Synthesis and structural characterization of tin analogues of N-heterocyclic carbenes

Article abstract of DOI:10.1021/ic801479g

The synthesis and X-ray crystal structures of five N-heterocyclic stannylenes are reported. These compounds, containing a variety of backbones, were prepared by the salt metathesis of the appropriate dilithiated diamide with SnCl₂ and show a high degree of thermal stability compared to the corresponding species with unsaturated backbones. If bulky diisopropylphenyl groups are attached to the nitrogen centers then the structures are monomeric, but when the less bulky mesityl groups are employed the solid-state structure was shown to be dimeric.

Full text of DOI:10.1021/ic801479g

Inorg. Chem. 2008, 47, 11367-11375
Synthesis and Structural Characterization of Tin Analogues of
N-Heterocyclic Carbenes
Stephen M. Mansell, Christopher A. Russell,* and Duncan F. Wass
School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K.
Received August 5, 2008
The synthesis and X-ray crystal structures of five N-heterocyclic stannylenes are reported. These compounds,
containing a variety of backbones, were prepared by the salt metathesis of the appropriate dilithiated diamide with
SnCl2 and show a high degree of thermal stability compared to the corresponding species with unsaturated backbones.
If bulky diisopropylphenyl groups are attached to the nitrogen centers then the structures are monomeric, but when
the less bulky mesityl groups are employed the solid-state structure was shown to be dimeric.
Introduction
supported complexes in catalytic reactions,1 have been
another driving force in the development of this area.
An alternative strategy to modification of this class of
compound is to alter the various components of the hetero-
cyclic ring framework. One commonly explored route has
been to probe the versions of these ligands with saturated
backbones, 4,5-dihydroimidazol-2-ylidenes (Chart 1, type B,
E ) C), which have been found to be less stable as free
carbenes and are more strongly basic than their unsaturated
analogues.6 In addition, several elegant studies have sought
to vary the groups directly adjacent to the divalent carbon
center. For example, the P-heterocyclic carbenes (PHCs)7
and cyclic (alkyl)(amino)carbenes (CAAC) systems8 devel-
oped by Bertrand and co-workers have been shown to have
rather different properties compared to the parent NHCs.8,9
A more commonly explored approach is to substitute the
carbene center with an isovalent p-block atom. Examples of
analogues of the Arduengo-type carbene have been success-
fully synthesized for many elements from groups 13 to 16
which requires the formal adjustment of charge to give
formally anionic (group 13),10 neutral (group 14),11-13
cationic (group 15),14 or dicationic (group 16)15 species. In
each case, the main-group center is two coordinate and
The quest for isolable carbenes has come to form part of
chemical folklore.1,2 The best known example of this class
of compound is based on the imidazol-2-ylidene framework
(Chart 1, type A, E ) C) which was first reported by
Arduengo et al. in 1991 using imidazolium salts as precur-
sors.3 This stimulated considerable interest in the preparation
of similar species, and shortly after, carbenes derived from
benzimidazole (Chart 1, type C, E ) C) and acyclic
representatives were added to the group.4 These N-hetero-
cyclic carbenes (NHCs) and their acyclic counterparts derive
their remarkable stability from the strong N-C carbene
π-donation that leads to a largely filled p(π) orbital at the
carbene carbon atom. It is no coincidence that most applica-
tions of stable carbenes involve the Arduengo-type imida-
zole-based carbenes as they are much more stable than the
benzimidazole- based or acyclic representatives.5 These
applications, particularly the distinctive performance of NHC-
* To whom correspondence should be addressed. E-mail: Chris.Russell@
bris.ac.uk. Fax: +44 (0)117 929 0509. Tel: +44 (0)117 928 7599.
(1) (a) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. 1997, 36,
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2008, 47, 3122–3172.
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10.1021/ic801479g CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 23, 2008 11367
Published on Web 11/07/2008

Mansell et al.
Chart 1a
a
E ) C, Si, Ge, Sn, Pb.
formally possesses a lone pair of electrons. To complement
these synthetic studies, molecular orbital calculations have
been employed to probe the similarities and differences
between the electronic structures of the various members of
this class of compounds.11,13,16,17 Within the more specific
class of neutral compounds that are the group 14 analogues
of NHCs which are the direct subject of this paper, most
attention has focused on the lighter elements Si11,17,18 and
Ge,13,19-22 and several structurally characterized examples
of these have been reported containing both saturated and
unsaturated backbones. Furthermore, several of the com-
pounds proved to be preparable on a sufficient scale to enable
their reactivity, in particular regarding their use as ligands
toward transition metals, to be investigated.18,21,23,24 In
contrast, NHC analogues of the heaviest group 14 elements
tin and lead are relatively sparse, despite the well-known
inert pair effect which suggests that the divalent group 14
species should become more stable upon descending the
group. Lead analogues of NHCs have recently been reported
by us and others.25-27 Only recently have tin analogues of
Arduengo-type carbenes and their benzannulated derivatives
been described.22,27-30 The chemistry of related tin(II)
amides has expanded considerably in the last few decades
following the pioneering studies of Lappert and co-workers,31
and we believe N-heterocyclic stannylenes to be particularly
attractive in extending the heavier group 14 analogues of
such compounds and eventually exploring their catalytic
chemistry. It is also noteworthy that simple Sn(II) halides
already have important applications as promoters for Pt-
catalyzed hydroformylation.32 In this paper we report the
synthesis and X-ray crystal structures of five examples of
N-heterocyclic stannylenes.
