The addition of organometallic reagents to tetramesityldigermene

Article abstract of DOI:10.1139/v02-128

The thermolysis and photolysis of hexamesitylcyclotrigermane in the presence of ethylmagnesium bromide has been investigated. Under photochemical conditions, ethyldimesitylgermane, 1,2-diethyl-1,1,2-trimesityldigermane and ethyl-1,1,2,2-tetramesityldigerm

Full text of DOI:10.1139/v02-128

1387
The addition of organometallic reagents to
tetramesityldigermene
Kyle L. Fujdala, David W.K. Gracey, Erica F. Wong, and Kim M. Baines
Abstract: The thermolysis and photolysis of hexamesitylcyclotrigermane in the presence of ethylmagnesium bromide
has been investigated. Under photochemical conditions, ethyldimesitylgermane, 1,2-diethyl-1,1,2-trimesityldigermane
and ethyl-1,1,2,2-tetramesityldigermane were isolated and, under thermal conditions, 1,2,2-triethyl-1,1-dimesityl-
digermane and 2,2-diethyl-1,1,1-trimesityldigermane were isolated. The photolysis of hexamesitylcyclotrigermane in the
presence of methyllithium has also been investigated. In both cases, the organometallic reagent adds to tetramesityl-
digermene and dimesitylgermylene formed by photochemical or thermal cleavage of the cyclotrigermane. In the case of
the addition of the Grignard reagent, the resulting germyl Grignard reagent undergoes a facile ligand exchange reaction.
Key words: digermene, germylene, Grignard reagents, alkyllithium reagents, germylmagnesium compounds,
germyllithium compounds.
Résumé : On a étudié la thermolyse et la photolyse de l’hexamésitylcyclotrigermane en présence de bromure
d’éthylmagnésium. Dans les conditions photochimiques, on a isolé de l’éthyldimésitylgermane, du 1,2-diéthyl-1,1,2-
trimésityldigermane et du éthyl-1,1,2,2-tétramésityldigermane alors, que dans des conditions thermiques, on a isolé du
1,2,2-triéthyl-1,1-dimésityldigermane et du 2,2-diéthyl-1,1,1-trimésityldigermane. On a aussi examiné la photolyse de
l’hexamésitylcyclotrigermane en présence de méthyllithium. Dans les deux cas, le réactif organométallique s’additionne
au tétraméthyldigermène et au dimésitylgermylène qui se forment par clivage photochimique ou thermique du
cyclotrigermane. Dans le cas de l’addition du réactif de Grignard, le réactif germylmagnésien qui en résulte subit une
réaction facile d’échange de ligands.
Mots clés : digermène, germylène, réactifs de Grignard, réactifs alkyllithium, composés germylmagnésium, composés
germyllithium.
[Traduit par la Rédaction] Fujdala et al. 1392
Introduction
Furthermore, we discovered that thermolysis of the result-
ing germyl Grignard reagent in the presence of excess
MeMgX leads to a facile ligand exchange reaction in which
the mesityl groups on the germanium bonded to the magne-
sium were exchanged for methyl groups (eq. [2]).
The reactivity of digermenes has been of interest since
stable and relatively stable derivatives of these compounds
were first synthesized twenty years ago (1). Despite two de-
cades of research in this area, the addition of organometallic
reagents remains a relatively unexplored area of digermene
chemistry. We have reported that the addition of
[2]
methylmagnesium halide to Mes2Ge=GeMes2 (Mes
=
mesityl = 2,4,6-trimethylphenyl) gives (dimesitylmethyl-
germyl)dimesitylgermylmagnesium bromide in good yield
(eq. [1]) (2).
A mechanism involving the ꢀ-elimination of MesMgX
followed by the addition of MeMgX to an intermediate
germylene has been proposed to explain the observed ligand
exchange (Scheme 1). Evidence in support of the proposed
mechanism was provided by trapping the initially formed in-
termediate germylene with 2,3-dimethylbutadiene. We have
also found that methylmagnesium halide adds regio-
selectively to germasilenes giving germyl Grignard reagents,
which undergo an analogous methyl-for-mesityl exchange
(2, 3). To investigate not only the generality of the Grignard
addition to digermenes and the generality of the ligand ex-
change reaction, we now describe the addition of
ethylmagnesium bromide to tetramesityldigermene gener-
ated either photochemically or thermally from hexamesityl-
cyclotrigermane. Furthermore, we also report on the addition
of methyllithium to tetramesityldigermene and the thermal
behaviour of the products obtained therefrom.
