An Unexpected “Step-Conjugated” Biphosphole via Unique P–P Bond Formation

Article abstract of DOI:10.1016/j.chempr.2018.08.022

The synthesis of a π-conjugated organophosphorus species with bridging P–P unit is reported. Because of the pyramidal geometry of the phosphorus centers, the molecular scaffold provides intriguing electronic communication throughout the three-dimensional structure via π-σ-π conjugation in stepwise fashion. The dimeric species was serendipitously found to be accessible via a reaction of the corresponding P-amino-phosphole precursor through mediation with the hard Lewis acid BF₃. We provide detailed mechanistic studies toward a suitable reaction mechanism that was also verified via computational means. Moreover, we elaborate the utility of the biphosphole via phosphorus functionalization that lends further proof for the step conjugation provided by the unique phosphorus-based molecular architecture. Fundamental chemistry lays the foundation for future innovation with considerable impact on next-generation technologies. Main-group chemistry in particular provides a unique angle with regard to structure and bonding, which makes it an intriguing state-of-the-art avenue for the development of readily accessible advanced materials with significantly value-added functionality, that is, optical and electronic properties. The need for innovative molecular architectures is particularly evident in the rapidly accelerating development of conjugated organic building blocks because their high utility for a variety of sustainable energy applications is now well established. Herein, we report a unique type of phosphorus-based “step-conjugated” building block via an unexpected reaction that we have elucidated mechanistically in our studies. Reaction of a P-amino-dithienophosphole with the hard Lewis acid, BF₃, leads to a dimeric species via a P–P bond-forming reaction, as supported through systematic mechanistic studies. Because of the inherent pyramidal geometry of the phosphorus centers, the dimer shows extended step-like conjugation through the scaffold via π-σ-π conjugation, which makes it an intriguing new building block for organic hybrid materials.

Full text of DOI:10.1016/j.chempr.2018.08.022

Article
An Unexpected ‘‘Step-Conjugated’’
Biphosphole via Unique P–P Bond Formation
Zisu Wang, Nayanthara Asok,
Joshua Gaffen, ..., Chris Gendy,
Roman Dobrovetsky, Thomas
Baumgartner
rdobrove@post.tau.ac.il (R.D.)
tbaumgar@yorku.ca (T.B.)
HIGHLIGHTS
P-amino-phospholes are homo-
coupled via exposure to BF3
Phosphorus enables step
conjugation, a new type of 3D
conjugation
Reaction of a P-amino-dithienophosphole with the hard Lewis acid, BF3, leads to a
dimeric species via a P–P bond-forming reaction, as supported through systematic
mechanistic studies. Because of the inherent pyramidal geometry of the
phosphorus centers, the dimer shows extended step-like conjugation through the
scaffold via p-s-p conjugation, which makes it an intriguing new building block for
organic hybrid materials.
Wang et al., Chem 4, 1–16
November 8, 2018 ª 2018 Elsevier Inc.
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Please cite this article in press as: Wang et al., An Unexpected ‘‘Step-Conjugated’’ Biphosphole via Unique P–P Bond Formation, Chem (2018),
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Article
An Unexpected ‘‘Step-Conjugated’’
Biphosphole via Unique P–P Bond Formation
Zisu Wang,1 Nayanthara Asok,2 Joshua Gaffen,2 Yael Gottlieb,3 Wenhua Bi,1 Chris Gendy,1
1,2,4,
Roman Dobrovetsky,3, and Thomas Baumgartner
*
*
SUMMARY
The Bigger Picture
Fundamental chemistry lays the
foundation for future innovation
with considerable impact on next-
generation technologies. Main-
group chemistry in particular
provides a unique angle with
regard to structure and bonding,
which makes it an intriguing state-
of-the-art avenue for the
The synthesis of a p-conjugated organophosphorus species with bridging P–P
unit is reported. Because of the pyramidal geometry of the phosphorus
centers, the molecular scaffold provides intriguing electronic communication
throughout the three-dimensional structure via p-s-p conjugation in stepwise
fashion. The dimeric species was serendipitously found to be accessible via a re-
action of the corresponding P-amino-phosphole precursor through mediation
with the hard Lewis acid BF3. We provide detailed mechanistic studies toward
a suitable reaction mechanism that was also verified via computational means.
Moreover, we elaborate the utility of the biphosphole via phosphorus function-
alization that lends further proof for the step conjugation provided by the
unique phosphorus-based molecular architecture.
development of readily accessible
advanced materials with
significantly value-added
functionality, that is, optical and
electronic properties.
