Ethylene Polymerization Using Discrete Nickel(II) Iminophosphonamide Complexes

The syntheses and structures of the discrete (π-allyl)nickel iminophosphonamide (PN2) complexes
2a−d from the reaction of (π-allyl)nickel bromide and the corresponding PN2 ligands 3a,b or from the
reaction of (π-allyl)2Ni and phosphorane 1 are reported. Complexes 2a,b are characterized by having
long Ni−N distances coupled with an acute bite angle for the PN2 ligand. The π-allyl ligands in complexes
2a−d are not fluxional on the NMR time scale at room temperature, although chemical exchange between
the isomeric complexes 2c,d occurs via PN2 ligand reorientation. Purified complexes 2a−d are not active
for ethylene polymerization; it is only when complexes 2c,d are generated in situ in the presence of
monomer that high-Mw branched poly(ethylene) is formed. A variety of indirect evidence suggests that
the active catalyst arises from the reaction of Ni(0)−alkene complexes with phosphorane 1, either
preformed or generated in situ through decomposition of (π-allyl)2Ni. A bona fide PN2NiPh(PPh3) complex,
5, was prepared from NiPh(PPh3)2Br and the PN2 ligand 3a and was structurally characterized. This
complex is active for 1-hexene isomerization in the absence of an activator. During hexene isomerization,
variable amounts of the paramagnetic bis(PN2) complex 4 are produced along with ligand 3a. In addition,
the fluxional intermediate 6, containing both a PN2 ligand and coordinated PPh3, is present during catalysis.
Reaction of 5 with an equimolar amount of propene provides α-methylstyrene, the product of 1,2-insertion
followed by β-H elimination. Complex 5 is not effective for polymerization or oligomerization of ethylene
under a variety of conditions. The reactions of complex 5 with various phosphine scavengers were studied,
and of these only Rh(acac)(C2H4)2 is both effective and selective for PPh3. Hard Lewis acids, including
AlMe3, B(C6F5)3, and PMAO, have a pronounced tendency toward abstraction of the PN2 or other anionic
ligands in these unhindered complexes. All of the complexes reported in this paper are extremely active
for ethylene dimerization in the presence of PMAO. In the presence of stoichiometric Rh(I), complex 5
rapidly isomerizes 1-hexene and in the presence of ethylene produces branched PE oligomers at modest
activity.