directly with H3NBH3 using THF and benzene as solvents. As
1
confirmed by H NMR spectroscopy, conversion of 3 was very
slow at room temperature, presumably due to the limited solubility
of H3NBH3. However heating to 60 ◦C greatly accelerated
the reaction, and after 2 h (THF) and 18 h (benzene) nearly
quantitative conversion of 3 to LGeH (2) and a little amount
of free ligand LH was observed (Fig. 1).
Fig. 2 13C NMR spectra of 13C-labelled 3 with 3 eq. H3NBH3 in C6D6
directly, 2 h and 10 h after heating to 60 ◦C.
The conversion of LGeOC(O)H (3) to LGeH (2) regenerates
the primary CO2-capturing agent. Here it is worth mentioning
that LGeH is stable towards water and can therefore be easily
separated from the other reaction products by extraction with
benzene. Currently we are trying to optimise the conditions for
the catalytic conversion of carbon dioxide to its derivatives under
ambient conditions. Initial results indicate that the reaction of the
tin(II) analogue LSnOC(O)H with ammonia borane is complete
within a few hours at room temperature and yields similar methyl
derivatives. However, in contrast to LGeH (2), the corresponding
tin hydride is not stable under the reaction conditions and
decomposes to the free ligand LH and metallic Sn. In conclusion
we have shown the possibility of a germanium(II) hydride mediated
synthesis of formic acid and methanol from gaseous carbon
dioxide using ammonia borane as the hydrogen source.
Fig. 1 Reaction of 3 with 3 eq. H3NBH3 at 60 ◦C in C6D6 monitored by
1H NMR (ꢀ: 3, : 2, : unidentified second product, : LH). The g-H
resonances at 5.07 ppm, 4.93 ppm, 4.94 ppm and 4.86 ppm, respectively,
were used for integration. Start of heating to 60 ◦C was at t = 0.
The workup was again accomplished with D2O, and the
1
resulting D2O phase contained CH3OD (5) (yield: 33–43%, H
NMR: singlet at 3.27 ppm, 13C NMR: quartet at 48.7 ppm, 1JCH
= 142 Hz) as the major product. To investigate the mechanism of
methanol formation, we ran several reactions in C6D6 and THF-
d8 at the NMR scale, using 13C-labelled formate 3 (obtained from
13CO2 as described above). Initially, with the formation of LGeH
(2) the 13C-label appears in several formate species (the resonances
are somewhat broad), suggesting that the Ge–O bond is cleaved
prior to reduction of the –OC(O)H group (Scheme 3).
We gratefully acknowledge financial support from the Deutsche
Forschungsgemeinschaft. G. T. is thankful to the Alexander von
Humboldt Stiftung for a research fellowship.
Notes and references
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3 W. Reutemann and H. Kieczka, ’Formic Acid’ in Ullmann’s Encyclo-
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4 K. Tominaga, Y. Sasaki, M. Kawai, T. Watanabe and M. Saito, J. Chem.
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Scheme 3 Proposed mechanism of the formation of methanol from 2 and
ammonia borane.
7 G. Me´nard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796–
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8 A. E. Ashley, A. L. Thompson and D. O’Hare, Angew. Chem., 2009,
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The reduction then presumably proceeds via several interme-
diates that are short-lived and could not be detected by NMR
spectroscopy. The finally 13C-labelled compounds are mostly
(B)–O–CH3 derivatives that appear around 3–3.5 ppm and 48–
1
52 ppm in the H and 13C NMR spectrum, respectively (Fig. 2).
These compounds are expected to yield methanol upon workup
with D2O. As side-products, small amounts of N-methylated
compounds are formed.
12 (a) A. N. C. Smythe and J. C. Gordon, Eur. J. Inorg. Chem., 2010,
509–521; (b) A. Paul and C. B. Musgrave, Angew. Chem., 2007, 119,
9488 | Dalton Trans., 2010, 39, 9487–9489
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