Metathesis of carbon dioxide and phenyl isocyanate catalysed by group(IV) metal alkoxides: An experimental and computational study

Article abstract of DOI:10.1007/s12039-011-0069-4

The insertion reactions of zirconium(IV) n-butoxide and titanium(IV) n-butoxide with a heterocumulene like carbodiimide, carbon dioxide or phenyl isocyanate are compared. Both give an intermediate which carries out metathesis at elevated temperatures by inserting a second heterocumulene in a head-to-head fashion. The intermediate metallacycle extrudes a new heterocumulene, different from the two that have inserted leading to metathesis. As the reaction is reversible, catalytic metathesis is feasible. In stoichiometric reactions heterocumulene insertion, metathesis and metathesis cum insertion products are observed. However, catalytic amounts of the metal alkoxide primarily led to metathesis products. It is shown that zirconium alkoxides promote catalytic metathesis (isocyanates, carbon dioxide) more efficiently than the corresponding titanium alkoxide. The difference in the metathetic activity of these alkoxides has been explained by a computational study using model complexes Ti(OMe) ₄ (1bTi) and Zr(OMe)₄ (1bZr). The computation was carried out at the B3LYP/LANL2DZ level of theory. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0069-4

J. Chem. Sci., Vol. 123, No. 1, January 2011, pp. 29–36. * Indian Academy of Sciences.
Metathesis of carbon dioxide and phenyl isocyanate catalysed
by group(IV) metal alkoxides: An experimental
and computational study
AKSHAI KUMAR and ASHOKA G SAMUELSON*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
e-mail: ashoka@ipc.iisc.ernet.in
MS received 8 October 2010; accepted 6 December 2010
Abstract.
The insertion reactions of zirconium(IV) n-butoxide and titanium(IV) n-butoxide with a
heterocumulene like carbodiimide, carbon dioxide or phenyl isocyanate are compared. Both give an
intermediate which carries out metathesis at elevated temperatures by inserting a second heterocumulene in a
head-to-head fashion. The intermediate metallacycle extrudes a new heterocumulene, different from the two
that have inserted leading to metathesis. As the reaction is reversible, catalytic metathesis is feasible. In
stoichiometric reactions heterocumulene insertion, metathesis and metathesis cum insertion products are
observed. However, catalytic amounts of the metal alkoxide primarily led to metathesis products. It is shown
that zirconium alkoxides promote catalytic metathesis (isocyanates, carbon dioxide) more efficiently than the
corresponding titanium alkoxide. The difference in the metathetic activity of these alkoxides has been
explained by a computational study using model complexes Ti(OMe)4 (1bTi) and Zr(OMe)4 (1bZr). The
computation was carried out at the B3LYP/LANL2DZ level of theory.
Keywords. Aryl isocyanates; insertion; metathesis; titanium n-butoxide; zirconium n-butoxide.
1. Introduction
In the case of titanium(IV) alkoxides, it has been
shown that metathesis cum insertion products can be
Reactions converting carbon dioxide to organic com- isolated when stoichiometric amounts of aryl isocya-
pounds not only help in scavenging carbon dioxide nates are reacted with them in refluxing toluene.4e
which is a green house gas, but also provide easy Double insertion of aryl isocyanate in a head-to-head
access to many useful compounds. Metathesis of fashion with subsequent elimination of carbon dioxide
carbon dioxide and carbodiimide is one such conver- was said to be responsible for the observed metathesis.
