Synthesis and structures of new oligomethylene-bridged double ladders. How far can the layers be separated?

Article abstract of DOI:10.1039/b316009b

The synthesis of the α,ω-bis[dichloro(trimethylsilylmethyl) stannyl]alkanes, (Me₃SiCH₂)Cl₂Sn(CH ₂) _nSnCl₂(CH₂SiMe₃) (13, n = 5; 14, n = 6; 15, n = 7; 16, n = 8; 17, n = 10; 18, n = 12) and the corresponding oligomethylene-bridged diorganotin oxides [(Me ₃SiCH₂)(O)Sn(CH₂)_nSn(O)(CH ₂SiMe₃)]_m (19, n = 5; 20, n = 6; 21, n = 7; 22, n = 8; 23, n = 10; 24; n = 12) is reported. The reaction of the diorganodichlorostannanes 13-18 with the corresponding diorganotin oxides 19-24 provided the spacer-bridged tetraorganodistannoxanes {[(Me₃SiCH ₂)ClSn(CH₂)_nSnCl(CH₂SiMe ₃)]O}₄ (25, n = 5; 26, n = 6; 27, n = 7; 28, n = 8; 29, n = 10; 30, n = 12). Compounds 13-30 have been identified by elemental analyses and multinuclear NMR spectroscopy. Compounds 25, 27, 29 and 30 have also been characterised by single crystal X-ray diffraction analysis and electrospray mass spectrometry. For the latter the essential double ladder motif is maintained for all n in the solid state, but subtle changes in alignment of the ladder planes occur. Separation between the two layers of the double ladder ranges from approx. 8.7 A (for 25, n = 5) to approx. 15 A (for 30, n = 12). In solution there is some dissociation of the double ladders into the corresponding dimers. The degree of dissociation is favoured by increasing oligomethylene chain length n.

Full text of DOI:10.1039/b316009b

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P A P E R
Synthesis and structures of new oligomethylene-bridged double
ladders. How far can the layers be separated?wz
Jens Beckmann,a Dainis Dakternieks,*a Andrew Duthie,a Fong Sheen Kuan,a
¨
Klaus Jurkschat,*b Markus Schurmannb and Edward R. T. Tiekink*c
a Centre for Chiral and Molecular Technologies, Deakin University, Geelong, 3217 Australia
b Lehrstuhl fur Anorganische Chemie II, Universitat Dortmund, 44221 Dortmund, Germany
¨
¨
c Department of Chemistry, National University of Singapore, Singapore 11754
Received (in Montpellier, France) 8th December 2003, Accepted 5th May 2004
First published as an Advance Article on the web 16th September 2004
The synthesis of the a,o-bis[dichloro(trimethylsilylmethyl)stannyl]alkanes, (Me3SiCH2)Cl2Sn(CH2)
nSnCl2(CH2SiMe3) (13, n ¼ 5; 14, n ¼ 6; 15, n ¼ 7; 16, n ¼ 8; 17, n ¼ 10; 18, n ¼ 12) and the corresponding
oligomethylene-bridged diorganotin oxides [(Me3SiCH2)(O)Sn(CH2)nSn(O)(CH2SiMe3)]m (19, n ¼ 5; 20, n ¼ 6;
21, n ¼ 7; 22, n ¼ 8; 23, n ¼ 10; 24; n ¼ 12) is reported. The reaction of the diorganodichlorostannanes 13–18
with the corresponding diorganotin oxides 19–24 provided the spacer-bridged tetraorganodistannoxanes
{[(Me3SiCH2)ClSn(CH2)nSnCl(CH2SiMe3)]O}4 (25, n ¼ 5; 26, n ¼ 6; 27, n ¼ 7; 28, n ¼ 8; 29, n ¼ 10; 30, n ¼ 12).
Compounds 13–30 have been identified by elemental analyses and multinuclear NMR spectroscopy.
Compounds 25, 27, 29 and 30 have also been characterised by single crystal X-ray diffraction analysis and
electrospray mass spectrometry. For the latter the essential double ladder motif is maintained for all n in the
solid state, but subtle changes in alignment of the ladder planes occur. Separation between the two layers
of the double ladder ranges from approx. 8.7 A (for 25, n ¼ 5) to approx. 15 A (for 30, n ¼ 12). In solution
there is some dissociation of the double ladders into the corresponding dimers. The degree of dissociation
is favoured by increasing oligomethylene chain length n.
