Reactivity of "GaI" toward N-substituted 1,4-diazabuta-1,3-dienes: Synthesis and characterization of gallium heterocycles containing paramagnetic diazabutadiene monoanions

Article abstract of DOI:10.1021/om0200510

The paramagnetic heterocycles N(R)C₂H₂N(R)GaI₂ (R = 2,6-dimethylphenyl (1), 2,4,6-trimethylphenyl (2), and 2,6-diisopropylphenyl (3)) were obtained by reaction of "GaI" with Xyl-dab (N,N′-bis[2,6-dimethylphenyl]-l,4-diazab

Full text of DOI:10.1021/om0200510

Organometallics 2002, 21, 3169-3172
3169
Rea ctivity of “Ga I” Tow a r d N-Su bstitu ted
1,4-Dia za bu ta -1,3-d ien es: Syn th esis a n d Ch a r a cter iza tion
of Ga lliu m Heter ocycles Con ta in in g P a r a m a gn etic
Dia za bu ta d ien e Mon oa n ion s
Thomas Pott,† Peter J utzi,*,† Wolfgang Kaim,‡ Wolfgang W. Schoeller,†
Beate Neumann,† Anja Stammler,† Hans-Georg Stammler,† and
Matthias Wanner‡
Fakulta¨t fu¨r Chemie, Universita¨t Bielefeld, Bielefeld, Germany, and Institut fu¨r
Anorganische Chemie, Universita¨t Stuttgart, Stuttgart, Germany
Received J anuary 24, 2002
The paramagnetic heterocycles N(R)C2H2N(R)GaI2 (R ) 2,6-dimethylphenyl (1), 2,4,6-
trimethylphenyl (2), and 2,6-diisopropylphenyl (3)) were obtained by reaction of “GaI” with
Xyl-dab (N,N′-bis[2,6-dimethylphenyl]-1,4-diazabuta-1,3-diene), Mes-dab (N,N′-bis[2,4,6-
trimethylphenyl]-1,4-diazabuta-1,3-diene), and Dipp-dab (N,N′-bis[2,6-diisopropylphenyl]-
1,4-diazabuta-1,3-diene), respectively. Compounds 1-3 were characterized structurally by
X-ray crystallography. ESR investigations of 3 and quantum chemical calculations for the
parent system NHC2H2NHGaI2 were carried out to provide information on the bonding. The
reaction of 1 with 1 equiv of the 1,4-dilithiated Xyl-dab led to the formation of Ga(Xyl-dab)2
(4) with one singly and one doubly reduced ligand; this compound was also characterized by
X-ray crystallography.
In tr od u ction
1,4-Diazabuta-1,3-diene (dab) ligands with bulky sub-
stituents in 1,4-position have been used as chelating
ligands in the chemistry of gallium(III) compounds. The
resulting heterocycles can be classified according to the
formal oxidation state of the dab ligand (Figure 1).
Whereas in compounds of type I the dab system is
present as an ene-diamido dianion, it occurs as a
monoanion in type II species. In compounds of type III
the dab system acts as a neutral ligand. Heterocycles
of type I are well known; they can be prepared by the
reaction of organogallium dihalides with the dilithiated
dab systems1-5 or by oxidative addition of neutral
diazabutadienes to Cp*Ga(I).6 In contrast, type II and
III heterocycles are less well known. The only example
of a type II heterocycle is Ga(But-dab)2 (But-dab ) N,N′-
di-tert-butyl-1,4-diazabuta-1,3-diene).6,7 In this com-
F igu r e 1. Possible oxidation states of the dab ligand in
gallium-dab heterocycles.
pound one dab unit has undergone a two-electron
reduction (type I), whereas the other dab unit is singly
reduced (type II). Comparable paramagnetic complexes
(bpy)GaR2 with the aromatic R-diimine 2,2′-bipyridine
were studied by ESR.8 The ionic compound [(But-dab)-
GaCl2][GaCl4] is the only example of a type III hetero-
cycle.9
“Green’s GaI”10 was employed as a facile starting
material for the synthesis of novel gallium compounds
during the last years.11a-e We report here the reactions
of “Green’s GaI” with differently N-substituted 1,4-
diazabuta-1,3-dienes in which gallium heterocycles
containing paramagnetic diazabutadiene monoanions
are formed. ESR studies and quantum chemical calcula-
§ Dedicated to Prof. H. Bu¨rger, University of Wuppertal, on the
occasion of his 65th birthday.
