A Stable Crystalline Triarylphosphine Oxide Radical Anion

Article abstract of DOI:10.1002/anie.201605963

Triarylphosphine oxides (Ar₃P=O) are being intensely studied as electron-accepting (n-type) materials. Despite the widespread application of these compounds as electron conductors, experimental data regarding the structural and electronic properties of their negatively charged states remain scarce owing to their propensity for follow-up chemistry. Herein, a carefully designed triarylphosphine oxide scaffold is disclosed that comprises sterically demanding spirofluorenyl moieties to shield the central phosphoryl (P=O) moiety. This compound undergoes chemical one-electron reduction to afford an exceptionally stable radical anion, which was isolated and characterized by X-ray crystallography for the very first time. The experimental data, corroborated by computational studies, shall allow for the construction of phosphine oxide materials with enhanced stability.

Full text of DOI:10.1002/anie.201605963

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201605963
German Edition: DOI: 10.1002/ange.201605963
Phosphorus Heterocycles
A Stable Crystalline Triarylphosphine Oxide Radical Anion
Tobias A. Schaub, Eva M. Zolnhofer, Dominik P. Halter, Tatyana E. Shubina, Frank Hampel,
Karsten Meyer, and Milan Kivala*
Abstract: Triarylphosphine oxides (Ar P=O) are being
chemical instability during operation has recently been
3
[
7,8]
intensely studied as electron-accepting (n-type) materials.
Despite the widespread application of these compounds as
electron conductors, experimental data regarding the structural
and electronic properties of their negatively charged states
remain scarce owing to their propensity for follow-up chemis-
try. Herein, a carefully designed triarylphosphine oxide scaf-
fold is disclosed that comprises sterically demanding spiro-
fluorenyl moieties to shield the central phosphoryl (P=O)
moiety. This compound undergoes chemical one-electron
reduction to afford an exceptionally stable radical anion,
which was isolated and characterized by X-ray crystallography
for the very first time. The experimental data, corroborated by
computational studies, shall allow for the construction of
phosphine oxide materials with enhanced stability.
identified as detrimental to the device lifetime.
The
observed device decay mostly originates from the consider-
2
ably lowered PꢀC(sp ) bond dissociation energies in the
negatively charged states, which are actively involved in the
[
7]
electron transport. In fact, initial research activities on the
[
9,10]
electrochemical
phine oxides with alkali metals date back to the 1950s
were most likely conducted because of chemical curiosity in
and chemical reduction of triarylphos-
[11–13]
and
[
14]
analogy to the ketyl radical anion. These early studies were
mainly based on electron paramagnetic resonance (EPR)
analysis of the one-electron reduced triarylphosphine oxides,
which indicated that the unpaired electron spin density is
[9,10]
distributed over the adjacent phenyl moieties.
However,
the formed radical anions were prone to subsequent chemical
reactions that led to numerous paramagnetic and non-para-
magnetic secondary products, which hampered their isolation
O
wing to the unique bonding situation at the pentavalent
[
11–13,15]
phosphorus center, the phosphoryl moiety acts as an electron-
and characterization in the solid state.
these early spectroscopic studies and theoretical calculations
later on,
Apart from
[1]
accepting unit. The formal double bond in the P=O
fragment is significantly polarized and most appropriately
described in terms of so-called negative hyperconjugation,
that is, the p back-donation from the lone pair at oxygen into
[16]
to the best of our knowledge, no structural
information regarding triarylphosphine oxide radical anions
that is based on X-ray crystallographic analysis has been
reported to date.
[2,3]
the antibonding s* orbital localized at phosphorus.
