Alternating polystannanes: Syntheses and properties

Article abstract of DOI:10.1039/c5cc00568j

A new condensation polymerization route leading to alternating polystannanes is presented. The stoichiometric reaction of tin dihydrides and tin diamides in diethyl ether or toluene under mild reaction conditions afforded three new moderate molecular weig

Full text of DOI:10.1039/c5cc00568j

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Alternating polystannanes: syntheses and
properties†
Cite this: Chem. Commun., 2015,
51, 7120
Shane Harrypersad and Daniel Foucher*
Received 20th January 2015,
Accepted 18th March 2015
DOI: 10.1039/c5cc00568j
www.rsc.org/chemcomm
A new condensation polymerization route leading to alternating with improved light stability were afforded by utilizing ethyl
polystannanes is presented. The stoichiometric reaction of tin spacers directly attached to tin were also described by Caseri
dihydrides and tin diamides in diethyl ether or toluene under mild et al.18–20,26 This was done in part to facilitate dehydrocoupling
reaction conditions afforded three new moderate molecular weight with a bulky Rh catalyst as a minimum of an ethyl spacer unit
alternating polystannanes, –[Ph2Sn-alt-Sn(n-Bu)2]n–, –[Ph2Sn-alt- must be present separating the tin atoms from an aryl carbon.26
SnMe2]n–, –[Me2Sn-alt-Sn(n-Bu)2]n–, in addition to a known homo-
polymer, –[(n-Bu)2]n–.
Our approach to preparing polystannanes employs condensa-
tion polymerization where both diaryl and dialkyl substituents
can be introduced alternatively into the backbone to ensure
For more than 20 years, dialkyl and diaryl homopolystannanes increased solubility27 and, surprisingly, an improved degree of
have been prepared by reductive coupling of either tin dichlor- light stability. Toluene or Et2O solutions of equimolar quantities
ides1–8 or tin dihydrides.9–23 These intriguing polymers possess of high purity tin dihydrides and tin diamides (NMR) were added
a backbone of s–s delocalized1–3,6–13,15–16 organotins envisioned together at 0 1C to facilitate optimal polycondensation condi-
for use as processable intrinsic semiconductors1–4,7,13,15,22 or tions.28,29 This condensation (Scheme 1) produces volatile
printable polymeric wires.2–6,12 Along with the considerable diethyl amine, which along with the reaction solvent, is removed
synthetic challenges in preparing later Group 14 polymeric in vacuo resulting in the recovery of a relatively pure polymer.
materials is their profound sensitivity to moisture and
Related examples of oligostannanes prepared by transmetala-
¨
light,1,2,7–13,15,18,24–27 increasing down the period. In the case of tion or condensation were described independently by both Drager
polystannanes, the Lewis acidic nature of formally Sn(IV) centers and Sita. Drager et al.30,31 prepared a series of oligostannanes with
¨
results in weak s–s overlap between neighbouring tin centers one to four (t-Bu)2Sn moieties as the central core capped on each
and a likely cause of their inherent susceptibility.11,13,15–20,22,25 end with a Ph3Sn unit. Sita et al.12 carried out a careful, stepwise
Despite the original promise of these materials, a relatively condensation of (n-Bu)2SnH2 with (n-Bu)2Sn(OCH2CH2OEt)(NMe2)
limited number of researchers have continued to pursue in the presence of a strong base to produce linear oligostannanes
¨
improved alternative synthetic methods and stabilization stra- containing 3–15 tin atoms. For both the Drager and Sita oligomers,
tegies with polystannanes. Dialkylpolystannanes, –[R2Sn]n–, a red shift of the s–s* transition was observed with increasing
(R = Et,8–13,15–20,22,24 Pr,8–13,18–20,22 n-Bu,2,8–13,15–20 hexyl,2,13,14–17 etc.) catenation.
