Angewandte
Chemie
equatorial nitrogen atoms (15.1%) of the pseudo-trigonal
À
bipyramid, indicating larger covalent character for the Bi N
bonds within the trigonal plane. In 3, only 6–7% of the
À
electron density of the Bi N bonds is found at the bismuth
atom, indicating a predominantly ionic bonding situation for
all six azido ligands attached to the bismuth atom.
The charge distribution for all three azides is character-
ized by alternating net charges along the Bi(d+)–N(dÀ)–N(d+)
–
N(dÀ) units, with a large positive charge at the bismuth atom (1
+ 1.70, 2 + 1.71, 3 + 1.90 e; Table 1). A closer look at the
À
charges reveals that upon further N3 complexation, the
charge at the Bi, Na, and Nb atoms does not change much,
whilst the terminal Ng atoms feature a significantly larger
negative charge (1 À0.08, 2 À0.23, 3 À0.43 e).
Figure 4. ORTEP representation of the anion of 3 in the crystal at
173 K (thermal ellipsoids set at 50% probability). Selected bond
lengths [ꢀ] and angles [8]: Bi–N1 2.487(2), Bi–N4 2.32(2), Bi–N7
2.364(2), Bi–N10 2.478(2), Bi–N13 2.331(2), Bi–N16 2.719(5), N1–N2
1.188(3), N4–N5 1.27(3), N7–N8 1.194(3), N10–N11 1.188(3), N13–
N14 1.203(3), N2–N3 1.149(3), N5–N6 1.10(2), N8–N9 1.140(3),
N11–N12 1.158(3), N14–N15 1.150(3), N17–N18 1.19(2); N1-Bi-N4
94.4(5), N1-Bi-N10 89.03(7), N1-Bi-N13 85.59(7), N1-Bi-N16 108.8(2),
N4-Bi-N7 86.8(5), N4-Bi-N13 84.3(4), N4-Bi-N16 78.2(4), N7-Bi-N10
88.53(7), N7-Bi-N13 88.47(7), N7-Bi-N16 77.4(2), N10-Bi-N13
84.10(7), N10-Bi-N16 111.9(1), N1-Bi1-N7 173.78(8), N4-Bi-N10
167.6(4), N13-Bi1-N16 158.8(2), N2-N1-Bi 119.5(2), N5-N4-Bi
124.5(1), N8-N7-Bi 123.2(2), N11-N10-Bi 120.7(2), N14-N13-Bi
119.1(2), N17-N16-Bi 133.4(9), N3-N2-N1 177.0(3), N6-N5-N4
177.5(2), N9-N8-N7 175.8(2), N12-N11-N10 177.6(3), N15-N14-N13
177.2(2).
As it was not possible to obtain experimental structural
data, the structure of 1 was calculated for the gas phase at the
B3LYP level of theory. For nitrogen, a standard 6-31G(d)
basis set was used and a quasi-relativistic pseudopotential
(ECP78MWB) and a (4s4p1d)/[2s2p1d] basis set for the
bismuth atom. The lowest-lying C3 symmetric isomer with the
three azido ligands in an anti configuration with respect to the
lone pair (Figure 1) was characterized as a minimum at the
potential energy surface by a frequency analysis. It can be
assumed that in the solid state owing to strong intermolecular
interactions, such as in E(N3)3 (E = As, Sb), a three-dimen-
sional network also exists.[1c] As shown on numerous occa-
sions, the azido groups are almost linear, with N-N-N angles
of 175.58.[1–7,11] The tetrahedral angles at the bismuth atom
decrease from an ideal value of 109.58 to 98.18.
(PPh4)[Bi(N3)4] crystallizes in the monoclinic space group
P21/c with four formula units per cell, whereas
(PPh4)[Bi(N3)4]·CH2Cl2 crystallizes in the triclinic space
¯
group P1 with two formula units per cell. As the structural
parameters for both compounds are very similar, we focus
only on the data for (PPh4)[Bi(N3)4]. The asymmetric unit
consists of separated (PPh4)+ and [Bi(N3)4]À units with no
significant cation–anion contacts. The [Bi(N3)4]À ion
(Figure 2) adopts a distorted bisphenoidal geometry with
Scheme 3. Synthesis of 3.
observation of only one set of azide signals and the absence of
the Na signal indicates strong quadrupole relaxation effects
and a rapid ligand exchange on the NMR timescale.[12]
The vibrational spectra of all three azides feature the
presence of covalently bonded azido ligands, as shown by the
asymmetrical stretching mode in the range 2200–2000 cmÀ1,
the symmetrical stretching mode at 1400–1200 cmÀ1, and the
deformation mode at 700–600 cmÀ1. The bismuth Bi–N
stretching modes are found in the range 400–250 cmÀ1 (cf.
352 cmÀ1 in Me2BiN3).[13] The presence of more than one
azido ligand results in in-phase and out-of-phase coupling.
The most prominent structural feature in all three
bismuth(III) azides is the stereochemically active lone pair
localized according to NBO analysis[9] in a mainly s-type
atomic orbital. The s character of the lone pair (Table 1)
increases in the order 1 (87.6%) < 2 (88.5%) < 3 (99.5%). All
À
À
two shorter (Bi N10 2.273(2), Bi N7 2.291(2) ꢀ) and two
À
À
À
longer Bi N bonds (Bi N1 2.377(2), Bi N4 2.449(2) ꢀ;
[14]
À
ꢀrcov(Bi N) = 2.2 ꢀ),
in accord with the valence-shell
electron-pair repulsion model. Both N7 and N10 are part of
the trigonal plane with a N7-Bi-N10 angle of 86.95(9)8,
whereas N1 and N4 sit on the axis with an N1-Bi-N4 angle of
159.45(8)8. For all four azido ligands, the typical trans-bent
structure with N-N-N angles of about 176–1798 is observed.[11]
À
The Na Nb bonds are in the range 1.198(3)–1.216(3) ꢀ
À
(average 1.207 ꢀ), and an average Nb Ng bond length of
1.146 ꢀ is observed.[11]
A closer look at the interanionic interactions (Figure 3)
reveals an expansion of the bismuth coordination number by
the formation of nitrogen bridges involving the a-nitrogen
atoms (N1, N4) of two of the azido ligands. This bridging
mode results in the formation of four-membered rings
composed of two hexacoordinated bismuth and two bridging
Na atoms. These rings are interconnected, with adjacent rings
almost perpendicular to each other, thus forming infinite
À
of the Bi N bonds are highly polarized and can be considered
dominantly ionic. The degree of polarization increases in the
À
order 1 < 2 < 3. Although all three Bi N bonds in 1 are
localized to 19.7% at the bismuth atom, in bisphenoidal 2
there is a considerable difference between the nitrogen atoms
occupying the axial positions (8.5%) compared to the
À
zigzag chains in the crystal. The Bi Nbridge bonds amount to
’
À
À
Bi N1 2.684(2) and Bi N4’’ 2.717(2) ꢀ, which are consid-
Angew. Chem. Int. Ed. 2010, 49, 8017 –8020
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim