10.1002/anie.201902226
Angewandte Chemie International Edition
COMMUNICATION
To better understand the formation and decay of adduct 1H,
calculations were performed at the DLPNO-CCSD(T)/
def2-TZVPP//PBE0-D3BJ/def2-TZVP level of theory (Table 1,
see SI). The formation of 1H (1,1-isomer) is a slightly exergonic
process (–4.3 kcal/mol, Scheme 2), while the release of N2, which
leads to 2H, is strongly exergonic (–87.1 kcal/mol). The activation
barrier to be overcome is 21.6 kcal/mol in accord with experi-
mental observation. The structure of the transition state (TS_H,
Scheme 2) is characterized by elongated N1-N2 and B1-C1
bonds. The imaginary frequency describes a movement of the N2-
N3 group away from N1 and a movement of N1 towards C1. Thus,
the formation of N2 and the migration of one C6F5 group from the
boron atom to the N1 atom is a concerted mechanism.
bond length (ca. 1.75 Å), in a concerted mechanism. With this
work, we want to close a gap in modern boron-nitrogen chemistry,
which maybe began with the synthesis of borazole by Stock and
Pohland[60] followed by the studies of boron azides by Paet-
zold[61,62] and later by Klapötke et al.,[14,63–65] and culminated with
metal-free N2 fixation in a borylene dinitrogen compound by the
Braunschweig group last year.[66]
Experimental Section
Caution! HN3 is highly toxic and can decompose explosively under various
conditions! Appropriate safety precautions (protection shield, face shield,
ear protection, Kevlar gloves, low temperatures, small amounts) should be
taken.
The 1,1-isomers of all adducts are significantly more stable
than the 1,3-isomers (relG° between 8.7 and 14.3 kcal/mol, Table
1). The calculated activation barriers for the release of molecular
nitrogen in 1R increase along the series Btp (19.1) < H (21.6) <
Ph (22.2) < TMS (36.7 kcal/mol) in agreement with experiment,
that is, 1TMS is most stable and 1Btp and 1H are most labile.
Consistent with this sequence, the calculated charge transfer
from azide to borane increases exactly along this series (cf. 1Btp:
0.346 vs. 1TMS: 0.379 e), i.e. higher charge transfer leads to
more stable azide-borane adducts 1R. The release of molecular
nitrogen, yielding 2R, is a highly exothermic process for all
considered 1R species (1TMS: –78.4 - 1Btp: –93.1 kcal/mol). It is
worthy to note that the computed Gibbs energy for the solvate
formation 1H + xyl → 1H·xyl is 0.2 kcal/mol and therefore an
equilibrium. As pointed out before, the adduct formation of 1H is
associated with a charge transfer of 0.37 e from the HN3 to the
B(C6F5)3 molecule. This, in turn, results in a significantly increased
Brønsted acidity of 1H (pKa,H2O = −7.3, pKa,MeCN = 3.7, see SI
Experimental and computational details including all spectra and ORTEP
representations of all experimentally studied species are given in the
supporting information.
Acknowledgements
„Deutsche Forschungsgemeinschaft“ (DFG SCHU 1170/12-1) is
acknowledged for financial support (K. Bläsing). We also like to
thank the University of Rostock for access to the cluster computer
and especially Malte Willert for technical support.
Keywords: Adduct • Azide • Aminoborane • Amine synthesis •
Hydrazoic acid
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