Charge-transfer complexes of PXX (PXX = 6, 12-dioxaanthanthrene). The formal charge and molecular geometry

Article abstract of DOI:10.1246/bcsj.74.53

Three kinds of charge-transfer (CT) complexes of PXX (PXX = 6, 12-dioxaanthanthrene) have been newly prepared and structurally characterized. In the 1:2 CT complex with TNP (TNP = 2,4,6-trinitrophenol), PXX is practically neutral. In the semiconducting partially oxidized salt with I₃, the PXX molecules form a trimer and each PXX is formally oxidized by e/3. In the 3:2 salt with [Ni(mnt)₂]^- (mnt = maleonitriledithiolate), the PXX molecules again form a trimer unit and each PXX is formally oxidized by 2e/3. Together with the structural data obtained from the PXX single component crystal, the molecular geometry change by the formal charge on PXX has been examined. A noticeable change has been found in the aromatic ring framework, which is consistent with the distribution of the HOMO coefficients obtained from the extended Hueckel calculation.

Full text of DOI:10.1246/bcsj.74.53

©
2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 53–58 (2001)
53
Charge-Transfer Complexes of PXX (PXX = 6, 12-Dioxaanthanthrene).
The Formal Charge and Molecular Geometry
Takehiro Asari, Norihito Kobayashi, Toshio Naito, and Tamotsu Inabe*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810
(Received August 11, 2000)
Three kinds of charge-transfer (CT) complexes of PXX (PXX = 6, 12-dioxaanthanthrene) have been newly pre-
pared and structurally characterized. In the 1:2 CT complex with TNP (TNP = 2,4,6-trinitrophenol), PXX is practically
neutral. In the semiconducting partially oxidized salt with I , the PXX molecules form a trimer and each PXX is formally
3
–
oxidized by e/3. In the 3:2 salt with [Ni(mnt) ] (mnt = maleonitriledithiolate), the PXX molecules again form a trimer
2
unit and each PXX is formally oxidized by 2e/3. Together with the structural data obtained from the PXX single compo-
nent crystal, the molecular geometry change by the formal charge on PXX has been examined. A noticeable change has
been found in the aromatic ring framework, which is consistent with the distribution of the HOMO coefficients obtained
from the extended Hückel calculation.
The degree of charge transfer in CT complexes of π-conju-
gated donors and/or acceptors is important for understanding
their chemical and physical properties. This information may
be obtained from measurements of the infrared spectra; the
spectrum becomes the superimposition of the component spec-
tra when the components are neutral, while it changes dramati-
cally when the components are ionized. When the component
molecules become π-radicals by removal of an electron from
the HOMO orbital or by addition of an electron to the LUMO
orbital, the bond strength should be changed according to the
population of the HOMO or LUMO coefficients on the bond,
producing shifts of the vibrational frequencies. However, this
method is applicable only when the molecule has a specific
band which is sensitive to the formal charge and is distinguish-
able from the other bands. On the other hand, the molecular
geometry is known to be sensitive to the formal charge on the
molecule, and for example, this relationship is well established
for TCNQ and is widely used for evaluation of the degree of
Chart 1.
are necessary. Therefore, we have intended to determine the
crystal structures in which PXX takes various formal charges.
We have carried out the crystal growth by combining with vari-
ous acceptors including very weak ones and that by the electro-
chemical oxidation using various counter ions. For the neutral
species, we have also tried an X-ray structure analysis of the
single component PXX crystal. Since PXX is a rather weak
donor (the redox potential for the first oxidation is +0.79 V vs.
Ag/Ag in acetonitrile), it is relatively difficult to obtain crys-
tals in which the formal oxidation state of PXX is more than
1/2. The crystals obtained so far are [PXX][TNP]
and [PXX] [Ni(mnt) . In this paper we describe their crystal
1
ionization.
