Phosphorus centers of different hybridization in phosphaalkene-substituted phospholes

Article abstract of DOI:10.1002/chem.201402406

Phosphole-substituted phosphaalkenes (PPAs) of the general formula Mes P=C(CH₃)-(C₄H₂P(Ph))-R 5a-c (Mes=2,4,6-tBu ₃Ph; R=2-pyridyl (a), 2-thienyl (b), phenyl (c)) have been prepared from octa-1,7-diyne-substituted phosphaalkenes by utilizing the Fagan-Nugent route. The presence of two differently hybridized phosphorus centers (σ²,λ³ and σ³, λ³) in 5 offers the possibility to selectively tune the HOMO-LUMO gap of the compounds by utilizing the different reactivity of the two phosphorus heteroatoms. Oxidation of 5a-c by sulfur proceeds exclusively at the σ³,λ³-phosphorus atom, thus giving rise to the corresponding thioxophospholes 6a-c. Similarly, 5a is selectively coordinated by AuCl at the σ³,λ³-phosphorus atom. Subsequent second AuCl coordination at the σ², λ³-phosphorus heteroatom results in a dimetallic species that is characterized by a gold-gold interaction that provokes a change in π conjugation. Spectroscopic, electrochemical, and theoretical investigations show that the phosphaalkene and the phosphole both have a sizable impact on the electronic properties of the compounds. The presence of the phosphaalkene unit induces a decrease of the HOMO-LUMO gap relative to reference phosphole-containing π systems that lack a P=C substituent.

Full text of DOI:10.1002/chem.201402406

DOI: 10.1002/chem.201402406
Full Paper
&
Hybridization
Phosphorus Centers of Different Hybridization in Phosphaalkene-
Substituted Phospholes
Elisabet ꢀberg,[a] Andreas Orthaber,[a] Christophe Lescop,[b] Rꢁgis Rꢁau,*[b] Muriel Hissler,*[b]
and Sascha Ott*[a]
Abstract: Phosphole-substituted phosphaalkenes (PPAs) of
the general formula Mes*P=C(CH3)ꢀ(C4H2P(Ph))ꢀR 5a–
c (Mes*=2,4,6-tBu3Ph; R=2-pyridyl (a), 2-thienyl (b), phenyl
(c)) have been prepared from octa-1,7-diyne-substituted
phosphaalkenes by utilizing the Fagan–Nugent route. The
presence of two differently hybridized phosphorus centers
(s2,l3 and s3,l3) in 5 offers the possibility to selectively tune
the HOMO–LUMO gap of the compounds by utilizing the
different reactivity of the two phosphorus heteroatoms. Oxi-
dation of 5a–c by sulfur proceeds exclusively at the s3,l3-
phosphorus atom, thus giving rise to the corresponding thi-
oxophospholes 6a–c. Similarly, 5a is selectively coordinated
by AuCl at the s3,l3-phosphorus atom. Subsequent second
AuCl coordination at the s2,l3-phosphorus heteroatom re-
sults in a dimetallic species that is characterized by a gold–
gold interaction that provokes a change in p conjugation.
Spectroscopic, electrochemical, and theoretical investiga-
tions show that the phosphaalkene and the phosphole both
have a sizable impact on the electronic properties of the
compounds. The presence of the phosphaalkene unit indu-
ces a decrease of the HOMO–LUMO gap relative to reference
phosphole-containing p systems that lack a P=C substituent.
Introduction
dination or oxidation reactions can be performed to modify
the physical properties of the p system.[1c,5] Recently, it has
been shown that the inclusion of phosphaalkenes (a s2,l3-
phosphorus center) into oligoacetylenes has a pronounced
effect on the energies of the frontier molecular orbitals.[6] Simi-
lar effects have been described for oligomeric and polymeric
p-conjugated systems in which s2,l3-phosphorus centers fea-
ture in the form of phosphaalkenes or diphosphenes.[7] A spe-
cific class of phosphorus-containing p-conjugated materials are
phospholes, that is, phosphorus analogues to pyrroles.[5] The
tricoordinated phosphorus atom of phospholes possesses
a pyramidal geometry with a lone pair of pronounced s charac-
ter. These geometric and electronic features prevent an effi-
cient endocyclic conjugation of the electron sextet. In fact, de-
localization within the phosphole ring arises from a hyperconju-
gation that involves the exocyclic PꢀR s bond and the p
system of the diene moiety.[8] As a consequence, the phosp-
hole ring exhibits a unique set of properties (low aromatic
character, reactive P atom, s–p hyperconjugation) that has mo-
tivated the development of new conjugated p systems that in-
tegrate the phosphole unit.[9] In particular, the intact reactivity
of the phosphorus heteroatom has proven to provide a unique
possibility for facile tuning of the optoelectronic properties of
phospholes (e.g., through oxidation or metal coordination to
the phosphorus), thereby making them suitable for applica-
tions within optoelectronic devices.[5b,9a,10] In an early study,
Mathey and co-workers combined two differently hybridized P
centers, namely, a phosphole and a phosphinine unit[11] or
a phosphole decorated with an imine functionality,[12] and ex-
plored their coordination behavior. In the present work, we
wish to chemically tune the electronic properties of a new
The field of organic p-conjugated materials has matured tre-
mendously over the last two decades.[1] More and more materi-
als, which range from single molecules to oligomers and poly-
mers, have found use in organic electronics devices, for exam-
ple, and a plethora of organic compounds with potential appli-
cations within the field of molecular electronics have been syn-
thesized.[2] Today, the art of organic synthesis is well
developed, and both the chemical and electronic properties of
synthesized organic compounds are easily modified.[3] In partic-
ular, the insertion of heteroatoms in the backbone of p sys-
tems has attracted great interest owing to the impact of the
heteroatom on the electronic properties of the molecules.[4]
Furthermore, postsynthetic manipulations such as metal coor-
[a] Dr. E. ꢀberg, Dr. A. Orthaber, Prof. S. Ott
Department of Chemistry, ꢁngstrçm Laboratories
Uppsala University, Box 523, 75120 Uppsala (Sweden)
Fax: (+46)18-471-6844
E-mail: sascha.ott@kemi.uu.se
[b] Dr. C. Lescop, Prof. R. Rꢂau, Prof. M. Hissler
Institut des Sciences Chimiques de Rennes, UMR6226
CNRS-Universitꢂ de Rennes 1
Campus de Beaulieu, 35042 Rennes Cedex (France)
E-mail: muriel.hissler@univ-rennes1.fr
regis.reau@univ-rennes1.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402406.
ꢃ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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class of compounds that contains two differently hybridized
phosphorus centers in the form of phosphaalkenes and phos-
pholes as integral parts of a conjugated system. Through the
unique reactivity of two differently hybridized phosphorus
atoms, chemical manipulations will offer possibilities to selec-
tively alter the optical and electronic properties of the novel p-
conjugated molecules. Herein, we present an unprecedented
synthetic approach towards phosphole-phosphaalkenes (PPAs).
Showcases for the differences in reactivity of the two phospho-
rus heteroatoms through metal coordination and oxidation are
presented.
methyl iodide and subsequent workup, Mes*P=CCH3Br (2) was
obtained in good yields.[18] The 1,7-octadiyne motif can be in-
troduced by using one of two procedures that basically differ
in the alkyne source. In the first case, 2 is coupled with 1,7-oc-
tadiynemagnesium bromide in THF by using [Pd(dba)2] (dba=
dibenzylideneacetone) and PPh3 under reflux conditions.[16]
This reaction gives octadiynephosphaalkene 3 in 98% yield
and is suitable for medium to large scales, preferably less than
or equal to one gram, mainly due to the fact that it is easier to
handle Grignard reagents on these scales. In accordance to the
cross-couplings presented by Bickelhaupt et al., an isomeriza-
tion occurs during the palladium-mediated coupling step, and
only the E isomer of 3 is obtained from (Z)-2 (see below).[16,19]
For smaller scales, 2 can be coupled directly with 1,7-octa-
diyne in Et2NH with [Pd(PPh3)2Cl2] and CuI as catalysts. The re-
action time is crucial for this reaction, since longer reaction
times result in homocoupling of the product. By using the
standard Sonogashira coupling conditions, that is, amine as
solvent and [Pd(PPh3)2Cl2] and CuI as catalysts in the presence
of 2-bromopyridine, 2-iodothiophene, or iodobenzene, com-
pound 3 can be converted into 4a–c in medium to good
yields. Higher overall yields of 4a can be obtained by coupling
2 directly with 1-(2-pyridyl)octa-1,7-diyne. The direct cross-cou-
pling approach also provides a larger functional group toler-
ance and would allow the introduction of substituents that are
reactive towards Grignard reagents. Compounds 3 and 4a–
Results and Discussion
Synthesis and X-ray crystallography of phosphole-phos-
phaalkenes
Phosphole-substituted phosphaalkenes (PPAs) were targeted
through a sequence of reactions in which the phosphole is
formed in the last step from phosphaalkene-containing 1,7-oc-
tadiyne compounds 4a–c by using the Fagan–Nugent protocol
(Scheme 1).[13] The strategy turned out to be a feasible synthet-
ic route, as sterically protected phosphaalkenes are compatible
with nBuLi and [Cp2ZrCl2] (Cp=cyclopentadienyl ligand), that
is, the required reagents for the phosphole assembly. The syn-
thetic sequence towards 4a–c starts from previously described
C,C-dibromophosphaalkene 1 (Scheme 1).[14] For subsequent
cross-coupling reactions, it is important to substitute one of
the bromide atoms by other groups as palladium catalysts that
are commonly used for cross-coupling reactions lead to a rear-
rangement of 1 and the formation of phosphaalkynes.[15] Using
this strategy, Bickelhaupt et al. previously presented cross-cou-
pling reactions on Mes*P=CHBr (Mes*=2,4,6-tri-tert-butylphen-
yl).[16] To increase the stabilities of the compounds, it was de-
cided to introduce a methyl group instead of one bromide
substituent in 1. Hence, Mes*P=CBr2 (1) was treated with nBuLi
at ꢀ1208C in the Trapp mixture (THF/diethyl ether/pentane
4:4:1) for 30 min.[17] After quenching the reaction mixture with
c
are isomerically pure, as is evident from one single
31P{1H} NMR spectroscopic resonance in the typical region for
P=C compounds between d=274.7 and 275.3 ppm. Figure 1
shows a depiction of the solid-state structures of 3 and 4b,c.
