Dalton Transactions View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
PAPER
Cite this: Dalton Trans., 2014, 43, 15990
View Journal | View Issue
Nickel promoted functionalization of CO2 to anhydrides and ketoacids† Zoe R. Greenburg, Dong Jin, Paul G. Williard and Wesley H. Bernskoetter* The reductive functionalization of carbon dioxide into high value organics was accomplished via the coupling with carbon monoxide and ethylene/propylene at a zerovalent nickel species bearing the 2-((dit-butylphosphino)methyl)pyridine ligand (PN). An initial oxidative coupling between carbon dioxide, olefin, and (PN)Ni(1,5-cyclooctadiene) afforded five-membered nickelacycle lactone species, which were produced with regioselective 1,2-coupling in the case of propylene. The propylene derived nickelacycle
Received 25th April 2014, Accepted 11th September 2014
lactone was isolated and characterized by X-ray diffraction. Addition of carbon monoxide, or a combi-
DOI: 10.1039/c4dt01221f
nation of carbon monoxide and diethyl zinc to the nickelacycle lactone complexes afforded cyclic anhydrides and 1,4-ketoacids, respectively, in moderate to high yields. The primary organometallic product of
www.rsc.org/dalton
the transformation was zerovalent (PN)Ni(CO)2.
Introduction Rising energy costs and climatological changes associated with carbon combustion have heightened society’s sensitivity to its dependence on non-renewable fossil fuels. Though less pervasive, the same reliance on coal, natural gas, and petroleum which constrains our energy supply is also found in the production of numerous carbon based commodity chemicals.1 These sustainability and economic influences have driven research into the utilization of inexpensive and renewable carbon feedstocks to satisfy the demand for industrial chemicals.2 The functionalization of carbon dioxide has emerged as a feedstock of particular interest due to its immense abundance, low toxicity, and identity as a greenhouse gas emission.3 Unfortunately the utilization of CO2 in chemical and related applications is currently limited to roughly 130 MT year−1, largely due to a lack of reactions which surmount the strong kinetic and thermodynamic stability of the gas.4 Our laboratory and others have been working to expand this scope of chemical CO2 utilization by developing transition metal mediated processes which couple CO2 with light olefins.5 The chemistry of CO2–olefin coupling has been an area of interest since the seminal reports of C–C bond formation between CO2 and ethylene by Hoberg and Carmona in the mid-1980’s.6 In particular, Hoberg’s isolation of a nickelacycle lactone (also termed nickelalactone) complex (Fig. 1) has Department of Chemistry, Brown University, Providence, RI 02912, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. CCDC 997717–997719. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01221f
15990 | Dalton Trans., 2014, 43, 15990–15996
Fig. 1
Hoberg’s nickelalactone synthesis from CO2.
inspired considerable effort to establish catalytic syntheses of acrylates from CO2 and ethylene which could alter the renewability of acrylic materials such as super water absorbing polymers.1 Recently researchers have reported progress in this endeavor by using strong external bases and Lewis acids to promote acrylate formation from diphosphine nickelalactone complexes.5,7 However, the reticence of nickelalactone species to undergo β-hydride elimination processes is still a limiting factor in the utilization of this coupling reaction.5 Thus, developing reactions of nickelalactone complexes which further functionalize the metallacycle without necessitating β-elimination may be an attractive method to leverage low-valent nickel species which couple CO2 with light olefins. The ring-expansion of nickelalactone complexes by insertion of organic unsaturates has previously been observed for a handful of diamine and diphosphine supported complexes.6 In most cases, saturated 5-membered nickelalactone species were treated with an alkyne, activated olefin, or carbon dioxide to afford a 7-membered metallacycle which was hydrolyzed to obtain carboxylate products.6 Unfortunately, the addition of acid makes development of these reactions into catalytic processes a significant challenge given the incompatibility of Bronsted acids and low-valent nickel. An alternative approach is the insertion of strong field ligands which not only functionalize the five-membered nickelalactone species, but also induce elimination of up-converted organics derived from
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
CO2. Herein, we describe the coupling of CO2 and light olefins to generate new amino-phosphine nickelalactones with good regioselectivity, and the use of strong field carbon monoxide to afford valuable cyclic anhydride and 1,4-ketoacid products.
Methods Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
General considerations All manipulations were carried out using standard vacuum, Schlenk, cannula or glovebox techniques. Ethylene and carbon dioxide were purchased from Corp Brothers and stored over 4 Å molecular sieves in heavy walled glass vessels prior to use. Argon and nitrogen were purchased from Corp Brothers and used as received. 2-((di-t-butylphosphino)methyl)pyridine (PN) was prepared according to literature procedure.8 All other chemicals were purchased from Aldrich, VWR, Strem, Fisher Scientific or Cambridge Isotope Laboratories. Volatile, liquid chemicals were dried over 4 Å molecular sieves and distilled prior to use. Solvents were dried and deoxygenated using literature procedures.9 1 H, 13C, and 31P NMR spectra were recorded on Bruker DRX 400 Avance, 300 and 600 MH Avance spectrometers. 1H and 13 C chemical shifts are referenced to residual solvent signals; 31 P chemical shifts are referenced to an external standard of H3PO4. Probe temperatures were calibrated using ethylene glycol and methanol as previously described.10 Unless otherwise noted, all NMR spectra were recorded at 23 °C. IR spectra were recorded on a Jasco 4100 FTIR spectrometer. GC-MS data were recorded using a Hewlett-Packard (Agilent) GCD 1800C GC-MS spectrometer. X-ray crystallographic data were collected on a Bruker D8 QUEST diffractometer. Samples were collected in inert oil and quickly transfered 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. CIF data for 1, 2c, 3b has been deposited with the CCDC under numbers 997718, 997717, and 997719, respectively. Elemental analyses were performed at Atlantic Microlab, Inc., in Norcross, GA. All geometry optimizations were performed using Gaussian 09 Revision A.02,11 using the M06 l functional. The LANL2DZ basis set was used for Ni and the 6-31G++(d,p) basis set was used for all other atoms. The LANL2DZ pseudo-potential was used for Ni. Frequency calculations were performed on all optimized structures to ensure that they were true minima. (PN)Ni(COD) (1). A 20 mL scintillation vial was charged with 0.058 g (0.26 mmol) of Ni(COD)2 and approximately 5 mL of tetrahydrofuran. The solution was chilled to −35 °C and 0.074 g (0.31 mmol) of 2-((di-t-butylphosphino)methyl)pyridine (PN) in diethyl ether was added slowly over 2 hours turning the solution dark blue. After stirring at ambient temperature for a further 30 minutes, the volatiles were removed, and the residue extracted with pentane. Concentrating and chilling the pentane extraction to −35 °C overnight afforded 49 mg (75%) of 1 as dark blue crystals. Anal. Calcd for
