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 iPr2 PNP (CH3) PdH ( iPr2 PNP (CH3) = N(2-P i Pr 2 -4-MeC 6 H 3 ) 2 ), iPr2 PNP (CH3) NiH, iPr2 PNP (F) PdH ( iPr2 PNP (F) = N(2-P i Pr 2 -4-C 6 H 3 F) 2 ), CyPh PNPPdH ( CyPh PNP = N(2-P(Cy)(Ph)-4-MeC 6 H 3 ) 2 ), tBu2 PCPPdH ( tBu2 PCP = 2,6-C 6 H 3 (CH 2 P t Bu 2 ) 2 ), tBu2 PCPNiH, Cy2 PCPPdH ( Cy2 PCP = 2,6-C 6 H 3 (CH 2 PCy 2 ) 2 ), and Cy2 PCPNiH, were prepared using literature methods. In addition, the new Ni and Pd hydrides Cy2 PSiPMH (M = Ni, Pd; Cy2 PSiP = Si(Me)(2-PCy 2 -C 6 H 4 ) 2 ) supported by PSiP ligands were synthesized. The analogous metal hydride complexes supported by the Ph2 PSiP ligand ( Ph2 PSiP = Si(Me)(2-PPh 2 -C 6 H 4 ) 2 ) could not be prepared. Instead, the Ni(0) and Pd(0)which have been proposed to be in equilibrium with Ph2 PSiPMH (M = Ni, Pd) and PPh 3 , were prepared. Facile carbon dioxide insertion into the metal−hydride bond to form the metal formate complexes tBu2 PCPM-OC(O)H (M = Ni, Pd) or Cy2 PCPM-OC(O)H (M = Ni, Pd) was observed for PCP-supported species, and a similar reaction was observed for Cy2 PSiP-supported hydrides to form Cy2 PSiPM-OC(O)H (M = Ni, Pd).No reaction with carbon dioxide was observed for any complexes supported by PNP ligands. The η 2 -silane complex Ph2 PSi H PPd(PPh 3 ) reacted rapidly with carbon dioxide to give Ph2 PSiPPd-OC(O)H and PPh 3 , while the corresponding Ni complex Ph2 PSi H PNi(PPh 3 ) 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 η 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 PPh 3 and the η 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.

A Biomimetic Nickel Complex with a Reduced CO2 Ligand Generated by Formate Deprotonation and Its Behaviour towards CO2

“…The 1 H NMR spectrum of 1 in C 6 D 6 shows resonances between δ =7.07 and 0.98 ppm, with a pattern expected for a C 2 v symmetric nickel centre with square‐planar coordination. The formate proton gives rise to a singlet at δ =5.40 ppm, which appears to be shifted to exceptionally high fields compared to other nickel formates; this is probably due to the donation of electron density from the occupied nickel d xy orbital to the formate carbon atom, which leads to a shielding of the formate proton.…”

Nickel and Iron Pincer Complexes as Catalysts for the Reduction of Carbonyl Compounds

“…The calculated ΔG ‡ values for CO 2 insertion into nickel hydrides with various pincer ligands agree reasonably well with the experimental data. 23 Meanwhile, the comparison between nickel and palladium systems through the computational study suggests the possibility that a judicious selection of ligand could lead to even better reactivity for a nickel hydride than a palladium hydride.…”

An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

“…Recently, both our research group7q and Mitton and Turculet7o investigated the insertion of CO 2 into [( Cy PSiP)PdH]8 ( Cy PSiP=Si(Me)(2‐PCy 2 ‐C 6 H 4 ) 2 , in which Cy is cyclohexyl), and demonstrated that a Pd formate was rapidly generated. We were interested in exploring the same reaction with the related complex [( Ph PSiP)PdH] ( Ph PSiP=Si(Me)(2‐PPh 2 ‐C 6 H 4 ) 2 ), which Takaya and Iwasawa proposed to be a crucial intermediate in allene carboxylation 7d.…”

“…The choice of hydride source does not appear to be crucial, as when [( Ph PSiP)PdCl] was treated with LiAlH 4 instead of LiEt 3 BH, [( Ph PSiP) 2 Pd 2 ] was observed in the 31 P NMR spectrum. Given that the analogous reaction between [( Cy PSiP)PdCl] and LiEt 3 BH results in the clean formation of [( Cy PSiP)PdH],7q it is probable that the formation of [( Ph PSiP) 2 Pd 2 ] occurs through a transient [( Ph PSiP)PdH] species, although even at low temperature there is no evidence for the formation of this intermediate. Presumably, the relative lack of steric bulk in [( Ph PSiP)PdH] results in bimolecular dimerization, whereas the more sterically protected complex [( Cy PSiP)PdH] is stable 8…”