Catalytic thermal and photo-induced CH and CC activation reactions of alkanes with ansa amido functionalized half-sandwich complexes and methylalumoxane

Article abstract of DOI:10.1016/j.jorganchem.2009.06.018

In the literature most of the dehydrogenation reactions of alkanes are described as CH activation reactions of cyclooctane. The best results of CH activation reactions have been found for the reaction of MAO activated metallocene complexes and cyclooctane

Full text of DOI:10.1016/j.jorganchem.2009.06.018

Journal of Organometallic Chemistry 694 (2009) 3270–3275
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Catalytic thermal and photo-induced CH and CC activation reactions of alkanes
with ansa amido functionalized half-sandwich complexes and methylalumoxane q
*
H.G. Alt , Christine E. Denner
Laboratorium für Anorganische Chemie, Universität Bayreuth, Universiätsstr. 30, D-95440 Bayreuth, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In the literature most of the dehydrogenation reactions of alkanes are described as CH activation reac-
tions of cyclooctane. The best results of CH activation reactions have been found for the reaction of
MAO activated metallocene complexes and cyclooctane at temperatures over 300 °C.
The application of ansa amido functionalized half sandwich compounds of the type Ind0Si(Me)2Nt-
BuMCl2 (Ind0= monosubstituted indenyl); M = Ti, Zr, Hf) for CH and CC activation reactions is completely
unknown in the literature.
In contrast to the dehydrogenation reactions of cyclooctane and the metallocene complexes of the
group 4 metals, where the zirconocene complexes give higher TONs than the titanocene complexes
the ansa amido functionalized titanium complexes give more than two times higher TONs than the cor-
responding Zr or Hf complexes. The ansa amido functionalized ligand increases the TONs for the Ti com-
plexes and decreases the TONs of the Zr complexes.
Received 21 April 2009
Accepted 9 June 2009
Available online 21 June 2009
Keywords:
Ansa half-sandwich complexes
Group IV metal
CH activation reactions
Cyclooctane
Thermally MAO activated
In contrast to the metallocene complexes, the ansa amido functionalized dichloride complexes of Ti
show also a higher activity than the corresponding Zr complexes. It is known that the photolysis of orga-
nometallic titanium, zirconium and hafnium (IV) compounds can give M(III) radicals. The formation of
the active Ti metal centre is easier than in the case of the corresponding Zr and Hf metal compounds.
Ó 2009 Published by Elsevier B.V.
1. Introduction
tuted indenyl; M = Ti, Zr, Hf) for CH and CC activation reactions is
completely unknown in the literature.
The cheap and abounding but nearly chemical inert alkanes of-
fer a huge potential for the synthesis of sophisticated products. The
possibility to convert aliphatic CH bonds into other functional
groups offers a wide application of alkanes and the resulting
alkenes.
Ansa amido functionalized half-sandwich complexes of tita-
nium are commercially used as homogeneous catalysts for ethyl-
ene polymerization reactions at temperatures over 130 °C in high
boiling solvents [17] and give polymers with interesting proper-
ties like high molecular weights and broad or bimodal molecular
weight distributions [18]. At temperatures over 200 °C decompo-
Presently, alkenes are produced by unselective thermal cracking
processes [1].
sition of these complexes occurs within
a very short time.
Another possibility to activate inert alkanes can be accom-
plished by catalytic dehydrogenation reactions.
‘‘Decomposition reactions” of metallocene complexes in the pres-
ence of MAO at temperatures over 250 °C indicate the formation
of a new product [19] that is able to activate linear and cyclic
alkanes.
Therefore it is worthwhile to test half-sandwich complexes for
catalytic CH activation reactions under various conditions.
Late transition metal complexes of iridium [2–8] and palladium
[9] gave good results as catalysts for CH activation reactions of al-
kanes. In the literature most of the dehydrogenation reactions of
alkanes are described as CH activation reactions of cyclooctane
[10–14]. The best results of CH activation reactions have been
found for the reaction of MAO activated metallocene complexes
and cyclooctane at temperatures over 300 °C [15,16].
