A π-Conjugated, Covalent Phosphinine Framework

Article abstract of DOI:10.1002/chem.201900281

Structural modularity of polymer frameworks is a key advantage of covalent organic polymers, however, only C, N, O, Si, and S have found their way into their building blocks so far. Here, the toolbox available to polymer and materials chemists is expanded

Full text of DOI:10.1002/chem.201900281

A Journal of
Accepted Article
Title: A π-conjugated, covalent phosphinine framework
Authors: Jieyang Huang, Ján Tarábek, Ranjit Kulkarni, Cui Wang,
Martin Dračínský, Glen J. Smales, Yu Tian, Shijie Ren, Brian
R. Pauw, Ute Resch-Genger, and Michael Janus Bojdys
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A π-conjugated, covalent phosphinine framework
Jieyang Huang,[a, b] Ján Tarábek, Ranjit Kulkarni,
[b]
[a, b]
Cui Wang,[c, f] Martin Dračínský, Glen J.
[b]
[
d]
[e]
[e]
[d]
[c]
[a, b]
Smales, Yu Tian, Shijie Ren, Brian R. Pauw, Ute Resch-Genger, and Michael J. Bojdys*
Abstract: Structural modularity of polymer frameworks is a key
advantage of covalent organic polymers, however, only C, N, O, Si
and S have found their way into their building blocks so far. Here, we
expand the toolbox available to polymer and materials chemists by
porous organic frameworks (POFs),[1] conjugated microporous
polymers (CMPs)[2] and porous aromatic frameworks (PAFs)[3]
have been extensively investigated in the fields of gas and energy
storage,[4] catalysis,[5] sensing,[6] and in various opto-electronic
devices.[7] Incorporation of heteroatoms such as nitrogen, sulphur
and silicon into the backbone of these polymers yields triazine-
based graphitic carbon nitride (TGCN), covalent triazine-based
frameworks (CTFs),[9] sulphur- and nitrogen-containing porous
polymers (SNPs),[10] and silicate organic frameworks (SiCOFs).[11]
This has not only led to recent breakthroughs in our
understanding of how to construct such extended systems, but it
has also uncovered new layers of complexity in material
properties. For example, nitrogen-containing CTFs can be used
one additional nonmetal, phosphorus. Starting with a building block
5
that contains a λ -phosphinine (C
5
P) moiety, we evaluate a number of
[
8]
polymerisation protocols, finally obtaining a π-conjugated, covalent
phosphinine-based framework (CPF-1) via Suzuki-Miyaura coupling.
2
-1
CPF-1 is a weakly porous polymer glass (72.4 m g
2
N BET at 77 K)
with green fluorescence (λmax 546 nm) and extremely high thermal
stability. The polymer catalyzes hydrogen evolution from water under
UV and visible light irradiation without the need for additional co-
-
1
-1
catalyst at a rate of 33.3 μmol h g . Our results demonstrate for the
first time the incorporation of the phosphinine motif into a complex
polymer framework. Phosphinine-based frameworks show promising
electronic and optical properties that might spark future interest in
their applications in light-emitting devices and heterogeneous
catalysis.
as heterogeneous Periana catalysts and enable low-temperature
oxidation of methane to methanol.[
12]
Further, sulphur- and
nitrogen-containing polymers feature intimately linked donor-
acceptor domains and a narrow bandgap that result in record-
breaking performance in photocatalytic water-splitting.[13] Despite
the confusing three-letter nomenclature in this field, CTFs, CMPs
PAFs and SNPs have chiefly in common that they are constructed
from covalently-linked π-conjugated building blocks;[1-3,
electron delocalization in polymer framework can improve
intriguing properties, especially electric storage and conductivity,
9-10]
Introduction
The unique selling point of covalently-linked organic materials –
according to the claims of polymer and materials chemists – is
their LEGO™-like design. That means, that there is hypothetically
an infinite number of organic building blocks (tectons) at our
disposal for their fabrication. However, in reality, only a small
subset of all available nonmetals has been incorporated into
complex polymers, chiefly aromatic carbon. Such materials as
[14]
bandgap properties, photoluminescence and light harvesting.
