Rhodium-catalyzed dehydrocoupling of fluorinated phosphine-borane adducts: Synthesis, characterization, and properties of cyclic and polymeric phosphinoboranes with electron-withdrawing substituents at phosphorus

Article abstract of DOI:10.1002/chem.200401296

The dehydrocoupling of the fluorinated secondary phosphine-borane adduct R₂PH·BH₃ (R = p-CF₃C₆H ₄) at 60°C is catalyzed by the rhodium complex [{Rh(μ-Cl)(1,5-cod)}2] to give the four-membered chain R<

Full text of DOI:10.1002/chem.200401296

Rhodium-Catalyzed Dehydrocoupling of Fluorinated Phosphine–Borane
Adducts: Synthesis, Characterization, and Properties of Cyclic and Polymeric
Phosphinoboranes with Electron-Withdrawing Substituents at Phosphorus
Timothy J. Clark,[a] JosØ M. Rodezno,[a] Scott B. Clendenning,[a, b] Stephane Aouba,[c]
Peter M. Brodersen,[d] Alan J. Lough,[a] Harry E. Ruda,[c] and Ian Manners*[a]
Dedicated to Professor Peter Paetzold on the occasion of his 70th birthday
Abstract: The dehydrocoupling of the
fluorinated secondary phosphine–
borane adduct R2PH·BH3 (R p-
CF3C6H4) at 608C is catalyzed by the
rhodium complex [{Rh(m-Cl)(1,5-
cod)}2] to give the four-membered
chain R2PH-BH2-R2P-BH3. A mixture
of the cyclic trimer [R2P-BH2]3 and tet-
ramer [R2P-BH2]4 was obtained from
the same reaction at a more elevated
temperature of 1008C. The analogous
rhodium-catalyzed dehydrocoupling of
the primary phosphine–borane adduct
RPH2·BH3 at 608C gave the high mo-
lecular weight polyphosphinoborane
polymer [RPH-BH2]n (Mw = 56170,
PDI = 1.67). The molecular weight
was investigated by gel permeation
chromatography and the compound
characterized by multinuclear NMR
spectroscopy. Interestingly, the elec-
tron-withdrawing fluorinated aryl sub-
stituents have an important influence
on the reactivity as the dehydrocou-
pling process occurred efficiently at the
mildest temperatures observed for
phosphine–borane adducts to date.
Thin films of polymeric [RPH-BH2]n
=
(R = p-CF3C6H4) have also been
shown to function as effective nega-
tive-tone resists towards electron beam
(e-beam) lithography (EBL). The re-
sultant patterned bars were character-
ized by scanning electron microscopy
(SEM), atomic force microscopy
(AFM) and time-of-flight secondary
ion mass spectrometry (TOF-SIMS).
Keywords: boranes
·
fluorinated
ligands · lithography · phosphanes ·
polymers
Introduction
The discovery of new and efficient synthetic methods for
the formation of bonds between main group elements is of
considerable interest for the general development of molec-
ular and polymeric main group chemistry.[1–3] Transition
metal catalyzed reactions are of widespread importance in
organic chemistry whereas their use in preparative inorganic
chemistry is much less explored. However, examples of the
use of transition metal catalyzed dehydrocoupling reactions
for the preparation of a variety of homo- and heteronuclear
bonds between main group elements have been demonstrat-
ed beginning with the use of titanocene complexes to con-
vert primary silanes to polysilanes in 1986.[4] New classes of
inorganic macromolecules such as polystannanes[5] have
been generated via catalytic dehydrocoupling methods while
[a] T. J. Clark, J. M. Rodezno, Dr. S. B. Clendenning, Dr. A. J. Lough,
Prof. I. Manners
Department of Chemistry, University of Toronto
80 St. George Street
Toronto, ON, M5S 3H6 (Canada)
Fax : (+1)416-978-2450
E-mail: imanners@chem.utoronto.ca
[b] Dr. S. B. Clendenning
Current Address: Intel Corporation
RA3-252, 2513 NW 229th Ave.
Hillsboro, OR 97124 (USA)
[c] Dr. S. Aouba, Prof. H. E. Ruda
Centre for Advanced Nanotechnology
University of Toronto, 170 College Street
Toronto, ON, M5S 3E3 (Canada)
[6]
[7]
[8]
[9]
[10]
À
À
À
À
À
several examples of P P, B Si, Sn Te, Si P, Si N
[11]
[d] Dr. P. M. Brodersen
À
and Si O bond-forming reactions have also been report-
ed.[12] As part of our continuing program to develop novel
extended structures based on main group elements,[13] we
have studied the late transition metal (e.g. rhodium) cata-
Surface Interface Ontario
Department of Chemical Engineering and Applied Chemistry
University of Toronto, 200 College Street
Toronto, ON, M5S 3E5 (Canada)
4526
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FULL PAPER
À
lyzed formation of phosphorus boron bonds, and its appli-
oil in 72% overall yield in four steps from PCl3 by using a
protection–deprotection sequence (Scheme 1).
The primary phosphine–borane adduct 2 was prepared in
an analogous manner to 1 and was isolated as a white solid
in 64% overall yield via the procedure shown in Scheme 2.
cation to the dehydrocoupling of phosphine–borane adducts
of the type R2PH·BH3 and RPH2·BH3. This has resulted in
the development of new synthetic routes to phosphine–
borane rings, chains and macromolecules.[14–17] In addition,
the catalytic dehydrocoupling approach has been extended
À
to amine–borane adducts to yield cyclic products with B N
skeletons.[18]
The electronic nature of the substituents at phosphorus in
phosphine–borane adducts might be expected to significant-
ly influence both the rate of dehydrocoupling and the poly-
mer properties. The observation that the catalytic dehydro-
coupling of PhPH2·BH3 and Ph2PH·BH3 is more facile com-
[16]
pared with that of iBuPH2·BH3 and iBu2PH·BH3 as well
[19]
as tBu2PH·BH3 is likely associated with differences in the
Scheme 1.
À
polarity of the P Hbonds. The strong ( +)-inductive effect
of the alkyl group attached to phosphorus is expected to de-
crease the polarity of the P-Hfunctionality, which in turn,
would be anticipated to render the bond less reactive for
dÀ
catalytic dehydrocoupling with Bd+
bonds.[16] Converse-
À
H
ly, the presence of electron withdrawing substituents on
phosphorus would be expected to make metal-catalyzed de-
hydrocoupling reactions more favourable. Furthermore, the
presence of the fluorinated substituents would represent a
significant new structural variation for polyphosphinobor-
anes as this would be expected to introduce additional
useful properties.[20] With these considerations in mind, in
this paper we report our work on the synthesis and catalytic
dehydrocoupling behaviour of the fluoroaryl-substituted
phosphine–borane adducts (p-CF3C6H4)2PH·BH3 and (p-
CF3C6H4)PH2·BH3. Furthermore, we describe preliminary
investigations into the use of polymeric fluorinated phosphi-
noboranes as resists towards electron beam (e-beam) lithog-
raphy (EBL).