INTRODUCTION
The exponential growth of organic electronics has fueled the search for new and
improved molecular building blocks since its inception. Organophosphorus building
blocks with highly desirable optoelectronic and morphological properties in partic-
ular have carved their own niche in this rapidly evolving field in recent years.1–4
The need for innovative molecular
architectures is particularly
evident in the rapidly accelerating
development of conjugated
organic building blocks because
their high utility for a variety of
sustainable energy applications is
now well established. Herein, we
report a unique type of
One of the most prominent examples of building blocks in this context is phosphole,
a five-membered heterocycle. Although structurally similar to pyrrole, the properties
of phosphole proved to be far from a simple analog of its nitrogen-based congener
(Figure 1). Whereas pyrrole is highly aromatic, the aromaticity of phospholes is
limited.5
phosphorus-based ‘‘step-
The strong s character of the phosphorus lone pair makes it unsuitable for conjuga-
tion with the p orbital of the butadiene moiety. The phosphole’s limited aromaticity
stems from the hyperconjugation of the s orbital of the exocyclic P–R moiety with the
butadiene p orbital.2,5 Therein lies the foundation for both the chemistry as well as
the beneficial optoelectronic properties of phospholes. Chemically, because the
lone pair is localized on the phosphorus, it is available for further modifications
such as oxidation and coordination.1 Our group and others have utilized these mod-
ifications to produce compounds that exhibit highly desirable properties in elec-
tronics, stimuli response, and self-organization.1–4,6–9 At the same time, because
of its conjugation with the p backbone, the exocyclic P–R moiety can affect the elec-
tronics of the system directly.
conjugated’’ building block via an
unexpected reaction that we have
elucidated mechanistically in our
studies.
The method of utilizing the P–R moiety for the design of electronic materials has
drawn considerable interest recently. One of the most intriguing features of the
phosphole is the s*-p* hyperconjugation, a corollary of s-p hyperconjugation.2
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Figure 1. Pyrrole and Phosphole
With s*-p* hyperconjugation, the lowest unoccupied molecular orbital (LUMO) en-
ergy of the molecule can be lowered by extension of conjugation (Figure 2). This has
enabled phospholes to be used as luminescent and electron-acceptor materials in a
range of applications.1,2
In this context, P–P-bridged biphospholes are particularly appealing because the
two phosphole units are connected electronically via s*-p* hyperconjugation
through the P–P bridge.5 These compounds thereby afford one-of-a-kind opportu-
nities for the study of conjugated systems mediated by s scaffolds, and the effective
doubling of the LUMO orbital of phospholes can lead to enhanced acceptor prop-
erties and new highly sought after n-type materials.2 Synthetically, P–P-bridged
biphosphines and biphospholes can also serve as useful precursors for highly func-
tional compounds.
However, because of the complicated nature of the synthesis of these compounds,
there are few reports on biphospholes in the literature. In the early 2000s, Re´ au and
coworkers reported the synthesis of a series of biphospholes involving the reductive
cleavage of the P–R s bond of a precursor molecule (Figure 3).10,11 The synthesized
compounds showed promising optoelectronic properties that confirmed theoretical
predictions. On the basis of these intriguing studies, we attempted to adapt this
method to the dithieno[3,2-b:20,30-d]phosphole system, which we introduced in
2004,12 but all attempts to access the P–P dimer along similar lines failed as a result
of the quite distinct chemistry of the dithienophosphole backbone compared with
that of the Re´ au system.
Here, we describe our serendipitous discovery of a reaction of P-amino-dithieno-
phospholes that efficiently produces bi(dithienophosphole) in high yields. More-
over, we provide computational support for the mechanism of the reaction and
also showcase the utility of the intriguing unique building block for the development
of phosphole-containing electronic materials.
RESULTS AND DISCUSSION
Discovery
P-amino-phospholes are a class of compounds that are quite useful in phosphole
research. The amino group can be easily cleaved to give a chlorophosphole, which
1Department of Chemistry, University of Calgary,
Calgary, AB T2N 1N4, Canada
Figure 2. Phosphole-Typical Features
Electronic structure of the phosphole
LUMO (A) and generic structure of a
P–P-bridged biphosphole (B).
2Department of Chemistry, York University,
Toronto, ON M3J 1P3, Canada
3School of Chemistry, Tel Aviv University,
PO Box 39040, Tel Aviv 6997801, Israel
4Lead Contact
*Correspondence:
rdobrove@post.tau.ac.il (R.D.),
tbaumgar@yorku.ca (T.B.)
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Figure 3. Re´ au’s Biphosphole
Molecular structure (A) and structure in the solid state (B). H atoms and second molecule from the
unit cell have been omitted for clarity.7
in turn acts as the precursor for convenient subsequent functionalization. In a previ-
ous study on the self-assembly behavior of p-conjugated phosphinamides, the
P-amino-dithienophosphole precursor 1 was synthesized according to our reported
methods (Scheme 1), and functionalization provided a series of phosphina-
mides with unique hydrogen-bond-based self-organization and photophysical
properties.13
To further expand the scope of the P-amino-phosphole chemistry, we functionalized
1 by oxidation to give 2, and reaction with BH3 provided 3. However, we always
encountered difficulties in obtaining the trivalent species 1 cleanly. In an attempt
to purify the freshly synthesized amino-dithienophosphole 1, we passed it through
an alumina flash column under nitrogen, as per the commonly used procedures in
our group. To our surprise, the eluent of the flash column produced crystals imme-
diately after exiting the column. Considering the low concentration of the solution
(<10 mg/mL), concentration-effect crystallization was unlikely. Because the crystals
obtained were of sufficient quality, single-crystal X-ray crystallography was
conducted. The crystallographic data (see below) confirmed that the species was,
in fact, the elusive biphosphole 4 (Scheme 2). Although the result was encouraging,
it was also rather puzzling because such P–P bond formation had no precedence in
the literature. We consequently decided to take up the challenge to elucidate the
mechanism of this unique reaction.
