frequency of 0.29 h21 was measured at 23 uC, suggesting catalyst
inhibition from THF coordination to titanium.
In summary, a family of organometallic titanium and zirconium
complexes have been evaluated for the catalytic dehydrogenation
of Me2NHBH3. In general, the titanium compounds are more
active than their zirconium counterparts, with the silyl-substituted
titanocene dinitrogen complex 4 being one of the most active for
Me2NHBH3 dehydrogenation. This compound is also effective for
parent NH3BH3 dehydrogenation, although lower activity is
observed in THF solution. These studies highlight the impact of
cyclopentadienyl substitution on the rate of amineborane dehy-
drogenation and demonstrate the ability to tune the rate of H2 loss
by substituent manipulation.
Fig. 3 Zirconocene borohydride hydride compounds recovered follow-
ing Me2NHBH3 dehydrogenation.
This work has been supported by the U. S. National Science
Foundation (CAREER award to P. J. C.). We thank Emily
Trunkley and Donald Knobloch for preparing
respectively.
5 and 6,
Notes and references
{ Crystal data for 13. C30H47BZr, M = 509.71, monoclinic, a = 9.226(3),
b = 19.364(4), c = 15.896(4) s, b = 99.197(9)u, U = 3082.9(2) s3, T = 173(2)
K, space group P2(1)/n, Z = 4, m(Mo-Ka) = 0.71073 mm21, 14059
reflections measured, 2963 unique (Rint = 0.0854) which were used in all
calculations. R1 = 0.0495. The final wR(F2) was 0.0873 (all data). CCDC
642675. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b704941b
Fig. 4 Molecular structure of 13 at 30% probability ellipsoids.
The solution dynamics of 13 were probed by EXSY NMR
spectroscopy. While the [BH4]2 ligand undergoes rapid exchange
of terminal and bridging hydride positions at 23 uC, no exchange
was detected between the Zr–H and borohydride positions up to
temperatures of 90 uC with a mixing time of 500 ms.
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The molecular structure of 13 was also established by X-ray
diffraction (Fig. 4) and confirmed the formation of a tetrahydro-
indenyl ligand from benzo group hydrogenation.{ The two rings
are in a near ideal anti configuration with a rotational angle of
169.1u. The other metrical parameters are as expected for a bent
zirconocene borohydride complex.20,21 Zirconocene borohydride
hydride 13 most likely arises from N–B cleavage and capture of the
BH3 by zirconocene dihydride. Confirmation of Me2NH forma-
tion was provided by collecting the volatiles and treatment with
PhC(O)Cl to yield PhC(O)NMe2. Similar N–B bond cleavage has
been identified as a catalyst deactivation pathway in iridium pincer
chemistry.7 However, unlike the iridium compound, warming 12,
13 or 14 to 65 uC in the presence of excess Me2NHBH3 produced
modest turnover, slightly slower (12 and 13) or similar (14) to the
parent compounds. Presumably, catalyst activation occurs via BH3
loss, possibly assisted by amineborane.
The most active precatalyst in the series, 4, was also assayed for
the dehydrogenation of H3NBH3. In benzene solution, a modest
turnover frequency of 0.23 h21 was observed at 65 uC. Most of the
substrate and resulting dehydrogenated products were insoluble
under these conditions. As a result, the reaction was also carried
out in THF at 65 uC. A slightly higher turnover frequency of
0.31 h21 was measured by monitoring the reaction by 11B NMR
spectroscopy. [H2NBH2]3 was observed as the initial dehydrogena-
tion product, which eventually converted to borazine over time.
To determine if THF was deleterious to turnover, dehydrogena-
tion of Me2NHBH3 with 4 was also performed in THF. Unlike
in benzene-d6, where rapid catalysis occurs, a modest turnover
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Chem. Commun., 2007, 3297–3299 | 3299