[1]
Received 5 February 2002. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 11 September 2002.
Dedicated to Professor Tristram Chivers in recognition of his
contributions to the field of inorganic chemistry.
K.L. Fujdala, D.W.K. Gracey, E.F. Wong, and
K.M. Baines.1 Department of Chemistry, University of
Western Ontario, London, ON N6A 5B7, Canada.
1Corresponding author (e-mail: kbaines2@uwo.ca).
Can. J. Chem. 80: 1387–1392 (2002)
DOI: 10.1139/V02-128
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Can. J. Chem. Vol. 80, 2002
Scheme 1.
Similarly, compound 4 is apparently derived from addition
of the ethyl Grignard reagent to tetramesityldigermene to
give the germyl Grignard reagent 5 followed by hydrolysis
(Scheme 2). Compound 3 is believed to arise from 5 via an
ꢀ-elimination of mesitylmagnesium bromide in the cold to
give the germylgermylene 6. Ethylmagnesium bromide
could then add to 6, giving 3 after hydrolysis (Scheme 2).
The reactivity observed in the addition of EtMgBr to
Mes2Ge=GeMes2 parallels that for the addition of the methyl
analog. However, unlike the methyl analog where a ligand
exchange was observed only upon thermolysis of the germyl
Grignard reagent at 105°C, the ethyl-for-mesityl exchange
was observed at low temperature. Presumably, the more fac-
ile ligand exchange is the result of greater steric congestion
in the germyl Grignard reagent, which is subsequently re-
lieved upon elimination of mesitylmagnesium bromide.
Ligand exchange in t-Bu2MeSi-GeMes2MgX was also ob-
served at lower temperatures compared to ligand exchange
in Mes2MeSi-GeMes2MgX (3).
Results and discussion
The photolysis of hexamesitylcyclotrigermane (1) in tolu-
ene at –60°C in the presence of excess ethylmagnesium bro-
mide, followed by hydrolysis using ammonium chloride, led
to the isolation of three products: ethyldimesitylgermane (2),
1,2-diethyl-1,1,2-trimesityldigermane (3), and ethyl-1,1,2,2-
tetramesityldigermane (4) in a 7.8:2.8:1 ratio (eq. [3]). All
new compounds were characterized by H, 13C NMR, and
1
IR spectroscopy and mass spectrometry, the details of which
are given in the experimental section. In general, the greater
shift dispersion of the 13C NMR spectrum of a given com-
pound was useful in determining the number of different
1
mesityl or ethyl groups and the H NMR spectrum gave the
ratio of mesityl:ethyl:GeH groups. Finally, the high resolu-
tion mass spectrum was critical in determining the molecular
formula of the compound.
The formation of compounds 2ꢁ4 can be understood on
the basis of our previous mechanistic proposal. The photo-
lysis of hexamesitylcyclotrigermane results in the formation
of dimesitylgermylene and tetramesityldigermene (4). The
formation of 2 is most easily accounted for by the quenching
of ethyldimesitylgermylmagnesium bromide formed by the ad-
dition of ethylmagnesium bromide to dimesitylgermylene.
Thermolysis of hexamesitylcyclotrigermane for extended
periods of time gave two major products: 1,2,2-triethyl-1,1-
dimesityldigermane (8) and 2,2-diethyl-1,1,1-trimesityldi-
germane (9) in a 1:1 to 2:1 ratio (eq. [4]). Compounds 8 and
9 were identified using standard spectroscopic techniques.
[3]
[4]
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Fujdala et al.
1389
Scheme 2.
Scheme 3.
