Si-C bond activation of ArMe2SiOH promoted by a bromoplatinum(II) complex and Ag2O. Aryl group transfer from silicon to platinum

Article abstract of DOI:10.1021/om010006c

[PtBr(PEt₃)₃]BF₄ (1) reacts with an excess of dimethyl{4-(trifluoromethyl)phenyl}silanol in the presence of Ag₂O and 4 ? molecular sieves at 60°C to afford trans-Pt(C₆H₄CF₃-4)₂ (PEt₃)₂ (2). The reaction without molecular sieves and that with addition of a small amount of water give a mixture of 2 and [Pt₂(μ-OH)₂-(PEt₃)₄] (BF₄)₂ (3). The organic products of the reaction are composed of ArSiMe₂-O-(SiMe₂-O)_n- SiMe₂Ar (Ar = C₆H₄CF₃-4, n = 0, 1, 2) and cyclooligodimethylsiloxanes in lesser amounts. The obtained complexes were characterized by NMR spectroscopy and X-ray crystallography.

Full text of DOI:10.1021/om010006c

Organometallics 2001, 20, 1243-1246
1243
Notes
Si-C Bon d Activa tion of Ar Me2SiOH P r om oted by a
Br om op la tin u m (II) Com p lex a n d Ag2O. Ar yl Gr ou p
Tr a n sfer fr om Silicon to P la tin u m
Neli Mintcheva, Yasushi Nishihara, Makoto Tanabe, Kazunori Hirabayashi,
Atsunori Mori, and Kohtaro Osakada*
Chemical Resources Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, J apan
Received J anuary 2, 2001
Summary: [PtBr(PEt3)3]BF4 (1) reacts with an excess of
dimethyl{4-(trifluoromethyl)phenyl}silanol in the pres-
ence of Ag2O and 4 Å molecular sieves at 60 °C to afford
trans-Pt(C6H4CF3-4)2(PEt3)2 (2). The reaction without
molecular sieves and that with addition of a small
amount of water give a mixture of 2 and [Pt2(µ-OH)2-
(PEt3)4](BF4)2 (3). The organic products of the reaction
are composed of ArSiMe2-O-(SiMe2-O)n-SiMe2Ar (Ar
) C6H4CF3-4, n ) 0, 1, 2) and cyclooligodimethylsilox-
anes in lesser amounts. The obtained complexes were
characterized by NMR spectroscopy and X-ray crystal-
lography.
natively unsaturated Pt(0),9 Rh(I),10 and Ru(0)11 com-
plexes occurs selectively to afford complexes with metal-
alkynyl carbon and/or metal-silicon bonds.
These reactions appear to be related to the mecha-
nism involved in transition metal-catalyzed synthetic
organic reactions involving the cleavage of Si-C(sp)
bonds.12,13 The cleavage of an Si-C bond is also involved
in a number of coupling reactions of chlorosilanes14,15
and fluorosilicates16,17 with aryl halides. Recently, we
found that a Pd complex-catalyzed cross-coupling reac-
tion of dimethyl(aryl)silanol with iodoarenes occurs in
the presence of Ag2O to give biaryls.18 Scheme 1 depicts
(11) Huang, D.; Heyn, R. H.; Bollinger, J . C.; Caulton, K. G.
Organometallics 1997, 16, 292.
In tr od u ction
(12) (a) Suzuki, T.; Mita, I. J . Organomet. Chem. 1991, 414, 311.
(b) Ito, H.; Arimoto, K.; Sensui, H.; Hosomi, A. Tetrahedron Lett. 1997,
38, 3977. (c) Ikegashira, K.; Nishihara, Y.; Hirabayashi, K.; Mori, A.;
Hiyama, T. Chem. Commun. 1997, 1039. (d) Nishihara, Y.; Ikegashira,
K.; Mori, A.; Hiyama, T. Chem. Lett. 1997, 1233. (e) Mori, A.; Fujita,
A.; Nishihara, Y.; Hiyama, T. Chem. Commun. 1997, 2159. (f) Nishi-
hara, Y.; Ikegashira, K.; Mori, A.; Hiyama, T. Tetrahedron Lett. 1998,
39, 4075. (g) Mori, A.; Fujita, A.; Kajiro, H.; Nishihara, Y.; Hiyama, T.
