctional, secondary amine pincer ligand.14 Given this prior art, our laboratory sought to enhance the CO2 functionalization activity of low-cost molybdenum metal sources via the application of the tertiary amine pincer ligand PNMeP (PNMeP = MeN(CH2CH2PPh2)2). Our efforts, described herein, have resulted in the successful synthesis of several zerovalent molybdenum complexes utilizing this pincer ligand and leveraging those complexes in the development of a modest catalyst for thermochemical CO2 hydrogenation.

CO2 functionalization, as extended removal of the dinitrogen atmosphere resulted in a shift in the coordination mode of the triphenylphosphine to an η6-arene and expulsion of the N2 ligands to form 1-PPh3 (Figure 3). Like several previously reported 18-electron zerovalent molybdenum arene complexes,16,18 1-PPh3 was found to be unreactive toward CO2, ethylene, or H2 and was thermally stable even above 110 °C. A detailed characterization of 1-PPh3 may be found in the Experimental Section. Given the stability of 1-PPh3, the pursuit of more reactive [(PNMeP)Mo0] complexes focused on nonchelating ancillary ligands which lacked arene substituents. Ethylene was selected as an attractive stabilizing ligand, since it may also prove useful in coupling with CO2 toward acrylates. Alkali-metal reduction of 1-Cl3 under an ethylene atmosphere generated an orange product with an empirical composition of (PNMeP)Mo(C2H4)2. Analysis of the product species by 1H, 31P−1H COSY, 1H−13C HSQC, and 1H NOESY NMR experiments indicated a structure in which a C−H bond of the N-methyl group was oxidatively added to molybdenum, (κ4-PNCH2P)Mo(H)(C2H4)2 (2-C2H4) (Figure 4).11g,16,18b,19 This assignment is supported in the 1H NMR spectrum by a characteristic triplet Mo−H resonance at −2.06 ppm (2JP−H = 19 Hz) and a complex multiplet signal from 1.42 to 1.48 ppm assigned to the cyclometalated N−CH2. (see the Supporting Information for details). 1H−13C HSQC and 13C DEPT NMR spectra indicate that this 1H signal originates from a methylene unit, and strong 16 Hz coupling to the equivalent phosphines can be detected via 31P decoupling experiments. The 1H COSY NMR spectrum also indicates coupling between the cyclometalated N−CH2 and the Mo−H resonances. {1H−1H} NOESY NMR spectra (600 ms mixing time; 23 °C) displayed an exchange correlation between the Mo−H peak and the more downfield of two pairs of multiplet resonances from bound ethylene at 2.88, −0.03 ppm and 0.86, −0.63 ppm. This chemical exchange likely

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RESULTS AND DISCUSSION Coordination Chemistry of [(PNMeP)Mo]. Functionalization reactions of CO2 are frequently mediated by low-valent forms of transition-metal complexes, owing to the stronger thermodynamic bias for reducing the CO bond. A strategy of ligand coordination to a metal halide precursor followed by alkali-metal reduction was employed to obtain low-valent [(PNMeP)Mo] complexes. Stirring a tetrahydrofuran solution of PNMeP with (THF)3MoCl3 afforded the trivalent (PNMeP)MoCl3 (1-Cl3) as a poorly soluble yellow powder in moderate yield. 1-Cl3 was then reduced by treatment with an excess of sodium amalgam in the presence of dinitrogen and triphenylphosphine, a common stabilizing ligand, to generate the zerovalent species trans-(PNMeP)Mo(PPh3)(N2)2 (1-N2). 1-N2 was characterized by a combination of infrared and multinuclear NMR spectroscopy. The 31P NMR spectrum of 1N2 exhibited apparent doublet and triplet resonances at 53.09 and 75.54 ppm, respectively. Solid-state infrared analysis revealed two strong bands at 1933 and 2009 cm−1, indicative of a bis(dinitrogen) complex.16 The average frequency for these stretching vibrations indicates a notably weaker electron donation to the metal in comparison to analogous triphosphine chelate complexes.17 Unfortunately, 1-N2 proved ineffective for B