This journal is © The Royal Society of Chemistry 2014
Paper
C22H36NPNi: C, 65.37; H, 8.98, N, 3.47. Found: C, 64.99; H, 8.28; N, 3.38. 1H NMR (C6D6): δ 1.15 (d, 11.6 Hz, 18H, PtBu), 2.20 (m, 4H, CH2–COD), 2.44 (m, 4H, CH2–COD), 2.67 (d, 2H, 6 Hz, 2H, CH2–P), 2.80 (m, 2H, CH2–COD), 4.21 (m, 2H, CH– COD), 4.82 (m, 2H, CH–COD), 6.35 (t, 1H, py), 6.51 (d, 1H, py), 6.80 (t, 1H, py), 8.95 (d, py). 31P {1H} NMR (C6D6): δ 68.5 (s). Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 29.39 (PtBu), 31.04, 31.75 (CH2–COD), 33.26 (P–CH2), 78.99, 82.37 (CH–COD), 121.22, 121.65, 129.98, 151.02 ( py). (PN)Ni(C2H4) (2a). A J. Young NMR tube was charged with 0.030 g (0.07 mmol) of 1a in approximately 500 µL of benzened6. On a high vacuum line 5 equivalents of ethylene were added via a calibrated gas bulb at −196 °C. The reaction was allowed to warm to ambient temperature, immediately changing color from dark blue to dark pink. Full conversion to 2a was confirmed by NMR spectroscopy. 1H NMR (C6D6): δ 1.13 (d, 12 Hz, 18H, PtBu), 2.36 (br, 4H, ethylene), 2.65 (d, 6 Hz, 2H, CH2–P) 6.23 (t, 1H, py), 6.63 (d, 1H, py), 6.93 (t, 1H, py), 9.35 (d, py). 31P {1H} NMR (C6D6): δ 71.99. Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 28.10 (PtBu), 31.04, 33.61 (C2H4), 33.24 (P–CH2), 121.51, 122.45, 132.77, 151.99 ( py). (PN)Ni(C2H3CH3) (2b). A J. Young NMR tube was charged with 0.0075 g (0.032 mmol) of Ni(COD)2, 0.076 g (0.029 mmol) of PN ligand and approximately 500 µL of benzene-d6. On a high vacuum line 10 equivalents (140 torr in 28.9 mL) of propylene were added via calibrated gas bulb at −196 °C. The reaction was allowed to warm to ambient temperature, quickly changing color from dark blue to an intense purple. Analysis by NMR showed near quantitative conversion to 2b, with two isomers apparent by 31P NMR in a 2 : 1 ratio. Only the major isomer was resolved in the 1H NMR and HSQC NMR spectra. The sample reverted to 1 upon removal of propylene. 1H NMR (C6D6): δ 1.15 (d, 12 Hz, 18H, PtBu), 1.89 (br, 3H, H2CvCHCH3), 2.18 (br, 1H, H2CvCHCH3), 2.43 (br, 1H, H2CvCHCH3), 2.64 (br, 2H, CH2–P), 3.13 (br, 1H, H2CvCHCH3) 6.26 (m, 1H, py), 6.63 (m, 1H, py), 6.93 (m, 1H, py), 9.34 (m, 1H, py). 31P {1H} NMR (C6D6): δ 72.41 (minor), 68.83 (major). Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 23.95 (H2CvCHCH3), 29.94 (PtBu), 32.09 (H2CvCHCH3), 33.67 (CH2–P), 121.83, 122.75, 132.88, 151.41 ( py), H2CvCHCH3 not located due to broadening from exchange with free propylene (exchange confirmed by 2D NOESY with 300 ms mixing time). (PN)Ni(C2H3CN) (2c). A 25 mL round bottom flask fitted with a vacuum adapter was charged with 0.066 g (0.278 mmol) of Ni(COD)2, 0.076 (0.276 mmol) of PN ligand and approximately 10 mL of toluene. On a high vacuum line 2 equivalents (76 torr in 101 mL) of acrylonitrile were added via calibrated gas bulb at −196 °C. The reaction was allowed to warm to ambient temperature, quickly changing color from dark blue to orange. After stirring for 3 hours, the volatiles were removed, the orange residue rinsed with pentane and extracted with 5 mL of 1 : 1 tetrahydrofuran–diethyl ether. The orange solution was chilled to −35 °C overnight to afford 0.043 g of 2c as orange powder. Concentrating the mother liquor produced a second 0.022 g fraction of product for a total yield of 0.065 g
Dalton Trans., 2014, 43, 15990–15996 | 15991
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Paper
(69%). Anal. Calcd for C17H27NiN2P: C, 58.49; H, 7.80; N, 8.02. Found: C, 58.09; H, 7.49; N, 8.02. 1H NMR (C6D6): δ 0.93 (d, 7.4 Hz, 18H, PtBu), 1.89 (br, 1H, H2CvCHCN), 2.16 (br, 1H, H2CvCHCN), 2.19 (br, 1H, H2CvCHCN), 2.51 (br, 2H, CH2– P), 6.27 (m, 1H, py), 6.50 (m, 1H, py), 6.79 (m, 1H, py), 9.29 (m, 1H, py). 31P {1H} NMR (C6D6): δ 77.52. Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 11.92 (H2CvCHCN), 28.51 (PtBu), 28.80 (H2CvCHCN), 32.38 (CH2–P), 122.07, 122.23, 134.86, 152.15 ( py). (PN)Ni(κC,κO-CH2CH2COO) (3a). A 50 mL Schlenk flask was charged with 0.200 g (0.00073 mol) of Ni(COD)2, approximately 15 mL of tetrahydrofuran and placed under 1 atm of ethylene. A tetrahydrofuran solution containing 0.172 g (0.00073 mol) of PN ligand was added via syringe and stirred for 20 minutes at ambient temperature. CO2 was then admitted to the reaction by sparging the solution for 10 minutes. The reaction mixture was stirred overnight, and the product, a light yellow powder, was collected by cannula filtration to obtain 245 mg (91%) of 3a following rinsing with additional tetrahydrofuran. Anal. Calcd for C17H28NiNPO2: C, 55.48; H, 7.67; N, 3.81. Found: C, 55.81; H, 7.00; N, 3.47. 1H NMR (acetone-d6): δ 0.86 (m, 2H, Ni-α-CH2) 1.43 (d, 12 Hz, 18H, tBu), 2.08 (m, 2H, Ni-β-CH2), 3.42 (d, 9 Hz, 2H, CH2–P), 7.29 (t, 1H, py), 7.56 (d, 1H, py) 7.87 (t, 1H, py), 8.86 (d, 1H, py). 31P {1H} NMR (acetone-d6): δ 71.01 (s). Partial 13C NMR taken from 1H–13C HSQC NMR (CD2Cl2): δ −1.7 (Ni-α-CH2), 29.1 (PtBu), 39.1 (Ni-β-CH2), 32.8 (CH2–P), 122.0, 122.2, 137.7, 149.3 ( py). 13C {1H} NMR of 3a labeled with 13CO2 (acetone-d6): δ 185.51 (CO2). (PN)Ni(κC,κO-CH2CH2(CH3)COO) (3b). A J. Young NMR tube was charged with 0.028 g of PN ligand and 0.019 g of Ni(COD)2 in tetrahydrofuran. On a high vacuum line, 3 equivalents of propene were added via calibrated gas bulb at −196 °C. The sample was thawed and allowed to stand at ambient temperature for 1 hour where it changed to a deep pink solution. Then an additional 3 equivalents of CO2 were added via calibrated gas bulb at −196 °C and the sample allowed to stand overnight at ambient temperature. Monitoring by 31P NMR spectroscopy showed complete disappearance of 2b after that time, and then 30 mg (76%) of 3b was collected as a yellow powder by filtration. Anal. Calcd for C18H30NiNPO2: C, 56.58; H, 7.91; N, 3.67. Found: C, 57.09; H, 7.57; N, 3.39. 1H NMR (CD2Cl2): δ 0.56 (br, 1H, Ni-α-CH2) 1.04 (br, 3H, Niγ-CH3), 1.22 (br, 1H, Ni-α-CH2), 1.35 (d, 14 Hz 18H, tBu), 2.36 (br, 1H, Ni-β-CH), 3.14 (br, 2H, CH2–P), 7.17 (br, 1H, py), 7.33 (br, 1H, py) 7.69 (br, 1H, py), 8.90 (br, 1H, py). 31P {1H} NMR (CD2Cl2): δ 71.97 (s). Partial 13C NMR taken from 1H–13C HSQC NMR (CD2Cl2): δ 9.42 (Ni-α-CH2), 21.15 (Ni-γ-CH3), 29.46 (PtBu), 33.45 (CH2–P), 44.18 (Ni-β-CH), 122.33, 122.66, 137.75, 149.93 ( py). 13C {1H} NMR of 3b labeled with 13CO2 (acetoned6): δ 186.71 (CO2). General procedure for the formation of 1,4-ketoacids A J. Young NMR tube was charged with 0.020 g of either 3a or 3b in approximately 0.5 mL of THF. The solution was frozen in a liquid nitrogen cold well and 15 µL of neat Et2Zn injected via
15992 | Dalton Trans., 2014, 43, 15990–15996
Dalton Transactions
microsyringe. The tube was quickly removed from the inert atmosphere box and chilled to −196 °C. On a vacuum line 260 torr carbon monoxide in a 28.9 mL volume was placed over the sample and allowed to equilibrate for 5 minutes. The J. Young tube was sealed and thawed in warm water. The sample was allowed to react at ambient temperatures overnight, then treated with excess HCl gas. The solution was dried under vacuum and the extracted into CDCl3 where the 1,4-ketoacid was quantified by NMR spectroscopy. Yield 46% from 3a; 14% from 3b. The product may be further purified by sequential aqueous base-acid extraction.