Tetrahedron 1999, 55, 4573. (h) Nishihara, Y.; Ikegashira, K.; Hira-
bayashi, K.; Ando, J .; Mori, A.; Hiyama, T. J . Org. Chem. 2000, 65,
1780. (i) Nishihara, Y.; Ikegashira, K.; Toriyama, F.; Mori, A.; Hiyama,
T. Bull Chem. Soc. J pn. 2000, 73, 985. (j) Nishihara, Y.; Ando, J .; Kato,
T.; Mori, A.; Hiyama, T. Macromolecules 2000, 33, 2779.
(13) Related synthetic organic reactions involving metal-promoted
Si-C(sp2) bond cleavage processes: (a) Seki, Y.; Takeshita, K.; Kawa-
moto, K. J . Organomet. Chem. 1989, 369, 117. (b) Wakatsuki, Y.;
Yamazaki, H. J . Organomet. Chem. 1995, 500, 349, and references
therein. (c) Mise, T.; Takaguchi, Y.; Umemiya, T.; Shimizu, S.;
Wakatsuki, Y. Chem. Commun. 1998, 699. (d) Marciniec, B.; Pietras-
zuk, C. J . Organomet. Chem. 1993, 447, 163. (e) Marciniec, B.;
Pietraszuk, C.; Foltynowicz, Z. J . Organomet. Chem. 1994, 474, 83. (f)
Marciniec, B.; Pietraszuk, C. J . Chem. Soc., Chem. Commun. 1995,
2003. (g) Marciniec, B.; Kujawa, M.; Pietraszuk, C. Organometallics
2000, 19, 1677. (h) Kakiuchi, F.; Yamada, A.; Chatani, N.; Murai, S.;
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Transition metal complex promoted Si-C bond acti-
vation is attractive because it leads to the formation of
new transition metal-carbon bonds under mild condi-
tions; however, the number of reports on this subject is
small, partly due to the large bond dissociation energy
(311 kJ mol-1)1 required for activation. An organosilyl
group bonded to metal center(s) of mono- and dinuclear
transition metal complexes can undergo intramolecular
Si-C bond cleavage,2-8 a process that has been reported
more frequently than intermolecular Si-C bond activa-
tion by transition metal complexes. Oxidative addition
of a Si-C(sp) bond of trialkyl(alkynyl)silane to coordi-
(1) Organometallics. A Concise Introduction, 2nd ed.; Elschenbroich,
C., Salzer, A., Eds.; Wiley-VCH: New York, 1992; Chapter 8, pp 96-
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115, 3022, and references therein. (b) Lin, W.; Wilson, S. R.; Girolami,
G. S. Organometallics 1994, 13, 2309.
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1995, 14, 3855.
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Organometallics 1998, 17, 5721.
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Angew. Chem., Int. Ed. 1990, 29, 880.
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Wadepohl, H. J . Chem. Soc., Dalton Trans. 1979, 1749.
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12, 1359. (b) Baum, M.; Windmu¨ller, B.; Werner, H. Z. Naturforsch.
1994, 49B, 859.
(14) Weber, W. P.; Felix, R. A.; Willard, A. K.; Koenig, K. E.
Tetrahedron Lett. 1971, 4701.
(15) Yamamoto, K.; Shinohara, K.; Ohuchi, T.; Kumada, M. Tetra-
hedron Lett. 1974, 1153.
(16) (a) Mu¨ller, R. Z. Chem. 1985, 25, 421. (b) Hatanaka, Y.; Hiyama,
T. J . Org. Chem. 1988, 53, 918. (c) Hatanaka, Y.; Matsui, K.; Hiyama,
T. Tetrahedron Lett. 1989, 30, 2403. (d) Hiyama T. Metal-Catalyzed
Cross-coupling Reactions. In Organosilicon Compounds in Cross-
Coupling Reactions; Diederich F., Stang P. J ., Eds.; Wiley-VCH: New
York, 1998; Chapter 10, pp 421-454.
(17) (a) Mowery, M. E.; DeShong, P. J . Org. Chem. 1999, 64, 1684.
(b) Denmark, S. E.; Choi, J . Y. J . Am. Chem. Soc. 1999, 121, 5821.
(18) (a) Hirabayashi, K.; Kawashima, J .; Nishihara, Y.; Mori, A.;
Hiyama, T. Org. Lett. 1999, 1, 299. (b) Hirabayashi, K.; Mori, A.;
Kawashima, J .; Suguro, M.; Nishihara, Y.; Hiyama, T. J . Org. Chem.