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Results and Discussion
Synthesis. The stannylenes 1-5 (Chart 2) were readily
synthesized using a salt metathesis procedure similar to those
that have previously proved successful for other tin(II)
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11368 Inorganic Chemistry, Vol. 47, No. 23, 2008

Structural Characterization of Tin Analogues
Chart 2a
a
Dipp ) 2,6-iPr2C6H3, Mes ) 2,4,6-Me3C6H2.
amides,33,34 that is, by the addition of equimolar quantities
of the dilithium salts of the appropriate diamide with SnCl2
in diethylether at -78 °C followed by gradual warming to
room temperature and stirring for approximately 2 h.
The low temperatures at which the reactions were carried
out were found to be essential as shown by the attempted
synthesis of 1 at room temperature which gave a large
mixture of products (1H NMR spectroscopy) and negligible
quantities of the desired product. All the products were
isolated by extraction into n-hexane followed by filtration
of the byproduct; highly air- and moisture-sensitive single
crystals of 1-5 were produced by storage of concentrated
n-hexane solutions at -18 °C for 16 h. The crystalline yields
of 1-5 varied widely between 15 and 81%, which, given
The 1H NMR spectra of 1, 2 and 4 show two doublets for
pairs of CH3 moieties of the diisopropyl groups (each
coupling to the CH), yet one septet for the CH proton. This
is a commonly encountered feature of complexes of this
bulky ligand due to restricted rotation around the N-aryl
bond, which leads to different environments for the methyl
groups within each CHMe2 unit, with one pointing relatively
toward the tin atom, and one pointing in the direction of the
backbone carbon atoms. In contrast, complex 5, which has
less steric congestion as a result of the smaller central four-
membered SnN2Si ring, exhibits magnetically equivalent
isopropyl groups, presumably on account of rotation about
1
the C-N bond being relatively fast on the H NMR time
1
scale. No 117/119Sn satellites could be discerned in the H
1
that the H NMR spectra of the solutions showed evidence
spectra of 1-5, presumably as a result of interaction through
the quadrupolar amide nitrogen centers.
for only the desired product and the free diamine in each
case, suggests that the isolated crystalline yield is a conse-
quence of the high air and moisture sensitivity of the product
and the high solubility of some of the complexes in n-hexane.
Compounds 1-5 have either a yellow (3 and 4) or orange
(1, 2, and 5) coloration. Moreover, 3 and 5 are thermochro-
mic; upon heating, 3 changed from yellow to red coloration
and 5 changed from an orange to red coloration. Similar
temperature-dependent color changes have been noted for
other low valent tin amides.30,33,35
Further information on the identities of the species was
gleaned from 119Sn NMR spectroscopy (in C6D6, see Figure
1), which showed a distinctive range of chemical shifts
{δ ) 499 (5), 386 (3), 366 (1), 291 (2) and 172 (4) ppm}.
The chemical shifts for 1, 2, 3, and 5 are all consistent with
two-coordinate tin centers and are of a similar order of
magnitude to those previously observed for benzannellated
tin(II) amides;29 these values are at higher frequencies than
the closely related 1,3,2λ2-diazastannoles (type A, E ) Sn -
these are the analogues of 1 and 3 containing unsaturated
backbones; for tert-butyl substituents on N, δ ) 237 ppm
and for mesityl groups, δ ) 259 ppm),28 which originates
from the greater shielding of the Sn(II) center in the latter
from π-conjugation. The lower frequency resonance for 4
suggests a higher coordination number at the tin center which
is unsurprising given the O-donor atom present in the ligand
backbone (vide infra). The formulation and structure of 4 is
closely related to a stannylene containing an N-donor atom
in the backbone.36
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Rheingold, A. L.; Fettinger, J. C.; Sita, L. R. Organometallics 1999,
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Inorganic Chemistry, Vol. 47, No. 23, 2008 11369

Mansell et al.
Figure 1. 119Sn NMR spectra of 1-5 in C6D6.
In contrast to some Sn(II) complexes, 1-5 are photolyti-
cally stable; thus, toluene solutions of 1-5 can be stored
under anaerobic conditions for many weeks without any sign
of deterioration of the sample. In addition, complexes 1-5
all show a relatively high degree of thermal stability,
decomposing between 115 and 250 °C, in marked contrast
to compounds of type A (E ) Sn) which decompose at 60
°C via chelotropic fragmentation to yield diazadienes and
elemental tin.28 Furthermore, a solution of 1 in C6D6 was
heated to 60 °C for 6 days without any sign of decomposi-
tion. The relative thermal stability of the N-heterocyclic
stannylene with a saturated backbone compared to an
unsaturated backbone is in stark contrast to the corresponding
N-heterocyclic carbenes for which the species with unsatur-
ated backbones show far higher stability. This interesting
difference is presumably a consequence of the much lower
aromatic stabilization of Sn analogues of Arduengo-type
carbenes which thus favors the chelotropic decomposition
pathway.
in Table 1; data relating to the structures and collection
parameters are shown in Table 2.
