Titanocene(iii) complexes with 2-phosphinoaryloxide ligands for the catalytic dehydrogenation of dimethylamine borane

Article abstract of DOI:10.1039/c5dt00275c

A study of the dehydrogenation of dimethylamine borane using different titanocene(iii) complexes with 2-phosphinoaryloxide ligands is presented. Complexes Cp₂Ti(κ²-O, P-O-C₆H₄-PR₂) (3a: R = i-Pr, 3b: R = Ph) (Cp = η⁵-cyclopentadienyl) and Cp?₂Ti(κ¹-O-O-C₆H₄-PR₂) (5a: R = i-Pr, 5b: R = Ph) (Cp? = η⁵-pentamethylcyclopentadienyl) were prepared by reactions of the 2-phosphinophenol ligand with different titanocene sources and fully characterised. Their catalytic activity depends on the steric influence of the cyclopentadienyl ligand, the coordination mode of the 2-phosphinoaryloxide ligand and on the used solvent. Complex 3a showed a turnover number of 43.2 in the neat substrate after 24 hours. EPR investigations were used to elucidate the fate of the Ti(iii) catalyst.

Full text of DOI:10.1039/c5dt00275c

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ARTICLE
Titanocene(III) complexes with 2-phosphinoaryloxide
ligands for the catalytic dehydrogenation of
dimethylamine borane†‡
Cite this: DOI: 10.1039/x0xx00000x
Marcus Klahn,a Dirk Hollmann,a Anke Spannenberg,a Angelika Brücknera and
Torsten Beweriesa*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
A study of the dehydrogenation of dimethylamine borane using different titanocene(III)
complexes with 2ꢀphosphinoaryloxide ligands is presented. Complexes Cp2Ti(
PR2) (3a: R = ꢀPr, 3b: R = Ph) (Cp =
5ꢀcyclopentadienyl) and Cp*2Ti( 1ꢀ
5a: R = ꢀPr, 5b: R = Ph) (Cp* =
5ꢀpentamethylcyclopentadienyl) were prepared by reactions
κ O,PꢀOꢀC6H4ꢀ
2ꢀ
κ OꢀOꢀC6H4ꢀPR2)
www.rsc.org/
i
η
(
i
η
of the 2ꢀphosphinophenol ligand with different titanocene sources and fully characterised.
Their catalytic activity depends on the steric influence of the cyclopentadienyl ligand, the
coordination mode of the 2ꢀphosphinoaryloxide ligand and on the used solvent. Complex 3a
showed a turn over number in neat substrate of 43.2 after 24 hours. EPR investigations were
used to elucidate the fate of the Ti(III) catalyst.
applications.2g Also, recent studies by Liu showed that the
formation of dehydrogenated products which retain their fluid
Introduction
During the last years, amine borane adducts attracted great
interest in the organometallic community since such
compounds are widely discussed as candidates for the storage
of hydrogen. Main benefits include the high gravimetric
hydrogen capacity (e.g. 19.6 % for ammonia borane, H3N·BH3,
AB) and the facile release of hydrogen under mild conditions.1
Several contributions describe the use of transition metal
complexes as catalysts for the dehydrogenation of such
compounds2 as well as the regeneration of spent fuels.3
Noteworthy, only a few examples are known for the use of nonꢀ
precious 3d metal complexes for the generation of hydrogen
from amine borane compounds, e.g. Ni,4 Fe5 and Ti.6,7 Some
major drawbacks still prevent a general applicability like the
production of the starting materials amines and boranes by
environmentally benign processes.8 Moreover, the circumstance
that most amine borane adducts are solid at operating
temperatures and thus cannot be used as a classical liquid “fuel”
without dilution and resulting loss of hydrogen storage capacity
is regarded as a disadvantage. An attempt to solve the latter
problem was made by Baker and coꢀworkers, who used amine
borane blends, which remained liquid throughout the whole
catalytic dehydrogenation process.2g Furthermore, it was
shown, that the addition of an ionic liquid impeded the
formation of insoluble dehydrogenation products such as linear
poly(aminoborane), which makes this approach an attractive
nature in the bulk is possible using 3ꢀmethylꢀ1,2ꢀBNꢀ
cyclopentane as hydrogen carrier.9
In our group, we have used different group 4 metallocene
sources for the dehydrogenation of dimethylamine borane
(Me2NH·BH3)7a,b and hydrazine borane7c and found significant
differences in catalytic activity between titanocene and
zirconocene complexes. Also, the groups of Manners and
Chirik highlighted the use of other titanocene and zirconocene
compounds for the catalytic dehydrogenation of dimethylamine
borane.6 As suggested by DFT calculations,10 it was found
experimentally that Ti(III) complexes play an important role in
the catalytic cycle.6c As an extension of the dehydrogenation
chemistry with early transition metal species, Wass et al. used
cationic zirconocene 2ꢀphosphinoaryloxide complexes,11 which
can be described as a mimic of wellꢀknown main group
frustrated Lewis pairs (FLPs12). These gave very high turnover
frequencies of up to 600 hꢀ1 for the hydrogen release from
dimethylamine borane. Later, this concept was extended by the
same authors using structurally similar titanocene 2ꢀ
phosphinoaryloxide complexes.13 In all cases, the 2ꢀ
phosphinoaryloxide ligand offers the possibility of a hemilabile
ligand which can additionally coordinate to the metal centre to
stabilise an active species such as a metallocene cation. The
hitherto known method to prepare titanocene(III) 2ꢀ
phosphinoaryloxides consists of multiple reaction steps, some
concept for
a technical realisation, e.g. in automotive
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VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5DT00275C
of them involving expensive reducing agents or precious power: ~7 mW, receiver gain: 1*103, modulation frequency:
intermediate species. Detailed information on the 100 kHz, modulation amplitude: 0.4 G, Sweep time: 61.44 s.
