Formation of isomeric tetrathiotungstate clusters [{Cp*Ru(CO)}2(WS4){W(CO)4}] by the reaction of [Cp2/*Ru2S4] with [W(CO)3(MeCN)3]

Full text of DOI:10.1002/(SICI)1521-3773(19980817)37:15<2126::AID-ANIE2126>3.0.CO;2-J

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were collected at 160 K with a Bruker AXS SMART CCD area-detector
diffractometer on the high-flux single-crystal diffraction station 9.8 at
CCLRC Daresbury Laboratory Synchrotron Radiation Source, with a
wavelength of 0.6849 Š selected by a horizontally focusing silicon (111)
monochromator and vertically focused by a cylindrically bent palladium-
coated zerodur mirror.[13] The data set covered more than a hemisphere of
reciprocal space with several series of exposures, each series with a
different crystal orientation and each exposure taken over 0.28 rotation.
Corrections were made for the decay of the synchrotron beam intensity as
part of standard interframe scaling procedures.[14] The SSZ-23 sample was
examined in the as-synthesized form and therefore included the template
and charge-balancing fluoride ions. The crystal structure of SSZ-23 was
solved with the direct methods program SHELXS-97[14] and refined with a
full-matrix least-squares technique with the program SHELXL-97.[14] The
final cycle of least-squares refinement included anisotropic displacement
parameters for silicon, oxygen, carbon, and nitrogen atoms, and fractional
occupancies for the fluorine atoms. The hydrogen atoms of the template
molecule were placed in chemically sensible positions, and their positions
were recalculated after each cycle of refinement. A total of 22014
integrated intensities were measured (1.79 < q < 28.278), of which 8314
were unique and 4665 observed (F2 > 2s(F2)). The large proportion of
unobserved reflections is due to the small size of the crystal. Final
refinement of the 571 least-squares parameters converged to wR(F2obs:) ˆ
0.1657, wR(F2all data) ˆ 0.2028, R(Fobs.) ˆ 0.0811, S(F2all data) ˆ 0.992; max. re-
sidual electron density ˆ 0.744 eŠ 3. Refinement on F2 was carried out
against all data; the observed criterion F2 > 2s(F2) was used only for the
calculation of wR(F2obs:) and R(Fobs.) and is not relevant to the choice of
reflections for refinement. The final unit cell was refined to a ˆ 12.9594(3),
b ˆ 21.7919(6), c ˆ 13.5980(4) Š, and b ˆ 101.855(2)8, Vˆ 3758.3(2) Š at
Tˆ 160 K, space group P21/n, 1calcd ˆ 2.13 gcm 3. Details of the final atomic
coordinates, equivalent isotropic displacement parameters, and fluoride ion
occupancies are given in Table 1. Further details of the crystal structure
investigation may be obtained from the Fachinformationzentrum Karls-
ruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: (‡49)7247-808-
666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the depository number
CSD-408348.
[12] M. E. Davis, R. F. Lobo, Chem. Mater. 1992, 4, 756 ± 768.
[13] R. J. Cernik, W. Clegg, C. R. A. Catlow, G. Bushnell-Wye, J. V.
Flaherty, G. N. Greaves, I. Burrows, D. J. Taylor, S. J. Teat, M.
Hamichi, J. Synchrotron Rad. 1997, 4, 279 ± 268.
[14] G. M. Sheldrick, Programs SADABS and SHELX-97, Göttingen,
1997.
Formation of Isomeric Tetrathiotungstate
Clusters [{Cp*Ru(CO)}2(WS4){W(CO)4}]
*
by the Reaction of [Cp2 Ru2S4] with
[W(CO)3(MeCN)3]
Masahiro Yuki, Masaaki Okazaki, Shinji Inomata, and
Hiroshi Ogino*
Dinuclear transition metal sulfur complexes of the type
5
*
[Cp2 M2S4] (Cp* ˆ h -C5Me5) are excellent starting materials
for the formation of multinuclear transition metal sulfur
clusters.[1] However, surprisingly little is known about reac-
tivity of the iron-[2] or ruthenium-containing complexes.[3] In
*
1994 our group reported the reactions of [Cp2 Fe2S4] with iron
and ruthenium carbonyl complexes to give closo-
[2]
*
[Cp2 (CO)3Fe2MS2] clusters (M ˆ Fe, Ru). Rauchfuss et al.