as montmorillonite, to give nanocomposite materials with
modified qualities.6
Introduction
Dimeric tetraorganodistannoxanes [R2(X)SnOSn(Y)R2]2 (X,
Y ¼ Hal, OH, OCOR, OSO2R, OSiMe3; R ¼ alkyl, aryl)1 that
possess a common ladder-like Sn4O2X2Y2 structural motif in
the solid state (A, Chart 1) are currently under investigation as
mild homogeneous catalysts for a variety of organic reactions,
including transesterifications, selective acylation of alcohols,
urethane formation and the ring-opening polymerisation of
lactones.2 The catalytic activity of these ladder compounds has
been attributed to the kinetic lability and partial dissociation of
the dimeric structure in solution to give monomers with a
greater Lewis acidity.3
In previous work, we demonstrated that double ladder
arrangements can be obtained by use of organic spacers and
that these double ladders can be considered as new types of
inorganic-organic hybrid oligomers featuring parallel
Sn4O2X2Y2 units (B, Chart 1). The basic layer structure of
double ladders is tolerant to a great variety of different organic
spacers, for example, –(CH2)n– (n ¼ 1–4), –(p- or m-CH2Si-
Me2C6H4CH2SiMe2)– and –[p- or m-(CH2)2C6H4(CH2)2]–.4
The choice of the organic spacers allows modifications of the
geometric features such as (i) layer separation, (ii) cavity size,
(iii) twist of the layers, (iv) chirality and may also allow
incorporation of functional groups, such as ether moieties,5
into the framework. In continuation of this work we now
describe the use of very long oligomethylene spacer-linked ditin
precursors with up to twelve carbon atoms. Inorganic-organic
hybrid clusters with tailor-made shapes and sizes may find
applications as intercalates for pillared clay minerals, such
Results and discussion
Synthetic aspects
The synthesis of ditin precursors was achieved following the
reaction sequence outlined in Scheme 1. Starting from a,o-
bis(triphenylstannyl)alkane, Ph3Sn(CH2)nSnPh3, one phenyl
group at each tin atom was selectively cleaved using iodine to
provide the respective a,o-bis(iododiphenylstannyl)alkanes,
IPh2Sn(CH2)nSnPh2I (1–6; n ¼ 5–8, 10, 12). To assist with
crystallisation of the double ladders, the trimethylsilylmethyl
group was introduced by reaction of 1–6 with 2 equiv of
Me3SiCH2MgCl, affording the corresponding a,o-bis[(tri-
methylsilylmethyl)diphenylstannyl]alkanes,
(Me3SiCH2)
Ph2Sn(CH2)nSnPh2(CH2SiMe3) (7–12; n ¼ 5–8, 10, 12). All
remaining phenyl groups were then cleaved, using an excess
of concentrated hydrochloric acid, to give the respective a,o-
bis[(trimethylsilylmethyl)dichlorostannyl]alkanes, (Me3SiCH2)
Cl2Sn(CH2)nSnCl2(CH2SiMe3) (13–18; n ¼ 5–8, 10, 12). A por-
tion of the latter was hydrolysed to produce the corresponding
spacer-bridged diorganotin oxides [(Me3SiCH2)(O)Sn(CH2)
nSn(O)(CH2SiMe3)]m (19–24; n ¼ 5–8, 10, 12).
Compounds 1–18 are liquid or low-melting materials that
are very soluble in most organic solvents, whereas compounds
19–24 are insoluble, high-melting solids, presumably due to
their polymeric nature.7 Compounds 1–24 were characterised
in most cases by elemental analysis and 119Sn NMR spectro-
scopy; these data are collected along with preparative yields in
1
Table 1. Additional H and 13C NMR data are given in the
w Dedicated to the occasion of the 70th birthday of Professor Reinhard
Schmutzler.
Electronic supplementary information (ESI).
The
z Electronic supplementary information (ESI) available: 1H and 13C
NMR data (in CDCl3) of compounds 1–18 and 25–30. See http://
www.rsc.org/suppdata/nj/b3/b316009b/
double
ladders
{[(Me3SiCH2)ClSn(CH2)nSnCl
(CH2SiMe3)]O}4 (25–30; n ¼ 5–8, 10, 12) were prepared
by the reaction of equimolar amounts of the a,o-bis
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
1268
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 6 8 – 1 2 7 6

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Chart 1
[(trimethylsilylmethyl)dichlorostannyl]alkanes 13–18 with the
corresponding spacer-bridged diorganotin oxides 19–24 of the
same space length [eqn. (1)]:
Molecular structures of 25, 27, 29 and 30
The molecular structure of 25 is shown in Fig. 1 and key
geometric parameters for this and for structures 27, 29 and 30
are collected in Table 2. The structure conforms to the expected
motif in that two tetraorganodistannoxane units are linked via
a common Sn2O2 core with additional bridges between the
endo- and exocyclic tin atoms being provided by two chlorides,
with the two remaining chlorides occupying terminal positions
at the two exocyclic tin atoms. Two such essentially planar
(mean deviation 0.14 A) Sn4O2Cl4 entities are linked via –
(CH2)5– spacers so that pairs of endocyclic tin atoms are
connected by common chains, as are the exocyclic tin atoms.
