Sequential Insertion Reactions of Carbon Monoxide and Ethylene into

Synopsis. The Ni(II) alkyl [(dtbpe)Ni(CH2tBu)][PF6] undergoes CO insertion to give the acyl complex [(dtbpe)Ni(η2-C(O)CH2tBu)][PF6] (2), and subseque...
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Organometallics 2009, 28, 2568–2571

Sequential Insertion Reactions of Carbon Monoxide and Ethylene into the Ni-C Bond of a Cationic Nickel(II) Alkyl Complex John J. Curley, Kristina D. Kitiachvili, Rory Waterman,‡ and Gregory L. Hillhouse* Department of Chemistry, Gordon Center for IntegratiVe Science, The UniVersity of Chicago, 929 E. 57th Street, Chicago, Illinois 60637 ReceiVed January 12, 2009

The cationic nickel(II) neopentyl complex [(dtbpe)Ni(CH2tBu)][PF6] (1; dtbpe ) 1,2-bis(di-tertbutylphosphino)ethane) sequentially inserts carbon monoxide and ethylene into the Ni-C bond to form isolable products. Upon exposure to carbon monoxide, 1 inserts 1 equiv of CO to give [(dtbpe)Ni(η2C(O)CH2tBu)][PF6] (2). This cationic acyl complex reacts productively with ethylene, resulting in [(dtbpe)Ni(κ2-CH2CH2C(O)CH2tBu)][PF6] (3), in which the oxygen atom is bound to Ni, forming a fivemembered metallacycle. Treatment of 2 with phenyl lithium results in deprotonation at an acidic methylene position adjacent to the carbonyl, affording the neutral ketene complex (dtbpe)Ni(η2-OCdCHtBu) (4). Complexes 2, 3, and 4 are stable at 20 °C and have been characterized by NMR and IR spectroscopy as well as single-crystal X-ray diffraction. Introduction The sequential insertions of carbon monoxide and olefins into either metal-carbon or metal-hydrogen bonds is an important method for C-C bond formation. These processes are used industrially for the synthesis of both commodity chemicals1-3 and functionalized polymers.4-7 The common C-C bond forming step in these two processes is migratory insertion of CO into the metal-carbon bond.8,9 Migratory insertion of carbon monoxide into metal-alkyl bonds is an essential step in copolymerization of CO and olefins that is typically mediated by Pd complexes,4-7 and it has been implicated in biological carbon-catenation, such as that mediated by the Ni-containing enzyme acetyl CoA synthase.10-13 Catalysts based on Pd remain the most active for CO/C2H4 copolymerization despite theoretical predictions that Ni-based catalysts ought to have lower kinetic barriers for the sequential insertions of CO and C2H4 that are responsible for chain growth.14 Brookhart and co-workers demonstrated that insertion of CO and ethylene into the Ni-C bond of [(L2)NiMe][BArF4] (L2 ) 1,2-bis(diphenylphosphino)propane; 1,2-bis(di-ortho* Corresponding author. E-mail: [email protected]. ‡ Current address: Department of Chemistry, University of Vermont, Burlington, VT 05405. (1) Halpern, J. Annu. ReV. Phys. Chem. 1965, 16, 103–124. (2) Cornils, B. Org. Process Res. DeV. 1998, 2, 121–127. (3) Snyder, G.; Tadd, A.; Abraham, M. A. Ind. Eng. Chem. Res. 2001, 40, 5317–5325. (4) Sen, A. Acc. Chem. Res. 1993, 26, 303–310. (5) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663–682. (6) Sommazzi, A.; Garbassi, F. Prog. Polym. Sci. 1997, 22, 1547–1605. (7) Bianchini, C.; Meli, A. Coord. Chem. ReV. 2002, 225, 35–66. (8) Kuhlmann, E. J.; Alexander, J. J. Coord. Chem. ReV. 1980, 33, 195– 225. (9) Cheng, T.-Y.; Southern, J. S.; Hillhouse, G. L. Organometallics 1997, 16, 2335–2342. (10) Ragsdale, S. W. J. Inorg. Biochem. 2007, 101, 1657–1666. (11) Evans, D. J. Coord. Chem. ReV. 2005, 249, 1582–1595. (12) Tucci, G. C.; Holm, R. H. J. Am. Chem. Soc. 1995, 117, 6489– 6496. (13) Tan, X.; Surovtsev, I. V.; Lindahl, P. A. J. Am. Chem. Soc. 2006, 128, 12331–12338. (14) Svensson, M.; Matsubara, T.; Morokuma, K. Organometallics 1996, 15, 5568–5576.

