Synthesis and structure of the chiral palladium-fullerene C60 and C70 complexes with enantiomeric ligand 2,2′,5,5′-tetramethyl-4,4′-bis(diphenylphosphino)-3, 3′-bithiophene [(-)tetraMe-BITIOP]

Article abstract of DOI:10.1016/j.jorganchem.2005.07.003

η²-Fullerene (C₆₀ and C₇₀) palladium optically active complexes with the axially chiral enantiomeric ligand of bithienyl series, [(-)tetraMe-BITIOP], have been synthesized and investigated by ³¹P-{¹H}

Full text of DOI:10.1016/j.jorganchem.2005.07.003

Journal of Organometallic Chemistry 690 (2005) 4330–4336
www.elsevier.com/locate/jorganchem
Synthesis and structure of the chiral palladium–fullerene
C60 and C70 complexes with enantiomeric ligand
2,20,5,50-tetramethyl-4,40-bis(diphenylphosphino)-3,30-bithiophene
[(ꢀ)tetraMe-BITIOP]
a
a
a
Vasily V. Bashilov , Feodor M. Dolgushin , Pavel V. Petrovskii ,
a,
b
c
d
Viatcheslav I. Sokolov *, Mara Sada , Tiziana Benincori , Gianni Zotti
a
‘‘Nesmeyanov’’ Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Street, 119991 Moscow, Russian Federation
b
Universita di Milano, Dipartamento Chimica Organica e Industriale, via Golgi 19, 20133 Milano, Italia
Universita dellꢀInsubria, Dipartamento Chimiche, Fisiche e Mathematiche, via Valleggio 11, 22100, Como, Italia
c
d
Istituto CNR per lꢀEnergetica e le Interfasi, Corso Stati Uniti 4, 35127 Padova, Italia
Received 20 May 2005; received in revised form 20 June 2005; accepted 4 July 2005
Available online 5 August 2005
Abstract
g2-Fullerene (C60 and C70) palladium optically active complexes with the axially chiral enantiomeric ligand of bithienyl series,
[(ꢀ)tetraMe-BITIOP], have been synthesized and investigated by 31P–{1H} NMR, electronic spectroscopy, CD spectroscopy, cyclic
voltammetry and by single crystal X-ray diffraction.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: [60]fullerene; [70]fullerene; Palladium–fullerene complexes; Optical activity; Axial chirality; (ꢀ)tetraMe-BITIOP; Circular dichroism;
NMR electronic spectroscopy; X-ray structure; Cyclic voltammetry
1. Introduction
to be the direct synthesis using Pd2dba3, fullerene and
a free phosphine ligand [6–8].
Coordinatively unsaturated phosphine complexes of
10 group metals (Pt, Pd) are known to add on the dou-
ble bonds of fullerenes to afford exo-metal g2-fullerene
complexes [1], however, a mixture of adducts is often
formed with different number of the metal fragments
up to six [2]; organometallic complexes of fullerenes
have been reviewed [3]. We have been interested for sev-
eral years in developing new routes leading selectively to
mono-metallated fullerenes. It was found that fullerenes
are able to abstract the ML2 moiety from such com-
pounds as RHgPtL2R0 [4] or R2PtL2 [5]. The most prac-
tical preparation of palladium g2-complexes appeared
Keeping in mind the catalytic activity of palladium
g2-fullerene complexes found in the hydrogenation reac-
tion [9,10], one can expect that the optically active ana-
logues might be interesting for enantioselective catalysis.
In addition, the fullerene core is a chromophore with
many absorption bands in the electronic spectra; at least
some of them may be optical active in a chiral environ-
ment and therefore interesting for chiroptical study.
However, just few optically active organometallic ful-
lerene derivatives have been reported till present time.