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molybdenum.4 Treating solutions of 2-C2H4 with approximately 1 atm of CO2 over 6 h afforded a single product, identified as (κ4-PNCH2P)Mo(C2H4)(κ2-O2CH) (2-O2CH) (eq 1). Crystals of 2-O2CH suitable for X-ray diffraction were

occurs via a fast reversible insertion of the hydride into the olefin coordinated adjacent to its position. Notably, no exchange correlation was observed between the Mo−H and the cyclometalated N−CH2 resonances (though a throughspace NOE cross peak was detected), suggesting that reductive elimination of the activated bond is not rapid on the NMR time scale. Interestingly, cooling and storing pentane solutions of the reduction product at −35 °C afforded X-ray diffraction quality crystals in which the molybdenum coordination sphere is completed by a β-agostic C−H bond from the N-methyl substituent, (PNMeP)Mo(C2H4)2 (1-C2H4), rather than an oxidative addition product (Figure 5). The crystal sizes were

obtained from chilled diethyl ether and revealed a molecular structure with a cyclometalated N-methyl group and κ2-formate bound approximately trans to the Mo−N bond (Figure 6). The

Figure 5. Molecular structure of (PNMeP)Mo(C2H4)2 (1-C2H4) with ellipsoids at the 30% probability level. Hydrogen atoms bound to C(33) are shown in calculated positions.

small, and the data were not of sufficient quality to locate and freely refine all hydrogens. Those shown attached to the Nmethyl carbon C(33) are in calculated positions. The Mo(1)− C(33) distance of 2.21(1) Å is typical of β-agostic C−H bonds at molybdenum20 and is longer than the 2.123(4) Å length reported by Mösch-Zanetti and co-workers for a closely related cyclometalated N-methyl ligand on molybdenum.21 Although the difference in Mo−C bond lengths is modest, it may be enough to support a conclusion that the N-methyl group in 1C2H4 is not coordinated to Mo as part of an oxidative addition of the C−H bond, as would be expected in the structure of 2C2H4. Weaker support of the β-agostic structure comes from the Mo(1)−H(33c) distance of 1.574 Å, which, even from its calculated position, is shorter than most reported β-agostic Mo−H bonds (1.88−2.27 Å)20d This β-agostic structure is a likely intermediate along the pathway for oxidative addition of the N-methyl C−H bond to produce 2-C2H4, though attempts to better characterize its solid-state structure by growing larger crystal samples were unsuccessful.22 Attempts to observe an equilibrium between 1-C2H4 and 2-C2H4 in solution were also unsuccessful, even after storing samples in arene solvent at −78 °C for 24 h. This evidence suggests that the concentration of the β-agostic structure in solution is at best fleeting, even at lower temperature. Carbon Dioxide Functionalization. The observation of the divalent cyclometalated complex 2-C2H4 suggested that this molybdenum system may be better suited for CO2 insertion than for reductive coupling with ethylene to generate acrylate, since acrylate formation has only be observed using zerovalent

Figure 6. Molecular structure of (PNCH2P)Mo(C2H4)(κ2-CHO2) (2O2CH) with ellipsoids at the 30% probability level. All hydrogen atoms are omitted for clarity.

