Iron-catalyzed dehydropolymerization: A convenient route to poly(phosphinoboranes) with molecular-weight control

Article abstract of DOI:10.1002/anie.201411957

The catalyst loading is the key to control the molecular weight of the polymer in the iron-catalyzed dehydropolymerization of phosphine-borane adducts. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism.

Full text of DOI:10.1002/anie.201411957

Angewandte
Chemie
DOI: 10.1002/anie.201411957
Polymerization
Hot Paper
Iron-Catalyzed Dehydropolymerization: A Convenient Route to
Poly(phosphinoboranes) with Molecular-Weight Control**
Andrꢀ Schꢁfer, Titel Jurca, Joshua Turner, James R. Vance, Kajin Lee, Van An Du,
Mairi F. Haddow, George R. Whittell, and Ian Manners*
Dedicated to Professor Peter Paetzold on the occasion of his 80th birthday
Abstract: The catalyst loading is the key to control the
molecular weight of the polymer in the iron-catalyzed dehy-
dropolymerization of phosphine–borane adducts. Studies
showed that the reaction proceeds through a chain-growth
coordination–insertion mechanism.
P
olymers based on main chains that contain p-block
elements other than carbon are of major current interest as
functional soft materials.[1] For example, polysiloxanes
[R2SiO]n and polyphosphazenes [R2PN]n possess remarkable
thermophysical properties with a wide range of applications,
and polysilanes [SiR2]n and polystannanes [SnR2]n offer
unusual electronic characteristics and photosensitivity.[1]
Polymers based on the Group 13 element boron have been
developed as easily processable thermal precursors to refrac-
tory ceramic fibers and shaped monoliths and are also
attracting attention as optoelectronic materials.[2]
Recently, our group and others have explored the catalytic
dehydrocoupling of amine–borane adducts (RNH2·BH3) to
high-molecular-weight poly(aminoboranes) ([RNH-BH2]n),
which are boron–nitrogen analogues of polyolefins.[3] These
materials are of interest for hydrogen storage, as piezo-
electrics, and as precursors to refractory BN-based materials.
Developments in this area have been facilitated by the
discovery of Ir, Ru, Rh, and Fe catalysts that are highly active
in solution at room temperature (Scheme 1). In contrast,
although a similar Rh-catalyzed approach using precatalysts
[{(1,5-cod)Rh(m-Cl)}2] and [(1,5-cod)2Rh]OTf (cod = cyclo-
octadiene) has been developed for the formation of their
phosphorus analogues, poly(phosphinoboranes) ([RPH-
BH2]n) were only accessible by melt reactions at temperatures
Scheme 1. Dehydropolymerization of Group 13/15 adducts with a) [Ir],
b) [Rh], and c) [Fe] (pre)catalysts.
of ca. 1308C, giving polydisperse, soluble, branched polymers
(Mn > 10000) and crosslinked, swellable, but insoluble mate-
rials, unless the phosphine–borane adduct is deliberately
activated by the presence of electron-withdrawing groups
(Scheme 1).[4,5] Although there have been key advances in the
À
mechanistic understanding of P B bond formation in model
examples of these dehydrocoupling reactions,[6] the challenges
of using the melt reaction with a precious-metal precatalyst
have held back the development of the poly(phosphinobor-
ane) field. Herein we report the discovery of an earth-
abundant, iron-based precatalyst for the synthesis of poly-
(phosphinoboranes) that proceeds in solution and also allows
insightful mechanistic investigations and control over the
molecular weight for the first time.
