Highly Active Ethene Polymerization Catalysts with Unusual Imine Ligands - Late Transition Metal Polymerization Catalysis - page 81

Fig. 3.9 Molecular structure of (2,6-di-
methylphenylimino-2-phenylisopropoxy)
NiPh(PPh 3 )( 123 ). Selected bond lengths
(pm): Ni(1)-O(1) =190.0(6), Ni(1)-C(1) =
191.7(7), Ni(1)-N(1) =194.4(6), Ni(1)-P(1) =
218.5(4). Selected angles ( ): O(1)-Ni(1)-
C(1) =166.6(3), N(1)-Ni(1)-P(1)=172.4(2),
C(1)-Ni(1)-P(1)=89.8(2), O(1)-Ni(1)-
P(1)=92.1(2), C(1)-Ni(1)-N(1) =96.0(3).
plexes was inactive in polymerization tests, as was found in other similar cases
[8c, d, 32]. Several efforts were undertaken to prepare isolable monomeric
(N,O)NiMe(donor) complexes that are useful precatalysts, or could be transformed
into such compounds. Nitriles, like acetonitrile or pivalonitrile do not coordinate
strongly enough to inhibit dimer formation, and on the other hand phosphanes
are not easily removed to afford a vacant coordination site for the incoming mono-
mer ( vide supra ). Nevertheless, in analogy to other (P,O)-Ni polymerization cata-
lysts [8 a, 8b], it was found that methylene phosphorane is a suitable donor to pre-
pare the monomeric complexes (N,O)NiMe(donor) (Chart 3.8). By this route, com-
plexes 126 - 131 were prepared with ligands 104 , 109 , 113 - 116 , respectively.
In some cases, a second product was also formed in the synthesis. This was
identified as a CH activated or ortho-metalated complex (N,O)Ni-( o -Ph)PPh 2 =
CH 2 , where an ortho C-H bond of a phosphorane phenyl group was deproto-
nated by the Ni-Me moiety with concomitant formation of methane. Fig. 3.10
shows the molecular structure of complex 132 obtained from ligand 114 . The cen-
tral square planar Ni atom is part of two five-membered metallacycles.
It was of importance to establish the formation route of the byproduct, in partic-
ular to investigate whether it is a thermal decomposition product of (N,O)-
NiMe(CH 2 =PPh 3 ). This is relevant for the polymerization performance because
productivity and/or activity are quite sensitive to the thermal stability of the cat-
alytically active species. However, heating (N,O)-NiMe(CH 2 =PPh 3 ) in benzene so-
lution to temperatures well above those employed in polymerization studies did
not result in the CH-activation. In fact, the parent nickel methyl phosphorane
complexes turned out to be quite stable under those conditions, indicating that
the CH-activated product forms during the synthesis along a separate reaction
channel. More importantly, it was also established that this byproduct is not poly-
merization active, nor can it be activated for the polymerization by the procedure
used for the (N,O)-NiMe(CH 2 =PPh 3 ) complexes.