Results and discussion The three decade history of nickel promoted coupling between CO2 and light olefins, in particular ethylene, has seen varied success with diamine and diphosphine ligands, but little investigation into the use of amino-phosphine chelates.12 Our attention was drawn to amino-phosphine ligands due to the dissymmetry in electron donation imposed on the metal center which, when matched to dissymmetry in the C,O chelate of a nickelalactone, could enhance opportunities for regio- or even enantioselectivity in CO2–olefin coupling reactions. The disparate trans-influence between the strong field phosphine coordination and weaker field amine coordination should affect a preference to orient the nickelalactone with the weak trans-influence carboxylate opposite the phosphine and the stronger field methine group opposite the amine. This net effect would allow judicious choice of the amino-phosphine ligand substituents to reliably control the regioselectivity in CO2–olefin coupling through steric interactions between the phosphine and olefin substituents. This arrangement would still leave the amino group tunable to permit ample access to the metal for CO2 activation. The pyridyl-phosphine ligand, 2-((di-t-butylphosphino)methyl)pyridine (PN), was selected as a target for this methodology due to the large steric impetus of the t-butyl phosphine substituents matched with the relatively open pyridyl fragment.13 This chelate has recently been employed on heavier group 10 metals for cross-coupling catalysis and dioxygen insertion reactions.8,14 Coordination of PN to nickel was accomplished by treatment with a toluene solution of bis(1,5cyclooctadiene)nickel (Ni(COD)2) to afford modest yields of (PN)NiCOD (1) as a dark blue-purple solid upon crystallization from pentane (eqn (1)).
ð1Þ (PN)NiCOD was characterized by NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction. The
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Dalton Transactions
Fig. 2 Molecular structure of 1 at 30% probability ellipsoids. All hydrogen atoms are omitted for clarity. Select bond distances (Å) and angles (°): Ni(1)–P(1) 2.1948(4), Ni(1)–N(1) 1.993(1), P(1)–Ni(1)–N(1) 87.62(4).
solid state structure (PN)NiCOD of (Fig. 2) reveals a slightly distorted tetrahedral coordination sphere common for four-coordinate nickel 1,5-cyclooctadiene species. The dissymmetry in the PN ligand steric environment is self-evident from the ellipsoid plot and the five-membered metallacycle created by the ancillary ligand is modestly buckled with the bridging methylene carbon (C6) displaced from the mean plane of the other metallacycle atoms by 0.32 Å. The (PN)NiCOD complex, either in isolated form or generated in situ, served as a source of “(PN)Ni0” for activation of light olefins such as ethylene, propylene, and acrylonitrile. For example, addition of excess ethylene gas (∼10 equiv.) to an NMR sample of (PN)NiCOD in benzene-d6 resulted in an immediate color change from blue-purple to an intense violet, signaling conversion to an η2-ethylene complex, (PN)Ni(C2H4) (2a) (Fig. 3). Analogous species were observed using propylene (2b) and acrylonitrile (2c), though only 2c was successfully isolated following evacuation of the reaction volatiles. Each of the olefin species were characterized by NMR spectroscopy, with 1 H NMR resonances for the vinyl hydrogens ranging between 1.89–3.13 ppm. Olefin species 2a and 2b exhibited broad 1H NMR resonances for both the free and bound alkene,
Fig. 3
Paper
suggesting a rapid exchange at ambient temperature. Propylene species 2b also exhibited two resonances in the 31P NMR spectrum in an approximate 2 : 1 ratio, likely originating from isomers which vary in the orientation of the asymmetric alkene. Complex 2c was further characterized by single crystal X-ray diffraction (Fig. 4). The solid-state structure reveals an approximate square planar geometry about the nickel with the –CN substituent oriented away from the large PtBu2 ligand. Additionally, the olefinic C(2)–C(3) bond length of 1.46(1) is substantially elongated from a typical double bond indicative of a strong metallacyclopropane resonance contribution. This feature is also observed in other structurally characterized nickel–acrylonitrile complexes.15 Following the characterization of complexes 2a–c, the olefin species were used to explore the potential for coupling with carbon dioxide. Treatment of 2a and 2b with 1 atm CO2 over 12 hours at ambient temperature resulted in successful oxidative coupling to afford (PN)Ni(κC,κO-CH2CH2COO) (3a) and (PN)Ni(κC,κO-CH2CH(CH3)COO) (3b) in good yields (Fig. 3). The acrylonitrile complex 3c proved unsuitable for CO2 activation even under 50 bar at 50 °C, likely due to the poor nucleophilicity engendered by the –CN substituent. Complexes 3a and 3b were both isolated as yellow solids with 3b showing greater solubility in non-polar solvents than 3a. In acetone-d6, the 1H NMR spectrum of 3a exhibited two multiple methylene resonances for the metal lactone at 0.86 and 2.08 ppm, which correlated to 13C NMR resonances of −1.7 and 39.1 ppm in HSQC NMR experiments. Similarly, the 1H NMR spectrum of 3b in dichloromethane-d2 exhibited two diastereotopic hydrogens for the α-CH2 at 0.54 and 1.22 ppm as well as methine and methyl resonances from the metallacycle at 2.36 and 1.04 ppm. 2D COSY and HSQC NMR spectra were used to establish these assignments and the regioselectivity of the CO2–propylene coupling confirmed by X-ray diffraction (Fig. 5). The crystal sample of 3b suffered from moderate twining problems, but the data offer clear indication of a methacrylate-type structure. A notable feature of 3b structure is the significant contortion of the metal lactone ring in which C(2) is 0.24 Å removed from the mean plane. This contrasts the highly planar arrangement observed for numerous unsubstituted γ-nickelalactone species.5,16 Geometry optimization performed 3b shows a structure consistent with that observed by X-ray diffraction.17 Additional calculations on the ground
Substitution with light olefins and coupling with carbon dioxide.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 15990–15996 | 15993
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Paper
Fig. 4 Molecular structure of 2c at 30% probability ellipsoids. All hydrogen atoms are omitted for clarity. Select bond distances (Å) and angles (°): Ni(1)–P(1) 2.137(2), Ni(1)–N(1) 1.968(3), C(2)–C(3) 1.46(1), P(1)–Ni(1)– N(1) 88.1(2).
state structure of 3a indicate a metal lactone ring distortion comparable to 3b, suggesting this feature originates more from an influence of the ancillary ligand than the alkylation of the lactone.17 The regioselectivity for 1,2-coupling between propylene and CO2 at ambient temperature (>90% by NMR) is remarkable compared to the mixtures previously obtained from nickel mediated α-olefin and CO2 oxidative cyclizations.6 In the case of propylene, this selectivity is crucial to distinguishing between precursors for methacrylate (methyl on the β-carbon) and crotonate (methyl on the α-carbon) structures. The observed preference for 1,2-coupling is particularly desirable as methacrylate species are much more widely used monomers with applications in durable thermoplastics such as Plexiglass, optical fibers, and paints.18 As noted earlier, the β-hydrogen elimination reactions required to liberate such acrylates from
Dalton Transactions
nickel remain challenging, which led to the exploration of other methods to leverage value-added products from nickelalactones 3a and 3b. Carbonylation of 3a and 3b under moderate pressures of CO (500 torr added at −196 °C) afforded high yields of succininc anhydride and methylsuccinic anhydride, respectively, along with (PN)Ni(CO)2 (Fig. 6).19 The organic products were identified by comparison to authentic samples and the yields determined by NMR spectroscopy. Careful monitoring of the reaction between 3a and CO in acetone-d6 by 31P NMR spectroscopy indicated the formation of a low concentration intermediate with a chemical shift of 68.20 ppm, which decayed as the reaction completed. Repeating this experiment using 3a derived from 13CO2 and 13CO resulted in splitting of this 31P resonance into a doublet (2JC–P = 22 Hz) and afforded two enhanced resonances in the 13C NMR spectrum at 252.44 (2JC–P = 22 Hz) and 174.75 ppm. This isotopic labeling also caused alteration in the splitting pattern of two 1H NMR peaks at 2.54 and 2.84 ppm. This spectral data led to tentative assignment of the intermediate as the six-membered metallacycle resulting from carbonyl insertion into the Ni–C bond of 3a (Fig. 6), analogous to species observed by Rovis and Coates during the dicarbonylation of succinic anhydride with nickel(0) sources. The preparation of succinic and methylsuccinic anhydride from CO2 functionalization offers a potentially attractive alternative to dehydration, oxidation, or bio-synthetic routes for accessing these cyclic products which are used in paper manufacturing and as pharmaceutical intermediates. Attempts to conduct these reactions catalytically using CO2, olefin, and CO were unsuccessful thermally, even at low CO partial pressures. This obstruction likely originates from the inability of the olefin to coordinate to the substitutionally inert (PN)Ni(CO)2. Such carbonyl substitution processes are often aided by high energy photolysis which our laboratory is currently investigating. However, the conversion of the nickelalactones to cyclic anhydrides eliminates much of the value added from regioselective CO2–olefin coupling. Given this factor, our focus
Fig. 5 Molecular structure of 3b at 30% probability ellipsoids. Full view (left) and partial view (right). All hydrogen atoms and a cocrystallized solvent molecule are omitted for clarity. Select bond distances (Å) and angles (°): Ni(1)–P(1) 2.136(6), Ni(1)–N(1) 2.00(1), Ni(1)–O(1) 1.87(1), Ni(1)–C(3) 1.94(3), P(1)–Ni(1)–N(1) 85.1(4), O(1)–Ni(1)–C3(1) 84.2(8).
