DIPPNH2BH3 and lower basicity of the Al–H compared to
early main group metal hydride compounds. Substitution
at B is a pathway that should be considered in mechanistic
evaluations.
Notes and references
1 H. V. K Diyablanage, R. P. Shrestha, T. A. Semelsberger,
B. L. Scott, M. E. Bowden, B. L. Davis and A. K. Burrell, Angew.
Chem., Int. Ed., 2007, 46, 8995.
2 Z. Xiong, C. K. Yong, G. Wu, P. Chen, W. Shaw, A. Karkamkar,
T. Autrey, M. O. Jones, S. R. Johnson, P. P. Edwards and
W. I. F. David, Nat. Mater., 2008, 7(2), 138.
3 Reviews: (a) Y. S. Chua, P. Chen, G. Wu and Z. Xiong, Chem.
Commun., 2011, 47, 5116; (b) A. Staubitz, A. P. M. Robertson and
I. Manners, Chem. Rev., 2010, 110, 4079.
4 J. Spielman, G. Jansen, H. Bandmann and S. Harder, Angew.
Chem., Int. Ed., 2008, 47, 6290.
5 J. Spielmann, M. Bolte and S. Harder, Chem. Commun., 2009, 6934.
6 J. Spielman and S. Harder, J. Am. Chem. Soc., 2009, 131, 5064.
7 J. Spielman, D. Piesik, B. Wittkamp, G. Jansen and S. Harder,
Chem. Commun., 2009, 3455.
8 J. Spielmann, D. F.-J. Piesik and S. Harder, Chem.–Eur. J., 2010,
16, 8307.
9 J. Spielmann and S. Harder, Dalton Trans., 2011, 40, 8314.
10 D. Y. Kim, N. J. Singh, H. M. Lee and K. S. Kim, Chem.–Eur. J.,
2009, 15, 5598.
11 A. T. Luedtke and T. Autrey, Inorg. Chem., 2010, 49, 3905.
12 D. Dou, D. R. Ketchum, E. J. M. Hamilton, P. A. Florian,
K. E. Vermillion, P. J. Grandinetti and S. G. Shore, Chem. Mater.,
1996, 8, 2839.
Scheme 4 Alternative catalytic scheme for dehydrocoupling of
HNR2BH3 (based on nucleophilic substitution).
[H2B(NH3)2+][BH4À] (DADB) which is proposed to be
formed by substitution reactions on B centers.29
In the light of these results, alternative mechanisms could
be operative in catalytic ammonia-borane dehydrogenation.
E.g. the recently reported dehydrogenation, 2HNMe2BH3 -
(H2BNMe2)2 + 2H2, by Al(NMe2)3 alternatively could proceed
through the coupled cycles shown in the general Scheme 4. The
difference with the earlier proposed mechanism (Scheme 2)14–16 is
that the [M]-NR2BH3 is not formed by deprotonation but by
substitution. Reaction of the leaving group HNR2 with [M]-H
would regenerate the metal amide catalyst [M]-NR2. Such species
could be formed in the reaction from any nucleophilic metal
complex with ammonia-borane. The importance of nucleophilic
substitution is naturally dependent on the nature of the sub-
stituents R and the Lewis-acidity of the metal but could be an
alternative to be taken into account in a discussion of reaction
mechanisms.
13 M. S. Hill, G. Kociok-Kohn and T. P. Robinson, Chem. Commun.,
¨
2010, 46, 7587.
14 M. S. Hill, G. Kociok-Kohn and T. P. Robinson, Chem. Commun.,
¨
2010, 46, 7587.
15 D. J. Liptrot, M. S. Hill, M. F. Mahon and D. J. MacDougall,
Chem.–Eur. J., 2010, 16, 8508.
16 (a) H. J. Cowley, M. S. Holt, R. L. Melen, J. M. Rawson and
D. S. Wright, Chem. Commun., 2010, 46, 7587; (b) M. H. Hansmann,
R. L. Melen and D. S. Wright, Chem. Sci., 2011, 2, 1554.
17 R. V. Genova, K. J. Fijalkowski, A. Budzianowski and
W. Grochala, J. Alloys Compd., 2010, 499, 144.
18 A. K. Burrell, B. J. Davis, D. L. Thorn, J. C. Gordon, R. T. Baker,
T. A. Semelsberger, W. Tumas, H. Vichalya, K. Diyabalanage and
R. P. Shrestha, US pat. 2008/031101.
As aluminium amidoborane complexes are apparently
not formed by attempted deprotonation of DIPPNH2BH3,
we also investigated the salt metathesis route. Reaction of
DIPPnacnacAlCl2 with two equivalents of KNR(H)BH3 (R =
iPr or DIPP) in THF gave in good yields DIPPnacnacAlH2.
This is likely formed by double b-hydrogen elimination in the
expected product. This not only demonstrates the instability of
aluminium amidoborane complexes but especially underscores
the usefulness of amidoboranes in the preparation of metal
hydride complexes (see ESIw for this alternative synthetic
route to DIPPnacnacAlH2).
19 S. P. Green, C. Jones and A. Stasch, Angew. Chem., Int. Ed., 2008,
47, 9079.
20 J. Spielmann and S. Harder, unpublished results.
21 C. Cui, H. W. Roesky, H. Hao, H. G. Schmidt and
M. Noltemeyer, Angew. Chem., Int. Ed., 2000, 39, 1815.
22 B. Twamley, N. J. Hardman and P. P. Power, Acta Crystallogr.,
2001, E57, m227.
23 S. Aldridge, A. J. Blake, A. J. Downs, R. O. Gould, S. Parsons and
C. R. Pulham, J. Chem. Soc., Dalton Trans., 1997, 1007.
24 D. E. Walmsley, W. L. Budde and M. F. Hawthorne, J. Am. Chem.
Soc., 1971, 93, 3150.
25 S. Toyota, T. Futawaka, M. Asakura, H. Ikeda and M. Oki,
Organometallics, 1998, 17, 4155.
In conclusion, despite high N–H acidity in DIPPNH2BH3,
reaction with DIPPnacnacAlH2 proceeds not through a
deprotonation protocol but nucleophilic substitution at B is
observed instead. This reaction pathway is likely favoured on
26 S.-Y. Yang, P. Fleurat-Lessard, I. Hristov and T. Ziegler, J. Phys.
Chem. A, 2004, 108, 9461.
27 A. Haaland, Angew. Chem., Int. Ed. Engl., 1989, 28, 992.
28 H. Noth and R. Rurlander, Inorg. Chem., 1981, 20, 1062.
¨
29 V. S. Nguyen, M. H. Matus, D. J. Grant, M. T. Nguyen and
¨
account of
a
combination of Lewis-acid activation of
D. A. Dixon, J. Phys. Chem. A, 2007, 111, 8844.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11945–11947 11947