Redox-active iron-containing polymers: Synthesis and anionic polymerization of a C-ferrocenyl-substituted phosphaalkene

Article abstract of DOI:10.1039/b704967f

The addition polymerization of a ferrocenyl-substituted P=C bond leads to new redox-active polymers with functional ferrocene and phosphine moieties. The Royal Society of Chemistry.

Full text of DOI:10.1039/b704967f

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Redox-active iron-containing polymers: synthesis and anionic
polymerization of a C-ferrocenyl-substituted phosphaalkene
Kevin J. T. Noonan, Brian O. Patrick and Derek P. Gates*
Received (in Cambridge, UK) 3rd April 2007, Accepted 25th May 2007
First published as an Advance Article on the web 21st June 2007
DOI: 10.1039/b704967f
known,14 however very few non-heteroatom-substituted PLC
systems have been reported. A rare example, BuPLCHFc, was
The addition polymerization of a ferrocenyl-substituted PLC
bond leads to new redox-active polymers with functional
ferrocene and phosphine moieties.
t
detected in solution but dimerized upon attempted isolation.14e
For polymerization studies, isolable monomers with no heteroa-
toms directly attached to the PLC bond are desired to avoid
possible side reactions during initiation. Consequently, methods to
access new ferrocene-substituted phosphaalkenes are needed. The
phospha-Peterson reaction is a general and convenient route to
phosphaalkenes.15,16 This convenient reaction involves the con-
densation of a silyl phosphide Li[RP(SiMe3)] with a ketone and
affords phosphaalkenes with a variety of substituents provided
that suitably bulky substituents are employed to protect the PLC
bond.
Macromolecules containing transition metals are known to possess
unique redox, magnetic, optical and electronic properties and are
attractive for use as ceramic precursors and catalyst supports.1 The
ferrocenyl moiety has played an integral role in the growth of
metal-containing polymer science. Macromolecules are known
where ferrocene is incorporated into the main chain, side-chain
and even into dendrimers.2 The earliest ferrocene-containing
polymers were prepared from the addition polymerization of the
CLC bond in vinyl ferrocene.3,4
We have recently reported that PLC bonds, like CLC bonds, can
be polymerized to afford macromolecules containing functional
phosphine moieties.5 Interestingly, copolymerizations of phos-
phaalkenes and olefins afford either random copolymers or block
copolymers depending on the method of initiation.6,7 The chemical
functionality of poly(methylenephosphine)s is demonstrated by
their effectiveness as polymeric ligands in Pd-catalyzed reactions.
The preparation of macromolecules composed of both phosphines
and ferrocenes is particularly attractive due to the prospect of
accessing bimetallic polymers with interesting electronic, magnetic
or catalytic properties. Primarily a consequence of the lack of
effective synthetic routes to phosphine–ferrocene hybrid polymers,
only a few examples have been reported.8–10 Herein we describe
the synthesis and addition polymerization of C-ferrocenyl
phosphaalkene 1 to afford a new redox active bifunctional
poly(methylenephosphine) (2).
The precursor to C-ferrocenyl phosphaalkene (1), benzoylferro-
cene, was prepared according to a modified literature procedure.17
A THF solution of in situ prepared Li[MesP(SiMe3)] was treated
with benzoylferrocene at 60 uC. Analysis of an aliquot removed
from the deep red reaction mixture using 31P NMR spectroscopy
revealed that Li[MesP(SiMe3)] (d 2187) had been completely
consumed. Two new signals were observed downfield at 218 and
211 ppm (70 : 30 ratio) which are consistent with the formation of
E/Z-1.{ Crystals suitable for X-ray diffraction were obtained from
the slow evaporation of a hexanes solution of E/Z-1.{ The
molecular structure of the C-ferrocenyl-substituted phosphaalkene
is shown in Fig. 1 and, interestingly, the Fc substituent is cis to the
bulky P-Mes substituent (i.e. Z-1). The 31P NMR spectrum of a
C6D6 solution of the crystals of Z-1 shows signals consistent with
Although metalla-phosphaalkenes are quite common,11–13 the
target monomers, ferrocenyl-substituted phosphaalkenes, are
not widely available. Becker-type phosphaalkenes [i.e.
RPLC(OSiMe3)R9] possessing C–Fc and P–Fc substituents are
Fig. 1 Molecular structure of Z-1 (hydrogen atoms omitted for clarity).
˚
Thermal ellipsoids at 30% probability. Selected bond lengths (A) and
angles (u): P1–C1 1.697(2), P(1)–C(18) 1.842(2), C(1)–C(2) 1.512(2), C(1)–
C(8) 1.468(2); C(8)–C(1)–C(2) 114.95(13), C(8)–C(1)–P(1) 130.69(11),
C(2)–C(1)–P(1) 114.16(11), C(1)–P(1)–C(18) 107.31(7).
