Stabilization of three-coordinated germanium(II) and tin(II) cations by a neutral chelating ligand

Article abstract of DOI:10.1021/om400082t

Treatment of the neutral 2-[C(CH₃)=N(C₆H ₃-2,6-ⁱPr₂)]-6-(CH₃O)C ₆H₃N ligand (hereafter assigned as L) with SnCl ₂ and GeCl₂ provided the ionic germanium(II) and tin(II) complexes [LGe^IICl]⁺[Ge^IICl₃] ^- (1) and [LSn^IICl]⁺[Sn^IICl ₃]^- (2), respectively, as the result of spontaneous dissociation of ECl₂ (E = Ge, Sn). The cationic parts [LE ^IICl]⁺ of 1 and 2 contain three-coordinated germanium(II) and tin(II) atoms. In comparison, treatment of the ligand L with GeCl ₄ and SnBr₄ yielded the germanium(IV) and tin(IV) complexes LGeCl₄ (3) and LSnBr₄ (4), respectively, and no dissociation process was observed. Compounds 1-4 were characterized by means of elemental analyses, ¹H, ¹³C, and ¹¹⁹Sn NMR spectroscopies, and single-crystal X-ray diffraction analysis in (compounds 1 and 4).

Full text of DOI:10.1021/om400082t

Article
pubs.acs.org/Organometallics
Stabilization of Three-Coordinated Germanium(II) and Tin(II) Cations
by a Neutral Chelating Ligand
Marek Bouska, Libor Dostal, Ales Ruzicka, and Roman Jambor*
̌
́
̌
̊
̌ ̌
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska
10, Pardubice, Czech Republic
́
95, CZ-532
S
* Supporting Information
ABSTRACT: Treatment of the neutral 2-[C(CH3)N(C6H3-2,6-iPr2)]-6-(CH3O)C6H3N
ligand (hereafter assigned as L) with SnCl2 and GeCl2 provided the ionic germanium(II) and
tin(II) complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]− (2), respectively, as the
result of spontaneous dissociation of ECl2 (E = Ge, Sn). The cationic parts [LEIICl]+ of 1 and 2
contain three-coordinated germanium(II) and tin(II) atoms. In comparison, treatment of the
ligand L with GeCl4 and SnBr4 yielded the germanium(IV) and tin(IV) complexes LGeCl4 (3)
and LSnBr4 (4), respectively, and no dissociation process was observed. Compounds 1−4 were
1
characterized by means of elemental analyses, H, 13C, and 119Sn NMR spectroscopies, and
single-crystal X-ray diffraction analysis in (compounds 1 and 4).
Chart 1
INTRODUCTION
■
The field of group 14 element(II) monocation RE:+ chemistry
remains a challenge for the inorganic chemists.1 The positive
charge of the central atom suggests that these species should be
electrophilic, but at the same time, they hold potential to have
nucleophilic character through their lone pair. The pioneering
works showed the possible stabilization of RSn:+ or RGe:+
cations by cyclopentadienyl, N-isopropyl-2-(isopropylamino)-
troponimine, or cyclophane groups.2 The area of divalent
cations RE:+ was further explored by Jutzi, Power, Driess, and
Muller, who reported the synthesis of two-coordinated RSi:+ or
̈
RGe:+ cations stabilized by sterically encumbered β-diketimi-
nate ligands.3 The use of the bulky terphenyl ligand provided
stable lead cation RPb:+ that is weakly coordinated by a
molecule of toluene.4 The recent work of Krossing and Jones
reported on the synthesis of one coordinated RGe:+ and RSn:+
cations stabilized by an extremely hindered amide ligand.5 All of
these strategies involved kinetic stabilization of corresponding
monocations RE:+ (Chart 1A). Another approach to stabilize
extremely electrophilic monocations RE:+ is the electronic
stabilization. This can be achieved by the coordination of
divalent cations of group 14 elements to the transition metal,