tolylphosphino)propane; ArF ) 3,5-bis(trifluoromethyl)phenyl) solvates has a lower barrier than the corresponding Pd-based systems and suggested that formation of kinetically stable resting states is responsible for the lower turnover rates of Ni-containing catalysts.15,16 During this study, the reactive four-coordinate complexes [(L2)Ni(Me)CO]+ and [(L2)NiC(O)Me]+ were observed as an equilibrium mixture by 1H NMR spectroscopy at low temperatures. Treatment of this mixture with C2H4 irreversibly formed a five-membered chelate, [(L2)NiC(κ2CH2CH2C(O)Me]+. Because these species are the first compounds required for the catalytic formation of strictly alternating CO/C2H4 copolymers, their structural features are of interest. However, the species described were not isolated as pure materials; therefore, structural studies could not be pursued. The Ni(II) alkyl complex [(dtbpe)NiCH2tBu][PF6] (1, dtbpe ) 1,2-bis(di-tert-butylphosphino)ethane) has previously been prepared by 1-e- oxidation of (dtbpe)NiCH2tBu.17 Complex 1 and its analogues are susceptible to deprotonation to form neutral metallacyclobutane complexes that reductively eliminate substituted cyclopropanes. We were interested in further studying the reactivity of these electron-deficient (formally 14 e-) alkyl species. One attractive avenue of investigation was the potential for the isolation of compounds that contain analogous structural features to those observed by Brookhart,15,16 but kinetically stabilized by the bulkier dtbpe ligand set. Here we report that the cationic neopentyl complex 1 undergoes sequential reactions with CO and C2H4 to form stable molecules that may be isolated and handled at 20 °C. The X-ray crystal structures of both insertion products have been obtained. These solid-state structures bear resemblance to the discrete intermediates involved in the CO/C2H4 copolymerization process.

Experimental Section General Considerations. Unless otherwise stated, all operations were performed in a M. Braun Labmaster drybox under an atmosphere of purified nitrogen or using high-vacuum and standard Schlenk techniques under an argon atmosphere.18,19 Celite, alumina, and 4 Å molecular sieves were activated under vacuum overnight at a temperature above 180 °C. CH2Cl2 and n-hexane were dried

