Synthesis and Structural Characterization of 2,5-Bis(N-aryliminomethyl)pyrrolyl Complexes of Aluminum

Article abstract of DOI:10.1246/bcsj.76.1965

Reaction of tridentate ligands, 2,5-bis(N-aryliminomethyl)pyrroles (1a-e), with 1 equiv of AlMe₃ in toluene afforded aluminum dimethyl complexes, AlMe₂[2,5-bis(N-aryliminomethyl)pyrrolyl] (2a-e) with the release of one equiv of methane. In solution the aluminum complexes 2a-e showed C_s-symmetry as characterized by their ¹H NMR spectra, while in solid the geometry around the Al atom as crystallographically determined for 2a and 2b was found to a tetrahedral by an amido nitrogen atom of the pyrrolyl anion, a nitrogen atom of one of two imino groups and two methyl groups. The aluminum complex 2e upon activated by one equiv of B(C ₆F₅)₃ became a catalyst for ethylene polymerization (Activity = 80 g-PE/mol-cat·h).

Full text of DOI:10.1246/bcsj.76.1965

Ó 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 1965–1968 (2003) 1965
Synthesis and Structural Characterization of 2,5-Bis(N-arylimino-
methyl)pyrrolyl Complexes of Aluminum
ꢀ
Yutaka Matsuo, Hayato Tsurugi, Tsuneaki Yamagata, Kazuhide Tani, and Kazushi Mashima
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
Received April 8, 2003; E-mail: mashima@chem.es.osaka-u.ac.jp
Reaction of tridentate ligands, 2,5-bis(N-aryliminomethyl)pyrroles (1a–e), with 1 equiv of AlMe3 in toluene af-
forded aluminum dimethyl complexes, AlMe2[2,5-bis(N-aryliminomethyl)pyrrolyl] (2a–e) with the release of one equiv
of methane. In solution the aluminum complexes 2a–e showed Cs-symmetry as characterized by their 1H NMR spectra,
while in solid the geometry around the Al atom as crystallographically determined for 2a and 2b was found to a tetra-
hedral by an amido nitrogen atom of the pyrrolyl anion, a nitrogen atom of one of two imino groups and two methyl
groups. The aluminum complex 2e upon activated by one equiv of B(C6F5)3 became a catalyst for ethylene polymer-
ization (Activity = 80g-PE/mol-cat h).
ꢁ
The chemistry of alkylaluminum bearing nitrogen-based
polydentate ligands has recently attracted much attention due
to the rich structural chemistry of this class of complexes1–5
and their applicability as unique catalyts.6–8 Although there
are many examples of monodentate amido and imido alkylalu-
minum complexes, only a few mononucelar alkylaluminum
complexes with bidentate and tridentate amido and imido li-
gands have been reported.2,9 We and other groups recently
reported that 2-(N-aryliminomethyl)pyrroles and 2,5-bis(N-ar-
yliminomethyl)pyrroles serve as unique didentate and triden-
tate monoanionic nitrogen ligands, respectively, and their ho-
moleptic and heteroleptic complexations with early transiton
metals were elucidated.10–14 As an extensiton of our countin-
uous interest, we have investigated the reactions of trialkylalu-
minum with these tridentate pyrroles. Here we report synthe-
ses and structures of mononuclear dimethylaluminum com-
plexes bearing 2,5-bis(N-aryliminomethyl)pyrrolyl ligands, as
well as their possible applications for transition metal-free eth-
ylene polymerization catalysts.
aluminum metal. Thus, the signals of both the CH3 group
and the pyrrolyl ring are consistent with the Cs symmetry of
the molecule in solution. In the 13C NMR spectra of 2a–e,
the carbon resonance of the iminomethyl moiety was observed
in the range of ꢀ 150.2–158.6, a chemical shift value which
was shifted to a lower field compared with that found for the
free ligand (ꢀ 146.7–151.8).
ð1Þ
The discrete structure of these aluminum complexes in solid
state was elucidated by X-ray crystallographic analyses for the
complexes 2a and 2b. Figures 1 and 2 show the crystal struc-
tures of 2a and 2b, respectively, and selected bond distances
and angles are listed in Table 1. Both complexes adopt the
same tetrahedral geometry around the aluminum center: the
pyrrolyl ligand chelates to the aluminum atom through the
Results and Discussion
Reaction of tridentate ligands, 2,5-bis(N-aryliminomethyl)-
pyrroles 1a–e, with 1 equiv of AlMe3 in toluene afforded the
corresponding mononuclear aluminum dimethyl complexes,
[Al(Me)2{2,5-bis(N-aryliminomethyl)pyrrolyl}] (2a–e), in
85–90% yields with the release of one equiv of methane (Eq.
