4,5-Diazafluorene-based Overcrowded Alkene: A New Ligand for Transition Metal Complexes

Article abstract of DOI:10.1021/ol017130c

matrix presented A new ligand system, where a 4,5-diazafluorene-type chelate and a methoxybenzoxanthene unit are coupled by a double bond has been synthesized and fully characterized including X-ray structure. The synthesis and UV-vis spectra of Ru(II), O

Full text of DOI:10.1021/ol017130c

ORGANIC
LETTERS
2002
Vol. 4, No. 7
1067-1070
4,5-Diazafluorene-Based Overcrowded
Alkene: A New Ligand for Transition
Metal Complexes
,†
Manel Querol,† Helen Stoekli-Evans,‡ and Peter Belser*
Department of Chemistry, UniVersity of Fribourg, Pe´rolles, CH-1700 Fribourg,
Switzerland, and UniVersite´ de Neuchaˆtel, Institute of Chemistry,
AVenue de BelleVaux 51, CH-2000 Neuchaˆtel, Switzerland
peter.belser@unifr.ch
Received November 27, 2001
ABSTRACT
A new ligand system, where a 4,5-diazafluorene-type chelate and a methoxybenzoxanthene unit are coupled by a double bond has been
synthesized and fully characterized including X-ray structure. The synthesis and UV−vis spectra of Ru(II), Os(II), and Re(I) complexes with the
above-mentioned ligand are also shown.
The use of photochromic materials for reversible data storage
is currently a growing field. This interest is based on their
potential applications in the “bottom-up” approach adopted
in nanotechnology.1 Among them, overcrowded bistricyclic
aromatic enes have been extensively studied since their
“helicity” can be inverted by means of CPL (circularly
polarized light) and such a change can be easily monitored
by means of CD techniques. They have also been used as
phototriggers in LCD technology.2 These kinds of systems
can be used either in a pure diastereomeric form (through a
photodestruction process) or as a racemic MP mixture
(through a photoenrichment process) as has been shown by
Feringa et al.3 To improve the photomodulation of these
systems, several synthetic modifications have been intro-
duced. These modifications are mainly oriented toward the
increase of the encumbrance around the central double bond
in order to achieve more and more overcrowding in the fjord
region of these systems.4
In this Letter we will show the synthesis of an over-
crowded polycyclic aromatic ene based on a 4,5-diazafluo-
rene moiety which fulfills the steric requirements to adopt a
helical shape. Moreover, the presence of a 4,5-diazafluorene
moiety would allow us to introduce different metallo-based5
sensitizers such as Ru(II), Os(II), and Re(I).
The synthesis of the target molecule was envisioned to
have as coordinating site a 4,5-diazafluorene moiety due to
its chelating ability. Keeping in mind this fragment, the upper
part (methoxybenzoxanthene unit) had to provide enough
substituents around the double bond in order to avoid thermal
conformational inversion processes.6 In this case, these
processes should even be more important due to the planar
geometry of the diazafluorene fragment (bottom part).7 In
† University of Fribourg, Department of Chemistry.
‡ Universite´ de Neuchaˆtel, Institute of Chemistry.
(1) (a) Special issue on Photochromism: Memories and Switches. Chem.
ReV. 2000, 1683. (b) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F.
Angew. Chem., Int. Ed. 2000, 39, 3348. (c) Martin, R. E.; Diederich, F.
Angew. Chem., Int. Ed. 1999, 38, 1351. (d) Drexler, K. E. Nanosystems.
Molecular Machinery, Manufacturing, and Computation; Wiley: New York,
1992.
(2) (a) Huck, N. P. M.; Jager, W. F.; de Lange, B.; Feringa, B. L. Science
1996, 273, 1686. (b) Feringa, B. L. Acc. Chem. Res. 2001, 34, 504. (c)
Chen, C.-T.; Chou, Y.-Chen. J. Am. Chem. Soc. 2000, 122, 7662.
(3) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38,
3418.
(4) Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron Lett. 1992,
33, 2887.
(5) For a related example of overcrowded alkenes as transition metal
ligands, see: Wietske, I. S.; Schoevaars, A. M.; Kruizinga, W.; Veldman,
N.; Smeets, W. J. J.; Spek, A. L.; Feringa, B. L. Chem. Commun. 1996,
2265.
10.1021/ol017130c CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/13/2002

