Sulfur-bonded 2-methylthiopyridine (2-MeSpy) complex of iridium(III). Crystal structure of [Cp*IrI2-(2-MeSpy-κS)] (Cp* = η5-C5Me5)

Article abstract of DOI:10.1246/bcsj.79.1897

A reaction of [Cp*Ir(2-Spy)(N₃)] (Cp* = pentamethylcyclopentadienyl; 2-Spy = 2-pyridinethiolate) with two equivalents of methyl iodide afforded the diiodo complex bearing 2-MeSpy, [Cp*IrI ₂(2-MeSpy)] (3). From X-ray crystallography and IR spectroscopy, the S atom of 2-MeSpy is coordinated to the metal ion in the solid state. In CD ₂Cl₂, complex 3 exhibited a dissociation equilibrium of 2-MeSpy similar to the corresponding thioanisole complex of [Cp*IrI ₂(MeSPh)].

Full text of DOI:10.1246/bcsj.79.1897

Short Articles
Bull. Chem. Soc. Jpn. Vol. 79, No. 12, 1897–1899 (2006) 1897
Sulfur-Bonded 2-Methylthiopyridine
(2-MeSpy) Complex of Iridium(III).
Ã
Crystal Structure of [Cp IrI2-
Ã
(2-MeSpy-ꢀS)] (Cp = ꢁ5-C5Me5)
ꢀ
Yusuke Sekioka and Takayoshi Suzuki
ꢀ
Fig. 1. Perspective views of (a) [Cp Ir(2-Spy-ꢀS,N)I] (2)
and (b) [Cp IrI2(2-MeSpy-ꢀS)] (3).
Department of Chemistry, Graduate School of Science,
Osaka University, Toyonaka 560-0043
ꢀ
N
Received June 1, 2006; E-mail: suzuki@chem.sci.osaka-u.ac.jp
I
I
N
MeI
AN
N
Ir
MeI
N
I
Ir
Ir
N
S
S
AN
N
S
ꢀ
ꢀ
– MeN3
A reaction of [Cp Ir(2-Spy)(N3)] (Cp = pentameth-
ylcyclopentadienyl; 2-Spy = 2-pyridinethiolate) with two
equivalents of methyl iodide afforded the diiodo complex
(1)
(2)
(3)
Scheme 2.
ꢀ
bearing 2-MeSpy, [Cp IrI2(2-MeSpy)] (3). From X-ray crys-
tallography and IR spectroscopy, the S atom of 2-MeSpy is
coordinated to the metal ion in the solid state. In CD2Cl2,
complex 3 exhibited a dissociation equilibrium of 2-MeSpy
similar to the corresponding thioanisole complex of
CD3CN) with two equivalent amounts of MeI at 30 ꢂC was
monitored by 1H NMR spectroscopy. As the peaks for complex
1 decreased in intensity, the peaks corresponding to the iodo
ꢀ
complex [Cp Ir(2-Spy)I] (2), of which the molecular struc-
ture (Fig. 1a) and chemical shifts were confirmed by the
ꢀ
[Cp IrI2(MeSPh)].
ꢀ
NaI, appeared. After standing the reaction mixture for 8 h,
sample prepared independently from [Cp Ir(2-Spy)Cl] and
Transiton-metal azide complexes often exhibit interesting
reactivities on irradiation with UV–vis light or on addition
of various electrophiles.1–7 For instance, it has been well docu-
mented that photooxidation of early transition-metal(III) azido
complexes gave the corresponding nitridometal(V) com-
plexes.2 In case of photolysis or acid decomposition of azido-
pentaammineiridium(III), a coordinated nitrene intermediate
was postulated.3 In addition, the azide complexes often react
with alkynes or nitriles, affording triazolate or tetrazolate com-
plexes via 1,3-dipolar cycloadditions.4 Furthermore, it has
recently been reported that a reaction of cis-di(azido)bis(2,20-
bipyridine)ruthenium(II) with methyl iodide produced a novel
N-bonded methyleneimine (NH=CH2-ꢀN) complex.5 We are
investigating the reactivity of azido(pentamethylcyclopenta-
dienyl)iridium(III) complexes incorporating short-bite diden-
tate ligands, such as N,N-dialkyldithiocarbamate6 or 2-pyri-
1
the H NMR spectrum indicated the formation of at least four
ꢀ
complexes having a Cp Ir fragment. The main product (3:
ꢃ60% by integration) exhibited both Cp and py resonances,
ꢀ
which were different from those of 2 (complex 2 still remained
in this mixture), together with a SMe resonance at ꢁ 2.53.