[*] Dr. A. Schꢀfer,[#] [+] Dr. T. Jurca,[+] J. Turner, Dr. J. R. Vance, Dr. K. Lee,
Dr. V. A. Du, Dr. M. F. Haddow, Dr. G. R. Whittell,
Prof. Dr. I. Manners
School of Chemistry, Bristol University
Cantock’s Close, BS8 1TS, Bristol (UK)
E-mail: ian.manners@bristol.ac.uk
Recently, iron-catalyzed dehydrocoupling of various
amine–borane adducts has been developed,[3e,k,7] and we
have shown that poly(aminoboranes) can be formed under
ambient conditions in the case of primary amine–borane
substrates, such as MeNH2·BH3. In our work, we used the
readily accessible iron(II) complex [Cp(CO)2FeI] (2a) as
a precatalyst.[3e,7b] While there are few known catalysts that
promote both amine–borane and phosphine–borane dehy-
drocoupling, this species served as a good starting point in our
search for a more convenient and improved alternative to the
[#] Current adress: Saarland University
Department 8.1—Chemistry, 66123 Saarbrꢁcken (Germany)
[+] These authors contributed equally to this work.
[**] We acknowledge Dr. Paul Gates for performing mass spectroscopy
experiments and for helpful discussions. I.M. thanks the EPSRC for
support. T.J. thanks the EU for a Marie Curie Fellowship and A.S. and
V.A.D. the DFG for postdoctoral fellowships.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411957.
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previously utilized Rh-based precatalysts.[8] Preliminary
experiments showed dehydrocoupling of phenylphosphine–
borane adduct (1a) to take place in the presence of 10 mol%
of 2a, in toluene at 1008C. Poly(phenylphosphinoborane)
(1b) was the exclusive product of this reaction and full
conversion of monomer 1a was observed by 11B and 31P NMR
spectroscopy after 24 h; the molecular weight of 1b was
however only moderate (Mn = 18000, PDI = 2.0; PDI = poly-
dispersity index). Replacement of iodide by the more weakly
coordinating trifluoromethanesulfonate gave a more active
precatalyst [Cp(CO)2Fe(OTf)] (OTf = OSO2CF3, 2b).[9] Thus,
when 1a was heated with only 1 mol% of 2b (toluene, 1008C,
24 h), quantitative conversion to high-molecular-weight 1b
was observed (Mn = 59000, PDI = 1.6).[10] The 31P NMR
spectrum of polymer 1b, which was isolated by precipitation
into cold pentane, showed a doublet at d(31P) = À49 ppm
(1JPH = 349 Hz), and the 11B NMR spectrum showed a broad
singlet at d(11B) = À35 ppm, similar to previous reports
(Figure 1).[4b]
smooth dehydropolymerization of 1a upon treatment with
1 mol% of 2b under identical conditions, under which the
typical darkening of the reaction solution that accompanies
the Fe nanoparticle formation was not observed. These
observations suggest that the dehydropolymerization of 1a
mediated by precatalyst 2b is a homogeneous process, an
assertion further supported by the lack of poisoning detected
with a substoichiometric amount of PMe3.[11]
In a typical dehydropolymerization of 1a (1 mol% of 2b,
1008C, toluene), a color change from red (corresponding to
2b) to yellow was observed early in the reaction. We also
studied the reaction of 1a with 2b on a stoichiometric scale
(1:1 ratio) under ambient conditions (Scheme 2a). Although
very slow, a clean conversion to the corresponding iron
Scheme 2. Synthesis of 2c by the reaction of a) 1a with 2b, and b) 1c
with 2a.
phosphidoborane species 2c was detected by 11B and
31P NMR spectroscopy.[11] This species was isolated as
a yellow solid and the structure was confirmed by multi-
nuclear NMR spectroscopy, single-crystal X-ray diffraction
(Figure 2), and an independent synthesis starting from 2a and
LiPhPH·BH3 (1c; Scheme 2b).[11] The analogous iron(diphe-
nylphosphido)borane complex 2d has been reported before
by our group[7] and also by Wagner and co-workers.[12]
Figure 1. a) 11B (128 MHz, 298 K, CDCl3) and b) 31P NMR spectrum
(162 MHz, 298 K, CDCl3) of 1b, isolated from the reaction of 1a with
1 mol% of 2b.