are soluble in most organic solvents where as diarylpolystannanes
Alternating polystannanes prepared by polycondensation of
(R = Ph22 etc.) are often considerably less soluble but demonstrate tin hydrides with tin amides are listed in Table 1. Four starting
a dramatic red shift of the s–s* transition in the UV-visible range materials, (n-Bu)2SnH2 (1), Ph2SnH2 (2), (n-Bu)2Sn(NEt2)2 (3)
in addition to their increased stability. Random dialkyl (n-Bu, and Me2Sn(NEt2)2 (4) were prepared in good yield following
octyl, dodecyl) and diphenyl polystannanes were recently pre- literature methods.19,32 The choice of these starting materials
pared by Caseri et al. via a two-step Na Wurtz coupling in liquid was based on ease and safety. Dimethyltin dihydride, Me2SnH2,
ammonia. The recovered polymers are only partially THF
soluble and of moderate molecular weight.25 Polystannanes
Department of Chemistry and Biology, Ryerson University, 350 Victoria Street,
Toronto, Ontario M5B 2K3, Canada. E-mail: daniel.foucher@ryerson.ca
† Electronic supplementary information (ESI) available: Experimental details,
Scheme 1 Preparation of alternating polystannanes.
NMR characterization etc. See DOI: 10.1039/c5cc00568j
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Table 1 Data for tin-containing condensation polymers
Cmpd
119Sn NMR d (C6D6) (ppm)
À191
Tg (1C)
Mw (kDa), PDI
EA
519
1
91, 2.53
Ccalc/found: 40.76/41.25
Hcalc/found: 7.79/7.61
Ccalc/found: 40.76/39.81
Hcalc/found: 7.79/7.13
Ccalc/found: 47.30/46.56
Hcalc/found: 5.95/5.39
Ccalc/found: 34.81/33.72
Hcalc/found: 5.50/5.63
Ccalc/found: 31.30/29.26
Hcalc/found: 6.28/6.37
5
6
7
8
À191
1
117
118
À6
18.1, 2.27
18.8, 2.86
242.5, 2.03
82.1, 2.86
À187, À208
À60, À201
À186, À235
is a gas at room temperature and is extremely toxic.33 All
possible combinations of polymers were then made incorpor-
ating the four compounds listed above.
The polystannane (–[(n-Bu)2Sn]n–), 5, was chosen in order to
validate the condensation polymerization process, as extensive
characterization of this homopolymer has been previously
carried out. Under an inert atmosphere in the absence of light,
an ethereal solution of 1 was added slowly at 0 1C to a Schlenk
flask containing 3 in Et2O and the mixture allowed to react for
3 hours at 0 1C. The solvent and diethylamine co-product were
then removed in vacuo.
Condensations were carried out at 0 1C in an effort to avoid
thermally promoted self-polymerization of the heat sensitive
alkyl or aryl stannyl dihydrides, as well as to limit possible
termination reactions. Polymer 5 was recovered as a brightly
yellow coloured solid and characterized by NMR (119Sn, 1H, 13C),
GPC, UV-Vis spectroscopy and DSC. GPC analysis showed the
polymer to be of modest molecular weight (Table 1). Three new
alternating polymers, 6 (–[Ph2Sn-alt-Sn(n-Bu)2]n–), 7 (–[Ph2Sn-alt-
SnMe2]n–), and 8 (–[Me2Sn-alt-Sn(n-Bu)2]n–) were also synthe-
sized using the same methodology. Their data is listed in
Table 1. The calculated elemental analysis was based on the
relative degree of polymerization of the sample and accounts for
the uncertainty in the process. For more details on the metho-
dology used for this estimation, see ESI.†
Fig. 1 Polystannanes 6 and 7 after exposure to ambient light for 7 days.
formation of a stannoxane. Polymer 5 degraded to a similar
white powder in much the same way as previously described by
Tilley and others.7–10,13,15,25,35
NMR (1H, 13C, 119Sn) analysis of the new alternating polymers
(6, 7, and 8) revealed resonances corresponding to both attached
R groups as well as two well separated 119Sn NMR resonances.