These
characteristics render the phosphoryl moiety a valuable
building block for the construction of versatile electron
acceptors, an aspect of vital importance in the context of the
In light of the continuously growing significance of
organic phosphine oxide materials, structural characterization
of the corresponding anionic radical species in the solid state
would be highly desirable as it could provide data essential for
the molecular design of phosphine oxide scaffolds with
enhanced stability. To this end, it is crucial to stabilize the
[
4]
vigorously developing field of organic electronics. Triaryl-
phosphine oxides have emerged as a promising class of
electron-conducting (n-type) materials particularly suited for
[5]
2
application in phosphorescent organic light-emitting diodes,
owing to their superior electron-transport ability, favorable
vulnerable PꢀC(sp ) bonds in the negatively charged species
by sufficient steric protection of the phosphorus-centered
radicals. Both requirements can be fulfilled by embedding the
phosphorus atom in a p-conjugated polycyclic system akin to
the previously reported dimethylmethylene-bridged phos-
[
6]
morphological stability, and high triplet energy. Although
triarylphosphine oxides appear to meet almost all of the key
requirements to serve as ideal n-type semiconductors, their
[
17]
[18]
[19]
phine A and its oxygen- and sulfur-bridged phosphine
oxide analogues B1 and B2 (Figure 1).
[*] T. A. Schaub, Dr. F. Hampel, Dr. M. Kivala
With this in mind, we synthesized triarylphosphine oxide
Chair of Organic Chemistry I
Department of Chemistry and Pharmacy
University of Erlangen-Nꢀrnberg
3
scaffold 1, in which the bridging C(sp ) spiro junctions are
decorated with rigid fluorenyl moieties to sterically protect
Henkestrasse 42, 91054 Erlangen (Germany)
E-mail: milan.kivala@fau.de
E. M. Zolnhofer, D. P. Halter, Prof. Dr. K. Meyer
Chair of General and Inorganic Chemistry
Department of Chemistry and Pharmacy
University of Erlangen-Nꢀrnberg
Egerlandstrasse 1, 91058 Erlangen (Germany)
Dr. T. E. Shubina
Computer Chemistry Center (CCC)
University of Erlangen-Nꢀrnberg
Nꢁgelsbachstrasse 25, 91052 Erlangen (Germany)
[17]
Figure 1. Bridged triarylphosphine A and triarylphosphine oxides
[
18]
[19]
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605963.
B1 and B2, which were reported previously. The spirofluorene-
bridged scaffold 1 synthesized in this work.
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the central phosphoryl moiety. We demonstrate that 1 under-
phosphorus center in 1 is reflected in the shortened average
goes chemical reduction to the corresponding radical anion
PꢀC bond of 1.762(2) ꢀ compared to that of Ph PO
3
ꢀ
[22]
1C
with unique stability, which enabled the isolation and
(1.803(1) ꢀ; Table 1). The phosphorus center in 1 is also
X-ray crystallographic characterization of the negatively
charged state of a triarylphosphine oxide for the very first
time. Experimental and computational studies revealed the
peculiar electronic situations in 1 and its anionic state 1C ,
which result from the steric constraint around the phosphoryl
Table 1: Selected bond lengths [ꢂ] and angles [8] of neutral 1 and the
ꢀ
+
radical anion 1C [Kꢁ18-crown-6] obtained from X-ray crystallographic
analysis (top) and theoretical calculations (bottom, in italics; see the
Supporting Information).
ꢀ
moiety.
[a]
[21]
PꢀCav
1.762(2)
PꢀO
ꢀC-P-C
Bowl depth
As shown in Scheme 1, complete lithiation of tris(2-
bromophenyl)phosphine (2) with t-BuLi in Et O/THF at
[20]
1
1
1.4853(13)
1.487
312.8(3)
310.2
1.50
1.60
2
1
.759(2)
Cꢀ
1.747(4)
.752
1.492(3)
1.501
308.7(6)
305.8
1.71
1.79
1
[
a] Average values are shown with standard deviations calculated
2
1/2
according to: {ꢀ(xꢀhxi) /(nꢀ1)}
.
i
characterized by increased pyramidalization, with the sum of
[
22]
the C-P-C angles being 312.8(3)8 (Ph PO: 318.6(3)8). The
Scheme 1. Synthesis of phosphine oxide 1. PPA=polyphosphoric acid,
THF=tetrahydrofuran.
