Electron-donating properties of p-phenylene phosphine imides: An electrochemical and spectroscopic investigation

Article abstract of DOI:10.1021/ol026065j

(matrix presented) Electronic properties of phosphine imide based organic electron donors have been investigated. N,N′-p-Phenylenebis(triphenyl)phosphine imide (Ph₃P=NC₆H₄N=PPh₃, 1) has two reversible single-ele

Full text of DOI:10.1021/ol026065j

ORGANIC
LETTERS
2002
Vol. 4, No. 13
2213-2216
Electron-Donating Properties of
p-Phenylene Phosphine Imides: An
Electrochemical and Spectroscopic
Investigation
Marcela Escobar, Zhou Jin, and Brett L. Lucht*
Department of Chemistry, UniVersity of Rhode Island, Kingston, Rhode Island 02881
blucht@chm.uri.edu
Received April 22, 2002
ABSTRACT
Electronic properties of phosphine imide based organic electron donors have been investigated. N,N′-p-Phenylenebis(triphenyl)phosphine
imide (Ph3PdNC H NdPPh3, 1) has two reversible single-electron oxidations (0.04 and 0.53 V vs SCE). Spectroscopic investigations of poly-
6
4
(p-phenylene phosphine imide)s (2) are similar to those of polymer model compounds, suggesting formation of localized radical cations on
the polymer chains and electronically insulating phosphorus atoms.
The properties of organic conducting materials have been
investigated for several years. The primary materials of
interest can be separated into two related areas: conjugated
polymers and organic donor-acceptor complexes. Conju-
gated polymers have a wide variety of optoelectric applica-
tions including sensors, light emitting diodes (LEDs) and
nonlinear optical materials.1 Conjugated polymers of interest
include poly(p-phenylene vinylene), polyaniline, and poly-
thiophene. Since the observation of superconductivity in
charge-transfers salts of organic electron donors and accep-
tors, interest in these materials has intensified. Organic
conducting materials typically include electron donors related
to tetrathiafulvalenes (TTF) and electron acceptors related
to tetracyanoquinodimethane (TCNQ).2 There have been
significant recent efforts to couple the two related areas via
the preparation of conjugated polymers that incorporate
π-donors or acceptors and conjugated oligimers capped with
electron-withdrawing or electron-donating groups.3 Whereas
many of the conjugated polymers and related organic donor
acceptor complexes have utilized both sulfur and nitrogen,
the investigation of the electronic properties of phosphorus
containing organic materials has been limited.4-6
In this article, we describe the synthesis and investigation
of phosphine imide containing organic materials. These
materials are related to the widely investigated poly- and
cyclophosphazenes, which have four covalent bonds to
phosphorus and thus no available p-orbital for involvement
in electronic delocalization.7 While the nature of the phos-
phorus-nitrogen double bond is debatable, there is strong
support for a π-bond resulting from the overlap of a 2p orbital
on nitrogen with a 3d orbital on phosphorus.8 The bond
(4) (a) Bevierre, M.-O.; Mercier, F.; Ricard, L.; Mathey, F. Angew.
Chem., Int. Ed. Engl. 1990, 29, 655. (b) Deschamps, E.; Ricard, L.; Mathey,
F. Angew. Chem., Int. Ed. Engl. 1994, 33, 1158.
(5) (a) Withers, H. P., Jr.; Seyferth, D.; Fellmann, J. D.; Garrou, P. E.;
Martin, S. Organometallics 1982, 1, 1283. (b) Honeyman, C. H.; Foucher,
D. A.; Dahmen, F. Y.; Rulkens, R.; Lough, A. J.; Manners, I. Organome-
tallics 1995, 14, 5503.
(6) (a) Lucht, B. L., St. Onge, N. O. Chem. Commun. 2000, 2097. (b)
Jin, Z.; Lucht, B. L. J. Organomet. Chem. 2002, in press.
(7) (a) Blonsky, P. M.; Shriver, D. F.; Austin, P.; Allcock, H. R. J. Am.
Chem. Soc. 1984, 106, 6854. (b) Allcock, H. R.; Olmeijer, D. L.; O’Connor,
S. J. M. Macromolecules 1998, 31, 753. (c) Gray, F. M. Solid Polymer
Electrolytes: Fundamentals and Technological Applications; VCH: New
York, 1991.
(1) (a) Conjugated Polymers: The NoVel Science and Technology of
Highly Conducting and Nonlinear Optically ActiVe Materials; Bredas, J.
L., Silbey, R., Eds.; Kluwer Academic Publishers: Dordecht, 1991. (b)
Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker:
New York, 1986.
(2) Introduction to Synthetic Electrical Conductors; Ferraro, J. R.,
Williams, J. M.; Academic Press: Orlando, 1987.
(3) Roncali, J. J. Mater. Chem. 1997, 12, 2307.
(8) Wisian-Neilson, P. Polyphosphazines. In Encyclopedia of Inorganic
Chemistry; King, B. R., Ed.; John Wiley & Sons: New York, 1994; Vol.
6, pp 3371-3389.
10.1021/ol026065j CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/23/2002