Scheme 1. Synthesis and Functionalization of Amino-dithienophosphole 1
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Scheme 2. Serendipitous
Formation of Biphosphole 4
Mechanistic Studies
The formation of the P–P biphosphole requires the cleavage of the P–N s bond in
one way or another. In conventional organophosphorus chemistry, the cleavage of
the P–N s bond can normally be accomplished through the addition of a Brønsted
acid.14 After the formation of an ammonium center, it is easily cleaved from the phos-
phorus center. Because the surface of alumina particles (used for the column chro-
matography) is covered with relatively ‘‘hard’’ acidic sites, both Brønsted but more
so Lewis in nature, it is thus reasonable to assume that the acidity of the alumina
would play a role in the formation of the biphosphole. To investigate this possibility
in more detail, 1 was treated overnight with the Lewis acid BF3 (as etherate) at room
temperature. 31P NMR spectroscopy of the resulting solution revealed two pairs of
doublets: one major (>85%) at 34 and ꢀ65 ppm and one minor (<15%) at 48
and ꢀ59 ppm (Figure 4). The close proximity of the two pairs of doublets, as well
as their similar coupling constants (JPP = 287 versus 285 Hz), suggests that both spe-
cies could be closely related (i.e., isomeric species). Further refluxing led to the for-
mation of the biphosphole 4 that could be isolated in good yield (75%). Although
this result confirmed our hypothesis that the Lewis acidity can contribute to the for-
mation of biphospholes in solution, no suitable mechanism for this transformation
had been known.
The large coupling constant of the two pairs of doublets indicated that the two phos-
phorus centers are connected through a single bond,15 which naturally became the
starting point for elucidating the mechanism of this reaction. This result was rather
Figure 4. 31P{1H} NMR Spectrum of the Intermediate S1
4
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Scheme 3. Proposed Formation of
BF3-Adduct Intermediate 1-BF3
encouraging because it clearly demonstrated that a hitherto unknown intermediate,
S1, with a P–P(E) bridge can form through Lewis acid mediation at room temperature
(Figure 4, inset). However, the exact nature of S1 was not known at this point.
Because of the ambidentate nature of 1, both P and N can potentially act as Lewis
bases, and a critical question was the coordination site for BF3 in the molecule. As
a result of the fast formation of the stable intermediate S1, any other potential inter-
mediate species between 1 and S1 could not be detected. However, several pieces
of supporting evidence can aid in the elucidation of the coordination site of the Lewis
acid BF3. First, the availability of the phosphorus lone pair in 1 proved to be key in the
formation of the dimeric species. As shown in Scheme 1, when 1 was oxidized, the
P-oxo species 2 could be isolated by column chromatography and was stable over
months, as was the case with related compounds. Perhaps the most revealing evi-
dence lies in the reaction between 1 and the Lewis acid BH3. When 1 was treated
with BH3SMe2, no biphosphole formation was observed. Second, the P–B adduct
that formed exclusively from this reaction could be isolated through column
chromatography and was also stable over months. In addition, the phosphorus
coordination of BH3 was unequivocally confirmed by single-crystal X-ray crystallog-
raphy (Scheme 1).
To make sense of all these results, the predictions of the hard-soft acid/base (HSAB)
theory can be used, wherein a phosphane is classified as a soft base and BH3 as a soft
acid.16 When viewed through the same lens, the coordination site of BF3 can only be
at the nitrogen (Scheme 3, 1-BF3). This not only follows the HSAB prediction of hard
acid (BF3)/hard base (amine) coordination, as well as a recent study using Verkade’s
base,17 but also makes the phosphorus lone pair available for further reactions.
The presence of a pair of broad doublets in the 31P NMR spectrum is indicative of an
asymmetric P–P bond in the intermediate, ‘‘P–P–E’’ (Figure 4, inset). Because the end
product 4 was a symmetrical molecule exhibiting a singlet in the 31P NMR spectrum,
the identity of the E moiety is key to understanding the mechanism behind this
unique reaction. Because of the transient nature of S1, its isolation was unfeasible.
As such, in situ solution NMR methods afforded some insight into the nature of
the intermediate. With 1H,31P heteronuclear multiple bond correlation NMR, the
downfield-shifted doublet (31P d = 34 ppm) correlates both with the phosphole pro-
tons and with the pyrrolidinyl protons (1H d = 3.4 ppm), whereas the upfield doublet
(
31P d = ꢀ65 ppm) only correlates with the phosphole protons (Figure 5A). It is thus
inherently plausible that, during the formation of S1, one of the pyrrolidinyl substit-
uents was shed.
Furthermore, the solution was also studied by high-resolution mass spectrometry
(HRMS). The positive-mode mass spectrometry produced one major peak
at m/z = 460. This peak corresponds closely with the cationic species S1+ (Figure 5A,
inset). This inference correlates well with the NMR results. The negative mode mass
spectrometry also produced a major peak at m/z = 206, which corresponds to the
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Figure 5. Characterization of Intermediates S1+ and A1ꢀ
(A) 1H,31P{1H} heteronuclear multiple bond correlation NMR of S1+.
