10.1002/anie.201811135
Angewandte Chemie International Edition
COMMUNICATION
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Na2[5] in a clean reaction (Scheme 3c). X-ray crystallography on
[Na(thf)2][Na(thf)][5] (Figure 1) revealed a B–O bond length of
1.562(6) Å, which is shorter by 0.027
Å than that of
[Na][Na3(thf)6][2]2, but still longer by 0.053 Å than that of
Harman’s CO2-inert adduct F. Upon exposure to CO2 (room
temperature, 1 atm), Na2[5] was converted further to Na2[6]
(Scheme 3c). The NMR spectra of this product are in line with
the uptake of 1 equiv of CO2: (i) all 1H, 11B, and 13C{1H}
resonances were shifted compared to the starting material
Na2[5], but the signal patterns remained compatible with a
bridged derivative of 1 with average Cs symmetry. (ii) A new
13C{1H} resonance appeared at = 159.1 ppm, which we assign
to the carbonyl carbon atom of the activated CO2 molecule (cf.
the [CO3]2– signal of Na2[4] at = 163.5 ppm). (iii) The IR
spectrum of Na2[6] (but not of Na2[5]) contains strong absorption
bands at = 1358 and 1397 cm–1, in the typical region of C=O
stretches. Notably, even if the reaction was carried out in a
sealed NMR tube, no resonance of CO could be detected. This
result strongly indicates that the CO released in the course of
the reaction between M2[1] and CO2 indeed originates from the
first incorporated molecule of CO2.
To conclude, the presence of 9,10-dimethyl-9,10-dihydro-9,10-
diboraanthracene (1) as a redox catalyst enables the clean
reduction of CO2 with lithium metal under ambient conditions
and with selective formation of CO and Li2CO3 (the main
products of the corresponding uncatalyzed reaction are Li2O and
elemental carbon[31]). Notwithstanding the relevance of Li2CO3
for the chemical and pharmaceutical industry,[32] it would be
desirable to prepare also Na2CO3 in the same way. The reaction
currently stops at the stage of an isolable Na2CO3–1 aggregate,
but our system offers multiple options to promote the release of
Na2CO3, e.g., by adjusting the steric demand of the boron-
bonded substituents.
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