sion. While metathesis of cumulenes is not known, the The reaction was shown to be reversible; forming
metathesis of heterocumulenes is better explored. unsymmetrical carbodiimides when carried out with a
Carbodiimide metathesis is catalysed by a number of mixture of different aryl isocyanates. Contrary to the
complexes leading to formation of unsymmetrical substituent effects in the head-to-tail double insertion
carbodiimides.1 Group 14 compounds are known to reactions observed at room temperature,6a electron
catalyse metathesis of phenyl isocyanates to give N, N′ donating groups favoured the metathesis reaction. It
diphenyl carbodiimides and carbon dioxide.2 The was also observed that steric factors slowed down the
reverse reaction, metathesis of carbon dioxide with metathesis reaction which is similar to that observed
isocyanides3 and carbodiimide4 leading to formation of with insertion reactions. The metathesis reactions of
isocyanates has also been demonstrated. Recently, heterocumulenes have been explored extensively only
Meyer and co-workers have shown that mid and high with titanium(IV) isopropoxide.
valent uranium amido and imido derivatives promote
No comparison of the metathetic reactivity of a
multiple bond metathesis with carbon dioxide to give common heterocumulene with titanium or with other
isocyanates and uranium oxo complexes.5
zirconium(IV) alkoxides is available. There have
been several examples where Ti and Zr behave
differently towards the same ligand system6b,7 and so
*For correspondence
29

30
Akshai Kumar and Ashoka G Samuelson
there is every possibility that different reactivity stirred and refluxed for 48 h. The solvent was then
would be exhibited by Zr towards metathesis. Hence evaporated to dryness under vacuum. The mixture was
it would be interesting to know whether such worked up by addition of a stoichiometric amount of
reactivity differences can be used to control the water to precipitate out ZrO2, followed by extraction of
metathesis reactivity and how a change in the the organic fraction with dichloromethane and removal
alkoxide from isopropoxide to butoxide affects me- of the solvent from the organic fraction. The yield of
tathesis. In this paper, we report the metathesis diphenyl carbodiimide was calculated by inverse gated
1
reaction of phenyl isocyante (PhNCO) catalysed by decoupled 13C NMR and H NMR using ferrocene as
titanium(IV) n-butoxide and zirconium(IV) n-butox- an internal standard. The yield of diphenyl carbodii-
ide. There are distinct differences in the reactivity of mide was further verified by isolating it as diphenyl
phenyl isocyante with group(IV) metal alkoxides at urea after hydrolysis.
room temperature and at elevated temperatures.
The organic fraction obtained in the catalytic
Computational studies performed on model com- metathesis reaction was run on preparative TLC plates
plexes Ti(OMe)4 and Zr(OMe)4 at the B3LYP/ using hexane, toluene and ethyl acetate in the ratio
LANL2DZ level of theory using the Gaussian 03 7:3:1 as eluent. Compound 2 separated out as diphenyl
package, explain these differences very well.
urea in this method, while compound 4 was obtained in
the pure form and compounds 3 and 6 remained as a
mixture. This mixture was run on preparative TLC
plates using 1% ethyl acetate in petroleum ether as
eluent, where compounds 3 and 6 were obtained in the
pure form.
2. Experimental
2.1 General
All manipulations were carried out under an inert
atmosphere of dry argon using a standard double 2.4 Compound 38b
manifold. Toluene and tetrahydrofuran were freshly
distilled from sodium/benzophenone prior to use. Butyl N-phenylcarbamate: mp = 62–63⋅5°C. 1H NMR
Zirconium(IV) n-butoxide (84wt% in 1-butanol), (400 MHz) δ 7⋅37 (d, 2H, J = 8 Hz), δ 7⋅30 (t, 2H,
titanium(IV) n-butoxide, titanium(IV) isopropoxide J = 8 Hz), δ 7⋅05 (t, 1H, J = 7⋅3 Hz), δ 6⋅63 (s, 1H, NH),
and phenyl isocyanate were procured from Aldrich δ 4⋅17 (t, 2H, J = 6⋅7 Hz), δ 1⋅65 (m, 2H), δ1⋅42 (m,
U.S.A. Zirconium(IV) n-butoxide solution was evap- 2H), δ 0⋅95 (t, 3H, J = 7⋅3 Hz). 13C NMR (100 MHz)
orated to dryness under vacuum at 110°C. Commer- δ153⋅78, δ 138⋅02, δ 129⋅10, δ 123⋅37, δ 118⋅63, δ
cial grade carbon dioxide was obtained from 65⋅18, δ 31⋅02, δ 19⋅14, δ 13⋅30.
