New fluoride derivative of a dinuclear titanium (III) fulvalene complex: Crystal structure of [(η5-C5H5)Ti(μ-F)] 2(μ-η5:η5-C10H 8)

Article abstract of DOI:10.1021/om9606808

Reduction of titanocene difluoride (Cp₂TiF₂) with 1.5 equiv of potassium in toluene at 60-100°C affords the dinuclear η⁵:η⁵-fulvalene complex [(η⁵-C₅H₅)Ti(μ-F)] ₂(μ-η⁵:η⁵-C₁₀H ₈) (2). 2 was also obtained via reaction of Me₃SnF with the corresponding chloride and hydride compounds [(η⁵-C₅H₅)Ti(μ-X)] ₂(μ-η⁵:η⁵-C₁₀H ₈) (X = Cl, H). 2 has been characterized by mass spectrometry, IR spectroscopy, and chemical analyses. The molecular structure of 2, as determined by the X-ray crystal structural analyses, reveals a Ti₂F₂ fragment symmetrically folded along the Ti-Ti direction and assuming a butterfly configuration with a dihedral angle of 27.6(2)°. A dihedral angle of 15.3(3)° is observed between the two C₅H₄ rings of the folded fulvalene ligand.

Full text of DOI:10.1021/om9606808

Organometallics 1997, 16, 313-316
313
Articles
New F lu or id e Der iva tive of a Din u clea r Tita n iu m (III)
F u lva len e Com p lex: Cr ysta l Str u ctu r e of
[(η5-C5H5)Ti(µ-F )]2(µ-η5:η5-C10H8)†
Peihua Yu, Eamonn F. Murphy, Herbert W. Roesky,* Paolo Lubini,
Hans-Georg Schmidt, and Mathias Noltemeyer
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany
Received August 9, 1996X
Reduction of titanocene difluoride (Cp2TiF2) with 1.5 equiv of potassium in toluene at 60-
100 °C affords the dinuclear η5:η5-fulvalene complex [(η5-C5H5)Ti(µ-F)]2(µ-η5:η5-C10H8) (2).
2 was also obtained via reaction of Me3SnF with the corresponding chloride and hydride
compounds [(η5-C5H5)Ti(µ-X)]2(µ-η5:η5-C10H8) (X ) Cl, H). 2 has been characterized by mass
spectrometry, IR spectroscopy, and chemical analyses. The molecular structure of 2, as
determined by the X-ray crystal structural analyses, reveals a Ti2F2 fragment symmetrically
folded along the Ti-Ti direction and assuming a butterfly configuration with a dihedral
angle of 27.6(2)°. A dihedral angle of 15.3(3)° is observed between the two C5H4 rings of the
folded fulvalene ligand.
In tr od u ction
In 1956, Wilkinson obtained bis(η5-cyclopentadienyl)-
titanium (titanocene) for the first time from the reaction
of TiCl2 with cyclopentadienylsodium.1 Titanocene was
later obtained by reduction of Cp2TiCl2 using sodium
naphthalene in THF, sodium metal, LiAlH4 in hydro-
carbons, or by hydrogenolysis of Cp2TiMe2.2,3 Titano-
cenes have been implicated as highly reactive interme-
diates in a wide variety of chemical reactions with
olefins, hydrogen, carbon monoxide, and dinitrogen.4
The chemical properties of titanocene were found,
however, to be at variance with the expected chemical
behavior of a monomeric titanocene. Though strong
evidence for a dimeric structure with one bridging
fulvalene ligand and two hydride bridges was given by
Brintzinger and Bercaw,5 it was not until 1992 that the
dimeric structure was confirmed by X-ray crystal-
lography.6 So far, only a few titanocenes with one
bridging fulvalene ligand and two X1X2 bridge atoms
(X1 ) X2 ) Cl, OH, H; or X1) H, X2 ) Cl, H2AlEt2)
(Figure 1) have been synthesized and characterized by
F igu r e 1. Dinuclear titanium fulvalene complexes; X1 )
X2 ) F (2), Cl (4), H (6), OH (7); X1 ) H, X2 ) Cl (3), H2AlEt2
(8).