Two other weak Cp resonances were assigned to [(Cp IrI)2-
ꢀ
ꢀ
ꢀ
(ꢂ-I)2]8 and [(Cp IrI)2(ꢂ-N3)2].9 In addition, the spectrum of
the reaction mixture showed a broad singlet resonance at ꢁ
2.98, which was probably due to methyl azide10 generated in
the reaction. When the reaction solution was allowed to stand
for 2 days, red prismatic crystals of product 3 were deposited
ꢀ
(together with a few red-brown platelet crystals of [(Cp IrI)2-
(ꢂ-N3)2]9). The crystal structure of 3 was determined by X-ray
analysis; complex 3 is a diiodo complex containing a sulfur-
ꢀ
Thus, the reaction of 1 with MeI can be outlined as Scheme 2
bonded 2-methylthiopyridine, [Cp IrI2(2-MeSpy-ꢀS)] (Fig. 1b).
ꢀ
dinethiolate (2-Spy).7 Photolysis of [Cp Ir(2-Spy)(N3)] (1) in
(the minor by-products are omitted).
The Ir–S bond length in 3 is 2.380(4) A, which is compara-
acetonitrile (AN) afforded a novel nitrogen-atom insertion
ꢀ
˚
product, [Cp Ir(1-N-2-Spy)], which reacted further with MeI
ꢀ
ble to those in the related Cp IrIII thioether-ꢀS complexes:
ꢀ
to give a N-methylated product, [Cp Ir(1-NMe-2-Spy)]I
ꢀ
ꢀ
[Cp IrCl2(dbt)] (dbt = dibenzothiophene) 2.375(2) A,11 [Cp -
˚
(Scheme 1).7 Here, we describe the reaction of complex 1 with
MeI prior to photolysis and structural characterization of the
product.
12
˚
IrCl2(PhSCH2SPh-ꢀS)] 2.385(1) A. The structural parame-
ters around the S atom in 3 are normal for a coordinated thio-
ether ligand:13 S1–C1 1.80(2), S1–C2 1.78(2) A, Ir1–S1–C1
˚
ꢀ
A reaction of [Cp Ir(2-Spy)(N3)] (1) (0.035 mol dmꢁ3 in
109.6(7), Ir1–S1–C2 111.4(5), and C1–S1–C2 101.7(9)ꢂ. The
˚
Ir–I bond lengths are 2.712(1) and 2.714(1) A, and the I1–
N
N
Ir1–I2 angle is 91.14(4)ꢂ. In contrast, in the chelating 2-Spy
complex, 2, the structural parameters associated with the
four-membered chelate ring are highly strained; the S1–
Ir1–N1, Ir1–S1–C2, and Ir1–N1–C2 bond angles are only
67.0(1), 80.4(2), and 102.5(4)ꢂ, respectively. The Ir1–S1 bond
N
+
I–
N
S
N
S
MeI
AN
h
ν
N
N
N
Ir
Ir
Ir
S
AN
(1)
[Cp*Ir(1-N-2-Spy)]
[Cp*Ir(1-NMe-2-Spy)]I
˚ ˚
Scheme 1.
in 2, 2.431(2) A, is 0.05 A longer than that in 3.
