Proof of Concept Studies Directed Towards Designed Molecular Wires: Property-Driven Synthesis of Air and Moisture-Stable Polystannanes

Article abstract of DOI:10.1002/chem.201703453

Polystannanes with azobenzene moieties designed to protect the Sn–Sn backbone from light- and moisture-induced degradation are described. The azo-stannyl precursor 3 (70 %) is converted in good yields (88–91 %) to the mono- (4), and dichlorostannanes (5),

Full text of DOI:10.1002/chem.201703453

A Journal of
Accepted Article
Title: Proof of Concept Studies Directed Towards Designed Molecular
Wires: Property Driven Synthesis of Air and Moisture-Stable
Polystannanes
Authors: Daniel Foucher, Jeffery Pau, Alan J Lough, Robert Steven
Wylie, and Robert A Gossage
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To be cited as: Chem. Eur. J. 10.1002/chem.201703453
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Proof of Concept Studies Directed Towards Designed Molecular
Wires: Property Driven Synthesis of Air and Moisture-Stable
Polystannanes
Jeffrey Pau,[a] Alan J. Lough,[b] R. Stephen Wylie, [a] Robert A. Gossage, [a] and Daniel A. Foucher*[a]
Abstract: Polystannanes with azobenzene moieties designed to
of tin, provides additional  delocalization, typically
protect the Sn-Sn backbone from light- and moisture-induced
resulting in a redshift of the visible absorption.[8] The
degradation is described. The azo-stannyl precursor 3 (70%) is
intriguing electronic properties of catenated stannanes come
at the expense of long and relatively weak bonds between tin
centres. Previous solution and solid state stability studies of
polystannanes reveal that they readily degrade into smaller
tin oligomers or cyclic species when exposed to visible light
(350–500 nm) even for very short time periods.[9],[10]
Although stable to oxidation, polystannanes are readily
hydrolyzed to linear and cyclic oligostannoxanes when
exposed to moist air.[11]
converted in good yields (88-91%) to the mono- (4), and dichloro- (5)
stannanes, by sequential chlorination, followed by further reduction of
5 to the dihydride (6) using NaBH4 (78%). All stannanes were
characterised by NMR (1H, 13C, 119Sn) spectroscopy and HRMS; in
addition, 3, 4 and 5 were structurally elucidated via X-ray diffraction
analysis. Metal-free dehydrocoupling of 6 at RT leads exclusively to
homopolymer (7-i) displaying an initial solution 119Sn NMR signal ( =
-196 ppm) that migrates to -235 ppm after 10 days (7-f). In contrast,
metal-catalyzed dehydrocoupling of 6 in toluene at RT leads directly
7-f. Random co-polymers formed from 6 and (n-Bu)2SnH2 at 4:1 (8a)
and 1:1 (8b) ratios were compared to the alternating polystannane (9)
prepared by the reaction of 6 with (n-Bu)2Sn(NEt2)2. DFT calculations
of 3-6 indicate that hypercoordination at Sn is influenced by
substituents and by solvation. Homopolymer 7 was found to have
unprecedented moisture and light stability in the solid state for > 6
months.
We have recently shown that the overall chemical and
photostability of polystannanes can be enhanced with the
incorporation of flexible hyper-coordinated (e.g., five-
coordinate) tin centers.[12] This change in coordination
number from four to five is surmised to decrease the Lewis
acidity at tin; this in turn leads to an increase in light stability
(> 90 d)[12] and a modest improvement with respect to
hydrolysis. This platform utilizes a non-rigid hydrocarbon-
based chain connected to an aryl or biaryl unit via an ether
linkage. This latter functionality was designed to promote O-
chelation and provide steric bulk.