angles and lengths support electronic delocalization.9 How-
ever, the delocalization is limited to P-N-P units as a result
of nodes in the molecular orbitals. The phosphorus atom is
considered an insulator atom between islands of electronic
delocalization. Infrared spectroscopic investigations of the
PdN bond stretch of p-substituted phenylphosphine imides
supports an expansion of conjugation due to the electron-
withdrawing effect of the phosphine imide, via increasing
quinoidal character of the aromatic ring (eq 1).10 The PdN
equiv of [bis(trifluoroacetoxy)iodo]benzene results in a shift
of the optical absorption bands of 1 (λmax ) 261, 269, 277,
and 365 nm). The new absorptions (λmax ) 393, 497, 540,
and 580 nm) are attributed to 1+•. Addition of a full
equivalent of (bis(trifluoroacetoxy)iodo)benzene results in
the loss of the bands consistent with the formation of 12+.
Both 1+• and 12+ are stable upon exposure to atmospheric
conditions for several hours. Since the oxidation potential
of 1 is comparable to that of many electron donors used in
conductive organic solids, we investigated the reaction of 1
with TCNQ. The addition of TCNQ to chloroform solutions
of 1 results in the formation of a deeply green-colored
solution. UV-visible spectroscopy reveals absorptions char-
acteristic of the radical anion of TCNQ (650-900 nm) and
1+• (450-620 nm). Ball milling of TCNQ with 1 in the solid
state results in the formations of a reflective black power.
These results provide support for the good electron-donating
properties of aromatic phosphine imides and the formation
of stable radical cationic and dicationic complexes. Com-
pound 1 possesses many characteristics of electron donors
in organic conductive solids including high symmetry, high
polarizability, low ionization potential, a small separation
between the first and second oxidation state, and electron-
withdrawing substituents at diametrically different points on
the molecule.2
stretching frequency is highly dependent upon the electron-
accepting or -donating ability of the para substituent. The
preparation and investigation of phosphine imide containing
π-conjugated materials will provide a better understanding
of the PdN bond and the degree of overlap with adjacent
π-systems. In addition, the electron-donating nature of the
phosphine imide will provide unique and interesting proper-
ties of these novel organic materials.
N,N′-p-Phenylenebis(triphenyl)phosphine imide (Ph3Pd
NC6H4NdPPh3, 1) was prepared via the reaction of triph-
enylphosphine with 1,4-diazidobenzene in high yields (94%)
as a yellow powder.11 Cyclic voltammetry of 1 in a 0.1 M
solution of tetrabutylammonium tetrafluoroborate in CH2-
Cl2 affords two single-electron quasi-reversible oxidations
at 0.04 and 0.53 V vs SCE (Figure 1). The low oxidation
We extended our investigation of the electronic properties
of phosphine imides and the electronic nature of the PdN
bond via incorporation into potentially conjugated polymers.
Poly(p-phenylene phosphine imide)s were first prepared in
1961 as red-orange solids (2a, Scheme 1).11,13 The synthesis
Scheme 1
and investigation of the poly(p-phenylene phosphine imide)
2a was limited by low yields of the monomer bis(diphe-
nylphosphino)benzene (3a, 19%) and poor solubility of the
polymer. Newly developed palladium-catalyzed carbon-
phosphorus bond forming reactions provide a high yield
synthesis of 1,4-bis(diphenylphosphino)benzenes. Addition
of 2.2 equiv of diphenylphosphine to 1.0 equiv of 1,4-
diiodobenzenes, 1.1 equiv of DABCO, and 0.05 equiv of
Pd(PPh3)4 in toluene followed by heating to 80 °C for 72 h
results in slight darkening of the solution along with the
formation of a white precipitate characteristic of HI-DABCO.
Removal of solvent followed by recrystallization of the crude
products in 1:4 degassed Et2O/MeOH affords 3a,b (76-88%
yield) as white crystalline solids.14 Incorporation of flexible
alkoxy substituents frequently results in a substantial increase
in the solubility of conjugated polymers. The addition of 1,4-
Figure 1. Cyclic voltammogram for 1.
potential indicates that 1 is a better electron donor than TTF
(0.30 and 0.66 V SCE).12 Chemical oxidation of 1 with 0.5
(9) Dewar, M. J. S.; Lucken, E. A. C.; Whitehead, M. A. J. Chem. Soc.
1960, 2423.
(10) Wiegrabe, W.; Bock, H. Chem. Ber. 1968, 101, 1414.
(11) Herring, D. L. J. Org. Chem. 1961, 26, 3998.
(12) Wheland, R. C.; Gillson, J. L. J. Am. Chem. Soc. 1976, 98, 3916.
(13) Pomerantz, M.; Victor, M. W. Macromolecules 1989, 22, 3512.
(14) (a) Hillhouse, J. H. Preparation of Arylalkyl Phosphines, Phosphine
Oxides or Phosphine Sulfides. U.S. Patent 5,550,295, 1996. (b) Herd, O.;
Hessler, A.; Hingst, M.; Tepper, M.; Stelzer, O. J. Organomet. Chem. 1996,
522, 69.
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Org. Lett., Vol. 4, No. 13, 2002