X-ray structure determination.7 All of these complexes
exhibit chloro, hydrido, or hydrogen bridging ligands;
however, the fluorine-bridged analogue has not been
reported to date. Since fluorine atoms demonstrate a
considerable tendency to act as bridging ligands,8 the
syntheses of corresponding fluoride-bridged complexes
from the titanocene difluoride should be possible. Fur-
thermore, we have found that trimethyltin fluoride,
Me3SnF, is an excellent fluorinating reagent. Access
to a wide range of cyclopentadienyl-substitued titanium
fluorides is possible via the reaction of Me3SnF with the
corresponding chloride in suitable solvents.9,10 We
report herein an extension of this fluorination procedure
and the properties and solid-state structure of the new
titanium fulvalene complex with two fluoride bridges
(X1 ) X2 ) F).
† Dedicated to Professor Gottfried Huttner on the occasion of his
60th birthday.
X Abstract published in Advance ACS Abstracts, J anuary 15, 1997.
(1) Fischer, A. K.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956, 2, 149.
(2) (a) Watt, G. W.; Baye, L. J .; Drummond, F. O. J . Am. Chem.
Soc. 1966, 88, 1138. (b) Salzmann, J .-J .; Mosimann, P. Helv. Chim.
Acta 1967, 50, 1831. (c) Van Tamelen, E. E.; Cretney, W.; Klaentschi,
N.; Miller, J . S. J . Chem. Soc., Chem. Commun. 1972, 481.
(3) Clauss, K.; Bestian, H. Liebigs Ann. Chem. 1962, 654, 8.
(4) Pez, G. P.; Armor, J . N. Adv. Organomet. Chem. 1981, 19, 1.
(5) (a) Calderazzo, F. J . Organomet. Chem. 1973, 53, 179. (b)
Brintzinger, H. H.; Bercaw, J . E. J . Am. Chem. Soc. 1970, 92, 6182.
(6) Troyanov, S. I.; Antropiusova, H.; Mach, K. J . Organomet. Chem.
1992, 427, 49.
(7) (a) Olthof, G. J . J . Organomet. Chem. 1977, 128, 367. (b)
Guggenberger, L. J .; Tebbe, F. N. J . Am. Chem. Soc. 1976, 98, 4137.
(c) Perevalova, E. G.; Urazovski, I. F.; Lemenovskii, D. A.; Slovokhotov,
Yu. L.; Struchkov, Yu. T. J . Organomet. Chem. 1985, 289, 319. (d)
Guggenberger, L. J .; Tebbe, F. N. J . Am. Chem. Soc. 1973, 95, 7870.
(8) Witt, M.; Roesky, H. W. Prog. Inorg. Chem. 1992, 40, 353.
(9) Herzog, A.; Liu, F. Q.; Roesky, H. W.; Demsar, A.; Keller, K.;
Noltemeyer, M.; Pauer, F. Organometallics 1994, 13, 1257.
(10) Murphy, E. F.; Yu, P.; Dietrich, S.; Roesky, H. W.; Parisini, E.;
Noltemeyer, M. J . Chem. Soc., Dalton. Trans. 1996, 1983.
S0276-7333(96)00680-2 CCC: $14.00 © 1997 American Chemical Society

314 Organometallics, Vol. 16, No. 3, 1997
Yu et al.
Sch em e 1a
a
Reactions and conditions: (a) +2Me3SnF, -2Me3SnCl/toluene; (b) +Me3SnF, -Me3SnCl/toluene, room temperature.
or [(η5-C5H5)Ti(µ-H)]2(µ-η5:η5-C10H8) (6) with 2 equiv of
Me3SnF in toluene in high yield (56%, 75%, respec-
tively), which indicates not only organometallic chlo-
rides but also hydrides may be fluorinated easily using
Me3SnF. It is very interesting to note that attempts to
prepare the compound with both fluorine and hydrogen
bridges [(η5-C5H5)2Ti2(µ-H)(µ-F)](µ-η5:η5-C10H8) by reac-
tion of compound 3 with Me3SnF in a molar ratio of 1:1
in toluene were unsuccessful. In this case a mixture of
compounds 2, 4, and [(η5-C5H5)2Ti2(µ-Cl)(µ-F)](µ-η5:η5-
C10H8) (5) was isolated, indicating that a ligand redis-
tribution reaction between Cl and F occurs. This
behavior also suggests that replacement of the bridging
hydrogen by fluoride occurs prior to that of bridging
chloride (Scheme 1).
