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Coexistence of Mobile and Localized Electrons in Bis(ethylene)dithiotetrathiafulvalene (BEDT-TTF) Radical Salts with Paramagnetic Polyoxometalates Synthesis and Physical Properties of (BEDT-TTF)8[CoW12O40]╖ 5.5 H2O

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2.74 (t, 8 H , H4, H4), 2.76 (m. 4H. H7'), 3.08 (triplet of doublets, 4H, Hl), 6.18
(septet, 4 H , H3), 9.35 (s, 8H , pyrrole H); ',C NMR (50 MHz, CDCI,, TMS,
20°C): 6 = 22.0 (CH,), 26.6 (CH,), 32.8 (C4). 33.4 (C7), 38.8 (C6), 40.7 (C5), 52.9
(c1), 119.4 (Cm), 128.9 (C3). 131.3 (Cg), 141.1 (Cx),147.3 (C2).
Received: May 5, 1993
Revised version: August 10, 1993 [Z 6059 IE]
German version: Angew. Chem. 1994, 106, 200
[l] For leading references on asymmetric porphyrin catalysts, see: a) D. Mansuy,
P. Battioni. J. P. Renaud, P. Guerin, .I Chem. Soc. Chem. Commun. 1985,
155-156; b)S. OMalley. T. Kodadek, .IA m . Chem. Soc. 1989, Iff, 91169117; c) J. T. Groves, P. Viski, J. Org. Chem. 1990, 55, 3628-3634; d) R. L.
Halterman, S. T. Jan, ibid. 1991, 56, 5253-5254; e) K . Konishi, K. I. Oda, K.
Nishida, T. Aida, S. Inoue, J. A m . Chem. Soc. 1992,114, 1313-1317; f ) P .
Maillard. J. L. Guerquin-Kern, M. Momenteau, Tetrahedron Lett. 1991, 32,
4901 -4904; g) J. P. Collman, V. J. Lee, X. Zhang, J. A. Ibers, J. I. Brauman, J.
Am. Chem. Soc. 1993, 115, 3834-3835; h)Y. Naruta, N. Ishihara, F. Tani, K.
Maruyama. Bull. Chem. Sot. Jpn. 1993.66, 158-166; i) G. Proess, L. Hevesi,
J. Mol. Catal. 1993,80, 395-401 ; for other properties of chiral porphyrins, see
also: j) H. Ogoshi, Y. Aoyama, Y. Okamoto, K. Saita, K. Sakurai, H. Toi,
T. Watanahe, Tetrahedron Letl. 1986, 27, 6365-6368; k) H. P. Pfeiffer,
E. Breitmaier, H . Sander, Liebigs Ann. Chem. 1987, 725-726; 1) B. Boitrel.
A. Lecas, E. Rose, J. Chem. Soc. Chem. Commun. 1989,349-350; m) Y Naruta, F. Tani, K. Maruyama. ibid. 1990, 1378-1380; n) S. Licoccia, M. Paci,
P. Tagliatesta, R. Paolesse, S. Antonaroli, T. Boschi, Magn. Reson. Chem. 1991,
29, 1084-1091; 0)S. OMalley, T. Kodadek, TefrahedronLett. 1991, 32, 24452448; p) K. Ohkubo. T. Sagawa. H. Ishidd, Inorg. Chem. 1992,31, 2682-2688;
q) N. Nishino, H. Mihara, R. Hasegawa, T. Yanai, T. Fujimoto, J. Chem. Soc.
Chem. Commun. 1992, 692-694; r)P. Maillard, C. Huel, M. Momenteau,
Tetrahedron Lett. 1992, 33, 8081-8084; s) P. Le Maux, H. Bahri, G. Simonneaux, Tetrahedron 1993,49,1401-1408; t) K. Konishi, Y Takahata, T. Aida,
S. Inoue, .
I
Am. Chem. Soc. 1993, 115, 1169-1170.
[2] A. Pfaltz, Acc. Chem. Res. 1993, 26, 339-345.
[3] a) J. T. Groves, T. E. Nemo, J. Am. Chem. Soc. 1983, 105. 5786-5791; b) M.
Tavarks, R. Ramasseul, J. C. Marchon, B. Bachet, C. Brassy, J. P. Mornon, J.
Chem. SOC.Perkin Trans. 2 1992, 1321-1329.
