Redox behavior of 2-substituted 1,3-benzoxaphospholes and 2,6-substituted benzo[1,2- d:4,5- d]bisoxaphospholes

Article abstract of DOI:10.1021/om200014f

The electrochemical behavior of a series of 2-substituted 1,3-benzoxaphospholes and 2,6-disubstituted benzo[1,2-d:4,5-d]bisoxaphospholes has been examined by cyclic voltammetry methods. One-electron reductions near -1.99 V vs SCE in THF (0.1 M [ⁿBu₄N][BF₄]) are observed for 2-aryl-1,3-benzoxaphospholes, with the exception of compounds where aryl = p-C₆H₄Br or p-C₆H₄Cl, which show irreversible reduction processes. 2,6-Dimesityl-benzobisoxaphosphole and 2,6-dixylyl-benzobisoxaphosphole show two reduction waves, with the first wave displaying reversibility (ca. E_1/2 ≈ -2.0 V vs SCE). By contrast, 2,6-di-tert-butyl-benzobisoxaphosphole and 2,6-diadamantyl-benzobisoxaphosphole show single reversible reductions near -2.36 V vs SCE. DFT calculations have been conducted in order to give a greater understanding of the electronic factors influencing the electrochemical results.

Full text of DOI:10.1021/om200014f

ARTICLE
pubs.acs.org/Organometallics
Redox Behavior of 2-Substituted 1,3-Benzoxaphospholes and
2,6-Substituted Benzo[1,2-d:4,5-d0]bisoxaphospholes
Marlena P. Washington,† John L. Payton,† M. Cather Simpson,‡ and John D. Protasiewicz*,†
†Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States
‡Departments of Chemistry and Physics and the Photon Factory, University of Auckland, Auckland, New Zealand
S Supporting Information
b
ABSTRACT: The electrochemical behavior of a series of
2-substituted 1,3-benzoxaphospholes and 2,6-disubstituted
benzo[1,2-d:4,5-d0]bisoxaphospholes has been examined by
cyclic voltammetry methods. One-electron reductions near
-1.99 V vs SCE in THF (0.1 M [nBu4N][BF4]) are observed
for 2-aryl-1,3-benzoxaphospholes, with the exception of com-
pounds where aryl = p-C6H4Br or p-C6H4Cl, which show
irreversible reduction processes. 2,6-Dimesityl-benzobisoxa-
phosphole and 2,6-dixylyl-benzobisoxaphosphole show two
reduction waves, with the first wave displaying reversibility
(ca. E1/2 ≈ -2.0 V vs SCE). By contrast, 2,6-di-tert-butyl-
benzobisoxaphosphole and 2,6-diadamantyl-benzobisoxa-
phosphole show single reversible reductions near -2.36 V vs
SCE. DFT calculations have been conducted in order to give a
greater understanding of the electronic factors influencing the electrochemical results.
’ INTRODUCTION
molecules that feature phosphaalkene units.18 Unlike other
PdC containing materials, these materials are highly fluorescent
with emission in the blue. As part of our investigations to more
deeply understand these materials and their photoelectrochem-
ical properties, we have performed cyclic voltammetric studies of
these benzoxaphospholes. These studies illuminate the relation-
ships among electrochemical properties and optical properties.
