Catalytic dehydrogenation of dimethylamine borane by group 4 metallocene alkyne complexes and homoleptic amido compounds

Article abstract of DOI:10.1039/c1dt10366k

Dehydrogenation of Me₂NH·BH₃ (1) by group 4 metallocene alkyne complexes of the type Cp₂M(L)(η²- Me₃SiC₂SiMe₃) [Cp = η⁵- cyclopentadienyl; M = Ti, no L (2Ti); M = Zr, L = pyridine (2Zr)] and group 4 metal amido complexes of the type M(NMe₂)₄ [M = Ti (8Ti), Zr (8Zr)] is presented.

Full text of DOI:10.1039/c1dt10366k

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Volume 40 | Number 30 | 14 August 2011 | Pages 7653–7792
ISSN 1477-9226
COVER ARTICLE
Rosenthal et al.
Catalytic dehydrogenation of dimethylamine borane by group 4 metallocene
alkyne complexes and homoleptic amido compounds

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COMMUNICATION
Catalytic dehydrogenation of dimethylamine borane by group 4 metallocene
alkyne complexes and homoleptic amido compounds†
Torsten Beweries, Sven Hansen, Monty Kessler, Marcus Klahn and Uwe Rosenthal*
Received 23rd March 2011, Accepted 4th April 2011
DOI: 10.1039/c1dt10366k
2
Dehydrogenation of Me2NH·BH3 (1) by group 4 metallocene
Cp¢2M(L)(h -Me3SiC2SiMe3) (Cp¢ = unsubstituted or substituted
5
2
h -cyclopentadienyl; 2Ti: Cp¢ = Cp, M = Ti, no L, 2Zr: Cp¢ =
alkyne complexes of the type Cp2M(L)(g -Me3SiC2SiMe3)
5
Cp, M = Zr, L = pyridine, 7Ti: Cp¢ = Cp*, M = Ti, no L, 7Zr: Cp¢ =
Cp*, M = Zr, no L)7,8 appeared to be interesting to study. These
compounds release the alkyne under relatively mild conditions
resulting in the formation of the highly reactive 14 electron
fragments [Cp¢2M], which can then undergo further stoichiometric
and catalytic reactions. Examples for the application of these
alkyne complexes include the formation of metallacycles, catalytic
hydroaminations and polymerisation reactions.9
In this contribution we present studies on the application
of group 4 metal complexes, in particular metallocene alkyne
complexes and metal amides for the catalytic dehydrogenation
of dimethylamine borane.
[Cp = g -cyclopentadienyl; M = Ti, no L (2Ti); M = Zr, L =
pyridine (2Zr)] and group 4 metal amido complexes of the
type M(NMe2)4 [M = Ti (8Ti), Zr (8Zr)] is presented.
Introduction
Along with the increasing current interest in hydrogen technology,
the research on suitable, safe and stable hydrogen storage materials
became more and more important. Amine borane adducts are
discussed to be promising candidates to meet the requirements for
such applications since they possess a high gravimetric hydrogen
capacity and release of the energy carrier is easily possible using
suitable catalysts.1 The use of metal hydrides and transition
metal complexes for this purpose was demonstrated on several
occasions.2 However, the most prominent disadvantage so far is
the regeneration of the dehydrogenation products. This problem
was addressed successfully by Dixon and Gordon and co-workers,
who used benzenedithiol and organotin hydrides for the reduction
of polyborazylene.3
Very recently, Manners et al. reported on the dehydrogenation
of Me2NH·BH3 (1) using group 4 metallocene complexes.4 They
found that starting from different Cp2MCl2/n-BuLi precursors
(M = Ti, Zr, Hf) the in situ formed [Cp2M] species can
perform catalytic dehydrogenation reactions. It became evident
that Cp2TiCl2/n-BuLi shows the highest activity whereas the
corresponding zirconocene system displays no activity at all. The
reason for this behavior could be found in the different stabilities
of the species formed during the catalytic cycle. This assumption
is corroborated by the results of Roesler and co-workers who
reported on the isolation and full characterization of the adduct
from the reaction of [Cp2Zr] with H3N·BH3.5 The mechanism of
the overall reaction was studied by Luo and Ohno using DFT
calculations,6 spectroscopic evidence supporting the theoretical
results was provided later by Manners et al.4
Results and Discussion
Catalytic dehydrogenation of 1 with 2M
For a direct comparison, all catalytic experiments were performed
using a catalyst concentration of 2 mol% (as used before by
Manners et al.). Solutions of the amine borane 1 were cannula-
transferred to stirred solutions of the corresponding metallocene
catalyst 2M, followed by immediate extensive gas evolution. The
colour of the reaction mixtures deepened during the reaction to
give dark reddish/black solutions in all cases. Analysis of the
reaction gases by gas chromatography confirmed the exclusive
formation of hydrogen.
11B NMR investigations revealed the formation of a mixture of
dehydrogenation products (Fig. 1). The constitution of the latter
is very close to the one observed before,4 however, in our case,
no product 6 was found. Instead, only the linear intermediate
Me2NH-BH2-NMe2-BH3 (3, 11B NMR: 2.2 ppm) and the products
cyclic dimer [Me2N-BH2]2 (4, 5.5 ppm) and dehydrogenated
aminoborane Me2N BH2 (5, 38.2 ppm) were found in solution
(Scheme 1).10
In this context, the use of other group 4 metallocene
sources such as the metallocene alkyne complexes of the type
Leibniz-Institut fu¨r Katalyse e. V. an der Universita¨t Rostock, Albert-
Einstein-Str. 29a, 18059, Rostock, Germany. E-mail: uwe.rosenthal@
catalysis.de; Fax: +49 381 128151176; Tel: +49 381 1281176
† Electronic supplementary information (ESI) available: Experimental
procedures, NMR spectra and ESI-MS of the dehydrogenation reactions.
See DOI: 10.1039/c1dt10366k
Scheme 1 Catalytic dehydrogenation of 1.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7689–7692 | 7689
©

