Benzannulated N-heterocyclic germylenes and stannylenes with sterically demanding N,N′-substituents

Article abstract of DOI:10.1039/c3dt51961a

Four N,N′-disubstituted 1,2-diaminobenzes (1a: R = t-Bu, 1b: R = adamantyl, 1c: R = Ph, 1d: R = Dipp) have been prepared and reacted with E[N(SiMe₃)₂]₂ (E = Ge, Sn) to give the benzannulated N-heterocyclic germylenes 2a-d

Full text of DOI:10.1039/c3dt51961a

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Benzannulated N-heterocyclic germylenes and
stannylenes with sterically demanding
N,N’-substituents†
Cite this: Dalton Trans., 2014, 43,
173
Sergei Krupski, Rainer Pöttgen, Inga Schellenberg and F. Ekkehardt Hahn*
Four N,N’-disubstituted 1,2-diaminobenzes (1a: R = t-Bu, 1b: R = adamantyl, 1c: R = Ph, 1d: R = Dipp)
have been prepared and reacted with E[N(SiMe3)2]2 (E = Ge, Sn) to give the benzannulated N-heterocyclic
germylenes 2a–d and stannylenes 3a–d. Germylene 2d and stannylenes 3a, 3c and 3d have been charac-
terized by X-ray diffraction showing different types of intermolecular interactions in the solid state. Depro-
tonation of 1b with n-BuLi yields the lithium stannyl anion Li[4] with an n-Bu group bound to the tin(II)
center.
Received 19th July 2013,
Accepted 19th September 2013
DOI: 10.1039/c3dt51961a
www.rsc.org/dalton
various applications for NHCs as spectator ligands in organo-
metallic chemistry and catalysis8 and very recently, for the con-
Introduction
Contrary to the case of N-heterocyclic carbenes (NHCs), the struction of metallosupramolecular assemblies9 have been
chemistry of their heavier divalent homologues, the N-hetero- described, stable N-heterocyclic germylenes, stannylenes and
cyclic germylenes, stannylenes and plumbylenes (generally plumbylenes have received less attention in these fields.10
named tetrylenes) has been less intensively explored. The first
We became interested in benzannulated N-heterocyclic car-
stable acyclic dialkyl1a and diamino tetrylenes1b,c were isolated benes and their heavier analogues. Both the carbenes of type
by Lappert et al.1 as early as 1973, long before the first acyclic B111 and their heavier analogues B2,12 B313 and B414 have
diaminocarbene was described in 1996.2 Later the heavier been described. In addition, benzobiscarbenes C115 and
cyclic analogues of N-heterocyclic carbenes A1,3 the N-hetero- related digermylenes and distannylenes C2 and C3 are
cyclic germylenes A2,4 stannylenes A35 and plumbylenes known16 as well as flexible di(tetrylenes) D1,11b,17 D218 and
A46 (Fig. 1.) were prepared in addition to a number of D319 which can coordinate to transition metals in a bidentate
derivatives containing subvalent group 14 atoms.7 While chelating fashion.
The heavier tetrylenes are of interest as they possess simul-
taneously a nucleophilic sp2-hybrid orbital and an electro-
philic p-orbital at the EII center.10 Dialkylgermylenes and
stannylenes have been shown to activate C–Cl bonds20 or
dihydrogen.21 Dihydrogen activation is particularly interesting.
It has also been demonstrated with cyclic (alky)(amino)carb-
enes (CCACs) but fails with classical diaminocarbenes of the
Arduengo-type A1.22 It has been proposed that the smaller
singlet–triplet gap of CAACs compared to the classical NHCs is
responsible for this difference in reactivity. Benzannulated
NHCs B1 also feature a reduced singlet–triplet gap when com-
pared to the unsaturated NHCs of the Arduengo-type A1 as can
Fig. 1 Monodentate (A1–A4 top left and B1–B4 top center) and
bidentate tetrylenes (C1–C3 top right and D1–D3 bottom).
be seen by their rapid dimerization when functionalized with
sterically non-demanding N,N′-substituents.11b We assumed
that benzannulated heavy tetrylenes also possess a small
singlet–triplet, enabling a subsequent activation of selected
substrates. Here we present the preparation of a series of new
benzannulated N-heterocyclic germylenes 2a–d and stanny-
lenes 3a–d bearing bulky N,N′-substituents, their aggregation
behavior and first studies of their reactivity.
Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität
Münster, Corrensstraße 30, D-48149 Münster, Germany.
E-mail: fehahn@uni-muenster.de; Fax: +49-251-8333108; Tel: +49-251-8333110
†Electronic supplementary information (ESI) available: NMR spectra for new
N-heterocyclic germylenes and stannylenes. CCDC 949605–949611. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt51961a
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Results and discussion
Previously, we had prepared benzannulated N-heterocyclic ger-
mylenes and stannylenes by
a transamination reaction
between E[N(SiMe3)2]2 (E = Ge, Sn) and suitable N,N′-substi-
tuted 1,2-diaminobenzenes 1 (Scheme 1). Compounds of type
1 were obtained by reduction of the corresponding amides
with LiAlH4 or AlH3.13c,16,18 This reduction is cumbersome and
requires large amounts of expensive LiAlH4 or AlH3. Alterna-
tively, compounds of type 1 can be obtained by the palladium
catalyzed reaction of 1,2-dibromobenzene with suitable
primary amines in the presence of a base.23 Contrary to the
N,N′-substituted 1,2-diaminobenzenes obtained by reduction
of the corresponding diamides with LiAlH4 or AlH3 the 1,2-di-
aminobenzenes 1a–d obtained as depicted in Scheme 1 exhibit
an enhanced resistance to atmospheric oxygen and can easily
be stored.
Single crystals of the diamines 1b and 1d were obtained by
slow evaporation of the solvent from a saturated THF solution.