Department of Chemistry, University of British Columbia, 2036 Main
Mall, Vancouver, BC, Canada V6T 1Z1. E-mail: dgates@chem.ubc.ca;
Fax: (+1) 604-822-2847; Tel: (+1) 604-822-9117
3658 | Chem. Commun., 2007, 3658–3660
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both E-1 and Z-1 isomers (ca. 85 : 15 ratio) and suggests facile E,
Z isomerization in solution. Selective 1H NMR NOE experiments
provided confirmation that the major isomer in solution is Z-1. In
particular, irradiating at the resonance frequency of the ortho CH3
groups on the mesityl ring (d 2.30) leads to enhancement of signals
assigned to the ortho hydrogens of the substituted Cp ring (d 4.61).
This signal enhancement is consistent with the Mes and Fc
substituents being cis configured and therefore the Z-1 isomer was
concluded to be the major isomer in solution.
The molecular structure of Z-1 is shown in Fig. 1. The PLC
˚
bond length in 1 (1.697(2) A) is in the long end of the range typical
Fig. 2 UV/Vis spectra of monomer 1 (top) and polymer 2 (bottom)
(0.1 mM in toluene).
˚
for phosphaalkenes (1.61–1.71 A), however is similar to the PLC
bond length in MesPLCPh2 (1.692(3) A).18 Interestingly, the cis-Fc
capable of electronic communication through p conjugation.20
The deep red solution of 1 exhibited two broad absorbances at
˚
substituent shows a larger angle to the PLC bond than the ideal sp2
angle (/PLC–CFc 130.69(11)u) and is greater than the typical cis
aryl substituent of P-mesityl phosphaalkenes (/PLC–Ccis 125–
128u).16 We speculate that this is a consequence of increased steric
repulsion between the cis configured Fc and Mes moieties in Z-1.
Remarkably, the angle between the best planes of the C5H4 moiety
and the PLC bond is just 13.2(1)u. For comparison, the analogous
angle in the only other crystallographically characterized
C-ferrocenyl phosphaalkene (Me3SiPLC(OSiMe3)Fc) is 36.6u,14d
whilst that in MesPLCPh2 is 42.9u.18,19 This data suggests that
significant p-conjugation is present between the Fc substituent and
the PLC bond in Z-1. Additional support for p-conjugation in Z-1
lmax = 512 and 331 nm which are assigned to the d-d transition of
the ferrocenyl moiety and the p–p* transition for the PLC bond,
respectively. Interestingly, the absorbance maximum at 512 nm is
significantly red-shifted when compared to that for vinylferrocene
(lmax = 442 nm) and ferrocene (lmax = 440 nm).4,21 The dramatic
bathochromic shift is most likely attributable to a favourable
electronic interaction between the ferrocene group and the PLC
bond. The second transition at 331 nm, assigned to the p–p*
transition of the PLC bond, is similar to that observed for
MesPLCPh2 (lmax = 324 nm).22 In contrast, the gold coloured
macromolecule 2 exhibits only a weak transition at 448 nm, which
is very similar to the d–d transition for ferrocene (440 nm).
The electrochemical properties of poly(methylenephosphine) 2
were analyzed by cyclic voltammetry. The observed reversible one
electron oxidation of 2 is consistent with the expected ferrocene–
ferrocenium couple. The detection of a single wave suggests that
the Fc moieties are electronically isolated; analogous to the case of
poly(vinylferrocene). The half-cell potential (vs. SCE) for polymer
2 is (DE1/2 = 0.41 V) is slightly lower than that for poly(vinylfer-
rocene) (DE1/2 = 0.44–0.48 V).23 For comparison, FcCH2PPh2, a
possible model compound for polymer 2, similarly exhibits a
reversible oxidation slightly lower than that for ferrocene (DE =
20.04 V).24
˚
is provided by the shortening of C(1)–C(8) bond (1.468(2) A) with
˚
respect to the typical C–C single bond length which is 1.54 A and
by the intense red colour of 1 compared with pale yellow
MesPLCPh2.
To determine whether anionic initiation of a C-ferrocenyl
phosphaalkene was feasible, monomer 1 was treated with nBuLi
(1 equiv) in glyme (1,2-dimethoxyethane). The 31P NMR spectrum
of the reaction mixture showed only a singlet at 218.7 ppm which
is consistent with the formation of Li[Mes(Bu)PC(Fc)Ph]. Once
the feasibility of anionic initiation had been demonstrated, a
solution of monomer 1 in 1,2-dimethoxyethane (glyme) was
treated with nBuLi (5 mol%) as the anionic initiator.§ The progress
of the polymerization was monitored by 31P NMR spectroscopy
and, after seven days, 50–60% of monomer 1 had been cleanly
converted to polymer 2 (d 25 br). Due to facile isomerization in
solution, the isomer ratio in monomer E/Z-1 (85 : 15) does not
change during the polymerization reaction. The polymerization
was terminated by quenching with methanol and polymer 2 was
separated from 1 by reprecipitation from THF into methanol
(64). The golden coloured polymer was analyzed by triple
detection GPC and an absolute number average molecular weight
(Mn) was determined to be 9500 with a polydispersity index (PDI)
of 1.21. The identical experiment was repeated and the same Mn
was achieved with a PDI of 1.18. Further studies are underway to
ascertain whether this polymerization may be conducted in a living
fashion.
