Solution structures and exchange phenomena of the new alkene polymerization initiators (η-C5Me5)TiMe(E)(μ-Me) B(C6F5)3 (E = C6F5, OC 6F5) and [(η-C5</s

Article abstract of DOI:10.1039/a700232g

The solution structures and dynamics of the new alkene polymerization initiators (η-C₅Me₅)TiMe(C₆F ₅)(μ-Me)B(C₅F₅)₃ 2, (η-C₅Me₅)TiMe(OC₆F _5<

Full text of DOI:10.1039/a700232g

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Solution structures and exchange phenomena of the new alkene
polymerization initiators (h-C5Me5)TiMe(E)(m-Me)B(C6F5)3 (E = C6F5,
OC6F5) and [(h-C5Me5)Ti(OC6F5)2][BMe(C6F5)3]
Tracey L. Tremblay, Sean W. Ewart, Mark J. Sarsfield and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
The solution structures and dynamics of the new alkene
polymerization initiators (h-C5Me5)TiMe(C6F5)(m-
Me)B(C5F5)3 2, (h-C5Me5)TiMe(OC6F5)(m-Me)B(C6F5)3
compounds (h-C5Me5)TiMe(C6F5)(m-Me)B(C6F5)3 2 and
(h-C5Me5)TiMe(OC6F5)(m-Me)B(C6F5)3 3, and to the achiral,
apparently ionic compound [(h-C5Me5)Ti(OC6F5)2][B-
Me(C6F5)3] 4. The new zwitterionic complexes 2 and 3, in
which one of the methyl ligands of 1 has been replaced by the
more electronegative ligands C6F5 and OC6F5, formed cleanly
on treatment of solutions of (h-C5Me5)TiMe2(C6F5)† and
(h-C5Me5)TiMe2(OC6F5)† with 1 equiv. each of the borane
B(C6F5)3 in CD2Cl2 at 195 K [eqn. (1)]. These complexes
decompose above 283 K and could not be isolated, but have
been fully characterized by 1H, 13C and 19F NMR spectroscopic
studies (CD2Cl2, 223–283 K).†
3
and [(h-C5Me5)Ti(OC6F5)2][BMe(C5F5)3] 4 are compared
and contrasted with those of the known initiator (h-C5Me5)-
TiMe2(m-Me)B(C6F5)3 1; compound 2 undergoes neither
spontaneous ion-pair dissociation to the solvent separated
[(h-C5Me5)TiMe(C6F5)]+ and [BMe(C6F5)3]2 nor borane
dissociation to its precursors (h-C5Me5)TiMe2(C6F5) and
B(C6F5)3; in contrast, 3 is more labile and does undergo ion-
pair dissociation, while 4 exists in solution as the separated
ion species in equilibrium with its precursors,
(h-C5Me5)TiMe(OC6F5)2 and B(C6F5)3.
1
The H, 19F and 13C{1H} NMR spectra† of 2 and 3 are all
consistent with structures as in Fig. 1, sharp terminal Ti–Me and
broadened m-MeB resonances being especially characteristic of
the zwitterionic structures proposed.3 Interestingly, however,
the 1H NMR spectrum of 2 at 223 K exhibits a doublet Ti–Me
resonance because of long-range coupling with one of the
o-fluorine atoms of the Ti–C6F5 ligand (JHF 3.1 Hz), while the
19F spectrum of 2 at 223 K exhibits five equal intensity
resonances attributable to the Ti–C6F5 group, On warming, the
pairs of o- and m-19F resonances broaden and coalesce at ca.
293 and 273 K, respectively, although the p-fluorine resonance
and all of the borate fluorine resonances remain well resolved in
the temperature range 223–283 K. These observations require
that both rotation about the Ti–C6F5 bond and inversion at the
chiral metal (involving ion-pair dissociation–recombina-
tion4a–d) be slow on the NMR timescale at 223 K, while
consideration of the coalescence temperatures suggests an
approximate DG‡ of 50.6 ± 2.1 kJ mol21 for the exchange
process(es).7 This would represent a lower limit for both
processes, and is to be compared with ca. 58 ± 19 kJ mol21 for
ion-pair dissociation–reorganization processes of similar zirco-
nocene and hafnocene complexes.4a–d
There is currently considerable interest in the utilization of
group 4 metal complexes of the types [(CpA)2MMe]+X2 and
[(CpA)MMe2]+X2 (CpA
=
substituted cyclopentadienyl;
M = Ti, Zr, Hf; X2 = poorly coordinating anion) as initiators
for alkene polymerization.1–3 Since most of the more active
molecular alkene polymerization initiators are cationic rather
than neutral,1–3 one might anticipate that increasing the Lewis
acidity of the metal atoms by incorporation of more electron-
withdrawing ligands would result in even hgher activities.