nearly identical C(32)−O(1) and C(32)−O(2) bond lengths of 1.247(6) and 1.257(7) Å indicate strong resonance delocalization of the CO bond in the formate. The Mo(1)−C(31) bond distance of 2.154(6) Å is consistent with oxidative addition of the N-methyl group and is significantly shorter than that observed in 1-C2H4. The most notable feature in the 1H NMR spectrum of 2-O2CH is a singlet resonance assigned to the formate proton at 7.70 ppm, which splits into a doublet (1JC−H = 203 Hz) upon labeling with 13 CO2. A corresponding resonance at 165.5 ppm was also enhanced in the 13C{1H} NMR spectrum of an isolated sample. The stoichiometric reduction of CO2 to formate via insertion into the Mo−H bond of 2-C2H4 offered the possibility of establishing a catalytic cycle for CO2 hydrogenation. As the hydrogenation of carbon dioxide to formic acid is thermodynamically disfavored, addition of an exogenous base or other trapping agent is required to drive the process. 1,8Diazabicycloundecene (DBU) is one such base which has been successfully applied to carbon dioxide hydrogenation in several nonaqueous catalytic systems23 and was selected for initial investigation of the [(PNMeP)Mo] platform. Treatment of a 2-O2CH tetrahydrofuran solution with 100 equiv of DBU and a 1:1 mixture of 69 atm of H2 and CO2 at 100 °C for 16 h in an autoclave reactor afforded free formate as detected by C

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NMR spectroscopy. However, less than 1 equiv of formate per Mo was observed, indicating that the reaction was noncatalytic. To better understand this reaction, a related experiment was conducted in an NMR tube. A sample of 2-O2CH in tetrahydrofuran-d8 was treated with 10 equiv of DBU for 24 h at ambient temperature, with no observable changes occurring. This observation suggests deprotonation of the Mo−H by base alone is insufficient to efficiently remove the formate. Addition of 1 atm each of H2 and CO2 also failed to induce a reaction over a similar time period. The removal of formate has been implicated as a limiting step in several catalytic CO2 hydrogenation reactions,11c,12,24 including our own laboratory’s recent work using iron catalysts.15 It was discovered that treatment with cocatalytic amounts of Lewis acidic alkali-metal salts dramatically improved formate extrusion from iron; thus, the influence of added lithium triflate (LiOTf) was explored in formate extrusion from 2-O2CH. Once again the reaction was first studied in NMR tube experiments in which 2-O2CH in tetrahydrofuran-d8 was treated with 10 equiv of DBU and 5 equiv of LiOTf. Although no reaction was initially observed, the addition of 1 atm each of H2 and CO2 immediately resulted in extrusion of formate as detected by 1H NMR spectroscopy and complete consumption of 2-O2CH in the 31P NMR spectrum (60−80% yield of free ligand was also observed). Encouraged by these results, we reinvestigated catalytic CO2 hydrogenation using 2-O2CH using LiOTf as a cocatalyst. The addition of 50 equiv of LiOTf to 2-O2CH along with 100 equiv of DBU and 69 atm of a 1:1 mixture of H2 and CO2 at 100 C for 16 h now produced formate in catalytic quantities. A turnover number (TON) of 16(2) was measured for this reaction, with similar reactions conducted for 24 h yielding no additional formate. Control experiments conducted in the absence of 2-O2CH produced only trace amounts of formate. A higher TON of 35(5) was achieved by changing the solvent to 1,4-dioxane, similar to the solvent effect reported for related iron-catalyzed CO2 hydrogenations.15

