Homoleptic low-valent polyazides of group 14 elements

Article abstract of DOI:10.1039/c5cc00259a

First examples of coordinatively unsaturated, homoleptic azido complexes of low-valent group 14 elements are reported. A simple strategy uses low-valent precursors, ionic azide transfer reagents and bulky cations to obtain salt-like compounds containing E(N₃)₃^- of Ge(ii)/Sn(ii) which are fully characterised, including XRD. Remarkably, these compounds are kinetically stable at r.t. and isolable in sub-gram quantities. This journal is

Full text of DOI:10.1039/c5cc00259a

Volume 51 Number 35 1 May 2015 Pages 7407–7566
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Peter Portius et al.
Homoleptic low-valent polyazides of group 14 elements

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Homoleptic low-valent polyazides of group 14
elements†
Cite this: Chem. Commun., 2015,
51, 7435
Benjamin Peerless, Theo Keane, Anthony J. H. M. Meijer and Peter Portius*
Received 11th January 2015,
Accepted 19th February 2015
DOI: 10.1039/c5cc00259a
www.rsc.org/chemcomm
First examples of coordinatively unsaturated, homoleptic azido com-
plexes of low-valent group 14 elements are reported. A simple strategy
uses low-valent precursors, ionic azide transfer reagents and bulky
ꢀ
cations to obtain salt-like compounds containing E(N3)3 of Ge(II)/Sn(II)
which are fully characterised, including XRD. Remarkably, these com-
pounds are kinetically stable at r.t. and isolable in sub-gram quantities.
Binary azides are known for all elements in group 14 and exist as
2ꢀ
covalent E(N3)4 compounds (E = C, Si),2 as hyper-coordinate E(N3)6
complexes (Si–Pb)1 and as E(N3)2 compounds (Sn, Pb).5c However, no
low-valent, homoleptic group 14 complex has yet been reported.
All known binary p-block azides are highly endothermic primary
Scheme 1 Synthesis of azido(chloro) germanates (1ꢀ) and stannates (1ꢀ), E =
Ge, n = 1 (1, 3); E = Sn, n = 0 (2, 4); WCC = AsPh4 (a), PPh4 (b), N(PPh3)2 (PPN, c).
explosives most of which possess exceedingly high electrostatic synthesis and characterisation of the first low-valent and
and friction sensitivities and a propensity to release N2. As homoleptic Ge and Sn azides described in this paper.
covalent, N-rich compounds, their isolation is generally challen-
Compounds already containing WCC ions and low-valent germ-
ging and experimental characterisation is limited.5 In contrast, anium, AsPh4GeCl3, PPh4GeCl3,9 PPNGeCl3 (ref. 10) (1a–c), were
stability-inducing effects of hyper-coordination and of bulky, prepared in high yield from the GeCl2(diox) adduct18 and WCC
weakly coordinating counter ions (WCC)6 allow many salt-like chlorides19 (Scheme 1, route A).‡11 These colourless, moderately air
homoleptic polyazides to be synthesised in bulk and charac- sensitive crystalline trichlorogermanates react readily with THF
terised fully, including via X-ray crystallography and IR spectro- suspensions of NaN3. In situ IR spectra of the reaction (2b) show
scopy,5a owing to azide groups (N3) giving rise to intense bands bands due to asymmetric NNN stretches, nas(N3), typical for coordi-
nated N3 groups at nꢀmax/(cmꢀ1) = 2092 and 2058, which have grown
ꢀ
in the mid-IR region. It has been shown that azide anions (N3
)
are able to coordinate to low-valent centres in compounds such fully after a reaction time of 1 h. Exposure of the reaction solution to