2000, 65, 5342.
10.1021/om010006c CCC: $20.00 © 2001 American Chemical Society
Publication on Web 02/15/2001

1244 Organometallics, Vol. 20, No. 6, 2001
Notes
Sch em e 1
a generally accepted mechanism for these cross-coupling
reactions that involve the transfer of the silicon-bonded
organic group to the arylpalladium intermediate. There
have been only a few reports19 that provide direct
evidence for the formation of transition metal-carbon
bonds via the transmetalation of such Si-containing
compounds. We have examined the reaction of silanols
with several group 10 metal complexes and report
herein the transmetalation of dimethyl(aryl)silanol to
a bromoplatinum(II) complex in the presence of Ag2O
to give a diarylplatinum complex.
Resu lts a n d Discu ssion
F igu r e 1. Structure of complex 2 determined by X-ray
crystallography with 50% thermal ellipsoidal plotting. The
molecule has crystallographic C2 and Cm symmetry within
the molecule. Atoms with asterisks are crystallographically
equivalent to those having the same number without
asterisks. Selected bond lengths (Å) and angles (deg): Pt1-
C1 2.089(8), Pt1-P1 2.287(3), P1-Pt1-C1 89.7(3), P1-
Pt1-C1* ) 90.3(3).
The reaction of dimethyl{4-(trifluoromethyl)phenyl}-
silanol with [PtBr(PEt3)3]BF4 (1)20 in 8:1 molar ratio in
the presence of Ag2O and 4 Å molecular sieves (30 h at
60 °C) in anhydrous acetone produces trans-Pt(C6H4-
CF3-4)2(PEt3)2 (2) in 56% isolated yield (eq 1). NMR
1
singlet at δ 8.43 with a coupling constant J PtP ) 2750
Hz, which is typical of Pt(II) complexes possessing two
trans phosphine ligands. The 13C{1H} NMR signal of
the PCH2 carbon at δ 13.9 appears as an apparent
triplet due to virtual coupling. The ipso carbon signal
of the aryl ligand is a triplet (δ 169.4) with J (PC) ) 11
Hz. These NMR data are similar to those of the
previously reported trans-PtAr2(PBu3)2 and trans-PtAr2-
(PPh3)2.21
analysis of the reaction mixture indicated that 2 is the
main product (∼80% yield). The reaction in 2:1 molar
ratio produces 2 in lower yield (24%) probably due to
accompanying dehydration of the silanol to the siloxane,
which does not react further with the Pt complex (vide
infra). Figure 1 shows the molecular structure of 2 as
determined by X-ray crystallography. The molecule has
crystallographic C2 symmetry around the metal center
and Cm symmetry at the coordination plane. The square-
planar coordination with two aryl and phosphine ligands
at trans positions is dominant also in solution. The 31P-
{1H} NMR spectrum of 2 in benzene-d6 consists of a
The reaction without addition of 4 Å molecular sieves
gives a mixture of 2 and [Pt2(µ-OH)2(PEt3)4](BF4)2 (3)
in 51% and 29% yields, respectively (eq 2). Complex 3
(19) Triorganosilyl group tethered to a Pd(II) complex reacts with
TASF ((Me2N)3S+SiMe3F2-), leading to Si-C bond cleavage and
metalation of the alkyl group. See: Mateo, C.; Ferna´dez-Rivas, C.;
Echavarren, A. M.; Ca´denas, D. J . Organometallics 1997, 16, 1997.
Mateo, C.; Ferna´dez-Rivas, C.; Ca´denas, D. J .; Echavarren, A. M.
Organometallics 1998, 17, 1998.
was identified by comparison of the spectroscopic and
(20) [PtBr(PEt3)3]BF4 was prepared by a slight modification of the
preparation of analogous [PtF(PEt3)3]BF4 and [PtCl(PEt3)3]BF4. See:
Cairns, M. A.; Dixon, K. R.; McFarland, J . J . J . Chem. Soc., Dalton
Trans. 1975, 1159.
(21) (a) Ertl, J .; Debaerdemaeker, T.; Brune, H. A. Chem. Ber. 1982,
115, 3860. (b) Brune, H. A.; Unsin, J .; Alt, H. G.; Schmidtberg, G.;
Spohn, K.-H. Chem. Ber. 1984, 117, 1606.