Published on the web December 13, 2006; doi:10.1246/bcsj.79.1897

1898 Bull. Chem. Soc. Jpn. Vol. 79, No. 12 (2006)
Ó 2006 The Chemical Society of Japan
ꢀ
equivalent amounts of MeI afforded the diiodo complex of
In summary, the reaction of [Cp Ir(2-Spy)(N3)] and two
ꢀ
[Cp IrI2(2-MeSpy)] (3), in which 2-MeSpy acted as a mono-
dentate S-bonded (ꢀS) ligand; neither in the solid state nor in
solution was detected any evidence for the ꢀN coordination
mode of 2-MeSpy. Contrary to the rich chemistry of transi-
tion-metal thiolate complexes, surprisingly few studies have
been reported so far for 2-MeSpy or 2-RSpy (R = alkyl) com-
plexes.16,17 In most reported complexes, the alkylthiopyridines
act as a monodentate N-bonded (ꢀN), a didentate chelating
(ꢀ2N,S), or a bridging (ꢂ-ꢀN:ꢀS) ligand. For the gold(I) com-
plexes of [Au(2-MeSpy)2]ClO4 and [Au(2-MeSpy)(tht)]ClO4
(tht = tetrahydrothiophene), a ꢀS-coordination mode of 2-
MeSpy was speculated but without enough structural eviden-
ꢀ
ces.16 To the best of our knowledge, the present Cp IrIII com-
Fig. 2. Variable-temperature 1H NMR spectra of complex 3
in CD2Cl2 in the pyridyl and the methyl region.
plex of 3 is the first example in which the S-bonded 2-MeSpy
was confirmed crystallographically and spectroscopically.
Experimental
Complex 3 was directly prepared by reacting 2 and MeI in a
nearly quantitative yield. Methylation at thiolate-S atom is a
commonplace reaction,14 but the resulting S-bonded 2-methyl-
thiopyridine complex is rare (vide infra). We have, also, con-
firmed by IR spectroscopy that the pyridyl-N atom of 2-MeSpy
in complex 3 does not coordinate to the IrIII center in the solid
state. In the spectrum of complex 2, the absorption band due to
an in-plane ring deformation of pyridyl moiety was observed
at 652 cmꢁ1, which was indicative of the N-bonded pyridyl
ligand, while complex 3 showed a band in the region for a free
pyridyl group (607 cmꢁ1).15
Caution. Transition-metal azido complexes are, in general,
potentially explosive, so that should be handled with great cau-
tion, although we did not have any accident with this Cp IrIII–
N3 system.
General. The preparations of the azido complex 1 and the
ꢀ
chloro analogue, [Cp Ir(2-Spy)Cl], were reported previously.7 So-
ꢀ
dium iodide, methyl iodide, thioanisole, 2-methoxypyridine, and
solvents were purchased from Nacalai Tesque, Inc. or Wako Pure
Chemicals Industry, Ltd., and used as received. Variable-temper-
ature 1H NMR spectra were acquired on a JEOL EX-270 spec-
trometer. Chemical shifts were referenced to the residual 1H NMR
signals of the deuterated solvents and reported vs TMS. Infrared
spectra were measured on a Jasco FT-IR 410 spectrophotometer
using Nujol mulls.
Complex 3 in CD3CN at 30 ꢂC exhibited sharp 1H NMR sig-
nals (see Experimental), while in CD2Cl2 at the same temper-
ꢀ
ature the Cp and py-3H resonances were broadened. On cool-
ꢀ
ing the Cp resonance (ꢁ 1.74 at 30 ꢂC) was split into two sig-
Ã
ꢀ
[Cp Ir(2-Spy)I] (2). A mixture of [Cp Ir(2-Spy)Cl] (280 mg,
0.59 mmol) and NaI (490 mg, 3.3 mmol) in methanol (60 cm3) was
stirred at ambient temperature for 3 h. After evaporation of the sol-
vent, the residue was extracted with dichloromethane. The filtered
extract was concentrated (to ca. 2 cm3) under reduced pressure,
and diethyl ether vapor was diffused into the concentrate to give
red block crystals. Yield: 303 mg (91%). Anal. Found: C, 31.63;
H, 3.36; N, 2.42%. Calcd for C15H19IIrNS: C, 31.92; H, 3.39; N.