In summary, a phosphaalkene analogue of vinyl ferrocene has
been prepared and polymerized anionically to afford new
bifunctional redox active polymers. This development opens the
door to the synthesis of bimetallic polymers possessing both Fc
moieties and phosphine–metal complexes which may have
interesting optoelectronic, magnetic and preceramic properties.
Notes and references
{ Synthesis of MesPLC(Fc)Ph (1): To a stirred solution of MesP(SiMe3)2
(5.03 g, 17 mmol) in THF was added MeLi in Et2O (12.2 mL, 1.4 M,
17 mmol) at 25 uC. The reaction mixture was heated to 60 uC for ca. 1 h
and analysis of an aliquot by 31P NMR spectroscopy suggested total
conversion to MesP(SiMe3)Li (d 2187). Subsequently, benzoylferrocene
(4.94 g, 17 mmol) was added and the reaction mixture was stirred at 60 uC
for ca. 30 min. The 31P NMR spectrum showed signals at 218 and 211 ppm
(70 : 30 ratio) assigned to Z-1 and E-1, respectively. To the solution was
added Me3SiCl (2.15 mL, 17 mmol) to quench the LiOSiMe3. The volatiles
were removed in vacuo, the residue was extracted into hexanes (3 6 50 mL),
In order to investigate the electronic properties of 1 and 2, both
were analyzed by UV-Vis spectroscopy (Fig. 2). Previous studies
on phosphaalkenes have demonstrated that PLC bonds are
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filtered through Celite, and the solvent removed in vacuo. Single crystals of
1 (4.16 g, 58%) were obtained from slow evaporation of a hexanes solution
of the crude product. 31P NMR (C6D6) d (ppm): 218 (Z, 85%), 211 (E,
15%). 1H NMR (CD2Cl2) d (ppm): 7.5 (m, 5H, E-o,m,p-Ph), 7.10 (m, 5H,
Z-o,m,p-Ph), 6.98 (s, 2H, E-m-Mes), 6.69 (s, 2H, Z-m-Mes), 4.61 (t, 2H,
2JH–H = 1.2 Hz, Z-o-Fc), 4.45 (s, 2H, Z-m-Fc), 4.23 (m, 5H, Z-Cp-Fc, 2H,
E-m-Fc), 4.10 (s, 5H, E-Cp-Fc), 3.79 (s, 2H, E-o-Fc), 2.44 (s, 6H, E-o-CH3),
2.37 (s, 3H, E-p-CH3), 2.30 (s, 6H, Z-o-CH3), 2.17 (s, 3H, Z-p-CH3). MS
(EI, 70 eV): 424 [8, 36; M+]. UV/Vis (C7H9): lmax/nm (e/M21 cm21) = 331
(4600), 512 (930). Anal. Calc. for C26H25PFe: C, 73.60; H, 5.94. Found: C,
73.30; H, 6.04%.
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{ Crystal data for 1: C26H25PFe, M = 424.30, triclinic, space group P1
˚
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˚
96.033(4), c = 96.270(4)u, U = 2547.2(5) A , T = 173(2) K, Z = 2, m(Mo-
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crystallographic data in CIF or other electronic format see DOI: 10.1039/
b704967f
§ Synthesis of [P(Mes)C(Fc)(Ph)]n (2): A typical polymerization procedure
is described. To a stirred solution of MesPLCPh(Fc) (1.00 g, 2.3 mmol) at
room temperature in glyme (7 mL) was added nBuLi (1.5 M, 80 mL,
0.12 mmol). The reaction mixture was stirred at room temperature and
monitored by 31P NMR spectroscopy. The growth of a broad signal in the
31P NMR spectrum was observed over 7 days and, subsequently, the
reaction mixture was removed from the glovebox, quenched and
precipitated using methanol (3 6 100 mL). Residual solvent was removed
1
in vacuo. Yield = 250 mg (25%). 31P NMR (C6D6) d (ppm): 25 ppm. H
16 M. Yam, J. H. Chong, C.-W. Tsang, B. O. Patrick, A. E. Lam and
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UV/Vis (C7H9): lmax/nm (e/M21 cm21) = 448 (210). Anal. Calc. for
C26H25PFe: C, 73.60; H, 5.94. Found: C, 73.50; H, 6.29%.
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This journal is ß The Royal Society of Chemistry 2007