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Molecular Structure and Spectroscopic Properties of Kekulene.

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the dimensions of the NO:- group are remarkably regular:
Td symmetry is present within the limits of error (cf. Table
1). The coefficients of the anisotropic temperature factors
can be reasonably interpreted assuming librational motion of
rigid NO:- ions[41;correspondingly corrected distances are
also listed in Table 1.
The N-0 distances are unexpectedly small; they lead to
the shortest known contacts (226 pm) between oxygen particles not directly linked together. Considering the coordination number of N, a length of ca. 150 pm would be estimated
for the N -0single bond. A value calculated approximately
for the equilibrium distance by the EHMO method is of the
same order of magnitude (151 pm)I5].The only possible reason for the significant shortening to 139 pm lies in polar interactions superimposed on the single bond. This result emphatically demonstrates that pd-v bonding contributions[61
need not be invoked in explaining the bond lengths in oxoanions such as PO:-, SO:-, or CIOT!
lar structure of kekulene by X-ray analysis was of special interest.
(1) crystallized from pyrene (purified by zone-melting) in
a tube sealed under high-vacuum on slow cooling from 450
to 350 "C. Removal of pyrene by high-vacuum sublimation
and washing the crystals with chloroform yielded yellow
monoclinic needles suitable for a structure analysis [crystal
size 0.05 x 0.08 x 0.4 mm; space group C2/c; a = 2799(3),
b=458.7(5), c=2271(2) pm, /3=109.6(1)", Z = 4 , pCalc.=1.45
g . cm- 3; MoKu radiation, 1596 observed reflections,
R=0.056].
Received: May 10, 1979 [Z 292 IEJ
German version: Angew. Chem. 91, 762 (1979)
111 M. Jansen, Angew. Chem. 89. 567 (1977); Angew. Chem. Int. Ed. Engl. 16,
534 (1977); Habilitationsschrift, Universitat Giessen 1978.
121 M. Junsen. Angew. Chem. 88, 410 (1976): Angew. Chem. Int. Ed. Engl. 15,
376 (1976): Z. Anorg. Allg. Chem. 435, 13 (1977).
I31 o=863.2(2), b=973.1(2), c=904.2(2) pm, space group Pbca, Z=8; 1073 observed intensities (diffractometer data), R = 0.05.
141 Y. Schomaker, K. N . Trueblood, Acta Crystallogr. B 24, 63 (1968).
151 D. K. Johnson. J. R. Wusson, Inorg. Nucl. Chem. Lett. 10, 891 (1974).
[61 Cf. D. W J. Cruickshunk, J. Chem. SOC.1961. 5486 L. Puuling: The Nature
of the Chemical Bond, Cornell University Press, Ithaca: Die Natur der chemischen Bindung. Verlag Chemie, Weinheim 1968.
Molecular Structure and Spectroscopic Properties of
Kekulene"'
By Clam Krieger, Francois Diederich, Dieter Schweitzer, and
Heinz A. Staab['l
Kekulene ( I ) , the synthesis of which was recently reported[*],is the first example of a new class of aromatic compounds in which the annellation of six-membered rings leads
to a cyclic system enclosing a cavity lined with hydrogen
atoms. Compound (l),for which 200 Kekule structures with
different arrangements of double and single bonds can be
formulated[31,was of interest with regard to n-electron delocalization and the related problem of the diatropicity in the
macrocyclic system for which, as long ago as 1951, different
theoretical approaches were shown to lead to contradictory
predictions in the specific case of
Experimentally, the
'H-NMR absorption of the internal hydrogens did not show
any evidence for a diatropicity in the macrocyclic system[*],
in qualitative agreement with MO calculations of chemical
s h i f t ~ [ ~In
. ~ this
l . context, and in connection with some spectroscopic properties of (l),the determination of the molecu-
Ill'
c li'
Fig. 1 . Lattice structure of ( I ) as projected along the a axis (the rear half of the
unit cell is not drawn).
In the crystal lattice of (1) the molecules are stacked along
the b axis with the stacking axis forming an angle of 42.9"
with the molecular planes. Neighboring molecules in such a
stack have an interplanar distance of 336 pm and are parallel-shifted by 312 pm. The molecular planes of adjacent
stacks are inclined by 86" to each other, resulting in the "herringbone pattern" shown in Figure 1 as a side view along the
a axis.
With a mean deviation of carbon atoms from the mean
plane through the 48 carbon atoms of only 3 pm and a maximum deviation of 7 pm, (1) has an almost perfectly planar
structure. This includes, with deviations of 5 and 10 pm, respectively, even the six internal hydrogens, although the
non-bonding distances of 196(2) pm between adjacent hydrogen atoms are unusually short.