The structures confirm the formation of E isomer 3 from Z
isomer 2 during the preparation of the octadiynephosphaal-
kenes. In all cases, the bond lengths of the P=C double bonds
and the CꢁC triple bonds are in the usual range.
With phosphaalkene-substituted octadiynes in hand, PPAs
were synthesized by utilizing the Fagan–Nugent route, which
is a well-established method for the preparation of five-mem-
bered heterocycles.[13] Thus, treating phosphole precursors 4a–
Scheme 1. Synthesis of octadiyne-phosphaalkenes 4a–c. i) 1) nBuLi, ꢀ1308C, Trapp mixture, 30 min; 2) MeI, ꢀ1208C, 30 min then to RT, 2 h. Compound 2
93%. ii) 1,7-octadiynemagnesium bromide, [Pd(dba)2], PPh3, THF, reflux, 4–5 h. Compound 3 98%. iii) 1,7-octadiyne, CuI, [Pd(PPh3)2Cl2], Et2NH, RT, 3 h. Com-
pound 3 65%. iv) 1-(2-pyridyl)octa-1,7-diyne, CuI, [Pd(PPh3)2Cl2], Et2NH, RT, 14 h. Compound 4a 74%. v) b) 2-iodothiophene, CuI, [Pd(PPh3)2Cl2], Et3N, RT, 14 h.
Compound 4b 96%. c) iodobenzene, CuI, [Pd(PPh3)2Cl2], Et2NH, RT, 14 h. Compound 4c 54%.
Chem. Eur. J. 2014, 20, 8421 – 8432
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c with [Cp2ZrCl2] and nBuLi in THF results in the formation of
zirconium metallacycle intermediates, which were subsequent-
ly treated with PhPBr2 to afford PPAs 5a–c (Scheme 2). Note-
worthy is a difference in stability between the three PPAs. PPA
5a is stable under ambient conditions and on silica, thereby al-
lowing purification by column chromatography without visible
decomposition. In contrast, 5b was found to be less stable,
and new red bands appeared during chromatographic purifica-
tion, even when a rapid flash procedure was applied. The ap-
pearance of the red bands is consistent with partial oxidation
of 5b during chromatographic workup. Compound 5c was not
isolated and was directly subjected to oxidation with elemental
sulfur (S8) (see below).
For a first indication of whether it would be possible to per-
form selective chemistry on either of the two phosphorus het-
eroatoms, PPAs 5a–c were subjected to oxidative conditions
by using an excess amount of sulfur in CH2Cl2. Although such
oxidation by sulfur is well established for phospholes[9,10d,h] it
has previously been shown that phosphaalkenes can also be
oxidized by sulfur to thiaphosphiranes.[20] However, procedures
for phosphaalkene oxidation usually suggest the addition of
base to accelerate the reaction, although sulfurization is also
described to proceed without a base but under extended reac-
tion times.[20] It was thus an objective of this work to investi-
gate whether selective oxidation of the phosphole in the series
of PPAs is possible while maintaining the intactness of the
phosphaalkene moiety. First indications of selective oxidation
of the phospholes to a s4,l5 center can be anticipated from
the 31P{1H} NMR spectra of the reaction products (Table 1). For
PPAs 6a–c, the 31P{1H} NMR spectroscopic signals of the phosp-
hole phosphorus centers are shifted downfield by approxi-
Table 1. 31P{1H} NMR spectroscopic chemical shifts [ppm] of the PPAs
(CD2Cl2) and 3J(P,P) coupling constants [Hz].
Entry Substituent
31P{1H} phosphole 31P{1H} phosphaalkene 3J(P,P)
5a
5b
5c
6a
6b
6c
7
pyridyl
thienyl
phenyl
pyridyl
thienyl
+10.4
+10.5
+12.2
+52.3
+51.7
+53.4
+38.3
+44.1
+249.0
+249.4
+246.2
+269.6
+268.6
+267.8
+279.4
+199.0
89.7
92.8
90.3
35.8
36.7
35.7
75.6
45.0
Figure 1. ORTEP plots of phosphaalkenes 3 and 4b,c (ellipsoids are drawn at
50% probability level). Hydrogen atoms and disorder in the alkyl groups are
omitted for clarity. Selected bond lengths [ꢃ]: Compound 3: P1ꢀC1 1.855(2),
P1=C19 1.693(2), C21ꢁC22 1.207(3), C27ꢁC28 1.194(4); compound 4b: P1ꢀ
C1, 1.852(3), P1=C19 1.686(3), C21ꢁC22 1.194(6), C27ꢁC28 1.183(6); Com-
pound 4c: P1ꢀC1 1.8449(16), P1=C19 1.6834(18), C21ꢁC22 1.198(11), C27ꢁ
C28 1.185(10).
phenyl
pyridyl*(AuCl)
pyridyl*(AuCl)2
8
Scheme 2. Synthesis of phosphole-phosphaalkenes (PPAs) 5,6a–c. i) 1) [Cp2ZrCl2], nBuLi (2 equiv), THF, ꢀ788C 1 h, then RT, 14 h; 2) PhPBr2, ꢀ788C, then for
5a; RT, 4 h, 5b,c; RT, 24 h. Compound 5a 42%, 5b 21%, 5c not isolated. ii) sulfur, CH2Cl2, RT, 14 h. Compound 6a 40%, 6b 19% (over 2 steps), 5c 29% (over
2 steps).
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mately 40 ppm to approximately 52 ppm, which is in good ac-
cordance with the chemical shifts of other s4,l5-thioxophos-
pholes.[9,10d,h] Interestingly, also the 31P{1H} NMR spectroscopic
resonances of the phosphaalkene phosphorus atoms are signif-
icantly shifted downfield by approximately 20 ppm. Neverthe-
less, the observed chemical shifts are still in the typical region
for phosphaalkenes, which indicates that the phosphaalkene is
left intact. Comparison with typical chemical shifts of oxidation
products of phosphaalkenes, that is, l5-phosphorane (dP =
10.0 ppm) and thiaphosphirane (dP =ꢀ34.9 ppm) observed
during oxidation of Mes*P=CPh2 with sulfur further supports
our assignment.[20b] HRMS and X-ray crystallography (see
below) further confirm that only the phosphole phosphorus is
oxidized, thus encouraging our strategy to utilize the different
reactivity of the two phosphorus centers in PPAs for the tuning
of their electronic properties. Despite their differences in
oxygen sensitivity, it is also evident that the 31P{1H} NMR spec-
troscopic chemical shifts of both P centers (see Table 1) of the
PPAs are only marginally influenced by the electronic nature of
the different aryl substituents in the 5-position, similar to previ-
ous observations of pyridyl-, thienyl-, or phenyl-substituted
phospholes.[21] Furthermore, it can be noted that the coupling
constants between the two phosphorus atoms decrease upon
sulfurization of the phospholes.
phorus atom. The nitrogen heteroatom of the pyridyl substitu-
ent in both 5a and 6a has an s-trans relationship with respect
to the phosphole phosphorus in the solid state, whereas the
phosphaalkene resides in an s-cis arrangement to form a biden-
tate binding pocket in the solid state. In contrast, the solid-
state structure of 6b,c shows an s-trans arrangement between
the two phosphorus atoms. The sulfur heteroatom of the
thienyl substituent in 6b has an s-trans relationship with re-
spect to the phosphole phosphorus in the solid state. The
bond lengths for the P=C double bonds in 5a and 6a–c are in
the usual range as are the bond lengths within the phosphole
core.[21] Furthermore the CꢀC bond bridging the phosphole
and the phosphaalkene in 5a and 6a–c is typical for a Csp2ꢀ
Csp2 single bond.