nals, and the py-3H and SMe resonances (ꢁ 7.69 and 2.80, re-
spectively, at 30 ꢂC) became much broader (Fig. 2). At 0 ꢂC
ꢀ
an integration ratio of ca. 2:1. The lower-field signal seems
two Cp resonances were observed at ꢁ 1.64 and 1.77 with
ꢀ
to correspond to [(Cp IrI)2(ꢂ-I)2], being indicative of the dis-
sociation of 2-MeSpy from the IrIII center. These variable-tem-
1
perature H NMR spectral behaviors were similarly observed
for the analogous thioanisole complex [Cp IrI2(MeSPh)] (4),
2.48%. 1H NMR (CD3CN, 303 K): ꢁ 1.82 (s, Cp , 15H), 6.54 (dt,
ꢀ
ꢀ
which was obtained as orange threadlike crystals. In CD2Cl2,
J ¼ 8:2, 1.1 Hz, py-3H, 1H), 6.81 (ddd, J ¼ 7:4, 5.7, 1.1 Hz, py-
5H, 1H), 7.33 (ddd, J ¼ 8:2, 7.4, 1.7 Hz, py-4H, 1H), 8.00 (ddd,
J ¼ 5:7, 1.7, 1.1 Hz, py-6H, 1H).
ꢀ
the Cp resonance of 4 was observed as a broad singlet (ꢁ
1.69) at 30 ꢂC and split into two signals (ꢁ 1.64 and 1.77) on
decreasing the temperature to 0 ꢂC. The SMe and ortho-H
(of Ph) resonances became broader on cooling (see Supporting
Information). This spectral similarity of complexes 3 and 4 im-
plies that the pyridyl-N atom of 2-MeSpy does not participate
[Cp IrI2(2-MeSpy)] (3). To an acetonitrile solution (10 cm3)
Ã
of complex 2 (105 mg, 0.186 mmol) was added MeI (200 mg,
1.4 mmol). After standing for a day at room temperature, the mix-
ture was allowed to evaporate slowly (to ca. 2 cm3), depositing red
prismatic crystals. Yield: 91 mg (69%). Anal. Found: C, 26.99; H,
3.13; N, 1.97%. Calcd for C16H22I2IrNS: C, 27.20; H, 3.14; N,
ꢀ
in coordination to the Cp IrI2 fragment either in solution.
We have also attempted to prepare the corresponding 2-
1.98%. 1H NMR (CD3CN, 303 K): ꢁ 1.81 (s, Cp , 15H), 2.53
ꢀ
ꢀ
did not react with (excess) 2-MeOpy to give the desired com-
methoxypyridine (2-MeOpy) complex, but [(Cp IrI)2(ꢂ-I)2]
(s, SMe, 3H), 7.04 (ddd, J ¼ 7:4, 4.9, 1.0 Hz, py-5H, 1H), 7.26
(dm, J ¼ 8:3 Hz, py-3H, 1H), 7.57 (ddd, J ¼ 8:3, 7.4, 1.9 Hz,
py-4H, 1H), 8.39 (dm, J ¼ 4:9 Hz, py-6H, 1H).
ꢀ
lated as an orange powdery solid by a reaction of [(Cp IrI)2-
plex. Instead, the pyridine complex [Cp IrI2(py)] (5) was iso-
ꢀ
Ã
[Cp IrI2(MeSPh)] (4). Diethyl ether vapor was diffused into
a dichloromethane (2 cm3) solution of [(Cp IrI)2(ꢂ-I)2]8 (130 mg,
ꢀ
(ꢂ-I)2] and pyridine in dichloromethane. These results suggest
that the unfavorable formation of the N-bonded 2-MeSpy or 2-
MeOpy complexes must result from a severe steric hindrance
0.11 mmol) and MeSPh (230 mg, 1.85 mmol) in a closed vessel.
The resulting orange threadlike crystals were collected by filtra-
tion and dried in vacuo. Yield: 121 mg (77%). Anal. Found: C,
28.77; H, 3.35%. Calcd for C17H23I2IrS: C, 28.94; H, 3.29%.
ꢀ
between the Cp IrI2 fragment and the MeS or MeO ortho-sub-
stituent of (coordinated) pyridine.

Y. Sekioka et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 12 (2006) 1899
ꢀ
1H NMR (CD3CN, 303 K): ꢁ 1.78 (s, Cp , 15H), 2.61 (s, SMe,
3H), 7.20–7.41 (m, SPh, 5H).
References
Z. Dori, R. F. Ziolo, Chem. Rev. 1973, 73, 247; J. Strahle,
[Cp IrI2(py)] (5). Pyridine (2 cm3) was added with sitirring
to a red solution of [(Cp IrI)2(ꢂ-I)2] (50 mg) in dichloromethane
Ã
1
¨
ꢀ
Comments Inorg. Chem. 1985, 4, 295.
