From Cyclic Iminophosphoranes to -Conjugated Materials

Article abstract of DOI:10.1002/anie.200904219

Join herel Phosphenium ion-mediated coupling of an annulated σ⁴-1λ⁵-[1,3,2]diazaphosphole (or cyclic iminophosphorane) affords a pseudo-quinoidal dication with extended π-conjugation (see scheme), which emits in the near-infrared (NI

Full text of DOI:10.1002/anie.200904219

Angewandte
Chemie
DOI: 10.1002/anie.200904219
Phosphorus Heterocycles
From Cyclic Iminophosphoranes to p-Conjugated Materials**
Dan A. Smith, Andrei S. Batsanov, Mark A. Fox, Andrew Beeby, David C. Apperley,
Judith A. K. Howard, and Philip W. Dyer*
The “closed” benzannulated s4-l5-[1,3,2]diazaphosphole 1c
has been shown to exist in valance tautomeric equilibrium
with its “open” form 1o (Scheme 1).[1] As a result of the small
Scheme 1. Valence tautomeric equilibrium between the “open” pyridyl-
N-phosphinoimine 1o and the “closed” s4-l5-[1,3,2]diazaphosphole
derivative 1c.
Scheme 2. Synthesis of diphosphonium salts cis- and trans-2.
energy difference between these two isomers, the air- and
moisture-sensitive compound 1 exhibits reactivity that is not
only commensurate with an iminophosphorane (e.g. cyclo-
sumption of both starting materials is evident from 31P NMR
spectroscopic analysis of the reaction mixture. Following
purification, compound 2 was isolated in good yield (61%) as
an air- and moisture-stable green solid, which was found to
contain both cis- and trans-isomers in a 1:1 ratio by multi-
nuclear NMR spectroscopy (d 31P = + 51.9 and + 51.4 ppm).[6]
No interconversion between the two isomers in solution was
observed using selective 31P NMR inversion experiments at
408C ([D2]tetrachloroethane).
Recrystallization of cis-/trans-2 from a CH2Cl2/hexane
solution gave crystals of trans-2·2CH2Cl2 suitable for an X-ray
diffraction study.[7] The dicationic component has crystallo-
graphic Ci symmetry (Figure 1) and is comprised of a 1H,1’H-
=
addition at the P N moiety and Lewis acid coordination at N
with 1c), but also with a P,N chelate ligand (e.g. metal
coordination of 1o), and with a 1,2-dihydropyridine (e.g.
cyclobuta[b]pyridine formation from 1c).
With a view towards exploring further the unusual
multifunctional character of 1, it was of interest to probe its
reactivity with an equally versatile substrate. In this regard,
s2-l2-phosphenium ions, R2P+, were an attractive choice for
study. These dicoordinate cations possess both a phosphorus-
centered lone pair and a formally vacant 3p orbital, rendering
them “ambiphilic” in terms of their reactivity, something that
gives rise to an extensive Lewis acid and cycloaddition
chemistry.[2–4]
Treatment of the equilibrium mixture 1c/1o with an
equimolar quantity of the phosphenium salt [(iPr2N)2P]-
[OTf][5] in CH2Cl2 affords a new compound 2 as the only
phosphorus-containing product (Scheme 2); complete con-
[*] D. A. Smith, Dr. A. S. Batsanov, Dr. M. A. Fox, Dr. A. Beeby,
Dr. D. C. Apperley, Prof. J. A. K. Howard, Dr. P. W. Dyer
Department of Chemistry, Durham University
South Road, Durham, DH1 3LE (UK)
Fax: (+44)191-384-4737
E-mail: p.w.dyer@durham.ac.uk
Homepage: http://www.dur.ac.uk/chemistry/staff/profile/
?id=1317
Figure 1. X-ray molecular structure of the dication of trans-2 (primed
atoms are generated by an inversion centre).[7] H atoms are omitted
for clarity. Thermal ellipsoids are drawn at the 50% probability level.
[**] Durham University and Sasol Technology UK Ltd. are warmly
acknowledged for financial support of this work. We thank Dr. A. M.
Kenwright, C. Heffernan, and I. McKeag for assistance with NMR
studies, Dr. P. J. Low for access to cyclic voltammetry facilities and
Prof. T. B. Marder for invaluable discussions.