Compound 1b forms colorless long needles upon crystalliza-
tion while 1d crystallized as colorless prisms. Both compounds
were studied by X-ray diffraction. The molecular structures
determined (Fig. 2) confirm the proposed connectivity. Appar-
ently for steric reasons, the two adamantyl substituents of 1b
Fig. 2 Molecular structures of 1b (top) and 1d (bottom) with hydrogen
atoms omitted for clarity and displacement ellipsoids are drawn at 50%
probability. Selected bond lengths (Å) and angles (°) for 1b [1d]: N1–C1
1.439(2) [1.4034(13)], N1–C7 1.487(2) [1.4312(12)], N2–C2 1.389(2)
[1.4130(12)], N2–C17 1.474(2) [N2–C19 1.4370(12)]; C1–N1–C7 118.09(12)
[122.64(8)], C2–N2–C17 126.18(13) [C2–N2–C19 120.92(8)].
are located at opposite sides of the aromatic ring. Steric Interestingly, the adamantyl substituted germylene 2b and
reasons are also responsible for the almost perpendicular stannylene 3b are almost insoluble in any solvent tried.
orientation of the central phenyl ring and the Dipp substitu-
The bulky N,N′-substituents in 2a, 2b, 2d and 3a, 3b, 3d
ents in 1d (dihedral angle C1–N1–C7–C8 80.7°). The N–Cphenyl lead to an enhanced stability of the compounds towards air
bonds are slightly shorter than the N–CAd bonds in 1b or the and moisture in comparison to previously described N-hetero-
N–CDipp bonds in 1d.
cyclic germylenes and stannylenes.10b However, all compounds
The reaction of the 1,2-diaminobenzenes 1a–1d with one still decompose when exposed to air for prolonged periods of
equivalent of Ge[N(SiMe3)2]2 or Sn[N(SiMe3)2]2 in THF gave the time and must therefore be handled under an inert atmos-
benzannulated N-heterocyclic germylenes 2a–d and stanny- phere. An exception is stannylene 3b which can be handled in
lenes 3a–d (Scheme 1) as intensely colored compounds in air and only gradually decomposes when exposed to air for
yields of 63 to 97%. Germylene 2a was obtained as an orange 24 h.
oil and decomposes rapidly in air. Stannylene 3a forms yellow-
All compounds with the exception of rather unstable 2a
orange crystals upon sublimation of the black crude reaction were characterized by mass spectrometry. The EI mass spectra
product under oil pump vacuum at 60 °C. Germylene 2c and featured the peaks for the molecular ions with the correct
stannylenes 3c and 3d are only sparingly soluble in THF and isotope pattern as intensive peaks. NMR spectra were recorded
toluene and almost insoluble in n-hexane. Stannylene 3a is for all new compounds with the exception of the nearly insolu-
1
soluble in polar or aromatic solvents like THF or benzene. Con- ble stannylene 3b. Only H NMR and EI mass spectra could be
trary to the poor solubility of stannylenes 3c and 3d, the ger- recorded for 2b.
mylenes 2a and 2d show a good solubility even in n-hexane.
The 13C{1H} NMR spectra for compounds 2a, 2c, 2d and 3a,
3c, 3d exhibit a significant downfield shift of the ipso-carbon
resonance signal from δ ≈ 137 ppm for the 1,2-diaminobenz-
enes to δ ≈ 143 ppm for the germylenes and δ ≈ 147 ppm for
the stannylenes. This downfield shift is typically observed
upon addition of the EII (E = Ge, Sn) atom to 1,2-diaminobenz-
enes and has been observed previously.13c,18a
The stannylenes 3a, 3c and 3d give distinctly different reso-
nances in their 119Sn NMR spectra. The 119Sn NMR resonance
for stannylene 3a was detected at δ = 267.9 ppm in the non-
polar C6D6. This resonance shifts upfield to δ = 215.5 ppm in
the polar solvent THF-d8. For the N,N′-Dipp-substituted stanny-
lene 3d the 119Sn resonances have been detected at δ =
216.1 ppm (C6D6) and at δ = 166.2 ppm (THF-d8), respectively.
Scheme 1 Preparation of the N,N’-substituted 1,2-diaminobenzene 1
and the benzannulated N-heterocyclic germylenes 2 and stannylenes 3.
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The dependence of the 119Sn resonance on the solvent used
has been observed for various N-heterocyclic stannylenes like
N,N′-dimethylbenzimidazolin-2-stannylene where values of δ =
225.3 ppm (in C6D6) and δ = 106.5 ppm (in THF-d8) have been
reported.13c The chemical shift of the 119Sn resonance reflects
the electron density at the tin atom. In THF solution, solvent
molecules can coordinate to the empty p-orbital at the tin(II)
atom. Such coordination has been confirmed crystallographi-
cally.19a,b,d The result of the THF coordination is an increase of
the electron density at the tin(II) center and a high-field shift
of the 119Sn resonance. The difference in the 119Sn resonances
is not as pronounced in 3a (Δδ = 52.4) and 3d (Δδ = 49.9 ppm)
as was observed for N,N′-dimethylbenzimidazolin-2-stannylene
(Δδ = 118.8 ppm). One possible explanation for this obser-
vation is a difference in the accessibility of the tin(II) center for
THF molecules which is certainly less pronounced in 3a and Fig. 4 Molecular structure of 3a (top) with hydrogen atoms omitted for
clarity (displacement ellipsoids are drawn at 50% probability) and
packing of 3a in the crystal (bottom). Selected bond lengths (Å) and
angles (°): Sn–N 2.0883(14), N–C1 1.385(2), N–C4 1.483(2); N–Sn–N**
79.16(8), Sn–N–C1 114.36(10), Sn–N–C4 122.77(11), C1–N–C4 122.87(14).
3d which are substituted with sterically demanding N,N′-sub-
stituents. In this context it is interesting to note that the less
bulky N,N′-diphenyl substituted derivative 3c in THF-d8 gives a
resonance at −86.4 ppm in the 119Sn NMR spectrum.