1). These complexes 2a–e were obtained as yellow air- and
moisture-sensitive crystals. The formulation and structure of
2a–e were characterized by NMR spectroscopy and combus-
tion analysis. The 1H NMR spectra of 2a–e in benzene-d6 ex-
hibited one singlet due to CH3 and one set of the pyrrolyl li-
gand in an exact 2:1 ratio. The singlet signal of –CH=N– ap-
peared in a higher field (ꢀ 7.33–7.92) compared with that of
the corresponding free ligand (ꢀ 7.81–8.22) and the pyrrolyl
ring protons were observed as a singlet signals in the olefinic
region (ꢀ 6.65–6.74), suggesting that the pyrrolyl anion and
the imino moiety coordinated in an ꢁ-N-coordination to the
Fig. 1. Molecular structure of 2a with the numbering
scheme. Hydrogen atoms are omitted for clarity.
Published on the web October 15, 2003; DOI 10.1246/bcsj.76.1965

1966 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
Bis(iminomethyl)pyrrolyl Complexes of Aluminum
Fig. 3. Aluminum ethylene polymerization catalyst systems.
of the pyrrolyl nitrogen in the Al–N1 bond.2,4
We preliminary tested the catalytic activity of aluminum
complexes for ethylene polymerization because some cationic
organoaluminum complexes have been utilized as unique ini-
tiators of ethylene polymerization (Fig. 3); dimethylaluminum
complexes (L)AlMe2 reacted with B(C6F5)3 or [Ph3C]-
[B(C6F5)4] to afford catalytically active cationic monomethyl-
aluminum complexes (L)AlMeþ.8 Treatment of 2e with 1
equiv of B(C6F5)3 in toluene under ethylene atmosphere (1
atm) for 1 h gave solid polyethylene (Eq. 2). The activity of
Fig. 2. Molecular structure of 2b with the numbering
scheme. Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Distances and Angles of 2a and 2b
2a
2b
ꢀ
Bond distances (A)
Al–N1
Al–N2
Al–C21
Al–C22
N1–C1
N1–C4
N2–C5
N3–C6
C1–C2
C1–C5
C2–C3
C3–C4
C4–C6
1.8899(17)
1.8977(11)
2.0368(11)
1.9590(13)
1.9505(13)
1.3708(16)
1.3635(16)
1.3006(17)
1.2816(17)
1.4013(18)
1.4228(18)
1.389(2)
2e/B(C6F5)3 (80g-PE/mmol-cat h) was found to be high
for the ethylene polymerization, but the value was lower than
those of imidinate–aluminum system (Fig. 3, 3/B(C6F5)3)
ꢁ
2.0582(17)
1.952(2)
1.951(2)
1.368(3)
1.360(3)
1.302(3)
1.286(3)
1.398(3)
1.424(3)
1.390(3)
1.406(3)
1.439(3)
(700 g-PE/mmol-cat h)8a and aminotroponiminate–aluminum
system (Fig. 3, 4/[Ph3C][B(C6F5)4]) (2600 g-PE/mmol-
ꢁ
cat h)8c (Fig. 1).
ꢁ
ð2Þ
In summary, we have demonstrated that aluminum com-
plexes 2a–e bearing 2,5-bis(N-aryliminomethyl)pyrrolyl li-
gands can be readily prepared by reactions of AlMe3 with free
ligands 1a–e. The aluminum complexes 2a–e were found to
have Cs-symmetry in solution, while they adopted dissymmet-
ric structure in solid state. In the presence of one equiv of
B(C6F5)3, the aluminum complex 2e was found to be an active
catalyst for ethylene polymerization.