À
À
Selected bond lengths [ꢀ] and angles [8]: P N(1) 1.687(2), P N2
À
À
À
À
1.657(2), P N3 1.619(2), P N4 1.612(2), N1 C2 1.407(3), N2 C1
À
À
À
À
1.306(4), C1 C2 1.468(4), C2 C3 1.355(4), C3 C4 1.433(4), C4 C5
À
À
À
À
1.457(4), C5 C6 1.343(4), C4 C4’ 1.394(5), C6 N1 1.402(3), C1 C11
1.478(4); N1-P-N2 95.6(1), N1-C2-C1 108.1(2), C2-C1-N2 115.5(2), C1-
N2-P 110.7(2).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904219.
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4,4’-bipyridylidene core bearing two benzannulated s4-l4-
[1,3,2]diazaphosphole fragments at each terminus; each
phosphorus atom has a distorted tetrahedral environment.
Both the 4,4’-bipyridylidene unit and the diazaphosphole
rings show substantially localized bonding patterns and are
planar within Æ 0.04 and Æ 0.03 ꢀ, respectively, with an angle
of 6.58 between the two planes of the ring systems. The phenyl
À
rings are inclined to the bipyridylidene core by 388. The C(4)
C(4’) distance, 1.394(5) ꢀ, is consistent with a pseudo-
À
quinoidal structure and compares well with the central C C
distance of 1.381(3) ꢀ reported for the two-electron reduced
bis(trimethylsilyl)dihydro-4,4’-bipyridine, (cf. localized 4,4’-
bipyridine, 1.486 ꢀ).[8]
X-Ray analysis of a selection of individual crystals
revealed each to be trans-2. However, the 31P NMR spectrum
(CD2Cl2) of this small sample of crystalline material consis-
tently showed the presence of both cis and trans isomers in an
approximately 1:1 ratio, with the exact proportions being
sample-dependent. Since the two isomers of 2 do not
interconvert in solution (see above), this suggests that cis-2
is present in amorphous form alongside crystalline trans-2.
Although repeated attempts were made to separate cis-/trans-
2 by fractional crystallization and by chromatography, these
proved unsuccessful.[6]
The mechanism for the formation of diphosphonium salt 2
from reaction of [(iPr2N)2P][OTf] with 1o/1c is not readily
apparent. Despite extensive efforts no phosphorus-containing
by-products could be detected in the reaction mixture, prior
to purification, by 31P NMR spectroscopy.[6] Most likely the
unaccounted for material is lost in the form of intractable
oligomeric products that are removed during the purification
Scheme 3. Proposed mechanisms for the formation of 2 from 1.
coupling followed by double deprotonation (pathway A) or,
reaction with a second equivalent of 1o/1c generating [2]+,
which is subject to elimination of H+ and oxidation to yield 2
(pathway B). Both pathways are supported by the observed
formation of sub-stoichiometric quantities of [iPr2NH2][OTf]
resulting from the generation of acid following reaction of 1o/
1c with either [(iPr2N)2P][OTf] or [Cp2Fe][OTf].[6] Further-
more, both paths A and B involve singly-bonded biaryl
species consistent with the formation of a 1:1 mixture of cis-/
trans-2; this is supported by a DFT study (B3LYP/6-31G*),
which revealed that trans-2 is only 1.8 kJmolÀ1 lower in
energy than cis-2.[6]
From a structural standpoint, the dihydro-4,4’-bipyridine
core of the dicationic component of 2 resembles that of the
doubly-reduced N,N’-dimethyl-4,4’-bipyridinium dication,
known commonly as “reduced viologen”, MV0.[16] Initially
developed as herbicides, the viologens constitute a well-
known class of redox-active compound, which give rise to
intensely colored, stable radical monocations.[16] The exten-
sive electrochromic behavior reported for the viologens has
led to their use in a range of applications including as electron
relays, redox mediators in catalytic cycles, and molecular-
scale devices.[17–19] Thus, it was of interest to further probe the
electronic character of dication 2 and the extent of its
similarity with MV0.
of 2, something consistent with the observation of [iPr2NH2]-
[6]
À
[OTf] formed by P N bond cleavage (see below).
Elucidation of the mechanistic pathway by which 2 is
formed is further complicated by the ambiphilic nature of the
phosphenium ion.[2] However, a hydride abstraction process
can be ruled out since: 1) no (iPr2N)2PH[9] was detectable
from the reaction of [(iPr2N)2P][OTf] with 1o/1c, and 2) no
reaction takes place on treating 1o/1c with an equimolar
quantity of the trityl salt [Ph3C][B(C6F5)4].[6] Thus, in order to
reduce the complexity of the system, an alternative cationic
Lewis acid/oxidant, namely a ferrocenium salt, was employed.