Introduction
As electronic components become increasingly reduced in
size, the need for “molecular wire”[1] interconnects in the
same dimension becomes essential.[2] Molecular or
polymeric wires would ideally be air-stable, easily processed
or printed, flexible nano-materials consisting of a single chain
of atoms.[3] This chain would be capable of electron transport
along the backbone and ideally exhibit excellent conductive
properties. A further advantage would be their production
from elements that are high in natural abundance and
potentially recyclable. The bonding in all later Group 14
homopolymers (i.e., Si, Ge or Sn-based) feature, along the
polymer backbone, extensive  overlap of the (formally)
large and diffuse sp3 metal-centred hybrid orbitals.[4–7] The
unique structural and electronic configuration(s) in these
homopolymers results in relatively narrow band gaps (2-4
eV) and a visible * absorption related to their pronounced
semi-conducting nature. The incorporation of aromatic
groups onto the Group 14 element, most notably in the case
To further enhance the stability of polystannanes, the
application of a similar bulky alkoxy group, however now
incorporating an azobenzene moiety, is herein investigated.
Azobenzenes are biaryl species linked by a -N=N- bridge,[13]
and absorb specific wavelengths of visible light while
transmitting or reflecting others.[14] Azobenzenes are highly
coloured and used extensively in the food, drug and
cosmetics industries and account for approximately 60% of
the worlds’ production of industrial dyes.[14],[15] Azobenzenes
have also recently been incorporated for use in dye-
sensitized solar cells and in the prototyping of high density
solar thermal cells.[16]
Barrett and others have also
the harnessing of
successfully demonstrated
photomechanical energy of azo-functionalized “walking”
polymers; and has further described their suitability as key
components for light driven plastic motors.[17] We envisioned
that this light absorbing unit might lead to greatly enhanced
polystannane stability with respect to the simple aryl or biaryl
groups that we previously studied.[9]
[a]
[b]
J. Pau, Prof. R. S. Wylie, Prof. R. A. Gossage, Prof. D. A. Foucher
Department of Chemistry and Biology
Ryerson University, Toronto, Ontario Canada
350 Victoria Street, M5B 2K3
E-mail: daniel.foucher@ryerson.ca
Dr. Alan J. Lough,
Department of Chemistry
University of Toronto, Toronto, Ontario Canada
80 St. George Street, 2M5S 3H6
Supporting information for this article is given via a link at the end of
the document.
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except we have found that a slight modification greatly improves
product yields. Hence, the reaction was carried out in a sealed
flask, recovering 1 as a white coloured solid in quantitative
yield.[16] The attempted tosylation of 1 using Christoffers et al.
procedure (employing pyridine: RT; 5h) was unsuccessful and
resulted in only the recovery of starting materials.[16] However,
tosylation of 1 to 2 can be achieved (60%) in a 2 h time frame
using Yoshida’s procedure (Scheme 1),[20] wherein a catalytic
amount of the ammonium salt, Me3N•HCl, along with NEt3, are
added to the reaction mixture.
Figure 1. The light-induced trans-/cis- (i.e., (E)/(Z)) isomerisation of azobenzene.
An important characteristic of the azobenzene class of
molecules is the photoisomerization between the cis- and
trans- (i.e., (Z) to (E)) forms (Figure 1). The (E)-isomer is
rapidly converted to the (Z)-form when exposed to light
wavelengths between 320–350 nm. Absorption bands
equivalent to the S0 → S2 state are employed, corresponding
to the energy gap of a * symmetry-allowed transition.
The reaction is fully reversible and the (E)-isomer is
regenerated when the more sterically hindered (Z)—
compound is exposed to either visible light (400–450 nm) or
by the application of heat.[18] This (Z)- to (E)- conversion
results in absorption bands equivalent to the S0 → S1 state
and correspond to the energy gap of a n* symmetry-
forbidden transition.
This investigation involves a multi-step preparation of an
azobenzene-stannyl dihydride monomer, which is
subsequently polymerised to azobenzene containing homo-
and co-polymers via dehydropolymerisation. A second
polystannane of the alternating copolymer class, also
containing an azobenzene functionality, was prepared by a
Scheme 1. Tosylation conditions used to prepare 2.