More than ten years ago we have synthesized the first
optically active organometallic g2-C60 complex with pal-
ladium [6], then with platinum [11] using a well-known
bidentate enantiomeric tertiary diphosphine ligands
(+)DIOP (1) (CD spectra of (g2-C60)M[(+)DIOP],
*
Corresponding author
E-mail address: sokol@ineos.ac.ru (V.I. Sokolov).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.003

V.V. Bashilov et al. / Journal of Organometallic Chemistry 690 (2005) 4330–4336
4331
M = Pd, Pt in different solvents were reported elsewhere
[12]). Later on the chiral palladium complex with race-
mic monodentate ligand phenylbenzylferrocenylphos-
phine (2) (g2-C60)Pd(PPhBnFc)2 has been prepared
[13]; for review of all chiral and optically active deriva-
tives of fullerenes see [14]. Recently our work [11] was
repeated by another research group with (ꢀ)DIOP
enantiomer to afford the same results in X-ray structure
and CD spectra [15]. In addition, optically active g2-C60
molybdenum and tungsten complexes have been re-
ported with enantiomeric bidentate ligands (g2-
C60)M(CO)3[L-L], where [L-L] is either the same DIOP
[15] or the optically active N,N-ligand (what is unusual
for g2-fullerenyl metal complexes) bpy*-(+) or (ꢀ) 4,5-
pinenobipyridine (M = Mo only) [16].
one of the most efficient chiral chelate ligands for enan-
tioselective homogeneous hydrogenation [17].
When a fullerene (C60 or C70) reacts with Pd2
(dba)3 ÆC6H6 (4), a known synthetic equivalent of palla-
dium atom, in toluene or benzene in the presence of 3, col-
our of the solution immediately turns to green or brown,
correspondingly, and crystals of the products 5 or 6 sep-
arate slowly in nearly quantitative yield (Scheme 1).
The crystalline samples of (g2-C60) Pd(tetraMe–BI-
TIOP)ÆEt2O (5) and (g2-C70)Pd(tetraMe-BITIOP) (6)
were characterized by elemental analysis. Complexes 5
and 6 are sparingly soluble in most common solvents
but are sufficiently soluble in 1,2-dichlorobenzene
(ODCB) to permit recording of their NMR spectra.
31P–1{H} NMR spectrum of 5 exhibited a single reso-
nance at 22.9 ppm that is deshielded as compared with
that of the free ligand, ꢀ21.1 ppm. Both phosphorus nu-
clei are equivalent. The coordination shift for 5 is
44.0 ppm. On the contrary, 31P–1{H} NMR spectrum
of 6 showed two doublets of magnetically nonequivalent
phosphorus nuclei (Fig. 1). This is a typical AB system
with d(PA) 23.6 ppm, d(PB) 23.2 ppm, J(PA,PB) =
23.2 Hz. The coordination shifts for 6 are 44.7 and
44.3 ppm. This pattern is characteristic of the metal g2
complexes formed on the most reactive Ca–Cb bond in
C70.
Ligands 1 and 2 have chiral centres at either carbon
or phosphorus atoms. Now we wish to report on the
synthesis of the palladium–fullerene complexes with
the enantiomeric ligand 3 with axial chirality.
The electronic absorption spectra of 5 and 6 in tolu-
ene (Fig. 2) are similar to those of C60 and C70. Broad
peaks occur at positions: [kmax (nm), loge, molar extinc-
tion coefficients]; 665 (3.50), 624 (3.88), 452 (4.10), 355
(4.59) (for 5), and 695 (3.74), 603 (3.88), 471 (4.29),
380 (4.31) (for 6). The most characteristic bands ob-
served in the spectra of 5 and 6 are the charge-transfer
bands 452 and 471 nm correspondingly.
2. Results and discussion
Enantiomeric
ligand
2,20,5,50-tetramethyl-4,40-
bis(diphenylphosphino)-3,30-bithienyl, (ꢀ)-(tetraMe-BI-
TIOP), 3, had been synthesized previously and its chir-
optical properties investigated. Chirality of this
molecule is of axial nature (atropoisomerism) due to
the restricted rotation around the thiophene–thiophene
bond assisted by four substituents in a,b-positions. tet-
raMe-BITIOP (in the form of Ru or Rh complexes) is
CD spectra of 5 and 6 were registered in toluene as
well (Fig. 3); they exhibited ill-resolved Cotton effects
shifted to longer wavelengths compared with the
Scheme 1.

4332
V.V. Bashilov et al. / Journal of Organometallic Chemistry 690 (2005) 4330–4336
Fig. 1. 31P–{1H} NMR spectrum of 6 in 1,2-dichlorobenzene (ODCB).
starting ligand 3 that probably points to the electron
density transfer from the ligand to fullerene as in other
palladium–fullerene complexes [14]. R(ꢀ) absolute con-
figurations for the ligand 3 previously deduced from CD
spectra only has been now confirmed by direct X-ray
study of the complex 5 (see below).