Catalytic dehydrogenation of 1 with 7M
Under the same conditions as used before for the 2M catalysed
reactions (2 mol% catalyst, toluene, 24 ◦C), permethylated metal-
2
locene alkyne complexes Cp*2M(h -Me3SiC2SiMe3) (7M) display
no activity for the catalytic dehydrogenation of 1. This behaviour
was observed before by Chirik and co-workers, who found that
sterically more demanding substituents at the cyclopentadienyl
ligand result in a significant decrease of activity in the catalytic
dehydrogenation of 1.14 Moreover, in the metallocene alkyne com-
plex catalysed dehydrogenative coupling of silanes to polysilanes
reduced reactivity was observed when increasing the steric demand
of the Cp ligand.15
1
Fig. 1 11B{ H} NMR spectra (96 MHz, 297 K, toluene, unlocked) of the
reaction of 1 with 2Ti (2 mol%). Last spectrum recorded at t = 26 h.
Catalytic dehydrogenation of 1 with homoleptic M(IV) complexes
Thus, the reaction mechanism should be similar as described
in literature. Most surprisingly and in contrast to the already
published results, the zirconocene alkyne complex shows activ-
ity in the catalytic dehydrogenation of dimethylamine borane.
Although being significantly less active compared to the ti-
tanocene complex (2Ti: 86% conversion after 16 h, 2Zr: 42%
conversion after 16 h, Fig. 2), it becomes clear that under these
conditions [Cp2Zr] is a suitable catalyst. Manners et al. used
Cp2ZrCl2/n-BuLi11 and found no conversion after 20 h. In this
context, it should be noted that the authors performed the
reaction to give the metallocene [Cp2Zr] at -15 ◦C, followed by
warming of the solution to room temperature and addition of
Me2NH·BH3. Dioumaev and Harrod showed that under these
conditions, no [Cp2Zr] is present in solution.12 Instead, a mix-
ture of zirconocene complexes including butylzirconocene(III),
zirconocene(III) hydride, butenylzirconocene(IV) hydride dimer
and 1,1-bis(cyclopentadienyl)-2-methyl-3-(zirconocenyl hydride)-
1-zirconacyclobutane(IV) dimer is formed upon warming of
dibutylzirconocene to room temperature.
To evaluate the influence of the oxidation state at the titanium
center on the performance of the catalyst, we tested homoleptic
Ti(IV) complexes. Manners et al. showed that Ti(0) and Ti(III)
compounds display no activity in dehydrogenation reactions,
however, Ti(IV) species were not investigated.
Similarly as for the dehydrogenation of 1 with 2M, in the
reactions with metal amides M(NMe2)4 (8M, M = Ti, Zr)
immediate and extensive hydrogen evolution was observed. The
solutions of the reaction mixtures turned dark red for 8Ti but
remained yellow for 8Zr. This might indicate subtle differences in
the reaction pathways.
One possible reaction mechanism includes the initial formation
of a metallaaziridine species from a metal amido complex as
suggested earlier by Nugent et al. (Scheme 2).16 This assumption
finds experimental support in the results of Schafer and co-workers
who isolated such metallacycles and highlighted their role in
catalytic reactions involving metal amido complexes.17 Another
possibility which was also mentioned very recently by Wright et al.
for the Al(NMe2)3 catalysed dehydrogenation of Me2NH·BH3 is
the initial deprotonation of the substrate along with formation of
Me2NH and b-hydride elimination to give a four-membered M–
N–B–N ring, which could then eliminate H2 or (Me2N)2BH (6),
respectively.18 This mechanism might explain the formation of
a relatively large amount of the latter compound, as indicated
1
by the signal at 29.1 ppm in 11B{ H} NMR spectra (Fig. 319).
Studies on the reaction mechanism and the fate of the catalyst
are still ongoing and will be published in due course. However,
from NMR investigations it becomes clear that the composition
of the mixture of dehydrogenation products – although containing
the same species – is different compared to the 2M catalysed
reactions. Volumetric analysis of the hydrogen evolution in the
reactions using 8M revealed that both metal amides display the
same catalytic activity (Fig. 4), indicating that in this case the
influence of the metal can be neglected. Most likely, this is due
to the similarities in M–N(alkyl) bond energies (e.g. Ti–NEt2
307 kJ mol-1, Zr–NEt2 337 kJ mol-1).20
Fig. 2 Hydrogen evolution curves of the catalytic dehydrogenation of 1
with 2M (top: M = Ti, bottom: M = Zr). Conditions: toluene, 2 mol%
catalyst, 24 ◦C.
An explanation for the lower activity of 2Zr compared to 2Ti
could be found in the stronger complexation of the alkyne to the
metal in the first case,7b which hinders the generation of the free
1
metallocene, most likely the catalytically active species. 11B{ H}
NMR spectra of this reaction also indicate the lower activity of
the zirconocene complex; details referring to this can be found in
the Electronic Supplementary Information (ESI†).13
Scheme 2 Metallaaziridines from metal amido complexes.16
7690 | Dalton Trans., 2011, 40, 7689–7692
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The Royal Society of Chemistry 2011
©

