Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Reactivity of Reduced α‑Diimine Nickel Complexes Relevant to Acrylic Acid Synthesis Matthew V. Joannou,† Mat́ e ́ J. Bezdek,† Khalid Albahily,‡ Ilia Korobkov,‡ and Paul J. Chirik*,† †
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States of America SABIC Corporate Research & Development, Fundamental Catalysis, Thuwal 23955-6900, Saudi Arabia
‡
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S Supporting Information *
ABSTRACT: The aryl-substituted α-diimine (DI) nickel vinyl complex (iPrDI)Ni(CHCH2) (iPrDI = [2,6-(iPr)2C6H3NC(CH3)]2) was synthesized and structurally characterized. The complex is dimeric in the solid state and has a distorted-square-planar geometry at nickel. A combination of single-crystal X-ray diffraction, EPR, magnetic susceptibility, and NMR analyses was used to elucidate the electronic structure of the compound, and it is best described as a low-spin Ni(II) derivative with a singly reduced α-diimine chelate. Addition of CO2 to the nickel vinyl complex resulted in insertion into the nickel−carbon bond to yield the corresponding nickel acrylate (iPrDI)Ni(κ2-O2CCHCH2). EPR spectroscopy coupled with DFT calculations established that the S = 1/2 product maintains the nickel(II) oxidation state with an α-diimine-centered radical. Addition of acrylic acid to (iPrDI)Ni(CHCH2) induced rapid, net bimetallic reductive elimination to release butadiene and produced the metastable olefin-bound acrylic acid complex (iPrDI)Ni(η2-CH2CHCO2H). Over the course of 2 h at 23 °C, this complex underwent a net oxidation to produce (iPrDI)Ni(κ2-O2CCHCH2), with concomitant loss of H2.
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tilization of carbon dioxide as an inexpensive and abundant C1 source in synthesis has received considerable attention in organometallic chemistry and homogeneous catalysis.1 Distinct from methodologies such as hydrocarboxylation, the direct, catalytic coupling of CO2 and ethylene (and other commodity α-olefins) to produce acrylic acid and other α,βunsaturated carboxylic acids has been a sought-after “dream reaction” by industrial and academic chemists alike.1a Currently, acrylic acid is produced on a 4 million ton/year scale by successive oxidations of propylene and is used as precursors for polymers utilized in absorbents, cosmetics, adhesives, and paints (Figure 1, top).1e While ethylene−CO2 coupling to acrylic acid would be an atom-economical process, it is thermodynamically unfavorable under standard conditions, with ΔG° = +14 kcal mol−1 in the gas phase and +5 kcal mol−1 in THF.2 To overcome this thermodynamic limitation, chemists have relied on linking CO2−ethylene coupling to other thermodynamically favorable processes, usually acrylate salt formation via deprotonation with an external base.1a Reports by Hoberg and co-workers described a stoichiometric, nickel-promoted ethylene−CO2 coupling with nickel by oxidative cyclization of the two molecules to form the metallalactone (dbu)2Ni(κ2-O,C-O(O)(CH2)2), which proved stable to β-hydride elimination and was isolated and characterized (Figure 1, middle).3 Carmona and co-workers reported a similar coupling with molybdenum and tungsten phosphine complexes, yet metallalactones in these reactions underwent rapid β-hydride elimination to form bridged acrylate−hydride complexes that were inert to successive © XXXX American Chemical Society
Figure 1. Strategies for the direct coupling of ethylene and CO2: metallalactone intermediates vs direct metal−vinyl addition to CO2.
Special Issue: In Honor of the Career of Ernesto Carmona Received: May 23, 2018
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DOI: 10.1021/acs.organomet.8b00350 Organometallics XXXX, XXX, XXX−XXX
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Organometallics reactivity relevant to catalytic turnover.4 Vogt5 and Limbach6 have recently developed nickel-catalyzed methods for the carboxylation of ethylene and other α-olefins, with the latter method producing sodium acrylate in up to 107 TON from ethylene and CO2. Both procedures utilize stoichiometric Lewis acid additives to activate the metallalactone and facilitate βhydride elimination, as well as external base to drive the reaction forward and remove acrylate from the catalyst. The inert nature of metallalactones inspired exploration into alternative pathways for carbon−carbon bond formation from ethylene and CO2. Carbon−carbon bond formation from organometallic reagents and CO2 has been known for over a century and ranges from Grignard’s discovery of the fixation of CO2 with alkylmagnesium halides to current methods that catalytically generate and add transition-metal−carbon bonds to CO2.1c Regeneration of the metal−vinyl and acrylic acid release could, in principle, be accomplished by concerted metalation− deprotonation (CMD), of which there is theoretical precedent with ethylene (Figure 1, bottom).7 Due to the prevalence of nickel in both catalytic and stoichiometric carboxylation and hydrocarboxylation chemistry,1c,3,5,6 our studies focused on nickel vinyl complexes. Here we describe the synthesis and electronic structure of an α-diimine nickel vinyl complex and its reactivity toward CO2 and acrylic acid. Because of the rich olefin insertion chemistry known with αdiimine nickel complexes,8 a nickel vinyl complex supported by the 2,6-diisopropylaryl diimine iPr DI ( iPr DI = [2,6(iPr)2C6H3NC(CH3)]2) was targeted. Two equivalents of vinylmagnesium bromide dissolved in THF was added to a thawing pentane slurry of (iPrDI)NiBr2 and the mixture warmed to ambient temperature over 20 min with stirring. Following filtration and removal of the volatiles in vacuo, the formally nickel(0) butadiene complex (iPrDI)Ni(η4-C4H6) was isolated in 92% yield as a diamagnetic, dark mauve solid (Scheme 1, top).