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Co-insertion of CO and C2H4 at Nickel by passage through activated alumina and Q-5 columns.20 THF was distilled from dark purple solutions of sodium benzophenone ketyl. Anhydrous grade Et2O was degassed in Vacuo and dried with activated alumina. C6D6, and CD2Cl2 purchased from Cambridge Isotope Laboratory were freeze-pump-thaw degassed and distilled from CaH2. All solvents were stored over 4 Å molecular sieves. Infrared data (Fluorolube mulls, CaF2 plates) were measured with a Nicolet Nexus 670 FT-IR. Elemental analyses were performed by Desert Analytics (Tucson, AZ). 1H, 13C, and 31P NMR spectra were recorded by using Bruker 500 and 400 MHz NMR spectrometers. 1H and 13C NMR data are reported with reference to solvent resonances (C6D6, δH ) 7.16 and δC ) 128.0; CD2Cl2, δH ) 5.32 and δC ) 53.8). 31P NMR spectra are reported with respect to external 85% H3PO4 (δ ) 0). [(dtbpe)NiCH2CtBu][PF6] (1) was prepared as previously described.17 X-ray Crystallography. Data were collected at 100 K on a Bruker Platform goniometer with a CCD detector (λ ) 0.71073 Å). Structures were solved by direct methods using the SHELXTL (version 5.1) program.21 The space groups were determined with the aid of XPrep software, based on systematic absences and intensity statistics. Hydrogen atoms were fixed at calculated positions and refined isotropically. All non-hydrogen atoms were refined anisotropically. Crystallographic data, including refinement statistics, are given in the Supporting Information (Tables 1S, 2S, and 3S). For 3, two chemically equivalent but crystallographically unique molecules were found in the unit cell. For one of the molecules, the carbon bound to nickel is disordered, and the disorder was not modeled. A molecule of diethyl ether is present in the unit cell of 3 and is disordered. Synthesis of [(dtbpe)Ni(η2-C(O)CH2tBu)][PF6] (2). A Schlenk flask equipped with a stir bar was loaded with 1 (167 mg, 0.281 mmol), and the solid was dissolved in 10 mL of CH2Cl2. The solution was degassed and cooled to -78 °C, and then 1 equiv of CO (7 mL, 760 Torr, 20 °C) was added via a syringe. The orangered solution was allowed to warm to room temperature over the course of 3 h. During this time the color of the solution changed to yellow. The reaction mixture was then concentrated under reduced pressure and taken into a glovebox. The mixture was filtered, layered with Et2O, and cooled to -35 °C for 1 day to afford yellow crystals of 2 (116 mg, 0.187 mmol, 67% yield). X-ray quality crystals were obtained from cold THF. 1H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 3.31 (s, CH2tBu, 2 H), 2.05 (m, CH2CH2, 2 H), 1.90 (m, CH2CH2, 2 H), 1.40 (d, PtBu, 18 H, 3JHP ) 14.0 Hz), 1.37 (d, PtBu, 18 H, 3JHP ) 14.0 Hz), 1.11 (s, CH2tBu), 9 H). 13C{1H} NMR (22 °C, 125.8 MHz, CD2Cl2): δ 248.30 (dd, NiCO, 2JCPtrans ) 48 Hz, 2JCPcis ) 10 Hz), 55.11 (dd, NiC(O)CH2, 3JCP ) 9 Hz, 5 Hz), 35.68 (d, PC(CH3)3, 1JCP ) 19 Hz), 34.88 (d, PC(CH3)3, 1JCP ) 16 Hz), 33.18 (s, CH2C(CH3)3), 30.28 (d, PC(CH3)3, 2JCP ) 6 Hz), 30.24 (d, PC(CH3)3, 2JCP ) 5 Hz), 29.30 (s, CH2C(CH3)3), 25.25 (m, CH2CH2), 19.77 (m, CH2CH2). 31P{1H} NMR (22 °C, 202.4 MHz, CD2Cl2): δ 103.26 (d, PtBu2, 2JPP ) 26 Hz), 85.02 (d, PtBu2, 2 JPP ) 26 Hz), -143.6 (sept, PF6, 1JPF ) 711 Hz). IR: νCO ) 1597 cm-1. Anal. Calcd for C24H51NiF6P3O: C, 46.55; H, 7.98. Found: C, 46.88; H, 8.07. (15) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 9172–9173. (16) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20, 16–18. (17) Kitiachvili, K. D.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2004, 126, 10554–10555. (18) Burger, B. J.; Bercaw, J. E. Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC, 1987; pp 79-98. (19) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-SensitiVe Compounds, 2nd ed.; John Wiley & Sons: New York, 1986. (20) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520. (21) Sheldrick, G. Acta Crystallogr. A 2008, A64, 112–122.

