Bis(phosphine imide)s: Easily tunable organic electron donors

Article abstract of DOI:10.1021/jo051196u

The electrochemical, structural, and spectroscopic properties of bis(phosphine imide)s have been investigated. p-Phenylenebis(phosphine imide)s Ar₃PNC₆H₄NPAr₃ (1a-d) have two reversible single-electron oxidation

Full text of DOI:10.1021/jo051196u

Bis(phosphine Imide)s: Easily Tunable Organic Electron Donors
Vanina V. Guidi, Zhou Jin, Devin Busse, William B. Euler, and Brett L. Lucht*
Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881
blucht@chm.uri.edu
Received June 13, 2005
The electrochemical, structural, and spectroscopic properties of bis(phosphine imide)s have been
investigated. p-Phenylenebis(phosphine imide)s Ar3PNC6H4NPAr3 (1a-d) have two reversible
single-electron oxidations. The first oxidation potentials can be varied from -0.05 to 0.15 V (versus
SCE) by modification of the substituents on phosphorus (Ar). Electron-donating substituents lower
the oxidation potential, while electron-withdrawing substituents increase the oxidation potential.
The difference between the first and second oxidation potential (∆E, 0.41-0.50) and the electronic
coupling (Hab, 1.1 eV) are similar for 1a-d. Computational (DFT) and UV-visible-NIR spectroscopic
investigations of 1a-d suggest that the first oxidation leads to a delocalized radical cation 1a•+
while the second oxidation leads to a quinonoidal dicationic state 1a2+. The aromatic linker between
phosphine imides has also been modified. Upon oxidation, N,N′-4,4′-biphenylene(bis(triphenyl)-
phosphine imide) (3) forms radical cationic and a dicationic species similar to 1a-d. While ∆E
(0.18 V) and Hab (0.63 eV) are smaller, suggesting weaker electronic communication between the
two PdN units in the radical cationic state, the presence of NIR absorptions with vibrational fine
structure (768, 861, and 983 nm) supports the formation of delocalized radical cation for 3•+.
Introduction
transport layer. The addition of the hole transport and
electron transport layers allows easier hole and electron
injection into the emissive layer, which provides devices
that operate at significantly lower voltages and have
longer lifetimes.3 The reversible generation of radical
cations is required for good organic hole transporting
materials. The most widely used hole transport materials
include triarylamines such as 4,4′-bis-(m-tolylphenylami-
no)biphenyl (TPD) and poly(N-vinylcarbazole) (PVK)
(Chart 1).4 While many polymer or conjugated organic
materials have been used as electron transport layers,
the number of good hole transport materials is limited.
Thus, the investigation of novel organic materials that
One of the important uses for conjugated organic
materials is the preparation of organic light emitting
diodes (OLEDs) and polymer light emitting diodes
(PLEDs).1 The initial investigations of PLEDs were
simple single-layer devices where the conjugated poly-
mer, typically poly(p-phenylene vinylene), was placed
between a positive and a negative electrode.2 As the field
of OLEDs and PLEDs developed, the preferred architec-
ture evolved into a three-layer device consisting of a hole
transport layer, light emitting layer, and an electron
(1) (a) Handbook of Conducting Polymers; Skotheim, T. A., Ed.;
Marcel Dekker: New York, 1986. (b) Fichou, D. In Conjugated
Polymers: The Novel Science and Technology of Highly Conducting
and Nonlinear Optically Active Materials; Bredas, J. L., Silbey, R., Eds.;
Kluwer Academic: Dordecht, The Netherlands, 1991.
(2) Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990,
347, 539.
(3) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402.
(4) Anderson, J. D.; McDonald, E. M.; Lee, P. A.; Anderson, M. L.;
Ritchie, E. L.; Hall, H. K.; Hopkins, T.; Mash, E. A.; Wang, J.; Padias,
A.; Thayumanavan, S.; Barlow, S.; Marder, S. R.; Jabbour, G. E.;
Shaheen, S.; Kippelen, B.; Peyghambarian, N.; Wightman, R. M.;
Armstrong, N. R. J. Am. Chem. Soc. 1998, 120, 9646.