The application of ansa amido functionalized half-sandwich
compounds of the type Ind0Si(Me)2NtBuMCl2 (Ind0 = monosubsti-
2. Results and discussion
2.1. Preparation of the complexes 1–28
The ansa amido functionalized indenyl complexes of titanium,
zirconium and hafnium dichloride were synthesized by the reac-
tion of the corresponding ligand precursor system TiCl3ꢀTHF, ZrCl4
or HfCl4.
q
Financial Support from ConocoPhillips, Bartlesville, USA, is gratefully acknowl-
edged.
* Corresponding author. Tel.: +49 (0)921 55 2555; fax: +49 (0)921 55 2044.
E-mail address: helmut.alt@uni-bayreuth.de (H.G. Alt).
0022-328X/$ - see front matter Ó 2009 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2009.06.018

H.G. Alt, C.E. Denner / Journal of Organometallic Chemistry 694 (2009) 3270–3275
3271
The substitution of the indenyl group is performed by a lithia-
tion reaction of indene [20] followed by the reaction with the cor-
responding alkyl, alkenyl or phenyl bromide [21].
The addition of the silyl group to the functionalized indenyl li-
gand is conducted with n-butyl lithium and the aquimolar amount
of the corresponding chloro silyl compound. The reaction of the
n
R´
n
R
Cl
Me
R
Me
Me
M
M
Si
Cl
Si
Me
R
N
N
25 M = Zr, n = 0, R = H
26 M = Zr, n = 2, R = Ph
27 M = Hf, n = 0, R = H
28 M = Hf, n = 2, R = Ph
1 - 24
Complex
n
0
2
3
4
2
3
4
4
0
2
3
4
2
3
4
4
0
2
3
4
2
3
4
4
M
Ti
R
R`
H
1
2
Me
Me
Me
Me
Me
Me
Me
Ph
Ti
Ph
3
Ti
Ph
4
Ti
Ph
5
Ti
CH=CH2
CH=CH2
CH=CH2
Me
6
Ti
7
Ti
8
Ti
9
Zr
Zr
Zr
Zr
Zr
Zr
Zr
Zr
Hf
Hf
Hf
Hf
Hf
Hf
Hf
Hf
Me
Me
Me
Me
Me
Me
Me
Ph
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Ph
Ph
Ph
CH=CH2
CH=CH2
CH=CH2
Me
Me
Me
Me
Me
Me
Me
Me
Ph
H
Ph
Ph
Ph
CH=CH2
CH=CH2
CH=CH2
Me
Fig. 1. Applied ansa amido functionalized complexes 1–28.

3272
H.G. Alt, C.E. Denner / Journal of Organometallic Chemistry 694 (2009) 3270–3275
indenyl chloro silyl compound with tBuNH2 in methylene chloride
gives the corresponding amino substituted indene derivative [22].
The synthesis of the corresponding zirconium and hafnium
complexes proceeds in the conventional manner [23].
Ansa amido functionalized titanium complexes were synthe-
sized by the reaction of TiCl3ꢀ3THF and the corresponding com-
pound, while PbCl2 was added as a mild oxidation agent [24,25]
(Fig. 1).
Table 1
TON numbers of the photo - induced reactions of cyclooctane with the MAO activated
complexes 1, 2, 9, 10, 17, 18, 25 and 27 at room temperature.
TON
6
5
4
3
2
1
0
The four tested dimethyl complexes 25–28 of Zr and Hf were
synthesized by the reaction of the corresponding dichloride com-
plex and 2 equiv. of MeLi at 0 °C.
2.2. Dehydrogenation reactions of cyclooctane
1
2
9
10
17
18
25
27
The ansa amido functionalized catalyst precursors 1–28 were
investigated for their catalytic CH activation potential. MAO was
used as an activator.
complexes
= Ti complexes
= Zr complexes
= Hf complexes
Cyclooctane is an ideal compound for CH activation reactions
due to its low dehyrogenation energy [26]. Dehydrogenation
experiments are mostly described with this alkane in the literature
[27,28].