Scheme 2.
Characterization of 1 and 2 was achieved by H, 31P, 13C,
1
19F and 11B NMR spectroscopy and mass spectrometry,
which afforded spectra completely consistent with the as-
signed structures. For example, the 31P NMR spectrum of 1
displayed a doublet (1JPH =386 Hz) due to coupling to one
PHproton while 2 exhibited a triplet (1JPH =370 Hz) due to
1
the presence of the PH2 moiety when the Hcoupled spec-
trum was recorded. The 1HNMR spectrum of 2 reflected
the greater acidity of the PH2 protons relative to those in
PhPH2·BH3 in accordance with the electron withdrawing tri-
fluoromethyl functionalities on the phenyl ring of 2. The
signal for the PH2 protons in PhPH2·BH3 was observed at d
5.51 ppm[21] while the signal for 2 was detected further
downfield at d 5.60 ppm.
Single crystals of 1 suitable for X-ray analysis were ob-
tained by cooling the neat compound to À308C. The geome-
try around phosphorus and boron is approximately tetrahe-
dral, and their substituents are oriented in a staggered con-
formation (Figure 1). The smallest angle at phosphorus is
101.1(9)8 (H(1P)-P(1)-C(1)), and the largest is 115.6(9)8
(H(1P)-P(1)-B)1)) while the angles at boron range from
Results and Discussion
Synthesis and characterization of secondary and primary
phosphine-borane adducts 1 and 2: We have previously
shown that primary phosphine–borane adducts RPH2·BH3
(R = Ph, iBu, p-nBuC6H4, p-dodecylC6H4) undergo rhodi-
um-catalyzed dehydrocoupling between 90–1308C to yield
moderate to high molecular weight polyphosphinobor-
anes.[15,16] The synthesis of these polymers was most effec-
tively achieved using rhodium (pre)catalysts [{Rh(m-Cl)(1,5-
cod)}2], [Rh(cod)2]OTf, anhydrous RhCl3, or RhCl3·xH2O
(ca. 1–2 mol% Rh). Previously, it was shown by 31P and
11B NMR spectroscopy that in the absence of Rh catalyst,
the reactions are very slow and yield poorly defined oligo-
meric materials that appear to have branched structures.[15]
To investigate whether aryl-substituted monomers bearing
electronegative groups are indeed more reactive than those
previously studied, we synthesized and investigated the
À
103.8(13) to 115(2)8. The P B bond length of 1.917(2) Š in
1 is typical for a P B single bond (1.90–2.00 Š)[22] and no in-
À
À
À
termolecular hydrogen bonding between P Hand B Hsub-
stituents was observed.
Single crystals of 2 suitable for X-ray analysis were also
obtained by cooling the neat phosphine–borane adduct (to
À358C) over several days (Figure 2a and b). Compound 2
was also found to possess a staggered geometry around the
para-substituted
phosphine–borane
CF3C6H4)2PH·BH3 1 and (p-CF3C6H4)PH2·BH3 2. The secon-
dary phosphine–borane adduct 1 was isolated as a colorless
adducts
(p-
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I. Manners et al.
Figure 1. Molecular structure (p-CF3C6H4)2PH-BH3 (1). Selected bond
À
À
lengths [Š] and angles [8]: P(1) B(1) 1.917(2), P(1) H(1P) 1.37(2),
À
À
À
B(1) H(1B) 1.12(2), B(1) H(2B) 1.07(3), B(1) H(3B) 1.09(3); H(1P)-
P(1)-B(1) 115.6(9), H(1B)-B(1)-P(1) 103.8(13), H(2B)-B(1)-P(1)
104.5(15), H(3B)-B(1)-P(1) 105.8(15).
À
tetrahedral phosphorus and boron atoms with a P B bond
length of 1.900(3) Š. In a similar manner to the molecular
structure of PhPH2·BH3,[15] close intermolecular P-Hd+
···dÀH-B contacts were found between two molecules of 2
creating a centrosymmetric dimer. The closest contacts were
found to be 2.308 Š, however, these distances are underesti-
mated due to the systematic error in determining hydrogen
positions in X-ray structures. After standardization with a
À
value of 1.21 Š used for B Hbonds (established by neutron
diffraction)[23] and 1.40 Š for P Hbonds (determined by mi-
À
crowave spectroscopy),[24] the H···H distances were found to
be 2.230 Š. This value is significantly shorter than the sum
of the van der Waals radii for two hydrogen atoms (2.4 Š)
suggesting an attractive P-Hd+···dÀH-B interaction exists as a
result of the oppositely charged hydrogen atoms. Further-
more, the P-H···H angle (165.86(2.37)8) is more linear rela-
tive to the B-H···H angle (134(3)8), which is also observed
for most compounds with an N-Hd+···dÀH-B interaction.[23]
Figure 2. a) Molecular structure p-CF3C6H4PH2-BH3 (2) and its symme-
À
try-related neighbour. Selected bond lengths [Š] and angles [8]: P(1)
À
À
À
B(1) 1.900(3), P(1) H(1P) 1.29(3), P(1) H(2P) 1.42(3), B(1) H(1B)
À
À
1.11(3), B(1) H(2B) 1.16(3), B(1) H(3B) 1.11(4); H(1P)-P(1)-B(1)
111.8(12), H(2P)-P(1)-B(1) 112.5(11), H(1B)-B(1)-P(1) 108.4(14), H(2B)-
B(1)-P(1) 108.2(16), H(3B)-B(1)-P(1) 98.9(19); b) molecular structure p-
CF3C6H4PH2-BH3 (2) and its symmetry-related neighbour (symmetry
code “A”: x, 0.5Ày, 0.5+z), showing weak intermolecular H···H interac-
tions; selected bond angles [8]: B(2)-H(5B)-H(4PA) 134(3), P(2A)-H-
(4PA)-H(5B) 165.9(2.4).