To investigate the selectivity of metal coordination toward
the two phosphorus heteroatoms, 5a was treated with [AuCl-
(tht)] (Scheme 3; tht=tetrahydrothiophene). With one equiva-
lent of [AuCl(tht)], coordination occurs exclusively at the
phosphole phosphorus to afford PPA 7. The 31P{1H} NMR spec-
troscopic signal of the s4,l4-phosphorus center is shifted
downfield by approximately 28 ppm to approximately 38 ppm.
Analogous to the oxidation of the phosphorus atom of the
phosphole ring by sulfur, the coordination by [AuCl(tht)] shifts
the 31P{1H} NMR spectroscopic signal of the phosphaalkene
phosphorus downfield (Table 1). The selective coordination of
the s3,l3-phosphorus is verified by X-ray crystallography as
shown in Figure 3. The solid-
Figure 2 shows the crystal structures of 5a and 6a–c. Struc-
tures 6a–c confirm the selective oxidation of the s3,l3-phos-
state structure of 7 shows similar
structural parameters to 5a with
a
typical P=C bond length
(1.698(8) ꢃ) and expected AuI co-
ordination. The P-Au-Cl fragment
shows
a
linear arrangement
(177.56(7)8) with typical bond
lengths (Au1ꢀP1: 2.234(2) ꢃ and
Au1ꢀCl1: 2.295(2) ꢃ). Interesting-
ly, the phosphaalkene, the
phosphole moiety, and the
pending pyridine fragment are
almost coplanar (P1-C8-C9-P2
ꢀ4.5(9)8) and calculated angles
of the least-squares plane (l.s.pl.)
between pyridine, phosphole,
and phosphaalkene are 13.3(4)
and 9.2(4)8, respectively. In con-
trast to the solid-state structures
of 5a and 6a, the pyridine ring
has an s-cis relationship with re-
spect to the phosphole phos-
phorus in the solid state. Inter-
estingly, treating 5a with two
equivalents of [AuCl(tht)] results
in an upfield shift of the
31P{1H} NMR spectroscopic signal
of 8 to d=199 ppm relative to
d=249 ppm for 5a of the phos-
phaalkene phosphorus concomi-
Figure 2. ORTEP plots of PPAs 5a and 6a–c (ellipsoids are drawn at 50% probability level). Hydrogen atoms and
disorder in the alkyl groups are omitted for clarity. Selected bond lengths [ꢃ] and angles [8]: Compound 5a: P1ꢀ
C1 1.7984(19), P1ꢀC8 1.811(2), P1ꢀC29 1.828(2), P2=C9 1.705(2), P2ꢀC11 1.8582(19), C1ꢀC2 1.372(3), C2ꢀC7
1.463(3), C7ꢀC8 1.378(3), C8ꢀC9 1.467(3); C1-P1-C8 91.28(9), C9-P2-C11 104.23(9), P2-C9-C8 115.61(15). Compound
6a; P1ꢀC1 1.802(4), P1ꢀC8 1.823(4), P1ꢀC29 1.821(3), P2=C9 1.706(4), P2ꢀC11 1.839(4), C1ꢀC2 1.367(5), C2ꢀC7
1.482(5), C7ꢀC8 1.377(5), C8ꢀC9 1.455(5), P1ꢀS1 1.9500(12); C1-P1-C8 94.17(17), C9-P2-C11 104.03(18), P2-C9-C8
114.67(3). Compound 6b; P1ꢀC1 1.813(2), P1ꢀC8 1.814(2), P1ꢀC21 1.822(2), P1ꢀS1 1.9493(11), P2=C13 1.697(2),
P2ꢀC15 1.846(2), S2ꢀC12 1.707(3), S2ꢀC9 1.741(2), C1=C2 1.364(3), C1ꢀC13 1.471(3), C2ꢀC7 1.487(3), C7=C8
1.362(3), C9=C10 1.377(3), C10ꢀC11 1.412(3), C11=C12 1.355(4), C13ꢀC14 1.509(3); C13-P2-C15 101.93(10), C1-P1-
C8 93.41(10), C1-C13-P2 115.99(16). Compound 6c: P1=C9 1.6889(12), P1ꢀC11 1.8580(12), P2ꢀC1 1.7999(12), P2ꢀ
C8 1.8214(12), P2ꢀC29 1.8150(13), C1ꢀC2 1.3487(17), C1ꢀC35 1.4782(16), C2ꢀC7 1.4955(16), C7ꢀC8 1.3571(17),
C8ꢀC9 1.4762(16), P2ꢀS1 1.9482(4); C1-P1-C8 94.17(17), C9-P2-C11 104.03(18), P2-C9-C8 114.67(3).
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Electronic absorption spectros-
copy and cyclic voltammetry
The electronic absorption spec-
tra of all PPAs are characterized
by a strong absorption in the
visible region, except for those
of 6c and the Au2 complex 8,
which feature additional absorp-
tions at higher energy (Figure 3
and Table 2). The observed ab-
sorption maxima are attributed
to p!p* transitions of the con-
jugated system on the basis of
quantum chemical calculations
(see below). The nature of the
2,5-substituents and the phosp-
hole-P oxidation state affect the
optical properties of phospholes
5a,b and 6a–c. The presence of
a thienyl group in 5b relative to
a pyridyl group in 5a leads to
a redshift of the lowest-energy
absorption maximum by 20 nm.
Sulfur oxidation of phospholes
Scheme 3. Coordination of AuCl to phosphole-phosphaalkene (PPA) 5a. i) [AuCl(tht)] (1 equiv), CH2Cl2, RT, 1 h. 7
98%. ii) [AuCl(tht)] (2 equiv), CH2Cl2, RT, 1 h. Compound 8 61%. ORTEP plot of PPAs 7 and 8 (ellipsoids are drawn
at 50% probability level). Hydrogen atoms and disorder in the alkyl groups are omitted for clarity. Selected bond
lengths [ꢃ] and angles [8]: Compound 7: Au1ꢀP1 2.234(2), Au1ꢀCl1 2.295(2), P1ꢀC1 1.813(7), P1ꢀC22 1.817(9), P1ꢀ
C8 1.831(7), P2ꢀC9 1.698(8); P1-Au1-Cl1 177.56(7). Compound 8: C1ꢀP9 1.79(2), C9ꢀP7 1.682(18), P7ꢀAu1 2.226(4),
P9ꢀAu2 2.228(5), Cl1ꢀAu1 2.280(4), Cl2ꢀAu2 2.280(4), Au1ꢀAu2 3.0692(11); P7-Au1-Cl1 170.85(19), P9-Au2-Cl2
175.6(2).
Table 2. UV/Vis spectroscopic data of PPAs 5a,b and 6a–c and gold com-
plexes 7 and 8 in CH2Cl2 at room temperature.
Entry
Substituent
lmax [nm]
e [mꢀ1 cmꢀ1
]
lonset [nm]
5a
5b
5c
6a
6b
6c
7
pyridyl
thienyl
phenyl
pyridyl
thienyl
415
435
407
429
397
396
378
415
14000
12000v
7700
14500
6400
6300
5900
14000
485
502
506
529
491
514
455
485
phenyl
pyridyl*(AuCl)
pyridyl*(AuCl)2
8
5a,b to 6a,b induces a small blueshift of the absorption maxi-
mum and a redshift of the onset absorption (Dlonset (5aꢀ6a)=
21 nm, Dlonset (5bꢀ6b)=27 nm). The longest wavelength ab-
sorption maximum of phenyl-substituted 6c is the highest in
energy of all the PPAs investigated.
Figure 3. UV/Vis spectra of PPAs 5a,b, 6a–c, and gold complexes 7 and 8 in
CH2Cl2 at room temperature. Synthesis of phosphole-phosphaalkenes (PPAs)
5,6a–c.
Coordination of one AuCl to 5a results in a blueshift of lmax
by 19 nm, and the lonset shifts by 29 nm to lower energies. In-
terestingly, coordination of a second AuCl fragment as in 8 re-
sults in a further hypsochromic shift, both relative to 5a and 7
for lmax. In contrast to the monoaurated complex 7, the lonset is
significantly shifted to higher energies upon complexation of
a second AuCl fragment. As described above, the Au–Au inter-
action in 8 twists the P=C unit out of the phosphole plane and
thereby breaks effective conjugation between the two chro-
mophores, which explains the hypsochromic shift that is ob-
served in the UV/Vis spectrum of 8. When comparing the UV/
Vis data of 5a with a related phosphole, 1-phenyl-2,5-bis(2-pyr-
idyl)phosphole (lmax =374 nm), it is noticeable that the incor-
tant with a similar chemical shift of the aurated phosphole.
The solid-state structure of 8 confirms the formation of a dime-
tallic species and shows that one AuCl is coordinated to each
phosphorus heteroatom; it also confirms the presence of
a gold–gold interaction (3.069(1) ꢃ) (Scheme 3). This metallo-
philic interaction twists the P=C out of the phosphole plane
(P9-C8-C7-P7 ꢀ67(2)8 and angle between l.s.pl. 64.9(10)8) and
thereby breaks the effective conjugation between the phosp-
hole moiety and the phosphaalkene in 8. Again, the pyridine
fragment is almost coplanar to the phosphole unit (9.7(10)8).
