Convenient and highly efficient synthesis of boron-dipyrrins bearing an arylboronate center

Article abstract of DOI:10.1016/j.tetlet.2009.02.094

Novel boron-dipyrrins bearing an aryl group and a phenyloxy group linked directly to the boron center have been prepared in high yields from arylboronic acid. The unique coordination geometry around the boron was determined by an X-ray crystallographic an

Full text of DOI:10.1016/j.tetlet.2009.02.094

Tetrahedron Letters 50 (2009) 3349–3351
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Convenient and highly efficient synthesis of boron–dipyrrins
bearing an arylboronate center
*
Chusaku Ikeda, Tetsuji Maruyama, Tatsuya Nabeshima
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel boron–dipyrrins bearing an aryl group and a phenyloxy group linked directly to the boron center
have been prepared in high yields from arylboronic acid. The unique coordination geometry around the
boron was determined by an X-ray crystallographic analysis. The complexes showed a strong fluores-
cence, and alkylation of the OH group shifted the emission to a longer wavelength with a larger Stokes
shift.
Received 15 January 2009
Revised 11 February 2009
Accepted 12 February 2009
Available online 15 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Dipyrrin
Boron
BODIPY
Boronic acid
Fluorescence
Boron dipyrrins containing a BF2 unit1 (F-BODIPYs) have been
attracting considerable interest in many research areas because
of their intense absorption and fluorescence.2 Various substituents
have been introduced into the F-BODIPY skeleton to modify the
original optical properties for applications such as biological label-
ing3 and electroluminescent devices,4 and laser dyes5. Substitution
of the fluorine atoms on the boron also changes the properties.6,7
To date, boron dipyrrins bearing alkyl,7c alkyloxy,6b aryl,7b,c aryl-
oxy,6a,b and ethynyl7a,c groups have been prepared on the basis of
this substitution. The dipyrrin complexes bearing one or two aryl
group(s) at the boron center (Ar-BODIPYs) are of great interest be-
cause (1) the optical properties would be modified by introducing
various aromatic chromophores, (2) fluorescent sensoring of an
appropriate guest is expected when the aromatic moiety has the
binding site, and because (3) the Ar-BODIPYs usually show an in-
tense fluorescence. However, substitution of the fluorine atoms
with an aryl-Grignard or aryl-lithium reagent is the only reported
way for the Ar-BODIPY preparation, but the yields are moderate.7b,c
Hence, the high-yield one-step synthesis of Ar-BODIPYs from the
dipyrrin ligands and readily available boron-containing com-
pounds has been strongly desired in order to obtain a variety of
boron dipyrrins with valuable properties.
the free ligand 1 (Scheme 1). We expected that the reaction of 1
with arylboronic acid afforded a variety of Ar-BODIPY derivatives
such as 2 in one-step by using the huge library of boronic acids.
Furthermore, the chiral boron center in the Ar-BODIPYs would be
utilized for chiral recognition or for circularly polarized
luminescence.
In this Letter, we describe the convenient high-yield synthesis,
characterization, and optical properties of novel Ar-BODIPYs 2–4
BBr3
N
O
N
O
N
N
OMe
MeO
B
B
F
F
5
6
B(OMe)3
Ar-B(OH)2
NH
N
OH
HO
N
N
OH
Recently, we synthesized the boron dipyrrin 5 from the dipyrrin
ligand 1 by the reaction with B(OMe)3,8 and not from F-BODIPY 6.9
Our synthetic method is very useful because the non-fluorinated
boron dipyrrin is readily obtained via a one-step reaction from
B
O
Ar
1
2 : Ar = Ph
3 : Ar = naphth-1-yl
4 : Ar = 4-methoxyphenyl
* Corresponding author. Tel./fax: +81 29 853 4507.
E-mail address: nabesima@chem.tsukuba.ac.jp (T. Nabeshima).
Scheme 1. Synthesis of the boron dipyrrins. Only one of the enantiomers is shown
for 2–5.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.094

3350
C. Ikeda et al. / Tetrahedron Letters 50 (2009) 3349–3351
and 7–9 bearing an aryl ring and a phenyloxy group at the boron
center. To the best of our knowledge, this is the first example of
dipyrrins with N, N, O, C atoms coordinating to the boron.
