A synthetic route to borylene-bridged poly(ferrocenylene)s

Article abstract of DOI:10.1002/anie.200503515

Iron chains: The highly soluble, ferrocene-containing polymer [-fc-B(Mes)-]_n (fc = Fe(C₅H₄)₂, Mes = mesityl), with an average chain length of about 16 repeat units (n = 16), is readily accessible by a novel polycondensation reaction starting from fc(BBr₂)₂ and HSiEt₃ (see scheme). The polymer contains three-coordinate boron centers, which are well-suited for the promotion of electron delocalization along the polymer chain. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200503515

Communications
Ferrocene-Based Polymers
DOI: 10.1002/anie.200503515
A Synthetic Route to Borylene-Bridged
Poly(ferrocenylene)s**
Julia B. Heilmann, Matthias Scheibitz, Yang Qin,
Anand Sundararaman, Frieder Jäkle,* Tonia Kretz,
Michael Bolte, Hans-Wolfram Lerner,
Max C. Holthausen,* and Matthias Wagner*
Dedicated to Professor Herbert W. Roesky
on the occasion of his 70th birthday
The chemistry of metal-containing polymers is receiving
increasing attention in the search for new materials that
possess useful optical, electronic, and magnetic properties.[1–4]
Among the compounds developed so far, organometallic
macromolecules have become particularly attractive targets
ever since the ring-opening polymerization (ROP) of strained
[*] Y. Qin, A. Sundararaman, Prof. Dr. F. Jäkle
Department of Chemistry
Rutgers University Newark
73 Warren Street, Newark, NJ07102 (USA)
Fax: (+1)973-353-5064
E-mail: fjaekle@andromeda.rutgers.edu
J. B. Heilmann, Dr. M. Scheibitz, T. Kretz, Dr. M. Bolte,
Dr. H.-W. Lerner, Prof. Dr. M. C. Holthausen, Prof. Dr. M. Wagner
Institut für Anorganische Chemie
Johann Wolfgang Goethe-Universität Frankfurt
Marie-Curie-Strasse 11, 60439 Frankfurt (Main) (Germany)
Fax: (+49)69-798-29260
E-mail: max.holthausen@chemie.uni-frankfurt.de
matthias.wagner@chemie.uni-frankfurt.de
[**] The authors thank Prof. Dr. Rainer Winter and Dipl.-Chem. Jörg
Maurer, Universität Regensburg, for helpful discussions. Financial
support through PhD grants from the Hessisches Ministerium für
Wissenschaft und Kunst (J.B.H.), the Fonds der Chemischen
Industrie, and the Bundesministerium für Bildung und Forschung
(M.S.) is gratefully acknowledged. M.C.H. wishes to thank Prof. Dr.
Gernot Frenking for his helpful discussions. Computer time was
provided by the CSC Frankfurt and the HHLR Darmstadt. This work
was supported by the Deutsche Forschungsgemeinschaft, and the
GPC and light-scattering instrumentation was in part provided by
the National Science Foundation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ring-tilted [1]ferrocenophanes[5] was used to synthesize the
first high-molecular-weight poly(ferrocenylene)s (I,
Scheme 1) with skeletal iron atoms. Within this particular
class of compounds, boranediyl-bridged poly(ferrocenylene)s
Scheme 1. Poly(ferrocenylene)s I generated by ring-opening polymeri-
zation of [1]ferrocenophanes; the boron-bridged target polymer II.
Scheme 2. Synthesis of 2Br, 4Br, 2Mes, and 4Mes. a) toluene, ꢀ788C
to RT; b) [CuMes]n, toluene, 808C; Mes=mesityl.
(II, Scheme 1) are of paramount interest because the three-
coordinate boron spacer offers a vacant p orbital for con-
jugative interaction with the cyclopentadienyl p-systems.
Boranediyl bridges should therefore be capable of mediating
electronic interactions between the iron centers very effi-
ciently.[6,7] The degree of electronic communication via the
boron atom can be fine-tuned by modifying the p-donor
ability of its substituent R, thereby offering a way for the
rational design of novel materials with potentially useful
electronic properties. Attempts to polymerize boron-bridged
[1]ferrocenophanes using the ROP approach have so far been
restricted to boron moieties bearing amino substituents.