Strategies to expand the properties and applications of π-
conjugated frameworks can be broadly divided into two
categories; post-synthetic modifications and initial reaction design.
Post-synthetic modifications rely on a number of tools such as
changes of the polymer topology (e.g. by template removal,
[15]
freeze-drying), or various reactions that introduce heteroatoms
into the backbone of the network.[ Although post-synthetic
modifications enable interesting enhancements of material
properties, they are all strongly dependent on initial framework
morphologies (compact or open) and suffer from randomness and
inhomogeneity in the case of diffusion limited processes. Hence,
a more rational pathway to improved, π-conjugated frameworks
is the initial design of building blocks (“tectons”).[1-2, 7, 14, 17] Design
of tectons is the most exciting and promising field for further
exploration, as there hitherto only tectons containing the
16]
[
[
a]
b]
Dr. J. Huang, R. Kulkarni, Dr. M. J. Bojdys
Department of Chemistry, Humboldt-Universität zu Berlin
Brook-Taylor-Str. 2, 12489 Berlin (Germany)
E-mail:m.j.bojdys.02@cantab.net
Dr. J. Huang, Dr. J. Tarábek, R. Kulkarni, Dr. M. Dračínský, Dr. M. J.
Bojdys
Institute of Organic Chemistry and Biochemistry of the CAS
Flemingovo nám. 2, 166 10 Prague (Czech Republic)
C. Wang, Dr. U. Resch-Genger
Bundesanstalt für Materialforschung und -prüfung (BAM)
Unter den Eichen 87, 12205 Berlin (Germany)
Dr. G. J. Smales, Dr. B. R. Pauw
[
[
c]
d]
[9-10, 18]
nonmetals C, N, O and S have been explored in any depth.
All main group elements in first 3 rows of periodic table with
four to six valence electrons can form aromatic rings. Lower
stability may be the reason why Si and P, which are light elements
in the third period of the periodic table and in the same groups as
C and N, have never been introduced into π-conjugated
frameworks.[19] Phosphorus is an outstanding candidate atom to
be incorporated into the backbone of materials for a number of
reasons: (1) P is electron rich with five valence electrons, like its
group Ⅴ neighbor nitrogen, while it has a larger atomic radius with
Division Biophotonics, Federal Institute for Materials Research and
Testing (BAM)
Richard- Willstätter-Straße 11, 12489 Berlin (Germany)
Y. Tian, Dr. S. Ren
College of Polymer Science and Engineering, State Key Laboratory
of Polymer Materials Engineering, Sichuan University
Chengdu 610065 (P. R. China)
[
e]
f]
[
C. Wang
Institute of Chemistry and Biochemistry, Free University of Berlin
Takustrasse 3, D-14195 Berlin (Germany)
weaker electronegativity;[ (2) The polarity of C-P bond makes P
20]
partially positively charged;[
20b]
(3) phosphorus has a wide range
Supporting information and the ORCID identification number(s) for
the author(s)of this article can be found under:
of bonding environments, valence states and coordination
numbers to achieve more probable spatial configurations of
http://dx.doi.org/10.1002/chem.2019xxxxx.
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repeating units;[
20a,
21]
(4) P can increase thermal stability of
synthesized by Märkl in 1966,[27] derivatives of these six-
polymers.[ Moreover, phosphorous-containing molecules are
interesting homogeneous catalysts and luminescent dyes.[23]
A variety of P-containing polymer frameworks or graphitic
materials have been prepared and applied in water treatment,
halogen-free flame retardant, transition metal-catalysis, photo-
catalysis, lithium batteries, gas-selective membranes and
biomedical field.[24] However, none of these P-containing
materials is rationally designed or incorporate phosphorous in an
overall aromatic, stable network.[25] For example, the P-doped
graphitic materials obtained in the past have an entirely random
distribution of phosphorus sites.[20b, 26] Although the effects of P-
doping are real, such statistical functionalizations make it
inherently difficult to rationalize any of the observed phenomena.
22]
membered C
P rings were studied and analyzed for decades.
5
3
Phosphinines, can contain λ -phosphinine (a trivalent phosphorus
atom with a coordination number of 2) and λ -phosphinine (a
5
pentavalent phosphorus atom with a coordination number of 4).
