Phosphole-triazole hybrids: A facile synthesis and complexation with Pd(II) and Pt(II) salts

Article abstract of DOI:10.1021/ol901194m

The copper-promoted Huisgen reaction of α-ethynylphosphole P-oxides with aryl azides affords α-(1-aryl-1,2,3-triazol-4-yl)phospholes in high yield. The α-(1-phenyl-1,2,3-triazol-4-yl)phosphole exhibits different coordination behavior in the complexation w

Full text of DOI:10.1021/ol901194m

ORGANIC
LETTERS
2009
Vol. 11, No. 15
3338-3341
Phosphole-Triazole Hybrids: A Facile
Synthesis and Complexation with Pd(II)
and Pt(II) Salts
Yoshihiro Matano,*,† Makoto Nakashima,† Arihiro Saito,† and Hiroshi Imahori†,‡,§
Department of Molecular Engineering, Graduate School of Engineering, and Institute
for Integrated Cell-Material Sciences (iCeMS), Kyoto UniVersity, Nishikyo-ku, Kyoto
615-8510, Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity,
Sakyo-ku, Kyoto 606-8103, Japan
matano@scl.kyoto-u.ac.jp
Received May 28, 2009
ABSTRACT
The copper-promoted Huisgen reaction of r-ethynylphosphole P-oxides with aryl azides affords r-(1-aryl-1,2,3-triazol-4-yl)phospholes in high
yield. The r-(1-phenyl-1,2,3-triazol-4-yl)phosphole exhibits different coordination behavior in the complexation with MCl2(cod) (M ) Pd, Pt). It
has been revealed that P,N-coordination to PdCl2 dramatically changes the optical properties of the newly constructed phosphole-triazole
hybrid π-system.
Phospholes bearing N-heteroaryl substituents exhibit char-
acteristic optical and coordinating properties endowed by the
P and N atoms.1 The synergetic effects of these two
heteroatoms are of particular interest to the application of
such ligands to the fields of materials, supramolecular, and
synthetic chemistry. For example, Re´au and co-workers have
demonstrated that the NLO property of R-(2-pyridyl)phos-
pholes and the chiroptical property of phosphole-modified
aza[6] helicenes were remarkably enhanced by P,N-chelation
with a cationic palladium salt.1c,k In addition, palladium and
ruthenium complexes bearing phosphole-containing P,N-
heteroditopic ligands have shown high catalytic activities in
olefin/CO copolymerization,1b transfer hydrogenation of
ketones,1d and cross-coupling reactions.1e,f Despite these
promising results, however, the number of N-heteroaryl
substituents examined is quite limited. In this context, it is
of significance to develop a reliable methodology for
introducing a series of N-heteroaryl groups onto the phos-
phole ring, as well as to reveal the optical, electrochemical,
and coordinating properties of the resulting phosphole-based
hybrid π-systems.
† Graduate School of Engineering.
‡ Institute for Integrated Cell-Material Sciences.
§ Fukui Institute for Fundamental Chemistry.
(1) For example, see: (a) Hay, C.; Hissler, M.; Fischmeister, C.; Rault-
Berthelot, J.; Toupet, L.; Nyula´szi, L.; Re´au, R. Chem.sEur. J. 2001, 7,
4222–4236. (b) Sauthier, M.; Leca, F.; Toupet, L.; Re´au, R. Organometallics
2002, 21, 1591–1602. (c) Fave, C.; Hissler, M.; Se´ne´chal, K.; Ledoux, I.;
Zyss, J.; Re´au, R. Chem. Commun. 2002, 1674–1675. (d) Thoumazet, C.;
Melaimi, M.; Ricard, L.; Mathey, F.; Le Floch, P. Organometallics 2003,
22, 1580–1581. (e) Melaimi, M.; Thoumazet, C.; Ricard, L.; Le Floch, P.
J. Organomet. Chem. 2004, 689, 2988–2994. (f) Thoumazet, C.; Melaimi,
M.; Ricard, L.; Le Floch, P. C. R. Chimie 2004, 7, 823–832. (g) Welsch,
S.; Lescop, C.; Scheer, M.; Re´au, R. Inorg. Chem. 2008, 47, 8592–8594.
(h) Matano, Y.; Miyajima, T.; Ochi, N.; Nakabuchi, T.; Shiro, M.; Nakao,
Y.; Sakaki, S.; Imahori, H. J. Am. Chem. Soc. 2008, 130, 990–1002. (i)
Matano, Y.; Nakabuchi, T.; Fujishige, S.; Nakano, H.; Imahori, H. J. Am.
Chem. Soc. 2008, 130, 16446–16447. (j) Nohra, B.; Rodriguez-Sanz, E.;
Lescop, C.; Re´au, R. Chem.sEur. J. 2008, 14, 3391–3403. (k) Graule, S.;
Rudolph, M.; Vanthuyne, N.; Autschbach, J.; Roussel, C.; Crassous, J.;
Re´au, R. J. Am. Chem. Soc. 2009, 131, 3183–3185. (l) Crassous, J.; Re´au,
R. Dalton Trans. (PerspectiVe) 2009, 6865–6876, and references therein.
Quite recently, we have established a convenient method
for the synthesis of silyl-capped 2-ethynylphosphole 1 and
2,5-diethynylphosphole 2 (Figure 1).2 Notably, 1 and 2 were
readily converted to the acetylene-linked phosphole-arene
10.1021/ol901194m CCC: $40.75
Published on Web 07/08/2009
 2009 American Chemical Society