The catalytic activation reaction of cyclooctane with the com-
plexes 1–28/MAO gave the isomerization product ethylcyclohex-
ane, the olefin cyclooctene, hydrogen and the CC coupling
product bi(3.3.0.)cyclooctane and hydrogen (Scheme 1).
With TONs of 4 and 6 the titanium complexes 1 and 2 showed
the highest conversion rates for cyclooctane, adequate to the ansa
amido functionalized dimethyl zirconium complex 25 (TON 4).
No difference of the TONs was found for the hafnium dimethyl
(27) and dichloride complexes (17).
The phenyl substituted indenyl complexes gave in all cases low-
er TONs than the unsubstituted derivatives. The dimethyl com-
plexes of zirconium (25) gave higher TONs than the
corresponding dichloride complexes.
A positive effect on the formation of cyclooctene was also ob-
served for the unsubstituted complexes 1, 9 and 25.
2.2.1. Photochemical CH activation reactions of cyclooctane with the
complexes 1, 2, 9, 10, 17, 18, 25 and 27
The MAO activated dichloride complexes 1, 2, 9, 10, 17 and 18
and the dimethyl complexes 25 and 27 (M:Al ratio 1:100) were dis-
solved in cyclooctane and the solution was irradiated for 16 h with
UV light (k > 300 nm) at room temperature. For these reactions it is
not necessary to use hydrogen acceptors like tert-butylethylene in
order to shift the equilibrium to the right side.
2.2.2. Thermal CH activation reactions of cyclooctane with complexes
1–28
The MAO activated half-sandwich complexes (M:Al ratio 1:100)
were heated to 300 °C. At this temperature they formed a new ac-
tive species. Mass spectroscopy and elemental analysis suggest an
analogous product that has been identified from the reaction of
Cp2ZrCl2 and MAO: An MAO cage with an incorporated CpZrMe
fragment [16,19]. The hydrolysis of this species at room tempera-
ture produces butenylindene and methane. These results confirm
the results of the reaction of MAO and the metallocene complexes
of Zr at 300 °C [16,19], where the organic ligand is still connected
to the metal at this high temperatures and plays an important role
for the activity of the CH activation reactions.
The results are given in Table 1.
In all CH activation reactions, the formation of the main product
cyclooctene could be detected by GC/MS. The isomerisation prod-
uct ethylcyclohexane was found with a selectivity of about 20%
in all conducted reactions.
n
R´
Reactions of cyclooctane and the catalyst precursors at temper-
atures lower than 200 °C gave no CH activation products.
Cl
Cl
R
R
M
Si
N
2.2.3. Comparison of the dichloride and dimethyl complexes of
titanium, zirconium and hafnium (1, 9, 17, 24 and 25–28) in catalytic
thermal CH activation reactions
To investigate the effect of the two methyl groups instead of the
chloride ligands, the activated complexes 1, 9, 17, 24, 25–28 were
reacted with cyclooctane in a handautoclave at various tempera-
tures from 200 °C to 320 °C. The CH activation process is an endo-
thermic reaction and higher temperatures should give higher
conversion rates but also lower selectivities of the products (Table
2).
300°C
+
There was no reaction at temperatures below 250 °C. The high-
est TONs were found at the highest temperature 320 °C. The zirco-
nium dichloride complexes 9 and 10 show higher TONs than the
corresponding Hf complexes 17 and 18. The dimethyl complexes
24–28 gave in all cases higher TON than the corresponding dichlo-
ride complexes. The highest TON was found for the dimethyl zirco-
nium complex 25 with a TON of 27. Lower temperatures gave
lower TONs. The increase of the temperature from 280 °C to
+ H2
+ H2
Scheme 1. Reaction pathway of the activated complexes (M:Al, 1:100) 1–24 and
cyclooctane at 300 °C.

