SYNTHESIS, STRUCTURES, AND PROPERTIES OF STRAINED, SILICON-BRIDGED [1]FERROCENOPHANES WITH METHYLATED CYCLOPENTADIENYL RINGS

A series of strained, silicon-bridged [1]ferrocenophanes with methylated cyclopentadienyl rings; which are prospective monomeric precursors of poly(ferrocenylsilanes) via ring-opening polymerization, has been synthesized and characterized. The dimethylated silicon-bridged [1]ferrocenophanes Fe(eta-C(5)H(3)Me)(2)SiMe(2) (4) were prepared as a mixture of isomers via the reaction of Fe(eta-C(5)H(3)MeLi)(2) . TMEDA (TMEDA = tetramethylethylenediamine) with Me(2)-SiCl2. The 3,3'-syn isomer 4a was cleanly isolated via low-temperature crystallization. The tetramethylated silicon-bridged [1l]ferrocenophane Fe(eta-C(5)Me(4))(eta-C5H4)SiMe(2) (6) and the octamethylated silicon-bridged [1]ferrocenophane Fe(eta-C(5)Me(4))(2)SiMe(2) (8) were prepared via the reaction of FeCl2 with Li-2[(C(5)Me(4))(C5H4)SiMe(2)] and Li-2[(C(5)Me(4))(2)SiMe(2)], respectively. X-ray diffraction studies of 4a and 8 revealed that these molecules possess highly strained ring-tilted structures. Comparison with the previously determined structure of Fe(eta-C5H4)(2)-SiMe(2) (1) revealed that, with increasing ring methylation, the tilt angle between the planes of the cyclopentadienyl ligands decreases significantly (1, 20.8(5)degrees; 4a, 18.6(3)degrees; 8, 16.1(3)degrees), the angle between the cyclopentadienyl ligand and the ipso cyclopentadienyl carbon-silicon bond increases (1, 37.0(6)degrees 4a, 39.1(2)degrees 8, 40.3(2)degrees), and the Fe-Si distance decreases (1, 2.690(3) Angstrom; 4a, 2.6767(8) Angstrom; 8, 2.652(1) Angstrom. Cyclic voltammetric analysis of 1, 4a, 6, and 8 in CH2Cl2 revealed that each ferrocenophane exhibits a reversible, one-electron oxidation. Analysis of 1 and 8 by Fe-57 Mossbauer spectroscopy in conjunction with the crystallographically determined trend in Fe-Si distances provided evidence for the presence of a weak dative Fe-Si interaction which is more substantial in the latter species than the former. Crystals of 4a (C14H18FeSi) are orthohombic, space group Pbca, with a 14.775(2) Angstrom, b = 10.655(2) Angstrom, c = 16.882(2) Angstrom, V = 2657.7(7) Angstrom(3), and Z = 8. Crystals of 8 (C20H30FeSi) are monoclinic, space group C2/c, with a = 15.687(3) Angstrom, b = 9.194(1) Angstrom, c = 26.612(5) Angstrom, beta = 98.65(2)degrees, V = 3653(6) Angstrom(3), and Z = 8.