Electrochemical experiments were performed at 25 ꢁC
under nitrogen in three-electrode cells. The compound 5
was dissolved in 1,2-dichlorobenzene (ODCB) with tet-
rabutylammonium perchlorate (TBAP) previously dried
under vacuum at 70 ꢁC as supporting electrolyte. The
counter-electrode was platinum and the reference elec-
trode was a silver/0.1 M silver perchlorate in acetonitrile
(0.39 V versus Ag/AgCl). The working electrode for cyc-
lic voltammetry was a platinum minidisk electrode
(0.003 cm2).
Fig. 2. Visible–ultraviolet absorption spectra of 5 (A) and 6 (B) in
toluene.
typical of C60 (Fig. 4). They are displayed at E1/2
=
ꢀ1.11; ꢀ1.49; and ꢀ1.95 V. The three waves are shifted
to more negative potential by about 0.05 V relative to
those of free C60 that is consistent with the presence of
a donating ligand. Analogous by sign shifts of the reduc-
The cyclic voltammogram of the compound 5
(1 · 10ꢀ3 M) in ODCB + 0.1 M TBAP displays in the
reductive scan three one-electron reversible processes
Fig. 3. Circular dichroism spectra of the complexes 5 and 6 together with the ligand 3.

V.V. Bashilov et al. / Journal of Organometallic Chemistry 690 (2005) 4330–4336
4333
Table 1
Selected bond lengths and angles for 5
˚
Bond lengths (A)
Pd(1)–P(1)
Pd(1)–P(2)
Pd(1)–C(1)
Pd(1)–C(2)
S(1)–C(61)
S(1)–C(64)
S(2)–C(67)
S(2)–C(70)
P(1)–C(62)
P(1)–C(73)
P(1)–C(79)
P(2)–C(68)
P(2)–C(85)
P(2)–C(91)
2.327(2)
2.299(2)
2.083(5)
2.135(6)
1.735(6)
1.726(6)
1.722(6)
1.710(6)
1.824(6)
1.802(6)
1.818(6)
1.812(6)
1.835(6)
1.818(6)
1.482(8)
1.461(8)
1.489(8)
1.464(8)
1.483(8)
1.358(8)
1.483(8)
1.451(7)
1.352(8)
1.487(8)
1.476(8)
1.347(8)
1.520(8)
1.452(8)
1.364(9)
1.520(9)
Fig. 4. Cyclic voltammogram for (g2-C60)Pd[(ꢀ)tetraMe-BITIOP] in
C(1)–C(2)
C(1)–C(9)
C(1)–C(6)
C(2)–C(3)
ODCB + 0.1 M TBAP. Scan rate: 0.1 V/s.
tion waves were recently observed [18] in the electro-
chemical study of palladium–fullerene complexes with
metallocene-phosphine ligands. In the positive scan the
compound 5 is irreversibly oxidized in a two-electron
process at Ep = 0.32 V.
The molecular structure of 5ÆEt2O was established by
a single-crystal X-ray study. The structure of complex 5
with the atomic numbering scheme is shown in Fig. 5;
important geometrical parameters are listed in Table 1.
C(2)–C(12)
C(61)–C(62)
C(61)–C(65)
C(62)–C(63)
C(63)–C(64)
C(63)–C(69)
C(64)–C(66)
C(67)–C(68)
C(67)–C(71)
C(68)–C(69)
C(69)–C(70)
C(70)–C(72)
Bond angles (ꢁ)
C(1)–Pd(1)–C(2)
C(1)–Pd(1)–P(2)
C(2)–Pd(1)–P(2)
C(1)–Pd(1)–P(1)
C(2)–Pd(1)–P(1)
P(2)–Pd(1)–P(1)
C(64)–S(1)–C(61)
C(70)–S(2)–C(67)
C(73)–P(1)–C(79)
C(73)–P(1)–C(62)
C(79)–P(1)–C(62)
C(73)–P(1)–Pd(1)
C(79)–P(1)–Pd(1)
C(62)–P(1)–Pd(1)
C(68)–P(2)–C(91)
C(68)–P(2)–C(85)
C(91)–P(2)–C(85)
C(68)–P(2)–Pd(1)
C(91)–P(2)–Pd(1)
C(85)–P(2)–Pd(1)
C(62)–C(61)–C(65)
C(62)–C(61)–S(1)
C(65)–C(61)–S(1)
C(61)–C(62)–C(63)
C(61)–C(62)–P(1)
C(63)–C(62)–P(1)
C(64)–C(63)–C(62)
C(64)–C(63)–C(69)
C(62)–C(63)–C(69)
C(63)–C(64)–C(66)
C(63)–C(64)–S(1)
41.1(2)
160.6(2)
119.6(2)
101.5(2)
142.6(2)
97.73(6)
93.0(3)
92.2(3)
108.8(3)
105.3(2)
102.9(3)
103.7(2)
114.0(2)
121.6(2)
106.2(3)
105.4(3)
104.0(3)
104.2(2)
121.8(2)
114.0(2)
133.0(6)
109.8(5)
117.1(4)
113.6(6)
128.0(5)
118.4(4)
112.3(5)
123.5(5)
123.2(5)
128.7(6)
111.1(4)
Fig. 5. ORTEP drawing of (g2-C60)Pd[(ꢀ)tetraMe-BITIOP]. Hydro-
(continued on next page)
gen atoms are omitted for clarity.

