Nickel Polymerization Catalysts with Ylide Steering Ligands - Late Transition Metal Polymerization Catalysis - page 8

The Ni-phenyl group(Ph)
This is the other C-bound ligand. It originates from an oxidative addition of a P-
phenyl moiety to nickel(0). In this way nickel binds to P and phenyl in a cis fash-
ion. Accordingly, the phenyl group has a heavily deshielded ipso C atom with a
two-bond cis coupling 2 J ( 31 P 13 C) cis =31 Hz. The highfield shifts of the ortho and
para C signals are indicative of a phenyl group bound to an electron-rich center
(Ni). The chemical shift behavior is reminescent of benzyl-Grignard reagents and
benzylidene phosphoranes [Eq. (8)].

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The PO Chelate Ligand (Ph 2 PCHCPhO)
The Ni-P complexation generates a four-coordinate phosphorus and the ligand may
be viewed as a metallated ylide structure in a bis(ylide)nickel environment. The

carbon of the stabilized ylide remains trigonal planar after complexation, as indi-
cated by the one-bond ( 13 C 1 H) coupling of 163 Hz. The double bond, which can
be formulated as a phosphinoenolate structure, is strongly polarized. Owing to a par-
tial negative charge, C- is shifted upfield by approximately 50 ppm compared with

“normal” sp 2 centers at 128 ppm. Accordingly, C-

is strongly deshielded.
The pronounced differences in the electronic structure of phosphanes R 3 P and
highly polar ylides R 3 PCH 2 are clearly visible in valence electron photoelectron
spectra (UV-PES) in that the outermost electrons of ylides (n C -) are energetically
more readily available than those of corresponding phosphanes (n P ) (see Tab. 1.1).
This feature is “sensed” by the nickel core electrons of related complexes [Eq. (9)]

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ESCA
The ESCA (X-ray photoelectron spectroscopy) of related nickel complexes shows
the 2p 3/2 binding energy of the nickel center of [NiPh(Ph 2 PCHCMeO)(Ph 3 PCH 2 )]
=853.4 eV is lower than that of [NiPh(Ph 2 PCHCMeO)(Ph 3 P)] =854.4 eV by 1 eV
and thus it falls into the range of zerovalent nickel complexes! Obviously, the
nickel center experiences an energetic destabilization similar to ylide C-substitu-
ents [Eq. (5)].

1.2.3
X-ray Structure Analysis [11]

The basic structure 1 is structurally related to complexes of types 2 and 3
(Tab. 1.3). A comparison is of interest because of the marked differences in activ-
ity and selectivity of ylide catalysts 1 , phosphane catalysts 2 , and cis- [Ni(PO) 2 ]
bis(chelate) complexes 3 . The ylide catalysts 1 are significantly more active and
produce higher molecular weight products than corresponding phosphane cata-
lysts 2 . The bis(chelate) complexes 3 can be detected as polymerization-inactive de-
composition products of 1 and 2 , e.g. after completed oligo- or polymerization.