Tetrazastannoles versus distannadiazanes-a question of the tin(ii) source

Article abstract of DOI:10.1039/C8DT04295K

Various tin(ii) compounds such as Mes*₂Sn (Mes* = 2,4,6-tri-tert-butylphenyl), Sn[N(SiMe₃)₂]₂ and TerSnCl (Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl) could be readily oxidised by organic azides to release N₂, forming nitrogen-tin compounds. Depending on the used Sn(ii) compound, the reactions with two equivalents of azide led to the formation of tetrazastannoles (R₂N₄SnR′₂) or cyclo-distannadiazanes [R₂Sn(μ-NR′)]₂. The reactivity was independent of the electronic situation of the organic azide. Additionally, Mes*₂Sn formed an amido-azido compound of the type R(R′)Sn(N(SiMe₃)₂)N₃ in the presence of Me₃SiN₃. Presumably, the corresponding tetrazastannole was formed in the first step followed by a ring opening reaction. The same holds true for the reaction of Sn[N(SiMe₃)₂]₂ with Me₃SiN₃ while TerSnCl showed no reaction in the presence of Me₃SiN₃, even after prolonged heating.

Full text of DOI:10.1039/C8DT04295K

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Tetrazastannoles versus distannadiazanes – a
question of the tin(II) source†
Cite this: Dalton Trans., 2019, 48, 125
Axel Schulz,
*
a,b Max Thomas a and Alexander Villingera
Various tin(II) compounds such as Mes*2Sn (Mes* = 2,4,6-tri-tert-butylphenyl), Sn[N(SiMe3)2]2 and TerSnCl
(Ter = 2,6-bis(2,4,6-trimethylphenyl)phenyl) could be readily oxidised by organic azides to release N2,
forming nitrogen-tin compounds. Depending on the used Sn(II) compound, the reactions with two
equivalents of azide led to the formation of tetrazastannoles (R2N4SnR’2) or cyclo-distannadiazanes
[R2Sn(μ-NR’)]2. The reactivity was independent of the electronic situation of the organic azide.
Additionally, Mes*2Sn formed an amido-azido compound of the type R(R’)Sn(N(SiMe3)2)N3 in the presence
of Me3SiN3. Presumably, the corresponding tetrazastannole was formed in the first step followed by a ring
opening reaction. The same holds true for the reaction of Sn[N(SiMe3)2]2 with Me3SiN3 while TerSnCl
showed no reaction in the presence of Me3SiN3, even after prolonged heating.
Received 26th October 2018,
Accepted 16th November 2018
DOI: 10.1039/c8dt04295k
rsc.li/dalton
an element such as tin in a formal low oxidation state by
azides. In fact, the first compound with a stable tetrazabuta-
Introduction
In recent years we have been interested in pnictogen-rich ring diene fragment was observed utilizing such
a reaction
systems such as neutral tetrazapnictoles (R-N4Pn; Pn = P, As, pathway.15 Today, many tetrazenides and tetrazabutadienes
Sb, and Bi), of which the corresponding phosphorus,1 are known, synthesised by the oxidation route as depicted in
arsenic2,3 and antimony4 derivatives could be successively pre- Scheme 2 (pathway A).15–17
pared. So far, however, the related bismuth compound is
Indeed, the oxidation of tin(II) compounds with organic
unknown, although several synthesis attempts have been azides is a useful technique to generate SnN4 compounds. For
made. This includes experiments for generating a bismadiazo- example, Weidenbruch’s group was able to synthesize a tetra-
nium salt (R-NBi+) by Lewis acid induced elimination of zastannole in the reaction of Mes*2Sn with two equivalents of
Me3SiCl5–7 or Me3SnCl8 and attempted [3 + 2] cycloadditions 1-azido-3,5-bis(trifluoromethyl)benzene (1b).18 An interesting
with azides (Scheme 1). But all these attempts proved unfruit- product of this oxidation reaction was the isolation of a stan-
ful and a new strategy had to be considered. For this reason, naimine from the reaction of Sn[N(SiMe3)2]2 with one equi-
we attempted [4 + 1] cycloadditions rather than [3 + 2] cyclo-
additions, thus avoiding the in situ generation of highly reac-
tive and sensitive bismuthenium or bismadiazonium cations.
For this purpose, a suitable N4 transfer reagent had to be
identified. Tetrazastannoles, consisting of a SnN4 heterocycle,
appeared to be ideal since SnN compounds are known to be
mild transfer reagents of nitrogen-containing groups.8–14
Therefore, we were interested in the synthesis of tetrazastan-
noles (tetrazenides of tin) as starting materials for the gene-
ration of BiN4 ring system. A convenient method for the syn-
thesis of tetrazenides or tetrazabutadienes is the oxidation of
aInstitute of Chemistry, 18059 Rostock, Albert-Einstein-Str. 3a, Germany.