The reaction of 1 with phenylboronic acid and 1-naphthylbo-
ronic acid quantitatively afforded Ar-BODIPYs 2 and 3, respectively
(Table 1, entries 1 and 2).10 The reaction with 4-methoxyphenylbo-
ronic acid also gave Ar-BODIPY 4 in high yield, although a small
amount of BODIPY 5 was also obtained (entry 3). In case of 4-dim-
ethylaminophenylboronic acid and anthracence-9-boronic acid, 5
was isolated as a single product instead of Ar-BODIPY (entries 4
and 5). 4 was gradually converted to 5 in chloroform under reflux.
This reaction involves the cleavage of the B–C bond and the forma-
tion of the B–O bond. The initial step is probably the electrophilic
attack of the phenol proton of 4 on the ipso-carbon.11 The proposed
mechanism agrees with the exclusive formation of 5 in the case of
highly electron-rich arylboronic acids.11
Since the optical properties of some boron–dipyrrins bearing a
phenol OH group are different from those of the corresponding alk-
oxy derivatives,12 we alkylated the residual OH group of 2. The
reaction of 2 with methyl iodide, butyl bromide, and benzyl bro-
mide afforded 7, 8, and 9 in 79%, 75%, and 98% yields, respectively
(Scheme 2).13 These alkoxy dipyrrins show different emission
properties when compared to 2 (vide infra).
Figure 1. Crystal structure of 2. Thermal ellipsoids are plotted at 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): B1–N1
1.562(3), B1–N2 1.542(3), B1–O2 1.479(3), B1–C31 1.611(3).
connected to the phenyl ring E. The chiral boron center adopts a
distorted tetrahedral geometry with the N–B–N, C–B–N, and C–
B–O bond angles of 104.3°, 108.0°, and 111.7°, respectively. The
A ring and the two B and C pyrrole rings are nearly coplanar with
the maximum mean plane deviation of 0.27 Å. The boron atom is
out of the bis-pyrrolic plane by 0.44 Å. The length of the boron–car-
bon bond (1.61 Å) is similar to that of the boron–dipyrrin bearing
two phenyl groups at the boron center.7b The phenol ring D is tilted
with respect to the pyrrole ring C, and the dihedral angle between
C and D is 54°. The distance between the two oxygens in the A and
D rings is 2.61 Å, indicative of OHÁ Á ÁO hydrogen bonding.
Compounds 2, 3, 4, 7, 8, and 9 were identified by spectroscopic
measurements (1H, 13C, and 11B NMR, ESI-mass) and elemental
analysis. The 1H NMR spectra of these compounds indicate a C1
symmetric dipyrrin skeleton. For example, the pyrrolic-b protons
of 2 afford four doublets at 6.35, 6.68, 6.79, and 6.84 ppm. The
methyl protons of the mesityl group of 2 appeared at 2.14, 2.32,
and 2.39 ppm as a singlet. The 11B NMR of 2 shows a broad singlet
at 2.55 ppm, suggesting a tetrahedral geometry around the boron
atom.14
In
the presence
of
europium tris[3-(hepta-
The absorption and fluorescence spectral data of the obtained
Ar-BODIPY are summarized in Table 2. All the compounds show a
strong absorption band around 590 nm and an intense fluores-
cence with a quantum yield of 0.59–0.68. Wavelengths of the
absorption band maximum (kmax) of 2–4 (588 nm) are longer than
that of the F-BODIPY 6 (kmax 550 nm). The B–O linkage in 2–4 may
cause the red shift. The A ring, which is nearly coplanar with the
pyrrole rings, probably contributes to extending the conjugation.6a
Compounds 7–9 bearing an alkoxy or a benzyloxy phenyl group
show larger Stokes shifts when compared to those of 2 (Table 2).
This fact suggests the larger conformational changes of 7–9
than those of 2 in the excited state. The rotational motion of the
phenol substituents in 7–9, and lack of the intramolecular hydro-
gen bond observed in 2 may account for the larger shifts.
fluoropropylhydroxymethylene)-(+)-camphorate] as a chiral shift
reagent, the methyl signal of 2 at 2.14 ppm split into two signals
due to the diastereotopic complex formation. This fact shows that
the chiral configuration of the boron center is stable at least on the
NMR timescale.