Initial efforts to prepare
3 (Scheme 2) via substitution of
4-hydroxyazobenzene (HAB) with allyl bromide, followed by
complimentary condensation method.
The bulky
hydrostannylation with Ph3SnH, were not successful.
azobenzene unit was designed to serve two purposes: 1) to
act as a “photoantenna” whose purpose is to absorb and/or
reflect visible and UV light energy with the function of
reducing photoscission events along the polystannane chain;
and 2) to incorporate steric bulk and potential ligation to
facilitate protection of the Sn backbone from nucleophilic
attack. In this regard, the characteristic (Z)/(E)-
photoisomerization of the azobenzene chromophore in
proximity to either a four- or five-coordinate geometry in all
tin species is revealed. In addition, oligomeric and polymer
models and complete characterization of monomer and pre-
monomer compounds is supplemented by DFT calculations
and UV-Vis spectroscopy where appropriate.
However, the synthesis of
3 can be achieved by reacting
HAB with
2
under SN2 displacement conditions.[21] This
process results in the recovery, after column
chromatography, of an air and moisture stable orange
coloured solid
3 (70%).
Results and Discussion
Synthesis
Alcohol 1 (Scheme 1) has been previously described by Wardell
et al.[19] using a preparation involving the treatment of Ph3SnH and
an excess of allyl alcohol (in a solventless environment) at 80-
95 °C in the presence of AIBN. This gives 1 in modest yields
(~58%). In our hands, the same reaction conditions were followed
Scheme 2. SN2 Displacement reaction leading to 3.
Compound
chlorination (Scheme 3) to the mono- (
stannanes ( ) in good yields (88% and 91%, respectively)
without the need for column chromatography.[22] Following
3
was subsequently converted via stepwise
4
), and dichloro-
5
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conditions traditionally used to obtain tin dihydrides,
5 was
Table 1. Select NMR data for compounds 3-6.
reacted with LiAlH4 (Et2O: 0 ºC; 3 h). However, this potent
source of H– appeared to decompose the starting material.
When a more moderate hydrogenating agent, NaBH4, was
1H NMR
(ppm)
Sn-CH2-CH2-
CH2O
2.25
119Sn NMR
(ppm)
Sn-CH2
CH2-O
used
5 was cleanly converted to 6 (78%) and thereafter
3*
4*
5*
1.66
1.94
2.18
1.02
4.03
4.14
4.22
3.46
-99.7
-15.3
-9.06
-214.5
isolated as an orange-yellow coloured powder (Scheme 3).
2.43
2.56
1.80
NMR Spectroscopy
6**
1
The H NMR (CDCl3) spectra of
3
-
6
(Table 1) display three
*CDCl3. **C6D6.
distinct sets of methylene resonances with large 119/117Sn-H
coupling constants (J) observed for the CH2-Sn resonance.