Organometallics, Vol. 28, No. 8, 2009 2569 Synthesis of [(dtbpe)Ni(K2-CH2CH2C(O)CH2tBu)][PF6] (3). A Schlenk flask equipped with a stir bar was loaded with 2 (71 mg, 0.114 mmol), and the solid was dissolved in 10 mL of CH2Cl2. The yellow solution was degassed and cooled to -78 °C, and then 1 equiv of C2H4 was added using a calibrated volume (54 mL, 40 Torr, 20 °C). The solution was allowed to warm to ambient temperature while stirring over the course of 4 h. No color change was noticeable. The reaction mixture was then concentrated under reduced pressure and taken into a glovebox. The mixture was filtered, layered with Et2O, and cooled to -35 °C for 1 day to afford yellow crystals of 3 (29 mg, 0.045 mmol, 39% yield). X-ray quality crystals were grown from a concentrated solution of 3 in THF layered with Et2O. 1H NMR (22 °C, 500.1 MHz, CD2Cl2): δ 2.76 (m, NiCH2CH2C(O), 2 H), 2.64 (s, C(O)CH2C(CH3)3), 2 H), 1.98 (m, CH2CH2, 2 H), 1.68 (m, CH2CH2, 2 H), 1.56 (m, NiCH2CH2C(O), 2 H), 1.42 (d, PtBu, 18 H, 3JHP ) 6 Hz), 1.39 (d, PtBu, 18 H, 3JHP ) 6 Hz), 1.06 (s, CH2tBu, 9 H). 13C{1H} NMR (22 °C, 125.8 MHz, CD2Cl2): δ 240.6 (dd, NiCH2CH2C(O), 2JCPtrans ) 11 Hz, 2JCPcis ) 2 Hz), 66.2 (s, NiC(O)CH2), 52.80 (t, 3JCP) 4 Hz, NiCH2CH2C(O)), 38.21 (d, PC(CH3)3, 1JCP ) 21 Hz), 35.60 (d, PC(CH3)3, 1JCP ) 11 Hz), 33.57 (s, CH2C(CH3)3), 30.81 (d, PC(CH3)3, 2JCP ) 3 Hz), 30.47 (d, PC(CH3)3, JCP ) 5 Hz), 30.25 (s, CH2C(CH3)3), 26.91 (m, CH2CH2), 18.78 (m, CH2CH2), 17.85 (dd, NiCH2CH2C(O), JCP ) 59 Hz, JCP ) 31 Hz). 31P{1H} NMR (22 °C, 202.4 MHz, CD2Cl2): δ 83.46 (d, PtBu2, 2JPP ) 4 Hz), 79.04 (d, PtBu2, 2JPP ) 4 Hz), -143.9 (sept, PF6, 1JPF ) 711 Hz). IR: νCO ) 1601 cm-1. Anal. Calcd for C26H55NiF6P3O: C, 48.17; H, 8.40. Found: C, 48.37; H, 8.25. Synthesis of (dtbpe)Ni(η2-OCdCHCtBu) (4). A 12 mL THF solution of 2 (105 mg, 0.169 mmol) was cooled to -35 °C, and a cold Et2O solution of phenyl lithium (14 mg, 0.169 mmol) was added. The resulting pale orange solution was stirred for 45 min and dried under reduced pressure. The solids were extracted with a minimum of n-hexane, filtered, and slowly cooled to give yellow blocks of 4 suitable for X-ray diffraction (72 mg, 0.151 mmol, 90% yield). 1H NMR (22 °C, 400.0 MHz, C6D6): δ 3.80 (d, CHtBu, 1 H, 3JHP ) 8 Hz), 1.66 (s, CHtBu, 9 H), 1.25 (m, CH2CH2, 4 H), 1.18 (d, PtBu, 18 H, 3JHP ) 14 Hz), 1.09 (d, PtBu, 18 H, 3JHP ) 12 Hz). 13C{1H} (22 °C, 100.6 MHz, C6D6): δ 80.6 (s, CHtBu), 34.4 (m, PC(CH3)3), 33.8 (m, PC(CH3)3), 32.6 (s, (CH3)3), 30.4 (s, (CH3)3), 30.3 (s, (CH3)3), 24.4 (m, CH2), 20.4 (m, CH2). 31P{1H} (22 °C, 161.9 MHz, C6D6): δ 100.2 (d, PtBu2, 2JPP ) 61 Hz), 80.4 (d, PtBu2, 3JPP ) 61 Hz). IR: νCCO ) 1655 cm-1. Anal. Calcd for C24H50ONiP2: C, 60.65; H, 10.60. Found: C, 59.29; H, 10.45.

