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Do Cp(CO)2Mn Fragments Stabilize Radicals.

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decomposition explain in a simple manner the product 7,
which is formed in a cheleotropic reaction from 2,3-dimethyl-l,3-butadiene and the sulfur dioxide already present.["] The formation of 8 has not yet been unequivocally
explained. 8 is probably formed by reduction of 5 by the
elemental sulfur that was formed earlier."31 The intermediacy of 9 is supported, moreover, by the fact that the
lachrymatory factor of onions, Z-thiopropionaldehyde Soxide 11, dimerizes to the stable E-3,4-diethyl-l,2-dithietane 1,l-dioxide 12."'' Furthermore, it was possible to demonstrate the existence of 9 in the crude product by mass
spectroscopy [m/z 412 (M')]. All attempts to obtain pure 9
have been unsuccessful and have led exclusively to 6.
When the reaction of 1 and 2 is carried out in the absence
of a trapping agent for sulfine, 6 is again the only product
isolated in pure form.
[I] R. W. Saalfrank, W. Rost, F. Schutz, U. Ross, Angew. Chem. 96 (1984)
597; Angew. Chem. Int. Ed. Engl. 23 (1984) 637; R. W. Saalfrank, F.
Schutz, U. Moenius, Synthesb. in press.
[21 Transallenation has played only a minor role in cumulene chemistry up
to now. Cf. S. L. Buchwald, R. H. Grubhs, J. Am. Chem. Sor. 105 (1983)
5490.
I31 R. W. Saalfrank, W. Rost, Angew. Chem. 95 (1983) 328; Angew. Chem.
I n ( . Ed. Engl. 22 (1983) 321; Angew. Chem. Suppl. 1983. 451.
[4] Tetraethoxyallene I is prepared from tetraethoxyethylene [5] in analogy
to the synthesis of tetramethoxyallene [6].
[51 D. Bellus, H. Fischer, H. Greuter, P. Martin, Helu. Chim. Aeta 61 (1978)
1784.
[6] R. W. Hoffmann, W. Schafer, U. Bressel, Chem. Ber. 105 (1972) 21 I I ; R.
D. Mackenzie, T. R. Blohm, J. M. Grisar, J. Med. Chem. 16 (1973)
688.
[7] Allene 1 (10 mmol) in 50 mL of toluene at - 78°C was treated with a solution of 2 (10 mmol). After 30 min, 2,3-dimethyl-1,3-hutadiene
(50
mmol) was added at - 50°C. The reaction mixture was allowed to warm
over 4 h to room temperature and chromatographed on silica gel 60 FZS4,
ether/n-hexane (1 :I). 5 and 8 were then distilled. 6 [ether/n-hexane
(2 : I ) ] and 7 (ether) were recrystallized. Only 15 was formed with disulfur dichloride [crystals from e t h e r h h e x a n e ( I : I)].
[8] Cf. P. A. T. W. Porskamp, M. van der Leij, B. H. M. Lammerink, B.
Zwanenburg, Red. Traa. Chim. Pays-Bas 102 (1983) 400: 101 (1982) 1.
(91 For the mechanism of dimerization of 4 to 9, see [lo].
[lo] E. Block, A. A. Bazzi, L. K. Revelle, J . Am. Chem. Soc. I02 (1980)
2490.
[ I l l Cf. S. S. Bhattacharjee, H. Ila, H. Junjappa, Synthesis 1984. 249.
1121 H . J. Backer, J. Strating, C. M. H. Cool, Red. Trao. Chim. Pays-Bas 58
(1939) 778.
[I31 Under the reaction conditions used, 5 is not reduced to 8 by Sa.
1141 Cf. the review on the possibility of a tautomeric equilibrium
CI-S-S-CI+S=SCl2
[15].
[IS] G . W. Kutney, K. Turnbull, Chem. Rev. 1982. 333.
[I61 A. Senning, Angew,. Chem. 91 (1979) 1006; Angew. Chem. I n t . Ed. Engl.
18 (1979) 941.
1171 Cf. G . W. Kutney, J. W. J. Still, Can. J . Chem. 58 (1980) 1233.
[I81 Conceivably, no thiosulfine 13 is formed and 15 arises via dimerization
of I . This possibility is unlikely because of the failure to detect 11 (1 mol
112 mol CI-S-S-CI)
or 111 ( I mol 1 / 0 3 mol Cl-S-S-Cl).