[4] a) R. J. Abraham, J. Plant, G. R. Bedford, Org. Magn. Reson. 1982, 19, 204210; b) M. J. Crossley, L. D. Field, A. J. Forster, M. M. Harding, S. Sternhell,
J. Am. Chem. Soc. 1987, 109. 341-348; c) The predicted multiplicities of the
proton signals, on symmetry grounds [Is], are the following for each atropisomer of a porphyrin tetrasuhstituted in the meso positions with a chirdl substituent. Pyrrolic protons: aag1(. s (2H), s' (2H), dd (4H); aaag, s (2H), s'
(2H), dd (2H), d d (2H); apag. s (4H), s' (4H); aaxa,dd (8H). Protons of the
meso substituents: oragfl. s (ZH), s' (2H); aaag, s, (1 H), s2 (1 H), s, (1 H), s4
(1 H); apag, s (4H); aama. s (4H).
[5] a) D. Bakshi, V. K. Mahidroo, R. Soman, S. Dev, Tetrahedron 1989, 45, 767774; b) This compound was prepared from (1R)-cis-hemicaronaldehyde
(= (1R)-cis-caronaldehydic acid hemiacetal) ( 299.5 % ee. J. Buendia, personal
communication). which was a generous gift from Roussel Uclaf.
[6] a) J. S. Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kedrney, A. M. Marguerettaz, J. Org. Chem. 1987,52,827-836; b) R. W. Wagner, D. S. Lawrence,
J. S. Lindsey, Tetrahedron Lett. 1987, 28, 3069-3070.
[7] Steric effects of meso substituents on atropisomer distribution have been noted
for several tetraarylporphyrins, see: a) K. Hatano, K. Anzai, T. Kubo, S.
Tamai, BUN. Chem. Soc. Jpn. 1981, 54, 3518-3521; b) T. Fujimoto, H.
Umekawa, N. Nishino, Chem. Lett. 1992,37-40; c) N. Nishino, K. Kobata, H.
Mihara, T. Fujimoto, ibid. 1992, 1991-1994.
[8] a )K. M. Barkigia, L. Chantranupong, K. M. Smith, J. Fajer, .IAm. Chem.
Soc. 1988, 110, 7566-7567; b) K. M. Barkigia, M. D. Berber, J. Fajer, C. J.
Medforth, M. W. Renner, K. M. Smith, ibid. 1990, 112, 8851 - 8857, 9675;
c) M. W. Renner, R. J. Cheng, C. K. Chang, J. Fajer, .IPhys. Chem. 1990,94,
8508-8511; d) C. J. Medforth, M. D. Berber, K. M. Smith, J. A. Shelnutt,
Tetrahedron Lett. 1990, 31, 3719-3722; e) H. K. Hombrecher, G. Horter, C.
Arp, Tetrahedron 1992, 48, 9451-9460; f) D. Mandon, P. Ochsenbein, J. Fischer, R. Weiss, J. Jayaraj, R. N. Austin, A. Gold, P. S. White, 0. Brigaud, P.
Battioni, D. Mansuy, Inorg. Chem. 1992, 31, 2044-2049; g) C. J. Medforth,
M. 0. Senge, K. M. Smith, L. D. Sparks, J. A. Shelnutt, .I A m . Chem. Soc.
1992, 114, 9859-9869.
[9] D. Cuvinot, P. Mangeney, A. Alexakis, J. F. Normant, J. P. Lellouche, J. Org.
Chem. 1989, 54,2420-2425.
[lo] a) F. Kaplan, C. 0. Schulz, D. Weisleder, C. Klopfenstein, .IOrg. Chem. 1968,
33, 1728-1730; b) R. B. Bates, V. P. Thalacker, ibid. 1968, 33, 1730-1732.
monoclinic, P2, (No. 4), 2 = 4,
[ l l ] Crystal data for 3: C,,H,,N,Ni;
a = 20.055(4), b = 11.753(1),c = 21.496(3) A, fl = 116.267(3)", V = 4543.4 A,,
pcalcd
= 1.24 g ~ m - F(000)
~,
= 1808. Data were collected at 293 K on an EnrdfNonius FAST area detector with Mo,, radiation. The structure was solved by
direct methods and refined anisotropically to give R, = 0.051, R, = 0.061 for
7396 unique data and 1098 variables. Further details of the crystal structure
investigation are available on request from the Director of the Cambridge
Angew. Chem. Int. Ed. Engl. 1994, 33, No. 2
0 VCH
Crystallographic Data Centre, 12 Union Road, GB-Cambridge CB2 1EZ
(UK), on quoting the full journal citation.
a) C. Kratky, R. Waditschatka, C. Angst, J. E. Johansen, J. C. Plaquevent, J.