J. Du Bois, C. S. Tomooka, J. Hong, E. M. Carreira, Acc.
Chem. Res. 1997, 30, 364; C. A. Grapperhaus, B. Mienert, E. Bill,
(6 cm3). The color of the mixture immediately turned orange, and
an orange precipitate formed after a few minutes. After stirring for
30 min, the product was collected by filtration, washed with
CH2Cl2 (2 cm3) and Et2O (3 cm3), and dried in vacuo. Yield:
42 mg (74%). Anal. Found: C, 27.35; H, 3.04; N, 2.19%. Calcd
for C15H20I2IrN: C, 27.28; H, 3.05; N, 2.12%. 1H NMR (dmso-d6,
2
T. Weyhermuller, K. Wieghardt, Inorg. Chem. 2000, 39, 5306; M.
¨
Tsuchimoto, N. Yoshioka, S. Ohba, Eur. J. Inorg. Chem. 2001,
1045.
3
B. C. Lane, J. W. McDonald, V. G. Myers, F. Basolo, R. G.
Pearson, J. Am. Chem. Soc. 1971, 93, 4934; E. D. Johnson, F.
Basolo, Inorg. Chem. 1977, 16, 554.
ꢀ
303 K): ꢁ 1.88 (s, Cp , 15H), 7.38 (ddd, J ¼ 7:6, 4.2, 1.6 Hz, py-
3,5H, 2H), 7.78 (tt, J ¼ 7:6, 2.0 Hz, py-4H, 1H), 8.57 (dt, J ¼ 4:2,
1.6 Hz, py-2,6H, 2H). IR (Nujol): ꢃ(py) = 639 cmꢁ1
.
4 W. Rigby, P. M. Bailey, J. A. McCleverty, P. M. Maitlis,
J. Chem. Soc., Dalton Trans. 1979, 371; C.-W. Chang, G.-H.
Crystallography. A red block crystal of 2 suitable for the X-
ray diffraction study (approximate dimension of 0:25 ꢄ 0:15 ꢄ
0:15 mm3) was mounted with a cryoloop and flash-cooled by
a cold nitrogen stream. The intensity data were obtained at
ꢁ73ð2Þ ꢂC on a Rigaku R-axis rapid diffractometer [8 ꢅ 2ꢄ ꢅ
Lee, Organometallics 2003, 22, 3107; K. S. Singh, C. Thone,
M. R. Kollipara, J. Organomet. Chem. 2005, 690, 4222.
¨
5
H. Nagao, T. Kikuchi, M. Inukai, A. Ueda, T. Oi, N.
Suzuki, M. Yamasaki, Angew. Chem., Int. Ed. 2006, 45, 3131.
T. Suzuki, A. G. Dipasquale, J. M. Mayer, J. Am. Chem.
Soc. 2003, 125, 10514.
ꢂ
˚
55 , ꢅ(Mo Kꢆ) = 0.71073 A], and numerical absorption collec-
tions from crystal shape were applied.18 The X-ray diffraction data
of a red prismatic crystal of 3 (0:24 ꢄ 0:24 ꢄ 0:16 mm3) sealed in a
glass capillary tube were collected on a Rigaku AFC5R diffrac-
6
7 Y. Sekioka, S. Kaizaki, J. M. Mayer, T. Suzuki, Inorg.
Chem. 2005, 44, 8173.
ꢂ
ꢂ
˚
tometer at 23(2) C [2ꢄ ꢅ 60 , ꢅ(Mo Kꢆ) = 0.71073 A]. Absorp-
tion effects were corrected by an empirical method based on three
sets of ꢀ-scan data.19 The structures were solved by the direct
method using SIR92 program,20 and refined on F2 by full-matrix
least-squares using SHELXL97 program.21 All non-H atoms were
refined anisotropically, and H atoms were treated as riding models.