15994 | Dalton Trans., 2014, 43, 15990–15996
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Dalton Transactions
Fig. 6
Paper
Carbon monoxide insertion and organic product liberation from nickelalactone species.
shifted toward the liberation of γ-ketoacids, which are another high value CO2 functionalization product.
ð2Þ
Ketoacids are important precursors to multiple biosyntheses and are interesting components of bio-energy storage mechanisms. Previous studies have shown that γ-ketoacids can be obtained from the nickel catalyzed ring opening of cyclic anhydrides in the presence of organonuclephiles, such as diethyl zinc (eqn (2)). This led to investigation of the possible synthesis of similar products sourced from CO2 and olefin coupling. Initial attempts to liberate γ-ketoacids using 3a and 3b with 3 equiv. of diethyl zinc under 0.5 atm of CO resulted in formation of the respective γ-ketoacids in low isolated yields (26% for 3a; 14% for 3b) following hydrolysis (Fig. 6). In situ monitoring of this reaction by 13C NMR spectroscopy using 13 CO2 derived 3a revealed that the low yield of γ-ketoacid was due, in part, to competitive formation of succinic anhydride. To minimize this secondary reaction, the procedure was repeated with a larger diethyl zinc to carbon monoxide ratio, using 10 equivalents of Et2Zn under 25 torr of CO. This reaction produced the zinc salt of the γ-ketoacid from 3a and 3b as the sole 13C containing product by NMR spectroscopy. (PN)Ni(CO)2 again appeared as a prominent organometallic product, obviating additional reaction. Acid hydrolysis and isolation gave improved yields of 46% (from 3a) and 33% (from 3b). While still modest, it is anticipated on the basis of the near quantitative conversion observed by NMR spectroscopy that the isolated yields from the acid/base extraction procedure could improve substantially with larger scale reactions.
This journal is © The Royal Society of Chemistry 2014
Conclusions Though the utilization of fossil fuel resources for meeting energy and commercial chemical demands are quite different in scale, both suffer for the same unsustainable paradigm. In the commercial chemical industry, circumventing this limitation will require the utilization of diverse renewable carbon feedstocks including carbon dioxide. Our investigations have discovered a new ligand–nickel platform which effectively couples carbon dioxide with ethylene or propylene with good regioselectivity. The addition of diethyl zinc and/or carbon monoxide induces the elimination of high value organic products, including cyclic anhydrides and ketoacids. Interestingly, this reaction also produces a zerovalent nickel dicarbonyl product, which creates the possibility of reactivating the metal for further coupling. Together, this sequence of reactions opens potential pathways to synthetic procedures for accessing commodity, biologically relevant, or chiral carboxylate products derived from CO2.
Acknowledgements This material is based upon work supported by Brown University. We would like to thank Nilay Hazari for computational assistance.
References 1 M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann and F. E. Kuhn, Angew. Chem., Int. Ed., 2011, 50, 8510; Renewable Raw Materials: New Feedstocks for the Chemical Industry, ed. R. Ulber, D. Sell and T. Hirth, Wiley-VCH, Weinheim, 2011.
Dalton Trans., 2014, 43, 15990–15996 | 15995
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Paper
2 Developments and Innovations in Carbon Dioxide Capture and Storage Technology. Vol 2: Carbon Dioxide Storage and Utilisation, ed. M. M. Maroto-Valer, Woodhead, Cambridge, 2010. 3 Carbon Dioxide as Chemical Feedstock, ed. M. Aresta, WileyVCH, Weinheim, 2011. 4 W. B. Tolman, Carbon Dioxide Reduction and Uses as a Chemical Feedstock. Activation of Small Molecules: Organometallic and Bioinorganic Perspectives, Wiley-VCH, Weinheim, 2006, pp. 1–35. 5 (a) M. L. Lejkowski, R. Lindner, T. Kageyama, G. E. Bodizs, P. N. Plessow, I. B. Muller, A. Schafer, F. Rominger, P. Hofmann, C. Futter, S. A. Schunk and M. Limbach, Chem. – Eur. J., 2012, 18, 14017; (b) C. Bruckmeier, M. W. Lehenmeier, R. Reichardt, S. Vagin and B. Rieger, Organometallics, 2010, 29, 2199; S. Y. T. Lee, M. Cokoja, M. Drees, Y. Li, J. Mink, W. A. Herrmann and F. E. Kuhn, ChemSusChem, 2011, 4, 1275; (c) J. M. Wolfe and W. H. Bernskoetter, Dalton Trans., 2012, 41, 10763; (d) P. N. Plessow, L. Weigel, R. Lindner, A. Schafer, F. Rominger, M. Limbach and P. Hofmann, Organometallics, 2013, 32, 3327; (e) W. H. Bernskoetter and B. T. Tyler, Oganometallics, 2011, 30, 520; (f ) S. Y. T. Lee, A. A. Ghani, V. D’Elia, M. Cokoja, W. A. Herrmann, J. M. Basset and F. E. Kuhn, New J. Chem., 2013, 37, 3512; (g) Y. Zhang, B. S. Hanna, A. Dineen, P. G. Williard and W. H. Bernskoetter, Organometallics, 2013, 32, 3969. 6 (a) H. Hoberg and D. Schaefer, J. Organomet. Chem., 1983, 251, C51; (b) H. Hoberg, D. Schaefer, G. Berukhart, C. Kruger and M. Romao, J. Organomet. Chem., 1984, 266, 203; (c) H. Hoberg, Y. Peres, C. Kruger and Y. H. Tsay, Angew. Chem., Int. Ed. Engl., 1987, 26, 771. 7 (a) J. Dong, P. G. Williard, N. Hazari and W. H. Bernskoetter, Chem. – Eur. J., 2014, 20, 3205; (b) J. Dong, T. J. Schmeier, P. G. Williard, N. Hazari and W. H. Bernskoetter, Organometallics, 2013, 32, 2152. 8 L. Zhang, D. Peng, X. Leng and Z. Huang, Angew. Chem., Int. Ed., 2013, 52, 3676. 9 A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J. Timmers, Organometallics, 1996, 15, 1518.
15996 | Dalton Trans., 2014, 43, 15990–15996
Dalton Transactions
10 J. Sandström, Dynamic NMR Spectroscopy, Academic Press, New York, 1982. 11 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. 12 H. Hoberg, A. Ballesteros, A. Sigan, C. Jegat, D. Barhausen and A. Milchereit, J. Organomet. Chem., 1991, 407, C23. 13 P. G. Edwards I. A. Fallis and B. S. Yong, British Patent Application GB, 2378182 A, 2003. 14 K. A. Grice and K. I. Goldberg, Organometallics, 2009, 28, 953. 15 L. J. Guggenberger, Inorg. Chem., 1973, 12, 499. 16 (a) J. Langer, D. Walther, A. Malassa, M. Westerhausen and H. Gorls, Eur. J. Inorg. Chem., 2010, 275; (b) J. Langer, D. Walther and H. Gorls, J. Organomet. Chem., 2006, 691, 4874; (c) J. Langer, H. Gorls, R. Fischer and D. Walther, Organometallics, 2005, 24, 272. 17 See ESI.† 18 W. Bauer Jr., Methacrylic Acid and Derivatives, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2011. 19 The identify of (PN)Ni(CO)2 was confirmed by independent preparation from addition of CO to 1.