Schreiber, A. Eschenmoser, Helv. Chim. Acta 1985, 68, 1312-1337; b) W. R.
Scheidt, Y. Lee, Struct. Bonding Berlin 1987, 64, 1-70,
0. Q. Munro, J. C. Bradley, R. D. Hancock, H. M. Marques, F. Marsicano,
P. W. Wade, J. Am. Chem. Soc. 1992, 114, 7218-7230.
Many nickel porphyrins with a four-coordinate metal center adopt a rumed
conformation, see for example: a) L. D. Sparks, C. J. Medforth, M. S. Park,
J. R. Chamberlain, M. R. Ondrias, M. 0. Senge, K. M. Smith, J. A. Shelnutt,
J. Am. Chem. SOC.1993, fl5,581-592, and references therein. However, several Ni" porphyrins with a planar structure are also known, see: b) J. C. Gallucci,
P. N. Swepston, J. A. Ibers, Acla CrystaNogr. Sect. B 1982,38,2134-2139, and
references therein.
Inversion of enantiomeric ruffled conformers of nickel(n) ccccc-octaethylpyrrocorphinate has been reported, see: R. Waditschatka, C. Kratky, B. Jaun,
J. Heinzer, A. Eschenmoser, J. Chem. Soc. Chem. Comm. 1985, 1604-1607.
a) R. Irie, K . Noda, Y Ito, T. Katsuki, Tetrahedron Lett. 1991,32, 1055-1058;
b) E. N. Jacobsen, W. Zhang, A. R. Muci, J. R. Ecker, L. Deng, J. A m . Chem.
Soc. 1991, 113, 7063-7064.
J. P. Collman, X. Zhang, V. J. Lee, E. S. Uffelman, J. I. Brauman, Science 1993,
261. 1404-1411.
Coexistence of Mobile and Localized Electrons
in Bis(ethy1ene)dithiotetrathiafulvalene
(BEDT-TTF) Radical Salts with Paramagnetic
Polyoxometalates: Synthesis and Physical Properties of (BEDT-TTF),[CoW,,O,,] * 5.5H,O **
Carlos J. Gomez-Garcia, Lahcene Ouahab,*
Carlos Gimenez-Saiz, Smail Triki, Eugenio Coronado,*
and Pierre Delhaes
One of the challenges in materials science is to prepare new
molecular compounds with unusual combinations of properties
as, for example, compounds coupling magnetic and conducting
sublattices. Choosing as molecular building block the n donor
bis(ethy1ene)dithiotetrathiafulvalene (BEDT-TTF or ET),
which is the most widely used molecule in the synthesis of molecular superconductors,''] and polyoxometalate anions,[*]which
show a very well-known ability to accommodate magnetic metal
ions at specific sites, we have prepared a radical salt in which
mobile electrons from the organic component coexist with localized electrons from the magnetic Co"-containing polyoxometalate [C011W,,0,,]6-, and other related polyanions.
[*I Dr. L. Ouahab, Dr. C. J. Gomez-Garcia [ + I
Laboratoire de Chimie du Solide et Inorganique Moleculaire
URA-CNRS 1495, Universite de Rennes I
Av. GBneral Leclerc. F-35042 Rennes Cedex (France)
Telefax: Int. code + (99)383487
Prof. Dr. E. Coronado. C. Gimenez-Saiz, Dr. S. Triki
Departamento de Quimica Inorginica
Universidad de Valencia
Dr. Moliner 50, E-46100 Burjasot (Spain)
Telefax: Int. code+ (6-)38643322.
