Nucleophilically assisted ring-opening polymerization of group 14 element-bridged [1]ferrocenophanes [10]

Full text of DOI:10.1021/ja9941227

J. Am. Chem. Soc. 2000, 122, 4231-4232
Nucleophilically Assisted Ring-Opening
4231
Polymerization of Group 14 Element-Bridged
[1]Ferrocenophanes
Frieder Ja¨kle, Ron Rulkens, Gernot Zech,
Jason A. Massey, and Ian Manners*
Department of Chemistry, UniVersity of Toronto
80 St. George Street
Toronto, Ontario M5S 3H6, Canada
polymerization is much slower (ca. 50% conversion, 15 d, 0.1
M).6 In either case high molecular weight polymer is formed at
low conversion (for 3a ca. 20% after 1 h, Mn ) 4.8 × 105, PDI
) 1.3; for 3b ca. 20% after 5 d, Mn ) 1.0 × 106, PDI ) 1.3),7
which is indicative of a chain growth process where the
concentration of propagating centers is extremely low. As a radical
mechanism was suspected, the effect of a variety of externally
added radical traps was investigated under similar conditions. The
observation that neither the traps (Galvinoxyl, TEMPO, BzSSBz,
BHT, 1,4-cyclohexadiene) nor irradiation (λ ) 365 nm) had a
substantial effect on the rate of polymerization of 3a or 3b
suggested that a homolytic ROP mechanism is unlikely.8,9 This
was supported by experiments in which the influence of stannyl
radicals on the ROP of 3b was investigated. Reaction of 3b with
an excess of Bu3SnH and AIBN at 60 °C, which is known to
generate Bu3Sn• species,10 afforded the ring-opened product 6.
This process presumably involves a radical reaction with attack
of Bu3Sn• on the ferrocenophane as the formation of 6 was not
detected when 3b was treated with Bu3SnH alone under the same
conditions. Importantly, treatment of 3b with either AIBN or
excess Bu3SnH at 60 °C did not significantly influence the ROP
rate. Moreover, no increase in the ROP rate was detected when
3b was treated with a deficiency (ca. 15 mol %) of AIBN and
excess Bu3SnH at 60 °C. An alternative ROP mechanism
involving sequential ring-fusion via σ-bond metathesis was
dismissed on the basis that ring-fusion of [1]stannaferro-
cenophanes 3a and 3b with the cyclic dimers 5a and 5b was not
detected.8 These results encouraged us to investigate the influence
of polar additives on the ROP of 3a and 3b and a dramatic
increase in the rate of ROP was observed on addition of amine
nucleophiles.11,12 Thus, on addition of excess pyridine, ROP of
3a (0.1 M in C6D6) was complete after less than 90 s and the
polymerization rate for 3b was dramatically increased (in C6D6
ca. 95% conversion after 24 h compared to <3% in a control
ReceiVed NoVember 29, 1999
Ring-opening polymerization (ROP) of strained ring-tilted [1]-
and [2]metallocenophanes has recently become a well-established
route to high molecular weight poly(metallocene)s which possess
a range of interesting properties.1,2 Thermally induced ROP is
currently the most general synthetic method. However, although
evidence for cleavage of the silicon-ipso Cp carbon bond in
silicon-bridged [1]ferrocenophanes has been presented, the de-
tailed mechanism of these reactions is unclear but it appears to
involve heterolysis.3 In contrast to thermal ROP, the mechanism
of anionic ROP is well-established. For silicon-bridged [1]ferro-
cenophanes (e.g., 1), after initial attack of the nucleophile at silicon
a cyclopentadienyl (Cp) anion is generated. The latter can attack
further silicon centers of other monomer molecules in the
propagation step and the chain ends of the resulting living anionic
polymer can be capped with, for example, SiMe3 groups by the
addition of Me3SiCl (to give 2) or used to prepare block
copolymers.4
The recent discovery that tin-bridged [1]ferrocenophanes 3a
and 3b can be successfully isolated if sterically demanding
substituents are present on tin5 and the observation of their
apparent “spontaneous” ROP in solution to afford high molecular
weight poly(ferrocenylstannane)s 4a and 4b (and small amounts
of cyclics 5a and 5b) allows for a convenient mechanistic
investigation of this ROP process.5a,b These studies may also
provide insight into the thermal ROP reactions for metallo-
cenophanes which proceed in the melt. In this paper we report
our initial results and, in particular, the discovery of a new and
potentially general method of polymerization which involves
nucleophilic assistance.