Complexes 1, 2, 4, and 5 which contain the bulky
diisopropylphenyl ligand are essentially monomeric, whereas
3, which contains the less bulky 2,4,6-trimethylphenyl
(mesityl) ligand, forms a dimer. In 1, 2 and 5, the tin(II)
center is two-coordinate, being bonded to two planar
N-centers and forming part of nonplanar five- (1), six- (2),
or planar four- (5) membered heterocyclic rings. In complex
4, the oxygen atom in the ligand backbone forms a contact
with the tin center {Sn-O 2.437(1) Å, cf. 2.527(5) Å in
SnCl2 · 1,4-dioxane}, which renders the tin center formally
three-coordinate, an observation which is consistent with the
119Sn chemical shift of this species. The diisopropylphenyl
groups align essentially perpendicular to the N-heterocyclic
stannylene ring in each case. The Sn atoms bond essentially
symmetrically to both N centers, although there are minor
variances which in some cases fall just outside of experi-
mental error.
Far more significant are the variations of Sn-N bond
lengths between compounds, for which the Sn-N distances
increase in the order 1 (av. 1.989 Å), 2 (av. 2.034 Å), 4 (av.
2.075 Å), and 5 (av. 2.090 Å). These distances compare to
average Sn-N bond lengths of 2.093 Å in Gudat’s tin
analogue of an Arduengo-type carbene,28 and 2.059 and
2.082 Å observed in the two structurally characterized
examples of monomeric annulated N-heterocyclic stan-
Solid State Structures. X-ray diffraction experiments
were performed on suitable single crystals of each of the
compounds 1-5 and their molecular structures are shown
in Figures 2-6. Key bond lengths and angles are summarized
in the legends of the respective figures, and a ready
comparison of some significant bond dimensions are collated
(36) Faure´, J. L.; Gornitzka, H.; Re´au, R.; Stalke, D.; Bertrand, G. Eur.
J. Inorg. Chem. 1999, 229, 5–2299.
11370 Inorganic Chemistry, Vol. 47, No. 23, 2008

Structural Characterization of Tin Analogues
Figure 3. Molecular structure of 2. The thermal ellipsoids are at 50%
probability and all hydrogen atoms have been omitted for clarity. The CH2
group of the backbone is disordered over two sites (SOF ) 0.73/0.27) which
are shown as C26A and C26B. Selected bond distances (Å) and angles (°)
for 2: Sn(1)-N(1) 2.035(1), Sn(1)-N(2) 2.032(1), N(1)-C(1) 1.435(2),
N(1)-C(25A) 1.464(2), N(2)-C(13) 1.435(2), N(2)-C(27A) 1.466(2),
C(25A)-C(26A) 1.503(3), C(26A)-C(27A) 1.489(3); N(1)-Sn(1)-N(2)
92.63(6), C(1)-N(1)-Sn(1) 118.1(1), C(1)-N(1)-C(25A) 114.9(1),
C(25A)-N(1)-Sn(1) 127.0(1), C(13)-N(2)-Sn(1) 120.1(1), C(13)-N(2)-
C(27A) 114.0(1), C(27A)-N(2)-Sn(1) 125.9(1), C(27A)-C(26A)-C(25A)
118.6(2),Σ angles at N(1) 360.0,Σ angles at N(2) 360.0.
Figure 2. Molecular structure of 1 showing the monomer (top) and
aggregation through long-range Sn· · ·aryl interactions (bottom). The
molecule was disordered over two sites that were related by a 2-fold rotation
axis oriented through the N1-N1A vector; thus atoms Sn, C(13) and C(14)
are disordered over two positions (with 50% occupancy); only one position
of the tin atom and carbon atoms in the ligand backbone are shown. Thermal
ellipsoids are at 50% probability and all hydrogen atoms have been omitted
for clarity. Selected bond distances (Å) and angles (°) for 1: Sn(1)-N(1)
1.973(2), Sn(1)-N(1A) 2.004(2), N(1)-C(13) 1.548(6), C(13)-C(14)
1.542(11), N(1A)-C(14) 1.543(6), N(1)-Sn(1)-N(1A) 84.15(8), C(1)-
N(1)-Sn(1) 130.6(2), C(1)-N(1)-C(13) 112.5(3), C(13)-N(1)-Sn(1)
113.8(3), C(1A)-N(1A)-Sn(1) 132.9(2), C(1A)-N(1A)-C(14A) 111.7(3),
C(14)-N(1a)-Sn(1) 113.0(3),Σ angles at N(1) 356.9,Σ angles at N(1A)
357.6, Sn-arylcentroid 3.51.
Figure 4. Molecular structure of 3. The thermal ellipsoids are at 50%
probability and all hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (°) for 3: Sn(1)-N(1) 2.259(5), Sn(1)-N(2)
2.101(6), Sn(1)-N(1A) 2.366(6), N(1)-C(1) 1.466(8), N(1)-C(19) 1.481(8),
N(2)-C(10) 1.423(8), N(2)-C(20) 1.460(8), C(19)-C(20) 1.513(9);
N(1)-Sn(1)-N(2) 79.7(2), N(1)-Sn(1)-N(1A) 85.8(2), N(2)-Sn(1)-N(1A)
96.1(2), C(1)-N(1)-Sn(1) 118.6(4), C(1)-N(1)-C(19) 111.0(5), C(19)-
N(1)-Sn(1) 106.4(4), C(1)-N(1)-Sn(1A) 117.3(4), C(20)-N(1)-Sn(1A)
107.7(4), Sn(1)-N(1)-Sn(1A) 94.2(2), C(10)-N(2)-Sn(1) 129.1(4),
C(10)-N(2)-C(20) 117.5(5), C(20)-N(2)-Sn(1) 113.0(4),Σ angles at N(1)
336.0,Σ angles at N(2) 359.6. Symmetry transformations used to generate
equivalent atoms labeled “A” -x + 2, -y + 1, -z + 1.
nylenes.29,30 This comparison serves to highlight the short
Sn-N distances within 1 and 2, although the origin of this
observation is not readily apparent.