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out using standard vacuum, Schlenk, cannula, or glovebox techniques. Ethylene, hydrogen, and carbon dioxide were purchased from Corp Brothers, of which ethylene and carbon dioxide were stored over 4 Å molecular sieves in heavy-walled glass vessels prior to use in NMR tube experiments. High-pressure catalytic trials employed the gases as received. (THF)3MoCl3 and (Ph2PCH2CH2)2NMe were prepared as previously described.27 All other chemicals were purchased from Aldrich, Fisher, VWR, Strem, or Cambridge Isotope Laboratories. Solvents were dried and deoxygenated using literature procedures.28 1 H, 13C, and 31P NMR spectra were recorded on Bruker DRX 400 Avance, 300 Avance, and 600 Avance MHz spectrometers. 1H and 13C chemical shifts are referenced to residual protio 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.29 IR spectra were recorded on Jasco 4100 FTIR and Metler Toledo React IR spectrometers. X-ray crystallographic data were collected on a D8 QUEST diffractometer using Mo 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 SHELXTL. Elemental analyses were performed at Atlantic Microlab, Inc., in Norcross, GA, or Robertson Microlit Laboratories in Ledgewood, NJ. Preparation of (PNMeP)MoCl3 (1-Cl3). A 25 mL scintillation vial was charged with 588 mg (1.29 mmol) of (Ph2PCH2CH2)2NMe, 421 mg (1.00 mmol) of (THF)3MoCl3, and approximately 8 mL of tetrahydrofuran. The mixture was stirred at ambient temperature for 12 h, affording copious yellow precipitate. The precipitate was collected by filtration, washed with cold tetrahydrofuran, and dried in vacuo to yield 347 mg (53%) of 1-Cl3. Anal. Calcd for (PNMeP)MoCl3, C29H31Cl3MoNP2: C, 52.95; H, 4.75; N, 2.13. Found: C, 52.78; H, 4.84; N, 2.03. 1H NMR (23 °C, CD2Cl2): δ −39.27, −19.51, −6.16, 1.32, 8.59, 9.19, 12.51, 12.92, 15.71, 18.46, 21.56. Preparation of trans-(PNMeP)Mo(PPh3)(N2)2 (1-N2). A 25 mL scintillation vial was charged with 7.035 g (1.59 mmol) of 0.5% sodium amalgam, 0.100 g (0.152 mmol) of 1-Cl3, 40 mg (0.152 mmol) of triphenylphosphine, and approximately 8 mL of tetrahydrofuran under a nitrogen atmosphere. The mixture was stirred at ambient temperature for 16 h, and the volatiles were removed in vacuo. The residue was extracted with a diethyl ether/tetrahydrofuran (3/1) mixture, filtered through Celite, and chilled at −35 °C to afford 139 mg (73%) of 1-N2 as a red-orange powder. Due to the loss of dinitrogen caused by the extended periods of evacuation required for removal of trace solvent, satisfactory elemental analysis could not be obtained. However, NMR spectra have been deposited in the Supporting Information to provide evidence of purity. 1H NMR (23 °C, C6D6): δ 1.99−2.12 (m, 2H, CH2), 2.19−2.27 (m, 2H, CH2), 2.46 (m, 2H, CH2), 2.51 (s, 3H, NCH3), 2.68 (m, 2H, CH2), 6.79−6.83 (m, 3H, PPh), 6.87−6.91 (m, 3H, PPh), 7.03−7.10 (m, 6H, PPh), 7.19−7.21 (m, 3H, PPh), 7.39 (m, 3H, PPh), 7.67 (m, 2H, PPh). 13C NMR (23 °C, C6D6): δ 35.44, 61.28 (CH2) 47.25 (NMe), 127.16, 127.41, 128.50, 128.5, 128.83, 133.02, 134.10, 134.20, 135.16, 138.07, 142.58, 142.92 (Ar). 31P{1H} NMR (23 °C, C6D6): δ 54.2 (d, 6.7 Hz, 2P, PPh2), 76.8 (t, 6.7 Hz, 1P, PPh3). IR (KBr): νN−N 1933, 2009 cm−1. Preparation of (PNMeP)Mo(η6-PPh3) (1-PPh3). Heating pure samples of 1-N2 under vacuum for 12 h at 80 °C in arene solvent resulted in conversion to 1-PPh3 in essentially quantitative yield. Pure material could be obtained by simple removal of the solvent under vacuum. Anal. Calcd for (PNMeP)MoPPh3, C47H46MoNP3: C, 69.37; H, 5.70; N, 1.72. Found: C, 69.50; H, 5.58; N, 1.47. 1H NMR (23 °C, C6D6): δ 1.86 (m, 4H, CH2), 2.00 (m, 2H, CH2), 2.10 (m, 2H, CH2), 2.52 (s, 3H, NCH3), 3.46 (m, 2H, η6-C6H5PPh2), 3.56 (m, 2H, η6C6H5PPh2), 3.76 (m, 1H, η6-C6H5PPh2), 6.90 (m, 6H, PPh), 6.98 (m, 2H, PPh), 7.05 (m, 8H, PPh), 7.10 (m, 6H, PPh), 7.56 (m, 8H, PPh). Partial 13C NMR from 1H−13C HSQC: δ 31.82, 61.87 (CH2), 55.58