Notes
Organometallics, Vol. 20, No. 6, 2001 1245
The preparation of 1 was carried out by slight modification
of the reported preparation method of analogous complexes
as follows.20 To an acetone (10 mL) solution of PtBr2(PEt3)2
(355 mg, 0.50 mmol) was added AgBF4 (97 mg, 0.50 mmol) in
one portion, which caused separation of a gray solid of AgBr
from the pale yellow solution. After stirring of the mixture for
1 h, the solid product was removed by filtration. Solvent was
reduced to 3-4 mL at room temperature under vacuum.
Addition of Et2O (10 mL) and subsequent cooling of the
mixture to below -10 °C for 24 h gave 1 as colorless crystals,
which were collected by filtration and dried in vacuo (228 mg,
64%). 1H NMR (300 MHz, benzene-d6): δ 0.90 (apparent
quintet due to virtual coupling, 18 H, CH3), 1.17 (dt, 9H, CH3,
J (PH) ) 15 Hz, J (HH) ) 8 Hz), 1.96 (m, 12H, PCH2), 2.17 (m,
6H, PCH2). 31P{1H} NMR (121.5 MHz, benzene-d6): δ 14.02
(d, J (PP) ) 18 Hz, J (PPt) ) 2270 Hz), 13.00 (t, J (PP) ) 18
Hz, J (PPt) ) 3410 Hz). Anal. Calcd for C18H45BBrF4P3Pt: C,
30.18; H, 6.33; Br, 11.16. Found: C, 29.99; H, 6.29; Br, 11.27.
Rea ction of SiMe2(C6H4CF 3-4)OH w ith 1 in th e P r es-
en ce of Ag2O. SiMe2(C6H4CF3-4)OH (441 mg, 2.0 mmol) was
added to a mixture of Ag2O (58 mg, 0.25 mmol), 1 (179 mg,
0.25 mmol), and 4 Å molecular sieves (500 mg, 25 particles)
in dried acetone (10 mL) at room temperature. After several
pump-and-thaw cycles, heating of the reaction mixture with
an oil bath was continued for 30 h at 60 °C under Ar. After
removing a gray solid by filtration and washing the solid with
acetone, the combined filtrate and washings were analyzed
by NMR spectroscopy. The 31P{1H} NMR spectrum in acetone-
crystallographic data of the product with those previ-
ously reported.22 The addition of a small amount of
water ([H2O]:[Pt] ) 7.5:1) changes the yields of the
products and leads to the formation of 2 and 3 in 8%
and 57% yields, respectively. Thus, the source of OH
groups in 3 is the water contained in the reaction
mixture.
The organic products in the above reactions contain
ArSiMe2-O-(SiMe2-O)n-SiMe2Ar (Ar ) C6H4CF3-4, n
) 0, 1, 2), octamethyltetracyclosiloxane, and decameth-
ylpentacyclosiloxane, which were characterized by NMR
(1H and 29Si) spectroscopy and GC-MS measurement
of the reaction mixture. Consumption of the silanol and
formation of ArSiMe2-O-SiMe2Ar during reaction 1
suggests that a simple dehydration of the silanol to the
siloxane takes place independently from the aryl ligand
transfer.23 Formation of the hydroxoplatinum complex
3 can be attributed to the reaction of water generated
in the dehydration with a platinum complex in the
reaction mixture. Heating of a mixture of the silanol,
1, and AgBF4 in acetone does not give 2, but results in
the exclusive formation of 3. These results suggest that
activation of the Si-C bond in the reactions 1 and 2 is
-
induced by Ag2O rather than by BF4 anion, although
the anion was reported to be able to fluorinate orga-
nosilyl compounds to give fluorosilanes or fluorosilicates
which are susceptible to Si-C bond cleavage promoted
by late transition metal complexes.24
This work demonstrates the formation of aryl-
platinum bonds via transmetalation of an arylsilanol
to a cationic platinum complex in the presence of Ag2O.
The selective cleavage of a Si-C(aryl) bond of silanol
and the intermolecular transfer of the aryl group to a
group 10 metal center were postulated to account for
the results of Pd-catalyzed coupling reactions and have
been realized by the present study.