As mentioned above, the use of the smaller mesityl group
in 3 compared to the diisopropylphenyl group in 1, 2, 4,
and 5 leads to aggregation of monomers in the solid state,
although the solution 119Sn NMR spectrum is suggestive of
a two-coordinate tin center in C6D6 solution (vide supra).
This fine balance between the formation of monomeric and
dimeric structures is further reinforced by comparison of the
structures of both 3 and 5 with literature precedents. Whereas
5 is essentially monomeric, closely related species containing
SnN2Si rings show a variety of aggregation; hence,
[Me2Si(tBuN)2Sn] contains a mixture of both monomers and
dimers37 in the solid state, whereas [Me2Si(iPrN)2Sn] is
exclusively dimeric;38 similarly imposing silacycloalkyl units
on the backbone in [(cyclo)Si(tBuN)2Sn] {cyclo ) (CH2)n
with n ) 3-5} realizes dimeric structures.39
In a similar vein, the version of 3 (type A, E ) Sn)
containing an unsaturated backbone was found to be a
monomer showing no evidence of intermolecular contacts.28
However, the solid-state structure of 3 shows two stannylene
units linking through amide moieties to form a dimer, in a
similar fashion to that which has been previously observed
in the related [R2Si(tBuN)2Sn] and [1,2-C6H4(NMe)2-
Sn]2.29,37,39 The Sn-N bond lengths in 3 fall into three
distinct groups: Sn(1)-N(2) and its symmetry equivalent are
the shortest as they play no part in intramolecular bonding;
the Sn(1)-N(1) and its symmetry equivalent are longer as
they are the endocylic Sn-N units which form the base unit
in each stannylene ring for dimerization; finally Sn(1)-N(1A)
(37) Veith, M. Z. Naturforsch. B 1978, 33, 7–13.
(38) Veith, M. Z. Naturforsch. B 1978, 33, 1–6.
(39) Kim, S. J.; Lee, Y. J.; Kim, S. H.; Ko, J.; Cho, S.; Kang, S. O.
Organometallics 2002, 21, 5358–5365.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11371

Mansell et al.
Figure 5. Molecular structure of 4. The thermal ellipsoids are at 50%
probability and all hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (°) for 1: Sn(1)-N(1) 2.075(2), Sn(1)-N(2)
2.075(2), Sn(1)-O(1) 2.4369(14), N(1)-C(1) 1.423(3), N(1)-C(25) 1.459(3),
N(2)-C(13) 1.419(3), N(2)-C(28) 1.463(3); N(1)-Sn(1)-N(2) 100.55(7),
N(1)-Sn(1)-O(1) 73.29(6), N(2)-Sn(1)-O(1) 74.88(6), C(1)-N(1)-Sn(1)
120.29(13), C(1)-N(1)-C(25) 116.72(16), C(25)-N(1)-Sn(1) 122.79(13),
C(26)-O(1)-Sn(1) 107.09(12), C(27)-O(1)-Sn(1) 108.15(12), C(13)-
N(2)-Sn(1) 121.78(13), C(13)-N(2)-C(28) 118.56(17), C(28)-N(2)-Sn(1)
115.84(13),Σ angles at N(1) 359.8,Σ angles at N(2) 356.2.
and its symmetry equivalent are the longest bonds as these
are the intramolecular contacts themselves. The oligomer-
ization is accompanied by a change in geometry at the linking
nitrogen centers which become distinctly pyramidal. The
Sn-N bond linking separate stannylene units {2.366(6) Å}
is longer than either of those within the individual heterocycle
(av. 2.18 Å), but it should be noted that in the heterocycle
there is a large difference in the Sn-N bond lengths
{2.101(6) and 2.259(5) Å}.
Figure 6. Molecular structure of 5 showing the monomer (top) and
aggregation through long-range Sn · · ·η2-aryl and Sn · · ·η6-aryl interactions
(bottom). The thermal ellipsoids are at 50% probability and all hydrogen
atoms have been omitted for clarity. The unit cell contains two crystallo-
graphically independent molecules. Selected bond lengths (Å) and angles
(°) are presented for both molecules of 5. first molecule: Sn(1)-N(1)
2.100(3), Sn(1)-N(2) 2.071(2), N(1)-C(1) 1.427(4), N(1)-Si(1) 1.726(3),
N(2)-C(13) 1.424(4), N(2)-Si(1) 1.733(3), N(1)-Sn(1)-N(2) 74.62(10),
C(1)-N(1)-Sn(1) 129.19(19), C(1)-N(1)-Si(1) 133.8(2), Si(1)-N(1)-Sn(1)
95.19(11), C(13)-N(2)-Sn(1) 133.1(2), Si(1)-N(2)-Sn(1) 96.01(11),
N(1)-Si(1)-N(2) 93.92(12),Σ angles at N(1) 358.18,Σ angles at N(2)
359.11, av. Sn-C(η2-arene) 3.745; second molecule: Sn(2)-N(3) 2.120(3),
Sn(2)-N(4) 2.069(2), N(3)-C(37) 1.411(4), N(3)-Si(2) 1.722(3), N(4)-
C(49) 1.425(4), N(4)-Si(2) 1.730(3), N(3)-Sn(2)-N(4) 74.04(10), C(37)-
N(3)-Sn(2) 121.70(19), Si(2)-N(3)-Sn(2) 95.11(11), C(49)-N(4)-Sn(2)
132.7(2), C(49)-N(4)-Si(2) 130.2(2), Si(2)-N(4)-Sn(2) 96.75(12), N(3)-
Si(2)-N(4) 93.92(12),Σ angles at N(3) 355.81,Σ angles at N(4) 359.65,
Sn-arenecentroid(η6-) 3.228.