They show superior fluorescent properties and with applications
in organophosphorus chemistry, coordination chemistry,
optoelectronics and homogeneous catalysis.[19b, 25a, 28] Though a
multitude of molecular reactions and their mechanisms are known,
to this day, not one polymer based on phosphinines has been
achieved.[
25a, 29]
We believe this is because of the high, relative
reactivity of the phosphinine P-atom which renders phosphinine
3
incompatible with most polymerisation protocols. For λ -
phosphinine, the typical electrophilic attack or nucleophilic attack
For the rational design of a π-conjugated, P-containing network, is more preferred at the P-atom.[ λ -phosphinine is more stable
30]
5
we chose phosphinine – a C
5
P ring, equivalent to pyridine – as
due to the absence of phosphorus lone pairs, but some strong
acids or bases can still attack the P-atom during some
the principle building block.[19b] Since phosphinine was firstly
Figure 1. (a) Synthetic route for the phosphinine-based tectons 3 and 4; (b) Suzuki-Miyaura coupling polymerization route of tecton 4 to covalent phosphinine-
based framework, CPF-1.
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polymerisations.[25a] Therefore, stable phosphinine-based
monomers and mild polymerisation routes need to be carefully
chosen to achieve P-containing π-conjugated frameworks.
Therefore, we chose Suzuki-Miyaura coupling in a first attempt to
obtain P-containing π-conjugated frameworks, which is described
above and in Figure 1b.
Structure, composition and morphology characterization of CPF-1
Benzene-1,4-diboronic acid, as the most simple, difunctional
monomer which is commercially available, was chosen for
coupling with tecton 4, aiming to form π-conjugated frameworks
5
In this work, we synthesize a stable λ -phosphinine-based
tecton, and evaluate its stability in
a series of common
polymerisation protocols. We identify Pd-catalyzed Suzuki-
Miyaura coupling as the best approach and thus, obtain the first
P-containing
fully-aromatic
covalent
phosphinine-based
with a classical honeycomb-like structure. Suzuki-Miyaura
framework (CPF-1). The structure and properties of CPF-1 were
analyzed in detail with a particular focus on optical and electronic
factors which are unique to the high content of aromatic
phosphorus in the structure.
coupling of tecton 4 yields a brown powder that is insoluble in all
solvents tested. We synthesized a corresponding molecular
5
phosphinine compound 1,1-dimethoxy-2,4,6-tri(4-biphenylyl)-λ -
phosphinine (PMC) by Suzuki-Miyaura coupling between
monomer 4 and phenylboronic acid (Figure S20 in SI) for
comparison. The structure of PMC after purification was
1
31
confirmed by mass, H NMR and P NMR spectra (Figure S21,
Results and Discussion
S22 and 2c). In 31P NMR spectrum of PMC, the signal at δ =
5
[28b]
6
8.22 ppm is characteristic for λ -phosphinine.
31P magic angle spinning (MAS) solid-state NMR (ssNMR) and
C cross polarization magic angle spinning (CP-MAS) ssNMR
Design of phosphinine-based monomers
We design phosphinine-based building blocks with C2v symmetric
bonding axes to achieve fully crosslinked frameworks in analogy
to triazine-based frameworks (CTFs).[9, 31] Since the phosphinine
P-atom influences the reactivity of directly attached substituents
considerably,[29c] we chose to include aryl spacers on which the
1
3
spectra of CPF-1 are shown in Figure 2a and 2b. The two peaks
at δ = 62.30 and 20.30 ppm in the 31P MAS ssNMR spectrum of
5
CPF-1 are assigned to the ring-P of the λ -phosphinine moiety
3 3
and to Ph PO, respectively. Residues of Ph PO are a side-
polymerizable reactive groups are situated. Combining several
3
product of the decomposition of Pd(PPh3) catalyst (see
reports about synthesis of aryl-halide substituted λ -
4
discussion in Section S4, Figure S23, S24, Section S5, Table S4
and Figure S26).
phoshpinines,[
32]
[33]
and the bromophenyl coupling reaction,
we
3
obtain 2,4,6-tri(4-bromophenyl)-λ -phosphinine (3) as the
The 13C NMR spectrum of PMC has characteristic signals at
115.82 ppm (para-C of P), 94.14/93.24 (ortho-C of P) and
51.85 ppm (C in MeO).[35] The other peaks are assigned to
principle P-containing tecton. Since previous reports suggest a
3
high reactivity (and hence low stability) of λ -phosphinine, we
5
further synthesize the corresponding, protected λ -phosphinine
compound,
1,1-dimethoxy-2,4,6-tri(4-bromophenyl)-λ5-
phenyl-carbons next to the signals of benzene-d (127-128 ppm).