the [Cp2Zr] fragment is active for such reactions, however being
significantly less active than the corresponding titanocene species.
Moreover, metal(IV) amide complexes have proved to be suitable
for dehydrogenation of Me2NH·BH3 as well. In contrast to the
[Cp2M] catalysed reactions, in this case no influence of the metal
was observed. It should be noted that in all cases exclusively
hydrogen was released from the reaction mixtures.
Acknowledgements
We thank our technical and analytical staff, in particular Dr
Christine Fischer and Andreas Koch for assistance. Financial
support by the BMBF is gratefully acknowledged.
1
Fig. 3 11B{ H} NMR spectra (96 MHz, 297 K, toluene, unlocked) of the
reaction of 1 with 8Ti (2 mol%). Last spectrum recorded at t = 30 h.
Notes and references
1 (a) T. B. Marder, Angew. Chem., Int. Ed., 2007, 46, 8116; (b) C. W.
Hamilton, R. T. Baker, A. Staubitz and I. Manners, Chem. Soc. Rev.,
2009, 38, 279.
2 Recent Examples: (a) R. J. Keaton, J. M. Blacquiere and R. T. Baker,
J. Am. Chem. Soc., 2007, 129, 1844; (b) M. Ka¨b, A. Friedrich, M.
Drees and S. Schneider, Angew. Chem., Int. Ed., 2009, 48, 905; (c) T.
Hu¨gle, M. F. Ku¨hnel and D. Lentz, J. Am. Chem. Soc., 2009, 131,
7444; (d) C. Boulho and J.-P. Djukic, Dalton Trans., 2010, 39, 8893;
(e) T. W. Graham, C.-W. Tsang, X. Chen, R. Guo, W. Jia, S.-M. Lu, C.
Sui-Seng, C. B. Ewart, A. Lough, D. Amoroso and K. Abdur-Rashid,
Angew. Chem., Int. Ed., 2010, 49, 8708; (f) M. Vogt, B. de Bruin, H.
Berke, M. Trincado and H. Gru¨tzmacher, Chem. Sci., 2011, 2, 723.
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Matus, B. Scott and F. H. Stephens, Angew. Chem., Int. Ed., 2009,
48, 6812. After acceptance of this communication, the same authors
reported on the efficient regeneration of spent ammonia borane fuels
using hydrazine and liquid ammonia: (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.
Fig. 4 Hydrogen evolution curves of the catalytic dehydrogenation of 1
with 8M (top: M = Ti, bottom: M = Zr). Conditions: toluene, 2 mol%
catalyst, 24 ◦C.
4 M. E. Sloan, A. Staubitz, T. J. Clark, C. A. Russell, G. C. Lloyd-Jones
and I. Manners, J. Am. Chem. Soc., 2010, 132, 3831.
5 T. D. Forster, H. M. Tuononen, M. Parvez and R. Roesler, J. Am.
Chem. Soc., 2009, 131, 6689.
We also performed catalytic tests with other homoleptic Ti(IV)
compounds such as Ti(OiPr)4 (9) and TiCl4 (10) to find out whether
the amido functionality plays a crucial role for the performance
of the catalyst. We found that both, 9 and 10 show no activity at
all; this indicates that the presence of a group which can undergo
insertion reactions to start a catalytic cycle is necessary.
6 Y. Luo and K. Ohno, Organometallics, 2007, 26, 3597.
7 (a) V. V. Burlakov, U. Rosenthal, P. V. Petrovskii, V. B. Shur and M. E.
Vol’pin, Organomet. Chem. USSR, 1988, 1, 526; (b) U. Rosenthal, A.
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Chem., Int. Ed. Engl., 1993, 32, 1193.
8 (a) V. V. Burlakov, A. V. Polyakov, A. I. Yanovsky, Y. T. Struchkov, V. B.
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9 U. Rosenthal and V. V. Burlakov, in Titanium and Zirconium in Organic
Synthesis (Ed.: I. Marek), Wiley-VCH, Weinheim, Germany, 2002, pp.
355.
Conclusions
We have demonstrated the homogeneous catalytic dehydrogena-
tion of Me2NH·BH3 by group 4 metallocene(II) alkyne complexes
and group 4 metal(IV) amides (Table 1). It became evident that
10 The nature of the dehydrogenation products was confirmed by proton
coupled 11B NMR as well as ESI-MS (for 8Ti) (see Supporting
Information† for details).
11 The mixture of Cp2ZrCl2 and n-BuLi forms the Negishi reagent
2
Cp2Zr(h -1-butene) at low temperatures. cf. (a) E. Negishi and T.
Table 1 Catalytic dehydrogenation of Me2NH·BH3. Conditions: toluene,
Takahashi, Bull. Chem. Soc. Jpn., 1998, 71, 755; (b) E. Negishi and
S. Hou, in Titanium and Zirconium in Organic Synthesis (Ed. I. Marek),
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therein; (c) T. Takahashi and Y. Li, in Titanium and Zirconium in Organic
Synthesis (Ed.: I. Marek), Wiley-VCH, Weinheim, Germany, 2002; pp.
50, and references cited therein.
24 ◦C, 2 mol% catalyst. Yields were determined by volumetric analysis
Catalyst
Time/h
Yield (H2) (%)
2Ti
2Zr
7Ti
7Zr
8Ti
8Zr
9
16
16
16
16
22
22
16
16
86
42
0
12 V. K. Dioumaev and J. F. Harrod, Organometallics, 1997, 16, 1452.
13 Elimination of bis(trimethylsilyl)acetylene takes place in both cases,
as indicated by a signal in the 1H NMR at 0.16 ppm (benzene-d6).
However, the intensity of this signal cannot be directly correlated with
the activities of 2M.
14 D. Pun, E. Lobkovsky and P. J. Chirik, Chem. Commun., 2007, 3297.
15 (a) N. Peulecke, A. Ohff, P. Kosse, A. Tillack, A. Spannenberg, R.
Kempe, W. Baumann, V. V. Burlakov and U. Rosenthal, Chem.–Eur. J.,
0
86
87
0
10
0
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©