The tin atoms of both conformers observed in the solid-state
of (Figure 2) are pseudo-tetrahedral ( 4 = 0.85,
4’ = 0.87)[25]
with a Sn to O distance of ~4.9 Å, which lies well outside of
the sum of the van der Waals radii (Sn-O 2.3 Å). In contrast,
The Sn-H resonance signal of
with distinct 119/117Sn satellites (1J119Sn-1H = 918 Hz, 1J117Sn-
1H = 878 Hz). The methylene protons in
are shifted upfield
in comparison to and the chloro-containing stannanes
and also indicates that two
. Interestingly, the 1H NMR of
isomeric species are present in solution. The H NMR of
6 is observed at 5.51 ppm,
3


6
3
4
the mono-
4
, and dichloro species
5
(Figures 3 and 4) are
5 = 0.46)[26] and
5
6
formally five-coordinate (
4
:
5 = 0.85, 5:
1
6
possess Sn-O distances of 2.775 Å and 2.729 Å, in line with
other five-coordinate Sn species which have a dative oxygen
interaction.[24]
taken initially t = 0 , 6 and 24 h are shown in Figure S1 and
displaying isomer ratios of 10:1, 4:1 and 1:1 respectively. In
contrast, the corresponding 119Sn NMR signals at t = 0, 6
and 24 h show no change in the tin resonance behaviour and
this value is typical of a four-coordinate tin dihydride
species.[23] We therefore ascribe these observations as likely
being related to a light induced (E)/(Z) isomerization of the
azobenzene fragment. The 119Sn NMR resonance for
found at = -100 ppm, a value typical of tetrahedrally
coordinated organotins.[21] By comparison, the 119Sn NMR
resonances for and are significantly deshielded ( = -15.3
ppm and = -9.06 ppm, respectively). These downfield
resonances are more consistent with a five-coordinate
geometry for both and
in solution.[24]
Further evidence for the preferred geometries of
compounds and was obtained by single crystal X-ray
diffraction studies (see appendices B1, B2 and B3).
3 is

4
5


4
5
3
,
4
5
Figure 2. ORTEP representation of one of the conformers of
3 found in
the unit cell. Selected interatomic distances [Å] and angles [º]: Sn(1)-C(16)
2.136(3), Sn(1)-C(22) 2.137(3), Sn(1)-C(28) 2.131(3), Sn(1)-C(1)
2.147(3), C(28)-Sn(1)-C(16) 112.06(11), C(28)-Sn(1)-C(22) 107.98(11),
C(16)-Sn(1)-C(22) 106.58(12), C(28)-Sn(1)-C(1)109.26(12), C(16)-Sn(1)-
C(1) 108.00(12), C(22)-Sn(1)-C(1) 113.01(12).
Scheme 3. Stepwise chlorination of
3 to 4 and 5, and hydrogenation to 6.
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Physical changes (colour change, trace precipitate formation
<10%) were observed and these changes thus characterized
(NMR). The transparent orange colour characteristic of
6 in
C6D6 noticeably darkened, along with the deposition of
precipitate from the solution, after 24 h. There was an
evident decrease in the relative integration of the 119Sn NMR
signal (i.e., the -215 ppm signal for
5) and a new resonance
appeared at -196 ppm (for ). This value is typical of mixed
7-i
aryl/alkyl polystannanes containing a propyloxy linkage.[12]
After 5 days, the Sn-resonance at -215 ppm had completely
vanished with only the resonance at -196 ppm remaining.
After 6 days, the colour of the observed semi-solid material
changed from a dark yellow-orange to a bright yellow colour
(Figure S2). Examination by 119Sn NMR spectroscopy
revealed that the -196 ppm (presumed) polymer resonance
was also decreasing in intensity and another new resonance
at -235 ppm (7-f
) began to appear. 119Sn NMR monitoring
from the 7th to 10th day showed a further decrease in the
intensity of the former resonance and a continued growth of
the latter. The identity and possible structure of the
polymeric component displaying the higher field 119Sn
resonance will be discussed in the next section (vide infra).
Figure 3. ORTEP representation of
4 found in the unit cell. Selected
interatomic distances [Å] and angles [º]: Sn(1)-C(16) 2.155(3), Sn(1)-
C(22) 2.126(3), Sn(1)-Cl(1) 2.4099(8), Sn(1)-C(1) 2.137(3), C(22)-Sn(1)-
C(1) 124.05(14), C(22)-Sn(1)-C(16) 111.35(12), C(1)-Sn(1)-C(16)
116.35(13), C(22)-Sn(1)-Cl(1) 102.43(8), C(1)-Sn(1)-Cl(1) 97.67(9),
C(16)-Sn(1)-Cl(1) 98.71(9).