However, binding of X2 to the cationic species [(CpA)2MMe]+
and [(CpA)MMe2]+ may also be enhanced by increasing the
Lewis acidity,4 resulting in inhibition of monomer coordination,
while the relative rates of initiation, propagation, termination
and chain transfer are affected unpredictably by ligand
substitution.1,5 The result is that electron-withdrawing ligands
sometimes result in both decreased catalytic activity and
reduced polymer molecular masses.5
Although detailed studies of ion-pairing phenomena and their
effects on catalytic activities of some metallocene systems
[(CpA)2MMe]+X2 are available,4 relatively little is known as yet
of the behaviour of monocyclopentadienyl systems [(CpA)M-
Me2]+X2. Interestingly, it has been shown that the zwitterionic
Consistent with this interpretation and in contrast to 1,3
attempted magnetization transfer experiments provided no
1
compound (h-C5Me5)TiMe2(m-Me)B(C6F5)3
1
(Fig. 1,
evidence in the H NMR spectrum of 2 for dissociation of the
E = Me), formed by treating (h-C5Me5)TiMe3 with B(C6F5)3
borate anion [BMe(C6F5)3]2; thus the (m-Me)B(C6F5) moiety is
relatively strongly bound to the titanium centre in 2. In contrast,
irradiation of the m-Me resonance of 3 did result in magnetiza-
tion transfer and appearance of a weak free borate resonance,
indicating an equilibrium between solvent separated ion pairs
and the methyl bridged species, as with 1.3 With neither 2 nor 3
[eqn. (1)],3 undergoes facile displacement of borate anion,
(h-C5Me5)TiMe2E + B(C6F5)3 "
(h-C5Me5)TiMeE(m-Me)B(C6F5)3 (1)
E = Me 1, C6F5 2, OC6F5 3
[BMe(C6F5)3]2, on reaction with other ligands.3 Furthermore,
magnetization transfer experiments have demonstrated a low
degree of reversible borate dissociation from 1 at 223 K [eqn.
(2); E = Me]3 but no exchange between terminal and bridging
methyl groups was detected, i.e. the borane in 1 does not
‘hop’ from one methyl to another [reverse of eqn. (2)],
Ti+
Me
H
(h-C5Me5)TiMeE(m-Me)B(C6F5)3 "
H
E
C6F5
–
[(h-C5Me5)TiMeE]+[BMe(C6F5)3]2 (2)
C
B
H
as occurs in some metallocene systems.4a–d
In order to better understand the effects of ligand electronic
properties on catalytic activities of this class of compounds, we
have extended our investigation to the methyl-bridged chiral
C6F5
C6F5
Fig. 1 Proposed structure for 1 (E
= Me), 2 (E = C6F5) and 3
(E = OC6F5)
Chem. Commun., 1997
831

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(h-C5Me5)TiMe2(OC6F5). 1H NMR (CD2Cl2, 223 K), d 1.86 (s, 15 H,
C5Me5), 0.55 (s, 3 H, Ti–Me); 13C{1H} NMR (CD2Cl2, 223 K), d 141.1 (d,
o-CF, JCF 231.5 Hz), 138.7 (d, m-CF, JCF 241.5 Hz), 135.5 (d, p-CF, JCF
241.5 Hz), 124.2 (C5Me5), 59.0 (Ti–Me), 12.1 (C5Me5); 19F NMR (CD2Cl2,
298 K, ref. CFCl3), d 2160.1 (m, 2 F, o-F), 2167.0 (m, 2 F, m-F), 2171.3
(t, 1 F, p-F).