Intermolecular distance Sn⋯C3(C3**) 3.467.
Compounds 2d, 3a, 3d and 3c could be crystallized and
were characterized by X-ray diffraction. Colorless crystals of 2d
were obtained by cooling a saturated n-hexane solution to
−40 °C. The molecular structure of germylene 2d is depicted for related benzannulated N-heterocyclic stannylenes. The Sn–N
in Fig. 3.
separation (2.0883(14) Å) and the N–Sn–N** angle (79.16(8)°),
The molecular parameters of 3a resemble those reported
Bond distances and angles in 2d fall in the range previously for example, compare well with equivalent parameters
observed for benzannulated N-heterocyclic germylenes.13d,e,18 observed in the N,N′-dineopentyl13a or the N,N′-dimethyl-sub-
As was observed for the diamine precursor 1d the N,N′-Dipp- stituted13c benzannulated N-heterocyclic stannylenes. The
substituents in 2d are oriented almost perpendicular relative bulky N,N′-t-Bu substituents influence the aggregation behav-
to the central phenyl ring. The N–Cphenyl distances are slightly iour of 3a in the solid state. The N,N′-dineopentyl substituted
shorter than the N–CDipp distances. No intermolecular inter- stannylene dimerizes in the solid state via intermolecular
actions between 2d molecules have been observed in the solid Sn⋯η6-C6H4 interactions13a and the N,N′-dimethyl-substituted
state.
stannylene dimerizes via short intermolecular Sn⋯N inter-
Crystals of stannylene 3a were obtained by sublimation actions of 2.361(2) Å.13c No such interactions were observed for
under vacuum. The X-ray diffraction analysis reveals a highly 3a. Here the tin atom interacts with two carbon atoms of the
symmetrical molecule which resides on a mirror plane and is phenyl ring of a neighboring stannylene. Apparently for steric
bisected by another mirror plane. The molecular structure of reasons caused by the bulky t-Bu substituents, these carbon
3a is depicted in Fig. 4 (top).
atoms are located at a maximum distance from the tin center.
This leads to the formation of an infinite chain of stannylenes
3a in the solid state (Fig. 4, bottom). The intermolecular
Sn⋯C3(C3**) separation measures 3.467 Å and is only slightly
shorter than the sum of the van der Waals radii of the two
elements.24
Stannylene 3c is N,N′-substituted with sterically less
demanding phenyl groups. This stannylene is only soluble in
polar solvents, which we take as an indication of strong inter-
molecular interactions in the solid state. Crystals of 3c were
obtained by slow evaporation of the solvent from a THF solu-
tion of 3c. The molecular structure of 3c is depicted in Fig. 5.
In the solid state stannylene 3c is bisected by a mirror plane.
An inspection of the bond distances reveals a slightly expanded
Sn–N separation of 2.1077(14) Å compared to 3a. The reasons
for the longer Sn–N separation become apparent upon evalu-
ation of the packing of 3c in the solid state (Fig. 5, bottom).
In the solid state stannylene 3c forms polymeric chains by
intermolecular interactions of the tin atom with the phenyl
Fig. 3 Molecular structure of 2d with hydrogen atoms omitted for clarity
and displacement ellipsoids are drawn at 50% probability. Selected bond
lengths (Å) and angles (°): Ge–N1 1.857(2), Ge–N2 1.862(1), N1–C1 1.394(2),
N1–C7 1.442(2), N2–C2 1.392(2), N2–C19 1.440(2); N1–Ge–N2 84.41(7),
Ge–N1–C1 114.90(12), Ge–N1–C7 124.79(12), C1–N1–C7 120.01(15),
Ge–N2–C2 114.77(11), Ge–N2–C19 124.26(13), C2–N2–C19 120.96(15).
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Fig. 6 Molecular structure of 3d (top) with hydrogen atoms omitted for
clarity (displacement ellipsoids are drawn at 50% probability) and packing
of 3d in the crystal (bottom, only the first atom of each N-substituent is
shown). Selected bond lengths (Å) and angles (°): Sn–N 2.0811(12), N–C1
1.384(2), N–C4 1.436(2); N–Sn–N* 77.95(6), Sn–N–C1 115.48(9), Sn–N–C4
124.78(9), C1–N–C4 118.68(11). Intermolecular distances Sn⋯C3(C3*) 3.621.
Fig. 5 Molecular structure of 3c (top) with hydrogen atoms omitted for
clarity (displacement ellipsoids are drawn at 50% probability) and
packing of 3c in the crystal (bottom). Selected bond lengths (Å) and
angles (°): Sn–N 2.1077(14), N–C1 1.383(2), N–C4 1.417(2); N–Sn–N*
77.66(7), Sn–N–C1 115.39(10), Sn–N–C4 118.67(10), C1–N–C4 123.64(14).
Intermolecular distances Sn⋯C3(C3*) 2.943, Sn⋯Sn# 3.669.
The shift of the substituents out of the plane of the heterocycle
together with the longer Sn–N bonds in 3d compared to the
Ge–N bonds in 2d lead to a sterically less shielded EII center in
3d. This situation enables the weak intermolecular Sn⋯C3-
(C3*) interactions (Fig. 6, bottom). This interaction is,
however, rather weak as judged by the long Sn⋯C3(C3*) separ-
ation of 3.621 Å. It compares well with the intermolecular
Sn⋯C interaction (3.467 Å) observed for the sterically demand-
ing N,N′-substituted stannylene 3a.
Stannylene 3b is essentially insoluble in all solvents tested.
