Anionic polymerisation of phosphaalkenes bearing polyaromatic chromophores: Phosphine polymers showing "turn-on" emission selectively with peroxide

Article abstract of DOI:10.1039/c4sc02133a

Three new phosphaalkenes bearing C-aryl chromophores, MesPCPh(Ar) (1a: Ar = 1-naphthyl, 1b: 9-phenanthryl, 1c: 5-dibenzosuberenyl; Mes = 2,4,6-trimethylphenyl), are reported. Each phosphaalkene was characterised by multinuclear NMR spectroscopy (¹

Full text of DOI:10.1039/c4sc02133a

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Anionic polymerisation of phosphaalkenes bearing
polyaromatic chromophores: phosphine polymers
showing “turn-on” emission selectively with
peroxide†
Cite this: Chem. Sci., 2014, 5, 4928
*
Benjamin W. Rawe, Cindy P. Chun and Derek P. Gates
Three new phosphaalkenes bearing C-aryl chromophores, MesP]CPh(Ar) (1a: Ar ¼ 1-naphthyl, 1b: 9-
phenanthryl, 1c: 5-dibenzosuberenyl; Mes ¼ 2,4,6-trimethylphenyl), are reported. Each phosphaalkene
was characterised by multinuclear NMR spectroscopy (1H, 13C, 31P), X-ray crystallography, mass
spectrometry, UV/Visible spectroscopy and elemental microanalysis (1b and 1c). Monomers 1a and 1b
were successfully polymerised using anionic methods of initiation (n-BuLi: 5 and 1.5 mol%, respectively)
to afford poly(methylenephosphine)s (PMPs: 2a: Mn ¼ 15 100 Da, PDI ¼ 1.14; 2b: Mn ¼ 17 500 Da; PDI ¼
1.39). Detailed NMR spectroscopic analysis of polymer 2a revealed that its microstructure is primarily
composed of [P(CHPhNaph)–(4,6-Me2C6H2)–2-CH2]n units rather than the expected [P(Mes)–CPh2]n.
Functional polymer 2a was oxidised to the phosphine oxide 2a$O or phosphine sulfide 2a$S, and
coordinated to borane to afford 2a$BH3 or gold(I) to afford 2a$AuCl. Significantly, 2a shows “turn-on”
emission properties selectively as its oxide 2a$O (lex ¼ 286 nm; lem ¼ 342 nm) which is prepared by
treatment with hydrogen peroxide.
Received 18th July 2014
Accepted 25th August 2014
DOI: 10.1039/c4sc02133a
www.rsc.org/chemicalscience
multiple bonds. For more than three decades, the synthesis and
reactivity of compounds featuring genuine (p–p)p bonds
Introduction
involving the heavier p-block elements has been a central theme
within main group chemistry.8 Phosphaalkenes, which contain
a (2p–3p)p bond, represent one of the most well-studied classes
of unsaturated main group compounds and remain an
intensely active area of research, particularly in the areas of p-
conjugation and ligand design.9,10
Of particular interest is the close analogy between the
molecular chemistry of the P]C bond of phosphaalkenes and
the C]C bonds of olens.11,12 Over the past decade, we have
been interested in expanding the P]C/C]C analogy to polymer
science (Scheme 1).13 To this end, we reported the rst addition
polymerisation of a heavy element multiple bond in 2003.14
Namely, MesP]CPh2 was polymerised in the melt to afford
poly(methylenephosphine) (PMP), a new class of chemically
functional phosphorus-containing macromolecule. The poly-
merisation of MesP]CPh2 can be accomplished by employing
either radical or anionic methods of initiation. Radical-initiated
copolymerisation of MesP]CPh2 with styrene permits the
Macromolecules with phosphorus atoms in the main chain are
of broad interest due to their unique properties, chemical
functionality and the prospect of nding specialty applications.1
Beyond the ubiquitous and important polyphosphazenes,2
several new classes of phosphorus-containing polymers have
attracted considerable recent attention, such as: ferrocene and
related Cp-iron-containing phosphorus polymers;3 p-conju-
gated phosphole-containing polymers;4 polyphosphines with
arylene and/or vinylene spacers;5 polyphosphinoboranes;6 and
p-conjugated phospha- and diphospha-PPVs with P]C or P]P
bonds in the main chain.7 Despite this growth in phosphorus
polymer chemistry, the incorporation of phosphorus atoms into
long chains still presents a formidable synthetic barrier. For
example, ring-opening routes require access to suitably strained
rings whereas commonly used step growth methods require
high purity monomers in precise stoichiometries to achieve
reasonable molecular weights.
Perhaps surprisingly, addition polymerisation, being the
most general and widespread method of organic polymer
synthesis, has been largely unexplored for hetero-element
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver,
V6T 1Z1, British Columbia, Canada. E-mail: dgates@chem.ubc.ca
† CCDC 1013549–1013551. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4sc02133a
Scheme 1 The phosphorus–carbon analogy between the polymeri-
sation of olefins and phosphaalkenes.
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development of randomly enchained copolymers that can be quantitatively [1a: d ¼ 238 (30%, E), 250 (70%, Z); 1b: d ¼ 238
employed at catalyst-supports.15 In contrast, the anionic poly- (40%, E), 252 (60%, Z); 1c: d ¼ 249].
merisation of MesP]CPh2 can be performed in a “living”
The rst step in the synthesis of 1a and 1b involves treating a
fashion and provides access to block copolymers that self- THF solution of MesP(SiMe3)2 with MeLi (1 equiv.) in Et2O to
assemble in block selective solvents.16–18
afford MesP(SiMe3)Li (dP ¼ ꢀ187). Subsequent addition of the
The versatile chemical functionality of the P(III) moiety corresponding ketone to the Et2O solution of MesP(SiMe3)Li at
within PMPs, which can be oxidised to P(V) or coordinated to ꢀ78 ꢁC affords a mixture of E/Z phosphaalkene with the Z-
metals or Lewis acids, is of considerable interest.19,20 Molecular isomer corresponding to the major product. Interestingly,
phosphines bearing p-conjugated chromophores are widely recrystallisation of the crude phosphaalkenes revealed that the
used as uorescent probes since they exhibit “turn-on” emis- minor stereoisomer in solution (E-1a) crystallised preferentially
sion upon oxidation or coordination.21 Therefore, we hypoth- as determined by 31P NMR spectroscopy and X-ray crystallog-
esised that similar behaviour may be observed for PMPs to raphy (vide infra). In contrast, the major Z-isomer of the phe-
provide a sensory-type behaviour for these novel materials.