The –CH2SiMe3 groups in effect cap the {Sn4O2Cl4[(CH2)5]4S-
n4O2Cl4} molecular box thus formed. The overall molecular
symmetry is centrosymmetric, with the centre being at the
centre of gravity of the molecule, so that the two Sn4O2Cl4
faces are parallel and directly superimposable. The average
separation between the Sn4O2Cl4 faces is 8.7 A. The chloride
bridges are to a first approximation symmetric and the Sn–Cl
terminal bond distances are shorter than the bridging Sn–Cl
distances (Table 2). The coordination geometry for the
ð1Þ
Scheme 1
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 6 8 – 1 2 7 6
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Table 1 Analytical and physical data for 1–30 (R ¼ CH2SiMe3)
C, H Analysis (%)
Compound
Yield (%)
Mp/1C
119Sn NMR/ppmd
C (calcd)
H (calcd)
c
IPh2Sn(CH2)5SnPh2I (1)
IPh2Sn(CH2)6SnPh2I (2)
495a
495a
495a
495a
495a
495a
495a
495a
495a
495a
495a
495a
68b
–
ꢁ51.5
c
–
ꢁ50.7
c
IPh2Sn(CH2)7SnPh2I (3)
IPh2Sn(CH2)8SnPh2I (4)
–
ꢁ50.7
c
–
ꢁ52.0
c
IPh2Sn(CH2)10SnPh2I (5)
IPh2Sn(CH2)12SnPh2I (6)
RPh2Sn(CH2)5SnPh2R (7)
RPh2Sn(CH2)6SnPh2R (8)
RPh2Sn(CH2)7SnPh2R (9)
RPh2Sn(CH2)8SnPh2R (10)
RPh2Sn(CH2)10SnPh2R (11)
RPh2Sn(CH2)12SnPh2R (12)
RCl2Sn(CH2)5SnCl2R (13)
RCl2Sn(CH2)6SnCl2R (14)
RCl2Sn(CH2)7SnCl2R (15)
RCl2Sn(CH2)8SnCl2R (16)
RCl2Sn(CH2)10SnCl2R (17)
RCl2Sn(CH2)12SnCl2R (18)
[R(O)Sn(CH2)5Sn(O)R]m (19)
[R(O)Sn(CH2)6Sn(O)R]m (20)
[R(O)Sn(CH2)7Sn(O)R]m (21)
[R(O)Sn(CH2)8Sn(O)R]m (22)
[R(O)Sn(CH2)10Sn(O)R]m (23)
[R(O)Sn(CH2)12Sn(O)R]m (24)
{[RClSn(CH2)5SnClR]O}4 (25)
{[RClSn(CH2)6SnClR]O}4 (26)
{[RClSn(CH2)7SnClR]O}4 (27)
{[RClSn(CH2)8SnClR]O}4 (28)
{[RClSn(CH2)10SnClR]O}4 (29)
{[RClSn(CH2)12SnClR]O}4 (30)
a
–
ꢁ51.9
c
–
ꢁ51.8
c
–
ꢁ58.4
c
–
ꢁ58.7
c
–
ꢁ58.6
c
–
ꢁ58.8
c
–
ꢁ58.6
c
–
41–43
ꢁ58.4
138.8
138.9
24.95 (25.03)
26.32 (26.36)
27.46 (27.64)
28.80 (28.86)
31.35 (31.15)
33.27 (33.50)
30.17 (30.38)
31.85 (31.95)
33.33 (33.24)
34.25 (34.56)
36.65 (37.01)
38.90 (39.24)
29.68 (29.93)
28.67 (28.85)
30.28 (30.18)
31.40 (31.45)
33.88 (33.83)
36.10 (36.01)
5.25 (5.17)
5.24 (5.37)
5.50 (5.57)
5.85 (5.75)
6.05 (6.10)
6.42 (6.50)
6.18 (6.28)
6.49 (6.15)
6.55 (6.69)
6.60 (6.89)
7.00 (7.25)
7.20 (7.57)
6.11 (5.79)
5.82 (5.88)
6.07 (6.08)
6.25 (6.27)
6.52 (6.62)
6.90 (6.95)
75b
49–52
43–45
71b
138.2
138.3
65b
27–28
45–46
73b
138.9
138.1
67b
94
36–37
4350
4270
4300
4350
Insoluble
Insoluble
93
99
Insoluble
Insoluble
96
92
4350
4350
Insoluble
Insoluble
95
96
294-295
245–247
296–297
245–246
229–231
216–218
ꢁ86.9/ꢁ130.9 (DL: 100%)
85
80
ꢁ71.8/ꢁ137.2 (DL: 90%); ꢁ81.0/ꢁ137.0 (L: 10%)
ꢁ84.3/ꢁ132.6 (DL: 85%); ꢁ81.0/ꢁ136.5 (L: 15%)
ꢁ75.6/ꢁ136.6 (DL: 75%); ꢁ75.9/ꢁ137.4 (L: 25%)
ꢁ78.6/ꢁ136.3 (DL: 67%); ꢁ78.9/ꢁ137.2 (L: 33%)
ꢁ80.5/ꢁ136.2 (DL: 60%); ꢁ80.6/ꢁ136.9 (L: 40%)
b
52
43
35
For compounds 1–6 and 7–12 yields are as measured by 119Sn NMR spectroscopy. Yields (isolated) for compounds 13–18 are for three synthetic
c
d
steps. Compound is an oil at room temperature. No mp was determined. L ¼ ladder and DL ¼ double ladder.
five-coordinate exocyclic tin atoms is trigonal bipyramidal with
the axial positions being occupied by the chlorides from the
C2OCl2 donor set. The geometry of the endocyclic tin atoms is
also distorted trigonal bipyramidal but with a chloride and an
oxygen atom occupying axial positions. The intermolecular
Sn1ꢀ ꢀ ꢀCl3 and Sn2ꢀ ꢀ ꢀCl4 distances are greater than 3.4 A and as
such are not considered to form bonding interactions.