Whereas 3 shows strong intermolecular contacts in forming
a dimeric structure in the solid state, molecules of 2 and 4
show little or no tendency to associate in their respective
solid-state structures. In contrast, complexes 1 and 5 assume
an intermediate position and show weak long-range interac-
tions between tin atoms and aryl rings of adjoining mol-
ecules. In 1, a quasi-dimeric structure (Figure 1, bottom) is
formed by the reciprocal interaction of tin atoms of a
stannylene unit with aryl rings of a second molecule, with
Sn-arylcentroid distances of 3.51 Å. A rather more complex
network of Sn-aryl contacts are seen for 5 where two distinct
types of Sn-aryl interactions are evident. The first is an η2-
interaction with one of the phenyl rings of a SiPh2 unit from
the backbone of a neighboring molecule. In addition η6-
interactions with the aryl ring of other diisopropylphenyl
groups are evident; the combination of these interactions link
individual molecules of 5 into a complex network. It is well
established that tin, and indeed most of the low-valent
p-block metals, is capable of forming coordination com-
pounds with aromatic hydrocarbons. However, the distances
observed in 1 and 5 are much greater than those seen in the
well-established examples of Sn · · · η6-aryl interactions,40
where distances as small as 2.53 Å have been observed.41
Hence, despite the alignment of monomers hinting at
oligomerization in the solid-state structures of 1 and 5, the
long intermolecular bond lengths would seem to suggest that
these are only very weak interactions.
Compounds 1-3 represent the first structurally character-
ized examples of N-heterocyclic stannylenes possessing
saturated organic backbones (type B); as noted earlier
examples are known for types A28 and C.29,30 Within the
wider context of heavier group 14 element (E) analogues of
N-heterocyclic carbenes, examples are now known for all
the elements although the volume of structural data is
relatively limited. For silicon, a handful of examples based
on five-membered rings have been reported for all types
A-C;24,42 the same is true of germanium although examples
(40) Schmidbaur, H.; Schier, A. Organometallics 2008, 27, 2361–2395.
(41) Probst, T.; Steigelmann, O.; Riede, J.; Schmidbaur, H. Angew. Chem.,
Int. Ed. 1990, 29, 1397–1398.
11372 Inorganic Chemistry, Vol. 47, No. 23, 2008

Structural Characterization of Tin Analogues
Table 1. Comparison of Significant Bond Lengths and Angles for Compounds 1-5
1
2
3
4
5a
intermolecular Sn-N
1.973(2)
2.004(2)
84.15(8)
356.9
2.035(1)
2.032(1)
92.63(6)
360.0
2.259(5)
2.101(6)
79.7(2)
336.0
2.075(2)
2.075(2)
100.52(9)
359.8
2.100(3)
2.071(2)
74.6(1)
358.2
2.120(3)
2.069(2)
74.0(1)
355.8
N-Sn-N
Σ angles at N
357.6
360.0
359.6
356.2
359.1
359.7
a There are two independent molecules in the unit cell of 5; each column shows the relevant dimensions for each independent molecule.