3
bridging fluorenyl moieties are bent towards the phosphoryl
unit, indicating an intramolecular hydrogen-bond interaction,
2
C(sp )ꢀH···O, with an average C52/72/92···O1 distance of
ꢀ958C resulted in a yellow solution, which was stable for
3.21(6) ꢀ. The observed elongation of the PꢀO bond in
[
22]
a few hours at this temperature. The lithiated species reacted
readily with 9-fluorenone to afford 3 in 53% yield on gram
scale (for X-ray structural analysis and the properties of 3, see
the Supporting Information). The final intramolecular Frie-
del–Crafts cyclization was achieved upon treatment of 3 with
polyphosphoric acid at 2008C for 5 min to deliver the
trispirocyclic phosphine oxide 1 as a white microcrystalline
solid in 68% yield after purification by column chromatog-
raphy. The harsh reaction conditions were found to be crucial
for the formation of 1 as at lower temperatures, the reaction
times were significantly prolonged, and below 1508C, only
partially cyclized products were identified in the reaction
1 (1.4853(13) ꢀ) compared to Ph PO (1.479(2) ꢀ) indicates
3
a slight weakening of this bond. However, a direct assessment
of the PꢀO bond strength in 1 in comparison to that of Ph PO
3
by means of infrared spectroscopy was not possible owing to
the pronounced dependency of the n(PꢀO) stretching fre-
[23]
quency on the environment,
that is, on the extent of
intermolecular interactions in solution and in the solid state
(see the Supporting Information).
For a better understanding of the structural and electronic
characteristics of 1, density functional theory (DFT) calcu-
lations at the PBE0/def2-QZVP level of theory were carried
out. In the optimized structure of 1, all of the PꢀC and PꢀO
3
1
mixture. Interestingly, the P NMR (162.0 MHz, CD Cl )
bond lengths are in good agreement with those determined by
X-ray crystallography (Table 1). To elucidate the consider-
able shielding of the phosphorus center, population, natural
bond orbital (NBO), natural localized molecular orbital
(NLMO), atoms in molecules (AIM), and electron local-
ization function (ELF) studies were conducted, which all
confirmed the presence of negative hyperconjugation at the
2
2
resonance of 1 was dramatically upfield-shifted to ꢀ21.0 ppm
compared to that of non-bridged triphenylphosphine oxide
(
Ph PO; d = 27.5 ppm).
3
X-ray crystallographic analysis of single crystals obtained
by slow evaporation of an ethyl acetate solution of 1 at room
temperature (RT) revealed that the triarylphosphine oxide
core adopts a bowl-shaped structure with a bowl depth of
[
1–3]
PꢀO moiety (see the Supporting Information).
The AIM
1
.50 ꢀ, which is surrounded by the three perpendicular
analysis showed that the bond critical point (bcp) of the PꢀO
bond is shifted towards the phosphorus atom, with a P–bcp
distance of 0.88 ꢀ and an O–bcp distance of 0.61 ꢀ, and is
[21]
fluorenyl flanks (Figure 2).
The steric congestion at the
2
characterized by a positive Laplacian electron density (r 1Þ
as well as a negative electronic energy density, which is in
[24]
agreement with previous results (Figure S35a). Moreover,
Mayerꢁs bond order of 1 is reduced to 1.59–1.76 (Ph PO: 2.05–
3
2
.10) depending on the level of theory. Reduced density
gradient analysis clearly illustrates the weak intramolecular
2
C(sp )ꢀH···O interaction (Figure S35b). Together with the
anisotropy of the induced current density (ACID) plot
(
Figure S32), these results provide a clear explanation for
3
1
the unusual P NMR upfield shift, namely strong electronic
interactions with the adjacent aromatic rings locked in plane
by the C(sp ) tethers, effective p back-donation from the
Figure 2. X-ray crystal structure of 1. Thermal ellipsoids set at 50%
probability. Hydrogen atoms and solvent molecules omitted for clarity.
3
2
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filled p orbital at oxygen into the antibonding s* orbital at
phosphorus (negative hyperconjugation), and steric effects in
the phosphorus-containing six-membered rings.