Table I . Physical data for the compounds 5 , 6, 7 , 8 , and 15
5 : IR (film): v= 1730 c m - ' (C=O); "C-NMR (CDCI?): 6 = 13.92 and 13.98
(CH?), 19.26 and 20.32 (CH,), 28.15 and 51.57 (CH,), 62.55 and 63.07
(OCH2), 70.53 (C,), 116.33 and 125.43 (C=), 165.36 and 165.75 (C=O);
b.p.= 117"C/O.I torr
6 : IR (KBr): v=1735 c m - ' (C=O); "C-NMR (CDCI,): 6=13.83 (4CH3),
62.52 (40CH2), 135.35 (2C=), 162.29 (4C=O): m.p.=56"C
7 :IR (KBr): v = 1300 and Ill0 c m - ' (SO2); ' T - N M R (CDCI,): 6 ~ 1 4 . 5 6
(2CH,), 60.61 (2CH2), 125.50 (2C=); m.p.= 135°C
8 : IR (film): ~ ~ 1 7 c2m5- ' (C=O); ',C-NMR (CDCI,): 6=13.89 (ZCH,),
19.05 and 19.96 (2CH3), 30.76 and 37.68 (CH2), 56.67 (CJ, 62.09 (20CH2),
122.83 and 125.31 (C=), 168.66 (2C=O): b.p.=97"C/0.1 torr
1 5 : IR (KBr): v = 1720 (C=O): I3C-NMR (CDCI?): 6 = 13.92 (4CH,), 62.52
(2C,), 63.77 (4CHZ), 165.84 (4C=O); m.p.=56"C
C0,Et
If tetraethoxyallene 1 is reacted with disulfur dichloridell'l instead of with thionyl chloride 2, no diethyl thioxomalonate S-sulfide 13 can be trapped with 2,3-dimethyl1,3-butadiene. (13 possibly exists in equilibrium with the
isomeric diethyl dithiiranedicarboxylate 14[15,l6].) Instead,
15["l
tetraethyl 1,2,4,5-tetrathiane-3,3,6,6-tetracarboxylate
is isolated in 83% yield (Table 1). We assume that 15 is
formed by dimerization of the isomers 13/14."*l Apparently, 13 and/or 14 d o not eliminate sulfur, since no diethyl
thioxomalonate 16 or its dimer can be dete~ted."'.'~]
OEt
/
I
EtO
cl\
*E
C
tl
c1
S-S
S-S
XCOZM
COzEt
II
CO E t
E t O ' k f 6 O E t
C02Et
OEt
rn
1191 K. Oka, J. Org Chem. 44 (1979) 1736
Do Cp(CO)*Mn Fragments Stabilize Radicals?**
(EtO,C),C=S=S
13
s-s
(EtOZC),
(s-s ) (CO,Et),
15
t-
It *
-s
(EtO,C),C=S
16
Received: February I I , 1985;
revised: April 9, 1985 [Z I164 IE]
Publication delayed at authors' request
German version: Angew. Chem. 97 (1985) 870
CAS Registry numbers:
1 , 85152-89-2: 2, 7719-09-7 4, 98194-64-0: 5 , 96745-90-3: 6, 6174-95-4; 7 ,
18214-56-7; 8 , 66950-00-3; 15, 98194-65-1; 2,3-dimethyl-1,3-butadiene,
5138 1-5; toluene, 108-88-3, sulfur dichloride, 10545-99-0.
856
0 VCH VerlagsgesellschaJi mbH, D-6940 Wemheim. 1985
By Renate Gross and Wolfgang Kaim*
Cp(CO)*Mn fragments [Cp =q'-CsH5(Cpo), q'-C5H4Me
(Cp'), q5-C,Mes(Cp')] can stabilize diamagnetic, unstable
molecules."] Three recent studies were concerned with the
formation of radical complexes involving these metal fragment~,'*-~'
and although each of the complexes described
has one unpaired electron, they differ significantly in the
distribution of spin. For anionic, binuclear coordination
compounds of pyrazine, the unpaired electron was shown
by ESR spectroscopy to be localized largely in the n-system of the heterocyclic ligand;[31 for the complexes
[Cp(CO),(L)Mn] (L = NH(m-C6H4CH,),[21 StBu, and
SePht4]), only a large "Mn coupling constant was ob["I Priv.-Doz. Dr. W. Kaim, Dipl.-Chem. R. Gross
lnstitut fur Anorganische Chemie der Universitat
Niederurseler Hang, D-6000 Frankfurt am Main SO (FRC)
[**I This work was supported by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, the Hermann-Willkomm-Stiftung, the
BASF AG, the Messer Griesheim GmhH, and the Karl-Winnacker-Stiftung of Hoechst AG.
