Syntheses, structures, and properties of biphosphinines tethered aromatic π-system

Article abstract of DOI:10.1246/cl.150103

The syntheses of a series of biphosphinine derivatives inserted an aromatic π-spacer have been achieved. Their absorption, emission properties, and electrochemical behavior are also disclosed.

Full text of DOI:10.1246/cl.150103

CL-150103
Received: February 2, 2015 | Accepted: February 19, 2015 | Web Released: February 28, 2015
Syntheses, Structures, and Properties of Biphosphinines Tethered Aromatic π-System
Noriyoshi Nagahora,*1 Tamaki Ogawa,1 Maki Honda,1 Motomi Fujii,1 Hiroshi Tokumaru,1
Takahiro Sasamori,2 Kosei Shioji,1 and Kentaro Okuma1
1Department of Chemistry, Faculty of Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180
2Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(E-mail: nagahora@fukuoka-u.ac.jp)
The syntheses of a series of biphosphinine derivatives
inserted an aromatic π-spacer have been achieved. Their
absorption, emission properties, and electrochemical behavior
are also disclosed.
The chemistry of phosphinine, a heavier analogue of
pyridine, has been studied extensively for a long time,1 since
Märkl and co-workers reported the synthesis of stable phosphi-
nine by the reaction of the corresponding pyrylium salt with
tris(trimethylsilyl)phosphine.2 Up to now, many examples of
phosphinines were synthesized, and their unique properties were
revealed, most remarkably, their low-lying LUMO level as
compared with the corresponding pyridine should be noted.3
Scheme 1. Syntheses of 4,4¤-biphosphinines 1a-1c. Reagents and
conditions: (a) PhCOCH3, Ba(OH)2¢8H2O, 2-PrOH, reflux, 4.5 h (for
Stable biphosphinines, which have two phosphinine moie-
3a), PhCOCH3, KOH, 2-PrOH, reflux, 2.5 h (for 3b); (b) PhCOCH3,
ties intramolecularly connected by an organic fragment, have
NaNH2, toluene, rt, 20 h (for 4a), PhCOCH3, NaNH2, o-Cl2C6H4,
100 °C, 4 h (for 4b); (c) PhCOCH3, NaNH2, toluene, rt, 72 h; (d) HBF4,
Ph3COH, Ac2O, rt, 20 h (for 5a and 5b), HBF4, Ph3COH, 70 °C, rt,
also been synthesized by Märkl,4,5 Mathey and Le Floch,6 and
Müller’s group,7 while some of them could have an important
role in the development of organometallic chemistry as bidentate
ligands. Although several biphosphinines were synthesized,
reporting of their fundamental properties in photophysics and
electrochemistry has been limited because attention has been
focused on their structural properties and skeletal functions,
and accordingly there has been less interest in their physical
properties.6b,6c,8 Thus, we would like to describe design and
syntheses of revisited Märkl’s 4,4¤-biphosphinine 1a having
a p-phenylene and novel biphosphinines 1b-1d bearing p-
phenylene-vinylene-p-phenylene (trans-stilbene-4,4¤-diyl), m-
phenylene, and p-phenylene backbone, respectively, in order to
investigate their structures and properties.
16 h (for 5c); (e) P(SiMe3)3, C6H6, CH3CN, 60 °C, 5 h (for 1a-1c).
Ph
Ph Ph
Ph
O
P
O
2BF4
(a)
(b)
O
P
O
Ph
Ph Ph
Ph
5d
1d
(25%)
(7%)
Scheme 2. Synthesis of 2,2¤-biphosphinine 1d. Reagents and
conditions: (a) PhCOCH=CHPh, HBF4, 1,2-Cl2C2H4, reflux, 18.5 h;
(b) P(SiMe3)3, C6H6, CH3CN, 60 °C, 5 h.
We have decided to use the methodology for the con-
struction of a phosphinine framework by the reaction of an
oxygen-containing six-membered ring heterocycle, pyrylium
salt, with tris(trimethylsilyl)phosphine reported by Märkl.1,2 The
synthetic route for biphosphinines 1a-1d is shown in Schemes 1
and 2. Aldol condensation reactions of benzaldehydes 2a and
2b9 with barium or potassium enolate, generated by treatment
of acetophenone with barium or potassium hydroxide, gave
compounds 3a and 3b in 90% and 94% yields, respectively.