Moreover, the products obtained proved to be mainly
insoluble in organic solvents and the minor soluble fraction
was shown by NMR spectroscopy and mass spectrometry to
consist primarily of small, cyclic oligomers (n = 2, 3).[8,9] The
purpose of this paper is to present a new synthetic strategy
that provides convenient access to the first soluble ferroce-
nylborane polymers containing electron-deficient, three-
coordinate boron spacers in the polymer main chain. Our
key discovery in this context is the facile and quantitative
coupling reaction that occurs upon treatment of FcBBr2 with
HSiEt3 to yield the diferrocenylborane Fc2BBr (Fc =
(C5H5)Fe(C5H4)). We show here that the same synthetic
protocol can successfully be applied to the preparation of the
corresponding polymers [-fc-B(Br)-]n starting from the
difunctional monomer fc(BBr2)2 (fc = Fe(C5H4)2).
We began our study with a detailed investigation of the
key coupling step. As mentioned above, Fc2BBr (2Br,
Scheme 2) can be obtained by treating two equivalents of
FcBBr2 (1)[10] with three equivalents of HSiEt3 in toluene;[11]
in addition, half an equivalent of B2H6 and three equivalents
of BrSiEt3 are liberated in the course of the reaction. The
NMR spectra of 2Br agree with those of a structurally
authenticated sample of Fc2BBr that was synthesized inde-
pendently from FcSnMe3 and FcBBr2.[12] Since there is no
indication of the presence of Fc2BH[13] in the crude reaction
mixture, ferrocenyl transfer obviously proceeds not only with
practically quantitative yield but also with excellent selectiv-
ity towards the formation of BBr bridges. A series of NMR
experiments showed that the reaction of FcBBr2 with HSiEt3
in a molar ratio of 2:1 leads to a mixture of FcBBr2, Fc2BBr,
and B2H6. For quantitative conversion, a minimum of
1.5 equivalents of HSiEt3 is required.
Details of the reaction mechanism for the formation of
2Br (G in Scheme 3) were elucidated by means of quantum
chemical calculations employing the ONIOM extrapolation
scheme (see Supporting Information[11] for details). These
calculations were performed with the real molecular models,
except for the use of HSiMe3 instead of HSiEt3 as the hydride
source. In line with the experimental finding that FcBBr2 does
not dimerize under any experimental conditions, we were
unable to locate a doubly bromide-bridged dimer of 1 (A in
Scheme 3). Instead, the reaction commences with formation
of FcBHBr (B) from FcBBr2 (A) via TSAB, the transition
structure of
a concerted Br/H exchange by HSiMe3
(Scheme 3). TSAB is connected with an energy barrier of
DG° = 25.4 kcalmolꢀ1, and formation of B is exergonic by
DGR = ꢀ6.5 kcalmolꢀ1. B can dimerize to the doubly hydride-
bridged cis/trans isomers C and D. We did not find reaction
pathways leading from C or D to the experimentally
characterized final product G. Instead, we identified alter-
native routes via the two isomeric (m-H)(m-C)-bridged dimers
E and F. Both are higher in energy than their doubly hydride-
bridged counterparts, but are formed with significantly lower
barriers. Subsequent product formation from E and F is an
exergonic process and occurs with elimination of BH2Br.
Assuming fast pre-equilibria between B and E and B and F,
the effective activation barriers for the formation of G from B
amount to DG° = 27.2 and 27.4 kcalmolꢀ1, respectively. Yet
another reaction pathway opens up for B after consumption
of a second equivalent of HSiMe3 to yield FcBH2 (H). With
DG° = 25.3 and DGR = ꢀ4.2 kcalmolꢀ1, the energetic charac-
teristics of this process are very similar to those of the initial
step A!B. Dimerization of H to I closely resembles the
corresponding elementary steps B!C and B!D and is
kinetically and thermodynamically feasible, but represents a
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Scheme 3. Computed reaction pathways leading to the formation of G (=2Br); gray C, light blue Fe, blue B, red Br, yellow Si.
dead end for the reaction cascade as well. Thus, also in this
region of the potential-energy surface, the productive path-
way (now leading to diferrocenylborane L with DG° =
28.5 kcalmolꢀ1) proceeds via a (m-H)(m-C)-bridged dimer (K).