Homopolymerisation of 6 with catalytic amounts of
RhCl(PPh3)3
The homopolymer
7-f was also prepared by the slow addition
of (over 15 min) to a stirring toluene solution containing 4
6
mol% of catalyst (RhCl(PPh3)3) at RT for 4 h (Scheme 4).
After this time, the sample volume was reduced in vacuo and,
using a double-tipped cannula syringe, the liquid was
transferred into a 10-fold excess of cold hexanes where an
orange-yellow coloured semi-solid thereafter precipitated.
This presumed polymer was recovered in 77% yield by
carefully decanting off the hexanes followed by drying (in
vacuo). The polymer is readily soluble in C6D6 and a 119Sn
NMR spectrum of this material revealed a chemical shift (-
236 [± 1] ppm: Table 2), similar to the value observed in the
final product of the metal-free dehydrocoupling reaction.
Polymer 7-f possesses a Tg of 22°C, with no evidence of a
melt transition. A GPC analysis indicated that a moderate
molecular weight polymer was indeed present (Table 2).
Figure 4. ORTEP representation of
5 found in the unit cell. Selected
interatomic distances [Å] and angles [º]: Sn(1)-C(16) 2.109(3), Sn(1)-
C(15) 2.120(3), Sn(1)-Cl(1) 2.3917(8), Sn(1)-Cl(2) 2.3469(7), C(15)-
Sn(1)-Cl(1) 101.40(9), C(15)-Sn(1)-Cl(2) 106.93(9), C(16)-Sn(1)-Cl(2)
107.04(7), C(16)-Sn(1)-Cl(1) 101.48(7).
Metal-Free Dehydrocoupling of 6
Metal-free dehydrocoupling of
6
(Scheme 4) was observed
in a Teflon sealed
over a 10 d timeframe. A C6D6 solution of
6
NMR tube protected from ambient light sources was
monitored initially by NMR (1H and 119Sn) spectroscopy.
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polystannanes. Such an alternating polymer, 9, consisting of
a 1:1 ratio of azo-stannyl and dibutyltin units was constructed
using this strategy. The polymerisation was carried out at
0°C for 4 h in Et2O solution, as shown in Scheme 6. The
purification and recovery was similar to that used for the
other polymers in this study. After washing with several
portions of cold hexanes and drying in vacuo, an orange
coloured, low molecular weight semi-solid was recovered
(53% yield). A noteworthy property of this material is that
three resonances were detected in the 119Sn NMR spectrum
(Table 2) indicating that two structural motifs may be present
in the polymer backbone (vide infra).
Characterization and Potential Structures of Polymers 7, 8a-
b, 9
Scheme 4: Dehydropolymerisation of
6 to 7.
As noted above, 119Sn NMR resonances for the polymers
(Table 2) indicate that at least two structural forms may be
present due perhaps to variations in the structural backbone
of such prepared under metal-free conditions. This
characteristic is also observed for the alternating polymer.
Polymers such as 10, with similar flexible propyloxy ligands
capable of datively bonding to Sn atoms, typically display
Synthesis of random co-polymer 8a (4:1) and 8b (1:1) using
catalytic amounts of RhCl(PPh3)3
Two random co-polymers 8a-b were also prepared (Scheme
5) by incorporating (n-Bu)2SnH2, at two different loadings
(4:1 and 1:1), during the metal-catalysed polymerisation.
This was intended to produce a material with a modified
119Sn resonances between -190 and -197 ppm.[12],[21]
plausible explanation for the observed spectroscopic
behaviour herein is that there are datively bonded (O Sn)
A
(improved) solubility profile.
polymer recovery were carried out under similar conditions
used to prepare homopolymer These polymers, also
The polymerisations and
7
.

semi-solid in nature, were dried under reduced pressure and
orange coloured gels were thus isolated (Figure S3). The
and unbound propyloxy species along the polymer chain.