electronic delocalization of the PdN bond in aromatic
phosphine imides. Incorporation of phosphine imides into
conjugated polymers followed by chemical or electrochemi-
cal oxidation can result in the formation of localized radical
cations related to 1+• or delocalized radical cations related
to those observed in polyaniline. It is well-known that poly-
and oligoanilines are readily oxidized to form quinoidal
structures along the polymer backbone.15 The uncharged
oxidized polyanilines can be doped by protonation to high
conductivities via the formation of delocalized radical cations.
Chemical and electrochemical oxidation of 2b supports a
stepwise conversion to 2b+• and 2b2+ via two single-electron
transfers. Titration of solutions of 2b with (bis(trifluoroac-
etoxy)iodo)benzene (a two-electron oxidant) affords spec-
troscopic changes similar to those observed for 1. The λmax
is shifted from 355 to 340 nm upon the addition of 0.5 equiv
of oxidant and is accompanied by the appearance of new
absorption bands at 497, 540, and 580 nm (Figure 3). The
Figure 2. UV-visible spectra of 1, 2b, and 4.
bis(diphenylphosphino)-2,5-dihexyloxybenzene (3b) to p-
diazidobenzene in benzene solution results in the evolution
of N2 and a color change of the solution to light orange.
The reaction mixture was stirred for 7 days, and during this
time a significant portion of orange solid precipitated from
the solution. The benzene solution was filtered to remove
the orange precipitate (2b, 57%), which was only partially
soluble in THF. The THF soluble fraction contains low
molecular weight oligomers (2b, Mn ) 2000-4000 GPC vs
polystyrene standards, 3-7 repeat units). 31P and H NMR
1
spectroscopy and IR spectroscopy support the proposed
structure of 2b. We observe several 31P NMR resonances
around 21 ppm consistent with different phosphine imides
in the oligomer backbone and resonances around -2.0 ppm
consistent with phosphine-terminated oligomers. We also
observe two strong infrared absorptions at 1352 and 1312
cm-1 consistent with PdN stretches and medium absorptions
at 2106 and 2197 cm-1 characteristic of residual azido groups
terminating the oligomers. IR spectra of the insoluble orange
precipitate are similar to the spectra of soluble oligomers
except that the absorptions characteristic of the azido groups
are much weaker. Thus the insoluble material is considered
to be higher molecular weight polymeric material. Thermal
gravimetric analysis (TGA) under N2 reveal decomposition
temperatures around 440 °C for 1, 2b, and 4. UV-visible
spectra of oligomers 2b (λmax ) 276, 285, 355 nm) and the
polymer model compounds 1 and 4 (λmax ) 353 nm) suggest
the presence of primarily isolated chromophores with weak
electronic communication in the ground state of the polymer
and a break in the conjugation at phosphorus (Figure 2). The
results are consistent with electronic delocalization in 2 being