(6) The exact molecular weights as well as the polydispersities of polymers
4a and 4b vary from sample to sample and from experiment to experiment.
In addition, we have studied the ROP of 3a in different solvents. The ROP
was found to be faster in coordinating and polar solvents (e.g. THF and
CH2Cl2) than in PhCl or benzene. Only in the case of CH2Cl2 is a substantial
amount of cyclic dimer 5a formed.
(7) Polymerization of 3a in benzene led to bimodal molecular weight
distributions with a large amount of high molecular weight polymer (ca. 80%)
and a small amount of low molecular weight polymer (ca. 20%; Mn ca. 10 000),
whereas 3b almost exclusively yielded high molecular weight polymer.
(8) For details see the Supporting Information.
The [1]stannaferrocenophane 3a polymerizes in benzene or
toluene solution at 25 °C forming high molecular weight polymer
4a (100% conversion, ca. 6 h, 0.1 M solution). For 3b the
(1) (a) Foucher, D. A.; Tang, B. Z.; Manners, I. J. Am. Chem. Soc. 1992,
114, 6246. (b) Manners, I. AdV. Organomet. Chem. 1995, 37, 131. (c) Manners,
I. Chem. Commun. 1999, 857.
(9) Homolytic cleavage of stannacycloalkanes with free radical sources has
previously been reported. See: Davies, A. G.; Roberts, B. P.; Tse, M.-W. J.
Chem. Soc., Perkin Trans. 2 1977, 1499.
(2) For other examples of poly(ferrocene)s obtained via ROP routes, see:
(a) Brandt, P. F.; Rauchfuss, T. B. J. Am. Chem. Soc. 1992, 114, 1926. (b)
Stanton, C. E.; Lee, T. R.; Grubbs, R. H.; Lewis, N. S.; Pudelski, J. K.;
Callstrom, M. R.; Erickson, M. S.; McLaughlin, M. L. Macromolecules 1995,
28, 8713. (c) Heo, R. W.; Somoza, F. B.; Lee, T. R. J. Am. Chem. Soc. 1998,
120, 1621. (d) Buretea, M. A.; Tilley, T. D. Organometallics 1997, 16, 1507.
(3) Pudelski, J. K.; Manners, I. J. Am. Chem. Soc. 1995, 117, 7265.
(4) (a) Ni, Y.; Rulkens, R.; Manners, I. J. Am. Chem. Soc. 1996, 118, 4102.
(b) Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger,
W. E. J. Am. Chem. Soc. 1996, 118, 12683.
(5) (a) Rulkens, R.; Lough, A. J.; Manners, I. Angew. Chem., Int. Ed. Engl.
1996, 35, 1805. (b) Ja¨kle, F.; Rulkens, R.; Zech, G.; Foucher, D. A.; Lough,
A. J.; Manners, I. Chem. Eur. J. 1998, 4, 2117. (c) Sharma, H. K.; Cervantes-
Lee, F.; Mahmoud, J. S.; Pannell, K. H. Organometallics 1999, 18, 399.
(10) Davies, A. G., Ed. Organotin Chemistry; VCH: Weinheim, 1997.