Rhodium-catalyzed dehydrocoupling of the secondary phos-
phine–borane adduct (p-CF3C6H4)2PH·BH3 (1)—Synthesis
and
characterization
of
(p-CF3C6H4)2PH-BH2-(p-
CF3C6H4)2P-BH3 (3): The dehydrocoupling of 1 was found
to be catalyzed by the rhodium complex [{Rh(m-Cl)(1,5-
cod)}2] at 608C over 15 h affording the linear dimer (p-
CF3C6H4)2PH-BH2-(p-CF3C6H4)2P-BH3 (3) (Scheme 3) in
69% yield. Pure 3 was obtained by recrystallization from di-
ethyl ether as a colorless, air- and moisture-stable crystalline
solid.
Scheme 3.
The 31P{1H} NMR spectrum of 3 in CDCl3 showed two
broad resonances at d À2.1 [(p-CF3C6H4)2PH] and at d
À14.7 ppm [(p-CF3C6H4)2P]. The resonance at d À2.1 ppm
showed a doublet of multiplets centered at d 6.98 ppm
(1JPH =425 Hz, PH proton) and two sets of broad resonances
assigned to the BH2 and BH3 protons centered around 1.00
and 2.40 ppm, respectively. When compared with Ph2PH-
BH2-PPh2-BH3, obtained from heating neat Ph2PH·BH3 at
908C with a Rh (pre)catalyst, we observed almost indistin-
1
was further split into a doublet in the Hcoupled 31P NMR
spectrum (1JPH =437 Hz). The 11B{1H} NMR spectrum of 3
also showed two broad lines centered at d À33.6 ppm [BH2]
and d À37.8 ppm [BH3]. The 1HNMR spectrum of
3
guishable HNMR spectroscopic data. [15]
1
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Phosphinoboranes
FULL PAPER
Single-crystal X-ray analysis of compound 3 confirmed
the assigned structure based on a PBPB chain (Figure 3).
The geometries around B(1), B(2), P(1) and P(2) are ap-
proximately tetrahedral, with bond angles varying from
104.1(18) to 112.4(16)8, 106.8(18) to 113(3)8, 103.3(12) to
115.46(18)8 and 102.83(15) to 112.95(19)8, respectively. All
three P–B distances are in the single bond range with P(1)–
B(1) 1.916(4), P(2)–B(1) 1.935(4) and P(2)–B(2) 1.929(4) Š.
Scheme 4.
1
À18.2 ppm.[15] However, the HNMR spectrum in [D 8]THF
showed two sets of aromatic protons that were assigned to
4a and 4b, respectively, in the ratio of 4:1.
It was possible to isolate pure samples of the cyclic tet-
ramer 4b, which is significantly less soluble in dichlorome-
thane than trimer 4a. Although we have been unable to iso-
late single crystals of 4a for X-ray analysis from diethyl
ether and THF solutions, single crystals of 4b were obtained
from the slow evaporation of a THF solution. From these
crystals, an X-ray diffraction study was carried out which
proved the eight-membered ring structure of 4b (Figure 4).
Figure 3. Molecular structure (p-CF3C6H4)2PH-BH2-(p-CF3C6H4)2P-BH3
À
À
(3). Selected bond lengths [Š] and angles [8]: P(1) B(1) 1.916(4), P(2)
À
À
B(1) 1.935(4), P(2) B(2) 1.929(4), P(1) H(1P) 1.30(3), B(2)-P(2)-B(1)
112.95(19), H(1P)-P(1)-B(1) 114.9(12), P(1)-B(1)-P(2) 108.7(2). Hydro-
gen atoms attached to phenyl rings are omitted.
Interestingly, no transformation of 1 into 3 occurs when
the compound is heated in the melt in the absence of Rh
catalyst under the same conditions (608C, 15 h) verifying
the crucial role the catalyst plays in the dehydrocoupling re-
action at this temperature. Moreover, the catalytic reaction
conditions employed to form Ph2PH-BH2-PPh2-BH3 from
Ph2PH·BH3 (908C, 15 h) are more forcing compared to
those required to form 3. This significant difference in reac-
tion temperature can likely be attributed to the presence of
the electron-withdrawing trifluoromethyl substituents on the
phenyl rings of the phosphorus atom of 1 compared with
Ph2PH-BH3 as this is the only variance in composition be-
tween the two compounds.
Figure 4. Molecular structure [(p-CF3C6H4)2P-BH2]4 (4b). Selected bond
À
À
lengths [Š] and angles [8]: P(1) B(1) 1.956(5), P(1) B(1)#1 1.933(5),
À
P(1)#2 B(1) 1.933(5), B(1)#1-P(1)-B(1) 120.5(3), P(1)#2-B(1)-P(1)
112.9(2). Hydrogen atoms attached to phenyl rings are omitted.
Synthesis and characterization of [(p-CF3C6H4)2P-BH2]3
(4a) and [(p-CF3C6H4)2P-BH2]4 (4b): When neat 1 and ꢀ
1–2 mol% [{Rh(m-Cl)(1,5-cod)}2] were heated at 1008C for
15 h, the linear compound 3 was no longer observed as the
reaction product. Instead a mixture of cyclic trimer 4a and
cyclic tetramer 4b was isolated (Scheme 4).
Multinuclear NMR studies of 4a and 4b revealed that
there are virtually no differences in their 31P and 11B NMR
spectra. For example, the 31P NMR spectrum of the mixture
revealed a broad signal centered at d À18.5 ppm, which is
similar to that of [Ph2P-BH2]3 and [Ph2P-BH2]4 centered at d
In an analogous manner to the structure of [Ph2P-BH2]4,
molecules of 4b were found to exist in a boat–boat confor-
mation and possess similar bond lengths and bond angles to
those found in the former compound.[15] The P–B single
bond lengths are 1.933(5) and 1.956(5) Š. The B-P-B and
the P-B-P bond angles are 120.5(3) and 112.9(2)8, respec-
tively. The B-P-B bond angle deviates from the expected tet-
rahedral value (ca. 1098) possibly due to steric interactions
between the aryl groups.
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Heating 1 alone at 1008C for 15 h in the absence of Rh
catalyst does not lead to the ring systems 4a and 4b. How-
ever, approximately 50% of the monomer does undergo de-
hydrocoupling at this temperature to afford 3. The reaction
conditions employed to prepare [Ph2P-BH2]x (x=3, 4) in-
volved heating Ph2PH·BH3 with RhI (pre)catalysts to
1208C.[15] In comparison, it is likely that the decrease in de-
hydrocoupling temperature by 208C reflects the increased
acidity and reactivity of the P-Hfunctionality in 1 compared
with that in Ph2PH·BH3.
ported for other polyphosphinoboranes,[15,16] polymer 5 is
also air- and moisture-stable in the solid state.