dehydrogenation mechanism of dimethylamine borane using factors have been calculated from the resonance field B0 and the
these Ti(III) compounds was not given. Together with the resonance frequency using the resonance condition
paramagnetic Ti(III) centre and the nuclei boron and nitrogen = g B0. The calibration of the values was performed using
of the substrate, the phosphorus atom of the 2ꢀ DPPH (2,2ꢀdiphenylꢀ1ꢀpicrylhydrazyl) ( = 2.0036±0.00004).
g
ν
h
ν
β
g
g
phosphinoaryloxide ligand provides an excellent spectroscopic Analysis of the Spin Hamiltonian parameters was performed
means for monitoring the progress and identifying elementary using the simulation program EPRSim32 of Sojka and coꢀ
steps of dimethylamine borane dehydrogenation using EPR workers.17
techniques. Thus, we aimed to use this class of ligands for the
Synthetic procedures
straightꢀforward
preparation
of
new
Ti(III)
2ꢀ
phosphinoaryloxide complexes of the type Cp'2Ti(OC6H4PR2) Preparation of complex 3a: Cp2Ti(η2ꢀMe3SiC2SiMe3) (
1
)
)
(Cp' = unsubstituted or substituted
Pr or Ph).
η
5ꢀcyclopentadienyl, R =
iꢀ
(0.804 g, 2.31 mmol) and 2ꢀ(diisopropylphosphino)phenol (2a
(0.500 g, 2.38 mmol) were dissolved in 30 mL of THF and
stirred for 24 hours, during that time the colour of the mixture
changed from brown to green. All volatiles were removed in
General information
vacuum. Extraction with nꢀhexane and concentration yielded
All operations were carried out under an oxygenꢀ and moistureꢀ
free argon atmosphere with standard Schlenk and glove box
techniques. Prior to use all solvents were freshly distilled from
sodium/benzophenone and stored under argon. Dimethylamine
the product 3a (0.737 g, 1.9 mmol, 82 %) as fine green crystals,
which were suitable for Xꢀray diffraction analysis. Mp. 139 °C
(decomp.) under Ar. Anal. Calcd. for C22H28OPTi: C, 68.23; H,
7.29; Found: C, 68.33; H, 7.34. NMR (298 K, 121 MHz,
borane
(6) and 2ꢀ(diphenylphosphino)phenol (2b) were
benzeneꢀd6) δ
31P ꢀ9.0 ppm. MS (70 ev, m/z): 387 [M]+, 211
purchased from Sigma Aldrich and purified by sublimation and
recrystallisation from toluene, respectively. Cp2Ti(η2ꢀ
[L+2H]+, 178 [MꢀL]+.
Preparation of complex 3b: Cp2Ti(η2ꢀMe3SiC2SiMe3) (
1
)
)
Me3SiC2SiMe3)
(
1
),
Cp*2TiMe
(
4
)
and
2ꢀ
(0.310 g, 0.889 mmol) and 2ꢀ(diphenylphosphino)phenol (2b
(diisopropylphosphino)phenol (2a) were synthesised according
to published procedures.14
The following instruments were used: NMR: Bruker AV 300.
1H, 13C and 31P chemical shifts are given in ppm (
δ) and were
(0.250 g, 0.898 mmol) were dissolved in 25 mL of THF and
stirred for 12 hours, during that time the colour of the mixture
changed from brown to green. All volatiles were removed in
vacuum. Extraction with nꢀhexane and concentration yielded
referenced using (residual) solvent resonances relative to SiMe4
[1H, 13C] or relative to an external standard [31P: 85 % H3PO4].
– Gas chromatography was performed using an Agilent
Technologies 7890A, Column: 60/80 Carboxen 1000 (Supelco),
Detection: TCD. – MS: Finnigan MAT 95ꢀXP (Thermoꢀ
Electron). – Elemental analysis: Leco TruSpec Micro CHNS. –
Melting points: METTLERꢀTOLEDO MP 70. Melting points
are uncorrected and were measured in sealed capillaries. – Xꢀ
ray analysis: Bruker Kappa APEX II Duo diffractometer. The
structures were solved by direct methods (SHELXSꢀ97) and
refined by least squares full matrix methods (SHELXLꢀ97).15
CCDC 1042510ꢀ1042513 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
the product (0.163 g, 0.358 mmol, 40 %) as fine green crystals
of complex 3b, which were suitable for Xꢀray diffraction
analysis. Mp. 138 °C (decomp.) under Ar. Anal. Calcd. for
C28H24OPTi: C, 73.86; H, 5.31; Found: C, 73.75; H, 5.44. NMR
(298 K, 121 MHz, benzeneꢀd6): 31P
m/z): 455 [M]+, 178 [MꢀL]+.
δ
ꢀ15.3 ppm. MS (70 ev,
Preparation of complex 5a:
A
solution of 2ꢀ
(diisopropylphosphino)phenol (2a) (0.300 g, 1.43 mmol) in 10
mL of ꢀhexane was added via cannula to Cp*2TiMe ( ) (0.461
g, 1.38 mmol), which was previously dissolved in 10 mL of
n
4
n
ꢀ
hexane. The mixture was allowed to stir over night at room
temperature; the formed methane was released by pressure
compensation. The reaction mixture was concentrated to 10 mL
in vacuum, upon standing at ꢀ78 °C the paramagnetic product
5a (0.622 g, 1.18 mmol, 86 %) was obtained as brown crystals
suitable for Xꢀray diffraction analysis. Mp. 142 °C (decomp.)
under Ar. Anal. Calcd. for C32H48OPTi: C, 72.85; H, 9.17;
Found: C, 72.87; H, 9.06. NMR (298 K, 121 MHz, benzeneꢀ
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
–
Volumetric analyses were carried out in a doubleꢀwalled
thermostatically controlled reaction vessel using an
automatically operating burette (MesSen Nord GmbH,
Stäbelow, Germany).16 – EPR spectroscopy: In situ EPR
spectra in Xꢀband were recorded by a Bruker EMX CWꢀmicro
spectrometer equipped with an ER 4131VT Digital
Temperature Control System and an ER 4119HSꢀWI highꢀ
sensitivity optical resonator. A solution of the Ti(III) complex
in n
ꢀhexane (c = 1.0 mg mLꢀ1, 0.050 mL) was filled into a J.