E-mail: axel.schulz@uni-rostock.de
bLeibniz-Institut für Katalyse e., 18059 Rostock, Albert-Einstein-Straße 29a, Germany
†Electronic supplementary information (ESI) available: X-ray data, synthesis of
all starting materials and discussed tin species along with all spectra and
analytical details. CCDC 1872841–1872847. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c8dt04295k
Scheme 1 Previously attempted syntheses of tetrazabismutholes.
Dalton Trans., 2019, 48, 125–132 | 125
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Because it was impossible to isolate pure Mes*2Sn, the
crude product was used in the further reactions with azides.
Adding
a
solution of 1-azido-4-dimethylaminobenzen
(p-(CH3)2N–C6H4–N3, 1a) in toluene to a Mes*2Sn solution in
the same solvent resulted in a moderate gas evolution. In con-
trast to the Weidenbruch reaction of 1-azido-3,5-di(trifluoro-
methyl)benzene (m,m-(CF3)2–C6H3–N3, 1b) with Mes*2Sn,
a
four-membered ring system – a cyclo-distannadiazan
(2, Fig. 1) – was formed in small yields amongst other un-
identified products. However, one of the Mes* groups attached
to the tin was isomerized as discussed before in the synthesis
of Mes*2Sn (Scheme 2). As we started from a mixture Mes*2Sn/
Mes*-Sn-Mes′, it could not be determined whether the isomeri-
zation occurred due to the reaction with the azide. Obviously,
in the reaction mixture of Mes*2Sn (or Mes*SnMes′) and 1a,
the [2 + 2] cycloaddition (dimerization) of the in situ formed
stannaimine (Scheme 2, pathway B) is preferred over the [3 + 2]
cycloaddition with another equivalent of azide.
Besides aryl azides, we were also interested in the reaction
of Mes*2Sn with readily available Me3SiN3. In this case, also
isomerization of one of the Mes* scaffolds was observed but
neither a tetrazastannole nor a cyclo-distannadiazane could be
identified. X-ray diffraction of a single crystal obtained from
this reaction revealed the formation of the azido-disilylamido
compound 3 (Fig. 2). It can be assumed that the formation of
3 occurred via the desired tetrazastannole. It is known that
Me3Si moieties can easily migrate. A similar behaviour was
already observed for zirconium,23 thorium24 and tin com-
pounds.20 In all these cases a ring opening mechanism was
proposed leading to the formation of open-chain like trans-
bent azido ligands and in our case to 3 after migration of one
Me3Si group.
Scheme 2 Reaction of tin(II) compounds with organic azides (pathway
A: +R-N3; pathway B: dimerization) yielding nitrogen–tin(IV) species.
valent of DippN3 (Dipp = 2,6-diisopropylphenyl), a compound
with a N–Sn double bond that is stable towards dimerization
in the solid state.19 In the event that two equivalents of the
less sterically encumbered 1-azido-2,6-diethylbenzene were
used, the reaction led to the respective tetrazastannole. Based
on the oxidation of Lappert’s stannylene (Sn[N(SiMe3)2]2) with
different organic azides, a whole series of tetrazastannoles was
already synthesised.20 The observed reactivity hinted at a two-
step reaction: in the first step a stannaimine was formed under
N2 evolution, which reacted in the second step with a second
equivalent azide in a [3 + 2]-cycloaddition to give a tetrazastan-
nole (Scheme 2, pathway A). Or, if no second equivalent of azide
was available for the cycloaddition, the stannaimine dimerized
to a cyclo-distannadiazane (Scheme 2, pathway B).20 The compe-
tition between reaction pathway A and B was already discussed
by Wiberg et al.21 and the group of Grützmacher.22
Sn[N(SiMe3)2]2. Since the synthesis of Mes*2Sn was rather
hindered due to the formation of side products that could not
be completely removed, another tin(II) compound,
Sn[N(SiMe3)2]2, was utilized. There are already some examples
in the literature of tetrazastannoles synthesized from Lappert’s
Results and discussion
Synthesis
Mes*2Sn. In a first series of experiments, Weidenbruch’s
stannylen Mes*2Sn was used as tin(II) source. The synthesis of
Mes*2Sn, however, turned out to be more complicated than
expected. At first, Mes*Br was transferred into Mes*Li in a
metal–halide-exchange reaction with n-BuLi followed by a reac-
tion with Sn[N(SiMe3)2]2. The two main difficulties of this
synthesis were to remove LiN(SiMe3)2 and the occurrence of
the isomerization product Mes*SnMes′ (Scheme 3).
Fig. 1 Molecular structure of 2. H atoms omitted for clarity. Selected
bond length (Å) and angles (°): Sn1–N1a 2.111(5), Sn1–N1a’ 2.013(4),
Sn1–C9 2.199(2), Sn1–C27 2.173(2), N1a–C1a 1.404(3), N1a’–Sn1–N1a
80.0(2), N1a–Sn1–C9 118.1(2), N1a–Sn1–C27 105.6(2), C27–Sn1–C9
117.88(8), Sn1’–N1a–Sn1 100.0(2), C1a–N1a–Sn1 122.4(2), C1a–N1a–
Sn1’ 135.5(2), C2a–C1a–N1a–Sn1–122.4(4).