Results and Discussion Exposing CH2Cl2 solutions of 1 to 1 equiv of CO causes a gradual color change from red to yellow. The yellow product, [(dtbpe)Ni(η2-C(O)CH2tBu)][PF6] (2), is isolated in 67% yield by recrystallization from CH2Cl2/Et2O (Scheme 1). The 1H and 31 P NMR spectra are consistent with approximate Cs symmetry on the NMR time scale; both phosphorus atoms are magnetically distinct. This behavior contrasts the starting material, 1, which is fluxional on the NMR time scale. The infrared spectrum of 2 exhibits a CO stretch at 1597 cm-1, in the expected range for a cationic, η2-bound acyl. In the 13C NMR spectrum, a downfield doublet-of-doublets is observed. The resonance is centered at 248.3 ppm, having both 2JCPcis ) 10 Hz and 2JCPtrans ) 48 Hz couplings. No evidence for reversible CO insertion, as is observed in the reaction of trans-(Me3P)2Ni(Cl)Me with CO,22 was noted at 20 °C on the NMR time scale. Crystals of 2 suitable for X-ray diffraction analysis were grown from THF at -35 °C. These crystals were orthorhombic (Pbca), and the asymmetric unit contains the complete molecule. (22) Klein, H.-F.; Karsch, H. H. Chem. Ber. 1976, 109, 2524–2532.

2570 Organometallics, Vol. 28, No. 8, 2009

Curley et al.

Scheme 1. Reactions of 1 and 2

Figure 2. 1H-1H COSY spectrum for 3.

Figure 1. Molecular structure of 2 with H-atoms omitted (50% probability ellipsoids). Table 1. Selected Bond Lengths and Angles for 2 bond lengths (Å) Ni(1)-C(10) O(1)-C(10) Ni(1)-O(1) Ni(1)-P(1) Ni(1)-P(2)

1.813(3) 1.229(4) 1.918(2) 2.2290(9) 2.1602(9)

angles (deg) C(10)-Ni(1)-O(1) C(10)-Ni(1)-P(1) C(10)-Ni(1)-P(2) O(1)-Ni(1)-P(1) O(1)-Ni(1)-P(2) P(1)-Ni(1)-P(2)

38.34(12) 151.01(11) 115.12(11) 113.39(7) 153.46(7) 92.89(4)

2

The crystal structure shows that the η -bound acyl ligand has an acute C(10)-Ni(1)-O(1) angle of 38.34(12)° and similar Ni-C (1.813(3) Å) and Ni-O (1.918(2) Å) bond lengths (Figure 1, Table 1). This structural feature is unusual and not present in the X-ray crystal structure of (Me3P)2Ni(Cl)C(O)Me, for which the acyl ligand is σ-bound through carbon with the oxygen atom unattached to Ni.23 The carbonyl-acyl motif, Ni(CO)C(O)Me, has been structurally characterized by singlecrystal X-ray diffraction for complexes of Ni and Pd.24,25 In the solid-state structure of 2 the short C(10)-O(1) bond of 1.229(4) Å is comparable to organic ketones. Moreover, the acyl carbon atom is planar with the O(1)-C(10)-C(11) angle of 127.6(3)°, consistent with sp2 hybridization. The bite angle of the bulky dtbpe ligand is retained in 2, 92.89(4)°, as compared to 1, 93.20(4)°. Addition of 1 equiv of C2H4 to the Ni(II) acyl complex 2 results in the formation of a four-membered, chelating ligand (23) Huttner, G.; Orama, O.; Bejenke, V. Chem. Ber. 1976, 109, 2533– 2536. (24) Shirasawa, N.; Nguyet, T. T.; Hikichi, S.; Moro-oka, Y.; Akita, M. Organometallics 2001, 20, 3582–3598. (25) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118, 4746–4764.