* Corresponding author.
† Universita¨t Bielefeld.
‡ Universita¨t Stuttgart.
(1) Brown, D. S.; Decken, A.; Cowley, A. H. J . Am. Chem. Soc. 1995,
117, 5421.
(2) Cowley, A. H.; Brown, D. S.; Decken, A.; Kamepalli, S. Chem.
Commun. 1996, 2425.
(3) Schmidt, E. S.; J okisch, A.; Schmidbaur, H. J . Am. Chem. Soc.
1999, 121, 9758.
(4) Schmidt, E. S.; J okisch, A.; Schmidbaur, H. J . Chem. Soc., Dalton
Trans. 2000, 1039.
(5) Schmidt, E. S.; Schier, A.; Schmidbaur, H. J . Chem. Soc., Dalton
Trans. 2001, 505.
(6) Pott, T.; J utzi, P.; Neumann, B.; Stammler, H.-G. Organome-
tallics 2001, 20, 1965.
(7) Cloke, F. G. N.; Hanson, G. R.; Henderson, M. J .; Hitchcock, P.
B.; Raston, C. L. J . Chem. Soc., Chem. Commun. 1989, 1002. See also:
Kaim, W.; Matheis, W. J . Chem. Soc., Chem. Commun. 1991, 597.
(8) Hasenzahl, S.; Kaim, W.; Stahl, T. Inorg. Chim. Acta 1994, 225,
23.
(9) Clyburne, J . A. C.; Culp, R. D.; Kamepalli, S.; Cowley, A. H.;
Decken, A. Inorg. Chem. 1996, 35, 6651.
(10) Green, M. L. H.; Mountford, P.; Smout, G. J .; Speel, S. R.
Polyhedron 1990, 9, 2763.
(11) (a) Kuchta, M. C.; Bonanno, J . B.; Parkin, G. J . Am. Chem.
Soc. 1996, 118, 10914. (b) Linti, G.; Rodig, A. J . Chem. Soc., Chem.
Commun. 2000, 127. (c) Kehrwald, M.; Ko¨stler, W.; Rodig, A.; Linti,
G. Organometallics 2001, 20, 860. (d) Linti, G.; Li, G.; Pritzkow, H. J .
Organomet. Chem. 2001, 626, 82. (e) Linti, G.; Rodig, A.; Ko¨stler, W.
Z. Anorg. Allg Chem. 2001, 627, 1465.
10.1021/om0200510 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/25/2002

3170 Organometallics, Vol. 21, No. 15, 2002
Pott et al.
F igu r e 3. Packing of 3 in the crystal.
The molecular structures of 2 and 3 resemble that of
1. However, there is a slight tendency from planarity
to pyramidality in the geometric environment of the
nitrogen atoms from 1 to 3. This effect is accompanied
by a displacement of the gallium atom from the NCCN
plane and by a change in gallium-iodine distances,
which differ most in 3 (2.4982(2) and 2.5397(2) Å). With
respect to the packing of molecules in the crystal, weak
iodine-iodine interactions seem possible, the iodine-
iodine distance between two adjacent molecules being
4.093 Å (Figure 3). Crystallographic data of compounds
1-3 are given in Table 1, and structural parameters
are collected in Table 2.
F igu r e 2. ORTEP plot of 1, thermal ellipsoids at 50%
probability.
tions have served to explain the bonding in the product
heterocycles. Furthermore, a derivatization reaction is
described which leads to a compound of the type
Ga(dab)2.
Resu lts a n d Discu ssion
The reaction of 2 equiv of “Green’s GaI” with 1 equiv
of Xyl-dab, Mes-dab, or Dipp-dab in benzene at room
temperature led to the formation of compounds 1-3 in
quantitative yield. The reaction could be followed by the
formation of a reddish-brown solution and by the
precipitation of gallium metal (eq 1).