The crystal structure of 25 comprises regular stacking of the
molecular parallelepipeds so that simply, molecules stack on
top of each other separated by hydrophobic interactions
involving, in the main, the CH2SiMe2 groups. Adjacent col-
umns are similarly connected via hydrophobic interactions so
that the lattice can be described as simple orthogonal packing
of boxes in three dimensions. Fig. 2(a) shows a representation
of the crystal lattice found for 25.
The molecular structure of 27, the –(CH2)7– analogue, is
shown in Fig. 3 and exhibits the same structural features as for
25, being centrosymmetric, but is not isomorphous. The mean
deviation of the atoms defining the Sn4O2Cl4 face is 0.12 A and
the average separation between these parallel faces is 12.3 A.
The crystal packing in 27 is quite distinct from that described
for 25. In the lattice of 27, the molecules within the columns
defined for 25, Fig. 2(a), are off-set laterally by half a molecule
and, similarly, the molecules in the columns are off-set by half a
molecule with respect to neighbouring columns, as represented
in Fig. 2(b).
Increasing the length of the spacer to ten methylene groups,
as in 29, sees retention of the basic molecular motif described
above, see Fig. 4, but with some differences that are probably
related to the increased flexibility in the spacer. The molecule
has crystallographic two-fold symmetry, with the axis passing
through the oligomethylene spacers. The two faces, each planar
Fig. 1 Molecular structure of centrosymmetric 25 showing the atomic
numbering scheme. Hydrogen atoms have been removed for reasons of
clarity. Symmetry transformation used to generate equivalent atoms:
i ¼ 1 ꢁ x, 1 ꢁ y, ꢁz.
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Table 2 Geometric parameters (1, A) for 25, 27, 29 and 30
25 27
29 ꢀ 5(toluene) 30 ꢀ 2(toluene)
Sn1–O1
Sn1–O2
2.039(10)
2.150(10)
2.641(5)
2.122(10)
2.028(10)
2.629(4)
2.048(10)
2.744(5)
2.455(5)
2.045(10)
2.780(4)
2.448(5)
2.046(4)
2.151(4)
2.658(2)
2.165(4)
2.047(4)
2.627(2)
2.009(5)
2.788(4)
2.438(5)
2.010(4)
2.820(2)
2.443(2)
2.037(6)
2.138(6)
2.660(3)
2.151(6)
2.042(6)
2.662(3)
2.023(6)
2.800(3)
2.454(3)
2.025(6)
2.744(3)
2.468(3)
73.9(2)
2.056(2)
2.120(3)
2.826(2)
2.106(3)
2.040(3)
3.090(1)
2.024(2)
2.650(1)
2.518(1)
2.016(2)
2.607(2)
2.547(2)
73.8(1)
Sn1–Cl1
Sn2–O1
Sn2–O2
Sn2–Cl2
Sn3–O1
Sn3–Cl1
Sn3–Cl3
Sn4–O2
Sn4–Cl2
Sn4–Cl4
O1–Sn1–O2
O1–Sn1–Cl1
72.6(4)
77.9(3)
74.3(1)
78.3(1)
152.4(1)
73.9(1)
153.2(1)
79.5(1)
75.7(2)
90.9(2)
166.4(2)
75.4(1)
89.5 (1)
164.6 (1)
79.1(2)
152.7(2)
73.5(2)
73.64(8)
147.46(7)
74.4(1)
O2–Sn1–Cl1 149.9(3)
O1–Sn2–O2 73.4(4)
O1–Sn2–Cl2 151.7(3)
151.2(2)
77.8(2)
75.9(2)
71.94(9)
146.38(7)
78.28(8)
83.42(8)
161.20(4)
80.34(9)
84.86(9)
164.17(4)
O2–Sn2–Cl2
O1–Sn3–Cl1
O1–Sn3–Cl3
78.9(3)
75.3(3)
89.6(3)
90.1(2)
Cl1–Sn3–Cl3 164.6(2)
165.15(10)
76.1(2)
88.7(2)
O2–Sn4–Cl2 75.0(3)
O2–Sn4–Cl4 91.5(3)
Cl2–Sn4–Cl4 165.9(2)
164.29(10)
to 0.06 A, are still effectively parallel but twisted approximately
411 with respect to each other. The average distance between
faces is 14.8 A. In all other respects, such as in the nature of the
Fig. 3 Molecular structure of centrosymmetric 27 showing the atomic
numbering scheme. Hydrogen atoms have been removed for reasons of
clarity. Symmetry transformation used to generate equivalent atoms:
i ¼ ꢁx, ꢁy, ꢁz.
molecular connectivity, the structure of 29 resembles those
described above (Table 2). The crystal lattice of 29 contains
solvent toluene molecules so that for each molecule of 29, there
are five toluene molecules. The solvent is not incorporated
within the molecular framework but rather exists in regions
surrounding the spacer ligands of adjacent molecules. In fact,
the crystal packing in 29 resembles almost exactly that de-
scribed for 25, Fig. 2(a), but with solvent molecules occupying
regions between the columns so that the global crystal packing
is not affected to any great extent by the inclusion of solvent.