Consequently, single crystals of 3b could not be obtained. In
order to gain information about its crystal structure, stanny-
lene 3b was investigated by 119Sn Mössbauer spectroscopy. The
Mössbauer spectrum shows a doublet with an isomer shift of
δ = 2.39(1) mm s−1 and a quadrupole splitting of ΔEQ = 1.87(1)
mm s−1 (Table 1). For comparison, the Mössbauer spectra of
stannylene 3d and of the previously described compound N,N-
dimethylbenzimidazolin-2-stannylene13c were also recorded
(Fig. 7).
ring of a neighboring stannylene and by intermolecular
Sn⋯Sn# interactions (Fig. 5, bottom). The Sn⋯C3(C3*) separ-
ation measures 2.943 Å which is significantly shorter than that
observed for 3a (3.467 Å) and well below the sum of the van
der Waals radii.24 In addition, intermolecular Sn⋯Sn# inter-
actions of 3.669 Å are observed which are also significantly
shorter than the sum of the van der Waals radii of approxi-
mately 4.4 Å.24 The strong intermolecular Sn⋯C interactions
are most likely responsible for the poor solubility of 3c as
these interactions have to be broken upon dissolving 3c. In
addition, the geometrical orientation of the stannylenes in the
solid state indicates an intermolecular interaction of a filled
sp2-orbital of one stannylene with an empty p-orbital of a
neighboring stannylene. This type of interaction, which was
not observed for 3a, appears to be reasonable for the slightly
enlarged Sn–N bond distances in 3c in comparison to 3a.
Stannylene 3d was crystallized by slow diffusion of n-hexane
into a saturated THF solution of the compound. The molecular
parameters found for the sterically demanding N,N′-substi-
tuted 3d (Fig. 6, top) resemble those of 3a bearing the bulky
N,N′-t-Bu-substituents. For example, the Sn–N separations are
very similar (2.0883(14) for 3a, 2.0811(12) Å for 3d) but shorter
than observed for 3c. This observation would indicate a
similar aggregation behavior of 3a and 3d.
Compared to N,N′-dimethylbenzimidazolin-2-stannylene,
the isomer shift and the quadrupole splitting recorded for 3b
and 3d are significantly smaller. In addition, the values
recorded for 3b and 3d are almost identical. Given the fact that
in the solid state N,N′-dimethylbenzimidazolin-2-stannylene
In the solid state stannylene 3d forms weak intermolecular
Sn⋯C3(C3*) interactions (Fig. 6, bottom). In contrast, no such
interactions were observed for the analogous germylene 2d
(Fig. 3). A closer inspection of the molecular structures of 2d
and 3d reveals that the N,N′-substituents of the stannylene 3d
are moved out of the plane of the heterocycle (the N–C4 vector
forms an angle of 12.3° with the plane of the heterocycle). This
deviation amounts to only 1.4° for the germylene 2d where the
N,N′-substituents and the heterocycle are essentially coplanar.
Table 1 119Sn Mössbauer spectroscopic parameters for N,N’-dimethyl-
benzimidazolin-2-stannylene, 3b and 3d at 77
K (δ, isomer shift;
Г, experimental line width; ΔEQ, quadrupole splitting parameter)
Compound
δ [mm s−1
]
Г [mm s−1
]
ΔEQ [mm s−1
]
N,N′-Dimethylbenz-
2.73(1)
0.97(1)
2.31(1)
imidazolin-2-stannylene
3b
3d
2.39(1)
2.43(1)
0.81(1)
0.80(1)
1.87(1)
1.88(1)
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Fig. 8 Molecular structure of Li[4]·2THF with hydrogen atoms omitted
for clarity and displacement ellipsoids are drawn at 50% probability.
Selected bond lengths (Å) and angles (°): Sn–N1 2.207(2), Sn–N2 2.212(2),
Sn–C35 2.229(3), Li–O1 1.972(5), Li–O2 1.983(5), Li–N1 2.144(5), Li–N2
2.164(5), N1–C1 1.407(3), N1–C7 1.474(3), N2–C2 1.407(3), N2–C17
1.477(3); N1–Sn–N2 71.52(7), N1–Sn–C35 96.13(10), N2–Sn–C35 95.46(9),
Sn–N1–C1 105.90(13), Sn–N1–C7 120.31(14), Sn–N1–Li 89.65(14),
C1–N1–C7 122.5(2), C1–N1–Li 87.7(2), C7–N1–Li 122.4(2), Sn–N2–C2
106.83(13), Sn–N2–C17 120.57(14), Sn–N2–Li 89.01(14), C2–N2–C17
121.2(2), C2–N2–Li 85.7(2), C17–N2–Li 125.0(2).
Fig. 7 119Sn Mössbauer spectra of N,N’-dimethylbenzimidazolin-
2-stannylene, stannylene 3b and 3d at 77 K.
dimerizes with the formation of strong intermolecular Sn⋯N
interactions13c and the similarity of the Mössbauer data for 3b
and 3d, it can be assumed that stannylene 3b exists in the
solid state in a similar aggregation state as 3d (Fig. 6) with the
formation of only weak intermolecular interactions. In con-
trast, the higher isomer shift observed for N,N′-dimethylbenz-
imidazolin-2-stannylene is most likely caused by the strong
intermolecular interaction of the free electron pair at the ring
nitrogen atoms with the empty p-orbital at the tin atom of a
neighboring molecule.13c
Attempts to synthesize stannylene 3b by deprotonation of
N,N′-diadamantyl-1,2-diaminobenzene 1b with n-butyl lithium
and subsequent reaction with tin(II) chloride (Scheme 2) gave
unexpected results. Complex Li[4] was isolated from this reac-
tion in low yield. Complex Li[4] only forms in the presence of
an excess of n-BuLi.
Compound Li[4] forms by attack of the n-butyl anion (pro-
vided in excess) at the stannylene 3b underlining again the
electrophilic nature of the tin center. The 13C{1H} NMR spec-
trum of Li[4] features the resonances of the ipso-carbon atoms
as expected low-field shifted at δ = 148.8 ppm when compared
to the free diamine 1b. Four resonances for the tin-bound
n-butyl group were observed.