6
The peaks at 119.17 and 117.84 ppm in 13C NMR spectrum of
phosphinine (4) as another P-containing tecton.[25a] The synthetic
monomer 4 (Figure S25), which are assigned to halogenated,
aromatic carbons (C-Br), cannot be observed in spectrum of PMC
nor in the spectrum of CPF-1, also corroborating the high yield of
the Suzuki-Miyaura coupling reaction.
routes to monomer 3 and monomer 4 and the subsequent,
successful polymerization of tecton 4 to CPF-1 are described in
detail in Figure 1.
Stability of phosphinine-based monomers under common
polymerization protocols
13C CP-MAS ssNMR spectrum of CPF-1 and its assignment
are shown in Figure 2b. By comparison with 13C NMR spectra of
PMC and monomer 4, it is found that the characteristic peaks for
We have investigated several conventional, mild polymerization
protocols to achieve phosphinine-based polymer networks, via
5
λ -phosphinine are at 121.80, 88.87, 86.34 and 45.87 ppm, while
reactions of bromophenyl, or its derivative groups of
ethynylphenyl and benzonitrile, as shown in Table S1. [9b, 34] Since
the signal of halogenated, aromatic carbon (C-Br) cannot be
detected.
3
λ -phosphinine in particular is very reactive, we evaluate whether
The combustion elemental analysis (EA) and inductively
coupled plasma - optical emission spectrometry (ICP-OES)
results reveal that the contents of Pd (0.19 wt%), Br (1.32 wt%)
and B (0.01 wt%) are quite low in CPF-1, and the mass ratio of
P:H =1.176 matches with the calculated ratio of 1.178 accurately
the solvent or catalyst attack the ring-phosphorus during the
polymerization reaction. After setting up the polymerization of 3 or
4
under conditions described in Table S1 the stability of the
3
1
phosphinine moiety was checked periodically via P NMR
spectroscopy. The stabilities of the phosphinine moieties 3 and 4
under different reaction conditions are listed in Table S1 and
discussed in more detail in the SI (section S3).
(
Section S5 and Table S3). Further, the ICP-OES results show a
mass ratio of C:P in as-received polymer sample of 12.96 which
is close to the theoretical value of 13.20 for perfect CPF-1. The
low content of Pd, Br and B in particular suggests that only few
unreacted end-groups remain after the polymerization and
purification steps, and that the observed residuals are trapped
within the polymer matrix rather than regularly built into the
framework. The FTIR spectra of monomer 4, PMC, and CPF-1
According to Table S1 and S2, the phosphinine ring is attacked
and decomposes under most classical polymerization conditions.
3
Especially for the λ -phosphinine compound, we were unable to
3
find a compatible polymerization protocol. After λ -phosphinine is
5
protected by MeO-groups, the λ -phosphinine compound is stable
3 4 2 3
in the presence of Pd(PPh ) and K CO , but not stable under
-
are shown in Figure 3a. The bands ranging from 2930 to 2830 cm
alkaline conditions.
Even though the options for polymerization are limited,
protocols such as Suzuki-Miyaura or coupling remain viable.
1
are assigned to C-H stretching vibrations of MeO,[36] and the
bands between 1005 and 1015 cm-1 are assigned to C-O / P-O
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Figure 2. (a) 31P magic angle spinning (MAS) solid-state NMR (ssNMR) spectrum of CPF-1; (b) 13C CP-MAS ssNMR spectrum of CPF-1; (c) 31P-NMR spectrum of
13
6 6
PMC in benzene-d ; (d) C NMR spectrum of PMC in benzene-d .
5
[37]
stretching vibrations of the λ -phosphinine (P-O-C).
