Chemistry Reference
In-Depth Information
teristic is also found for the solid state structure of Brookhart-type diimine cata-
lysts that produce high molecular weight polyolefins [9].
2.5.3
NMR Studies on the Palladium Dichloro Species
By formation of the complex
14 c
, the pattern of resonances of the bis(phosphine)
13 c
(
1
H- and
13
C-NMR) significantly changed in such way that additional signals
appeared in the aromatic as well as in the aliphatic region. The
1
H-absorptions of
14 c
are broad and do not display a clear splitting of the NMR signals. The
500 MHz
1
H-NMR spectrum revealed eight resonances which belong to aromatic
hydrogens (two of them are superimposed). These observations suggest that two
different orientations of the aromatic groups are realized in solution which inter-
convert on the NMR time scale [57]. This is confirmed by
1
H,
1
H-COSY experi-
ments. The signals are divided into two sets, each resembling one of the sug-
gested arrangements. Accordingly, the methylene (bridge) and the methine (iso-
propyl) resonances are split into two signals. If the
1
H-NMR spectra are recorded
at elevated temperatures (up to 90
C), the absorbences will gradually change their
line shape and position. At 90
C, molecular motion seems to proceed almost un-
restrictedly. NMR spectra of complex
14b
,
d
, however, exhibit well-defined
1
H-ab-
sorptions with a clear splitting pattern. In case of
14d
the absence of fluxional
phenomena is caused by additional isopropyl units in the 6-positions of the aro-
matic rings. This prevents the type of molecular motion that is still found for
14 c
. As a consequence, the theoretical number of resonances in
14d
is doubled
compared to the ligand
13d
, although some of them are superimposed, too. In
contrast, the methyl substituents in
14b
are not bulky enough to provide any visi-
ble effect on the molecular flexibility of the complex. The number of resonances
of
14b
is equal compared to the free ligand (
13b
). The solid state structures sup-
port these interpretations.
2.5.4
Influence of Substitution on Ethene Polymerization Reactions
In order to investigate the influence of the substitution pattern on the polymeriza-
tion behavior, compounds
15a-d
were treated with sodium tetrakis[3,5-bis(tri-
fluoromethyl)phenyl]borate [NaB(Ar
f
)
4
,Ar
f
=3,5-(CF
3
)
2
C
6
H
3
]. After chlorine ab-
straction the generated monocationic species were tested in ethene polymerization
experiments [9]. The large anion B(Ar
f
)
4
is known to have a reduced tendency to
coordinate to the electrophilic palladium(II) center. Thereby, small amounts of
acetonitrile, which can be easily replaced by incoming monomers, are added to
stabilize the monocationic species through coordination on the free sites. The
quantitative formation of the monocationic species was proved by
31
P-NMR spec-
troscopy [58]. The solution of the catalytically active species was diluted with
20 mL dichloromethane and transferred into a 100 mL stainless steel autoclave
equipped with a magnetic stirring bar and a glass inlay. The autoclave was pres-
Search WWH ::
Custom Search