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.