[*] Prof. Dr. H. A. Staab, C. Krieger, Dr. F. Diederich
Abteilung Organische Chemie
Dr. D. Schweitzer
Abteilung Molekulare Physik
Max-Planck-lnstitut fur medizinische Fonchung
Jahnstrasse 29. D-6900 Heidelberg 1 (Germany)
Angeu. Chem. I n ( . Ed. Engl. I8 (1979) No. 9
Fig. 2. Bond lengths of ( I ) in pm (standard deviatlons in units of the last figure
given in brackets) and valence angles (as mean values of equivalent angles).
Q Verlug Chemie. GmbH, 6940 Weinheim, I979
0570-0833/79/0909-0699
IE 02.50/0
699
Figure 2 shows clearly that there is a remarkable difference between the bond lengths in the two groups of six-membered rings A, C, E, G, I, K and B, D, F, H, J, L. Only in the
last-mentioned group, with mean values of 138.8 pm for the
bonds of the internal (1)-perimeter and of 139.8 pm for the
bonds of the external perimeter, are “normal” arene bond
lengths observed; even here, all six radial bonds, having a
mean value of 142.3 pm, are significantly stretched. Drastic
deviations from the bond lengths of these “aromatic rings”
(B, D, . . .), however, are found for the second group of six
equivalent six-membered rings (A, C, ...): The six peripheral
CH-CH bonds of these rings have a mean value of 134.6
pm, i.e. almost the length of a normal C-C double bond;
correspondingly, a mean length of 144.5 pm for the bonds
between these “double bonds” and the “aromatic rings”
(B, D, . ..) indicates a relatively high single bond character.
An even stronger extension is found for the bonds (about 146
pm) which link the “aromatic rings” in the inner perimeter;
they accordingly approach single bonds of the type found in
polyphenyl systems.
Considering the number of 200 possible Kekule structures
of (l),the considerable bond localization evident from the
bond lengths might seem surprising. The observed variation
in bond lengths was, however, predicted in its qualitative
trend-though not in the extent actually found-by MOSCF calculations[61. Excellent agreement with the experimental bond lengths was found on evaluation of the Kekule
structures with regard to Pauling bond orders of the individual bonds in (1)[’1. On the basis of the structure analysis, a
formulation employing Clar’s sextet notation (la) is undoubtedly the best representation of the actual bonding situation in kekulene.
3). The symmetry-forbidden a-band of (I) is obviously hidden under the long-wavelength part of the p-band; however,
the strongly vibrationally structured fluorescence band at
453 nm evidently has to be assigned to this a-transition (Fig.
4a).
Just as hexabenzocoronene shows a strong phosphorescence at 575 nm (in 1,2,4-trichlorobenzene) at low temperatures[*], compound (1) in a [D2]-1,2,4,5-tetrachlorobenzene
matrix at 1.3 K exhibits a strong phosphorescence emission
in the range 585-595 nm with several narrowly spaced
“site”-dependent maxima and a vibrational structure resulting from combination with molecular vibrations in the range
1280-1630 c m-’ (Fig. 4b).
I
,
590 600 6 1 0 620 630
e
LO
650 660 nm
Fig. 4. a) Fluorescence spectrum of (I) (in 1,2,4-trichlorobenzene, 298 K. excitation 326 nm); b) phosphorescence spectrum of ( I ) (in [D,]-l,2,4.5-tetrachlorobenzene. 1.3 K,excitation 365 nm); intensities in arbitrary units.
The strong phosphorescence of (1) permits application of
the ODMR method for the determination of zero field splitting parameters IEI and ID1 of the excited triplet state[’I. For
(1) in a [D2]-1,2,4,5-tetrachlorobenzene
matrix, an IEl value
of 0.001935 cm ’ was obtained which, as expected from the
Dbh symmetry of (I), is very low (benzene: IEI=0.0064
cm- I). However, the JDI value-which measures the dipolar
coupling of the triplet electrons and, therefore, is inversely
proportional to the third power of the average distance between these electrons-found
for kekulene [ID/=0.10622
cm-’1 is only slightly lower than for benzene and even
greater than I
D1 for naphthalene and anthracene (cf. benzene
0.1581, naphthalene 0.0994, anthracene 0.0694 cm I ) . This
D1 in (I)[’’’ is very striking
unexpectedly small reduction of I
at first sight, considering the wide spatial extension of the Telectron system in kekulene. We seek to explain this result
qualitatively as an effect of the partial compartmentization
of the .rr-electron system in (1) as suggested by the X-ray
structure analysis‘’‘1.