Scheme 3 Synthesis of Mes*2Sn along with Mes*-Sn-Mes’ as side
products.
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Fig. 2 Molecular structure of 3. H atoms omitted for clarity. Selected
bond length (Å) and angles (°): Sn1–N1 2.068(2), Sn1–N2 2.101(2), Sn1–
C1 2.167(2), Sn1–C36 2.169(2), N2–N3 1.198(3), N3–N4 1.147(3), N1–Si1
1.749(2), N1–Si2 1.771(2), N1–Sn1–N2 98.51(7), N1–Sn1–C1 105.88(7),
N2–Sn1–C1 106.56(7), N1–Sn1–C36 118.92(7), C1–Sn1–C36 118.23(7),
Si1–N1–Sn1 123.77(9), Si1–N1–Si2 116.95(9), Si2–N1–Sn1 117.59(8), N2–
N3–N4 175.5(3), N3–N2–Sn1 122.3(2).
Fig. 4 Molecular structure of 5a and 5b. H atoms omitted for clarity.
Selected bond length (Å) and angles (°), 5a: Sn1–N1 2.032(2), Sn2–N2
2.039(2), Sn1–C17 2.138(3), Sn1–Cl1 2.3499(8), Sn2–N1 2.031(2), Sn2–
N2 2.035(2), Sn2–C41 2.153(3), Sn2–Cl2 2.3591(8); N1–Sn1–N2 81.61(9),
N2–Sn1–C17 119.2(1), C17–Sn1–Cl1 107.96(8), N1–Sn2–N2 81.74(9),
N2–Sn2–C41 125.7(1), C41–Sn2–Cl2 113.46(8), Sn2–N1–Sn1 97.9(1). 5b:
Sn1–N1 2.042(2), Sn1–N2 2.049(2), Sn1–C1 2.145(3), Sn1–Cl1 2.3471(7),
Sn2–N1 2.043(2), Sn2–N2 2.046(2), Sn2–C25 2.128(3), Sn2–Cl2
2.3354(3), N1–C49 1.391(3), N2–C57 1.395(3); N1–Sn1–N2 81.10(9),
N1–Sn1–C1 121.88(9), N2–Sn1–C1 131.6(1), N1–Sn1–C1 102.19(6),
N2–Sn1–Cl1 100.19(6), C1–Sn1–Cl1 113.29(8), N1–Sn2–N2 81.14(8),
N1–Sn2–C25 121.59(9), N2–Sn2–C25 117.56(9), N1–Sn2–Cl2 109.82(7),
N2–Sn2–Cl2 111.33(6), C25–Sn2–Cl2 111.91(7), C49–N1–Sn1 130.2(2),
C49–N2–Sn2 130.7(2), Sn1–N1–Sn2 98.41(9), C57–N2–Sn2 128.9(2),
C57–N2–Sn1 130.7(2), Sn2–N2–Sn1 98.10(9).
stannylene.19,20 As shown by Meller et al., the synthesis of tet-
razastannoles can be started from relatively small phenyl
azides and the readily available Sn[N(SiMe3)2]2. With regard to
the results with Mes*2Sn, it was interesting to see if the elec-
tronic situation in the azide has an influence on the product
formed. For this purpose, Sn[N(SiMe3)2]2 was reacted with
both the electron rich azide 1a (formation of 4a, Fig. 3) as well
with the electron poor azide 1b (formation of 4b, Fig. 4). In
both case the corresponding tetrazastannole was formed
(Scheme 4) and could be isolated in reasonable yields (4a 60%,
4b 45%) as pure substances.
In another experiment, the aryl azide 1-azido-4-nitro-
benzene (p-O2N–C6H4–N3, 1c) was also reacted with Sn[N
(SiMe3)2]2, however, only decomposition was observed (no for-
mation of 4c). Hence, it can be concluded that the nitro group
is not stable under the applied reaction conditions. In the reac-
tion of Sn[N(SiMe3)2]2 with an excess of Me3SiN3, an azido-tri-
bis(trimethylsilyl)amidostannane (4d) was obtained in almost
quantitative yield (Scheme 5), similar to the reaction of
Mes*2Sn with Me3Si-N3 (Fig. 2). Presumably, in the first step a
stannaimine is formed upon release of molecular nitrogen and
Scheme 4 Reaction of Sn[N(SiMe3)2]2 with 1a and 1b.