The ability to perform the dehydropolymerization in
solution rather than in molten monomer provided an
opportunity to further probe the mechanism of the Fe-
catalyzed process. A key question is whether the catalyst is
homogeneous or heterogeneous. In our previous study on the
Fe-catalyzed dehydrocoupling of amine–borane Me2NH·BH3,
the reaction with precatalyst 2a proceeded through a homo-
geneous mechanism, whereas the FeI dimers [Cp2(CO)2Fe2(m-
CO)2] and [Cp2(CO)(CH3CN)Fe2(m-CO)2] gave rise to very
small Fe nanoparticles, which were potent heterogeneous
catalysts.[7b] We found that treatment of 1a with these
nanoparticles did not result in detectable catalytic dehydro-
coupling at ambient temperature or upon heating (1008C,
toluene, 24 h).[11] This observation is in marked contrast to the
Figure 2. Molecular structure of 2c in the crystal. Thermal ellipsoids
drawn at 75% probability level. Selected bond distances: Fe-P:
225.39(1) pm, P-B: 195.14(1) pm (color code: grey: carbon, white:
hydrogen, purple: boron, brown: iron, red: oxygen, orange: phospho-
rus).
2
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When species 2c rather than 2b was used as a precatalyst
under identical conditions (1 mol%, 1008C, toluene, 24 h), 1b
was obtained with similar molecular weight (Mn = 80000;
PDI = 1.6). This result suggests that initiation with 2b
À
involves triflate dissociation followed by P H bond activation
À
and [Fe] P bond formation. The previous observation of
Scheme 3. Photolysis of 2c,d to give the corresponding monocarbonyl
derivatives 3a,b.
facile CO substitution in the diphenylphosphido analogue of
2c[13] at ambient temperature suggests that this is likely to be
followed by CO dissociation, substrate coordination, and the
À
formation of the P B bond. This process is mechanistically
similar to that demonstrated by Weller and co-workers in
model reactions of secondary phosphine–borane adducts at
Rh centers.[6a,b] To further probe the postulated CO dissoci-
ation step following the initial formation of 2c, we turned to
DFT investigations. The relative energy differences for the
loss of one CO ligand from 2c and the complexation of 1a in
a m-H s borane fashion to the Fe center were calculated at the
M06-2X/6-311 + G(d,p)(C, H, B, O, P), SDD(Fe) level of
theory (Figure 3).[11]
Figure 4. Molecular structure of 3b in the crystal. Thermal ellipsoids
drawn at 75% probability level. Selected bond distances: Fe-P:
218.08(5) pm, Fe-H: 128.64(252) pm, B-H:. 108.77(238) pm, 110.40-
(254) pm (color code: grey: carbon, white: hydrogen, purple: boron,
brown: iron, red: oxygen, orange: phosphorus).
À
In order to provide evidence for the P B bond-forming
step that follows CO dissociation and substrate binding, 2c
was reacted with 1a in a stoichiometric manner (1:1 ratio).
The reaction proved to be extremely slow at ambient
conditions, but, after four weeks, the 11B and 31P NMR
spectra showed new signals, which we tentatively assigned to
the iron-bound phosphinoborane dimer 3c (Scheme 4, Fig-
Figure 3. Calculated relative Gibbs free energies (in kJmolÀ1) of 2c
(left), for CO dissociation forming 3a (center), and subsequent
coordination of 1a to 3a (right) at the M06-2X/6-311+G(d,p)(C, H, B,
O, P), SDD(Fe) level of theory (relative Gibbs free energies given for:
C19H24B2FeO2P2; all H atoms except P-H and B-H omitted for clarity;
color code: grey: carbon, white: hydrogen, purple: boron, brown: iron,
red: oxygen, orange: phosphorus).
ure S6).[16] This assignment is further supported by the
[11]
À
observation of metal-bound P B species by ESI-MS.