The observed bond lengths within the heterocycles
require the description (dab)-IGaIIII2 for 1-3. ESR
studies support this assumption. The ESR spectrum of
3 at room temperature in toluene is only partially
resolved due to the coupling of the unpaired electron
1
with H, 14N, 69Ga, 71Ga, and 127I nuclei (1320 theoretical
lines, excluding the N-substituents).13 The smallest
observable coupling is found at 0.41 mT, which is a
typical value for the coupling of the imine protons in
paramagnetic dab complexes.14 A quartet structure with
about 2.7 mT spacing is attributed to the coupling with
the 69,71Ga isotopes, both with I ) 3/2.8,13 Assuming a
14N coupling of about 0.7 mT,14 and considering the total
spectral width of 19.9 mT, we estimate a value of about
0.8 mT for the 127I coupling (I ) 5/2, 100% nat.
abundance). A possible hyperconjugative interaction of
the gallium-iodine bonds with the π-system of the dab
unit would give rise to such a large value.15 At lower
temperatures the ESR spectrum of 3 is less well
resolved, revealing only the quartet coupling of about
2.7 mT due to the gallium isotopes (Figure 4). The
anisotropy of the g-factor is not strongly developed,
which is in agreement with a predominantly organic
radical. The observed ESR data prove the paramagnetic
character of 3 and reveal a considerable influence of the
cationic diiodogallium(III) fragment on the dab radical
ligand.
Compounds 1-3 were isolated as air-sensitive, brown,
crystalline solids, which readily dissolve in aprotic
organic solvents such as n-hexane, toluene, and tet-
rahydrofuran (THF).
Single crystals suitable for X-ray crystallography were
obtained by recrystallization from a concentrated solu-
tion of the respective compound in toluene at room
temperature. 1 crystallizes in the space group P21/n
(Figure 2). The molecular structure is pseudosymmetric
with respect to a mirror plane spanned out by the
gallium and the two iodine atoms. Small deviations from
the ideal symmetry are caused by the aromatic substit-
uents at the nitrogen atoms; the planes are twisted
against each other by approximately 15°. The gallium-
iodine distances are 2.5329(16) and 2.5207(15) Å, re-
spectively. The GaN2C2 ring is planar within experi-
mental error, and the nitrogen atoms adopt a slightly
distorted trigonal planar geometry. The carbon-carbon
and the average carbon-nitrogen and gallium-nitrogen
distances are 1.421(17), 1.325(16), and 1.954(10) Å,
respectively. These values are comparable to those
found for the singly reduced dab-unit in Ga(But-dab)2
(dab-: C-C ) 1.393(6) Å, C-N ) 1.332(5) Å, Ga-N )
1.968(3) Å; dab2-: CdC ) 1.344(5) Å, C-N ) 1.406(3)
Å, and Ga-N ) 1.865(2) Å).7 For free, nonreduced
dab ligands the typical values are C-C ≈ 1.45 Å and
CdN ≈ 1.23 Å.12
Density functional calculations (B3LYP/ECP++g-
(d,p))16,17 for the parent compound NHC2H2NHGaI2
were performed in order to analyze the bonding in more
detail. They yield bonding parameters (Figure 5, left)
that are close to the experimentally determined values.
The calculations predict C2v symmetry of the structures
(distorted tetrahedral shape), in accord with the previ-
ous analysis of the main group diazabutadiene com-
(13) Weil, J . A.; Bolton, J . R.; Wertz, J . E. Electron Paramagnetic
Resonance; Wiley: New York, 1994.
(14) Klein, A.; Hasenzahl, S.; Kaim, W. J . Chem. Soc., Perkin Trans.
2 1997, 2573.
(12) Berger, S.; Baumann, F.; Scheiring, T.; Kaim, W. Z. Anorg. Allg.
Chem. 2001, 627, 620.
(15) Kaim, W. J . Am. Chem. Soc. 1984, 106, 1712.