Electrochemical studies on such heterocyclic molecules hav-
ing low-coordinate phosphorus atoms without complexation by
transition metals are rather limited, and no reports on the
electrochemistry of 1,3-benzoxaphospholes and benzobisoxa-
phospholes have appeared previously. Electrochemical studies
on phosphinine and biphosphinine have been reported,
however.19
An important aspect of organophosphorus compounds having
PdP and PdC pπ-pπ bonds is that they are often more
susceptible to electrochemical reduction than analogous CdC
containing counterparts. This fact can be attributed to the
presence of relatively low lying π*-orbitals, which are character-
istic of molecules possessing these types of pπ-pπ bonds.1
Figure 1 provides an overview to compare energies of the frontier
orbitals across the fundamental series of NdN, NdC, CdC,
CdP, and PdP pπ-pπ bonded systems. In addition to reduced
HOMO-LUMO gaps, the relative energies of the lone pair on N
or P atoms change, as does the nature of the orbital (from
HOMO to HOMO-1). For phosphaalkenes the HOMO orbital
is predicted to be the π-bond and relatively nonpolar. These facts
make phosphaalkenes more analogous in behavior to olefins as
compared to imines.2 In this series, diphosphenes (RPdPR)
should be most susceptible to electrochemical reduction due to
the lowest lying π*-orbitals, and this has indeed been observed.
Many diphosphenes have been shown to undergo reversible one-
electron reduction to radical anions, some of which can even be
isolated and can exhibit multiredox activity .3-11 Phosphaalkenes
(RPdCR2) have somewhat higher lying LUMOs than those of
diphosphenes, but still lower lying energies than for many simple
olefins and can also be redox active.11-16
’ RESULTS AND DISCUSSION
Synthesis. Compounds 1-9 (Table 1) and 11-14 (Table 2)
were reported previously and prepared according to published
procedures.17,18 The new benzoxaphosphole 10 was synthesized
by analogous methods, i.e., via a cyclocondensation reaction be-
tween mesitylimidoyl chloride and 4-isopropyl-2-phosphinophenol
in THF after refluxing for two days (Scheme 1). After removal of
THF under vacuum, the remaining solid was extracted into hexane
We recently reported on 2-substituted 1,3-benzoxaphosp-
holes (Chart 1, left) and 2,6-disubstituted benzo[1,2-d:4,5-d0]-
bisoxaphospholes (Chart 1, right),17 interesting heterocyclic
Received: January 8, 2011
Published: March 10, 2011
r
2011 American Chemical Society
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and platinum wire as the counter electrode, was utilized. All scans
were performed with 0.1 M tetrabutylammonium tetrafluorobo-
rate, [nBu4N][BF4], in THF as the supporting electrolyte, with a
scan rate of 0.1 V/s. Ferrocene was utilized as an internal
reference because of the use of a quasi-reference electrode
(silver wire) during analyses. Ferrocene (final concentration
0.001 M) was added after the initial scans of compounds. The
reduction potentials were thus referenced to the ferrocene/
ferrocenium redox couple versus saturated calomel electrode
(E1/2 = 0.55 V vs SCE).20
A representative cyclic voltammogram of compound 1 in the
range of 1.5 to almost -2.5 V is shown in Figure 2. For each of
the compounds 1-10 a reduction wave (Epc) was recorded
between -1.8 and -2.3 V. Table 1 summarizes the reduction
potentials for this series of 1,3-benzoxaphospholes.
Compounds 1-3, 6, and 8-10 show reversible, one-electron
reduction waves (E1/2) between = -1.9 and -2.2 V vs SCE.
Reversibility was ascertained by scanning compound 1 at various
scan rates (25 to 200 mV) and generating linear plots of scan
rates versus ΔEp for both ferrocene and compound 1 (see
Supporting Information), confirming adherence to the Nernst
equation.20
Figure 1. Frontier molecular orbitals of some double-bonded systems.
Chart 1
The impact of the addition of substituents on the 2-aryl ring of
the nonhalogenated 2-aryl-benzoxaphospholes and the 5-isopro-
pyl-2-aryl-benzoxaphospholes is portrayed in Figures 3 and 4,
respectively. The reduction potential for 1 (E1/2 = -1.90 V)
shifts negative by 60 and 130 mV on changing to compounds 2
(p-MeC6H4) and 3 (p-MeOC6H4), respectively. Similarly, the
5-isopropyl-2-aryl-benzoxaphospholes 6 (Ar = C6H5), 9 (Ar =
p-MeC6H4), 8 (Ar = p-MeOC6H4), and 10 (Ar = 2,4,6-
Me3C6H2) show increasing resistance to reduction. The impact
of the 5-isopropyl substituent on the benzoxaphosphole ring can
be assessed by looking at the pairs of compounds 1 and 6, 2 and 9,
and 3 and 8, where the 2-aryl group remains constant. For these
pairs, the reduction process is more difficult for the 5-isopropyl-
substituted material by 30-80 mV. Very small shoulders on
some the negative scans (ca. -1.8 to -2.0 V) can be descerned.