1
d6 showed the existence of 2 as the major Pt complex. The H
NMR spectrum contains signals at δ 0.42-0.37 and 0.14-0.05
in a 77:23 peak area ratio and are assigned to the methyl
hydrogens of the -OSiMe2(C6H4CF3-4) group and of the
-OSiMe2O- group of the organic products, respectively. The
29Si{1H} NMR spectra are also consistent with this observa-
tion.26 The Pt complex was extracted with hexane (20 mL) from
the product and subsequent Celite filtration of the hexane
solution. Concentration of the hexane extract to 3-4 mL and
keeping the resulting solution at -10 °C for 24 h led to
separation of 2 as colorless crystals. The crystals of 2 collected
by filtration and the second crop of the crystals obtained from
the filtrate were combined (101 mg, 56%).
A similar reaction of an acetone (8 mL) solution of 1 (287
mg, 0.40 mmol) with SiMe2(C6H4CF3-4)OH (176 mg, 0.80
mmol) in the presence of Ag2O (92 mg, 0.40 mmol) but without
4 Å molecular sieves gave 2 (51%) and [Pt2(µ-OH)2(PEt3)4]-
(BF4)2 (3). Recrystallization of the hexane-insoluble fraction
of the product from CH2Cl2-hexane afforded 3 as colorless
crystals (62 mg, 29%). GC-MS analysis of the solution showed
the presence of ArSiMe2-O-(SiMe2-O)n-Ar (Ar ) C6H4CF3-4
n ) 0, 1, 2) and octamethylcyclotetrasiloxane (EIMS m/z 281
(M - Me)+) and decamethylcyclopentasiloxane (m/z 355 (M -
Me)+) in a much smaller amount. The reaction in the presence
of added water (50 µL, [H2O]:[Pt] ) 7.5:1) resulted in formation
of 2 and 3 in 8% and 57% yields, respectively.
Exp er im en ta l Section
Gen er a l Con sid er a tion , Mea su r em en t, a n d Ma ter ia ls.
All manipulations of the Pt complexes were carried out under
nitrogen or argon using standard Schlenk techniques unless
otherwise stated. Glassware was dried in the oven and heated
under reduced pressure by a heat gun before use. Acetone was
dried over 4 Å molecular sieves, which were preactivated by
heating at 160 °C for 6 h in vacuo. Silver(I) oxide purchased
from Wako Pure Chemical Ind., Ltd. and AgBF4 purchased
from Aldrich Chemical Co., Inc. were dried at 160 °C for 3 h
and 60 °C for 1 h in vacuo, respectively, and stored under Ar
atmosphere. NMR spectra (1H, 13C{1H}, 31P{1H}, and 19F{1H})
were recorded on Varian 300 and J EOL EX-400 spectrometers.
Chemical shifts were referenced to external SiMe4 (1H and 13C-
{1H}), 85% H3PO4 (31P{1H}), and CFCl3 (19F{1H}). GC-MS
Da ta for 2. 31P{1H} NMR (121.5 MHz, benzene-d6): δ 8.43
(s, 1J (PtP) ) 2750 Hz). 1H NMR (300 MHz, benzene-d6): δ
0.71 (dt, 9 H, 3J (PH) ) 16 Hz, 3J (HH) ) 8 Hz, PCH2CH3), 1.00
(m, 6 H, PCH2CH3), 7.46 (d, 4 H, 3J (HH) ) 8 Hz, Pt-C6H4-
analysis was carried out on
a Shimadzu QP-5000 mass
spectrometer. Elemental analyses were carried out with a
Yanaco MT-5 CHN autocorder. The silanol was prepared
according to the literature.25
3
3
m), 7.63 (d, 4 H, J (HH) ) 8 Hz, J (PtH) ) 33 Hz, Pt-C6H4-
o). 13C{1H} NMR (75.3 MHz, benzene-d6): δ 7.7 (s, J (PtC) )
3
25 Hz, PCH2CH3), 13.9 (apparent triplet due to virtual
coupling, 1+3J (PC) ) 34 Hz, 2J (PtC) ) 70 Hz, PCH2), 123.4 (q,
3J (FC) ) 3 Hz, 3J (PtC) ) 44 Hz, Pt-C6H4-m), 124.4 (q, 2J (FC)
(22) (a) Bushnell, G. W.; Dixon, K. R.; Hunter, R. G.; McFarland, J .