F igu r e 2. Molecular structure of [(η5-C5H5)Ti(µ-F)]2(µ-η5:
We also observed a similar redistribution reaction of
(RCp)2TiCl2 (R ) H, Me) with Me3SnF in a molar ratio
of 1:1 in toluene, CH2Cl2, and CH3CN (eq 1). The
η5-C10H8).
Resu lts a n d Discu ssion
reactions between Cp2MX2 and Cp2MR2 (M ) Ti, Zr; X
Syn th esis a n d Sp ectr a . Reduction of Cp2TiF2 (1)
in toluene with 1.5 molar equiv of potassium at 60-
100 °C in toluene gives the dark green crystalline µ-η5:
η5-fulvalene complex [(η5-C5H5)Ti(µ-F)]2(µ-η5:η5-C10H8)
(2) in moderate yield (37%). The mass spectrum of 2
confirmed its dinuclear structure [EI-MS: m/z 392
(M+)]. In contrast when 1 was reacted with 1 molar
equiv of potassium, only the fluorine-bridged compound
) F, Cl, Br, I; R ) Me, Ph) (eq 2) have been studied in
detail by J ordan.11
2(RCp)2TiCl2 + 2Me3SnF f 2(RCp)2TiClF f
(RCp)2TiCl2 + (RCp)2TiF2 (1)
Cp2MX2 + Cp2MR2 h 2Cp2MRX
(2)
9
[Cp2Ti(µ-F)]2 was obtained. Attempts to prepare the
The IR spectrum of 2 is similar to those of 4 and 6,5b
containing two medium absorptions in the range 1100-
zirconium analogue by reduction of Cp2ZrF2 with 1.5
molar equiv of potassium were unsuccessful. In this
instance an insoluble solid formed due to the facile
polymerization of zirconocene difluoride under reflux.
It is interesting to note that under the same conditions
reaction of titanocene dichloride (Cp2TiCl2) with 1.5
molar equiv of potassium proceeds differently, giving a
compound with two bridging heteroatoms (H, Cl), [(η5-
C5H5)2Ti2(µ-H)(µ-Cl)](µ-η5:η5-C10H8) (3).7c The failure to
isolate a corresponding fluoride-hydride compound is
attributed to the greater bond strength of the Ti-F bond
compared with the Ti-Cl bond and the propensity of
fluorine to form stable fluorine-bridged compounds.
Me3SnF is particularly suited for the synthesis of
cyclopentadienyl-substitued titanium fluorides. Thus,
the (fulvalene)titanium fluoride 2 can be obtained in
good yield by reaction of the corresponding chloride [(η5-
C5H5)Ti(µ-Cl)]2(µ-η5:η5-C10H8) (4) with 2 molar equiv of
Me3SnF (Scheme 1). Moreover, we found that com-
pound 2 can be obtained alternatively by reaction of 3
-1
-1
1000 cm
and a strong absorption ca. 780 cm
.
Compound 2 is stable up to ca. 300 °C and decomposes
without melting on further heating. To our knowledge,
it is difficult to record the 1H NMR spectrum of the
compounds due to the paramagnetism of titanium(III)
compounds.6,7c,12 It was found in the 1H NMR of
compound 2 at room temperature that the cyclopenta-
dienyl ring proton signals are broad and appear at
relatively high field [δ(H(C5H4) 5.1 and 7.9 ppm;
δ(H(C5H5) 6.7 ppm] as compared with complex 6
[δ(H(C5H4) 5.5 and 8.1 ppm; δ(H(C5H5) 6.9 ppm].12 In
the 19F NMR spectrum, however, no signal was ob-
served.
X-r a y Cr ysta l Str u ctu r e of 2. The X-ray analysis
reveals that the crystal of 2 is composed of discrete
(11) J ordan, R. F. J . Organomet. Chem. 1985, 294, 321.
(12) Lemenovskii, D. A.; Urazowski, I. F.; Grishin, Yu. K.; Roznya-
tovsky, V. A. J . Organomet. Chem. 1985, 290, 301.