-
Ilal
Some spectroscopic properties of kekulene also agree well
with the sextet formulation (la): The electronic spectrum of
(I) shows an absorption at remarkably short wavelength as is
generally observed, according to CZur[xll,for arenes of the
“condensed polyaryl type”, such as hexabenzocoronene (Fig.
-
Received: July 31, 1979 [Z 293 IE]
German version: Angew. Chem. 91, 733 (1979)
,
I
300
LOO
-
h[nml
Fig. 3. Absorption spectra of (I) and hexahenzocoronene (in I ,2,4-trichlorobenzene).
700
0 Verlag
Chemie, G n b H , 6940 Weinhrim, 1979
[ l ] Conjugation in Macrocyclic Systems, Part 30.-Part 29: H. A. Sraab, U. E.
Meissner, A . Gender, Chem. Ber., in press.
[2] F. Diederich, H . A. Slaab, Angew. Chem. 90,383 (1978); Angew. Chem. Int.
Ed. Engl. 17, 372 (1978).-W. Jenny. P. Baumgartner, R. Paoni (ISNAProc., Sendai/Japan, August 1970) claimed, without giving any details, to
have prepared kekulene (I)-which they named “[I 2lcoronaphene”-as
long ago as 1970. Since other authors (cf., for example. 131) refer to this
claim we are compelled to make the following statement: The authors
named have until now published neither a synthesis of ( I ) nor any account
of its properties in the chemical literature. Although they have not repeated
their claim to the synthesis of (I) in later topically related publications, they
have failed so far to withdraw this claim.
[3] J. Aihara, Bull. Chem. Soc. Jpn. 49, 1429 (1976).
[4] R. M c Weeny, Proc Phys. Soc. London A 64, 261,921 (19511, and references
given therein.
[5] G. Ege, H. Vogler. Theor. Chim. Acta 26. 5 5 (1972); H. Vogler, Tetrahedron
Lett. 1979. 229; C. Wilcox, personal communication.
[6] G. Ege, H. Fischer. Tetrahedron 23. 149 (1967).
[7] We thank Prof. W. C. Herndon, University of Texas at El Paso, for stimulating discussions.
0570-0833/79/0909-0700
$ 02.50/0
Angew. Chem. Inl. Ed. EngI. I8 (1979) No. 9
181 E. Clar: Polycyclic Hydrocarbons, Val. 2. Academic Press, New York/
Springer-Verlag, Berlin 1964. p. 97.
191 Concerning the ODMR method, see references in D. Schweifrer, K . H .
Hausser, V. Taglieber, H . A . Staab, Chem. Phys. 14, 183 (1976).
[lo] Regarding these results with ( I ) it is interesting that similar ODMR results
have been obtained recently with hexahenzocoronene and related aromatic
hydrocarbons of the “polyaryl type” with high molecular symmetry: J. Yoitlander, personal communication.
[ l I ] Attempted theoretical treatment of this hypothesis: H. Vogler, Symposium
on Aromaticity, Duhrovnik. September 1979.
tion of Ru(bpy):+ by TI3+have been described in the literaturel5’. An advantage of the T13+ system is the high quantum
yield of Ru(bpy):+ oxidation (4=2); however, the pH of the
solution has to be kept below 1.7 to avoid precipitation of hydroxide complexes. Higher O2 yields are obtained at pH between 4 and 5. It is noteworthy that even after long irradiation periods there is practically no depletion in sensitizer, indicating that the reverse reaction of Ru(bpy):
to
Ru(bpy): (Scheme 2) occurs faster than thermal decompositionI6l. (The turnover number of the sensitizer must be at
least 100.) Table 1 also shows that colloidal RuOz is much
more effective in mediating O2 evolution than the powdered
compound, since almost the same yields are obtained at 50100 times smaller concentrations. The colloidal dispersion
also has the advantage of being completely transparent.
The experiments for the simultaneous light-induced production of H2 and O2were performed with dimethylviologen
(N,N-dimethylbipyridine dication, MV’ +) as an acceptor.
MV2+ is reduced by photo-excited Ru(bpy):+ in a diffusion-controlled process["^'^. The reaction products, i. e.