Gallium Heterocycles
Organometallics, Vol. 21, No. 15, 2002 3171
Ta ble 1. Cr ysta llogr a p h ic Da ta for 1-3
empirical formula
fw
C18H20GaI2N2, 1
587.88
C20H24GaI2N2, 2
615.93
C26H36GaI2N2, 3
700.09
cryst color, habit
cryst size
temperature
wavelength
dark red, plate
red, plates
brown, needles
0.29 × 0.20 × 0.02 mm3
0.30 × 0.20 × 0.08 mm3
0.30 × 0.30 × 0.15 mm3
100(2) K
0.71073 Å
space group
unit cell dimensions
P21/n
P21/c
Pnma
a ) 8.2030(2) Å
b ) 14.1840(4) Å
c ) 16.8650(5) Å
â ) 94.659(1)°
1955.78(9) Å3
4
a ) 7.3865(1) Å
b ) 17.8440(1) Å
c ) 17.4850(1) Å
â ) 108.8780(3)°
2180.491(17) Å3
4
a ) 12.5080(1) Å
b ) 21.2160(1) Å
c ) 10.4060(2) Å
â ) 90°
volume
Z
density (calcd)
θ range for data collection
no. of reflns collected
no. of unique reflns
abs corr
final R indices [I>2σ(I)]
no. of reflns [I>2σ(I)]
no. of params
2761.44(6) Å3
4
1.997 Mg/m3
3 to 25°
32 769
1.876 Mg/m3
3 to 30°
65 899
6351 (Rlnt ) 0.111)
multiscan
RF ) 0.0250, wRF ) 0.0618
5701
233
1.684 Mg/m3
3 to 30°
65 299
4107 (Rlnt ) 0.034)
3395 (Rlnt ) 0.140)
2
2
2
RF ) 0.0666, wRF ) 0.1427
2366
213
RF ) 0.0150 wRF ) 00384
3906
217
largest diff peak and hole
2.051 (1.07 Å near I(1))
1.226 (1,1 Å near I(2))
and -1.025 e Å-3
Nonius Kappa CCD
0.780 and -0.556 e Å-3
and -1.216 e Å-3
diffractometer used
structure refinement
full-matrix least-squares on F2 (SHELXL-97)
Ta ble 2. Selected Bon d Len gth s (Å) for 1-3
C18H20GaI2N2, 1 C20H24GaI2N2, 2 C26H36GaI2N2, 3
C-C
C-N
1.421(17)
1.313(15)
1.337(17)
1.952(10)
1.956(9)
1.408(3)
1.337(3)
1.338(3)
1.9384(17)
1.9388(18)
2.5173(2)
2.5261(3)
1.406(2)
1.3386(15)
Ga-N
Ga-I
1.9449(10)
2.5207(15)
2.5329(16)
2.4982(2)
2.5397(2)
plexes El(dab)2 with group 13 elements.18 It is of further
interest to inspect the charge distribution within the
molecules.19 The results for the parent compound are
as follows (Figure 5, right): the chelated gallium atom
carries a positive charge of 0.946, considerably less than
computed for Ga(dab)2 (1.655)18 with a comparable basis
set (and the same density functional). This effect may
be attributed to the iodine atoms bonded to the gallium
atom, which are less electronegative than the nitrogen
atoms. The Wiberg bond indices (values in italics) reveal
a fractional bond order between the gallium and the
neighboring nitrogen atoms. At the same time the C-N
bonds are shortened and between a single and a double
bond, while the C-C bond is lengthened toward a single
F igu r e 4. ESR spectrum of 3 in toluene at 210 K (top)
(16) All calculations were performed with the Gaussian set of
programs. All calculations were performed with the Gaussian set of
programs. Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G.
E.; Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J .
A.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B. G.; Chen,
W.; Wong, M. W.; Andres, J . L.; Head-Gordon, M.; Replogle, E. S.;
Pople, J . A. Gaussian 98 (Revision A.1); Gaussian Inc.: Pittsburgh,
PA, 1998.
with computer simulation (bottom).