1H NMR spectroscopy for polymers 6–8 support the integration of
both monomer units at an approximately 1 : 1 ratio (6, 1 :0.98
SnPh2 to Sn(n-Bu)2, 7, 1: 0.99 SnPh2 to SnMe2, 8, 1 :0.96 of SnMe2
to Sn(n-Bu)2). The alternating polymers 7 and 8 showed a distinct
downfield 119Sn resonance (À60 and À186 ppm) for the Me
substituents, whereas the Ph (À200 ppm) and n-Bu (À235 ppm)
substituents are found in the region more typical of known cyclic
stannanes and polystannanes. Evidence of strong Sn–Sn coupling
(Fig. 2) was also observed for methylated tin center of 7 (119/117J =
A hydrostannolysis34 reaction of the two difunctional mono-
mers results in the release of HNEt2 and formation of Sn–Sn
bonds; thus promoting a facile condensation polymerization.
This type of condensation has previously been demonstrated in
part by others for tin systems, including oligostannanes,12 but
to our knowledge has not been used exclusively for the pre-
paration of polystannanes. This methodology allows for the
reaction to be carried out in a single vessel, at modest tem-
perature, with short polymerization period(s) and proceeds
without the aid of an external catalyst.
1
4421 Hz) and is similar to reported J couplings for methylated
The phenyl-containing polymer derivatives 6 and 7 (Fig. 1)
showed increased chemical stability to light and moisture in
the solid state with exposure up to 5 days with no apparent loss
of colour or change in the 119Sn NMR spectroscopic profile.
Similar stability of homopolystannanes bearing phenyl deriva-
tives as side groups was reported by Tilley.10 Under these same
conditions, the alkylated polymer 8 experienced a more rapid
degradation with a noticeable colour change from a yellow
semisolid texture to a white powder; possibly this represents
Fig. 2 119Sn NMR (C6D6) of 7.
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Fig. 3 GPC of polymer 7 in THF.
Fig. 4 UV-Vis (normalized) of 6–8 alternating polymers compared to the
homopolymer 5 in THF.
stannanes32 and a polyferrocenyldistannane.36 Polymers 5, 6 and
8 are found in a region more typical of most polystannanes
(d = À190 to À210 ppm) with one exception.1,2,8–11,13,15,18–20,22,24 absorb light energy that might damage Sn–Sn bonds. As the
The Sn(n-Bu)2 chemical shift of 8 (d = À235 ppm) was displaced repeat unit consists of at least two dialkyl substituents (Me or
upfield by a further 45 ppm relative to 5 (d = À190 ppm).
n-Bu), the polymer remains reasonable soluble in solution.
There is no evidence (NMR) for either end group in any of Fig. 4 shows the UV-Vis spectra of the alternating polymers as
the polymers prepared by condensation. As all efforts were well as for the homopolymer 5 prepared by condensation,
made to keep samples away from reactive solvents or moisture, which is in good agreement with literature values.19
it is presumed that one of the terminal ends contains a Sn–H
Polystannanes 6 and 7 comprised of alternating alkyl and
bond and, the other a Sn–NEt2 containing unit. Alternatively, aryl substituents display a small redshift (10–15 nm) of their
termination maybe the result of macrocyclic cyclization and an s–s* transition relative to 5 (lmax = 379 nm) while a blue shift
absence of end groups.
of similar magnitude (16 nm) was observed for the mixed alkyl
DSC analysis of 5 produced by our condensation polymer- polymer 8. The redshift of polymers 6 and 7 is likely a con-
ization shows a nearly identical glass transition (Tg = 1 1C) to sequence of s + p conjugation from the Ph substituents
polymers of 5 produced by reductive coupling.11 Polymer 5 attached to Sn.27
recovered from the catalyst free condensation polymerization
A new condensation route to homopolystannanes and pre-
described earlier is a yellow coloured solid absent of 5- and viously unknown alternating polystannanes has been demon-
6-membered cyclic stannanes (GPC, NMR).4,9,18 The alternating strated. This thermally favourable condensation reaction offers
polystannanes 6 and 7 (Table 1) display higher glass transitions a relatively straightforward process and enables design features
(6, Tg = 117 1C, 7 Tg = 118 1C) compared to the homopolymer that could lead to even more light and moisture stable
(5, Tg = 1 1C), while a slightly lower glass transition (Tg = À6 1C) polystannanes.