π-systems by using cross- and homocoupling methods. We
anticipated that the acetylene functions in 1 and 2 would
also be applicable to Huisgen-type [2 + 3] cycloaddition
reactions3 affording the corresponding five-membered het-
erocycles. Here we report the first facile synthesis of R-(1-
aryl-1,2,3-triazol-4-yl)phospholes from 1/2 and aryl azides.
As we expected, the dipolar cycloaddition proceeded ef-
ficiently with the assistance of a copper salt. The substituent
and complexation effects on the optical properties of the
newly developed phosphole-triazole-arene π-systems are also
reported.
Scheme 1. Synthesis of 5, 7, and 9
Figure 1. Silyl-capped 2-ethynylphosphole 1 and 2,5-diethy-
nylphosphole 2.
The starting materials, silyl-capped R-ethynylphospholes
1 and 2, were prepared according to the reported procedure.2
Treatment of 1 with tetra-n-butylammonium fluoride (TBAF)
in THF followed by addition of m-chloroperbenzoic acid
(mCPBA) generated R-ethynylphosphole P-oxide 3 [δP )
52.8 ppm; δH (acetylene) ) 3.46 ppm (d, J ) 3.9 Hz); m/z
) 316.1017 (M+)] in high NMR yield (Scheme 1). As 3
slowly decomposed in solution, the crude product was
subsequently treated with aryl azides 4a-c in the presence
of CuSO4·5H2O and sodium ascorbate. As expected, the
Huisgen dipolar cycloaddition occurred efficiently to give
the corresponding 2-(1-aryl-1,2,3-triazol-4-yl)phospholes
5a-c in 81-86% yields based on 1.4 Similarly, 2,5-bis(1-
aryl-1,2,3-triazol-4-yl)phospholes 7a-c were prepared in
74-77% yields from 2 via terminal-free phosphole P-oxide
6 [δP ) 51.0 ppm; δH (acetylene) ) 3.49 ppm (d, J ) 3.9
Hz); m/z ) 264.0704 (M+)]. When 2,7-diazido-9,9-dihexyl-
fluorene (8) was used as the N3 source, fluorene-bridged
phosphole-triazole hybrid 9 was isolated in 86% yield.
Overall, the Huisgen reaction has proven to be applicable to
the synthesis of phosphole derivatives bearing triazole
substituents at the R positions.
ence in 31P chemical shifts among 5a, 5b, and 5c, implying
that the electronic effects of the para-substituents on the
phosphorus nuclei are negligible (Table 1). The same is true
for 7a-c and 10a-c.
The crystal structure of 7a was further elucidated by X-ray
crystallography.5 As shown in Figure 2, one triazole ring
(N1-N3-C8) is somewhat twisted as compared to the other
Deoxygenation of 5a-c with trichlorosilane proceeded in
refluxing toluene to give the respective σ3-P type compounds
10a-c in 65-71% yields (eq 1).
(2) Matano, Y.; Nakashima, M.; Imahori, H. Angew. Chem., Int. Ed.
2009, 48, 4002–4005.
(3) (a) Huisgen, R. Proc. Chem. Soc. 1961, 357. (b) Huisgen, R. In
1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York,
1984; pp 1-176.
(4) The phosphorus center was masked with the oxo group to prevent
undesirable Staudinger reaction.
j
(5) 7a: C29H23N6OP, triclinic, P1, a ) 8.7372(15), b ) 9.6953(16), c )
16.196(3) Å, R ) 73.121(8)°, ꢀ ) 82.282(11)°, γ ) 64.087(7)°, V )
1180.8(4) Å3, Z ) 2, Fcalcd ) 1.413 g cm-3, collected 9212, independent
5248, parameters 335, Rw ) 0.0865, R ) 0.0412 (I > 2.00σ(I)), GOF )
j
1.067. 11: C27H22Cl2N3PPd·2(C2H2Cl4), triclinic, P1, a ) 8.1743(11), b )
13.1609(19), c ) 17.591(3) Å, R ) 80.241(4)°, ꢀ ) 76.545(4)°, γ )
84.462(5)°, V ) 1810.7(4) Å3, Z ) 2, Fcalcd ) 1.710 g cm-3, collected 21770,
independent 7954, parameters 416, Rw ) 0.0850, R ) 0.0389 (I > 2.00σ(I)),
GOF ) 1.086.
Compounds 5, 7, 9, and 10 were fully characterized by
conventional spectroscopic techniques. There is little differ-
Org. Lett., Vol. 11, No. 15, 2009
3339