NiBr]210 followed by warming to room temperature, stirring for 20 min, filtering, and recrystallizing furnished a dark purple, paramagnetic solid identified as (iPrDI)Ni(CHCH2) in 85% yield (Scheme 1, bottom).11 The benzene-d6 1H NMR spectrum of (iPrDI)Ni(CHCH2) exhibits resonances for the iPrDI ligand and the vinyl group between 0 and 47 ppm. A solid-state magnetic moment of 2.3 μB was measured at 23 °C by magnetic susceptibility balance, consistent with an S = 1/2 ground state for each nickel complex with negligible antiferromagnetic coupling between them. This is a rare example of an unsubstituted nickel vinyl species that is isolable; to our knowledge, the only other example is a nickel(II) bis-phosphine bromide vinyl complex synthesized by Lu and co-workers.12 The X-band EPR spectrum of (iPrDI)Ni(CHCH2) (Figure 2, left) collected in fluid benzene solution at 296 K exhibits an
Scheme 1. Addition of Vinyl Metal Reagents to (iPrDI)Ni Bromides in Different Oxidation States
Figure 2. (left) X-band EPR spectrum of (iPrDI)Ni(CHCH2) at 296 K in fluid benzene. (right) Solid-state representation of (iPrDI)Ni(CHCH2) with 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
On the basis of work by Cardaci and co-workers, the expected nickel bis-vinyl complex is likely formed at the outset of the reaction, but rapid reductive coupling of the two vinyl groups produces the reduced nickel butadiene complex.9 Because the proposed [(iPrDI)Ni(CHCH2)2] intermediate underwent rapid C−C reductive coupling below ambient temperature, lower oxidation nickel halide precursors were targeted. Addition of a diethyl ether solution containing 2 equiv of vinyllithium to a thawing pentane slurry containing [(iPrDI)-
isotropic signal with g1,2,3 = 2.021, 2.012, 2.015, with coupling to both 14N nuclei in the α-diimine chelate (Aiso(14N, n = 2) = 20 MHz). The deviation of the calculated g value from that of a free electron (g = 2.002) and the presence of 14N nuclei coupling indicate that the unpaired electron in (iPrDI)Ni(CHCH2) is ligand-based, with detectable contributions from Ni d orbitals. This likely arises from spin−orbit coupling due to d−π* orbital mixing resulting from the highly π acidic character of the ligand (see the Supporting Information for details). For comparison, the X-band EPR spectrum of [(iPrDI)NiBr]2 in fluid benzene solution at 23 °C, which was shown to have a completely Nibased SOMO, exhibits an isotropic signal with g1,2,3 = 2.20, 2.24, 2.17 (see the Supporting Information for details). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a pentane solution of (iPrDI)Ni(CHCH2) at −35 °C over 48 h. A representation of the dimeric solid-state structure is presented in Figure 2 (right), where the geometry is best described as distorted square planar at each nickel (τ4 = 0.16). The Cimine−Nimine and Cimine−C′imine bond distances are established metrics for establishing the redox state of the αdiimine ligand as a consequence of the transition metal.13 For (iPrDI)Ni(CHCH2), Cimine−Nimine distances of 1.343(3) and 1.338(4) Å and a Cimine−C′imine value of 1.403(4) Å are consistent with a singly reduced form of the chelate. Together B
DOI: 10.1021/acs.organomet.8b00350 Organometallics XXXX, XXX, XXX−XXX