1.4083(18)
1.442(2)
Bond angles (ꢂ)
N1–Al–N2
N1–Al–C21
N1–Al–C22
N2–Al–C21
N2–Al–C22
C21–Al–C22
Al–N1–C1
Al–N1–C4
C1–N1–C4
Al–N2–C5
81.69(7)
117.05(10)
113.70(10)
107.41(10)
103.41(9)
123.19(12)
107.55(17)
137.02(15)
107.55(17)
111.02(14)
127.33(13)
121.62(17)
360.0
82.34(5)
114.94(5)
118.03(5)
107.53(5)
106.91(5)
119.23(7)
114.05(8)
139.25(9)
106.38(10)
111.25(9)
127.06(8)
121.66(11)
359.7
Experimental
General Procedures. All manipulations involving air- and
moisture-sensitive organometallic compounds were carried out us-
ing the standard Schlenk techniques under argon. Hexane, THF,
and toluene were dried and deoxygenated by distillation over so-
dium diphenyl ketyl under argon. Benzene-d6 was distilled from
Na/K alloy and thoroughly degassed by trap-to-trap distillation
before use. 2,5-bis{N-(4-methoxyphenyl)iminomethyl}pyrrole
(1a), 2,5-bis{N-(4-methylphenyl)iminomethyl}pyrrole (1b), 2,5-
bis{N-(2-methylphenyl)iminomethyl}pyrrole (1c), 2,5-bis{N-
(2,6-dimethylphenyl)iminomethyl}pyrrole (1d), and 2,5-bis{N-
(2,6-diisopropylphenyl)iminomethyl}pyrrole (1e) were prepared
according to the literature method.11,14
Al–N2–C7
C5–N2–C7
Sum of around N1 atom
Sum of around N2 atom
360.0
360.0
amido nitorogen atom of the pyrrolyl anion and the nitrogen
atom of one of two imino groups, and hence the nitrogen atom
of the other imino group is coordinatively free. In contrast,
when we observed 1H NMR spectra due to Cs symmetry of
aluminum compounds in the temperature range of 213–308
K, we did not observe any fluxionality. Some mononuclear di-
alkyl aluminum complexes with a tetrahedral Al–C2N2 core
have been reported so far.3,9 The ligand in both complexes
is essentially planar and the Al atom is coplanar with the li-
gand plane. Two methyl carbon atoms are nearly symmetrical-
ly bisected by the plane of the ligand. The bond distance
1
The H (500, 400, 300, and 270 MHz) and 13C (125, 100, 75,
and 68 MHz) NMR spectra were measured on a VARIAN Unity
Inova-500, a JEOL JNM-AL400, a VARIAN Mercury-300, or a
JEOL GSX-270spectrometer. When benzene- d6 was used as
the solvent, the spectra were referenced to the residual solvent
protons at ꢀ 7.20in the 1H NMR spectra and to the residual sol-
vent carbons at ꢀ 128.0in the 13C NMR spectra. Assignments
for 1H and 13C NMR peaks for some complexes were aided by
1
1
1
2D H–1H COSY, 2D H–1H NOESY, 2D H–13C HMQC, and
2D 1H–13C HMBC spectra. Elemental analyses were recorded
by Perkin Elmer 2400. All melting points were measured in
sealed tubes under argon atmosphere and were not corrected.
ꢀ
(1.8899(17) A for 2a and 1.8977(11) A for 2b) of Al–N1 is
ꢀ
ꢀ
significantly shorter than that (2.0582(17) A for 2a and
ꢀ
2.0368(11) A for 2b) of Al–N2 and the known Al–N(amido)
bond distances, indicative of the participation of the lone pair