acetylene-arene2 and phosphole-vinylene-arene6 hybrid π-sys-
tems. To get more insight into the π-conjugation structures,
we performed density functional theory (DFT) calculations
on model compounds of 7a-c. As shown in Figure S2 in
Supporting Information, HOMO and LUMO of 7am, 7bm,
and 7cm consist of those originated mainly from the
phosphole-triazole subunits, and the contribution of the
N-aryl rings is little. This reasonably explains the experi-
mentally observed, small para-substituent effects.
Table 1. 31P NMR Chemical Shifts, UV-vis Absorption and
Fluorescence Maxima, and Redox Potentials of 5, 7, 9, and 10
compd δP/ppma λabs/nm (log ε)b λem/nm (Φf)b c Eox/Vd e Ered/Vd e
,
,
,
5a
5b
5c
53.4
53.4
53.3
52.6
52.6
52.4
53.3
31.8
31.7
32.7
386 (4.18)
388 (4.19)
384 (4.19)
390 (4.20)
393 (4.21)
388 (4.23)
389 (4.61)
370 (4.28)
371 (4.29)
372 (4.29)
489 (0.03)
492 (0.02)
486 (0.03)
492 (0.04)
495 (0.03)
486 (0.03)
490 (0.03)
448 (0.09)
451 (0.14)
448 (0.11)
+1.02
+0.97
+1.07
+1.03
+0.97
+1.11
n.d.
-2.04
-2.05
-2.02
-1.99
-2.01
-1.95
n.d.
7a
7b
7c
9
10a
10b
10c
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Scheme 2. Complexation of 10a with MCl2(cod) (M ) Pd, Pt)
a Measured in CDCl3, vs H3PO4. b Measured in THF. λex ) 365 nm.
Fluorescence quantum yield relative to that of Re´au’s 3,4-C4-bridged
1-phenyl-2,5-bis(2-thienyl)phosphole (Φf ) 0.050). d Measured in CH2Cl2,
vs Fc/Fc+. Reference electrode Ag/Ag+ [0.01 M AgNO3, 0.1 M n-Bu4NPF6
(MeCN)]. e n.d. ) not determined.
c
With new phosphole-containing P,N-ligands in hand, we
examined the complexation with Pd(II) and Pt(II) salts
(Scheme 2). Compound 10a reacted with PdCl2(cod) (cod
) 1,5-cyclooctadiene) in THF at room temperature to give
Pd-complex 11 in 88% yield. Spectroscopic and X-ray
crystallographic analyses (vide infra) have elucidated that
the phosphole-triazole subunit in 11 coordinates to the
palladium(II) center as the P,N-bidentate ligand. On the other
hand, 10a underwent complexation with a half amount of
PtCl2(cod) to yield PtCl2-bisphosphine complex 12 as a sole
product. The formation of 12 was exclusive even when an
equimolar amount of the Pt source was employed. The 31P
resonance of 11 appeared at δ 80.0, whereas that of 12
appeared at δ 34.0 with Pt-P satellites. The 195Pt-31P
coupling constant of 3287 Hz indicates cis orientation of the
Figure 2. Top (upper) and side (lower) views of 7a. Hydrogen
atoms (except for triazole-CH) are omitted for clarity. Selected bond
lengths (Å): N1-N2 ) 1.312(2), N2-N3 ) 1.356(2), N4-N5 )
1.306(2), N5-N6 ) 1.3578(19).
(N4-N6-C16). The twisted conformation may be caused
by the steric repulsion between the P-substituents and the
syn-oriented triazole-CH group. The N1-N2 and N4-N5
bond lengths are shorter by ca. 0.04-0.05 Å than the N2-N3
and N5-N6 bond lengths, representing that the double bond
character of the former bonds is larger than that of the latter.
To reveal the π-conjugation efficiency of the phosphole-
triazole-benzene hybrids, the absorption and fluorescence
spectra of 5a-c, 7a-c, and 10a-c were compared (Table
1 and Figure S1 in Supporting Information). The P-oxidation
1
two phosphine ligands in 12. The H NMR peaks of the
triazole-methine protons of 11 and 12 were observed at δ
8.05 and 8.78, respectively, which also reflects their different
coordination modes.
causes bathochromic shifts of λabs and λem values (∆λabs
)
12-17 nm and ∆λem ) 38-41 nm for 5a-c vs 10a-c) as
observed for typical phosphole-based π-systems.1 Among
5a-c, there are very small differences in λabs and λem values
(∆λmax e 6 nm). The same is true for 7a-c (∆λmax e 9 nm)
and 10a-c (∆λmax e 3 nm), indicating that the electronic
effects of para-substituents on the phosphole-triazole-arene
hybrid π-systems are not so significant. Indeed, there are
slight differences in redox potentials among 5a-c (∆E )
0.03-0.10 V) and 7a-c (∆E ) 0.06-0.14 V). These are in
sharp contrast to the results observed for the phosphole-
Figure 3 depicts the crystal structure of 11.5 The palladium
center adopts a square planar geometry with the phosphorus
and nitrogen atoms at cis orientation. Due to the P,N-
chelation, the phosphole and triazole rings are twisted
significantly as observed for palladium complexes bearing
heteroditopic R-(2-pyridyl)phosphole ligands.1b,f,7 The Pd-P
and Pd-N bond lengths of 11 [2.2401(8) and 2.055(2) Å]
(6) Matano, Y.; Miyajima, T.; Imahori, H.; Kimura, Y. J. Org. Chem.
2007, 72, 6200–6205.
3340
Org. Lett., Vol. 11, No. 15, 2009