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Organometallics with the EPR data, (iPrDI)Ni(CHCH2) is best described as a Ni(II) center, supported by a singly reduced α-diimine. While (iPrDI)Ni(CHCH2) is dimeric in the solid state, the observed EPR spectrum and paramagnetically shifted signals of the 1H NMR spectrum suggest either formation of a detectable amount of monomer in solution or a persistent dimeric structure in solution with negligible coupling between the two metal complexes (Figure 2, bottom).14 While there is close contact between the α-vinylic carbon atoms in the X-ray structure (1.482(3) Å), the experimental data do not support the existence of a carbon−carbon bond. This was supported by degradation analysis: ethane was observed upon addition of 4 atm of H2 to the complex, and ethylene was observed upon addition of MeOH. tom Dieck and co-workers have isolated and characterized bimetallic α-diimine nickel complexes of similar atomic composition,15 yet these are distinctly different from [(iPrDI)Ni(CHCH2)]2, as they are derived from alkyne, contain planar C4 bridging ligands, and are diamagnetic. With an isolated nickel−vinyl complex in hand, its reactivity with CO2 was examined. Addition of 1 atm of CO2 to a benzened6 solution containing (iPrDI)Ni(CHCH2) was monitored by 1 H NMR spectroscopy (Figure 3, top). Over the course of 2 h,
to produce a new product, which exhibited an isotropic signal with g1,2,3 = 2.008, 1.994, 2.003 and sharp/resolved coupling to both 14N nuclei in the α-diimine chelate (Aiso(14N, n = 2) = 14 MHz) (Figure 3, bottom). The hyperfine coupling pattern, along with the observed g value that does not appreciably deviate from that of a free electron, suggests that the unpaired electron is almost entirely ligand based, consistent with a singly reduced αdiimine chelate. Full-molecule density functional theory studies (see the Supporting Information for details) were conducted on (iPrDI)Ni(κ2-O2CCHCH2) to simulate and verify the experimentally determined EPR parameters. The DFT-computed g values of 2.009, 1.995, and 2.004 are in excellent agreement with the experimentally determined values. A Mulliken spin density plot (Figure 3, bottom) was also generated and supports spin density almost exclusively on the α-diimine chelate, with only minimal involvement of the metal, which corroborates the EPR measurements. The combined EPR measurements and DFT computations demonstrate that (iPrDI)Ni(κ2-O2CCHCH2) is best described as a low-spin Ni(II) center bound to a singly reduced α-diimine ligand. The localization of spin density almost exclusively on the α-diimine ligand likely arises from the weaker ligand field at nickel, a result of replacing the vinyl group for a κ2-bound carboxylate. To understand how nickel vinyl and acrylate complexes would behave in a potential catalytic setting, their reactivity with acrylic acid18 was studied. A frozen benzene-d6 solution of (iPrDI)Ni(CHCH2) was charged with 1 equiv of acrylic acid and monitored by 1H NMR spectroscopy after warming to ambient temperature. After 10 min, free butadiene was observed along with >98% conversion to a new, diamagnetic Cs symmetric complex, which was identified as ( iPr DI)Ni(η 2 -CH 2 CHCO2H) (Scheme 2, top). This implies that coordination of Scheme 2. Reaction of (iPrDI)Ni(CHCH2) with Acrylic Acid: Observation of a Ni(0) Acrylic Acid Complex
Figure 3. (top) Addition of (iPrDI)Ni(CHCH2) to CO2. (bottom left) X-band EPR spectrum of (iPrDI)Ni(κ2-O2CCHCH2) at 296 K in fluid benzene. (bottom right) Spin density plot of computed (iPrDI)Ni(κ2-O2CCHCH2). Mulliken spin populations: iPrDI, 0.94; Ni, −0.05 + 0.05.