Y. Matsuo et al.
Table 2. Crystal Data and Data Collection Parameters of 2a and 2b
Bull. Chem. Soc. Jpn., 76, No. 10 (2003) 1967
Complex
Formula
Formula weight
Crystal system
Space group
2a
C22H24AlN3O2
389.42
orthorhombic
Pbca (No. 61)
12.7468(4)
28.2110(9)
11.5848(5)
4165.9(3)
8
2b
C22H24AlN3
357.43
orthorhombic
Pna21 (No. 33)
19.4940(2)
7.3050(1)
13.7841(1)
1962.90(4)
4
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
V/A
ꢀ 3
Z
No. of refl. for cell det. (ꢂ range)
Dcalcd/g cmÀ3
F(000)
47534 (2.15–27.47 deg)
1.242
1648
33107 (2.79–27.48 deg)
1.209
760
ꢃ[Mo Kꢄ]/mmÀ1
Diffractometer
T/K
0.119
R-AXIS RAPID
153(1)
0.113
R-AXIS RAPID
153(1)
Crystal size/mm
No. of images
Total oscillation angles/deg
Exposure time/min degÀ1
2ꢂmax/deg
No. of reflections measured
Unique data (Rint
Completeness to ꢂ ¼ 27:48/%
No. of observations
Max and min transmission
No. of variables
0:43 Â 0:37 Â 0:25
65
260.0
1.00
55.055.0
44572
4778 (0.0954)
99.9
3213
0.9788, 0.8413
349
0:55 Â 0:35 Â 0:27
90
270.0
1.50
19684
)
4465 (0.0248)
100
4252
0.9887, 0.9106
330
R1, wR2 (all data)
0.0873, 0.1183
0.0482, 0.1003
—
1.025
0.296, À0:242
0.0300, 0.0733
0.0276, 0.0749
À0:05ð9Þ
1.044
R1, wR2 (I > 2:0ꢅðIÞ)
Flack parameter (ꢆ)
GOF on F2
ꢀ À3
Áꢇ/e A
0.169, À0:178
ꢂ
ꢂ
Preparation of 2a. A solution of 1a (500 mg, 1.50 mmol) in
toluene (15 mL) was added to a solution of AlMe3 (1.50mmol) in
toluene (5 mL) at À78 ꢂC. After evolution of CH4 gas was com-
pleted, the reaction mixture was allowed to warm to room temper-
ature and further stirred for 6 h at room temperature. All volatiles
were removed in vacuo. The resulting yellow solid was recrystal-
lized from a mixture of THF and hexane to give yellow crystals of
2a (504 mg, 1.35 mmol, 90%), mp 178–181 ꢂC. 1H NMR (C6D6,
2c: 85% yield, mp 175–178 C. 1H NMR (C6D6, 35 C): ꢀ
À0:22 (s, 6H, Al–CH3), 2.22 (s, 6H, Ar–CH3), 6.68 (s, 2H, 3,4-
pyr), 6.76 (d, 2H, 6-C6H4), 7.02 (d, 2H, 5-C6H4), 7.03 (d, 2H,
4-C6H4), 7.03 (d, 2H, 3-C6H4), 7.56 (s, 2H, N=CH). 13C NMR
(C6D6, 35 ꢂC): ꢀ À10:8 (q, JC{H ¼ 113 Hz, Al–CH3), 18.3 (q,
1
1JC{H ¼ 127 Hz, Ar–CH3), 118.7 (d, JC{H ¼ 170 Hz, 3,4-pyr),
1
1
1
121.7 (d, JC{H ¼ 160 Hz, 6-C6H4), 126.4 (d, JC{H ¼ 161 Hz,
1
5-C6H4), 126.9 (d, JC{H ¼ 159 Hz, 4-C6H4), 130.9 (s, 2-C6H4),
ꢂ
1
35 C): ꢀ 0.01 (s, 6H, Al–CH3), 3.34 (s, 6H, OCH3), 6.74 (s, 2H,
3,4-pyr), 6.75 (m, 4H, m-C6H4), 7.30(m, 4H, o-C6H4), 7.90(s,
131.2 (d, JC{H ¼ 157 Hz, 3-C6H4), 140.9 (s, 2,5-pyr), 148.6 (s,
1
1-C6H4), 156.3 (d, JC{H ¼ 167 Hz, N=CH). Anal. Calcd for
1
2H, N=CH). 13C NMR (C6D6, 35 ꢂC): ꢀ À9:44 (q, JC{H
¼
C22H24AlN3: C, 73.93; H, 6.77; N, 11.76. Found: C, 74.03; H,
7.03; N, 11.78.
