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

Abstract

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(CH2PtBu2)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) η2-silane complexes Ph2PSiHPM(PPh3) (M = Ni, Pd; Ph2PSiHP = (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–hydride 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 η2-silane complex Ph2PSiHPPd(PPh3) reacted rapidly with carbon dioxide to give Ph2PSiPPd-OC(O)H and PPh3, while the corresponding Ni complex Ph2PSiHPNi(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–hydride bond. In contrast, carbon dioxide insertion is thermodynamically unfavorable for the PNP-supported species. In the case of the η2-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 η2-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.