Though the vibrational frequency and molecular geometry
are convenient probes for determining the formal charge on
molecules, the available data are rather limited. For the title
molecule, PXX shown in Chart 1 (this abbreviation comes
from its informal name; peri-xanthenoxanthene), data are few;
only one piece of structural information on the TCNQ complex
+
+
2 3 3
, [PXX] I ,
3
2 2
]
structures, along with that of the single component PXX crys-
tal, and some of their physical properties.
2
has been reported. Recently, we have found that PXX can
III
form a highly conducting 1:1 complex with [Co (Pc)(CN)
2
III
]
Experimental
(
dicyano(phthalocyaninato)cobalt(III)), in which the [Co -
Materials. PXX was prepared by a slight modification of the
reported method. 1,1′-Bi-2-naphthol was dissolved in an aqueous
3
(
Pc)(CN)
2
] units form a ladder chain. For such a ladder−chain
5
conductor, the electronic structure should strongly depend on
the position of the Fermi level. Information about this comes
from the degree of charge transfer. The Pc framework is rather
3 2
NaOH solution; then an aqueous Cu(CH COO) solution was add-
ed. After stirring for 1 hour the solid products were filtered out
and dried in a vacuum. PXX was isolated by fractional sublima-
tion and finally purified by vacuum sublimation. TNP was pur-
4
insensitive to the charge, and thus the structural data for PXX

5
4
Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Charge-Transfer Complexes of PXX
chased and recrystallized from ethanol. Bu N[Ni(mnt)
4
2
] was pre-
on request, free of charge, by quoting the publication citation and
the deposition numbers 151672–151675.
6
pared following the method reported. When the single crystals of
PXX were grown from the solution, it was not possible to com-
plete the X-ray structure analysis due to the anomalously uneven
Results and Discussion
7
unit cell. Crystal growth by vacuum sublimation was found to
Crystal Structure of PXX. The crystal structure of the
single component PXX crystal obtained from vacuum sublima-
tion is shown in Fig. 1. Molecular packing in this crystal may
be regarded as a slight modification of those in polycyclic aro-
matic hydrocarbons. The PXX molecules form a one-dimen-
sional column along the b axis with slipped stacking. The
chain arrangement along the c axis is a herringbone type, while
that along the a axis occurs by a parallel shift. Interplanar dis-
tance is 3.36 Å, and there are no specific contacts between the
one-dimensional columns.
give a polymorph which is suitable for the diffraction study. Sin-
gle crystals of [PXX][TNP] were grown by slow cooling and pro-
2
longed slow evaporation of a saturated CHCl solution. Single
3
crystals of [PXX] I were obtained by electrochemical oxidation
3
3
of PXX in acetonitrile with Bu
4
N•
3
I as an electrolyte. Single
crystals of [PXX] [Ni(mnt) ] were obtained by electrochemical
3
2 2
oxidation of PXX in a chloroform/acetonitrile mixed solvent with
Bu N[Ni(mnt) ] as an electrolyte. The crystals were obtained only
4
2
when the applied current is relatively high (ca. 10 µA).
Measurements. Electrical-conductivity measurements were
carried out using a four-probe method for low-resistance samples
and a two-probe method for high-resistance samples. The infrared
spectra were recorded on a Perkin−Elmer 1650 FT-IR spectrome-
ter. The visible-near-IR (vis-NIR) spectra were recorded on a
JASCO V-570 spectrometer. For the solid sample, the diffuse re-
flectance diluted with KBr was measured. The spectra were then
plotted using the Kubelka−Munk function.