4334
V.V. Bashilov et al. / Journal of Organometallic Chemistry 690 (2005) 4330–4336
˚
Table 1 (continued)
tance for the rest of the fullerene carbons is 3.521 A,
interval being 3.488–3.549 A.
˚
C(66)–C(64)–S(1)
C(68)–C(67)–C(71)
C(68)–C(67)–S(2)
C(71)–C(67)–S(2)
C(67)–C(68)–C(69)
C(67)–C(68)–P(2)
C(69)–C(68)–P(2)
C(70)–C(69)–C(68)
C(70)–C(69)–C(63)
C(68)–C(69)–C(63)
C(69)–C(70)–C(72)
C(69)–C(70)–S(2)
C(72)–C(70)–S(2)
120.2(4)
130.6(6)
112.1(5)
117.2(4)
111.9(5)
125.3(4)
121.4(4)
112.3(5)
119.6(5)
127.9(5)
127.0(6)
111.5(5)
121.4(5)
Planes of two thiophene cycles form the dihedral an-
gle equal to 75.5ꢁ (torsion angle C(62)–C(63)–C(69)–
C(68) is 76.9ꢁ).
Intermolecular distances in the crystal correspond the
usual van-der-Waals contacts.
Absolute configurations of two enantiomers of free
tetraMe-BITIOP have been previously assigned by com-
parison of their CD curves with other biheteroaryl
diphosphines [17]. This work establishes by anomalous
dispersion effects in diffraction measurements on the
crystal (see Section 3) that (ꢀ)tetraMe-BITIOP in com-
plex 5 has R configuration that is in agreement with pre-
vious assignment.
Palladium atom is g2-coordinated to C60 on (6:6)
bond C(1)–C(2) with non-equal bond lengths Pd–C(1)
˚
˚
2.083(5) A and Pd–C(2) 2.135(6) A. Due to coordination
˚
C(1)–C(2) bond is much longer [1.482(8) A] than the
non-coordinated (6:6) bonds having the average length
1.391 A.
3. Experimental
˚
Palladium atom has a slightly distorted planar-square
coordination, dihedral angle between planes P(1)–Pd–
P(2) and C(1)–Pd–C(2) being 2.6ꢁ. Two similar Pd–P
bonds have also different lengths (see Table 1), in addi-
tion, they are inclined to the fullerene core in a different
way (angles are following: P(1)–Pd–C(2) 142.6(2)ꢁ and
P(2)–Pd–C(1) 160.6(2)ꢁ). This may be due to the non-
identical orientations of phenyl groups C(73)–C(78)
and C(91)–C(96) at P(1) and P(2) correspondingly, tor-
sion angles being C(74)–C(73)–P(1)–Pd 60.5ꢁ and
C(96)–C(91)–P(2)–Pd 15.6ꢁ. Thus the phenyl group
C(91)–C(96) is directed by its ‘‘edge’’ to fullerene and
situated nearly cis to the Pd–P(2) bond; torsion angle
C(91)–P(2)–Pd–X, where X is a midpoint of C(1)–
C(2), is 7.7ꢁ whereas phenyl groups at P(1) are situated
in gauche position to fullerene concerning Pd–P(1) bond
with torsion angle C(73)–P(1)–Pd–X equal to 40.8ꢁ.