(Scheme 3)
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poration of a phosphaalkene motif induces a bathochromic
shift of Dlmax =41 nm.[21] The same trend is observed for 6c
when comparing with 1,2,5-triphenylthiooxophosphole
(Dlmax =17 nm), whereas 6b shows almost the same end ab-
5a,b and their corresponding thiooxophospholes 6a,b stand
in contrast to the observation of, for example, 1-phenyl-2,5-
bis(2-thienyl)phosphole, in which oxidation by sulfur was
found to lead to a shift of the mildest oxidation potential by
280 mV.[21] The above comparisons imply that the HOMOs of 5
and 6 reside mainly on the extended p system that includes
the phosphaalkenes and possibly the butadiene of the phosp-
hole, but with only marginal contributions of the auxiliary pyr-
idyl or thienyl substituent. The rich oxidative chemistry of 5
and 6 at relatively mild potentials indicates closely spaced oc-
cupied frontier orbitals, which was also found in DFT calcula-
tions. The first oxidation potentials of gold complexes 7 and 8
are observed at more positive potentials than both 5a and 6a.
sorption
as
1-phenyl-2,5-bis(2-thienyl)thiooxophosphole
(Dlmax =3 nm),[22] thereby suggesting an electronically similar
effect of the phosphaalkene and the thiophene moiety.
PPAs 5a,b and 6a–c exhibit a rich electrochemistry and their
cyclic voltammograms feature multiple reduction and oxida-
tion processes (Table 3). The first reduction of 5a and 5b
Table 3. Electrochemical data for 1 mm solutions of the PPAs in CH2Cl2
(0.1m NBu4PF6); n=100 mVsꢀ1 (all potentials are given versus Fc+/0).
Entry
Ep,c [V]
Ep,a [V]
+0.81
+0.62
+0.90
+0.59
+0.70
+1.09
–
Density functional theory (DFT) calculations
5a
5b
6a
6b
6c
7
–
–
–
–
–
ꢀ2.16
ꢀ2.18
ꢀ1.90
ꢀ1.99
ꢀ2.05
ꢀ1.86
ꢀ1.57
ꢀ2.27
ꢀ2.29
ꢀ2.20
ꢀ2.28
ꢀ2.27
ꢀ2.24
ꢀ1.69
+0.48
+0.37
+0.50
+0.39
+0.51
+0.66
+1.04
+0.99
+0.99
+1.08
+1.02
+1.07
–
+1.26
+1.08
–
DFT calculations of the different PPAs were performed to gain
further insight into the conformations of the PPA, delocaliza-
tion through the p systems, and the influence of the phos-
phaalkene moiety. Computations were performed at the DFT
B3LYP/6-311G** level of theory, which is known to perform
well for main group systems.[7k,23] As a first step, we decided to
investigate the energy difference between the s-cis and s-trans
conformations of PPAs 5c and 6c with respect to the torsion
angle between the phosphole and the phosphaalkene moiety
(Figure 4). Since both conformations were observed in solid-
+1.21
–
–
–
ꢀ1.53
ꢀ0.97
8
–
occurs at very similar potential of ꢀ2.16 and ꢀ2.18 V, respec-
tively, and is thus not influenced by the pyridyl or thienyl auxil-
iary substituent. The situation is somewhat different in the re-
duction potentials observed for oxidized 6a–c, which can be
detected over a broader potential range of ꢀ1.90 to ꢀ2.05 V
depending on the substituent (all potentials are measured in
CH2Cl2 and given versus the ferrocene/ferrocenium (Fc+/0
)
couple). In general, electron uptake by oxidized 6a–c occurs at
milder potentials than those of 5a,b.
When comparing PPAs 5a,b to literature-known phospholes
that contain an additional pyridyl or thienyl group instead of
the phosphaalkene,[21] a large difference in reduction potential
can be observed. Whereas 5a displays a first reduction at
ꢀ2.16 V, the equivalent process occurs at significantly more
negative potential in 1-phenyl-2,5-bis(2-pyridyl)phosphole at
ꢀ2.45 V.[21] The electronic effect of the phosphaalkene on the
reduction potential of 6a,b is, however, negligible as they are
almost identical (the difference is 20–40 mV) to those of 1-
phenyl-2,5-bis(2-pyridyl)thiooxophosphole and 1-phenyl-2,5-
bis(2-thienyl)thiooxophosphole, respectively. The first reduction
potentials of gold complexes 7 and 8 are observed at signifi-
cantly milder potentials than both 5a and 6a. It should be
noted, however, that 8 was not stable during the voltammetry
experiments. The reported values in Table 3 are from the first
scan, whereas new reduction and/or oxidation waves are visi-
ble in the second scan, possibly due to cleavage of the highly
twisted structure or demetalation.
Figure 4. The relative conformations between the P=C double bond and the
orientation of the phosphole. The designation s-cis is used when the torsion
angle between the two phosphorus heteroatoms is close to 08 (top left) and
anti when the torsion angle between the two phosphorus heteroatoms is
close to 1808 (top right). The designation s-cis/s-trans is used when the tor-
sion angle between the two phosphorus heteroatoms is close to 08 and the
torsion angle between the phosphole phosphorus and the nitrogen (or
sulfur for 5,6b) is close to 1808 (bottom left); s-cis/s-cis is used when the tor-
sion angle between the two phosphorus heteroatoms is close to 08 and the
torsion angle between the phosphole phosphorus and the nitrogen (or
sulfur for 5,6b) is close to 08 (bottom right).
state structures of 5 and 6, the expectation was that the
energy difference between the two arrangements of the phos-
phorus heteroatoms would be very small. For both 5c and 6c,
the s-trans arrangement is favored by 0.53 and 1.60 kJmolꢀ1,
respectively, which is in good agreement with the observations
from X-ray crystallography and also indicates that both con-
formers can be expected to be present in solution.
The first oxidation potentials of 5a and 5b are almost identi-
cal to those of 6a and 6b, respectively, which points towards
the absence of any contribution of the phosphole phosphorus
in the HOMO and large contributions of the auxiliary substitu-
ents, interconnected by the butadiene unit within the phosp-
hole. The similarities of the first oxidation potentials between
Chem. Eur. J. 2014, 20, 8421 – 8432
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As the pyridyl-phospholes in 5a and 6a are potential biden-
tate metal-binding sites, the conformational preference of
these two subunits relative to each other was investigated. Ac-
cording to the X-ray crystallographic structures of the com-
pounds, the s-cis conformations of the two phosphorus heter-
oatoms were kept, and the torsion angles between the phosp-
hole phosphorus and the nitrogen were set either close to 08
in s-cis/s-trans and close to 1808 in the s-cis/s-cis. The struc-
tures were then allowed to fully relax during geometry optimi-
zation (Figure 4). In both 5a and 6a, the s-cis/s-trans confor-
mations were favored by 1.35 and 2.08 kJmolꢀ1, respectively,
values that correlate well with the X-ray crystallographic struc-
tures. However, we assume that the rotational barrier is very
low, which might also explain the pyridine conformation found
in gold complexes 7 and 8, both of which show the s-cis/s-cis
conformation. Owing to the small energy difference found be-
tween the different conformers for 5a,c and 6a,c, it was decid-
ed that the geometries for geometry optimization 5b and 6b
were based on the structures obtained from the X-ray crystal-
lographic analysis.
tions from the auxiliary substituents. These results are thus in
excellent agreement with the conclusions from the electro-
chemical studies. In general, PPAs 6a,b are more difficult to ox-
idize than 5a,b; this is reflected well in the energies of the cal-
culated HOMOs, which are higher in energy for 5a,b than for
6a,b, even though the quantitative agreement is not excellent.
As for the HOMOs, the LUMOs are located on the phosphaal-
kene and the butadiene unit with contributions from the sub-
stituents. However, for PPAs 5, there is a contribution from the
phosphorus lone pair of the phosphole, whereas the contribu-
tion of the P=S is smaller for the oxidized PPAs 6. The LUMO
lies higher in energy for 5a,b relative to 6a,b, which reflects
the experimentally obtained reduction potentials in which
6a,b are reduced at milder potentials than 5a,b.
Gold complexes 7 and 8 were calculated at the PBE1PBE/6-
311G**/LANL2DZ level of theory. Similar to their precursor 5a,
both the HOMOs and the LUMOs of 7 and 8 are localized on
the phosphole and phosphaalkene motifs with small contribu-
tions from the pyridyl substituent. The AuCl moiety connected
to the phosphaalkene of 8 contributes to the HOMO with a dxy
orbital localized on the Au and a p orbital localized on the
chlorine atom. The contribution of the phosphaalkene moiety
in 8 is decreased owing to the twist between the phosphaal-
kene and the phosphole ring (dihedral angle of 78.38) relative
to the situation in 5 and 6 in which the two moieties are gen-
erally are more coplanar (dihedral angles of (150ꢂ15)8 both in
the experimental and the theoretical data). This twist is also re-
sponsible for the relatively high-energy transition in the UV/Vis
spectrum of 8. The HOMO and the LUMO are lower in energy
for 7 and 8 than those of 5a and 6a, which is in good agree-
ment with the electrochemical data in which both 7 and 8 are
more difficult to oxidize than 5 and 6. Diaurate complex 8 is
the most difficult compound to be oxidized of the series
(Figure 6).