Table 2. Selected Crystallographic and Data Collection Parameters for Compounds 1-5
compound
1
2
3
4
5
color, habit
size (mm)
empirical formula
Mw
orange plate
yellow block
colorless plate
yellow needle
orange prism
0.40 × 0.25 × 0.05 0.50 × 0.40 × 0.10 0.40 × 0.30 × 0.05 0.30 × 0.05 × 0.05 0.45 × 0.20 × 0.20
C26H38N2Sn
497.27
100
C27H40N2Sn
511.30
173
C40H52N4Sn2
826.28
100
C28H42N2OSn
541.33
100
C36H44N2SiSn
651.53
100
T/K
crystal system
space group
monoclinic
C2/c
1.291
monoclinic
C2/c
1.267
triclinic
P1
1.562
monoclinic
P21/c
1.335
orthorhombic
Pbca
1.318
j
Fcalc (g cm-3
)
a/Å
b/Å
c/Å
20.165(4)
6.5412(13)
19.886(4)
90.00
102.82(3)
90.00
2557.7(9)
4
1.012
31.3536(10)
11.3904(4)
17.3236(6)
90.00
119.9150(10)
90.00
5362.5(3)
8
0.967
8.828(4)
9.957(4)
11.879(5)
66.30(3)
74.13(3)
68.21(5)
878.3(7)
1
12.5897(14)
17.5722(18)
12.2886(13)
90
97.805(5)
90
2693.4(5)
4
0.97
18.014(4)
21.158(4)
34.457(7)
90.00
90.00
90.00
13133(5)
16
0.841
R/°
ꢀ/°
γ/°
V/A-3
Z
µ/mm-1
1.456
reflections: total /independent/Rint
θ range (°)
13985/2937/0.0178 27821/6151/0.0283 9983/4006/0.0601
2.07 to 27.48
34851/6665/0.0408 90410/15070/0.0695
1.94 to 27.48
0.0234, 0.0627
0.293, -0.462
1.89 to 27.48
0.0666, 0.1469
1.926, -3.118
3.18 to 28.35
0.0262, 0.0829
0.429, -0.391
1.18 to 27.48
0.0434, 0.1171
0.917, -0.706
final R1 {I > 2σ(I)} and wR2 (all data) 0.0417, 0.0987
largest peak, hole
1.151, -0.674
are relatively more common;20-22,43 examples for E ) Pb
are extremely rare.25,26 The heterocycle 2 is particularly
noteworthy as it represents only the second example of a
heavier homologue of a N-heterocyclic carbenes as part of
a six-membered ring with a saturated organic backbone, the
first being our recently reported N-heterocyclic plumbylene;26
in this respect, we note examples of analogues for all the
heavier group 14 elements save lead involving six-membered
rings with unsaturated backbones.44
Given such limited structural data, it would be inappropri-
ate to attempt to identify any trends; however, a few
observations are pertinent. The heterocycles of type B appear
to adopt nonplanar geometries in the solid state, and the
structures do not show considerable changes upon coordina-
tion to a transition metal. Furthermore, the angle subtended
at E by the nitrogen centers becomes increasingly acute as
the group is descended. Whereas it is tempting to ascribe
this to being a consequence of the inert pair effect, it may
simply reflect the geometric effect that is a consequence of
increasing E-N bond lengths as the group is descended.
Conclusions
In summary, we have shown that tin analogues of
N-heterocyclic carbenes containing a variety of backbone
units may be prepared by salt metathesis of the appropriate
dilithiated diamide with SnCl2. The compounds show a far
greater degree of thermal stability than those reported for
the corresponding species with unsaturated backbones. When
the substituent on the nitrogen centers is the bulky diiso-
propylphenyl group then the structures are monomeric, but
when the less bulky mesityl group is employed then the solid-
state structure was shown to be dimeric, although 119Sn NMR
spectroscopy suggests a monomeric structure in solution.
Experimental Section
General Experimental. Many of the compounds described
herein are extremely air- and moisture-sensitive. All air- and
moisture-sensitive materials were weighed out, isolated, and stored
in an argon-filled Saffron Beta glovebox. Solvents used were
distilled HPLC grade and further dried and degassed using a
commercially available solvent purification system (Anhydrous
Engineering). All chemicals used were purchased from commercial
sources and used as supplied with the exception of the diamine
ligand precursors which were synthesized according to established
literature procedures.45 Melting points were determined by sealing
the sample in melting point tubes under argon in a glovebox prior
to determination using conventional apparatus. Elemental composi-
tions (C, H and N) were determined by sealing samples in airtight
aluminum boats in a glovebox and recorded on a Carlo Erba
EA1108 CHN elemental analyzer. Solution NMR spectroscopy
(42) (a) Moser, D. F.; Guzei, I. A.; West, R. Main Group Metal Chemistry
2001, 24, 811–812. (b) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.
Z. Anorg. Allg. Chem. 2005, 631, 1383–1386. (c) Gehrhus, B.; Lappert,
M. F.; Heinicke, J.; Boese, R.; Blaser, D. J. Chem. Soc., Chem.
Commun. 1995, 1931–1932.
(43) Kuhl, O.; Lonnecke, P.; Heinicke, J. Polyhedron 2001, 20, 2215–
2222.
(44) (a) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc.
2001, 123, 11162–11167. (b) Bazinet, P.; Yap, G. P. A.; DiLabio,
G. A.; Richeson, D. S. Inorg. Chem. 2005, 44, 4616–4621. (c) Driess,
M.; Yao, S. L.; Brym, M.; van Wuellen, C.; Lentz, D. J. Am. Chem.
Soc. 2006, 128, 9628–9629. (d) Driess, M.; Yao, S. L.; Brym, M.;
van Wullen, C. Angew. Chem., Int. Ed. 2006, 45, 4349–4352. (e)
Driess, M.; Yao, S. H.; Brym, M.; van Wullen, C. Angew. Chem.,
Int. Ed. 2006, 45, 6730–6733. (f) Avent, A. G.; Drost, C.; Gehrhus,
B.; Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem. 2004, 630,
2090–2096.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11373

Mansell et al.
samples were prepared using dry and degassed deuterated solvent
in airtight NMR tubes sealed with a Young’s tap; 119Sn and 29Si
NMR spectra were run on a Jeol Eclipse and/or Lambda 300 MHz
spectrometers and referenced to external samples of SnMe4 and
Elemental analysis: Found: C 58.22 H 6.85 N 7.20%. Required:
C 58.14 H 6.34 N 6.78%.
Analytical Data for O(CH2CH2NDipp)2Sn (4). Synthesized
n
using O(CH2CH2N(H)Dipp)2(0.642 g, 1.5 mmol), BuLi (1.6 M,
1.9 cm3, 3.0 mmol) and anhydrous SnCl2 (0.287 g, 1.5 mmol)
yielding the product as yellow crystals.