F igu r e 5. Equilibrium geometry of NHC2H2NHGaI2 (bond
lengths in Å, bond angles in deg [left]; natural atomic
charges and Wiberg bond indices [right]).
(17) Relativistic corrected effective core potentials basis set described
by Stevens and co-workers were utilized. Stevens, W. J .; Basch, H.;
Krauss, M. J . Chem. Phys. 1984, 81, 6026. Stevens, W. J .; Krauss, M.;
Basch, H.; J asien, P. Can. J . Chem. 1992, 70, 612. Cundari, T.; Stevens,
W. J . J . Chem. Phys. 1993, 98, 5555.
(18) Schoeller, W. W.; Grigoleit, S. J . Chem. Soc., Dalton Trans.
2002, 405.
bond. The Ga-I bonds are single bonds. As a conse-
quence, the unpaired electron resides preferentially in
the five-membered ring system. We note that our
calculations do not explicitly account for spin-orbit
coupling. Hence we cannot give a further analysis of the
anisotropy of the g-factor in the ESR measurement.
(19) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.

3172 Organometallics, Vol. 21, No. 15, 2002
Pott et al.
techniques. The solvents were purified by conventional means
and distilled immediately prior to use. Xyl-dab, Mes-dab, and
Dipp-dab were prepared according to the literature.20 “GaI”
was prepared by a modified procedure according to the
literature.10 Sonication experiments were performed using a
Bandelin Sonorex Super 10 P ultrasonic bath (50% power).
Elemental analyses were performed by the Microanalytical
Laboratory of the Universita¨t Bielefeld. IR data were collected
using a Bruker Vektor 22-FT spectrometer. The samples were
measured as KBr pellets. The melting point determinations
were performed using a Bu¨chi 510 melting point apparatus.
The ESR spectrum was recorded on a Bruker ESP 300 system.
P r ep a r a tion of 1. A mixture of gallium particles (697 mg,
10 mmol) and iodine (1269 mg, 10 mmol) in 50 mL of benzene
was sonicated for 12 h at 50 °C. To the resulting suspension
of pale green “GaI” in benzene was added 5 mmol of Xyl-dab
(1322 mg). Instantaneously a brown solution was formed and
gallium metal precipitated. The reaction mixture was stirred
for another 2 h. After filtration, all volatile components were
removed in vacuo to yield 2675 mg of 1 (4.6 mmol, 91% based
on Xyl-dab) as a brown crystalline solid. Single crystals
suitable for X-ray crystallography were obtained by recrystal-
lization from toluene at room temperature. Mp: 170-180 °C
(dec). IR (cm-1, KBr): 2962(s), 2918(m), 1450(s), 1378(w), 1326-
(m), 1261(m), 1231(s), 1096(m), 1030(m), 881(w), 801(w), 772-
(m). Anal. Calcd for C18H20GaI2N2 (M ) 587.88 g mol-1): C,
36.78; H, 3.43; N, 4.77. Found: C, 36.70; H, 3.48; N, 4.60.
P r ep a r a tion of 2. The same procedure was used in the
reaction of “GaI” (10 mmol) with Mes-dab (1462 mg, 5 mmol)
in benzene (50 mL). Compound 2, 2864 mg (4.7 mmol, 93%
based on Mes-dab), was obtained as a brown crystalline solid.
Single crystals suitable for X-ray crystallography were ob-
tained by recrystallization from toluene at room temperature.
Mp: 170-180 °C (dec). IR (cm-1, KBr): 2961(s), 2920(m), 1452-
(s), 1380(w), 1324(m), 1263(m), 1227(m), 1091(m), 1026(m),
881(w), 800(w), 776(m). Anal. Calcd for C20H24GaI2N2 (M )
615.93 g mol-1): C, 39.00; H, 3.93; N, 4.55. Found: C, 38.76;
H, 3.91; N, 4.43.
F igu r e 6. ORTEP plot of 4, thermal ellipsoids at 50%
probability.
The reaction of 1 with 1 equiv of the 1,4-dilithiated
Xyl-dab led to the formation of Ga(Xyl-dab)2 (4) (eq 2).