Dr. P. Delhaes
Centre de Recherche Paul Pascal
Universite de Bordeaux I-CNRS
Av. A. Schweitzer, F-33600 Pessac (France)
['I Permanent address: Departamento de Quimica Inorginica
Universidad de Valencia (Spain)
[**I This work is supported by the Comision Interministerial de Ciencia y Tecnologia (CICYT), the Generalitat de Cataluiia (CIRIT) (Grant QFN91-4220), the
CNRS, and the EC. C.J.G-G and S.T. thank the Ministerio de Educacion y
Ciencia for a postdoctoral fellowships.We thank C. Garrigou-Lagrange for her
helpful discussions about the IR spectra. We thank H. Noel, J. Amiell, and R.
Canet for assistance with the physical measurements.
Verlagsgesellschajt mbH. 0-69451 Weinheim, 1994
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Diamagnetic polyoxoanions have been recently used by usL3]
and by other groups[41as inorganic electron acceptor components of charge-transfer salts, with the aim of examining the
novel structural and electronic properties in the organic moiety
induced by the sizes, shapes, and charges of these inorganic
clusters. In some cases the polyoxoanion is reduced by the organic molecule to give a material in which delocalized electrons
coexist in both the organic and the inorganic components.[51
However, no structural analyses have been reported for these
salts with paramagnetic polyoxoanions.[61In this communication we report the structure and physical properties of the first
example of this kind.
The structure['] of the title compound shows the presence of
alternating layers of the anions and the ET molecules (Fig. 1).
oxygen atoms of the anion (shortest S - 0 distance of 3.04(3) A).
The second type of interaction is a hydrogen bond between
oxygen atoms of the anion and the carbon atoms of both
ethylene groups in the ET molecules of the A type (shortest
C - 0 distance, 3.08(5) A; Fig. 1). Note that interactions
through the ethylene groups have been shown to play an important role in the conducting properties of solid ET.['] Indeed, this
donor arrangement is one of the classical types, called the a
phase, in 2-D molecular superconductors.["]
a
I1
I
I1
I
I1
b
Fig. 1. Perspective view of the organic and inorganic layers showing the chains of
eclipsed ET molecules and chains of dimers, as well as the interactions between the
anions and the organic C-type molecules [A]: d, (0-S) = 3.04(3), d2 (0-C) =
3.08(5).
This structure is similar to that found for ET8[SiW,,0,,], but in
that case the structure is acentric[4a1and the compound does not
contain water molecules. In our case the anion displays the
well-known Keggin structure containing a Co" ion in its tetrahedral cavity. The anion is disordered (rotation of 90" around the
C, axis), which makes it appear to be a centrosymmetric unit.
This kind of disorder has been unambiguously described by
Pope and Evans.18]The organic layers are formed by three crystallographically independent molecules of ET noted as A, B,
and C (Fig. 2a). These molecules are arranged in two different
kinds of chains (types I and 11, Fig. 2a). Type I chains are
formed by alternating B and C type molecules packed eclipsed
with S-S distances of 4.04(2) A. Chains of type I1 consist of A
type molecules packed as dimers with S-S distances within a
dimer of 3.86(2) and between dimers of 3.98(1) 8, (Fig. 2b). The
two types of chains alternate parallel to the direction [TOl] and
give rise to an a phase[la]with a dihedral angle of 46.5" between
the ET molecules. The interchain connections (Figure 2a)
(ranging from 3.46(2) to 3.52(2) A), which are much shorter
than the intrachain ones, are close to those commonly observed
in the quasi 2-D materials based on ET.['"] Moreover, two kinds
of interactions between the organic and inorganic layers can be
observed (Fig. 1). The first occurs between the sulphur atoms of
the C-type molecules (in the eclipsed chain) and the terminal
224
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VerlagsgeselischaftmbH. 0-69451 Weinheim. 1994
Fig. 2. a) Projection of the organic layer onto the ac plane showing the mode of
packing of the a phase and the intrachains interactions [A]: d3 = d, = 3.51(2),
d, = d6 = 3.52(2), and d, = 3.46(2). b) View of the positions of the anions in the
unit cell and the A-type molecules of ET stacked in the chains of dimers (type 11)
along the [I011 direction.
The electrical properties of a single crystal of ET,[CoW,,O,,]. 5.5H,O are illustrated through a plot to the normalized resistance versus T in Figure 3. The compound is a
semiconductor ( r ~I 0.1 Scm-' at room temperature) showing
two different semiconducting behaviors with activation energies
of 120 and 27 meV (at high and low temperatures, respectively),
and a transition temperature of about 170 K (estimated from
the intersection of the two semiconductive regimes).