The dissociation of one CO ligand from 2c was found to
be quasi thermoneutral. The calculations suggested that the
phosphidoborane moiety acts as a bidentate ligand, with the
BH3 group involved in an agostic interaction with the metal
center, thereby promoting CO loss through chelation. Bind-
ing of 1a to the resulting monocarbonyl complex, as a s-
borane complex, was slightly endergonic (DG298 =+
48.7 kJmolÀ1).[14] However, taking into consideration the
large excess of monomer 1a present in solution under
catalytic conditions, this is still reasonable (indeed, analogous
s-borane complexes, such as [Cp(Me3P)2Ru(h1-H3B·PMe3)]+,
have been reported for ruthenium).[15] Moreover, we were
able to obtain key evidence for the stability of the mono-
carbonyl complex 3a by 11B and 31P NMR spectroscopy and
by the synthesis and isolation of the analogous diphenyl
derivative 3b from 2d, through photolysis of a solution of 2d
in toluene at room temperature (Scheme 3).[11] Compound 3b
Scheme 4. Reaction of 2c with 1a to give a metal-bound phosphine–
borane dimer complex (3c; [Fe]=Cp(CO)xFe).
Another key mechanistic consideration is whether the
polymerization proceeds through a step-growth or a chain-
growth mechanism.[17] To probe this issue, the molecular
weight of 1b in a typical reaction of 1a with 5 mol% 2b was
analyzed at a low conversion (6 h/ ꢀ 35%). High-molecular-
weight polymer (Mn = 40000, PDI = 1.7) was detected, indi-
cating a chain-growth mechanism. Significantly, for a chain-
growth polymerization, the molecular weight should decrease
with increased catalyst loading. This tendency offers conven-
ient control over molecular weight and the use of 0.1, 1, 5 or
10 mol% precatalyst 2b gave a smooth decrease of both Mn
and Mw under otherwise identical conditions (24 h, 1008C,
toluene; Figure 5). In addition, allowing the reaction of 1a
with 5 mol% of 2b to continue for another 24 h led to no
1
was isolated as a red solid and was characterized by H, 11B,
and 31P NMR spectroscopy as well as single-crystal X-ray
diffraction, which showed the presence of a 3-center-2-
electron [Fe]-H-B agostic interaction (Figure 4).[11]
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further increase in molecular
weight, which suggests that the
polymer cannot condense further.
Based on our findings in this
work as well as previous studies of
dehydrocoupling mechanisms and
model species,[6] we propose
a
chain-growth coordination-type
mechanism (Scheme 5).[18] The first
proposed step after monomer coor-
dination to iron (in I) is the activa-
À
tion of the B H bond and the
À
formation of a new P B bond by
À
insertion into the [Fe] P bond. We
postulate an intermediate (II), in
which the polymer chain is bound to
the iron center through the BH2
À
group. In a second step, P H acti-
vation is proposed to form H2 and
À
a species with a new [Fe] P bond
(III). Finally, elimination of coordi-
nated H2 can open up the vacant
coordination site for binding further
monomer.
Scheme 5. Postulated catalytic cycle for the chain-growth coordination polymerization of 1a by the
iron catalyst derived from 2c (scheme showing one insertion event).
Figure 6. a) Isolated 1b, and b–d) SEM images of micropatterns
fabricated from 1b on silicon wafers by soft-lithography.
with excellent definition were imaged by scanning electron
microscopy (SEM; Figure 6b–d).[11] The facile synthesis and
fabrication of this material should enable future studies of
Figure 5. Dependence of the molecular weight of 1b (y axis: a) Mn,
b) Mw) derived from the reaction of 1a with different catalyst loadings
of 2b (x axis).
properties and applications.
