Poly(di-n-propylsilylene) and poly(diethylsilylene-co-di-n-propylsilylene) have been investigated by Raman, IR, and UV spectroscopy, TGA, DSC, wide angle X-ray diffraction, and solid state l8C and MSi NMR. No IR-Raman coincidences were found in the Si-Si or Si-C stretching regions for (n-Pr2Si)", demonstrating that the main chain has a planar all-trans configuration, T" consistent with the UV absorption at 355 nm. From the Si-C stretching bands, all of the n-propyl groups appear to have the same conformation.(n-Pr2Si)n(Et2Si)m also has the T. configuration at room temperature, and a tetragonal crystal lattice. Both polymers undergo a first-order transition to a mesomorphic phase, above 222 °C for (n-Pr2Si)" and above 113 °C for the copolymer. ylsilylene) [ ( Me2Si) "] 3 poly (diethylsilylene) [(Et^SDJ,3-5 and poly(di-n-propylsilylene) [(n-Pr2Si)"]4 all have been shown to have a planar, all-trans zigzag (T") structure for the main polymer chain. Poly(di-n-butylsilylene)6-7 and poly(di-n-pentylsilylene)7•8 however, adopt 7/3 helical structures under ambient conditions. The all-trans structure returns with six-carbon side chains; the wellstudied poly(di-n-hexylsilylene) has the T" structure up to its transition temperature at 42 °C and a mesomorphic structure above that.9-11 Poly(di-n-heptylsilylene) also takes the T" structure at ambient temperatures.10'12Here we report a study of (n-Pr2Si)" and the related polysilane copolymer poly(diethylsilylene-co-di-n-propylsilylene), (n-Pr2Si) "(Et2Si) m. Our results confirm the T" conformation for poly(di-n-propylsilylene), reported from earlier electron and X-ray diffraction studies,4 and show that at room temperature the copolymer also has a planar zigzag conformation of the silicon chain. These results also allow a closer description of the structures, especially at elevated temperatures where both polymers have a firstorder phase transition.
II. Results and DiscussionA. Poly(di-n-propylsilylene) at Room Temperature. The ultraviolet spectrum of (n-Pr2Si)" was previously reported to consist of three peaks at 312, 324, and 360 nm, but the earlier authors considered that impurities
The pyrolysis of [NEt 4 ] 2 [Bi 4 Fe 4 (CO) 13 ] in MeCN cleanly produces the square-pyramidal cluster [NEt 4 ] 2 [Fe 3 (CO) 9 -Bi 2 ]. An X-ray crystallographic study was carried out on this compound at 223 K (orthorhombic space group P2 1 2 1 2 1 (No. 19) with a ) 7.280(1) Å, b ) 20.642(4) Å, c ) 22.365(4) Å, V ) 3360.9(10) Å 3 , and Z ) 4) showing it to belong to the square-pyramidal class of 50 electron E 2 M 3 clusters. When the E atom bears a substituent, the E‚‚‚E distances are shorter, and the M basal -E-M basal angles are larger than when a lone pair of electrons resides on E. It has become fashionable to attribute such changes in bond parameters to the formation of E-E bonding, but rehybridization of the main group element could also explain this phenomenon. In order to shed light on this fundamental aspect of bonding in E-M clusters, a detailed extended Hückel molecular orbital analysis was undertaken. It appears that weak E-E bonding may be present in some cases but that this is not the dominant effect.
The reaction of PPN[Bi2Co4(C0),,(p-CO)] with Mo(C0)3(toluene) in THF yields, after slow oxidation, the large clusters, ( ~~~) ~[ ~~~~O ~( ~~) ~( ~-~~) ~] and (PPN)2[Bi8Co14(C0)12(p-C0)8] [PPN = bis(triphenylphosphine)iminiuml which exhibit close-packed arrays of main group elements and transition metal atoms.
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