The UV/Vis absorption spectrum of 1, recorded in CH Cl2
2
at RT, features three bands at 275, 298, and 310 nm whereas
the emission spectrum shows two maxima at 314 and 324 nm
with a quantum yield of 0.07 (Figure 3a). As corroborated by
Figure 4. a) Chemical reduction of 1 with potassium and 18-crown-6
or [2.2.2]cryptand. b) Evolution of the UV/Vis absorption bands upon
reduction of 1. c) Cyclic voltammogram of 1 (in THF with 0.1m n-
+
ꢀ
Bu NPF , RT, vs. Fc/Fc ). d) EPR spectrum of 1C in the presence of
[
4
6
[
27]
2.2.2]cryptand in THF at RT (black) and its simulation (blue).
d) Mulliken spin densities derived from DFT calculations (blue: “ꢀ”,
red: “+” spin density, isovalue ꢀ0.0004).
electroactive polymer film on the working electrode, as
reported in the literature for other fluorenyl-containing
compounds (see the Supporting Information).
Figure 3. a) Experimental UV/Vis absorption (red) and emission spec-
tra (blue, lexc =280 nm) in CH Cl at room temperature along with the
[
25]
2
2
oscillator strengths (red and blue bars) obtained by TD-DFT calcula-
tions (B3PW91/def2-TZVP//PBE0/def2-TZVPP). For comparison, the
computed spectra were red-shifted by 20 nm. b) Kohn–Sham molecular
orbitals (DFT, PBE0/def2-QZVP).
Treatment of 1 with an excess of potassium metal in THF
in the presence of 18-crown-6 or [2.2.2]cryptand at RT
resulted in a deep red solution of radical anion 1Cꢀ (Fig-
ure 4a). The color change was concomitant with the emer-
gence of a UV/Vis absorption maximum at 378 nm and
a broad band centered at 485 nm, reaching saturation after
14 hours (Figure 4b). The spectral features remained largely
unchanged even after prolonged treatment with an excess of
potassium for three days, indicating the sufficient stability of
time dependent (TD) DFT computational studies, the
observed UV/Vis absorption and emission features originate
from the electronic transitions from HOMO and HOMOꢀ2
to LUMO+1 LUMO+2, and LUMO+3, respectively, with-
,
[
26]
out involvement of the LUMO, which is localized exclusively
at the triarylphosphine oxide core (Figure 3b). Hence, all
transitions are of p–p* nature and occur at the electronically
the charged species under these conditions.
As TD-DFT calculations completely failed to reproduce
ꢀ
the UV/Vis spectrum of 1C , the ZINDO/S approach with
3
independent fluorenyl moieties connected through the C(sp )
150 roots was applied instead (Figure S33). Although the
peak positions are still off by 40–60 nm, it is suggested that the
absorption band around 485 nm corresponds to multiple
excitations from the SOMO to the LUMO and higher lying
LUMO+n orbitals, and that the band around 378 nm
originates from multiple HOMOꢀn to LUMO+n excitations.
The continuous wave X-band EPR spectrum of the deep
red THF solution, which was obtained by reduction of 1 with
elemental potassium in the presence of [2.2.2]cryptand,
recorded at RT, exhibits a sharp signal at g ꢂ 2 with a well-
spiro junctions (Tables S5 and S6).
Cyclic voltammetry (CV) studies provided further insight
into the electronic nature of 1 (Figure 4c). In THF, the
compound undergoes a reversible reduction event at ꢀ2.85 V
+
(
all potentials are reported vs. Fc /Fc) centered at the
phosphoryl moiety. Furthermore, in CH Cl , two independent
2
2
oxidation events are observed, which were ascribed to
) oxidation of the peripheral fluorenyl units at a half-wave
1
potential of + 1.44 V and 2) subsequent deposition of an
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resolved hyperfine structure (Figure 4d). The addition of
2.2.2]cryptand was found to significantly increase the reso-
[
lution of the EPR spectra, most likely by interrupting the
interactions between the radical anion and the potassium
cation through its incorporation within the cryptand (see the
Supporting Information). The observed fine structure results
from hyperfine coupling of the unpaired electron with the
3
1
internal P atom (I = 1/2, 100%) in addition to superhyper-
1
fine coupling to the three equivalent H atoms (I = 1/2,
9
9.9%) in the para positions of the adjacent phenyl rings. The
[
27]
ꢀ
+
spectrum was simulated with an effective g value centered
at giso = 2.0001, a linewidth of W = 0.10 mT, and hyperfine
Figure 5. X-ray crystal structure of 1C [Kꢁ18-crown-6] and illustration
of the C-P-C angles. Thermal ellipsoids set at 50% probability. C atoms
of THF shown in light blue. Disordered THF molecules and H atoms
omitted for clarity.