successfully applied for the synthesis of trans-[(MeCN)(Ph2-
PCH2CH2PPh2)2WGe(η1-Cp*)][B(C6F5)4],6 [{NPhC(Me)-
CHC(Me)NPh}(OTf)Ge]W(CO)5,7 and [(Ph2PCH2CH2-
PPh2)2WSnC6H3-2,6-Mes2]PF6.8 The groups of Reid and
Baines used bi-, or multidentate ligands for autoionization of
GeX2 (X = Cl, Br) to stabilize monocation [RXGe:]+[GeCl3]−
or even dication [(crown-ether)Ge:]2+2[GeCl3]−.9
structurally and electronically resembling Y,C,Y−chelating
ligands, has been applied in main group chemistry.11 Recently,
Roesky et al. reported on the spontaneous dissociation of ECl2
in the presence of the Diimp ligand, yielding cation
[(Diimp)EIICl]+ containing four-coordinated metal centers E
(E = Ge, Sn) (Chart 1C).12
As part of a comprehensive study on the chemistry of
coordinated monocation RE:+ of group 14 element(II), we
report here the synthesis of three-coordinated tin and
germanium monocations by the use of neutral chelating ligand
2-[C(CH3)N(C6H3-2,6-iPr2)]-6-(CH3O)C6H3N (hereafter,
assigned as L). Treatment of ligand L with SnCl2 and GeCl2
provided complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+-
[SnIICl3]− (2), respectively, as the result of spontaneously
The electronic stabilization was also applied in the synthesis
of Y → Sn coordinated organotin(II) cations (Chart 1B).10 The
literature search revealed that neutral N,N,N-ligand (Diimp),
Received: January 30, 2013
Published: March 5, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/om400082t | Organometallics 2013, 32, 1995−1999

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Article
dissociation of ECl2 (E = Ge, Sn). In comparison, when more
Lewis acidic halides GeCl4 and SnBr4 were treated with ligand
L, no autoionization had occurred and the complexes LGeCl4
(3) and LSnBr4 (4) were isolated, respectively.
1; the crystallographic data are given in Table S1 (see the
Supporting Information).
RESULTS AND DISCUSSION
■
Chelating ligand 2-[C(CH3)N(C6H3-2,6-iPr2)]-6-(CH3O)-
C6H3N was synthesized by treatment of the 2-acetyl-6-
methoxypyridine with diisopropylaniline to give ligand L in
quantitative yield as a yellowish powder soluble in CH2Cl2 and
toluene. Treatment of L with 2 equiv of ECl2 provided the ionic
complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]−
(2), respectively (Scheme 1).
Scheme 1. Synthesis of Compounds 1 and 2
Figure 1. Molecular structure of 1 together with selected bond lengths
(Å) and angles (deg): Ge1−N1 2.045(4), Ge1−N2 2.070(4), Ge1−
Cl1 2.215(2), N2−C6 1.289(6); N1−Ge1−Cl1 94.12(1), N2−Ge1−
Cl1 94.92(1), N1−Ge1−N2 76.50(2).
Compounds 1 and 2 are highly soluble in chlorinated
solvents. Both compounds have been characterized by
elemental analysis and NMR spectroscopy and compound 1
1
by single-crystal X-ray structural analysis. In the H NMR
The molecular structure of 1 reveals the presence of an ionic
pair consisting of the cation [L*GeIICl]+ and anion [GeIICl3]−.
The germanium center is three-coordinated in the cation
[L*GeIICl]+, as the result of two strong N → Ge interactions
from ligand L and one chlorine atom. The Ge1−O1 bond
distance (2.984(4) Å) suggests the absence of a donor O → Ge
interaction (ΣcovGe,O = 1.84 Å),14 leaving the oxygen atom of
ligand L uncoordinated to the Ge atom. The nitrogen atoms
N1 and N2 of the ligand L and chlorine atom Cl1 form a basal
plane and define a distorted trigonal-pyramidal geometry of the
Ge1 atom when the electron lone pair of Ge1 is considered.