as E(L)(N3) and E(L0)(N3)2, E = Ge, Sn.3,4 On the other hand, the fresh NaN3 results in no further spectral change. From the solution,
stability of low valent molecules, e.g. carbenes, silylenes, germy- a highly air sensitive, colourless solid (3b) was precipitated, the IR
lenes, stannylenes,7 increases by saturating the electron deficient spectrum of which exhibits the finger print of PPh4+ and the nas(N3)
centre with sterically demanding, electron donating groups, bands. The nas(N3) frequencies lie within the range of those reported
such as N-based C(NiPr)2(NiPr2) and HC{(CMe)(2,6-iPr2C6H3N)}2 previously for semi-covalent germanium(II) azides (2027–2077 cmꢀ1
ligands.7a,d This insight has led to tri- and tetracoordinate complexes Fig. 1), below those of Ge(IV) azides (Ge(N3)4, PPN2Ge(N3)6, 5c)1b and
,
ꢀ
1
bearing uni-, bi- and terdentate ligands, e.g. E(NHC)X2, Ge(NHC)2Cl+ above that of the N3 ion. While solution H, 13C and 31P NMR
and Ge{HB(Me2pz)3}Cl, E = Si, Ge; X = Cl-I, N3; NHC = N-heterocyclic spectra of 3b show signals of the WCC cations only, two peaks are
carbene.4a,8 Exploitation of these concepts has permitted the observed in the 14N NMR spectra at ꢀ263 and ꢀ207 ppm next to the
solvent (ꢀ136 ppm) with FWHM line widths of 552, 147 and 24 Hz,
respectively. These characteristics are typical for the Na and Ng nuclei
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HF, UK.
of coordinated N3 groups while the signal for Nb is obscured by
E-mail: p.portius@sheffield.ac.uk
solvent.5a
† Electronic supplementary information (ESI) available: Spectra, thermograms, full
Alternative routes to 3 and 4 use WCC azides as azide
crystallographic data and computational details. CCDC 1030031 and 1030032. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc00259a transfer reagents. GeCl2(diox) reacts directly also with (PPN)N3
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small (cf. GeH3N3, B1 kJ molꢀ1),13 fast interconversion involves
all significantly thermally populated rotamers above the minimum
energy conformation (Erel o 5.8 kJ molꢀ1). This process is likely to
result in averaged absorption bands weighted by the rotamer
population (rotamers may have more than one degenerate, absolute
spatial configuration, and inter-rotamer vibrational energy transfer is
unaccounted for). Taking account of the theoretical equilibrium
mole fractions, absorption intensities and scaled vibrational
frequencies,24 approximate average frequencies of the in-phase
and out-of-phase nas(N3) stretches and the qualitative intensity
ratios could be determined (Table S1, ESI†), which match those
ꢀ
observed (e.g. Ge(N3)3ꢀ, 2059, 2091 vs. 2060, 2093; Sn(N3)3
,
2051, 2080 vs. 2050, 2078 cmꢀ1). This approach leads to the
assignment of the observed bands of intermediates to
GeCl2(N3)ꢀ, GeCl(N3)2ꢀ, SnCl2(N3)ꢀ and SnCl(N3)2ꢀ. Calculations
using the Gauge-Independent Atomic Orbital method25 verify the
assignment of 14N NMR data (see ESI†).
ꢀ
ꢀ
Fig. 1 IR spectra of Ge(N3)3 (black), Sn(N3)3 (red) in THF; nas(N3) frequency
ranges of related azides, Ge(N3)4,12 HN3,10 Ge(N3)6
,
LnGe(II) azides L1 =
2ꢀ 1b
{Me2(tBuO)Si}2N;4c L2
=
nPr2ATI, Mes2DAP,4f,g (NHC)2;3 L3 = HB(R2pz)3,
(C5R5)Co{P(O)(OEt)2}3,4a,e,10 (2,6-iPr2C6H3)2C2H2N2CGe(N3)2 (ref. 8d) and
ꢀ 4e
Crystals of azido germanates were grown from THF–Et2O
(1 : 10) solutions at ꢀ18 1C (3b, needles) or by diffusion of Et2O
N3
,
are indicated in the top bar; see Table S1 (ESI†) for exact values.