Young EPR tube in a glove box and introduced into the EPR
spectrometer. The following parameters were used:
temperature: 27 °C, microwave frequency: 9.4 GHz, microwave
d6): 31P
δ
ꢀ7.0 ppm. MS (70 ev, m/z): 527 [M]+, 391 [MꢀCp*]+,
211 [L+2H]+.
Preparation of complex 5b:
A
solution of 2ꢀ
(diphenylphosphino)phenol (2b) (0.324 g, 1.17 mmol) in 10
mL of ꢀhexane was added via cannula to Cp*2TiMe ( ) (0.377
g, 1.13 mmol), which was previously dissolved in 10 mL of
n
4
n
ꢀ
hexane. The mixture was allowed to stir over night at room
temperature; the formed methane was released by pressure
compensation. The reaction mixture was concentrated to 10 mL
2 | J. Name., 2012, 00, 1-3
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in vacuum, upon standing at ꢀ78 °C the paramagnetic product
5b (0.384 g, 0.645 mmol, 55 %) was obtained as fine green
crystals suitable for Xꢀray diffraction analysis. Mp. 192 °C
(decomp.) under Ar. Anal. Calcd. for C38H46OPTi: C, 76.63; H,
7.45; Found: C, 76.87; H, 7.32. NMR (298 K, 121 MHz,
R2
SiMe3
SiMe3
R2P
HO
P
III
(a) Cp2Ti
+
Cp2Ti
O
- 0.5 H2
- Me3SiC2SiMe3
R = i-Pr (3a), Ph (3b)
1
2a b
/
benzeneꢀd6): 31 ꢀ 18.8 ppm. MS (70 ev, m/z): 595 [M]+.
P δ
R2P
R2P
General procedures for the dehydrogenation of
dimethylamine borane
III
(b) Cp*2Ti Me
+
Cp*2Ti
O
- CH4
HO
In toluene: A solution of the catalyst (0.08 mmol, 2 mol%) in 2
mL of toluene was stirred at 25 °C. To this, a solution of
dimethylamine borane (6) (0.242 g, 4.1 mmol) in 2 mL of
4
2a/b
5a 5b
R = i-Pr ( ), Ph ( )
Scheme 1. Synthesis of titanocene(III) 2-phosphinoaryloxide complexes.
toluene was added. The reaction vessel was closed, the flow of
Argon was stopped and the excess pressure was released. The
measurement was started and hydrogen evolution curves were
recorded. Gas samples were taken at the end of the experiment
and analysed by gas chromatography using a TC detector.
Solvent-free: A sample of the catalyst (0.08 mmol, 2 mol%)
was placed in a Teflon crucible and carefully introduced into
Notably, the nature of the reaction products is strongly
dependent on the steric demand of the cyclopentadienyl ligand;
an influence of the different substituents at the phosphorus
atoms of the ligand was not observed. Additional coordination
of P to Ti is present for both complexes with unsubstituted
cyclopentadienyl groups 3a and 3b, whereas for fully
methylated complexes 5a and 5b no additional Ti−P
interactions were observed in the solid state. The molecular
structure of complex 3a is depicted in Figure 1.
the reaction vessel, containing solid dimethylamine borane (6)
(0.242 g, 4.1 mmol). The vessel was closed and heated at 50 °C
to obtain liquid phase. Once all substrate was liquefied, the
magnetic stirrer was started to dissolve the catalyst.
Immediately afterwards, the flow of Argon was stopped and the
excess pressure was released. The measurement was started and
hydrogen evolution curves were recorded. Gas samples were
taken at the end of the experiment and analysed by gas
chromatography using a TC detector.
Results and Discussion
Synthesis of titanocene(III) 2-phosphinoaryloxide complexes
Unsubstituted titanocene(III) aryloxide species Cp2Ti(
OꢀC6H4ꢀPR2) (3a ) were prepared starting from the titanocene
source Cp2Ti( ) by reaction with the free
2ꢀMe3SiC2SiMe3) (
2ꢀphosphinophenol (2a ). Liberation of the alkyne and a
formal redox reaction that results in the formation of hydrogen
takes place. Compounds 3a (R = ꢀPr) and 3b (R = Ph) were
κ O,Pꢀ
2ꢀ
/
b
η
1
Figure 1. Molecular structure of compound 3a. The thermal ellipsoids correspond
to 30% probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°]: Ti1−O1 2.0488(17), Ti−P1 2.7127(8); Ti1−O1−C1
127.51(15).
/
b
i
obtained in moderate to good yields as green needles. Both
complexes are paramagnetic due to the presence of a Ti(III)
centre with one covalently bound O atom as well as the
phosphine moiety, which forms a dative interaction with the
metal fragment. In general, this method is also feasible for the
synthesis of fully methylated complexes starting from
The trivalent titanium centre is coordinated by two
cyclopentadienyl
ligands
and
the
2ꢀ
(diisopropylphosphino)aryloxide in
a
distorted tetrahedral
coordination geometry due to the additional interaction with the
phosphorus atom (Ti−P 2.7127(8) Å). This contact results in an
elongated Ti−O distance for 3a (2.0488(17), cf. 1.9114(10) Å
for 5a) and a significantly decreased Ti−O−Cipso angle (3a
127.51(15), cf. 151.65(10)° for 5a).
The molecular structure of 5a (Figure 2) reveals a distorted
trigonalꢀplanar coordination geometry around the titanium atom
which is surrounded by two Cp* groups and the 2ꢀ
(diisopropylphosphino)aryloxide.
Cp*2Ti(η2ꢀMe3SiC2SiMe3)
(Cp* = η
5ꢀ
pentamethylcyclopentadienyl), however, elevated temperatures
are required to get acceptable yields within reasonable reaction
times. Thus, we prepared the permethylated complexes
Cp*2Ti(
κ
1ꢀ
O
ꢀOꢀC6H4ꢀPR2) (5a: R =
iꢀPr, 5b: R = Ph) from
Cp*2TiMe (
4
) and the free ligand, resulting in methane
elimination by reaction of the methyl group with the acidic
proton of the phenol group (Scheme 1).