This journal is © The Royal Society of Chemistry 2014
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
PAPER
Cite this: Dalton Trans., 2014, 43, 15990
View Journal | View Issue
Nickel promoted functionalization of CO2 to anhydrides and ketoacids† Zoe R. Greenburg, Dong Jin, Paul G. Williard and Wesley H. Bernskoetter* The reductive functionalization of carbon dioxide into high value organics was accomplished via the coupling with carbon monoxide and ethylene/propylene at a zerovalent nickel species bearing the 2-((dit-butylphosphino)methyl)pyridine ligand (PN). An initial oxidative coupling between carbon dioxide, olefin, and (PN)Ni(1,5-cyclooctadiene) afforded five-membered nickelacycle lactone species, which were produced with regioselective 1,2-coupling in the case of propylene. The propylene derived nickelacycle
Received 25th April 2014, Accepted 11th September 2014
lactone was isolated and characterized by X-ray diffraction. Addition of carbon monoxide, or a combi-
DOI: 10.1039/c4dt01221f
nation of carbon monoxide and diethyl zinc to the nickelacycle lactone complexes afforded cyclic anhydrides and 1,4-ketoacids, respectively, in moderate to high yields. The primary organometallic product of
www.rsc.org/dalton
the transformation was zerovalent (PN)Ni(CO)2.
Introduction Rising energy costs and climatological changes associated with carbon combustion have heightened society’s sensitivity to its dependence on non-renewable fossil fuels. Though less pervasive, the same reliance on coal, natural gas, and petroleum which constrains our energy supply is also found in the production of numerous carbon based commodity chemicals.1 These sustainability and economic influences have driven research into the utilization of inexpensive and renewable carbon feedstocks to satisfy the demand for industrial chemicals.2 The functionalization of carbon dioxide has emerged as a feedstock of particular interest due to its immense abundance, low toxicity, and identity as a greenhouse gas emission.3 Unfortunately the utilization of CO2 in chemical and related applications is currently limited to roughly 130 MT year−1, largely due to a lack of reactions which surmount the strong kinetic and thermodynamic stability of the gas.4 Our laboratory and others have been working to expand this scope of chemical CO2 utilization by developing transition metal mediated processes which couple CO2 with light olefins.5 The chemistry of CO2–olefin coupling has been an area of interest since the seminal reports of C–C bond formation between CO2 and ethylene by Hoberg and Carmona in the mid-1980’s.6 In particular, Hoberg’s isolation of a nickelacycle lactone (also termed nickelalactone) complex (Fig. 1) has Department of Chemistry, Brown University, Providence, RI 02912, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. CCDC 997717–997719. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01221f
15990 | Dalton Trans., 2014, 43, 15990–15996
Fig. 1
Hoberg’s nickelalactone synthesis from CO2.
inspired considerable effort to establish catalytic syntheses of acrylates from CO2 and ethylene which could alter the renewability of acrylic materials such as super water absorbing polymers.1 Recently researchers have reported progress in this endeavor by using strong external bases and Lewis acids to promote acrylate formation from diphosphine nickelalactone complexes.5,7 However, the reticence of nickelalactone species to undergo β-hydride elimination processes is still a limiting factor in the utilization of this coupling reaction.5 Thus, developing reactions of nickelalactone complexes which further functionalize the metallacycle without necessitating β-elimination may be an attractive method to leverage low-valent nickel species which couple CO2 with light olefins. The ring-expansion of nickelalactone complexes by insertion of organic unsaturates has previously been observed for a handful of diamine and diphosphine supported complexes.6 In most cases, saturated 5-membered nickelalactone species were treated with an alkyne, activated olefin, or carbon dioxide to afford a 7-membered metallacycle which was hydrolyzed to obtain carboxylate products.6 Unfortunately, the addition of acid makes development of these reactions into catalytic processes a significant challenge given the incompatibility of Bronsted acids and low-valent nickel. An alternative approach is the insertion of strong field ligands which not only functionalize the five-membered nickelalactone species, but also induce elimination of up-converted organics derived from
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
CO2. Herein, we describe the coupling of CO2 and light olefins to generate new amino-phosphine nickelalactones with good regioselectivity, and the use of strong field carbon monoxide to afford valuable cyclic anhydride and 1,4-ketoacid products.
Methods Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
General considerations All manipulations were carried out using standard vacuum, Schlenk, cannula or glovebox techniques. Ethylene and carbon dioxide were purchased from Corp Brothers and stored over 4 Å molecular sieves in heavy walled glass vessels prior to use. Argon and nitrogen were purchased from Corp Brothers and used as received. 2-((di-t-butylphosphino)methyl)pyridine (PN) was prepared according to literature procedure.8 All other chemicals were purchased from Aldrich, VWR, Strem, Fisher Scientific or Cambridge Isotope Laboratories. Volatile, liquid chemicals were dried over 4 Å molecular sieves and distilled prior to use. Solvents were dried and deoxygenated using literature procedures.9 1 H, 13C, and 31P NMR spectra were recorded on Bruker DRX 400 Avance, 300 and 600 MH Avance spectrometers. 1H and 13 C chemical shifts are referenced to residual solvent signals; 31 P chemical shifts are referenced to an external standard of H3PO4. Probe temperatures were calibrated using ethylene glycol and methanol as previously described.10 Unless otherwise noted, all NMR spectra were recorded at 23 °C. IR spectra were recorded on a Jasco 4100 FTIR spectrometer. GC-MS data were recorded using a Hewlett-Packard (Agilent) GCD 1800C GC-MS spectrometer. X-ray crystallographic data were collected on a Bruker D8 QUEST diffractometer. Samples were collected in inert oil and quickly transfered 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. CIF data for 1, 2c, 3b has been deposited with the CCDC under numbers 997718, 997717, and 997719, respectively. Elemental analyses were performed at Atlantic Microlab, Inc., in Norcross, GA. All geometry optimizations were performed using Gaussian 09 Revision A.02,11 using the M06 l functional. The LANL2DZ basis set was used for Ni and the 6-31G++(d,p) basis set was used for all other atoms. The LANL2DZ pseudo-potential was used for Ni. Frequency calculations were performed on all optimized structures to ensure that they were true minima. (PN)Ni(COD) (1). A 20 mL scintillation vial was charged with 0.058 g (0.26 mmol) of Ni(COD)2 and approximately 5 mL of tetrahydrofuran. The solution was chilled to −35 °C and 0.074 g (0.31 mmol) of 2-((di-t-butylphosphino)methyl)pyridine (PN) in diethyl ether was added slowly over 2 hours turning the solution dark blue. After stirring at ambient temperature for a further 30 minutes, the volatiles were removed, and the residue extracted with pentane. Concentrating and chilling the pentane extraction to −35 °C overnight afforded 49 mg (75%) of 1 as dark blue crystals. Anal. Calcd for