0570-0833/85/1010-0856 $ 02.50/0
Angew. Chem. In,. Ed. Engl. 24 (1985) N o 10
served. We report here on the ESR spectroscopic distinction between genuine radical complexes and those compounds that must be described as low-spin Mn" systems.
We were able to obtain the radical anions of 4-cyanopyridine 1 , 4,4'-bipyridine 2, and terephthalonitrile 3 with
coordinated Cp'(CO),Mn' fragmentsfs1analogously to the
preparation of pyrazine radical complexes.[31At room temperature, 1 ' forms an N ' - c o ~ r d i n a t e d ~mononuclear
~~I
radical complex (Fig. 1). 2" and 3" yield binuclear
complexes, even though the neutral ligand 3 only affords
the mononuclear complexes [Cp(CO),( 3)Mnl
this is
due to the increased basicity of reduced ligands.[61 In all
these cases, the ESR data (Table 1) indicate that radical
complexes are present, i.e., the spin distribution is little
changed compared with that in the free radical anions"]
and the y factors are only slightly smaller than gelectron=
2.0023."'
'
4
5
g
u(I4N) o('H)
Radical anion complexes [b]
[ ( p y r a ~ i n e ~ ~ ) ( C p ' ( C O ) ~ M[3]
n J 2 11.9997 0.750
[Cp'(CO)2(1'")MnI
2.0005 0.655
0.863
0.725
[(2 0e)K'~'(CO)2Mn121
0.463
2.001 1 0.463
I
7
6
Table 1. ESR data of paramagnetic Cp(CO)>Mn complexes [a].
Compound
plexes: [CpCI(D),Mn"] ( D = amine or phosphane) are
high-spin
and a high-spin/low-spin equilibrium is observed in the manganocenes, Cp,Mn,["] so that
a low-spin complex is expected for [Cp(CO),(L)Mn"] with
the strong ligands CO and L. Electrochemical investigat i o n ~ [ ~ . show that the one-electron oxidation of
[Cp(CO),(L)Mn'] is extremely facilitated by nucleophilic
ligands L: the difference between E,, = + 1.24 V for
L = C O and E,, = + 0.09 V for L = piperidinefI2] suggests
that the extremely strong nucleophiles 'NR2 and 'SR
should effectively stabilize the oxidized state, that is, Mn" ;
indeed, negative potentials relative to the saturated calomel electrode are found for [Cp5(C0)2(L)Mn""]:
E = -0.87 V ( L = 5[I3l)and E = -0.98 V (L= 'StBd4I).
10 mT
Fig. 1. The ESR spectra of [Cp5(CO)2(5)Mn]( A ) and [Cp'(C0)2(I"o)Mn]
( B ) at room temperature in toluene and THF, respectively, illustrate the difference between localization of the unpaired electron on the metal and in the
n-system of the radical ligand. In A, only a large "Mn coupling constant is
observed ( I = 5/2, 100% natural abundance), whereas spectrum B exhibits
extensive hyperfine coupling of the numerous magnetically active nuclei of
the radical ligand (486 theoretical lines).