Compound Li[4] was not obtained from the reaction of 3b
with one equivalent of n-BuLi at −78 °C in THF. This reaction
leads to a product mixture (four signals in the 119Sn NMR
spectrum). The major components in the product mixture
were identified by mass spectrometry to be unreacted 3b and
the doubly alkylated stannylene. Only small amounts of Li[4]
were detected in the mass spectrum of the product mixture.
Similar results were obtained when the amine 1b was reacted
with GeCl2·dioxane in the presence of an excess of n-BuLi. This
reaction also yielded a product mixture containing mostly ger-
mylene 2b and the dialkylated germylene with the monoalky-
lated germylene only present in a small amount.
Cooling of an n-hexane solution of Li[4] to −18 °C for one
week led to the precipitation of orange crystals of Li[4]·2THF
which were investigated by X-ray diffraction. The molecular
structure of Li[4]·2THF is shown in Fig. 8. The major differ-
ence between the previously presented benzannulated
N-heterocyclic stannylenes and [4]− is the non-planar geometry
of the stannyl anion. The tin atom is moved out of the plane of
the heterocycle by 1.274 Å.
The n-butyl group binds to the tin atom via the p-orbital.
Consequently, the Sn–C35 vector is directed almost perpen-
dicular relative to the N1–Sn–N2 plane. This type of bonding
reduces the Lewis-acidity of the p-orbital at the tin center and
the (Np → Snp)π interactions typically observed for tetrylenes.
As a consequence, the nitrogen atoms are no longer trigonally
surrounded but become pyramidalized and bind with the free
electron pair to the lithium cation. This type of coordination
leads to a significant elongation of the Sn–N bonds (Sn–N1
2.207(2) and 2.212(2) Å) compared to the Sn–N bond lengths
in the stannylenes 3a,c,d.
Conclusion
Benzannulated N-heterocyclic germylenes 2a–d and stanny-
lenes 3a–d have been prepared from N,N′-disubstituted 1,2-di-
aminobenzenes and E[N(SiMe3)2]2 (E = Ge, Sn). The bulky N,N-
Scheme 2 Preparation of the benzannulated N-heterocyclic lithium
stannyl salt Li[4].
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substituents enhance the stability of the germylenes and stan- 7.38 (m, 2H, Ar–H), 7.02 (m, 2H, Ar–H), 1.61 (s, 18H, CH3).
nylenes. In addition, the steric bulk of the N,N′-substituents 13C{1H} NMR (50.3 MHz, C6D6): δ 142.9 (Ar–Cipso), 117.1
determines the mode of aggregation of the tetrylenes in the (Ar–Cmeta), 114.7 (Ar–Cortho), 56.2 (NC(CH3)3), 32.5 (C(CH3)3).
solid state. The chemical shift of the tin atom recorded for
N,N′-Di(adamantyl)benzimidazolin-2-germylene
2b. The
the stannylenes dissolved in THF serves as an indicator for the crude reaction product was isolated by filtration from the reac-
accessibility of the tin atom for coordination by THF mole- tion mixture. It was washed twice with THF (2 mL each) yield-
cules. Attempts to deprotonate N,N′-diadamantyl-1,2,diamino- ing the pure compound as an off-white powder. Yield: 0.78 g
1
benzene with an excess of n-BuLi followed by reaction with (1.74 mmol, 87%). H NMR (200 MHz, THF-d8): δ 7.47 (m, 2H,
SnCl2 yielded the salt Li[4]·2THF containing a stannyl anion Ar–H), 6.69 (m, 2H, Ar–H), 2.44 (m, 12H, Ad–CH2), 2.21 (m,
obtained by tin(II) alkylation of stannylene 3b with an n-Bu 6H, Ad–CH), 1.72 (m, 12H, Ad–CH2). The compound was not
group.
soluble enough to record a 13C{1H} NMR spectrum. MS (EI):
m/z (%, ion) 448 (100, [2b]+), 376 (10, [2b − Ge]+).
N,N′-Di(phenyl)benzimidazolin-2-germylene 2c. The red
solid crude reaction product was washed with n-hexane to
yield a yellow powder. Yield: 0.60 g (1.81 mmol, 91%). 1H NMR
(400 MHz, C6D6): δ 7.24–7.16 (m, 10H, Ph–H); 7.09–7.05 (m,
2H, Ar–H), 6.97–6.93 (m, 2H, Ar–H). 13C{1H} NMR (100.6 MHz,
C6D6): δ 143.1 (Ar–Cipso), 142.1 (Ph–Cipso), 129.7, 126.6, 126.1,
119.8, 111.7 (Ar–C and Ph–C). MS (EI): m/z (%, ion) 332 (54,
[2c]+), 261 (11, [[2c] − Ge]+).
Experimental
General
All manipulations were carried out under an argon atmosphere
employing standard Schlenk or glove-box techniques. Solvents
were dried over sodium/benzophenone under argon and were
freshly distilled prior to use. The N,N′-substituted 1,2-diamino-
benzens of type 1 were prepared according to the literature
procedures.23 Compounds 1a–1c were prepared according to
the method described by Bielawski et al. from 1,2-dibromo-
benzene, Pd(OAc)2/N,N′-(Dipp)2imidazolium chloride, NaOtBu
and the appropriate primary amine. Compound 1d was
obtained in a similar fashion, but by utilizing Pd(OAc)2/P(tBu)2
as a catalyst for the C–N coupling reaction. The spectroscopic
data for the diamines of type 1 obtained in this way are identi-
N,N′-Bis(2,6-diisopropylphenyl)-benzimidazolin-2-germylene
2d. The solid crude reaction product was dissolved at ambient
temperature in a small amount of n-hexane (2 mL) and after
cooling to −18 °C compound 2d precipitated as colorless crys-
tals which were isolated by filtration. Yield: 0.63 g (1.26 mmol,
1
63%). H NMR (400 MHz, C6D6): δ 7.29 (m, 6H, Dipp–Ar–H),
3
6.82 (m, 2H, Ar–H), 6.60 (m, 2H, Ar–H), 2.95 (sept, JH,H
=
3
6.9 Hz, 4H, CH(CH3)2), 1.22 (d, JHH = 6.9 Hz, 12H, CH3), 1.05
cal to the values reported in the literature (1a,23b 1b,23c 1c,23a
(d, JHH = 6.9 Hz, 12H, CH3). 13C{1H} NMR (100.6 MHz, C6D6):
3
1b
1d23a). The germylene Ge[N(SiMe3)2]2
and stannylene Sn-
δ 146.4 (Dipp–Ar–Cipso), 142.8 (Ar–Cipso), 137.6 (Ar–C–CMe2),
127.9, 124.1, 119.8, 111.7 (Ar–C and Dipp–Ar–C), 28.7
(CH(CH3)2), 26.4, 23.9 (CH(CH3)2). MS (EI): m/z (%, ion)
500 (100, [2d]+).