1
soluble polymers were characterized by H, 13C and 119Sn
NMR, elemental analysis, and GPC (Table 2). Characteristic
of these random copolymers are two distinct 119Sn
resonances (-168 and -243 ppm) that have been assigned to
the -(n-Bu)2Sn- and -(Ph)Sn-(CH2)3-O-(C6H4N=NC6H5)-
segments, respectively.
Scheme 6. Preparation of alternating polymer 9.
Further, exposure to ambient light sources, even for a short
time, may also cause (E)/(Z)-isomerization in the
azobenzene moiety, and the additional crowding of this
organic fragment in the (Z)-conformer could facilitate its
release from dative interactions. Alternatively, dative
interactions may also include lone pair electrons found on the
-N=N- unit of the azobenzene. In cases where small
molecule stannanes are found in tetrahedral environments
Scheme 5. Preparation of random copolymers 8a and 8b
.
by X-ray crystallography, such as in
shift (-100 ppm) is typically observed.[12] Similarly, it is
expected that species such as dihydride likely adopt a
similar distorted tetrahedral geometry because of the
3, an upfield chemical
Alternating polymerisation of 6 and (n-Bu)2SnH2
We have previously demonstrated the facile condensation
polymerisation of diaryl- or dialkyl tin dihydrides with dialkyl
tin diamides.[27] This led to the first examples of alternating
6
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electronic environment at Sn; this is assumed to be more
Lewis basic due to the attached hydrides. Resonances
signals for this class of Sn atoms are also found upfield.[28]
These factors lead one to the suggestion that the non-metal
catalysed route to the initially observed homopolymer 7-i
forms in a presumed kinetically unstable conformation. This
may be due to interactions such as those described above
along the polymer backbone. Over time in solution, these
fragments could re-organise to give a thermodynamically
more stable end-product 7-f. In a constrained system, such
as a highly substituted polystannane, it is logical to
hypothesise that this process could be kinetically slow as
observed. The metal-catalysed route to
dispose these polymer groups to give 7-f directly: as
observed by 119Sn NMR spectroscopy.
A plausible
7 appears to pre-
Figure 5. Bound and unbound polystannanes.
explanation for this is that the Rh-bound stannyl groups are
required to arrange themselves into a preferred conformation
before the next Sn-Sn bond can be formed during
polymerisation. For random co-polymers 8a
alternating polymer , the downfield signal at
,
8b and the
9

= -167 ppm
most likely represents the -(n-Bu)2Sn- unit and the
resonances at = -243 ppm the azo-stannyl building block.
For
, the alternating polymer presents three 119Sn NMR
resonances (Table 2) when the polymerisation was carried
out by adding the diamide-tin to
The 119Sn NMR
resonances for therefore suggest that there may be both

9
6
.
9
Figure 6. GPC chromatogram (RI) of polymers 7, 8a, 8b, and 9.
coordinated and free propyloxy units (Figure 5: vide supra).
Computational studies
Table 2. Characterization data for 7, 8a, 8b, 9 and 10.
DFT calculations of 3-6 were carried out to gain insight into
substituent and solvation effects on hypercoordinate bonding at
the Sn centre. B3LYP geometry optimizations of 3-5 using the
crystallographic molecular structures as initial guesses converged
to equivalent conformations, with structural parameters in
reasonable agreement with experiment. The calculated Sn-O
separation distances were larger in each case (e.g. for 5, Sn-O
was 3.04 Å (calc.) as compared to 2.73 Å (expt.)), but reproduced
the trend in diminishing Sn-O separation distance observed in 3-
5, as well as other structural differences.
119Sn NMR
ppm (C6D6)
Mw (Da)
× 104
Tg
(°C)
Polymer
Ð
7-i, 7-f
No Cat.
7-f Cat.
8a (4:1)
-196, -235
-
-
-
-236
-168, -243
2.85
3.53
1.1
2.4
4.1
2.0
2.1
2.0
1.8
22.0
N/A
1st 7.80
2nd 2.84
1st 1.60
2nd 1.22
2.45
8b (1:1)
-167, -241
N/A
9
-167, -197, -240
-195
N/A
64
10[21]
The GPC and DSC data are also provided in Table 2.