The X-ray crystallographic analysis of 2 confirmed the unique
structure of the Ar-BODIPY (Fig. 1).15,16
The compound crystallized as a racemate in the centrosymmet-
ꢀ
ric space group P. The two pyrrole nitrogen atoms and phenol oxy-
gen O(2) of the dipyrrin ligand bind to the boron atom, which is
Table 1
Condensation of 1 and arylboronic acid for the synthesis of Ar-BODIPYs
The results in this study would provide a facile, efficient, and
versatile way to afford boron–dipyrrins bearing various aryl groups
at the boron center. The chiral Ar-BODIPYs should be used for pro-
ducing interesting chiral properties and functions such as circu-
larly polarized luminescence and chiral recognition. The
separation of the enantiomers and the investigation of the proper-
ties are the subject of a current study.
Entry
Ar
Products
1
2
3
4
5
Phenyl
Naphth-1-yl
4-Methoxyphenyl
4-Dimethylaminophenyl
Anthr-9-yl
2(quant)
3(quant)
4(92%), 5(8%)
5(85%)
5(quant)
RX (X = I or Br)
Table 2
K2CO3
Spectroscopic data for the various Ar-BODIPYs in chloroform at 298 K
2
CH3CN or acetone
N
N
Stokes shift (cmÀ1
)
Ufa
OR
kmax (nm)
kflu (nm)
B
O
2
3
4
7
8
9
588
588
588
595
597
596
614
615
613
644
643
642
730
750
690
1280
1200
1200
0.65
0.68
0.63
0.59
0.60
0.62
Ph
7: R = Me
8: R = nBu
9: R = CH2Ph
a
Determined under aerated conditions.
Scheme 2. Alkylation of 2. Only one of the enantiomers is shown for 7–9.

C. Ikeda et al. / Tetrahedron Letters 50 (2009) 3349–3351
3351
observed m/z 563.29 ([M+H]+), calcd for C37H32BN2O3 m/z 563.25. Anal. Calcd
for C37H31BN2O3Á0.5H2O: C, 77.76; H, 5.64; N, 4.90. Found: C, 78.06; H, 5.78; N,
4.91.
Acknowledgment
This work was supported by Grants-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science,
and Technology, Japan.
11. (a) Kuivila, H. G.; Nahabedian, K. V. J. Am. Chem. Soc. 1961, 83, 2159–2163; (b)
Coopper, J. N.; Powell, R. E. J. Am. Chem. Soc. 1963, 85, 1590–1592; (c) Nishida,
H.; Takada, N.; Yoshimura, M. Bull. Chem. Soc. Jpn. 1984, 57, 2600–2604.
´
12. Baruah, M.; Qin, W.; Basaric, N.; De Borggraeve, W. M.; Boens, N. J. Org. Chem.
2005, 70, 4152–4157.
13. Compound 7: To a stirred solution containing 2 (53.2 mg, 0.100 mmol) and
K2CO3 (0.138 g, 0.100 mmol) in acetonitrile (20 mL) was added methyl iodide
References and notes
(7.4 lL, 1.2 mmol), and the stirred mixture was heated to reflux under N2 for
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A.; Izzet, G.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10868–10875; (d) Ulrich, G.;
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10 h. The resulting mixture was poured into water and the organic layer was
separated. The aqueous phase was extracted with chloroform. The combined
organic phases were dried over sodium sulfate, filtered, and the solvents were
evaporated in vacuo. The obtained black solid was purified by column
chromatography over silica gel with chloroform/hexane (2:1) to give 7 (43.0
mg, 79%). Black solid. Mp 194–195 °C. 1H NMR (CDCl3, 400 MHz) d 2.15 (s, 3H),
2.34 (s, 3H), 2.39 (s, 3H), 3.48 (s, 3H), 6.47 (d, J = 4.0 Hz, 1H), 6.64 (d, J = 4.0 Hz,
1H), 6.68 (d, J = 4.0 Hz, 1H), 6.72 (d, J = 4.0 Hz, 1H), 6.77 (t, J = 7.8 Hz, 1H), 6.81
(d, J = 8.2 Hz, 2H), 6.87–6.93 (m, 5H), 6.98 (s, 1H), 7.00 (s, 1H), 7.05 (t, J = 7.8 Hz,
1H), 7.23 (td, J = 7.8, 1.6 Hz, 1H), 7.41–7.45 (m, 2H), 7.68 (dd, J = 7.8, 1.6 Hz,
1H). 13C NMR (CDCl3, 100 MHz) d 20.1, 20.5, 21.2, 55.7, 110.7, 114.6, 118.4,
119.0, 119.2, 119.8, 120.8, 122.3, 125.8, 125.9, 126.6, 126.8, 127.6, 128.2, 130.6,
131.1, 132.5, 133.1, 133.2, 136.6, 136.8, 137.0, 138.3, 142.1, 149.4, 154.6, 156.8,
158.0. 11B NMR (CDCl3, 128 MHz) d 2.80 (s). MS(ESI) observed m/z 547.29
([M+H]+), calcd for C37H32BN2O2 m/z 547.26. Anal. Calcd for
C37H31BN2O2Á0.2CHCl3: C, 78.34; H, 5.51; N, 4.91. Found: C, 78.05; H, 5.77; N,
4.62.