N,N′-Di(t-butyl)benzimidazolin-2-stannylene 3a. Compound
3a was obtained in pure form as orange crystals by sublimation
of the crude reaction product at 60 °C in vacuo. Yield: 0.58 g
(1.72 mmol, 86%). 1H NMR (400 MHz, C6D6): δ 7.36 (m, 2H,
Ar–H), 7.00 (m, 2H, Ar–H), 1.59 (s, 18H, CH3). 13C{1H} NMR
(100.6 MHz, C6D6): δ 146.8 (Ar–Cipso), 116.1, 115.8 (Ar–C), 56.9
[N(SiMe3)2]2 1b were synthesized by published methods. Samples
for NMR spectroscopy were prepared using dry and degassed
deuterated solvent in air-tight NMR tubes sealed with a
1
Young’s tap. C6D6 and THF-d8 were dried over Na/K alloy. H,
13C{1H} and 119Sn NMR spectra were measured using Bruker
AC-400 or AC-200 spectrometers at 400 or 200 MHz for 1H,
100.6 or 50.3 MHz for 13C{1H} and 149.2 MHz for 119Sn.
Syntheses
General procedure for the synthesis of germylenes 2a–2d (C(CH3)3), 33.5 (CH3). 119Sn NMR (149.2 MHz, C6D6): δ 267.9.
and stannylenes 3a–d. To a solution containing 2 mmol of 119Sn NMR (149.2 MHz, THF-d8): δ 215.5. MS (EI): m/z (%, ion)
one of the diamines 1a–d in THF (5 mL) were added 1.01 338 (22, [3a]+), 220 (100, [3a − Sn]+).
eq. of Ge[N(SiMe3)2]2 (795 mg, 2.02 mmol) or Sn[N(SiMe3)2]2
N,N′-Di(adamantyl)benzimidazolin-2-stannylene
3b. The
(888 mg, 2.02 mmol) under an argon atmosphere. The reaction crude reaction product was isolated by filtration from the reac-
mixture was then stirred for 36 h at 65 °C for the synthesis of tion mixture. It was washed twice with THF (2 mL each) yield-
the germylenes and at ambient temperature for the synthesis ing the pure compound as a yellow powder. Yield: 0.96 g
of the stannylenes. Subsequently, the solvent and the volatile (1.94 mmol, 97%). NMR spectra could not be recorded due to
HN(SiMe3)2 were removed in vacuo and the crude reaction the insolubility of the compound. MS (EI): m/z (%, ion)
product was purified as described below for the individual 494 (35, [3b]+), 376 (100, [[3b] − Sn]+).
compounds. Satisfactory microanalytical data could not
be obtained for the extremely air-sensitive GeII and SnII solid reaction product was washed with n-hexane. Recrystalliza-
compounds. tion by slow evaporation of the solvent from a THF solution
N,N′-Di(phenyl)benzimidazolin-2-stannylene 3c. The crude
1
N,N′-Di(t-butyl)benzimidazolin-2-germylene 2a. The crude yielded 3c as red needles. Yield: 0.56 g (1.48 mmol, 74%). H
reaction product was obtained as an orange oil and was used NMR (400 MHz, THF-d8): δ 7.28 (m, 8H, Ph–H), 7.00 (m, 2H,
without further purification for the spectroscopic analysis. Phpara–H), 6.92 (m, 2H, Ar–H), 6.43 (m, 2H, Ar–H). 13C{1H}
Yield: 0.52 g (1.79 mmol, 90%). 1H NMR (200 MHz, C6D6): δ NMR (100.6 MHz, THF-d8): δ 149.1 (Ph–Cipso), 145.1 (Ar–Cipso),
178 | Dalton Trans., 2014, 43, 173–181
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Paper
130.0, 124.3, 122.3, 117.1, 112.1 (Ar–C and Ph–C). 119Sn NMR 10 mg Sn cm−2. Spectra were fitted with the Normos-90
(149.2 MHz, THF-d8): δ −86.4. MS (EI): m/z (%, ion) 378 program system.25
(34, [3c]+), 260 (100, [[3c] − Sn]+).