Figure 3. Molecular structure of 3 with H-atoms omitted (50% probability ellipsoids). Table 2. Selected Bond Lengths and Angles for 3 bond lengths (Å) Ni(1)-C(10) O(1)-C(12) C(10)-C(11) Ni(1)-O(1) Ni(1)-P(1) Ni(1)-P(2)

1.953(3) 1.239(4) 1.507(4) 1.975(2) 2.2789(10) 2.1650(9)

angles (deg) C(10)-Ni(1)-O(1) C(10)-Ni(1)-P(1) C(10)-Ni(1)-P(2) O(1)-Ni(1)-P(1) O(1)-Ni(1)-P(2) P(1)-Ni(1)-P(2)

82.63(12) 173.13(11) 91.89(10) 95.24(7) 171.90(7) 90.88(4)

that is wholly composed of the two small molecules that have been inserted into the Ni-C bond: CO and C2H4 (Scheme 1). Although β-hydrogen atoms are present in 3, β-hydride elimination is not rapid at 20 °C and was not observed during routine manipulations of this complex. The 13C NMR spectrum shows a resonance for the carbonyl carbon at 240.6 ppm with smaller JCP couplings (11 Hz, 2 Hz) than were present in 2. The JPP coupling of 4 Hz is smaller than the coupling in either 1 or 2 and may be an effect of a less constrained geometry about the Ni center. It was of interest to unambiguously assign resonances corresponding to the two sets of chemically inequivalent, ethylene-derived protons along the ring in the 1H NMR spectrum. Therefore, a 1H homonuclear COSY spectrum was obtained (Figure 2). This spectrum shows coupling between the protons in the Ni-C-C-C-O ring (2.76, 1.58 ppm) and between the two proton pairs on the dtbpe ligand backbone

Co-insertion of CO and C2H4 at Nickel

(1.98, 1.68 ppm). This assignment was supported by selective NOE experiments. Cooling solutions of 3 in THF/n-pentane at -35 °C afforded yellow plates that were used in X-ray diffraction studies. As expected, the Ni-C-C-C-O ring is nonplanar (Figure 3), with puckering at the carbon atom bound to nickel. This C-C-C-O chelating ligand has a structure similar to that for the first insertion products of CO and an olefin into Pd-C bonds in catalytic model systems containing bipyridine ancillary ligands.26,27 The C(12)-O(1) bond in 3 (1.239(4) Å; Table 2) is similar to that found in 2. The bite angle of the dtbpe ligand is relatively unchanged by insertion of the C2-unit into the other chelating ligand. Complex 3 does not react with 1 equiv of carbon monoxide at room temperature, and addition of a 10-fold excess (and up to 1 atm) of CO to solutions of 3 results in decomposition, giving a complex mixture of unidentified nickel-containing products as judged by 1H and 31P NMR. In view of the positive charge of the acyl complex cation in 2, it seemed likely that the addition of an external base might result in deprotonation at an acidic methylene position R to the CdO group. Indeed, addition of PhLi to a THF solution of 2 resulted in the precipitation of LiPF6 and isolation of a soluble Ni-containing product in 90% yield (Scheme 1). This product was identified as the η2-ketene complex (dtbpe)Ni(η2OCdCHtBu) (4), in which the ketene is bound to Ni through its CdO bond (Vide infra). Ketene complexes of Ni have previously been synthesized via a Wittig reaction with coordinated CO228 and by addition of free ketenes to a Ni source.29-31 A stretching mode assigned to the cumulene system was located in the IR spectrum at 1655 cm-1. This value compares well with the corresponding value of 1643 cm-1 reported for (dtbpm)Ni(η2-OCdCPh2) (dtbpm ) 1,2-bis(di-tert-butylphosphino)methane).29 In order to verify that the ketene ligand is CdO rather than CdC bound to Ni, crystals suitable for X-ray diffraction were grown from Et2O at -35 °C. The X-ray crystal structure of 4 confirms CdO coordination (Figure 4). For compounds such as (Ph3P)2Ni(η2-OCC(Ph)R) (R ) Me, Et) there is a fluxional process that interconverts Ni coordination between the CdC and CdO bonds;31 however, we did not observe evidence for such a process in 4 as determined by variable-temperature 1H and 31P NMR. The observation of sharp, pairwise-inequivalent t Bu resonances for the dtbpe ligand in the 1H NMR spectrum of 4 is consistent with Cs symmetry; a structure featuring CdC ketene coordination would be expected to exhibit four inequivalent dtbpe tBu resonances due to its C1 symmetry. In the 31P NMR spectrum, two sharp sets of resonances (δ 100.2 and 80.4, 3 JPP ) 61 Hz) corresponding to the two inequivalent phosphorus environments were observed, consistent with the unsymmetrical ketene ligand coordinated to square-planar Ni(II). Both (dtbpm)Ni(η2-OCdCPh2) and (Cy3P)2Ni(η2-OCdCH2) contain similar η2-O,C-bound ketene ligands, although the latter has not been (26) Shen, H.; Jordan, R. F. Organometallics 2003, 22, 1878–1887. (27) Stoccoro, S.; Minghetti, G.; Cinellu, M. A.; Zucca, A.; Manassero, M. Organometallics 2001, 20, 4111–4113. (28) Wright, C. A.; Thorn, M.; McGill, J. W.; Sutterer, A.; Hinze, S. M.; Prince, R. B.; Gong, J. K. J. Am. Chem. Soc. 1996, 118, 10305–10306. (29) Hofmann, P.; Perez-Moya, L. A.; Steigelmann, O.; Riede, J. Organometallics 1992, 11, 1167–1176. (30) Hoberg, H.; Korff, J. J. Organomet. Chem. 1978, 152, 255–264. (31) Miyashita, A.; Sugai, R.; Yamamoto, J. J. Organomet. Chem. 1992, 428, 239–247.