Ru(bpy):+ and M V + , have the thermodynamic ability to
produce oxygen and hydrogen from water. However, to mediate these processes, very active redox catalysts have to be
employed since the back reaction can occur with a rate constant of k = 2.4 x lo9 1 mol- ‘ s - ‘[‘‘I. We recently found that a
finely dispersed, centrifuged Pt sol
mol/l) reoxidizes
MV’ within microsecondsl’l. This is therefore used to catalyze H2 formation: R u 0 2 , on the other hand, can mediate efficiently the O2 evolution step. When illuminating a solution
containing
mol/l of Ru(bpy):+ and 2 x
mol/l of
MV2+ at a p H of 4.7 in the presence of R u 0 2 powder (50
mg/150 ml) or colloidal R u 0 2 (1 mg/150 ml) and colloidal
Pt (4.5 mg/150 ml), simultaneous evolution of H2 and O2is indeed observed. Typically, we obtained 0.3 ml of O2 and 0.6
ml of H2 after 3 hours’ illumination of 150 ml of solution
with a 250-W projector lamp. Blank experiments showed
that the presence of both redox catalysts is mandatory for
production of both gases. The depletion of Ru(bpy):’ during 3 hours’ irradiation is less than 5%, illustrating the cyclic
nature of this cleavage of water. The two catalysts seem to be
sufficiently specific to avoid short-circuiting of the back reaction. Also, it appears that charge transfer from MV+ to Pt
can compete efficiently with O2 reduction according to
+
+
Cyclic Cleavage of Water into H2 and O2 by Visible
Light with Coupled Redox Catalysts[**’
By Kuppuswamy Kalyanasundararn and Michael Gratzel1‘I
Hydrogen evolution from water can be catalyzed by suitable noble metal (oxide) dispersions[’]. Recently, we succeeded in developing redox catalysts which are capable of
mediating oxygen production from water‘’]. R u 0 2 proved to
be a judicious choice of material since it has an extremely
low overpotential for O2 evolution131.This paper reports on a
system of two coupled redox catalysts permitting cyclic
cleavage of water by light.
The water contained Ru(bpy):+ as a sensitizer, which after light excitation reduces an electron acceptor (A). Initially
we added only R u 0 2 to the solution and chose the acceptor
such that it undergoes rapid subsequent reaction once it has
been reduced. Such a system is suitable for separate investigation of the light-induced evolution of oxygen from water
(Scheme 1).
Rulbpyli’
+
A
hv
Rulbpyl;’
A-
+
1
\1,
irreversible p r o d u c t s
Scheme I
Table 1 . Light induced evolution of O2 from water with Ru(hpy):+ as sensitizer and R u 0 2 ascatalyst. t=irradiation time. Variant A: 50 ml ofsolution with 20
mg of R u 0 2 powder or 0.3 mg of RuOl colloid. Variant B: 150 ml of solution
with 50 mg of RuO, powder or 1.0 mg of RuO: colloid.
MV’
Acceptor
Formula
c [mol/ll
pH
TICI,
8x
1.6
[CO(NH,)~CI]’+
10
4.8 [a]
[CO(NH,)~B~]”
CO(C204)1
RuO,
form
10.’
10-2
4.8
4.8
Powder
Colloid
Powder
Colloid
Powder
Colloid
Powder
Colloid
Variant
f [h]
A
0.5
0.33
1.5
3
9
9
1.5
1.5
A
A
B
B
B
A
A
+ O2 + 0;+ MV”
O2
Wl
0.3
0.4
From these findings we deduce the following reaction sequence shown in Scheme 2.
1.2
12
3
3.2
0.3
0.4
[a] The pH value of the solutions with cobalt complexes increases during irradiation, leading to formation of a brown precipitate. Precipitation can he avoided by
buffering with acetate; cf. also 191.
Results are shown in Table 1. Oxygen is obtained with all
the acceptors employed. The pathway of decomposition of
the cobalt(m) complexes141and the irreversible photooxida[‘I
Prof. Dr. M. Gratzel, Dr. K. Kalyanasundaram
Institut de chimie physique, Ecole Polytechnique Federale
CH-1015 Lausanne (Switzerland)
[**I
Acknowledgment is made to the Swiss National Foundation for supporting
this work under grant No. 4.061.076.04, and to Ciha-Geigy Ltd., Basel, Switzerland.
Angcw Chem. Inl. Ed. Engl. 18 (1979) No. 9
LO2 +
H’
Scheme 2
According to these findings, the cleavage of water by visible light is undoubtedly possible in a four-quantum process.
0 Verlag Chcmte, GmbH, 6940 Wernheim. 1979
05 70-0X33/79/0909-0701
S 02.50/0
701
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