[4]
*
reported that the ruthenium analogue [Cp2 Ru2S4] reacts
with [Cp*Ru(MeCN)3](PF6) and [Cp*Rh(MeCN)3](PF6)2 to
give [Cp*3Ru3S4](PF6)[3a] and [Cp*3RhRu2S4(MeCN)]-
(PF6)2,[3c] respectively. To examine whether this cluster con-
struction methodology can be applied generally, the thermal
Received: February 27, 1998 [Z11528IE]
German version: Angew. Chem. 1998, 110, 2234 ± 2239
*
reaction of [Cp2 Ru2S4] with [W(CO)3(MeCN)3] was inves-
tigated, and is described here. In the reaction, there was a
dramatic redistribution of CO and S ligands between the W
and Ru centers to give geometric isomers of tetrathiotung-
state clusters [{Cp*Ru(CO)}2(WS4){W(CO)4}]. Furthermore,
we observed thermal and photochemical interconversion
between the two isomers.
Keywords: heterogeneous catalysis ´ microporosity ´ syn-
chrotron radiation ´ zeolites
[1] W. M. Meier, D. H.Olson, C. Baerlocher, Atlas of Zeolite Structure
Types, Elsevier, New York, 1996.
[2] M. E. Davis, C. Saldarriaga, C. Montes, J. M. Garces, C. Crowder,
Nature 1988, 331, 698 ± 699.
[3] M. D. Shannon, J. L. Casci, P. A. Cox, S. J. Andrews, Nature 1991, 353,
417 ± 420.
[4] M. E. Leonowicz, J. A. Lawton, S. L. Lawton, M. K. Rubin, Science
1994, 264, 1910 ± 1913.
*
A solution of [Cp2 Ru2S4] in toluene was added to a solution
of [W(CO)3(MeCN)3][5] (2 equiv) in acetonitrile (Scheme 1).
The mixture was heated at 508C with stirring for 40 minutes.
Evaporation of the volatile components and subsequent flash
chromatographic separation of the residue over silica gel gave
the two red-brown clusters 1 and 2 in 58% and 11% yield.
Elemental analysis and mass spectrometry data indicated that
[5] C. C. Freyhardt, M. Tsapatsis, R. F. Lobo, K. J. Balkus, M. E. Davis,
Nature 1996, 381, 295 ± 298.
[6] a) S. I. Zones, R. A. van Nordstrand, D. S. Santilli, D. M. Wilson, L.
Yuen, L. D. Scampavia in Zeolites: Facts, Figures, Future (Eds.: P. A.
Jacobs, R. A. van Santen) Elsevier, Amsterdam, 1989, pp. 299 ± 309;
b) S. I. Zones, Eur. Pat. Appl. 231018, 1987 [Chem. Abstr. 1987, 107,
179456f]; c) the pattern was indexed with the program TREOR90 and
gave the space group symmetry P21/n, with unit-cell dimensions a ˆ
13.13(1), b ˆ 21.78(1), c ˆ 13.75 (1) Š, and b ˆ 102.61(6)8.
[7] G. Horvath, K. Kawazoe, J. Chem. Eng. Jpn. 1983, 16, 470 ± 475.
[8] C. Y. Chen, L. W. Finger, R. C. Medrud, P. A. Crozier, I. Y. Chan, T. V.
Harris, S. I. Zones, Chem. Commun. 1997, 1775 ± 1776.
[9] J. M. Thomas, J. Klinowski, S. Ramdas, B. K. Hunter, D. T. B.
Tennakoon, Chem. Phys. Lett. 1983, 102, 158 ± 161.
1 and 2 have the same formula Cp2 Ru2W2S4(CO)6,[6] which is
*
consistent with a cubane-type mixed metal ± sulfur cluster
[(Cp*Ru)2{W(CO)3}2(m3-S)4]. However, X-ray diffraction
study disclosed that this is not the case.[7] Cluster 1 was
unequivocally determined to be [{Cp*Ru(CO)}2{W(m3-S)2(m2-
S)2}{W(CO)4}] (Figure 1). Interestingly, according to the
crystal structure of 1, redistribution of CO and S ligands
[*] Prof. H. Ogino, M. Yuki, Dr. M. Okazaki, Dr. S. Inomata
Department of Chemistry, Graduate School of Science
Tohoku University
[10] G. van de Goor, C. C. Freyhardt, P. Behrens, Z. Anorg. Allg. Chem.