Compound 8: To a stirred solution containing 2 (29.1 mg, 0.055 mmol) and
K2CO3 (11.4 mg, 0.083 mmol) in acetonitrile (20 mL) was added 1-
bromobutane (0.50 mL of a 0.12 M solution in acetonitrile, 0.061 mmol). The
resulting mixture was treated as described for 7 to give 8 (24.4 mg, 75%). Black
solid. Mp 176–177 °C. 1H NMR (CDCl3, 400 MHz) d 0.70 (t, J = 7.6 Hz, 3H), 1.05–
1.15 (m, 2H), 1.32–1.40 (m, 2H), 2.14 (s, 3H), 2.30 (s, 3H), 2.38 (s, 3H), 3.39–
3.46 (m,1H), 3.67–3.72 (m, 1H), 6.49 (d, J = 4.0 Hz, 1H), 6.61 (d, J = 4.0 Hz, 1H),
6.68 (d, J = 4.0 Hz, 1H), 6.72 (d, J = 4.0 Hz, 1H), 6.72–6.91 (m, 8H), 6.97 (s, 1H),
7.00 (s, 1H), 7.07 (t, J = 7.6 Hz, 1H), 7.21 (t, J = 8.0 Hz, 1H), 7.42–7.37 (m, 2H),
7.78 (d, J = 7.2 Hz, 1H). 13C NMR (CDCl3, 100 MHz) d 13.8, 19.0, 20.0, 20.3, 21.2,
31.2, 68.3, 112.1, 114.3, 118.5, 119.0, 119.1, 119.9, 121.4, 122.7, 125.7, 125.9,
126.5, 126.9, 127.3, 128.2, 130.6, 131.1, 132.4, 132.9, 133.1, 136.7, 136.8, 137.1,
138.2, 142.0, 149.0, 155.3, 156.8, 157.7. 11B NMR (CDCl3, 128 MHz) d 2.85 (s).
MS(ESI) observed m/z 588.29 ([M+H]+), calcd for C40H38BN2O2 m/z 588.31. Anal.
Calcd for C40H37BN2O2Á0.5H2O: C, 80.40; H, 6.41; N, 4.69. Found: C, 80.67; H,
6.60; N, 4.48.