limited to P-Ar-P and P-N-Ar-N-P units, as expected
from investigations of phosphazines.7
Figure 3. Oxidation of 2b with PhI(OTf)2 in CHCl3.
spectral changes are consistent with the generation of radical
cations along the polymer backbone. As the concentration
of oxidant is raised to 1.0 equiv the new bands at 540 and
580 disappear while the absorption at 350 nm is retained,
consistent with the conversion of radical cations to dications.
Cyclic voltammetry of 2b provides results similar to those
observed for 1. We observe two quasi-reversible oxidations
at 0.07 and 0.38 V vs SCE consistent with the two single-
electron transfers (Scheme 2). The oxidation peaks are
significantly broader in 2b than in 1 as would be expected
for a mixture of oligomers. While the first oxidation is shifted
to slightly higher oxidation potential, the second oxidation
potential is significantly lowered. Oxidation of 4 was
observed at potentials greater than 1.0 V vs SCE supporting
the oxidations depicted in Scheme 2. We do not have a
thorough understanding of the source of this shift but suspect
We investigated the electrochemical and spectroscopic
properties of 2b to provide further insight into the nature of
(15) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss. Chem. Soc.
1989, 317.
Org. Lett., Vol. 4, No. 13, 2002
2215

phosphorus despite the contraction of the d-orbitals upon
formation of radical cationic and dicationic phosphonium
complexes supports the presumption that the nodal properties
of the phosphorus d-orbitals do not allow delocalization
through phosphorus along the polymer chain.9
Scheme 2
We will continue our investigation of the spectroscopic
and electronic properties of the incorporation of phosphine
imides into π-conjugated materials. The electron-donating
properties, ease of synthetic modification, and structural
dependence of the oxidation potentials make this new class
of organic materials particularly interesting.
Acknowledgment. The authors of this paper would like
to thank the University of Rhode Island and the donors of
the Petroleum Research Fund, administered by the American
Chemical Society, for financial support of this research, and
Professor William Euler for helpful discussions.
that it may be the result of the incorporation of the electron-
donating alkoxy substituents in 2b. The similarities of the
electrochemical and optical properties of 1 and 2b suggest
that there is little electronic communication through phos-
phorus in the neutral, radical cationic, or dicationic forms
of 2b. The lack of electronic communication through
Supporting Information Available: Experimental pro-
cedures and characterization data of 2b, 3b, and 4, CV of
2b, and UV-visible spectra of 1 and TCNQ. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL026065J
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Org. Lett., Vol. 4, No. 13, 2002