Infrared and Vis-NIR Spectra of [PXX][TNP]2. The ob-
tained black crystal is expected to have a neutral ground state,
since the difference of the redox potentials between the first ox-
idation of PXX and the first reduction of TNP is large (∆Eredox
=
11
1.14 V) and far from the neutral-ionic boundary. This expec-
X-ray Structure Analyses. An automated Rigaku AFC-7R
diffractometer (PXX, [PXX][TNP] , and [PXX] I ) or a Rigaku R-
2
3 3
AXIS Rapid area detector ([PXX] [Ni(mnt) ] ) with graphite-
3
2 2
monochromated Mo Kα radiation (λ = 0.71073 Å) was used for
data collection. The data were corrected for Lorentz and polariza-
tion effects, and an absorption correction was applied for [PXX]3I3
and [PXX] [Ni(mnt) ] . In all the measurements, no intensity de-
3
2 2
cay was observed. The data-collection conditions and crystal data
are summarized in Table 1. The crystal structures were solved by a
8
direct method (SIR 92 ), and the positions of all the hydrogen at-
oms were placed at the calculated ideal positions. A full-matrix
9
least-squares technique (teXsan ) with anisotropic thermal param-
eters for non-hydrogen atoms and isotropic ones for hydrogen
atoms (1.2 times of the attached carbon) was employed for a struc-
1
0
ture refinement.
Crystallographic data have been deposited at the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK and copies can be obtained
Fig. 1. Crystal structure of PXX.
Table 1. Data-Collection Conditions and Crystal Data
PXX
C H O
282.30
20 9. 75 0×0.30×0.20
monoclinic
[PXX][TNP]2
[PXX]3I3
[PXX] [Ni(mnt) ]
3 2 2
Chemical formula
Formula weight
Crystal size/mm3
Temperature/K
Crystal system
C H N O
C H O I
C H N O S Ni
6
76 30 8 8 2
1525.00
20 9. 75 5×0.50×0.45
triclinic
2
0
10
2
32 13
6
16
60 30 6 3
737.49
20 9. 75 0×0.75×0.10
1227.61
20 9. 15 0×0.10×0.60
monoclinic
triclinic
Space group
Cc
P2 /n
1
P1
P1
a/A˚
b/A˚
c/A˚
15.928(3)
4.983(3)
16.170(3)
7.257(5)
26.267(8)
8.010(4)
10.595(3)
12.833(2)
8.586(2)
14.422(1)
15.066(1)
7.448(1)
α/deg92.56(2)
β/deg91.33(2)
γ /deg83.39(2)
V/A˚
Z
95.10(1)
106.08(4)
73.24(1)
1283.0(8)
4
112.96(2)
95.42(1)
3
1467(1)
2
1.38
3711
1067.8(5)
1
22.57
5170
1539.4(2)
1
9.51
−1
µ(Mo Kα)/cm
No. of reflections measured 1478
0.94
6951
No. of reflections observed
1059 [I > 2.5σ(I))] 2352 [I > 3.0σ(I)] 3454 [I > 3.0σ(I)] 5574 [I > 3.0σ(I)]
R
RW
0.042
0.041
0.047
0.049
0.032
0.028
0.036
0.036

T. Asari et al.
Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
55
tation has been confirmed from the infrared spectra. Figures
2(a), 2(b), and 2(d) show the IR spectra of PXX, TNP, and
[
PXX][TNP] , respectively. The last spectrum is well repro-
2
duced by superimposition of the first two spectra. From this re-
sult, the formal charge of PXX is reasonably assigned to be 0.
The vis-NIR spectra shown in Fig. 3 also clearly support this
assignment. The bands in the visible region of the spectrum of
[
PXX][TNP]
The position of the charge-transfer band, around 15000 cm
1.86 eV), is also in good agreement with the simple model
2
well correspond to the absorption bands of PXX.
–
1
(
1
1
proposed by Torrance, and supports the suggestion that this
complex is located deep inside the neutral ground state region.
The crystal is practically an insulator (ρ > 10 Ω cm at room
8
temperature).
Crystal Structure of [PXX][TNP]
2
.
As shown in Fig. 4,
the crystal comprises [TNP]−[PXX]−[TNP] trimer units.