nanthryl phosphaalkene 1b was crystallised exclusively. In
Herein, we report the synthesis and full characterisation of addition to 31P NMR and X-ray crystallography, both new
phosphaalkenes 1a–c bearing aromatic chromophores and the phosphaalkenes were characterised by 1H and 13C{1H} NMR
anionic polymerisation of two of these monomers to afford spectroscopy, mass spectrometry and elemental analysis (1b
PMPs 2a and 2b. The fact that anionic polymer 2a adopts an only). A very slow E/Z-isomerisation is observed by 31P NMR
alternative microstructure that involves an addition–isomer- spectroscopy when crystals of pure E-1a or Z-1b are dissolved
isation mechanism similar to that reported recently for the in CDCl3 and their spectra recorded daily (Fig. 1). The inter-
radical polymerisation of MesP]CPh2 is of particular signi- conversion of E- and Z-isomers in solution is commonly
cance.22 Remarkably, naphthyl-substituted 2a exhibits “turn- observed for phosphaalkenes, however it oen requires UV
on” emission selectively towards peroxide oxidation.
irradiation.23,26–29
Due to the potential reactivity of the C]C bond in 5-diben-
zosuberenone, the milder base-catalysed phospha-Peterson
reaction was employed to prepare 1c. To a stoichiometric
mixture of MesP(SiMe3)2 and 5-dibenzosuberenone in THF
solvent was added KOH (10 mol%). The reaction mixture was
monitored by 31P NMR spectroscopy and, over a period of two
days, the signal assigned to MesP(TMS)2 (d ¼ ꢀ161) diminished
in intensity in favour of a new signal assigned to the desired
phosphaalkene 1c (d ¼ 249). Yellow crystals of compound 1c
were isolated from a saturated hexanes solution and the identity
of the product was conrmed by X-ray crystallography (vide
infra). Additional evidence of the identity of 1c was provided by
1H and 13C{1H} NMR spectroscopy, mass spectrometry and
elemental analysis.
Results and discussion
Synthesis of phosphaalkenes (1a–1c)
New phosphaalkenes bearing p-conjugated C-substituents such
as 1-napthyl, 9-phenanthryl and 5-dibenzosuberenyl were tar-
geted for the present study. We10e,23 and others24,25 have exploi-
ted the phospha-Peterson reaction as a general and convenient
route to phosphaalkenes from silylphosphines and ketones.
The P]C bond is formed by reaction of the ketone either
directly with a lithium silylarylphosphide or indirectly with a
mixture of RP(SiMe3)2 and a base catalyst (e.g. KOH). The milder
base-catalysed method was suitable for the preparation of 1c
whilst the standard phospha-Peterson reaction was employed to
access 1a and 1b due the sluggish rate of the base-catalysed
version (>2 weeks to completion) (Scheme 2). In each case, 31P
NMR spectroscopic analysis of the reaction mixture suggested
that the desired phosphaalkenes were formed cleanly and
Fig. 1 31P NMR spectra (CDCl3, 121.5 MHz, 298 K) showing the iso-
merisation of pure E-1a to a mixture of E/Z-1a over a period of 14 d.
Scheme 2 Synthetic route to phosphaalkenes 1a–c.
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molecules in the solid state (Fig. 3). A similar intermolecular
arrangement has also been observed in the crystal structure of
phenanthrene itself.34
Structural and electronic properties of phosphaalkenes
The molecular structures of phosphaalkenes 1a–c are shown in
Fig. 2 with selected metrical parameters being given in Table 1.
Selected data collection parameters, cell constants and rene-
ment details are given in Table 2. The P]C bond lengths in 1a–c
In 1a, the naphthyl and phenyl moieties are twisted out of
the plane of the P]C bond by 59.3ꢁ and 41.8ꢁ, respectively.
Similarly, 5-dibenzosuberene-substituted 1c shows signicant
twisting of the C-aryl moieties with respect to the plane of the
P]C bond and one another. For instance, the angles between
the P]C bond plane and the cis and trans C-aryl moieties are
53.6ꢁ and 55.7ꢁ, respectively. In addition, the seven-membered
suberene ring is not planar with the benzo moieties lying on two
different planes twisted by 41ꢁ with respect to one another. In
the Mes*P]C(C14H10) analogue,35 this angle is signicantly
larger (49ꢁ) whilst in 5-dibenzosuberenone the analogous angle
is 37ꢁ.36 These data for 1b and 1c are consistent with moderate
p-conjugation between the C-substituents and the P]C bond.
To gain further insight into the p-conjugative effects in 1a–
1c, the electronic properties of these new phosphaalkenes were
investigated by UV/Vis spectroscopy. The absorption spectra are
shown in Fig. 4. Emission spectra were also recorded but no
emission was observed for any of the monomers. Remarkably,
each of the absorption spectra show quite similar features with
one fairly sharp band (lmax: 298 nm, 1a; 302 nm, 1b; 294 nm, 1c)
that overlaps with a broad lower energy band (ca. 330–360 nm).