We have investigated the molecular weight of polymer 5
through gel permeation chromatography (GPC) experi-
ments. Previous studies on solutions of high molecular
weight polymers [RPH-BH2]n (R=Ph, iBu) in THF by using
GPC for molecular weight characterization were found to
be problematic due to facile aggregation and/or adsorption
of the polymer chains to the GPC columns. This behavior is
not surprising as low angle light scattering studies showed
that THF is a q solvent under these conditions.[15,16] Howev-
er, in contrast, GPC analysis of other polyphosphinoboranes
(R = p-nBuC6H4, p-dodecylC6H4) in THF revealed Mw
values that were in fairly good agreement with estimates
provided by hydrodynamic radius data obtained from dy-
namic light scattering.[16] Initial GPC studies of 5 were con-
ducted in THF but consistently indicated long elution times
consistent with column adsorption effects. However, increas-
ing the ionic strength of the eluent by addition of
[Bu4N]Br[27] was found to lead to well-defined GPC peaks
which showed that the material was of high molecular
weight (Mw = 56170, PDI = 1.67). We presume that the
use of the 3 mm salt solution lowers the affinity of polymeric
5 for the column.[27]
Rhodium-catalyzed dehydrocoupling of the primary phos-
phine-borane adduct (p-CF3C6H4)PH2·BH3 (2): Synthesis
and characterization of polyphosphinoborane [(p-
CF3C6H4)PH-BH2]n (5): Upon treatment of neat 2 with a
catalytic amount (2.5 mol%) of [{Rh(m-Cl)(1,5-cod)}2] at
608C for 9 h, dehydrocoupling was observed with the reac-
tion mixture gradually becoming viscous after several hours.
Dissolution of the orange residue in THF and subsequent
precipitation into pentane gave light brown fibers, which
were identified as the polyphosphinoborane 5 (Scheme 5).
Due to the step-growth nature of the polycondensation
reaction used to form polyphosphinoboranes, very high con-
version (>99%) of the phosphine–borane adducts is re-
quired to obtain high molecular weights. Unfortunately,
there was a general tendency for the neat reaction mixture
to become increasingly viscous during the course of the de-
hydrocoupling reaction and for stirring (using a magnetic
stir bar) to ultimately cease. It was found that for Rh cata-
lyst loadings greater than 5 mol%, rapid solidification of the
reaction mixture occurred at 608C and resulting GPC analy-
sis revealed the presence of only oligomeric soluble materi-
al. However, for loadings of 1–3 mol%, high molecular
weight polymer was generated. This may be due to a smaller
number of initiation sites, which would favour the formation
of longer chains before monomer was consumed.
In the absence of catalyst, no reaction was detected at
608C under the same conditions. In addition, as noted for
the formation of the linear dimer 3 and cyclic systems 4a
and 4b from fluorinated adduct 1, the dehydrocoupling tem-
perature of 608C for the conversion of 2 to 5 in the presence
of a Rh catalyst is much lower than that previously reported
for the conversions of PhPH2·BH3 to [PhPH-BH2]n (90–
1308C for 6 h) and iBuPH2·BH3 to [iBuPH-BH2]n (1208C
for 15 h).[16] This demonstrates the significant influence of
changing the substituent at phosphorus in the phosphine–
borane adduct on the reaction conditions. In particular, this
result is important as a high molecular weight polyphosphi-
noborane 5 can now be accessed at a relatively low tempera-
ture. Once again it is likely that the electron-withdrawing p-
Scheme 5.
The 31P NMR spectrum of 5 showed three resonances that
resemble a 1:2:1 triplet at d À46.9 ppm, which are in close
proximity to the resonance of monomer 2 (d À48.3 ppm).
The appearance of three peaks is consistent with the forma-
tion of an atactic polymer with resolution of the triad struc-
ture. Thus, the possible triad configurations in 5 involve four
distinct tacticity environments composed of two successive
dyads, namely mm, mr, rm, and rr, where m and r are the
meso (adjacent units of the same configuration) and racemic
(adjacent units of opposite configuration) forms, respective-
ly. Since mr and rm configurations are mirror images, they
are indistinguishable by NMR and this leads to the expected
statistical distribution of 1:2:1 in the 31P NMR spectrum.[25]
The resonance centered at d À46.9 ppm split into a broad
doublet (1JPH =354 Hz) when the Hcoupled 31P NMR spec-
1
trum was recorded, which supported the presence of a single
hydrogen substituent at the phosphorus.
1
The HNMR spectrum of 5 is also consistent with the as-
signed structure and contains broad peaks for the aromatic
protons (d 7.9–6.9) and the BH2 protons (d 1–2), as well as a
broad doublet centered at d 4.42 for the PHgroup ( JPH
1
=
354 Hz). The 11B NMR spectrum of 5 showed a single broad
resonance at d À34.3, which is consistent with a four-coordi-
nate boron center attached to two phosphorus atoms.[26] The
19F NMR resonance for 5 at d À62.5 ppm differs slightly
from that of monomer 2 (d À63.7 ppm). As previously re-
À
CF3C6H4 group increases the polarity of the P Hbond com-
pared with the unsubstituted Ph substituents, rendering the
proton more acidic and therefore more reactive towards de-
À
hydrocoupling with the hydridic B Hbonds. With a conven-
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Phosphinoboranes
FULL PAPER
ient synthetic route to polyphosphinoborane 5 firmly estab-
lished, we performed a preliminary investigation into litho-
graphic applications of thin films of this material.
(AFM) and time-of-flight secondary ion mass spectrometry
(TOF-SIMS). As can be seen by SEM, the vast majority of
the bars are well-formed and intact (Figure 5).
Analysis by AFM over a 20 mm”20 mm area of bars gave
an average height of 113 Æ 2 nm and an average width at
half height of 1.19 Æ 0.02 mm (Figure 6). As can be seen by
the cross sectional analysis in Figure 6b, the bars exhibit ex-
cellent uniformity both in height and width.
Direct writing of patterned bars by using electron-beam lith-
ography (EBL) and [(p-CF3C6H4)PH-BH2]n 5 as a resist:
Patterning is of intense current interest in many areas of sci-
ence and technology. This has led to the development of a
wide variety of fabrication methods that accompany a wide
range of available feature sizes.[28,29] For example, techniques
employing focused beams of high energy particles such as
electrons can be used to generate high resolution patterns in
the sub 100 nm range.[30] Furthermore, direct-write EBL is a
mask-less lithography in which a processible resist is re-
quired. A variety of substances have been shown to act as
resists towards EBL including inorganic materials[31,32] and
polymers.[33] An example of the latter is poly(methyl metha-
crylate) (PMMA) which has been used extensively. For in-
stance, monolayer films ranging from one to nine monolay-
ers of PMMA have been shown to act as positive resists by
using relatively low electron beam energies.[33]
We decided to explore the use of 5 as a resist towards
EBL in an effort to modify the characteristics of the surface.