Scheme 1. General Synthetic Scheme for the Preparation of
Compounds 1 and 2a
Scheme 2. General Synthetic Scheme for the Obtention of the
Upper Part of Compounds 1 and 2a
a Conditions: (a) toluene; (b) PPh3/toluene.
a Conditions: (a) Cu(I), base, toluene; (b) PCl5, benzene; (c)
SnCl4, benzene; (d) P2S5, toluene.
this sense, the upper part was envisioned to be a benzo[a]-
xantone in order to provide enough rigidity to the system as
well as enough encumbrance around the double bond. In
addition, substitution at the 2 position of the benzo[a]xantone
unit could minimize the possibility of conformational inver-
sion processes.
material and the preparation of the ether derivative 9 was
attempted in two different ways. The first attempt was carried
out by heating at 190 °C a mixture of the iodobenzoic acid
7 with a 3-fold excess of the sodium salt of the corresponding
7-methoxy-2-naphthol. Under these conditions, compound
9 was obtained in 37% yield. The relatively low yield
obtained was correlated with the concomitant isolation of
the biaryl lactone type compound 8 with a yield of 23%.
The second path chosen for the synthesis of the ether-
type compound 9 was carried out by following the method
described by Snieckus et al.14 The method employs a toluene-
or xylene-soluble Cu(I) complex as catalyst and Cs2CO3 as
base. Under these conditions, compound 9 was obtained in
67% yield after two recrystallizations from toluene. When
copper bronze was used as catalyst15 and maintaining the
same conditions, yields were always below 40%. The
obtention of the benzo[a]xantone skeleton 11 was ac-
complished using SnCl4 in benzene at 0 °C in a “one-pot”
reaction starting from the corresponding ether 9; this
compound was transformed into the corresponding acyl
chloride 10, using an excess of PCl5, which was further
treated with SnCl4 to afford the xantone-type skeleton in 76%
yield after two recrystallizations using EtOH. This strategy
can be an alternative not only to the use of polyphosphoric
acid15 (when acid sensitive groups are present) but also to
the photooxidative cyclization of styrilchromones. The
sulfurization step (Scheme 1) was achieved using P2S5 in
boiling toluene for 7 h.
The synthesis of such highly hindered alkenes has been
mainly obtained by means of Barton’s 2-fold extrusion
process.8 Following this methodology, in principle, both the
diazofluorene/xanthenethione and the fluorenethione/diazo-
xanthene could be adopted. Different attempts to obtain the
fluorenethione over the 4,5-diazafluorenone starting material,
using either P2S5 or Lawsson reagent9 under different
conditions including new procedures based on the use of
microwaves,10 were unsuccessful. In every case the obtention
of the bis(4,5-diazafluore-9-ylidene) was the main product.11
Therefore, the distribution of the functionalities needed was
done as shown Scheme 1 The synthesis of the 9-diazo-4,5-
diazafluorene 5 was accomplished as reported previously by
Belser et al.,12 taking as starting material 4,5-diazafluoren-
9-one. The synthesis of the xanthone skeleton 11 (Scheme
2) has already been published in an overall yield below 10%
using a methodology based upon a photooxidative cyclization
of the corresponding substituted styrilchromone.13 Due to
this low yield, we decided to attempt the synthesis of such
a moiety in a different way as shown in Scheme 2. In this
sense, 7-methoxy-2-naphthol 6 was chosen as starting
(6) For a definition of the conformational inversion process in over-
crowded alkenes, folding angles, and pyramidalization angles, see: Bied-
ermann, P. U.; Stezowski, J. J.; Agranat, I. Eur. J. Org. Chem. 2001, 15.
(7) Levy, A.; Biedermann, P. U.; Agranat. I. Org. Lett. 2000, 2, 1811.
(8) (a) Back, T. G.; Barton, D. H. R.; Britten-Kelly, M. R.; Guziec, F.
S., Jr. J. Chem. Soc., Perkin Trans. I 1976, 2079. (b) Krebs, A.; Kaletta,
B.; Nickel, W.-U.; Ru¨ger, W.; Tikwe, L. Tetrahedron 1986, 42, 1693.
(9) Scheibye, S.; Kristensen, J.; Lawesson. S.-O. Tetrahedron 1979, 35,
1339.
The diazo-thioketone coupling reaction was tested under
various conditions. Conditions using low temperatures to
avoid possible decomposition of compound 5 did not lead
(13) (a) Kumar, K. A.; Srimannarayana, G. Indian J. Chem., Sect. B
1980, 615. (b) Ichiro, Y.; Kyoko, H.; Yoshiaki; S.; Manki, K. Chem. Pharm.
Bull. 1981, 29(9), 2670.
(10) Varma, R. S.; Kumar, D. Org. Lett. 1999, 1, 697.
(11) For synthesis description and X-ray structure of bis(4,5-diazafluore-
9-ylidene), see: Riklin, M.; von Zelewsky, A.; Bashall, A.; McPartlin, M.;
Baysal, A.; Connor, J. A.; Wallis, J. D. HelV. Chim. Acta 1999, 82, 1666.
(12) Bernhard, S.; Belser, P. Synthesis 1996, 192.
(14) Kalinin, A. V.; Bower, J. F.; Riebel, P.; Snieckus, V. J. Org. Chem.
1999, 64, 2986.
(15) Barf, T.; Jansen, J. F. G. A.; van Bolhuis, F., Antony, L.; Feringa,
B. L. Recl. TraV. Chim. Pays-Bas 1993, 112, 376.
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Org. Lett., Vol. 4, No. 7, 2002