into concentrated THF solutions (3c).§¶ According to single
+
in MeCN solution (Scheme 1, route B). Intriguingly, equimolar crystal X-ray diffraction studies, 3b consists of PPh4 and
reactant mixtures produce only one nas(N3) band (2078 cmꢀ1); Ge(N3)3 ions (Fig. 2). The shortest interionic Geꢁ ꢁ ꢁN and
ꢀ
increasing the stoichiometric ratio (1 : 2) results in two additional Nꢁ ꢁ ꢁN distances were found to be 4.13 and 5.07 Å, respectively
bands (2088, 2066 cmꢀ1), while at ratios of 1 : 9 and above only (Fig. 4, left), hence, covalent {[Ge(N3)3]ꢀ}ꢁ ꢁ ꢁ{[Ge(N3)3]ꢀ} inter-
ꢀ
bands at 2095, 2064 cmꢀ1 and that of N3 were detected. These actions are absent (Fig. 3). Germanium is coordinated by three,
observations are interpreted tentatively in terms of the formation of essentially linear N3 ligands and occupies the apical position in
mono, di- and triazido complexes.
a trigonal-pyramidal Ge[N]3 framework. The ligands are bound
Tin dichloride was subjected to a similar treatment as GeCl2(diox) in the fashion typical of covalent azides and adopt Ge–Na–Nb
using WCC(N3) and NaN3; however, complete Cl/N3 exchange angles between 1161 and 1211. All inter-ligand angles are close
requires a larger excess of azide transfer reagent. Similar observa- to 901 which indicates stereochemical inactivity of the lone
tions as with 3b were made, including the intermediate rise and electron pair at germanium.14
decay of a nas(N3) band (2064 cmꢀ1) and the ultimate rise of bands of
This structural feature has been found in the valence isoelectronic
the final product 4b (2081, 2050 cmꢀ1) in the expected region complexes of Ge2(m-pz*)3 GeCl3 (ref. 11f ) and pilocarpine-
+
ꢀ
2ꢀ
between Sn(N3)6 and charge-neutral Sn(II) monoazides (Table S1, trichlorogermanate hemihydrate.11g The Ge–Na bonds of 3b are
ESI†), and 14N resonances at ꢀ218.5 ppm (FWHM E 32 Hz) and shorter than those of tetracoordinate Ge(II) azides 2.088(6)–
ꢀ260.0 ppm (166 Hz). The 14N NMR signals of 3b and 4b, in 2.094(7) Å (Fig. 1), longer than those of the homoleptic Ge(IV)
particular those assigned to Na, are deshielded in comparison to azide PPN2Ge(N3)6 (5c, Table S1, ESI†) and rather within the
those of E(N3)6 dianions.1b,5a,17 3b and 4b are soluble in MeCN, range of previously investigated, tricoordinate Ge(II) azides
2ꢀ
THF and CH2Cl2.
+
The synthetic strategy was extended to AsPh4 and PPN+
counter ions affording compounds 3a,c and 4c (Scheme 1A)
which all have spectroscopic properties analogous to those of
3b and 4b described already. The combined analytical evidence,
including the absence of chlorine in 3b and the 119Sn NMR
signal of 4b (d = ꢀ220 ppm, see ESI†) point to the formation of
anionic complexes in compounds of the type (WCC)E(N3)3 as
the final products of Cl/N3 exchange.
Further insight into the nature of intermediates and pro-
ducts of the exchange reactions was obtaineꢀd from quantum
chemical calculations20 on the ECl(3ꢀn)(N3)n species, which
were performed at the B3LYP/cc-pVTZ level21 with effective core
ꢀ
potentials22 for Ge and Sn. Solvent (THF) was described using
Fig. 2 Thermal ellipsoid plot (50%) of Ge(N3)3 in the crystal of
PPh4Ge(N3)3 (3b). Bond lengths [Å] Ge1–N7 1.984(2), Ge1–N1 1.988(3),
Ge1–N4 2.011(3), N1–N2 1.213(3), N2–N3 1.148(3), N4–N5 1.209(3), N5–
PCM.23 The calculations found conformational isomerism
resulting in several minima for n = 1, 2, 3, that were close in
N6 1.142(3), N7–N8 1.206(3), N8–N9 1.140(3), angles [1] N7–Ge1–N1
energy. These conformers are related by rotation of ligands.