Bhuruka Gas India. It was dried by passing through
a column containing alternative layers of fused 2.5 Compound 68c
calcium chloride and P2O5 prior to use. Diphenyl
1
carbodiimide was synthesized by the literature Butyl 2,4-diphenylallophanate: mp = 64⋅5–65⋅5°C. H
procedure.8a
NMR (400 MHz) δ 10⋅91 (s, 1H, NH), δ 7⋅53 (d, 2H,
J = 7⋅6 Hz), δ 7⋅41 (m, 3H), δ 7⋅31 (t, 2H, J = 7⋅6 Hz), δ
7⋅22 (d, 2H, J = 7⋅6 Hz), δ 7⋅09 (t, 1H, J = 7⋅6 Hz), δ
4⋅15 (t, 2H, J = 6⋅8 Hz), δ 1⋅48 (m, 2H), δ 1⋅18 (m, 2
2.2 Physical measurements
1H NMR, 13C{H} NMR, inverse gated decoupled 13C H), δ 0⋅82 (t, 3H, J = 7⋅6 Hz). 13C NMR (100 MHz) δ
NMR were recorded on Bruker AMX 400 operating at 156⋅08, δ 151⋅58, δ 137⋅77, δ 137⋅10, δ 128⋅96, δ
1
400 MHz for H NMR and 100 MHz for 13C NMR, 128⋅70, δ 128⋅26, δ 123⋅96, δ 119⋅92, δ 67⋅22, δ 30⋅31,
with tetramethylsilane as internal reference. All spectra δ 18⋅78, δ 13⋅47. HRMS: Found m/z=335⋅1356 (M +
+
were recorded in CDCl3.
Na) , calcd. mass for C18H20N2O3Na is 335⋅1372.
2.3 General procedure for the metathesis of phenyl
2.6 Compound 2
isocyanate
1
N,N′ diphenyl carbodiimide: H NMR (100 MHz) δ
To a solution of zirconium(IV) n-butoxide (0⋅134 g, 7⋅27 (t, 2H, J = 7⋅4 Hz), δ 7⋅14 (d, 2H, J = 7⋅4 Hz), δ
0⋅35 mmol) dissolved in 15 ml toluene, phenyl isocy- 7⋅11 (t, 1H, J = 7⋅4 Hz).13C NMR (100 MHz) δ 138⋅47,
anate (0⋅38 ml, 3⋅5 mmol) was added. The mixture was δ 135⋅21, δ 129⋅42, δ 125⋅53, δ 124⋅15.

Metathesis of carbon dioxide and phenyl isocyanate
31
2.7 Compound 4
mixture was heated at 110°C for 48 h in a sealed vial
using toluene as solvent. The reaction mixture obtained
Carbamimidic acid, N,N′-diphenyl;n-butyl ester: 1H after hydrolytic work up was found to contain
NMR (400 MHz) δ 7⋅30 (m, 5H), δ 7⋅03 (m, 5H), δ significant amounts of the metathesis product, phenyl
5⋅83 (s, 1H, NH), δ 4⋅35 (t, 2H, J = 6⋅8 Hz), δ 1⋅79 (m, isocyanate, that was isolated as butyl N-phenylcarba-
2H), δ 1⋅48 (m, 2H), δ 0⋅97 (t, 3H, J = 7⋅2 Hz). 13C mate, the insertion product 3 (entry1, table 1). In
NMR (100 MHz) δ 150⋅31, δ 129⋅78, δ 128⋅94, addition, formation of carbamimidic acid, N,N′-
δ123⋅06, δ120⋅39, δ 66⋅92, δ 30⋅84, δ19⋅40, δ13⋅93.
diphenyl;n-butyl ester 4 was observed which arises
from the insertion of diphenyl carbodiimide into the
butoxide (entry1, table 1). Metathesis reaction occurred
to a greater extent as the amount of carbon dioxide
content was increased (entry 2 and 3, table 1).