Noting that the extinction coefficients (>104 L molꢀ1 cmꢀ1) for
the sharper band in 1a–1c are signicantly greater than those of
the substituents alone (e.g. naphthalene, phenanthrene or
derivatives: 103–104 L molꢀ1 cmꢀ1),37 these bands are most
likely attributable mainly to the p–p* and n–p* transitions
involving the P]C bond. For comparison, the absorption
spectrum of MesP]CPh2 was also recorded and shows bands at
285 nm and 326 nm which are similar to those reported previ-
ously by Bickelhaupt.31 It is noteworthy that the absorbance
maxima in phosphaalkenes undergo a red shi of the high
energy band in the order MesP]CPh2, 1a and 1b. This trend is
perhaps reective of the increasing p-conjugation between the
˚
[avg. 1.691(4) A] are on the longer side of that expected for
30
˚
C-substituted phosphaalkenes (1.61–1.71 A), but are charac-
teristic of phosphaalkenes bearing P-Mes substituents
(1.67–1.70 A).23,31,32 In contrast, the P–CMes bonds [avg. 1.829(4)
˚
˚
A] lie at the shorter end of the range typical of P–C single bonds
33
˚
(1.85–1.90 A). We have previously suggested that the slight
elongation of the P]C bond accompanied by the shortening of
the P–CMes bond suggests a degree of p-conjugation between
the Mes group and the P]C bonds.23 The C–Ctrans and C–Ccis
˚
bond lengths in 1a–1c (ca. 1.49 A) are shorter than typical C–C
single bonds (1.54 A) but are typical of Csp2–CAr (1.48 A) bonds,
perhaps also reecting p-conjugative effects between the P]C
bond and the C-substituents. Of course, more insight into the
nature of p-conjugation within these phosphaalkenes may be
gathered by evaluation of the angles between the planes of the
substituents and that of the P]C bond.
33
˚
˚
In P-Mes phosphaalkenes, the angle between the P]C bond
and the Mes group are consistently ca. 71ꢁ.23 It is therefore
striking that the present systems, bearing large aromatic
C-substituents, show signicantly different angles between the
plane of the P]C bond and the Mes substituent (E-1a: 64.8ꢁ,
Z-1b: 85.3, 1c: 66.4). Although this may simply be due to crystal
packing effects, it is consistent with a higher degree of
p-conjugation between Mes and the P]C bond in E-1a and 1c
and a lower degree of conjugation in Z-1b. Interestingly, the cis-
congured 9-phenanthryl moiety in Z-1b is similarly bent out of
the best plane of the P]C bond by 63.2ꢁ. This is accompanied
by smaller than usual :CMes–P]C and :P]CꢀCcis angles as
the mesityl and phenanthryl substituents seemingly bend
towards one another. In addition, there are clear intermolecular
interactions between the phenanthryl moieties of adjacent Z-1b
Fig. 2 Molecular structure of E-1a (a), Z-1b (b) and 1c (c) (thermal ellipsoids are displayed at 50% probability level). Hydrogen atoms are omitted
for clarity. Selected distances (A˚) and angles (ꢁ): E-1a: P1–C10 1.6876(15), P1–C1 1.8307(16), C10–C17 1.4933(19), P1–C10–C11 129.23(10), P1–
C10–C17 114.15(10), C1–P1–C10 108.36(7); Z-1b: P1–C10 1.693(2), P1–C1 1.827(2), C10–C17 1.482(3), P1–C10–C11 117.7(1), P1–C10–C17
123.9(1), C1–P1–C10 105.76(9); 1c: P1–C10 1.691(3), P1–C1 1.829(3), C10–C11 1.492(4), C10–C24 1.476(4), C17–C18 1.330(5), P1–C10–C11
114.4(2), P1–C10–C24 126.6(2), C1–P1–C10 108.5(2).
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Table 1 Selected metrical parameters for phosphaalkenes bearing P-Mes and C–Ar substituents
Compound
E-1a
Z-1b
1c
MesP]CPh2
MesP]CPh2
˚
Bond lengths (A)
P]C
P–CMes
C–Ctrans
C–Ccis
1.688(2)
1.831(2)
1.493(2)
1.482(2)
1.693(2)
1.827(2)
1.483(3)
1.482(3)
1.691(3)
1.829(3)
1.492(4)
1.476(4)
1.692(3)
1.828(3)
1.491(5)
1.487(4)
1.693(2)
1.830(2)
1.493(5)
1.489(2)
Bond angles (ꢁ)
:CMes–P]C
:P]C–Ctrans
:P]C–Ccis
108.37(7)
114.2(1)
129.2(1)
116.6(1)
105.76(9)
117.7(1)
123.9(1)
118.1(2)
108.5(2)
114.4(2)
126.6(2)
119.0(3)
107.5(2)
116.2(2)
127.2(2)
116.6(2)
107.6(2)
118.0(2)
124.8(2)
117.1(3)
:Ccis–C–Ctrans
Angles between planesa (ꢁ)
Mes
Artrans
Arcis
64.79
59.33
41.82
This work
85.33
20.45
63.16
This work
66.41
55.71
53.61
This work
71
72.2
21.4
59.2
32
36.6
42.9
31
Ref.
a
The angle between the mean plane of the specied aryl ring atoms to the best plane of CMes–P]C–(Ctrans)(Ccis).
Table 2 X-ray crystallographic data for E-1a, Z-1b and 1c
E-1a
Z-1b
1c
Empirical formula
FW
C
26H23
P
C30H25
416.47
P
C24H21
340.38
P
366.41
Crystal system
Space group
Monoclinic Monoclinic
Monoclinic
P21/c
7.6281(5)
29.8493(19)
8.0370(5)
90
P21/n
C2/c
˚
a [A]
12.054(5)
9.298(5)
18.724(5)
90
19.514(2)
13.3010(14)
20.028(2)
90
˚
b [A]
˚
c [A]
a [ꢁ]
b [ꢁ]
g [ꢁ]
99.348(5)
90
119.5190(10) 95.421(4)
90
90
3
˚
V [A ]
2070.7(15)
173(2)
4
4523.6(8)
173(2)
8
1821.79
173(2)
4
T [K]
Z
F (000)
776
1760
720
No. of reections collected 13 929
47 661
1.054
0.0488
0.1372
1013551
2381
1.198
0.056
0.137
1013549
Goodness of t on F2
R1 [I > 2s(I)]a
wR2 [all data]b
CCDC
1.01
0.042
0.104
1013550
a
b
2
R1 ¼ S||Fo| ꢀ |Fc||/S|Fo|. wR2(F2[all data]) ¼ {S[w(Fo ꢀ Fc2)2]/
S[w(Fo2)2]}1/2
.