By contrast to the preceding three structures, some signifi-
cant structural changes are evident for 30. As apparent from
Fig. 5, the description of the molecular structures of 25, 27 and
29 as molecular rectangular boxes is no longer strictly applic-
able for 30 as there is definite skewing in the molecular
polygon. Molecules of 30 are centrosymmetric so that the
Sn4O2Cl4 planes are parallel but there is some significant
buckling in the links at the Sn1 and Sn4 atoms so that there
is no superimposition of the two faces. The relationships
between the four structures in terms of the relative orientations
of the Sn4Cl4O2 faces are conveniently summarised in Fig. 6
where the top Sn4Cl4O2 face is projected normal to the plane
through these atoms. For structure 25 (and 27), it is evident
that the faces are superimposable. By contrast, in molecule 29,
the faces are of opposite orientation but lie on top of each
other. Finally, in 30, the faces, while of the same orientation as
found in 25 and 27, are somewhat displaced with respect to
each other. The displacement can be rationalised by consider-
ing the conformation of the linking alkyl groups. The indivi-
dual links within the alkyl spacers all have the open or trans
conformation; the maximum deviation of any of the C–C–C–C
torsion angles from 1801 is 171.9(4)1. This pattern extends to
Fig. 2 Schematic diagrams representing the molecular packing in (a)
25 and 29 ꢀ 5(toluene), (b) 27, and (c) 30 ꢀ 2(toluene).
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 6 8 – 1 2 7 6
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Fig. 4 Molecular structure of 29 in 29 ꢀ 5(toluene), showing the atomic
numbering scheme. The molecule has crystallographically imposed
two-fold symmetry. Hydrogen atoms have been removed for reasons
of clarity. Symmetry transformation used to generate equivalent
1
atoms: i ¼ ꢁx, y, ꢁ ꢁ z.
2
Fig. 5 Molecular structure of centrosymmetric 30 in 30 ꢀ 2(toluene),
showing the atomic numbering scheme; the C5 atom is obscured.
Hydrogen atoms have been removed for reasons of clarity. Symmetry
transformation used to generate equivalent atoms: i ¼ ꢁx, 1 ꢁ y, 2 ꢁ z.
include the Sn2 and Sn3 atoms, as seen in the respective
Sn–C–C–C torsion angles of ꢁ167.8(3)1 and 175.6(3)1, but
not to the Sn1 and Sn4 atoms, as reflected in the respective
Sn–C–C–C torsion angles of ꢁ67.3(4)1 and 74.7(6)1. This has
the effect of tilting the faces as shown in Fig. 5. The question
then arises: is there a chemical reason behind this arrangement?
The major difference in the molecular dimensions defining
the Sn4O2Cl4 plane pertains to the nature of the Sn–Cl con-
tacts. In each of 25, 27 and 29 there are two almost symmetric
Sn–Cl–Sn bridges and no significant interactions between the
endocyclic tin atoms and the terminal chlorides. By contrast, in
30, a different situation pertains as the Snendocylic–Cl distances
are Sn1–Cl1 2.826(2) A, Sn1–Cl4 3.139(2) A, Sn2–Cl2 3.090(1) A
and Sn2–Cl3 2.942(2) A; these are not shown in Fig. 5. Taken to
an extreme, that is if these contacts are not considered sig-
nificant, the endocyclic tin atoms may be considered as being
four-coordinate, distorted tetrahedral as opposed to octa-
hedral if they were considered significant. A similar situation
is observed for the simple ladder compound [Me2(Cl)
SnOSn(Cl)Me2]2 in the co-crystallisate with [PtClMe2(SnCl3)
(bu2bpy)] (Bu2bpy ¼ 4,40-di-tert-butyl-2,20-bipyridine), where it
shows four crystallographically related Snendocylic–Cl distances
of 3.015(2) A.8 However, by itself the structure of [Me2(Cl)
SnOSn(Cl)Me2]2 reveals two crystallographically related
Snendocylic–Cl distances of 2.702(2) and 3.409(2) A.9 The pat-
tern observed for 30 is distinctly different from that found in
the structures of the lower congeners; the exocyclic tin atoms
remain five-coordinate in all four structures. However, the
tilting in the structure involves centrosymmetrically related
pairs of endo- (Sn1) and exocyclic (Sn4) tin atoms so the
difference in molecular connectivity revolving around the
chlorides is unlikely to be the source of the disparity between
the overall shapes of the molecular polygons. Compound 30
crystallises with two solvent molecules of toluene per molecule
of 30, but there are no noteworthy interactions between these
components of the crystal. The crystal packing of 30 is based
upon that described for 29, which was, in turn, related back to
25. The only difference is the tilting evident in the molecules as
represented schematically in Fig. 2(c). Given that there is great
scope for molecular flexibility in these molecules and that there
are no inherent reasons for the adoption of molecular rectan-
gles, it is possible that the energetics of crystal lattice formation
may play a role in determining the molecular structure, but
there is no experimental evidence to support this conjecture.