Michael addition reactions of 3a and 3b with the sodium enolate
in toluene or o-dichlorobenzene gave tetraones 4a and 4b in
48% and 98% yields, respectively. In the case of synthesis of 3c,
we found that the reaction of isophthalaldehyde (2c) of the
barium or potassium enolate afforded a complicated mixture.
Thus, one-pot operation of the synthesis of 4c was performed,
that is, the reaction of 2c with acetophenone with sodium amide
gave tetraone 4c in 14% isolated yield. Treatment of tetraones
4a-4c with tetrafluoroboric acid in the presence of triphenyl-
methanol in acetic anhydride afforded bipyrylium tetrafluoro-
borates 5a-5c in 76%, 92%, and 59% yields, respectively.10 The
reaction of 1,4-diacetylbenzene with 1,3-diphenyl-2-propen-1-
one in the presence of tetrafluoroboric acid afforded compound
5d in 25% yield (Scheme 2). The molecular structure of 5a
was determined by X-ray crystallographic analysis, and we
confirmed the formation of six-membered ring framework
(Figure 1).11
Syntheses of 4,4¤-biphosphinines 1a-1c inserting a p-
phenylene, trans-stilbene-4,4¤-diyl, or m-phenylene moiety,
respectively, were performed by the reactions of bipyrylium
tetrafluoroborates 5a-5c with P(SiMe3)3 in benzene/acetonitrile
solution at 60 °C for 5 h (Scheme 1). Similarly, 2,2¤-biphosphi-
nine 1d bearing p-phenylene, a regioisomer of 1a, was also
synthesized by treatment of 5d with P(SiMe3)3 in benzene/
acetonitrile solution (Scheme 2). After chromatographic purifi-
cation of the reaction mixture, biphosphinines 1a-1d were
obtained as pale yellow crystals in 3%, 2%, 25%, and 7%
isolated yields, respectively. All compounds can be handled
under air without any decomposition, and were thermally stable
under inert atmosphere.
706 | Chem. Lett. 2015, 44, 706–708 | doi:10.1246/cl.150103
© 2015 The Chemical Society of Japan

Figure 2. UV-vis absorption spectra of 1a-1d and 6 in CH2Cl2
¹1
¹1
(concentration; 2.15 © 10¹5 mol L for 1a, 1.95 © 10¹5 mol L for
1b, 2.20 © 10¹5 mol L for 1c, 2.15 © 10¹5 mol L for 1d, and
¹1
¹1
¹1
2.05 © 10¹5 mol L for 6).
Figure 1. Molecular structures of (a) 5a and (b) 1a with thermal
ellipsoid plot (30% probability). The solvated acetonitrile molecules of
5a are omitted for clarity. Selected bond lengths [¡] and angles [°]
of 5a: O(1)-C(1) 1.356(3), O(1)-C(5) 1.352(3), C(5)-O(1)-C(1)
122.54(19). Selected bond lengths [¡] and angles [°] of 1a: P(1)-
C(1) 1.752(3), P(1)-C(5) 1.743(3), P(2)-C(12) 1.743(3), P(1)-C(16)
1.746(3), C(5)-P(1)-C(1) 101.84(14), C(12)-P(2)-C(16) 101.63(14).
Table 1. Photophysical properties of biphosphinines 1a-1d and
monophosphinine 6
Absorption
¾/M¹1 cm
Emission Stokes shift
Compd. Solvent
¹1
¹1
-/nm
k/cm
-/nm
a
1a
1b
1c
1d
6
hexane 282, 323(sh)
CH2Cl2 335
hexane 354
CH2Cl2 368
hexane 280, 311(sh)
®
36000
465
®
9080
b
Biphosphinines 1a-1d showed a singlet signal at +186.2
(for 1a), +185.2 (for 1b), +185.3 (for 1c), +183.5 ppm (for 1d)
®
6700
4400
10200
®
a
®
64000
464
439
455
in the 31P NMR spectra, and a doublet signal at 8.25 (3JPH
5.6 Hz) (for 1a), 8.22 (3JPH = 6.0 Hz) (for 1b), 8.23 (3JPH
=
a
®
=
b
6.0 Hz) (for 1c), and 8.21/8.26 (3JPH = 5.2/5.6 Hz) ppm (for
1d), which can be assigned to protons at the 3-position for
CH2Cl2 278, 321(sh) 61000, 12000
®
472
a
hexane 296, 326(sh)
®
9490
®
b
CH2Cl2 303, 337(sh) 63000, 13000
hexane 277, 317(sh) 39000, 4900
CH2Cl2 277, 319(sh) 39000, 6000
®
352
1
the phosphinine ring, in the H NMR spectra. In the 13C NMR
3100
®
spectra, biphosphinines 1b-1d in CDCl3 (CS2:CDCl3 = 9:1 was
used for 1a) showed characteristic doublet signals at 171.5
(1JPC = 56 Hz) (for 1a), 171.8 (1JPC = 52 Hz) (for 1b), 172.0
(1JPC = 53 Hz) (for 1c), and 171.0 (1JPC = 53 Hz) (for 1d),
attributable to carbons at the 2-position for the phosphinine ring.