J . Can. J . Chem. 1972, 50, 3694. (b) Bushnell, G. W. Can. J . Chem.
1978, 56, 1773. The reported lattice parameters of 3 are a ) 16.04(1),
b ) 18.47(1), c ) 13.36(1) Å and b ) 101.6(1)° with a space group P21/
c, while those of the crystals obtained in the present reaction were a
) 16.108(5), b ) 18.406(4), c ) 13.348(6) Å, and â ) 101.57(3)° with
the same space group.
1
) 31 Hz, Pt-C6H4-p), 126.3 (q, J (FC) ) 270 Hz, CF3), 139.8
(t, 2J (PtC) ) 21 Hz, Pt-C6H4-o), 169.4 (t, 2J (PC) ) 11 Hz,
1J (PtC) ) 575 Hz, Pt-C). 19F{1H} NMR (188.2 MHz, benzene-
(23) Goikhman, R.; Aizenberg, M.; Shimon, L. J . W.; Milstein, D. J .
Am. Chem. Soc. 1996, 118, 10894.
(24) Hiyama, T. Synlett 1991, 845, and references therein.
(25) Hirabayashi, K.; Ando, J .; Kawashima, J .; Nishihara, Y.; Mori,
A.: Hiyama, T. Bull. Chem. Soc. J pn. 2000, 73, 1409.
(26) (a) Williams, E. A. NMR Spectroscopy of Organosilicon Com-
pounds. In The Chemistry of Organic Silicon Compounds Part 1; Patai,
S., Rappoport, Z., Eds.; J ohn Wiley & Sons: Chichester; Chapter 8, p
547. (b) Williams, E. A.; Wengrovius, J . H.; VanValkenburgh, V. M.;
Smith, J . F. Macromolecules 1991, 24, 1445.

1246 Organometallics, Vol. 20, No. 6, 2001
Notes
d6): δ -61.5 (s). Anal. Calcd for C26H38F6P2Pt: C, 43.28; H,
5.31; F, 15.80. Found: C, 43.45; H, 5.37; F, 15.54.
group C2/m (No. 12); a ) 16.362(3) Å, b ) 9.432(2) Å, c )
10.422(3) Å, â ) 109.80(2)°, V ) 1513.3(6) Å3, Z ) 2, Dcalcd
)
Da ta for 3. 31P{1H} NMR (121.5 MHz, CDCl3): δ 6.87 (s,
1.583 g cm-3; 293 K; 2θ range 5-55°; number of independent
reflections 1852 (Rint ) 0.048). The final cycle of full-matrix
least-squares refinement on F was based on 1800 observed
reflections (I > 3.00σ(I)) and 92 variable parameters: R )
0.044 and Rw ) 0.046; GOF ) 3.36.
1
1J (PtP) ) 3475 Hz). H NMR (300 MHz, CDCl3): δ 1.22 (dt,
36 H, 3J (PH) ) 18 Hz, 3J (HH) ) 7 Hz, PCH2CH3), 1.83 (m, 24
H, PCH2CH3), 2.17 (s, 2 H, OH). Anal. Calcd for C24H62B2F8O2P4-
Pt2: C, 26.93; H, 5.84. Found: C, 27.34; H, 5.41.
Cr ysta l Str u ctu r e Deter m in a tion . Crystals of 2 were
mounted in glass capillary tubes under Ar. Intensities were
collected for Lorentz and polarization effects on a Rigaku AFC-
7R automated four-cycle diffractometer by using Mo Ka
radiation (λ ) 0.71069 Å) and the ω-2θ scan method, and an
empirical absorption correction (Ψ scan) was applied. Calcula-
tions were carried out by using the program package teXsan
for Windows. Atomic scattering factors were obtained from the
literature. A full-matrix least-squares refinement was used for
non-hydrogen atoms with anisotoropic thermal parameters.
Hydrogen atoms were located by assuming the ideal geometry
and were included in the structure calculation without further
refinement of the parameters.27 Crystal data for 2: C26H38F6P2-
Pt, 721.61, colorless, 0.30 × 0.25 × 0.05 mm; monoclinic, space
Ack n ow led gm en t. This work was financially sup-
ported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports, and Culture,
J apan.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
results of 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM010006C
(27) International Tables for X-ray Crystallography; Kynoch: Bir-
mingham, England, 1974; Vol. 4.