Fig. 3 A portion of the crystal structure of Z-1b showing intermo-
lecular distances between the planes of 9-phenanthryl moieties of
adjacent molecules.
P]C bond and the aryl substituents as they increase in size (i.e.
Ph, Naph, Phen). Similar p-conjugative effects on UV/Vis
absorption spectra have been observed with related aryl- or
alkynyl-substituted systems.7a–d,9b,9f
initiator to the pale yellow monomer solution resulted in a deep
red solution analogous to that observed for MesP]CPh2 poly-
merisation. Aer 5 d, the 31P NMR spectrum of the reaction
mixture revealed that signals assigned to E/Z-1a were replaced
by a broad resonance at ꢀ9 ppm. This signal was assigned to
Anionic polymerisation of monomers (1a–c)
The polymers resulting from 1a–c are of interest due to the polymer 2a. The addition of three drops of methanol to the red
prospect that the emissive properties of the uorophore may be polymerisation mixture afforded a yellow coloured solution.
sensitive to functionalisation of the phosphine moiety. Aer concentration of the THF solution, the polymer was
Following procedures developed for the anionic polymerisation precipitated with hexanes. The precipitation was repeated (ꢂ2)
of MesP]CPh2,16,18 monomer 1a was treated with n-BuLi (5 to afford the colourless solid polymer 2a (yield ¼ 53%). Analysis
mol%) in THF in a glovebox (Scheme 3). The addition of the of 2a using gel permeation chromatography (GPC) equipped
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Fig. 5 31P NMR spectra (121.5 MHz, 298 K) of: (a) the crude reaction
mixture in the n-butyllithium-initiated polymerisation of E/Z-1b
{[M] : [I] ¼ 50 : 1} in THF (ca. 73% monomer conversion after 5 d); (b)
poly(methylenephosphine) (2b) (Mn ¼ 27 100; PDI ¼ 1.37) isolated after
precipitation.
Fig. 4 UV/Vis spectra of 1a, 1b and 1c in THF solution (all concen-
trations were 2 ꢂ 10ꢀ5 mol Lꢀ1).
close to that predicted from the monomer-to-initiator ratio
[Mn (calc.) ¼ 27 900 Da, n ¼ 67].
Efforts to polymerise 5-dibenzosuberenyl-substituted 1c
using n-BuLi as initiator have thus far proven unsuccessful. The
presence of the strained seven-membered ring of the dibenzo-
suberene group suggested an alternate ring-opening metathesis
polymerisation pathway might be available. In an NMR-scale
experiment, a solution of 1c in C6D6 was treated with Grubb's
(generation 1) catalyst, RuCl2(PCy3)2(]CHPh) (6 mol%). Anal-
Scheme 3 The anionic polymerisation of 1a and 1b to afford PMPs 2a
and 2b, respectively.
1
ysis of the reaction mixture by H and 31P NMR spectroscopy
with a multi-angle light scattering (MALS) detector revealed a
narrow, monomodal molecular weight distribution (Mn
¼
showed only unrꢁeacted phosphaalkene, even aer several days
15 100 Da; PDI of 1.14, DPn z 41). The observed molecular
weight is signicantly higher than that predicted from the
monomer-to-initiator ratio [Mn (calc.) ¼ 7400 Da, n ¼ 20]. We
speculate that a portion of the initiator was consumed by traces
of electrophilic impurities present in the monomer 1a and the
solvent used for the polymerisation resulting in a larger
monomer-to-initiator ratio. It should be noted that monomer
1a, although spectroscopically pure, did not meet the stringent
standards of purity that are required for the anionic polymeri-
sation of MesP]CPh2 to be living.
The polymerisation of 9-phenanthryl phosphaalkene 2b fol-
lowed a similar procedure to that described above with the only
difference being that the monomer to initiator ratio was 67 : 1
rather than 20 : 1 for 1a (Scheme 3). Over the course of several
days the dark coloured reaction mixture became increasingly
viscous. The progress of polymerisation was monitored by 31P
NMR spectroscopy. Aer ve days the spectrum revealed only a
broad resonance at ꢀ10 ppm assigned to polymer 2b and sharp
signals assigned to E/Z-1b with a conversion ratio of ca. 73% (by
integration of 31P NMR signals; see Fig. 5a). Aer an additional
two days the ratio of monomer to polymer was unchanged and,
consequently, the dark polymerisation mixture was terminated
with MeOH (3 drops). A concentrated THF solution of the crude
polymerisation mixture was precipitated into hexanes (ꢂ3) to
afford 2b free from monomer (Fig. 5b). GPC analysis of the
isolated 2b revealed a monomodal polymer of molecular weight
(Mn ¼ 27 100 Da; PDI ¼ 1.37, DPn z 65). Coincidently, this Mn is
of heating to 70 C.
We have recently discovered that the radical-initiated poly-
merisation of MesP]CPh2 affords PMP with an alternative
microstructure to that initially reported.22 It was therefore of
interest to investigate the microstructure of the polymer 2a
derived from 1a using an anionic initiator. The microstructure
of 2a, as depicted in Scheme 3, shows both the alternating
P–C–P–C structure and the alternate structure where the mesityl
group is part of the main chain presumably resulting from an
isomerisation process. The key experiments that permitted the
elucidation of this alternative microstructure resulted from 13
C
1
APT and H–13C HSQC NMR experiments performed on a 600
MHz NMR spectrometer with cryoprobe. Analogous experi-
ments were performed with polymer 2a in CDCl3 solution.
1
Signicantly, the H–13C HSQC NMR spectrum (Fig. 6) reveals
cross-correlations that are assigned to –CH2– and –CH(Naph)Ph
groups of the alternative microstructure. Thus, we conclude
that a similar isomerisation polymerisation occurs with the
anionic polymerisation of 1a. However, these experiments do
not permit quantifying the ratio of x to y in polymer 2a and
furthermore the quaternary carbon atom (P–CPh2–P) will likely
be quite difficult to identify spectroscopically especially if
present in small quantities. Unfortunately, insufficient sample
of 2b was available and, consequently, analogous experiments
could not be performed with the phenanthryl-polymer.