1995, 621, 311 ± 314.
[11] H. Koller, A. Wölker, H. Eckert, C. Panz, P. Behrens, Angew. Chem.
1997, 109, 2939 ± 2940; Angew. Chem. Int. Ed. Engl. 1997, 36, 2823 ±
2825.
Sendai 980 ± 8578 (Japan)
Fax : (‡ 81)22-217-6543
E-mail: ogino@agnus.chem.tohoku.ac.jp
2126
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Scheme 1. Synthesis of 1 and 2 in toluene/acetonitrile (508C, 40 min).
Figure 2. Structure of 2 (ellipsoids at the 50% probability level). Selected
bond lengths [Š] and angles [8]: W1 W2 2.997(1), W1 Ru1 2.900(2),
W1 Ru2 2.892(2), W1 S1 2.125(7), W1 S2 2.261(6), W1 S3 2.274(6),
W1 S4 2.276(5), W2 S3 2.558(6), W2 S4 2.566(6), Ru1 S2 2.394(6),
Ru1 S3 2.421(6), Ru2 S2 2.373(6), Ru2 S4 2.408(6), O4 ´´´ O5 3.07(3),
O4 ´´´ O6 3.09(3), O5 ´´´ O6 3.14(3); W2-W1-Ru1 95.45(5), W2-W1-Ru2
95.69(5), Ru1-W1-Ru2 94.55(6), S1-W1-S2 109.9(2), S1-W1-S3 110.2(3),
S1-W1-S4 110.4(3), S2-W1-S3 107.4(2), S2-W1-S4 106.8(2), S3-W1-S4
112.0(2).
between the Ru and W centers was again observed, without
the loss of any ligands. Unlike 1, 2 has a terminal S ligand. The
WS4 fragment is bound to one W atom by two S atoms and to
two Ru atoms by three S atoms, two of which bridge one Ru
and two W atoms in a m3 fashion; the third S atom is bound to
two Ru atoms and one W atom also in a m3 fashion. Three
metal ± metal bonds (W1 W2 2.997(1), W1 Ru1 2.900(2),
and W1 Ru2 2.892(2) Š) are somewhat longer than those in
1. The elongation of the metal ± metal bonds is attributable to
the large steric repulsion between CO ligands on W2 and the
two Ru atoms in 2 (O4 ´´´ O5 3.07(3), O4 ´´´ O6 3.09(3), and
O5 ´´´ O6 3.14(3) Š). All spectroscopic data indicate that the
molecular structure of 2 in the crystal is maintained in
solution.
Figure 1. Structure of 1 (ellipsoids at the 50% probability level). Selected
bond lengths [Š] and angles [8]: W1 W2 2.9053(7), W1 Ru1 2.874(2),
W1 Ru2 2.869(2), W1 S1 2.212(3), W1 S2 2.196(3), W1 S3 2.268(4),
W1 S4 2.272(3), W2 S3 2.551(3), W2 S4 2.553(3), Ru1 S1 2.403(4),
Ru1 S3 2.387(3), Ru2 S2 2.395(3), Ru2 S4 2.384(4), O3 ´´´ O5 3.18(2),
O4 ´´´ O6 3.19(2); W2-W1-Ru1 98.82(3), W2-W1-Ru2 98.98(3), Ru1-W1-
Ru2 162.08(4), S1-W1-S2 111.8(1), S1-W1-S3 106.5(1), S1-W1-S4 109.1(1),
S2-W1-S3 107.6(1), S2-W1-S4 106.9(1), S3-W1-S4 115.0(1).