Compound 9: To a stirred solution containing 2 (18.2 mg, 0.034 mmol) and
K2CO3 (7.1 mg, 0.051 mmol) in acetone (20 mL) was added benzyl bromide
(0.50 mL of a 84 mM solution in acetonitrile, 0.042 mmol). The resulting
mixture was treated as described for 7 to give 9 (20.7 mg, 98%). Black solid. Mp
165–166 °C. 1H NMR (CDCl3, 400 MHz) d 2.14 (s, 3H), 2.35 (s, 3H), 2.39 (s, 3H),
4.46 (d, J = 11.9 Hz, 1H), 4.74 (d, J = 11.9 Hz, 1H), 6.50 (d, J = 4.0 Hz, 1H), 6.62 (d,
J = 4.0 Hz, 1H), 6.70 (d, J = 4.0 Hz, 1H), 6.76 (d, J = 4.0 Hz, 1H), 6.77 (t, J = 7.2 Hz,
1H), 6.80-6.86 (m, 5H), 6.92 (dd, J = 8.4, 2.1 Hz, 2H), 6.98–7.01 (m, 4H), 7.10–
7.16 (m, 4H), 7.23 (td, J = 7.6, 1.6 Hz, 1H), 7.40–7.45 (m, 2H), 7.80 (dd, J = 7.6,
1.6 Hz, 1H). 13C NMR (CDCl3, 100 MHz) d 20.0, 20.3, 21.2, 70.6, 112.9, 114.7,
118.5, 119.2, 119.6, 119.8, 121.2, 123.2, 125.8, 126.0, 126.6, 126.7, 126.9, 127.1,
127.6, 127.9, 128.2, 130.6, 131.1, 132.5, 133.0, 133.2, 136.6, 136.8, 137.0, 137.3,
138.3, 142.1, 149.4, 154.6, 156.7, 157.4. 11B NMR (CDCl3, 128 MHz) d 2.68 (s).
MS(ESI) observed m/z 622.27 ([M+H]+), calcd for C43H36BN2O2 m/z 622.29. Anal.
Calcd for C43H35BN2O2: C, 81.59; H, 5.58; N, 4.42. Found: C, 81.74; H, 5.96; N,
4.22.
9. The reported compound possesses a 4-iodophenyl group at the meso position.
See Ref.6a
.
10. General procedure for the synthesis of Ar-Bodipys 2–4: To a stirred solution
containing (44.4 mg, 0.100 mmol) in chloroform (20 mL) was added
1
arylboronic acid (0.120 mmol), and the mixture was heated to reflux for 3 h.
After being cooled, the solvent was evaporated and the resulting residue was
purified by column chromatography on silica gel using chloroform as the
eluent to give Ar-Bodipy.
Compound 2: Black solid. >99% yield. Mp 230 °C. 1H NMR (CDCl3, 400 MHz) d
2.14 (s, 3H), 2.32 (s, 3H), 2.39 (s, 3H), 6.35 (d, J = 4.0 Hz, 1H), 6.68 (d, J = 4.0 Hz,
1H), 6.75 (dd, J = 8.0, 1.3 Hz, 2H), 6.79 (d, J = 4.0 Hz, 1H), 6.84 (d, J = 4.0 Hz, 1H),
6.86–6.98 (m, 5H), 6.99 (s, 1H), 7.02 (s, 1H), 7.06 (dd, J = 7.8, 1.8 Hz, 1H), 7.12
(dd, J = 7.8, 1.0 Hz, 1H), 7.24 (dd, J = 7.8, 1.0 Hz, 1H), 7.32 (td, J = 7.8, 1.6 Hz, 1H),
7.41 (td, J = 7.8, 1.8 Hz, 1H), 7.49 (dd, J = 7.8, 1.6 Hz, 1H). 13C NMR (CDCl3, 100
MHz) d 20.1, 20.4, 21.2, 115.7, 118.5, 119.9, 120.1, 120.7, 120.8, 120.9, 124.6,
126.2, 126.5, 126.8, 127.9, 128.2, 128.3, 130.1, 130.8, 130.9, 131.6, 133.1, 133.8,
136.7, 136.9, 137.1, 138.6, 143.3, 149.6, 153.7, 154.7, 155.1. 11B NMR (CDCl3,
128 MHz)
d
2.55 (s). MS(ESI) observed m/z 533.28 ([M+H]+), calcd for
C36H30BN2O2 m/z 533.24. Anal. Calcd for C36H29BN2O2Á0.1CHCl3: C, 79.65; H,
5.69; N, 5.15. Found: C, 79.63; H, 5.68; N, 5.15.