These
5
absorption bands appear in all FTIR spectra including the λ -
phosphinine containing CPF-1 structure.
The powder X-ray diffraction (PXRD) patterns of CPF-1 in
Figure 3b shows only a glassy polymer profile, which result is
consistent with previous reports of amorphous polymer networks
polymerized by Suzuki-Miyaura coupling.[38] Transmission
electron microscopy (TEM) images and corresponding selected
area diffraction (SAED) patterns (Figure 3d and 4e) corroborate
amorphous, “cauliflower”-like aggregates with sizes ranging from
0.7 to 1.5 μm with no discernible internal structure, which says
two-fold: (1) the “cauliflower”-like aggregates of CPF-1 are
indicative for point-nucleation growth, and (2) residual inorganics
are dispersed in atomic (rather than macroscopic) aggregates
throughout the polymer network.[9, 31] SEM analysis (Figure 3c)
indicates, Energy-dispersive X-ray (EDX) results (Figure S27 and
Table S5) match with the composition data obtained by EA/ICP-
OES.
A nitrogen sorption isotherm obtained at 77 K for CPF-1 is seen
in Figure S28a, revealing only a weak porosity nature by
Brunauer–Emmett–Teller (BET) analysis with a surface area of
2
-1
7
2.4 m g . The surface area of CPF-1 is comparable to
structurally analogous, carbon-only conjugated microporous
polymers (CMPs).[38] NL-DFT pore size distribution analysis
shows predominantly pore sizes ranging between 2 to 9 nm
(
Figure S28b). These pores are also evident by analysis of small
Figure 3. (a) FTIR spectra of monomer 4, PMC and CPF-1; (b) PXRD pattern of
CPF-1; Electron microscopic investigation of CPF-1: (c) SEM image; (d) TEM
image; (e) SAED image.
angle X-ray scattering (SAXS) data patterns (Figure S29). This
data suggests that there are some pores with average radii of
3.156 nm ranging from 1 to 5 nm – much smaller than any of the
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particles observed in SEM surveys. Compared with the BET pore
size distribution results, the SAXS confirms the presence of
micropores. Thermogravimetric analysis (TGA) data shows that
CPF-1 is stable up to 350 °C under air and up to 700 °C under
nitrogen; in fact, under non-oxidative conditions up to 67% of the
mass are retained even at 1000 °C (Figure S30). This is an
extraordinarily high thermal and oxidative stability outperforming
any of the S- and N-containing polymer frameworks. [9-10]
and stable phosphinine-based framework was successfully
prepared, although some catalyst residues remain trapped in the
pore structure.
Electronic and optical properties of CPF-1
We find that although CPF-1 is π-conjugated, it is not very
conductive in macroscopic, pressed-pellet form with conductivity
values of 1.16 × 10- S cm (see Section S6). Hence, any new,
electronic effects will be found on a local, microscopic level. We
see from 13C CP-MAS ssNMR spectroscopy that CPF-1 has a
relatively low signal-to-noise ratio even at increased MAS rates
12
-1
The comprehensive structure characterization of CPF-1, as
well as monomer 4 and PMC, demonstrates that a π-conjugated
Figure 4. EPR spectra of the polycrystalline (powder) samples of (a) monomer 4 and (b) CPF-1 together with simulations of their corresponding radical cation; (c, d)
Solid-state UV-vis diffuse-reflectance spectrum of CPF-1; (e) solid-state photoluminescence emission spectrum (λexc = 400 nm) and solid-state photoluminescence
excitation spectrum (λem = 560 nm) of CPF-1; (f) Images of CPF-1 dispersed in various solvents and monomer 4 illumination with UV and visible light.
