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Synthesis of PCP-Supported Nickel Complexes and their Reactivity with Carbon Dioxide

Authors

  • Timothy J. Schmeier,

    1. The Department of Chemistry, Yale University, P. O. Box 208107, New Haven, Connecticut, 06520 (USA)
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  • Dr. Ainara Nova,

    Corresponding author
    1. Institute of Chemical Research of Catalonia (ICIQ), Ave Països Catalans 16, 43007 Tarragona (Spain)
    • Institute of Chemical Research of Catalonia (ICIQ), Ave Països Catalans 16, 43007 Tarragona (Spain)
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  • Prof. Dr. Nilay Hazari,

    Corresponding author
    1. The Department of Chemistry, Yale University, P. O. Box 208107, New Haven, Connecticut, 06520 (USA)
    • The Department of Chemistry, Yale University, P. O. Box 208107, New Haven, Connecticut, 06520 (USA)
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  • Prof. Dr. Feliu Maseras

    1. Institute of Chemical Research of Catalonia (ICIQ), Ave Països Catalans 16, 43007 Tarragona (Spain)
    2. Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra (Spain)
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  • PCP=bis-2,6-di-tert-butylphosphinomethylbenzene

Abstract

The Ni amide and hydroxide complexes [(PCP)Ni(NH2)] (2; PCP=bis-2,6-di-tert-butylphosphinomethylbenzene) and [(PCP)Ni(OH)] (3) were prepared by treatment of [(PCP)NiCl] (1) with NaNH2 or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H2O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2-Me-imidazole)] (4), [(PCP)Ni(dimethylmalonate)] (5), [(PCP)Ni(oxazole)] (6), and [(PCP)Ni(CCPh)] (7), respectively. The hydroxide compound 3, could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN3 generated [(PCP)Ni(CN)] (8) or [(PCP)Ni(N3)] (9), respectively. Compounds 3–7, and 9 were characterized by X-ray crystallography. Although 3, 4, 6, 7, and 9 are all four-coordinate complexes with a square-planar geometry around Ni, 5 is a pseudo-five-coordinate complex, with the dimethylmalonate ligand coordinated in an X-type fashion through one oxygen atom, and weakly as an L-type ligand through another oxygen atom. Complexes 2–9 were all reacted with carbon dioxide. Compounds 24 underwent facile reaction at low temperature to form the κ1-O carboxylate products [(PCP)Ni{OC(O)NH2}] (10), [(PCP)Ni{OC(O)OH}] (11), and [(PCP)Ni{OC(O)-2-Me-imidazole}] (12), respectively. Compounds 10 and 11 were characterized by X-ray crystallography. No reaction was observed between 59 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 29 to form a κ1-O carboxylate product and understand the pathways for carbon dioxide insertion into 2, 3, 6, and 7. The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7, but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.

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