Scheme 5 Reaction of Sn[N(SiMe3)2]2 with an excess of Me3Si-N3 yielding
azido-tri-bis(trimethylsilyl)amidostannane (4d) in a formal 2-step synthesis.
then a formal oxidative addition of a second equiv. of Me3SiN3
along with a 1.4 shift of the Me3Si+ ion occurs as shown in
Scheme 5. Amido-azido-stannane 4d was already reported by
Fig. 3 Molecular structure of 4a and 4b in the crystal. H atoms omitted
for clarity. Selected bond length (Å) and angles (°), 4a: N1–Sn1 2.044(2),
N4–Sn1 2.045(2), N7–Sn1 2.036(2), N8–Sn1 2.028(2), N1–N2 1.375(3), Ruzicka et al.20 Unfortunately, the tetrazastannoles 4a and 4b
N2–N3 1.272(3), N3–N4 1.384(3), N8–Sn1–N7 113.2(1), N8–Sn1–N1
126.6(1), N7–Sn1–N1 105.0(1), N8–Sn1–N4 103.8(1), N7–Sn1–N4
129.19(9), N1–Sn1–N4 76.66(9). 4b: N3–Sn1 2.003(1), N1–Sn1 2.062(1),
N1–N2 1.381(2), N2–N2’ 1.264(2), N3–Sn1–N3’ 123.76(6), N3–Sn1–N1
are so well kinetically protected by the N(SiMe3)2 groups that it
was impossible to generate a corresponding BiN4 heterocycle
by using these tetrazastannoles as R2N4 transfer reagent.
TerSnCl. Other interesting tin(II) compounds related to the
synthesis of tetrazastannoles are Power’s TerSnCl and
116.99(5), N3’–Sn1–N1 106.91(4), N1–Sn1–N1’ 75.65(6), N2–N1–C1
114.4(1), N2–N1–Sn1 114.88(8), C1–N1–Sn1 128.81(8).
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hyperconjugation effects.20,27–30 As expected for a covalently
bonded azide, it exhibits a trans-bent arrangement (N2–N3–N4
175.5(3)°, N3–N2–Sn1 122.3(2)°). As in compound 2, there was
an isomerization of one of the Mes* groups leading to a tin
bond with one carbon atom of a methylene group (Sn1–C36
2.169(2) Å) besides a bond between the tin and an aryl carbon
bond (Sn1–C1 2.167(2) Å). These two different C–Sn bond
lengths are equal within the standard deviation.
4a and 4b could be crystallised from toluene (Fig. 3). While
Scheme 6 Reaction of synthesis of 5a and 5b.
ˉ
4a crystallises in the triclinic space group P1 with a complete
molecule in the asymmetric unit, 4b crystallises in the mono-
Ter2Sn.25 Prior to the synthesis, we modelled a hypothetical clinic space group C2/c with only half a molecule in the asym-
tetrazastannole by quantum chemical methods starting from metric unit. All N–Sn distances are similar for 4a and 4b. (4a:
Ter2Sn, but the terphenyl substituent proved to be too bulky to N1–Sn1 2.044(2) Å, N4–Sn1 2.045(2) Å, N7–Sn1 2.036(2) Å, N8–
allow the formation of a terphenyl-substituted tetrazastan- Sn1 2.028(2) Å; 4b: N3–Sn1 2.003(1) Å, N1–Sn1 2.062(1) Å) and
noles. For this reason, all experiments were performed only in the expected range for polarised N–Sn bonds (∑rcov(N–Sn) =
with TerSnCl as starting material. In contrast to the reactions 2.11 Å).26 The structure parameters of the N4 unit can be used
with Sn[N(SiMe3)2]2, four-membered ring systems with an as indicators to determine whether a tetrazenide or a tetraza-
Sn2N2 skeleton could be isolated in reactions with the aryl butadiene is present in the solid state.16,17,24,31–37,38,39 It
azides 1a and 1b, respectively (Scheme 6 and Fig. 4). The should be noted that there are other spectroscopic methods to
corresponding tetrazastanoles were not observed.
distinguish between these forms,40 but in some cases no
To complete the studies on TerSnCl, at first the reactivity to unambiguous determination is possible. The most prominent
Me3SiN3 was also investigated at ambient temperatures. Since structural feature is the planar 5-membered ring with a formal
no reaction was observed after 24 hours, we performed tem- double bond in the backbone and two formal single bonds, as
perature-variable NMR studies. For this reason, TerSnCl was shown by the comparison of the experimentally determined
added to an NMR tube and dissolved in C6D6. After addition N–N bond lengths with the sum of the covalent radii
1
of Me3SiN3, the reaction was monitored by H and 29Si INEPT (4a: N1–N2 1.375(3) Å, N2–N3 1.272(3) Å, N3–N4 1.384(3) Å; 4b:
NMR spectroscopy. Even after heating the reaction mixture at N1–N2 1.381(2) Å, N2–N2′ 1.264(2) Å, cf. ∑rcov(N–N) = 1.42 Å;
60° C for 6 hours, no reaction between TerSnCl and Me3SiN3 ∑rcov(NvN) = 1.20 Å).26 In this regard, the compounds can
was observed.
be discussed as tetrazastannoles with a
formal Sn(IV)
(Scheme 7, A) rather than tetrazabutadiene complexes with a
formal Sn(II) center (Scheme 7, B, vide infra).