Since rotational barriers of sterically unhindered N3 groups are
93.59(10), N7–Ge1–N4 94.29(10), N1–Ge1–N4 91.05(11), N2–N1–Ge1
116.4(2), N5–N4–Ge1 118.7(2), N8–N7–Ge1 121.3(2).
7436 | Chem. Commun., 2015, 51, 7435--7438
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ꢀ
4b also contains a E(N3)3 moiety; however, the packing is at
variance with 3b, which allows Sn(N3)3ꢀ to interact via two long
2ꢀ
Eꢁ ꢁ ꢁNa bonds and form {Sn(N3)3}2
dimers (Fig. 3). The
interaction involves asymmetric m1,1-N3 bridges with short
(2.207(3) Å) and long (2.674(3) Å) Sn–Na bonds, the latter being
considerably shorter than the sum of the van der Waals radii
(3.72 Å).16 Weak intermolecular interactions have been
observed previously between neutral Sn(nPr2ATI)N3 complexes
(vide supra),4b where a slightly longer Snꢁ ꢁ ꢁNa bond (2.87 Å) was
found. The sum of bond angles involving the bridging Na
indicates planarity and effective sp2 hybridisation. As in the
crystal of 3b, the primary E(II)–Na bonds are significantly longer
(2.193(3)–2.262(3) Å) than those found in the homoleptic E(IV)
azide 6 (2.125 Å).15 The potential for dimerisation was studied
2ꢀ
by DFT using the geometry of {E(N3)2(m1,1-N3)}2 in crystalline
4b as a starting point. Optimisation results in separate anions
devoid of covalent interionic interactions in the case of
Fig. 3 Thermal ellipsoid plot (50%) of {Sn2(N3)6}2ꢀ in crystals of
PPh4Sn(N3)3 (4b). Bond lengths [Å] Sn–N1 2.262(3), Sn–N4 2.193(3), Sn–
N7 2.207(3), 2.674(3), N1–N2 1.203(4), N2–N3 1.148(5), N4–N5 1.200(5),
N5–N6 1.143(5), N7–N8 1.185 (5), 1.193(5), N8–N9 1.157(6), 1.168(6) Sni–
N6 3.567(4), angles [1] N1–Sn–N4 88.35(12), N4–Sn–N7 89.14(13), N1–Sn–
N7 88.59(12), N2–N1–Sn 123.6(3), N5–N4–Sn 119.4(2), N8–N7–Sn
128.2(9), 120.2(8), N7–Sn–N7 68.32(16), Sn–N7–Sn 111.68(18).
ꢀ
Ge(N3)3ꢀ, whereas a dimeric structure was found for Sn(N3)3
that resembles the molecular structure in the crystal. Estimates
of the basis set superposition error for the solution phase were
obtained from BSSE calculations26 in the gas phase. After BSSE
2ꢀ
correction, {Sn(N3)2(m1,1-N3)}2 was found to be at least 4 kcal
molꢀ1 less stable than two monomers, rendering the existence
of a dimer in solution highly unlikely.
According to differential scanning calorimetry measure-
ments (Fig. S11 and S12, ESI†), compound 3b decomposes in
two exothermic processes with (DH = ꢀ270 and ꢀ467 J gꢀ1).8
ex1
on
Remarkably, step 1 occurs at temperatures (T = 99 1C) that
are drastically below the decomposition onset of the related,
ex1
hypercoordinate azide 5c (T = 256 1C),1b whereas step 2 sets
on
in at T epx2 = 310 1C, which is nearly identical with the tempera-
ture found in 5c (312 1C).** Furthermore, the molar enthalpies
of step 2 (ꢀ251 vs. ꢀ482 kJ molꢀ1) scale approximately with the
complex charge; however, step 1 releases much less energy than
expected (145 kJ molꢀ1, 3b vs. 705 kJ molꢀ1, 5c). This phenom-
enon is still under investigation. Further experiments show that
heating of 3b at 1ꢀ50 1C produces PPh4N3, which suggests thꢀat
Fig. 4 Packing diagrams of PPh4Ge(N3)3 (3b, left) and PPh4Sn(N3)3 (4b,
right); H (bright grey), C (dark grey), N (light blue), P (orange), Ge and Sn
(turquoise).