2.8 General procedure for reactions with carbon
dioxide
Into 6 ml toluene in a sealable vial flushed with
The yield of the phenyl isocyanate inserted product,
nitrogen, carbon dioxide from a gas burette was butyl N-phenylcarbamate, 3 obtained using zirconium
condensed. A mixture of zirconium(IV) n-butoxide (IV) n-butoxide (entry 3, table 1) was more than that
(0⋅239 g, 0⋅6 mmol) and diphenyl carbodiimide obtained in a similar reaction with titanium(IV) n-
(0⋅118 g, 0⋅6 mmol) in 2 ml toluene was added while butoxide (entry 4, table 1). The metathesis reaction
maintaining the vial at liquid nitrogen temperature. proceeds by the coordination of carbon dioxide to the
The vial was subsequently evacuated and sealed. The metal followed by insertion into the C–N bond of the
sealed vial was heated in an oil bath maintained at carbodiimide, which then eliminates an isocyanate
115°C for 48 h. The vial was cooled, broken open and (scheme 1). So it is observed that metathesis of carbon
the reaction mixture evacuated to dryness under dioxide is catalysed more efficiently by zirconium(IV)
vacuum. The mixture was then worked up by addition n-butoxide than by titanium(IV) n-butoxide. Since it
of stoichiometric amount of water to precipitate out has been shown that double insertion of phenyl
ZrO2, followed by extraction of the organic fraction isocyanate takes place in a head-to-tail fashion at room
with dichloromethane. The solvent was removed from temperature and is more favoured with titanium,6b the
the organic fraction and the ratio of 3 and 4 in the present result is counterintuitive. It suggests that head-
1
residue was determined by H NMR.
to-head double insertion is favoured by zirconium
rather than by titanium. These results were further
confirmed by performing a reverse reaction with
phenyl isocyanate.
2.9 Computational details
For theoretical studies, Ti(OMe)4 and Zr(OMe)4 were
taken as model complexes in place of Ti(OnBu)4 and 3.2 Reaction of phenyl isocyanate with titanium(IV)
n
Zr(OnBu)4 where bulky Bu groups are replaced by n-butoxide and zirconium(IV) n-butoxide
Me groups for computational efficiency. All structures
were optimized using the DFT method (B3LYP/ The reaction of phenyl isocyanate (PhNCO) was
LANL2DZ), based on Becke’s three-parameter func- carried out with sub-stoichiometric amounts of
tional.9 The LANL2DZ basis set uses the effective core titanium(IV) n-butoxide (Ti(OnBu)4 1aTi) and the
potentials (ECP) of Hay and Wadt.10 The nature of the products were analysed after work up. The crude
stationary points was characterized by vibrational reaction mixture was found to contain a significant
frequency calculations. The Gaussian 03 program amount of metathesis product N,N′ diphenyl carbo-
package was used for all calculations.11
diimide 2 in addition to insertion products butyl N-
phenylcarbamate 3, butyl N,N′-diphenylallophanate 6
and carbamimidic acid, N,N′-diphenyl;n-butyl ester 4
(entry1, table 2).
3. Results and discussions
3.1 Metathesis with carbon dioxide
The products 3 and 6 arise from the mono and
double insertion of PhNCO into Ti(OnBu)4 and have
In an attempt to test the metathesis reaction of carbon been reported earlier.6 Compound 4 is a metathesis
dioxide, equimolar amounts of diphenyl carbodiimide cum insertion product and is formed by insertion of
2 and zirconium n-butoxide 1aZr were mixed with metathesis product 2 into 1aTi. The ratio of the
1
half a mole equivalent of carbon dioxide. The reaction products formed were determined by H NMR and

32
Akshai Kumar and Ashoka G Samuelson
Table 1. Metathesis products obtained from carbon dioxide and carbodiimide mediated by titanium(IV) n-butoxide
and zirconium(IV) n-butoxide.
Ratio of productsb (%)
Entry
M
Time (h)
na
3
4
.
1
2
3
4
Zr
Zr
Zr
Ti
48
48
48
48
0 5
46
60
89
80
54
40
11
20
.
1 0
.
3 0
.
3 0
a n = Equivalents of carbon dioxide used
b Ratio of products were determined by1H NMR
inverse gated decoupled 13C{1H} NMR. Interestingly, um(IV) n-butoxide can be utilized to convert
there was substantial improvement in the amount of isocyanates to carbodiimides catalytically. Clearly,
metathesis product formed when titanium(IV) n-but- zirconium(IV) n-butoxide is more efficient for the
oxide was used in catalytic amounts (entry2, table 2). conversion.
There was no formation of the metathesis cum
insertion product 4. Further, the amount of diphenyl 3.3 A mechanistic hypothesis
carbodiimide produced was more when Zr was used
instead of Ti (entry 3 and 4, table 2). These experi- The mechanism of the reaction essentially involves the
ments show that zirconium(IV) n-butoxide and titani- insertion of PhNCO into metal alkoxide 1a to generate

Metathesis of carbon dioxide and phenyl isocyanate
33
Scheme 1. The steps involved in the metathesis of carbon dioxide.
a metal carbamate species 7a (scheme 2). The second tion of a molecule of carbon dioxide from the
molecule of PhNCO can either insert in a head-to-tail intermediate 8aHH leads to 9. The intermediate 9 can
fashion leading to complex 8aHT or in a head-to-head then extrude carbodiimide regenerating the metal
fashion giving rise to species such as 8aHH. Elimina- alkoxide 1a.
Table 2. Products formed in the catalytic metathesis of phenyl isocyanate with Ti and Zr alkoxides.
Ratio of productsa (%)
Entry
Catalyst
Mol%
Time ( h)
4
12
-
2
3
6
1
4
1
1
Ti(OnBu)4
Ti(OnBu)4
Zr(OnBu)4
Zr(OnBu)4
1
2
3
4
10
1
48
48
48
48
62
90
88
97
25
6
10
9
2
-
1
2
1
a Ratio of products were determined by H and inverse gated decoupled 13C{1H} NMR

34
Akshai Kumar and Ashoka G Samuelson
Scheme 2. Proposed mechanism for the metathesis of phenyl isocyanate mediated by M(OR)4.
4. Computational studies
4.1 Comparison of energies of intermediates for the
two possible insertion modes
Computational methods have been successfully used to
predict the reactivity patterns in group(IV) metal
alkoxides.6b,7 This encouraged us to seek reasons for
the differences between Ti and Zr in the preference for
double insertion via head-to-head and head-to-tail
fashion.
The structures numbered 1 and 7 to 9 in scheme 1
were investigated. For this purpose Ti(OnBu)4 (1aTi)
and Zr(OnBu)4 (1aZr) were modelled by Ti(OMe)4
n
(1bTi) and Zr(OMe)4 (1bZr) where bulky Bu groups
are replaced by Me groups to reduce computational
Figure 1. Computed energy profiles for the reaction of
PhNCO with M(OR)4 [M=Ti, Zr and R=nBu or Me] for
load. The letters a and b are used to indicate metal
complexes with OnBu ligand and OMe ligand respec- Path X and Path Y at the B3LYP/LANL2DZ level of theory.

Metathesis of carbon dioxide and phenyl isocyanate
35
tively. Ti and Zr are added to indicate the metal B3LYP/LANL2DZ level attribute the observed differ-
present in the complex. HH and HT indicate the mode ence in reactivity between Ti and Zr to the energetics
of insertion. For example, 8aTiHH indicates structure of the reaction. These calculations have also been
8 with M=Ti, OnBu as the alkoxy group and a structure helpful in explaining the reversal of preferences for
where PhNCO has undergone double insertion in a both Zr and Ti towards double insertion at room
head-to-head fashion.
temperature and at elevated temperature.