In addition, a thin patterned film of 5 on a Si substrate
could yield excellent control over hydrophobic versus hydro-
philic surfaces. One would expect that the p-CF3 substitu-
ents at phosphorus of 5 would render it relatively hydropho-
bic. Also, preliminary work in our group has indicated that
polyphosphinoboranes are promising preceramic polymers
as they can give high ceramic yields upon pyrolysis at
10008C.[16] Consequently, we were motivated to extend this
to a pre-fabricated design of 5 in order to lay the ground-
work for the preparation of patterned boron–phosphide ce-
ramics.
Figure 5. SEM image of bars fashioned by EBL using 5 as a resist.
Solutions of 5 in toluene were spin-coated onto silicon
substrates affording uniform films. The thickness and refrac-
tive index of the films were characterized using ellipsometry
and were determined to be 121 Æ 2 nm and 1.53, respective-
ly. The refractive index value is very close to the fluoroaryl
organic analogue poly(4-trifluoromethylstyrene) which has
been determined as 1.50.[34] E-beam lithography was carried
out by using a converted thermionic DSM 940 (Zeiss) scan-
ning electron microscope (SEM) with an accelerating volt-
age of 25 kV and controlled by a Raith ELPHY lithography
system. Samples were developed through 1 min sonication
in THF prior to characterization. Indeed, resists of 5 were
found to act in a negative-tone fashion as the material that
was exposed to the e-beam was firmly attached to the sub-
strate while unexposed polymer was removed by sonication.
This type of behavior may result from crosslinking of poly-
mer chains via homolytic cleavage of sigma bonds and radi-
cal dimerization in the presence of the e-beam rendering
them insoluble in organic solvents.
Figure 6. a) Tapping-mode AFM image of lithographically patterned bars
of 5 and b) cross-sectional analysis.
The elemental composition of the array of bars was inves-
tigated using TOF-SIMS. Boron and fluorine elemental
maps over a 40 mm”40 mm area of the array of bars on the
Si substrate were obtained and clearly revealed that boron
and fluorine were concentrated within the bars (Figure 7).
These results show the promising use of 5 as a resist for
EBL. In addition, polyphosphinoboranes are potential pre-
cursors to boron-based ceramics.[16] Future work will involve
Well-ordered arrays of micron-scale bars about 4.0 mm”
0.7 mm (as determined by SEM) were prepared with a dose
of 25 mCcmÀ2 and a beam current of 5.4 nA. They were
characterized by using SEM, atomic force microscopy
Chem. Eur. J. 2005, 11, 4526 – 4534
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4531

I. Manners et al.
Experimental Section
General methods: All reactions were performed under an atmosphere of
nitrogen using dry solvents. Workup of all phosphinoborane compounds
was carried out in air. PCl3, HNEt2, p-CF3C6H4Br, nBuLi (1.6m solution
in hexanes), HCl (2.0m solution in diethyl ether), Li[BH4] (Aldrich) were
purchased and used as received. [{Rh(m-Cl)(1,5-cod)}2][37] was prepared
following literature procedures. Hexanes was purified using the Grubbs
method.[38] Diethyl ether was dried over Na/benzophenone and distilled
prior to use. Pentane, tetrahydrofuran and dichloromethane were ACS
grade and used as received from ACP Chemicals Inc. NMR spectra were
recorded on a Varian Gemini or Mercury 300 MHz spectrometer. Chemi-
cal shifts are referenced to residual protonated solvent resonances (1H,
13C) or external BF3·Et2O (11B), CFCl3 (19F) or H3PO4 (31P) standards.
Mass spectra were obtained with a VG 70–250S mass spectrometer oper-
ating in electron impact (EI) mode. The molecular weight of 5 was esti-
mated by gel permeation chromatography (GPC) using a Viscotek GPC
MAX liquid chromatograph equipped with a Viscotek Triple Detector
Array consisting of a differential refractometer and Ultrastyragel col-
umns with pore sizes of 103–105 Š. Polystyrene standards were purchased
from American Polymer Standards and were used for calibration purpos-
es. A flow rate of 1.0 mLminÀ1 was used, and the eluent was a 3 mm solu-
tion of tetra-n-butylammonium bromide in THF.
Figure 7. TOF-SIMS elemental maps for B and F of a 40 mm”40 mm
area of bars patterned by EBL using 5 as a resist.
X-ray structural characterization: Diffraction data were collected on a
Nonius Kappa-CCD with graphite-monochromated MoKa radiation (l=
0.71073 Š). The data were integrated and scaled with the Denzo-SMN
package.[39] The structures were solved and refined with the SHELXTL-
PC V5.1 software package.[40] Refinement was by full-matrix least-
squares on F2 of all data (negative intensities included). All molecular
structures are presented with thermal ellipsoids at a 30% probability
level. In all structures, hydrogens bonded to carbon atoms were included
in calculated positions and treated as riding atoms. For structures 1, 2, 3
and 4b the hydrogen atoms attached to boron and phosphorus were lo-
cated and refined with isotropic thermal parameters.
studies of the conversion of micro- and nanopatterned
arrays of 5 and related polyphosphinoboranes into ceramic
replicas.[32,35,36]
Conclusion
The desire to induce catalytic dehydrocoupling of phos-
phine–borane adducts under more mild conditions prompted
our investigations into the use of electron-withdrawing fluo-
rinated phenyl substituents at phosphorus. We anticipated
that the (+)-inductive effect of these substituents on the
protons bound to the phosphorus renders them more acidic
and therefore more reactive towards dehydrocoupling. This
approach was successful, as supported by the Rh catalyzed
dehydrocoupling of neat (p-CF3C6H4)2PH·BH3 1 at 608C to
CCDC-258352 (2), -258353 (1), -258354 (3) and -258355 (4b) contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
Substrate preparation: Silicon (100) substrates were purchased from
Wafer World Inc. and cleaned by successive sonication for 10 min in
CH2Cl2, 2-propanol, piranha solution (30% H2O2/concentrated H2SO4,
1:3 vol%, CAUTION: extremely corrosive!) and deionized water prior
to drying in a jet of filtered air.
afford
the
linear
dimer
(p-CF3C6H4)2PH-BH2-(p-
Spin-coating: [(p-CF3C6H4)PH-BH2]n (5) was dissolved in dry toluene
(20 mgmLÀ1) that was filtered with a Whatman 13 mm GD/X nylon sy-
ringe filter (0.45 mm pore size) immediately prior to spin-coating. The
ꢀ2 cm2 Si wafer was coated with the polymer solution and immediately
accelerated to 600 rpm for 18 s followed by an increase to 1000 rpm for
60 s.