N,N′-Bis(2,6-diisopropylphenyl)-benzimidazolin-2-stannylene
3d. The crude reaction product was washed with n-hexane and
X-ray diffraction studies
recrystallized from THF/n-hexane to give 3d as orange crystals. X-ray diffraction data were collected using a Bruker AXS APEX
Yield: 0.87 g (1.6 mmol, 80%). 1H NMR (400 MHz, C6D6): (MoKα radiation, λ = 0.71073 Å) diffractometer equipped with
δ 7.29 (m, 6H, Dipp–Ar–H), 6.74 (m, 2H, Ar–H), 6.52 (m, 2H, a rotation anode at 153(2) K using graphite monochromated
3
3
Ar–H), 3.00 (sept, JHH = 6.9 Hz, 4H, CH(CH3)2), 1.20 (d, JHH
=
radiation. Diffraction data were collected over the full sphere
6.9 Hz, 12H, CH3), 1.05 (d, JHH = 6.9 Hz, 12H, CH3). 13C{1H} and were corrected for absorption. The data reduction was per-
NMR (100.6 MHz, C6D6):
146.2 (Dipp–Ar–Cipso), 145.8 formed with the Bruker SMART26 program package. Structure
3
δ
(Ar–Cipso), 140.6 (Ar–C–CMe2), 127.2, 124.0, 118.9, 112.9 (Dipp– solutions were found with SHELXS-9727 using the heavy-atom
Ar–C and Ar–C), 28.3 (CH(CH3)2), 26.8, 24.1 (CH(CH3)2). 119Sn method and were refined with SHELXL-9726 against all F2
NMR (149.2 MHz, C6D6): δ 216.1. 119Sn NMR (149.2 MHz, THF- using first isotropic and later anisotropic displacement para-
d8): δ 166.2. MS (EI): m/z (%, ion) 546 (100, [3d]+), 428 (37, meters for all non-hydrogen atoms. Hydrogen atoms were
[[3d] − Sn]+).
added to the structure models on calculated positions.
Lithium-N,N′-di(adamantyl)benzimidazolin-2-butylstanny-
Crystal data for 1b. C26H36N2, M = 376.57, colorless crystal,
lenat(−1) Li[4]. Diamine 1b (382 mg, 1.01 mmol) was dis- 0.15 × 0.04 × 0.02 mm3, a = 11.7125(6), b = 6.5108(3), c =
solved in THF (5 mL). To this solution was added at −78 °C a 27.4895(13) Å, β = 101.9330(10)°, V = 2051.0(2) Å3, monoclinic,
solution of 1.6 M n-BuLi in n-hexane (1.31 mL, 2.1 mmol). The P21/n, Z = 4, ρcalcd = 1.22 g cm−3, μ = 0.1 mm−1, semiempirical
resulting mixture was stirred for one h at −78 °C. Sub- absorption correction (0.989 ≤ T ≤ 0.999), ω- and φ-scans,
sequently, SnCl2 (192 mg, 1.01 mmol) in THF (4 mL) was 20 971 measured intensities (3.0° ≤ 2θ ≤ 57.0°), 5193 indepen-
added dropwise. After the addition, the cooling was termi- dent (Rint = 0.0360) diffraction data, refinement of 253 para-
nated and the deep-brown reaction mixture was allowed to meters against all F2, R = 0.0576, wR = 0.1474 for 3861
warm up to ambient temperature. After one hour, the solvent observed data (I ≥ 2σ(I)), Rall = 0.0794, wRall = 0.1636 for all
was removed. The brown solid residue was taken up in dry data. The asymmetric unit contains one formula unit.
toluene (5 mL) and the resulting suspension was filtered
Crystal data for 1d. C30H40N2, M = 428.64, pale pink crystal,
under argon. The clear solution obtained was brought to 0.21 × 0.10 × 0.09 mm3, a = 12.2811(4), b = 12.3784(4), c =
dryness in vacuo. The resulting brown solid was dissolved in 17.0783(5) Å, β = 97.7570(10)°, V = 2572.49(14) Å3, monoclinic,
n-hexane and cooled to −18 °C. After one week intense orange P21/c, Z = 4, ρcalcd = 1.11 g cm−3, μ = 0.1 mm−1, semiempirical
crystals of Li[4]·2THF formed which were collected by fil- absorption correction (0.987 ≤ T ≤ 0.994), ω- and φ-scans,
tration. The precise yield was not determined since the com- 27 712 measured intensities (3.3° ≤ 2θ ≤ 59.0°), 7152 indepen-
pound is extremely air-sensitive and only a small amount of dent (Rint = 0.0194) diffraction data, refinement of 305 para-
1
Li[4]·2THF was obtained. H NMR (400 MHz, THF-d8): δ 6.58 meters against all F2, R = 0.0482, wR = 0.1264 for 5849
(m, 2H, Ar–H), 6.04 (m, 2H, Ar–H), 3.61 (m, 8H, OCH2CH2), observed data (I ≥ 2σ(I)), Rall = 0.0591, wRall = 0.1369 for all
2.10 (m, 12H, Ad–CH2), 2.06 (m, 6H, Ad–CH), 1.77 (m, 8H, data. The asymmetric unit contains one formula unit.
OCH2CH2), 1.68 (m, 12H, Ad–CH2), 1.51 (quint, 3J = 8.0 Hz,
Crystal data for 2d. C30H38N2Ge, M = 499.21, colorless
2H, Sn–CH2–CH2), 1.28 (m, 2H, Sn–CH2–CH2–CH2), 0.79 (t, 3J = crystal, 0.13 × 0.05 × 0.03 mm3, a = 9.0671(15), b = 10.948(2),
7.3 Hz, 3H, Sn–CH2–CH2–CH2–CH3), 0.41(t, 3J = 8.0 Hz, 2H, c = 27.971(5) Å, β = 91.713(3)°, V = 2775.4(8) Å3, monoclinic,
Sn–CH2–CH2). 13C{1H} NMR (100.6 MHz, THF-d8): δ 148.8 (Ar– P21/c, Z = 4, ρcalcd = 1.20 g cm−3, μ = 1.1 mm−1, semiempirical
Cipso), 115.2, 114.0 (Ar–C), 68.2 (OCH2CH2), 54.4 (Ad–Cq), 45.6 absorption correction (0.868 ≤ T ≤ 0.967), ω- and φ-scans,
(Ad–Cs), 38.1 (Ad–Cs), 33.5 (Sn–CH2–CH2), 31.5 (Ad–Ct), 30.8 25 109 measured intensities (2.9° ≤ 2θ ≤ 60.0°), 8008 indepen-
(Sn–CH2–CH2–CH2), 29.2 (Sn–CH2–CH2), 26.4 (OCH2CH2), 14.5 dent (Rint = 0.0443) diffraction data, refinement of 306 para-
(Sn–CH2–CH2–CH2–CH3). MS (EI): m/z (%, ion) 552 (100, [4]+); meters against all F2, R = 0.0426, wR = 0.0953 for 5528
495 (37, [[4] − C4H9]+).
observed data (I ≥ 2σ(I)), Rall = 0.0749, wRall = 0.1077 for all
data. The asymmetric unit contains one formula unit.