0.226 (4 H)
0.21 1 (2 H, 1 N)
0.154(2H)
0.207 (4 H)
0.050 (4 H)
0.154 (4H)
Further evidence for strongly differing electronic configurations
is provided by the opposite deviations of the g
[(3°0)K~'(CO)2Mn121
2.0001 0.457
0.283
factors from gelecIrOn
(Table 1): whereas the radical anion
Mn" systems
complexes experience only a small lowering of the g fac[Cp1(C0)d4)MnI
2.0086 4.24
[cl
tor, on account of low-lying unoccupied metal orbitals,[31
[Cpo(CO)2(m-C7H7NH)Mn]
[2]
2.0114 5.0
the manganese(r1) complexes exhibit significantly higher g
[Cp5(C0)2(5)Mn1
2.014 5.09
values, which-in agreement with theoretical investiga2.022 5.5
[CP '(COM 6)Mnl
tions on Cp(CO),M c~mplexes['~~-indicate
doubly occu~ICp'(COh(7)MnI
2.035 6.1
[CP~(CO)~(~B~S
PI) M ~ I
2.031 5.14
pied levels close to the singly occupied orbital.[3. The re[ C ~ " ( C o ) ~ ( P h s e ) M Pn lI
2.069 4.61
latively strong absorptions of some of these Mn" compounds
in the v i ~ i b I e [ ' ~can
~ ~ 'accordingly
~
be interpreted as
[a] Coupling constants u [mTJ [b] In THF, counterion K O . Coupling conLMCT (ligand-to-metal charge-transfer) transitions, in
stants determined by computer simulation. [c] Insufficiently resolved superhyperfine coupling.
analogy to corresponding Cu" and Ru"'
It is
only in this charge-transfer excited state ( d 6 / 5 n t d 5 / 6 n )
that the unpaired electron is localized on the ligand (radical complex) and, as expected, the LMCT excitation enerThe complexes of Cp(CO)2Mn fragments with the degies, the g factors, and the 55Mn coupling constants deprotonated amine ligands 4-7,['l prepared according to the
crease as the electrons on the ligands become more availmethod of Sellmann, Miiller et a1.,[21and the corresponding
able, such as in the series 7 , 6 , 5 , 4. In the compound
sulfur and selenium
show much larger 55Mncou[Cp'(CO)2(4)Mn],the separation between the ground state
pling constants than d o radical complexes (Fig. I), exhibit
and the LMCT excited state has already decreased to
no detectable interaction of the unpaired electron with the
= 11 500 crn-I, so that the first evidence of a super-hypercoordinated ligands, except for the special case
fine interaction with the ligand 4 is recognizable (Table 1).
[Cp'(CO)2(4)Mn]J'1 and have isotropic g factors that are
This, species, which is stabilized by the extreme donor efsignificantly larger than gelrEirOn
(Table 1). These results d o
fect of a deprotonated p-phenylenediamine ligand, must
not support their formulation as "radical c ~ m p l e x e s , " [ ~ ~ ~ ] nonetheless be viewed as a Mn" system.
but rather indicate the presence of low-spin rnangaThe Cp(CO)2Mn fragments thus form two different
nese(ii),["' i.e., d' configuration of the metal. Further arguclasses of stable paramagnetic complexes: as neutral and
ments in favor of this interpretation are provided by comdiamagnetic Mn' fragments with d6 metal configuration,
parison with other cyclopentadienyl manganese(1r) cornthey can stabilize radical anion ligands by z back-bonding.
~
Angew. Chrm I n t . Ed. Engl. 24 (1985) No. I0
0 VCH VerlagsgesellschaJi mbH, 0-6940 Weinherm. 1985
0570-0833/85/1010-0857
S 02.S0/0
857
In contrast, the less common oxidized form of the organometal fragment with the metal center present as low-spin
Mn" is
if very nucleophilic diamagnetic ligands are employed.
Received: June 27, 1985;
supplemented: July 25, 1985 [Z 1367 IE]
German version: Angew. Cbem. 97 (1985) 869
CAS Registry numbers:
4 - H t , 99-98-9; 5 - H t , 106-49-0; 6 - H f , 504-24-5; 7 - H + , 288-32-4;
[Cp '(CO),( l)Mnl,
98 104-71-3;
[( Z){Cp'(CO),Mn},],
9 1798-54-8 :
[(3)[Cp'(CO),MnJ,I,
98104-72-4;
[Cp'(CO)2(4)Mn],
98104-73-5;
[CpS(CO),(5)Mnl,
98104-74-6;
[ C ~ ' ( C 0 ) ~ ( 6 ) M n l , 98104-75-7;
[Cp ' ( C 0 ) 23)Mn],
(
98 104-76-8;
[Cp '(C0),(7 )Mnl,
98 126- 12-6;
[Cp'(CO),(thf)Mn],
51922-84-0;
[Cp'(C0),(4-Ht)Mn],
98104-77-9;
[Cp'(CO),(S-H +)Mn], 98 104-78-0; [Cp1(CO),(6-H +)Mn], 98 104-79- 1 ;
[Cp'(CO)2(7-H+)Mnl,
98104-80-4;
[Cps(CO),(thf)Mn],
86155-78-4;
Cp(CO),Mn, 38548-46-8.