Compound 3: Black solid. >99% yield. Mp 270–271 °C. 1H NMR (CDCl3,
400 MHz) d 2.13 (s, 3H), 2.40 (s, 3H), 2.48 (s, 3H), 6.23 (d, J = 4.0 Hz, 1H),
6.65 (d, J = 4.0 Hz, 1H), 6.79 (d, J = 4.5 Hz, 1H), 6.82 (d, J = 4.0 Hz, 1H), 6.83–6.89
(m, 2H), 6.90–6.95 (m, 2H), 7.01 (s, 1H), 7.02 (d, J = 7.3 Hz, 1H), 7.05 (s, 1H),
7.17 (t, J = 7.6 Hz, 1H), 7.19 (d, J = 8.6 Hz, 1H), 7.23–7.32 (m, 2H), 7.40–7.45 (m,
3H), 7.56 (d, J = 7.6 Hz, 1H), 7.95 (d, J = 8.6 Hz, 1H). 13C NMR (CDCl3, 100 MHz) d
20.1, 20.8, 21.2, 116.3, 119.0, 119.8, 120.6, 120.9, 121.0, 123.9, 124.2, 124.5,
126.2, 127.4, 127.9, 128.1, 128.3, 128.4, 128.5, 130.2, 130.8, 131.5, 131.6, 133.1,
133.2, 134.4, 135.9, 136.9, 137.0, 137.2, 138.7, 143.2, 150.3, 153.2, 154.1, 155.1.
11B NMR (CDCl3, 128 MHz) d 2.96 (s). MS(ESI) observed m/z 583.30 ([M+H]+),
calcd for C40H32BN2O2 m/z 583.26. Anal. Calcd for C40H31BN2O2Á0.05CHCl3: C,
81.74; H, 5.32; N, 4.76. Found: C, 81.63; H, 5.61; N, 4.76.
14. Islam, T. M. B.; Yoshino, K.; Sasane, A. Anal. Sci. 2003, 19, 455–460.
15. Crystallographic analysis of 2: deep red prism (0.5 Â 0.2 Â 0.2 mm3), C36H29BN2O2
(M = 532.42), triclinic, a = 8.1638(6), b = 12.7262(11), c = 13.3910(12) Å,
Compound 4: Black solid. 92% yield. Mp 225–226 °C. 1H NMR (CDCl3, 400 MHz)
d 2.13 (s, 3H), 2.31 (s, 3H), 2.39 (s, 3H), 3.60 (s, 3H), 6.35 (d, J = 4.0 Hz, 1H), 6.43
(d, J = 8.6 Hz, 2H), 6.67 (d, J = 8.6 Hz, 2H), 6.68 (d, J = 4.0 Hz, 1H), 6.82 (d,
J = 4.0 Hz, 1H), 6.88 (d, J = 4.0 Hz, 1H), 6.90 (td, J = 7.8, 1.0 Hz, 1H), 6.96 (td,
J = 7.8, 1.0 Hz, 1H), 6.99 (s, 1H), 7.01 (s, 1H), 7.10 (m, 2H), 7.25 (dd, J = 7.8,
1.0 Hz, 1H), 7.32 (td, J = 7.8, 1.6 Hz, 1H), 7.41 (td, J = 7.8, 1.6 Hz, 1H), 7.50 (dd,
J = 7.8, 1.6 Hz, 1H). 13C NMR (CDCl3, 100 MHz) d 20.1, 20.4, 21.2, 54.7, 112.3,
115.6, 118.5, 119.9, 120.1, 120.7, 120.8, 120.9, 124.7, 126.2, 127.9, 128.1, 128.2,
128.3, 130.1, 130.8, 131.6, 132.1, 133.0, 133.7, 136.7, 136.9, 137.1, 138.6, 143.2,
149.5, 153.7, 154.7, 155.2, 158.3. 11B NMR (CDCl3, 128 MHz) d 2.68 (s). MS(ESI)
a
= 83.874(3)°, b = 81.820(2)°,
c P
= 81.777(2)° V = 1357.6(2) Å3, space group
(No. 2), Z = 2,
q
calcd = 1.302 g/cm3, T = 120 K,
l
(Mo K
a
) = 0.092 mmÀ1, collected
(I)), wR2 = 0.1302
refections, 13,321, unique, 6140 (Rint = 0.0438), R1 = 0.0539 (I/2
r
(all data) GOF (F2) = 1.078. CCDC 716494 contains the supplementary
crystallographic data for this Letter. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
16. Sheldrick, G. M. SHELXL-97. Program for crystal structure refinement, University
of Göttingen, 1997.