With the scenario outlined in Scheme 3, key elementary
steps leading to diferrocenylboranes G (= 2Br) and L have
been unveiled. The experimentally observed exclusive for-
mation of G can be explained by a H/Br exchange reaction
according to L + 1= (BH2Br)2!G + 1= B2H6, for which we
reaction meets all the requirements for an efficient polymer-
ization process.
Accordingly, the bromo-substituted polymer 4Br was
prepared from fc(BBr2)2 (3)[14] and three equivalents of
HSiEt3 in a very clean reaction. This polymer is highly
sensitive to air and moisture and does not dissolve in common
inert solvents. It was therefore transformed into the corre-
sponding mesityl-substituted polymer 4Mes by treating a
slurry of 4Br in toluene with [CuMes]n (n = 4, 5).[15–17] The
mesityl substituent was chosen because it aids in the
solubilization of the material and also provides steric
protection to the boron sites (Scheme 2).[18] This new polymer
is only moderately sensitive to air and has a high thermal
stability, according to thermogravimetric analysis (TGA)
under nitrogen. A single-step weight loss of 72%, with an
inflection point at 4038C, gave a brown ceramic material in
28% yield. Differential scanning calorimetry[11] revealed a
glass-transition temperature, Tg, of 1108C, which is similar to
2
2
compute a thermodynamic driving force of DGR = ꢀ7.9 kcal
molꢀ1. In addition, we found a thermodynamically viable
shunt pathway that competes for the formation of L via
regeneration of B from H according to H + 1= (BH2Br)2!B +
2
1
= B2H6 (DGR = ꢀ7.7 kcalmolꢀ1). In conclusion, theory indi-
2
cates that the overall reaction 2A + 3HSiMe3!G +
1
= B2H6 + 3BrSiMe3 is highly exothermic (by DGR =
2
ꢀ26.2 kcalmolꢀ1) and that competing alternative reaction
pathways converge to the sole product G, which is fully in line
with the experimental observations. Hence, this condensation
that
reported
for
poly(ferrocenylphenylphosphane)
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([-fcP(Ph)-]n, 1268C), which also contains tricoordinate
bridging units.[19] The molecular weight and dispersity of
4Mes were analyzed by gel permeation chromatography
(GPC) on a chromatograph equipped with refractive index
(RI) and multi-angle laser light scattering (MALLS) detec-
tors, and further studied by MALDI TOF mass spectrome-
try.[11] The mesityl-stabilized polymer 4Mes was found to be
well compatible with GPC separation in THF. The GPC trace
shows a monomodal distribution with an average molecular
weight, Mn, of 5160 and a polydispersity index (PDI) of 1.45
based on in-line static light scattering detection. In order to
investigate the structure of these low-molecular-weight poly-
mers further, and to determine the nature of the end-groups, a
MALDI TOF–TOF mass spectrum of 4Mes was acquired in
reflectron (+) mode with benzo[a]pyrene as the matrix.[20]
The spectrum is characterized by three series of peaks
(Figure 1), which, by comparison with the peak patterns of
model systems representing different segments of the poly-
mer chain, the dinuclear compound 2Mes and the 1,1’-
diborylated ferrocene fc(BMes2)2 (5Mes). While 2Mes is
readily available using the synthesis protocol developed for
4Mes (Scheme 2), compound 5Mes is best prepared from
1,1’-dilithioferrocene[21] and Mes2BF.[11]
In the temperature range between 25 and 708C, the
11B NMR resonances of 2Mes, 4Mes and 5Mes are broad-
ened beyond detection. The proton NMR spectrum of 2Mes
shows a singlet at d = 6.81 ppm (C6H2), two virtual triplets at
d = 4.58 and d = 4.42 ppm (C5H4), and resonances for the para
and ortho methyl groups at d = 2.31 and 2.23 ppm, respec-
tively. The 1H NMR spectrum of 5Mes is characterized by the
following resonances: d = 6.75 (s, C6H2), 4.73/4.41 (2 ” vtr,
C5H4), 2.27 ppm (o,p-CH3). Correspondingly, five major
NMR signals [d(1H) = 6.79, 4.48/4.28, 2.30/2.14 ppm] are
found in the spectrum of 4Mes and assigned to internal
[-(C5H4)Fe(C5H4)-B(Mes)-] fragments constituting the poly-
mer chain. These major signals are accompanied by several
smaller resonances attributable to the chain end groups
[-(Mes)B-Fc] and [-BMes2]. For an infinitely long polymer,
the ratio between the cyclopentadienyl protons and aromatic
mesityl protons would be 4:1. In the case of 4Mes, the ratio
between the integral values of all cyclopentadienyl signals on
one hand and all aromatic mesityl resonances on the other is
4:1.14.