Formation of these two compounds can be easily ex-
plained based on our current mechanism. Compound 8 appears
to be the result of an ꢀ-elimination of mesitylmagnesium
bromide from germyl Grignard 7 to give ethyldimesityl-
germylethylgermylene, followed by the addition of ethyl-
magnesium bromide to give 8 after hydrolysis. The
formation of compound 9 is interesting since an analogous
product was not observed in the addition of MeMgBr to the
digermene. Tetramesityldigermene is known to undergo a
facile mesityl migration to give trimesitylgermyl-
mesitylgermylene (5). Addition of ethylmagnesium bromide
to this germylene, followed by ꢀ-elimination of mesitylmag-
nesium bromide and the addition of a second equivalent of
ethylmagnesium bromide, would result in the formation of
germyl Grignard 10, which upon quenching would give the
observed product (Scheme 3). As in the analogous reaction
with methylmagnesium halide, no apparent product derived
from dimesitylgermylene was isolated. Presumably,
ethylmagnesium bromide adds to Mes2Ge: to give
ethyldimesitylgermylmagnesium bromide; however, under
the reaction conditions, complete ligand exchange occurs to
give triethylgermane (after hydrolysis), which was not iso-
lated. Thermolysis for shorter periods of time (2 h) gave a
more complex reaction mixture. In addition to compounds 8
and 9, compounds 2 and 3 were formed, providing evidence
for the intermediate formation of ethyldimesitylgermylmag-
nesium bromide and the germyl Grignard 7.
Again, there are both similarities and differences observed
in comparison to the analogous thermolysis of hexamesityl-
cyclotrigermane with the methyl Grignard reagent. A facile
ligand exchange was also observed in this system; however,
the rate of addition of the bulkier ethyl group appears to
compete with the 1,2-mesityl shift at 105°C.
Given the interesting reactivity of germyl Grignard re-
agents and the absence of any reported additions of
alkyllithium reagents to Group 14 dimetallenes, we also in-
vestigated the addition of methyllithium to Mes2Ge=GeMes2
under similar conditions.
The photolysis of hexamesitylcyclotrigermane in the pres-
ence of methyllithium at –60°C followed by quenching the
reaction mixture with a solution of ammonium chloride gave
two products: methyl-1,1,2,2-tetramesityldigermane (2) (11)
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Can. J. Chem. Vol. 80, 2002
[5]
and dimesitylmethylgermane (6) (12) in modest isolated yields
(eq. [5]). The compounds were identified by comparison of
their spectroscopic data to that of authentic samples. Given
that methyllithium does not react with the cyclotrigermane
in the dark, it is reasonable to assume that methyllithium
adds to both tetramesityldigermene and dimesitylgermylene
(generated by photolysis of the cyclotrigermane) in the same
manner as Grignard reagents. Thermolysis of hexamesityl-
cyclotrigermane in the presence of methyllithium gave only
a complex reaction mixture, in contrast to the analogous re-
action with methyl Grignard reagent.
In summary, the results described here for the addition of
ethylmagnesium bromide to tetramesityldigermene and
dimesitylgermylene parallel those previously observed for
the addition of methylmagnesium bromide to digermenes
and germasilenes. Hence, it has become increasingly appar-
ent that the addition of Grignard reagents to both diger-
menes and germylenes and the ligand exchange of germyl
Grignard reagents are all general reactions. Furthermore,
alkyllithium reagents also appear to readily add to diger-
menes and germylenes, although thermolysis of the resulting
mixture of germyllithium reagents does not lead to isolable
species. We continue to explore the addition of a variety of
organometallic reagents to digermenes and the chemistry of
the resulting germyl metallic reagents.
Photolysis of hexamesitylcyclotrigermane in the
presence of excess ethylmagnesium bromide
A solution of Ge3Mes6 (48 mg, 0.052 mmol) and freshly
prepared ethylmagnesium bromide (1 mL, 1.5 mmol in
Et2O) in toluene (3 mL) was photolyzed (350 nm) for 22 h
at –60°C. At the end of the photolysis, the solution was
bright yellow-orange. The cold reaction mixture was
quenched with aqueous ammonium chloride. The aqueous
and organic phases were then separated. The aqueous phase
was extracted with three portions of hexanes and the ex-
tracts were combined with the organic phase. The organic
phase was dried over anhydrous magnesium sulfate and
then filtered to remove the solid. Removal of the solvent in
vacuo yielded a clear, colourless oil. 1H NMR spectro-
scopic analysis of the reaction mixture showed three major
products in a 7.8:1:2.8 ratio (2:4:3). The reaction mixture
was separated by TLC using 8% CH2Cl2 in hexanes as the
eluent to give ethyldimesitylgermane (2, 15.3 mg, 87%)
and ethyl-1,1,2,2-tetramesityldigermane (4, 8.5 mg, 25%).
Compound 3 was not isolated in pure form.