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Figure 2. Molecular structure of compound 5a. The thermal ellipsoids correspond
to 30% probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°]: Ti1−O1 1.9114(10); Ti1−O1−C1 151.65(10).
Figure 3. EPR spectra of 3a (g=1.9808; ATi=10.0G, AP=13.6G,
(g=1.9827; ATi=10.0G, AP=17.8G, B=2.4G), 5a (g=1.9724; ATi=7.0G,
B=3.2G). Black: experimental spectra, Red:
ꢀ
B=2.4G), 3b
ꢀ
ꢀ
B=2.9G),
and 5b (g=1.9715; ATi=7.0G,
ꢀ
simulated spectra.17
Similar differences in the bonding patterns can also be found
for the phenyl substituted analogues due to distorted tetrahedral
Catalytic dehydrogenation of dimethylamine borane
The catalytic dehydrogenation of with group 4 metallocene
(
3b) and trigonalꢀplanar (5b) coordination modes (Figure S1
6
and S2).
complexes has been studied before by several research
groups.6,7,11 In general, different BN dehydrocoupling products
A comparison of the intramolecular Ti−P coordination reveals
that complex 3a shows a weaker interaction than 3b. However,
in both complexes, these Ti−P interactions are more
pronounced than in the neutral Ti(III) compound Cp2TiꢀOC6H4ꢀ
were observed in these studies: monomeric aminoborane (
cyclodiborazane ), linear diborazane and
bis(dimethylamino)borane (10), whereby in most cases the
7),
(
8
(9)
PtꢀBu2 (Ti–P 2.907(2) Å) and in the cationic species
[Cp2Ti(IV)ꢀOC6H4ꢀPt
ꢀBu2]+ (Ti–P 2.785(2) Å) described by
cyclic dimer
8
was found to be the major product (Scheme 2).
Wass et al.13 A general comparison with other titanocene
aryloxides is compiled in Table 1.
H
B
Me2N
NMe2
Table 1. Comparison of selected bond lengths and angles of different
titanocene aryloxide complexes.
10
Me2N BH2
H2B NMe2
Ti−O/ Å
2.0488(17)
2.0497(10)
1.9114(10)
1.9200(9)
2.028(6)
1.901(5)
1.9283(17)
1.895(2)
TiꢁꢁꢁP/ Å
2.7127(8)
2.6171(6)
—
Ti−O−Cipso/ °
127.51(15)
128.25(10)
151.65(10)
147.59(9)
124.3(5)
122.5(4)
180
144.2(2)
2 Me2NH BH3
2 Me2N BH2
Cp2Ti(OC6H4PiꢀPr2) (3a)
Cp2Ti(OC6H4PPh2) (3b)
Cp*2Ti(OC6H4PiꢀPr2) (5a)
Cp*2Ti(OC6H4PPh2) (5b)
Cp2Ti(OC6H4PtꢀBu2)13a
[Cp2Ti(IV)OC6H4PtꢀBu2]+ 13b
Cp*2Ti(OC6F5)18a
6
7
8
—
2.907(2)
2.785(2)
—
Me2N BH2 NMe2 BH2
9
Cp2Ti(Oꢀ2,6ꢀMe2C6H3)18b
CpTi(Cl)2(Oꢀ6ꢀtꢀ
—
Scheme 2. Possible dehydrocoupling products of 6.
1.860(5)
2.624(3)
134.9(4)
BuC6H3PPh2)18c
We tested the performance of the new Ti(III) 2ꢀ
phosphinoaryloxides for the catalytic dehydrogenation of
EPR spectra of compounds 3a and 3b recorded in nꢀhexane are
6
depicted in Figure 3 (Table 3). The signals at 300 K show
typical hyperfine structure (hfs) splitting which results from the
coupling of the single electron of TiIII to the nuclear spin of the
isotopes 47Ti (I = 5/2, 7.44 % natural abundance) and 49Ti (I =
7/2, 5.41 % natural abundance). This is characteristic for
isolated Cp*2TiIIIOR complexes.19 The small coupling constant
to phosphorus (31P) of 13.6 G (3a) and 17.8 G (3b) indicates a
weak but distinct coordination of the P atom to the Ti
centre.6c,20 Furthermore, the smaller hfs for 3a points to an
elongated Ti−P distance. Compared to these Cp complexes, the
corresponding Cp* complexes 5a and 5b show no 31P
superhyperfine structure (shfs) which supports information
obtained from Xꢀray analysis.
using either toluene as solvent at a temperature of 25 °C or
using neat substrate as solvent at 50 °C (Figure S3ꢀS11). A
similar approach was published before by Baker et al., who
used a mixture of secꢀbutylamine borane and ammonia borane
for the metal catalysed dehydrogenation at 60 °C in order to
obtain liquid dehydrogenation products.2f The results of our
investigations are compiled in Figure 4. In all cases, hydrogen
was found to be the only gaseous product, as confirmed by GC
analysis (see Figure S12). The highest activity is observed for
3a without solvent with a superior TON of 43.2 after 24 hours.
Most remarkably, the activity drops significantly from 3a to 3b
and again from 3b to almost catalytically inactive compounds
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5a and 5b, respectively. This could be explained by an The TOFs for the reactions in toluene (entry 1ꢀ4) are very
increasing steric hindrance caused by the phenyl groups at the similar, thus indicating the presence of similar phosphine
phosphorus atom and the fully methylated cyclopentadienyl environments in aromatic solvents. This assumption finds
ligands in 5a/b. A similar relationship between steric demand support in the very similar 31P NMR values of the catalyst
and catalytic dehydrogenation activity was observed before.6,7
complexes obtained in benzeneꢀd6
comparable to most titanium(III)
Cp2Ti(PPh2BH3)6c and Cp2Ti(OC6H4P ꢀBu2).13b However,
.
