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

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.