10.1021/jo051196u CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/20/2005
J. Org. Chem. 2005, 70, 7737-7743
7737

Guidi et al.
TABLE 1. Oxidation Potentials of Bis(phosphine
Imide)sa
CHART 1
compound
E1
E2
∆E ) (E1 - E2)
1a
1b
1c
1d
3
0.04
-0.05
-0.02
0.15
0.55
0.40
0.47
0.56
0.54
0.50
0.45
0.49
0.41
0.18
0.36
a 1 mM solution in 0.1 M TBATFB in CH2Cl2. E1,2: Electrode
potentials (volts) referenced to SCE. E1: Potential of radical cation.
E2: Potential of dication.
form stable radical cations is important for improvements
in OLEDs and PLEDs. Although many of the conjugated
polymers and related conjugated organic materials have
utilized both sulfur and nitrogen,1,4,5 the investigation of
the electronic properties of phosphorus containing organic
materials has only recently received significant atten-
tion.6-9
Phosphine imides are related to the widely investigated
phosphazenes. Phosphazenes have four σ-bonds to phos-
phorus and thus no available p-orbital for involvement
in π-conjugation. However, the bond angles and lengths
support delocalization limited to P-N-P units.10 The
nature of the phosphorus-nitrogen double bond is debat-
able, but recent results argue against significant involve-
ment of the d-orbitals on phosphorus and support a
combination of σ-bonding and some degree of π-back-
bonding from N into the σ* orbitals of P.11 Infrared
spectroscopic investigations of the PdN bond stretch of
p-substituted phenylphosphine imides support an expan-
sion of conjugation due to the electron-donating effect of
the phosphine imide and increasing quinonoidal charac-
ter of the aromatic ring (eq 1).12
V versus saturated calomel electrode (SCE)). This sup-
ports the formation of stable radical cationic and dicat-
ionic species. Results of spectroscopic and electrochemical
investigations of poly(p-phenylene phosphine imide)s (2)
are similar to 1a, suggesting formation of localized
radical cations on the polymer chains and electronically
insulating phosphorus atoms. The formation of stable
radical cationic species at potentials comparable to
tetrathiafulvalene (TTF) indicates that bis(phosphine
imide)s are good electron donors and may be useful as
hole transport materials.
This report is an expansion of our initial studies of the
electronic properties of 1a and the related polymers (2).
We have investigated a variety of bis(phosphine imide)s
via modification of both the substitutents on phosphorus
and the aromatic linking unit between bis(phosphine
imide)s. The properties of the bis(phosphine imide)s have
been characterized by a combination of cyclic voltamme-
try (CV), absorption spectroscopy (UV-vis-NIR), single-
crystal X-ray diffraction, and density functional theory
(DFT) calculations.
The electrochemical and spectroscopic investigation of
N,N′-p-phenylene(bis(triphenyl)phosphine imide) (Ph3-
PdNC6H4NdPPh3, 1a) has been recently reported.13
Phosphine imides are prepared in high yields via the
Staudinger reaction of aryl azides with triaryl phos-
phines.14 Investigation of the electronic properties of
phosphine imide-based materials suggests that p-phen-
ylenebis(phosphine imide)s are good organic electron
donors. 1a is stable to atmospheric conditions and has
two reversible single-electron oxidations (0.043 and 0.547
Results
Bis(phosphine imide)s 1a-d15,16 and 317 were prepared,
purified, and identified following literature procedures.
(5) Ferraro, J. R.; Williams, J. M. Introduction to Synthetic Electrical
Conductors; Academic Press: Orlando, FL, 1987.