The IR and visible absorption spectra recorded at room temperature shows the presence of two charge transfer bands and a
doublet for the associated vibronic modes. The electronic bands
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is in a delocalized mixed-valence state. In fact, magnetic measurements carried out on the related silicon-containing salt,
ET,[SiW,,O,,], show a maximum in x at about 60 K, which
indicates the presence of significant antiferromagnetic interactions in the organic s~blattice!'~~
200
0
50
100
150
T
-
200
El
250
S [GI
300
15C
z
I
1LX
Angew.. Chem. Int. Ed. EngI. 1994. 33, No. 2
g
i
Fig. 3. Plot of the normalized resistance versus T for a single crystal of the title
compound. Straight lines represent the fit for the two semiconductive regimes (see
text).
are located at 4500 and 9900 cm-': the first one is associated
with a mixed-valence system, whereas the second one accounts
for a doubly occupied site." Besides, we observe vibronic lines
due to the classical electronic-molecular vibrational coupling
with two pairs of lines at 1405 and 1344 cm-' for the agv4C=C
central stretching mode and at 450 cm-' with a shoulder
around 480 cm-' for the agv10C-S-C stretching mode. In
analogy to assignments made for other ET salts[''] we conclude
that two different organic stacks are present, as already observed from the X-ray crystal structure data: on one hand, the
stack including B- and C-type ET molecules appears to be completely ionized, and on the other, the A-type dimeric molecular
arrangement is be a mixed valence system with + 112 charge on
each ET molecule (to yield the total anionic charge of -6).
In a wide temperature range (40-300 K), single-crystal EPR
measurements show one homogeneous signal at g % 2 that appears to be mainly related with the organic radical cations. A
second signal becomes observable at lower temperatures, which
is very sensitive to the orientation of the crystal (the g value of
this line varies between 4 and 6 when the static magnetic field is
perpendicular or parallel to the long axis of the needle-shaped
crystal). This signal is due to the tetrahedral Co" ions. The
observation of two signals indicates that the exchange interaction between the two spin sublattices is small. Further indication
of magnetic interactions can be obtained from the plot of the
dependence of temperature on the spin susceptibility, the g value, and the signal width derived from the radical signal (Fig. 4).
Thus, the plot of the X Tvs. T decreases continuously on lowering the temperature, and at 4 K reaches a value that is 3 % of the
room temperature value, which indicates short-range antiferromagnetic interactions in the organic sublattice. On the other
hand, both g and line-width parameters do not change, within
the experimental error, down to about 80 K, and at lower temperatures a progressive increase of these parameters is observed.
This behavior is reminiscent of that previously observed in two
other organic/inorganic salts for which magnetic interactions
between the localized moments of the inorganic part and the
itinerant electrons of the organic moiety have been proposed.['']
However, in our case the presence of a very small organic-inorganic exchange interaction does not seem to be sufficient to
justify the strong variability of the experimental parameters. A
more plausible explanation arises from the possibility of magnetic interactions between the two kinds of organic chains
present in the structure: one probably contains completely ionized ET molecules, and therefore localized spins, while the other
t
t '
!.0100
!.0050
X~X3rnK
t
Fig. 4. Temperature dependence of the linewidth S of the ESR signal, the g factor,
and the relative spin susceptibility y, for a given position of the single crystal within
the resonance cavity.
From the above results we conclude that the two spin sublattices are quasi-isolated. The only significant interaction seems to
occur within the organic component. The bulk susceptibility
measurements confirm this conclusion : The product X Tremains
constant in the range 150-300 K and shows a slight decrease
from 3 to 2.2 emu mole K - ' on lowering the temperature to
4 K. This decrease should arise from the antiferromagnetic interaction in the organic component, while the low temperature
value is in good agreement with the presence of isolated Co"
( S = 312 and g z 2.2).
The material presented here constitutes the first example of
organic/inorganic salts made with both magnetic polyoxometalates and n-type organic donors. It opens the possibility of synthesizing new molecular compounds in which the coupling between magnetic and conducting sublattices should lead to new
and interesting properties. Radical cation salts containing other
polyoxometalates XW,,O,, are under current investigation in
our groups. Preliminary results show that other paramagnetic
Zn",
(X = Co"', Fe"', Cu", etc.) and diamagnetic (X = H:',
B"') anions with the Keggin structure present the same a phase.