The reaction of the first molecule of PhNCO with
1bTi/1bZr will generate the metal carbamate interme-
diate 7bTi/7bZr.6b The second molecule of PhNCO
can insert either in a head-to-head fashion to give the
intermediate 8bTiHH/8bZrHH (Path X) or in a head-
to-tail fashion giving 8bTiHT/8bZrHT (Path Y). The
double insertion proceeds only though Path Y at room
temperature. At higher temperatures head-to-head
insertion was found to occur in greater amounts
through Path X. The preference of Zr and Ti towards
Path X and Path Y can be understood if we look into
the thermodynamic stabilities of the intermediates
involved (figure 1).
For both Ti and Zr, path Y is an exothermic process
whereas path X is endothermic. This explains why head-
to-head insertion is not observed at room temperature.
For path Y it is seen that though 8bZrHT is more
stabilized, the formation of 8bTiHT is more exothermic
than 8bZrHT. Hence the double insertion in a head-to-
tail mode is more favoured for Ti. Path X is endothermic
for both Ti and Zr, however the formation of 8bHHZr
is less endothermic than that of 8bHHTi. This explains
the higher efficiency of zirconium alkoxides over
titanium alkoxides towards head-to-head insertion re-
quired for metathesis of phenyl isocyanates.
Supporting information
Total energies in a. u. at B3LYP/LANL2DZ level of theory,
energy and cartesian co-ordinates of atoms in each structure
are given in the website (http://www.ias.ac.in/chemsci).
Acknowledgements
AGS thanks the Department of Science and Technol-
ogy (DST) for the award of a research grant and AK
gratefully acknowledges a senior research fellowship
from Council of Scientific and Industrial Research
(CSIR) and a PDF from Indian Institute of Science
(IISc). Authors also thank Prof. ED Jemmis, Susmita
De and Shrabani Dinda for their valuable suggestions
with Gaussian calculations. We thank DST, New Delhi,
for providing us funds through its FIST program for
purchase of a 400MHz NMR spectrometer.
References
1. (a) Weiss K and Kindl P 1984 Angew. Chem. 96 616;
(b) Weiss K 1984 Stud. Surf. Sci.Catal. 19 397; (c)
Meisel I, Hertel G and Weiss K 1986 J. Mol. Catal. 36
159; (d) Weiss K and Hoffmann K 1987 Z. Naturforsch.
42b 769; (e) Holland A W and Bergman R G 2002 J.
Am. Chem. Soc. 124 9010
2. (a) Birdwhistell K R, Boucher T, Ensminger M, Harris
S, Johnson M and Toporek S 1993 Organometallics 12
1023; (b) Babcock J R and Sita L R 1998 J. Am. Chem.
Soc. 120 5585; (c) Babcock J R, Incarvito C, Rheingold
A L, Fettinger J C and Sita L R 1999 Organometallics
18 5729; (d) Babcock J R, Liable-Sands L, Rheingold
A L and Sita L R (1999) Organometallics 18 4437
3. Delaet D L, Fanwick P E and Kubiak C P 1987 J.
Chem. Soc. Chem. Commun. 1412
One can assume that the reverse reaction of carbon
dioxide with carbodiimide inserted intermediate 9b
follows the same path to give the intermediate 8bHH.
In this case, the process 8bHH→9b is energetically
more costly for Ti compared to Zr. Hence the
metathesis of carbon dioxide is also better with Zr
than with Ti. The computational result suggests that
thermodynamic stabilities of intermediates are respon-
sible for the experimental outcome.
5. Conclusions
4. (a) Kim W Y, Chang J S, Park S E, Ferrence G and
Kubiak C P 1998 Chem. Lett. 1063; (b) Kim W Y,
Chang J S, Park S E and Kubiak C P 1999 Res. Chem.
