β-Hydrogen Elimination and Reductive Elimination from a κ³-PPC Nickel Complex

Abstract

The incorporation of a PPC donor ligand set onto nickel is described. While C–H activation routes using Ni­(II) precursors and benzyl- and phenethyl-substituted diphosphines But 2PCH2CH2P­(But)­R failed, success was achieved via oxidative addition of the 2-bromo-benzyl-ligand precursor But 2PCH2CH2P­(But)­(CH2-o-C6H4Br) with Ni­(COD)2 to generate [κ3-BnPPC]­NiBr. Subsequent reaction with KBEt3H resulted in decomposition unless PPh3 was present, which allowed isolation of the tricoordinate Ni(0) complex [κ2-BnPP]­Ni­(PPh3). Reaction of [κ3-BnPPC]­NiBr with EtMgCl also resulted in the formation of the Ni(0) ethylene complex, [κ2-BnPP]­Ni­(η2-C2H4), via β-elimination followed by reductive elimination. Deuterium-labeling studies are consistent with a reversible β-elimination process prior to reductive elimination, which scrambles the deuterium isotopes. Reaction of [κ3-BnPPC]­NiBr with CH3Li results in the formation of the Ni­(II) methyl complex, [κ3-BnPPC]­Ni­(CH3). Heating this species in the presence of PPh3 results in the formation of the Ni(0) derivative, [κ2-Bn‑o‑MePP]­Ni­(PPh3), in which reductive elimination of the methyl and aryl units has occurred. Reaction of [κ3-BnPPC]­NiBr with heteroatom-containing species such as sodium isopropoxide, sodium 2-propanethiolate, and lithium methylphenylamide resulted in different outcomes depending on the heteroatom; with isopropoxide, the products isolated were geometric isomers of Ni(0) η2-acetone adducts, which presumably arise via a β-elimination–reductive elimination sequence; the analogous sulfur reagent generated isolable Ni­(II) isopropyl thiolate species that was resistant to β-elimination upon thermolysis; with methylphenylamide, the Ni­(II) amido complex could be isolated and structurally characterized; subsequent heating to 80 °C resulted in β-elimination followed by reductive elimination to generate a Ni(0) phenylimine complex. While the κ3-PPC donor set generates stable Ni­(II) derivatives, further functionalization at nickel with hydride, alkyl, and heteroatom-containing moieties can lead to Ni(0) products via a combination of β-elimination and/or reductive elimination.

Ancillary Ligand Modification via Reductive Elimination at Nickel(II)

“…The fact that nickel­(II) trimethylsilylmethylene complex 6 does not reductively eliminate unless heated is reminiscent of the previously reported nickel­(II) methyl complex, [κ 3 - Bn PPC]­Ni­(CH 3 ), which cleanly undergoes reductive elimination in the presence of PPh 3 to generate Ni(0) derivative 8 but only when heated at 50°C (eq ). While the two trapping ligands for 6 and 8 are different, the key point is that both Ni­(II) precursors are isolable and stable and require heating for the reductive elimination to proceed, which contrasts the diphenylmethyl and benzyl complexes that undergo spontaneous reductive elimination as shown in Schemes and .…”

“…We recently reported the synthesis of 1-(iodo- tert -butylphosphino)-2-(di- tert -butylphosphino)­ethane, Bu t 2 PCH 2 CH 2 P­(Bu t )­I, which is a useful synthon to generate a range of bulky bidentate and tridentate ligand systems by simple modification of the iodo-phosphine unit. Of particular relevance was the introduction of the o -bromobenzyl moiety (via the Grignard reaction) and subsequent oxidative addition to nickel(0) to produce [κ 3 - Bn PPC]­NiBr ( 1 ), a square-planar Ni­(II) derivative, which we have already investigated in the context of β-H elimination followed by reductive elimination sequences. In this work we examine various alkyl and amido derivatives that lack β-hydrogens in an effort to promote an α-abstraction process to try to access nickel–alkylidene, −imido and −phosphinidene analogs of the iconic trophy complexes shown in Figure .…”

Nickel and Palladium Complexes with Reactive σ‐Metal‐Oxygen Covalent Bonds

“…In line with the previously discussed investigations for Ir and Pt complexes, our main conclusion was that the ease of β‐hydrogen elimination in methylNi(II) systems decreases in the order alkyl>amide≫alkoxide. However, Fryzuk has reported a nickel system closely related to ours, [( tBu PPC)Ni‐ECHRR′], that contains a cyclometallated diphosphine ligand ( tBu PPC) . In this system, β‐hydrogen elimination for ECHRR′=alkyl, amido or alkoxide ligands, is followed by irreversible H transfer to the metallacycle, to give Ni(0) olefin, imino and ketone complexes, respectively.…”

“…However, Fryzuk has reported a nickel system closely related to ours, [( tBu PPC)Ni-ECHRR'], that contains a cyclometallated diphosphine ligand ( tBu PPC). [69] In this system, β-hydrogen elimination for ECHRR' = alkyl, amido or alkoxide ligands, is followed by irreversible H transfer to the metallacycle, to give Ni(0) olefin, imino and ketone complexes, respectively. Fryzuk reports that attempts to introduce the alkoxo moiety led directly to the Ni(0) aldehyde complex, suggesting that β-hydrogen from the alkoxide is very fast, whereas the β-elimination from an amido ligand was considerably slower.…”

Density functional theory study on the reductive elimination of ancillary ligand at PPC nickel complexes

“…1 Gibbs free energy of activation for diphenylmethyl substrate is smaller than that for methyl substrate, which in good agreement with the experimental results. 2,3 For the reductive elimination reaction, C sp3 -C sp2 coupling (R 2 = Me) is slower than Reductive elimination C sp2 -C sp2 coupling (R 2 = Ph), which is found for the nonpincer type complexes. The rate of C sp3 -C sp2 coupling is comparable to or faster than that of C sp2 -C sp2 coupling for symmetric tridentate pincer complexes.…”