1
114 Hz, Al–CH3), 55.0(q, JC{H ¼ 144 Hz, OCH3), 114.6 (d,
1
ꢂ
1JC{H ¼ 159 Hz, m-C6H4), 117.4 (d, JC{H ¼ 174 Hz, 3,4-pyr),
2d: 87% yield, mp 220–225 C (dec). 1H NMR (400 MHz,
1
122.8 (d, JC{H ¼ 159 Hz, o-C6H4), 140.4 (s, 2,5-pyr), 140.9 (s,
C6D6, 35 ꢂC): ꢀ À0:28 (s, 6H, Al–CH3), 2.16 (s, 12H, Ar–
1
ipso-C6H4), 150.2 (d, JC{H ¼ 166 Hz, N=CH), 159.1 (s, p-
CH3), 6.65 (s, 2H, 3,4-pyr), 6.96 (m, 6H, C6H3), 7.33 (s, 2H,
N=CH). 13C NMR (100 MHz, C6D6, 35 ꢂC): ꢀ À9:3 (q, 1JC{H
C6H4). Anal. Calcd for C22H24AlN3O2: C, 67.85; H, 6.21; N,
10.79. Found: C, 67.68; H, 6.04;ꢂN, 10.79.
¼
1
114 Hz, Al–CH3), 18.6 (q, JC{H ¼ 127 Hz, Ar–CH3), 116.0(d,
2b: 87% yield, mp 180–182 C. 1H NMR (C6D6, 35 C): ꢀ
1JC{H ¼ 170 Hz, 3,4-pyr), 125.4 (d, JC{H ¼ 159 Hz, p-C6H3),
ꢂ
1
1
À0:02 (s, 6H, Al–CH3), 2.13 (s, 6H, Ar–CH3), 6.72 (s, 2H, 3,4-
128.5 (d, JC{H ¼ 158 Hz, m-C6H3), 129.6 (s, o-C6H3), 140.4
1
pyr), 6.98 (m, 4H, m-C6H4), 7.26 (m, 4H, o-C6H4), 7.92 (s, 2H,
1
(s, 2,5-pyr), 148.2 (s, ipso-C6H3), 158.6 (d, JC{H ¼ 168 Hz,
N=CH). 13C NMR (C6D6, 35 ꢂC): ꢀ À9:66 (q, JC{H ¼ 114
N=CH). Anal. Calcd for C24H28AlN3: C, 74.78; H, 7.32; N,
10.90. Found: C, 74.99; H, 7.19; N, 10.89.
1
Hz, Al–CH3), 20.9 (q, JC{H ¼ 126 Hz, Ar–CH3), 117.8 (d,
1JC{H ¼ 170 Hz, 3,4-pyr), 121.5 (d, JC{H ¼ 159 Hz, o-C6H4),
2e: 88% yield, mp 185–186 ꢂC (dec). 1H NMR (C6D6, 35
1
1
3
130.2 (d, JC{H ¼ 157 Hz, m-C6H4), 136.6 (s, p-C6H4), 140.4
ꢂC): ꢀ À0:22 (s, 6H, Al–CH3), 1.17 (d, JH{H ¼ 6:9 Hz, 24H,
1
3
(s, 2,5-pyr), 145.4 (s, ipso-C6H4), 151.3 (d, JC{H ¼ 167 Hz,
CH(CH3)2), 3.23 (sept, JH{H ¼ 6:9 Hz, 4H, CH(CH3)2), 6.67
N=CH). Anal. Calcd for C22H24AlN3: C, 73.93; H, 6.77; N,
11.76. Found: C, 74.18; H, 6.91; N, 11.83.
(s, 2H, 3,4-pyr), 7.15 (s, 6H, C6H3), 7.89 (s, 2H, N=CH).
1
13C NMR (C6D6, 35 ꢂC): ꢀ À10:4 (q, JC{H ¼ 114 Hz, Al–

1968 Bull. Chem. Soc. Jpn., 76, No. 10 (2003)
Bis(iminomethyl)pyrrolyl Complexes of Aluminum
CH3), 24.5 (q, 1JC{H ¼ 126 Hz, CH(CH3)2), 28.2 (d, 1JC{H ¼ 128
6
T. Ooi and K. Maruoka, ‘‘Lewis Acids in Organic Synthe-
1
Hz, CH(CH3)2), 118.8 (d, JC{H ¼ 171 Hz, 3,4-pyr), 123.9 (d,
sis,’’ ed by H. Yamamoto, Wiley-VCH, Weinheim (2000), Vol. 1,
pp. 191–281.
1
1JC{H ¼ 157 Hz, m-C6H3), 126.5 (d, JC{H ¼ 159 Hz, p-C6H3),
129.3 (s, o-C6H3), 140.7 (s, 2,5-pyr), 145.5 (s, ipso-C6H3),
1
7
W. D. Wulff, ‘‘Lewis Acids in Organic Synthesis,’’ ed by
158.0(d, JC{H ¼ 167 Hz, N=CH). Anal. Calcd For C32H44AlN3:
H. Yamamoto, Wiley-VCH, Weinheim (2000), Vol. 1, pp. 283–
354.
C, 77.23; H, 8.91; N, 8.44. Found: C, 77.38; H, 8.54; N, 8.43.