Yield: 0.380 g, 0.70 mmol, 46%.
1
SiMe4 respectively. H and 13C NMR spectra were referenced to
the internal solvent peaks.
General Procedure for the Synthesis of 1-5. The appropriate
diamine was dissolved in Et2O (30 cm3) and 2 equiv of nBuLi (1.6
M in hexane) was added dropwise at 0 °C. The solution was stirred
for 2 h and then added to a -78 °C suspension of anhydrous SnCl2
(0.304 g, 1.6 mmol) in Et2O (10 cm3). This was allowed to warm
to room temperature and then stirred for 2 h before removal of the
solvent and extraction into n-hexane (40 cm3). The extract was
filtered (porosity 3 sinter with celite); the filtrate was reduced in
volume in vacuo and storage at -18 °C yielded crystalline product.
Analytical Data for (CH2NDipp)2Sn (1). Synthesized using
Mp: ca 200 °C (dec).
i
1H NMR (270.17 MHz, C6D6): 7.15-7.09 (m, 6H, Pr2C6H3),
3
3
3.75 (septet, 4H, JH-H ) 6.9 Hz, CHMe2), 3.48 (br. t, 4H, JH-H
3
) 5.3 Hz, OCH2), 3.28 (br. t, 4H, JH-H ) 5.3 Hz, NCH2), 1.27
3
3
(d, 12H, JH-H ) 6.9 Hz, MeCHCH3), 1.24 (d, 12H, JH-H ) 6.9
Hz, CH3CHMe) ppm.
13C NMR (75.4 MHz, C6D6): δ 148.9 (Ar ipso-C), 147.6 (Ar
o-C), 124.8 (Ar m-C), 124.1 (Ar p-C), 74.7 (OCH2), 60.1 (CH2N),
28.7 (CHMe2), 25.8 (CH(CH3)2) ppm.
119Sn NMR (111.9 MHz, C6D6): δ 172 ppm.
n
(CH2-NH(2,6-iPr2C6H3))2 (0.610 g, 1.6 mmol), BuLi (1.6 M in
Elemental analysis: Found: C 60.89 H 7.78 N 4.99%. Required:
C 62.12 H 7.82 N 5.17%.
hexane, 2.0 cm3, 3.2 mmol) and anhydrous SnCl2 (0.304 g, 1.6
mmol) giving the product as orange crystals.
Yield: 0.48 g, 0.96 mmol, 60%.
Analytical Data for Ph2Si(DippN)2Sn (5). Synthesized using
n
Ph2Si(N(H)Dipp)2 (0.962 g, 1.8 mmol), BuLi (1.6M, 2.25 cm3,
Mp: ca. 250 °C (dec).
3.6 mmol) and anhydrous SnCl2 (0.341 g, 1.8 mmol) yielding the
product as orange crystals.
Yield: 0.341 g, 0.52 mmol, 29%.
i
1H NMR (300 MHz, C6D6): δ 7.25-7.18 (m, 6H, Pr2C6H3),
3
3.95 (s, 4 H, CH2N), 3.74 (septet, 4H, JH-H ) 7.0 Hz, CHMe2),
1.34 (d, 12H, 3JH-H ) 7.0 Hz, MeCHCH3), 1.27 (d, 12H, 3JH-H
7.0 Hz, CH3CHMe) ppm.
)
Mp: 178 °C (turned darker orange); 190-195 °C (melts); 230
°C (dec).
13C NMR (75.4 MHz, C6D6): δ 146.8 (Ar ipso-C), 146.2 (Ar
o-C), 128.0 (Ar p-C), 126.2 (Ar m-C), 62.2 (CH2N), 28.3, 26.2,
24.5 (iPr) ppm.
1H NMR (300 MHz, C6D6): δ 7.42-6.99 (collection of m, 16H,
Ar-H), 3.83 (septet, 4H, 3JH-H ) 6.8 Hz, CHMe2), 1.05 (d, 24H,
3JH-H ) 6.8 Hz, CHMe2) ppm.
119Sn NMR (111.9 MHz, C6D6): δ 366 ppm.
Elemental analysis: Found: C 61.95 H 7.84 N 6.43%. Required:
C 62.79 H 7.70 N 5.63%.
Analytical Data for CH2(CH2NDipp)2Sn (2). Synthesized using
CH2(CH2-NH(2,6-iPr2C6H3))2 (1.25 g, 3.15 mmol), nBuLi (1.6 M
in hexane, 3.9 cm3, 6.3 mmol) and anhydrous SnCl2 (0.598 g, 3.15
mmol) giving an orange crystalline product.
Yield: 1.30 g, 2.54 mmol, 81%
13C NMR (75.4 MHz, C6D6): 144.4, 143.6, 139.1, 134.9, 129.5,
127.5, 123.6, 123.4 (collection of Ar C signals), 28.4 (CHMe2),
25.6 (CHMe2) ppm.
119Sn NMR (111.9 MHz, C6D6): δ 499 ppm.
29Si NMR (59.6 MHz, C6D6): δ -14.8 ppm.
Elemental analysis: Found: C 66.00 H 6.94 N 4.31%. Required:
C 66.36 H 6.81 N 4.30%.