(6) Cosmina, D.; Shashin, S.; Smith, R. C.; Choua, S.; Berclaz, T.;
Geoffroy, M.; Protasiewicz, J. D. Inorg. Chem. 2003, 42, 6241.
(7) (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.
(8) (a) Lucht, B. L.; St. Onge, N. O. Chem. Commun. 2000, 2097.
(b) Jin, Z.; Lucht, B. L. J. Organomet. Chem. 2002, 653, 167. (c) Jin,
Z.; Lucht, B. L. J. Am. Chem. Soc. 2005, 127, 5586.
Cyclic Voltammetry. Investigation of substituted bis-
(phosphine imide)s suggests that the electron-donating
substituents can stabilize the positive charge on phos-
phorus and improve the electron-donating ability while
electron-withdrawing substituents reduce the donor prop-
erties (Table 1, Figure 1). The electron-donating p-
methoxyphenyl substituents of 1b significantly lower the
oxidation potentials of the bis(phosphine imide). Two
single-electron oxidations are observed at -0.05 and 0.40
V versus SCE (oxidation potentials for 1a are 0.04 and
(9) (a) Wright, V. A.; Gates, D. P. Angew. Chem., Int. Ed. 2002, 41,
2389. (b) Smith, R. C.; Chen, X.; Protasiewicz, J. D. Inorg. Chem. 2003,
42, 5468.
(10) (a) Blonsky, P. M.; Shriver, D. F.; Austin, P.; Allcock, H. R. J.
Am. Chem. Soc. 1984, 106, 6854. (b) Gray, F. M. Solid Polymer
Electrolytes: Fundamentals and Technological Applications; VCH:
New York, 1991. (c) Neilson, R. H. Phosphorus-Nitrogen Compounds.
In Encyclopedia of Inorganic Chemistry; King, B. R., Ed.; Wiley &
Sons: New York, 1994; Vol. 6, pp 3180-3199.
(11) Sudhakar, P. V.; Lammertsma, K. J. Am. Chem. Soc. 1991, 113,
1899.
(12) Wiegrabe, W.; Bock, H. Chem. Ber. 1968, 101, 1414.
(13) Escobar, M.; Jin, Z.; Lucht, B. L. Org. Lett. 2002, 2213.
(14) Abel, E. W.; Mucklejohn, S. A. Phosphorus Sulfur 1981, 9, 235.
(15) Herring, D. L. J. Org. Chem. 1961, 26, 3998.
(16) Khmurova, I. N.; Yurchenko, V. G.; Kukhar, V. P.; Zolotareva,
L. A. Zh. Obshch. Khim. 1974, 44, 74.
(17) Homer, L.; Oediger, H. Ann. 1959, 627, 142.
7738 J. Org. Chem., Vol. 70, No. 19, 2005

Bis(phosphine Imide)s: Tunable Organic Electron Donors
TABLE 2. UV-Visible-NIR Data of Bis(phosphine
Imide)sa
neutral
radical cation X•+
compound λmax nm (ꢀmax × 10-3
)
λmax nm (ꢀmax × 10-3
)
1a
1b
1c
1d
3
263 (17), 319 (sh)
394 (46), 492 (sh), 531 (31),
576 (31)
395 (30), 539 (22), 491 (sh),
580 (22)
396 (42), 496 (sh), 530 (28),
577 (30)
392 (34), 491 (sh), 533 (22),
576 (24)
317 (sh)
262 (sh), 318 (sh)
262 (sh), 319 (sh)
318 (14)
458 (sh), 499 (12), 768 (sh),
861 (8), 983 (17)
a UV-visible spectra were recorded in either CHCl3 or CH2Cl2.
Both solvents gave similar spectra. The wavelength maximum and
absorption coefficient (ꢀmax × 10-3) are provided for each major
absorption. (sh) Indicates absorption is a shoulder.
FIGURE 1. Cyclic voltammograms for compounds 1a, 1b, and
1d.