In summary, we report the first earth-abundant, iron-
based (pre)catalyst for the synthesis of poly(phosphinobor-
ane) 1b in solution. The molecular weight of 1b can be
controlled by varying the catalyst loading. The catalysis
appears to be homogenous in nature and to occur through
Poly(phosphinoboranes) are of interest as inorganic soft
materials with unusual and useful properties. Preliminary
evidence for their use as precursors to the luminescent
semiconductor boron phosphide[4d] and as electron-beam
resists for lithography[4e] have been reported. With the ability
to now readily synthesize colorless, essentially metal-free,
high-molecular-weight poly(phenylphosphinoborane) (Fig-
ure 6a) using precatalyst 2b, we were able to show that the
resulting material was easily patterned on silicon wafers using
soft lithographic techniques.[11] Micrometer-scale patterns
a
chain-growth coordination–polymerization mechanism.
These discoveries should facilitate the future development
of the field of poly(phosphinoboranes).
Received: December 12, 2014
Revised: January 29, 2015
Published online: && &&, &&&&
4
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Keywords: dehydrocoupling · dehydropolymerization ·
iron catalysis · phosphine–borane adducts ·
poly(phosphinoboranes)
.
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precatalysts for the dehydrocoupling of secondary phosphine –
borane Ph2PH·BH3 in the melt at 1208C to form the linear dimer
Ph2PH·BH2-Ph2P·BH3. See: K. Lee, T. J. Clark, A. J. Lough, I.
Manners, Dalton Trans. 2008, 2732 – 2740.
[9] D. J. Liston, Y. J. Lee, W. R. Scheidt, C. A. Reed, J. Am. Chem.
Soc. 1989, 111, 6643 – 6648.
[10] a) The efficiency of the catalysis appears to be unaffected by
light, as the polymerization proceeded at a similar rate, whether
the reaction mixture was exposed to or protected from ambient
light. In addition, no polymerization was observed when
a mixture of 1a with 5 mol% of 2b in toluene was photo-
irradiated at 208C for 16 h. b) Although 2b is active for the
catalytic dehydropolymerization of other primary phosphine–
borane adducts, no polymerization reaction was observed for the
secondary phosphine–borane adducts Ph2PH·BH3 and
PhMePH·BH3 (synthesis: O. Metters, unpublished results)
under similar conditions (5 mol% 2b, 1008C, toluene, 24 h).
Full details will be published in due course.
[11] See the Supporting Information for further details.
[12] T. I. Kꢀckmann, F. Dornhaus, M. Bolte, H.-W. Lerner, M. C.
Holthausen, M. Wagner, Eur. J. Inorg. Chem. 2007, 1989 – 2003.
[13] See Ref. [8].
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Communications
[14] The entropic contribution to G298 is often overestimated in DFT
calculations in the gas phase, making DG298 more endergonic
than experimentally observed in solution.
[15] Y. Kawano, M. Hashiva, M. Shimoi, Organometallics 2006, 25,
4420 – 4426.
[16] Performing the reaction at 408C leads to the formation of the
same product with about 70% conversion after 2 weeks, while
performing the reaction at > 508C gave a mixture of different
products.
[17] It is generally assumed that in an ideal chain-growth polymer-
ization, the maximum molecular weight of the polymer is
reached at low conversion, whereas for an ideal step-growth
mechanism a continuous increase of molecular weight over time
would be observed, with high-molecular-weight polymers only
formed at high conversion.
[18] Comprehensive DFT calculations support this mechanism. See
Section 3 in the Supporting Information for further details.
6
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Chemie
Communications
Polymerization
Loading matters: The catalyst loading is
the key to control the molecular weight of
the polymer in the iron-catalyzed dehy-
dropolymerization of phosphine–borane
adducts. Studies showed that the reac-
tion proceeds through a chain-growth
coordination–insertion mechanism.
A. Schꢁfer, T. Jurca, J. Turner, J. R. Vance,
K. Lee, V. A. Du, M. F. Haddow,
G. R. Whittell,
I. Manners*
&&&& — &&&&
Iron-Catalyzed Dehydropolymerization: A
Convenient Route to
Poly(phosphinoboranes) with Molecular-
Weight Control
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!