the desired alkene-type compound 2 and the thiirane-type
precursor 4. When the reaction was allowed to react for
longer times, decomposition of the thiirane-type compound
toward the thioxanthene starting material was detected. After
this observation, the reaction was repeated using the same
conditions for a period of 7 h. After this time, the solvent
was removed in vacuo and after two crystallizations of the
crude product using EtOH the thiirane-type compound was
isolated in 78% yield. The structure of this compound was
further confirmed by X-ray crystallography (Figure 1a).
Desulfurization of compound 4 was achieved using PPh3 in
boiling toluene for a period of 12 h. The resulting alkene 2
structure was also confirmed by X-ray crystallography
(Figure 1b).
Crystals suitable for X-ray analysis were obtained from
compounds 4, 2, and 1. Figure 1a shows one of the two
possible enantiomers. All three structures showed a butterfly
shape in the benzoxanthene part of the structure and a boat-
like conformation for the central six-membered ring. Com-
pound 4 shows as the main features folded conformation6
for the upper part with a folding angle of 47.0° and a nearly
planar conformation for the lower part (folding angle 3.9°).
This folding in the upper part of the molecule could be
attributed to the small angle defined by C(20)-C(19)-C(31)
of 104.6° that pushes up the benzoxanthene-based moiety.
The dihedral angles around the thiirane ring [C(6)-C(7)-
C(19)-C(31) and C(8)-C(7)-C(19)-C(20)] are -1.48° and
8.73°, respectively, and the thiirane ring plane is twisted by
2.2° from the plane defined by C(20)-C(31)-C(8)-C(6).
The crystal structure of compound 2 shows three independent
molecules in the asymmetric unit; all of them show anti-
folded conformations with pyramidalization angles of 5.1/-
5.0°, 10.1/-4.2°, and 7.3/-8.6°, respectively, and folding
angles for the upper/lower part of 47.3/15.8°, 41.9/12.9°, and
43.3/12.9°, respectively. The crystal structure of compound
1 showed a half molecule of hexane per molecule of 1 in
the asymmetric unit. This compound in the solid state also
shows an anti-folded conformation with pyramidalization
angles of 7.44/-3.43° and folding angles of 44.2/14.7° for
the upper and lower part, respectively. In either case,
compounds 2 and 1 show overcrowding6 in the fjord region
as can be seen in Table 1.
1H NMR spectra in CDCl3 (Figure 2) show as the main
feature a completely asymmetric pattern for the diazafluorene
protons as well as for the fluorene ones.18 This spectra shows
that protons a and b are strongly upfield shifted when
compared with the homologous protons c and d. This can
be rationalized by considering the strong shielding effect due
to the aromatic rings located close to these protons. This
effect can be also seen in the resonance of the methoxy group
protons. In this case those protons are shifted upfield by as
much as 0.6 ppm when compared with those of the
xanthione-based precursor. These features are consistent with
Figure 1. Crystal structures Platon14 representation of compound
(R)-4 (a), (P)-2 (b), and (P)-1 (c). See ref 16.
to thiirane-type compound 3. The thioxanthene-type com-
pound 12 was recovered in quantitative yield. When the
reaction was carried out in toluene at 110 °C for a minimum
of 48 h, and using a 2-fold excess of the 9-diazo-4,5-
diazafluorene 5 moiety, the corresponding alkene 1 was
isolated without detection of the thiirane intermediate.
Different attempts to isolate this intermediate, consisting of
temperature/time tuning, never showed the presence of this
compound. The structure of the newly formed molecule was
confirmed by X-ray crystallography (Figure 1c). To under-
stand the behavior of this system in further photophysical
studies, the model compound 2 was also synthesized. In this
case the diazafluorene moiety was replaced by the simple
fluorene moiety 13. The synthesis of this fragment following
the previous synthetic idea was carried out by oxidation of
the corresponding hydrazone to the azo compound using an
excess of MnO2 in THF at 0 °C.
(16) Only one of the isomers of the racemate has been represented.
(17) Defined as deviation from the sum of the van der Waals radii. For
the values of van der Waals radii of C and H, see: Levy, A.; Biedermann,
P. U.; Cohen, S.; Agranat, I. J. Chem. Soc., Perkin Trans. 2 2001, 2329
and references therein.
(18) Full assignments of the aromatic protons of systems 1 and 2 are
provided in the Supporting Information.
The previous coupling conditions applied to this new
molecule afforded, after 24 h, a mixture mainly formed by
Org. Lett., Vol. 4, No. 7, 2002
1069