SCHEME 1
The difference between the first and second oxidation
potential (∆E) for 1a-d is between 0.41 and 0.50 V.
While ∆E has been used as measure of the degree of
electronic communication, it can be misleading.21 How-
ever, the similarity of ∆E values for 1a-d suggests
comparable delocalization of radical cations 1a•+-d•+.
The ∆E values of 1a-d are similar to those reported for
N,N,N′,N′-tetra(4-methoxyphenyl)-1,4,-phenylenedia-
mine, 0.49 V,22 and N,N,N′,N′-tetramethylphenylenedi-
amine (TMPD), 0.57 V,23 and support strong electronic
coupling between PdN units via the phenylene linker
consistent with a delocalized radical cation.
In addition to modification of the substituents on
phosphorus, the aromatic linker between phosphine
imides can be easily modified. The investigation of 3,
which has been prepared from benzidene and triphen-
ylphosphine, is particularly interesting since 3 has the
same bridging group as TPD. Cyclic voltammetry of 3
provides results similar to those observed for 1a-d. Two
reversible single-electron oxidations are observed for 3.
The higher first oxidation potential (0.36 versus SCE) and
smaller ∆E of 3 (0.18 versus 0.45 V for 1a) suggest
weaker electronic coupling between phosphine imides.22
UV-Visible-NIR Absorption Spectroscopy. The
absorption spectra of 1a-d were investigated in the
neutral, cationic, and dicationic states. The absorption
maxima are summarized in Table 2. The absorption
spectra for neutral compounds 1a-d are similar, with
two high-energy absorption maxima characteristic of the
localized aromatic rings on phosphorus and in the linking
phenylenediamine (λmax ) 260-265 nm, 345-355 nm).
Chemical oxidation of the bis(phosphine imide)s was
conducted under an inert atmosphere and followed by
UV-visible-NIR absorption spectroscopy. Representa-
tive spectra of the chemical oxidation of 1a are shown in
Figure 2. Upon oxidation to the radical cation, several
new absorption bands are observed for 1a-d at longer
wavelength (350-600 nm). The new low energy bands
result from absorptions of the radical cation and are
similar to those observed for TMPD24,25 and bis(3-oxo-9-
0.55 V versus SCE). Alternatively, the electron-with-
drawing p-fluorophenyl substituents of 1d increase the
oxidation potential (0.15 and 0.56 V versus SCE). As
expected, 1c has an oxidation potential lower than that
of 1a due to the superior electron-donating properties of
an alkyl group versus hydrogen. The results suggest that
the oxidation potentials of bis(phosphine imide)s can be
easily tuned via modification of the substituents on
phosphorus. The oxidation potentials of 1a-d are com-
parable to the most widely investigated organic electron
donors, such as TTF.18 However, TTF derivatives require
much more complicated syntheses, making chemical
modification of the oxidation potentials difficult.19
Both the radical cation and dications of 1a-d are
stable. Chloroform solutions of 1a•+ can be stored at room
temperature in the presence of atmospheric oxygen and
moisture for weeks with no apparent degradation. The
structure of the oxidized bis(phoshine imide) 1a has been
investigated by DFT, as discussed later in the article, and
is consistent with the stepwise formation of a radical
cation and dication as depicted in Scheme 1. The qui-
nonediimine structure of 1a2+ is consistent with the
observed stability and low oxidation potential for forma-
tion.
In all cases, the two single-electron oxidations of 1a-d
appear reversible. The reversibility of the oxidations was
confirmed for 1b by monitoring the intensity of the anodic
peaks at different scan rates. A linear plot of Ip versus
the square root of the scan rate was observed, confirming
the reversibility of the redox process.20
(21) Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U.
T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262.
(22) Lambert, C.; Noll, G. J. Am. Chem. Soc. 1999, 121, 8434.
(23) Nelsen, S. F.; Kessel, C. R.; Brien, D. J. J. Am. Chem. Soc. 1980,
102, 702.