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up to a maximum of 20 kHz, long contact times (τ = 10 ms) and
acquisition times (34.4 ms). λ -phosphinine can lose an electron
photocatalytic hydrogen evolution. For aerobic oxidation
experiments, we found that CPF-1 is unable to activate the
aerobic oxidation of benzyl alcohol in acetonitrile at 80 °C even
after 120 h (Section S7). Similarly, photocatalytic hydrogen
evolution under visible light (wavelength 380 to 780 nm) with
triethanolamine as the sacrificial agent and 3wt% of Pt co-catalyst
5
and be oxidized to a radical cation at a lower oxidation potential
than the λ3-phosphinine.[39] We probe this by electron
paramagnetic resonance (EPR) spectroscopy of monomer 4 and
CPF-1. Relevant DFT computations and simulations of EPR
5
-1 -1
spectra for the λ -phosphinine radical cation are discussed in
yields hydrogen evolution rates lower than 1 μmol h g , below
Section S6. EPR spectra of paramagnetic monomer 4 and
polymer CPF-1 are shown in Figure 4a and 4b together with the
simulations of their corresponding radical cation. Both spectra
display rhombic symmetry (simulation parameters are
summarized in Table S6) showing that radical cationic centers
within the polymer as well as within monomer do not possess a
well-defined symmetry of the paramagnetic centers. DFT
calculation results indicate that maximum electron density is
indeed situated on the P-atom ring (see Table S6 and S7 for
hyperfine coupling constants and Table S8 for spin population).
Taking into account the DFT calculations, simulation of the EPR
spectra and DFT computed NBO spin population analysis
the detection limit. Hydrogen evolution under UV and visible light
(300 to 2500 nm) is shown in Figure S34. Here, the hydrogen
evolution rate of 29.3 μmol h-1 g-1 is reached using 3 wt% of Pt co-
catalyst. Hydrogen evolution rates stayed roughly constant at
33.3 μmol h-1 g-1 when no Pt co-catalyst was added. We assume
that residual palladium from the network-forming reaction
(0.19 wt% by ICP-OES; Table S3) acts as a co-catalyst in this
case.
Conclusions
(
Section S6 and Table S8), the EPR spectra confirm that the
In summary, we have synthesized an extended, π-conjugated,
covalent phosphinine-based framework (CPF-1), and we
addressed two gaps in research: (1) since phosphinine was first
synthesized in 1966, we have achieved for the first-time a
successful incorporation of the six-membered phosphinine ring
into a polymer via the Suzuki-Miyamura protocol; (2) we have
successfully expanded the family of potential nonmetal-containing
tectons available for the rational design of π-conjugated
frameworks to C, N, O, S and now P. Two new Br-functionalized
highest spin density is localized on the central phosphinine moiety
for both the tecton 4 and CPF-1. The results match some reports
which state that there are radicals localized on the central λ -
phosphinine moiety.[39-40] The radicals remain localized on the
ring-phosphorus even after formation of the CPF-1 network,
leading to relative broad ssNMR signals with low signal-to-noise
ratio. Furthermore, EPR results show that the Suzuki-Miyaura
coupling reaction does not change the environment of the λ -
phosphinine moieties.
According to diffuse reflectance UV-vis spectroscopy, CPF-1
has a discernible absorption edge at around 620 nm (Figure 4c).
According to the Kubelka-Munk function this equates to a direct
optical band gap of 2.19 eV and an indirect optical band gap of
5
5
3
5
monomers containing λ -phosphinine and λ -phosphinine were
obtained in the process and evaluated as building blocks in
conventional polymerization protocols. Suzuki-Miyaura coupling
was chosen as a conservative polymerization route with no side-
5
reactions on the central P-atom of the λ -phosphinine tecton.
1.64 eV (Figure 4d). Photoluminescence (PL) spectroscopy of
We found CPF-1 not only shows remarkable thermal stability
CPF-1 shows one prominent peak observed at excitation
transition energy of 2.27 eV, corresponding to a wavelength of
(
under inert atmosphere) but also retains the green fluorescence
of its phosphinine building blocks, guiding towards potential
application of CPF-1 as OLEDs. As this is a first step in the
investigation of the phosphinine-based polymers, it becomes
clear that highly-efficient catalysis, and potential electronic
applications will require a reliable preparation protocol of λ -
phosphinine based polymers in the future.