[ I ] K. G. Caulton, Coord. Cbem. Reu. 38 (1981) I ; for another example, see
I.-P. Lorenz, J. Messelhauser, W. Hiller, K. Haug, Angew. Chem. 97
(1985) 234; Angew. Chem. In/. Ed. Engl. 24 (1985) 228.
[21 D. Sellmann, J. Miiller, P. Hofmann, Angew. Chem. 94 (1982) 708; Angew. Chem. I n t . Ed. Engl. 21 (1982) 691; D. Sellmann, J. Miiller, J. Organomet. Chem. 281 (1985) 249.
[ 3 ] R. Gross, W. Kaim, Angew. Chem. 96 (1984) 610; Angew. Chem. Int. Ed.
Engl. 23 (1984) 614; Inorg. Chem., in press.
[4] A. Winter, G. Huttner, L. Zsolnai, P. Kroneck, M. Gottlieb, Angew.
Chem. 96 (1984) 986; Angew. Chem. 1111. Ed. Engl. 23 (1984) 975; A.
Winter, G. Huttner, M. Gottlieb, I. Jibril, J. Organomet. Chem. 286
(1985) 317.
[5] a) Synthesis of radical complexes by reduction of neutral complexes
with potassium in tetrahydrofuran (THF) under high vacuum. b)
[Cp'(CO)2(1)Mn]: R. Gross, W. Kaim, J . Organomet. Cbem., 292 (1985)
C21; c) [(2){Cp'(C0)2Mn)2]:P. L. Gaus, N. Marchant, M. A. Marsinek,
M. 0. Funk, Inorg. Chem. 23 (1984) 3269; c) [Cp'(CO)2(3)Mn]: cf. M.
Herberhold, H. Brabetz, Chem. Ber. 103 (1970) 3896, 3909.
[6] Cf. K. Wieghardt, H. Cohen, D. Meyerstein, Angew. Chem. YO (1978)
632; Angew. Chem. I n t . Ed. Engl. 17 (1978) 608.
[7] Cf. F. Gerson: Hocbaujlosende ESR-Spektroskopie. Verlag Chemie,
Weinheim 1967, and references cited therein.
[8] a) Preparation of the Mn" complexes by oxidative deprotonation of amine complexes with PbO, in toluene [2] or THF. Stability and amount of
synthesized Mn" complex decrease in the series 4, 5 , 6, 7 with decreasing donor ability of the ligand. lsolable are [Cp'(CO)2(4)Mn]: IR
(THF): v(C0)=1940, 1883 c m - ' ; UV (THF): d,,,=870
nm;
[CpS(CO),(5)Mn]: IR (THF): v(CO)= 1940, 1880 c m - ' ; UV (THF):
A,, = 687 nm; b) amine complexes were synthesized from photolytically generated [Cp(CO),(thf)Mn] and the neutral ligands. [ C P ' ( C O ) ~ ( ~ H@)Mn]:IR(THF): v(CO)= 1900, 1820cm-'; UV(THF):dm,,=385 sh,
448 sh nm: [CpS(C0),(5-He)Mn]: IR (THF): v(CO)=1895, 1820 c m - ' ;
(cf. [2]); [Cp'(CO),(6-He)Mn]: IR(THF): v(CO)= 1915, 1838 c m - ' : UV
(THF): 1,,,=380 sh, 450 sh nm; ' H - N M R (C,D,): S = 1.34 (3H), 2.64
(2H, br.), 4.03 (2H). 4.26 (2H), 5.23 (2H), 7.99 (2H), each s ;
[Cp'(CO):(7-Ha)Mn]: IR (THF): v(CO)= 1918, 1845 c m - ' ; UV (THF):
A,,,=375 sh, 440 sh nm; 'H-NMR (C2D,SO): 6 = 1.35 (3H), 4.14 (2H),
4.48 (2H), 6.70 (1 H), 7.06 (1 H), 7.65 (1 H), 12.4 (1 H, br.), each s.