(24) Shida, T. Electronic Absorption Spectra of Radical Ions; Else-
vier: Amsterdam, 1988.
(18) Torrance, J. B. Mol. Cryst. Liq. Cryst. 1985, 126, 55.
(19) Roncali, J. J. Mater. Chem. 1997, 12, 2307.
(20) Laboratory Techniques in Electroanalytical Chemistry; Kiss-
inger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1985.
J. Org. Chem, Vol. 70, No. 19, 2005 7739

Guidi et al.
FIGURE 3. UV-visible-NIR spectra of the stepwise oxida-
tion of 3 (50 µM in CHCl3) to 3•+ (a) and 3•+ to 32+ (b).
FIGURE 2. UV-visible-NIR spectra of the stepwise oxida-
tion of 1a (40 µM in CHCl3) to 1a•+ (a) and 1a•+ to 1a2+ (b).
azabicyclo[3.3.1]non-9-yl)benzene).25,26 Further oxidation
to the dication results in a loss of the low energy
absorptions characteristic of the radical cation. The
dications have short wavelength <300 nm absorption
maxima, but the absorptions are difficult to quantita-
tively characterize due to overlapping absorptions of the
chemical oxidant, bistrifluoroacetoxyiodobenzene, utilized
in these investigations.
Chemical oxidation of 3 provided similar results (Fig-
ure 3). Upon removal of a single electron to generate 3•+,
several new absorptions were observed. In addition to the
longer wavelength absorption bands in the visible spec-
trum (458 and 499 nm), three new absorption bands are
observed in the NIR (768, 861, and 983 nm). The new
low energy absorptions are similar to those observed in
N,N,N′,N′-tetramethylbenzidene (TMB).24 The presence
of vibrational fine structure is characteristic of delocal-
ized radical cations.27 The steric interactions present in
the biphenyl unit result in a decrease in the electronic
coupling between PdN units as evidenced by the lower
energy absorptions. Changing the solvent from CH2Cl2
to acetonitrile did not result in a significant change in
the NIR absorption bands (768, 859, and 978), consistent
with delocalized species.28 The addition of a second
FIGURE 4. X-ray structure of 1b.
equivalent of oxidant to 3•+ to generate the dication
results in the loss of low energy absorption bands,
consistent with the formation of a dication.
Single-Crystal X-ray Diffraction. Suitable crystals
of 1b were prepared via crystallization from benzene and
analyzed by single-crystal X-ray diffraction (Figure 4).
The structure of 1b has an inversion center in the middle
of the central aromatic ring and PdN bonds (156.2(2) pm)
comparable to those previously reported for N-p-bro-
mophenyl triphenylphosphine imide (4, 156.7 pm).29 The
C-P bond lengths are not equivalent. Two short bonds
(P(1)-C(10) 179.9(2) and P(1)-C(4) 180.6(2) pm) and one
long bond (P(1)-C(16) 181.6(2) pm) are observed. In
addition, the C(4)-P(1)-C(10) bond angle is large, 106.2-
(10)°, while the C(4)-P(1)-C(16) and C(10)-P(1)-C(16)
(25) Bailey, S. E.; Zink, J. I.; Nelsen, S. F. J. Am. Chem. Soc. 2003,
125, 5939.
(26) Nelsen, S. F.; Weaver, M. N.; Telo, J. P.; Lucht, B. L.; Barlow,
S. J. Org. Chem. Submitted.
(27) Nelsen, S. F. Chem.-Eur. J. 2000, 6, 581.
(28) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002,
31, 168.
(29) Hewlins, M. J. E. J. Chem. Soc. B 1971, 942.