(B) Proposed structure of A1ꢀ
.
lost pyrrolidinyl and BF3 adduct in anionic form (A1ꢀ, Figure 5B). Another piece of
the mechanistic puzzle was the identity of the final by-product. After the formation
of biphosphole 4, the solvent was removed under vacuum. The resulting residue
was then extracted with Et2O then pentane. The combined Et2O and pentane solu-
tion was slowly evaporated to produce a crystalline compound. Single-crystal X-ray
crystallography revealed the compound to be the pyrrolidine-BF3 adduct 5
(Figure 6).
The generation of the intermediate necessitates the (at least transient) formation of a
phosphenium cation, which we were, however, unable to detect directly by any
means. We therefore validated the feasibility of this process via theoretical
calculations.
Figure 6. Compound 5
Molecular structure (A) and structure in the solid state (B).
6
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Figure 7. Proposed Reaction Mechansim
(A) Calculated reaction of 1 with BF3.
(B) Calculated reaction of salt [S1+A2ꢀ] with BF3.
(C) Reaction coordinates of the proposed mechanism; the energies are relative to the starting materials. Gibbs free energies and enthalpies (in
parentheses) are given in kcal molꢀ1
.
Mechanistic Computations
To get a better understanding of the reaction mechanism, we performed density
functional theory (DFT) calculations at the BP86(D3)/def2-SVP18–20 level of theory
for the reaction by using the experimentally observed evidence as input parameters.
On the basis of these calculations, the formation of the 1ꢀBF3 adduct is exergonic
with DG of ꢀ9.1 kcal molꢀ1 (Figure 7A).
Because both 1 and 1ꢀBF3 are required for this reaction, we believe that the
breaking of the 1-BF3 adduct in situ enables the formation of S1+ with the concerted
or subsequent formation of A1ꢀ (potentially via A2ꢀ). The nucleophilic substitution
of the trifluoro(pyrrolidin-1-yl)-borate anion (A2ꢀ) by 1 leading to salt [S1+A2ꢀ] is
slightly endergonic (DG = 7.5 kcal molꢀ1), with a relatively low activation barrier
(DGz) of 8.2 kcal molꢀ1 (Figure 7C, TS1). It is thus reasonable to assume that A2ꢀ
can bind another equivalent of BF3 leading to salt [S1+A1ꢀ], which is an exergonic
process with DG of ꢀ20.0 kcal molꢀ1 (Figure 7B), and should thus strongly inhibit
the reaction. This would also explain why the reaction to form 4 ultimately needs
to be heated; the availability of [S1+A2ꢀ] is critical to drive the reaction forward.
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Figure 8. Solid-State Structure of 4
(50% Probability)
Note the head-to-head conformation
(A) and the ‘‘step’’ arrangement of the
two halves of the molecule (B).
Once (re-)formed, we suggest that S1+ and A2ꢀ, which are held together by the
Coulombic forces, react with one another by the abstraction of the proton from
the pyrrolidinyl group in S1+ by A2ꢀ, which is the rate-determining step with DGz
of 26.3 kcal molꢀ1 (Figure 7C, TS2), leading to intermediate
23.7 kcal molꢀ1). The last step of this reaction, the dissociation of 6 to 4, 5, and
6 (DG =
7
21 upon the approach of BF3, is exergonic (DG = ꢀ26.4 kcal molꢀ1). Overall, this re-
action is strongly exergonic and exothermic (DH = ꢀ24.5 kcal molꢀ1), which supports
the experimentally observed reaction outcome.
X-Ray Crystallography and Solid-State Organization of 4
Most electronic materials are ultimately used as solids in practical applications. The
structure and organization of the molecules in the solid state are thus good indica-
tors as to their potential utility. The molecular structure of 4 in the solid state was
confirmed by single-crystal X-ray crystallography (Figure 8). The most prominent
feature of the structure lies in its step-like shape. The two phosphole moieties are
connected through an out-of-plane P–P s bond. Because of the anticipated strong
p character of the P–P s bond, it is close to perpendicular (75ꢁ) to the conjugated
phosphole planes. This observation indeed supports the expected, biphosphole-
typical ‘‘step conjugation.’’10,11
Another important characteristic of the molecular structure of 4 is the head-to-head
conformation of the two phosphole moieties. In contrast to the previous biphosp-
hole example reported by Re´ au’s group, where the two phosphole units were ar-
ranged in the twisted ‘‘gauche-like’’ fashion, the head-to-head conformation
enables very efficient packing of the molecules. In the solid state, 4 packs in a
‘‘herringbone’’ motif (Figure 9). In this packing order, the tight overlap between
˚
the chromophores (p-stacking distance, 3.5 A) is known to promote high charge-car-
rier mobilities in the bulk.22 To validate this potential, theoretical studies (B3LYP/6-
31G(d) level of theory) were conducted on a segment of the crystal lattice of 4.23 It
can be clearly seen that significant intermolecular overlap of molecular orbitals exists
(Figure 10), thus suggesting that 4 should exhibit efficient charge transfer (CT) in the
bulk.
Figure 9. Space-Fill Model of 4 in
Aggregate Form
Note the herringbone
arrangement of the molecules (A)
and layered superstructure (B).