(11) Other nucleophilically assisted or accelerated reactions with tin
compounds include hydrogenation reactions with tin hydrides (homolytic and
heterolytic reactions), the reaction of tin dihydrides to form tin-tin bonds,
scrambling reactions of Sn2R6 with Sn2R′6, and oxidative cleavage of Sn-C
bonds. See ref 10 and, for example: Suga, S.; Manabe, T.; Yoshida, J. J.
Chem. Soc., Chem. Commun. 1999, 1237 and references therein.
(12) An increased susceptibility of stannacyclopentanes and stannacyclobu-
tanes toward oligomerization in polar solvents had been noted previously
without mechanistic investigations or discussions. See, for example: (a) Bulten,
E. J.; Budding, H. A. J. Organomet. Chem. 1977, 137, 165. (b) Seetz, J. W.
F. L.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F. J. Am. Chem. Soc. 1983,
105, 3336.
10.1021/ja9941227 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/13/2000

4232 J. Am. Chem. Soc., Vol. 122, No. 17, 2000
Scheme 1. Proposed Mechanism
Communications to the Editor
(Scheme 1). Crucially, heterolytic cleavage of the Sn-C bond
trans to the coordinating amine does not generate a free anion
but results in a tin ate complex as an intermediate. Pentacoordinate
stannate complexes are known to show decreased reactivity toward
electrophiles in comparison to the corresponding lithium carban-
ions,18 which may account for the fact that silicon halides have
no significant influence on the polymerization process. The
formation of cyclic dimer 5 can be explained by backbiting
reactions.
Although the silicon centers of silyl halides appear insufficiently
electrophilic to react with the nucleophilic Cp propagating centers,
organotin halides such as Me3SnCl should possess a Group 14
center of comparable (or greater) electrophilicity to that in 3b
and would therefore be expected to successfully compete with
the monomer for the propagating sites. However, Me3SnCl was
found to react directly with 3b to afford ring-opened addition
products such as 7 and higher oligomers.8,19
To explore the generality of the new nucleophilically assisted
ROP methodology we have also investigated the influence of
amines on the ROP of germanium-bridged [1]ferrocenophane 8
and the silicon-bridged [1]ferrocenophanes 9 and 1, for which
the electrophilicity of the silicon center is 9 . 1.1b,c When pyridine
was added to solutions of 1, 3a, 3b, 8, and 9 in C6D6, all apart
from 1 underwent ROP; the order of decreasing rate was 3a >
3b ≈ 8 . 9. The relative ROP rates can be rationalized by the
influence of the steric accessibility and electrophilicity of the
bridging atom on the ease of formation of the proposed penta-
coordinate intermediate.15
experiment without pyridine addition).8,13 The use of the more
basic 4-(dimethylamino)pyridine led to a further increase of the
reaction rate for 3b in benzene solution (ca. 95% conversion in
3.5 h), whereas for the more bulky NEt3 the rate was much slower
and for the highly sterically encumbered amine 2,6-di-tert-
butylpyridine no accelerating effect was detected.
These observations initially suggested a mechanism, in which
coordination of the amine to tin might induce heterolytic cleavage
of the Sn-Cp bond to generate free anionic Cp sites in a manner
similar to the anionic ROP mechanism. HoweVer, crucially, the
addition of silyl halides such as Me3SiCl, which terminate chain
ends in anionic ROP, had no significant effect on the rate of
polymerization of 3a or 3b in the presence (or absence) of added
amine. This indicated a mechanism (Scheme 1) in which amine
coordination to tin increases the nucleophilicity of the Cp carbon
bonded to tin without generating a free anion. In the initiation
step a small equilibrium concentration of a pentacoordinate tin
species may form.14,15 Although examples of amine coordination
to tetraorganotin compounds are still rare, evidence for the
formation of stable pentacoordinate tetraorganostannanes has been
reported.16 Moreover, intramolecular coordination of amines is
known to lead to elongated tin-carbon bonds trans to the
incoming nucleophile, which display enhanced reactivity.17 The
increased nucleophilicity of the Cp carbon bonded to tin may
allow for attack at the tin center of another monomer molecule 3
In summary, we report a new method for the ROP of
metallocenophanes that operates at ambient temperature in
solution, and which involves nucleophilic assistance by amines
without the formation of free cyclopentadienyl anions. Intrigu-
ingly, the presence of trace amounts of amine (or other nucleo-
philic) impurities might be anticipated in the monomers as a result
of their synthesis from amine (TMEDA) adducts of dilithioferro-
cene. Therefore, an analogous mechanism also needs to be
considered for the thermal ROP of [1]ferrocenophanes in the melt.