Investigations of the reactivity of 1 and 2 revealed that
isomerization between 1 and 2 occurred both thermally and
photochemically in solution [Eq. (1)]. When a solution of 1 in
takes place between Ru and W centers without the loss of any
ligands. The W1 atom, with four S ligands, adopts a slightly
distorted tetrahedral geometry with S-W-S angles of
106.5(1) ± 115.0(1)8. The WS4 fragment is bound to one W
atom by two S atoms and to two Ru atoms by four S atoms,
two of which bridge one Ru and two W atoms in a m3 fashion;
the other S atoms are bound to one Ru and one Watom in a m2
fashion. The Ru1 and Ru2 atoms each bear a Cp* and a CO
ligand. The W2 atom, with four CO ligands and two S ligands,
has a distorted octahedral geometry. The three metal ± metal
bond lengths (W1 W2 2.9053(7), W1 Ru1 2.874(2), and
W1 Ru2 2.869(2) Š) lie in the normal range expected for
metal ± metal single bonds. The W S bond lengths in
[D6]benzene was irradiated, the amount of 1 decreased while
the amount of 2 increased. Cluster 1 disappeared completely
after one hour of irradiation, and 2 was formed in approx-
imately 50% yield with the deposition of an unidentified
brown precipitate. Thermolysis of a solution of 2 in [D6]ben-
zene at 808C for two hours caused 2 to disappear completely
to yield 1 in approximately 90% yield. The difference in
thermodynamic stability between 1 and 2 may be due to steric
reasons: The steric crowding between CO ligands on W and
the two Ru atoms is less severe in cluster 1 than in 2.
The tetrathiometalates [MS4]2 have been employed as
building blocks for the synthesis of transition metal clusters
for the last three decades. However, there are still few
examples of [MS4]2 coordinated to organometallic frag-
ments.[8] Our system is the first example in which the [MS4]2
unit combines with three organometallic moieties. The
VI
ˆ
W ( S)4 of 2.196(3) ± 2.272(3) Š are similar to those in
tetrathiotungstate clusters previously reported, and confirm
the unsaturated bonding character. In contrast, the bonds of
the two m3-S atoms (S3, S4) to the octahedral W2 atom
(2.551(3), 2.553(3) Š) are 0.28 Š longer than those to the
tetrahedral W1 atom. This reflects the dative bond character
of the former bonds. The NMR spectroscopic data of 1 are
consistent with the crystal structure (Figure 1).
ˆ
The structure of [{Cp*Ru(CO)}2{W(m3-S)3( S)}{W(CO)4}]
(2) is shown in Figure 2. Redistribution of CO and S ligands
Angew. Chem. Int. Ed. 1998, 37, No. 15
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2127

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density 1.09 eŠ 3. Crystal data for 2 ´ 2CH2Cl2: C28H34Cl4O6Ru2S4W2,
Mr ˆ 1306.47, crystal dimensions 0.30  0.20  0.15 mm3, a ˆ 15.642(8),
b ˆ 11.132(6), c ˆ 23.463(4) Š, b ˆ 96.64(2)8, Vˆ 4058(2) Š3, Tˆ
mechanism and general scope of application of this reaction
are now under investigation.
293 K, monoclinic, space group P21/c (no. 14), Z ˆ 4, 1calcd
ˆ
2.138 gcm 3, 1found ˆ 2.1 gcm 3, F(000) ˆ 2472, m ˆ 68.92 cm 1, Rigaku
AFC6S diffractometer, MoKa radiation (l ˆ 0.71069 Š) graphite mono-
chromator, scan mode w 2q, 2qmax ˆ 55.08, 10167 measured reflec-
tions, Lorentz-polarization, decay (27.4%) and absorption correction
(transmission factors: 0.7988 ± 1.0000), 3634 observed reflections with
(I > 3s(I)), Patterson methods (DIRDIF92 PATTY), full-matrix least-
Experimental Section
*
1 and 2: A solution of [Cp2 Ru2S4] (161 mg, 0.268 mmol) in toluene (15 mL)
was added to a solution of [W(CO)3(MeCN)3] in acetonitrile, which was
prepared by refluxing a solution of [W(CO)6] (189 mg, 0.536 mmol) in
acetonitrile (5 mL). The mixture was heated with stirring at 508C for
40 min. The volatile components were removed under reduced pressure,
and the residue was extracted with toluene/hexane (1/1, 10 mL). Com-
pounds 1 and 2 were obtained from the extracted solution and the insoluble
material as follows: the extracted solution was subjected to flash column
chromatography (silica gel, eluent toluene/hexane (1/1)). A red-brown
fraction was concentrated to give 1 in 58% yield. An analytically pure
sample was obtained by recrystallization from dichloromethane/hexane.