Indeed, treatment of a CH2Cl2 solution of 1o/1c with one
equivalent of [Cp2Fe][OTf][10] (Cp = cyclopentadienyl)
resulted in the near-instantaneous formation of a blue-green
solution from which cis-/trans-2 (1:1) was isolated in 39%
yield following purification.[6]
Together, these observations suggest that the formation of
2 from 1o/1c involves a radical-based process, which may be
regarded as a variant of the intermolecular Scholl reaction
(i.e. a Friedel–Crafts aryl coupling).[11] Although the mecha-
nism for this type of transformation is not well-established,
the intermediacy of a radical cation has been implicated.[12–14]
Indeed, such a process was invoked to account for the
formation of N,N’-disilyldihydropyridines through silylation
of pyridine in the presence of Pd/C.[15] Consequently, two
reaction pathways for the formation of 2 are proposed here
(Scheme 3). Both depend on the initial formation of the
radical cation [1]C, which then undergoes either a two-radical
A computational study using time-dependent density
functional theory (TD-DFT) methods (B3LYP/6-31G*)
gave an optimized structure for the dication of 2 that was in
excellent agreement with that obtained by X-ray crystallog-
raphy.[6,20] The computed absorption maxima (lmax = 761 nm,
e = 20000 dm3 molÀ1 cmÀ1) correspond well with those deter-
mined experimentally (see below), with the band at 761 nm
being comprised largely of a transition from the HOMO to
the LUMO (73%). As expected, the computed HOMO and
LUMO for 2 extend across the whole of the dication and are
consistent with a quinoidal-type structure, although only a
small orbital component is located at the two formally
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cationic phosphorus centers as would be expected (Figure 2).
For the HOMO–LUMO transition it is calculated that the
electron density shifts from the central quinoidal core
(P,N1,C6,C5,C4) to the imine moiety (C1,N2), which is
Figure 3. Cyclic voltammogram of cis-/trans-2, 10À4 m in CH2Cl2/10À1
Bu4NBF4 at m=100 mVsÀ1 (versus ferrocene/ferrocenium, Fc/Fc+).
m
Figure 2. HOMO (left) and LUMO (right) for trans-2 dication deter-
mined by DFT methods at the B3LYP/6-31G* level of theory.
consistent with previous reports of the reduction of imino-
pyridines.[21] The HOMO–LUMO gap of 1.7 eV for 2 is
considerably smaller than those reported for other N,N’-R2-
dihydropyridines (e.g. R = Me, 3.0 eV;[22] R = Ph,[23] 2.7 eV;
R = B(mesityl)2, 2.6 eV[24]), consistent with the p-conjugation
extending across the two imino groups as well as the
bipyridylidene moiety. Overall, this DFT study highlights
the remarkable similarity between the composition of the
HOMO of 2 with the HOMO of the doubly-reduced N,N’-
dimethyl-4,4’-bipyridinium dication (MV0),[6,22] although the
LUMOs of the two compounds differ as would be expected.
The UV/vis absorption spectra of dication 2 measured in a
variety of solvents were found to be anion-independent, but
solvatochromic, with a hypsochromic shift being displayed
with increasing solvent dielectric constant (Table 1).[6] In all
solvents two bands are observed in the visible region at ca. 400
Figure 4. UV/vis spectroelectrochemistry of the two one-electron
reduction processes for 2 in CH2Cl2 (dotted line) affording 3 (solid
line), and [2]C/3 (dashed line). Due to the small difference in reduction
potential between [2]C and 3, a clean spectrum of the 3 could not be
obtained.
Table 1: UV/vis spectroscopic data for 2 in various solvents.
[2]C with lmax = 925 nm (Scheme 4). This is followed by a
further one-electron reduction to afford the neutral linked
benzannulated s4-l5-[1,3,2]diazaphosphole 3, giving rise to a
feature centered at lmax = 610 nm. In order to probe the
identity of 3, chemical reduction of 2 was undertaken using
metallic Na in THF solution.[6] Following this reaction by
31P NMR spectroscopy revealed the clean formation of a
single product 3 after 2 d at ambient temperature, accom-
Solvent
er[a]
lmax [nm]
loge
lmax [nm]
loge
THF
CH2Cl2
MeOH
7.5
9.1
32.6
398
392
380
4.00
3.96
3.96
772
740
720
4.30
4.31
4.33
[a] Dielectric constant data from ref. [25].
and ca. 740 nm, with the latter agreeing well with that
determined by TD-DFT.[6] Molar extinction coefficients of
log e ꢀ 4.3 were determined in each of the different solvents.