One of the key questions with regard to the electronic
properties of 4Mes is whether a conformation is possible that
allows for efficient p-conjugation of the ferrocenes via the
empty p orbital of the boron bridge. To this end, the model
compound 2Mes was investigated by X-ray crystallography
(Figure 2).[11] An inspection of the dihedral angles between
the cyclopentadienyl rings and the B1C1C11C31 plane
Figure 1. a) MALDI TOF-TOF MS of 4Mes (reflectron (+) mode with
benzo[a]pyrene as matrix); b) region between m/z 2845 and 3355
enlarged.
the respective calculated molecular ion peaks, can confidently
be assigned to polymers bearing two -BMes2 termini, one
-BMes2 and one -Fc end group, and two -Fc caps, respectively
(see Figure 1b: m/z 2880, [Mes2B{-fc-B(Mes)-}8Mes]+; m/z
2946, [H{-fc-B(Mes)-}9Mes]+; m/z 3012, [H{-fc-B(Mes)-}9-
Fc]+). The series of peaks representing polymers with two
ferrocenyl end groups possesses by far the lowest intensity,
while Mes2B[-fc-B(Mes)-]nMes molecules bearing two boryl
end groups are clearly the most abundant ones. Within each
series the difference between adjacent peaks, D(m/z), is
always 314, which corresponds to one [-fc-B(Mes)-] repeat
unit. The highest molecular weight peak that was unambig-
uously detectable appears at m/z 6962 and belongs to a
polymer containing 21 ferrocenylene fragments (Mes2B[-fc-
B(Mes)-]21Mes).
Figure 2. Molecular structure and numbering scheme of compound
2Mes; thermal ellipsoids shown at the 50% probability level; hydrogen
atoms omitted for clarity. Selected bond lengths [Š], angles [8], torsion
angles [8], and dihedral angles [8]: B1–C1 1.590(4), B1–C11 1.554(4),
B1–C31 1.555(4); C1-B1-C11 119.3(2), C1-B1-C31 117.3(2), C11-B1-C31
123.4(2); C1-B1-C11-C12 ꢀ163.2(2), C1-B1-C31-C32 ꢀ173.2(2), C11-
B1-C31-C32 7.0(4), C31-B1-C11-C12 16.7(4), C11-B1-C1-C6 ꢀ119.8(3),
C31-B1-C1-C6 60.4(3); C1 to C6 ring//C1B1C11C31 plane 61.2, C11 to
C15 ring//C31 to C35 ring 23.4.
To facilitate the analysis of the NMR and UV/Vis spectra
as well as the electrochemical data of 4Mes we prepared two
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revealed values of 21.88 [C11 to C15] and 14.18 [C31 to C35] in
line with a substantial overlap of the corresponding p-
electron clouds.
B(Mes)-Fc+] in the case of the dimer).[23] The large splitting
D(E8’) = 705 mV for 4Mes further suggests very pronounced
electronic interactions in this polymer.
p-Overlap between the p orbital on boron and the
ferrocene units is expected to influence the UV/Visible
spectra, and the ensuing electronic communication between
the ferrocene moieties should be reflected in the redox
properties of 4Mes. Parent ferrocene has its longest wave-
length absorption maximum at l = 440 nm in the UV/Vis
spectrum. This band is pronouncedly shifted to the red upon
going to diferrocenylborane 2Mes (l = 490 nm) and experi-
ences a further bathochromic shift in polymeric 4Mes (l =
In summary, borylene-bridged poly(ferrocenylene)s [-fc-
B(Br)-]n are readily prepared from fc(BBr2)2 and HSiEt3 via a
novel polycondensation reaction. Treatment with [CuMes]n
gives a highly soluble polymer [-fc-B(Mes)-]n with three-
coordinate boron centers, which, due to their empty p orbital,
are well-suited for the promotion of electron delocalization
along the polymer chain.