H.G. Alt, C.E. Denner / Journal of Organometallic Chemistry 694 (2009) 3270–3275
3273
Table 2
Table 4
TONs of the reaction of the complexes 9, 10, 17, 18 and 25–28 and cyclooctane at
various temperatures.
TONs of the reaction of cyclooctane and the complexes 2–4, 10–12 and 18–20 at
300 °C.
30
45
40
35
30
25
20
15
10
5
280°C
25
300°C
320°C
20
15
10
5
0
2
3
4
10
11
12
= Zr
18
19
20
= Hf
0
9
10
17
18
25
26
27
28
= Ti
Table 3
Selectivities of the reaction products of the zirconium complex 9 and cyclooctane at
various temperatures (reaction time: 5 h).
Table 5
TONs of the reaction of cyclooctane and the activated complexes 4, 7, 12, 15, 20 and
23.
90
cyclooctene
ethylcyclohexane
octahydropentalene
Complex
TON
80
70
60
50
40
30
20
10
0
4
7
12
15
20
23
45
22
23
8
11
4
%
of the chain. The number of the CH2 spacer groups had an effect
on the TONs but a trend was not obvious (Table 4).
The Ti complexes 2– 4 show the highest TON, followed by the Zr
complexes 10–12. The lowest TONs were found for the Hf com-
plexes 18–20.
280°C
300°C
320°C
0
. . .
2.2.5. Influence of the indenyl substituent R (Ph, CH CH2) of the
•
320 °C tripled the TON for the Hf complex 28. The dimethyl com-
plexes of zirconium (25 and 26) and hafnium (27 and 28) showed
no significantly higher TONs than the corresponding dichloride
complexes 9, 10 and 17, 18 at various temperatures.
The highest selectivities were found at lower temperatures.
When cyclooctane was activated with 9/MAO at 280 °C, the selec-
tivity for cyclooctene was 82%, at 300 °C 73% and at 320 °C 55%
when cyclooctane was activated with 9/MAO. The other applied
complexes gave a similar product distribution.
For the reaction of the metallocene complexes of Ti, Zr and Hf
and cyclooctane, no cyclooctene was found at temperatures over
280 °C [16,19]. In contrast, the ansa amido functionalized com-
plexes9, 10, 17, 18, 25–28 gave also cyclooctene at higher temper-
atures. The higher the temperature and the longer the reaction
time the higher was the amount of octahydropentalene and ethyl-
cyclohexane. At 320 °C, 10% selectivity of octahydropentalene and
35% of the isomerization product ethylcyclohexane were found
(Table 3).
complexes 4, 7, 12, 15, 20 and 23 (n = 4) on the catalyst activity
The influence of the substituents on the indenyl ligand precur-
sor with a constant spacer length of the CH2 group (n = 4) was
investigated. The end groups varied between a phenyl and an
alkenyl group.
x-
The positive effect of the phenyl group is shown in Table 5. In
. . .
contrast to the small
x-alkenyl group –CH2 CH2, the bulky Ph
•
group is able to increase the TON. The influence of the substituent
is so remarkable that the zirconium (12) and hafnium (20) com-
plexes show nearly three times higher TON than the corresponding
x
-alkenyl complexes (Table 5). This behaviour suggests that the
active species still contains the bulky substituent.
The influence of the ligand is so drastic that TONs – more than
10 times higher – could be found for the same metals under the
same conditions.
3. Summary and conclusion
2.2.4. Effect of the CH2 chain length of the indenyl phenyl substituent
complexes 2–4, 10–12 and 18–20
In contrast to the dehydrogenation reactions of cyclooctane and
the metallocene complexes of the group 4 metals, where the zirco-
nocene complexes give higher TONs than the titanocene com-
plexes [16,19] the ansa amido functionalized titanium complexes
give more than two times higher TONs than the corresponding Zr
or Hf complexes. The ansa amido functionalized ligand increases
the TONs for the Ti complexes and decreases the TONs of the Zr
complexes.