Structure and bonding
ˉ
Compound 2 crystallized in the triclinic space group P1 as
centrosymmetric dimer. The centre of inversion is in the
middle of the planar Sn2N2 scaffold (Fig. 1). There is a shorter
(Sn1–N1a′ 2.013(4) Å) and one slightly longer (Sn1–N1a
2.111(5) Å) N–Sn distance within the Sn2N2 ring system, but
both distances are in the expected range for a (polarized)
single bond (∑rcov(N–Sn) = 2.11 Å).26 As shown in Fig. 1, the
C–Sn distance to the Mes* moiety (Sn1–C9 2.199(2) Å) as well
as to the methylene carbon atom of the Mes′ moiety (Sn1–C27
2.173(2) Å) are in the typical range of C–Sn single bonds
(∑rcov(C–Sn) = 2.15 Å).26 Interestingly, the C–Sn bond to the
methylene carbon atom (C27) is slightly shorter than to the
aryl carbon (C1) hinting at a larger steric strain of the Mes*
compared to the Mes′ moiety. Both tin atoms exhibit a dis-
torted tetrahedral environment with rather short N–Sn–N
angles (80.0(2)°) and significantly larger Sn–N–Sn′ angles
(100.0(2)°).
The amido–azido compound 3 also crystallized in the space
ˉ
group P1 with two molecules per unit cell (Fig. 2). Although
both N–Sn bonds are in the expected range for polarized single
bonds, the distance to the amido ligand (Sn1–N1 2.068(2) Å) is
slightly shorter than to the azido ligand (Sn1–N2 2.101(2) Å).
The amido nitrogen N1 is nearly trigonal-planar coordinated
Scheme 7 Lewis representations of tetrazastannole(IV) (A) and a tetra-
zabutadiene tin(II) complex (B). Not all ionic representations are shown.
Only lone pairs, which are part of the π bonding system within the
(∑∡(N1) = 358°), which is typical for (Me3Si)2N moieties due to heterocycle are depicted.
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The cyclo-distannadazanes 5a and 5b crystallized in mono-
clinic space groups (Fig. 4). But while 5a crystallized in P21/n
with one independent molecule in the asymmetric unit, 5b
crystallized in C2/c with two independent molecules. Both
independent molecules show the same structural parameters
within the standard deviation. Therefore, only one of the two
independent molecules is considered in structure discussions.
In the four-membered ring systems of 5a and 5b, the N–Sn dis-
tances are all equal within the standard deviation (5a: Sn1–N1
2.032(2), Sn2–N2 2.039(2), Sn2–N1 2.031(2), Sn2–N2 2.035(2);
5b: Sn1–N1 2.042(2), Sn1–N2 2.049(2), Sn2–N1 2.043(2), and
Sn2–N2 2.046(2) Å). Intriguingly, there is no inversion centre
in the middle of the Sn2N2 scaffold as in the closely related
Mes* compound 3. This is because the two terphenyl groups
are twisted by approximately 90° in both compounds.
In order to better understand the structure and bonding of
compounds 2–5 and to determine whether they are formal
Sn(II) or Sn(IV) species, in particular compounds 4a and 4b,
density functional calculations along with NBO analyses
(NBO = natural bond analysis)41–43 were performed at the
PBE1PBE/def2svp utilizing the X-ray structural data (single
point computations). A summary of selected NBO charges are
listed in Table 1. For all considered tin species, NBO analysis
finds a Lewis representation with tetravalent tin(IV) atoms as
the most favoured Lewis formula. All Sn–N bonds are highly
polar and usually only localized with ca. 10–15% at the tin
atoms. Therefore, a rather large partial charge is found for the
tin atoms ranging between +2.0 and +2.3e. In accord with our
structural data for 4a and 4b (vide supra), NBO analysis finds
as best Lewis representation a structure with four highly polar
Sn–N single bonds and one N–N double bond as depicted in
formula A (Scheme 7), clearly indicating the preference of a
Fig. 5 Two-dimensional cross section through the Sn–N1–N2 plane of
the electron localization function (ELF). Left: 4a and right: 4b.
Conclusions
In the oxidation reaction of tin(II) compounds with various
azides, the nature of the product formed, strongly depends on
the tin compound used as starting material. With Mes*2Sn
several difficulties arose, starting with the isolation of the pure
compound and the problem of the isomerization reactions of
the Mes* units. The reaction of Sn[N(SiMe3)2]2 with two
equivalents of aryl azides led, on the one hand, to the for-
mation of the desired five-membered SnN4 ring system, a tetra-
zastanole. On the other hand, the same reaction with TerSnCl
as the tin(II) source led to the formation of four-membered
ring systems, cyclo-distannadiazanes, which contain a Sn2N2
skeleton. Regardless of whether the azide was added to the tin
compound (local excess of TerSnCl) or whether the tin com-
pound was given to the azide (local excess of azide), the for-
mation of cyclo-distannadiazanes was always observed. The
use of only one equivalent of azide in the reaction with
TerSnCl is a convenient method for the synthesis of com-
pounds of the type [TerSn(Cl)NR]2 (R = aryl). Since the by-
product is N2, the work-up of the reaction mixture is easily
achieved. Furthermore, there is still a chlorine atom present at
the tin atom allowing further functionalisation. The bulky ter-
phenyl group at tin is also helpful in the synthesis of highly
sensitive compounds. In addition, the electronic situation
within the Sn2N2 ring can be adjusted by using differently sub-
stituted aryl groups. Tetrazastannoles could be easily syn-
thesised from Sn[N(SiMe3)2]2 and their use as N4 transfer
reagents has yet to be demonstrated. In particular, we want to
test the use of tetrazastannoles in the synthesis of PnN4
species (Pn = P, As, Sb, Bi) in the future. It could be shown that
Me3SiN3 is not suitable for the synthesis of tetrazastannoles
because the Me3Si group can migrate easily. This behavior
causes a ring-opening reaction and the formation of amido-
azido compounds. Although all attempts to generate a BiN4
heterocycle by using the discussed tetrazastannoles as transfer
reagent failed, because of the high kinetic stability of the tetra-
zastannoles, many new interesting aspects of tin-nitrogen
chemistry are reported in this work (see above).