Solution studies of 25–30
The oligomethylene-bridged double ladder clusters {[(Me3SiCH2)
ClSn(CH2)nSnCl(CH2SiMe3)]O}4 (25–30; n¼ 5–8, 10, 12) are mod-
erately soluble in slightly polar organic solvents such as chloro-
form or toluene. For freshly prepared samples of 25–30 in
CDCl3, 119Sn NMR spectroscopy reveals at first only two
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N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 6 8 – 1 2 7 6

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It is worth mentioning that a fluorine- as well as a ben-
zoate-substituted analogue of this class of compound, namely
11
{[(Me3SiCH2)2CH(F)Sn(CH2)3Sn(F)CH(CH2SiMe3)2]O}2 and
{[Me3SiCH2(PhCOO)Sn(CH2)3Sn(OCOPh)CH2SiMe3]O}2 [4f],
were described recently. After about 10 days the integral ratio
of the 119Sn NMR signals remains unchanged, which apparently
suggests that a steady state concentration of the reactants
(equilibrium) is reached at that time (Table 1). While the
equilibrium for 26 and 31 (n ¼ 6) stabilizes at 90 : 10, the
proportion of ladder increases as the spacer length increases,
so that for n ¼ 12 the ratio between 30 and 35 is 60 : 40.
Presumably this is due to longer spacers having a higher degree
of flexibility. In order to confirm that water was not involved in
the above equilibrium, a sample of 29 in dry toluene was
prepared under inert conditions and flame-sealed. The 119Sn
NMR spectrum of this sample (5 min after dissolving) showed
a DL-to-L ratio of 95 : 5. The 119Sn NMR spectrum was again
recorded after the sample had been maintained at 70 1C for 2 h.
After heating the DL-to-L ratio changed to 70 : 30 (similar to
that obtained for the sample kept at room temperature for
10 days). The IR spectrum (KBr) of the solid obtained after the
removal of toluene contained no bands that could be asso-
ciated with hydroxy groups.
It is also interesting to note that the preparative yield of the
double ladders consistently decreases with increasing spacer
length and goes from 96% to 35% on going from 25 (n ¼ 5) to
30 (n ¼ 12) (Table 1). In turn, there is evidence to suggest that
the amount of higher oligomers and polymers in the crude
reaction mixtures increases with increasing spacer chain length.
The 119Sn NMR spectra of crude reaction mixtures of 25–27
(n ¼ 5–7) exhibit only sharp signals belonging to the corre-
sponding ladder and double ladder; the reaction mixtures of
28–30 (n ¼ 8, 10, 12) reveal broad signals in the regions ꢁ75 to
ꢁ90 and ꢁ130 to ꢁ145, which are tentatively assigned to
oligomeric and polymeric species. Integration shows that the
amount of these species increases from about 40% for n ¼ 8 to
60% for n ¼ 12, suggesting that random cross-linking becomes
more favoured with longer spacer lengths. It was necessary to
perform the reactions for n ¼ 8, 10, and 12 at high dilution (2–3
g Lꢁ1) in order to obtain a higher proportion of ladder/double
ladder to oligomer/polymer. The crude reaction mixtures from
which the products 29 and 30 (n ¼ 10, 12) had been removed
were investigated to determine if there is an equilibrium at
higher temperatures that would result in further product
formation. However, after prolonged heating in refluxing
toluene no changes in composition were observed (by 119Sn
NMR spectroscopy).
Fig. 6 View normal to the Sn2O2 plane of the superimposition of the
top to bottom Sn4Cl4O2 faces for: (a) 25 (the view for 27 is essentially
identical), (b) 29, and (c) 30.
equally intense signals centred at d ꢁ 79 (deviation ꢂ8 ppm)
and ꢁ134 (deviation ꢂ4 ppm), which are assigned to the
exocyclic and endocyclic tin sites of the clusters, respectively.10
2
Each of these signals shows identical J(119Sn–O–117Sn) cou-
plings of 65 Hz (deviation ꢂ1 Hz), consistent with the observed
connectivity of the tin atoms.10 The inequivalence of the
exocyclic and endocyclic tin sites is further reflected by two
1
different sets of H and 13C resonances (see ESI). After a few
Further indication for equilibria that generate ladders, and
even monomeric distannoxane units, was obtained using elec-
trospray mass spectrometry (ESMS). Thus, the ESMS spectra
(positive mode, cone voltage 200 V) of the double ladder
clusters {[(Me3SiCH2)ClSn(CH2)nSnCl(CH2SiMe3)]O}4 (M)
(25–30; n ¼ 5–8, 10, 12) revealed mass clusters for [14 M ꢁ Cl]1(a),
[12 M ꢁ 3Cl þ O]1 (b), [21 M ꢁ Cl]1(c), [Mꢁ 2Cl þ OH]1 (d), and
[M ꢁ Cl]1 (e) with the expected isotope patterns. Proposed
structures for ions a–e for 25–30 are depicted in Chart 2.