The origin of these humps is uncertain at this time.
For the benzoxaphospholes possessing 2-p-XC6H4 groups, the
initial reduction waves (Epc) occur more readily. The resulting
radical anions appear to be unstable under the experimental
conditions, as shown in Figure 5. Varying the scan rates during
the analysis of compound 4 from 25 to 200 mV did not
significantly change the nature of the CV waves (see Supporting
Information). It seems probable that these radical anions are
prone to lose halide (either Cl or Br), and the subsequently
generated radicals react with solvent, by analogy with the
processes shown for other aryl halides.21-25
Table 1. Reduction Potentials for 1,3-Benzoxaphospholes
(V vs SCE)
R0
R
Epc
Epa
E1/2
1
2
H
C6H5
-2.02
-2.08
-2.16
-1.91
-1.81
-2.06
-1.97
-2.25
-2.16
-2.33
-1.77
-1.84
-1.90
-1.90
-1.96
-2.03
H
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-BrC6H4
C6H5
3
H
4
H
5
H
6
iPr
iPr
iPr
iPr
iPr
-1.80
-1.93
7
p-ClC6H4
p-MeOC6H4
p-MeC6H4
2,4,6-Me3C6H2
8
-1.96
-1.87
-2.09
-2.11
-2.02
-2.21
9
10
The reduction potentials are similar to other phosphaalkenes
with no electron-donating and electron-withdrawing substituent
effects reported in the literature. For example, the phosphaalkene
Mes*PdC(H)Ph (Mes* = 2,4,6-tritert-butylphenyl) displays a
reversible one-electron reduction with an E1/2 = -1.98 V vs SCE
in THF.12a In addition, the impact of remote substituents on the
electrochemistry of phosphaalkenes has been amply demon-
strated previously in several studies,12-15 for example, the effect
of electron-withdrawing groups, specifically phenyl, thiophene,
and furan, on the cyclic voltammetry of phosphaalkenes.12d
Analyses on diphosphathienoquinones revealed reversible one-
electron reduction at E1/2 = -1.55 V vs Ag/Agþ for these
materials having a PdC unit.14c More recently electrochemical
analyses on phosphaalkenes featuring acetylenic components
and the solution treated using consecutive aqueous acid and base
washes, followed by filtration through basic alumina to yield a
colorless, moderately air-sensitive solid 10 in 25% yield. The 31P
NMR shift was determined to be δ 96.9 ppm, which is similar to
other 1,3-benzoxaphospholes reported in the literature.
Cyclic Voltammetry: Reductions. 1,3-Benzoxaphospholes of
the form 2-R-1,3-benzoxaphosphole (1-5) and 5-isopropyl-2-
R-1,3-benzoxaphosphole (6-10) were studied by cyclic voltam-
metry under anhydrous conditions inside a nitrogen-filled dry-
box (Table 1). Solutions with a concentration of 0.001 M
benzoxaphosphole in THF were used for reduction analyses of
these compounds. A three-electrode system, with glassy carbonas
the working electrode, silver wire as the quasi-reference electrode,
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Scheme 1
Table 2. Reduction Potentials for Benzobisoxaphospholes
(in V vs SCE)
R
Epc
Epa
E1/2
Epc
11
12
13
14
tBu
Ad
-2.49
-2.49
-2.12
-2.07
-2.20
-2.22
-1.89
-1.82
-2.35
-2.36
-2.01
-1.95
2,4,6-Me3C6H2
2,6-Me2C6H3
-2.48
-2.41
Figure 3. Overlay of cyclic voltammograms for compounds 1, 2, and 3.
display two, one-electron reduction waves, in which the first
wave occurs near E1/2 = -1.98 V vs SCE and a second wave near
Epc = -2.44 V vs SCE. A priori, one might anticipate that if the
benzoxaphospholes have one reduction wave, the benzobisox-
aphospholes might have two such waves, especially since these
two PdC units are firmly locked into conjugation with one
another.