was there evidence from spin-transfer experiments for Ti–Me/
B–Me exchange, as occurs in zirconocene systems.4a–d
Compounds 1,3 2 and 3 all behave as ethene and propene
polymerization catalysts under strictly anhydrous conditions in
toluene. While the polyethylene formed is generally too
insoluble for even high-temperature GPC measurements, the
molecular masses of the polypropylene formed at 195 K
decrease in the order 2 (Mw = 2.3 3 106, Mw/Mn = 1.7) > 3
(Mw = 2.0 3 106, Mw/Mn = 1.7) > 1 (Mw = 0.3 3 106, Mw/Mn
= 1.3). The low dispersities observed are consistent with single
site catalysts in all cases,1 but the most Lewis acidic catalyst
gives the highest molecular mass polymer, just the opposite to
apparent trends in metallocene systems.5 On the other hand, the
yields of polymers obtained with 2 and 3 are about 30% lower
than those obtained with 1, consistent with stronger borate
coordination to the more Lewis acidic catalytic sites. Thus
combination of the types of Lewis acidic complexes described
here with counter anions which, perhaps for steric reasons,8
coordinate less weakly than [BMe(C6F5)3]2, may well lead to
alkene polymerization catalysts of high activity.
(h-C5Me5)TiMe(OC6F5)2. 1H NMR (CD2Cl2, 223 K), d 1.89 (s, 15 H,
C5Me5), 1.09 (s, 3 H, Ti–Me); 13C{1H} NMR (CD2Cl2, 223 K), d 139.9 (d,
o-CF, JCF 251.5 Hz), 138.1 (d, m-CF, JCF 241.5 Hz), 135.2 (d, p-CF, JCF
251.5 Hz), 127.0 (C5Me5), 61.5 (Ti–Me), 10.9 (C5Me5); 19F NMR (CD2Cl2,
223 K), d 2161.4 (m, 2 F, o-F), 2167.0 (m, 2 F, m-F), 2171.3 (t, 1 F,
p-F).
1
(h-C5Me5)TiMe(C6F5)(m-Me)B(C6F5)3 2. H NMR (CD2Cl2, 223 K), d
2.61 (d, 3 H, Ti–Me, JHF 3.1), 2.10 (s, 15 H, C5Me5), 1.36 (br s, 3 H, m-Me);
13C{1H} (CD2Cl2, 223 K), d 138.2 (C5Me5), 109.9 (Ti–Me), 13.6 (C5Me5);
19F (CD2Cl2, 223 K), d 2118.6 (m, 1 F, o-F of Ti–C6F5), 2124.3 (m, 1 F,
o-F of Ti–C6F5), 2135.1 (m, 6 F, o-F of B–C6F5), 2150.1 (t, 1 F, p-F of Ti–
C6F5), 2160.3 (m, 1 F, m-F of Ti–C6F5), 2160.8 (t, 3 F, p-F of B–C6F5),
2161.7 (m, 1 F, m-F of Ti–C6F5), 2166.0 (m, 6 F m-F of B–C6F5).
(h-C5Me5)TiMe(OC6F5)(m-Me)B(C6F5)3 3. 1H NMR (CD2Cl2, 223 K), d
2.04 (s, 15 H, C5Me5), 1.89 (s, 3 H, Ti–Me), 0.62 (br s, 3 H, m-Me); 13C{1H}
(CD2Cl2, 223 K), d 134.3 (C5Me5), 82.2 (Ti–Me), 12.2 (C5Me5); 19F
(CD2Cl2, 223 K), d 2135.4 (m, 6 F, o-F of B–C6F5), 2159.7 (m, 2 F, o-F
of Ti–OC6F5), 2160.9 (t, 3 F, p-F of B–C6F5), 2164.5 (m, 2 F, m-F of Ti–
OC6F5), 2165.0 (m, 2 F, m-F of Ti–OC6F5), 2166.0 (m, 6 F, m-F of
B–C6F5).
The solution behaviour observed for 4 was most unexpected.
