Metal-Catalyzed Dehydropolymerization of Secondary Stannanes to High Molecular Weight Polystannanes

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 (M_w/M_n = 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 λ_max 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 λ_max (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 M_w/M_n = 2400/1200.