The effect of different chain lengths at the indenyl ligand
showed no significant influence on the TONs and the selectivities
of the products. The highest TON (45) showed complex 4 with Ti
as the centre metal and four CH2 spacers. The lowest TON (7)
was found for the corresponding Hf complex 18 with two CH2
spacer groups. No trend was found for the TONs and the length

3274
H.G. Alt, C.E. Denner / Journal of Organometallic Chemistry 694 (2009) 3270–3275
Table 6
R
The TONs of the photo-induced reaction of complexes 1, 2, 9, 10, 24, 25 and 27 and
β-H-elim.
cyclooctane.
M
H
H
- olefin
+ RH
M
M
TON
H
20
photo-induced
15
10
5
ν
h
*
or
Δ T
- H2
M
= complex fragment
Scheme 3. Catalytic reaction pathway of an active metal centre and an olefin.
tive addition of the alkane to the metal center. The requirement
for this oxidative addition is a low electron density at the metal.
Acceptors, which are able to remove the CH bonding electrons
and to initiate an electrophile addition reaction are AlCl3 [35],
H2SO4, HF and super acids [36–38].
Metal surfaces, radicals, carbene and low valent metal com-
plexes are able to act as an acceptor as well as a donor. The com-
bination of the acceptor and donor properties can be very useful
[27]. The decomposition products of the group 4 metal complexes
1–33 seem to be subcompounds, this means metal rich com-
pounds. In the GC of the gaseous organic decomposition products
methane, CO, NO and C2H4 could be found. We assume that these
products are formed because of a low oxidation level of the metal
centre and a low valence electron concentration. The structure of
the active centre depends on the kind of the ligand. Often these
compounds show unexpected physical and chemical behaviour
(Scheme 3).
0
1
2
9
10
17
18
25
27
In contrast to the metallocene complexes, the ansa amido func-
tionalized dichloride complexes of Ti show also a higher activity
than the corresponding Zr complexes. It is known that the photol-
ysis of organometallic titanium, zirconium and hafnium (IV) com-
pounds can give M(III) radicals [29–31]. The formation of the active
Ti metal centre is easier than in the case of the corresponding Zr
and Hf metal compounds. The reactions of the Zr and Hf complexes
9, 10, 17, 18, 25 and 27 and cyclooctane proceed with lower TONs
than those of the corresponding Ti complexes 1 and 2. Therefore,
the photolysis of the MAO activated ansa amido complexes seems
to be initiated by a radical reaction. The elimination of methane
and hydrogen was observed, that may indicate the formation of a
low valent complex in the first stage of the CH activation reaction
similar to the photolysis reaction of Cp2TiMe2 [31,32]. In the pres-
ence of cyclooctane, the complexes 1, 2, 9, 10, 17, 18, 25 and 27
could transfer a radical from the metal M (III) to the cyclooctane
to give cyclooctene and hydrogen (Scheme 2). Low TONs (see Table
6) of the photo-induced reactions of complexes1, 9, 17, 24, 25 and
27 and cyclooctane confirm this assumption.
The b-hydrogen elimination step can be induced thermally or
photochemically to produce the olefin and hydrogen in a catalytic
cycle. Thermally activated compounds are able to form the CH acti-
vation product cyclooctene and furthermore the CC coupling com-
pound octahydropentalene.
Further reaction products, like octahydropentalene – found at
the thermolysis reactions – could not be observed. These results
support the mechanism of a radically induced catalytic reaction
(Scheme 2).
For the thermally activated complexes, the hydrogen substi-
tuted dimethyl Zr complex 25 (TON 25) showed the highest TON
of the zirconium complexes. The highest TON at all (45) was ob-
served for the titanium containing complexes 4 and 8.
Intramolecular non catalytic CH activation reactions at 120 °C
are known of several substituted cyclopentadienyl complexes of
Zr [33,34].