This journal is © The Royal Society of Chemistry 2014
Paper
C22H36NPNi: C, 65.37; H, 8.98, N, 3.47. Found: C, 64.99; H, 8.28; N, 3.38. 1H NMR (C6D6): δ 1.15 (d, 11.6 Hz, 18H, PtBu), 2.20 (m, 4H, CH2–COD), 2.44 (m, 4H, CH2–COD), 2.67 (d, 2H, 6 Hz, 2H, CH2–P), 2.80 (m, 2H, CH2–COD), 4.21 (m, 2H, CH– COD), 4.82 (m, 2H, CH–COD), 6.35 (t, 1H, py), 6.51 (d, 1H, py), 6.80 (t, 1H, py), 8.95 (d, py). 31P {1H} NMR (C6D6): δ 68.5 (s). Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 29.39 (PtBu), 31.04, 31.75 (CH2–COD), 33.26 (P–CH2), 78.99, 82.37 (CH–COD), 121.22, 121.65, 129.98, 151.02 ( py). (PN)Ni(C2H4) (2a). A J. Young NMR tube was charged with 0.030 g (0.07 mmol) of 1a in approximately 500 µL of benzened6. On a high vacuum line 5 equivalents of ethylene were added via a calibrated gas bulb at −196 °C. The reaction was allowed to warm to ambient temperature, immediately changing color from dark blue to dark pink. Full conversion to 2a was confirmed by NMR spectroscopy. 1H NMR (C6D6): δ 1.13 (d, 12 Hz, 18H, PtBu), 2.36 (br, 4H, ethylene), 2.65 (d, 6 Hz, 2H, CH2–P) 6.23 (t, 1H, py), 6.63 (d, 1H, py), 6.93 (t, 1H, py), 9.35 (d, py). 31P {1H} NMR (C6D6): δ 71.99. Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 28.10 (PtBu), 31.04, 33.61 (C2H4), 33.24 (P–CH2), 121.51, 122.45, 132.77, 151.99 ( py). (PN)Ni(C2H3CH3) (2b). A J. Young NMR tube was charged with 0.0075 g (0.032 mmol) of Ni(COD)2, 0.076 g (0.029 mmol) of PN ligand and approximately 500 µL of benzene-d6. On a high vacuum line 10 equivalents (140 torr in 28.9 mL) of propylene were added via calibrated gas bulb at −196 °C. The reaction was allowed to warm to ambient temperature, quickly changing color from dark blue to an intense purple. Analysis by NMR showed near quantitative conversion to 2b, with two isomers apparent by 31P NMR in a 2 : 1 ratio. Only the major isomer was resolved in the 1H NMR and HSQC NMR spectra. The sample reverted to 1 upon removal of propylene. 1H NMR (C6D6): δ 1.15 (d, 12 Hz, 18H, PtBu), 1.89 (br, 3H, H2CvCHCH3), 2.18 (br, 1H, H2CvCHCH3), 2.43 (br, 1H, H2CvCHCH3), 2.64 (br, 2H, CH2–P), 3.13 (br, 1H, H2CvCHCH3) 6.26 (m, 1H, py), 6.63 (m, 1H, py), 6.93 (m, 1H, py), 9.34 (m, 1H, py). 31P {1H} NMR (C6D6): δ 72.41 (minor), 68.83 (major). Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 23.95 (H2CvCHCH3), 29.94 (PtBu), 32.09 (H2CvCHCH3), 33.67 (CH2–P), 121.83, 122.75, 132.88, 151.41 ( py), H2CvCHCH3 not located due to broadening from exchange with free propylene (exchange confirmed by 2D NOESY with 300 ms mixing time). (PN)Ni(C2H3CN) (2c). A 25 mL round bottom flask fitted with a vacuum adapter was charged with 0.066 g (0.278 mmol) of Ni(COD)2, 0.076 (0.276 mmol) of PN ligand and approximately 10 mL of toluene. On a high vacuum line 2 equivalents (76 torr in 101 mL) of acrylonitrile were added via calibrated gas bulb at −196 °C. The reaction was allowed to warm to ambient temperature, quickly changing color from dark blue to orange. After stirring for 3 hours, the volatiles were removed, the orange residue rinsed with pentane and extracted with 5 mL of 1 : 1 tetrahydrofuran–diethyl ether. The orange solution was chilled to −35 °C overnight to afford 0.043 g of 2c as orange powder. Concentrating the mother liquor produced a second 0.022 g fraction of product for a total yield of 0.065 g
Dalton Trans., 2014, 43, 15990–15996 | 15991
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Paper
(69%). Anal. Calcd for C17H27NiN2P: C, 58.49; H, 7.80; N, 8.02. Found: C, 58.09; H, 7.49; N, 8.02. 1H NMR (C6D6): δ 0.93 (d, 7.4 Hz, 18H, PtBu), 1.89 (br, 1H, H2CvCHCN), 2.16 (br, 1H, H2CvCHCN), 2.19 (br, 1H, H2CvCHCN), 2.51 (br, 2H, CH2– P), 6.27 (m, 1H, py), 6.50 (m, 1H, py), 6.79 (m, 1H, py), 9.29 (m, 1H, py). 31P {1H} NMR (C6D6): δ 77.52. Partial 13C NMR taken from 1H–13C HSQC (C6D6): δ 11.92 (H2CvCHCN), 28.51 (PtBu), 28.80 (H2CvCHCN), 32.38 (CH2–P), 122.07, 122.23, 134.86, 152.15 ( py). (PN)Ni(κC,κO-CH2CH2COO) (3a). A 50 mL Schlenk flask was charged with 0.200 g (0.00073 mol) of Ni(COD)2, approximately 15 mL of tetrahydrofuran and placed under 1 atm of ethylene. A tetrahydrofuran solution containing 0.172 g (0.00073 mol) of PN ligand was added via syringe and stirred for 20 minutes at ambient temperature. CO2 was then admitted to the reaction by sparging the solution for 10 minutes. The reaction mixture was stirred overnight, and the product, a light yellow powder, was collected by cannula filtration to obtain 245 mg (91%) of 3a following rinsing with additional tetrahydrofuran. Anal. Calcd for C17H28NiNPO2: C, 55.48; H, 7.67; N, 3.81. Found: C, 55.81; H, 7.00; N, 3.47. 1H NMR (acetone-d6): δ 0.86 (m, 2H, Ni-α-CH2) 1.43 (d, 12 Hz, 18H, tBu), 2.08 (m, 2H, Ni-β-CH2), 3.42 (d, 9 Hz, 2H, CH2–P), 7.29 (t, 1H, py), 7.56 (d, 1H, py) 7.87 (t, 1H, py), 8.86 (d, 1H, py). 31P {1H} NMR (acetone-d6): δ 71.01 (s). Partial 13C NMR taken from 1H–13C HSQC NMR (CD2Cl2): δ −1.7 (Ni-α-CH2), 29.1 (PtBu), 39.1 (Ni-β-CH2), 32.8 (CH2–P), 122.0, 122.2, 137.7, 149.3 ( py). 13C {1H} NMR of 3a labeled with 13CO2 (acetone-d6): δ 185.51 (CO2). (PN)Ni(κC,κO-CH2CH2(CH3)COO) (3b). A J. Young NMR tube was charged with 0.028 g of PN ligand and 0.019 g of Ni(COD)2 in tetrahydrofuran. On a high vacuum line, 3 equivalents of propene were added via calibrated gas bulb at −196 °C. The sample was thawed and allowed to stand at ambient temperature for 1 hour where it changed to a deep pink solution. Then an additional 3 equivalents of CO2 were added via calibrated gas bulb at −196 °C and the sample allowed to stand overnight at ambient temperature. Monitoring by 31P NMR spectroscopy showed complete disappearance of 2b after that time, and then 30 mg (76%) of 3b was collected as a yellow powder by filtration. Anal. Calcd for C18H30NiNPO2: C, 56.58; H, 7.91; N, 3.67. Found: C, 57.09; H, 7.57; N, 3.39. 1H NMR (CD2Cl2): δ 0.56 (br, 1H, Ni-α-CH2) 1.04 (br, 3H, Niγ-CH3), 1.22 (br, 1H, Ni-α-CH2), 1.35 (d, 14 Hz 18H, tBu), 2.36 (br, 1H, Ni-β-CH), 3.14 (br, 2H, CH2–P), 7.17 (br, 1H, py), 7.33 (br, 1H, py) 7.69 (br, 1H, py), 8.90 (br, 1H, py). 31P {1H} NMR (CD2Cl2): δ 71.97 (s). Partial 13C NMR taken from 1H–13C HSQC NMR (CD2Cl2): δ 9.42 (Ni-α-CH2), 21.15 (Ni-γ-CH3), 29.46 (PtBu), 33.45 (CH2–P), 44.18 (Ni-β-CH), 122.33, 122.66, 137.75, 149.93 ( py). 13C {1H} NMR of 3b labeled with 13CO2 (acetoned6): δ 186.71 (CO2). General procedure for the formation of 1,4-ketoacids A J. Young NMR tube was charged with 0.020 g of either 3a or 3b in approximately 0.5 mL of THF. The solution was frozen in a liquid nitrogen cold well and 15 µL of neat Et2Zn injected via
15992 | Dalton Trans., 2014, 43, 15990–15996
Dalton Transactions
microsyringe. The tube was quickly removed from the inert atmosphere box and chilled to −196 °C. On a vacuum line 260 torr carbon monoxide in a 28.9 mL volume was placed over the sample and allowed to equilibrate for 5 minutes. The J. Young tube was sealed and thawed in warm water. The sample was allowed to react at ambient temperatures overnight, then treated with excess HCl gas. The solution was dried under vacuum and the extracted into CDCl3 where the 1,4-ketoacid was quantified by NMR spectroscopy. Yield 46% from 3a; 14% from 3b. The product may be further purified by sequential aqueous base-acid extraction.