8
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Figure 10. Calculated Frontier Molecular Orbitals (B3LYP/6-31G(d)) of a Stack of Three Molecules of 4
X-ray crystallographic data were used as input.23
Functionalization of the Phosphorus Center
One of the inherent advantages of incorporating phosphorus within chromophore
building blocks is the easy tunability through the modification of the phosphorus
center.1,24 To provide a representative example to this end, we targeted phos-
phorus oxidation. Because there are two phosphorus centers in the biphosphole,
both mono- and bis-oxidation of 4 were attempted. Mono-oxidation can be
achieved by treating 4 with 1 equiv of bis(trimethylsilyl)peroxide ((Me3SiO)2). How-
ever, further oxidation of the monoxide 8 led to the cleavage of the P–P bond,
which is known for related species from the literature,25 and the formation of the
previously reported phosphinic acid 913 was observed (Scheme 4, top). Oxidation
of 8 with air was not successful either, given that prolonged exposure to air only
led to its decomposition. On the other hand, sulfidation of 4 with the mild Lawes-
son’s reagent successfully led to the bisulfide species 10 without cleavage of the
P–P s bond (Scheme 4, bottom).
Photophysical Properties
With the two P-functionalized compounds 8 and 10, in addition to 4, in hand, we
proceeded with investigating their photophysical properties to elucidate their
S
S
S
S
S
S
(Me3SiO)2
P
P
P
P
(Me3SiO)2
‘H+’
O
P
HO
O
S
S
S
S
S
4
8
9
S
S
S
S
Lawesson’s reagent
P
P
P
P
S
S
S
S
S
4
10
Scheme 4. Phosphorus Functionalization of the Biphosphole 4
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Figure 11. UV-Vis Spectra of the Biphospholes and 11
electronic properties. UV-visible (UV-vis) spectroscopy was utilized to study the
photo-transitions of the compounds. Compared with the parent phosphole 11,12
the absorption onsets of 4, 8, and 10 are significantly red-shifted (lonset
>
425 nm; cf. 11: 374 nm), indicating that the P–P s bond is effective in extending
the conjugation within the dimers (Figure 11).
When comparing the absorption spectra of 4, 8, and 10, some surprising insights
can be gained. First, the absorption features of the biphosphole system appear
to be influenced more by the overall geometrical symmetry of the biphosphole
than by the oxidation states of the phosphorus atoms. The absorption spectra
of 4 and 10 bear striking similarities to each other (Figure 11). Even though 4 con-
tains two trivalent phosphorus centers, and 10 contains two pentavalent phos-
phorus centers, the peak positions of the two compounds are rather similar
(Table 1). This is in contrast to conventional dithienophospholes, where significant
differences in photophysical features can be found between the P(III) and the P(V)
species.1,12 Second, although not very pronounced, the peak positions are also
quite close to those of 8, which by contrast shows two fairly pronounced
absorption peaks at 315 and 365 nm. The latter peak coincides with the p-p*
transition of previously studied dithienophosphole oxides,3,12,13 suggesting a
localized transition within this half of the molecule; the peak at 315 nm appears
to originate from a localized transition likely involving the trivalent phosphole
subunit.
To shed more light on the unexpected photophysics of the new species, we eval-
uated the features of the species via DFT studies at the B3LYP/6-31G(d) level of
theory. As per the theoretical calculations, and despite representing different
rotamers, the nearly degenerate highest occupied molecular orbital (HOMO) and
HOMO-1, as well as the LUMOs of 4 and 10, respectively, are evenly distributed
across the entire molecule (Figures 12A, S7, and S9), supporting the extended
conjugation across the whole scaffold in both cases.23 By contrast, the HOMO of
8 is only located on the P(III) dithienophosphole moiety (Figure 12B). The LUMO,
however, is primarily located on the oxo-dithienophosphole moiety with significant
delocalization on the P(III) moiety. The spatial separation of the HOMO and LUMO
suggests the potential presence of some CT in the optical transitions of 8; to exper-
imentally validate the hypothesis, solvatochromic studies were conducted. Five
10
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Table 1. Photophysical Properties of the Biphosphole Derivatives
Compound
lmax (nm)
lonset (nm)
430
3 (Mꢀ1cmꢀ1 a
)
4
310, 329, 372
315, 365
3,400; 3,300; 2,050
12,500; 5,850
8
425
10
11
293, 324, 380
295, 336
428
9,600; 6,800; 4,150
11,000; 14,000
374
Solution photophysical data were collected as CH2Cl2 solutions (10ꢀ5 M).
aMolar extinction coefficient.
solvents were chosen across a wide range of polarity (see Table S16 and Fig-
ure S10). However, when the solvent polarity progressed from non-polar (pentane)
to polar (acetonitrile), the peak at 365 nm experienced only small red shift (Dl =
8 nm), which still made a clear correlation of the experimental photophysical
data with specific transitions difficult. For that reason, we extended the theoretical
studies to also include time-dependent DFT (TD-DFT). To our satisfaction, the TD-
DFT data for the trivalent dimer 4 correlate well with the optical spectroscopy data.
The calculations reveal three main transitions that correspond to two p-p* transi-
tions at 307 and 381 nm and an n-p* transition at 354 nm (see Table S9). Notably,
all three transitions involve the entire molecular scaffold, which is again in line with
the expected step conjugation.