CF3C6H4)2P-BH3 (3) while at 1008C, the cyclic species [(p-
CF3C6H4)2P-BH2]3 (4a) and [(p-CF3C6H4)2P-BH2]4 (4b)
were obtained. The temperatures required for all of these
transformations are 20–408C less than that of the corre-
sponding dehydrocoupling of Ph2PH·BH3. Analogously,
whereas high molecular weight poly(phenylphosphinobor-
ane) is formed upon heating PhPH2·BH3 at 90–1308C for 6 h
in the presence of a small quantity of Rh (pre)catalyst,[15,16]
heating (p-CF3C6H4)PH2·BH3 (2) with the same (pre)catalyst
at 608C for 9 h affords high molecular weight polyphosphi-
noborane 5. Lastly, we have shown that thin films of 5 func-
tion as negative-tone resists for EBL as well-ordered, pat-
terned arrays of micron-sized bars were generated by the
direct-write method. AFM indicated that the bars have ex-
cellent structural uniformity while TOF-SIMS confirmed the
elemental composition as the boron and fluorine maps
proved that these elements were localized within the bars.
Future work will involve studies of the pyrolysis of these
and related materials in order to prepare patterned boron
phosphide ceramics.
Ellipsometry: A Sopra (GES-5) spectroscopic ellipsometer acquired y
and D data from 0.62 to 4.00 eV by using the analyzer in a previous
tracking mode and with the integration time for each data point deter-
mined by either the minimum threshold of 2000000 detector counts or
10 s. A Levenberg–Marquardt algorithm was used to fit a three-layer
model of the sample to the experimentally acquired cos(2y)/(sin(2y)-
cos(D)) curves. The three-layer model consisted of a homogeneous crys-
talline Si substrate with 2 nm of native oxide underneath the polymer
layer that was described by a Cauchy dispersion law.
Electron-beam lithography (EBL): The e-beam lithography system was a
converted thermionic DSM 940 (Zeiss) scanning electron microscope.
The system had been converted by implementing an electrostatic beam
blanker and a low hysteresis beam scanning coil. Exposures were carried
out at room temperature on a stage without temperature control capabil-
ities in a vacuum of 10À5 Torr or better. A remote computer (PC), using a
fast IEEE interface drove the beam and the system stage motor through
the Raith Elphy-Plus Pattern generator (32 bits architecture). The system
stage also had a joystick for manual control and its position was moni-
4532
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Chem. Eur. J. 2005, 11, 4526 – 4534

Phosphinoboranes
FULL PAPER
tored by a laser interferometer (with 5 nm resolution) and read by the
computer. A Faraday cup located on the stage connected to a multimeter
allowed measurement of the beam current by using the remote computer.
was distilled (858C, 11 mmHg) to yield a colorless liquid (20.9 g, 77%).
1HNMR (300 MHz, C 6D6): d=7.30 (dd, 2H, 3JPH =8.02 Hz, Ar-Hpara),
7.10 (d, 2H, 3JHH =7.95 Hz, Ar-Hmeta); 31P{1H} NMR (121 MHz, C6D6):
d=157 (s, (p-CF3C6H4)PCl2).
Satisfactory results were obtained by using an accelerating voltage of
25 kV and a current of 5.4 nA with a dose of 25 mCcmÀ2. The sample was
a small silicon wafer (1 cm2) with a [(p-CF3C6H4)PH-BH2]n (5) layer with
a thickness of 121 nm. The sample was developed through 1 min sonica-
tion in THF. The selected pattern was an array of bars (0.5 mm”4.0 mm)
spaced 2.0 mm apart horizontally and 3.0 mm apart vertically.
(p-CF3C6H4)2PH·BH3 (1): Neat (p-CF3C6H4)2PCl (1.70 g, 4.60 mmol) was
added dropwise to a diethyl ether (20 mL) suspension of LiBH4 (0.10 g,
4.80 mmol) cooled to 58C with an ice bath. The mixture became cloudy
immediately and was allowed to stir for 1 h. The diethyl ether was re-
moved in vacuo, the residue dissolved in hexanes (20 mL) and filtered
through Celite. Removal of all volatiles under reduced pressure afforded
(p-CF3C6H4)2PH·BH3 (1) as a colourless oil (1.47 g, 4.37 mmol, 95%).
Single crystals of 1 for X-ray analysis were obtained by cooling the neat
compound to À308C. 1HNMR (300 MHz, CDCl 3): d = 7.9–7.7 (m, Ar-
H), 6.41 (dm, 1JPH =340 Hz, PH), 1.8–0.6 (brm, BH); 31P NMR
Scanning electron microscopy (SEM): SEM images were obtained a Hita-
chi S-5200 field-emission electron microscope.
Scanning probe microscopy: Tapping mode atomic force microscopy
(AFM) was performed on a Digital Instruments Multimode IIIa Nano-
scope system using silicon cantilevers with resonant frequencies near
166 kHz. The substrates were mounted on steel sample pucks and
imaged in air by using an E-scanner. Image analysis was performed by
using the Digital Instruments Nanoscope software. All AFM data sets
were collected as 512”512 pixel data sets with a scan rate of ca. 1 Hz.
Images were processed using Digital Instruments Nanoscope software
version 4.42r9 and were flattened using a zero order flatten command.
Noise was filtered out using a low-pass filter command.
(121 MHz, CDCl3):
d =
3.12 (brd, 1JPH =386 Hz, (p-CF3C6H4)2PH);
11B{1H} NMR (160 MHz, CDCl3): d = À40.6 (br, BH3); EI-MS (70 eV):
m/z (%): 322 (100) [M +ÀBH3].