iso
3
1
1
coupling constants A( P) and A( Hpara) of 99.9 MHz
3.57 mT) and 8.96 MHz (0.32 mT), respectively. These find-
(
ings are in accordance with the EPR spectra reported
previously for persistent radical anions derived from various
acylphosphine oxides, in which, however, the negative charge
was additionally stabilized by delocalization into the electron-
2
central (C(sp )) PꢀO moiety, which is in agreement with the
3
EPR results. Comparable changes in the bonding situation
have been reported for the CꢀO moiety in benzophenone
[28]
withdrawing carbonyl groups.
In agreement with the literature, no coupling to meta hy-
upon one-electron reduction to the corresponding potassium
ꢀ
+
ketyl radical anion, (Ph CO)C K , where an elongation of the
2
[
9]
drogen atoms was observed. Conclusively, the recorded
CꢀO bond from 1.222 ꢀ to 1.311(5) ꢀ and concomitant
ꢀ
2
EPR spectrum of 1C in the presence of [2.2.2]cryptand clearly
shortening of the OCꢀC(sp ) bond from 1.496 ꢀ to 1.460-
[
30]
indicates the formation of a discrete triarylphosphine oxide
radical anion, with the unpaired electron spin delocalized
over the phosphoryl moiety and the three adjacent phenyl
(6) ꢀ were observed.
Based on the orbital picture, it can be envisaged that the
addition of one additional electron to 1 to afford the
corresponding radical anion should occur with an elongation
[
29]
rings. The experimental EPR data are supported by DFT-
calculated Mulliken spin densities, which confirm that only
hydrogen atoms in para position with respect to the phos-
phorus atom possess unpaired spin density (Figure 4e and
Table S1). The calculated g value of 2.0028 and the hyperfine
coupling constants (ꢀ0.25, ꢀ0.30, and ꢀ0.32 mT) for the three
para hydrogen atoms are in satisfactory agreement with the
experiment although the calculations suggest very weak
coupling of the unpaired phosphorus-centered electron with
the meta hydrogen atoms of the adjacent phenyl moieties (six
values ranging from 0.03 to 0.08 mT).
ꢀ
of the PꢀO bond. Indeed, our computational studies of 1C are
in good agreement with the experiment, that is, the calculated
PꢀO bond length is 1.501 ꢀ (exp. 1.4853(13) ꢀ), the average
PꢀC bond length is 1.752 ꢀ (exp. 1.747(4) ꢀ), and the C-P-C
angles add up to 305.88 (exp. 308.7(6)8; Table 1). The SOMO
ꢀ
of 1C strongly resembles the parent LUMO of 1, with the
unpaired electron localized on the central triarylphosphine
oxide fragment (Figure 3b). The Mulliken charge distribution
in 1C indicates that the phosphoryl oxygen atom becomes
more negatively charged (ꢀ0.66e) upon reduction, while, at
the same time, the the positive charge on the phosphorus
atom decreases (0.39e) as compared to neutral compound
1 with ꢀ0.62e (O) and 0.50e (P), respectively. In accordance
with this charge evolution, Mayerꢁs bond order decreases
ꢀ
ꢀ
+
Single crystals of 1C [Kꢁ18-crown-6] , obtained by slow
diffusion of n-pentane into a THF solution of the compound
at ꢀ358C under a nitrogen atmosphere, provided the very
first opportunity to analyze the structural changes arising
from the one-electron reduction of the triarylphosphine oxide
moiety (Figure 5; for further information, see the Supporting
Information). The overall geometry of the neutral parent
ꢀ
from 1.59 in 1 to 1.48 in 1C , which corresponds well with the
calculated elongation of the PꢀO bond by 0.014 ꢀ (Table 1,
for calculated characteristics of Ph PO and its radical anion,
3
compound 1 is largely retained in the spirocyclic scaffold of
see the Supporting Information). In comparison to 1, with
P–bcp = 0.88 ꢀ and O–bcp = of 0.61 ꢀ, the AIM analysis
ꢀ
1C (Table 1). However, significant deviations become appar-
ꢀ
ent for the environment of the phosphorus center. The
stronger pyramidalization of the phosphorus atom in the
anionic state is illustrated by the lower sum of the C-P-C
shows that the bcp of the PꢀO bond in 1C , with a P–bcp
distance of 0.82 ꢀ and an O–bcp distance of 0.68 ꢀ, is shifted
towards the phosphorus atom, which is also reflected in the
observed shortening of the PꢀC bonds upon reduction.