As a result of coordination with metal atom the ful-
lerene core undergoes the substantial distortion which
reflects in the increase of not only C(1)–C(2) length
but also of all C–C bonds involving either C(1) or
C(2) atom. Their lengths are within the interval 1.461–
All the reactions were carried out using standard
Schlenk technique under argon atmosphere. The sol-
vents were dried, degassed, and distilled under argon
prior to use. 31P–{1H} (at 162 MHz) and 1H NMR
spectra (at 400 MHz) were recorded on a Bruker
AMX-400 FT instrument; the chemical shifts were mea-
sured relative to 1% H3PO4 and TMS, respectively.
Electronic absorption spectra were registered on ÆÆUV-
2201 SHIMADZUææ, circular dichroism spectra on
JASCO-720. Electrochemical measurements were made
using the voltammetric apparatus (AMEL, Italy)
including a 551 potentiostat modulated by a 568 pro-
grammable function generator and coupled to a 731
digital integrator.
3.1. Preparation of (g2-C60)Pd[(ꢀ)tetraMe-BITIOP]
(5)
A mixture of C60 (72 mg, 1·10ꢀ4 mol), Pd2(dba)3
ÆC6H6 (50 mg, 5·10ꢀ5 mol), and tetraMe-BITIOP
(60 mg, 1.01·10ꢀ4 mol) was stirred in benzene (10 ml
of C6H6 and 1 ml of C6D6) at room temperature. From
time to time aliquots of the solution were examined by
31P NMP spectroscopy. Colour of the solution gradually
turned to green and dark-green precipitate formed. It
was separated after 5 days, thoroughly washed with
diethyl ether and dried in vacuo. The yield of complex
5 was 137 mg (97%). NMR 31P–{1H}(ODCB, d, ppm):
22.87, singlet. To obtain single crystals suitable for X-
ray study 5 was dissolved in 1,2-dichlorobenzene
(ODCB), solution was filtered into Schlenk vessel, pen-
tane and diethyl ether were added and solution was left
for 40 days at room temperature isolated from daylight.
Then crystals were separated from mother liquor,
washed with dry ether and dried in argon stream to give
80.9 mg of 5 monosolvate (g2-C60)Pd[(ꢀ)tetraMe-BI-
TIOP]. Et2O as dark green plate crystals. Analysis.
˚
˚
1.489 A (in average 1.474 A) whereas the average length
for the rest of (5:6) bonds of the fullerene amounts
˚
1.448 A. Besides, 5- and 6-membered cycles involving
C(1) and C(2) atoms are declined of planarity to the
conformations of envelope and sofa, respectively.
Atoms C(1) and C(2) go out of the planes of these cycles
˚
by 0.14 A in average while the rest of 5- and 6-mem-
bered cycles of the fullerene core are planar better than
˚
0.008A.
Distortion of the fullerene ligand in complex 5 can be
described by its ‘‘radius’’ in different directions. So, dis-
tances from the fullerene centre (determined without
taking into account C(1), C(2) and symmetrically oppo-
site C(59) and C(60) positions) to C(1) and C(2) are
˚
3.694 and 3.683 A, respectively, whereas the average dis-

V.V. Bashilov et al. / Journal of Organometallic Chemistry 690 (2005) 4330–4336
4335
Found (%): C 79.95; H 2.72; S 4.08. C96H32P2PdS2 Æ
Acknowledgements
C4H10O calc. (%): C 80.51; H 2.84; S 4.30%. 31P–{1H}
1
NMR (ODCB-C6D6, d, ppm): singlet, 22.86. H NMR
This work had been supported by grants from Rus-
sian Foundation for Basic Research (RFBR), project
03-03-32695, Ministry of Science and Technology RF,
Chemistry Division of Russian Academy of Sciences,
and Leading Scientific Schools of the President of the
Russian Federation SS-1060.2003.03.
(ODCB-C6D6, d, ppm): singlet, 1.995 (6H, 2CH3); sin-
glet, 2.038 (6H, 2CH3).