1
However, we will note that the H NMR spectrum of 2b shares
very similar features to that shown in Fig. 6 for 2a.
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Fig. 7 31P{1H} NMR spectra (121.5 MHz, 298 K) of reaction mixtures
showing the formation of PMPs: 2a from 1a and BuLi (5 mol%) in THF;
2a$O from 2a and H2O2 (aq.) in CH2Cl2; 2a$S from 2a and S8 in toluene;
2a$BH3 from 2a and BH3$SMe2 in THF; 2a$AuCl from 2a and tht$AuCl
in CH2Cl2.
Fig. 6 1H–13C HSQC NMR spectrum (600 MHz for 1H, CDCl3, 298 K)
of poly(methylenephosphine) 2a. The ordinate axis shows the 13C{1H}
NMR spectrum and the abscissa axis shows the H NMR spectrum. *
indicates CDCl3.
prepared and characterised by 31P and 1H NMR spectroscopy
(Scheme 4). The 31P NMR spectra of the reaction mixtures are
shown in Fig. 7 and are consistent with the assigned products
(2a$BH3: d ¼ 27; 2a$AuCl: d ¼ 24). Interestingly, the reaction of
2a with Au(tht)Cl (1 equiv.) proceeded smoothly at room
temperature whilst a large excess of BH3$SMe2 (10 equiv.) was
required to achieve full conversion to 2a$BH3. Furthermore,
prolonged heating of the borane adduct 2a$BH3 (ca. 12 h at
1
Chemical functionality of new PMPs
The presence of phosphine moieties within the polymer back-
bone bestows unique chemical functionality and properties
upon PMPs when compared to typical organic polymers. With
this in mind, naphthyl-substituted 2a was treated with oxidising
agents (H2O2 and S8) to afford the phosphine oxide and phos-
phine sulde 2a$O and 2a$S, respectively (Scheme 4). The
resultant polymers were characterised by 31P and 1H NMR
spectroscopy. The 31P{1H} NMR spectra, shown in Fig. 7, reveal
signals assigned to the oxide 2a$O (d ¼ 48 ppm) and 2a$S (d ¼ 53
ppm) that are shied downeld with respect to unoxidised 2a (d
¼ ꢀ9 ppm). The oxidation of 2a with sulfur required reux
conditions in toluene for 22 h and still a small amount of
unoxidised 2a (ca. 10%) remains as suggested by the 31P NMR
spectrum. It is also interesting that the signals assigned to 2a$O
and 2a$S, albeit still broad (fwhm ¼ 445 Hz and 590 Hz,
respectively), are signicantly sharper than that assigned to 2a
(fwhm ¼ 1210 Hz). Similar spectral features were observed for
PMPs derived from MesP]CPh2.14,19 The coordination
complexes of polymer 2a with gold(I) and borane were also
ꢁ
60 C) in vacuo resulted in loss of borane and reformation of
uncomplexed 2a.
Each of the new polymers were analysed by GPC-MALS in
THF solution to determine their molecular weight moments.
Expectedly, the molecular weight of 2a (Mn ¼ 15 100 Da)
increased upon oxidation (2a$O: Mn ¼ 17 500 Da) and coordi-
nation to gold(I) (2a$AuCl: Mn ¼ 26 000 Da) or borane (2a$BH3:
Mn ¼ 23 100 Da). In contrast, the sulfurisation of 2a resulted in
a polymer with a signicantly lower molecular weight (2a$S:
Mn ¼ 12 800 Da). This suggests that there may be some degra-
dation of the polymer chain due to the forcing conditions
necessary to achieve ca. 90% sulfurisation. It is gratifying that
all functionalised polymers have relatively narrow PDIs (range:
1.07–1.13) consistent with their precursor 2a.
The absorption and emission properties of naphthyl polymer
2a and its derivatives were evaluated to assess the impact of the
phosphorus chemical environment on the electronic properties
of the naphthyl chromophore. The UV/Vis spectra of naphthyl-
substituted polymer 2a and its derivatives are shown in Fig. 8
(top). Very similar features and absorption proles are observed
for each polymer. In particular, the absorbance maximum
(lmax ¼ 286 nm) is identical for each polymer and is very similar
to 1-methylnaphthalene (lmax ¼ 283 nm).37 Moreover, the
extinction coefficients for the polymers are much lower than
that observed for monomer 1a due to the absence of the strongly
absorbing P]C moiety in the polymers. Therefore, we speculate
that the polymer absorbances mainly result from p–p* transi-
tions within the naphthyl substituent. A broad absorbance
Scheme 4 Chemical functionalisation of naphthyl-substituted poly-
(methylenephosphine) 2a.
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were deoxygenated with nitrogen and dried by passing through
a column containing activated alumina. Tetrahydrofuran (THF)
was dried over sodium and benzophenone. 1-Bromonaph-
thalene, 9-bromophenanthrene, dibenzosuberenone, chloro-
trimethylsilane, benzaldehyde and borane dimethylsulde
complex were purchased from Aldrich and used as received.
Hydrogen peroxide was purchased as a 50% solution with water
from Aldrich and used as received. Methyllithium and n-butyl-
lithium were purchased from Aldrich and used as received and
titrated by using N-benzylbenzamide.39 MesP(SiMe3)2,40 Au(tht)
Cl,41 1-benzoylnaphthalene,42 and 9-benzoylphenanthrene43
were made following literature procedures.