The Ge center is twisted by 0.335 Å from the N1−N2−C6−C1
plane defined by the ligand L. Both Ge−N bond lengths are
similar (Ge1−N1 is 2.045(4) Å and Ge1−N2 is 2.070(4) Å)
and define the presence of strong N → Ge interactions that is
practically unknown for structurally characterized Ge(II)
cations with N-donor ligands, where the range of 2.071(2)−
2.4286(5) Å was established for Ge−N bonds.9c,12,15 The
recently published chlorogermyliumylidene complex {[1,8-
bis(tri-n-butylphosphanzenyl)naphthalene]GeCl}+Cl−, with
Ge−N bond lengths being 1.981(1) and 1.960(3), is the only
example with a stronger Ge−N interaction reported to date.16
The coordination of two nitrogen atoms to the Ge atom
provided one five-membered ring N1−Ge1−N2−C1−C6 with
an acute angle N1−Ge1−N2 being 76.50(2)°.
spectrum of both compounds 1 and 2, the aromatic protons are
shifted downfield (δ 7.63, 8.25, and 8.67 ppm for 1, δ 7.48,
8.12, and 8.37 ppm for 2) when compared with the free ligand
(δ 6.75, 7.68, and 7.95 ppm). Similarly, the signal of the CH3
protons of the (CH3)CN group are shifted downfield (δ 2.66
ppm for 1, δ 2.87 ppm for 2) in comparison to ligand L (δ 2.10
ppm). On the other hand, protons of the OMe group revealed
1
similar chemical shifts in the H NMR spectrum of 1 and 2 (δ
4.32 for 1 and 4.05 ppm for 2) in comparison to ligand L (δ
i
3.99 ppm). The CH3 protons of Pr groups showed two
different resonances (δ 1.13, 1.21 ppm for 1 and δ 1.17, 1.29
ppm for 2) in comparison to the free ligand L, where CH3
i
protons of Pr groups resonate at 1.14 ppm. The 119Sn NMR
spectrum of 2 revealed two resonances at −73.2 and −330.4
−
ppm. While the signal at −73.2 ppm corresponds to the SnCl3
anion,13 a second resonance at −330.4 ppm was assigned to the
[LSnCl]+ cation. This value is, however, shifted downfield when
compared to the related cationic complex [(Diimp)SnIICl]+
(−435.0 ppm), where a four-coordinated tin center was
found.12 The NMR spectra of 1 and 2 thus indicate that the
ligand L strongly coordinates metal atoms E (E = Ge, Sn) by N
→ E coordination of the (CH3)CN moiety and suggest the
absence of O → E coordination of the OMe group in solution.
This coordination behavior of L provided three-coordinated
metal centers, as suggested by the downfield shift in the 119Sn
NMR spectrum.
To see whether similar autoionization will take place with
more Lewis acidic halides of 14 group elements, ligand L was
further treated with GeCl4 and SnBr4. As no autoionization
occurred, complexes LGeCl4 (3) and LSnBr4 (4) were isolated
(Scheme 2).
Whereas compound 3 is soluble in nonpolar solvents,
compound 4 is soluble in polar solvents, such as CH2Cl2 or
THF only. Both compounds have been characterized by the
The existence of ionic structures of [L*GeIICl]+[GeIICl3]−
(1), as predicted by NMR spectroscopy, was unambiguously
established by the single-crystal X-ray diffraction analysis of 1.
Suitable single crystals of 1 were obtained from saturated
toluene solutions at room temperature. The molecular structure
of 1 and selected bond lengths and angles are shown in Figure
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Article
is 2.320(12) Å) and indicate the existence of strong Sn ← N
interactions. While both nitrogen atoms N1 and N2 of ligand L
are coordinated to the tin atom, the Sn1−O1 bond distance
(3.205(13) Å) suggests the absence of Sn−O interaction,
leaving the oxygen atom of ligand L uncoordinated to Sn atom.
In conclusion, we have prepared neutral chelating ligand L
and showed that this ligand is able to stabilize cationic
complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]−
(2), as the result of spontaneous dissociation of ECl2. In
contrast, similar reaction of L with EX4 provided the neutral
complexes LGeCl4 (3) and LSnBr4 (4), respectively. Molecular
structures of complexes 1 and 4 revealed that the neutral ligand
L behaved as a 4e− N,N-chelating ligand and no E ← O
donation was established. This coordination behavior of the
ligand L allowed the stabilization of three-coordinated EII
cations (E = Ge, Sn).
Scheme 2. Synthesis of Compounds 3 and 4
elemental analysis and NMR spectroscopy and compound 4 by
single-crystal X-ray diffraction analysis. The 1H NMR spectra of
compounds 3 and 4 revealed one set of signals of ligand L,
indicating a highly symmetrical arrangement around central E
atoms. In addition, the aromatic protons and the signals of the
CH3 protons of the CH3CN group did not showed
reasonable downfield shifts when compared with the free
ligand L, which hints that no ionization occurred in 3 and 4. All
attempts to get a 119Sn NMR signal of 4 were unsuccessful,
although long experiments were measured even at 220 K.