Table 1 Bond lengths D/Å in the salt-like homoleptic azides of the type
the release of N3 initiates the decomposition of Ge(N3)3
.
(WCC)+n[E(N3)3n
]
nꢀ, E = Ge, Sn; n = 1, WCC = PPh4+; n = 2, WCC = N(PPh3)2
+
The tin homologue 4b melts at Ton = 115 1C and decays at
215 1C and 308 1C and thus behaves as expected relative to 6c.15
No sensitivity was noted during preparation and analysis
of compounds 3b and 4b on the stated reaction scale. The
material remains unchanged when struck by a hammer. Upon
lighting up, it burns rapidly with an orange flame leaving black
residues.
a
E, n E–Na
Na–Nb
Nb–Ng
DNNav
Ge, 1 1.984(3)–2.011(2) 1.206(4)–1.213(4) 1.140(4)–1.148(4) 6.6(5)b
Ge, 2 1.969(2)–1.980(3) 1.210(4)–1.214(3) 1.143(3)–1.151(3) 6.5(4)
c
d
c
Sn, 1 2.193(3)–2.262(3) 1.189(6)–1.203(4) 1.143(5)–1.163(6) 4.6(17)
Sn, 2 2.117(3)–2.134(2) 1.182(3)–1.213(3) 1.111(4)–1.148(3) 5.7(10)
e
ꢅ
ꢄ
ꢂ ꢀ
ꢁ
ꢀ
ꢁ ꢃ
P
P
1
1
n
i¼1
n
a
DNN ¼
d Na ꢀ Nb ꢀd Nb ꢀ Ng , s ¼
DNNiꢀ
i¼1
i
i
n
n ꢀ 1
In solution, Ge(N3)3ꢀ and Sn(N3)3ꢀ react with hydrazoic acid
leading to a precipitate with the IR nas(N3) absorptions char-
ꢆ i
2
0:5
b
DNNav
in parentheses.
s
{
s, error estimated by
ꢅ
ꢇ
0:5
ꢄ
ꢆ
2ꢀ
2ꢀ
2
P
si
N
n
acteristic for Ge(N3)6 and Sn(N3)6 complexes, respectively,
c
d
e
s ¼
.
This work. Ref. 1b. Ref. 15.
i¼1
(1.969–2.047(2) Å, Table 1). All other bond lengths and angles
are close to those of 5c (Table 1). The crystallographic structure
ꢀ
of Ge(N3)3 is consistent with one of the geometries predicted
by DFT (vide supra).
Single crystalline needles of 4b were obtained and investi-
gated with the methods used for 3b.¶ The asymmetric unit of
Scheme 2 Oxidation of the tri(azido) complexes 3b and 5b.
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and H. V. R. Dias, Inorg. Chem., 2000, 39, 4147; (g) A. E. Ayers,
which can be verified by comparison with spectra of the fully
characterized salts 5c, 6c (Scheme 2, Fig. S13 and S14, ESI†).
The first low-valent homoleptic azido complexes of group 14
have been synthesized and fully characterised. The preparative
approach to salt-like compounds containing these complexes has
been demonstrated on a 0.2–0.7 g scale for a range of weakly
¨
T. M. Klapotke and H. V. R. Dias, Inorg. Chem., 2001, 40, 1000.
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639, 1053, ref. cited; (c) T. Mu¨ller, F. Karau, W. Schnick and F. Kraus,
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coordinating cations and involves chloro(azido) species, EClx(N3)yꢀ
.