The TD-DFT data for compounds 8 and 10, on the other hand, provided a multitude
of fairly weak transitions involving orbitals largely ranging from HOMOꢀ2 to
LUMO+1 for 8 and HOMOꢀ6 to LUMO+1 for 10 (Tables S10 and S11). This is in
line with an increased density of states, as also observed for the P–P dimers re-
ported by Re´ au et al.10 and experimentally supported by our electrochemical
studies (Figures S13–S16). The data clearly provide a strong testament to the conju-
gation across the whole scaffold in both species. TD-DFT for compound 8 suggests
a series of transitions between 299 and 418 nm in close proximity to each other for
the first six excited states that as a whole involve the entire molecular scaffold
(Table S10). Notably, three groups of transitions can be identified and include bidi-
rectional CT, locally excited (LE), and n-p* transitions. In fact, the first excited state
at 418 nm corresponds to the HOMO-LUMO CT transition. Its oscillator strength,
however, is low, which would suggest that this transition is hidden underneath
the broad 365 nm absorption peak in the experimental spectrum. The latter transi-
tion is also reproduced by the TD-DFT and corresponds to an LE state within the
pentavalent half of the molecule, in line with the data for the corresponding
monomer.12
The TD-DFT data for compound 10 suggest an even larger number of (weak) transi-
tions that occur between 279 and 423 nm (Table S11) and prominently feature
several p-p* and n-p* transitions involving the entire p-system and the sulfur lone
pairs. The first excited state at 423 nm, within the experimental spectrum envelope,
corresponds to a weak n-p* transition, and the excited states between 368 and
402 nm can be grouped together as p-p* transitions across the whole molecular
scaffold. These transitions are then followed by a series of n-p* transitions from
325 to 361 nm and finally a stronger p-p* transition at 279 nm. Although these
data appear fairly complex, they in fact match the observed experimental spectrum
with its broad features quite nicely.
The emissive properties of the compounds were also studied experimentally.
However, in contrast to 11, which is highly emissive,12 all the new compounds are
Chem 4, 1–16, November 8, 2018
11

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Figure 12. Frontier Molecular Orbitals
Calculated frontier orbitals of the biphospholes 4 (A) and 8 (B) B3LYP/6-31G(d) level of theory.
non-emissive. The rotational flexibility of the P–P s bond potentially opens up a non-
radiative decay pathway when the chromophore is excited, leading to loss of photon
emissions.24 Although not immediately evident, this phenomenon can be assigned
to a symmetry-breaking intramolecular CT that is induced by the solvent polarity,
particularly in the case of compounds 4 and 10. We were recently able to establish
a similar feature in a previous study that was based on conformation-dependent
behavior between two identical halves of a perfluorophenylene-bridged bisphosp-
hole.26 Considering that the 31P{1H} NMR spectrum shown in Figure 4 does indeed
suggest the presence of two isomeric species (i.e., cis/trans), it is thus inherently
plausible that a similar process applies the new biphosphole series as well. However,
in the case of 10, another straightforward mechanism also comes into play. Here, the
soft, heavy sulfide substituents that play a prominent role in all frontier orbitals and
several optical transitions (see above) lead to the quenched fluorescence, as is well
established in the literature.27,28
Conclusions
We have discovered a convenient method of synthesizing P–P-bridged biphosp-
holes via effective coupling from P-amino-phospholes. Our fundamental
mechanistic studies revealed, by combining the results of various characterization
methods, that a Lewis-acid-mediated polar mechanism via an unsymmetrically
substituted dimer applies to the formation of the P–P s bond in the biphosphole 4.
Although only explicitly demonstrated for our dithienophosphole system, we never-
theless surmise that this mechanism could likely be a generally applicable, new, and
efficient way toward selective P–P bond formation. We are currently exploring the
potential of this reaction.
To further showcase the phosphole-typical versatility of the new biphosphole,
we also exposed 4 to common phosphorus chemistry reactions. Although we
were unable to afford the dioxide species, the mono-oxide and the disulfide were
synthesized in satisfactory yields. In addition to the synthesis, the optoelectronic
properties, as well as the solid-state structure of the new biphosphole and its
12
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derivatives, were studied and provided credible evidence for the existence of step
conjugation throughout the scaffold involving the p-systems linked through the
P–P s bond via s*-p* hyperconjugation. Importantly, the step shape of the molecule
not only promotes intramolecular electronic communication but also facilitates well-
defined intermolecular interactions. The tight herringbone superstructure of 4 in the
solid state is a good indicator of the material’s potential in organic electronic
applications. Corresponding exploratory studies are currently underway in our
laboratory.
EXPERIMENTAL PROCEDURES
General
All manipulations were carried out under a dry nitrogen atmosphere using stan-
dard Schlenk techniques. Solvents were dried with an MBraun Solvent Purification
System. Unless noted otherwise, starting materials were used as received. 31P(1H)
NMR, 1H NMR, and 13C(1H) NMR were recorded on Bruker DRX400 and Avance (II,
III) 400 MHz spectrometers. Chemical shifts were referenced to external 85%
H3PO4 (31P), BF3(OEt2) (11B), and external tetramethylsilane (13C, 1H). Mass spectra
were run on a Finnigan SSQ 7000 spectrometer or a Bruker Daltonics AutoFlex
III system. All photophysical experiments were recorded on a Jasco FP-6600 spec-
trofluorometer and a UV-vis Cary 5000 spectrophotometer. Cyclic voltammetry
was performed on an Autolab PGSTAT204 instrument with a polished glassy car-
bon electrode as the working electrode, a Pt-wire as the counter electrode, and
an Ag wire as the reference electrode. Cyclic voltammetry experiments were per-
formed in tetrahydrofuran (THF) solution with tetrabutylammonium hexafluoro-
phosphate (1.0 M) as supporting electrolyte. Theoretical calculations were carried
out at the B3LYP/6-31+G(d) and BP86(D3)/def2-SVP levels of theory with the
Gaussian 09 suite of programs.23 Diffraction data were collected on a Nonius
Kappa CCD diffractometer equipped with a graphite monochromator with Mo
˚
Ka radiation (l = 0.71073 A). The structure was solved by direct methods (SHELXS
and SHELXL) and refined anisotropically on F by full-matrix least-squares tech-
niques. Hydrogen atoms were included with the use of a riding model and refined
isotropically.