Results and discussion The three decade history of nickel promoted coupling between CO2 and light olefins, in particular ethylene, has seen varied success with diamine and diphosphine ligands, but little investigation into the use of amino-phosphine chelates.12 Our attention was drawn to amino-phosphine ligands due to the dissymmetry in electron donation imposed on the metal center which, when matched to dissymmetry in the C,O chelate of a nickelalactone, could enhance opportunities for regio- or even enantioselectivity in CO2–olefin coupling reactions. The disparate trans-influence between the strong field phosphine coordination and weaker field amine coordination should affect a preference to orient the nickelalactone with the weak trans-influence carboxylate opposite the phosphine and the stronger field methine group opposite the amine. This net effect would allow judicious choice of the amino-phosphine ligand substituents to reliably control the regioselectivity in CO2–olefin coupling through steric interactions between the phosphine and olefin substituents. This arrangement would still leave the amino group tunable to permit ample access to the metal for CO2 activation. The pyridyl-phosphine ligand, 2-((di-t-butylphosphino)methyl)pyridine (PN), was selected as a target for this methodology due to the large steric impetus of the t-butyl phosphine substituents matched with the relatively open pyridyl fragment.13 This chelate has recently been employed on heavier group 10 metals for cross-coupling catalysis and dioxygen insertion reactions.8,14 Coordination of PN to nickel was accomplished by treatment with a toluene solution of bis(1,5cyclooctadiene)nickel (Ni(COD)2) to afford modest yields of (PN)NiCOD (1) as a dark blue-purple solid upon crystallization from pentane (eqn (1)).
ð1Þ (PN)NiCOD was characterized by NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction. The
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Dalton Transactions
Fig. 2 Molecular structure of 1 at 30% probability ellipsoids. All hydrogen atoms are omitted for clarity. Select bond distances (Å) and angles (°): Ni(1)–P(1) 2.1948(4), Ni(1)–N(1) 1.993(1), P(1)–Ni(1)–N(1) 87.62(4).
solid state structure (PN)NiCOD of (Fig. 2) reveals a slightly distorted tetrahedral coordination sphere common for four-coordinate nickel 1,5-cyclooctadiene species. The dissymmetry in the PN ligand steric environment is self-evident from the ellipsoid plot and the five-membered metallacycle created by the ancillary ligand is modestly buckled with the bridging methylene carbon (C6) displaced from the mean plane of the other metallacycle atoms by 0.32 Å. The (PN)NiCOD complex, either in isolated form or generated in situ, served as a source of “(PN)Ni0” for activation of light olefins such as ethylene, propylene, and acrylonitrile. For example, addition of excess ethylene gas (∼10 equiv.) to an NMR sample of (PN)NiCOD in benzene-d6 resulted in an immediate color change from blue-purple to an intense violet, signaling conversion to an η2-ethylene complex, (PN)Ni(C2H4) (2a) (Fig. 3). Analogous species were observed using propylene (2b) and acrylonitrile (2c), though only 2c was successfully isolated following evacuation of the reaction volatiles. Each of the olefin species were characterized by NMR spectroscopy, with 1 H NMR resonances for the vinyl hydrogens ranging between 1.89–3.13 ppm. Olefin species 2a and 2b exhibited broad 1H NMR resonances for both the free and bound alkene,
Fig. 3
Paper
suggesting a rapid exchange at ambient temperature. Propylene species 2b also exhibited two resonances in the 31P NMR spectrum in an approximate 2 : 1 ratio, likely originating from isomers which vary in the orientation of the asymmetric alkene. Complex 2c was further characterized by single crystal X-ray diffraction (Fig. 4). The solid-state structure reveals an approximate square planar geometry about the nickel with the –CN substituent oriented away from the large PtBu2 ligand. Additionally, the olefinic C(2)–C(3) bond length of 1.46(1) is substantially elongated from a typical double bond indicative of a strong metallacyclopropane resonance contribution. This feature is also observed in other structurally characterized nickel–acrylonitrile complexes.15 Following the characterization of complexes 2a–c, the olefin species were used to explore the potential for coupling with carbon dioxide. Treatment of 2a and 2b with 1 atm CO2 over 12 hours at ambient temperature resulted in successful oxidative coupling to afford (PN)Ni(κC,κO-CH2CH2COO) (3a) and (PN)Ni(κC,κO-CH2CH(CH3)COO) (3b) in good yields (Fig. 3). The acrylonitrile complex 3c proved unsuitable for CO2 activation even under 50 bar at 50 °C, likely due to the poor nucleophilicity engendered by the –CN substituent. Complexes 3a and 3b were both isolated as yellow solids with 3b showing greater solubility in non-polar solvents than 3a. In acetone-d6, the 1H NMR spectrum of 3a exhibited two multiple methylene resonances for the metal lactone at 0.86 and 2.08 ppm, which correlated to 13C NMR resonances of −1.7 and 39.1 ppm in HSQC NMR experiments. Similarly, the 1H NMR spectrum of 3b in dichloromethane-d2 exhibited two diastereotopic hydrogens for the α-CH2 at 0.54 and 1.22 ppm as well as methine and methyl resonances from the metallacycle at 2.36 and 1.04 ppm. 2D COSY and HSQC NMR spectra were used to establish these assignments and the regioselectivity of the CO2–propylene coupling confirmed by X-ray diffraction (Fig. 5). The crystal sample of 3b suffered from moderate twining problems, but the data offer clear indication of a methacrylate-type structure. A notable feature of 3b structure is the significant contortion of the metal lactone ring in which C(2) is 0.24 Å removed from the mean plane. This contrasts the highly planar arrangement observed for numerous unsubstituted γ-nickelalactone species.5,16 Geometry optimization performed 3b shows a structure consistent with that observed by X-ray diffraction.17 Additional calculations on the ground
Substitution with light olefins and coupling with carbon dioxide.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 15990–15996 | 15993
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Paper
Fig. 4 Molecular structure of 2c at 30% probability ellipsoids. All hydrogen atoms are omitted for clarity. Select bond distances (Å) and angles (°): Ni(1)–P(1) 2.137(2), Ni(1)–N(1) 1.968(3), C(2)–C(3) 1.46(1), P(1)–Ni(1)– N(1) 88.1(2).
state structure of 3a indicate a metal lactone ring distortion comparable to 3b, suggesting this feature originates more from an influence of the ancillary ligand than the alkylation of the lactone.17 The regioselectivity for 1,2-coupling between propylene and CO2 at ambient temperature (>90% by NMR) is remarkable compared to the mixtures previously obtained from nickel mediated α-olefin and CO2 oxidative cyclizations.6 In the case of propylene, this selectivity is crucial to distinguishing between precursors for methacrylate (methyl on the β-carbon) and crotonate (methyl on the α-carbon) structures. The observed preference for 1,2-coupling is particularly desirable as methacrylate species are much more widely used monomers with applications in durable thermoplastics such as Plexiglass, optical fibers, and paints.18 As noted earlier, the β-hydrogen elimination reactions required to liberate such acrylates from
Dalton Transactions
nickel remain challenging, which led to the exploration of other methods to leverage value-added products from nickelalactones 3a and 3b. Carbonylation of 3a and 3b under moderate pressures of CO (500 torr added at −196 °C) afforded high yields of succininc anhydride and methylsuccinic anhydride, respectively, along with (PN)Ni(CO)2 (Fig. 6).19 The organic products were identified by comparison to authentic samples and the yields determined by NMR spectroscopy. Careful monitoring of the reaction between 3a and CO in acetone-d6 by 31P NMR spectroscopy indicated the formation of a low concentration intermediate with a chemical shift of 68.20 ppm, which decayed as the reaction completed. Repeating this experiment using 3a derived from 13CO2 and 13CO resulted in splitting of this 31P resonance into a doublet (2JC–P = 22 Hz) and afforded two enhanced resonances in the 13C NMR spectrum at 252.44 (2JC–P = 22 Hz) and 174.75 ppm. This isotopic labeling also caused alteration in the splitting pattern of two 1H NMR peaks at 2.54 and 2.84 ppm. This spectral data led to tentative assignment of the intermediate as the six-membered metallacycle resulting from carbonyl insertion into the Ni–C bond of 3a (Fig. 6), analogous to species observed by Rovis and Coates during the dicarbonylation of succinic anhydride with nickel(0) sources. The preparation of succinic and methylsuccinic anhydride from CO2 functionalization offers a potentially attractive alternative to dehydration, oxidation, or bio-synthetic routes for accessing these cyclic products which are used in paper manufacturing and as pharmaceutical intermediates. Attempts to conduct these reactions catalytically using CO2, olefin, and CO were unsuccessful thermally, even at low CO partial pressures. This obstruction likely originates from the inability of the olefin to coordinate to the substitutionally inert (PN)Ni(CO)2. Such carbonyl substitution processes are often aided by high energy photolysis which our laboratory is currently investigating. However, the conversion of the nickelalactones to cyclic anhydrides eliminates much of the value added from regioselective CO2–olefin coupling. Given this factor, our focus
Fig. 5 Molecular structure of 3b at 30% probability ellipsoids. Full view (left) and partial view (right). All hydrogen atoms and a cocrystallized solvent molecule are omitted for clarity. Select bond distances (Å) and angles (°): Ni(1)–P(1) 2.136(6), Ni(1)–N(1) 2.00(1), Ni(1)–O(1) 1.87(1), Ni(1)–C(3) 1.94(3), P(1)–Ni(1)–N(1) 85.1(4), O(1)–Ni(1)–C3(1) 84.2(8).