The crystallographic data for 2: C15H19IIrNS, MW ¼ 564:47,
8
9
M. R. Churchill, S. A. Julis, Inorg. Chem. 1979, 18, 1215.
The crystal structure of the ꢂ-N3 dimer was also analyzed
by X-ray, but the results were not sufficient to report here, because
of the highly efflorescent nature of the crystal and the partial
co-crystallization of the ꢂ-I dimer. However, the resulting atomic
ꢀ
arrangement of [(Cp IrI)2(ꢂ-N3)2] was believed to be correct.
10 A. Mertens, K. Lammertsma, M. Arvanaghi, G. A. Olah,
˚
T ¼200ð2Þ K, orthorhombic, Pbca, a¼8:5551ð4Þ A, b¼16:7071ð7Þ
J. Am. Chem. Soc. 1983, 105, 5657.
11 K. M. Rao, C. L. Day, R. A. Jacobson, R. J. Angelici,
Inorg. Chem. 1991, 30, 5046.
12 M. Valderrama, R. Contreras, V. Arancibia, D. Boys,
J. Organomet. Chem. 2001, 620, 256.
13 S. Ye, W. Kaim, M. Albrecht, F. Lissner, T. Schleid, Inorg.
Chim. Acta 2004, 357, 3325.
14 M. Lissel, S. Schmidt, B. Neumann, Synthesis 1986, 382;
N. Furukawa, F. Takahashi, T. Kawai, K. Kishimoto, S. Ogawa,
S. Oae, Phosphorus, Sulfur 1983, 16, 167.
15 K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds, 5th ed., John Wiley & Sons,
New York, U.S.A., 1997, Part B, p. 23; T. Suzuki, M. Kita, K.
Kashiwabara, J. Fujita, Bull. Chem. Soc. Jpn. 1990, 63, 3434.
16 J. Vicente, M.-T. Chicote, C. Rubio, Chem. Ber. 1996, 129,
327; J. Vicente, M.-T. Chicote, S. Huertas, M. C. R. de Arellano,
P. G. Jones, Eur. J. Inorg. Chem. 1998, 511.
17 A. J. Deeming, M. N. Meah, P. A. Bates, M. B.
Hursthouse, Inorg. Chim. Acta 1988, 142, 37; G. F. de Sousa,
C. A. L. Filgueiras, Trans. Met. Chem. 1990, 15, 290.
18 T. Higashi, Shape, Rigaku Corporation, Akishima, Tokyo,
1999.
3
˚
˚
˚
A, c ¼ 23:6078ð9Þ A, V ¼ 3374:3ð2Þ A , Z ¼ 8, Dcalcd ¼ 2:222
Mg mꢁ3
,
Rint ¼ 0:040, Fð000Þ ¼ 2096, ꢂ(Mo Kꢆ) = 9.855
mmꢁ1, No. of independent reflns = 3765, No. of param = 173,
R1[F2: F2 > 2ꢇðF2Þ] = 0.032, wR2(F2: all data) = 0.071. For
3: C16H22I2IrNS, MW ¼ 706:41, T ¼ 296ð2Þ K, orthorhombic,
˚
˚
˚
Pbca, a ¼ 16:710ð2Þ A, b ¼ 16:706ð2Þ A, c ¼ 14:044ð2Þ A, V ¼
3920:6ð9Þ A , Z ¼ 8, Dcalcd ¼ 2:394 Mg mꢁ3
,
Rint ¼ 0:052,
3
˚
Fð000Þ ¼ 2592, ꢂ(Mo Kꢆ) = 10.06 mmꢁ1, No. of independent
reflns = 5721, No. of param = 191, R1[F2: F2 > 2ꢇðF2Þ] = 0.056,
wR2(F2: all data) = 0.173.
Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition numbers CCDC-
613445 and 613446 for compounds 2 and 3, respectively. Copies
of the data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystal-
lographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ,
UK; Fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
This work was supported by a Grant-in-Aid for Scientific
Research No. 16550055 from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
19 A. C. T. North, D. C. Phillips, F. S. Mathews, Acta
Crystallogr., Sect. A 1968, 24, 351.
20 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi,
M. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 1994, 27,
435.
Supporting Information
Variable-temperature 1H NMR spectra of complex
CD2Cl2. This material is available free of charge on the Web at:
http://www.csj.jp/journals/bcsj/.
4
in
21 G. M. Sheldrich, SHELXL97, University of Gottingen,
Germany, 1997.
¨