(p-CF3C6H4)PH2·BH3 (2): Neat (p-CF3C6H4)PCl2 (1.04 g, 4.21 mmol) was
added dropwise to a slurry of Li[BH4] (182 mg, 8.35 mmol) in diethyl
ether (20 mL) at 58C. The mixture became cloudy immediately and was
stirred for 1.5 h. The solvent was removed in vacuo, the residue re-dis-
solved in hexanes (20 mL) and then filtered in the presence of Celite. Re-
moval of hexanes under vacuum afforded a colourless oil. Sublimation of
this oil overnight at 258C under vacuum gave (p-CF3C6H4)PH2·BH3 (2)
as a white solid (0.654 g, 3.41 mmol, 81%). Single crystals of 2 suitable
for X-ray analysis were obtained by cooling the oil to À358C. 1HNMR
Time-of-flight secondary ion mass spectrometry (TOF-SIMS): SIMS anal-
ysis was carried out with a TOF-SIMS IV instrument (Ion-TOF GmbH,
Munster). Surfaces were pre-sputtered with 6 nA Ar+ (500 eV) over a
300”300 mm2 area while monitoring contaminant signals (polydimethyl-
siloxane) for the purposes of cleaning. Analysis was performed with a
pulsed primary ion beam of focused 25 keV 69Ga+ ions incident on the
sample at 458. Boron was mapped by analyzing for the 10B and 11B +
10BHfragments. Fluorine was mapped by analyzing for F À. High spectral
resolution spectra (m/Dm=10000) and high spatial resolution images
(Dl=200 nm) were collected separately. Both high spectral resolution
spectra (“bunched mode”) and high-spatial-resolution spectra (“burst
alignment”-exhibiting unit mass resolution) were collected with a pri-
mary ion current of 2.5 pA over a 40 mm”40 mm area. For both experi-
ments, principal ion dose exceeded the static limit.
1
(300 MHz, CDCl3): d=7.90–7.74 (m, Ar-H), 5.60 (dm, 2H, JPH =370 Hz,
PH2), 0.91 (br (1:1:1:1) q, 3H, 1JBH =104 Hz, BH3); 31P NMR (121 MHz,
C6D6): d=À48.3 (brt, 1JPH =370 Hz, PH2); 11B{1H} NMR (160 MHz,
C6D6): d=À33.6 (brd, 1JPB =49 Hz, BH3); 11B NMR (160 MHz, C6D6):
d=À33.6 (d of q, 1JBH =126 Hz, BH3); 19F NMR (282 MHz, CDCl3): d=
À63.7 (s, CF3); EI-MS (70 eV): m/z (%): 178 (100) [M +ÀBH3].
(p-CF3C6H4)2PH-BH2-(p-CF3C6H4)2P-BH3 (3): Neat 1 (0.60 g, 1.79 mmol)
and [{Rh(m-Cl)(1,5-cod)}2] (ca. 10 mg, 2 mol% Rh) were stirred at 908C
for 14 h. During this period the orange solution gradually solidified. Re-
crystallization from diethyl ether gave colourless crystals of 3 (0.828 g,
1.24 mmol, 69%), which were suitable for single crystal X-ray analysis.
(p-CF3C6H4)2PCl: Et2NPCl2 required for this experiment was prepared
according to a literature procedure.[41] nButyllithium (35.4 mL of a 1.6m
solution in hexanes, 56.7 mmol) was slowly added to a solution of p-
CF3C6H4Br (12.8 g, 56.7 mmol) in diethyl ether (400 mL) at 58C. The
mixture was stirred for 3 h, and Et2NPCl2 (4.93 g, 28.3 mmol) was slowly
introduced into the flask at 58C. The contents of the flask were brought
to room temperature and the mixture was stirred overnight. (p-
CF3C6H4)2PNEt2 was not isolated but rather used in situ. 31P{1H} NMR
(121 MHz, D2O insert in Et2O): d=61.5.
1
M.p. 155–1578C; HNMR (400 MHz, CDCl 3): d = 7.74–7.45 (m, aromat-
ic), 6.98 (dm, 1JPH =425 Hz, PH), 1.9–2.8 (br, BH2), 1.5–0.5 (br, BH3);
31P{1H} NMR (121 MHz, CDCl3): d
=
À2.13 (br, (p-CF3C6H4)2PH),
À14.7 (br, (p-CF3C6H4)2P); 31P NMR (121 MHz, CDCl3): d
=
À2.13
(brd, 1JPH =437 Hz, (p-CF3C6H4)2PH); 11B{1H} NMR (160 MHz, CDCl3):
d = À33.6 (br, BH2), À37.8 (br, BH3); 19F NMR (282 MHz, CDCl3): d =
À63.6 (s, (p-CF3C6H4)2P-BH3), À64.0 (s, (p-CF3C6H4)2PH); EI-MS
(70 eV): m/z (%): 656 (21) [M +ÀBH3], 127 (100) [C6H4CF2+].
HCl (35 mL of a 2.0m solution in diethyl ether, 71 mmol) was added to
the reaction mixture containing (p-CF3C6H4)2PNEt2 at À788C. The mix-
ture was allowed to warm to room temperature and the solvent was re-
moved in vacuo. The residue was dissolved in hexanes (400 mL) and fil-
tered. The filtrate was concentrated and the remaining residue was distil-
led (858C, 0.001 mmHg) to yield a colorless oil (16.25 g, 80%). 1HNMR
(300 MHz, C6D6): d=7.17–7.13 (brm, Ar-H); 31P{1H} NMR (121 MHz,
C6D6): d=76.7 (s, (p-CF3C6H4)2PCl); 19F NMR (282 MHz, C6D6): d=
À63.3 (s, (p-CF3C6H4)2PCl).
(p-CF3C6H4)PCl2: (Et2N)2PCl required for this experiment was prepared
according to a literature procedure.[42] nButyllithium (69 mL of a 1.6m so-
lution in hexanes, 110 mmol) was slowly added to a solution of p-
CF3C6H4Br (25 g, 110 mmol) in diethyl ether (500 mL) at 58C. The mix-
ture was stirred for 1 h and (Et2N)2PCl (23.2 g, 110 mmol) was slowly
added to the mixture at 58C. The contents of the flask were brought to
room temperature and the mixture was stirred for 2 h. (p-CF3C6H4)P-
(NEt2)2 was not isolated but rather used in situ. 31P{1H} NMR (121 MHz,
D2O insert in Et2O): d=130.
Catalytic dehydrocoupling of (p-CF3C6H4)2PH·BH3 at 1008C: Neat 1
(0.41 g, 1.22 mmol) and [{Rh(m-Cl)(1,5-cod)}2] (4 mg, ca. 1 mol% Rh)
were heated at 1008C for 15 h, and the resulting solid material was puri-
fied and analyzed as mentioned below.