Also, activities are
catalysts like
t
Cp2Ti(NMe2BH3)6c showed a better activity as it represents an
intermediate of the catalytic dehydrogenation reaction. Only
with formally Ti(II) and Ti(IV) complexes, especially cationic
compounds, higher TOFs could be obtained under comparable
reaction conditions. Using
6 as both substrate and solvent led to
remarkably better performance, in particular for 3a, which has a
TOF in the range of the best solventꢀbased systems shown in
Table 2.
The good activity of 3a in neat
its longꢀterm stability by adding new portions of
4.1 mmol and 2nd addition 3.6 mmol) into the reaction vessel.
As shown in Figure 5, the initial hydrogen production activity
is restored and further substrate is fully consumed.
6
encouraged us to investigate
(1st addition:
3
Figure 4. Compilation of the catalytic activities after two and 24 hours (2 mol%
cat., T = 25 °C for toluene as solvent, T = 50 °C for neat substrate).
In order to put our results into the context of systems known
from literature, we compared our findings with the given
turnover frequencies (TOFs, mostly after 2 h) of similar
titanocene compounds, which have been used for the
dehydrogenation of
6 (Table 2).
Table 2. Comparison of several titanocene complexes for the catalytic
dehydrogenation of 6.
Reaction
conditions
TOF (hꢀ1) after
2h
toluene, 25 °C,
2 mol % cat.
toluene, 25 °C,
2 mol % cat.
toluene, 25 °C,
2 mol % cat.
toluene, 25 °C,
2 mol % cat.
neat, 50 °C,
2 mol % cat.
neat, 50 °C,
2 mol % cat.
neat, 50 °C,
2 mol % cat.
neat, 50 °C,
2 mol % cat.
C6D6, 23 °C,
10 mol% cat.
PhCl, 23 °C,
2 mol% cat.
toluene, 20 °C,
2 mol% cat.
toluene, 20 °C,
2 mol% cat.
toluene, 24 °C,
2 mol% cat.
toluene, 20 °C,
2 mol% cat.
Cp2Ti(OC6H4PiꢀPr2) (3a)
Cp2Ti(OC6H4PPh2) (3b)
Cp*2Ti(OC6H4PiꢀPr2) (5a)
Cp*2Ti(OC6H4PPh2) (5b)
Cp2Ti(OC6H4PiꢀPr2) (3a)
Cp2Ti(OC6H4PPh2) (3b)
Cp*2Ti(OC6H4PiꢀPr2) (5a)
Cp*2Ti(OC6H4PPh2) (5b)
Cp2Ti(OC6H4PtꢀBu2)13a
[Cp2Ti(IV)OC6H4PtꢀBu2]+ 13a
Cp2Ti(PPh2BH3)6c
0.39
0.16
0.16
0.10
10.56
0.90
0.03
0.02
0.8
Figure 5. Investigation of long-term stability for 3a by adding new portions of
(2 mol% cat., T = 50 °C, neat substrate).
6
In order to elucidate the dependence of hydrogen formation on
the catalytic loading, we tested our most active catalyst
complex 3a with concentrations of 1 and 5 mol% in addition to
the standard condition of 2 mol% (Figure 6). As expected, a
higher loading gave a faster dehydrogenation reaction. The
most remarkable difference could be found at the beginning of
the reactions with the curves for a 1 mol% and 2 mol% catalyst
loading showing an induction period before reaching the
highest hydrogen evolution rate.
12
0.13
10.7
5.6
Cp2Ti(NMe2BH3) 6c
Cp2Ti (η2ꢀMe3SiC2SiMe3) 7a
Cp2TiCl2/2 nBuLi6b
11.1
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DOI: 10.1039/C5DT00275C
boron atom of a RnBH3ꢀn moiety (n = 0 or 1, denoted as 11
,
Scheme 3) which influences catalytic activity of the Ti(III)
complexes in forming very stable intermediates. Isolation and
full characterisation of this species was however not possible.
The use of the neat substrate at 50 °C seems to destabilise this
state either by temperature induced cleavage and/or a better
availability of further substrate to displace the produced
dehydrogenated adduct.
H
H
O
R
R2
P
B
+RnBH3-n
n = 0 or 1
III
III
Cp2Ti
Cp*2Ti
PR2
O
Figure 6. Comparison for different catalyst loading of 3a. (T = 50 °C, neat
substrate).
3a
11
Scheme 3. Assumed reaction of 3a with the RnBH3-n moiety.
Mechanistic investigations
The activity for the dehydrogenation performed in toluene is
very low for all compounds. Each run shows relatively constant
gas evolution in the beginning which suddenly ends in a zeroꢀ
slope curve. From this finding, it is difficult to ascertain a
proper explanation, however, one can speculate that an
intermediate species is formed which impedes the further
Finally we were interested in elucidating the role of
paramagnetic Ti(III) species in the mechanism of catalytic
dehydrogenation of
6. Thus EPR spectroscopy was applied
which is a unique, highly sensitive and selective method for the
detection of paramagnetic species. Analysing the shfs, which
could results from the coupling of the single Ti(III) electron to
reaction with additional
6 to liberate hydrogen. In order to
the nuclear spins of 31P (
I
=1/2, abundance: 100%), 14N
=1/2), or
=3/2,), may allow
investigate whether hydrogen activation products play a role
here, C6D6 solutions of 3a and 5a, respectively, were
1
(abundance: 99.6%;
I
=1), H (abundance: 99.9%;
I
boron (abundance 10B: 20% 11B: 80%; both
I
pressurised with 4 bar of hydrogen in the presence and in the
the identification of the coordination sphere around the Ti(III)
centre.
1
absence of substrate
6
. Reactions were monitored with H and
31P NMR spectroscopy despite the fact that the presence of
paramagnetic Ti(III) compounds usually leads to undesired line
broadening. Activation of molecular hydrogen using similar
complexes was reported before,13a however, in our case no
conversion of the starting material into hydrogen activation
products was observed. The lack of reactivity with hydrogen
rules out the formation of potentially deactivating hydrogen
In situ EPR experiments were performed using complex 3a
.