The methyl abstraction reaction of (h-C5Me5)TiMe(OC6F5)2
with 1 equiv. of B(C6F5)3 in CD2Cl2 was monitored by 1H and
19F NMR spectroscopy in the temperature range 223–298 K,
and it was found that the 1H and 19F spectra at 223 K exhibited
resonances attributable only to the solvent separated species of
4, [(h-C5Me5)Ti(OC6F5)2]+ and [BMe(C6F5)3]2; none were
attributable to coordinated borate as in 1–3. Remarkably,
warming the NMR solution of 4 resulted in the reversible
reappearance of the resonances of the neutral precursor
(h-C5Me5)TiMe(OC6F5)2, and at 263 K there was a substantial
amount of both species present. Spin saturation transfer and
variable-temperature experiments showed them to be in equilib-
rium, with DH = 21.25 ± 0.1 kJ mol21, DS = 246 ± 4
J K21 mol21 for conversion to the non-ionic species. The ability
of 4 to engage in Ti–Me/B–Me exchange stands in contrast to
1–3 and even to methylzirconocene systems,4a–d for which
variable-temperature NMR studies imply similar exchange
but not the major shift in equilibrium noted here. Since
[(h-C5Me5)Ti(OC6F5)2]+ is expected to be a relatively strong
Lewis acid, its unusual disinclination to bind the borate anion
must be attributed to steric hindrance by the three bulky ligands
on the titanium hindering close approach of the borate anion. In
4, moreover, it appears that the strong but sterically hindered
Lewis acid [(h-C5Me5)Ti(OC6F5)2]+ effectively competes with
B(C6F5)3 for possession of the methyl group, presumably via a
transient methyl bridged species although none was detected in
the spin saturation experiments. An alternative structure for 4
such as {[(h-C5H5)Ti(OC6F5)(m-OC6F5)]2}2+ seems unlikely
since (a) the pentafluorophenoxy groups are equivalent in the
19F NMR spectrum and (b) 3 clearly does not contain such a m-
OC6F5 group.
[(h-C5Me5)Ti(OC6F5)2][BMe(C6F5)3] 4. 1H NMR (CD2Cl2, 223 K), d
2.17 (s, 15 H, C5Me5), 0.37 (br s, 3 H, B–Me); 13C{1H} NMR (CD2Cl2, 223
K), d 139.3 (C5Me5), 12.3 (C5Me5), 19F NMR (CD2Cl2, 223 K), d 2133.7
(m, 6 F, o-F of B–C6F5), 2159.2 (m, 4 F, o-F of O–C6F5), 2165.1 (t, 3 F,
p-F of B–C6F5), 2163.5 (m, 4 F, m-F of O–C6F5), 2162.4 (t, 2 F, p-F of
O–C6F5), 2168.4 (m, 6 F, m-F of B–C6F5).
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We thank the Natural Sciences and Engineering Research
Council of Canada (Research and Strategic Grants to M. C. B.
and Graduate Scholarship to S. W. E.) and Alcan (Graduate
Scholarship to S. W. E.) for financial support.
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Footnotes
* E-mail: bairdmc@qucdn.queensu.ca
† Reactions of the compounds (h-C5Me5)TiCl2Me6 and (h-C5Me5)-
TiMe2Cl6 with appropriate amounts of LiC6F5 or LiOC6F5 in hexanes
yielded the thermally robust, yellow compounds (h-C5Me5)TiMe2(C6F5),
(h-C5Me5)TiMe2(OC6F5) and (h-C5Me5)TiMe(OC6F5)2, all of which have
been fully and satisfactorily characterized by elemental analyses and
spectroscopic methods.
(h-C5Me5)TiMe2(C6F5). 1H NMR (CDCl3, 298 K), d 1.98 (s, 15 H,
C5Me5), 1.41 (t, 6 H, Ti–Me, JHF 2.0); 13C{1H} NMR (CDCl3, 298 K); d
127.0 (C5Me5), 80.0 (t, Ti–Me, JCF 3.3 Hz), 12.4 (C5Me5); 19F NMR (C6D6,
298 K, ref. CFCl3), d 2121.4 (m, 2 F, o-F), 2155.6 (t, 1 F, p-F), 2163.0 (m,
2 F, m-F).
Received in Bloomington, IN, USA, 10th January 1997; Com.
7/00232G
832
Chem. Commun., 1997