hours, 119Sn NMR spectroscopy begins to show additional
signals at ꢁ79 (deviation ꢂ3 ppm) and ꢁ137 (deviation ꢂ1 ppm)
for n Z 6, which are assigned to the ladder (L) compounds
{[(Me3SiCH2)ClSn(CH2)nSnCl(CH2SiMe3)]O}2 (31–35; n ¼ 6–8,
10, 12), which are in equilibrium with the double ladder (DL)
clusters 26–30 [eqn. (2)]:
Conclusions
In an effort to answer the question posed in the title, oligo-
methylene-bridged double ladders {[(Me3SiCH2)ClSn(CH2)n
SnCl(CH2SiMe3)]O}4 (25–30; n ¼ 5–8, 10, 12) with up to twelve
carbon atoms in the spacer were synthesised. It was noted that
the yields decreased with increasing spacer lengths from 96% (
n ¼ 5) to 35% (n ¼ 12) in favour of oligomeric or polymeric
products, which presumably have random cross-linked struc-
tures. In solution the double ladder compounds 26–30 are in
equilibrium
with
the
ladder
compounds
{[(Me3-
SiCH2)ClSn(CH2)nSnCl(CH2SiMe3)]O}2 (31–35; n ¼ 6–8, 10,
ð2Þ
12), whereby the population of the latter increases along with
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 6 8 – 1 2 7 6
1273

View Article Online
Chart 2
the spacer length n. In the context of this equilibrium, it may be
Harvard 22 syringe pump delivered the solutions to the vapor-
isation nozzle of the electrospray ion source at a flow rate of
10 mL minꢁ1. Nitrogen was used as both a drying gas and for
nebulisation with flow rates of approximately 200 and
20 mL minꢁ1, respectively. Pressure in the mass analyser region
was usually about 4 ꢃ 10ꢁ5 mbar. Typically 10 signal-averaged
spectra were collected.
interesting for a future investigation to compare the catalytic
activity of 25–30 in organic reactions, such as transesteri-
fications or the formation of urethane, to get further insights
into the operative mechanisms for tetraorganodistannoxane
catalysts.3
Experimental
Syntheses
General information
a,x-Bis(iododiphenylstannyl)alkanes IPh2Sn(CH2)nSnPh2I
(1–6; n ¼ 5–8, 10, 12). A solution of iodine (50.8 g, 200.0 mmol)
in CHCl3 (2 L) was added dropwise to a magnetically stirred
solution of the appropriate Ph3Sn(CH2)nSnPh3 (100.0 mmol)
in 200 mL of CHCl3 at 0 1C. After complete addition the
solution was allowed to warm to room temperature and was
stirred overnight. Removal of the solvent in vacuo and the
phenyl iodide at 80 1C/10ꢁ3 Torr for 2 h gave a light brown oil
in quantitative yields that was used without purification.
Solvents were dried and freshly distilled prior to use. Air-
sensitive compounds were handled under vacuum or argon
using standard vacuum line and Schlenk techniques. The a,o-
bis(triphenylstannyl)alkanes, (Ph3Sn)2(CH2)n (n ¼ 5–8, 10, 12),
were synthesised according to literature procedures.12 The H,
1
13C, and 119Sn NMR spectra were recorded using Jeol GX 270,
Varian 300 Unity Plus and Jeol Eclipse Plus 400 spectrometers
and were referenced to SiMe4 (1H, 13C) and SnMe4 (119Sn). The
elemental analyses were performed on an instrument from
Carlo Erba Strumentazione (Model 1106). The IR spectra were
recorded using a BioRad FTIR spectrometer. Electrospray
mass spectra were obtained with a Platform II single quadru-
pole mass spectrometer (Micromass, Altrincham, UK) using
an acetonitrile mobile phase. Acetonitrile solutions (0.1 mM)
of the compounds were injected directly into the spectrometer
via a Rheodyne injector equipped with a 50 mL loop. A
a,x-Bis[(trimethylsilylmethyl)diphenylstannyl]alkanes (Me3-
SiCH2)Ph2Sn(CH2)nSnPh2(CH2SiMe3) (7–12; n ¼ 5–8, 10, 12).
