Experimental and Computational Studies of the Reaction of Carbon Dioxide with Pincer-Supported Nickel and Palladium Hydrides

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• 作者：Hee-Won Suh ; Timothy J. Schmeier ; Nilay Hazari ; Richard A. Kemp ; Michael K. Takase
• 刊名：Organometallics
• 出版年：2012
• 出版时间：December 10, 2012
• 年：2012
• 卷：31
• 期：23
• 页码：8225-8236
• 全文大小：576K
• 年卷期：v.31,no.23(December 10, 2012)
• ISSN：1520-6041

文摘

A series of Ni(II) and Pd(II) hydrides supported by PNP and PCP ligands, including ^iPr2PNP^(CH3)PdH (^iPr2PNP^(CH3) = N(2-PⁱPr₂-4-MeC₆H₃)₂), ^iPr2PNP^(CH3)NiH, ^iPr2PNP^(F)PdH (^iPr2PNP^(F) = N(2-PⁱPr₂-4-C₆H₃F)₂), ^CyPhPNPPdH (^CyPhPNP = N(2-P(Cy)(Ph)-4-MeC₆H₃)₂), ^tBu2PCPPdH (^tBu2PCP = 2,6-C₆H₃(CH₂P^tBu₂)₂), ^tBu2PCPNiH, ^Cy2PCPPdH (^Cy2PCP = 2,6-C₆H₃(CH₂PCy₂)₂), and ^Cy2PCPNiH, were prepared using literature methods. In addition, the new Ni and Pd hydrides ^Cy2PSiPMH (M = Ni, Pd; ^Cy2PSiP = Si(Me)(2-PCy₂-C₆H₄)₂) supported by PSiP ligands were synthesized. The analogous metal hydride complexes supported by the ^Ph2PSiP ligand (^Ph2PSiP = Si(Me)(2-PPh₂-C₆H₄)₂) could not be prepared. Instead, the Ni(0) and Pd(0) 畏²-silane complexes ^Ph2PSi^HPM(PPh₃) (M = Ni, Pd; ^Ph2PSi^HP = (H)Si(Me)(2-PPh₂-C₆H₄)₂), which have been proposed to be in equilibrium with ^Ph2PSiPMH (M = Ni, Pd) and PPh₃, were prepared. Facile carbon dioxide insertion into the metal鈥揾ydride bond to form the metal formate complexes ^tBu2PCPM-OC(O)H (M = Ni, Pd) or ^Cy2PCPM-OC(O)H (M = Ni, Pd) was observed for PCP-supported species, and a similar reaction was observed for ^Cy2PSiP-supported hydrides to form ^Cy2PSiPM-OC(O)H (M = Ni, Pd). No reaction with carbon dioxide was observed for any complexes supported by PNP ligands. The 畏²-silane complex ^Ph2PSi^HPPd(PPh₃) reacted rapidly with carbon dioxide to give ^Ph2PSiPPd-OC(O)H and PPh₃, while the corresponding Ni complex ^Ph2PSi^HPNi(PPh₃) did not react with carbon dioxide. DFT calculations indicate that carbon dioxide insertion is thermodynamically favorable for PSiP- and PCP-supported hydrides because the strong trans influence of the anionic carbon donor destabilizes the metal鈥揾ydride bond. In contrast, carbon dioxide insertion is thermodynamically unfavorable for the PNP-supported species. In the case of the 畏²-silane complexes, carbon dioxide insertion is thermodynamically favorable for Pd and unfavorable for Ni. This is because the equilibrium between the metal hydride and PPh₃ and the 畏²-silane complex more strongly favors the metal hydride for Pd than for Ni. In the cases of metal hydrides, the thermodynamic favorability of carbon dioxide insertion can be predicted from the natural bond orbital charge on the hydride. The pathway for carbon dioxide insertion into the metal hydride is concerted and features a four-centered transition state. The energy of the transition state for carbon dioxide insertion decreases as the trans influence of the anionic donor of the pincer ligand increases.