These suggestions were proved by the help of single-crystal X-
ray diffraction analysis of 4. Suitable single crystals of 4 were
obtained from saturated CHCl3 solution at room temperature.
The molecular structure of 4 and selected bond lengths and
angles are shown in Figure 2; the crystallographic data are given
EXPERIMENTAL SECTION
■
General Methods. The starting compounds 2-acetyl-6-methox-
ypyridine, diisopropylaniline, SnCl2, GeCl2*dioxane, SnBr4, and GeCl4
were purchased by Sigma Aldrich. All reactions were carried out under
argon, using standard Schlenk techniques. Solvents were dried by
standard methods and distilled prior to use. The H, 13C, and 119Sn
1
NMR spectra were recorded on a Bruker Avance500 spectrometer at
1
300 K in CDCl3 or THF-d8. The H, 13C, and 119Sn NMR chemical
shifts δ are given in parts per million and referenced to external Me4Sn
1
(
119Sn) and Me4Si (13C, H). Elemental analyses were performed on
an LECO-CHNS-932 analyzer.
Synthesis of Ligand L. To a solution of 2-acetyl-6-methoxypyr-
idine (5 g, 33.1 mmol) in methanol (30 mL) was added 2,6-
diisopropylaniline (6.24 mL, 33.1 mmol) and 10 drops of formic acid.
The reaction mixture was heated for 24 h at 70 °C, and yellow powder
was precipitated. The yellow powder was filtered off and washed with
hexane. Yield: 8.9 g (87%). mp: 155.8−156.1 °C. Anal. Calcd for
C20H26N2O (310.4 g/mol): C, 77.38; H, 8.44. Found: C, 77.35; H,
8.42. 1H NMR (CDCl3, 500.18 MHz): δ = 1.14 (d, 12H, CH(CH3)2),
2.10 (s, 3H, (CH3)CN), 2.66 (h, 2H, CH(CH3)2), 3.99 (s, 3H,
OCH3), 6.75 (d, 1H, ArH) 7.01−7.20 (m, 3H, ArH), 7.68 ppm (t, 1H,
ArH), 7.95 ppm (d, 1H, ArH). 13C NMR (CDCl3, 125.77 MHz): δ =
17.2 CH(CH3)2, 22.9 CH(CH3)2, 23.1 CH(CH3)2, 28.2(CH3)CN,
53.2 OCH3, 111.6, 113.9, 122.9, 123.4, 135.8, 138.9, 146.6 (CH3)C
N, 153.9, 163.2, 166.8.
Synthesis of [LGeIICl]+[GeIICl3]− (1). GeCl2·dioxane powder (0.37
g, 1.6 mmol) was added with stirring to the solution of L (0.25 g, 0.8
mmol) in CH2Cl2 (20 mL) at room temperature. The reaction mixture
was stirred for an additional 24 h. The solution was filtered, and the
filtrate was concentrated to a volume of approximately 10 mL. Storage
overnight at room temperature gave yellow crystals. Yield: 0.40 g
(83%). mp: 197.0−200.3 °C. Anal. Calcd for C20H26N2OGe2Cl4
(601.4 g/mol): C, 39.94; H, 5.03. Found: C, 39.99; H, 5.01. 1H
NMR (CDCl3, 500.18 MHz): δ = 1.13 (d, 6H, CH(CH3)2), 1.21 (d,
6H, CH(CH3)2), 2.66 (s, 3H, (CH3)CN), 2.75 (h, 2H, CH(CH3)2),
4.32 (s, 3H, OCH3), 7.32−7.42 (m, 3H, ArH), 7.63 (d, 1H, ArH), 8.25
ppm (d, 1H, ArH), 8.67 ppm (t, 1H, ArH). 13C NMR (CDCl3, 125.77
MHz): δ = 19.1 CH(CH3)2, 25.0 CH(CH3)2, 29.3(CH3)CN, 59.0
OCH3, 114.6, 123.5, 130.1, 132.9, 137.8, 141.7, 145.6 (CH3)CN,
149.2, 163.7, 176.9.