These spectral features were comparable to those of 2,4,6-
triphenylphosphinine (6),12 indicating that phosphinine moieties
of 1a-1d did not display remarkable electronic perturbations
when connecting the π-electron frameworks, such as p-, m-
phenylene, and trans-stilbene-4,4¤-diyl.
X-ray crystallographic analysis of the single crystals of 1a
revealed the solid-state structure, the molecular structure of
which was depicted in Figure 1.11 The P-C bond lengths of 1a
lie in a range of 1.743(3)-1.752(3) ¡, being slightly longer than
those of typical P=C double bonds (1.61-1.71 ¡),13 and are
comparable to those of previously reported phosphinines
(1.728-1.758 ¡).8a,14 The C-P-C bond angles of 101.84(14)
and 101.63(14)° are similar to those of known phosphinines
(98.38-105.10°).8a,14 The torsion angles of 1a showed C(4)-
C(3)-C(6)-C(11) of ¹35.6(4)° and C(8)-C(9)-C(14)-C(13) of
30.1(4)°, the dihedral angles between the phosphinine and
inserted phenylene rings were ca. 30-36° in the solid state.
The UV-vis absorption measurement of biphosphinines 1a-
1d together with monophosphinine 6 was performed (Figure 2),
the data are shown in Table 1. They were shifted bathochromi-
cally in the order of 1b > 1a µ 1d > 1c µ 6. These results
indicate that trans-stilbene-4,4¤-diyl moiety in 1b can be
effectively extension of the π-conjugation, whereas m-phenylene
unit of 1c cause not varying absorption wavelength but
enhancing absorption coefficient as compared with 6.
b
®
aThe molar absorption coefficient could not be determined because of
b
its low solubility. The emission spectrum was not detected.
calculations, it was found that both the HOMO and LUMO of
1a-1d dominantly delocalized on the phosphinines and inserted
p-phenylene (1a and 1d), trans-stilbene-4,4¤-diyl (1b), and m-
phenylene (1c) fragments, respectively, except for the phenyl
substituents (Figures S1-S4 in Supporting Information). Assign-
ment of the absorption was estimated based on TD-DFT
calculations of 1a-1d.15,16 The absorption maxima of 1a, 1b,
and 1d at the lowest energy area (- = 339 nm; f = 0.728 (for
1a), - = 396 nm; f = 1.929 (for 1b), and - = 345 nm; f = 0.439
(for 1d)) are dominantly assignable to the symmetry-allowed
π-π* (HOMO to LUMO) electron transitions for phosphinine
chromophore and the connected fragment moieties. Meanwhile,
in the case of m-phenylene derivative 1c, -abs corresponds to
combinations of the π-π* electron transitions from HOMO¹2/
HOMO¹3 to LUMO/LUMO+1 (- = 331.81 nm; f = 0.0610).
Moreover, in hexane solution, the emission spectra of 1a-1d
appeared emission maxima at 465, 464, 455, and 472 nm,
respectively. The Stokes shifts of 1a-1d were found to be 9080,
6700, 10200, and 9490 cm¹1, respectively. These values are
larger than that of 6, it is reasonable to assume that 1a-1d do not
have entirely flat π-system in the ground state but their geometry
may become altered in the excited state.
The electrochemical behavior of 1a-1d has been discovered
by cyclic and differential pulse voltammetry. (Figures 3 and S5-
S7 in Supporting Information) In dichloromethane solution,
while an irreversible one-electron oxidation wave was observed
at +1.09 V (for 1c) and +0.94 V (for 1d), an irreversible
stepwise one-electron oxidation waves were displayed at +0.88/
To gain insight into the electronic structure, the theoretical
calculations were carried out.15,16 The absorptions of 1a-1d
were estimated on the basis of the results of TD-DFT
Chem. Lett. 2015, 44, 706–708 | doi:10.1246/cl.150103
© 2015 The Chemical Society of Japan | 707

F. Mathey, P. Le Floch, Thieme, 2004, Vol. 15, pp. 1097-1156.
G. Märkl, F. Lieb, A. Merz, Angew. Chem., Int. Ed. Engl. 1967, 6, 944.
a) P. Le Floch, Coord. Chem. Rev. 2006, 250, 627. b) C. Müller, D.