Only homo-polymer possessed a detectable Tg value,
The flexible propyloxy linkage between Sn and the
azobenzene chromophore allows more than 30 possible
conformations for each compound. A complete determination of
the extent of hypercoordination for each compound would require
a comprehensive conformational search followed by geometry
optimization, frequency calculations and Boltzmann weighting for
each conformer and then partitioning into fractions of
hypercoordinated and non-hypercoordinated conformers.
Examining the effect of solvation would necessitate repeating
these calculations for each solvent considered. For the sake of
expediency, we adopted the simpler approach of comparing the
properties of two extremes: the fully extended conformer in which
the propyloxy linkage has a trans-planar geometry (the “open”
7
lower than that observed for polystannanes such as 10. The
GPC chromatograms for all four new polymers in this study
are shown in Figure 6. All polymers were bimodal and of
moderate molecular weight. The corresponding values of
the lower Mw fractions for
7 and 8b could not be determined.
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Table 3. B3LYP Calculated Relative Free Energies for 3-6
conformer) and the conformer in which the linkage O atom most
closely approaches the Sn atom (the “closed” conformer), as
shown for 5 in Figure 7. These correspond, respectively, to the
longest and shortest Sn-O separation distances within the set of
conformers for each compound. In the absence of any stabilizing
interaction between the Sn and O atoms, the open conformer
should be the lowest energy conformer, since torsional strains
and van der Waals repulsions are minimized. The closed
conformer has substantial torsional strain due to the ring-like
structure involving the propyloxy linkage and Sn.
Boltzmann
weight of
closed
conformer
relative to
open
Gopen
–
Compound
Solvent
Gclosed
(kJ mol-1)
conformer at
298K
–
-8.2
-12.3
-15.8
-1.3
0.038
3
4
5
6
0.0073
0.0017
0.59
C6H6
CHCl3
–
0.17
C6H6
CHCl3
–
-4.3
0.55
-1.5
0.72
-0.82
1.33
3.02
-9.3
1.7
C6H6
CHCl3
–
3.4
0.024
0.56
C6H6
CHCl3
-1.4
1.7
1.3
Figure 7. Two conformational extremes for 5, (H atoms omitted for clarity)
optimized structures (B3LYP/6-31(d,p):H,C,O,N,Cl; LANL08d:Sn).
Summed electronic and thermal free energies were
determined for both conformers for each compound in the gas
phase and in benzene and chloroform solvents using the
As a rough first approximation to modelling hypercoordination in
the polymer 7, we calculated the relative free energies (B3LYP)
for open and closed conformations of a Sn trimer: Me3Sn-Sn(Ph)(-
(CH2)3-O-(C6H4N=NC6H5)-SnMe3. In the gas phase, Gopen - Gclosed
is -8.0 kJ mol-1, while in C6H6 solution the difference is -10.1 kJ
mol-1. These results suggest that conformers exhibiting
hypercoordination may not predominate in C6H6 solutions of the
hypothetical Sn trimer. The extent to which a simple trimeric
model is an adequate representation for 7 may be questioned; the
development of more sophisticated computational models for Sn
polymers is currently in progress. The above result is best viewed
as a benchmark for such hypercoordination phenomena in
oligostannane models.
Polarizable Continuum Model (PCM).[29]
The gas phase
calculations (Table 3) show the effect of substitution on the Sn
centre on the relative stabilities of the open and closed
conformers: as phenyl substituents are replaced with chloro
ligands in the series 3-5, the closed form becomes increasingly
stable. This is consistent with greater Lewis acidity at the Sn
centre resulting from substitution with the more electronegative
chloro substituent. Like 3, the dihydride compound 6 has a large
gas-phase calculated free energy difference between the open
and closed conformers, with the open conformer significantly
more stable, and the closed conformer calculated to make only a
very small contribution to Boltzmann weighted average properties.