Ethyldimesitylgermane (2):
mp 76.5–78°C. IR (thin film) (cm–1): 2045 (Ge-H).
1H NMR (ppm) ꢂ: 6.73 (s, 4H, Ar-H), 5.44 (t, J = 4.2 Hz,
1H, Ge-H), 2.35 (s, 12H, Mes o-CH3), 2.11 (s, 6H, Mes
p-CH3), 1.34–1.45 (m, 2H, Et-CH2), 1.12 (t, J = 7.7 Hz,
3H, Et-CH3). 13C NMR (ppm) ꢂ: 143.46 (Mes-C), 138.12
(Mes-C), 134.25 (Mes-C), 129.08 (Mes-CH), 23.89 (Mes
o-CH3), 21.02 (Mes p-CH3), 11.58 (CH2), 11.26 (CH3). MS
m/z: 342 ([M]+, 10), 313 ([HGeMes2], 100), 222 ([GeEtMes],
34), 191 ([GeMes], 42), 119 ([Mes], 18), 105 (25). HR-MS
calcd. for C20H2874Ge: 342.1403; found: 342.1393.
Experimental
All experiments were carried out in flame-dried glassware
under an inert argon atmosphere. Toluene, diethyl ether, and
THF were freshly distilled from sodium/benzophenone.
Ethylmagnesium bromide was prepared from commercially
available ethyl bromide and magnesium (Lancaster Chemi-
cal Co.). The concentration of the Grignard reagent was de-
termined by titration, prior to use. Chromatographic separations
and purifications were carried out using a Chromatotron
(Harrison Research) or on conventional silica gel preparative
plates. Photolyses were carried out at 350 nm in a Rayonet
photochemical reactor. The solution to be photolyzed was
placed in a Schlenk tube and suspended in an immersion
well filled with methanol at –60°C. The temperature of the
methanol was maintained using a NESLAB ULT-80 low
temperature bath circulator.
NMR spectra were recorded on a Varian Gemini 200, an
XL-300, or a Varian Gemini 300 NMR spectrometer using
benzene-d6 as a solvent. IR spectra were recorded (cm–1) as
thin films on a PerkinElmer 2000 FT-IR spectrometer. A
Finnegan MAT model 8200 mass spectrometer, with an ion-
izing voltage of 70 eV, was used to obtain electron impact
mass spectra (recorded in mass-to-charge units (m/z) with
ion identity and peak intensities relative to the base peak in
parentheses).
Ethyl-1,1,2,2-tetramesityldigermane (4):
1
IR (thin film) (cm–1): 2032 (Ge-H). H NMR (ppm) ꢂ:
6.72 (s, 4H, Ar-H), 6.70 (s, 4H, Ar-H), 5.92 (s, 1H, Ge-H),
2.36 (s, 12H, Mes o-CH3), 2.21 (s, 12H, Mes o-CH3), 2.10
(s, 6H, Mes p-CH3), 2.09 (s, 6H, Mes p-CH3), 1.92 (q, J =
7.8 Hz, 2H, Et-CH2), 1.1 (t, J = 7.8 Hz, 3H, Et-CH3).
13C NMR (ppm) ꢂ: 143.80, 143.73, 137.83, 137.48, 137.45,
136.58 (all C), 129.44 (Mes-CH), 128.96 (Mes-CH), 24.90,
24.85, 21.00, 20.91 (all CH3), 15.20 (CH2), 11.06 (CH3).
CI-MS m/z (isobutane): 651 ([M – H]+, 5), 533 ([M – Mes]+,
13), 431 ([GeMes3], 17), 341 ([GeMes2Et], 100), 311
([GeMes2 – H], 22), 223 ([GeEtMesH], 18), 191 ([GeMes],
10), 121 ([MesH
+
H], 88). HR-MS calcd. for
C38H5072Ge74Ge-H: 651.2267; found: 651.2276.
Thermolysis of hexamesitylcyclotrigermane in the
presence of excess ethylmagnesium bromide
A solution of Ge3Mes6 (50.3 mg, 0.055 mmol) and freshly
prepared ethylmagnesium bromide (1 mL, 1.65 mmol in
Et2O) in toluene (4 mL) was heated for 68 h at 105°C. After
a few minutes of heating, the clear and colourless solution
© 2002 NRC Canada

Fujdala et al.