was detected for the mixed dialkyl polystannane 8. Like 5,
This work supported by Ryerson University as well as an
polymers 6, 7 and 8 appear to be amorphous with no detectable NSERC Discovery grant. We would also like to thank the
Chemistry and Biology Department for their support and the
melt transition in the scan range (À50 to 225 1C).
The GPC analysis (Table 1) for the condensation polymers editorial support provided by Dr Robert Gossage, Dr Russell
(5–8) show PDI’s between 2 and 3 with the value expected for an Viirre, Dr Timothy Burrow (NMR). DF would like to thank the
ideal condensation polymerization to be closer to 2. With the Department of Chemistry at the Universidade Federal De Santa
exception of the 7, the molecular weights of polymers prepared Catarina, Brazil for their support.
by condensation are lower than polystannanes prepared by
dehydrocoupling reactions. The triple detection GPC traces of
Notes and references
1 W. Zou and N. Yang, Polym. Prepr., 1992, 33, 188.
2 N. Devylder, M. Hill, K. C. Molloy and G. J. Price, Chem. Commun.,
1996, 711–712.
3 M. Okano, N. Matsumoto, M. Arakawa, T. Tsuruta and H. Hamano,
Chem. Commun., 1998, 1799–1800.
4 P. R. Deacon, N. Devylder, M. S. Hill, M. F. Mahon, K. C. Molloy and
G. J. Price, J. Organomet. Chem., 2003, 687, 46–56.
5 A. Mustafa, M. Achilleos, J. Ruiz-Iban, J. Davis, R. E. Benfield,
R. G. Jones, D. Granjean and S. J. Holder, React. Funct. Polym.,
2006, 66, 123–135.
polymers (Fig. 3) appear to be monomodal for the most part.
30,31
Sita et al.,12 and others7–13,14–20,22–27 have
¨
Drager et al.,
observed a red shift of the s–s* lmax transition with increasing
catenation. What differs in the previous studies with this work
is that two of the polymers (6, 7) have two phenyl rings near
each Sn–Sn bond with one tin possessing a direct Sn–Caromatic
bond. The aromatic rings are themselves capable of s + p
orbital mixing with Sn, but can also act as a chromophore to
7122 | Chem. Commun., 2015, 51, 7120--7123
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6 D. Miles, T. Burrow, A. Lough and D. Foucher, J. Inorg. Organomet. 26 M. Trummer, T. Nauser, M.-L. Lechner, F. Uhlig and W. Caseri,
Polym. Mater., 2010, 20, 544–553. Polym. Degrad. Stab., 2011, 96, 1841–1846.
7 M. Trummer, D. Solenthaler, P. Smith and W. Caseri, RSC Adv., 27 High molecular weight polystannanes 6 and 7 containing alternat-
2011, 1, 823–833.
ing alkyl (n-Bu, or Me) and Ph substituents attached to tin centers in
the backbone display ready solubility (420 mg mLÀ1) in common
organic solvents (DCM, toluene, THF, C6H6). This is in contrast to
an earlier report of poly(diphenylstannane) which is effectively
insoluble in organic solvents even at low molecular weights
(B7000 Da). See V. Lu and T. D. Tilley, Macromolecules, 1996, 29,
5763–5764.
8 M. Trummer, J. Zemp, C. Sax, P. Smith and W. Caseri, J. Organomet.
Chem., 2011, 696, 3041–3049.
9 T. Imori and T. D. Tilley, J. Chem. Soc., Chem. Commun., 1993,
1607–1609.
10 T. Imori, V. Lu, H. Cai and T. D. Tilley, J. Am. Chem. Soc., 1995, 117,
9931–9940.