The electrochemistry of compounds with one or more phos-
phaalkene units was first reported for compounds of the form ortho-,
para-, and meta-{Mes*PdC(H)}2C6H4.12b,c,e In one study, it was
determined that two reduction waves (ΔEp = 0.083 V, THF) could
be seen in the voltammograms for para-{Mes*PdC(H)}2C6H4.12e
The extended diphosphaalkenes {Mes*PdC(Ph)-CtC-CtC}2-
Ar (Ar = 1,4-phenyl or 9,10-anthracenyl) clearly show two resolved
reduction waves for the anthracene-bridged system (ΔEp = 0.21 V,
CH2Cl2), while the phenyl-bridged system in the corresponding two
waves are less well resolved (ΔEp = 0.10 V, CH2Cl2).13c
Figure 2. Cyclic voltammogram of 1.
While a number of comparisons to acyclic phosphaalkene electro-
chemistry can be drawn, comparisons to similar studies involving
heterocycles having PdC units are rarer. A notable example is the
pair of phosphinines P1 and P2 in Chart 2.19 Phosphinine P1
displays partially reversible reduction at E1/2 = -2.27 V vs SCE
(DME). The biphosphinine P2 displays multiple reductions in
DMF, the first two at E1/2 = -1.85 V and -2.42 V vs SCE. The
latter process is reversible only at high scan rates (2000 V s-1 and
above). The electrochemistry of the benzoxaphospholes and benzo-
bisoxaphospholes are thus not so different than that observed for
these two materials.
Cyclic Voltammetry: Oxidations. While some phosphaalkenes
display reversible reduction waves in cyclic voltammetry studies, few,
if any, display reversible oxidation waves. Notable exceptions include
phosphaalkenes that bear one or more N-substituents (Chart 3).2h,16
In fact, the radical cation produced upon oxidation of phosphaalkene
having reduction potentials ranging from E1/2 = -1.84 V to -2.04 V
vs Fc/Fcþ, depending on the types of substituents, have
appeared.13
By contrast to the 2-aryl-substituted 1,3-benzoxaphospholes,
when 2-tert-butyl-1,3-benzoxaphosphole was examined by CV
under the same conditions, no reduction wave was observed.
This result was quite surprising and suggests that the LUMO in
these materials is much higher in energy than for the 2-aryl-
substituted benzoxaphospholes (vide infra).
The reduction of benzobisoxaphospholes, 11-14 (Table 2),
was also studied by cyclic voltammetry using the same conditions
as the benzoxaphospholes 1-10. The voltammograms obtained
are depicted in Figure 6, with the data presented in Table 2.
Two different types of behavior are observed. Compounds 11
and 12 feature a single, reversible, one-electron reduction wave
near E1/2 = -2.35 V vs SCE, whereas compounds 13 and 14
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4 and 5 are more difficult to oxidize. This trend is consistent with
the electron-donating or electron-withdrawing capability of the
particular substituent. A comparison between 1 and 6 illustrates
that there is little or no effect with the addition of the 5-isopropyl
group on the oxidation potential.
Oxidation studies were also performed on benzobisoxaphosp-
holes 12 and 13 (Figure 8 and Table 3). In general, compounds
12 and 13 displayed irreversible oxidation waves near Epa
=
1.59 V vs SCE. The difference in oxidation potential between
these two compounds is 130 mV, which is similar to changes seen
in the reduction potentials. The dynamics between alkyl versus
aromatic is displayed again, where 13, having increased conjuga-
tion, results in a higher HOMO.