X-ray Crystallography. Crystals suitable for X-ray diffraction
analysis of 1-5 were grown from their saturated n-hexane solutions,
mounted in an inert oil and then transferred to the cold gas stream
of the diffractometer. Experiments were performed on 1, 3, and 5
using a Bruker-AXS SMART APEX three circle diffractometer;46
the corresponding measurements for 2 were made on a Bruker-
AXS SMART three circle diffractometer, while the corresponding
measurements for 4 were made with a Bruker-AXS Kappa-APEX-
II four circle diffractometer.47 All diffractometers employed Mo
KR radiation (λ ) 0.71073 Å). Intensities were integrated48 from
several series of exposures, each exposure covering 0.3° in ω.
Absorption corrections were applied based on multiple and sym-
metry-equivalent measurements.49 The structures were solved by
direct methods and refined by least-squares on weighted F2 values
Mp: 143-149 °C; 248 °C (dec).
i
1H NMR (300 MHz, C6D6): δ 7.25-7.13 (m, 6H, Pr2C6H3),
3
3
3.69 (septet, 4H, JH-H ) 7.0 Hz, CHMe2), 3.49 (br. t, 4H, JH-H
) 5.1 HZ, CH2(CH2NDipp)2), 2.27 (m, 2H, CH2(CH2NDipp)2),
1.34 (d, 12H, 3JH-H ) 7.0 Hz, MeCHCH3), 1.27 (d, 12H, 3JH-H
7.0 Hz, CH3CHMe) ppm.
)
13C NMR (75.4 MHz, C6D6): δ 147.3 (Ar ipso-C), 147.0 (Ar
o-C), 125.8 (Ar p-C), 123.7 (Ar m-C), 56.6 (CH2(CH2NDipp)2),
36.6 (CH2(CH2NDipp)2), 27.9, 26.6, 24.7 (iPr) ppm.
119Sn NMR (111.9 MHz, C6D6): δ 291 ppm.
Elemental analysis: Found: C 63.84, H 7.90, N 5.66%. Required:
C 63.42, H 7.88, N 5.48%.
Analytical Data for [(CH2NMes)2Sn]2 (3). Synthesized using
n
(CH2-NH(2,4,6-Me3C6H2))2 (0.897 g, 3.0 mmol), BuLi (1.6M,
(45) (a) Schrock, R. R.; Schattenmann, F.; Aizenberg, M.; Davis, W. M.
Chem. Commun. 1998, 19, 9–200. (b) Murugavel, R.; Palanisami, N.;
Butcher, R. J. J. Organomet. Chem. 2003, 675, 65–71. (c) Scollard,
J. D.; McConville, D. H.; Vittal, J. J. Organometallics 1995, 14, 5478–
5480. (d) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig,
H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron
1999, 55, 14523–14534.
(46) SMART diffractometer control software; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(47) Apex II diffractometer control software; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 2007.
(48) (a) SAINT integration software; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1994. (b) SAINT integration software V7.34A;
Siemens Analytical X-ray Instruments Inc.: Madison, WI, 2007,
(49) Sheldrick, G. M. SADABS: A program for absorption correction with
the Siemens SMART system; University of Go¨ttingen: Germany, 1996.
3.8 cm3, 6.1 mmol) and anhydrous SnCl2 (0.574 g, 3.0 mmol)
yielding the product as yellow crystals.
Yield: 0.184 g, 0.22 mmol, 15%.
Mp: 113-116 °C (turned redder); 120-123 °C (melts); 155 °C
(dec).
1H NMR (300 MHz, C6D6): δ 6.95 (s, 4H, C6H2Me3), 3.70
(s, 4H, CH2NMes), 2.37 (s, 12H, Ar o-Me), 2.24 (s, 6H, Ar
p-Me) ppm.
13C NMR (75.4 MHz, C6D6): δ (148.4 (Ar ipso-C), 134.3
(Ar o-C), 132.4 (Ar p-C), 129.3 (Ar m-C), 59.1 (CH2NMes), 20.7
(p-Me), 19.4 (o-Me) ppm.
119Sn NMR (111.9 MHz, C6D6): δ 386 ppm.
11374 Inorganic Chemistry, Vol. 47, No. 23, 2008

Structural Characterization of Tin Analogues
for all reflections.50 All non-hydrogen atoms were assigned
anisotropic displacement parameters and refined without positional
constraints. Hydrogen atoms were constrained to ideal geometries
and refined with fixed isotropic displacement parameters. Refine-
ment proceeded smoothly to give the residuals shown in Table 2.
Complex neutral-atom scattering factors were used.51
As described in the main text, the structure of 1 was disordered
with atoms Sn, C21 and C22 disordered over two positions (refined
with SOF ) 0.5). In addition, the central CH2 group of the backbone
of 2 was disordered over two positions (refined with SOF )
0.73/0.27).
CCDC reference numbers 706511 (1), 706512 (2) 706513 (3),
706514 (4) and 706515 (5).
Acknowledgment. We thank the University of Bristol for
financial support and Dr. Mairi Haddow (Bristol) for her
assistance in dealing with disorder in the X-ray crystal
structures.
Supporting Information Available: Crystallographic informa-
tion file is available free of charge via the Internet at
http://pubs.acs.org.
(50) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(51) International Tables for Crystallography; Kluwer: Dordrecht, 1992;
Vol C.
IC801479G
Inorganic Chemistry, Vol. 47, No. 23, 2008 11375