Figure 2. Molecular structure of 4 together with selected bond lengths
(Å) and angles (deg): Sn1−N1 2.304(14), Sn1−N2 2.320(12), Sn1−
O1 3.205(13); N1−Sn1−N2 71.7(5), N1−Sn1−Br4 158.6(4), N2−
Sn1−Br1 177.3(3).
Synthesis of [LSnIICl]+[SnIICl3]− (2). SnCl2 powder (0.35 g, 1.84
mmol) was added with stirring to the solution of L (0.29 g, 0.92
mmol) in CH2Cl2 (20 mL) at room temperature. The reaction mixture
was stirred for an additional 24 h, the CH2Cl2 suspension was filtered,
and the filtrate was concentrated to a volume of approximately 10 mL.
Storage overnight at 5 °C gave a yellow solid. Yield: 0.46 g (71%). mp:
184.2−187.4 °C. Anal. Calcd for C20H26N2OSn2Cl41(MW = 689.6): C,
34.83; H, 3.80. Found: 34.90 C, %; 3.86 H, %. H NMR (CDCl3,
500.18 MHz): δ = 1.17 (d, 6H, CH(CH3)2), 1.29 (d, 6H, CH(CH3)2),
2.78 (bs, 2H, CH(CH3)2), 2.87 (s, 3H, (CH3)CN), 4.05 (s, 3H,
OCH3), 7.21−7.33 (m, 3H, ArH), 7.48 (bs, 1H, ArH), 8.12 ppm (bs,
in Table S1 (Supporting Information). Although the periphery
carbon atoms of the phenyl rings are disordered, the primary
coordination sphere of the tin atom can be described. The
molecular structure of 4 reveals the presence of a
hexacoordinated tin atom. A slightly distorted octahedral
geometry is formed by two N atoms from ligand L and four
bromine atoms, showing the symmetrical arrangement of the
tin atom, as suggested by NMR spectroscopy. Both Sn−N
bond lengths are similar (Sn1−N1 is 2.304(14) Å and Sn1−N2
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1H, ArH), 8.37 ppm (bs, 1H, ArH). 13C NMR (CDCl3, 125.77 MHz):
δ = 16.9 CH(CH3)2, 21.3 CH(CH3)2, 27.4 (CH3)CN, 53.3 OCH3,
112.4, 119.8, 122.8, 127.8, 130.0, 139.8, 141.6 (CH3)CN, 145.2,
162.2, 180.8. 119Sn NMR (CDCl3, 186.49 MHz): δ = −73.2, −330.4
ppm
ASSOCIATED CONTENT
■
S
* Supporting Information
Further details of the structure determination of compounds 1
and 4, including atomic coordinates, anisotropic displacement
parameters, and geometric data. This material is available free
of charge via the Internet at http://pubs.acs.org.
Synthesis of LGeCl4 (3). GeCl4 (0.22 mL, 1.9 mmol) was added
with stirring to a solution of L (0.58 g, 1.9 mmol) in CH2Cl2 (20 mL),
at room temperature. The reaction mixture was stirred for an
additional 24 h. The suspension was filtered, and the filtrate was
concentrated to a volume of approximately 5 mL. Storage overnight at
5 °C gave a yellow solid. Yield: 0.90 g (92%). mp: 149.3−151.8 °C.
Anal. Calcd for C20H26N2OGeCl4 (MW = 534.8): C, 45.77; H, 4.99.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: roman.jambor@upce.cz.
1
Notes
Found: 45.85 C, %; 5.05 H, %. H NMR (CDCl3, 500.13 MHz): δ
The authors declare no competing financial interest.
1.26 (d, 6H, CH(CH3)2, 1.30 (d, 6H, CH(CH3)2, 2.34 (s, 3H, CH3C
N), 2.88 (septet, 2H, CH(CH3)2), 4.10 (s, 3H, OCH3), 6.97 ppm (d,
2H, ArH), 7.23−7.29 (m, 3H, ArH), 7.80 ppm (t, 1H, ArH), 8.20 ppm
(d, 2H, ArH). 13C NMR (CDCl3, 125.77 MHz): δ = 17.5 CH(CH3)2,
22.9 CH(CH3)2, 23.3 CH(CH3)2, 28.3 (CH3)CN, 53.3 OCH3,
112.6, 114.8, 122.8, 124.6, 136.6, 139.1, 145.4 (CH3)CN, 153.1,
163.1, 168.6.