Since much of the importance of the viologens stems from
their electrochemistry,[16,26] it was of interest to explore the
redox properties of 2. The cyclic voltammogram of 2 in
CH2Cl2 solution (Figure 3) showed that it readily undergoes
two reversible, one-electron reduction steps, with reduction
occurring at À0.55 and À0.46 V.[6] An irreversible oxidation
was observed at + 0.79 V, most likely as a result of decom-
position.
These two reduction processes were monitored for 2 in
CH2Cl2 solution by means of spectroelectrochemistry
(Figure 4). In an initial reaction the pseudo-quinoidal dication
2 is reduced to what is proposed to be the radical monocation
Scheme 4. Redox reactions associated with dication 2: formation of
radical cation [2]C and bis(benzannulated diazaphosphole) 3. For clarity,
only trans-2 is shown.
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Communications
panied by a change in color of the solution from green-blue to
1
dark blue. The H and 31P NMR spectra of 3 are entirely
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consistent with its formulation as the neutral bis(benzannu-
lated diazaphosphole), giving rise to a singlet resonance at
d(31P) = + 39.1 ppm ([D8]THF), which is in excellent agree-
ment for the chemical shift for the annulated s4-1l5-
[1,3,2]diazaphosphole derivative 1c, d(31P) = + 40.4 ppm
([D8]THF).[1]
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solution of 2 at 635 nm induces a comparatively weak,
anion-independent emission in the near-infrared (NIR)
centered at 922 nm, with a Stokes shift of 2670 cmÀ1; the
normalized emission spectrum is given in the Supporting
Information.[6] Unfortunately, the low sensitivity of the
detector in this spectral region and, hence large correction
factors required, precludes a determination of the quantum
efficiency.
[6] See Supporting Information for details.
[7] Crystal data: trans-2: [C48H72N8P2][CF3O3S]2·2CH2Cl2; M =
1291.07; monoclinic; space group P21/c (No. 14); a =
9.6074(15), b = 17.592(3), c = 18.132(3) ꢀ; b = 90.94(2)8; V=
3064.1(8) ꢀ3; Z = 2; m(MoKa) = 0.33 mmÀ1; T= 120 K; 35703
reflections collected; Rint = 0.069; R(F) = 0.051 for 4485 data
with I > 2s(I), wR(F2) = 0.147 for all 5392 independent data.
CCDC 686757 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
In summary, this work demonstrates that the “multifunc-
tional”
pyridyl-N-phosphinoimine/s4-1l5-[1,3,2]diazaphos-
phole 1o/1c equilibrium mixture may be homocoupled in a
novel variant of a Scholl-type reaction, which exploits a
phosphenium salt as an oxidant. The resulting air- and water-
stable salt 2 possesses a 4,4’-dihydropyridine core bearing two
conjugated imino moieties, giving an extended p-conjugated
material, which emits in the NIR spectral region. Notably, 2 is
one of only a very small number of stable, neutral “extended
viologens”.[27] This work continues to highlight the impor-
tance of phosphorus-containing building blocks for the
preparation of “tuneable” extended p-conjugated materials,
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systems are starting to find application as components of
sensors and of single- and multilayer OLEDs (organic light-
emitting diodes), something that in light of their photo-
physical properties is particularly attractive for compounds 2
and 3, respectively.[31–33] These areas, together with an in-
depth mechanistic study, are being actively explored.
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Experimental Section
All reactions/manipulations were carried out under an atmosphere of
dry nitrogen using standard Schlenk and glovebox techniques.
cis-/trans-2: A solution of 1o/1c (207 mg, 0.50 mmol) in CH2Cl2
(5 mL) was added slowly to a solution of [(iPr2N)2P][OTf] (190 mg,
0.50 mmol) in CH2Cl2 (5 mL). After 7 d the solution was layered with
hexane causing a green solid to precipitate slowly. This material was
isolated by filtration and washed with H2O (3 ꢁ 3 mL) then Et2O (3 ꢁ
3 mL) and dried in vacuum affording a 1:1 mixture of cis-/trans-2 as a
green solid (168 mg, 61%).
À
À
À
the optimised geometry were the P N(1), P N(3) and P N(4)
distances, which were approximately 0.04 ꢀ larger than those
determined crystallographically.
3: A solution of cis-/trans-2 (50 mg, 0.02 mmol) in [D8]THF
(0.5 mL) was treated with excess finely divided Na metal (250 mg)
and left to stand for 2 d at ambient temperature. Complete conversion
of 2 to 3 was indicated by 31P NMR spectroscopy.
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Received: July 29, 2009
Revised: September 16, 2009
Published online: October 30, 2009
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Keywords: aryl coupling · conjugated materials ·
electrochemistry · NIR emitters · phosphenium ions
.
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