Graphical representation of the calculated FMOs of com-
pounds 5a–c and 6a–c are shown in Figure 5. The FMOs have
almost exclusively a pp character. In agreement with the elec-
tronic absorption spectra, both the phosphaalkene moiety and
the p-conjugated backbone of the phosphole are participating
in both the HOMO and the LUMO. The lowest energy absorp-
tions in the UV/Vis spectra can therefore be expected to result
from p!p* transitions. The trends observed for the PPAs from
the electronic absorption spectroscopy are also observed from
the LUMO–HOMO energies of the DFT calculations. The lon-
gest-wavelength absorption maximum of phenyl-substituted
5c and 6c are highest, and thienyl-substituted 5b and 6b are
the lowest in energy of all PPAs. The bathochromic shifts of
the lonset upon oxidation by sulfur is also reflected in the
values of the calculated LUMO–HOMO gaps. The calculated
LUMO–HOMO gaps for 6a,b of 3.06 and 2.91 eV are in excel-
lent agreement with the values obtained for the lowest energy
absorption maxima of 3.05 (407) and 2.89 eV (429 nm), respec-
tively, obtained from the UV/Vis absorption experiments.
The calculations also show that the phosphole-phosphorus
does not participate in the HOMOs of 5 and 6, which are local-
ized on the phosphaalkene and the butadiene with contribu-
Conclusion
In summary, we have established a synthetic versatile route to
the first examples of phosphole-phosphaalkenes that unite
two differently hybridized (s3,l3, and s2,l3) phosphorus hetero-
atoms as integral parts of one fully p-conjugated framework.
We have also demonstrated that selective modifications of
Figure 5. Calculated HOMO and LUMO with orbital energies [eV] for PPAs 5 and 6 at the DFT B3LYP/6-311G** level of theory; DH–L HOMO–LUMO splitting in
eV.
Chem. Eur. J. 2014, 20, 8421 – 8432
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sidual solvent signal (CHCl3, dc =77.1 ppm, central signal, CH2Cl2,
dc =54.00 ppm). 31P{1H} NMR spectra were recorded using a JEOL
Eclipse 400 MHz spectrometer (operating at 161.8 MHz), referenced
externally to 85% H3PO4 (aqueous).
UV/Vis spectroscopy
UV/Vis measurements were performed using a Varian Cary 50 in-
strument or a Varian Cary 5000 instrument at room temperature.
The measurements were performed in 1ꢄ1 cm2 optical quartz cells
as solutions in CH2Cl2.
Electrochemistry
The electrochemical measurements were conducted in dry CH2Cl2
(distilled from calcium hydride). An Autolab potentiostat with
a GPES electrochemical interface (Eco Chemie) was used for cyclic
voltammetry measurements with a glassy carbon disc (diameter
3 mm, freshly polished) as the working electrode. The counter elec-
trode was a glassy carbon rod and the reference electrode a nona-
queous Ag+/Ag electrode (a silver wire immersed into 10 mm
AgNO3 in acetonitrile) with a potential of ꢀ0.08 V versus the ferro-
cenium/ferrocene (Fc+/Fc) couple in CH2Cl2 as an external stan-
dard. The cyclic voltammograms were conducted at a scan rate (n)
100 mVsꢀ1. All solutions were conducted on 1 mm solutions of the
phospholes in dry CH2Cl2 with 0.1m tetrabutylammonium hexa-
fluorophosphate (NBu4PF6) as supporting electrolyte. Before all
measurements, oxygen was removed by bubbling with argon and
during the measurement all samples were kept under argon.
Figure 6. Calculated HOMO and LUMO with orbital energies [eV] for PPAs 7
and 8 at the DFT PBE1PBE/6-311G**/LANL2DZ level of theory; DLUMO–HOMO
HOMO–LUMO splitting in eV.
these compounds at the phosphole unit can be achieved by
using oxidation (S8) and metal coordination (AuCl). Ultimately,
a diaurated species was obtained with intermetallic distances
of 3.069(1) ꢃ. The Au–Au interaction induces a twist in the mo-
lecular structure that is responsible for a significant decrease in
the conjugation path. UV/Vis spectroscopic studies show that
the phosphaalkene is part of the extended p system and
thereby shifts the lowest-energy absorption maxima towards
lower energies than similar phosphole reference compounds
that lack the phosphaalkene unit. Cyclic voltammetry and ab
initio calculations both indicate that the phosphaalkene
moiety is mainly involved in the HOMO, whereas the phosp-
hole phosphorus participates in the LUMO of PPAs 5. This
result encourages further manipulations of the PPAs to tune
the optical properties of this new class of p-conjugated materi-
als and the use of such materials for organic electronics appli-
cations.
Mass spectrometry
Low-resolution mass spectrometry was performed using a Thermo
LCQ Deca XP Max with direct injection electrospray ionization (ESI).
A small amount of the compounds were dissolved in a solution of
approximately 2.5 mm AgBF4 in MeOH. High-resolution mass spec-
trometry (HRMS) were performed using a high-resolution and
FTMS+pNSI mass spectrometer (orbitrapXL), using a MicroTOF
spectrometer with ESI core at University of Muenster, or using
a Varian MAT 311, Waters Q-TOF 2, or ZabSpec TOF Micromass in-
strument at CRMPO of Rennes 1. MALDI-MS spectra were obtained
in a Ditranol matrix using a nitrogen laser accumulating 50 laser
shots and a Bruker Microflex LT MALDI-TOF mass spectrometer.
Experimental Section
Materials and general methods
Computational details
Chemicals were purchased from Sigma–Aldrich, VWR, ABCR, and
Fisher Scientific and were used as received. THF and Et2O were dis-
tilled from sodium/benzophenone. CH2Cl2 was distilled from calci-
um hydride or freshly purified using MBraun SPS-800 drying col-
umns. All reactions were performed under an inert atmosphere of
N2 or Ar using standard Schlenk techniques. Column chromatogra-
phy was performed using Merck silica gel Si-60 ꢃ (35–70 or 0.063–
0.200 mm) or on basic alumina (Aldrich type 5016A, 150 mesh,
58 ꢃ).
Calculations were carried out with the Gaussian 09 program pack-
age (G09 rev. A.02). The PPAs were fully optimized at the B3LYP/6-
311G** level of theory and were determined as true minima by in-
spection of their harmonic frequencies with no imaginary frequen-
cies. Molecular orbitals were calculated at the same level of theory.
Gold complexes were calculated at the PBE1PBE level of theory
with a 6-311G** basis set for CHNPCl and a LANL2DZ with ECP for
gold.[24]
NMR spectroscopy
Synthesis
1H NMR spectra were recorded at ambient temperature using
a JEOL Eclipse 400 MHz spectrometer (operating at 399.8 MHz),
Phosphaalkene 3: Mg turnings (350 mg, 14.4 mmol) were
added to 25 mL three-necked round-bottomed flask equipped
with a condenser, a stirring bar, and a dropping funnel. THF
(5 mL) was added and the suspension was heated to reflux.
EtBr (0.25 mL, 3.4 mmol) was added in one portion, and EtBr
(0.75 mL, 10.2 mmol) dissolved in THF (20 mL) was added to
the dropping funnel. The solution was added dropwise to the
a
Varian Mercury Plus 300 MHz spectrometer (operating at
300.03 MHz) or Bruker AM300, AM400, or AM500 instruments.
Chemical shifts are given in ppm and referenced internally to the
residual solvent signal (CHCl3, dH =7.26 ppm or CH2Cl2, dH =
5.32 ppm). 13C NMR spectra were recorded on the same instrument
(100.5 or 75.4 MHz) and were also referenced internally to the re-
Chem. Eur. J. 2014, 20, 8421 – 8432
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Mg suspension that was heated to reflux, and the Mg turnings
started to disappear. After stirring for 30 min, the solution was
transferred to a 250 mL three-necked round-bottomed flask
that contained THF (100 mL). The pale gray solution was
brought to reflux, and 1,7-octadiyne (1.5 mL, 11.3 mmol) in
THF (40 mL) was added dropwise to the solution that was
heated to reflux. The pale gray solution became clear and
a white precipitate was formed. Mes*P=CCH3Br 2 (1.80 g,
4.70 mmol), [Pd(dba)2] (269 mg, 0.47 mmol), and PPh3 (123 mg,
0.47 mmol) were added to the white suspension. The reaction
mixture turned dark green and was stirred at reflux for 4–5 h
until all the starting material had disappeared by TLC (silica,
pentane). If no change could be detected by TLC and there
was still starting material left, additional [Pd(dba)2] was added
to complete the reaction. The reaction mixture was cooled to
room temperature and filtered through a plug of silica, which
was rinsed with CH2Cl2. The solvent was removed under
vacuum and subsequently subjected to column chromatogra-
phy on silica with pentane as eluent to yield an off-white solid.