Irreversible electrochemical oxidation of phosphaalkenes seem to
be the norm. For example, Schoeller et al. reported on the
irreversible oxidation of nonaromatic-substituted phosphaalkenes
in 1991.11 Their oxidation potentials were determined to be
between Eox = 1.07 and 2.94 V vs SCE in butyronitrile. Ott et al.
demonstrated that acetylenic phosphaalkenes result in irreversible
oxidations as well. In CH2Cl2, their results showed oxidation
potentials near Eox = 1.02 V vs Fc/Fcþ.13
Figure 4. Overlay of cyclic voltammograms for compounds 6, 8, 9, and
10.
Computational Studies. The electrochemical data we have
obtained is mostly in accord with our expectations, with the
exception that there is a rather large difference in electrochemical
behavior between the aryl- and alkyl-substituted compounds.
Previously we have noticed significant differences in the photo-
luminescence between these classes of compounds.17 In order to
probe these differences, the electronic structures of benzoxa-
phospholes and benzobisoxaphospholes were studied using DFT
calculations. Kohn-Sham formalized density functional theory
(DFT) was used to calculate the ground-state electronic and
geometric structures with the B3LYP hybrid functional and
6-31þG** basis sets (i.e., B3LYP/6-31þG**) using Gaussian26
and with B3LYP/6-31-G* using SPARTAN.27 The results were
essentially the same for both methods, and the SPARTAN results
for the key frontier orbitals of select model and real compounds
are summarized in Figure 9. Several features merit comment.
First, the significant impact of the p-XC6H4 substituent on the
LUMO of the 2-aryl benzoxaphospholes is entirely consistent
with our experimental observations that show that electron-
donating substituents make reduction more difficult, while
electron-withdrawing substituents make the reduction more
facile. The predicted impact of a 5-isopropyl substutuent on
the 2-phenylbenzoxaphosphole is relatively small (Ph* in
Figure 9). The LUMOs for the 2-H- and 2-alkyl-substituted
benzoxaphospholes are notably higher in energy than for the
2-aryl benzoxaphospholes. A line (red dashed) can be drawn to
indicate the reductions that are reachable within our particular
electrochemical window. The essential nature of the LUMO and
HOMO orbitals across the series is similar and is composed of
large contributions from the PdC π- and π*-orbitals. Figure 10
portrays frontier orbitals for two examples. Importantly, the
2-aryl-substituted species show significant contributions from
the 2-aryl π-system into the frontier orbitals and exhibit proper-
ties that indicate that the PdC unit is π-conjugated throughout
the molecule.
Figure 5. Overlay of cyclic voltammograms for compounds 4, 5, and 7.
c (Chart 3) was sufficiently stable to allow isolation and structural
characterization.16 As our compounds also have heteroatom substit-
uents (oxygen instead of nitrogen), we sought to examine some of
these materials to ascertain if they might display some reversible
oxidations.
Compounds 1-6 were thus examined by cyclic voltammetry
for their propensity to undergo oxidation. For this purpose, the
solvent was changed to dichloromethane (DCM) at concentra-
tions of 0.001 M with 0.1 M tetrabutylammonium hexafluor-
ophosphate, [nBu4N][PF6], as the supporting electrolyte. A
three-electrode system, with glassy carbon as the working
electrode, silver wire as the quasi-reference electrode, and
platinum wire as the counter electrode, was used for analysis.
As above, ferrocene was utilized as an internal reference due to
the use of a quasi-reference electrode (silver wire).
Cyclic voltammograms are shown in Figure 7. Table 3 sum-
marizes the oxidation potentials for 1,3-benzoxaphospholes.