7740 J. Org. Chem., Vol. 70, No. 19, 2005

Bis(phosphine Imide)s: Tunable Organic Electron Donors
FIGURE 5. Orbital energy diagram for 1a, 1a•+, and 1a2+. The zero of energy was set to be mid gap in each case.
bond angles are small (104.2(10) and 104.8(10)°, respec-
tively). Similar bond asymmetry is observed for C-P
bonds in 4.29 The C-P-C bond angles and C-P bond
lengths are consistent with π-back-bonding from N to P,
which weakens and lengthens the P-C bond.30 The
asymmetry of P-C bond lengths suggests that the
π-back-bonding from N is largely oriented into the σ*
orbital of the P(1)-C(16) bond. Interestingly, the P(1)-
C(16) bond is nearly perpendicular to the P1-N1-C2
plane, consistent with this model.
Density Functional Theory Calculations. DFT
calculations were conducted on bis(phosphine imide)s 1a,
1a•+, and 1a2+. The structures were optimized using the
B3LYP functional and the 6-31G** basis set in the
Spartan quantum chemical program suite.31 The calcula-
tions strongly agree with many of the spectroscopically
and crystallographically observed properties of 1a-d.
Neutral 1a has localized PdN double bonds (155.1 pm),
compared to 156.2 pm in the X-ray structure for 1b
(Figure 4), long C-N bonds (140.3 pm), and a large
dihedral angle between the PdN bond and the central
aromatic ring (∼78°), indicating little conjugation be-
tween the PdN double bond and the benzene ring. Upon
one-electron oxidation, the P-N bonds become longer
(162.3 and 158.5 pm), the C-N bonds become shorter
(131.5 and 134.4 pm), and the P-N-C plane and the
central p-phenylenediamine ring become nearly coplanar
(dihedral angles less than 5°), indicating the development
of the quinonediimine structure. The charge and spin
densities for 1a•+ demonstrate that the oxidation has
occurred primarily from the N lone pairs and the ipso C
atom π-orbitals (∼70% of the spin density and about 45%
of the positive charge is located on these four atoms). In
1a•+, the charge density of P is nearly the same as that
in 1a and there is no spin density found on the P atoms,
indicating that the P atom is unaffected by the removal
of the electron. Upon further oxidation to 1a2+, the
quinonediimine structure fully develops. The C-N bonds
are found to be 126.5 pm, clearly a double bond, while
the P-N bond increases to 166.6 pm, close to a single
bond length. Again, there is little increased charge
density on the P atoms: the ionic charge resides on the
N atoms and the central benzene ring. Finally, the
calculated P-C bond angles and lengths for 1a are
similar to those of 1b. Upon oxidation to 1a•+ and 1a2+
,
the P-C bond inequivalency is lost and the P-C bond
lengths are reduced, consistent with a loss of electronic
donation into the σ* orbitals of the P-C bonds.
(30) Dunne, B. J.; Morris, R. B.; Orpen, A. G. J. Chem. Soc., Dalton
Trans. 1991, 653.
(31) Spartan for Linux; Wavefunction, Inc.: Irvine, CA.
J. Org. Chem, Vol. 70, No. 19, 2005 7741

Guidi et al.
SCHEME 2
The orbital energy diagram for 1a, 1a•+, and 1a2+ is
shown in Figure 5. In the reduced form, the highest
occupied molecular orbital (HOMO) is a π-orbital located
on the central ring and the N atoms while the lowest
unoccupied molecular orbital (LUMO) is a π-orbital
located on the peripheral phenyl groups. The HOMO-
LUMO gap is calculated to be 3.6 eV. Upon one-electron
oxidation to 1a•+, the singly occupied molecular orbital
(SOMO) is an N-ring-N π-orbital and is slightly lower
in energy and the LUMO is slightly raised in energy so
that the SOMO-LUMO gap is ∼3.8 eV. In terms of the
optical spectrum, however, the lowest energy excitation
will be the HOMO to SOMO. These orbitals have a
calculated energy difference of ∼1.9 eV. Finally, in the
dication the filled N-ring-N orbitals become much lower
in energy so that in 1a2+ the HOMO is located on the
peripheral phenyl rings and the LUMO is on the central
ring and N atoms. The HOMO-LUMO gap in 1a2+ is
found to be ∼2.9 eV.
compounds are completely delocalized and have two
equivalent redox centers with an intermediate oxidation
state. Compounds 1a•+-d•+ and 3•+ have properties
consistent with Robin and Day class III systems. The
electronic coupling between redox sites for class III
species can be measured from the energy of the optical
transition using eq 2.28
hνmax ) 2Hab
(2)
Hab ) 1.08 and 0.63 eV for 1a•+ and 3•+, respectively.