Vogt, C. R. Chim. 2010, 13, 1127.
2
3
4
Synthetic scheme of 1a and its melting point were described in
footnote of the following literature. See: G. Märkl, D. E. Fischer, H.
Olbrich, Tetrahedron Lett. 1970, 11, 645.
5
6
G. Märk, S. Dorsch, Tetrahedron Lett. 1995, 36, 3839.
a) H. Trauner, P. Le Floch, J.-M. Lefour, L. Ricard, F. Mathey,
Synthesis 1995, 717. b) P. Le Floch, D. Carmichael, L. Ricard, F.
Mathey, A. Jutand, C. Amatore, Organometallics 1992, 11, 2475. c) L.
Cataldo, S. Choua, T. Berclaz, M. Geoffroy, N. Mézailles, L. Ricard, F.
Mathey, P. Le Floch, J. Am. Chem. Soc. 2001, 123, 6654.
a) C. Müller, Z. Freixa, M. Lutz, A. L. Spek, D. Vogt, P. W. N. M.
van Leeuwen, Organometallics 2008, 27, 834. b) C. Müller, D.
Wasserberg, J. J. M. Weemers, E. A. Pidko, S. Hoffmann, M. Lutz,
A. L. Spek, S. C. J. Meskers, R. A. J. Janssen, R. A. van Santen, D.
Vogt, Chem.®Eur. J. 2007, 13, 4548. c) A. Loibl, I. de Krom, E. A.
Pidko, M. Weber, J. Wiecko, C. Müller, Chem. Commun. 2014, 50,
8842. d) C. Müller, J. A. W. Sklorz, I. de Krom, A. Loibl, M. Habicht,
M. Bruce, G. Pfeifer, J. Wiecko, Chem. Lett. 2014, 43, 1390.
a) S. Choua, C. Dutan, L. Cataldo, T. Berclaz, M. Geoffroy, N.
Mézailles, A. Moores, L. Ricard, P. Le Floch, Chem.®Eur. J. 2004, 10,
4080. b) F. Gerson, P. Merstetter, S. Pfenninger, G. Märkl, Magn.
Reson. Chem. 1997, 35, 384.
Figure 3. Cyclic (black lines) and differential pulse (red lines)
voltammograms of 1a in n-Bu4NPF6/CH2Cl2 at room temperature.
7
+1.12 V (for 1a), and +0.78/+0.94 V (for 1b) versus Fc/Fc+,
respectively (see Supporting Information). The first oxidation
potentials of 1a and 1b were somewhat lower than those of 1c
and 1d as well as monophosphinine 6 (Epa = +0.91 V versus
Fc/Fc+). Difference between the oxidation potentials of 1a and
1b are 0.24 and 0.16 V, respectively, which are somewhat
smaller than that of 2,2¤-biphosphinine (0.40 V),6b indicating
electronic interaction between the phosphinine units.
8
9
On the other hand, irreversible one-electron reduction waves
were found, and their reduction potentials were ¹2.25/¹2.46 V
(for 1a), ¹2.34 V (for 1b), ¹2.48 V (for 1c), and ¹2.27/
¹2.45 V (for 1d), which were higher than those of 2,2¤-
biphosphinine (E1/2 = ¹1.78/¹2.18 V),6b and comparable to
those the previously reported parent phosphinine (¹2.10 V,
Compound 2b has been synthesized according to the following
literature. See: A. Osuka, N. Tanabe, S. Kawabata, I. Yamazaki, Y.
Nishimura, J. Org. Chem. 1995, 60, 7177.
10 Syntheses of bipyrylium salts consisting of p- and m-phenylenes were
also reported. See: a) Y. Sakaguchi, F. W. Harris, Polym. J. 1992, 24,
1147. b) K. Okada, K. Matsumoto, M. Oda, H. Murai, K. Akiyama, Y.
Ikegami, Tetrahedron Lett. 1995, 36, 6689.