The phosphorus lone pair in these (as well as for the
benzobisoxaphospholes below) is predicted to be substantially
below the HOMO in energy. For compound 4 the lone pair
shown in Figure 11 is HOMO (-4) at -7.89 eV (vs -5.75 eV
for HOMO). Another noteworthy feature of these calculations
is that they are in line with our results on the optical
Compounds 1-6 show irreversible oxidation waves near Epa
=
1.3 V vs SCE. As with the reduction potentials, the oxidation
potentials are dependent upon the specific substituent. Com-
pared to compound 1, compounds 2 and 3 undergo more facile
oxidation (ΔE = 50 and 200 mV, respectively), while compounds
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Figure 7. Overlay of cyclic voltammograms of 1-6.
Table 3. Oxidation Data for Benzoxaphospholes and Ben-
zobisoxaphospholes (in V vs SCE)
R0
R
R00
Epa
1
H
H
H
H
H
iPr
C6H5
1.31
1.26
1.11
1.35
1.36
1.31
1.65
1.52
2
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-BrC6H4
C6H5
3
4
5
6
12
13
Ad
2,4,6-Me3C6H2
Figure 6. Overlaid cyclic voltammograms for compounds 11 and 12
(top) and for compounds 13 and 14 (bottom).
Chart 2
Chart 3
Figure 8. Overlay of cyclic voltammograms for 12 and 13.
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Figure 9. Frontier orbital energies for benzoxaphospholes and benzbisoxaphospholes as calculated by DFT theory. Ph* is data for the 5-isopropyl-2-
phenylbenzoxaphosphole.
Figure 10. Frontier molecular orbitals for benzoxaphosphole and compound 4.
spectroscopy of these materials. For the series of aryl-substi-
tuted compounds, both the LUMO and HOMO drop in energy,
yielding a relatively constant HOMO-LUMO gap. While
we have not yet performed TDDFT calculations to explore
the excited states, it is interesting to note that the absorption
and emission maxima vary by less that 10 nm across compounds
1-4.
The results of computations on benzobisoxaphospholes are
also given in Figure 9. As there are two PdC units, there are two
low-lying orbitals (LUMO and LUMOþ1) that are of PdC
π*-character (Figure 12). As for the calculation on benzoxa-
phospholes, the aryl-substituted compounds feature lower en-
ergy LUMOs than alkyl-substituted compounds. The
significance of this finding is that only the LUMO, but not the
LUMO(þ1), falls below the proposed limit for accessibility in
our electrochemical window (red dashed line). For the aryl-
substituted species, both the LUMO and LUMO(þ1) are below
this arbitrary line, suggesting the possibility for additional
reductions to be accessed by electrochemical reduction. This is
a rather simple model, as it lacks computations on the radical
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ARTICLE
strategy, in conjunction with the use of heterocycles (such as
benzoxaphospholes or phosphinines), thus offers many opportu-
nities for maximally π-conjugated multiply bonded phosphorus
compounds and materials.
’ EXPERIMENTAL SECTION
General Procedures. Experimental procedures were performed
under nitrogen either using Schlenk line techniques or in a nitrogen-
filled MBraun drybox. All benzoxaphospholes were prepared according
to the literature, with the exception of 5-isopropyl-2-mesityl-1,3-ben-
zoxaphosphole (10).17,18 Tetrahydrofuran, diethyl ether, and hexanes
were dried by distilling over metallic sodium and benzophenone.
Dichloromethane was dried over phosphorus pentoxide. NMR spectra
(1H and 31P{1H}) were recorded in CDCl3 on a Varian INOVA AS-400
spectrometer operating at 399.7 and 161.8 Hz, respectively. 31P{1H}
NMR spectra were referenced to 85% H3PO. 13C{1H} NMR spectra
were recorded in CDCl3 on a Varian INOVA AS-600 spectrometer
operating at 150.0 Hz. High-resolution mass spectrometry was per-
formed by the University of Michigan Mass Spectrometry facility using a
VG (Micromass) 70-250-S magnetic sector spectrometer with EI
technique at 70 eV. Elemental analyses were performed by Robertson
Microlit Laboratories.
