The polymerization behavior of [1]- and [2]ferrocenophanes containing silicon atoms in the bridge: comparison of the molecular structure of the strained, polymerizable cyclic ferrocenylsilane Fe(.eta.-C5H4)2(SiMe2) with that of the cyclic ferrocenyldisilane Fe(.eta.-C5H4)2(SiMe2)2

The strained cyclic ferrocenylsilane Fe(η-C5H4)2(SiMe2) (1), containing a single silicon atom in the bridge, undergoes thermal ring-opening polymerization at 130 °C to yield the high-molecular-weight poly(ferrocenylsilane)[Fe(η-C5H4)2(SiMe2)]n (2). In contrast, the analogous cyclic ferrocenyldisilane Fe(η-C5H4)2(SiMe2)2 (3), with two silicon atoms in the bridge structure, is resistant to polymerization under the same conditions. In order to probe the reasons for the dramatic difference in polymerization behavior between 1 and 3, the molecular structures of these species have been determined by single-crystal X-ray diffraction. This has allowed the first structural comparison of [1] - and [2]ferrocenophanes with the same bridging atoms (silicon) and substituents (methyl). The cyclic ferrocenylsilane 1 was found to possess a strained, ringtilted structure with an angle between the planes of the cyclopentadienyl rings of 20.8(5)°. In addition, considerable distortion from planarity was detected for the cyclopentadienyl carbon atom bonded to the bridging silicon moiety. In contrast, the cyclic ferrocenyldisilane 3 possesses a much less distorted structure and the corresponding tilt angle between the cyclopentadienyl ligands is only 4.19(2)°. Crystals of 1 are monoclinic, space group P21/a, with a = 7.438(3) Å, b = 10.322(4) Å, c = 15.575(6) Å, β = 99.04(3)°, V = 1180.9(8) Å3, and Z = 4. Crystals of 3 are monoclinic, space group P21/n, with a = 10.433(2) Å, b = 8.703(1) Å, c = 17.243(2) Å, β = 107.32(1)°, V = 1494.6(12) Å3, and Z = 4.