1998, 4, 1852; (b) N. Peulecke, D. Thomas, W. Baumann, C. Fischer
and U. Rosenthal, Tetrahedron Lett., 1997, 38, 6655.
16 W. A. Nugent, D. W. Ovenall and S. J. Holmes, Organometallics, 1983,
2, 161.
17 Some recent examples: (a) J. A. Bexrud, P. Eisenberger, D. C. Leitch,
P. R. Payne and L. L. Schafer, J. Am. Chem. Soc., 2009, 131, 2116; (b) P.
Eisenberger, R. O. Ayinla, J. M. P. Lauzon and L. L. Schafer, Angew.
Chem., Int. Ed., 2009, 48, 8361.
18 H. C. Cowley, M. S. Holt, R. L. Melen, J. M. Rawson and D. S. Wright,
Chem. Commun., 2011, 47, 2682.
1
19 Please note the following in the 11B{ H} NMR spectrum: (a) Conver-
sion was not complete because the reaction was stopped after a reaction
time of 22 h for a better comparison with the volumetric data; (b) In
the first two spectra the signals for 1 were broad, however, analysis of
the ratio 1:3, 1:5 and 1:6 shows a decrease of the amount of 1.
20 H. A. Skinner and J. A. Connor, Pure Appl. Chem., 1985, 57, 79.
7692 | Dalton Trans., 2011, 40, 7689–7692
This journal is
The Royal Society of Chemistry 2011
©