Crystallographic Data Collection and Structure Determina-
tion of 2a and 2b. Crystals of 2a and 2b suitable for the X-ray
diffraction study were mounted on glass filers. All measurements
were made on a Rigaku R-AXIS-RAPID Imaging Plate diffrac-
tometer with graphite monochromated Mo Kꢄ radiation
(ꢈ ¼ 0:71069). Crystal data and data statistics are summarized
in Table 2. Indexing was performed from 2 oscillations. The
camera radius was 127.40mm. Readout was performed in the
0.100 mm pixel mode. A symmetry-related absorption correction
using the program ABSCOR15 was applied. The data were cor-
rected for Lorentz and polarization effects.
8
a) M. P. Coles and R. F. Jordan, J. Am. Chem. Soc., 119,
8125 (1997). b) M. P. Coles, D. C. Swemspm, R. F. Jordan, and
V. G. Young, Jr., Organometallics, 16, 5183 (1998). c) E. Ihara,
V. G. Young, Jr., and R. F. Jordan, J. Am. Chem. Soc., 120,
8277 (1998). d) C. E. Radzewich, M. P. Coles, and R. F. Jordan,
J. Am. Chem. Soc., 120, 9384 (1998). e) S. L. Aeilts, M. P. Coles,
D. C. Swenson, R. F. Jordan, and V. G. Young, Jr., Organometal-
lics, 17, 3265 (1998). f) M. P. Coles, D. C. Swenson, R. F. Jordan,
and V. G. Young, Jr, Organometallics, 17, 4042 (1998). g) M.
Bruce, V. C. Gibson, C. Redshaw, G. A. Solan, A. J. P. White,
and D. J. Williams, Chem. Commun., 1998, 2523. h) C. E.
Radzewich, I. A. Guzei, and R. F. Jordan, J. Am. Chem. Soc.,
121, 8673 (1999). i) A. V. Korolev, I. A. Guzei, and R. F. Jordan,
J. Am. Chem. Soc., 121, 11605 (1999). j) S. Dagorne, I. A. Guzei,
M. P. Coles, and R. F. Jordan, J. Am. Chem. Soc., 122, 274 (2000).
k) A. V. Korolev, E. Ihara, I. A. Guzei, V. G. Young, Jr, and R. F.
Jordan, J. Am. Chem. Soc., 123, 8291 (2001).
The structures were solved by direct methods (SIR97)16 and re-
fined on F2 by full-matrix least-squares methods, using SHELXL-
97.17 The non-hydrogen atoms were refined anisotropically by the
full-matrix least-squares method. All hydrogen atoms of 2a and
2b were isotropically refined. The function minimized was
2
2
2
2
2
[ÆwðFo À Fc Þ ] (w ¼ 1=½ꢅ2ðFo Þ þ ðaPÞ þ bPŠ), where P ¼
ðMaxðFo2; 0Þ þ 2Fc Þ=3 with ꢅ2ðFo Þ from counting statistics.
9
J. Ashenhurst, L. Brancaleon, S. Gao, W. Liu, H.
2
2
The function R1 and wR2 were ðÆjjFoj À jFcjjÞ=ÆjFoj and
Schmider, S. Wang, G. Wu, and G. G. Wu, Organometallics,
17, 5334 (1998).
10Y. Matsuo, K. Mashima, and K. Tani, Chem. Lett., 2000,
1114.
11 Y. Matsuo, K. Mashima, and K. Tani, Organometallics, 20,
3510 (2001).
2
½ÆwðFo À Fc Þ =ÆðwFo ފ1=2. All calculations of least-squares
refinements were performed with SHELXL-97 programs on an
Origin 3400 computer of Silicon Graphics Inc. at the Research
Center for Structural Biology Institute for Protein Research,
Osaka University. Structural parameters and X-ray structure anal-
ysis for 2a and 2b are summarized in Table 2. Crystallographic
data have been deposited at the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK and copies can be obtained on request,
free of charge, by quoting the publication citation and the deposi-
tion numbers 213112 for 2a and 213113 for 2b.
2
2
4
12 a) V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw, G.
A. Solan, A. J. P. White, and D. J. Williams, Chem. Commun.,
1998, 1651. b) V. C. Gibson, S. Mastroianni, C. Newton, C.
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This research was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (A) ‘‘Exploitation of Multi-Ele-
ment Cyclic Molecules’’ from the Ministry of Education, Cul-
ture, Sports, Science and Technology.
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