Organometallics, Vol. 28, No. 8, 2009 2571

Figure 4. Molecular structure of 4 with H-atoms omitted (50% probability ellipsoids). Table 3. Selected Bond Lengths and Angles for 4 bond lengths (Å) Ni(1)-C(10) O(1)-C(10) C(10)-C(11) Ni(1)-O(1) Ni(1)-P(1) Ni(1)-P(2)

angles (deg)

1.864(2) 1.295(3) 1.339(3) 1.876(2) 2.1430(7) 2.2162(7)

C(10)-Ni(1)-O(1) C(10)-Ni(1)-P(1) C(10)-Ni(1)-P(2) O(1)-Ni(1)-P(1) O(1)-Ni(1)-P(2) P(1)-Ni(1)-P(2)

40.51(9) 114.17(8) 152.86(8) 154.59(6) 112.40(5) 92.96(2)

crystallographically confirmed. A molecular orbital model explaining the preference for coordination along the CdO bond has been discussed.29 In its solid-state structure, 4 has a C-O distance of 1.295(3) Å, significantly longer than the corresponding distances in 2 and 3, and may be interpreted as indicative of Ni back-bonding into the π* orbital of the ketene CO bond (Table 3).29 The C(10)-Ni-O(1) angle of 40.51(9)° is nearly the same as that found for 2 and reflects the steric strain imposed by the η2 coordination mode. The ketene ligand’s CdC bond at 1.339(3) Å is consistent with the fact that deprotonation at carbon creates unsaturation at this site.

Conclusions We have reported the sequential insertions of CO and C2H4 into the Ni-C bond of the cationic Ni(II) neopentyl complex 1. These insertions give new ligands that contain both Ni-C and Ni-O bonds. These complexes resemble those that were observed in solution by Brookhart using 1H NMR spectroscopy for a cationic Ni complex that catalytically forms CO/C2H4 copolymers. Herein we have presented the first structurally characterized examples of catalytically relevant species for nickel. Finally, deprotonation of the cationic Ni(II) acyl complex 2 results in formation of a neutral ketene complex, 4, which was also structurally characterized and compared to related literature examples.

Acknowledgment. This work was supported by the National Science Foundation through grant CHE-0615274 (to G.L.H.) and a predoctoral GAANN Fellowship from the Department of Education (to R.W.). Supporting Information Available: X-ray crystallographic data for 2, 3, and 4 (CIF files) and summary tables of the details of structure refinements and statistics (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. OM900023Y