The insoluble dark brown residue was dissolved in dichloromethane and
subjected to flash column chromatography (silica gel, eluent dichloro-
methane). The red-brown fraction was collected, and the solution was
concentrated to give red-brown 2 in 11% yield. Analytically pure 2 was
obtained by recrystallization from toluene/hexane.
squares refinement, 415 parameters, H atoms not located, R ˆ 0.056
3
and Rw ˆ 0.075 (w ˆ 1/(s2Fo)), max. residual electron density 2.12 eŠ
.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no.
CCDC-101083. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
[8] a) K. E. Howard, T. B. Rauchfuss, S. R. Wilson, Inorg. Chem. 1988, 27,
3561 ± 3567; b) P. A. Shapley, Z. Gebeyehu, N. Zhang, S. R. Wilson,
ibid. 1993, 32, 5646 ± 5651; c) S. Ogo, T. Suzuki, K. Isobe, ibid. 1995, 34,
1304 ± 1305; d) B. Zhuang, P. Yu, L. Huang, L. He, J. Lu, Polyhedron
1994 13, 125 ± 131; e) W. J. Evans, M. A. Ansari, J. W. Ziller, S. I. Khan,
1: Elemental analysis calcd for C26H30O6Ru2S4W2: C 27.48, H 2.66; found C
27.83, H 2.74; MS (FAB, Xe, m-nitrobenzyl alcohol matrix): m/z: 1138
Â
Organometallics 1995, 14, 3 ± 4; f) J. Ruiz, V. Rodríguez, G. Lopez, P. A.
Chaloner, P. B. Hitchcock, J. Organomet. Chem. 1995, 493, 77 ± 82; g) Y.
Mizobe, M. Hosomizu, Y. Kubota, M. Hidai, J. Organomet. Chem. 1996,
507, 179 ± 185.
‡
1
[M ]; IR (KBr): nÄmax [cm 1] ˆ 2031, 1975, 1959, 1921, 1848 n(CO) ; H NMR
([D6]benzene): d ˆ 1.68 (s, 30H, Cp*); 13C NMR ([D6]benzene): d ˆ 207.8,
204.3, 200.5 (CO), 100.5 (C5Me5), 10.2 (C5Me5); UV/Vis (toluene): lmax [nm]
(e[cm 1m 1]) ˆ 314 (1.7  104), 349 (1.5  104), 408 (1.2  104), 480 (sh), 535
(sh).
2: Elemental analysis calcd for C26H30O6Ru2S4W2: C 27.48, H 2.66; found C
‡
27.71, H 2.95; MS (FAB, Xe, m-nitrobenzyl alcohol matrix): m/z: 1138 [M ];
IR (KBr): nÄmax [cm 1] ˆ 2019, 1975, 1905(sh), 1882 n(CO); 1H NMR ([D6]ben-
zene): d ˆ 1.59 (s, 30H, Cp*); 13C NMR ([D2]dichloromethane): d ˆ 209.7,
208.1, 205.3, 196.2 (CO), 100.4 (C5Me5), 10.3 (C5Me5); UV/Vis (toluene):
lmax [nm] (e[cm 1m 1]) ˆ 325 (2.4  104), 429 (1.1  104), 510 (8.2  103), 682
(2.3 Â 103).
[Re5(m-H)4(CO)20] and [Re5(m-H)5(CO)20],
Two Isolobal Analogues of Cyclopentane
Mirka Bergamo, Tiziana Beringhelli, Giuseppe
DꢁAlfonso,* Pierluigi Mercandelli, Massimo Moret,*
and Angelo Sironi
Received: February 13, 1998 [Z11478IE]
German version: Angew. Chem. 1998, 110, 2232 ± 2234
The ReH(CO)4 fragment, isoelectronic with d8 M(CO)4,
can be considered isolobal with (singlet) methylene,[1] as far as
the formation of metal ± metal interactions is concerned, since
its frontier orbitals allow the interaction with two metal
centers. The known [ReH(CO)4]n oligomers (n ˆ 2 ± 4)[2] are
therefore isolobal analogues of the corresponding (CH2)n
species: [Re2(m-H)2(CO)8] is an ethylene-like molecule,[2b]
and the triangular and square-planar clusters [Re3(m-
H)3(CO)12] and [Re4(m-H)4(CO)16] ªcorrespondº to cyclo-
propane and cyclobutane, respectively. Interestingly, until
now no organometallic analogue of the most stable (CH2)n
oligomers (i.e., those with n ˆ 5 or 6) was known. The
pentanuclear cyclic clusters [Re5(m-H)5 n(CO)20]n (n ˆ 0, 1)
reported here fill this gap, at least partially.