5
46 nm and green fluorescence (Figure 4e). Figure 4f shows the
color of CPF-1 and tecton 4 under visible light and under UV light
exc = 365 nm). Because of the electronic interactions between
(
λ
conjugated polymer chains, the fluorescence efficiency of CPF-1
decreases dramatically.[38] When CPF-1 is dispersed in organic
solvents, especially in THF, it shows again green fluorescence
under UV light illumination (λexc = 365 nm), while monomer 4
shows extraordinarily intense green fluorescence.[28a]
3
[42]
Experimental Section
Under 365 nm UV light illumination the absolutely measured
quantum yield (QY) of monomer 4 is 43.2% (at 78.8% absorption)
in the solid-state (ss). CPF-1 only has a QY in the solid state of
less than 0.01% (at 81.9% absorption). Dispersed in THF, CPF-1
reveals a QY of 19.0% (at 54.3% absorption), while the QY of
tecton 4 is 81.9% (at 28.2% absorption) (Figure S33). Hence, the
fluorescence of CPF-1 is a consequence of the incorporation of
Synthesis of phosphinine-based precursors: The synthesis of the
precursor materials 1,3-Bis(4-bromophenyl)prop-2-en-1-one (1) and 2,4,6-
tri(4-bromophenyl)pyrylium tetrafluoroborate (2) is given in the Supporting
Information (Section S1).
3
Synthesis of 2,4,6-tri(4-bromophenyl)-λ -phosphinine (3): The
preparation procedure is a modified method based on previous reports for
phosphinine compounds.[28b, 32] Compound 2 (5.21 g, 8.20 mmol) and
5
the λ -phosphinine moiety into the polymer backbone. This
fluorescence is diminished in the solid state by π-π stacking
induced quenching, as observed in other π-conjugated, layered
polymer systems.[
4
0 mL anhydrous toluene were added to a 100 mL one-neck flask under
inert atmosphere in a glovebox. P(SiMe (2.06 g, 8.20 mmol) was added
3 3
)
28a, 41]
dropwise to the flask under continuous stirring. The flask was connected
to argon atmosphere on a Schlenk line, and the mixture was stirred and
refluxed for 18 h. After volatiles were removed under vacuum, the residue
Finally, we tried to utilize CPF-1 for some common catalytic
applications such as aerobic oxidation of benzyl alcohol and
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was purified by column chromatography on silica gel (eluent
Dr. Glen J. Smales0000-0002-8654-9867
Yu Tian 0000-0002-7888-1421
Dr. Shijie Ren 0000-0002-7562-8666
Dr. Brian R. Pauw 0000-0002-8605-838X
Dr. Ute Resch-Genger 0000-0002-0944-1115
Dr. Michael J. Bojdys 0000-0002-2592-4168
hexane/CH
2
Cl
2
= 9/1) to give 3.10 g (5.52 mmol for C23
H
14Br
3
P, 67.3%
yield) of 2,4,6-tri(4-bromophenyl)-λ -phosphinine (3) as a white solid. 1H
3
NMR (400 MHz, (CD
.85-7.78 (4H, m), 7.78-7.70 (6H, m) (see Figure S4). P NMR (162 MHz,
CD
CO): 186.27 (see Figure S5). Note: 2,4,6-tri(4-bromophenyl)-λ3-
phosphinine is not stable in CDCl
3 2
) CO): 8.63 (2H, d, J = 6.0 Hz), 7.91-7.86 (2H, m),
31
7
(
3 2
)
3
.
5
Synthesis of 1,1-dimethoxy-2,4,6-tri(4-bromophenyl)-λ -phosphinine
4): The preparation procedure was adapted from K. Dimruth et al.[43]
Compound 3 (3.10 g, 5.52 mmol), mercury(II) acetate (1.76 g, 5.53 mmol),
50 mL anhydrous MeOH and 150 mL anhydrous toluene were stirred
(
1
under argon atmosphere at room temperature for 12 h. After the liquid was
removed under vacuum, the residue was purified by column
chromatography on silica gel (eluent hexane/CH
2 2
Cl = 8/2) to give 2.40 g
Acknowledgements
(3.85 mmol) crude product. The crude product was recrystallized from
acetonitrile overnight to form green/yellow product 1,1-dimethoxy-2,4,6-
5
J.H. carried out the synthetic experiments, analyzed the data and
wrote the paper. J.T. carried out the EPR measurements and
analysis, EPR simulation and DFT computation. R.K. carried out
the BET measurements and analysis, and the aerobic oxidation
experiments. C.W. and U.R. carried out the optical properties
measurements and analysis. M.D. carried out the ss-NMR
measurements and NMR analysis. G.J.S. and B.R.P. carried out
the SAXS-WAXS measurements and analysis. Y.T. and S.R.
carried out the hydrogen evolution experiments and analysis.