First, a J. Young 3 mm tube was charged with (0.1 mmol) and
6
a solution of the catalyst 3a in THF22 (0.1 mL of a 26 mmol Lꢀ1
solution) in the glove box at room temperature. After being
implemented in the EPR cavity, the sample was heated and
EPR spectra were recorded continuously. Notably, we found
that the EPR signal of the catalyst 3a (g=1.9808; ATi=10.0G,
activation products during the catalytic dehydrocoupling of
In a second approach, we conducted NMR studies with 3a and
20 equivalents of in C6D6 at 80 °C (see Figure S13 and S14,).
The H NMR spectra show the generation of hydrogen and the
formation of . Unequivocal evidence for the release of the
6.
AP=13.6G, ꢀB=3.2G) was stable at temperatures below 50 °C.
Conversion into a species that does not show Ti−P coupling is
observed at 60 °C (Figure 7, g=1.9813; ATi=not resolved,
6
1
ꢀ
B=11.2G). Also, the overall integral of the EPR resonance
8
was constant throughout the experiment, thus indicating, that no
aryloxide ligand was not found as no characteristic resonance
for OH protons was observed. The 11B NMR spectra indicate
conversion of 3a into an EPR silent species takes place. This is
1
corroborated by H NMR spectra (Figure S19), which were
the formation of the dehydrocoupled dimer
8 and three
collected from the same sample and do not show defined
resonances that can be assigned to a Ti(IV) complex.
Unfortunately, the structure of this final complex is so far
unidentified.
significantly smaller signals due to and 10 as well as an
7
unidentified boron containing species at ꢀ43.9 ppm. This
chemical shift is in the range of B–P adducts (cf. ꢀ34.7 ppm for
21
BH3ꢁPPh3 and ꢀ45.1 ppm for BH3ꢁP(OMe)321b). Also, the 31P
NMR spectra (Figure S15) show the formation of a new signal
at 41.9 which is in reasonable agreement with the chemical
shift of BH3ꢁPPh3 at 20.8 ppm.21c It should be noted that after
reaction of complex 3a with BH3ꢁTHF in THFꢀd8, similar
resonances were observed (Figures S16ꢀS18). We thus propose
that an adduct is formed with an interaction between the
phosphorus atom of the coordinated aryloxide ligand and the
6 | J. Name., 2012, 00, 1-3
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ARTICLE
DOI: 10.1039/C5DT00275C
Figure 7. EPR spectra for dehydrogenation of 6 by 3a in THF-d8: (a) after 1 h at 60
°C, (b) after 3 h at 60 °C, (c) after 24 h at 60 °C).
Figure 9. In situ EPR experiments with 3a and
6 in THF.
The corresponding 11B NMR spectra of the same sample
(Figure 8) indicate that is fully consumed after 24 hours. In
addition to the main cyclic product and 10 could be
observed. Other than in tolueneꢀd8 (Figure S20), no linear
dimer was detected.
6
However, due to a smaller linewidth two new Ti(III)
intermediates were identified. First, a Ti(III) complex, denoted
as 3a-II (Figure 10, Table 3), with a g value of 1.9742 and a
shfs to 31P of 9.8 G was detected.
8, 7
9
Figure 10. Experimental (black) and simulated (red) EPR spectra involving Ti(III)
intermediates in dehydrogenation of 6 using 3a.
Figure 8. 11B NMR spectra (96 MHz) for dehydrogenation of
after 1 h at 60 °C, (b) after 3 h at 60 °C, (c) after 24 h at 60 °C).
6 by 3a in THF-d8: (a)
The high concentration of Ti(III) and thus the resulting strong
spinꢀspin coupling resulted in a large linewidth in the EPR
spectra making the determination of coupling between the Ti
centre and P and H atoms difficult. However, this would help to
further elucidate the structure of the formed Ti(III)
intermediates. To overcome this problem we repeated the EPR
Table 3. EPR Spin Hamiltonian parameter.
g value
1.9808
1.9827
1.9724
1.9715
1.9742
1.9786
ATi /G
10.0
10.0
7.0
AP /G
13.6
17.8
∆B /G
2.4
2.3
2.9
3.2
Cp2Ti(OC6H4PiꢀPr2) (3a)
Cp2Ti(OC6H4PPh2) (3b)
Cp*2Ti(OC6H4PiꢀPr2) (5a)
Cp*2Ti(OC6H4PPh2) (5b)
3a-II
ꢀ
ꢀ
7.0
experiment using lower concentrations of
6 (0.07 mmol) and 3a
12.1
9.8
ꢀ
2.5
2.5
(0.5 mL of a 0.86 mmol Lꢀ1 solution) (Figure 9). After heating
the mixture at 323 K, EPR spectra were collected. We found
that – other than for higher concentrated samples – a gradual
decrease of the signal corresponding to 3a was observed
indicating a continuous conversion into an EPR silent species
(Figure 9).
3a-III
n.d.a
a n.d. ꢀ not determined
The smaller g value points to a stronger spinꢀorbit coupling
induced by a stronger Ti−O bond. The smaller shfs to P may
suggest an elongated Ti−P bond (see also EPR discussion of 3a
and 5a). This could be explained by a further coordination of
the substrate 6. As no nitrogen hfs (theoretically splitting into
three lines of equal intensity),23 as well as no hydrogen hfs
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5DT00275C
(theoretically splitting into three lines of equal intensity) were
detected, coordination of might take place via the electronꢀ
poor boron atom, which is attracted by the electronꢀrich Ti(III).
Note that no shfs to boron is observed which can be explained
by a very small coupling constant of boron.24
‡ This work is dedicated to Dr. sc. Vladimir V. Burlakov on the occasion
of his 60th birthday.
6
Once the coordination of
6 is more pronounced, the Ti−P
1
(a) T. B. Marder, Angew. Chem. Int. Ed. 2007, 46, 8116; (b) F. H.
Stephens, V. Pons and R. T. Baker, Dalton Trans., 2007, 2613; (c) C.
