Metal-Catalyzed Dehydropolymerization of Secondary Stannanes to High Molecular Weight Polystannanes
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Source and publish data:
Journal of the American Chemical Society p. 9931 - 9940 (1995)
Update date:2022-08-16
Topics:
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Authors:
Imori, Toru
Lu, Victor
Cai, Hui
Tilley, T. Don
Article abstract of DOI:10.1021/ja00145a001
The first high molecular weight polystannanes, H(SnR2)nH (R = nBu, nHex, nOct), result from dehydropolymerization of secondary stannanes R2SnH2 by zirconocene catalysts.Good catalysts include zirconocenes based on both CpCp*Zr (Cp* = η5-C5Me5) and Cp2Zr fragments, and the most active catalyst with respect to production of high molecular weight polystannanes was Me2C(η5-C5H4)2ZrMe.The latter catalyst produced H(SnnBu2)nH chains (Mw/Mn = 66 900/20 300) that were contaminated by ca. 18percent (by weight) low molecular weight cyclic oligomers.Lower molecular weights resulted from dehydropolymerizations of Me2SnH2, PhMeSnH2, and Ph2SnH2.At room temperature, H(SnR2)nH (R = alkyl group) polystannanes have λmax values at ca. 380-400 nm, attributed to ? --> ?* transitions.Thermal gravimetric analyses on the polystannanes reveal that these polymers are as thermally stable as related poly(dialkylsilane)s and have onset temperatures for thermal decomposition in the range 200-270 deg C, under both nitrogen and air.The H(SnnBu2)nH polymer has been shown to be a good precursor to SnO2, as shown by bulk pyrolysis in air (ceramic yield: 56percent).Preliminary results also indicate that these polymers may be useful as precursors to elemental tin.The polystannanes are photosensitive, and their photobleaching behavior has been characterized by UV-vis spectrometry and 119Sn NMR spectroscopy, which demonstrated that H(SnnBu2)nH is photochemically depolymerized to a 2:1 mixture of cyclo-(SnnBu2)5 and cyclo-(SnnBu2)6.The polymers H(SnnHex2)nH and H(SnnOct2)nH exhibit thermochromic behavior which is visibly evident as a discoloration from yellow to colorless upon warming above room temperature.This reversible behavior is associated with an abrupt change in λmax (e.g., from 402 to 378 nm for films of H(SnnOct2)nH) and a phase change at ca. 40 deg C (by differential scanning calorimetry).Thin films of H(SnnBu2)nH and H(SnnOct2)nH on glass slides were doped by exposure to SbF5 vapor to conductivities of 10-2 and 0.3 S cm-1, respectively.Preliminary experiments suggest that the dehydropolymerization occurs by a ?-bond metathesis mechanism involving four-center transition states.A previous report on production of high molecular weight poly(dibutylstannane) by the reductive coupling of nBu2SnCl2 by Na/15-crown-5 was reinvestigated and found to produce only low molecular weight material with Mw/Mn = 2400/1200.
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Full text of DOI:10.1021/ja00145a001