505 nm), in agreement with an increasing degree of p- Experimental Section
Synthesis of 4Br: To 3 (3.990 g, 7.60 mmol) in toluene (20 mL) was
delocalization within this series of compounds. Cyclic vol-
tammograms were recorded in CH2Cl2 with [NBu4][B(C6F5)4]
(0.1m) as supporting electrolyte and are referenced against
the FcH/FcH+ couple.[11] The mononuclear model system
5Mes is reversibly oxidized at a potential of E8’ = 196 mV.
This anodic shift with respect to parent ferrocene may be
explained by the p-electron withdrawing nature of the BMes2
substituents. In the case of the dinuclear compound 2Mes, two
well resolved redox waves of equal intensity and features of
chemical reversibility appear at E8’ = 45 mV and 467 mV.
These transitions are assigned to the successive one-electron
oxidations of the two iron centers. The large separation of
D(E8’) = 422 mV between the two redox waves indicates
pronounced electronic communication between the two
ferrocenyl moieties in 2Mes.[22] The first oxidation event of
polymer 4Mes (one ferrocenylene donor per BMes acceptor)
is reversible on the CV timescale and occurs at 140 mV
(Figure 3). This value lies between the first redox potential of
added neat HSiEt3 (3.888 g, 33.44 mmol) at ꢀ788C with stirring
(note: even though 3 equiv of HSiEt3 are sufficient for full con-
version, we found it convenient to use the hydride transfer reagent in
some excess when working on a preparative scale). The resulting red
solution was allowed to warm to room temperature and stirred
overnight, whereupon a red precipitate gradually formed. The
microcrystalline solid was collected on a frit, triturated with toluene
(10 mL) and dried in vacuo. Yield: 1.914 g (92% repeat units).
Synthesis of 4Mes: A solution of [CuMes]n (0.162 g, 0.89 mmol)
in toluene (10 mL) was added slowly at room temperature to a slurry
of 4Br (0.255 g, 0.93 mmol repeat units) in toluene (20 mL). The
resulting mixture was stirred overnight and then heated to 808C for
8 h, whereupon its color changed from red to purple accompanied by
the formation of a greyish precipitate. After filtration and evapo-
ration of solvent under reduced pressure, a purple microcrystalline
solid was obtained. The crude product was dissolved in toluene and
repeatedly precipitated from toluene into hexanes. Yield: 0.404 g
(81% repeat units).
1H NMR (250.1 MHz, CDCl3): d = 6.79 (2H, C6H2), 6.76, 4.63,
4.57, 4.48 (4H, C5H4), 4.42, 4.28 (4H, C5H4), 4.23, 4.08, 2.36, 2.34, 2.30
(6H, o-CH3), 2.26, 2.24, 2.18, 2.14 (3H, p-CH3), 2.09 ppm. Note: The
five major peaks are printed boldface. UV/Vis (0.1 mm, CH2Cl2):
lmax = 300, 380, 505 nm. GPC-RI (THF, PS Standards): Mw = 3880,
Mn = 2890, PDI = 1.34; GPC-MALLS (THF; dn/dc = 0.167): Mw =
7480, Mn = 5160; PDI = 1.45.
Received: October 4, 2005
Published online: December 30, 2005
Keywords: boranes · density functional calculations · ferrocene ·
.
organic–inorganic hybrid composites · polymers
Figure 3. Cyclic voltammogram of 4Mes (CH2Cl2, 0.1m [NBu4]-
[B(C6F5)4], v=0.2 Vsꢀ1, vs. FcH/FcH+; starred wave:
(C5Me5)2Fe/(C5Me5)2Fe+).
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2Mes possessing two donors/one acceptor (45 mV) and the
redox potential of 5Mes with one donor/two acceptors
(196 mV). A second redox wave with features of chemical
reversibility is recorded for 4Mes at E8’ = 845 mV. This
transition is absent in the cyclic voltammograms of the model
systems and tentatively assigned to a second FeII/FeIII
oxidation. This second redox event is observed at a much
higher potential than that of the diferrocene 2Mes (467 mV),
which is probably due to the presence of two neighboring
ferrocenium moieties adjacent to each neutral ferrocene in
the polymer chain -fc+-B(Mes)-fc-B(Mes)-fc+-B(Mes)- fol-
lowing oxidation of alternating ferrocenylene moieties ([Fc-
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the theoretical calculations as well as of the DSC- and GPC
measurements, the MALDI mass spectrometry, the electro-
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values of 3 and 266 mV with D(E8’) = 263 mV. For electro-
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