Synthesis of a series of new, highly electrophilic,
monocyclopentadienyltitanium olefin polymerization
initiators
Abstract
The new compounds
Ti(η-C
5
Me
5
)Me
2
E
(E = C
6
F
5
or
OC
6
F
5
) and
Ti(η-C
5
Me
5
)Me(OC
6
F
5
)
2
have been synthesized and characterized by a variety of
techniques, including
47/49
Ti NMR spectroscopy. All three
compounds react with the borane
B(C
6
F
5
)
3
to form the highly
electrophilic but thermally unstable species
Ti(η-C
5
Me
5
)Me(E)(µ-Me)B(C
6
F
5
)
3
and
[(η-C
5
Me
5
)Ti(OC
6
F
5
)
2
][BMe(C
6
F
5
)
3
], the solution
structures and dynamics of which are investigated and compared with
those of the known compound
Ti(η-C
5
Me
5
)Me
2
(µ-Me)B(C
6
F
5
)
3
. Interestingly,
Ti(η-C
5
Me
5
)Me(C
6
F
5
)(
µ-Me)B(C
6
F
5
)
3
undergoes neither
significant ion-pair dissociation to the solvent separated ions
[(η-C
5
Me
5
)TiMe(C
6
F
5
)]
+
and
[BMe(C
6
F
5
)
3
]
-
nor
borane dissociation to its precursors
(η-C
5
Me
5
)TiMe
2
(C
6
F
5
) and B(C
6
F
5
)
3
; indeed, both
rotation about the Ti–C
6
F
5
bond and
inversion at the chiral metal are slow on the NMR time-scale. In
contrast,
Ti(η-C
5
Me
5
)Me(OC
6
F
5
)(
µ-Me)B(C
6
F
5
)
3
is more labile and,
like
Ti(η-C
5
Me
5
)Me
2
(µ-Me)B(C
6
F
5
)
3
, undergoes ion-pair dissociation, while
[(η-C
5
Me
5
)Ti(OC
6
F
5
)
2
][BMe(C
6
F
5
)
3
] exists in
solution as the solvent separated ion species
[(η-C
5
Me
5
)Ti(OC
6
F
5
)
2
]
+
and
[BMe(C
6
F
5
)
3
]
-
in
equilibrium with its precursors,
(η-C
5
Me
5
)TiMe(OC
6
F
5
)
2
and B(C
6
F
5
)
3
.
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