W. Hamilton, R. T. Baker, A. Staubitz and I. Manners, Chem. Soc.
Rev., 2009, 38, 279; (d) A. Staubitz, A. P. M. Robertson, M. E. Sloan
and I. Manners, Chem. Rev., 2010, 110, 4023; (e) A. Staubitz, A. P.
M. Robertson and I. Manners, Chem. Rev., 2010, 110, 4079.
interaction is fully split to give an isolated Ti(III) centre. This
species is detected in the final stage of the experiment, denoted
as 3a-III. However, the reason for the decline (conversion to
Ti(IV) in the diluted sample remains unclear.
Regarding the mechanism of the dehydrogenation of
catalysed by the titanocene(III) 2ꢀphosphinoaryloxide complex,
we believe that in a first step, coordination of without
involvement of hydrogen atoms (no shfs to hydrogen) takes
place. Decrease of the Ti−P shfs (Figure 10) could suggest the
presence of a species with a weakened Ti−P contact, which
upon release of H2 could give an aminoborane complex.
6
2
Selected recent examples: (a) M. Käß, A. Friedrich, M. Drees and S.
Schneider, Angew. Chem. Int. Ed., 2009, 48, 905; (b) T. W. Graham,
C.ꢀW. Tsang, X. Chen, R. Guo, W. Jia, S.ꢀM. Lu, C. SuiꢀSeng, C. B.
Ewart, A. Lough, D. Amoroso and K. AbdurꢀRashid, Angew. Chem.
Int. Ed., 2010, 49, 8708; (c) A. Staubitz, M. E. Sloan, A. P. M.
Robertson, A. Friedrich, S. Schneider, P. J. Gates, J. Schmedt auf der
Günne and I. Manners, J. Am. Chem. Soc., 2010, 132, 13332; (d) C. J.
Stevens, R. Dallanegra, A. B. Chaplin, A. S. Weller, S. A.
Macgregor, B. Ward, D. Mckay, G. Alcaraz and S. SaboꢀEtienne,
Chem. Eur. J., 2011, 17, 3011; (e) J. R. Vance, A. P. M. Robertson,
K. Lee and I. Manners, Chem. Eur. J., 2011, 17, 4099; (f) S. S. Mal,
F. H. Stephens and R. T. Baker, Chem. Commun., 2011, 2922; (g) W.
R. H. Wright, E. R. Berkeley, L. R. Alden, R. T. Baker and L. G.
Sneddon, Chem. Commun., 2011, 3177; (h) A. E. W. Ledger, C. E.
Ellul, M. F. Mahon, J. M. J. Williams and M. K. Whittlesey, Chem.
Eur. J., 2011, 17, 8704; (i) B. L. Conley, D. Guess and T. J.
Williams, J. Am. Chem. Soc., 2011, 133, 14212.
6
Subsequent elimination of the aminoborane
7
(which dimerises
to give
8) could regenerate a catalytically active Ti(III)
fragment.
Conclusions
In summary, we have demonstrated a convenient method to
synthesise titanocene (III) 2ꢀphosphinoaryloxide complexes
which contain iꢀpropyl and phenyl moieties, respectively, at the
phosphorus atom. The complexes were fully characterised also
by means of Xꢀray diffraction analysis. These compounds were
tested for the catalytic dehydrogenation of
comparably low performance in toluene. In contrast, the
reaction in neat substrate showed good activity for 3a, which is
in the same range as the best known systems. EPR
investigations using complex 3a showed that Ti(III) species are
present throughout the dehydrogenation process. Additionally,
the weakening of the Ti−P interaction could be derived from
the change in the Ti−P coupling, thus indicating that this is an
3
4
(a) B. L. Davis, D. A. Dixon, E. B. Garner, J. C. Gordon, M. H.
6, showing
Matus, B. Scott and F. H. Stephens, Angew. Chem. Int. Ed., 2009, 48
,
6812; (b) A. D. Sutton, A. K. Burrell, D. A. Dixon, E. B. Garner III,
J. C. Gordon, T. Nakagawa, K. C. Ott, J. P. Robinson and M. Vasiliu,
Science, 2011, 331, 1426.
(a) R. J. Keaton, J. M. Blacquiere and R. T. Baker, J. Am. Chem.
Soc., 2007, 129, 1844; (b) M. Vogt, B. de Bruin, H. Berke, M.
Trincado and H. Grützmacher, Chem. Sci., 2011, 2, 723.
5
6
J. R. Vance, A. Schäfer, A. P. M. Robinson, K. Lee, J. Turner, G. R.
Whitell and I. Manners, J. Am. Chem. Soc., 2014, 136, 3048.
essential step in the dehydrogenation of
titanocene phosphinoaryloxide complexes.
6 using these
(a) D. Pun, E. Lobkovsky and P. J. Chirik, Chem Commun., 2007,
3297; (b) M. E. Sloan, A. Staubitz, T. J. Clark, C. A. Russell, G. C.
LloydꢀJones and I. Manners, J. Am. Chem. Soc., 2010, 132, 3831; (c)
H. Helten, B. Dutta, J. R. Vance, M. E. Sloan, M. F. Haddow, S.
Sproules, D. Collison, G. R. Whittell, G. C. LloydꢀJones and I.
Manners, Angew. Chem. Int. Ed., 2013, 52, 437.
Acknowledgements
We would like to thank our technical and analytical staff, in
particular Andreas Koch, for assistance. T. B. would like to
thank Prof. Uwe Rosenthal (LIKAT) for his support and for
fruitful discussions regarding titanocene chemisty. Financial
support by the Federal Ministry for Education and Research
(BMBF, Project Light2Hydrogen) is gratefully acknowledged.
7
(a) T. Beweries, S. Hansen, M. Kessler, M. Klahn and U. Rosenthal,
Dalton Trans., 2011, 40, 7689; (b) T. Beweries, J. Thomas, M.