Crystal data for 3a. C14H22N2Sn, M = 337.03, yellow crystal,
119Sn Mössbauer spectroscopy
A Ca119mSnO3 source was available for the 119Sn investigations 0.12 × 0.07 × 0.06 mm3, a = 6.7920(3), b = 10.7367(4), c =
of N,N′-dimethylbenzimidazolin-2-stannylene, 3b and 3d and a 19.9685(8) Å, V = 1456.18(10) Å3, orthorhombic, Cmcm, Z = 4,
palladium foil of 0.05 mm thickness was used to reduce the ρcalcd = 1.54 g cm−3, μ = 1.7 mm−1, semiempirical absorption
tin K X-rays concurrently emitted by this source. The measure- correction (0.819 ≤ T ≤ 0.903), ω- and φ-scans, 8755 measured
ments were performed in the usual transmission geometry in intensities (4.1° ≤ 2θ ≤ 62.0°), 1279 independent (Rint
=
a commercial helium flow cryostat. The temperature of the 0.0163) diffraction data, refinement of 57 parameters against
absorber was 77 K. The source was kept at room temperature all F2, R = 0.0176, wR = 0.0475 for 1228 observed data (I ≥
in all experiments. The samples were placed within a thin- 2σ(I)), Rall = 0.0184, wRall = 0.0480 for all data. The asymmetric
walled PMMA container at a thickness corresponding to about unit contains a 1/4 formula unit. One molecule 3a resides on a
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Dalton Transactions
crystallographic mirror plane and is bisected by another
mirror plane.
J. R. Goerlich and W. J. Marshall, J. Am. Chem. Soc., 1995,
117, 11027–11028.
Crystal data for 3c. C18H14N2Sn, M = 377.0, dark red crystal,
0.23 × 0.06 × 0.04 mm3, a = 24.0650(9), b = 9.4662(3), c =
6.1652(2) Å, V = 1404.46(8) Å3, orthorhombic, Cmc21, Z = 4,
ρcalcd = 1.78 g cm−3, μ = 1.8 mm−1, semiempirical absorption
correction (0.681 ≤ T ≤ 0.931), ω- and φ-scans, 8236 measured
4 W. A. Herrmann, M. Denk, J. Behm, W. Scherer,
F.-R. Klingan, H. Bock, B. Salouki and M. Wagner, Angew.
Chem., Int. Ed. Engl., 1992, 31, 1485–1488.
5 T. Gans-Eichler, D. Gudat and M. Nieger, Angew. Chem., Int.
Ed., 2002, 41, 1888–1891.
intensities (3.4° ≤ 2θ ≤ 61.0°), 2173 independent (Rint
=
6 J. P. H. Charmant, M. F. Haddow, F. E. Hahn, D. Heitmann,
R. Fröhlich, S. M. Mansell, C. A. Russell and D. F. Wass,
Dalton Trans., 2008, 6055–6059.
0.0159) diffraction data, refinement of 97 parameters against
all F2, R = 0.0176, wR = 0.0460 for 2151 observed data (I ≥
2σ(I)), Rall = 0.0178, wRall = 0.0462 for all data. The asymmetric
unit contains a 1/2 formula unit related to the other half by a
crystallographic mirror plane.
Crystal data for 3d. C30H38N2Sn, M = 545.31, dark orange-
brown crystal, 0.31 × 0.14 × 0.14 mm3, a = 11.6500(5), b =
21.5505(9), c = 10.9679(5) Å, V = 2753.6(2) Å3, orthorhombic,
Pnma, Z = 4, ρcalcd = 1.32 g cm−3, μ = 0.9 mm−1, semiempirical
absorption correction (0.758 ≤ T ≤ 0.879), ω- and φ-scans,
23 836 measured intensities (3.8° ≤ 2θ ≤ 60.0°), 4112 indepen-
dent (Rint = 0.0265) diffraction data, refinement of 155 para-
meters against all F2, R = 0.0245, wR = 0.0628 for 3691
observed data (I ≥ 2σ(I)), Rall = 0.0283, wRall = 0.652 for all data.
The asymmetric unit contains 1/2 formula unit related to the
other half by a crystallographic mirror plane.
7 (a) M. Grenz, E. Hahn, W.-W. du Mont and J. Pickardt,
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Crystal data for Li[4]·2THF. C38H59N2LiO2Sn, M = 701.50,
orange crystal, 0.17 × 0.06 × 0.03 mm3, a = 10.7805(4), b =
11.2513(4), c = 14.5939(6) Å, α = 97.0290(10), β = 95.4150(10),
3
ˉ
γ = 94.3510(10)°, V = 1742.16(11) Å , triclinic, P1, Z = 2, ρcalcd
=
1.34 g cm−3, μ = 0.8 mm−1, semiempirical absorption correc-
tion (0.880 ≤ T ≤ 0.977), ω- and φ-scans, 20 491 measured
intensities (2.8° ≤ 2θ ≤ 60.0°), 10 076 independent (Rint
0.0160) diffraction data, refinement of 425 parameters against
all F2, R = 0.0402, wR = 0.1045 for 9094 observed data (I ≥
2σ(I)), Rall = 0.0452, wRall = 0.1084 for all data. The asymmetric
unit contains one formula unit. One of the THF molecules and
the Sn–butyl group are partly disordered.
=
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft
(SFB 858) for financial support.
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