Crystallization from CH2Cl2/CH3CN gave colorless crystals. Yield:
1.88 g, 4.60 mmol (98%).
(m, 4H; CH2), 7.18 (m, 1H; pyridine H5), 7.38 (m, 1H; pyridine
H4), 7.40 (s, 2H; Mes*), 7.60 (m, 1H; pyridine H4), 8.54 ppm (d,
1H; pyridine H6); 31P{1H} NMR (161.8 MHz, CDCl3): d=
274.7 ppm; 13C NMR (100 MHz, CDCl3): d=19.0 (s; Py-Cꢁ
4
2
CCH2CH2), 19.9 (d, J(P,C)=4.8 Hz; CꢁCCH2CH2), 25.4 (d, J(P,C)=
5
14.5 Hz; CH3), 27.6 (s; Py-CꢁCCH2CH2), 28.0 (d, J(P,C)=4.6 Hz;
CꢁCCH2CH2), 31.4 (s; p-C(CH3)3), 32.6 (d, 4J(P,C)=6.8 Hz; o-
C(CH3)3), 35.0 (s; o-C(CH3)3), 38.0 (s; p-C(CH3)3), 80.6 (s; CꢁCPy),
85.9 (d, 3J(P,C)=18.6 Hz; P=CꢀCꢁC), 90.6 (s; CꢁCPy), 97.5 (d,
2J(P,C)=29.3 Hz; P=CꢀCꢁC), 121.6 (s; meta-Ph), 122.3 (s; C5,
Py), 126.8 (s; C3, Py), 136.8 (s; C4, Py), 136.2 (d, 1J(P,C)=
60.3 Hz; PꢀC), 143.9 (s; C2, Py), 149.8 (s; C6, Py), 150.3 (s; para-
Ar), 153.7 (s; ortho-Ar), 162.1 ppm (d, 1J(P,C)=32.3 Hz; P=C);
MS (ESI): m/z: 592.5 (100) [M+Ag]+; HRMS (ESI): m/z calcd for
C33H44NNaP: 508.3109; found: 508.3106 [M+Na]+; m/z calcd for
C33H45NNaP: 486.3290; found: 486.3303 [M+H]+; elemental
analysis calcd (%) for C33H44NP: C 81.61, H 9.13; found: C 81.59,
H 8.92. The assignment was based on the assignment of 1-(2-
pyridyl)octa-1,7-diyne.[25]
Phosphaalkene 4b and 4c were prepared in a procedure analo-
gous to that used for the preparation of 4a in 96 and 54% yield,
respectively (see the Supporting Information).
Alternative procedure: Mes*P=CCH3Br 2 (1.50 g, 3.90 mmol)
was dissolved in Et2NH (30 mL), and the solution was deaerat-
ed by bubbling with argon. CuI (70 mg, 0.37 mmol), [Pd-
(PPh3)2Cl2] (223 mg, 0.32 mmol), and finally 1,7-octadiyne
(1.0 mL, 8.0 mmol) were added. The reaction mixture was
stirred for 3 h at room temperature until no Mes*P=CCH3Br
could be detected by TLC. The reaction mixture was concen-
trated and the solid extracted with pentane. Column chroma-
tography as above. Yield: 1.04 g, 2.54 mmol (65%). 1H NMR
Phosphaalkene 5a: Phosphaalkene 4a (250 mg, 0.51 mmol)
and [Cp2ZrCl2] (149 mg, 0.51 mmol) were added to a 150 mL
Schlenk flask, and the two compounds were dried for 2 h. THF
(10 mL) was added, and the yellow solution was cooled to
ꢀ788C. nBuLi (0.41 mL, 1.02 mmol, 2.5m in hexanes) was
added, and the resulting green solution was stirred at this tem-
perature for 1 h and then warmed to room temperature upon
which it turned red and left overnight. The following morning
the solution was cooled to ꢀ788C and PhPBr2 (0.11 mL,
3
(400 MHz, CDCl3): d=1.28 (d, J(P,H)=13.6 Hz, 3H; CH3), 1.33 (s,
9H; p-tert-Bu), 1.48 (s, 18H; o-tert-Bu), 1.69 (m, 4H; Cꢁ 0.56 mmol) was added. The red solution was warmed to room
4
CCH2CH2), 1.93 (t, J(P,H)=2.6 Hz, 1H; CꢁCCH), 2.25 (m, 2H; Cꢁ temperature and stirred for 4 h followed by a filtration through
CCH2), 2.47 (m; CꢁCCH2), 7.40 ppm (s, 2H; Ar); 31P{1H} NMR
basic alumina. The alumina was rinsed with THF (3ꢄ15 mL)
until the color of the eluent disappeared. The yellow-orange
solution was concentrated to yield 5a as a yellow solid. In
(161.8 MHz, CDCl3): d=275.3 ppm; 13C NMR (100 MHz, CDCl3):
4
d=18.0 (s; CꢁCCH2CH2), 19.9 (d, J(P,C)=3.8 Hz; CꢁCCH2CH2),
2
25.4 (d, J(P,C)=14.4 Hz; CH3), 27.7 (s; CꢁCCH2CH2), 27.8 (s; Cꢁ cases in which purification was needed, the phosphole was pu-
4
CCH2CH2), 31.3 (s; p-C(CH3)3), 32.5 (d, J(P,C)=6.2 Hz; o-C(CH3)3),
rified by column chromatography on silica with 10% Et2O in
35.0 (s; o-C(CH3)3), 37.9 (s; p-C(CH3)3), 68.4 (s; CꢁCH), 84.3 (s; Cꢁ pentane or by recrystallization from either pentane or CH2Cl2/
CH), 85.8 (d, 3J(P,C)=29.1 Hz; P=CꢀCꢁC), 97.5 (d, 2J(P,C)=
18.6 Hz; P=CꢀCꢁC), 121.6 (s; meta-Ph), 136.1 (d, 1J(P,C)=
60.3 Hz; PꢀC), 150.3 (s; para-Ar), 153.7 (s; ortho-Ar), 162.0 ppm
CH3CN. Yield: 127 mg, 0.21 mmol (42%). 1H NMR (400 MHz,
CDCl2): d=1.23 (s, 9H; tert-Bu) 1.30 (s, 9H; tert-Bu), 1.37 (s, 9H;
tert-Bu), 1.52 (d, 3J(P,H)=13.5 Hz, 3H; CH3), 1.70 (m, 2H;
CCH2CH2), 1.82 (m, 2H; CCH2CH2), 2.81 (m, 2H; CCH2CH2), 3.11
1
(d, J(P,C)=32.4 Hz; P=C); MS (ESI): m/z: 515.4 (100) [M+Ag]+;
3
HRMS (ESI): m/z calcd for C28H41NaP: 431.28436; found:
431.2843 [M+Na]+; elemental analysis calcd (%) for [C28H41P+
1/3CH3CN]: C 81.53, H 10.02; found: C 81.73, H 10.02.