tetrazastannole over
a
tetrazabutadiene complex (B).
Additionally, a closer look at the delocalisation effects within
the SnN4 5-ring (hyperconjugation), clearly indicates resonance
between structures A, C, and D that describes delocalisation of
the π bonds within the N4 frangment (e.g. 4a: nN1 → π*N2–N3
and nN4 → π*N2–N3, E = 50.6 kcal mol−1). Moreover, also
ionic resonance (E, F and G) plays an important role, in
accord with the electron localization function (Fig. 5)
which displays a formal Sn4+ center and a [RNNNNR]2– and
two [NR2]– ligands.
Table 1 Selected NBO data of all tin compounds, partial charges in e,
charge of a specific molecular fragment (Qfrag) in ea
q(Sn)
q(N1)
q(N2)
q(N3)
q(N4)
Qfrag
2
3
4a
4b
5a
5b
2.10
2.08
2.28
2.31
2.03
2.04
−1.32
−0.70
−0.71
−0.69
−1.28
−1.27
−1.32
0.24
−0.06
−0.04
−1.27
−1.26
—
—
—
−0.18
−0.06
−0.04
—
−1.75
−0.72
−0.69
—
−0.65b
−1.16c
−1.26c
—
Experimental
—
—
—
a Only N atoms of the heterocycle are listed. b Fragment = azido ligand.
c Fragment = RNNNNR.
All manipulations were carried out under oxygen- and moist-
ure-free conditions under argon using standard Schlenk or
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drybox techniques. Full experimental (including the synthesis (q, 2J{13C–19F} = 33 Hz, CCF3), 148.5 (s, arom. i-C). 19F{1H}
and full characterization of all starting materials, spectra etc.) NMR (298.2 K, C6D6, 282.40 MHz): −62.4 (s, CF3). 29Si-INEPT
and computational data are available in the ESI.† ‡
NMR (298.2 K, C6D6, 59.63 MHz): 10.8 (m, Si(CH3)3). 119Sn{1H}
NMR (298.2 K, C6D6, 111.92 MHz): −227.5 (s, N4SnN2). IR
(ATR-IR, 32 scans, cm−1): 3104 (w), 2966 (w), 1764 (w), 1620
(w), 1608 (w), 1461 (w), 1420 (w), 1377 (m), 1274 (s), 1253 (s),
1181 (m), 1165 (s), 1140 (m), 1125 (s), 1043 (w), 975 (s), 860 (s),
839 (s), 796 (s), 752 (m), 719 (s), 701 (s), 680 (s), 620 (m),
598 (w), 573 (w), 526 (w), 497 (w), 478 (w), 441 (w), 429 (m).
Raman (laser: 633 nm, accumulation time: 10 s, 20
scans, cm−1): 3105 (1), 3059 (1), 2976 (1), 2971 (1), 2911 (2),
2645 (1), 2595 (1), 1623 (2), 1611 (3), 1474 (1), 1423 (5),
1385 (4), 1323 (1), 1252 (2), 1184 (1), 1169 (1), 1144 (1),
1134 (1), 1111 (1), 1093 (1), 1042 (2), 999 (10), 896 (1), 875 (1),
845 (1), 800 (1), 754 (1), 729 (1), 682 (4), 672 (1), 638 (2),
619 (1), 597 (1), 526 (1), 497 (1), 428 (1), 391 (1), 362 (1),
354 (1), 342 (2), 321 (1), 286 (1), 254 (1), 225 (1), 202 (1).
Synthesis of 4a
Sn[N(SiMe3)2]2 (1.758 g, 4.00 mmol) was dissolved in toluene
(10 mL) and cooled to −40 °C. A solution of 1a (1.298 g,
8.00 mmol) in toluene (10 mL) was added dropwise, which
resulted in an immediate gas evolution. The reaction mixture
was stirred in the cold for 45 min and then warmed to room
temperature. After stirring the reaction mixture at this temp-
erature for another 45 min, the gas evolution ceased and the
solution was filtered (F4). The filtrate was concentrated to
induce crystallization. Yield: 1.741 g (2.37 mmol, 59.1%) of 4a
as yellow crystalline needles in three crops.