2ꢀ
Unlike their hypercoordinate E(N3)6 analogues and despite the
presence of innocent cations, the new class of compounds is highly
reactive, exhibiting low thermal stability and a propensity to
oxidation. Crystallographic analysis revealed that in the solid state,
ꢀ
E(N3)3 complexes of group 14 may dimerise via azido ligand
bridges. Neither of the low-valent coordination centres exhibits a
stereochemically active lone electron pair. DFT calculations cor-
rectly predict the dimerisation and suggest furthermore that the
dimers are unstable in polar solvents.
¨
¨
(d) B. Lyhs, D. Blaser, C. Wolper, S. Schulz, R. Haack and G. Jansen,
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9 U. M. Tripathi, G. L. Wegner, A. Schier, A. Jockisch and H. Schmidbaur,
Z. Naturforsch., B: J. Chem. Sci., 1998, 53, 939.
The authors thank the EPSRC (EP/E054978/1), the University
¨
of Sheffield and Humboldt-Universitat zu Berlin for support
¨
10 P. Portius, PhD thesis, Humboldt-Universitat zu Berlin Weißensee
Verlag, Berlin, 2002, ISBN 3-934479-63-4.
11 (a) X. Tian, T. Pape and N. W. Mitzel, Z. Naturforsch., B: J. Chem. Sci.,
2004, 59, 1524; (b) S. Nogai, A. Schriewer and H. Schmidbaur, Dalton
and Prof. A. C. Filippou for advice.
¨
Trans., 2003, 3165; (c) G. Kociok-Kohn, J. G. Winter and
Notes and references
A. C. Filippou, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
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22, 2541.
‡ GeCl3ꢀ (ref. 9 and 11) and SnCl3ꢀ salts (ref. 11c and e–g) with various
organic counter ions have been reported previously.
§ All attempts to crystallize compound 3a have been futile.
¶ Crystallographic data: 3b, CCDC 1030032, C24H20GeN9P, 538.07 g molꢀ1
,
%
P1, a = 7.7712(2) Å, b = 11.4711(4) Å, c = 14.2003(4) Å, a = 93.278 (2)1, b =
99.357(2)1, g = 100.865(2)1, Z = 2, V = 1221.59(6) Å3, Dc = 1.463 g cmꢀ3, T =
120(2) K, F(000) = 548, R1 = 0.0376 (316 param.), wR2 = 0.0807, GOOF = 1.090.
3c, P21, a = 10.7640(11) Å, b = 12.732(2) Å, c = 25.713(3) Å, a = g = 901, b ꢀ=
100.682(12)1, Dc = 1.414 g cmꢀ3, T = 180(2) K, extensive disorder of Ge(N3)3
part (see ESI† and ref. 10). 4b, CCDC 1030031, C24H20N9PSn, M =
12 J. E. Drake and R. T. Hemmings, Can. J. Chem., 1973, 51, 302.
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15 R. Campbell, B. Peerless and P. Portius, unpublished results.
16 A. Bondi, J. Phys. Chem., 1964, 68, 441.
584.15 g molꢀ1, P1, a = 10.7560(6) Å, b = 11.0605(6) Å, c = 12.3540(7) Å,
%
a = 91.668(4)1, b = 108.414(4)1, g = 116.545(3)1, Z = 2, V = 1222.11(12) Å3, Dc =
1.587 g cmꢀ3, T = 100(2) K, F(000) = 584, R1 = 0.0521 (334 param.), wR2 =
0.1020, GOOF = 1.053.
17 (a) W. Beck, W. Becker, K. F. Chew, W. Derbyshire, N. Logan,
D. M. Revitt and D. B. Sowerby, Dalton Trans., 1972, 245;
8 Estimated error approx. ꢂ10%.
** This step is assigned tentatively to the thermolysis of PPN(N3) and
¨
(b) W. Beck, W. P. Fehlhammer, P. Pollmann, E. Schuierer and
PPh4 (N3) since enthalpies and onset temperatures are comparable with
those of genuine samples of these salts: PPh4N3, mp = 250 1C, T
291 1C, ref. 15.
ex
on
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