Further experiments are being performed to test this possibility
and to explore the generality of this new ROP method, which
should be applicable to many other strained rings containing
Group 14 elements.
(13) (a) The observed reaction rate can vary depending on the sample of
3a and 3b. Control experiments without addition of other reagents were
performed to estimate the relative rate. (b) At 60 °C strongly accelerated
polymerization of 3a and 3b was observed even with an equimolar or catalytic
amount of pyridine. (c) A nucleophilically assisted radical mechanism appears
unlikely as treatment of 3b with amines in the presence of radical traps or
under irradiation led to no significant change in the rate of ROP.
(14) In solution a pentacoordinate intermediate formed by the coordination
of pyridine to 3b could not be detected by 1H, 13C, or 119Sn NMR. This may
be attributed to a very low concentration of the adduct, which is consistent
with the observed chain growth mechanism involving only a very small amount
of propagating species. Furthermore, examples of tetraorganotin compounds
which display a pentacoordinate geometry in the solid state without significant
change of the NMR spectra in solution in comparison to tetracoordinate
analogues have been previously reported. See, for example: (a) Podesta, J.
C.; Chopa, A. B.; Koll, L. C. J. Chem. Res. (S) 1986, 308. (b) Jousseaume,
B.; Villeneuve, P.; Dra¨ger, M.; Roller, S.; Chezeau, J. M. J. Organomet. Chem.
1988, 349, C1. (c) Jastrzebski, J. T. B. H.; Boersma, J.; Esch, P. M.; van
Koten, G. Organometallics 1991, 10, 930.
Acknowledgment. We thank the donors of the Petroleum Research
Fund (PRF), administered by the American Chemical Society, for financial
support of this work. We also thank Prof. S. A. Mabury, D. Ellis, and
Prof. A. J. Poe for experimental assistance and helpful discussions.
Supporting Information Available: Tables of representative NMR
experiments with 3a and 3b and experimental details and synthetic
procedures (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org. See any current masthead page for ordering
information and Web access instructions.
JA9941227
(15) We have recently isolated a silicon-bridged analogue with an intramo-
lecular Si-N interaction; in the solid-state Fe(η-C5H4)2SiMe(2-C6H4NMe2)
possesses a trigonal-bipyramidal Si center with an elongated Cp-Si bond trans
to the amine substituent: Ja¨kle, F.; Lough, A. J.; Manners, I. Manuscript in
preparation.
(16) See, for example: (a) Tzschach, A.; Jurkschat, K. Pure Appl. Chem.
1986, 58, 639. (b) Das, V. G. K.; Mun, L. K.; Wei, C.; Blunden, S. J.; Mak,
T. C. W. J. Organomet. Chem. 1987, 322, 163.
(17) See, for example: (a) Steenwinkel, P.; Jastrzebski, J. T. B. H.;
Deelman, B.-J.; Grove, D. M.; Kooijman, H.; Veldman, N.; Smeets, W. J. J.;
Spek, A. L.; van Koten, G. Organometallics 1997, 16, 5486. (b) Yoshida, J.;
Izawa, M. J. Am. Chem. Soc. 1997, 119, 9361.
(18) Reich, H. J.; Phillips, N. H. J. Am. Chem. Soc. 1986, 108, 2102.
(19) Ring-opening reactions were also detected with species such as AliBu3
or AlMe3.