These values are similar to those determined for TMPD•+
and TMB•+, 1.01 and 0.61 eV, respectively.33 3 undergoes
two reversible single-electron oxidations, but the first
oxidation occurs at a higher potential and the ∆E is
smaller (0.18 V) than that observed for 1a (0.50 V).
Changing the conjugated linker from phenyl in 1a to
biphenyl in 3 results in a decrease of ∆E and Hab,
suggesting weaker electronic coupling between PdN
units via the biphenyl linker.
The combination of DFT calculations and single-crystal
X-ray diffraction provides insight into the nature of the
PdN bond in phosphine imides. The bond angles and
lengths for the calculated structure of 1a and the crys-
tallographically determined structure of 1b are similar.
Upon oxidation, calculations indicate that the central
aromatic ring shifts from a benzenoid structure in 1a to
a quinonoid structure in 1a2+ (Scheme 1). The change in
structure of the central aromatic ring is accompanied by
an 8% increase in the P-N bond length. However, there
is little change in the charge on P going from 1a to 1a•+
and 1a2+. The increase of the P-N bond length is
accompanied by a modest decrease in the P-C bond
lengths (2-3%), consistent with a loss of electronic
donation into the σ* orbitals on P. The results provide
additional support for the recently proposed model for
PdC, PdN, and PdO bonds, π-back-bonding from N into
the σ* orbitals on P.34
The computational results support little change of the
charge on phosphorus upon oxidation. This suggests that
the standard structure depicted for phosphine imides
may be flawed. Recent investigations into the structure
of phosphine oxides and phosphine imides support a
highly polarized bond with significant π-back-bonding
interaction between nitrogen and phosphorus. Our com-
putational results support a highly polarized P-N bond
with almost complete charge separation in 1a. In fact,
the stepwise oxidation of 1a to 1a•+ and 1a2+ might best
be represented by Scheme 2. An electron-rich dianionic
diamine is capped by cationic phosphonium ions. Elec-
tronic donation of the imine into the σ*-orbitals of the
P-C bonds provides some additional stability to 1a. The
stepwise oxidation removes electrons from the central
diamine unit with little change to the phosphonium ions.
Discussion
The tunability of the electron donor properties of 1a-d
is displayed in Table 1. Electron-donating substituents
shift the oxidations to lower potential while electron-
withdrawing substituents increase the oxidation poten-
tials. The ready availability of many different phosphines
allows straightforward synthetic modification of p-phen-
ylenebis(phosphine imide)s and excellent control over the
oxidation potential. The development of OLEDs requires
hole injection materials with electronic structures that
match the work function of the electrode materials
allowing efficient hole transfer. Thus, p-phenylenebis-
(phosphine imide)s may make good hole transport ma-
terials for OLEDs.
Conclusions
The electrochemical, structural, and spectroscopic prop-
erties of aromatic bis(phosphine imide)s have been
investigated. Aromatic bis(phosphine imide)s have two
reversible one-electron oxidations at low potentials. The
oxidation potentials of these novel organic electron donors
can be easily modified via substituents on phosphorus.
Electron-donating substituents lower the oxidation po-
tential while electron-withdrawing substituents increase
the oxidation potential. The aromatic linker between bis-
(phosphine imide) units can also be varied, allowing
control over the electronic coupling between PdN units.