Figure 11. Computed HOMO(-4) for compound 4.
5-Isopropyl-2-mesityl-1,3-benzoxaphosphole (10). In a drybox,
508 mg (2.94 mmol) of 4-isopropyl-2-phosphinophenol, 726 mg
(3.24 mmol) of N-(mesityl)benzimidoyl chloride, and a stir bar were
added to a 100 mL round-bottom flask. To this flask was added
approximately 50.0 mL of THF, and the flask was sealed and removed
from the drybox. The flask was outfitted with a condenser and refluxed
under nitrogen for 36 h, giving a yellow, cloudy solution. The solvent was
then removed in vacuo, and the flask was taken into the drybox. The
resulting solid was then extracted with hexanes, and filtration removed
the insoluble material, yielding a yellow solution. This solution was
removed from the drybox and washed successively with degassed
solutions of 10% aqueous NaOH, 10% aqueous H2SO4, and distilled
H2O. The organic solution was then dried over Na2SO4 and then CaCl2.
The solvent was removed in vacuo, and the flask was taken into the
drybox. The solid was dissolved in hexanes, and the solution was filtered
through Celite and then filtered through basic alumina. The solvent was
removed in vacuo to yield 251.4 mg of a colorless solid. Yield: 25%. 1H
NMR: δ 7.86 (m, 1H), 7.64 (m, 1H), 7.34 (m, 1H), 6.98 (s, 2H), 3.06
Figure 12. Frontier molecular orbitals for model 2,20-diphenyl-
benzbisoxaphosphole.
anions, which are the actual species receiving the second electron.
Nevertheless, it is consistent with experimental data.
’ CONCLUSION
3
(m, 1H), 2.34 (s, 3H), 2.25 (s, 6H), 1.34 (d, 6H, JHH = 6.8 Hz).
A series of 1,3-benzoxaphospholes and benzobisoxaphosp-
holes were examined by cyclic voltammetry. Many of the 2-aryl-
1,3-benzoxaphospholes display reversible, one-electron reduc-
tions, with the exception of halogenated derivatives, which show
irreversible reduction waves. By contrast, the 2-alkyl-1,3-benzox-
aphosphole derivative did not show any reduction behavior. In
the case of benzobisoxaphospholes, alkyl-substituted derivatives
are harder to reduce than their aryl-substituted counterparts,
which feature an additional reversible, one-electron reduction
wave. Computational studies on benzoxaphospholes offer a
working model to explain the number of electrochemical waves
observed as well as the character of the HOMOs and LUMOs.
With this contribution, and those appearing from other groups,
the opportunities for π-conjugated materials featuring multiply
bonded phosphorus atoms has reached a very exciting stage.
Polymers having conjugated PdC and PdP groups have now been
realized.28,29 These materials, however, face some loss of maximal
π-conjugation, as the PdC units are often twisted out of the optimal
configuration for conjugation owing to steric interactions. With
careful design, one can engineer bulky ligands that favor conjugation,
such as seen for a recently reported phosphasilene.30 This type of
31P{1H} NMR: δ 98.9 (s, 1P). 13C{1H} NMR: δ 196.3 (d, JPC = 53.7
Hz), 158.2 (d, JPC = 3.6 Hz), 143.4 (d, JPC = 9.0 Hz), 138.8, 137.5 (d, JPC
= 5.3 Hz), 137.0, 130.8 (d, JPC = 13.4 Hz), 128.3, 125.8 (d, JPC = 2.7 Hz),
125.5 (d, JPC = 18.9 Hz), 113.0, 33.8, 29.5, 22, 20.8 (d, JPC = 53.0 Hz).
Mp: 64-70 °C. HRMS m/z: 296.1333 (calc 296.1330). Anal. Calcd for
C19H21PO (296.13): C 77.06, H 7.15. Found: C 77.13, H 7.34.