Klahn, A. Schulz, D. Heller and U. Rosenthal, ChemCatChem, 2011,
3
, 1865; (c) J. Thomas, M. Klahn, A. Spannenberg and T. Beweries,
Notes and references
LeibnizꢀInstitut für Katalyse e.V. an der Universität Rostock, Albertꢀ
Dalton Trans., 2013, 42, 14668.
a
8
9
S. Hausdorf, F. Baitalow, G. Wolf and F. Mertens, Int. J. Hydrogen
Energy, 2008, 33, 608.
EinsteinꢀStr. 29a, 18059 Rostock, Germany, Fax: (+49) 381 1281 51104,
Eꢀmail: torsten.beweries@catalysis.de
a) W. Luo, P. G. Campbell, L. N. Zakharov and S.ꢀY. Liu, J. Am.
Chem. Soc., 2011, 133, 19326. b) W. Luo, D. Neiner, A. Karkamkar,
† Electronic Supplementary Information (ESI) available: Crystallographic
details, volumetric curves, NMR studies and sample gas chromatograms.
See DOI: 10.1039/b000000x/
8 | J. Name., 2012, 00, 1-3
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K. Parab, E. B. Garner III, D. A. Dixon, D. Matson, T. Autrey and S.ꢀ 23 TiꢀN and TiꢀH coupling should be observable in this titanocene
Y. Liu, Dalton Trans., 2013, 42, 611.
system. Typical values can be found in reference 20b.
10 Y. Luo and K. Ohno, Organometallics, 2007, 26, 3597.
11 (a) A. M. Chapman, M. F. Haddow and D. F. Wass, J. Am. Chem.
Soc., 2011, 133, 8826; (b) A. M. Chapman, M. F. Haddow and D. F.
Wass, J. Am. Chem. Soc., 2011, 133, 18463.
24 A free electron which is directly located at a boron atom shows
coupling constants around 4 G. (a) H. Braunschweig, V. Dyakonov,
J. O. C. JimenezꢀHalla, K. Kraft, I. Krummenacher, K. Radacki, A.
Sperlich and J. Wahler, Angew. Chem. Int. Ed. 2012
(b) P. Bissinger, H. Braunschweig, A. Damme, C. Hörl, I.
Krummenacher and T. Kupfer, Angew. Chem. Int. Ed. 2015 54, 359.
, 51, 2977ꢀ2980;
12 The concept of frustrated Lewis pairs was widely discussed in the
organometallic community in recent years. The key issue is the
sterical hindrance of the Lewis acids and bases, which prevents the
formation of a classical adduct but gives rise to a unique reactivity
due to the interaction of the basic and acidic centres. This has been
shown by reactions with a variety of reagents. An overview can be
found in D. W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2010,
49, 46.
,
Thus, the coupling of the free electron at the Ti(III) to the boron
ligand would be substantially smaller.
13 (a) A. M. Chapman and D. F. Wass, Dalton Trans., 2012, 41, 9067;
(b) A. M. Chapman, M. F. Haddow and D. F. Wass, Eur. J. Inorg.
Chem., 2012, 1546.
14 (a) U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann and A.
Spannenberg, Organometallics, 2003, 22, 884; (b) G. A. Luinstra, L.
C. ten Cate, H. J. Heeres, J. W. Pattiasina, A. Meetsma, J. H. Teuben,
Organometallics, 1991, 10, 3227; (c) A. Suárez, M. A. MéndezꢀRojas
and A. Pizzano, Organometallics, 2002, 21, 4611.
15 G. M. Sheldrick, Acta Crystallogr., 2008, A64, 112.
16 Analogous equipment is used for the measurement of gas
consumption in hydrogenation reactions. This equipment has been
developed at the LIKAT Rostock together with MesSen Nord
(Stäbelow) and has been described in: H.ꢀJ. Drexler, A. Preetz, T.
Schmidt and D. Heller, in The Handbook of Homogeneous
Hydrogenation, Vol. 1 (Eds.: J. G. De Vries, C. J. Elsevier), Wileyꢀ
VCH, Weinheim, 2007, pp. 257.
17 (a) T. Spalek, P. Pietrzyk and Z. Sojka, J. Chem. Inf. Model. 2005,
45, 18; (b) F. E. Mabbs and D. Collison, Mol. Phys. Rep. 1999, 26
,
39.
18 (a) R. Gyepes, V. Varga, M. Horácek, J. Kubista, J. Pinkas and K.
Mach, Organometallics, 2010, 29, 3780; (b) S. Niibayashi, K. Mitsui,
Y. Motoyama and H. Nagashima, J. Organomet. Chem., 2005, 690
,
276; (c) C. A. Willoughby, R. R. Duff Jr., W. M. Davis and S. L.
Buchwald, Organometallics, 1996, 15, 472.
19 (a) J. W. Pattaisina, H. J. Heeres, F. van Bolhuis, A. Meetsma and J.
H. Teuben, Organometallics 1987,
6, 1004; (b) M. Horacek, R.
Gyepes, J. Kubista and K. Mach, Inorg. Chem. Commun. 2004,
7,
155; (c) V. Varga, I. Cisarova, R. Gyepes, M. Horacek, J. Kubista
and K. Mach, Organometallics 2009, 28, 1748; (d) R. Gyepes, V.
Varga, M. Horacek, J. Kubista, J. Pinkas and K. Mach,
Organometallics 2010, 29, 3780.
20 (a) C. D. Schmulbach, C. H. Kolich, C. C. Hinckley, Inorg. Chem.
1972, 11, 2841. (b) J. G. Kenworthy, J. Myatt, M. C. R. Symons, J.
Chem. Soc. A: Inorganic, Physical, Theoretical 1971, 3428.
21 (a) B. M. Graybill and J. K. Ruff, J. Am. Chem. Soc., 1962, 84, 1062;
(b) H. Nöth and H. Vahrenkamp, Chem. Ber., 1966, 99, 1049; (c) M.
Van Overschelde, E. Vervecken, S. G. Modha, S. Cogen, E. Van der
Eycken and J. Van der Eycken, Tetrahedron, 2009, 65, 6410.
22 THF was chosen for reasons of solubility of catalyst and substrate.
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