Mp.: 104 °C (decomp.). CHN calc (found) in %: 45.70
(45.362), H 7.67 (6.564), N 15.23 (15.324). 1H NMR (298.2 K,
benzene-d6, 300.13 MHz): 0.30 (s, 36 H, 2J{1H–29Si} = 6.2 Hz, Si
(CH3)3), 2.52 (s, 12 H, N(CH3)2), 6.74 (m, 4 H, arom. CH), 7.68
(m, 4 H, arom. CH). 13C{1H} NMR (299.0 K, benzene-d6,
75.48 MHz): 5.8 (s, Si(CH3)3), 41.2 (s, N(CH3)2), 114.1 (s, arom.
CH), 122.9 (s, arom. CH), 138.6 (s, arom C), 148.3 (s, arom. C).
29Si-INEPT NMR (298.2 K, benzene-d6, 59.63 MHz): 8.5 (dec,
2J{1H–29Si} = 6.5 Hz, Si(CH3)3). 119Sn{1H} NMR (298.3 K,
benzene-d6, 111.85 MHz): −215 (s, N4Sn(N(SiMe3)2)2). IR
(ATR-IR, 32 scans, cm−1): 3074 (w), 3045 (w), 2949 (w), 2893
(w), 2883 (w), 2837 (w), 2791 (w), 1610 (w), 1566 (w), 1510 (s),
1477 (w), 1441 (m), 1406 (w), 1342 (w), 1277 (m), 1265 (m),
1252 (s), 1217 (m), 1188 (w), 1161 (w), 1122 (w), 1057 (w),
1020 (m), 997 (m), 960 (m), 949 (m), 895 (s), 877 (s), 835 (s),
818 (s), 796 (s), 756 (s), 719 (s), 673 (s), 634 (m), 619 (m),
592 (m), 534 (m). Crystals obtained from toluene were suitable
for X-ray diffraction. As 4a was sensitive towards light, no
Raman spectra could be collected.
Synthesis of 5a
A solution of TerSnCl (0.234 g, 0.50 mmol) in toluene (4 mL)
was cooled to −40 °C resulting in an orange suspension. 1a
(0.162 g, 1.00 mmol) was dissolved in toluene (3 mL) and
added to the TerSnCl suspension dropwise. The now dark red,
clear solution was stirred in the cold for 1 h. After concen-
tration to approximately 1 mL, the solution was stored at +5 °C
for 10 h. The supernatant was removed from the precipitate
and discarded. The precipitate was washed with n-hexane
(1 mL) and recrystallized from hot benzene (ca. +50 °C). Two
crops of crystals yielded 0.080 g (0.06 mmol, 25.0%) of X-ray
quality crystals.
Mp.: 228 °C. CHN calc. (found) in %: C 63.87 (63.294), H
5.86 (5.638), N 4.65 (4.354). 1H NMR (298.2 K, thf-d8,
300.13 MHz): 1.73 (s, 24 H, o-CH3), 2.16 (s, 12 H, p-CH3), 2.88
(s, 12 H, N(CH3)2), 6.12 (m, 4 H, arom. CH from 4-dimethyl-
aminophenyl), 6.31 (m, 4 H, arom. CH from 4-dimethyl-
aminophenyl), 6.50 (s, 8 H, C6H2(CH3)3), 6.87 (m, 4 H, m-CH),
Synthesis of 4b
7.30 (s, 3 H, 1C6H6), 7.42 (m, 2 H, p-CH). 13C{1H}-NMR
2
To a stirred solution of Sn[N(SiMe3)2]2 (0.149 g, 0.34 mmol) in
(298.2 K, thf-d8, 75.48 MHz): 21.7 (s, CH3), 21.9 (s, CH3), 42.3
(s, N(CH3)2), 114.9 (arom. CH from 4-dimethylaminophenyl),
124.1 (arom. CH from 4-dimethylaminophenyl), 129.2
(s, C6H6), 129.6 (s, C6H2(CH3)3), 130.8 (s, m-CH), 132.4 (s,
p-CH), 136.7 (s, arom. C), 138.1 (s, arom. C), 139.3 (s, arom. C),
144.1 (s, arom. C), 145.2 (s, arom. C), 146.5 (s, arom. C), 150.2
(s, arom. C). 119Sn{1H}-NMR (298.2 K, thf-d8, 111.89 MHz):
−151 (s, N2Sn2(Cl)2Ter2). IR (ATR-IR, 32 scans, cm−1): 3088 (w),
3059 (w), 3018 (w), 2974 (w), 2947 (w), 2912 (m), 2875 (w),
2850 (w), 2831 (w), 2785 (w), 2733 (w), 1610 (w), 1568 (w),
1502 (s), 1477 (m), 1444 (s), 1377 (m), 1325 (w), 1315 (w),
1294 (w), 1263 (s), 1209 (m), 1180 (m), 1161 (m), 1132 (m),
1088 (w), 1055 (m), 1032 (m), 1011 (w), 987 (w), 945 (m),
912 (w), 881 (s), 847 (s), 810 (s), 802 (s), 775 (m), 731 (s),
704 (m), 692 (s), 679 (s), 625 (m), 590 (m), 573 (m), 548 (m).
