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associated with hydroboration (δ 45–55 ppm), but most boron-
containing products were similar to those seen in the dehydro-
coupling catalysis (vide supra). Hydroboration products were
not detected by 13C NMR spectroscopy or identified by GC/MS,
which suggests these are minor byproducts. With respect to
competitive hydroboration, the best substrate in this proof-of-
concept trial was 2-vinylpyridine, where only dehydrocoupling
products were observed by 11B{1H} NMR spectroscopy, and
complete conversion of the substrate to 2-ethylpyridine was
measured within the detection limits of 13C{1H} NMR spec-
troscopy and GC. All hydrogenation reactions gave nearly iden-
tical results whether they were performed in a PTFE-sealed
reaction tube or in an open Schlenk flask fitted with a conden-
ser. A control reaction of 2 mM of 1 in THF with 50 equiv. of
styrene was heated to 65 °C under one atmosphere of H2. No
detectible reaction occurred even after reaction times up to 7
days, which is consistent with transfer hydrogenation rather
than hydrogenation from ambient H2 generated by dehydro-
coupling of NH3BH3. Peters’s PBP-supported cobalt compound
also engages in transfer hydrogenation and uses dimethyl-
amine borane as the hydrogen.13 It should be noted that yields
of hydrogenated products were nearly quantitative in that
study, showing more selective reactivity than the systems
described here. However, that catalyst and reaction are not
reported to be air stable.
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ð2Þ
In conclusion, two half sandwich cobalt complexes 1 and 2
were shown to be effective catalysts for the dehydrogenation of
ammonia borane. These represent two rare examples of homo-
geneous cobalt complexes that can catalyze this reactivity. In
addition, these catalysts are able to effectively transfer hydro-
genate unsaturated organic substrates using NH3BH3 as a
sacrificial hydrogen source. Finally, these compounds display
unique robustness with respect to aerobic conditions for both
dehydrogenation and transfer hydrogenation reactions.
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Acknowledgements
This work was supported by the U. S. National Science Foun-
dation (CHE-1265608 to R.W.). We would like to thank
Dr Monika Ivancic for assistance with 11B NMR experiments
and Bruce O’Rourke for assistance with GC data collection.
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Notes and references
1 (a) A. Staubitz, A. P. M. Robertson, M. E. Sloan and 13 T.-P. Lin and J. C. Peters, J. Am. Chem. Soc., 2013, 135,
I. Manners, Chem. Rev., 2010, 110, 4023; (b) A. Staubitz,
15310.
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Dalton Trans.