Equipment
1H, 31P{1H}, 13C{1H} NMR spectra were recorded at 25 ꢁC on
Bruker Avance 300, 400 or 600 MHz spectrometers. H3PO4 (85%)
was used as an external standard (d ¼ 0 for 31P). 1H NMR spectra
were referenced to the residual protonated solvent signal and
13C{1H} NMR spectra were referenced to the deuterated solvent
signal. Elemental analyses were performed in the University of
British Columbia Chemistry Microanalysis Facility. Mass
spectra were recorded on a Kratos MS 50 instrument in EI mode
(70 eV). Polymer molecular weights were determined by triple
detection gel permeation chromatography (GPC-MALS) using
an Agilent liquid chromatograph equipped with an Agilent 1200
series isocratic pump, Agilent 1200 series standard autosam-
pler, Phenomenex Phenogel 5 mm narrow bore columns (4.6 ꢂ
Fig. 8 Top: UV/Vis absorption spectra of 2a–2e in THF (c ¼ 3 ꢂ 10ꢀ5
mol Lꢀ1). Bottom: emission spectra of 2a–2e in THF (lex ¼ 286 nm; c ¼
3 ꢂ 10ꢀ5 mol Lꢀ1).
4
3
˚
˚
˚
300 mm) 10 A (5000–500 000), 500 A (1000–15 000), and 10 A
(1000–75 000), Wyatt OptilabrEx differential refractometer (l ¼
658 nm, 40 ꢁC), Wyatt TriStar miniDAWN (laser light scattering
detector at l ¼ 690 nm) and a Wyatt ViscoStar viscometer. A
ow-rate of 0.5 mL minꢀ1 was used and samples were dissolved
in THF (ca. 2 mg mLꢀ1). The molecular weights were deter-
mined using 100% mass recovery methods from Astra soware
version 5 with each polymer sample being run at least three
times to ensure reproducibility of the calculated refractive index
increment (dn/dc). Solution absorption spectra were obtained in
THF on a Varian Cary 5000 UV-Vis-near-IR spectrophotometer
using a 1 cm quartz cuvette. Fluorescence and excitation spectra
were obtained in THF on a Horiba scientic uoromax-4 uo-
rospectrometer using a 1 cm quartz cuvette.
between 320 and 400 nm is observed for 2a that is not observed
in any of the P-functionalised polymers. This broad absorbance
is possibly associated with an electronic transition involving the
P-lone pair although such bands in molecular phosphines
typically show smaller molar extinction coefficients.38
It is well established that trivalent organophosphines
bearing uorescent substituents are not emissive due to
photoinduced electron transfer involving the P-lone pair.21
Upon oxidation or coordination of the P-lone pair, such systems
oen become highly emissive. Thus, the emission spectra for
each of the new naphthyl-containing PMP polymers were
recorded and are shown in Fig. 8 (bottom). Expectedly, the
unfunctionalised 2a showed no signicant emission when
irradiated at the absorbance maximum (lex ¼ 286 nm).
Remarkably, only the oxide polymer 2a$O showed a strong
emission (lem ¼ 342 nm) upon irradiation at 286 nm. This
observation of selective “turn-on” emission is particularly
exciting and hints at a strong potential for sensor applications
for PMPs which will be the subject of future investigations.
Syntheses
MesP]C(Naph)(Ph) (1a). To
a
stirred solution of
MesP(SiMe3)2 (8.01 g, 26.9 mmol) dissolved in THF (100 mL)
was added MeLi (17.6 mL, 28.2 mmol, 1.60 M) in ether. The
reaction mixture was heated under reux overnight. An aliquot
removed for analysis by 31P NMR spectroscopy revealed a single
resonance at
d
¼ ꢀ187 ppm, which was assigned to
MesP(SiMe3)Li. The reaction mixture was cooled to ꢀ78 ꢁC, aer
which a solution of 1-benzoylnaphthalene (6.43 g, 27.7 mmol)
was added dropwise to the reaction mixture. Aer 5 minutes,
the reaction mixture was warmed to room temperature and was
Experimental
Materials and methods
All manipulations of air- and/or water-sensitive compounds stirred for a further 20 min over which time the reaction mixture
were performed under a nitrogen atmosphere by using standard turned yellow. An aliquot was taken for 31P NMR spectroscopy,
Schlenk or glovebox techniques. Hexanes and dichloromethane which indicated an E/Z-mixture of phosphaalkene (d ¼ 250 and
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ꢁ
239). The reaction mixture was cooled to ꢀ78 C and SiMe3Cl vacuo and the product was extracted using hexanes. The
(4.1 mL, 32.3 mmol) was added to quench LiOSiMe3. The hexanes were removed under reduced pressure to leave a yellow
solvent was removed in vacuo leaving an orange residue. To this solid. Crystals of 1c were isolated from a saturated hexanes
residue was added hexanes (3 ꢂ 100 mL) and the suspension solution on standing (1.32 g, 57%); dP (121.5 MHz, C6D6): 249;
was ltered and the solvent removed in vacuo. The residue was dH (300 MHz, C6D6) 7.7–6.4 (m, 12H, Ar + alkene), 2.3 (s, 6H, o-
heated (55 ꢁC) in vacuo for 6 h to remove other volatiles e.g. CH3) 2.2 (s, 3H, p-CH3); dC (75.5 MHz, C6D6) 194.0 (d, 1JCP ¼ 45
(Me3Si)2O from the reaction mixture. From the resulting Hz, P]C), 143.2 (d, JCP ¼ 28 Hz), 141.6 (d, JCP ¼ 14 Hz), 141.3 (d,
residue, small yellow crystals of 1a formed from the slow JCP ¼ 11 Hz), 139.8 (s), 138.0 (s), 135.7 (d, JCP ¼ 45 Hz), 133.9 (d,
evaporation of hexanes (while maintaining a nitrogen atmo- JCP ¼ 11 Hz), 131.9 (s), 131.5 (s), 131.4 (s), 128.7–127.6 (m), 127.0
sphere) over 1 week (2.46 g, 25%). dP (162 MHz, CDCl3) 250 (s), 22.0 (m, o-CH3), 20.9 (s, p-CH3); LRMS (EI, 70 eV): m/z (%):
(70%, Z-1a), 238 ppm (30%, E-1a); dH (400 MHz, CDCl3) (E-1a) 342, 341, 340 (4, 29, 100) [M+], 339 (22) [M+ ꢀ H], 220 (12) [M+ ꢀ
7.9–6.8 (m, 14H, Ar-H), 2.4 (s, 6H, o-CH3), 2.3 ppm (s, 3H, Mes ꢀ H], 189 (23) [M+ ꢀ PMes ꢀ H]; found: C, 84.8; H, 6.2. Calc.