11 T. D. Tilley and T. Imori, High molecular weight polystannanes by 28 W. H. Carothers, Trans. Faraday Soc., 1936, 32, 39–49.
metal catalysed dehydropolymerization, US Pat., 5488091A, 1996.
12 J. R. Babcock and L. R. Sita, J. Am. Chem. Soc., 1996, 118,
12481–12482.
29 M. Trummer, D. Solenthaler, P. Smith and W. Caseri, RSC Adv.,
2011, 1, 823–833.
¨
30 S. Adams and M. Drager, Angew. Chem., Int. Ed. Engl., 1987, 26, 1255–1256.
¨
31 S. Adams and M. Drager, Main Group Met. Chem., 1988, 11, 151–180.
13 V. Lu and T. D. Tilley, Macromolecules, 1996, 29, 5763–5764.
14 H.-G. Woo, J.-M. Park, S.-J. Song, S.-Y. Yang, I.-S. Kim and W.-G. 32 M. Herberhold, U. Steffl, W. Milius and B. Wrackmeyer, Angew.
Kim, Bull. Korean Chem. Soc., 1997, 18, 1291–1295.
15 V. Y. Lu and T. D. Tilley, Macromolecules, 2000, 33, 2403–2412.
16 N. R. Neale and T. D. Tilley, Tetrahedron, 2004, 60, 7247–7260.
17 S. M. Thompson and U. Schubert, Inorg. Chim. Acta, 2004, 357,
1959–1964.
18 F. Choffat, P. Smith and W. Caseri, J. Mater. Chem., 2005, 15,
1789–1792.
Chem., Int. Ed. Engl., 1996, 35, 1803–1804.
33 S. Dobson, P. D. Howe and P. Floyd, Mono-and disubstituted methyltin,
butyltin and octyltin compounds, World Health Organization (WHO),
2006, www.inchem.org/documents/cicads/cicads/cicad73.pdf.
34 The hydrostannolysis reaction leading to the formation of a Sn–Sn
bond from the reaction of tin hydrides with tin amides was first
described and the kinetics and mechanism investigated in detail by
H. M. J. C. Creamers, F. Verbeek and J. G. Noltes, J. Organomet.
Chem., 1967, 8, 469–477 who proposed the reaction proceeds by an
electrophilic hydrostannolysis of the tin(IV)–amide bond from the
reaction with tin(IV) hydride. They also conclusively ruled out a
radical mechanism by carrying out reactions in the presence of
radical traps in which no radical products were observed. The
authors of this study suggest the formation of the Sn–Sn bond
proceeds by either an SE2 (solvent assisted) or SF2 (four center)
mechanism.
¨
19 F. Choffat, S. Kaser, P. Wolfer, D. Schmid, R. Mezzenga, P. Smith
and W. Caseri, Macromolecules, 2007, 40, 7878–7889.
20 F. Choffat, P. Smith and W. Caseri, Adv. Mater., 2008, 20, 2225–2229.
21 J. Beckmann, A. Duthie, M. Grossman and A. Semisch, Organo-
metallics, 2008, 27, 1495–1500.
22 F. Choffat, Y. Buchmu¨ller, C. Mensing, P. Smith and W. Caseri,
J. Inorg. Organomet. Polym. Mater., 2009, 19, 166–175.
23 A. Khan, R. A. Gossage and D. A. Foucher, Can. J. Chem., 2010, 88,
1046–1052.
24 F. Choffat, P. Wolfer, P. Smith and W. Caseri, Macromol Mater. Eng., 35 M. Trummer, F. Choffat, P. Smith and W. Caseri, Macromol. Rapid.
2010, 295, 210–221. Commun., 2012, 33, 448–460.
25 M. Trummer, D. Solenthaler, P. Smith and W. Caseri, RSC Adv., 36 J. Ward, S. Al-Alul, S. Harrypersad and D. A. Foucher, Can. J. Chem.,
2011, 1, 823–833.
2014, 92, 525–532.
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