Table 1. Degree of Overcrowding in the Fjord Region for
Compounds 1 and 217
compound
1
2a
Ca-Ca′
Ca-Ha′
Ca′-Ha
Cb-Cb′
12%
16%
4%
11%
15%
7%
Figure 3. UV-vis absorption of compound 1 and its Ru(II), Os-
(II), and Re(I) complexes.
12%
13%
a Average value from three different molecules in the asymmetric unit.
the molecule. At about 300 nm a transition associated with
the diazafluorene part can be observed. In the UV part of
the spectra of the ruthenium and osmium complexes an
intense absorption band (280 nm) can be attributed to a 1LC
transition from the bpy ligand. Moderately intense metal-
to-ligand charge transfer (1MLCT) bands are observed for
both complexes in the 400-500 nm region. In the osmium-
containing complex, spin-orbit coupling gives rise to broad
and weak absorptions at wavelengths higher than 600 nm,
corresponding to the spin forbidden, formally 3MLCT
transition. The rhenium complex Re(1)(CO)3Cl shows a large
a system in which the “bottom half” and “top half” are in
close proximity as has been shown previously by X-ray
analysis. These NMR features are constant even when the
sample was heated at 80 °C. This last observation can be
correlated with the blocking of the conformational inversion
processes associated with these systems.
[Ru(bpy)2(1)](PF6)2, [Os(bpy)2(1)](PF6)2, and Re(1)-
(CO)3Cl were prepared20 by refluxing compound 1 with the
appropriate metal source (Ru(bpy)2Cl2, Os(bpy)2Cl2, and
Re(CO)5Cl, respectively) for several hours using methoxy-
ethanol for the ruthenium and osmium complexes and toluene
for the rhenium complex formation. The ruthenium and
osmium complexes were purified by recrystallization of their
hexafluorophosphate salts from a mixture of acetone/hexane
and a second recrystallization from methanol. For the
rhenium complex the purification was achieved by crystal-
lization from a mixture toluene/acetone.
1
absorption maximum at 370 nm attributed to a MLCT
transition. This study shows that the possibility of a selective
irradiation over the metal center can be done and therefore
helicity tuning could be achieved trough a triplet-triplet
sensitization mechanism.
In conclusion, the helical character of compounds 1 and
2 has been proven by means of X-ray analysis. These
compounds are potential candidates for chiroptical switches
following a photoenrichment process. Moreover, the ability
of compound 1 to form metal complexes with Ru(II), Os-
(II), and Re(I) has also been proven and the UV-vis
absorption spectra shows well-differentiated bands where a
metal-based irradiation can be performed and therefore the
sensitization could be tested.
UV-vis absorption spectra of compound 1 and its metal
complexes with Ru(II), Os(II), and Re(I) are shown in Figure
3. The absorption spectrum of ligand 1 shows a broad peak
at 360 nm corresponding to a ligand-centered (1LC) transi-
tion, mainly located in the methoxybenzoxanthene part of
Acknowledgment. We thank the National Swiss Science
Fundation for funding through the NFP 47 program.
Supporting Information Available: Experimental pro-
1
cedures for the synthesis of compounds 1 and 2. Full H
NMR aromatic region assignments for compounds 1 and 2.
Crystallographic data for compounds 1, 2, and 4. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystal structure data for compounds 1 (CCDC
173789), 2 (CCDC 173790), and 4 (CCDC 173788) have
been deposited in the Cambridge Crystalographical Database.
OL017130C
(19) Taken at 25 °C using the racemic mixture (M/P) of compounds 2
and 1.
(20) For synthesis of related complexes, see: (a) Beyeler, A.; Belser,
P.; De Cola, L. Angew. Chem., Int. Ed. Engl. 1997, 36, 2779. (b) De Cola,
L.; Bazani, V.; Barigelletti, F.; Flamigni, L.; Belser, P.; Bernhard, S. Recl.
TraV. Chim. Pays-Bas 1995, 114, 534. All complexes gave satisfactory MS
data.
Figure 2. 1H NMR (400) of the aromatic region of compounds 2
(top) and 1 (bottom). See ref 19.
1070
Org. Lett., Vol. 4, No. 7, 2002