1
p-CH3); dC (101 MHz, CDCl3) (E-1a) d¼ 191.1 (d, JCP ¼ 44 Hz, for C24H24P: C, 84.7; H, 6.2.
P]C), 143.3 (d, JCP ¼ 15 Hz), 141.7 (d, JCP ¼ 27 Hz), 140.5 (d, JCP
Polymer 2a. The polymerisation was performed in a glove-
¼ 7 Hz), 138.9 (s), 136.7 (d, 1JCP ¼ 40 Hz, i-Mes), 134.1 (s), 132.0 box. To a stirred solution of 1a (2.61 g, 7.13 mmol) in THF, n-
(d, JCP ¼ 9 Hz), 129.4–124.8 (m), 22.2 (d, 3JCP ¼ 9 Hz, o-CH3), 21.3 BuLi (0.22 mL, 1.59 M, 0.36 mmol) was added. Upon the addi-
(s, p-CH3); LRMS (EI, 70 eV): m/z (%): 368, 367, 366 (2, 21, 91) tion of initiator the reaction mixture turned dark red immedi-
[M+]; 365 (100) [M+ ꢀ H], 246 (21) [M+ ꢀ H ꢀ Mes], 119 (8) [M+ ꢀ ately. The reaction mixture was stirred at room temperature for
H ꢀ Mes ꢀ Naph].
5 d. At this time an aliquot was taken for 31P NMR spectroscopy,
MesP]C(Phen)(Ph) (1b). A stirred solution of MesP(SiMe3)2 which revealed the replacement of phosphaalkene signals with
(2.38 g, 8.0 mmol) in THF (10 mL) was treated with MeLi in a very broad signal centred at ꢀ9 ppm, indicative of polymer
Et2O (5.35 mL, 1.5 M, 8.0 mmol) at 25 ꢁC. Aer heating the formation. The polymer was quenched with degassed methanol
reaction mixture under reux for 12 h, analysis of an aliquot (3 drops) to which the reaction mixture turned yellow. Polymer
removed from the reaction by 31P NMR spectroscopy suggested 2a was puried by precipitation from THF into ice cold hexanes
the complete lithiation of the starting material to MesP(SiMe3) (3 ꢂ 25 mL) and dried in vacuo (1.38 g, 53%). dP (121.5 MHz,
Li (d ¼ ꢀ187 ppm). To this reaction mixture was added CDCl3) ꢀ9 (br); dH (600 MHz, CDCl3) 9.0–6.5 (br, 14H, Ar-H),
9-benzoylphenanthrene (2.27 g, 8.0 mmol) dissolved in THF 6.5–5.0 (br, 1H, CH(Naph)Ph), 4.2–2.8 (br, 2H, CH2), 2.8–0.5 (br,
ꢁ
(ca. 10 mL) at ꢀ78 C. Aer 10 min an aliquot was taken for 6H, CH3); dC (151 MHz, CDCl3) 147 (br, Ar-C), 142.5 (br, Ar-C),
analysis by 31P NMR spectroscopy, which indicated an E/Z- 137.0 (br, Ar-C), 133.9 (br, Ar-C), 128.0 (br, Ar-C), 124.9 (br, Ar-C),
mixture of phosphaalkene (d ¼ 252 and 238 ppm). The reac- 122.8 (br, Ar-C), 46.1 (br, CH(Naph)Ph), 29.3 (s, CH2), 23.1 (br,
tion mixture was then treated with Me3SiCl (1.0 mL, 8.0 mmol) CH3), 20.6 (br, CH3). GPC-MALS (THF): Mn ¼ 15 100 Da, PDI ¼
to quench LiOSiMe3. Aer the removal of solvent, the crude oil 1.14, dn/dc ¼ 0.21.
was extracted with hexanes (3 ꢂ 40 mL) and ltered through
Polymer 2b. The polymerisation of 1b was performed inside
Celite. The solvent was removed in vacuo to afford a red oil. a glovebox. A stirred solution of 1b (0.39 g, 1.3 mmol) dissolved
Yellow crystals of 1b suitable for X-ray diffraction were in THF (3 mL) was treated with n-BuLi (1.2 M, 16 mL,
obtained from the slow evaporation of a concentrated hexanes 0.02 mmol). The reaction mixture was stirred at room temper-
solution (0.50 g, 15%). dP (162 MHz, CDCl3) 252 (60%, Z-1b), ature and monitored by 31P NMR spectroscopy, which showed
238 (40%, E-1b); dH (400 MHz, C6D6) (Z-1b) 8.3–7.0 (m, 14H, Ar- the gradual increase in intensity of a broad signal over 8 d. The
H), 6.5 (br s, 1H, Mes-H), 5.1 (br s, 1H, Mes-H), 2.5 (br s, 3H, viscous reaction mixture was subsequently removed from the
o-CH3), 2.0 (br s, 3H, o-CH3), 1.6 (s, 3H, p-CH3); dC (101 MHz, glovebox and quenched with methanol (3 drops). The solvent
CDCl3) (E/Z–1b) 193.7 (d, 1JCP ¼ 44 Hz, P]C), 191.0 (d, 1JCP
¼
was removed in vacuo, leaving an orange oil. Polymer 2b was
44 Hz, P]C), 144.2 (d, JCP ¼ 24 Hz), 142.7 (d, JCP ¼ 15 Hz), dissolved in minimal THF and isolated by hexanes precipitation
ꢁ
140.5 (d, JCP ¼ 6 Hz), 140.1 (d, JCP ¼ 28 Hz), 138.9 (s), 138.1 (s), (3 ꢂ 100 mL), then dried in a vacuum oven at 65 C for 24 h
1
138.0 (s), 136.6 (d, JCP ¼ 7 Hz), 136.6 (d, JCP ¼ 49 Hz, i-Mes), (0.12 g, 32%). dP (121.5 MHz, CDCl3) ꢀ10 (br); dH (300 MHz,
135.9 (d, 1JCP ¼ 41 Hz, i-Mes), 131.6–122.5 (m), 22.6 (br, o-CH3), CDCl3) 9–5 (br m), 3.5–0.5 (br m). GPC-MALS (THF): Mn
¼
3
22.1 (d, JCP ¼ 9 Hz, o-CH3), 21.3 (s, p-CH3), 21.0 (s, p-CH3). 27 100 Da, PDI ¼ 1.39, dn/dc ¼ 0.22.