A µ-F- Derivative of a Ti(III) Fulvalene Complex
Organometallics, Vol. 16, No. 3, 1997 315
Ta ble 1. Selected Bon d Dista n ce (Å) a n d An gles
(d eg) for Com p ou n d 2
Å) (see Table 3). The Ti-F distances (average 2.046 Å)
are comparable with the relative Ti(III) compound
(Cp2TiF)3TiF3 [Ti-Ti 3.257 Å, Ti-F 1.975 and 2.094 Å,
respectively].13 The angles subtended at the Ti by the
centroid of the two staggered individual Cp rings and
fulvalene [Cpa-Ti1-Cpc 136.4(3)°, Cpb-Ti2-Cpd
138.7(3)°, Cp ) ring centroid] are in the region expected
for a normal bent η5-Cp-metal system.14 The Ti-F-
Ti (101.5, 101.4°) and F-Ti-F (75.9°) angles are com-
parable to those with bridging bis(µ-H), (µ-H)(µ-Cl),
bis(µ-Cl), and bis(µ-OH) dititanium fulvalene com-
pounds (see Table 3). Similarly, the plane containing
the Ti2F2 fragment is symmetrically folded along the
Ti-Ti direction. The bridging F atoms lie lower than
the Ti atoms, farther from the fulvalene ligand and
assuming a butterfly configuration with a dihedral angle
of (27.6(2)°. All the other bond lengths and angles are
as expected.
Bond Distances
Ti(1)-F(2)
Ti(1)-C(11)
Ti(1)-C(4)
Ti(1)-C(15)
Ti(1)-C(14)
Ti(1)-C(13)
Ti(2)-F(2)
Ti(2)-C(16)
T(2)-C(9)
Ti(2)-C(10)
Ti(2)-C(8)
Ti(2)-C(18)
C(11)-C(16)
2.046(4)
2.333(5)
2.357(6)
2.369(6)
2.388(6)
2.397(6)
2.044(4)
2.336(6)
2.349(7)
2.354(8)
2.367(6)
2.393(6)
1.464(8)
Ti(1)-F(1)
Ti(1)-C(3)
Ti(1)-C(5)
Ti(1)-C(12)
Ti(1)-C(2)
Ti(1)-C(1)
Ti(2)-F(1)
Ti(2)-C(20)
Ti(2)-C(7)
Ti(2)-C(17)
Ti(2)-C(6)
Ti(2)-C(19)
2.046(4)
2.351(6)
2.359(7)
2.371(6)
2.391(7)
2.402(7)
2.047(4)
2.346(6)
2.349(7)
2.361(5)
2.372(8)
2.396(6)
Angles
F(2)-Ti(1)-F(1)
Ti(1)-F(1)-Ti(2)
75.9(2)
101.4(2)
F(2)-Ti(2)-F(1)
Ti(2)-F(2)-Ti(1)
75.9(2)
101.5(2)
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 2
empirical formula
fw
C20H18F2Ti2‚C6H6
470.25
Exp er im en ta l Section
temp, K
wavelength, Å
cryst system
space group
a, Å
193(2)
0.710 73
orthorhombic
P212121
5.9592(12)
18.792(4)
19.989(4)
90
Gen er a l Da ta . All experimental manipulations were car-
ried out under an atmosphere of dinitrogen rigorously exclud-
ing air and moisture. Solvents were purified according to
conventional procedures and were freshly distilled prior to use.
Me3SnF and Cp2TiF2 (1),9 [(η5-C5H5)2Ti2(µ-H)(µ-Cl)](µ-η5:η5-
b, Å
c, Å
C
10H8) (3),7c [(η5-C5H5)Ti(µ-Cl)]2(µ-η5:η5-C10H8) (4), and [(η5-
C5H5)Ti(µ-H)]2(µ-η5:η5-C10H8) (6)15 were prepared as previously
described. IR spectra were recorded on a Bio-Rad FTS-7
spectrometer. Mass spectra were obtained on Finnigan MAT
8230 and Varian MAT CH5 spectrometers. Elemental analy-
ses were performed at the Analytical Laboratory of the
Institute of Inorganic Chemistry of Go¨ttingen University.