A Grignard solution prepared from Me3SiCH2Cl (26.99 g,
220.0 mmol) and Mg (5.88 g, 242.0 mmol) in 200 mL THF
was added via cannula to a solution of the appropriate
IPh2Sn(CH2)nSnPh2I (100.0 mmol) in 400 mL of THF. The
1274
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 6 8 – 1 2 7 6

View Article Online
Table 3 Crystal data for 25, 27, 29 and 30
25
27
29
30
Formula
M
C52H128Cl8O4Si8Sn8
C60H144Cl8O4Si8Sn8
C72H168Cl8O4Si8Sn8 ꢀ 5 C7H8
C80H184Cl8O4Si8Sn8 ꢀ 2 C7H8
2275.38
Monoclinic
P21/c
2387.59
Monoclinic
C2/c
2878.37
Monoclinic
C2/c
2852.60
Triclinic
P ꢁ 1
Crystal system
Space group
a/A
b/A
18.351(2)
24.823(2)
12.2041(9)
90
38.6254(7)
12.3153(2)
21.5662(5)
90
49.801(1)
25.1497(7)
12.1328(3)
90
13.169(2)
22.458(6)
11.489(1)
94.79(1)
98.670(9)
76.67(2)
3264(1)
1
c/A
a/1
b/1
94.768(5)
90
101.3916(7)
90
100.687(2)
90
g/1
U/A3
5540.0(8)
2
2.079
10 056.6(3)
4
2.295
14 932.5(7)
4
1.561
Z
m/mmꢁ1
1.780
No. reflns collected
No. indep. reflns
No. obsd reflns [I 4 2s(I)]
R1 (F) [I 4 ns(I)]
wR2 (F2) (all data)
a
26 734
9720
2828
41 657
9194
4994
70 387
17 007
5254
15 561
14 913
9223
0.074 (n ¼ 2)
0.215
0.046 (n ¼ 2)
0.063
0.045 (n ¼ 3)
–
0.053 (n ¼ 3)
–
0.0911
–
0
–
0.048
–
0.051
0.00001
wR [I 4 3s(I)]
g
–
–
–
0
reaction mixture was heated at reflux overnight. After cooling to
room temperature and hydrolysing, initially with saturated
NH4Cl solution (50 mL) and then water (200 mL), most of
the THF was removed in vacuo. The reaction mixture was
extracted with diethyl ether (2ꢃ 400 mL) and the combined
organic phases dried over Na2SO4. The solution was filtered and
the solvent removed in vacuo. After heating at 80 1C and 10ꢁ3
Torr for 2 h to remove all volatiles, a light brown oil
was obtained in quantitative yields that was used without
purification.
Crystallography
Single crystals of 25, 27, 29 and 30 suitable for X-ray analysis
were obtained from toluene solutions at 4 1C. Crystallographic
parameters and refinement details are given in Table 3.y
Intensity data for colourless crystals of 25, 27, 29 ꢀ 5(toluene)
and 30 ꢀ 2(toluene) were measured at 173 K on a Nonius Kappa
CCD for 25, 27 and 29 and a Rigaku AFC7R for 30 (measured
at the University of Adelaide), using Mo Ka radiation. The
structures were solved by heavy-atom methods13 and refined
(anistropic displacement parameters and H atoms in their
calculated positions) with a weighting scheme w ¼ 1/[s2(Fo2) þ
aP2] where P ¼ (Fo2 þ 2F2c)/3) on F2 with SHELXL-97 for 25
and 27,14 and a weighting scheme of the form w ¼ 1/[s2(Fo) þ
g|F|2] for 29 ꢀ 5(toluene) and 30 ꢀ 2(toluene) on F using teX-
san.15 Disordered solvent toluene molecules were located in the
structures of 29 ꢀ 5(toluene) and 30 ꢀ 2(toluene) and were mod-
elled as idealised groups.15 The molecular structures (OR-
TEP,16 35% displacement parameters) are shown in Figs. 1,
3–5. Data manipulation was performed using teXsan.15
a,x-Bis[(trimethylsilylmethyl)dichlorostannyl]alkanes (Me3-
SiCH2)Cl2Sn(CH2)nSnCl2(CH2SiMe3) (13–18; n ¼ 5–8, 10, 12).
A suspension of the appropriate (Me3SiCH2)Ph2Sn(CH2)nSnPh2
(CH2SiMe3) (100.0 mmol) in 500 mL concentrated HCl was
stirred overnight at 60 1C. The crude product was extracted with
400 mL of dichloromethane, washed with 1 M HCl (2ꢃ 100
mL) and dried over Na2SO4. After filtering, the solvent was
removed in vacuo and the residue recrystallised from hexane at
ꢁ20 1C to give a colourless solid.
Acknowledgements
The Australian Research Council, the Deutsche Forschungsge-
meinschaft, and the National University of Singapore (R-143-
000-186-112) are thanked for their support of this work.
a,x-Bis[(trimethylsilylmethyl)stannoxy]alkanes Me3SiCH2(O)
Sn(CH2)nSn(O)CH2SiMe3 (19–24; n¼ 5–8, 10, 12). A solution
of the appropriate (Me3SiCH2)Cl2Sn(CH2)nSnCl2(CH2SiMe3)
(30.0 mmol) in 200 mL toluene was combined with a solution
of KOH (13.5 g, 240 mmol) in 200 mL H2O and stirred at reflux
overnight. After cooling, the precipitate was collected by filtra-
tion, washed with water (1 L) and dried at 80 1C to give a
colourless solid.
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