(m, 1H; CCH2CH2), 3.33 (m, 1H; CCH2CH2), 6.95 (ddd, J(H,H)=
7.4 Hz, 4.8 Hz, 5J(H,H)=1.1 Hz, 1H; pyridyl, H5) 7.15–7.34 (m,
3H; Ph), 7.34–7.41 (m, 4H; Mes*, Ph), 7.41–7.43 (m, 1H; pyridyl,
3
Phosphaalkene 4a: 1-(2-Pyridyl)octa-1,7-diyne (367 mg,
2.0 mmol) and Mes*P=CCH3Br 2 (500 mg, 1.30 mmol) were
H3), 7.52 (ddd, J(H,H)=7.9, 7.4 Hz, J(H,H)=1.9 Hz, 1H; pyridyl
3
4
5
H4), 8.47 ppm (ddd, J(H,H)=4.8 Hz, J(H,H)=1.9 Hz, J(H,H)=
1.0 Hz, 1H; pyridyl H6); 31P NMR (161.8 MHz, CD2Cl2): d=10.4
(d, 3J(P,P)=89.7 Hz), 249.0 ppm (d, 3J(P,P)=89.7 Hz); 13C NMR
(100 MHz, CDCl2): d=23.7 (s; CCH2CH2), 24.0 (s; CCH2CH2), 26.6
(dd, J(P,C)=14.4, 14.6 Hz; P=CCH3), 29.7 (s; CCH2CH2, C9), 30.6
(d, J(P,C)=4.9 Hz; CCH2CH2, C12), 31.6 (s, CH3, para-tBu), 32.4
(d, J(P,C)=6.4 Hz; CH3, ortho-tBu), 32.9 (d, J(P,C)=7.4 Hz; CH3,
ortho-tBu), 35.4 (s; para-C(CH3)3), 38.2 (s; ortho-C(CH3)3), 38.4 (s;
ortho-C(CH3)3), 120.7 (s; C5, pyridyl), 122.2 (d, J(P,C)=13.7 Hz;
CH, phenyl), 123.8 (d, J(P,C)=9.5 Hz; C3, pyridyl), 128.9 (d,
added to
a deaerated solution of [Pd(PPh3)2Cl2] (71 mg,
0.10 mmol) and CuI (10 mg, 0.05 mmol) in Et2NH (10 mL). Upon
addition of the reagents, the yellow solution turned to a sus-
pension. The mixture was stirred overnight, and following
morning it was filtered and concentrated. Column chromatog-
raphy (40% pentane in Et2O (Rf =0.40)) gave the product as
1
a pale yellow solid. Yield: 466 mg, 0.96 mmol (74%). H NMR
3
(400 MHz, CDCl3): d=1.28 (d, J(P,H)=13.6 Hz, 3H; CH3), 1.32 (s,
9H; p-tert-Bu), 1.48 (s, 18H; o-tert-Bu), 1.78 (m, 4H; CH2), 2.50
Chem. Eur. J. 2014, 20, 8421 – 8432
www.chemeurj.org
8429 ꢂ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
J(P,C)=8.5 Hz; CH, phenyl), 130.0 (s; CH, phenyl), 132.4 (d,
J(P,C)=11.0 Hz; ipso-C, phenyl), 135.4 (s; CH, Mes*), 135.2 (s;
CH, Mes*), 136.3 (s; C4, pyridyl), 138.8 (dd, J(P,C)=6.5 Hz,
61.9 Hz; ipso-C, Mes*), 143.5 (dd, J(P,C)=11.0, 3.0 Hz;
C14),144.5 (dd, J(P,C)=18.4, 11.4 Hz; C7), 149.6 (dd, J(P,C)=
10.0, 4.5 Hz; C13), 149.7 (s; C6, pyridyl), 150.5 (s; para-C, Mes*),
152.7 (d, J(P,C)=24.2 Hz; C8), 154.2 (s; ortho-C, Mes*), 154.7 (s;
ortho-C, Mes*), 156.5 (d, J(P,C)=18.6 Hz; C2, pyridyl),
179.7 ppm (dd, J(P,C)=16.2 Hz, 43.3 Hz; P=C); MS (ESI): m/z:
700.4 [M+Ag]+; elemental analysis calcd (%) for [C39H49NP2 +
CH2Cl2]: C 70.79, H 7.57, N 2.06; found: C 70.80, H 7.50, N 1.88.
Phosphaalkene 5b was prepared analogously to 5a.
was rinsed with THF (3ꢄ20 mL) until the color of the eluent
disappeared. The yellow-orange solution was concentrated, re-
dissolved in CH2Cl2, and an excess amount of sulfur was
added. The solution was stirred overnight at room tempera-
ture, filtered, and the solvent was removed under vacuum.
Column chromatography was performed on silica using pure
pentane to elute the remaining starting material and byprod-
ucts, and then the polarity was increased to 5% Et2O in pen-
tane. This gave 6b as an orange solid. Yield 74 mg, 0.12 mmol
1
(19%). H NMR (300 MHz, CDCl2): d=1.24 (s, 9H; tert-Bu), 1.29
3
(s, 9H; tert-Bu), 1.40 (d, J(P,H)=1.9 Hz, 3H; CH3), 1.45 (s, 9H;
tert-Bu), 1.87 (m, 4H; CH2), 2.92 (m, 2H; CH2), 3.08 (m, 2H; CH2)
6.96 (dd, 1J(H,H)=3.8, 5.1 Hz, 1H; H4, thienyl H4), 7.24 (d,
1
Phosphaalkene 6a: An excess amount of sulfur was added
to PPA 5a (100 mg, 0.17 mmol) in CH2Cl2 (20 mL), and the red
reaction mixture was stirred overnight. The solvent was re-
moved under vacuum. The mixture was subjected to column
chromatography in pentane to elute the excess amount of
sulfur. The polarity was then increased to 40:60 pentane/Et2O
to elute the product. After removal of the solvent, 6a was ob-
tained as a yellow solid. Yield: 42 mg, 0.067 mmol (40%).
1H NMR (400 MHz, CD2Cl2): d=1.31 (s, 9H; tert-Bu) 1.37 (s, 9H;
1J(H,H)=3.5 Hz, 1H; thienyl, H5), 7.33 (d, J(H,H)=5.7 Hz, 1H;
thienyl, H3), 7.34 (s, 1H; Mes*) 7.38 (s, 1H; Mes*), 7.41 (m, 3H;
m-/p-H Ph), 7.87 ppm (ddd, J=1.5, 8.0, 13.9 Hz, 2H; ortho-
3
phenyl); 31P{1H} NMR (161.8 MHz, CD2Cl2): d=51.7 (d, J(P,P)=
36.7 Hz), 268.6 ppm (d, 3J(P,P)=36.7 Hz); 13C NMR (75 MHz,
CDCl2): d=23.1 (s; CCH2CH2), 23.5 (d, J(P,C)=2.3 Hz; CCH2CH2),
25.6 (dd, J(P,C)=14.3, 4.6 Hz; P=CCH3), 29.6 (d, J(P,C)=13.0 Hz;
CCH2CH2), 30.5 (dd, J(P,C)=9.1, 13.3 Hz; CCH2CH2), 31.6 (s; CH3,
para-tBu), 32.8 (d, J(P,C)=7.0 Hz; CH3, ortho-tBu), 33.1 (d,
J(P,C)=7.1 Hz; CH3, ortho-tBu), 35.4 (s; para-C(CH3)3), 38.2 (s;
ortho-C(CH3)3), 38.4 (s; ortho-C(CH3)3), 122.3 (s; CH, phenyl),
127.2 (s; thienyl C5), 127.7 (s; thienyl C4), 128.2 (dd, J(P,C)=5.2,
1.9 Hz; thienyl C3), 129.2 (d, J(P,C)=72.7 Hz; CH, ipso-C,
phenyl), 129.2 (d, J(P,C)=12.4 Hz; CH, phenyl), 131.4 (s; CH,
Mes*), 131.6 (s; CH, Mes*), 132.3 (d, J(P,C)=3.0 Hz; phenyl),
135.5 (dd, J(P,C)=1.7, 16.8 Hz; thienyl, C2), 137.0 (d, J(P,C)=
63.4 Hz; ipso-C, Mes*), 138.9 (d, J(P,C)=8.7 Hz; Cb), 140.2 (dd,
J(P,C)=20.2, 78.0 Hz; Ca), 145.8 (dd, J(P,C)=6.6, 22.0 Hz; Ca),
148.2 (dd, J(P,C)=18.8, 23.0 Hz; Cb), 150.8 (s; para-C Mes*)
154.5 (s; ortho-C Mes*), 174.0 ppm (dd, J(P,C)=8.2, 46.3 Hz; P=
C); HRMS (ESI): m/z 653.2567 [M+Na]+; calcd for C38H48NaP2S2
653.25649.
3
tert-Bu), 1.39 (d, J(P,H)=10.1 Hz, 3H; CH3), 1.54 (s, 9H; tert-Bu),
1.88 (m, 4H; CCH2CH2), 3.10–3.40 (m, 4H; CH2), 6.95 (m, 1H;
pyridyl, H5), 7.43 (s, 1H, Mes*), 7.47 (s, 1H, Mes*), 7.48–7.55 (m,
3H, Ph), 7.58–7.60 (m, 2H, Ph), 7.58–7.60 (m, 2H; Ph), 7.61 (m,
1H; pyridyl, H3), 7.94–7.99 (m, 1H; pyridyl, H4), 8.59–8.61 ppm
(m, 1H; pyridyl H6); 31P NMR (161.8 MHz, CD2Cl2): d=52.3 (d,
3J(P,P)=35.8 Hz), 269.6 ppm (d, 3J(P,P)=35.8 Hz); 13C NMR
(75 MHz, CD2Cl2): d=23.0 (s; CCH2CH2), 23.5 (s; CCH2CH2), 25.4
(dd, J(P,C)=4.2 Hz, 14.3 Hz; P=CCH3), 30.0 (d, J(P,C)=12.5 Hz;
CCH2CH2), 30.4 (dd, J(P,C)=7.0, 20.7 Hz; CCH2CH2), 31.6 (s;
para-tBu), 32.8 (d, J(P,C)=7.0 Hz; CH3, ortho-tBu), 33.1 (d,
J(P,C)=6.9 Hz; CH3, ortho-tBu), 35.4 (s; para-C(CH3)3), 38.2 (s;
ortho-C(CH3)3), 38.5 (s; ortho-C(CH3)3), 122.2 (s; C5, pyridyl),
122.4 (s; CH, phenyl), 124.6 (d, J(P,C)=2.5 Hz; C3, pyridyl),
129.2 (d, J(P,C)=73.2 Hz; ipso-C, phenyl), 129.2 (d, J(P,C)=
12.4 Hz; CH, phenyl), 131.5 (s; CH, Mes*), 131.5 (s; CH, Mes*),
132.1 (d, J(P,C)=3.0 Hz; phenyl), 136.2 (s; C4, pyridyl), 136.9 (d,
J(P,C)=63.7 Hz; ipso-C, Mes*), 141.4 (d, J(P,C)=21.0 Hz; C14/
C7), 142.2 (d, J(P,C)=21.1 Hz; C14/C7), 147.8 (dd, J(P,C)=18.3,
23.9 Hz; C13), 149.8 (s; pyridyl C6), 150.8 (s; Mes*), 152.9 (d,
J(P,C)=16.8 Hz; C8), 153.7 (dd, J(P,C)=20.9, 5.4 Hz; pyridyl C2),
154.6 (s; Mes*), 174.0 ppm (dd, J(P,C)=8.2, 46.5 Hz; P=C);
HRMS (ESI): m/z calcd for C78H98AgP4S2: 1359.51936; found:
1359.51927 [2M+Ag]+.