3.2. Preparation of (g2-C70)Pd[(–)tetraMe-BITIOP]
(6)
A mixture of C70 (92.4 mg, 1.1·10ꢀ4 mol), Pd2
(dba)3 ÆC6H6 (50 mg, 5·10ꢀ5 mol), and tetraMe-BI-
TIOP (61 mg, 1.03·10ꢀ4 mol) was stirred in benzene
(7 ml of C6H6 and 0.56 ml of C6D6) at room tempera-
ture with monitoring by 31P NMP spectroscopy. After
12 h only non-complexed starting diphosphine 3, d(P)
21.11 ppm, could be revealed in solution. The product
precipitated was filtered off, washed with diethyl ether,
pentane and dried in vacuo to afford quantitative yield
(154 mg) of complex 6 as dark-brown small crystals.
Found (%): C 82.62; H 2.10; P 3.56. C106H32P2PdS2 calc.
(%): C 82.78; H 2.10; P 4.03. 31P–1H NMR (ODCB-
C6D6, d, ppm): AB-system, d(PA) = 23.57, d(PB) =
23.18, J(PA,PB) = 23.2 Hz.
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[15] L.C. Song, P.C. Liu, J.T. Liu, F.H. Su, G.F. Wang, Q.M. Hu,
P. Zanello, F. Laschi, M. Fontani, Eur. J. Inorg. Chem. (2003)
3201.
[16] H. Zhang, C.F. Zhu, L. Li, W. Zou, Y.Q. Huang, J.X. Gao,
Chin. Chem. Lett. 15 (2004) 1411.
[17] (a) T. Benincori, E. Cesarotti, O. Piccolo, F. Sannicolo, J. Org.
Chem. 65 (2000) 2043;
C96H32P2S2PdÆC4H10O (M = 1491.80), orthorhom-
˚
bic, space group P212121, a = 14.5760(9) A, b =
3
˚
˚
˚
17.087(1) A, c = 24.919(2) A, V = 6206.0(7) A , Z = 4,
l = 0.480 mmꢀ1, crystal size 0.40 · 0.20 · 0.05 mm. Sin-
gle-crystal X-ray diffraction experiment was carried out
with a Bruker SMART 1000 CCD area detector, using
graphite
monochromated
˚
Mo
Ka
radiation
(k = 0.71073 A, x-scan with 0.3ꢁ step in x and 20 s
per frame exposure, 2h < 54ꢁ) at 120 K. Reflection inten-
sities were integrated using SAINT software [19] and semi-
empirical method SADABS [20] was applied for absorption
correction (Tmin/Tmax = 0.831/0.976). A total of 42773
reflections were measured, 13595 (Rint = 0.0847) inde-
pendent reflections were used in further calculations
and refinement. The structure was solved by direct
methods and refined by the full-matrix least-squares
against F2 in anisotropic (for non-hydrogen atoms)
approximation. All hydrogen atoms were placed geo-
metrically and included in the structure factor calcula-
tion in the riding motion approximation. Absolute
configuration was determined from the value of Flack
parameter (x = 0.00(3)) [21]. The final refinement was
converged to R1 = 0.0633 (for 9395 observed reflections
with I > 2r(I)) and wR2 = 0.1242 (for all unique reflec-
tions); the number of the refined parameters is 955. All
calculations were performed on an IBM PC/AT using
the SHELXTL software [22]. Crystallographic data for
the structural analysis of 5 has been deposited with the
Cambridge Crystallographic Data Centre, CCDC No.
272221.
(b) P. Antognazza, T. Benincori, S. Mazzoli, F. Sannicolo, G.
Zotti, Sulfur Silicon 146 (1999) 405.

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V.V. Bashilov et al. / Journal of Organometallic Chemistry 690 (2005) 4330–4336
[18] T.V. Magdesieva, V.V. Bashilov, D.N. Kravchuk, F.M. Dolgu-
shin, K.P. Butin, V.I. Sokolov, Izv. AN, Ser. Khim. (2004) 759
[Russ. Chem. Bull., 53 (2004) 795 (Engl. Transl.)].
[19] SMART V5.051 and SAINT V5.00, Area detector control and integra-
tion software, Bruker AXS Inc., Madison, WI 53719, USA, 1998.
[20] G.M. Sheldrick, SADABS, Bruker AXS Inc., Madison, WI 53719,
USA, 1997.
[21] H.D. Flack, Acta Crystallogr. A39 (1983) 876–881.
[22] G.M. Sheldrick, SHELXTL-97, Version 5.10, Bruker AXS Inc.,
Madison, WI 53719, USA, 1997.