1391
became light yellow and some solid precipitated from solu-
tion. At the end of the thermolysis, the solution was cloudy
grey in colour with a white solid precipitate. Following the
thermolysis, the reaction mixture was quenched with
aqueous ammonium chloride. The aqueous and organic
phases were then separated. The aqueous phase was ex-
tracted with three portions of hexanes and the extracts were
combined with the organic phase. The organic phase was
dried over anhydrous magnesium sulfate and then filtered to
remove the solid. Removal of the solvent in vacuo yielded a
Photolysis of hexamesitylcyclotrigermane in the
presence of methyllithium
Methyllithium (1.0 mL of a 1.4 M solution in diethyl
ether, 1.4 mmol) was added to a Schlenk tube containing a
clear, pale yellow solution of Ge3Mes6 (53 mg, 0.057 mmol)
dissolved in toluene (4 mL). The Schlenk tube was then
sealed and placed in an immersion well, cooled to –60°C,
and then irradiated for 13 h. After photolysis, the clear, deep
yellow solution was warmed to room temperature and was
quenched with aqueous ammonium chloride. The aqueous
and organic phases were then separated. The aqueous phase
was extracted with four portions of hexanes and the extracts
were combined with the organic phase. The organic phase
was washed three times with water and then dried over an-
hydrous magnesium sulfate. After filtration to remove the
solid, removal of the solvent in vacuo yielded a pale yellow
solid. The product mixture was separated by preparative
TLC using hexanes–dichloromethane (99:1) as the eluent to
give 1,1,2,2-tetramesitylmethyldigermane (2) (12, 16 mg,
44%) and dimesitylmethylgermane (6) (11, 7 mg, 38%),
1
clear, colourless oil. H NMR spectroscopic analysis of the
reaction mixture showed two major products (8 and 9) in a
1:1 or 2:1 ratio. The product mixture was separated by
TLC using 10% CH2Cl2 in hexanes as the eluent to give 2,2-
diethyl-1,1,1-trimesityldigermane (9, 1.1 mg, 3.8%) and
1,2,2-triethyl-1,1-dimesityldigermane (8, 4.8 mg, 18.5%).2
1,2,2-Triethyl-1,1-dimesityldigermane (8):
1
Oily solid. IR (thin film) (cm–1): 1996 (Ge-H). H NMR
(ppm) ꢂ: 6.72 (s, 4H, Mes-H), 4.28 (m, 1H, Ge-H), 2.35 (s,
12H, Mes o-CH3), 2.11 (s, 6H, Mes p-CH3), 1.65 (q, J =
7.8 Hz, 2H, Et-CH2), 0.95–1.15 (m, 13H, 2 Et + Et-CH3).
13C NMR (ppm) ꢂ: 142.80, 137.64, 137.57, 129.26 (CH),
24.89 (CH3), 20.94 (CH3), 14.86 (CH2), 11.98 (CH3), 11.03
(CH3), 5.76 (CH2). MS m/z: 472 ([M]+, 2), 443 ([M – Et]+,
4), 415 ([M – 2Et + H]+, 2), 385 ([M – 3Et]+, 1), 341
([GeMes2Et], 100), 313 ([GeMes2 + H], 20), 221 ([GeEtMes –
H], 6), 193 ([GeMes], 36), 119 ([Mes], 14). HR-MS calcd.
for C24H4372Ge74Ge: 472.1406; found: 472.1390.
1
which were identified by comparison of their H NMR spec-
tra with authentic samples.
Attempted reaction of hexamesitylcyclotrigermane with
methyllithium in the dark
Methyllithium (1.0 mL of a 1.4 M solution in diethyl
ether, 1.4 mmol) was added to a Schlenk tube containing a
clear, pale yellow solution of Ge3Mes6 (50 mg, 0.054 mmol)
dissolved in toluene (4 mL). The Schlenk tube was covered
in aluminum foil and left undisturbed for 14 h at room tem-
perature (rt). No change in the colour of the solution was ob-
served. Saturated aqueous ammonium chloride was then
added to the solution. The aqueous and organic phases were
then separated. The aqueous phase was extracted with four
portions of hexanes and the extracts were combined with the
organic phase. The organic phase was washed three times
with water and then dried over anhydrous magnesium sul-
fate. After filtration to remove the solid, removal of the sol-
vent in vacuo yielded a pale yellow solid. The solid was
identified as hexamesitylcyclotrigermane (41.6 mg, 83%) by
1H NMR spectroscopy.