Furthermore, we have also found the existence of two other
related phases for almost all the mentioned p o l y a n i o n ~ . [ ' ~ ~
These results now open the possibility of trying other Keggin
polyanions having magnetic ions on the surface in order to favor
the magnetic interactions with the organic part. Moreover, they
show the rich structural polymorphism of this kind of inorganic/
organic association and the existence of isostructural series in
which different anionic charges (from -4 to - 8) and different
spins can be accommodated with ease, thus constituting ideal
supports for the investigation of systems combining conducting
and magnetic behaviors.
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Experimental Procedure
Single crystals of the title compound were obtained from the electrochemical oxidalo-' M in a mixture of CH,CI,/CH,CN/H,O 10:2:1) in the
tion of ET ( 3 . 5 ~
presence of the tetrabutylammonium salt of the polyoxoanion [COW,,O,,,-~ 1161
M in a mixture of CH,CI,/CH,CN 1 :1) in a U-shaped cell with Pt elec(5 x
trodes (0.5 mm diameter) separated by a sintered-glass frit. The intensity of the
current was fixed at 1 PA. After 2 o r 3 weeks. amber, platelike single crystals of the
title compound were collected carefully and air dried.
Magnetic measurements were carried out on single crystals with a magnetometer
(905 VTS, SEH Corporation) equipped with a SQUID sensor (SQUID = superconducting quantum interference device). EPR measurements of the X band at
variable temperature were recorded on a Bruker ER 200D spectrometer equipped
with a helium cryostat. In the experiment the static magnetic field is perpendicular
to the long axis of the needlelike single crystal. Infrared spectra in the range 4004700cm-' were made on KBr pellets with a FT-IR interferometer Nicolet MX1.
Conductivity measurements were performed using the four-probe method on approximately 4-mm long-needlelike single crystals.
Received: July 14, 1993
Revised version: August 7, 1993 [Z 6139 IE]
German version: Angew. Chem. 1994. 106. 234
[l] See, for instance: a) J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson,
U. Geiser, H. H . Wang, A. M. Kini. M. H. Whangbo in Organic Superconductors. SynrhesD, Structure. Properties and Theory (Ed.: R. N. Grimes), Prentice
Hall, Englewood Cliffs, NJ. USA, 1992; b) Proc. ICSMPU (Tiibingen): Synrh.
Mer. 1991, 41-43 and Proc. f C S M ' 9 2 (Goreborg): ibid. 1993, 55-57.
[2] a) M. T. Pope. Heteropoly and fsopoly 0.xomerolutes. Springer, Berlin, 1983; b)
M. T. Pope, A. Miiller. Angeir.. Chem. 1991. 103, 56; Angew. Chem. Inr. Ed.
Engl. 1991,30,34; c) Polyoxomeralares: from P h o n i c So/ids IO Anti-retrovird
octiviry (Eds.: M. T. Pope, A. Miiller), Kluwer, Dordrecht, in press.
[31 a) L. Ouahab, M. Bencharif, D. Grandjean, C. R. Aced. Sci. Ser. II. 1988,307,
749; b) S.Triki, L. Ouahab, J. Padiou, D. Grandjean. J. Chem. Soc. Chem.
Commun. 1989, 1068; C) C. J. Gomez-Garcia. E. Coronado, S. Triki. L. Ouahab, P. Delhahs, Adv. Muter. 1993, 4, 283.
[41 a) A. Davidson, K. Boubekeur, A. Pknicaud, P. Auban, C. Lenoir, P. Batail.
G. Herve. J. Chem. Soc. Chem. Commun. 1989,1373; b) C. Bellitto, D. Attanasio, M. Bonamico, V. Fares, P. Imperatori. S. Patrizio, Muter. Res. Soc. Svmp.
Proc. 1990. 173. 143.
[51 a) A. Mhanni, L. Ouahab, 0. Peiia, D. Gradjean, C. Garrigou-Lagrange. P.
Delhaes, Sjnth. Mer. 1991, 41-43, 1703, b) L. Ouahab, M. Bencharif, A.
Mhanni. D. Pelloquin, J. F. Halet, 0. Peiia, J. Padiou, D. Grandjean, C.