Syn th esis of [(η5-C5H5)Ti(µ-F)]2(µ-η5:η5-C10H8) (2) Meth od
1. A suspension of 1 (0.324 g, 1.5 mmol) and potassium (0.088
g, 2.25 mmol) in toluene (40 mL) was vigorously stirred at 60-
70 °C for 24 h. Then the reaction mixture was heated to 100
°C for at least 10 h, resulting in a dark green solution. After
cooling of the mixture to room temperature and separation of
the insoluble solid, partial removal of the solvent, and standing
at -20 °C for 1 week, large-size, air-sensitive, dark green
crystals of 2 separated in 37% yield (0.11 g). Mass spectrum:
m/z 392 (M+), 372 (M+ - 65). IR (Nujol, cm-1): 1226 (w),1067
(w), 1019 (m), 787 (s), 722 (m). Anal. Calcd for C20H18F2Ti2:
C, 61.22; H, 4.59; F, 9.69. Found: C, 61.96; H, 4.78; F, 9.12.
R, deg
â, deg
90
90
γ, deg
V, Å3
2238.5(8)
4
1.395
0.739
Z
D
calcd, g‚cm-3
µ, mm-1
F(000)
968
cryst size, mm
θ range, deg
index range
0.8 × 0.4 × 0.3
3.57-22.54
-6 e h e 6, -20 e k e 20,
-21 e l e 21
3594
tot. reflcns
indepdt reflcns
abs corr
refinement method
data/restraints/params
Flack param
2946 (Rint ) 0.0176)
none
full-matrix least squares on F2
2946/63/271
0.0307
S
1.003
R1, wR2 [I > 2σ(I)]
R1, R2 (all data)
largest diff peak and
hole, e‚Å-3
0.0496, 0.1311
0.0543, 0.1370
0.528, -0.420
Meth od 2. Rea ction of 4 w ith Me3Sn F . A mixture of 4
(0.21 g, 0.5 mmol) and Me3SnF (0.183 g, 1 mmol) in toluene
(30 mL) was stirred at room temperature overnight. After
filtration of the dark green reaction solution the solvent was
removed in vacuo, and the solid residue was recrystallized from
toluene/hexane (1:2) to give a 61% yield of 2 (0.12 g). Mass
spectrum: m/z 392 (M+), 372 (M+ - 65).
Meth od 3. Rea ction of 6 w ith Me3Sn F . A suspension
of 6 (0.356 g, 1 mmol) and Me3SnF (0.366 g, 2 mmol) in toluene
(30 mL) was stirred at room temperature for 12 h. After
filtration, the solvent was evaporated in vacuo and the dark
green solid obtained washed with hexane to give compound 2
in 75% yield (0.29 g).
molecules of [(η5-C5H5)Ti(µ-F)]2(µ-η5:η5-C10H8) (Figure
2), containing two Ti atoms bonded to a bridging
fulvalene ligand. The coordination around each Ti atom
can be described as a distorted tetrahedron if we
consider the Cp ring centroids as each occupying one
coordination site around the metal. The fulvalene
ligand is folded, forming a dihedral angle (15.3(3)°)
between the two ring planes. The inter-ring carbon-
carbon bond distance of the fulvalene ligand (C(11)-
C(16) 1.464(8) Å) and the distance of the Ti atoms from
the fulvalene rings (Ti(1)-Cpd 2.047(3) Å, Ti(2)-Cpc
2.041(3) Å) and from the cyclopentadienyl rings (Ti(1)-
Cpb and Ti(2)-Cpa 2.055(3) Å) have normal values
(Table 1), as have the Ti-C distances which average
2.372 and 2.358 Å, respectively, for the fulvalene ligand
and 2.372 and 2.358 Å, respectively, for the cyclopen-
tadienyls.6,7 The Ti-Ti distance (3.167(1) Å) that
indicates no formation of a metal-metal bond is shorter
than that of compounds 4 (3.638 Å) and 8 (3.374 Å) and
similar to that of compounds 3 (3.125 Å) and 7 (3.195
Meth od 4. Rea ction of 3 w ith Me3Sn F (1:2 Ra tio). A
mixture of 3 (0.5 g, 1.28 mmol) and Me3SnF (0.47 g, 2.56 mmol)
in 30 mL of toluene was stirred at ambient temperature
overnight. The solution was filtered, and the solvent was
removed in vacuo from the filtrate. The resulting dark green
(13) Liu, F. Q.; Gornitzka, H.; Stalke, D.; Roesky, H. W. Angew.