In our case it seems that the formation of the decomposition
products depends on the structure of the ligand and the metal. It
could be possible that metals of group 4 and the Al from the
MAO form the active species at these high temperatures. The key
step for CH activation reactions is the formation of an electroni-
cally and coordinatively unsaturated species to enable the oxida-
4. Experimental
The air and moisture sensitive reactions were carried out under
an atmosphere of purified argon in a hand autoclave.
An amount of 10–50 mg of the corresponding half-sandwich
complex was activated with MAO (30% in toluene, ratio M:Al
1:100). The activated complex was heated to 300 °C for 5 h. The
yellow residue was filtered, washed with THF and pentane and
dried in vacuo.
Methylalumoxane was purchased from Albemarle (Baton
Rouge, USA/Louvain, La Neuve, Belgium). All other starting materi-
als were commercially available and were used without further
purification. THF, n-pentane and methylene dichloride were re-
fluxed over the appropriate drying agents and distilled under ar-
gon. Cyclooctane was degassed and stored under argon. The
organic starting materials were purchased from Aldrich or Acros
and used without purification.
The mass spectroscopy was performed with a Varian MAT CH7
instrument of the University of Bayreuth. The GC/MS spectra were
obtained from an instrument of the THERMO company.
- 1/2H2
- 1/2H2
.
.
.
M(III)
M(III)
4.1. Mass spectroscopy
Mass spectra were routinely recorded at the Zentrale Analytik
of the University of Bayreuth with a VARIAN MAT CH-7 instrument
(direct inlet, EI, E = 70 eV) and a VARIAN MAT 8500 spectrometer.
Post-processing and data analyses were performed using the soft-
ware ‘‘Maspec II32 Data system”.
radical transition state
M = Ti, Zr, Hf
Scheme 2. CH activation reaction of cyclooctane and M(III) radicals formed from
the ansa amido functionalized metal complexes.

H.G. Alt, C.E. Denner / Journal of Organometallic Chemistry 694 (2009) 3270–3275
3275
4.2. GS/MS
Plateau phase
Total running
time
1 min at 180 °C
19
GC/MS spectra were recorded with a Focus DSQ of the Thermo
company. Helium (4.6) was applied as carrier gas. Technical data
and temperatures programs were used as followed:
Column flow
Split ratio
200 ml/min
200:1
4.4. Preparation of the dialkyl ansa amido complexes 1 – 25
Type of the column
TR-5MS (5% phenyl(equiv.)
polysilphenylene–siloxane)
30 m; (0.25 lm)
start phase: 2 min at 50 °C
10 °C/min
The preparation of the corresponding dichloride complexes was
performed according to the literature [11,12].
Length of the column
Temperature program
Heating phase
4.5. Preparation of the dialkyl ansa amido complexes 25–33
Plateau phase
Total running time
15 min at 290 °C
41 min
In 30 ml of toluene 1.5 mmol of the ansa amido complexes 1–33
was dissolved. Corresponding Grignard bromide (3 mmol) was
added and stirred overnight. After filtration of the solution over so-
dium sulfate, the solvent was evaporated. The residue was washed
with 70 ml of pentane and the solvent evaporated. The yields were
45–55%.
4.3. Gas chromatography
For the analysis of the organic compounds and the reaction gas
of the CH activation reactions the gas chromatograph 6890 of the
agilent company was used. Argon (5.0) was used as carrier gas.
Technical data and temperature programs were used as
followed:
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037772 B 3.
Investigation of the gas phase
Detector
Heat conductivity detector
30.9 ml/min
10:1
HP-Plot Q (Ethan, Ethen,
a.s.o.)
30 m (1.5 lm)
Column flow
Split ratio
Type of the
column
Length of the
column
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[31] H.G. Alt, Angew. Chem. 96 (1984) 752.
HP-MOLSIV (for N2, O2, CO,
H2, CH4 a.s.o.)
Length of the column: 15 m
(25 lm)
Start phase: 6 min at 35 °C
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Temperature
program
[38] A. Leon, J. Rothe, M. Fichtner, J. Phys. Chem. C 11 (2007) 16664.
Heating phase
12 °C/min