These units form a pseudo body-centered lattice. There are
several short contacts within the face-to-face overlapped trimer
(interplanar distance, 3.31 Å), and the intermolecular interac-
tion is in the range of typical charge-transfer complexes with a
neutral ground state. The hydroxy group forms an intra-
molecular hydrogen bond with one of the nitro groups in the
Fig. 3. Vis-NIR spectra of PXX, TNP, and Bu
Ni(mnt) ] in each chloroform solution (a), diffuse re-
flectance spectra plotted by the Kubelka–Munk func-
tion, f (R), of [PXX][TNP] (b), [PXX] (c), and
PXX] [Ni(mnt) (d).
4
N-
[
2
2
3 3
I
[
3
2 2
]
same molecule, and does not participate in the intermolecular
hydrogen bonding.
Infrared and Vis-NIR Spectra and Electrical Conductiv-
ity of [PXX]
I
3 3
.
Since this compound is a partially oxidized
salt, the fractional charge can be estimated from the
stoichiometry. The iodine atoms are observed as a discrete tri-
halide anion (vide infra), so the formal charge is estimated to
be +1/3. Some information on whether the charge is distribut-
ed uniformly or disproportionally may be obtained from the
optical and electrical measurements. The infrared spectrum of
3 3
[PXX] I is shown in Fig. 2(e). There are vibrational bands
most of which are not coincide with the bands observed for
neutral PXX in Fig. 2(a), in addition to the broad electronic ab-
sorption band. This feature is characteristic of partially oxi-
dized salts in which the charge delocalization occurs. Indeed,
2
the crystal is conducting; ρRT = 10 Ω cm with the activation
energy of 0.19 eV, which is in good agreement with the gap en-
ergy estimated from the electronic transition in the IR region.
The vis-NIR spectrum is considerably different from that of
[
2
PXX][TNP] , as shown in Fig. 3(c). There are extra bands
Fig. 2. Infrared spectra of PXX (a), TNP (b),
Bu N[Ni(mnt) ] (c), [PXX][TNP] (d), [PXX] I (e),
–
1
around 11000 cm , which may be assigned to the intramolecu-
lar electronic transition in the PXX cation radical.
4
2
2
3 3
and [PXX] [Ni(mnt) ] (f).
3
2 2

5
6
Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Charge-Transfer Complexes of PXX
posed of one-dimensional columns of PXX with ··B−
A−B−B−A−B·· sequence and discrete I anions. The B−A
3
overlap mode is equal to theA−B mode, while the B−B overlap
mode is different from them. The interplanar spacing between
A and B (3.385 Å) is shorter than that between B and B (3.422
Å). These differences result in a large difference in the overlap
–
3
–3
integral (s); s = 13.0 × 10 for the A−B overlap and 2.13 × 10
for the B−B overlap. The trimeric deformation divides the one-
dimensional band for uniformly stacked structure into three
subbands. When the trimer accommodates one electron (or
one hole in this case), one subband becomes half-filled. If the
on-site Coulomb repulsion energy (U) is sufficiently small, the
material becomes metallic. However, if U is rather large, the
half-filled band further splits into one filled band and one emp-
ty band with an appreciable energy gap between them. This
situation is assumed to occur in [PXX]
conducting property.
3
I
3
because of its semi-
Infrared and Vis-NIR Spectra of [PXX]
3
2 2
[Ni(mnt) ] .
The IR spectrum is shown in Fig. 2(f). The spectrum is compli-
cated due to the contribution from both two chemical species.
–
1
The important feature is the CN stretching band at 2207 cm ,
which has been reported to be a marker band of the formal
1
2
charge on the [Ni(mnt)
same position as that in the spectrum of Bu
Fig. 2(c)), the formal charge on [Ni(mnt) ] can be assigned as
1. The formal charge on PXX is thus +2/3.
The vis-NIR spectrum is rather similar to that of [PXX]
2
] unit. As this band is observed at the
Fig. 4. Crystal structure of [PXX][TNP]2.