Chem. 1993, 105, 447; Angew. Chem., Int. Ed. Engl. 1993, 32, 442.
(14) Gambarotta, S.; Chiang, M. Y. Organometallics 1987, 6, 897.
(15) (a) Cuenca, T.; Herrmann, W. A.; Ashworth, T. V. Organo-
metallics. 1986, 5, 2514. (b) Cano, A.; Cuenca, T.; Rodriguez, G.; Royo,
P. J . Organomet. Chem. 1993, 447, 51.

316 Organometallics, Vol. 16, No. 3, 1997
Yu et al.
Ta ble 3. Im p or ta n t Geom etr ica l P a r a m eter s in Ditita n iu m F u lva len e [(C5H5)TiX]2(C10H8) Com p lexes
interatomic dist (Å)
angle (deg)a
no. Ti-Ti Ti-C(fulv) Ti-C(Cp) Ti-X
compd
C11-C16
R
â
θ
ω
refs
137.7 (136.0) 15.3 this work
93 (77.6)b 137.3 (137.3)b 15.6 12
Cp2Ti2(C10H8)(F2)
2
3
4
6
7
8
3.167 2.35-2.37
3.125 2.32-2.39
3.638 2.37-2.40
2.989 2.30-2.39
3.195 2.34-2.39
3.374 2.33-2.39
2.37
2.36
2.38
2.36
2.41
2.35
2.04
1.46
1.45
1.46
1.43
1.43
1.45
75.9 101.5
92
81.3 92.5
57 120
75.4 100.7
138
Cp2Ti2(C10H8)(H)(Cl)
Cp2Ti2(C10H8)(Cl)2
Cp2Ti2(C10H8)(H)2
Cp2Ti2(C10H8)(OH)2
Cp2Ti2(C10H8)H(H2AlEt2)
2.17 (2.49)b
2.52
135.2
138.6
135.1
134.1
5.3 10
17.7
15.1 11
12.5 13
1.71
2.07
1.80
9
a
R, angle X-Ti-X; â, angle Ti-X-Ti; θ, dihedral angle between the C5H5 and C5H4 planes; ω, dihedral angle between the C5H4 planes.
Ti-Cl bond length or Ti-Cl-Ti bond angle.
b
solid was recrystallized from toluene/hexane (1:2) to give a 56%
yield of 2 (0.28 g).
model. One molecule of solvent was also found and refined
anisotropically using restraints. Other details related to the
data collection, structure solution, and refinement are listed
in Table 2.
Rea ction of 3 w ith Me3Sn F (1:1 Ra tio). A mixture of 3
(0.39 g, 1 mmol) and Me3SnF (0.183 g, 1 mmol) in toluene (30
mL) was stirred at room temperature overnight and filtered.
After removal of the solvent, the residue was washed with
hexane and dried in vacuo. Mass spectrum: m/z 392 [M+, for
compound 2], 408 [M+, for compound 5], 424 [M+, for compound
4].
X-r a y Str u ctu r e Deter m in a tion of 2. Diffraction data
were collected on a Siemens-Stoe AED2 four-circle instrument
(at -80 °C) using Mo KR radiation (λ ) 0.710 73 Å). The
structure was solved by direct methods using SHELXS-90,16
and refinement was made by full-matrix least squares on all
F2 values using SHELXL-93.17 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included at
geometrically calculated positions and refined using a riding
Ack n ow led gm en t. This work has been financially
supported by the Deutsche Forschungsgemeinschaft, the
BMBF, and the Hoechst AG. P.L. thanks the European
Community for a postdoctoral grant (BBW94.0162,
CHBG CT 940731).
Su p p or tin g In for m a tion Ava ila ble: Listings of crystal
data, atomic coordinates and equivalent isotropic displacement
parameters for all non-hydrogen atoms, hydrogen positional
and thermal parameters, anisotropic displacement parameters,
and bond distances and angles (7 pages). Ordering informa-
tion is given on any current masthead page.
(16) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(17) Sheldrick, G. M. SHELXL-93, Program for crystal structure
refinement, Go¨ttingen, 1993.
OM9606808