beta-Hydrogen Elimination and Reductive Elimination from a K-3-PPC Nickel Complex

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 (Bu2PCH2CH2P)-P-t(Bu-t)R failed, success was achieved via oxidative addition of the 2-bromo-benzyl-ligand precursor (Bu2PCH2CH2P)-P-t(Bu-t)(CH2-o-C6H4Br) with Ni(COD)(2) to generate {[}kappa(3)-(PPC)-P-Bn]NiBr. Subsequent reaction with KBEt3H resulted in decomposition unless PPh3 was present, which allowed isolation of the tricoordinate Ni(0) complex {[}kappa(2)-(PP)-P-Bn]Ni(PPh3). Reaction of {[}kappa(3)-(PPC)-P-Bn]NiBr with EtMgCl also resulted in the formation of the Ni(0) ethylene complex, {[}kappa(2)-(PP)-P-Bn]Ni(eta(2)-C2H4), via beta-elimination followed by reductive elimination. Deuterium-labeling studies are consistent with a reversible beta-elimination process prior to reductive elimination, which scrambles the deuterium isotopes. Reaction of {[}kappa(3)-(PPC)-P-Bn]NiBr with CH3Li results in the formation of the Ni(II) methyl complex, {[}kappa(3)-(PPC)-P-Bn]Ni(CH3). Heating this species in the presence of PPh3, results in the formation of the Ni(0) derivative, {[}kappa(2)-Bn-o-MePP]Ni(PPh3), in which reductive elimination of the methyl and aryl units has occurred. Reaction of {[}K-3-(PPC)-P-Bn]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) eta(2)-acetone adducts, which presumably arise via a beta-elimination-reductive elimination sequence; the analogous sulfur reagent generated isolable Ni(II) isopropyl thiolate species that was resistant to beta-elimination upon thermolysis; with methylphenylamide, the Ni(II) amido complex could be isolated and structurally characterized; subsequent heating to 80 degrees C resulted in beta-elimination followed by reductive elimination to generate a Ni(0) phenylimine complex. While the kappa(3)-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 beta-elimination and/or reductive elimination.