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CONCLUDING REMARKS The pincer ligand MeN(CH2CH2PPh2)2 was successfully used to stabilize a small family of formally zerovalent molybdenum complexes, including 1-PPh3, 1-N2, and 1-C2H4. Low-temperature X-ray diffraction experiments suggest that 1-C2H4 possessed an unexpected β-agostic C−H interaction with the N-methyl moiety, although in solution only the oxidative addition product, 2-C2H4, was directly observed. The Mo−H in 2-C2H4 proved sufficiently nucleophilic to insert CO2 and yield the molybdenum(II) formate complex 2-O2CH, bearing a cyclometalated PNMeP ligand. Complex 2-O2CH was found to be catalytically active for CO2 hydrogenation to formate in the presence of Bronsted base and Lewis acid. The maximum TON of 35 is quite modest in comparison to the activity of recent Fe, Co, and precious-metal catalyst discoveries; however, catalytic CO2 to formate hydrogenations at molybdenum are exceedingly rare.11c,13,15,24,25 In fact, to the best of our knowledge, 2O2CH represents the first defined homogeneous Mo catalyst for this process.26 Future studies into further development of this catalyst platform, the possible mechanistic role of the ligand cyclometalation, and the utility of these species for other CO2 reduction reactions could enhance the utilization of low-cost molybdenum in reversible CO2 hydrogenation processes. D