resonances corresponding to the nickel−vinyl complex disappeared and produced a featureless 1H NMR spectrum. To elucidate the identity of the new NMR-silent complex, HCl· (dioxane) was added to the reaction mixture. After the solution was degassed, the volatile components were vacuum-distilled and analyzed by 1H NMR spectroscopy, which established the formation of acrylic acid, supporting the insertion of CO2 into the nickel−vinyl bond. The infrared spectrum of the product contained a C−N stretch of the diimine ligand at ν(KBr) 1641 cm−1, consistent with other (iPrDI)Ni complexes of this type.16 A C−O stretch of the acrylate ligand was located at ν(KBr) 1621 cm−1, a value that shifted to 1584 cm−1 when the reaction was conducted with 13CO2. These values are consistent with other κ2acrylate and carboxylate ligands,17 which taken together with the other IR and degradation data suggest that the product of the reaction is (iPrDI)Ni(κ2-O2CCHCH2). The reaction was also analyzed by X-band EPR spectroscopy, and revealed >98% consumption of the starting material after 2 h
acrylic acid to (iPrDI)Ni(CHCH2) induces net bimetallic reductive elimination of the vinyl groups to form butadiene. Coordination-induced reductive elimination has been demonstrated with nickel using electron-poor olefins and other πaccepting ligands.19 1 H NMR resonances for the imine methyl groups of iPr ( DI)Ni(η2-CH2CHCO2H) were located at δ 0.13 and −0.04 ppm, characteristic of Cs-symmetric Ni(0) α-diimine C
DOI: 10.1021/acs.organomet.8b00350 Organometallics XXXX, XXX, XXX−XXX
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Organometallics olefin complexes.20,21 The carboxylic acid proton resonance appears at δ 12.6 ppm, while the olefinic protons of acrylic acid are upfield shifted to δ 2.45, 2.00, and 1.51 ppm. (iPrDI)Ni(η2CH2CHCO2H) was also formed in >98% conversion when (iPrDI)Ni(η4-C4H6) was exposed to 1 equiv of acrylic acid by straightforward ligand substitution. (iPr DI)Ni(η 2 -CH 2 CHCO2H) is metastable and underwent clean oxidation to (iPrDI)Ni(κ2-O2CCHCH2) (with observed release of H2) over the course of 2 h in benzene-d6 at ambient temperature. The identity of the resulting metal complex was confirmed by both 1 H NMR and EPR spectroscopy. Due to the limited stability of (iPrDI)Ni(η2-CH2CHCO2H), the methyl acrylate derivative, previously characterized by Brookhart and co-workers,21 was synthesized for comparison and exhibited remarkably similar 1H NMR resonances and corroborates the assignment of the η2acrylic acid complex (see the Supporting Information for synthetic details). Single crystals of (iPrDI)Ni(η2-CH2 CHCO2Me) suitable for X-ray diffraction were obtained from a concentrated toluene solution at −35 °C for 48 h. A representation of the solid-state structure is presented in Scheme 2, where, on the basis of the bond distances, the nickel is bound to a neutral form of the α-diimine chelate. The C(sp2)−C(sp2) bond of the η2-bound methyl acrylate ligand is significantly elongated to 1.431(3) Å, indicating a high degree of π backdonation from nickel. In summary, an α-diimine nickel vinyl complex was synthesized and spectroscopically and structurally characterized and is a rare example of an isolable unsubstituted transitionmetal−vinyl compound. Its reactivity with CO2 and acrylic acid was demonstrated where insertion of CO2 into (iPrDI)Ni(CH CH2) yielded (iPrDI)Ni(κ2-O2CCHCH2), which was also characterized and its electronic structure determined. Addition of acrylic acid to either (iPrDI)Ni(CHCH2) or (iPrDI)Ni(η4C4H6) produced metastable (iPrDI)Ni(η2-CH2CHCO2H), which slowly converted to the nickel acrylate by a currently unknown oxidative pathway that releases H2. Efforts to remove acrylate from the nickel center to render this process catalytic are currently under investigation.
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Paul J. Chirik: 0000-0001-8473-2898 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank SABIC for financial support.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00350. Experimental procedures and data, NMR spectra, crystal data, and details of the calculations (PDF) Cartesian coordinates of calculated structures (TXT) Accession Codes
CCDC 1844963−1844964 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for P.J.C.:
[email protected]. ORCID
Matthew V. Joannou: 0000-0002-0079-7107 Máté J. Bezdek: 0000-0001-7860-2894 D
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Organometallics 3425−3432. (b) Bernskoetter, W. H.; Tyler, B. T. Organometallics 2011, 30, 520−527. (18) A benzene-d6 mixture of acrylic acid and (iPrDI)Ni(κ2-O2CCH CH2) displayed only broadened free acrylic acid signals, indicating potential carboxylate exchange. No further reactivity was observed. (19) (a) Tatsumi, K.; Nakamura, A.; Komiya, S.; Yamamoto, A.; Yamamoto, T. J. Am. Chem. Soc. 1984, 106, 8181−8188. (b) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J. Am. Chem. Soc. 1971, 93, 3350−3359. (c) Xu, H.; Diccianni, J. B.; Katigbak, J.; Hu, C.; Zhang, Y.; Diao, T. J. Am. Chem. Soc. 2016, 138, 4779−4786. (20) Zhao, Y.; Wang, Z.; Jing, X.; Dong, Q.; Gong, S.; Li, Q.-S.; Zhang, J.; Wu, B.; Yang, X.-J. Dalton Trans. 2015, 44, 16228−16232. (21) Johnson, L. K.; Killian, C. M.; Arthur, S. D.; Feldman, J.; McCord, E. F.; McLain, S. J.; Kreutzer, K. A.; Bennett, M. A.; Coughlin, E. B.; Ittel, S. D.; Parthasarathy, A.; Tempel, D. J.; Brookhart, M. S. Alpha-olefins and Olefin Polymers and Processes Thereof. WO1996023010 A3, December 5, 1996.
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