Phosphaalkene 6c was prepared from 4c in 29% isolated yield
analogously to the procedure described for 6b (see the Supporting
Information).
Phosphaalkene 7: [AuCl(tht)] (22 mg, 0.068 mmol) was
added to PPA 5a (40 mg, 0.068 mmol) in CH2Cl2 (10 mL), and
the reaction mixture turned immediately red. The mixture was
stirred under argon for 1 h and the solvent was removed
under vacuum. Recrystallization from CH2Cl2/CH3CN. Com-
pound 7 was obtained as a red-orange solid. Yield: 55 mg,
0.067 mmol (98%). 1H NMR (400 MHz, CD2Cl2): d=1.23 (brs,
9H; tert-Bu), 1.30 (s, 9H; tert-Bu), 1.39 (s, 9H; tert-Bu) 1.54 (d,
3J(P,H)=14.9 Hz, 3H; CH3), 1.81 (m, 4H; CCH2CH2), 2.94 (m, 1H;
CCH2CH2) 3.05 (m, 2H; CCH2CH2), 3.23 (m, 1H; CCH2CH2), 7.09
(m, 1H; pyridyl, H5), 7.38 (m, 5H; Mes*, Ph), 7.62 (m, 2H; pyrid-
yl H3 and/or H4 and/or phenyl), 7.75 (m, 2H; pyridyl H3 and/or
H4 and/or phenyl), 8.45 ppm (m, 1H; pyridyl H6); 31P NMR
Phosphaalkene 6b: Phosphaalkene 4b (300 mg, 0.62 mmol)
and [Cp2ZrCl2] (224 mg, 0.77 mmol) were added to a 100 mL
Schlenk flask, and the two compounds were dried for 4 h. THF
(20 mL) was added, and the yellow solution was cooled to
ꢀ788C. nBuLi (0.50 mL, 1.25 mmol, 2.5m in hexanes) was
added, and the resulting red solution was stirred at this tem-
perature for 1 h and then warmed to room temperature and
left overnight. The following morning the solution was cooled
to ꢀ788C, and PhPBr2 (0.18 mL, 0.9 mmol) was added. The red
solution was warmed to room temperature and stirred for 24 h
followed by a filtration through basic alumina. The alumina
3
(161.8 MHz, CD2Cl2): d=38.3 (d, J(P,P)=75.6 Hz), 279.4 ppm (d,
3J(P,P)=75.6 Hz); 13C NMR (101 MHz, CDCl3): d=22.3 (s;
CCH2CH2), 22.8 (s; CCH2CH2), 26.4 (brm; P=CCH3), 29.7 (d,
J(P,C)=9.9 Hz; CCH2CH2), 30.6 (brm; CCH2CH2), 31.4 (s; para-
tBu), 32.5 (brm; CH3, ortho-tBu, both groups), 35.1 (s; para-
C(CH3)3), 37.7 (s; ortho-C(CH3)3), 37.9 (s; ortho-C(CH3)3), 121.8 (s;
Chem. Eur. J. 2014, 20, 8421 – 8432
www.chemeurj.org
8430 ꢂ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
C5, pyridyl), 122.0 (s; CH, phenyl), 123.8 (brm, 2C; C3, pyridyl
and ipso-C Ph), 129.2 (d, J(P,C)=13.0 Hz; CH, phenyl), 129.2 (s;
CH, Mes*), 129.3 (s; CH, Mes*), 132.1 (s; phenyl), 134.6 (d,
J(P,C)=15.3 Hz; C4, pyridyl), 136.4 (2brm; ipso-C, Mes* and C7,
C8, C13, or C14), 148.1 (brm; C7, C8, C13, or C14), 149.3 (brm,
2C; C7, C8, C13, or C14), 150.7 (s; pyridyl C6), 153.6 (s; Mes*),
154.0 (brm under Mes* signals, pyridyl C2) 154.6 (s; Mes*),
174.0 ppm (dd, J(P,C)=9.2, 42.8 Hz; P=C); APCI/ASAP: m/z
calcd for C39H50NClP2Au: 826.2760; found: 826.2767 [M+H]+;
m/z calcd for C39H50NP2: 594.3413; found: 594.3414
[MꢀAuCl+H]+.
and Prof. Jin Qu (Nankai University, China) for X-ray diffraction
studies and Dr. P. Bouit and W. Delaunay for their contribution
to this project.
Keywords: conjugation
·
electronic
structure
·
phosphaalkenes · phosphorus · X-ray diffraction
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Phosphaalkene 8: [AuCl(tht)] (22 mg, 0.069 mmol) was
added to PPA 5a (20 mg, 0.034 mmol) in CH2Cl2 (5 mL), and
the reaction mixture turned immediately yellow to orange. The
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1
22 mg, 0.021 mmol (61%). H NMR (400 MHz, CD2Cl2): d=1.31
3
(s, 9H; tert-Bu) 1.36 (s, 9H; tert-Bu), 1.56 (d, J(P,H)=28.6 Hz,
3H; CH3), 1.65 (s, 9H; tert-Bu), 1.89 (m, 4H; CCH2CH2), 2.70 (m,
1H; CCH2CH2) 3.20 (m, 3H; CCH2CH2), 7.14 (m, 1H; pyridyl, H5),
7.43–7.60 (m, 5H; Mes*, Ph), 7.60 (m, 1H; pyridyl H3), 7.67 (m,
1H; pyridyl H4), 7.82 (m, 2H; phenyl), 8.45 ppm (m, 1H; pyridyl
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3
199.0 (d, J(P,P)=45.0 Hz); 13C NMR (101 MHz, CD2Cl2): d=22.6
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(s; CCH2CH2), 22.8 (s; CCH2CH2), 27.4 (dd, J(P,C)=3.5, 7.4 Hz; P=
CCH3), 29.7 (dd, J(P,C)=9.3, 63.2 Hz; CCH2CH2), 31.1 (s;
CCH2CH2), 31.3 (s; para-tBu), 34.2 (d, J(P,C)=1.3 Hz; CH3, ortho-
tBu), 34.3 (m; CH3, ortho-tBu), 35.8 (s; para-C(CH3)3), 39.0 (s;
ortho-C(CH3)3), 39.3 (s; ortho-C(CH3)3), 123.1 (s; C5, pyridyl),
123.7 (d, J(P,C)=23.0; C2, pyridyl), 123.7 (d, J(P,C)=23.0; C2,
pyridyl), 124.0 (d, J(P,C)=5.9 Hz; C3 pyridyl or phenyl), 124.2 (d,
J(P,C)=17.6 Hz; C3 pyridyl or phenyl), 124.2 (s; CH, phenyl),
125.0 (d, J(P,C)=60.6 Hz; ipso-C, phenyl), 130.2 (s; CH, Mes*),
130.3 (s; CH, Mes*), 137.3 (s; C4, pyridyl), 139.1 (dd, J(P,C)=5.8,
54.5 Hz; ipso-C, Mes*), 149.7 (s; pyridyl C6), 151.8 (d, J(P,C)=
12.9 Hz; C7 or C8 or C13 or C14), 152.8–153.2 (3m overlap-
ping; C7 or C8 or C13 or C14), 155.2 (d, J(P,C)=2.4 Hz; Mes*),
157.0 (s; Mes*), 167.2 ppm (dd, J(P,C)=9.4, 72.2 Hz; P=C);
APCI/ASAP: m/z calcd for C39H50NClP2Au: 826.2767; found:
826.2761 [MꢀAuCl+H]+; m/z calcd for C39H50NP2: 594.3413;
found: 594.3414 [Mꢀ2AuCl+H]+. The PꢀAu bond of the P=C
is unstable.
Acknowledgements
This work was supported by the Swedish Research Council, the
Gçran Gustafsson Foundation, Uppsala University through the
U3 MEC molecular electronics priority initiative, le Ministꢅre de
la Recherche et de l’Enseignement Supꢁrieur, the CNRS, and
the Rꢁgion Bretagne. E.ꢀ. would like to thank Lennanders stif-
telse for financial support and the COST action CM1302 on
Smart Inorganic Polymers, and the COST action CM0802 for
granting a STSM in Rennes. A.O. would like to thank the Austri-
an Science Fund (FWF) for financial support through an Erwin-
Schrçdinger fellowship (J3193-N17). We thank Dr. Jin-Li Cao
Chem. Eur. J. 2014, 20, 8421 – 8432
www.chemeurj.org
8431 ꢂ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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Received: March 27, 2014
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