Compound 4 was isolated in high yield as an air-
sensitive, dark brown, crystalline solid, which readily
dissolves in aprotic organic solvents such as n-hexane,
toluene, and THF.
Single crystals suitable for X-ray crystallography were
obtained by recrystallization from a concentrated solu-
tion in toluene at room temperature. 4 crystallizes in
the space group Pbcn (Figure 6).
The molecule lies on a crystallographic symmetry axis
(C2), which results from the intersecting NCCN planes.
As in Ga(But-dab)2,7 the bonding in 4 shows all the
characteristics of one singly and one doubly reduced
P r ep a r a tion of 3. The same procedure was used in the
reaction of “GaI” (10 mmol) with Dipp-dab (1883 mg, 5 mmol)
in benzene (50 mL). Compound 3, 3290 mg (4.7 mmol, 94%
based on Dipp-dab), was obtained as a brown crystalline solid.
Single crystals suitable for X-ray crystallography were ob-
tained by recrystallization from toluene at room temperature.
Mp: 170-180 °C (dec). IR (cm-1, KBr): 2963(s), 2922(m), 2863-
(w), 1451(s), 1382(w), 1360(w), 1322(m), 1264(m), 1251(m),
1224(m), 797(m), 786(m), 753(m). Anal. Calcd for C26H36GaI2N2
(M ) 700.09 g mol-1): C, 44.61; H, 5.18; N, 4.00. Found: C,
44.30; H, 5.11; N, 3.94.
dab ligand, the bond lengths being as follows: dab2-
:
CdC ) 1.331(4) Å, C-N ) 1.413(2) Å, Ga-N ) 1.8831-
(13) Å; dab-: C-C ) 1.402(3) Å, C-N ) 1.337(2) Å,
Ga-N ) 1.9678(13) Å. Obviously, the orthogonality of
the dab ligands precludes any rapid intramolecular
electron transfer and delocalization of charge between
dab2- and dab-.
P r ep a r a tion of 4. A suspension of 1,4-dilithio-1,4-bis[2,6-
dimethylphenyl]-1,4-diazabuta1,3-diene in toluene (40 mL),
prepared from the diazabutadiene (1322 mg, 5 mmol) and
lithium metal (69 mg, 10 mmol), was added to a solution of 1
(2939 mg, 5 mmol) in toluene (40 mL). After filtration, all
volatile components were removed in vacuo to yield 2693 mg
of 4 (4.5 mmol, 90% based on Xyl-dab) as a brown crystalline
solid. Single crystals suitable for X-ray crystallography were
obtained by recrystallization from toluene at room tempera-
ture. Mp: 190 °C. IR (cm-1, KBr): 2964(s), 1630(w), 1550(w),
1478(m), 1344(m), 1262(m), 1226(s), 1101(s), 781(w). Anal.
Calcd for C36H40GaN4 (M ) 598.44 g mol-1): C, 72.25; H, 6.74;
N, 9.36. Found: C, 72.18; H, 6.45; N, 9.33.
Con clu sion
In the reaction of 1,4-diazabutadienes possessing
bulky substituents in 1,4-position with “Green’s GaI”,
neither simple complex formation (to type III com-
pounds) nor clean oxidative addition (to type I com-
pounds) is observed. Instead, a more complex reaction
sequence takes place, which finally leads to compounds
1-3 of the composition (dab)-IGaIIII2 containing the
paramagnetic monoanion of the respective dab species
(type II compounds) and to the formation of elemental
gallium. ESR measurements and quantum chemical
calculations explain the bonding within the novel het-
erocycles. The diiodo substituents can be replaced by
ene-diamido dianions, as shown by the synthesis of
Ga(Xyl-dab)2 (4).
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
structure data, positional and thermal parameters, and bond
lengths and angles of 1-4. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM0200510
Exp er im en ta l Section
Gen er a l Com m en ts. All manipulations were carried out
under a purified argon atmosphere using standard vacuum
(20) tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. 1980,
36b, 823.