4
N[Ni(mnt)
2
]
(
–
2
Crystal Structure of [PXX]
two crystallographically independent PXX’s; one molecule
molecule A) lies on the inversion center and the other (mole-
3
I
3
.
In this crystal, there are
3 3
I
(
–
2
except for the additional hump due to [Ni(mnt) ] , as shown in
–
1
cule B) is located at the general position. The molecular geom-
etry is not so much different for these two molecules (vide in-
fra). This is consistent with the optical and electrical proper-
ties, and the charge is assumed to be delocalized to some ex-
tent.
Fig. 3(d). There are again bands around 11000 cm , which can
be assigned to the PXX cation radical.
4
5
The crystal is a poor conductor (10 –10 Ω cm at room tem-
perature), which is consistent with the crystal structure (vide
infra).
The crystal structure is shown in Fig. 5. The crystal is com-
Crystal Structure of [PXX] [Ni(mnt) ]
3 2 2
.
The crystal
structure is shown in Fig. 6. In this crystal, again there are two
crystallographically independent PXX’s; one (molecule C) lies
on the inversion center and the other (molecule D) at the gener-
al position. The resultant D−C−D trimers are rather isolated in
the lattice; the [Ni(mnt) ] dimers are inserted between the PXX
2
Fig. 5. Crystal structure of [PXX] I . A and B are crys-
Fig. 6. Crystal structure of [PXX] [Ni(mnt) ] . C and D
3 2 2
3
3
tallographically independent molecules.
are crystallographically independent molecules.

T. Asari et al.
Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
for the oxidation state +0.67.
57
2
trimers. Since the [Ni(mnt) ] unit holds a charge –1, three
PXX molecules share the charge +2. As discussed in the next
section, there appear to be some geometrical differences be-
tween C and D, meaning that the charge disproportionation oc-
curs within the trimer to some extent.
Comparison of the Molecular Geometry of PXX. The
bond lengths of PXX in the complexes studied and in
The schematic diagram of the HOMO coefficients of PXX
obtained from the extended Hückel calculation (based on the
structural data of [PXX] I ) is shown in Scheme 1. From the
3 3
distribution of the HOMO coefficients, it is expected that the
bonds m and n must be sensitive to the fractional charge on
PXX. As shown inTable 2, with increasing the formal charge,
the bond m is lengthened and the bond n is shortened, which is
consistent with each bond character as expected from the
HOMO coefficients (bonding for m and anti-bonding for n).
[PXX][TCNQ] are summarized in Table 2. The formal charge
of PXX in [PXX][TCNQ] was estimated to be about 0.1 from
2
the molecular geometry of TCNQ. Though the accuracy in the
0
PXX crystal is relatively poor due to the small reflection num-
ber, the bond lengths are in good agreement with those in
These data are plotted in Fig. 7 (for PXX , data of
2
[PXX][TNP] are adopted). Linear correlation with similar
[
PXX][TNP]
more reliable as the data for neutral PXX. For [PXX]
PXX] [Ni(mnt) , the values are averaged for two crystallo-
2
. The bond lengths in [PXX][TNP]
2
are thus
slopes but opposite signs can be seen. Though the bond b also
shows some correlation that is consistent with the HOMO co-
efficients, the sensitivity is less. The other bond lengths are
nearly constant or irregularly changed by the formal charge.
From the fit in Fig. 7, we can deduce the relation between the
3 3
I
and
[
3
2 3
]
graphically independent molecules. In the former salt, the dif-
ferences are relatively small (see δmax inTable 2). On the other
hand, the differences between C and D in the latter salt are
found to be much larger (most of δmax values exceed σmax). This
implies that the charge is not uniformly distributed. The charge
m n
formal charge ρ and the bond lengths at m and n, d and d in
+
ρ
0
+1
distribution is expressed as PXX ꢀ (1 – ρ)PXX + ρPXX ,
where ρ is the formal charge (molar fraction of PXX ). The
ꢁ
+
1
apparent uniform charge distribution occurs when the reso-
0
+1
nance between PXX and PXX is so fast that only its aver-
aged feature is observable. In [PXX] [Ni(mnt) , the right-
3
2 3
]
hand-side state has considerably lower potential energy, so that
the uneven charge distribution becomes observable. Since the
averaged bond lengths in Table 2 correspond to the bond
+
0.67
lengths of the hypothetical PXX , we can take these values
Scheme 1.