Raman (633 nm, accumulation time: 20 s, 10 scans, cm−1):
3066 (1), 3043 (1), 2914 (2), 2855 (1), 2834 (1), 2821 (1),
2788 (1), 2734 (1), 1607 (3), 1508 (1), 1479 (1), 1441 (1),
1395 (1), 1382 (1), 1328 (1), 1303 (3), 1291 (3), 1265 (1),
toluene (5 mL) was added
a solution of 1b (0.173 g,
0.68 mmol) in toluene (3 mL) dropwise at −40 °C. The golden,
clear solution was warmed to room temperature over a period
of 30 min (slight gas evolution) and stirred at this temperature
for additional 30 min. The reaction mixture was filtered (F4)
and concentrated until the crystallization started (ca. 3 mL).
Fractionated crystallization yielded 0.140 g (0.15 mmol, 44.6%)
4b as yellowish crystals in two crops. Crystals obtained from
toluene were suitable for single crystal diffraction.
Mp.: 181.0 °C. CHN calc. (found): C 36.49 (36.353), H 4.59
1
(4.378), N 9.12 (9.357). H NMR (298.2 K, C6D6, 300.13 MHz):
0.07 (s, 36 H, 2J{1H–29Si} = 6.4 Hz, [N(Si(CH3)3)2]2), 7.52 (m,
2 H, p-CH), 8.04 (m, 4 H, o-CH). 13C{1H} NMR (298.2 K, C6D6,
75.47 MHz): 5.4 (s, 1J{13C–29Si} = 56.7 Hz), 116.7 (m, p-CH),
119.4 (m, o-CH), 124.3 (q, 1J{13C–19F} = 273 Hz, CF3), 133.5
‡Experimental and spectral data, as well as crystallographic data are given in the
ESI.†
130 | Dalton Trans., 2019, 48, 125–132
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To a stirred solution of TerSnCl (0.234 g, 0.50 mmol) in
toluene (3 mL) was added a solution of 1b (c = 0.2 M, V = 1 mL,
1.00 mmol) in toluene dropwise at −40 °C. The resulting clear,
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and warmed to room temperature afterward. The solvent was
removed in vacuo and recrystallization from a minimum
amount of hot benzene resulted in the deposition of pale-
yellow to colorless crystals of 5b (0.165 g, 0.09 mmol, 37.5%).
Mp.: 315 °C (decom.). CHN calc. (found): C 55.32 (55.437),
H 4.06 (3.834), N 2.02 (2.647). 1H NMR (298.2 K, thf-d8,
300.13 MHz): 1.72 (s, 24 H, o-CH3), 2.13 (s, 12 H, p-CH3),
6.57 (s, 8 H, C6H2(CH3)3), 6.79 (broad, 4 H, o-C6H3(CF3)2),
6.96 (m, 4 H, m-CH), 7.28 (broad, 2 H, p-C6H3(CF3)2), 7.55 (m,
2 H, p-CH). 13C{1H} NMR (298.2 K, thf-d8, 75.47 MHz): 20.9
(s, o-CH3), 21.7 (s, p-CH3), 114.1 (s, p-C6H3(CF3)2), 122.4 (s,
o-C6H3(CF3)2), 124.8 (q, 1J{13C-19F} = 272 Hz, CF3), 129.9 (arom.
CH), 131.6 (s, p-CH), 132.3 (q, 2J{13C-19F} = 32 Hz, C(CF3)),
134.1 (s, arom. C), 137.0 (s, arom. C), 138.6 (arom. C),
139.2 (arom. C), 141.6 (arom. C), 150.0 (s, arom. C), 154.3 (s,
arom. C). 19F{1H} NMR (298.2 K, thf-d8, 282.40 MHz): −65.4 (s,
CF3). 119Sn{1H} NMR (298.2 K, thf-d8, 111.92 MHz): −144.4 (s,
N4SnN2). IR (ATR-IR, 32 scans, cm−1): 3030 (w), 2982 (w),
2951 (w), 2920 (w), 2856 (w), 2737 (w), 1610 (w), 1601 (w),
1568 (w), 1462 (m), 1448 (m), 1363 (s), 1271 (s), 1171 (s),
1124 (s), 1032 (w), 997 (m), 966 (s), 912 (w), 870 (m), 849 (s),
804 (m), 769 (w), 735 (s), 723 (s), 698 (s), 679 (s), 640 (m),
625 (w), 596 (m), 573 (m), 548 (w). Raman (laser: 633 nm,
accumulation time: 10 s, 20 scans, cm−1): 3122 (1), 3083 (1),
3065 (1), 3040 (1), 3016 (1), 2922 (2), 2859 (1), 2737 (1),
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Conflicts of interest
There are no conflicts to declare.
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We thank the University of Rostock for financial support.
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132 | Dalton Trans., 2019, 48, 125–132
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