UV-Vis spectroscopy and Polymer stability
UV-Vis spectroscopy of all the azo-containing tin compounds
(5-9) were performed using benzene solutions of the
materials. Unsurprisingly, all compounds showed nearly
identical absorption characteristics assigned to azo-
containing units presumably absorbing energy to initiate
The calculations indicate that solvation significantly alters
the relative stability of the open and closed conformers and that
the magnitude of the effect varies by compound. In the case of 3,
the open conformer becomes substantially more stable in C6H6
and CHCl3, while for 5, the opposite is observed. The largest
predicted changes in relative stability are observed for the
dihydride 6, with the closed conformer strongly disfavoured in the
gas phase but roughly comparable in stability in C6H6 or CHCl3
solution. The differences in solvation behaviour likely arise from
variations in molecular charge distribution in open and closed
conformers as well as between compounds. The significantly
different dipole moments calculated for the compounds and
conformations studied are consistent with this.
(E)/(Z) transformations in solution.
More intense
absorbances were observed for all polymers in comparison
with the monomeric species. Five consecutive scans of the
all azo-containing tin polymers showed no evidence of
degradation or photoscission when left in solution.
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10.1002/chem.201703453
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also provides information as to the likely stability profile of
polystannanes which contain Sn atoms devoid of such
protecting groups (i.e., the -R2Sn- units). Neighbouring azo-
benzene fragments do not appear to impart stability over the
global polymer network. This suggests that the future design
of mixed-polystannane polymers should incorporate
monomers that are rich in protective functionalities such as
light absorbing groups and fragments with chelation
potential. Such concepts are currently being explored by us.
Experimental Section
Electronic Supplementary Information (ESI) available:
Figure 8. UV-Vis (C6D6) spectroscopy of polymers 8b and 9.
Crystallographic data for compounds
3 (CCDC1549269), 4
(CCDC 1550051), and (1549271) have been deposited with
5
Further evidence of the stability of the azo-containing
polymers was observed for a 100 mg sample of solid 7-f
which was left exposed to moist air and sunlight for extended
intervals. After 6 months, the colour of the semi-solid
darkened from a light orange-colour to a somewhat darker
shade of orange. However, there was no observable change
in the solubility properties of this material nor the NMR
the Cambridge Crystallographic Data Centre. See
DOI: 10.1039/b000000x/
All synthetic details and experimental results are provided in
the ESI.
(
119Sn) resonances (-236 ppm only). This suggests long term
stability with only the outer surface of this solid material being
adversely affected. By comparison, when the gel-like orange
Acknowledgements
coloured polymers 8a, 8b and 9 were left inside an open
This work is supported by the NSERC Discovery program.
The computational studies were made possible by the
facilities of the Shared Hierarchical Academic Research
Computing Network (SHARCNET:www.sharcnet.ca) and
amber vial for slightly over one month, they transformed to
insoluble darker brown coloured semi-solids which could not
be further characterized. UV-Vis characterization of solutions
Compute/Calcul Canada
.
of polymers 8b and
9 (C6D6) exposed to daylight, but sealed
from moisture, displayed a decrease in signal intensity (57%
and 23%, respectively) over a 120 day period, whereas 7-f
was unchanged over the same time period (see Figure 8).
Keywords: Polystannanes, Azobenzenes, hypercoordination
The decrease in absorption intensity for 8b and
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Chemistry - A European Journal
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Layout 1:
FULL PAPER
Polystannanes with azobenzene
ligands demonstrate improved
moisture and light stability
Jeffrey Pau, Alan J. Lough, R. Stephen
Wylie, Robert A. Gossage, and Daniel
A. Foucher*
Page 1 – Page 9
Proof of Concept Studies Directed
Towards Designed Molecular Wires:
Property Driven Synthesis of Air and
Moisture-Stable Polystannanes
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