2,2-Diethyl-1,1,1-trimesityldigermane (9):
mp 176–180°C. IR (thin film) (cm–1): 1982 (Ge-H).
1H NMR (ppm) ꢂ: 6.75 (s, 6H, Mes-H), 4.18 (tt, J = 1.0 Hz,
J = 5.0 Hz, 1H, Ge-H), 2.27 (s, 18H, Mes o-CH3), 2.11 (s, 9H,
Mes p-CH3), 1.21 (pseudo t, 6H, Et-CH3), 0.8–1.1 (m, 4H,
Et-CH2). 13C NMR (ppm) ꢂ: 143.82, 138.86, 137.85, 129.57,
24.92, 20.95, 13.42, 8.14 (CH2). MS m/z: 561 ([M – H]+,
17), 533 ([M – Et]+, 4), 443 ([M – Mes]+, 47), 431
([GeMes3], 100), 311 ([GeMes2 – H], 7), 191 ([GeMes], 12),
119 ([Mes], 10). HR-MS calcd. for C31H4372Ge74Ge ([M – H]):
561.1797; found: 561.1807.
The experiment was repeated (49 mg Ge3Mes6, 1 mL of
EtMgBr in Et2O, toluene (1.5 mmol, 3 mL), 105°C), except
the reaction mixture was heated for 2 h only. Under these
conditions the following products were obtained: Mes3Ge-
GeHEt2 (9), Mes2GeEtH (2), Mes2EtGe-GeEt2H (8), and a
fourth product tentatively identified as Mes2EtGe-GeHEtMes
(3) in a 3.3:2.9:1:1.2 ratio. The products were separated by
TLC using 6% CH2Cl2 in hexanes as the eluent. Compound
3 was obtained as a mixture with Mes3Ge-GeHEt2.
Thermolysis of hexamesitylcyclotrigermane in the
presence of methyllithium
Methyllithium (1.0 mL of a 1.4 M solution in diethyl
ether, 1.4 mmol) was added to a Schlenk tube containing a
clear, pale yellow solution of Ge3Mes6 (50 mg, 0.054 mmol)
dissolved in toluene (4 mL). The Schlenk tube was then
placed in an oil bath at 100°C for 14 h. The orange-brown
solution was allowed to cool to rt. The colour of the solution
changed to pale yellow upon quenching with saturated am-
monium chloride; a gas was evolved. The aqueous and or-
ganic phases were then separated. The aqueous phase was
extracted with four portions of hexanes and the extracts were
combined with the organic phase. The organic phase was
washed three times with water and then dried over anhy-
drous magnesium sulfate. After filtration to remove the solid,
removal of the solvent in vacuo yielded a yellow oil. Analysis
1,2-Diethyl-1,1,2-trimesityldigermane (3):
1H NMR (ppm) ꢂ: 6.74 (s), 6.70 (s, 2H, Mes-H), 5.21 (dd,
J = 3 Hz, J = 6 Hz, 1H, Ge-H), 2.31 (s, 6H, Mes CH3), 2.20
(s, 6H, Mes CH3), 2.18 (s, 6H, Mes CH3), 2.12 (s, 6H, Mes
CH3), 2.11 (s, 3H, Mes CH3), 1–1.8 (m, Et). 13C NMR
(ppm) ꢂ: 144.20, 142.99, 138.50, 137.65, 137.60, 133.33,
129.25 (CH), 129.17 (CH), 128.81 (CH), 25.05 (CH3), 24.91
(CH3), 24.88 (CH3), 20.94 (CH3), 14.69 (CH2), 12.62 (CH3),
11.00 (CH3), 9.87 (CH2). MS m/z: 562 ([M]+).
2 Due to the small quantities of samples, it was impossible to obtain enough material for elemental analysis.
© 2002 NRC Canada

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Can. J. Chem. Vol. 80, 2002
of the oil by 1H NMR spectroscopy revealed a complex mix-
ture of products, which could not be separated or identified.
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Acknowledgements
3. M.S. Samuel, K.M. Baines, and D.W. Hughes. Can. J. Chem.
The financial support of the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) is gratefully
acknowledged.
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© 2002 NRC Canada