Ipa/Ipc = 0.56 in n-Bu4NClO4/dimethoxyethane)17 and 6 (Epc
=
¹2.25 V). The results suggested that π-electrons of the phos-
phinine framework of 1a-1d can be slightly perturbed by the
peripherally connected aromatic π-systems. We found that
biphosphinine 1a showed novel and unique stepwise two-
electron oxidation/reduction behavior.
Finally, we succeeded in the syntheses of a series of
biphosphinines 1a-1d, and their photophysical properties and
electrochemical behavior have been disclosed. Due to expansion
of the π-system consisting of phosphinines, biphosphinines 1a,
1b, and 1d have absorption maxima at longer wavelength and
luminesces with larger Stokes shift in comparison with mono-
phosphinine 6. Research of further properties of 1a-1d is in
progress.
11 Crystallographic data for 1a; C40H28P2, M = 570.56, monoclinic,
P21/c (#14), a = 17.770(2) ¡, b = 7.2108(7) ¡, c = 23.780(3)¡,
¡ = 90°, ¢ = 110.031(4)°, £ = 90°, V = 2862.6(6) ¡3, Z = 4,
¹3
μ
calcd = 1.324 g cm , 2ªmax = 55.9°, 30424/5170 measured/inde-
pendent reflections, 379 refined parameters, R1 (wR2) = 0.0588
(0.1515) [I > 2·(I)], R1 (wR2) = 0.0868 (0.1763) (for all data), T =
93(2) K, GOF = 1.076. Crystallographic data for 5a; C40H28B2F8O2¢
ꢀ
2(CH3CN), M = 796.35, triclinic, P1 (#2), a = 7.889(6) ¡, b =
9.663(8) ¡, c = 13.868(11) ¡, ¡ = 102.558(9)°, ¢ = 103.216(9)°,
¹3
£ = 106.519(11)°, V = 940.4(13) ¡3, Z = 1,
μcalcd = 1.406 g cm ,
2ªmax = 52.0°, 9840/3653 measured/independent reflections, 263
refined parameters, R1 (wR2) = 0.0622 (0.1462) [I > 2·(I)], R1
(wR2) = 0.1048 (0.1732) (for all data), T = 113(2) K, GOF = 1.067.
Crystallographic data reported in this manuscript have been deposited
with Cambridge Crystallographic Data Centre as supplementary
publication nos. 1043686 (1a) and 1043685 (5a). Copies of the
data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge, CB2 1EZ, UK; fax: +44 1223 336033;
or deposit@ccdc.cam.ac.uk).
This work was partially supported by the Collaborative
Research Program of Institute for Chemical Research, Kyoto
University (grant No. 2013-68) and Grant-in-Aid for Young
Scientists (B) (No. 23750051) from JSPS. The synchrotron
radiation experiments were performed at the BL40XU beamline
of the SPring-8 with the approval of the Japan Synchrotron
Radiation Research Institute (JASRI; proposal No. 2014A1717).
We are grateful to Dr. N. Yasuda (JASRI) for the X-ray
crystallographic analyses at the SPring-8.
12 NMR data of monophosphinine 6; ¤H = 8.22 (d, 3JPH = 5.6 Hz), ¤C
=
1
171.4 (d, JPC = 52 Hz), ¤P = +185.8.
13 Multiple Bonds and Low Coordination in Phosphorus Chemistry, ed.
by M. Regitz, O. J. Scherer, Georg Thieme Verlag Thieme Medical
Publishers, New York, 1990.
14 L. E. E. Broeckx, S. Güven, F. J. L. Heutz, M. Lutz, D. Vogt, C.
Müller, Chem.®Eur. J. 2013, 19, 13087.
Supporting Information is available electronically on J-STAGE.
15 All theoretical calculations were carried out using Gaussian 09. The
results are shown in Supporting Information.
References and Notes
16 M. J. Frisch, et al., Gaussian 09 (Revision D.01), Gaussian, Inc.,
Wallingford CT, 2013.
17 C. Elschenbroich, M. Nowotny, B. Metz, W. Massa, J. Graulich, K.
Biehler, W. Sauer, Angew. Chem., Int. Ed. Engl. 1991, 30, 547.
1
a) K. B. Dillon, F. Mathey, J. F. Nixon, Phosphorus: The Carbon Copy,
John Wiley & Sons, 1998, pp. 235-257. b) Science of Synthesis: Six-
Membered Hetarenes with One Nitrogen or Phosphorus Atom, ed. by
708 | Chem. Lett. 2015, 44, 706–708 | doi:10.1246/cl.150103
© 2015 The Chemical Society of Japan