Intermed. 25 459; (c) Sita L R, Babcock J R and Xi R
1996 J. Am. Chem. Soc. 118 10912; (d) Xi R and Sita L
R 1998 Inorg. Chim. Acta 270 118; (e) Ghosh R and
Samuelson A G 2005 Chem. Commun. 115 2017
5. Bart S C, Anthon C, Heinemann F W, Bill E, Edelstein
N M and Meyer K 2008 J. Am. Chem. Soc. 130 12536
The metathesis reaction of heterocumulenes has been
carried out with zirconium(IV) n-butoxide and titanium
(IV) n-butoxide and the former was found to be the
most efficient catalyst. This is due to the fact that at
elevated temperatures, double insertion which takes
place in a head-to-head fashion is more favoured for
zirconium(IV) alkoxide compared to titanium(IV)
alkoxide. Computational studies carried out at the

36
Akshai Kumar and Ashoka G Samuelson
6. (a) Ghosh R, Nethaji M and Samuelson A G 2005 J. 11. Frisch M J, Trucks G W, Schlegel H B, Scuseria G E,
Organomet. Chem. 690 1282; (b) Kumar A, De S,
Samuelson A G and Jemmis E D 2008 Organometallics
27 955
Robb M A, Cheeseman J R, Montgomery Jr J A,
Vreven T, Kudin K N, Burant J C, Millam J M, Iyengar
S S, Tomasi J, Barone V, Mennucci B, Cossi M,
Scalmani G, Rega N, Petersson G A, Nakatsuji H, Hada
M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida
M, Nakajima T, Honda Y, Kitao O, Nakai H, Klene M,
Li X, Knox J E, Hratchian H P, Cross J B, Bakken V,
Adamo C, Jaramillo J, Gomperts R, Stratmann R E,
Yazyev O, Austin A J, Cammi R, Pomelli C, Ochterski
J W, Ayala P Y, Morokuma K, Voth G A, Salvador P,
Dannenberg J J, Zakrzewski V G, Dapprich S, Daniels
A D, Strain M C, Farkas O, Malick D K, Rabuck A D,
Raghavachari K, Foresman J B, Ortiz J V, Cui Q,
Baboul A G, Clifford S, Cioslowski J, Stefanov B B,
Liu G, Liashenko A, Piskorz P, Komaromi I, Martin R
L, Fox D J, Keith T, Al Laham M A, Peng C Y,
Nanayakkara A, Challacombe M, Gill P M W, Johnson B,
Chen W, Wong M W, Gonzalez C and Pople J A 2004
Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford
CT
7. (a) Pavan Kumar P N Vand Jemmis E D 1988 J. Am. Chem.
Soc. 110 126; (b) Jemmis E D and Giju K T 1997 Angew.
Chem. Int. Ed. Engl. 36 606; (c) Jemmis E D and Giju K T
1998 J. Am. Chem. Soc. 120 6952; (d) Jemmis E D,
Phukan A K and Giju K T 2002 Organometallics 21 2254
8. (a) Appel R, Kleinstueck R and Ziehn K D 1971 Chem.
Ber. 104 1335; (b) McGhee W, Riley D, Christ K, Pan
Y and Parnas B 1995 J. Org. Chem. 60 2820; (c) Wong
S -W and Frisch K C 1986 J. Polym. Sci. Part A: Polym.
Chem. 24 2867
9. (a) Becke A D 1993 J. Chem. Phys. 98 5648; (b) Becke
A D 1988 Phys. Rev. A 38 3098; (c) Lee C, Yang W and
Parr R G 1988 Phys. Rev. B 37 785
10. (a) Hay P J and Wadt W R 1985 J. Chem. Phys. 82 270;
(b) Wadt W R and Hay P J 1985 J. Chem. Phys. 82
284; (c) Hay P J and Wadt W R 1985 J. Chem. Phys. 82
299