Syntheses
Compound 1
n-BuLi (4 mL, 2.5 M, 10 mmol) was added to a solution of 3,30-dibromobithio-
phene (1.62 g, 5 mmol) in Et2O (250 mL) at ꢀ78ꢁC. The solution was stirred
at ꢀ78ꢁC for 30 min and then warmed to room temperature and stirred for another
30 min. The reaction mixture was cooled again to ꢀ78ꢁC. Dichloropyrrolidinylphos-
phane (0.86 g, 5 mmol) was added to the reaction quickly at ꢀ78ꢁC. After addition,
the reaction mixture was warmed with a warm water bath to a slight reflux. The
water bath was left to cool naturally to room temperature, and the reaction
was stirred for another 2 hr. Subsequently, all volatile materials were removed
under vacuum, and the residue was suspended in pentane. The suspension was
filtered via cannula to remove a brown precipitate. The crude product was obtained
as an orange oil (1.03 g, 78% yield) after the solvent was removed in vacuo. 1H NMR
(400 MHz, chloroform-d) d 7.23 (dd, J = 4.9, 2.4 Hz, 2H), 7.17 (d, J = 4.8 Hz, 2H),
2.95–2.88 (m, 4H), 1.77–1.72 (m, 4H); 31P{1H} NMR (162 MHz, chloroform-d)
d 23.3. 13C{1H} NMR (101 MHz, chloroform-d) d 146.28 (d, J = 12.1 Hz), 141.74
(d, J = 3.8 Hz), 127.10 (d, J = 19.1 Hz), 125.42 (d, J = 5.5 Hz), 50.04
(d, J = 9.4 Hz), 26.46 (d, J = 5.1 Hz); HRMS (ESI+) m/z: [M + H]+ calcd 266.0222, found
266.0220.
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Compound 2
(Me3SiO)2 (0.18 g, 1 mmol) was added to a solution of 1 (0.27 g, 1 mmol) in CDCl3
under nitrogen. The reaction was monitored by 31P NMR. When complete conver-
sion of 1 was observed, the reaction mixture was concentrated and purified
by column chromatography (ethyl acetate). Compound 2 was obtained as light
orange oil, which solidified when left for several hours (0.212 g, 75% yield).
Single crystals of 2 were obtained by slow evaporation of acetone. 1H NMR
(400 MHz, chloroform-d) d 7.26 (dd, J = 4.9, 3.3 Hz, 2H), 7.15 (dd, J = 4.9,
2.6 Hz, 2H), 3.22–3.10 (m, 4H), 1.90–1.76 (m, 4H); 31P{1H} NMR (162 MHz, chloro-
form-d) d 20.78; 13C{1H} NMR (100 MHz, chloroform-d) d 144.6 (d, J = 28.3 Hz),
136.57 (d, J = 132.6 Hz), 127.69 (d, J = 15.4 Hz), 125.47 (d, J = 13.8 Hz), 46.39
(d, J = 3.9 Hz), 26.27 (d, J = 8.2 Hz); HRMS (ESI+) m/z: [M+1]+ calcd 282.0171,
found 280.0164.
Compound 3
BH3SMe2 (0.084 g, 1.1 mmol) was added to a solution of 1 (0.27 g, 1 mmol) in CDCl3
under nitrogen. The reaction was monitored by 31P NMR. When complete conver-
sion of 1 was observed, the reaction mixture was concentrated and purified by col-
umn chromatography (ethyl acetate:hexane = 4:6). Compound 3 was obtained as a
light-yellow solid (0.172 g, 62% yield). Single crystals of 3 were obtained by slow
diffusion of hexane into dichloromethane solution. 1H NMR (400 MHz, chloroform-d)
d 7.31 (dd, J = 4.9, 3.0 Hz, 2H), 7.17 (dd, J = 4.8, 1.2 Hz, 2H), 3.04 (tdd, J = 6.6, 3.7,
2.3 Hz, 4H), 2.17–1.27 (m, 4H), 1.43–0.38 (m, 3H); 31P{1H} NMR (162 MHz, chloro-
form-d) d 47.2 (d, J = 77.8 Hz); 11B{1H} NMR (128 MHz, chloroform-d) d ꢀ39.6
(d, J = 60.2 Hz); 13C{1H} NMR (100 MHz, chloroform-d) d 144.68 (d, J = 13.1 Hz),
138.38 (d, J = 66.0 Hz), 127.63 (d, J = 11.7 Hz), 126.16 (d, J = 15.0 Hz), 47.56
(d, J = 1.5 Hz), 26.32 (d, J = 6.9 Hz); HRMS (ESI+) m/z: [M+1]+ calcd 280.0555, found
280.0551.