Experimental Procedures
Robin and Day developed a classification system of
mixed valence compounds with three classes:32 class I
contains redox centers that are completely localized and
behave separately, class II species have intermediate
coupling between the two redox centers, and class III
Bis(phosphine imide)s 1a-d15,16 and 317 were prepared,
purified, and identified following literature procedures. A
summary of the H, 13C NMR, and IR spectroscopic data for
1
(33) Nelsen, S. F.; Tran, H. Q.; Nagy, M. A. J. Am. Chem. Soc. 1998,
120, 298.
(34) Gilheany, D. G. Chem. Rev. 1994, 94, 1339.
(32) Robin, M.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247.
7742 J. Org. Chem., Vol. 70, No. 19, 2005

Bis(phosphine Imide)s: Tunable Organic Electron Donors
1b-d and 3 and a description for the general synthetic
procedure for 1a-d are included in the Supporting Informa-
tion. Phosphines were purchased from a commercial supplier.
All the solutions were prepared under N2 using anhydrous
solvents purchased from a commercial supplier and used as
received.
CV was performed on a Perkin-Elmer EG&G instruments
Potentiostat and Galvanostat model 263A from Princeton
Applied Research products, using the model K0264 micro-cell
kit, K0265 silver/silver chloride reference electrode, K0266
counter electrode assembly (platinum 0.3-mm diameter counter
electrode wire), and G0225 platinum microelectrode (working
electrode). The scan rate was set at 50 mV/s. Two cycles were
conducted for each system. No calibrations were made to
compensate IR drops. A solution (1 mM) of compounds 1a-d
and 3 was prepared in a 0.1 M solution of tetrabutylammo-
niumtetrafluoroborate (TBATFB, 99%) in anhydrous CH2Cl2.
TBATFB and CH2Cl2 were purchased from Aldrich and used
as received. Electrode potentials were referenced to SCE by
use of TTF (1 mM solution in the same electrolyte) as an
external standard (0.30 and 0.66 V versus SCE).35 The values
of electrode potentials reported in Table 1 (E1 and E2) cor-
respond to the average between the cathodic electrode poten-
Theoretical calculations were run using Spartan 02 (Build
114) for Linux 2.2.31 No negative frequencies were found,
indicating that the structures were minima. The electronic
structures were calculated using DFT using the B3LYP hybrid
functional and the 6-31G** basis set.
Crystals of compound 1b for single-crystal X-ray diffraction
were grown from anhydrous benzene at room temperature. The
CIF is included in Supporting Information.
Acknowledgment. We thank the Donor of the
Petroleum Research Fund for partial support of the
work, A. Chandrasekaran and D. Venkaraman at the
University of Massachusetts, Amherst, for acquisition
of the X-ray structure of 1b, and S. F. Nelsen, University
of Wisconsin, Madison, for helpful discussions.
tial (Ec) and the anodic electrode potential (Ea), Ei
)
1
[Ea + Ec]/2. ∆E corresponds to the gap between the electrode
potentials as reported in Table 1, ∆E ) [E1 - E2].
UV-vis-NIR spectra were acquired using a Perkin-Elmer
Lambda 900 UV-visible-NIR spectrometer. Spectra of 1a-d
and 3 were taken for the neutral compound, radical cation,
and dication. Chemical oxidation using bistrifluoroacetoxy-
iodobenzene (PhI(OTF)2) was performed for each compound.
Supporting Information Available: Listings of the H
and 13C NMR and IR absorptions for compounds 1b-d and 3,
1H NMR spectra of 1b and 1d, 13C spectrum for 1d, CIF for
1b, figure of molecular packing of 1b, and complete details of
the computational methods for 1a including Cartesian coor-
dinates and computed total energies. This material is available
free of charge via the Internet at http://pubs.acs.org.
(35) Wheland, R. C.; Gillson, J. L. J. Am. Chem. Soc. 1976, 98, 3916.
JO051196U
J. Org. Chem, Vol. 70, No. 19, 2005 7743