Keywords: clusters ´ isomerizations ´ ruthenium ´ sulfur ´
tungsten
[1] J. Wachter, Angew. Chem. 1989, 101, 1645 ± 1658; Angew. Chem. Int.
Ed. Engl. 1989, 28, 1613 ± 1626, and references therein.
[2] T. Mitsui, S. Inomata, H. Ogino, Inorg. Chem. 1994, 33, 4934 ± 4936.
[3] a) E. J. Houser, H. Krautscheid, T. B. Rauchfuss, S. R. Wilson, J. Chem.
Soc. Chem. Commun. 1994, 1283 ± 1284; b) Q. Feng, T. B. Rauchfuss,
S. R. Wilson, J. Am. Chem. Soc. 1995, 117, 4702 ± 4703; c) A. Venturelli,
T. B. Rauchfuss, A. K. Verma, Inorg. Chem. 1997, 36, 1360 ± 1365.
[4] T. B. Rauchfuss, D. P. S. Rodgers, S. R. Wilson, J. Am. Chem. Soc. 1986,
108, 3114 ± 3115.
[5] D. P. Tate, W. R. Knipple, J. M. Augl, Inorg. Chem. 1962, 1, 433 ± 434.
[6] Only one example of structurally characterized geometric isomers is
known for sulfur clusters: a) P. Braunstein, J. M. Jud, A. Tiripicchio, M.
Tiripicchio-Camellini, E. Sappa, Angew. Chem. 1982, 94, 318 ± 319;
Angew. Chem. Int. Ed. Engl. 1982, 21, 307 ± 308; b) P. D. Williams, M. D.
Curtis, D. N. Duffy, W. M. Butler, Organometallics 1983, 2, 165 ± 167.
[7] Crystal data for 1: C26H30O6Ru2S4W2, Mr ˆ 1136.60, crystal dimensions
0.50  0.40  0.25 mm3, a ˆ 22.41(1), b ˆ 13.432(6), c ˆ 11.148(3) Š,
b ˆ 90.07(3)8, Vˆ 3355(2) Š3, Tˆ 293 K, monoclinic, space group P21/
We recently exploited the s-donor capability of transition
metal hydrides[3±5] in the synthesis of open-chain tri- and
[*] Prof. G. DꢁAlfonso, Dr. M. Bergamo, Prof. T. Beringhelli
Dipartimento di Chimica Inorganica,
Metallorganica e Analitica e Centro CNR CSSMTBO
Via Venezian 21, I-20133 Milano (Italy)
Fax: ( ‡ 39)2-2362748
3
3
n
(no. 14), Z ˆ 4, 1calcd ˆ 2.249 gcm
,
1found ˆ 2.23 gcm
,
F(000) ˆ
2136, m ˆ 80.08 cm 1, Rigaku AFC6S diffractometer, MoKa radiation
(l ˆ 0.71069 Š), graphite monochromator, scan mode w 2q, 2qmax
ˆ
E-mail: dalf@csmtbo.mi.cnr.it
55.08, 8247 measured reflections, Lorentz-polarization and absorption
correction (transmission factors: 0.7316 ± 1.0000), 4762 observed re-
flections with (I > 3s(I)), Patterson methods (DIRDIF92 PATTY),
full-matrix least-squares refinement, 361 parameters, H atoms not
located, R ˆ 0.045 and Rw ˆ 0.065 (w ˆ 1/(s2Fo)), max. residual electron
Dr. M. Moret, Dr. P. Mercandelli, Prof. A. Sironi
Dipartimento di Chimica Strutturale e
Stereochimica Inorganica e Centro CNR CSSMTBO,
Via Venezian 21, I-20133 Milano (Italy)
2128
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1433-7851/98/3715-2128 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1998, 37, No. 15