Compound 4
Compound 1 (1.03 g, 4 mmol) was dissolved in CDCl3 (20 mL). BF3OEt2 (0.55 g,
4 mmol) was added to the solution to produce an intensely yellow solution. The
mixture was heated to reflux and stirred overnight. The reaction was monitored
by 31P NMR. When 1 was completely consumed, solvent was removed under vac-
uum. The residue was re-dissolved in dichloromethane (200 mL). The solution was
passed through a silica flash column under nitrogen. Removal of the solvent under
vacuum gave the product as a yellow powder (0.50 g, 66%). 1H NMR (400 MHz,
THF-d8) d 7.23 (dt, J = 4.9, 1.3 Hz, 1H), 6.57 (dt, J = 4.9, 0.7 Hz, 1H); 31P{1H} NMR
(162 MHz, THF-d8) d ꢀ37.0; 13C{1H} NMR (100 MHz, THF-d8) d 141.63, 140.36,
123.74 (t, J = 2.1 Hz), 123.35 (t, J = 10.0 Hz); HRMS (MALDI+) m/z: [M]+ calcd
389.8979, found 389.8997.
Compound 8
Compound 4 (0.10 g, 0.3 mmol) was suspended in dichloromethane (10 mL).
(Me3SiO)2 (0.046 g, 0.3 mmol) was added. The mixture was stirred at room tem-
perature overnight. The mixture was chromatographed to give the product as a
yellow amorphous solid (yield: 0.085 g, 82%). 1H NMR (400 MHz, chloroform-d)
d 7.21 (dd, J = 4.8, 2.6 Hz, 2H), 7.10 (dd, J = 4.9, 3.6 Hz, 2H), 6.81 (d, J = 5.0 Hz,
2H), 6.62 (dd, J = 4.8, 1.8 Hz, 2H). 31P{1H} NMR (162 MHz, chloroform-d) d 34.0
(d, J = 216.5 Hz), ꢀ49.65 (d, J = 216.5 Hz); 13C{1H} NMR (100 MHz, chloroform-d)
d 144.86 (d, J = 4.5 Hz), 144.49–143.74 (m), 139.91 (dd, J = 17.2, 4.1 Hz), 134.92
(dd, J = 95.5, 8.5 Hz), 127.28 (d, J = 14.8 Hz), 126.80 (dd, J = 6.1, 2.2 Hz), 125.97
14
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(dd, J = 18.7, 2.0 Hz), 125.37 (d, J = 14.2 Hz). HRMS (EI+) m/z: [M]+ calcd 405.8933,
found 405.8932.
Compound 10
Compound 4 (0.20 g, 0.5 mmol) was suspended in dichloromethane (10 mL) under
nitrogen. Lawesson’s reagent (0.20 g, 0.5 mmol) was added. The reaction was stirred
at room temperature overnight. The mixture was chromatographed to give the
product as a yellow amorphous solid (yield: 0.15 g, 65%). 1H NMR (400 MHz, chlo-
roform-d) d 7.24–7.21 (m, 4H), 7.09–6.51 (m, 4H); 31P{1H} NMR (162 MHz, chloro-
form-d) d 26.0; 13C{1H} NMR (100 MHz, chloroform-d) d 146.47–143.71 (m), 135.70
(dd, J = 82.7, 10.6 Hz), 130.14–127.45 (m), 126.16–124.55 (m); HRMS (ESI+) m/z:
[M + H]+ calcd 454.8498, found 454.8493.
DATA AND SOFTWARE AVAILABILITY
Crystallographic data have been deposited in the Cambridge Crystallographic Data
Center under accession numbers CCDC: 1811752, 1811753, 1811754, and
1811755.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 42 fig-
ures, 16 tables, and 4 data files and can be found with this article online at https://
doi.org/10.1016/j.chempr.2018.08.022. Specific elements include crystal data and
structure refinement parameters for compounds 2–5, frontier orbitals and energies
of the new species obtained via DFT calculations, computational details on the reac-
tion mechanism, Cartesian coordinates for minimized structures obtained via DFT
for the new species, solvatochromic studies of 8, high-resolution ESI mass spectra
for S1+ and A1ꢀ, heteronuclear NMR spectra for all new species, and cyclic voltam-
mograms of compounds 4 and 8.
ACKNOWLEDGMENTS
We are grateful to the Natural Sciences and Engineering Research Council of Can-
ada, the Canada Foundation for Innovation, the Canada Research Chairs program,
and the Israeli Ministry of Science, Technology, and Space for financial support of
this work. Z.W. thanks Alberta Innovates – Technology Futures for a graduate
fellowship.
AUTHOR CONTRIBUTIONS
Z.W., N.A., J.G., and Y.G. performed the synthetic studies. N.A. and J.G. performed
the cyclic voltammetry. W.B. and C.G. performed the single-crystal X-ray crystallog-
raphy. Z.W. and R.D. performed the computational experiments. T.B. and R.D.
directed the project and wrote the manuscript with input from all authors. All authors
analyzed the results and commented on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: December 17, 2017
Revised: July 11, 2018
Accepted: August 21, 2018
Published: September 20, 2018
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