are comparable to those of Re´au’s PdCl2-R-(2-pyridyl)phos-
phole complex [2.2353(11) and 2.076(4) Å].7 The Pd-Cl1
bond length [2.2768(8) Å] is appreciably shorter than the
Pd-Cl2 bond length [2.3429(8) Å], which is attributable to
a difference in trans influence between the P and N donors.
Although the refined data are not at the publishable level,
the cis coordination of two phosphole rings in 12 was
confirmed by X-ray crystallography.
11. As a whole, the chelation-induced modulation of π-π*
transitions of the phosphole-triazole π-system is achieved
by adding a Pd(II) salt.
Figure 4. UV-vis absorption spectra of 10a (black), 11 (red), and
12 (blue) in Cl2CHCHCl2.
In summary, we have explored a conceptually new method
for the synthesis of phospholes bearing N-heterocyclic
substituents, which relies on Huisgen dipolar [2 + 3]
cycloaddition reactions. Using this method, R-(1-aryl-1,2,3-
triazol-4-yl)phospholes were readily accessible from the
corresponding R-ethynylphospholes and aryl azides through
the assistance of the copper promoter. Although the electronic
effects of the para-substituents of the terminal benzene rings
are small, the metal complexation as well as the oxidation
at the phosphorus center has proven to perturb the optical
properties of the phosphole-triazole π-conjugated system. In
particular, the cis-P,N-chelation to the Pd(II) center has a
significant impact on the electronic structure of this hybrid
π-system. The present results feature a new aspect of
π-conjugated phosphole-containing heteroditopic ligands,
which may be applicable to the construction of unique metal-
assisted supramolecular π-networks.
Figure 3. Top and side views of 11. Hydrogen atoms (except for
triazole-CH) and solvent molecules are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Pd-P, 2.2401(8); Pd-N1,
2.055(2); Pd-Cl1, 2.2768(8); Pd-Cl2, 2.3429(8); N1-N2, 1.302(3);
N2-N3, 1.337(3); P-Pd-N1, 80.63(7); P-Pd-Cl1, 94.77(3);
N1-Pd-Cl2, 91.82(7); Cl1-Pd-Cl2, 92.69(3).
To reveal the effect of metal coordination on the electronic
structure of the phosphole-triazole π-system, we measured
absorption spectra of 10a, 11, and 12 in 1,1,2,2-tetrachlo-
roethane (Figure 4). The absorption maximum of platinum-
bisphosphine complex 12 is red-shifted as compared to that
of 10a without a significant change in the spectral shape,
implying that the Pt-complexation basically affects the energy
levels of the frontier orbitals. On the other hand, the
palladium chelate complex 11 displays a completely different,
broad spectrum with relatively low extinction coefficients
at the visible region. This may be attributable to charge-
transfer as well as orbital interactions between the metal and
the π-ligand and also to the twisted π-conjugated system of
Acknowledgment. This work was partially supported by
the Asahi Glass Foundation.
Supporting Information Available: Experimental details,
1
DFT computational results, H NMR spectra, and CIF files
for 7a and 11. This material is available free of charge via
the Internet at http://pubs.acs.org.
(7) Leca, F.; Lescop, C.; Toupet, L.; Re´au, R. Organometallics 2004,
23, 6191–6201.
OL901194M
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