a)
3
Table 2. Averaged Bond Length (d, A˚ ), Maximum Standard Deviation (σmax,×10 A˚ ), and Maximum Deviation from the
3
Average Value (δmax,×10 A˚ ) of PXX
b)
c)
d (σmax , δmax
)
+0.1
+0.33
+0.67
Bond
PXX0
PXX0
([PXX][TNP]2)
PXX
PXX
PXX
d)
(PXX)
([PXX][TCNQ])
([PXX] I )
([PXX] [Ni(mnt) ] )
3 3
3
2 3
a
b
c
d
e
1.391(9, 12)
1.390(9, 9)
1.40(10, 20)
1.37(10, 30)
1.42(10, 20)
1.42(10, 60)
1.37(10,10)
1.41(10, 20)
1.37(10, 30)
1.412(9, 17)
1.41(10, 50)
1.407(9, 27)
1.366(9, 29)
1.425(3, -)
1.391(3, -)
1.386(3, -)
1.397(3, -)
1.367(3, -)
1.426(3, -)
1.410(3, -)
1.373(3, -)
1.402(4, -)
1.362(3, -)
1.418(3, -)
1.414(3, -)
1.406(3, -)
1.371(3, -)
1.427(4, -)
1.394(2, -)
1.382(2, -)
1.399(3, -)
1.365(3, -)
1.418(3, -)
1.414(3, -)
1.357(3, -)
1.401(3, -)
1.362(3, -)
1.412(3, -)
1.416(3, -)
1.410(2, -)
1.375(2, -)
1.426(3, -)
1.392(4, 4)
1.381(4, 4)
1.396(5, 6)
1.369(5, 7)
1.427(5, 1)
1.411(5, 6)
1.373(6, 4)
1.402(6, 7)
1.365(5, 6)
1.415(5, 4)
1.410(5, 4)
1.407(5, 1)
1.380(4, 3)
1.424(6, 7)
1.390(2, 8)
1.377(2, 17)
1.398(3, 3)
1.377(3, 7)
1.423(3, 4)
1.408(3, 8)
1.371(3, 5)
1.399(3, 9)
1.374(3, 7)
1.408(3, 5)
1.413(3, 2)
1.412(2, 4)
1.385(2, 14)
1.413(3, 16)
f
g
h
i
j
k
l
m
n
a) Under centrosymmetric conditions. b) The value of the maximum σ among the bonds averaged. c) The value of the maximum
difference from the average value among the bonds averaged. d) Ref. 2.

5
8
Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Charge-Transfer Complexes of PXX
PXX have been prepared and structurally characterized. Some
correlation between the bond length and the formal charge on
PXX has been observed, and the trend of the bond length
change has been found to be consistent with that expected from
the HOMO coefficient distribution.
This work was partly supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Science, Sports
and Culture.
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1
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Fig. 7. Bond length (m and n in Table 2) vs. formal
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6
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7
cell dimensions are a = 42.650(7), b = 7.583(7), c = 3.814(10) Å,
and β = 92.53(10)°.
n m
ρ = 1.370 – 24.46 × (d – d ), 0 ≤ ρ ≤ 1.
8
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limited, since the band has to be well separated from the other
bands and can be assigned undoubtedly. In the case of PXX,
the bands which are expected to be sensitive to the formal
charge (vibrations accompanied by the motion of atoms with
the large HOMO coefficients) all lie in the finger print region.
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9
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1
0
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Chem. Soc. Jpn.
1
1
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2
In conclusion, the single crystals of three new complexes of