[9] Arylaminyl radicals exhibit considerable spin delocalization into the
aromatic n-system: F. A. Neugebauer in Landolt-Bornstein New Series,
11/9c 1, p. 9.
[lo] a) F. H. Kohler, N. Hebendanz, U. Thewalt, B. Kanellakopulos, R.
Klenze, Angew. Chem. 96 (1984) 697; Angew. Chem. Int. Ed. Engl. 23
(1984) 721; b) J. Heck, W. Massa, P. Weinig, ibid. 96 (1984) 699 and 23
(1984) 722.
[ I I] a) M. E. Switzer, R. Wang, M. F. Rettig, A. H. Maki, J. Am. Chem. Sor.
96 (1974) 7669; b) J. H. Ammeter, R. Bucber, N. Oswald, ibid. 96 (1974)
7833.
[I21 J. W. Hershberger, R. J. Klingler, J. K. Kochi, J . Am. Chem. Soc. 105
(1983) 61.
1131 CH,CN, electrolyte 0.1 M nBurCI04: glassy carbon working electrode vs.
saturated calomel reference electrode; T = 2 5 T , u = 100 mV/s.
[I41 B. E. R. Schilling, R. Hoffmann, D. L. Lichtenberger, J . Am. Chem. Sac.
I01 (1979) 585.
[I51 W. Kaim, Inorg. Chem. 23 (1984) 3365.
[I61 K. Krogh-Jespersen, H. J. Scbugar, Inorg. Chem. 23 (1984) 4390.
[I71 For stabilization of organometal fragments in high oxidation states, see
W. A. Herrmann, R. Serrano, H. Bock, Angew. Cbem. 96 (1984) 3 6 5 ; Angew. Chem. Int. Ed. Engl. 23 (1984) 383.
[IS] A binuclear Mn"/Mn' compound with thiolate ligands was recently
characterized at -40°C; J. W. McDonald, Inorg. Chem. 24 (1985) 1734;
the first Mn"/Mn' complexes that are stable at room temperature can
be detected in the oxidative reaction of [Cp'(CO),(thf)Mn] with alkoxides: R. Gross, W. Kaim, unpublished results.
858
0 VCH Verlagsgesellschaj2 mbH. 0-6940 Weinheim. 1985
Reactions in the Ligand Sphere of Iron([[):
Synthesis of Crown Ethers
By Kurt Meier* and Grety Rihs
Cationic arene-iron complexes are thermally and chemically very stable, but, upon excitation with light, can undergo ligand exchange reactions with high quantum
yields.['-31 Thus, irradiation of the toluene complex 1 in
aprotic solvents in the presence of the ligands L = C O ,
R N C results in the formation of the isolable complexes 2 ;
in the absence of suitable ligands, one-half molar equivalent of both ferrocene 3 and iron(i1) hexafluorophosphate
4 are formed.
2
1
1/2
J
9
&+
1 / 2 Fe(PF6)2
3
4
The Lewis acids generated by irradiation are active for
the polymerization of bi- and multifunctional epoxides to
give insoluble pol yet her^.[^.^] These photoinitiator properties of arene-iron complexes have found technical applications in resist and surface-protection technology.
When complex 1 is irradiated in dichloromethane in the
presence of a tenfold excess of ethylene oxide, one-half
molar equivalent of ferrocene 3 is again formed along with
Y F ,
A/
Fig. 1. ORTEP plot f: the structure of the cation of 5 . Aba 2, a = 17.819,
b=20.663, c=8.882 A, Z = 4 : R=0.112 for 1910 observed reflections
(1>2u(l)). The cation bas C 2 symmetry. The mean Fe-0 bond length is
2.40
The relatively poor R factor is due to disorder of the -CH2-CH2bridges. Further details on the crystal structure investigation are available
from the director of the Cambridge Crystallographic Data Centre, University
Chemical Laboratory, Lensfield Road, Cambridge CB2 I E W (England) by
giving the full journal citation.
A.
[*I Dr. K. Meier
Zentrale Forschungslaboratorien der Ciba-Geigy AG
CH-4002 Basel (Switzerland)
G. Rihs
Zentrale Funktion Forschung, Abteilung Physik, Ciba-Geigy AG
CH-4002 Basel (Switzerland)
0570-0833/8S/l0l0-0858 3! 02.50/0
Angew. Chem. Inr. Ed. Engl. 24 (1985) No. 10
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