Garrigou-Lagrange. J. Amiell. P. Delhaes. Chem. Muter. 1992,4,666; c) see L
Ouahab in ref. [2c].
[6] Powder materials formed by T T F and paramagnetic polyoxoanions of the type
[Co,(H,O),(PW,O,,),]l o and [CrMo,0,,HJ3
have very recently been prepared and magnetically characterized: C. J. Gomez-Garcia, J. J. Borras-Almenar, E. Coronado. P. Delhaes, C. Garrigou-Lagrange, L. C. W. Baker, Svnrh.
Met. 1993, 55-57, 2023.
[7] (BEDT-TTF),[CoW,,O,,] . 5.5: amber platelike crystal: 0.41 x 0.34 x
0.01 mm', Enraf-Nonius CAD4 diffractometer. M = 6081.8, monoclinic.
space group f 2 / m with u = 13.971(9), b = 43.117(7), c =14.042(5) A, fl =
107.25(3), V = 8078.5 A'. and Z = 2 (pea,cd
= 2.496 gem-'; Mo,, radiation,
E. = 0.71073 A. p = 96.554 cm-I), structure solved by direct methods and successive Fourier difference synthesis. Refinement of 329 variables with anisotropic thermal parameters (for Co, W and S atoms) gave R ( F ) = 0.068 and
R w ( F ) = 0.102 by using 2614 absorption corrected reflections with f 2 6rr(I).
Further details of the crystal structure investigation may be obtained from the
Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen
(FRG), on quoting the depository number CSD-57908. the authors, and the
journal citation.
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226
0 VCH
Verlugsgesellschuft mbH. 0-69451 Weinheim. 1994
Amides as a New Type of Backbone
Modification in Oligonucleotides**
Alain De Mesmaeker,* Adrian Waldner,
Jacques Lebreton, Pascale Hoffmann, ValCrie Fritsch,
Romain M. Wolf, and Susan M. Freier
Considerable efforts have been made recently in the structural
modification of oligonucleotides."] Several properties of the natural 2'-deoxyoligoribonucleotidesmust be improved for potential
therapeutic application of the antisense strategy. For example,
the stability of the antisense oligonucleotides towards nucleases
must be substantially increased, while the affinity and the
specificity for the complementary RNA target is maintained (or
even increased). So far, most of the replacements of the phosphodiester linkage that lead to an increased resistance towards
nucleases are also connected with a decrease in the affinity for
a complementary RNA strand.". 'I Two exceptions to this were
recently reported in which either an N-methylhydroxylamine or
a thioformacetal unit was used as a PO,
We proposed to replace the natural phosphodiester linkage in
1 (Scheme 1 ) by an amide function (as in 2-4), which offers
several advantages compared to previously reported modification~.'~
The
] amide unit is compatible with conditions required
Scheme 1. Structural isomers of oligonucleotide backbones modified by amide
linkages.
for solid-phase oligonucleotide synthesis and is also more stable
under physiological conditions than the phosphodiester linkage.
Furthermore, the lower overall charge of oligonucleotides containing neutral amide groups should facilitate penetration of the
negatively charged cell membrane^.'^] The amide moiety is readily accessible by simple synthetic methods and is achiral. Thus
the formation of diastereomeric mixtures after incorporation
into oligonucleotides is avoided.
We disclose here the synthesis of the thymidine dimers 10 (see
Scheme 2) having the same substitution pattern in the nucleotide
backbone as that in 2. The incorporation of 10 into oligodeoxynucleotides is described, together with the thermodynamic
stability (melting temperatures T,) of the corresponding duplexes
[*I
Dr. A. De Mesmaeker, Dr. A. Waldner, Dr. J. Lebreton, P. Hoffmann,
Dr. V. Fritsch, Dr. R. M. Wolf
Central Research Laboratories, Ciba-Geigy
CH-4002 Basel (Switzerland)
Telefax: Int. code (61)679-8252
Dr. S. M. Freier
ISIS Pharmaceuticals
2280 Faraday Avenue, Cartsbad. CA 92008 (USA)
We thank Dr. H. Moser for helpful discussions, and Dr. U. Pieles and Dr. D.
Hiisken for the synthesis and the purification of the oligonucleotides.
+
[**I
0570-0833/Y4j0202-02263 10.00+ ,2510
Angew. Chem. Inr. Ed. Engl. 1994, 33, No. 2
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