ACKNOWLEDGMENTS
■
The authors wish to thank the Ministry of Education of the
Czech Republic for financial support.
REFERENCES
Synthesis of LSnBr4 (4). SnBr4 powder (1.3 g, 2.9 mmol) was
added with stirring to a solution of 1 (0.90 g, 2.9 mmol) in CH2Cl2
(20 mL), at room temperature. The reaction mixture was stirred for an
additional 24 h. The suspension was filtered, and the filtrate was
concentrated to a volume of approximately 5 mL. Storage overnight at
room temperature gave yellow crystals. Yield 1.95 g (90%). mp: 162
°C-decomp. Anal. Calcd for C20H26N2OSnBr4 (MW = 748.7): C,
■
(1) Reviews: (a) Muller, T. Adv. Organomet. Chem. 2005, 53, 155.
̈
(b) Zharov, I.; Michl, J. In The Chemistry of Organic Germanium, Tin
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(f) Panisch, R.; Bolte, M.; Muller, T. J. Am. Chem. Soc. 2006, 128,
̈
32.08; H, 3.50. Found: 32.13 C, %; 3.44 H, %. H NMR (THF-d8,
9676.
500.13 MHz): δ 1.10 (d, 6H, CH(CH3)2, 1.14 (d, 6H, CH(CH3)2, 1.73
(s, 3H, CH3CN), 2.74 (septet, 2H, CH(CH3)2), 3.58 (s, 3H,
OCH3), 6.85 ppm (d, 2H, ArH), 6.88−7.13 (m, 3H, ArH), 7.73 ppm
(t, 1H, ArH), 7.95 ppm (d, 2H, ArH). 13C NMR (THF-d8, 125.77
MHz): δ 17.8 CH(CH3)2, 23.6 CH(CH3)2, 24.1 CH(CH3)2,
29.7(CH3)CN, 53.9 OCH3, 113.4, 115.1, 124.1, 124.8, 136.8,
140.3, 148.1 (CH3)CN, 155.2, 164.9, 167.8. 119Sn NMR (THF-d8,
186.49 MHz): δ not found.
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̈
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and slow diffusion of solvent produced material that was suitable for X-
ray analysis and characterized as compounds 1 and 4·CHCl3.
The X-ray data (Table S1, Supporting Information) for colorless
crystals of 1 and 4·CHCl3 were obtained at 150 K using an Oxford
Cryostream low-temperature device on a Nonius KappaCCD
diffractometer with Mo Kα radiation (λ = 0.71073 Å), a graphite
monochromator, and the ϕ and χ scan mode. Data reductions were
performed with DENZO-SMN.17 The absorption was corrected by
integration methods.18 Structures were solved by direct methods
(Sir92)19 and refined by full-matrix least-squares based on F2
(SHELXL97).20 Hydrogen atoms were mostly localized on a
difference Fourier map; however, to ensure uniformity of the
treatment of the crystal, all hydrogen atoms were recalculated into
idealized positions (riding model) and assigned temperature factors
Hiso(H) = 1.2 Ueq(pivot atom) or of 1.5Ueq for the methyl moiety with
C−H = 0.96, 0.97, and 0.93 Å for methyl, methylene, and hydrogen
atoms in aromatic rings, respectively. Only bad quality crystals of 4
were measured, giving positional disorder of the phenyl ring with low
precision of C−C bond distances and about 200 Å large solvent
accessible voids. We tried to treat these disorders and voids by
standard methods implemented in SHELXL or Platon software
packages, but these tries were unsuccessful. Crystallographic data for
structural analysis have been deposited with the Cambridge Crystallo-
graphic Data Centre. CCDC nos. 919630 and 919631 for 1 and
4·CHCl3, respectively. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EY, U.K. (Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.
uk or www: http://www.ccdc.cam.ac.uk).
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̈
G.; Huber, B. Chem. Ber. 1989, 122, 871. (h) Dias, H. V. R.; Jin, W. J.
Am. Chem. Soc. 1996, 118, 9123. (i) Dias, H. V. R.; Wang, Z. J. Am.
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