Cyclic Voltammetry for Reduction Scans. Cyclic voltammetry
experiments were performed in a nitrogen-filled MBraun drybox out-
fitted with a CH Instrument workstation (CHI630C) at room tempera-
ture. Tetrabutylammonium tetrafluoroborate was recrystallized five
times using ethyl acetate and ether, dried thoroughly under vacuum,
and stored in the drybox. Ferrocene was purified via sublimation under
vacuum and stored in the drybox. All glassware was oven-dried overnight
before use. A glassy carbon working electrode was polished with 0.05 μm
alumina and thoroughly cleaned and dried before use. A silver wire was
utilized as a quasi-reference electrode, and a platinum wire was the
counter electrode. All scans were performed at a scan rate of 0.1 V/s
unless otherwise stated. All spectra were referenced to SCE using
ferrocene as an internal standard. Samples were analyzed without
ferrocene initially and then analyzed again with ferrocene for
referencing.
1981
dx.doi.org/10.1021/om200014f |Organometallics 2011, 30, 1975–1983

Organometallics
ARTICLE
Typical Procedure for 1-10. In a drybox, 2.20-3.50 mg of com-
pounds 1-10 and 329 mg of tetrabutylammonium tetrafluoroborate
were dissolved in dry THF in a 10.0 mL volumetric flask to give a 0.001
M solution. The solution was loaded into an electrochemical cell with
electrodes, and electrical leads were attached for analysis. After initial
scanning, approximately 1.70 mg of ferrocene was added to the cell and
the solution was scanned again.
Typical Procedure for 11-14. In a drybox, 2.20-3.50 mg of
compounds 11-14 and 329 mg of tetrabutylammonium tetrafluorobo-
rate were dissolved in dry THF in a 10.0 mL volumetric flask to give a
0.001 M solution. The solution was loaded into an electrochemical cell
with electrodes, and electrical leads were attached for analysis. After
initial scanning, approximately 1.70 mg of ferrocene was added to the
cell and the solution was scanned again.
Cyclic Voltammetry for Oxidation Scans. Cyclic voltammetry
experiments were performed in a nitrogen-filled MBraun drybox outfitted
with a CH Instrument workstation (CHI630C) at room temperature.
Tetrabutylammonium hexafluorophosphate was recrystallized five times using
ethyl acetate and ether, dried thoroughly under vacuum, and stored in the
drybox. Ferrocene was purified via sublimation under vacuum and stored in
the drybox. All glassware was oven-dried overnight before use. A glassy carbon
working electrode was polished with 0.05 μm alumina and thoroughly cleaned
and dried before use. A silver wire was utilized as a quasi-reference electrode,
and a platinum wire was the counter electrode. All scans were performed at a
scan rate of 0.1 V/s unless otherwise stated. All spectra were referenced to SCE
using ferrocene as an internal standard. Samples were analyzed without
ferrocene initially and then analyzed again with ferrocene for referencing.
Typical Procedure for 1-6. In a drybox, 2.20-3.50 mg of com-
pounds 1-6 and 387 mg of tetrabutylammonium hexafluorophosphate
were dissolved in dry DCM in a 10.0 mL volumetric flask to give a 0.001
M solution. The solution was loaded into an electrochemical cell with
electrodes, and electrical leads were attached for analysis. After initial
scanning, approximately 1.70 mg of ferrocene was added to the cell and
the solution was scanned again.
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’ ASSOCIATED CONTENT
S
Supporting Information. Scan rate vs ΔEp plots for
b
€
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: protasiewicz@case.edu.
’ ACKNOWLEDGMENT
We thank the National Science Foundation for support (CHE-
0748982), and Prof. Daniel Scherson (CWRU) for helpful discussions.
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dx.doi.org/10.1021/om200014f |Organometallics 2011, 30, 1975–1983