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(NMe), 66.77, 75.34, 79.09 (η6-C6H5PPh2), 127.34, 127.81, 127.84, 128.12, 128.33, 131.21, 133.80 (Ar). 31P{1H} NMR (23 °C, C6D6): δ −3.12 (3, 1P, PPh3), 61.2 (t, 2P, PPh2). Preparation of (κ4-PNCH2P)Mo(H)(C2H4)2 (2-C2H4). A heavywalled glass vessel was charged with 100 mg (0.152 mmol) of [(Ph2PCH2CH2)2NMe]MoCl3, 10 equiv of 0.5% sodium amalgam, and approximately 8 mL of tetrahydrofuran. On a vacuum line 1 atm of ethylene was added via calibrated gas bulb at −196 °C. The reaction mixture was stirred at ambient temperature for 18 h, resulting in a near-colorless solution. After removal of volatiles in vacuo, the residue was extracted with toluene under a nitrogen atmosphere and filtered through Celite, resulting in an orange solution. Layering pentane on top of the above solution and chilling at −35 °C afforded 101 mg of orange crystals (73% yield). Analysis of the crystals by X-ray diffraction conclusively determined the structure of the product to be the βagostic species 1-C2H4; however, dissolving the material in ambienttemperature benzene-d6 indicated that the species present in solution is the cyclometalated product 2-C2H4. Due to the loss of ethylene caused by the extended periods of evacuation required for removal of trace solvent, satisfactory elemental analysis could not be obtained. However, NMR spectra have been deposited in the Supporting Information to provide evidence of purity. Characterization data for 2C2H4 are as follows. 1H NMR (23 °C, C6D6): δ −2.06 (broad t, 19 Hz, 1H, Mo-H), −0.63 (m, 2H, C2H4), −0.01 (m, 2H, C2H4), 0.89 (m, 2H, C2H4), 1.42−1.48 (m, 2H, Mo−CH2), 1.86 (m, 2H, CH2), 2.12 (m, 2H, CH2), 2.30−2.41 (m, 4H, CH2), 2.89 (m, 2H, C2H4), 6.74−6.82 (m, 2H, C6H5), 6.87 (m, 6H, C6H5), 7.20−7.25 (m, 6H, C6H5), 7.69 (m, 2H, C6H5), 8.14 (m, 2H, C6H5), 8.28 (m, 2H, C6H5). Partial 13C NMR from 1H−13C HSQC: δ 26.0 (PCH2), 33.8 (MoCH2), 40.6 (C2H4), 46.1 (C2H4), 53.9 (NCH2), 127.2 (C6H5), 127.3 (C6H5), 127.9 (C6H5), 131.0 (C6H5), 135.3 (C6H5). 31P{1H} NMR (23 °C, C6D6): δ 87.5 (s, 1P, PPh2). Preparation of (κ4-PNCH2P)Mo(C2H4)(κ2-CHO2) (2-O2CH). A heavy-walled glass vessel was charged with 100 mg (0.17 mmol) of 2C2H4 and approximately 8 mL of tetrahydrofuran. On a vacuum line 4 equiv of carbon dioxide was added via a calibrated gas bulb (1060 Torr in 28.9 mL) at −196 °C. The reaction mixture was stirred for 8 h at ambient temperature, and then the volatiles were removed in vacuo. The residue was extracted with diethyl ether, concentrated, and chilled to −35 °C, affording 87 mg (85%) of 2-O2CH as a red-orange microcrystalline powder. Anal. Calcd for (PNCH2P)Mo(C2H4)(κ2CHO2), C32H35MoNO2P2: C, 61.64; H, 5.66; N, 2.25. Found: C, 60.82; H, 5.98; N, 2.52. 1H NMR (23 °C, C6D6): δ 0.69 (m, 2H, C2H4), 1.25 (m, 2H, C2H4), 1.46 (m, 2H, CH2), 1.84 (m, 2H, CH2), 2.10 (apparent t, 2H, 10 Hz, Mo−CH2), 2.17 (m, 2H, CH2), 2.30 (m, 2H, CH2), 7.00 (t, 2H, C6H5), 7.05 (t, 2H, C6H5), 7.10 (t, 4H, C6H5), 7.19 (t, 4H, C6H5), 7.45 (m, 4H, C6H5), 7.70 (s, 1H, O2CH), 8.29 (m, 4H, C6H5). When 13CO2 was employed in the synthesis, the resonance at 7.70 ppm became a doublet with 1JC−H = 203 Hz. 1 Partial 13C NMR from 1H−13C HSQC: δ 24.7 (PCH2), 39.3 (MoCH2), 41.0 (C2H4), 51.1 (NCH2), 127.4 (C6H5), 128.3 (C6H5), 131.0 (C6H5), 134.0 (C6H5), 165.4 (O2CH). 31P{1H} NMR (23 °C, C6D6): δ 60.7 (s, 1P, PPh2). General Procedure of Catalytic CO2 Hydrogenation Studies. In a typical experiment, a 50 mL glass reactor liner was charged with 2O2CH (5.3 mg, 10 μmol) inside an inert-atmosphere glovebox. The sample was then dissolved in 5 mL of THF or 1,4-dioxane and treated with 100 equiv (per Mo) of DBU and (where applicable) 50 equiv (per Mo) of LiOTf. The reactor liner was then sealed inside a Parr reactor and removed from the glovebox. The reactor was pressurized with 69 atm of a 1/1 CO2/H2 mixture at ambient temperature and then heated and stirred at 100 °C for 16 h. The reaction was stopped by removal from the heat source and venting of the gases. The product solution and D2O washing of any residual solids were then quickly transferred to a 50 mL round-bottom flask, and the volatiles were removed under reduced pressure. The residue was dissolved in approximately 2 mL of D2O. A 10 μL portion of DMF was then added as an internal standard to quantify the formate product by 1H NMR spectroscopy.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00955. Selected NMR spectra (PDF) X-ray data for 1-C2H4 (CIF) X-ray data for 2-O2CH (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for W.H.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the Air Force Office of Scientific Research (Award No. FA9550-11-1-0041) and the Curators of the University of Missouri. W.H.B. is a fellow of the Alfred P. Sloan Foundation.

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DOI: 10.1021/acs.organomet.5b00955 Organometallics XXXX, XXX, XXX−XXX