ꢀ
angles (308.7(6)8) for 1C than for 1 (312.8(3)8), which, in turn,
ꢀ
leads to an increased bowl depth of 1.71 ꢀ for 1C in
Taken together, we have obtained a new type of organo-
3
comparison to 1.50 ꢀ for the neutral compound. Despite
phosphorus framework by the introduction of C(sp )-hybrid-
ꢀ
the increased pyramidalization of 1C , the intramolecular
ized spirofluorenyl bridges into the triarylphosphine oxide
moiety. The incorporation of the electron-withdrawing phos-
phoryl moiety into a p-conjugated polycyclic framework—
most likely in conjunction with the sterically shielded
environment provided by the bridging fluorenyl groups—
effectively stabilized the corresponding one-electron-reduced
species. This phosphine oxide radical anion could thus be
isolated and characterized by X-ray crystallography for the
2
hydrogen bond length, C(sp )ꢀH···O, remains virtually
unchanged (average C52/72/92···O1 distance of 3.21(6) ꢀ for
both compounds). More importantly, upon one-electron
reduction of 1, a slight shortening of the average PꢀC
av
bond length from 1.762(2) ꢀ to 1.747(4) ꢀ and the concurrent
elongation of the PꢀO bond from 1.4853(13) ꢀ to 1.492(3) ꢀ
suggest delocalization of the unpaired electron over the
4
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Angewandte
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520.
1
tional studies, we provided fundamental insight into the
structural and electronic properties of negatively charged
triarylphosphine oxides, which are anticipated to be formed
during electron transport in organic electronics. We believe
that the described molecular design will enable the construc-
tion of phosphine oxide materials with enhanced operational
stability as well as the stabilization of other heteroatom-
centered radical species that are hitherto believed to be too
unstable for isolation.
[
[
[
2
5
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This work was supported by the Deutsche Forschungsge-
meinschaft (DFG) as part of SFB 953 “Synthetic Carbon
Allotropes”. The “Solar Technologies Go Hybrid” (SolTech)
initiative of the Free State of Bavaria, the Cluster of
Excellence “Engineering of Advanced Materials” (EAM) at
FAU Erlangen-Nꢂrnberg, the German Fonds der Chemi-
schen Industrie (FCI), and the Graduate School Molecular
Science (GSMS) of FAU Erlangen-Nꢂrnberg are gratefully
acknowledged for generous support. T.E.S. acknowledges
support by the Bavarian Equal Opportunities Sponsorship for
promoting equal opportunities for women in research and
teaching.
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Information, Figure S22.
[
[
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Keywords: cyclization · phosphorus heterocycles ·
2
polycyclic systems · radical anions
ꢀ
+
[
26] Whereas a THF solution of 1C [Kꢁ18-crown-6] kept under Ar
at ꢀ358C was stable for several weeks, at room temperature,
gradual decomposition towards EPR-silent products was
ꢀ
observed within several days. Decomposition of 1C [Kꢁ18-
+
crown-6] in the solid state was indicated by rapid decoloration
[
[
[
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[
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[
obtained by chemical reduction of Ph PO were performed under
3
the same conditions; for details, see the Supporting Information.
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1
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Received: June 20, 2016
Revised: August 18, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Phosphorus Heterocycles
The spin within: A carefully designed
triarylphosphine oxide scaffold with ster-
ically demanding spirofluorenyl moieties
undergoes chemical one-electron reduc-
tion at its phosphoryl moiety. The unique
stability of the formed radical anion
enables the isolation and X-ray crystallo-
graphic characterization of this hitherto
elusive species.
T. A. Schaub, E. M. Zolnhofer,
D. P. Halter, T. E. Shubina, F. Hampel,
K. Meyer, M. Kivala*
&&&& — &&&&
A Stable Crystalline Triarylphosphine
Oxide Radical Anion
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!