[(p-CF3C6H4)2P-BH2]3 (4a): Dichloromethane (5 mL) was added to the
above reaction mixture and the suspension was filtered followed by sol-
vent removal giving 4a as a light yellow solid (0.978 g, 0.976 mmol,
80%). M.p. 206–2108C; 1HNMR (300 MHz, [D 8]THF): d
= 7.8–7.6
(brm, Ar-Hmeta), 7.53 (d, 3JHH =8.1 Hz, Ar-Hortho), 2.5–1.8 (br, BH2);
31P{1H} NMR (121 MHz, CDCl3): d = À18.1; 11B{1H} NMR (160 MHz,
CDCl3): d = À33.7 (br, BH2); EI-MS (70 eV): m/z (%): 681 (100) [M +
À(p-CF3C6H4)2P].
[(p-CF3C6H4)2P-BH2]4 (4b): To a mixture containing [(p-CF3C6H4)2P-
BH2]3 and [(p-CF3C6H4)2P-BH2]4, prepared by the above procedure, was
added CH2Cl2 (ꢀ10 mL) and the mixture stirred for 10 min. The suspen-
sion was filtered and
a colourless solid was collected (0.326 g,
0.244 mmol, 20%). Single crystals of 4b for X-ray analysis were obtained
from the slow evaporation of a THF solution. M.p. 2608C (decomp);
1HNMR (300 MHz, [D 8]THF): d = 7.56–7.48 (brm, Ar-Hmeta), 7.30 (d,
3JHH =8.1 Hz, Ar-Hortho), 3.2–2.2 (br, BH2); 31P{1H} NMR (121 MHz,
CDCl3): d = À17.8; 11B{1H} NMR (160 MHz, CDCl3): d = À31.3 (br,
BH2); EI-MS (70 eV): m/z (%): 1334 (19) [M +], 300 (100) [PC6H3CF3-
(C6H3CF2)+].
HCl (275 mL of a 2.0m solution in diethyl ether, 550 mmol) was added to
the reaction mixture containing (p-CF3C6H4)P(NEt2)2 at À788C. The mix-
ture was allowed to warm to room temperature and stirred for 12 h. The
solvent was removed in vacuo, the solid dissolved in hexanes (500 mL)
and then filtered. The filtrate was concentrated and the remaining oil
Chem. Eur. J. 2005, 11, 4526 – 4534
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4533

I. Manners et al.
High molecular weight [(p-CF3C6H4)PH·BH2]n (5): In a typical experi-
ment, neat (p-CF3C6H4)PH2·BH3 (2) (0.155 g, 0.81 mmol) and [{Rh(m-Cl)-
(1,5-cod)}2] (ca. 0.010 g, 2.5 mol% Rh) were stirred for 9 h at 608C, gas
evolution was observed and the contents of the flask slowly solidified.
After cooling to room temperature, the resulting dark yellow solid was
dissolved in THF (1 mL), filtered to remove residual catalyst and precipi-
tated into pentane (ca. 150 mL). The light brown fibrous product 5 was
dried under vacuum at 258C for 18–24 h and isolated (0.109 g,
0.567 mmol, 70%). GPC analysis (3 mm solution of [Bu4N]Br in THF,
polystyrene standards): Mw =56170, PDI=1.67; 1HNMR (300 MHz,
CDCl3): d = 7.9–6.5 (br, Ar-H), 4.42 (brd, 1JPH =354 Hz, PH), 2.2–0.8
[14] H. Dorn, R. A. Singh, J. A. Massey, A. J. Lough, I. Manners, Angew.
Chem. 1999, 111, 3540; Angew. Chem. Int. Ed. 1999, 38, 3321.
[15] H. Dorn, R. A. Singh, J. A. Massey, J. M. Nelson, C. A. Jaska, A. J.
Lough, I. Manners, J. Am. Chem. Soc. 2000, 122, 6669.
[16] H. Dorn, J. M. Rodezno, B. Brunnhçfer, E. Rivard, J. A. Massey, I.
Manners, Macromolecules 2003, 36, 291.
[17] For recent theoretical work on polyphosphinoboranes see: a) D. Jac-
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b) D. Jacquemin, V. Wathelet, E. A. Perpꢂte, Macromolecules 2004,
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[18] a) C. A. Jaska, K. Temple, A. J. Lough, I. Manners, J. Am. Chem.
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[19] H. Dorn, E. Vejzovic, A. J. Lough, I. Manners, Inorg. Chem. 2001,
40, 4327.
(br, BH2); 11B{1H} NMR (160 MHz, CDCl3): d
=
À34.3 (br, BH2);
31P{1H} NMR (121 MHz, CDCl3): d = À46.9 (t); 19F NMR (282 MHz,
CDCl3): d = À62.5 (s, CF3).
No reaction was detected by 31P NMR when neat 2 was heated at 608C
for 9 h. However, approximately 20% conversion to polyphosphinobor-
ane 5 was observed when neat 2 was heated at 758C for 9 h.
[20] J. Scheirs, Modern Fluoropolymers: High Performance Polymers for
Diverse Applications, Wiley, New York, 1997.
[21] K. Bourumeau, A.-C. Gaumont, J.-M. Denis, J. Organomet. Chem.
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[22] J. C. Huffman, W. A. Skupinski, K. G. Caulton, Cryst. Struct.
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[23] W. T. Klooster, T. F. Koetzle, P. E. M. Siegbahn, T. B. Richardson,
R. H. Crabtree, J. Am. Chem. Soc. 1999, 121, 6337.
[24] P. S. Bryan, R. L. Kuczkowski, Inorg. Chem. 1972, 11, 553.
[25] We have previously observed this phenomenon in the 29Si NMR
spectrum of a polyferrocenylsilane [Fe(h-C5H4)2SiRR’]n where R =
Me and R’ = Cl see: D. L. Zechel, K. C. Hultzsch, R. Rulkens, D.
Balaishis, Y. Ni, J. K. Pudelski, A. J. Lough, I. Manners, D. A.
Foucher, Organometallics 1996, 15, 1972. For a general overview of
dyad resolution using NMR spectroscopy see: F. Heatley in NMR
Spectroscopy of Polymers (Ed.: R. N. Ibbett), Blackie, London,
1993, pp. 1–49.
Acknowledgements
T.J.C. is grateful for a University of Toronto Open Fellowship, S.B.C.
thanks NSERC for a PDF and I.M. thanks the Canadian government for
a Canada Research Chair. We thank David Rider and Kun Liu for help
with AFM, Chantal Paquet for ellipsometry measurements, Alex Bar-
tole-Scott for GPC measurements and Dr. Hendrik Dorn and Dr. Eric
Rivard for helpful discussions.
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[27] Previous reports have mentioned similar problems with the GPC
analysis of polyphosphazenes that were circumvented by the addi-
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Received: December 16, 2004
Published online: May 18, 2005
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