15994 | Dalton Trans., 2014, 43, 15990–15996
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Dalton Transactions
Fig. 6
Paper
Carbon monoxide insertion and organic product liberation from nickelalactone species.
shifted toward the liberation of γ-ketoacids, which are another high value CO2 functionalization product.
ð2Þ
Ketoacids are important precursors to multiple biosyntheses and are interesting components of bio-energy storage mechanisms. Previous studies have shown that γ-ketoacids can be obtained from the nickel catalyzed ring opening of cyclic anhydrides in the presence of organonuclephiles, such as diethyl zinc (eqn (2)). This led to investigation of the possible synthesis of similar products sourced from CO2 and olefin coupling. Initial attempts to liberate γ-ketoacids using 3a and 3b with 3 equiv. of diethyl zinc under 0.5 atm of CO resulted in formation of the respective γ-ketoacids in low isolated yields (26% for 3a; 14% for 3b) following hydrolysis (Fig. 6). In situ monitoring of this reaction by 13C NMR spectroscopy using 13 CO2 derived 3a revealed that the low yield of γ-ketoacid was due, in part, to competitive formation of succinic anhydride. To minimize this secondary reaction, the procedure was repeated with a larger diethyl zinc to carbon monoxide ratio, using 10 equivalents of Et2Zn under 25 torr of CO. This reaction produced the zinc salt of the γ-ketoacid from 3a and 3b as the sole 13C containing product by NMR spectroscopy. (PN)Ni(CO)2 again appeared as a prominent organometallic product, obviating additional reaction. Acid hydrolysis and isolation gave improved yields of 46% (from 3a) and 33% (from 3b). While still modest, it is anticipated on the basis of the near quantitative conversion observed by NMR spectroscopy that the isolated yields from the acid/base extraction procedure could improve substantially with larger scale reactions.
This journal is © The Royal Society of Chemistry 2014
Conclusions Though the utilization of fossil fuel resources for meeting energy and commercial chemical demands are quite different in scale, both suffer for the same unsustainable paradigm. In the commercial chemical industry, circumventing this limitation will require the utilization of diverse renewable carbon feedstocks including carbon dioxide. Our investigations have discovered a new ligand–nickel platform which effectively couples carbon dioxide with ethylene or propylene with good regioselectivity. The addition of diethyl zinc and/or carbon monoxide induces the elimination of high value organic products, including cyclic anhydrides and ketoacids. Interestingly, this reaction also produces a zerovalent nickel dicarbonyl product, which creates the possibility of reactivating the metal for further coupling. Together, this sequence of reactions opens potential pathways to synthetic procedures for accessing commodity, biologically relevant, or chiral carboxylate products derived from CO2.
Acknowledgements This material is based upon work supported by Brown University. We would like to thank Nilay Hazari for computational assistance.
References 1 M. Cokoja, C. Bruckmeier, B. Rieger, W. A. Herrmann and F. E. Kuhn, Angew. Chem., Int. Ed., 2011, 50, 8510; Renewable Raw Materials: New Feedstocks for the Chemical Industry, ed. R. Ulber, D. Sell and T. Hirth, Wiley-VCH, Weinheim, 2011.
Dalton Trans., 2014, 43, 15990–15996 | 15995
View Article Online
Published on 18 September 2014. Downloaded by New York University on 16/10/2014 14:08:23.
Paper
2 Developments and Innovations in Carbon Dioxide Capture and Storage Technology. Vol 2: Carbon Dioxide Storage and Utilisation, ed. M. M. Maroto-Valer, Woodhead, Cambridge, 2010. 3 Carbon Dioxide as Chemical Feedstock, ed. M. Aresta, WileyVCH, Weinheim, 2011. 4 W. B. Tolman, Carbon Dioxide Reduction and Uses as a Chemical Feedstock. Activation of Small Molecules: Organometallic and Bioinorganic Perspectives, Wiley-VCH, Weinheim, 2006, pp. 1–35. 5 (a) M. L. Lejkowski, R. Lindner, T. Kageyama, G. E. Bodizs, P. N. Plessow, I. B. Muller, A. Schafer, F. Rominger, P. Hofmann, C. Futter, S. A. Schunk and M. Limbach, Chem. – Eur. J., 2012, 18, 14017; (b) C. Bruckmeier, M. W. Lehenmeier, R. Reichardt, S. Vagin and B. Rieger, Organometallics, 2010, 29, 2199; S. Y. T. Lee, M. Cokoja, M. Drees, Y. Li, J. Mink, W. A. Herrmann and F. E. Kuhn, ChemSusChem, 2011, 4, 1275; (c) J. M. Wolfe and W. H. Bernskoetter, Dalton Trans., 2012, 41, 10763; (d) P. N. Plessow, L. Weigel, R. Lindner, A. Schafer, F. Rominger, M. Limbach and P. Hofmann, Organometallics, 2013, 32, 3327; (e) W. H. Bernskoetter and B. T. Tyler, Oganometallics, 2011, 30, 520; (f ) S. Y. T. Lee, A. A. Ghani, V. D’Elia, M. Cokoja, W. A. Herrmann, J. M. Basset and F. E. Kuhn, New J. Chem., 2013, 37, 3512; (g) Y. Zhang, B. S. Hanna, A. Dineen, P. G. Williard and W. H. Bernskoetter, Organometallics, 2013, 32, 3969. 6 (a) H. Hoberg and D. Schaefer, J. Organomet. Chem., 1983, 251, C51; (b) H. Hoberg, D. Schaefer, G. Berukhart, C. Kruger and M. Romao, J. Organomet. Chem., 1984, 266, 203; (c) H. Hoberg, Y. Peres, C. Kruger and Y. H. Tsay, Angew. Chem., Int. Ed. Engl., 1987, 26, 771. 7 (a) J. Dong, P. G. Williard, N. Hazari and W. H. Bernskoetter, Chem. – Eur. J., 2014, 20, 3205; (b) J. Dong, T. J. Schmeier, P. G. Williard, N. Hazari and W. H. Bernskoetter, Organometallics, 2013, 32, 2152. 8 L. Zhang, D. Peng, X. Leng and Z. Huang, Angew. Chem., Int. Ed., 2013, 52, 3676. 9 A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J. Timmers, Organometallics, 1996, 15, 1518.
15996 | Dalton Trans., 2014, 43, 15990–15996
Dalton Transactions
10 J. Sandström, Dynamic NMR Spectroscopy, Academic Press, New York, 1982. 11 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. 12 H. Hoberg, A. Ballesteros, A. Sigan, C. Jegat, D. Barhausen and A. Milchereit, J. Organomet. Chem., 1991, 407, C23. 13 P. G. Edwards I. A. Fallis and B. S. Yong, British Patent Application GB, 2378182 A, 2003. 14 K. A. Grice and K. I. Goldberg, Organometallics, 2009, 28, 953. 15 L. J. Guggenberger, Inorg. Chem., 1973, 12, 499. 16 (a) J. Langer, D. Walther, A. Malassa, M. Westerhausen and H. Gorls, Eur. J. Inorg. Chem., 2010, 275; (b) J. Langer, D. Walther and H. Gorls, J. Organomet. Chem., 2006, 691, 4874; (c) J. Langer, H. Gorls, R. Fischer and D. Walther, Organometallics, 2005, 24, 272. 17 See ESI.† 18 W. Bauer Jr., Methacrylic Acid and Derivatives, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2011. 19 The identify of (PN)Ni(CO)2 was confirmed by independent preparation from addition of CO to 1.
This journal is © The Royal Society of Chemistry 2014