M.J.B conceived the project and wrote the paper.
tri(4-bromophenyl)-λ -phosphinine (4) 2.06 g (3.31 mmol for C25
H
20Br
3
O
2
P,
CO): 8.00 (2H, d, J = 36.6 Hz),
.64-7.57 (8H, m), 7.57-7.44 (4H, m), 3.50 (6H, d, J = 13.8 Hz) (see Figure
CO): 66.44 (see Figure S7). MS (APCI):
P] 620.88238, found 620.88248 (see Figure
6
7
0.0% yield). 1H NMR (400 MHz, (CD
)
3 2
S6). 31P NMR (162 MHz, (CD
3
)
2
21 2 3
calculated for [C25H O Br
S8).
Synthesis of π-conjugated covalent phosphinine-based framework
(
CPF-1): Polymerization was achieved with a palladium-catalyzed Suzuki-
Miyaura coupling process.[34c, 38, 44]
Monomer 4 (732 mg, 1.17 mmol),
catalyst
102 mg, 0.0880 mmol) were added to a 250 mL three-neck flask and
3 4
benzene-1,4-diboronic acid (292 mg, 1.76 mmol) and Pd(PPh )
(
purged under argon. 80 mL 1,4-dioxane was degassed by argon bubbling
for 20 min and then added to the flask under continuous stirring. 12.5 mL
aqueous K CO solution were degassed by argon bubbling for 20 min and
2 3
then added to the flask. The mixture was degassed by argon bubbling for
a further 30 min, and subsequently stirred and refluxed under argon
atmosphere for 80 h. The precipitate was collected by filtration and washed
2
We thank Barbora Balcarova for N sorption measurements, Dr.
Lucie Bednárová for FTIR spectroscopy, Dr. Radek Pohl for NMR
measurements, Romana Hadravová for TEM measurements,
Stanislava Matějková for EA/ICP-OES, Dr. Yu Noda for
conductivity measurements, Yi Xiong for graphical abstract
design, Pengbo Lyu and Prof. Dr. Petr Nachtigall for of DFT
with THF, CHCl
out by Soxhlet extraction for two days using THF and MeOH. The product
was washed with 200 mL water at 90 °C three times to remove K CO , and
the brown powder was finally dried at 120 °C for 12 h in vacuum to afford
CPF-1 0.4558 g (0.458 mmol for C68 , 78.3% yield). Calculated for
CPF-1: C, 82.07; H, 5.28; O, 6.43; P, 6.22. Found: C, 73.47; H, 4.82; P,
.67; K, 0.3; Pd, 0.19; Br, 1.32; B, 0.01.
3
, hot water and MeOH, and further purification was carried
calculated EPR parameters.
M.J.B. thanks the European
2
3
Research Council (ERC) for funding under the Starting Grant
Scheme (BEGMAT-678462).
52 4 2
H O P
5
Conflictof interest
The authors declare no conflict of interest.
AUTHOR INFORMATION
Corresponding Author
Keywords: phosphinine • π-conjugated frameworks • Suzuki-
Miyaura coupling • polymers • fluorescence
*
E-mail: m.j.bojdys.02@cantab.net
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Chemistry - A European Journal
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Phosphinine in π: For the first time
since its synthesis in 1966, the six-
membered phosphinine ring is
incorporated into a π-conjugated
polymer. The phosphinine-based
framework (CPF-1) has a high
concentration of paramagnetic
species, high thermal stability, green
fluorescence and shows activity in
photocatalytic splitting of water.
Jieyang Huang, Ján Tarábek, Ranjit
Kulkarni, Cui Wang, Martin Dračínský,
Glen J. Smales, Yu Tian, Shijie Ren,
Brian R. Pauw, Ute Resch-Genger,
and Michael J. Bojdys*
(
(Insert TOC Graphic here: max.
Page No. – Page No.
width: 5.5 cm; max. height: 5.0 cm))
A π-conjugated, covalent
phosphinine framework
This article is protected by copyright. All rights reserved.