LRMS (EI, 70 eV): m/z (%): 418, 417, 416 (3, 29, 98) [M+], 415
Polymer 2a$O. To a rapidly stirred solution of 2a (0.14 g, 0.37
(100) [M+ ꢀ H], 296 (29) [M+ ꢀ H ꢀ Mes], 220 (5) [M+ ꢀ Mes ꢀ mmol P) in CH2Cl2 (2 mL), H2O2 (30% 1.5 mL) was added
Phen], 119 (13) [M+ ꢀ H ꢀ Mes ꢀ Phen]; found: C, 86.2; H, 6.1. dropwise at RT. The reaction was stirred rigorously for 1 h, at
Calcd for C30H25P: C, 86.5; H, 6.1.
which time a 31P NMR spectrum of the reaction mixture
MesP]C(C14H12) (1c). To a solution of dibenzosuberenone revealed quantitative conversion to a new product. An addi-
(1.39 g, 6.7 mmol, 1 equiv.) in THF (20 mL) was added tional 5 mL of CH2Cl2 was added and the organic layer was
MesP(SiMe3)2 (1.99 g, 6.7 mmol, 1 equiv.) and KOH (0.04 g, 10 taken to give 2a$O as an off-white solid that was dried in a
mol%). Aer 1 h the reaction mixture appeared dark green. vacuum oven for 24 h (0.14 g, 98%). dP (121.5 MHz, CDCl3) 48
Aer 3 d the 31P NMR spectrum of the reaction mixture showed (br); dH (300 MHz, CDCl3) 9.0–6.4 (br, 14H), 6.4–5.5 (br, 1H), 2.7–
complete consumption of starting material, which was replaced 3.5 (br, 2H), 2.7–0.8 (br, 6H); GPC-MALS (THF): Mn ¼ 17 500 Da,
by a new resonance at 249 ppm. The solvent was removed in PDI ¼ 1.07, dn/dc ¼ 0.15.
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Polymer 2a$S. To a toluene (ca. 5 mL) solution of 2a (0.14 g,
0.38 mmol P) was added S8 (0.08 g, 0.31 mmol) and the reaction
Conclusions
mixture was stirred under reux for 2 h. An aliquot of the Phosphaalkenes that incorporate emissive chromophores as the
reaction mixture was taken and analysed by 31P NMR which C-substituents are monomers that are amenable to anionic
revealed almost quantitative formation of a new product (d ¼ 53 polymerisation. Detailed characterisation of the naphthyl-
ppm) with a small ꢃ10% of starting material present. Heating substituted polymer shows that its microstructure results from
the reaction mixture for a further 22 h failed to complete the the isomerisation of a proton from the ortho-Me group of Mes to
reaction. Aer this time the reaction mixture was cooled to the –CPh2 anion aer initial anion addition across the P]C
room temperature and the solvent was removed under reduced bond. This implies a highly unusual polymerisation mecha-
pressure. The reaction mixture was dissolved in minimal (1 mL) nism for P-mesityl phosphaalkenes. Despite this complex
CH2Cl2 and precipitated into hexanes (15 mL.) The precipitate mechanism, the phosphine moiety of naphthyl-substituted
was ltered and dried under reduced pressure to afford 2a$S PMP 2a displays a high degree of chemical functionality
(0.13 g, 83%). dP (121.5 MHz, CDCl3) 53 (br); dH (300 MHz, through either oxidation or coordination. Perhaps most inter-
CDCl3) 9.0–6.5 (br, 14H), 6.5–5.0 (br, 1H), 3.0–2.5 (br, 2H), 2.5– esting, this novel PMP shows selective “turn-on” emission upon
1.0 (br, 6H); GPC-MALS (THF): Mn ¼ 12 800 Da, PDI ¼ 1.09, dn/ oxidation to 2a$O by using hydrogen peroxide. This exciting
dc ¼ 0.22.
observation points towards potential sensor properties for
Polymer 2a$BH3. To a THF (ca. 3 mL) solution of 2a (0.20 g, PMPs bearing emissive chromophores, a subject that will be
0.55 mmol P) was added BH3$SMe2 (0.13 mL, d ¼ 0.80 g mLꢀ1
,
pursued in more detail in a future study.
1.37 mmol, 5 equiv.) at ꢀ78 ꢁC. The reaction mixture was
allowed to warm slowly to room temperature. Aer 24 h a 31
P
Acknowledgements
NMR spectrum of an aliquot from the reaction mixture
showed the presence of a new broad resonance (27 ppm, 50%)
in addition to starting material. The reaction mixture was
treated with additional BH3$SMe2 (0.13 mL, 1.37 mmol) at RT
and was stirred for 36 h. 31P NMR analysis at this time showed
complete conversion to product. The reaction mixture was
ltered and subsequently precipitated into hexanes (25 mL) to
afford 2a$BH3 as a white solid (0.14 g, 67%). dP (162 MHz,
CDCl3) 27 (br); dH (400 MHz, CDCl3) 9.0–5.5 (br, 15H), 3.0–0.5
(br, 11H); GPC-MALS (THF): Mn ¼ 23 100 Da, PDI ¼ 1.13, dn/
dc ¼ 0.10.
We thank the Natural Sciences and Engineering Research
Council (NSERC) of Canada for support of this work. We are
grateful to Prof. Michael O. Wolf for valuable discussions and
suggestions regarding the photophysical analysis.
Notes and references
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APEX II diffractometer with graphite-monochromated MoKa
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